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Isotropic rubber moulding
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ISOTROPIC RUBBER MOULDING
by
Timothy M. D. Buffham B. Eng. M. Sc.
A Doctoral thesis submitted in partial fulfilment of the requirements for the award of
the degree of Doctor of Philosophy
of the
Loughborough University
August, 1999
-,
Supervisor:j philip K: Freakley; Ph', D., FPRI . ..
Institute of Polymer Technology and Materials Engineering
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For my parents, Bryan and Dolly. Thank you.
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Acknowledgements
I received much help, support and encouragement during the course of this work. I
. would like to acknowledge the input from, and thank, Roger W Collins with whom I
had many lengthy conversations regarding the moulding system and its operation.
I would like to thank all the members of IPTME that have helped me during the
course of this work especially the members of RuPEC, my supervisor Philip
Freakley, Jane Clarke and Oavid Southwart whose suggestions, criticism and
knowledge of rubber and the rubber industry have been invaluable.
Other staff who deserve my thanks and without whose help things would have been
much more difficult are: Barry Clarke who helped so much in the RuPEC lab. and
IPTME pilot plant, R P Owens for his help with the physical testing equipment and
demonstrating its use, T Atkinson for his help with the photography for the project
and this thesis.
The engineering skills and knowledge of C Lines, K Ellison and M Hallam in IPTME
workshop and A. Trotter in the pilot plant were indispensable for the project, the
commissioning and modification of the prototype and helping to keep it in operation.
I would like to thank James Walker & Co. Ltd. For supplying compounds and O-rings
for the trials and comparison.
I would also like to acknowledge the contributions of the project partners Iddon Bros.
Ltd. and Euro-projects (L TTC) Ltd. and the OTI and EPSRC for funding the under
the EEM Link scheme.
Finally, I would like to thank my family for all their help and support over the last 32
years.
Abstract
The current work was initiated to develop, understand and optimise a novel
computer controlled, automated, flexible compression moulding system primarily for
the production of fluid seals. A prototype moulding system was designed and built
for the study. It was used to process a range of rubber compounds for process
evaluation.
The rheological properties of the compounds were determined with a Negretti TMS
biconical-rotor rheometer and the cure characteristics were obtained with a Wallace
Shawbury precision cure analyser. The data was used in fluid flow module of a
finite element analysis package (EMRC NISAII) to simulate the flow of uncured
elastomer in the system enabling the prediction of temperature rise and pressure
drop to aid the design and development. Temperature predictions currently
over-estimate the measured values by 10 - 20%.
The prototype moulding system, consisting of a preforming dispenser, press and
computer control system, was used to produce mouldings with the range of
selected compounds. The dispensed preforms and mouldings were studied for
signs of anisotropy. These preforms and mouldings were, however, still some way
from ideal, the preforms exhibiting an elongated lozenge shaped cross-section with
a typical shape factor of 3 and a large degree of post preforming shrinkage due to
the largely circumferential molecular orientation. The dispenser was redesigned
giving further consideration to managing the flow history and therefore molecular
orientation, relaxation and recovery of the material. The second-generation
dispenser produced considerably lower post preform shrinkage and a more
desirable cross section with a shape factor much closer to 1 (1.25).
A comparison between the conventionally moulded parts and those produced with
the prototype system was undertaken. The measurement of mechanical properties
! .of vulcanisates by tensile testing and solvent swelling methods, and mould
•. shrinkage were used to determine the extent of the anisotropy caused by molecular
" orientation.
:
Due to the elimination of in-mould flow isotropic O-rings were produced showing
uniform properties under testing and substantially better properties under testing
and substantially better properties than conventionally moulded O-rings. Sheet
samples produced by both systems were however, similar in all respects because
the elimination of flow in a flat mould during closure and compression was not
achieved.
The secondary objectives of automating the compression mould in process and the
production of flash free components, utilising the inherent viscoelastic properties of
elastomers to advantage, were also met.
Table of Contents
1. Introduction ......................................................................................................... 1 1.1 Why is Reduction of Anisotropy Important? .................................................... 1 1.2 Objectives ........................................................................................................ 2
2. Literature Review ................................................................................................ 3 2.1 Introduction ...................................................................................................... 3 2.2 Types of Polymer ............................................................................................ 4
2.2.1 Rubbers (Elastomers) .............................................................................. 4 2.2.2 The Structure and Morphology of Rubber Molecules ............................... 5 2.2.3 The Structure and Properties of Rubber (and Rubberlike MateriaL) ........ 7
2.2.3.1 Mechanical Properties .......................................................................... 7 2.2.3.1.1 Elasticity ...................................................................................... 10 2.2.3.1.2Viscoelasticity and Stress Relaxation .......................................... 11
2.2.3.2Thermal Properties ............................................................................. 14 2.2.3.2.1 Thermal Conductivity(A. or !<c) and Thermal Expansivity(ae) ........ 14
2.2.3.3Electrical Properties ........................................................................... 15 2.3 Reinforcement (and State of Mix) .................................................................. 16
2.3.1 Reinforcing Materials (and Particle Size) ............................................... 17 2.3.2 Reinforcing Mechanisms ........................................................................ 19
2.3.2.1 Bound and Occluded Rubber ............................................................. 19 2.3.2 .2Hydrodynamic Theories ...................................................................... 26 2.3.2.3Rubber Elasticity and Strain Amplification .......................................... 28 2.3.2.4lnterparticle Chain Breakage and Chain Slippage Mechanisms ......... 30
2.4 Cross-linking (Vulcanisation and Cu re) ......................................................... 32 2.4.1 Vulcanisation Mechanisms, Rates and Times ........................................ 34
2.5 Production and Manufacturing Processes ..................................................... 37 2.5.1 Pre-moulding Processes - Mixing and Milling ......................................... 38
2.5.1 .1 The Mixing Process ............................................................................ 38 2.5.1.2Mixing Equipment ............................................................................... 39
2.5.1.2.1 The Internal Mixer ........................................................................ 39 2.5.1.2.2 The External Mixer ...................................................................... 40 2.5.1 .2.3 The Continuous Mixer ................................................................ .40
2.5.1.3Milling and Calendering ...................................................................... 41 2.5.1.4Anisotropy and Pre-moulding Processes .......................................... .41
2.5.2 Moulding Processes ............................................................................... 41 2.5.2.1 Moulding Methods ............................................................. : ................ 42
2.5.2.1 .1 Compression Moulding ................................................................ 42 2.5.2.1 .2Transfer Moulding ........................................................................ 43 2.5.2.1 .3lnjection Moulding ........................................................................ 45
2.5.2.2Anisotropy in Moulding Processes .................................................... .46 2.5.2.2.1 Anisotropy in Compression Moulding ......................................... .47 2.5.2.2.2Anisotropy in Transfer and Injection Moulding ............................ .48
2.5.3 An isotropy in Special Cases ................................................................... 51 2.6 Summary and Comments on Anisotropy and Orientation in Rubber ............. 52
2.6.1 Causes of Anisotropy in Products and Mouldings .................................. 53 2.7 References .................................................................................................... 54
3. The FORM System ........................................................................................... 60 3.1 FORM System Concept - Overview ............................................................... 60
3.1 .1 The Dispenser Concept ......................................................................... 61
3.1.2 The Press Concept ................................................................................ 62 3.1.3 The Mould Concept.. .............................................................................. 63 3.1.4 The Computer Control System ............................................................... 64 3.1.5 Sequence of Operation .......................................................................... 64 3.1.6 Reduction in the Number of Processing Operations Compared with Conventional Compression Moulding ............................................................... 65 3.1.7 System Configurations ........................................................................... 65
3.1.7.1 Stand-Alone Configuration (Single-Station) ........................................ 66 3.1 .7 .2M ulti-Station Configu ration ................................................................. 67
3.2 Prototype FORM System Description ............................................................ 68 3.2.1 Description of the Dispenser .................................................................. 68 3:2.2 Description of the Press ......................................................................... 71 3.2.3 Description of the Control System .......................................................... 71
3.2.3.1Temperature Control and Set Points .................................................. 72 3.2.3.2Hydraulic Pressure Control ................................................................. 75 3.2.3.3Position Control .................................................................................. 75
3.3 System Operation .......................................................................................... 76 3.3.1 Programming the Dispenser and Press Operation Cycles ..................... 76
3.3.1.1Temperature and Pressure Setting .................................................... 76 3.3.1.2Dispenser Cycle ................................................................................. 77 3.3.1.3Press Cycle ........................................................................................ 78
3.4 References .................................................................................................... 79 4. Experimental ..................................................................................................... 80
4.1 Rubber Compounds ...................................................................................... 80 4.1.1 The Raw Polymers ................................................................................. 81 4.1.2 The Filler - Carbon Black ........................................................................ 82 4.1.3 The Additives ................................. : ................. ; ..................................... 82
4.1.3.1 Activator and Processing Aids ............................................................ 82 4.1.3.2Curatives ............................................................................................ 83 4.1.3.3Antidegradants ................................................................................... 84 4.1.3.4Plasticiser ........................................................................................... 84
4.1.4 Material Preparation ............................................................................... 84 4.1.4.1 Mixing Equipment ............................................................................... 84 4.1.4.2Mixing Procedure and Conditions ....................................................... 85
4.2 Material Properties ........................................................................................ 89 4.2.1 Determination of Rheological Properties ................................................ 89
4.2.1.1 Negretti TMS Biconical Rheometer ................................................... 90 4.2.1.2Measuring Viscous Flow in the TMS - Test Procedure ....................... 91
4.2.2 Determination of Scorch Safety and Cure Times ................................... 92 4.2.2.1 Equipment - The Wallace-Shawbury Precision Cure Analyser (PCA) 93 4.2.2.2Method - Determination of Scorch and Cure Times. with the PCA .... 94
4.2.3 Determination of Specific Heat Capacity at Constant Pressure (Cp) ...... 94 4.2.3.1 Equipment - Differential Scanning Calorimeter (DSC)' ....................... 94 4.2.3.2Method - Determination of Specific Heat Capacity (Cp) ...................... 95
4.2.4 Measurement of Density ........................................................................ 96 4.3 The FORM System Trials .............................................................................. 97
4.3.1 Methods and Procedures for the Operation of the Form Machine (Optimum Operation Procedures) ..................................................................... 97
4.3.1 .1 Dispenser Operation - Procedure for Preforming ............................... 97 4.3.1.1.1 Filling The Meter Cavity ............................................................... 97
ii
4.3.1.1.2 Preform Dispensing ..................................................................... 98 4.3.1.1.3Preform Size Range (Weight) ...................................................... 99 4.3.1.1.4Preform Consistency (Accuracy of Shot Weight) ........................ 99 4.3.1.1.5 Dispensed Preform Temperature ................................................ 99
4.3.1.2Press Operation and Moulding ........................................................... 99 4.3.1.2.1 Press Moulding/Forming Procedure ............................................ 99 4.3.1.2.2 Flash-Free Moulding .................................................................. 100
4.3.2 Test Specimen Production ................................................................... 101 4.3.2.1 Moulding Temperature Offsets ......................................................... 101 4.3.2.2Moulding O-ring Specimens ............................................................. 1 02 4.3.2.3Moulding Sheet Specimens .............................................................. 102
4.4· Physical Testing and Observations ............................................................. 103 4.4.1 General Observations - Preforms and Preforming ............................... 103
4.4.1.1 Preform Shape and Size .................................................................. 103 4.4.2 Mouldings (Product) ............................................................................. 104
4.4.2.1 Product Examination/Inspection ....................................................... 1 04 4.4.2.1.1 Rings ......................................................................................... 104 4.4.2.1.2 Sheet ......................................................................................... 105
4.4.2.2Mould Shrinkage .............................................................................. 105 4.4.2.2.1 Rings ......................................................................................... 105 4.4.2.2.2Sheet ......................................................................................... 106
4.4.2.3Swelling in Good Solvent... ............................................................... 106 4.4.2.3.1 Ring Shape ................................................................................ 107 4.4.2.3.2Volume ...................................................................................... 107
4.4.2.3.2.1 Sheet Samples .................................................................... 107 4.4.2.3.2.20-ring Samples ................................................................... 108
4.4.2.4Compression Set ....... , ...................................................................... 109 4.4.2.5Tensile Testing of Dumbbells Cut from Sheet .................................. 109
4.5 References .................................................................................................. 110 5. Finite Element Modelling (FEA) ...................................................................... 113
5.1 Model Construction ..................................................................................... 113 5.1.1 Geometric Modelling ............................................................................ 114 5.1.2 Meshing (Finite Element Modelling) ..................................................... 115 5.1.3 Boundary and Initial Conditions ............................................................ 118 5.1.4 Units ..................................................................................................... 119 5.1.5 Post-Processing ................................................................................... 119
5.2 Static Heat Transfer .................................................................................... 119 5.2.1 Conditions and Assumptions ................................................................ 119
5.3 Heat Transfer with Incremental Flow (Pseudo-flow) .................................... 120 5.4 Flow Modelling with NISAlFLUID ................................................................. 121
5.4.1 Dispenser Fill Modelling ....................................................................... 121 5.4.1.1 Ring Dispenser Geometry ................................................................ 122
5.4.1 .1 .1 Ring Dispenser Geometry I ....................................................... 122 5.4.1.1.2 Ring Dispenser Geometry II ...................................................... 123
5.4.1.2Sheet Dispenser Geometry .............................................................. 123 5.4.2 Dispense Modelling .............................................................................. 125
5.4.2.1 Ring Dispense Modelling .................................................................. 130 5.4.2.1.1 Ring Dispenser Altemative Geometries ..................................... 130
5.4.2.2Sheet Dispense Modelling ................................................................ 131 5.5 FLUID - STATIC Interface ........................................................................... 132
iii
5.6 References .................................................................................................. 133 6. Results and Discussion ................................................................................... 133
6.1 Mixing and Material Characterisation .......................................................... 133 6.1.1 The Factors Affecting Processibility ..................................................... 133 6.1.2 Mixing ................................................................................................... 134 6.1.3 Compound Rheology (Negretti TMS Biconical-Rotor Rheometer) ....... 135 6.1.4 Physical Constants ............................................................................... 139
6.1 .4.1 Specific Heat Capacity ..................................................................... 139 6.1 .4.2Density Measurement ...................................................................... 140
6.1.5 Scorch and Cure (Vulcanisation) Time ................................................. 140 6.2 Finite Element Modelling ............................................................................. 141
6.2.1 Heat Transfer Modelling ....................................................................... 142 6.2.1.1 Reservoir Geometry ......................................................................... 143 6.2.1.2Static Heat Transfer (Pseudo-Flow Simulation) ............................... 144
6.2.2 Fluid Flow Modelling ............................................................................. 146 6.2.2.1 Dispenser Flow (To Fill the Meter Cavity) ......................................... 146
6.2.2.1.1 Initial Design (Flow Geometry I of Chapter 5) ............................ 146 6.2.2.1.2 Modified Dispenser (Flow Geometry 11 of Chapter 5) ................ 150
6.2.2.2Dispense Flow (Metering and Preforming) ....................................... 156 6.2.2.2.1 What if? Modelling of Other Possible Meter Cavity Configurations ............................................................................ 162
6.2.3 Prediction of Preform Shape Change Using Fluid Flow and Static Finite Element Modelling in Combination ................................................................. 164
6.3 Experimental Work with the FORM System ................................................ 165 6.3.1 Preforming with the FORM Dispenser .................................................. 165
6.3.1.1 Filling the Meter Cavity and Preforming ........................................... 165 6.3.1.2Preform Consistency (Shot-to-Shot Repeatability) ........................... 166 6.3.1.3Preform Size Range (Weight) .......................................................... 168 6.3.1.4Dispensed Preform Temperature ..................................................... 168 6.3.1.5Preforming - Observations ................................................................ 172
6.3.1.5.1 Curtaining .................................................................................. 172 6.3.1.5.2Preform Shape .......................................................................... 172 6.3.1.5.3Lobing ........................................................................................ 173
6.3.1.5.3.1 Lobing and Molecular Orientation in O-ring Preforms ......... 176 6.3.1.5.4Preform Shrinkage ..................................................................... 179
6.3.2 Elimination of Lobes and Preform Shrinkage with a Modified Dispenser ............................................................................................. 180
6.3.2.1 Preforming with the Modified Dispenser ........................................... 182 6.3.2.1.1 Preform Shape .......................................................................... 182 6.3.2.1.2Preform Shrinkage ..................................................................... 184
Moulding - Observations, Problems and Defects ............................................ 185 6.3.3.1 Flash Free Moulding ......................................................................... 186
6.3.3.1.1 Phased Closure .................................................... , .................... 187 6.3.4 Product Testing .................................................................................... 189
6.3.4.1 Examination of Moulded Product ..................................................... 189 6.3.4.2Physical Testing of O-Rings ............................................................. 190
6.3.4.2.1 Swelling in solvent ..................................................................... 190 6.3.4.2.2 Compression Set ....................................................................... 194 6.3.4.2.3Mould shrinkage ........................................................................ 194
6.3.4.3Physical Testing of Sheet ................................................................. 195
iv
6.3.4.3.1 Mould shrinkage ........................................................................ 195 6.3.4.3.2Tensile Testing ................................................. : ........................ 196
6.3.4.4Summary of Preform Production and Physical Testing .................... 199 6.4 References .. ................... , ..................................................... ....................... 199
7. Conclusions .................................................................................................... 203 7.1 Suggestions for Further Work ..................................................................... 204
v
Appendix A Appendix B Appendix C Appendix D Appendix E Appendix F Appendix G Appendix H Appendix I
Chapter One
1. Introduction
Anisotropy (or 'grain') in moulded parts has long been known about. It was studied
in the 1940's and B.S. 902 Part A2 recommends that tensile test dumbbells be cut in
the direction of the grain. The term 'grain' seems to encompass orientation, residual
stresses, the anisotropy of shrinkage during cooling and other mechanical effects
that vary depending on the direction in which the test is carried out. This anisotropy
is induced into the rubber compound in the various processing, shaping and cross
linking stages of the rubber production process.
The control of polymer processing is a notoriously difficult problem. Rubber product
manufacture is typically low technology batch production and suffers from all the
common faults associated with this type of industry. The type of problem these
processes suffer from are: (i) batch-to-batch variation, (ii) poor process monitoring, it
being difficult if not impossible to cure 'on-line' non destructively, and (iii) the non
linear behaviour of the process.
In this project an attempt has been made to relate anisotropy of moulded rubber
parts to their processing history and to develop a rubber moulding process for
producing products with little or no anisotropy. A novel compression moulding
system was designed, manufactured and evaluated during the project.
1
1.1 Why is Reduction of Anisotropy Important?
Anisotropy is particularly relevant to the production of moulded precision parts
where the requirement for dimensional accuracy calls for careful control of moulding
'shrinkage'. Such critical components are used in the oil industry where physical
properties and dimensional accuracy requirements are strict. Another product area
of particular interest to this project is the production of large diameter O-rings which
are notoriously difficult to manufacture to high tolerances.
Mould design, and manufacturing cost and time, are affected by the anisotropic
behaviour of rubber. Differential directional shrinkage in mouldings can lead to
moulds being 'jiggled', modified or re-manufactured so that the parts produced have
the desired finished shape (Le. an accurate circular component may have been
produced in a mould cavity that is non-circular).
An isotropic molecular structure will provide a basis for consistent physical
properties and dimensions of finished mouldings.
1.2 Objectives.
The main purposes of this research are:
[1] to devise models which relate the anisotropy of rubber mouldings to the
process strain and heat history;
[2] to evaluate a novel compression moulding system that has been designed to
minimise anisotropy in moulded rubber products.
[3] to automate the compression moulding process into a highly automated, high
volume manufacturing process to compete with injection moulding.
[4] to develop a proposed method of producing flash-free mouldings which will
eliminate the need for post-demoulding finishing operations.
2
Chapter Two
2. Literature Review
2.1 Introduction
In this chapter it is proposed to review both the factors which determine the intensity
and amount of orientation and to explore the properties which are affected by
orientation. Consideration is given to the molecular structure and morphology and
properties of elastomers, highlighting some specific properties of rubber and
rubberlike materials, their elasticity, viscoelasticity and the phenomenon of stress
relaxation. It is this type of behaviour which sets elastomers apart from other
polymers and engineering materials and makes them unique, useful, unusual and
interesting. This is followed by a detailed look at the controversial, yet highly
important, topic of particulate 'reinforcement' which is one of the factors that
contribute to the physical properties, the in-service life and behaviour characteristics
of both un-cured and vulcanised elastomers. Cross-linking and cross-link structure is
also considered briefly. A section discussing the manufacturing processes of rubber
products is also included. Such operations as mixing and milling (and calendering)
are considered as it is from these premoulding operations that a portion of the
anisotropy in components stems. Moulding, in its various different forms, is
considered because flow in processing especially 'in-mOUld flow' is deemed by
many to be a major contributor to anisotropy in rubber (and other polymer) products.
Anisotropy and molecular orientation in elastomers is a theme that runs throughout
this review with a view to determining causes, effects and, hence, possible
remedies.
3
2.2 Types of Polymer
Thermoplastics are materials which soften and flow (melt) upon the application of
heat and pressure and can be processed and formed by a number of extrusion and
moulding techniques. The thermoplastic melt is then cooled and the desired shape
frozen in. Most thermoplastic materials can be remoulded many times. Thermosets
and rubbers have to undergo an irreversible chemical process, cross-linking, usually
initiated by the application of heat, forming an interconnected network of polymer
molecules, to give them their ultimate properties. In thermoplastics molecular
orientation is frozen in by cooling whereas in rubber and thermosets orientation is
fixed in by cross-linking in a heating process which increases molecular activity and
therefore aids the recovery of orientation but the low modulus of rubber means that
the effects of orientation will be much more apparent than in thermoplastics or
thermosets.
2.2.1 Rubbers (Elastomers)
Elastomers are perhaps best known for their capacity for very large and rapid
recovery of deformation, a phenomenon that is familiar to almost everybody and the
term 'rubber', broadly, covers any of the materials that stretch considerably and
recover more or less completely. The major polymers that occupy this group are:
natural rubber (NR) and its synthetic counterpart stereoregular polyisoprene (IR or
cis-1,4-polyisoprene), styrene-butadiene rubber (SBR), butyl rubber (IIR),
polychloroprene (CR or neoprene), acrylonitrile rubber (NBR or nitrile). Elastic
behaviour can also be seen in lightly cross-linked amorphous polymers, such as
poly(methyl methacrylate) (PMMA) and polystyrene (PS) above their Tg, albeit only
in short term experiments. Elastomers are network polymers and require curing (or
cross-linking) to form an entire network that could be considered a single, large
network molecule (or macromolecule). Cross-links are chemical bonds; long polymer
chains are connected together by small molecules. In the case of sulphur
vulcanisation, links between the polymer chains consist of one or more (up to about
eight) sulphur atoms. It is the C=C double-bond unsaturation in linear chain
polydienes and random copolymers that allows the irreversible cross-linking of the
polymer chains through liberation of one of the valences.
4
2.2.2 The Structure and Morphology of Rubber Molecules
The simplest and most basic form a polymer takes and therefore the easiest to
visualise is the linear polymer which, as the name suggests, is a long continuous
chain of the repeated unit monomer (formed by linking carbon atoms together or by
linking carbon and other atoms together). Many thousands of the monomer units
may be connected together in such a way that they are analogous to a piece of
string or cooked spaghetti (Figure 2-1 (a)). To give an idea of the aspect ratio, the
strand of spaghetti would have to be two to three metres long in order to represent a
polymer molecule. It is this shape, with a large aspect ratio in the order of 103, which
makes rubber susceptible to anisotropy and orientation. The chains may be
branched (Figure 2-1 (b)) or interconnected to form three dimensional tree or
network structures by cross-linking.
Covalent bonds are formed when two atoms share one or more pairs of valence
electrons and therefore they are associated with high energies and very small
Figure 2-1 Schematic of (a) linear, and (b), branched molecular chains
interatomic distances. Covalent bonds between the atoms in a monomer unit and
between the atoms in two adjacent monomer units hold the polymer molecule
together in its configuration. Each carbon atom has four valences or bonds
connecting it to its neighbouring atoms which are angled symmetrically towards the
comers of a tetrahedron at 109.5°. Although the interatomic distance (bond length)
and the bond angles are fixed the chain is flexible because of rotational motion
about the carbon-to-carbon bonds (and each carbon-to-hydrogen bond where
5
present). This flexibility can be used to help account for polymer behaviour such as
creep, melt flow, elasticity and orientation.
In solution and in melt the molecule possesses kinetic energy which is exhibited as
a combination of the following kinds of motion: translation of the molecule as a
whole (macro-Brownian motion), rotation of parts of the molecule (crankshaft motion
or reptation) and vibration of individual bonds which causes the conformation to
change continually. The molecule can, subject to the bond angle and length
restriction and any extemal force, adopt a random arrangement in space.
Branched molecules are more complex than linear molecules and have one or more
side chains (Figure 2-1 (b)). These side chains or branches interfere with the
ordering of the molecules so that crystallinity and orientation will tend to be
decreased. The processability of the branched polymers can also be slightly more
difficult'. Polymer networks (Figure 2-2) can be formed from both linear and
branched molecules cross-linked to other molecules (or sections of the same
molecule). Natural rubber, for example, is predominantly a linear polymer that can
form a polymer network with the addition and reaction with sulphur.
Rubber molecules generally have a carbon 'backbone', they are built up of
monomers which contain carbon-to-carbon bonds and hydrogen atoms. There are
other rubbers which retain the carbon-to-carbon 'backbone' but substitute other
elements, chlorine (Cl), fluorine (F), nitrogen (N) or other groups for hydrogen (H)
atoms.
The separate molecules, or even segments of the same molecule, are attracted to
each other and held in the mass by secondary van der Waals (intermolecular)
forces not primary chemical bonds, metallic, co-ordinate or ionic as are in metals or
ionic solids nor the covalent bonds which are predominant in individual polymer
molecules. The forces causing these secondary bonds are orders of magnitude
smaller than those of covalent bonds and this permits relative molecular motion.
They are much more difficult to define as they are not constant. These force
6
increase in the presence of polar groups and decrease with an increase in the
distance between the molecules.
If, as in the melt, there is a large number of molecules, the spaghetti analogy can be
extended to a saucepan of spaghetti in boiling water. Each piece of spaghetti is
entwined with others and in continuous motion but there are no actual physical
connections between the individual strands. In the case of the polymer molecules
they are entangled but not actually joined to each other and are free to move.
/ i
(a)
I ,
Figure 2-2 Network polymers: (a) loose cross-link network, and (b) tight cross-link network
2.2.3 The Structure and Properties of Rubber (and Rubberlike Material.)
2.2.3.1 Mechanical Properties
The variables stress, strain, time and temperature and their interrelationship are
important in describing the mechanical behaviour of elastomers. Below the glass
transition temperature (Tg) elastomers cannot support tensile strains of more than a
few percent. The microstructure does not allow the molecules to rearrange
internally, consequently failure is clean and sudden and there is no gross prefracture
yield.
7
At any strain below that of failure the deformation is largely reversible and subject to
spontaneous recovery on removal of the applied stress. Hooke's law is obeyed
approximately and this behaviour is described as elastic and the energy expended
to produce the original deformation is totally recoverable. This characteristic is much
the same as that observed in many metals, glasses and ceramics. This relationship
is often written,
cr = Ea (2-1)
where cr is stress, E is Young's modulus (or elastic modulus) and a is strain.
Figure 2-3 (and Figure 2-5) shows typical behaviour that follows the relationship
given in Equation (2-1). Strain, a, is small; stress, cr, however, is large and therefore
Young's modulus, E, is large and indicates that the forces needed to move atoms
60.---------------~--, Failure
50
or 40 E z ::E - 30 t:> .. (f)~ 20
10
O+----+----+----+--~
o 0.01 0.02 0.03 0.04
Strain a
Figure 2-3 Typical stress-strain curve for a polymer such as PM MA at a low temperature (Le. well below its T g.)
from their equilibrium positions by bond stretching, bond compression or by altering
the distance that separates non-bonded, adjacent atoms must also be large.
8
A typical elastomer, in its glassy state, below Tg. has a Young's modulus in the
order of 103 MNm·2 whereas other typical engineering materials (e.g. metals and
carbon fibre) have a Young's modulus that is in the region of 2-3 orders higher at
105_106 MNm·2 .
3-,--------------.
.:-2 'E z ~ t:l In
~ -III 1
O+--~-_+-_+--r_--4
o 1 2 3 Strain ex
4 5
Figure 2-4 Typical stress-strain curve for an unfilled rubber
Considerably different behaviour can be observed in a lightly cross-linked (above Tg)
rubber or a non-elastomer in the rubbery state, a phenomenon that many polymers
exhibit in a region just above their Tg, where chain entanglements restrict the
movement of chains and act, for a short time, as effective cross-links.
If a similar tensile test is carried out above Tg (as above) the stress-strain behaviour
is very different from that below (Figure 2-4); it is still described as elastic because it
will return to its original state when the load is removed. Hooke's law is NOT
obeyed. The extension is gigantic and Young's modulus is orders lower in
comparison to the solid state (below Tg) value.
9
2.2.3.1.1 Elasticity
The topic of rubber (or rubberlike) elasticity has been a subject for many
researchers and many investigations over many years and it is covered adequately,
from both theoretical and empirical points of view, in the literature. Treloar's classic
book2 and numerous reviews concerned with both cured3,4 and uncureds elastomers
have been published.
The property of high elasticity is the result of a particular molecular structure and is
not a special characteristic of a particular substance or chemical type. For a material
to exhibit these elastic properties it is necessary for6.7
.s: (a) the molecules to be long
and flexible with rapid free rotation about many of the covalent bonds between
adjacent atoms in the molecular chain as a result of thermal agitation; (b) there must
cross-links between the long chain molecules, due to either chemical bonds which
are permanent or physical molecular entanglements; (c) the cohesive forces
between the individual chain molecules must be low permitting a high degree of
molecular movement excepting, of course, where the molecules are cross-linked as
in (b) above. However, a similar arrangement exists in unvulcanised rubber gum and
rubber compounds, which also show elastic properties, where no permanent
chemical cross-links exist, due to purely molecular entanglements and molecular
entanglements and physical adsorption of molecules to the surface of filler particles
for gum and filled compound respectively creating 'effective' cross-links.
10
If a piece of rubber is stretched the molecules will become aligned, to a greater or
lesser extent, in the direction of stretching thus becoming more ordered and moving
away from the random tangled configuration. Thermodynamically the random state
is more likely (or probable) than the deformed state and therefore work has to be
done to create this departure from the disorderly state and the rubber will resist
being stretched. On release the rubber will revert to its more natural state of disorder
and therefore it will retract, returning to a state that is indistinguishable from its
original. In the undeformed state the rubber has its maximum entropy and it will
always return to or tend towards this state if there are no external constraints. For
visualisation the simple analogy is that of a spring. The molecules uncoil and
Input Stress
Time
c ;; ;;
Output Strain
Figure 2-5 Ideal elastic stress-strain response
elongate then, in recovery, 'snap' back and regain their original form.
2.2.3.1.2 Viscoelasticity and Stress Relaxation
Time
Elastomers exhibit both elastic solid and viscous liquid behaviour in both the cross
linked and uncross-linked states. Uncross-linked polymers behave largely as non
Newtonian liquids of high viscosity. They can be deformed irreversibly (Le. their
response to force is flow) and their flow can exhibit a considerable amount of elastic
behaviour. The flow behaviour is most important and desirable in the processing and
forming of the component and elastic behaviour is the most important characteristic
in the vulcanisate where the plastic behaviour becomes an undesirable quantity.
11
An ideal elastic solid will obey a time-independent stress-strain relationship such as
Hooke's law (Equation (2-1)). An instantaneous constant stress imposed will give an
immediate strain response in the form of a constant deformation. If the stress is then
Input Stress
Output Strain
Time Time
Figure 2-6 Viscoelastic stress strain response
returned to zero in a similar fashion, the strain deformation will also follow (Figure 2-
5). In a polymer the response to the same instantaneous stress is very different. At
first there is an instantaneous strain deformation response after which the material
continues to deform under constant load. This time-dependant or viscoelastic
behaviour is known as 'creep'. Removal of the stress shows an immediate elastic
recovery followed by a less rapid reduction in strain which is not wholly recovered
indicating viscous or irreversible deformation (Figure 2-6).
On a molecular level this response can be accounted for by the individual elastic
and viscous components. The elastic response (the vertical portions of the strain
curve (Figure 2-6) is driven first by the elongation and alignment of the chains and
then by the recovery of the strained long chain molecules as they return to the
preferable unstrained state. The time-dependent viscous component reflects the
difficulties these long chain molecules have in moving past one another due to
intermolecular friction and entanglements which progressively 'release' under stress.
Similarly when the stress is released there are frictional losses and 'new'
entanglements to overcome.
12
In the above the time-dependent strain response of polymer to an imposed constant
stress has been discussed. Stress relaxation on the other hand is the time
dependent stress response to an imposed constant strain (Figure 2-7). At moderate
Input Strain Output Stress
Time Time
Figure 2-7 Stress relaxation: Time dependent stress response to imposed strain
strains and brief hold times, a proportion of the higher molecular weight chains will
not slip at entanglement junctions9• When a sample is kept under constant strain,
the stress (or retraction force) decreases and eventually, when the strain is
removed, the sample will not return to its original state. The mechanism responsible
for this is similar to that described above. The molecules uncoil and elongate on
stretching and slip past each other. As entanglements are progressively released, a
new equilibrium state of random coiling is created as molecules take up new
positions and new entanglements are formed.
The time-dependent effects are, in certain circumstances, related as was shown by
Gent 10 who found a relationship between creep, stress relaxation and recovery (the
return to unstressed dimensions after the removal of a load) indicating that the
same, similar or linked processes are responsible for the behaviour. The stress
strain behaviour described above is affected considerably by chain length 11 (as
viscosity is known to be). The longer the chains in uncross-linked elastomer or the
distance between links in cross-linked elastomer the more difficult it is to release
entanglements and the more difficult it is for the molecules to work their way past
each other and thus reach their desired ideal state. This leads to greater elasticity
and longer recovery.
13
2.2.3.2 Thermal Properties
The transfer of heat is an important consideration in both the in-service (use) and
processing of polymers. For example, thermal insulation (the control of heat flow) is
a primary function of some products and in others, such as tyres, the rubber will
become hot, due to hysteresis effects (viscoelastic loss), which is an incidental
effect.
Somewhat less well known is the fact that orientation can have an effect on thermal
properties of polymers, Plomanteer, Thorne and Helmer12 and Wetton 13 have
reported that Tg can shift due to molecular orientation, reducing with increasing
orientation. Different values of Tg can be obtained depending on the direction of
measurement, being up to 15°C lower in the direction parallel with the orientation
than in the direction normal to orientation.
2.2.3.2.1 Thermal Conductivity(J.. or kc) and Thermal Expansivity(ae)
Heat is conducted through materials via atomic vibrations. The application of heat to
a cold polymer will have the effect of increasing the amplitude of the thermal
vibrations in the microstructure. This will be conducted to neighbouring atoms at a
rate which is dependent on the strength of the bonding between the adjacent atoms.
Crystalline solids and covalently bonded materials are good thermal conductors
because they have strong bonds between the atoms and molecules. Heat is
conducted poorly in amorphous solids in which secondary forces bind the
molecules. The weaker bonding does not transfer the thermal vibrations as
effectively. The thermal conductivity of polymers is in the order of 0·22 Wm·1K"1. For
comparison typical values of thermal conductivity, for a range of materials, are given
in Table 2-1. Polymers show anisotropic thermal conductivity if the molecules are
aligned; heat flow parallel with orientation is greater than heat flow normal to
orientation, this effect was clearly demonstrated by Hands 14 for stretched polymer
samples. Increases in chain length will tend to give an increase in thermal
conductivity because thermal energy flows more freely along polymer chains than
between them.
14
The coefficient of linear thermal expansion is higher for polymers than it is for metals
or ceramics and is not generally a truly linear function of temperature thus
complicating the design of moulds for precision parts and the design of metal inserts
in polymer parts. Thermal expansion is not necessarily isotropic, Muller'5.,e.'7
showed considerable anisotropy in hydrocarbons (CnHn+2 for n=5 to -50) with
negligible expansion in the direction parallel with orientation and up to 6·8% normal
to the direction of orientation. Later, Polmanteer et al.'2 showed similar results with
correspondingly higher values for thermal expansion coefficients of elastomers with
up to 86% difference between directions parallel and normal to the direction of
orientation. Changes in the composition can produce significant changes in thermal
expansion, for example, the replacement of polymer by fillers that, usually, have
lower expansivity will reduce the resultant expansion.
Thermal Linear Thermal Specific Heat Conductivity (A. or Expansivity( a,,) Capacity(cr) !<c)Wm"K" K" kJ kg" K'
Steel -63 -1·5 x 10'0 -0·42 Aluminium -201 -2·3 x 10'0 -0·9
Water (liquid) -0·6 -7 -4·19 (ice) -2 -5 -2·1
Solid Polymers 0·1 - 0·45 4 - 20 x 10'0 1 - 2
Table 2-1 Thermal properties of polymers compared to other (engineering) materials'··1 •. 2 ••
NB: Published thermal property data shows inconsistency. The table above is a composite of data from several sources and should be considered as an illustrative guide rather than firm or exact.
2.2.3.3 Electrical Properties
Most electrical properties of polymers are determined by primary chemical structure,
in a similar way to thermal expansivity, and are generally less varied than the
mechanical behaviour. Polymers are dielectric (non-conductors of direct electric
current) with very high electrical resistivity (1010 to 10'8 Qcm). Some carbon blacks
and metal powders used as fillers can impart conductivity to a polymer due to the
formation of a network of pathways of conductive filler particles2'.22.23 that are either
contacting or less than a 1 nm apart, although alternative suggestions have been
15
made on the theme of quantum mechanical electron tunnelling by Meyer4 and
others.
The measures of polymer electrical properties are: volume and surface resistivity,
dielectric strength, arc resistance, dielectric constant (relative permittivity) and
dissipation and loss factors. In this work reference is made only to resistivit!5 and
permittivit!6 as their measurement gives some clues to the structure of filled rubber
compounds21 .22,27.28 and yield anisotropic effects29.3o.31 with differences in
conductivity varying by a factor of 30: 1 parallel with and normal to the direction of
orientation.
2.3 Reinforcement (and State of Mix)
Reinforcement is a phenomenon that occurs when a finely divided filler is mixed with
rubber, a process which is as old as the industry itself. Fillers were originally used to
dilute the rubber and lower the cost and for increased rigidity and hardness.
However, it was soon noticed that the use of certain fillers gave extra strength and
toughness thereby reinforcing the properties of the rubber vulcanisate. The term
'reinforcement' refers to the striking changes in stress-strain properties that are
noticeable in rubber vulcanisates. These changes can be characterised as: (1) an
increase in tensile strength; (2) an increase in the modulus; and (3) an increase in
the elongation at break. Much of the work done in the study of reinforcement has
been carried out using amorphous materials, such as SBR, as they do not crystallise
or undergo strain crystallisation, an effect which may hide the reinforcement. There
is evidence, reported by Wheelans32, that the presence of carbon black enhances
anisotropy in moulded rubber products.
The increase in modulus is expected because the carbon particles are rigid but
reasons for the ability of the carbon black to permit an increased elongation at break
and the ultimate tensile strength of the composite are not clear. There is a very large
body of literature on the subject of rubber reinforcement and reinforcing fillers but
there seems to be little agreement on the fundamental mechanism of reinforcement,
apart from the logical conclusion that the bond between the particle surface and the
16
rubber phase must be responsible for the reinforcing effect. In a practical sense
reinforcement may be considered as the increased stiffness, modulus, rupture
energy, tear strength, tensile strength, cracking resistance, fatigue resistance or
abrasion resistance, any of which would tend to increase the service life of a
product. It is probable that the particulate reinforcement of rubbers is not the result
of anyone phenomenon, molecular process or mechanism33.
2.3.1 Reinforcing Materials (and Particle Size)
The list of particulate materials that have been added as fillers to rubber is long and
diverse and includes such materials as carbon blacks, aluminium hydroxide, calcium
carbonates and oxides (limestone and chalk etc.), calcium sulphate (gypsum),
silicas, zinc oxide, wood flour etc. They are added to reduce cost, improve
processing, to improve appearance and colour and to reinforce the polymer by
enhancing the mechanical properties in some way (e.g. hardness, tensile strength
etc.). Horn34 gives detailed information on carbon black fillers and Simmons35 has
produced an exhaustive list of non-black fillers including information on the
reinforcing ability and the properties imparted to the compound.
Among these fillers several have been shown to produce a significant reinforcing
effect and they include carbon blacks and graphitised carbon blacks36.37
,
precipitated silicas and silicates38, anhydrous silicas39
, esterified silicas4o, the
organic polymer Iignin41 and formaldehyde resins42 in particulate form.
The chemical nature of the surfaces of these particles varies widely and therefore
the only common feature between them is that they all have an extremely small
particle size. It is difficult then, for anyone studying the subject, not to conclude that
any finely divided solid material will reinforce rubber, as did Schmidt43 when he
wrote "small particle size of the pigment is of prime importance in elastomer
reinforcement, whereas the chemical nature of the pigment appears to be of
secondary importance" after having reported the strong reinforcing effects of stannic
oxide, silica, Prussian blue, polystyrene and casein.
17
In Figure 2-8 the results of Boonstra44 have been plotted. This clearly shows the
effect of carbon black particle size on reinforcement in SBA. It is generally accepted
in the literature that particle size has an effect, with the amount of reinforcement
increasing with decreasing particle size for a given volume fraction of filler particles.
The size ranges are broadly divided, for carbon blacks, as follows: at 300nm
diameter and above are thermal blacks which show little reinforcing effect (these
might be called 'dilutent'); 'semi-reinforcing' furnace grade blacks 100nm to 200nm
25~~------------------------~
20
.. Do
!. ~ c: 15 e -11)
.!!
.iij c: ~
10
5+-------~------~------~ o 100 200 300
Average Particle Size (nm)
Figure 2-8 Effect of particle size of carbon black at 50 phr. on tensile strength of SBR. Graph plotted from Boonstra data".
in diameter; 'high abrasion' fumace grade blacks at about 40nm diameter; and
'intermediate super abrasion' and 'super abrasion' grades with particle sizes below
35nm. For non-black fillers similar groupings can be identified45: soft clays etc. with
particle sizes in the range of 1000nm to 8000nm diameter are 'dilutent' fillers; hard
clays, zinc or titanium oxides and precipitated calcium carbonates with sizes ranging
from 100nm to 1 OOOnm diameter are 'semi-reinforcing' fillers; and 'reinforcing' fillers,
18
consisting of precipitated calcium carbonates, silicas, calcium silicates and
anhydrous silicas etc., at 10nm to 100nm diameter. The greatest reinforcement is
noticed in the range 10nm diameter to 100nm diameter (Figure 2-8). Very little work
on fillers with particles below 10nm has been reported.
Particle size is inversely proportional to the surface area of a particulate filler. The
effect of smaller particles might actually show the extent to which the filler can
interface with the polymer phase.
The carbon blacks and silicas are the most important reinforcing fillers for use in
industry. This is born out in the literature by the fact that polymer textbooks and
reviews, when discussing reinforcement, often concentrate on carbon blacks and/or
silicas and only mention other fillers in passing. Carbon black is the most common
and, perhaps, the most important because of its heavy use in the tyre industry.
Many other areas of the rubber industry also utilise carbon black, and its reinforcing
effect, in the manufacture of their products.
2.3.2 Reinforcing Mechanisms
This reinforcing phenomenon is the subject of several theories and explanations
although there is not really any single method which explains fully the mechanism of
reinforcement. The following sections highlight some of the more popular concepts
of particulate reinforcement in elastomers and rubber.
2.3.2.1 Bound and Occluded Rubber
On mixing rubber with a reinforcing filler, particularly fine carbon black, a change
comes about in the mixture and an increase in viscosity can be noted. When
exposed to a good solvent only a proportion of the rubber can be extracted from the
(unvulcanised) mixture. The portion that cannot be drawn into solution is known as
bound rubber and exists in the form of a rubber-filler gel that is swollen without loss
of shape. Twiss46 was the first to report (1925) observations that a mixture of natural
rubber and carbon black shows a resistance to solvent that is related to an
improvement in mechanical properties. A test was developed some twelve years
19
later by Fielding47• After the observations of Twiss, Stamberger48 noted fresh "killed"
rubber (after recent mastication) has no strength but after a time of standing gains
such strength that it can no longer be pulled apart by hand and freshly made rubber
dissolves readily but the stiff material, often standing only a few days, just swells.
Blow49 reported similarly the phenomenon that a batch of a fresh mixture of natural
rubber and channel black dissolves readily in a good solvent but if a batch is allowed
to stand before exposure to the solvent it will dissolve less readily. If the period of
standing is prolonged, solution of the rubber will be impossible and it will swell to a
gel retaining its shape, behaving as if it were vulcanised. Such storage maturation
effects have been shown by BlowsO and Leblanc and HardyS1 to evolve over a period
in the order of about 30 days. Blow49 also noted a structure viscosity and thixotropy
of the solutions.
It is clear that rubber-filler systems exhibit time-dependent characteristics. Fresh
rubber (or that recently freshened by mastication) is more easily processed and has
a lower viscosity than a compound that has been rested. It is believed that this is
due to the formation of bound or immobilised rubber over time as the material is
rested in storage. This structure can be broken down by work and it will reform if the
material is then allowed to rest again. Similarly if a rubber solution is allowed to
stand and is then worked the apparent viscosity will decrease with time due to a
'structural' (intermolecular attractions and entanglements) breakdown, structural
reformation occurs concurrently and an equilibrium point is reached. If the solution is
allowed to stand, then a structural reformation to the original state will occur due to
Brownian motion. McBain52 put forward the suggestion that this structure was due to
an orientation of the rubber "particles" on the carbon-black particles. Cotton53 also
makes mention of rubber molecules being oriented between filler particles, however,
no mention of the immobilisation of rubber is made. This orientation, if it exists,
could be a cause of anisotropy seen in products.
Occluded rubber is that part of the rubber phase that takes up the internal volume of
a filler aggregate. In doing so it becomes shielded from deformation of the
composite mixture as a whole and, therefore, many of the mechanical properties are
20
influenced by the amount of rubber which is shielded by such occlusion because
this has the effect of increasing the volume fraction of filler and decreasing the
volume of the rubber. This effect was initially pointed out by Medalia54 and
Kraus55.56.57.
The concept of a three-dimensional, two-phase lattice structure in which the longer
molecules were adsorbed, at different points along their length, onto the surface of
several different filler particles, was suggested in early studies by Stickney and
Falb58, Gessler59 (for carbon black systems) and by Southwart60 (for silica systems).
The two phases consisted of mobile rubber and semi-mobile or immobile rubber
bound to the surface of one or more filler particles, the latter creating a three
dimensional lattice and giving a reinforcing effect. It was also found that the amount
of bound rubber could be related to the observed mechanical properties of the
composite and that bound rubber was not only affected by particle size and
structure but also by the surface activity of the particle. Baker and Walker61 , in
reporting an insoluble bound rubber effect occurring in SBR-carbon black mixes,
also reported that the amount of bound rubber increases with increasing molecular
weight. The concept of "bound polymer" has also been used by Akal2 in the study
of highly filled thermoplastics.
A mathematical model for the description of filled elastomers, the "double network
model", has been developed with a similar, two phase, structure in mind by Reichert,
G6ritz and Duschl63• They describe a superposition of two independent networks.
The first network is the rubber matrix, the second network contains the filler and
connecting strands in a "supernetwork" these are analogous to the bound and
immobile phases described above. The model gives the stress atotal as the sum of
the partial stresses 0"1 and 0"2 which represent the stress in the rubber matrix and the
stress in the filler-supemetwork respectively,
O"total = (1 - <ll) 0"1 + <ll 0"2, (2-2)
21
where the partial stresses are,
2 1 0", = G, ("" -1n, )1 () ,
- '1', "', (2-3)
for the rubber matrix and,
(2-4)
for the supernetwork. <I> is the volume fraction of the supernetwork, G, and G2 are
the Gaussian moduli of the networks, '1', and '1'2 represent the proportion of fully
extended chains and appear in the empirical factor 1 ( ) in equations (2-3) and 1-'1' A
(2-4) (when all the chains in both networks are fully extended a limit is reached and
there can be no further extension), "', and "'2 are the macroscopic strains of the
networks calculated as a probability function of the microscopic strain 11. The
predictions of the double network model of Reichert et al.63 were compared to the
stress-strain results of Mark64 (for Si02 filled Polydimethylsiloxane (PDMS)
networks) giving a good correlation. Later Frey, Goritz and Freund65 showed a good
correlation between the model and carbon black - SBR systems with various carbon
blacks and loadings. The stress/strain predictions of this theoretical model show
good agreement with the stress/strain data obtained by experimental measurement
and can be viewed as good supporting evidence that a two-phase
(polymer/supernetwork) type structure exists.
Other evidence to support a two phase structure of a polymer filler composite exists
in the literature that covers electrically conductive rubber. Changes in the
conductivity (or its reciprocal resistivity) of carbon-black-filled rubber vulcanisates
with stressing such as flexing, elongation or compression etc. have been described
by Lane and Gardener66 and by Bulgin22 and Dannenberg67 among others. The work
by shows an initial rapid rise in resistivity to high values and a slow decrease as
soon as a stationary condition is established. This return to equilibrium is faster
22
when the temperature is higher which tends to support the idea of a structure
breakage and reformation mechanism. Wack, Anthony and Guth27 and Boonstra
and Dannenberg21 show a rapid increase in resistivity to a maximum (up to 50%
elongation) then a less rapid decrease (from approx. 50%-150% elongation) for
carbon black filled rubber compounds during stretching of tensile specimens. This is
explained by assuming the breakdown of a carbon structure at lower elongation and
a realignment at the higher elongation. Their apparatus was only capable of
elongation of up to 200% but they speculated that this decrease in resistivity would
not continue until breakage occurs, but would reach a minimum and then would rise
rapidly up to rupture. Later Voet and Morawski68 conducted experiments to higher
elongation. That there is a large rise in conductivity (fall in resistivity) above 200%
elongation is clear, reaching a maximum with a very rapid decrease thereafter as
predicted by Boonstra and Dannenberg.
These results support the two-phase, bound rubber structure proposed above, the
rubber-carbon black phase (or 'super-network') being the pathway supporting
conduction by containment and alignment of the conductive filler particles, (Le.
holding the conductive particle chain together in a distinct rubber-filler phase). The
changes in conductivity (resistivity) concur with the formation and destruction of
such a network. The exact mechanism for conduction, which could be via either the
direct contact or very close proximity (less than 1 nm) of filler particles or the later
"electron tunnelling" ideas69 (a few nano-metres) between filler particles is
unimportant because the distances proposed are the same order of magnitude and
each would be sustained by this three-dimensional bound rubber phase.
O'Brien et 81.70 suggested that the degree of mobility of the bound rubber was a
proportional function of its distance from the surface of the filler particle, the greater
the distance the greater the mobility. Kaufman, Slichter and Davis7\ Ban, Hess and
Papazian72 and later Leblanc73 point to the bound rubber network consisting of three
parts: tightly bound rubber at the filler surface which will not deform, a more loosely
bound rubber layer that surrounds the filler particles and forms connections in the
form of filaments to other particles in the compound and totally unbound rubber (Le.
freely mobile rubber). This latter structure, if it exists as proposed by these authors,
is similar enough to be reasonably approximated by the two-phase model used by
23
G6ritz and co-workers63,65 and explains why they found good correlation between
model and experiment. Interestingly, Ban et al. suggested that there may be a level
of molecular ordering such as an epitaxial type orientation of the polymer molecules
on the carbon black surface and alignment of the molecules in the network (similar
to the ideas of McBain51 and Cotton53 30 years previously). This is likely to be close
range ordering and the extent to which this could affect the more longer range order
is not clear. They also thought it likely that this orientation would persist in the final
vulcanisate. This could, possibly, have a contribution to the anisotropic effects
exhibited by moulded parts.
!:!. ~ ;;
~ ii ~
RII ... .fiIl ... !rd RII ... · F'd}rrer irter.DiCfl
, , [S/\ 0.5 (.0.25) ,
~~------------------~~-------------~ H,<tcxt,<mic elfeds ci.e 10 RIIer
Figure 2-9 Based on Payne75 plotlPayne effect dynamic test. Filler-filler and polymer filler intereraction is almost completely broken down by a dynamic strain amplitude of ±O·25.
There seems to be general agreement that bound rubber is an important factor in
reinforcement. Bound rubber could also be a factor in anisotropy, if it is accepted
that the three dimensional (2 or 3 phase) rubber-filler network structure exists and
persists from the uncured stock through into the vulcanisate. Leblanc clearly thinks
that it is possible that this structure could persist in the vulcanisate when saying ''this
portion of the elastomer can undergo very large deformation during flow whilst
remaining essentially attached to the filler particles". It would be possible for this
three-dimensional filamentous structure to contribute a component to anisotropy of
the product if it were cured with this phase in a state of strain. However Leblanc
24
does go on to describe a breakdown of the three dimensional structure at high
stresses by ruptures of the connective filaments or decohesion between rubber
chains and filler particles, this happening at a point where the stress is high enough
to cause melt fracture. The question must therefore arise: Does the structure persist
in the vulcanisate? Ban et al.72 and others73.74 think that the persistence of such a
structure is likely, although its existence would be difficult to prove, being largely
hidden by the cross-links that are formed during vulcanisation.
The writer believes that a three-dimensional rubber-filler network does exist in the
vulcanisate as a distinct phase, but that this is not necessarily the persistence of a
network that was formed prior to the last working and shaping of the uncured stock
but one which starts forming after shaping is complete. The work on dynamic
properties of rubber by Payne7S.76 shows a drop in shear modulus as dynamic strain
amplitude (DSA) is increased; the effects were shown as additive. A strain amplitude
independent residual value of shear modulus was shown due to pure gum modulus,
to hydrodynamic effects due to the filler and to filler-rubber linkages (additional
cross-linkages). A strong strain amplitude dependence was shown to be due to filler
filler interaction. In this classical view of the "Payne effect" it has widely been
accepted that the shear modulus loss is mainly, if not only, related to the filler
network formed in the rubber matrix. Figure 2-9 shows a schematic idealisation of
the effect of strain amplitude on shear modulus similar to that of Payne. Here the
strain amplitude independence is considered to be due only to pure gum modulus
and hydrodynamic effects. The strain amplitude dependence is a combination of
filler-polymer linkages and filler-filler interaction. This view is supported by Wang77
who states ''the Payne effect can serve as a measure of filler-filler interaction as well
as polymer-filler interaction". The size of the contribution of each of the interaction
components, filler-filler and filler-polymer, to the fall in shear modulus is not known.
When applied to flow, the Payne effect indicates that the three-dimensional
structure (rubber-filler network) is broken down relatively easily when it is worked
(i.e. freshened) or in a state of flow (i.e. processing). The reformation of the network
is time dependent and occurs when flow ceases and takes about 30 days, as
indicated by the storage maturation effects shown by BlowsO and Leblanc and
HardySl.
25
The breakdown and reformation of the filler network allows a mechanism for the
formation of anisotropy to be proposed. It is well known that polymer molecules can
be oriented by deformation, such as by flow. The macromolecules (long polymer
chains) tend. to be extended (un-coiled) and aligned when stressed in such a
manner. When the stress is removed the molecules recover and return to their
relaxed (coiled) state. Short molecules recover quickly but the medium-to-Iong
molecules have a recovery time that is in the order of, or longer than, the rubber
filler network reformation time. The rubber-filler network would therefore be
reformed before the recovery of the medium to long chain molecules is complete,
preventing them from recovering fully. This process creates anisotropy in the sample
due to the orientation of, and the stress in, the molecules that are not fully
recovered.
2.3.2.2 Hydrodynamic Theories
Einstein78 calculated the viscosity of a liquid with uniformly dispersed rigid spherical
particles,
11 = 110 (1 + 2·5c), (2-5)
where "0 is the viscosity of the liquid, " is the viscosity of the mixture and c is the
volume fraction of the particles. The particles should be incompressible, should not
interact, should be completely wettable and uniformly sized and distributed.
The relationship was shown to also apply to elastic modulus (Young's modulus) of
vulcanisates with large filler particles and low filler concentrations by Smallwood79,
E = Eo (1 +2·5c), (2-6)
26
where E and Eo are the Young's moduli of the mixture and the unfilled gum
respectively.
The assumptions that are made in the derivation of equation (2-5) do not apply to
most filler particles and Guth and Gold8o added a term to account for particle
interaction to give,
11 = 110 (1 + 2·5c + 14.1c2), (2-7)
which agrees well with experiment for larger particles of about 500nm and with a
volume fraction of 0·3 or less but not for fine reinforcing fillers. The reason was
thought to be asymmetry of the filler aggregates and Guth81 proposed the addition of
a shape factor, f, being the ratio of the length to the width of the aggregate, in a
modified equation,
11 = 110 (1 + 0·67 fc + 1·62 f2c\ (2-8)
For reinforcing blacks a good fit is obtained with a shape factor, f, of around 6 for a
high-abrasion furnace black. However it should be noted that shape factors for
carbon black aggregates are much closer to a value of 2 when measured using
electron rnicroscopl2. Guth81 and others, also assumed, like Srnallwood79, that the
change in the elastic constant of the rubber caused by the inclusion of spheres
would be entirely analogous to the theory of viscosity therefore the equations would
apply to the elastic modulus, E, giving,
E = Eo (1 + 2·5c + 14.1c2), (2-9)
for spherical particles and,
(2-10)
27
for non-spherical particles.
The Guth equation is useful for expressing the mechanical properties at low
extensions. Better correlations for some situations can be obtained by the use of an
"effective filler volume fraction" (EFVF), that is, by including the volume of
immobilised rubber "as filler" within the term c (similar to <1>, the volume fraction of
the supernetwork in Equation (2-2) from the Reichert et al. double network modeI63).
The EFVF term could then incorporate the volume of bound and/or the volume of
occluded rubber which does not have any interaction with the matrix and therefore
cannot be deformed as established by Medalia83 and KrausS5 (Also see Medalia54
and Kraus56•S7
).
2.3.2.3 Rubber Elasticity and Strain Amplification
A number of authors have developed methods based on the statistical theory of
rubber elasticity for unfilled gum vulcanisates. This relates the Young's modulus of
an ideal elastomer network directly to absolute temperature. Stress <J and strain
ratio ex are related by the equation,
<J = vkT(ex - 11 ex\ (2-11 )
where v is the number of elasticity effective chains per unit volume, k is Boltzmann's
constant and T is absolute temperature. As the predictions did not agree well with
experimental evidence, a later empirically modified, and much used, expression was
suggested by Mooney84 and subsequently modified by Rivlin and Saunders85 to
(2-12)
where Cl and C2 are constants characteristic of the rubber. C2 is the term that
makes Equation 2-12 differ from the classical ideal network behaviour. If, as in the
28
case of highly swollen gum vulcanisates, the C2 term is reduced so that it is close to
zero then 2C1 is equivalent to vkT.
The concept of strain amplification has also been proposed to describe the
behaviour of filler-elastomer composites at high strains. It is assumed that filler
particles are rigid and do not deform and therefore that the actual strain in the
rubber phase of the composite is not equal to the overall strain. In effect the particle
phase is thought to increase the strain in the rubber phase.
Several methods have been proposed to determine a value of effective strain, a', in
the rubber phase. This too, is a controversial area in terms of reinforcement. A. M.
BuecheB6 suggested the expression,
a' = 1 + (a-1).,Jc, (2-13)
where a' is the effective strain ratio in the rubber itself, a is the overall strain ratio
and c is the volume fraction of filler. A modified expression was later proposed by F.
BuecheB7:
a' = 1 + (a - Vc)/(1- Vc). (2-14)
Mullins and TobinBB suggested that the volume concentration factor from the Guth -
Gold and Guth equations could be used, as this would account for the disturbed
strain distribution and the lack of deformation in the fraction of the composite which
is filler:
a' = a(1 + 2·5c + 14.1c2). (2-15)
29
This, however, only agrees with experimental evidence for large particle sizes and
moderate extensions, so for finer more reinforcing particles and higher extension the
following should be used:
(2-16)
However, for this to fit experimental results, unusually large shape factors have to
be assumed. Here too, as with the modification of the Guth equation, the use of the
EFVF that includes occluded rubber can help bring the theoretical prediction more
into line with the experimental evidence.
2.3.2.4 Interparticle Chain Breakage and Chain Slippage Mechanisms
Another explanation of particulate reinforcement was proposed by Blanchard and
Parkinson89,who suggested that there are two types of bond, strong and weak, that
form between the rubber molecules and the carbon-black filler particles. This is
thought to be assisted "by the presence of a coherent chain structure of the filler
particles themselves". The strong type is due to chemisorptive attachment and the
weak due to a physical, van der Waals, type bonds. There are thought to be
relatively few of the chemisorptive bonds and many of the physical type. The
phenomenon of stress softening can be explained by the progressive breakage of
the weaker links. As the stress is increased, the strong bonds remain intact. The
reinforcement and ultimate strength are provided by this small number of the
stronger bonds which persist until the point of rupture. This explanation has been
extended by Blanchard9o•91 who added strain hardening at large deformations due
to: (1) tightening of short molecular chains between close particles; (2) strain
alignment of molecular segments between particles and crystallisation in
crystallisable rubbers; and, (3) slip rearrangements giving greatly increased
alignment and stress sharing by highly stretched chains. Blanchard himself points
out that there are similarities with F. Bueche's interparticle chain breakage
mechanism, and with Dannenberg's molecular slippage mechanism.
30
The Dannenberg92 molecular slippage mechanism uses a combination of molecular
slippage and bond rupture and reformation to explain how reinforcement is achieved
in a rubber-filler system. The assumption is made that it is possible for rubber
molecules attached to filler particles to undergo a surface slippage or other type of
rearrangement when a stress is applied. The shortest chains are extended to a point
where it is not possible for them to extend any further without scission or without the
bond at the surface of the filler desorbing or the chain slipping across the surface of
the filler particle. It is suggested that the most probable of these alternatives is
molecular surface slippage. The process continues in this manner when the
extension limit of the next shortest chain is reached and so on. Eventually chain
lengths equalise and the stress is redistributed between the chains that are now
oriented, aligned and equal in length, giving greater strength due to this stress
sharing and increased intermolecular association. On relaxation of the stress, the
chains of equal length will, initially, for a subsequent extension exhibit a lower
modulus (Le. stress softening). A random distribution, such as that which existed
before the rubber was stretched, is recovered on standing for some time as the
normal kinetic motion of the molecules is allowed to resume.
The interparticle chain breakage mechanism proposed by F. Bueche93 states that as
the interparticie chains in a rubber filler network have various chain lengths, the
shorter chains will break first at relatively small deformations and that, since the
stress on the chain immediately before breakage is large, it will contribute greatly to
the stiffness or modulus. Strength is also influenced by the filler particles acting as a
vehicle for distributing the tensile load, more equally, over a wide number of chains
(Le. stress sharing). Chains, once broken, will not be able to affect stiffness on a
second extension hence, a softening effect will be observed. Although a particular
mechanism is not explained the replacement of the broken chains with other similar
chains is postulated for stress recovery. A process is also proposed for stress
relaxation where the filler particle is able to move through the rubber; even a very
small motion would be sufficient to release most of the tension in extended chains.
As can be seen, except for the concentration on slippage rather than breakage of
chains and the inclusion of chain alignment effects, Dannenberg's and Bueche's
explanations are similar.
31
2.4 Cross-linking (Vulcanisation and Cure)
Vulcanisation (or curing) is the process in which a molecular network is created in an
elastomer by the formation of cross-links between its polymer chains. Before 1840
the use of rubber had been confined to those applications in which it could be
supported by a substrate (e.g. cloth). This was necessary because raw natural
rubber (NR) has poor mechanical properties existing as a tacky, weak thermoplastic
mass highly subject to the problem of heat softening. The properties are
transformed, on vulcanisation, to a useful level as NR is converted into a non-tacky,
strong, highly elastic and tough material suitable for a wide range of engineering
purposes. Vulcanisation (or vulcanising or cross-linking or curing), generally, is such
an important subject in the process of rubber manufacture and production that it is
covered in numerous texts by Hofmann94•95
, Hills96, Coran97
, Morrell98, and Alliger
and Sjothun99, to name but a few among many.
Hancock's and Goodyear's method of vulcanisation requires the addition of a small
amount of sulphur to the rubber and the resulting compound to be heated to bring
about the striking property changes highlighted in Table 2-2. Cross-linking by this
method was also found to be useful for the synthetic elastomers, developed later,
such as SBR, IIR, NBR etc. but the synthetic rubbers required different proportions
of the vulcanising additives (Le. sulphur and accelerator, see below), as synthetic
elastomers are slower curing than NR therefore greater amounts of the accelerators
needed to be used.
It was later found that ingredients other than elemental sulphur cause cross-linking.
Various sulphur compounds (sulphur donors) can also be used, their sulphur being
liberated at vulcanisation temperatures. Further investigations showed that, although
rubber could be cross-linked solely with sulphur and heat, the addition of certain
other compounds would increase the rate of cross-linking which is, on its own, slow.
These other compounds, 'accelerators', can be added to the polymer-sulphur
(polymer-sulphur-filler) mix to greatly enhance the rate at which cross-linking takes
place, enabling a reduction of cure time, hence, increasing production output and
improving vulcanisate properties. Presumably this improvement was due to shorter
exposure to heating causing less degradation. Zinc oxide was also found to have an
32
effect, enhancing the action of the accelerator and it became known as an
'activator'. Later a combination of zinc oxide and stearic acid became commonplace.
Property RawNR Vulcanised NR Non-reinforced Reinforced
Tensile Strength 2.1 20.1 31 (MPa) Elongation at 1200 BOO 600 Break(%) Modulus (MPa) - 2.B 17.3 Permanent Set High Low Snap Hioh Very Hioh Water absorption High Low Solvent Resistance Soluble Swells only (partially soluble on extended (hvdrocarbons) immersion)
Table 2-2 Typical properties of raw and vulcanised natural rubber (NR)
Conversely, vulcanisation inhibitors (retarders) are used to prevent scorch, or
premature cure. These additives delay the onset of cure and slow the rate of
vulcanisation thereby increasing scorch safety (scorch time) and the time to reach
final cure. This is a consideration that is particularly important for large, thick parts
where extended heating times are necessary to obtain cure throughout. The extent
of cure in parts of widely varying thickness could be uneven because a thin section
will warm more quickly and vulcanisation will occur more rapidly. The longer
effective time at the vulcanisation temperature compared to a thicker part in the
moulding may result in non-uniformity in the state of cure in the respective portions
of the moulding. Hofmann 100 has produced an extensive work that covers additives
for rubber vulcanisation and processing in great detail.
It was also discovered that cross-linking may be achieved without heat or elemental
sulphur. Other compounds exist that can be used to cross-link rubber components:
Oxidising agents can be used instead of sulphur but vulcanisates are weaker than
those from sulphur and price and toxicity tend to rule them out as an alternative or
replacement. Free radical generators like organic peroxides, azo compounds and
33
many accelerators and phenolic resins can also be used. A cold process using
sulphur chloride has been developed to cross-link rubber. It is therefore obvious that
there are several mechanisms that form cross-links.
LS...J
~!
~ ~
il (or lee)
rS-s-, (a) (b) (c) (d) (e) (f) (g)
Figure 2-10 Different Cross-link Formations: (a) monosulphidie cross-links, (b) disulphidie cross-links, (c) polysulphidic cross-links, (d) cyclic monosulphidic and disulphidic "cross-links", (e) vicinal cross-links, (f) sulphidic chains (pendants) and (g) C-C cross-links (i.e. peroxide cure systems).
2.4.1 Vulcanisation Mechanisms, Rates and Times
Dibb0101 describes a mechanism for vulcanisation and suggests the intermediate
steps in the reaction that occur in the formation of cross-links in sulphur-accelerator
and sulphur-accelerator-zinc oxide-stearic acid systems. Vulcanisation is described
as a continuous process that begins as soon as the ingredients are mixed and the
temperature elevated. It continues long after the heating has finished and persists
into the ageing phase of a rubber component's life. Dibbo also points out that a
number of types or structures of cross-link are formed as illustrated in Figure 2-10
(a) - (e), mono- (a), di- (b) and polysulphidic (c) cross-links, cyclic links (d) and
vicinal cross-links, which are so close in proximity to each other that they act,
effectively, as a single cross-link. It can also be seen that some of the sulphur does
not contribute to actual cross-links between different molecules but links to the same
molecule or to neither in the case of a pendant sulphur chain (Figure 2-10 (d) and (f)
respectively). Pendant chains can be formed with accelerator residues at the ends.
These links are not particularly harmful, as they are low in concentration compared
to cross-links10
2, although it is conceivable that they may hinder local molecular or
segmental motion by proximity. On the other hand, these sites, especially the
pendant sulphur chains, have the potential to be formed into cross-links on
34
continued heating. It is the formation of some or all of these types of cross-link that
will, progressively, prevent the material from flowing.
Before the onset of vulcanisation the polymer molecules can move relatively freely.
There may hindrance to movement from intermolecular entanglements but this is
slight, especially at elevated temperatures, where molecular energy permits free
molecular bond rotation allowing the molecules to slip past one another easily
(macro-Brownian motion). The formation of the desired type of cross-link can be
controlled by the choice of cure system and additives for a given polymer. Figure 2-
11 shows a schematic representation of a curemeter trace for a number of cure
schemes, these can apply to both sulphur and non-sulphur vulcanisation systems: A
shows a rapid onset of cure where cross-links start to form instantly on the
application of heat. The change in the properties can be seen immediately (typical of
peroxide cure systems). In most cases this is undesirable as it will interfere with the
safe processing of the compound. If cross-linking occurs in parts of the material
Onset of Cure (vt*:aniSabon) Under-cure • i •
OpomumCUre ., . ~ : ::::: ::::: :::::: ::: ::::: ::: :::::::: ::::: f: :::: :::::: ::::::: ::::: .. .1 .......... . o
I
E
Vulcanisation Tlm, a' b·
Figure 2-11 Stages of cure (vulcanisation); Modulus vs. Time
before it has been formed into the desired shape the molecules in that section of the
35
component could suffer from residual vulcanisation stresses and/or flow-induced
molecular orientation that is "frozen-in" by curing in a similar way to that described
by White and co_workersl03.104 and Isayev105 in a birefringence study of amorphous
polymer mouldings: B shows a more desirable curve indicating that there is a delay
between the application of heat and the start of cross-linking which allows time for
the material time to flow and take up its shape, say, in moulding operations. This
delay, known as the "scorch time" is constituent dependent. Its magnitude can be
controlled by the selection of cure system and additives. The bottom of curve B is
very important: it allows time for the material to flow and take up the shape of the
former (be it a mould cavity or extrusion die). If all flow forming the component
shape can be contained within the scorch time (i.e. before the onset of cross-linking)
the component is less likely to suffer from the residual stress and molecular
orientation effects that occur in A. The other end of the cure trace shows the region
where the vast majority or all of the possible cross-links form. C shows a gradually
increasing modulus value typical of the so-called 'marching cure' that could be
obtained with SBR. 0 is plateau cure: the modulus reaches a maximum level and,
once there, it remains constant. Other properties (e.g. elongation at break) might not
follow this trend. E shows decreasing torque with increasing time, after a maximum
at b - b', and represents 'reversion'; this is the breakdown of the cross-links (and/or
polymer degradation) caused by an extended heating time. Between the sections
already discussed there exists a region where the cross-linking reaction rate is
slowed dramatically but the point of optimum cure has not been reached. The lines
a - a' and b - b' show the x and y intercepts at 90% cure and 100% cure,
respectively. That is a and b are at 90% and 100% of the maximum torque value
attained by curves 0 and E (curve C never reaches a maximum torque value) and
a'(or t90) and b'(or hoo) are the times at which 90% and 100% cure is reached. A
value of t95 is often used for the determination of best 'technical cure' time. It is
worth noting, however, that if a curemeter measures only very small strains the trace
obtained for a given sample of compound might show up the carbon-black network
effect that would be lost or hidden if larger strains were used. A strain is usually
selected to eliminate most of the structure effect.
The number and type of cross-links will dictate the properties of the
vulcanisatel06.107, and this in turn will be dictated by cure system, the amounts of the
36
additives (vulcanising agent, accelerators, inhibitors etc.) and their activity. For
example, hard rubber ('Ebonite') is cured with 30 - 50phr of sulphur and forms a tight
network of many close cross-links producing a vulcanisate that is rigid and deforms
little, if at all, when stressed. It could easily be described as hard and strong. A small
amount of sulphur, say, 2 - 4phr will yield a lower cross-link density, a much smaller
number of cross-links, and the resulting vulcanisate (soft rubber) will be elastic and
deform comparatively easily when stressed and return to its original state when the
stress is removed.
G6ritz, Sommer and Duschl10B suggested that cross-linking is not random but that a
polymer network with 'islands' of concentrated cross-links connected by molecular
chains that are lightly or not cross-linked is formed; due to the chemical bonding
process being exothermic and the mobility of the chains near a recently formed
cross-link being impeded, thus increasing the probability of a second (and third and
so on) cross-link forming in close proximity. This structure could explain some of the
anisotropic effects that are noticed in vulcanised rubber products, for example,
anisotropic voids formed during drawing caused by an inhomogenious cross-link
distribution. A similar result could be obtained if the curative/cross-linking agent
were not properly mixed into the polymer. This would seem an unlikely, though, as
mixing and distribution of the curatives in the rubber compound is not as difficult as,
say, mixing carbon-black or other parliculate filler because only dispersion
(dispersive mixing) is required and the constituents tend to melt as temperature
rises thus making dispersion easier.
2.5 Production and Manufacturing Processes
It is believed that anisotropy is induced in the manufacture of most, if not all, rubber
products. The production sequence in their manufacture can be clearly divided into
three stages: mixing, forming and curing. The major components of the
manufacturing process are reviewed with respect to how and where anisotropy is
induced in rubber products.
37
- ---------
2.5.1 Pre-moulding Processes - Mixing and Milling
The object of the mixing process is to produce a compound that has its ingredients
sufficiently thoroughly incorporated and dispersed so the later processing (Le.
forming and curing) will ensure that the product has the desired properties for its end
use. In this section the process and purpose of mixing and mixing equipment are
briefly covered.
2.5.1.1 The Mixing Process.
The mixing process consists of three simultaneous processes (i) simple mixing, (ii)
laminar mixing and (iii) dispersive mixing. There is no single formula which
determines the importance of these different types of mixing. Anyone, depending
on the particular compound, may be the critical (or efficiency determining) process.
Simple mixing (i) is the process of moving particles around in the mix whereas
laminar mixing (ii) and dispersive mixing (iii) are much more drastic: the material will
flow in laminar mixing and particles will fracture in dispersive mixing if the stresses
are sufficiently large.
During mixing several physical changes take place:
• Incorporation - The incorporation stage is where the ingredients of the compound
are 'joined together' from being initially separate. The rubber is forced between
pairs of rotors and the mixing chamber wall or rolls with the affect of destroying
the original form of the rubber. As the material is deformed under shear, fresh
new surface is created allowing it to accept, surround and encapsulate the filler
forming agglomerates.
• Dispersion - Agglomerates are broken down and distributed through the rubber
(simple mixing). Then they are dispersed to give the required fine level of mixing
when the particles have been broken down to their ultimate size (the aggregate).
• Distribution - takes place throughout the mixing cycle to increase the
homogeneity of the compound.
38
• Plasticisation - The rheological properties of the compound are determined for
further processing operations. This process is ongoing from the very early stages
of polymer mastication.
Figure 2-12 Different internal mixer configurations: (a), non-intermeshing (tangential) and, (b), intermeshing.
2.5.1.2 Mixing Equipment
Mixing machines and equipment are covered comprehensively in the literature by
White109.11o, who includes reference to a large number of patents, and others111.
There are basically three broad types of mixing machine, the internal mixer, the
external (open roll mill) mixer and the continuous internal mixer.
2.5.1.2.1 The Internal Mixer
Internal mixers can either be of the intermeshing or non-intermeshing type, both
types rely on high local shear stresses and a lower shear-rate stirring action. The
basic configurations of internal mixers are shown in Figure 2-12. The rotors travel in
opposite directions. The non-intermeshing type relies upon a shearing action
between the rotor and the mixing chamber wall; however there is a certain amount
of shear between the rotors. The wings of an inter-meshing internal mixer are
synchronised so that they come close to those on the other rotor but do not collide.
This action causes the necessary shear rates and stresses to facilitate mixing. The
majority of the mixing action occurs between the rotors rather than between the rotor
and the wall. It is difficult to conceive that anisotropy or molecular orientation could
be caused on anything other than a localised micro scale.
39
2.6.1.1.2 The External Mixer Extemal (or open roll) mixers generally consist of two rolls that are parallel and
horizontal. These too rotate in opposite directions causing the material to be pulled
through the clearance (nip or bite) between the rolls. The rolls are rotated at
different speeds to create a shearing action. The surface of the roll is used to
transport the material, through banding, back to the clearance where more mixing is
undertaken. The action of the rolls is essentially to cause uniaxial shearing of the
material and stretching of the rubber molecules in one direction only. It is easy to
visualise how this process would leave the compounded rubber in a state where
there is significant molecular orientation. This anisotropy is referred to as 'grain'.
2.6.1.1.3 The Continuous Mixer
Continuous mixers generally rely on the use a screw or screws to mix and transport
material in a similar way to that employed in conventional extruders. In some cases
these machines are a hybrid between the previously described intemal mixer and an
injection or extruder mechanism. In some machines the majority of the mixing is
carried out by an intemal mixer mechanism which dumps out directly into a screw
mechanism. Others are just screw mechanisms. The screws are often staged with
first part having coarse 'blade-like' flights resembling the rotors of an intemal batch
mixer before the geometry alters to resemble the more familiar screw. This then can
be used to feed a machine that converts the material into a useable form such as
sheet, strip or pellet. In some cases, forming through a die is the final shaping
operation. It is the flow near the exit that seems to be the cause of anisotropy that is
similar to that produced by injection moulding. The exit of the mixing chamber is
usually some type of former or preformer. The molecules get orientated in the
direction of flow through this forming process.
2.6.1.2 Milling and Calendering
A rubber calender is a machine that is used to produce rubber sheet, in various
profiles, widths and thicknesses. In some cases a calender can be used to
incorporate reinforcing materials such as fabric or wire. A calender resembles a mill,
but will be much stiffer to be able to obtain and control the required thickness of
40
material. It will consist of two or more rolls, each of the adjacent rolls rotating in
opposite directions. Rubber is passed between the rolls where the nip (clearance) is
set to squeeze the material into sheets of the desired thickness. The anisotropy
induced by milling and calendering is similar to that of open-roll mixing.
2.5.1.4 Anisotropy and Pre-moulding Processes
It is very difficult to see how grain or anisotropy could be induced into the rubber
structure in an intemal intermeshing type mixer112 because of the dynamics of the
mixing process. However, other types of mixing (open roll mixing because of its
similarity to milling and calendering and continuous mixing, because of this similarity
to the secondary mix-transport phase to injection moulding transports) could be a
source of anisotropy if the action of the mixing is to orient the polymer molecules in
a particular direction. Milling and calendering have long been known to cause
anisotropy. This can be kept to a minimum by using as high a temperature as the
material will stand without the onset of cross-linking ("setting up") 113.
2.5.2 Moulding Processes
Moulding has been defined as the act or process of shaping in or on a mould, or
anything cast in a mould. These moulded parts are successfully used in a wide
variety of consumer, industrial and engineering applications. Moulding is covered
reasonably in most polymer and rubber textbooks114.11s.116. Sommer117.118 and
White119 pay particular attention to rubber moulding and Wheelans12o.121 to injection
moulding of rubber. Moulding processes are covered here to the extent necessary to
highlight orientation and isotropic effects.
2.5.2.1 Moulding Methods
In this work only compression, transfer and injection moulding will be considered.
Casting, reaction injection moulding (RIM), blow moulding and bladder moulding are
not considered at the present time, although it is should be noted that these forms of
moulding are likely to suffer from anisotropic effects similar to those to be described.
41
2.5.2.1.1 Compression Moulding
Compression moulding is the simplest form of moulding and is what most people
think of when 'moulding' is mentioned. A charge of un-cured rubber is placed in a
mould cavity prior to closure of the mould. Usually a slight excess volume of material
is used to ensure that the mould cavity is completly full on mould closure. Figure 2-
13 shows a compression mould before and after closure. The rubber preform is
squeezed between the top and bottom parts of the mould. Excess rubber is
squeezed between the split line and is captured in the spew (or flash) groove. This
obviously means that there will be flow in the mould even if the billet is a preform
that is near in shape to that of the finished article. For many years moulders have
controlled the flow in moulds to some extent by intuitively shaping charges. Silva
Neto, Fisher and Birley'22 used a finite difference approach to calculate pressure
and velocity fields and hence to determine optimum charge shapes for filling sheet
moulds.
r::"'lr-l'0~"""~"""::+,:,,,,,,"'1':"<:v' Top P I at e Slot
. ","VII v Plate
Figure 2-13 Schematic of typical compression moulding System
Compression moulds vary widely in terms of size, shape and number of cavities.
Obviously this depends on the size, shape of the product. Construction of the
moulds also varies. Two-plate moulds are common for thin and uncomplicated
geometry. Three or more plates are used for thicker and more complicated
geometry. The more complicated the geometry the more 'in-mould' flow will be
42
required and, generally, a greater excess volume of material will be required for
complete cavity filling (up to 50% excess can be needed for some complex parts).
Mould closure, generally, is attained with a large rigid press. It takes considerable
force to close a mould and force the material to the extremities of a cavity,
especially if it contains a high viscosity rubber. The most common types are the four
(tie-) bar (or four post) press and the side-frame (or side-plate) press.
2.5.2.1.2 Transfer Moulding
Transfer moulding is essentially injection moulding in a compression press. A typical
transfer moulding mechanism is shown by the schematic in Figure 2-14. The rubber
preform is heated by contact with the pot and the plunger. If sufficient force is
applied to the plunger in a press, as in compression moulding, the rubber is forced
to flow through the sprue into the mould cavity. A gap is deliberately left between the
transfer pot and the plunger, the size of which should be large enough to prevent
binding between the plunger and the pot and small enough to minimise the backflow
of rubber between the plunger and the side wall of the pot. The sprue is wider at the
transfer pot than at the mould cavity. Thus, when the flash pad is removed after
cure, the sprue breaks near the moulding and the majority of the sprue remains with
the flash pad. The diagram (Figure 2-14) shows a single cavity mould. However
multi-cavity mould configurations are also common.
Considerable flow is necessary to fill the mould and for the rubber is required to
pass through a relatively small orifice. Therefore there is a substantial increase in
the potential to induce molecular orientation in comparison with compression
moulding.
43
Plunger
Rubber Preform
.b,;>;"'-- Transfer Pot
~~~~~~~t-- S~rue ,,"-_ Cavity Plate
~~~~~~~-- Mould Cavity
""",....,--"<:""<"""""""""''''~_~ Piu n g er
.!t;.:>:*" __ Transfer Pot
Flash Pad
~~L_ Moulded Part
Figure 2-14 Schematic of transfer moulding set
2.5.2.1.3 Injection Moulding
Injection moulding is commonly used to obtain high production rates. The major
difference between injection moulding and compression or transfer moulding is that
compression and transfer moulding require a billet or preform to be placed directly
into a compression mould cavity or a transfer pot. An injection moulding machine
can be continually fed with granules or rubber strip. Injection moulding machines are
much more complex in terms of both geometry and machine control. An injection
moulding machine may have several temperature and pressure controls whereas
compression or transfer moulding systems normally do not. However, many
moulded parts are too complex to be made by compression moulding123• The clamp
(or press) is an integral part of an injection moulding system and must be capable of
providing high pressure clamping. The moulds need to be able to withstand high
temperatures, pressures and forces with very low levels of deformation during many
high speed closures. Figure 2-15 shows a simple injection moulding system. This
example has a screw-type injector but a ram or separated screw and ram systems
may be used.
44
Throat Nozzle Barrel Screw
Mould
Injection Chamber
Heating Fluid
Figure 2-15 Typical injection moulding system
The rubber is heated to its working temperature during its travel to the nozzle, by
heat transfer from the heated body of the ram and by work done on the rubber. The
rubber is forced through the nozzle and sprue into the mould cavity. The rubber will
become hotter as it passes through the nozzle and sprue due to the high shear
rates in the material. Hendrick and Fraser124 maintain that the physical properties of
injection moulded parts are better than those of compression moulded parts and
that, because the rubber enters a closed mould in a turbulent manner, strain lines
and stresses caused by flash or overflow are absent from injection moulded
products. As with transfer moulding, there will be considerable material flow during
mould filling. This has been investigated (for thermoplastic melts) by Spenser and
Gilmore125, White and Oee126 and Oda et al. 127, among others, but there are few
studies of rubber compound mould filling of injection machines save those of Isayev
and co-workers128•129
. There seem to be only two distinguishable mechanisms for
mould filling. In the first the melt emerges from the gate and fills the mould
progressively and evenly and in the second a 'jet' of material is projected from the
gate in filling the mould. Spenser and Gilmore, and White and Oee, observed mould
filling with special glass-windowed moulds and observed that small-diameter gates
and high injection rates could be associated with the phenomenon of jetting. A
similar jetting phenomenon was also reported by Lee, Griffith and Sommer130 for
polymer entering the cavity in transfer moulding. Oda et al. found that jetting was
related to the size of the extrudate melt entering the mould. If the extrudate swells to
less than the thickness of the mould after its exit from the gate (Le. it does not
45
contact the top and bottom of the mould) it would jet and this could also occur at low
injection rates. Oda et al. also found that by altering the geometry of the mould
entrance by placing barriers near the gate jetting could be completely eliminated.
With reference specifically to the injection moulding of rubber Deng and Isayev129
produced quantitative models for the simulation of flow dynamics, thermal history
and cure of rubber compounds in the mould. Their subsequent experimental work
for verification of the models revealed considerable anisotropy of the tensile
modulus of samples taken parallel with and normal to the direction of flow in the
mould.
2.5.2.2 Anisotropy in Moulding Processes
The is a reasonable body literature dealing with anisotropy in rubber moulding,
although it is not extensive when compared to the large body of literature which
exists relating to anisotropy in thermoplastics.
Gurney and Gough13\ Blow, Demirili and Southwart132, Chang, Yang and
Salovey133, Dinzburg and Bond134 and Hamed135 have studied anisotropy in
compression mOUlding. Lee, Griffith and Sommer136 and Dinzburg and Bond134 have
discussed transfer moulding and Lavebratt and Stenberg137,138,139,140,14\ Nakajima,
Fukata and Mineki142, Deng and Isayev129, Wheelans143 and Tsai144 injection
mOUlding. These works, and some other special cases, are reviewed in the following
sections.
2,5.2.2.1 Anisotropy in Compression Moulding
Anisotropy in rubber parts can be caused both in manufacture (of components) and
in the laboratory. This is highlighted by the tear-down test which is used to assess
the adhesion of a tyre tread to its base by measuring the force required to tear away
a strip of rubber tread from the tyre carcase. It was noticed that the force required
was different in opposite directions 131. Investigations using specially designed
compression moulds131 ,132,135, which force material in the mould to flow in a known
46
direction have shown anisotropy in swelling131.132.133, shrinkage132 and tensile
tests 132.134.135.
The experiments that were carried out in these studies were quite diverse. The
geometry of the mould cavities was generally simple and designed to produce sheet
from which test specimens could be cut. An exception was the study by Chang et al.
who used a more complex mould to investigate flow. Blow et al. experimented with
different billet placements in a circular mould to create different flow conditions (Le.
convergent, divergent and no flow) in a circular sheet mould. Anisotropy was noticed
in all of their flow conditions. The convergent flow condition showed significantly
more anisotropy than the other conditions indicating that there is probably greater
molecular alignment as the molecules converge in the centre of the mould. As
expected circumferential swelling is greater than radial swelling because the
molecules tend to be extended radially.
Hamed135 noted that secondary crack propagation occurred in different directions for
specimens cut parallel with the grain, indicating a directional failure mode. Gurney
and Gough showed that anisotropy was only evident in their samples when some
cross-linking had taken place and this, allied to the conclusion of Blow et al. that
peroxide cured samples show more anisotropy, is consistent with the idea that
anisotropy is due to residual stresses and molecular orientation that are 'fixed-in' by
curing. If it is not, the molecules can recover to a random isotropic configuration.
Chang et al. discovered that, depending on the geometry of the mould, flow does
not necessarily occur in straight lines. This means that at corners, or pins, where the
flow is curved, the local properties may have different directions from that of the
main body of the material in the moulding.
The experimental results generally concur with each other and with the in
manufacture results; anisotropy occurs in the direction of flow.
47
2.5.2.2.2 Anisotropy in Transfer and Injection Moulding
Injection moulding of rubber products has been growing in popularity because of the
level of automation that it allows for high volume production runs 142.143. Transfer
moulding has been commonplace in the production of rubber products for much
longer but it requires a similar level of labour intensity to compression moulding and
the moulds are unwieldy. Transfer moulding is included in this section because of its
similarity to injection moulding with regard to flow, from a point just before the
material enters the mould cavity through to the completion of mould filling. It is worth
mentioning at this point that the transport and warm-up system on most injection
moulding machines resembles the screw pump type mechanism used on many
extrusion machines. There have been several investigations covering the extrusion
of elastomer or rubber compounds; these are reviewed by White145. Brzoskowski et
al. 146 used two different coloured rubbers to investigate the flow of rubber in a screw
pump and produced good evidence of a re-circulating flow in between the flights. An
important point is that the rubber remembers the orientation induced in the screw
channel. This orientation could persist into the mould cavity and affect the overall
anisotropy of the product, especially if the runner and gate system is short in length
and large in bore. The longer and narrower this screw-to-cavity feed system is, the
less likely the circulating orientation is to persist.
The majority of studies of injection moulding have used a centre-gated mould to
produce disks138.139.14o.141 or sheets142.143 from which test specimens could be cut.
Deng and Isayev129 used an end-gated dumbbell and side-gate disk and Nakajima
et al. 142 used a long, narrow, centre-gated 'snake-like' cavity as well as a sheet
cavity. Again, a number of different methods were used to highlight the anisotropic
effects, tensile properties, shrinkage, etc. All the parts in the injection and transfer
moulding studies showed considerable anisotropy. Injection speed and mould
temperature were also considered as significant factors, the rate of flow being a
variable that is easier to control with an injection machine than it is, say, with a
compression moulding press. Deng's and Isayev's rubber mOUldings were found to
be highly anisotropic and the anisotropy could be reduced significantly by using a
higher injection speed. They explained this by saying that higher stress relaxation
speed would be the cause. The current author is of the opinion that there may also
48
be an explanation of this phenomenon in 'jetting' which has been shown to occur at
high injection speeds in the moulding of thermoplastics125.126 and this is possibly why
Hendrick and Fraser124 reported better properties in rubber injection mouldings than
compression mouldings in the mid 1940's. Figure 2-16 (a) shows a mechanism of
regular ordered flow filling of the mould cavity with smooth flow fronts and (b) show
jetting which is random.
11 11 II
(a)
11 11 11
(b)
Figure 2-16 Mould filling mechanisms, (a) regular ordered flow, and (b) random irregular flow or 'jetting'
Lavebratt et 81.'s studies showed anisotropy in swelling and mechanical properties.
They also observed that the presence of carbon black increased the anisotropy. The
anisotropy was thought to be due to residual molecular orientation from the mould
filling process. This filling however may not be simple filling flow of their disk cavity.
By delaminating the disks with a water jet-cutting technique, it was possible to carry
out further tests on the different layers. They concluded that the surfaces were
formed of radially oriented molecules produced by shear and fountain flow and that
the core orientation was the result of expanding flow147 giving tangential molecular
orientation.
49
Nakajima et al. showed that the shrinkage effects exhibited by the product from
their snake-like mould showed anisotropy that varied with distance from the gate,
reaching a peak and then tailing off gradually. This effect can easily be explained by
orientation. As the long mould was not completely filled before a no-flow situation
(flow stopped) was reached, the molecules elongate as they flow along the mould
away from the gate. In the mould, when the flow becomes static, the molecules at
the flow front (farthest from the gate) will start to recover. The molecules closest to
the gate will not have had time to extend and orient greatly in the direction of the
mould. The farther from the gate the greater the orientation of molecules at the peak
shrinkage point will be fully extended. Molecular recovery between this point and the
gate is prevented by molecular packing and, ultimately, cross-linking. The molecules
farthest from the gate will also have had the greatest recovery time.
Another problem caused by localised anisotropy, although it is not a direct measure,
in mouldings is backrinding or cut back148. This ragged indentation at the spilt line of
a mould is most likely caused by large escape flows at the mould split line caused
by mould closure pressure and thermal expansion orienting the molecules in the
direction of the escape flow. On release of mould pressure and cooling there is
greater retraction (shrinkage of these extended molecules) causing the material to
tear at that point. Control of this phenomenon is achieved by reducing the billet
volume or increasing the resistance to the escape flow in the design of the
mould149.15o, for example by using a plunger type mould.
2.5.3 Anisotropy in Special Cases
Anisotropy can be found in polymer networks produced by methods other than flow,
Hamed and Song151 have studied anisotropy induced by straining uncross-linked
rubber samples. The anisotropy is thought to be due to the rearrangement of the
molecules in the direction of strain and set due to slip rearrangement of molecular
entanglements. Theoretical models predicting the changes in properties of
elastomers cross-linked in states of flow (strain) have been produced by Berry,
Scanlan and Watson 152 and Flor/53 and these support the 'two network' scheme
proposed by Andrews, Tobolsky and Hanson154 in which there are two
interpenetrating networks, one formed in the unstrained state before flow and the
50
other in the strained state, during or after flow. Kramer, Carpenter, Ty and Ferry155
cross-linked strained samples near Tg (00) with y-radiation to minimise the
entanglement slippage in their investigation of entanglement networks. The samples
were mechanically anisotropic, but contrary to expectations the modulus was lower
parallel with the direction of stretch than normal to it. Similarly, linear swelling was
greater parallel with the direction of stretch than normal to it. The reason for this is
unclear, but could possibly be due to the difference between elastically effective
entanglements and chemical cross-links.
Alternatively Rigbi and Mark156 have shown that the anisotropy revealed by swelling
and stress-strain properties can be induced into rubber by incorporating a magnetic
filler and vulcanising in a magnetic field. The samples show slight increases of linear
elongation in the direction parallel to the magnetic field and marked differences in
stress in directions parallel and normal to the direction of the magnetic field. The
orientation of the filler in the magnetic field provided greater reinforcement in the
direction of the field, even though the particles were basically spherical.
Columnar discotic liquid crystals 157 are a special case because they are naturally
anisotropic. These materials are very highly oriented because of their molecular
shape. Results of swelling experiments on such a network are reported by Disch et
al. 158. These results are interesting because they mirror those of normal elastomers
in that there is less swelling in the direction of 'grain' (Le. the direction of the
director). Nematic polymers are also intrinsically anisotropic159.16o in character.
2.6 Summary and Comments on Anisotropy and Orientation in Rubber
Anisotropy (or 'grain') in rubber parts has been known about and its effects have
been worked around in the rubber industry for many years. However, suggestions
for control of anisotropy are vague. The term 'grain' seems to encompass
orientation, residual stresses, the anisotropy of shrinkage during cooling and other
mechanical effects. These effects vary directionally depending on the direction in
which the test is carried out, or show a significant directional component when
testing is multi-axial (e.g. immersion in solvent). It is significant to note at this point
that SS 903 Part A2: 1989161 (ASTM D_412162is a similar standard), says "Cut dumb-
51
bells, wherever possible parallel to the grain of the material unless grain effects are
to be studied, in which case cut a set of dumb-bells perpendicular to the grain", and
that the following standards all make mention of anisotropy or 'grain': BS 903 Part
A3 (ASTM 0-624), BS 903 Part A16 (ASTM 0-1460), BS 903 Part A51, ASTM 0-
378, ASTM 0-454, ASTM 0-518 and ASTM 0-797.
As described in the previous sections, anisotropy in moulded rubber can be
observed and measured in most aspects of its mechanical behaviour. Tensile
stress-stain behaviour, mould shrinkage and swelling due to the action of good
solvents seem to be the most popular methods. Tear strength and permanent set, in
tension or compression, also show differences due to grain or molecular orientation.
Lavebratt et al. 137 have shown that it is possible to use x-ray scattering to indirectly
assess the orientation of the rubber molecules in a test piece by studying the
orientation of zinc oxide crystallites which are often used in sulphur curing systems.
Tg and linear thermal expansion12.13 and electrical properties can also be used as a
measure of anisotropic behaviour9.3o.31.
Birefringence can be used to study the molecular orientation of polymer and
elastomer during forming and straining although this is of limited application to filled,
especially black filled polymers.
2.6.1 Causes of Anisotropy in Products and Mouldings
From this literature survey a few main points can be highlighted as the causes of
anisotropy in moulded rubber parts.
• Molecular orientation caused by flow in the mould seems to be the greatest
contributing factor to anisotropy131.132.133.134.135.138.140.
• Vulcanisation during flow (scorch) is an important factor giving anisotropy in
moulded parts 131.132.142.
• Anisotropic behaviour is due to the orientation rather than a difference in the
state of cure in various directions 144.
52
• The rate of vulcanisation is an important factor which determines the degree
of shrinkage anisotropy142.
• The presence of carbon black enhances anisotropy138,139.14o.141.143.
• Higher anisotropy observed in injection and transfer moulding than in
compression moulding134.142,143.
• Increased injection speed reduces anisotrop/29.
The dominating factor giving rise to anisotropy is molecular orientation occuring
during in-mould flow or mould filling; and the distribution of orientation direction is
due to a combination of different types of flow in thicker and (geometrically) more
complicated parts.
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112p. Whitaker, J. IRI, 4,153 (1970)
113M. M. Heywood, The Applied Science of Rubber Ch 5 Pt2 p342, W. J. S. Nauton Ed.(Edward Arnold, PLACE, 1961)
114B. G. Crowther, Rubber Technology and Manufacture 2nd ed. Ch8 pp340-346, C. M. Blow and C. Hepburn Eds., (Butterworth Scientific, London, 1982)
115J. G. Sommer, Basic Compounding and Processing of Rubber, H. Long Ed.Ch6 (Rubber Division, American Chemical Soc., Akron, 1985)
11BF. Rodriguez Principles of Polymer Systems: International Student Edition 2nd ed., Ch12 pp348-362, (McGraw-HiII, Tokyo, Japan 1983)
117J. G. Sommer, Rubber Chem. Technol., 51, 738 (1978)
11BJ. G. Sommer, Rubber Chem. Technol., 58, 662 (1985)
57
119J. L. White, Rubber Processing: Technology - Materials - Principles Ch20 pp519-535 (Hanser, Munich, 1995)
12°M. A. Wheelans, Injection Moulding of Rubber (Butterworth, London, 1974)
121 M. A. Wheelans, J. IRI, 4,160 (1970)
12'R. J. Silva-Neto, B. C. Fisher and A. W. Birley, Polym. Comp., 1,14 (1980)
123G. C. Hessney, Rubber Age, 70, 825 (1958)
124J. V. Hendrick and D. F. Fraser, Rubber Age, 56, 277, (1944)
125R. S. Spenser and G. D. Gilmore, J. Coil. Sci., 6, 118, (1951)
12BJ. L. White and H. B. Dee, Polym. Eng. Sci., 14, 212, (1974)
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129J. S. Deng and A. I. Isayev, Rubber Chem. Technol., 64, 296 (1991)
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14BR. Brzoskowski, J. L. White, F. C. Weissert, N. Nakajima and K. Min, Rubber Chem. Technol. 59, 634 (1986)
58
147W. Woebeken, Mod. Plas!., 40, 146 (1962)
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59
Chapter Three
3. The FORM System
This chapter introduces the isotropic moulding system concept. It describes the
novel computer-controlled, flexible, automated compression moulding system that
was designed and built for the project; the individual components that make up the
system and their interaction. The control system, the sequence of operation of the
system and the overall system operational methodology are discussed after a brief
mention of conventional compression moulding.
Compression moulding of rubber parts is a highly labour intensive process, in
common with much of the rubber industry and has very little automation. This is
especially true for the manufacture of high precision parts such as seals for the
aerospace and oilfield sectors. Generally, for seals, a rubber chord is extruded, cut
to length and placed into the hot mould by hand. For less critical and generally
smaller seals, injection or injection-compression moulding is used to attain the high
volumes required but some of the precision is lost. These products often exhibit a
significant degree of anisotropy due to molecular orientation, mould design can be
difficult due to directional post demoulding shrinkage effects. In the service
environment seals are often exposed to solvents and components can exhibit
anisotropic swelling which may lead to a premature failure.
3.1 FORM System Concept - Overview
The isotropic moulding system (Appendix A), or FORM system, as it has come to be
known, consists of a preforming dispenser with interchangeable metering inserts, an
60
----------------
up-stroking compression press and a computer control system. These three items
coupled with a set of plunge moulds provide a flexible modular system that can be
used to produce isotropic or near isotropic mouldings from a wide range of
elastomers and, possibly, other polymeric materials.
As described previously in Chapter 2 the common phenomenon of anisotropy in
rubber mouldings is largely attributed to molecular orientation that is induced into the
material during the various processing stages and 'fixed-in' when the part is cross
linked (cured). The combination of the dispenser, press and moulds is used to
minimise molecular orientation by reducing in-mould flow and allowing sufficient time
for molecular recovery before the onset of cross-linking. thus producing a random
molecular structure that will be substantially isotropic in nature and have consistent
properties. Additionally, automation of the compression moulding process
(eliminating labour and production processing steps), repeatability and the ability to
produce flash-free products are worthwhile but secondary advantages of the
process. The individual elements of the system and their function are described in
the following sections.
3.1.1 The Dispenser Concept
The objectives of the dispenser, a positive displacement valve, are several. The
primary function of the dispenser is as a preformer, repeatably producing a precise
and uniform preformed blank that is near both the shape and volume of the mould
cavity. The secondary functions are: to pre-work the material and break down any
residual molecular orientation ('grain' or primary orientation) from previous
production processes such as milling, calendering or extrusion, etc.; to deliver the
preform directly in to the open mould (Le. a material transport system); and, to pre
warm the material.
Producing a blank that is already close to the required size and shape of the
finished product is considered the most important function of the dispenser because
the primary cause of anisotropy in moulded parts is trapped in molecular orientation
61
due to flow in the mould during forming1,2,3,4,5,6,7, a blank of the correct shape, size
and weight will reduce to a minimum the need for such in-mould flow.
Molecular orientation will initially be partially randomised on a macro scale by the act
of division that is required in feeding the bulk stock into the machine and, perhaps
more importantly, pre-working the material (Le. making it flow) will further break
down8,9,1o,11 any molecular orientation or intermolecular/interparticulate structure that
is present in the uncured stock from previous preparation processes.
Pre-warming the material will occur due to direct heat transfer from the body of the
dispenser, which is heated, and due to heat being generated by viscous flow
(intermolecular friction). The resulting temperature rise in the material will allow it to
be more easily processed and formed by reducing viscosity. It will also bring the
material closer to moulding temperature and reduce the 'in-mould' time.
(Conventional compression moulding is often carried out starting at ambient
temperature and the time to warm the material through can be significant). The
tendency of the molecules to recover due to increased macro-Brownian motion 12, 13
will increase.
The dispenser unit is mobile and can be moved into the press daylight to a position
directly above the mould cavity where the preforming operation (dispense cycle) is
initiated and a preform dispensed directly on to the lower half of the mould cavity.
The dispensing operation forms the blank by 'extruding' material into free space and
parts it off, thus removing the material from any constraint of the material-flow path
and allowing molecular recovery in the short time it takes the preform to fall onto the
hot mould, and before complete mould closure. Cross-linking should not take place
until the part has been completely formed.
3.1.2 The Press Concept
The press is used for the final shaping operation, closing the moulds which form the
part into its ultimate configuration. A rigid press is required to ensure that the platens
and the mating halves of the mould are parallel and do not flex. The platens need to
be capable of bringing the mould to up to the temperature that is required for curing
62
the elastomer compound. The press closure needs to be precisely controllable in
terms of position and the initial closure speed should be rapid to minimise the length
of time that the preform is resting on the bottom half of the hot mould and therefore
susceptible to scorch at the contact points. If cross-linking starts before shaping has
been accomplished, anisotropy will be increased14.15.16. The press will be required to
hold the mould closed under high pressure until the desired cure time is complete.
Certain defects (e.g. backrinding) can be prevented to some degree if the press has
a high clamp force and the mould and platens are stiff and do not move or deform
with the thermal expansion of the rubber17•18. This helps to minimise escape flows.
3.1.3 The Mould Concept
Plunge type moulds were selected for use as an integral part of the FORM system
because they offer considerable advantages over conventional two-plate moulds in
terms of both material handling and the production of isotropic and flash-free parts.
A recessed cavity in the lower half of the mould pre-Iocates the preform, effectively
increasing the target area into which the preform can be dropped from the
dispenser. A step around the cavity will, when the two halves are mated, help to
control the escape flow of excess material from the mould cavity by providing an
impeded flow path. This design also makes it possible to have a large contact
support area that does not get fouled by the excess material escaping and affecting
the dimensional integrity of the product.
It is also possible to produce flash-free components that require very little or no post
demoulding. This is not a new concept as methods have been proposed for flash
free injection 19.20 and transfe~1 moulding. However, elimination of post-mould
trimming seems to have been accomplished in compression moulding by moving the
mould split line to 45° from horizontaI22.23. This is achieved by a combination of, first,
good mould design and careful manufacture and, second, a controlled mould
closure sequence. To achieve flash-free components it is necessary to have direct
metal-to-metal contact at the split line of the mould and to have even compression
covering the whole area of the mould and as near as possible truly parallel mould
closure (Appendix 8). To make possible metal-to-metal contact at the split line, the
mould was designed with a land 1 mm wide and 0·3 - 0·4mm in height adjacent to
63
both the inner and outer diameters of the upper mould cavity. The plunge has a
clearance, to impede escape flow, of between 0·25 - 0·3mm, opening to 0·75mm
and connecting a larger spew cavity. Further metal-to-metal contact is obtained over
a large contact support area out-board of the spew cavity.
3.1.4 The Com puter Control System
A computer system is used to control the operation of both the dispenser and the
press and is essential for the automation of the system because of the need for a
high degree of accuracy for position control, speed, timing and the sequencing of
the interaction between the dispenser and the press. The quality of the moulding
and the extent to which it is isotropic and flash-free depends ultimately on a
combination of these factors. In the future it is expected that the system could be
extended to log data for compliance with quality assurance standards such as SS
5750 and ISO 9000.
3.1.5 Sequence of Operation
The dispenser and press are controlled by the computer system to give the following
sequence of operations:
(i) Feed the dispenser with fresh material, this could be strip or cut sheet.
(ii) Pre-heat the fresh material to reservoir holding temperature (60-90°C) by
heat transfer and viscous work. The reservoir should provide sufficient
residence time to heat the fresh stock to the required temperature before it is
dispensed.
(iii) Open the press, demould products, clear spew.
(iv) Traverse the dispenser into the daylight between the platens.
(v) Dispense a metered preform and part into lower cavity of the plunge
mould.
64
(vi) Traverse the dispenser from between the press platens. Operation (i)
may be repeated as soon as the dispense operation (v) is completed.
(vii) Close press and cure part. This includes the phased closure to give
isotropic and flash-free components.
(viii) Repeat operations from (iii).
3.1.6 Reduction in the Number of Processing Operations Compared with
Conventional Compression Moulding
Conventional compression moulding usually requires as many as six processing and
storage steps of work in progress to reach the finished product. The FORM system
reduces the number of operations to three (Figure 3-1). Work in progress is
minimised by eliminating storage stages and combining preforming, forming and de
flashing into the moulding operation.
Pre-Form -+ Store f-+ Mould -+ De-flash
Blank
(a)
Mix H Store H Mould
(b)
Figure 3-1 Stages in the manufacturing process of (a), conventional compression moulding and (b), Form system moulding.
3.1.7 System Configurations
The system is versatile and flexible because its modular nature allows the creation
of large number of configurations which are only limited by such factors as the
speed of the traverse between the various moulding stations, required material
65
residence time (for warming) and rate of cure (the in mould cure time which is
dependent on component size).
3.1.7.1 Stand-Alone Configuration (Single-Station)
The single-station configuration comprises one of each of the main components, a
dispenser (with a number of interchangeable preforming heads), a press (with a
number of interchangeable moulds) and a computer control system. The dispenser
feeds only one press (Figure 3-2) and is consequently idle for most of the time it
takes to cure the component. This is not the most flexible arrangement of the
system, because only one component type can be manufactured at anyone time,
although it may be possible to produce a range of like components. Nor does it
make the most efficient use of capital, but it does give a level of automation that is
not seen in compression moulding, although an increased pre-warming residence
time for the raw stock in the dispenser unit is a consequence of this particular
configuration. More efficient use of a dispenser unit is made in multi-station
systems.
f---- Dispenser Press I--
Figure 3-2 Stand-alone (single station) system configuration. A single dispenser feeds a single press.
3.1.7.2 Multi-Station Configuration
It is envisaged that the FORM system in a production/manufacturing environment
will be a flexible, fully automated, manufacturing cell consisting of a dispenser (or
66
dispensers) and a number of presses. Such a configuration will add a great deal of
flexibility to the system. One dispenser unit could feed as many as eight presses
traversing between them in sequence dispensing preforms directly into the mould of
each (Figure 3-3(a)). If the cure time of the component is equal or less than the time
to dispense preforms in all eight presses and return to the start point of the
sequence, then dispenser and press utilisation and cell productivity will approach
100% efficiency. Figure 3-3(b) shows another variation which is yet more flexible.
Two completely different products could be manufactured in this cell configuration.
In both configurations it would also be possible to remove any single press from the
dispenser cycle for maintenance, manual operation or product/mould proving trials
etc.
a a a: a 1 Dis~ensei
Press
a:··········;
a: a IOisp:ensel
a a,··········· a
a a a ~
-r ;r:s~ -1- ~i~~~~:f :: : ·1· -a (b) : .......••. : L..::J (a)
Figure 3-3 Two different possible manufacturing cell configurations: (a), single dispenser -multi-press configuration for medium to high volume production and, (b), multi-dispenser -multi-press configuration for volume and flexibility.
3.2 Prototype FORM System Description
The prototype FORM system used in the research reported in this thesis is a single
station machine. It was constructed as a stand-alone system to prove the concept of
isotropic moulding. Unlike a production system, the dispenser and press are
inextricably linked.
67
3.2.1 Description of the Dispenser
The dispenser (Figure 3-4) consists of a material feed mechanism, the main body
carriage and interchangeable metering and preforming inserts. The feed mechanism
is, in this case, a double-acting hydraulic ram (050·8mm bore and 025·4mm rod)
and ('stuffer') piston (030mm in 030mm bore with tight clearance fit) which is used
to prime the system with polymer through the feed pocket and advance it through
the runner system of the dispenser to the meter cavity in the insert.
Dispense Actuator Ram (2 oil)
Heating and
Dispenser - Main Body (fixed)
Cooling Water ~~'\"_
Main Body - Outer (free)
Preform - Outer (free)
Insert Lock Ring
Dispenser - Preform Insert (fixed)
Material Feed Pocket
Feed 'Stuller' Piston
Feed Runners (6 oil)
"-''''+---+~-+-
'---Meter Cavity
Figure 3-4 Schematic section of the O-ring preform dispenser. The main body is shown in dark grey and the insert or 'head' in light grey. The meter cavity can clearly be seen formed between the fixed and 'free' (movable) parts.
The main body is a cast carcass with intemal voids for heating and cooling water, it
consists of two main parts which can be moved relative to one another and held at
any given position by means of two double acting hydraulic rams (063·5mm bore
025·0mm rod) and a continuously variable servovalve. The dispenser insert or
'head' is fitted into the main dispenser body. It, also, consists of two parts which can
68
be moved relative to each other. The meter cavity is formed in a void created
between the inner (fixed) and the outer (free) parts of the dispenser head. When the
inner and outer parts of the dispenser head are in the relative position shown in
Figure 3-4 and Figure 3-6 (a) and (b) the void has its maximum volume. The inner
and outer parts of the head are fixed to the inner and outer parts of the dispenser
main body, respectively. Heating and cooling of the insert is carried out by direct
contact with the main body which in turn takes heat from water recirculating through
a Churchill water heater.
Imagine that the system is filled with material and the meter cavity is full (Figure 3-6
(b)). The outer part of the dispenser head can move relative to the inner part which
remains stationary. The outer part can move up to give the displacement required to
shut off the feed runners and force a known volume of material out of the aperture
that is simultaneously opened at the bottom of the dispenser (Figure 3-5 (a) and
Figure 3-6 (c)). The outer part is then moved down to dispense and crop the preform
away from dispenser into the mould (Figure 3-5 (b) and Figure 3-6 (d)) before
returning to the initial position (Figure 3-4 and Figure 3-6 (a)) ready for re-filling the
meter cavity. The gap clearance at the opening of the point of the meter cavity is of
the order 0.05 - O.OBmm. This is generally small enough to prevent material from
escaping during filling. The whole dispenser unit is on a carriage that can be
traversed, by a hydraulic ram, back and forth into the daylight of the press for the
purpose of dispensing a preform directly into the hot mould.
69
~~~~-------------~--------~ (a)
I
(b)
Figure 3-5 Relative Movement of Dispenser Fixed Inner and Free Outer Component: (a), Typical dispense action (outer moved up); (b), Typical material purge position (outer moved down).
(a) (b)
(c) (d)
Figure 3-6 Dispense sequence for an O-ring preform: (a) the meter cavity filling; (b) when the meter cavity is full the narrow gap at the bottom is usually enough to prevent material escaping; (c) the outer part of the meter cavity (shown here on the right) moves upwards shutting off the filling flow and expelling the preform simultaneously; (d) the outer part is then moved down to crop preform away from dispenser, the preform then fall directly into the mould.
70
3.2.2 Description of the Press
The press is a four column (or (tie-)bar) upstroking hydraulic press. Four large
columns pass through both of the platen support plates, the upper plate being fixed.
The lower plate slides along the columns to meet the upper plate and provide good
guidance and aid parallel closure. Three hydraulic rams are used for press closure.
Two high-speed double acting jack rams (025mm bore and 018mm rod), are used
to effect near closure and opening and a large single acting main ram (0152-4mm
bore) is used for final high-pressure closure (i.e. forming and curing). The maximum
delivery from the hydraulic pump to feed the rams described in this, and the
previous, section is 23·65Mpa (~3400psi).
The platens are 400mm x 400mm. The effective moulding area is limited to the
300mm x 300mm central area. Each of the platens is heated by a 3·6kW resistance
heater. The maximum daylight between the platens (i.e. without moulds) is relatively
large at 500mm and can accommodate, with good clearance, the height of the
current dispenser, its inserts and moulds. This also allows room for modifications,
applications and adaptations.
3.2.3 Description of the Control System
The machine, as a stand-alone unit, has an integrated control system the backbone
of which is a GE-Fanuc 90-30 series programmable logic controller (PLC) and its
daughter sUb-systems. The PLC holds a sequential program that executes in a
repetitive manner until stopped by a command from the user, another device or
extemal event. The PLC program constantly scans the inputs from sensors,
actuators, limit switches etc. (reading input), and calculates solutions based on input
(decision making!) data. It sends output based on program logic (output update),
and checks for user input or override and CPU and subsystem diagnostics. One
complete cycle of these operations is often called a sweep. Each sweep will take in
instructions and information, act according to the logic of the program and send
output signals to the various elements of the system.
71
A personal computer runs a dedicated machine-control interface program which has
been written as part of this project, and is used to access PLC program and change
settings (i.e. temperature set points) and to upload/download position programming
sequences for the dispenser and the press. A hierarchical structure for the control
program was devised, with two levels of access, a 'user' level and a 'supervisor'
level. Each level is password protected. The user level (Figure 3-7) has limited
access and allows the operator only to load, program and run cycles and change
processing variables such as temperature set points and cure time. The supervisor
level (Figure 3-8) allows the operator to use all the functions from the operation of a
complete dispense and mould cycle to reading and writing individual control bits to
the PLC memory registers.
3.2.3.1 Temperature Control and Set Points
Precise temperature control of both the dispenser and platens is necessary for
successful and repeatable processing and moulding with the FORM system. The
dispenser is heated by recirculating water heated by a Churchill water heater and
the each platen is heated by a resistance heater. A set point is specified for each of
the three heater units.
(i) Dispenser Temperature Control
The control of the dispenser body temperature is devolved almost completely to the
Churchill unit. The PLC sends the set point to a Eurotherm controller. Temperature
is maintained within a ±1°C range, once set the point is attained, by water heating. If
temperature reduction is necessary, fresh cooling water is used to achieve rapid
active cooling. Such rapid cooling is used for dispenser cooling on shutdown to
increase the longevity of the material resident in the dispenser.
(ii) Platen (Press) Temperature Control
The platens are heated by resistance heaters that are controlled directly by a PID
(proportional/integral/derivative) closed-loop subroutine in the PLC. The controller
72
.., cl;" I: ~
CD W , .... 0 0
" I START-UP ~ ...
2- Start-Up Routine "lI ~
List alarms 0
"" ~ AI Temperature set and 3 " " up to temp indICator.
-..j c: w Cl> m ;;1:! r ~ ~ » " " CD .. .. :z: iD" ~ AI tl ". '<
Limited Function and Access (Level - 1)
1 OPEN 1
RECIPE
r I 1
I Ram 11 Dispenser 11 Pressures 1 Construct and Construct and edit ram edit dispenser (press) cycles. cycles.
Run and step Run and step through cycle through cycle
Including cure Including time and temp. dispenser
temp . Save and Restore Save and
Restore
No control of current pressure control (i.e. Register ROO11)
List and set the various pressures for the machine operations.
OPERATION
I I
1 Auto 11 Manual I Manual Run. Ex!. Dispense
Ram cycle Auto Run. Stuffer cycle
Stuffer in Temperature Stuffer out read and abort. Enable Disp.
axis control
Temperature read and abort.
"Y1 cC' I:
01 w clo o o " [ 'lI Cl ID iil 3 en c: 'lI m
~ en o ~
i" !!.
~ n CD
= ::t iD' Ol Cl ::r '<
START-UP
Start-Up Routine Ust alarms
Temperature set and up to temp indicator.
Setting
Mould Calibration
Full Function and Access (Level - 2)
RECIPE 1------------1 OPERATION
Registers
AI-Read R - Read R - Write Q - Read Q - Write I - Read
On-line list of thecommon register variables.
Ram Dispenser
Construct and Construct and edit ram edit dispenser (press) cycles. cycles.
Run and step Run and step through cycle through cycle
Including cure Including time and temp. dispenser
temp. Save and Restore Save and
RF!~tnrp.
Current pressure control (i. e. Register Roo11) should have hot-key instant access.
Pressures Auto Manual
Ust and set the Manual Run Int. Dispense various Ext Dispense pressures for Auto Run. Move disp. in the machine Move disp out operations. Temperature Ram cycle
read and abort. Stuffer cycle Stuffer in
Hotkey access Stuffer out to pressure(R Disp. Zero 11). (SoU)
Disp Max (Top) Enable Disp. axis control
Hotkey access to pressure(R 11 ).
Temperature read and abort.
parameters are recalculated every 0·01 s. An iron-constantan thermocouple
embedded in each platen supplies the measured temperature input for each
calculation and the output power variable necessary to maintain temperature is
altered accordingly. Temperature is maintained to ± 3°C (from observation) however
further tuning could reduce this if necessary.
3.2.3.2 Hydraulic Pressure Control
Hydraulic line pressure is maintained by direct proportional control of power to the
hydraulic pump. Line pressure is maintained to a current value set in the pressure
current-value register in the PLC memory. Set points are copied by the PLC to the
current-value register during the program sweep. Hydraulic fluid is continually
displaced and excess is dumped back to the storage tank via non-return relief
valves.
3.2.3.3 Position Control
The position control of both the dispenser and the press uses comparative-relation
function loops. The PLCs axis position module (APM) carries out an initial
comparative calculation based on the current position and the required set-point to
determine the direction of travel. Then the APM simply compares the set position
value with the current position value and drives toward the set value, sampling at
0.01 s intervals until the values are equal or greater than (or less than, depending on
the direction of travel) the set-point value. Both the dispenser and the press have a
Moire-fringe linear displacement encoder attached to provide position information.
(i) Dispenser Position Control
Control of the dispenser position is accomplished with a servovalve which ensures
accurate control and maintenance of the set-point position. The encoder has 7000
divisions for a maximum stroke length of 38mm (0·0054mm per division
(calculated)). Overshoot of set position was recorded at a maximum of 5 divisions
(0·027mm).
75
(ii) Press Position Control
Position control of the press closure is carried out by the APM, as accurate
maintenance of position does not have to be attained for long time periods,
excepting during cure when position control is effected by the natural stop that
mould closure provides. The encoder on the press has 85000 divisions over a
possible stroke length of 500mm (0·0059mm per division (calculated)). Overshoot of
the set position was recorded at a maximum of 10 divisions (0·059mm).
3.3 System Operation
3.3.1 Programming the Dispenser and Press Operation Cycles
The moulding system relies on the PLC to effect the production of mouldings. The
sequential operation of the dispenser in producing and parting-off the preform and
delivering it to the mould cavity and the phased closure of the mould are essential
processes in the moulding of isotropic components as well as in the production of
flash-free mouldings that require no, or little, finishing.
The inputs required from the operator are operation pressure, temperature and
position set points for the motion control of the dispenser and press. The structure
of the control program separates these inputs from each other, each having its own
menu.
3.3.1.1 Temperature and Pressure Setting
Set points for temperature and pressure are simply entered into the control program
via a menu option for each. The three temperatures (dispenser and upper and lower
platen) are set and, usually, not changed unless or until another change is made
(e.g. material or product change).
The pressure is defined in a similar way but there are more variables. Each ram or
set of rams has to have a defined working pressure. The value is stored in memory
and then copied to the current-pressure register at the beginning of the PLC
76
subroutine function call. This can be overridden for some functions and set manually
by directly writing a value to the current-pressure register.
3.3.1.2 Dispenser Cycle
The dispenser motion-control cycle consists of seven position set points and their
corresponding dwell times. The values that must be entered for position are in the
range 0 - 7000 (0 displacement is the lowest point the dispenser can reach and
7000, corresponds to the maximum dispenser stroke (Le. maximum preform
volume)), corresponding to the resolution of the Moire-fringe type linear encoder.
The dispenser cycle 'program' is entered into the PLC via the PC interface program.
A typical set of dispenser position control values are given in Table 3-1. This
represents a single crop-dispense motion. Positions 4, 5 and 6 are redundant. The
dwell times are used sparingly in this cycle; 1s for Dwell 2 is used to ensure that
enough material is expelled and the slight delay of 0·5s (Dwell 3) at the lowest
position set point (Position 3) is to aid cropping.
Dispenser Position Value and Dwell Register (encoder divisions/
dwell Cs» Position 1 3450 Dwell 1 0 Position 2 5850 Dwell 2 1 Position 3 200 Dwell 3 0·5 Position 4 3600 Dwell 4 0 Position 5 3600 Dwell 5 0 Position 5 3600 Dwell 6 0 Position 7 3450 Dwell 7 0
Table 3-1 Typical set point values for an O-ring dispense cycle. The first and last positions (Position 1 and 7) are set at the point of maximum cavity volume (for ease of operation only). Position 2 is the main dispense ('extrusion') stroke and Position 3 is the crop. Positions 4, 5, and 6 are set at a point above flush to aid parting although this is often not necessary.
77
----------
3.3.1.3 Press Cycle
The press motion-control cycle is very similar to the dispenser control cycle. Again, it
consists of seven position set points and corresponding dwell times. However, there
is one extra dwell, cure time, that is not strictly part of the position-control sequence
but was included with the press sequence for ease of operation. The press
displacement is also set by encoder value, in this case the range is 0 - 85000. The
full press stroke can only be obtained without moulds fitted. To determine and set
the maximum allowed stroke a separate operation has to be undertaken and the
maximum encoder value automatically read and stored by the PLC. This must not
be exceeded.
A press cycle usually consists of a complete forming closure and a single' breathe'
step followed by full closure and cure. The cure time dwell does not require a
position setting as the press will close to maximum allowed displacement under the
set pressure for the specified time immediately on completion of the press cycle.
Press Position and Value Dwell Register (encoder divisions/
dwell (s» Cure Time (s) 600 Position 1 82586 Dwell 1 0 Position 2 82570 Dwell 2 6 Position 3 82586 Dwell 3 0·5 Position 4 82586 Dwell 4 0 Position 5 82586 Dwell 5 0 Position 5 82586 Dwell 6 0 Position 7 82586 Dwell 7 0
Table 3-2 Typical press cycle for an o-ring. A single 'breathe' or 'bump off step is included (Position 2) and the mould is parted for a period of 6 seconds. The following positions are redundant but could be used if necessary for further 'bumping off or in special cases the full closure step could be shifted to, say, Position 4 and the preceding steps used a careful approach to initial full closure.
78
---- -----------
3.4 References
'w. A. Gurney and V. E. Gough, Trans. IRI. 22,132 (1946): RubberChem. Technol. 20, 863 (1947)
2C. M. Blow, H. B. Demirili and D. W. Southwart, J. IRI. 8, 244 (1974): Rubber Chem. Technol. 48, 236 (1975)
3W . V. Chang, P. H. Yang and R. Salovey, Rubber Chem. Technol. 54, 449 (1981)
4B. N. Dinzburg and R. Bond, Int. Polym. Proc., 6, 3 (1991)
5G. R. Hamed, J. Appl. Polym. Sci., 27, 4081, (1982)
6H. Lavebratt and B. Stenberg, Plast. Rubb. Comp. Proc. Appl., 20, 3 (1993)
7H. Lavebratt and B. Stenberg, Polym. Eng. Sci., 34, 905, (1994)
Bp. Stamberger, Kolloid Z., 42, 295 (1928)
9A. R. Payne, 2nd Int. Rubb. Symp., London, 11·13 Qct. 1960; Rubb. Plast. Age, 42, 963 (1961)
,oA. R. Payne, J. Appl. Polym. Sci., 9, 2273, (1965); Rubber Chem. Technol., 39, 365 (1966)
"C.M. Blow, IRI Trans., 5, 417 (1929)
'"w. Holmann, Rubber Technology Handbook, Ch4 (4.2.1), p222 (Carl Hanser Verlag, Munich, 1989)
"c. Hall in Polymer Materials: An Introduction lor Technologists and Scientists 2nd ed., Ch2 (2.9) pp49·50 (MaCmillan Education Ltd., Basingstoke, 1989)
"K. Qda, J. L. White and E. S. Clarke, Polym. Eng. Sci., 18,53, (1978)
"w. Dietz, J. L. White and E. S. Clarke, Polym. Eng. Sci., 18, 273, (1978)
,sA. I. Isayev, Polym. Eng. Sci., 23, 271, (1983)
"N. L. Catton, The Neoprenes: principles 01 compounding and processing Appendix 8 p199 (E. I Dupont de Nemours, Wilmington, 1953)
'BE. L. Stangor, Rubber Age, 60, 439 (1947)
'9H. F. Jurgeleit, Rubber Age, 90,763 (1962)
2°H. F. Jurgeleit, British Patent, 1022084 (1964)
21H. G. Gilette, Rubber World, 157, 1, 67 (1967)
22T . A. Harris and J. Lucas Ltd., British Patent, 654509 (1948)
23 J. E. Collins, British Patent, 759666 (1954)
79
Chapter Four
4. Experimental
In this chapter the materials, equipment and methods used in the evaluation, the
Finite Element (FE) modelling, and the determination of operating procedures for the
FORM system prototype are described in detail.
4.1 Rubber Compounds
Seven compounds were used in the determination of the FORM machine operating
procedure, process evaluation, and optimisation trials. Four, labelled NR, SBR1,
SBR2 and NBR (or NBR ASTM) throughout, were produced in-house and three,
labelled PB80, EOl (or Elast-0-Lion/85) and FR58 (or FR58/90) were samples of
commercially produced material kindly donated by James Walker & Co. Ltd. a
manufacturer of fluid seals.
Three of the in-house compounds, NR, SBR1 and SBR2, were selected because they
would exhibit a range of processing properties (e.g. viscosity, elasticity etc.). The
fourth, NBR, was based on the sulphur cured NBR I compound in ASTM D 2934 -
891. This was chosen to behave like a material used in the production of fluid seals.
The formulation of each of the in-house compounds is given in Table 4-1.
The exact formulation of the three commercial compounds was not divulged to the
author as this information was deemed to be commercially sensitive and therefore
only the polymer types are known. PB80 and Elast-o-Lion are NBR and H-NBR
(hydrogenated nitrile) compounds respectively and the FR58/90 is FKM (fluorocarbon
elastomer).
80
Ingredient NR SBR1 SBR2 NBRASTM (phr) (g) (phr) (0) (phr) (g) (phr) (0)
NR (SMR 10) 100 2608 - - - - - -SBR (1502) - - 100 1871 100 2496 - -NBR(45·5% ACN) - - - - - - 100 1895 CB - N330 20 522 60 1122 - - - -CB - N660 - - - - 40 998 609 1137 Stearic Acid 2 52 2 37 1 25 0·5 9 Zinc Oxide (ZnO) 5 130 5 94 3 75 5 95 Sulphur (S) 1·5 39 1·8 34 2 50 0·5 9 OPG - - 0·2 4 - - - -CBS 1 26 1·2 22 1 25 1 19 TMTD - - - - - - 2 38 IPPO 2 52 2 37 - - - -Flectol - H - - - - - - 2 38 OOP - - - - - - 5 95
Table 4-1 Formulations of the four in-house compounds produced for use in evaluating the Form system with the weight of each ingredient corresponding to the mixer fill factor.
4.1.1 The Raw Polymers
All the polymers used in the trials were standard grades.
(i) Natural Rubber (NR):
SMR 10 is Standard Malaysian Rubber with 0·1 % (by wt.) maximum dirt content after
straining2•3
. The SMR 10 used in the trials was labelled as from Dynamic Plantation
BHD. Malaysia (obtained from the MRPRA).
(ii) Styrene-butadiene rubber (SBR)
SBR 1502 (INTOL 1502) is a cold-polymerised non-pigmented rUbber4 as classified
by the International Institute of Synthetic Rubber Producers (IISRP). INTOL is
manufactured by Enichem and supplied by Enichem Elastomers Ltd., Southampton.
§ ASTM D2934 - 89 calls for the use of N539 carbon black which is a fast extrusion furnace black with low structure (FEF - LS). However this was not easily available and for this work and N660 a general purpose furnace (GPF) black was substituted in its place.
81
----- ----------
(iii) Acrylonitrile Butadiene Rubber (NBR or Nitrile Rubber)
The NBR (a Nipol N) used in the trial had an acrylonitrile content of 46% and was
manufactured by Nippon Zeon and supplied by Zeon Chemicals Europe Ltd., South
Glamorgan.
4.1.2 The Filler - Carbon Black
Two carbon blacks, Vulcan 3 and Sterling V, which are N330 and N660 respectively,
as classified in ASTM D 1765·96a5, were used in the compounds for the trials, both
were manufactured by Cabot Corporation and sourced from Cabot Carbon Ltd.,
Manchester.
The indicators of reinforcing potential and the processing behaviour, particle size6,7
(Figure 2-8) and surface area, which are obviously related, and structure (or
bulkiness) are given in Table 4-2.
Carbon Slack Particle size (nm) Surface Area (m'/g) Structure (ml/100g) Designation CATS· DSPA9
N330 29 83 102 N660 50 35 90
Table 4-2 Indicators of reinforcement and processability for N330 and N660 grades of carbon black
4.1,3 The Additives
The additives used in the rubber compounds for the trials are detailed in the following
sections, they are broadly classified by function.
4,1,3,1 Activator and Processing Aids
(i) Zinc Oxide (ZnO)
A rubber industry standard zinc oxide, supplied by BD Technical Polymer Ltd" COrby,
Northamptonshire, was used as a vulcanisation activator enabling the vulcanisation
accelerator to reach its full potential. Cross-linking efficiency can be increased by
60%.
82
(ii) Stearic Acid
BD Technical Polymer Ltd., also supplied a rubber industry standard stearic acid
which was used in the compounds as: (a) processing aid facilitating the dispersion of
filler and smooth processing, and (b) as a secondary activator increasing still further
the effect of the zinc oxide.
4.1.3.2 Curatives
(i) Sulphur (8)
In the early batches a rubber industry standard ground crystalline sulphur supplied by
Anchor Chemical (UK) Ltd. was been used. Later batches used Hays - 120 mesh
sulphur with 2·5% Mg coating, supplied by Schill and Seilacher (UK) Ltd., as the
curing agent.
(ii) Accelerators
(a) DPG
DPG or Diphenyl guanidine is a vulcanisation accelerator. DPG manufactured by
Monsanto Chemicals Ltd. and in later batches, Perkacit-DPG manufactured by
Flexsys Rubber Chemicals Ltd. was used in the trial batches of SBR1.
(b) CBS
Santocure (Monsanto Chemicals Ltd.) and later, Santocure (Flexsys Rubber
Chemicals Ltd.) (N-Cyclohexyl-2-benzothiazole sulphenamide (CBS)) was used as a
vulcanisation accelerator in all the trial batches.
(c) TMTD
The accelerator (and sulphur donor) tetramethylthiuram disulphide, under the trade
names of Thiurad (Monsanto Chemicals Ltd.) and Perkacit TMTD (Flexsys Rubber
Chemicals Ltd.), was used in the trial compound NBR.
All of the accelerators ((a), (b) and (c)) were supplied by supplied by Flexsys Rubber
Chemicals Ltd. (Formerly Monsanto Chemicals Ltd.), Wrexham.
83
4.1.3.3 Antidegradants
(i) IPPD
Santoflex IP (Monsanto Chemicals Ltd.) and Santoflex IPPD (Flexsys Rubber
Chemicals Ltd.) (N-Isopropyl-N'-phenyl p-phenylene diamine) was used in NR and
SBR1 compounds as an antioxidant.
(ii) TMO
Flectol H (Monsanto Chemicals Ltd.) and Flectol TMO (Flexsys Rubber Chemicals
Ltd.) is an antioxidant, the chemical name is 2,2,4-Trimethyle-1,2-dihydroquinoline
(polymerised) .
The antidegradants were both supplied by Flexsys Rubber Chemicals Ltd.
4.1.3.4 Plasticiser
DOP
Dioctyl phthalate (DOP) under the name Jayflex DOP (from Exxon Chemicals) is a
plasticiser. The source of supply for the DOP is unknown.
4.1.4 Material Preparation
4.1.4.1 Mixing Equipment
All batches of rubber compound were mixed in an automated, computer-controlled
Francis Shaw K1 (Intermix Mk. 4) internal mixer. The mixer has a chamber consisting
of two 'Siamesed' cylinders through each of which a rotor with wings (or nogs) passes
axially. The rotors are positioned and synchronised through a gearbox in such a way
that the wings intermesh and pass close to one another but do not contact, producing
rates of high shear in the material drawn between them. The configuration of the
internal mixer is shown in Figure 4-1. The rotor speed can be varied up to a maximum
of 150rpm. The capacity of the mixing chamber is 5·5 litres. Fill factors in the order of
0·5 - 0·6 giving a batch volume of 2·75 - 3·3 litres were chosen, from experience, for
efficient mixing. The temperature of the mixer is controlled by re-circulating water from
two Conair Churchill heat exchanger units unit. During mixing the chamber is sealed
84
and the discharge door is kept closed. A pneumatic ram under a pressure of 300kPa
(3 bar) is used to maintain pressure on the material being mixed. The filler is pre
weighed into a hopper and fed via fluidised air slide, oil can be injected directly into
the mixing chamber and the rubber and minor ingredients are added via conveyor
feed.
4.1.4.2 Mixing Procedure and Conditions
The procedure used for mixing batches of each of the various compounds was
similar, except that the NBR which was mixed in two stages. The required amount of
the polymer, filler and rubber chemicals were carefully weighed out. A warm-up batch
was mixed, dumped and discarded, if starting from cold. The polymer, cut into uniform
pieces approximately 50 x 50 x 50mm was added to the mixer with the activators,
process aids and anti-degradants and masticated for a time before the carbon black
was added automatically from a feed hopper. A further period of time was allowed for
the carbon black to be incorporated and dispersed before the addition of the
curatives. The compound was then mixed for a pre-determined time and dumped
from the mixing chamber automatically. The compound was then sheeted out on a
two-roll mill, heated to 50a C, with a nip of 3 - 5mm, and allowed to cool. A period of at
least 24hrs from mixing was allowed to elapse before the material was used for
further operations.
The NBR compound was prepared in two stages, a masterbatch containing the
polymer, filler, activators, anti-degradants and plasticiser was mixed, sheeted and
allowed to cool for 24hrs before the curatives were added in a second mixing
85
-- ----- -------------
./"--------Feed Hopper
.,-------Ram
.,-_____ Mixing Chamber
,..--:;:::::===== Intermeshing· ... Rotors
Discharge ~---"dump" Door
Figure 4-1 Schematic Cross Section of a Francis Shaw Intermix Intermeshing Rotor Internal Mixer'·
86
procedure. The compound formulations and weights used (corresponding to the
respective fill factors) for the batches are given in Table 4-2. The mixing cycles and
conditions for all of the in-house compounds are detailed below.
(i) Compound NR
The mixing cycle and mixing conditions used for the production of the compound NR
are given below in Table 4-3 and Table 4-4 respectively.
InQredient(s)/Operation Time (sec.) NR, ZnO, Stearic Acid and IPPD 0 Carbon Black (N330) 180 S, CBS and DPG 240 DischarQe 330
Table 4-3 The mixing cycle for the compound NR in the Francis Shaw K1 Intermix
Mixing Parameter Parameter value Mixer Temperature 30°C Mixer Rotor Speed 30 rpm Fill Factor 0·6
Table 4-4 The mixing conditions for compound NR in the Francis Shaw K1 Intermix
(ii) Compound SBR1
The mixing cycle and mixing conditions for compound SBR1 are given in Table 4-5
and Table 4-6 respectively.
InQredient(s)/Operation Time (sec.) SBR(1502), ZnO, Stearic Acid and IPPD 0
Carbon Black _(N330) 90 S, CBS and DPG 330 Discharge 420
Table 4-5 The mixing cycle for compound SBR1 in the Francis Shaw K1 Intermix
87
Mixing Parameter Parameter value Mixer Temperature 40°C Mixer Rotor Speed 45 rpm Fill Factor 0·5
Table 4-6 The mixing conditions for compound SBR1 in the Francis Shaw K1 Intermix
(iii) Compound SBR2
The mixing cycle and conditions for compound SBR2 are given in Table 4-7 and
Table 4-8 respectively.
Ingredient(s)IOperation Time (sec.) SBR(1502), ZnO and Stearic Acid 0 Carbon Black (N660) 90 Sand CBS 210 Discharqe 300
Table 4-7 The mixing cycle for SBR2 compound in the Francis Shaw K1 Intermix
Mixing Parameter Parameter value Mixer Temperature 30°C Mixer Rotor Speed 30 rpm Fill Factor 0·6
Table 4-8 The mixing conditions for SBR2 compound in the Francis Shaw K1 Intermix
(iv) Compound NBR
The NBR formulation, based on ASTM D 2934 - 89\ was mixed in two stages. The
mixing cycles for each of the mixing stages are given below in Table 4-9. The mixing
conditions used for both the masterbatch (stage 1) and the final mixing cycle (stage 2)
are given in Table 4-10.
88
I nqredient( s )/Operation Time (sec.) Stage 1 (Masterbatching)
NBR (NIPOL N (46%)), ZnO, 0 Stearic Acid, TMQ and DOP
Carbon Black (N660) 120 Discharge 180
Stage 2 (Final mix) Masterbatch 0 Sand CBS 60 Discharqe 90
Table 4-9 The mixing cycles (both stages) for NBR compound in the Francis Shaw K1 Intermix
Mixing Parameter Parameter value Mixer Tem~erature 30°C Mixer Rotor Speed 45 rpm Fill Factor 0·6
Table 4-10 The mixing conditions for both masterbatching and final mix of NBR compound in the Francis Shaw K1 Intermix
4.2 Material Properties
The measurement of the material properties for all the compounds used in the trials
was undertaken using a variety of equipment. Rheological measurements were made
using a Negretti TMS Biconical Rheometer. Moulding conditions, scorch and cure
times, were determined with a Wallace-Shawbury Precision Cure Analyser (PCA) and
the density of the materials was determined with a standard laboratory balance.
4.2.1 Determination of Rheological Properties
It was necessary to understand the flow behaviour of the materials in order to be able
to model the FORM system, for predictive (design) and verification purposes, using
finite element analysis (FEA). The rheological properties of the trial compounds were,
therefore, studied using a Negretti TMS Biconical Rheometer. The values produced
provided material data inputs for the FEA models.
89
- - - - -------
4.2.1.1 Negretti TMS Biconical Rheometer11,12
The Negretti TMS Biconical Rheometer has a similar configuration to that of the more
well known Mooney shearing disc or rotational viscometer. In both viscosity is
measured by applying a strain to the sample and measuring stress. However, the
TMS differs from the Mooney in the design of the rotor (disc). The TMS utilises a
biconical rotor (Figure 4-2) to give constant shear, but in a Mooney the shear rate
varies across the radius from zero at the centre to a maximum at the periphery13.
Another departure from the traditional Mooney design is that the rubber compound is
injected into a closed rotor chamber, after a suitable warm-up period, via a transfer
mechanism (Figure 4-3). This eliminates the formation of flash or spew that would,
effectively, alter the test chamber geometry, which is an important factor in constant
shear rate systems. The biconical design also means that the shear rate is equal to
the speed (rpm) because the cone angle has been selected to be 6°. The shear
stress is directly related to the applied torque and K, the constant of proportionality.
The apparent viscosity is, therefore, simply K x torque/rpm at any given speed (rpm).
22.5 mm
a.5m
6· I 2mm : m II .., r-
..... ;......0 .... - .... - ... ( r;" 'J ~m
1.5mm
Figure 4-2 TMS Rotor Dimensions"
90
- - - -- - - - ---------
Piston
Testing Cavity
Figure 4-3 Schematic of the TMS Transfer Pot, Rotor and Rotor Cavity Configuration
4.2.1.2 Measuring Viscous Flow in the TMS - Test Procedure
The test procedure used to discover the steady-state flow properties for all samples is
a standard, seven step test used in the Rubber Process and Engineering Centre
(RuPEC) Laboratory at Loughborough. The rheometer controller requires
temperatures to be set for the ram (piston), the upper die (transfer chamber) and the
lower die (rotor cavity). Also required are the time to pre-warm the sample before
injection into the cavity, the fill time, the test mode, the sampling rate and number of
steps (where each step consists of a shear rate and a duration for which for which it is
applied). A number of runs was carried out for each specimen material, using the
following test conditions (Table 4-11).
91
Parameter/set-pointlRate Value/option Temperature (upper and lower dies and ram (piston) 100°C Pre-heat Time 240 s Fill-time 120 s Pre-cure delay 10 s Shear Rates VarvinCl SamplinCl Rate 5 readinCls/s No of Steps 7
Step Number Step Duration (s) Shear Rate (s-') Mode of test 1 10 0·1 continuous 2 15 0·4 continuous 3 20 1 continuous 4 15 4 continuous 5 10 10 continuous 6 5 40 continuous 7 10 100 continuous
Table 4-11 Parameters and Settings for the TMS Rheometer Test
Once the test parameters have been entered and the machine has reached the set
operating temperature approximately 209 of compound are placed in the transfer
chamber and the machine set to run. The piston closes down to compact the sample
and ensure even heating of the transfer chamber.
The duration of each step is long enough for a steady state to be reached. This was
checked for each sample by examining the raw data on the personal computer which
acts an interface and data logger.
4.2.2 Determination of Scorch Safety and Cure Times.
A Wallace-Shawbury Precision Cure Analyser (PCA) was used to determine the
scorch safety time (or scorch time) and the cure time of all of the materials used in the
trials. The former time is the time a material can exposed to heating before the onset
of crOSS-linking, and the latter is the time taken to reach a given degree of cure (Le.
form the required number of cross-links). These data were use to determine the
moulding conditions, time and temperature, of the moulding trials.
92
~ - ---------
4.2.2.1 Equipment - The Wallace-Shawbury Precision Cure Analyser (PCA)
The Wallace-Shawbury PCA (or Curometer) is a 'rotorless' curemeter in which near
isothermal conditions are obtained by heating a small specimen rapidly. This makes
the material heat-up time negligible in comparison to the cure time. The temperature
at which measurements can be taken can be set at any value up to a maximum of
300°C. The sample is compressed between an upper plunger (male) containing a
torque transducer and a matching lower (female) cup which oscillates at a constant
frequency of 1·7 HZ15.
~ __ Plunger Measuring Torque
~--Gap 1mm approx.
'----Oscilating Cup (1·7Hz)
Figure 44 Wallace-Shawbury peA cavity configuration 16
The system has a Zylog Z80A microprocessor at its heart, which can monitor such
parameters as displacement amplitude and frequency, time and force in addition to
torque. Data is logged continually and torque-time curves can be plotted either during
(near real time) or after the test is complete. The display on the machine can show
instantaneous real time data. The system, once a run is complete, can scale the data
and produce a plot of cure (%) verses time and calculate the time taken to reach a
number of pre-set cure values.
93
4.2.2.2 Method - Determination of Scorch and Cure Times. with the PCA
The parameters for a cure test are entered through a numeric/function keypad. These
are test temperature, required output, C1 (Chart 1 - Torque versus Time) and/or C2
(Chart 2 - Cure(%) versus Time), strain, test duration and the cure (%) points of
interest. The PCA is then left until it has reached the test temperature. 1·5 - 3g of
material are placed in the female cavity of the lower former and the test started.
Plots of both C1 and C2 were generally obtained for each test, the test set points
were standardised at 5%, 50%, 95% and 100% (or t5, t5o, t95 and t100). t95 (Le. 195 time
to form 95% of cross-links or time to reach 95% of measured torque) is often used to
determine an acceptable cure time or 'best technical cure'. Cure times at t95 were
determined from plots of at least three consecutive runs. Compounds were frequently
retested, should they have been left for more that a month without being used.
Tests were carried out at 150°C and 160°C for all of the trial compounds and also at
185°C for the three commercial compounds (PB80, E-o-L and FR58/90) from James
Walker & Co. because 185°C is the cure temperature used for these compounds in
production.
4.2.3 Determination of Specific Heat Capacity at Constant Pressure (Cp)
A TA Instruments DSC 10 Differential Scanning Calorimeter'? was used to determine
the specific heat capacities of the trial samples, as is usual in the rubber industry'8
except when the highest precision is required.
4.2.3.1 Equipment - Differential Scanning Calorimeter (DSC)19.2o
The DSC is, as the name suggests, essentially a calorimeter as the measurement is
always proportional to the amount of heat taken up or emitted by the sample. Figure
4-5 shows the configuration of the DSC cell fitted to the DSC 10 unit. Two sample
holders, which are thermally isolated from one another, contain the sample and
reference and are heated in parallel at a pre-determined rate. If the temperature of the
sample and reference differ (Le. one takes up more heat than the other) energy is
94
applied to the appropriate sample. The difference in the heat input to maintain both
samples at the same temperature is recorded.
4.2.3.2 Method - Determination of Specific Heat Capacity (Cp)
The instrument programmer (an IBM PC compatible computer) was set to hold the
sample and reference pans at the starting temperature (30°C) for five minutes and
then to heat the samples at a rate of 10°C/min from 30°C to the upper limit
temperature (170°C) and hold it constant for two minutes. The samples were weighed
accurately and placed in aluminium sample pans, 10 - 15mg of material was required
for each test, the reference sapphire weighed 60·6mg.
SAMPLE PAN
1l1'ERMAl RADtATKlN SHI£LO
FAlRING
RUBBER O-RING
~\.L-CHI'OMI!LWlRE ....... OSC CEll CROSS-SECTION
PURGE GAS Coot.ANT YM:UUM PLATE
Figure 4-5 Schematic of the DSC 10 cell
To determine the specific heat capacity of the trial samples, a baseline measurement
and reference measurement were taken using an empty sample pan and a pan
containing sample with a known heat capacity. In this case sapphire (AI20 3) was used.
The heat flow versus temperature traces of the blank (baseline) sample and the
sapphire were compared to give the calorimetric differential (difference in V-axis
displacement) at 60°C. (Any temperature between about 45°C and 170°C could have
95
been chosen). E, the calibration coefficient, was calculated from the known specific
heat of sapphire over a range of temperatures21.22.23 by using.
C = [60E.ilqS]ilY P Hr m
(4-1)
Here E is the dimensionless calibration coefficient at the temperature of interest
(60cC), ilqs is the Y axis range scaling in mW/cm, Hr is the heating rate in cC/min, ilY
is the difference in Y-axis deflection between reference (sapphire) and baseline (pan)
curves in cm, m is the mass of sample in mg and Cp is the heat capacity in J/gCC.
Runs were carried out following the same heating profile used in the above procedure
with pans loaded with samples of trial compound. E having been determined, Cp was
calculated for the samples.
4.2.4 Measurement of Density
Densities of all the trial compounds, preforms and products, (moulded conventionally
and with the FORM system) were measured by weighing accurately (± 1 mg) in air and
water (ASTM D 297 - 81 24) at 23cC (±1 CC) and calculated by means of
(4-2)
where Ps is the density of the sample, Pw is the density of water at the test
temperature (kglm\ A is the mass of the sample in air, B is the mass of the sample in
water and C is the mass of the supporting thread in water. However a sufficiently thin
thread was used that the effect of the thread was negligible and C = o.
4.3 The FORM System Trials
Initially, a limited set of basic machine control instructions were available for the
operation of the machine. These were used to determine the full requirements of the
96
PLC control system and the dedicated computer control program in operating
conditions. The structure and requirements of the final control system are detailed
Figure 3-7 and Figure 3-8.
4.3.1 Methods and Procedures for the Operation of the Form Machine (Optimum Operation Procedures)
4.3.1.1 Dispenser Operation - Procedure for Preforming
4.3.1.1.1 Filling The Meter Cavity
The automatic fill mechanism proved inoperable and a method of filling the meter
cavity and dispenser runner system had to be devised. Lack of any type of continuous
pressure control on the flow of material in the system (apart from the intermittent
pressure applied to the material by the 'stuffer' piston which was dictated by hydraulic
line pressure, a controllable variable) made the filling of the meter cavity inconsistent.
However, two methods for consistently filling the meter cavity by manually feeding
milled strip into the feed pocket were empirically determined. Both of the following
methods were applied to the ring and sheet dispenser inserts. Initially each dispenser
had to be positioned in such a way that the maximum meter cavity volume was
available to be filled. This was achieved by implementing a series of step movements
and holding the dispenser in position until it is incremented to the next position setting
by a key stroke at the computer. The dispenser is in the correct position (i.e.
maximum cavity volume) when the lowermost faces of the dispenser insert inner and
outer parts are flush (Figure 3-4 and Figure 3-6 (a) and (b)). This point could be
detected simply by touch. Once in the correct position the axis control servovalve is
activated to maintain precise position control during filling.
(i) Fill Method I
The first method of filling the meter cavity and runner system used the entrance to the
'stuffer' piston bore as a datum for filling. Material (strip) was put into the feed pocket
and nipped of by the 'stuffer' piston at low pressure (10,000 - 15,000kPa). by means
of either a direct manual push-button control or selecting the menu option from the
computer control program. The process was repeated until the material being forced
into the 'stuffer' piston bore could be seen relaxing just into view in the feed pocket on
97
retraction of the piston. The pressure was then increased to maximum or near
maximum ( 21,000 - 23,637 kPa) and the piston operated for a set number (2-7) of
cycles. The pressure setting and the number of final 'stutter' cycles required varied
depending on the individual material.
(ii) Fill Method 11
The second method requires the system to be filled in a similar manner to that
described above with the hydraulic pressure set to maximum (23,637 kPa) but the
state of cavity fill is checked periodically by operating the dispense cycle. When the
cavity is full, the material will start extruding from the meter cavity as soon as the
dispenser operation is actuated. The preform is then weighed and that amount of
material added to the dispenser prior to and each and every subsequent dispense
thus ensuring the meter cavity is filled.
4.3.1.1.2 Preform Dispensing
As mentioned in Chapter 3 the dispenser cycle is controlled by the PLC in conjunction
with a hydraulic servovalve (continuous feedback flow control). The dispenser driven
to (up to seven) predetermined set points and held at those points for a pre-set dwell
time (dwell could be zero). For convenience the first and last position set points were,
generally, set equal to the dispenser fill position in order that the dispenser was
always ready for filling before the next dispense operation. Of the remaining five
available set points the second was set to a value corresponding to the stroke
required to expel the desired volume of material. The remaining set points were
reserved for movements to crop the material from the dispenser. In most cases only
one cropping stroke (downward motion of the dispenser outer part to a point below
flush with the inner part) was required. Initially all dwell times were set to zero. The
dispense cycle was initiated and the preform produced examined and weighed. The
position value and dwell time were altered to provide more or less material if required.
98
4.3.1.1.3 Preform Size Range (Weight)
To determine the possible maximum variation in dispensed preform size an
experiment was conducted, using the compounds NR and SBR1, in which the stroke
of dispenser was increased from near zero to maximum. The starting point of the
range was defined, for the ring dispenser, as the minimum stroke required to
consistently form and crop a complete ring and, for the strip dispenser, as the
minimum stroke to produce a preform that would crop completely and fall from the
dispenser and not hang up. The former test being more discerning than the latter.
4.3.1.1.4 Preform Consistency (Accuracy of Shot Weight)
Dispense routines, consisting of fill sequence and a dispenser stroke sequence, were
set up for a given material, dispenser insert and mould combinations. A minimum of
ten preforms were produced and weighed on a laboratory balance and the weights
recorded. The utmost care was taken to repeat the fill sequence faithfully without
deviation or error.
4.3.1.1.5 Dispensed Preform Temperature
The temperatures of the dispensed preforms were measured with a thermocouple
connected to a digital meter. The measurements were made on at least five preforms
at several points (least three) on the surface of the preform as it was being dispensed.
Care was taken to avoid false readings caused by contacting the metal surface of the
dispenser, which was heated (60°-90°C). The results were averaged.
4.3.1.2 Press Operation and Moulding
4.3.1.2.1 Press Moulding/Forming Procedure
Once the production of consistent preforms had been achieved. The moulding
sequence was ascertained. As with the dispenser, seven position set points and dwell
times could be set to control the closure of the press. A separate instruction from the
PLC invokes final press closure, which was maintained, under maximum pressure, for
99
----------
a specified cure time. The press set-up routine must be run to enable accurate
position control and determine the maximum displacement encoder value for press
closure, with moulds in situ, under maximum pressure (the value differs with mould
height and very slightly over time, with increasing use, as machine, tie bars, platens,
moulds etc. relax and/or settle). The value is automatically stored in the PLC and
needs to be noted for use when creating a moulding cycle.
The press is, essentially, a standard up-stroking compression press used for carrying
and heating the forming moulds save for the fact that sophisticated position control
can be achieved. This control over the press closure was used to aid the production of
isotropic (or near isotropic) parts and to enable the production of flash-free mouldings
as well as the more standard moulding practice of 'bumping-off' to expel trapped air.
4.3.1.2.2 Flash-Free Moulding
The ability to control press closure was used, in conjunction with specially designed
moulds, to produce flash-free mouldings, one of the significant advances of the
FORM system . The preform, of approximately the mould volume was dispensed into
the plunge mould cavity and the press closed to form the part. After a very brief
duration, in some cases just enough time to complete mould closure, the mould is
'cracked' open for a 'breathe' (perhaps 0·03-0·06mm) in the order of a few seconds.
The resulting O-rings are separated from any flash and need no further cleaning or
trimming to comply easily with SS 6442:198425.
For the optimisation of flash-free moulding a number of press cycles were developed
where the 'breathe' opening distance was known (0·038, 0·05, 0·01, 0·16, 0·26 and
0-4mm nominally) having been measured with feeler gauges during dry runs and the
positional accuracy of the press was ±Q·0294mm (±5 encoder divisions, usually as
overshoot). The 'stuffer' sequence was kept constant for each material and several
mouldings were produced at each 'breathe' distance and 'breathe' time (Le. the press
open time) which was altered from 1-8s in 1 s steps. On mould opening the ring was
examined in situ and then again on removal from the mould after cooling.
100
4.3.2 Test Specimen Production
Both O-rings and sheet were produced, for testing, with the FORM system and by
conventional compression moulding. Conditions for moulding (Le. cure time and
temperature), for all compounds, were determined with a Wallace-Shawbury PCA,
however, test specimens moulded using the commercial compounds were also cured
using the vulcanisation conditions specified by the manufacturer (Table 4-12). The
conventionally manufactured O-rings were commercially produced by James Walker
& Co. Ltd. and cured to their specification.
Compound Cure Time at Post Cure 185°C (mins) Time (hrs) Temp.(OC)
PB80 4 - -EOl 4 6 150 FR58 6 12 230
Table 4-12 Vulcanisation conditions used for the commercially produced O-rings and some of the in-house moulding of PBSO, EOl and FR58.
4.3.2.1 Moulding Temperature Offsets
Before moulding samples the moulding temperature offsets were determined. The
measurement of temperature in many moulding systems is achieved by a dedicated
thermocouple mounted in the platen. This is true for both the conventional hydraulic
press and the FORM system press. There is often a difference between temperature
at the point of measurement and the surface temperature of the mould. This needs to
be taken into account when moulding.
The offsets were established by measuring the temperature of a mould surface over a
range of temperatures. Sufficient time was left after a setting change to allow the
temperature to stabilise. In order not to damage the moulds and ensure good mould
contact the thermocouple was placed in contact with the metal surface of a flash or
spew groove covered with compound and the wire positioned in a vent groove for
closure so neither the mould nor the thermocouple was damaged during mould
closure. Temperature readings were compared to those of the set points and an offset
determined. It is assumed that the difference in PCA cavity temperature and set point
are negligible. It is further assumed that the conventionally produced O-rings were
moulded at a temperature close to that specified.
101
4.3.2.2 Moulding O-ring Specimens
Conventionally manufactured O-rings were produced in three compounds PB80, EOl
and FR58. As is common in industry, the rings were manufactured from extruded
chord which was cut to length at 45° to its extrusion axis. The chord was then placed,
by hand, into a hinged two plate mould, the mould and press were closed and the ring
cured. The ring was then removed from the mould after the prescribed cure time and
post cured if necessary.
O-rings were produced with the FORM system using an automatic cycle. The
dispenser producing a metered preform and cropped it directly into the mould. The
mould closure sequence for all samples includes a single 'breathe' step. The mould
dimensions are given in Table 4-13.
Production Mould Type Inner Diameter Cross Section Diameter Method (mm) (mm)
Form System Plun~e 199·53 8·64 Conventional Two plate 198·0 8·3
Table 4-13 C-ring mould dimensions.
4.3.2.3 Moulding Sheet Specimens
Test sheets, nominally 2 mm thick, were moulded conventionally and with the FORM
system. The conventionally moulded sheets were prepared in a three-plate (picture
frame) mould. A billet of milled sheet of approximately correct weight was placed on
the bottom plate, in the centre of the frame, covered with the top plate and placed into
the upper daylight of a double daylight, up-stroking, hydraulic press powered by an
electric pump. Press closure included a single bump-off or 'breathe' to expel trapped
air and keep all moulding conditions, for both conventional and FORM processes, as
consistent as possible. Vulcanisation times and press temperatures were consistent
with those specified and/or determined in the PCA. Post curing, where appropriate,
was carried out in a standard laboratory oven.
102
Specimen sheets made with the FORM system were produced with a dual meter
cavity dispenser and two cavity mould combination. The preforms were dispensed
and cropped into the mould cavity simultaneously.
Production Mould Type Length Width Thickness(mm) Method (mm) (mm) (2mm nom.)
Form System PlunQe(2 cavity) 152·9 90·3 1·94 Conventional Three plate(frame) 122·5 120.0 1·97
Table 4-14 Sheet mould dimensions.
4.4 Physical Testing and Observations
4.4.1 General Observations - Preforms and Preforming
The preforms were observed during and after cropping from the dispenser.
4.4.1.1 Preform Shape and Size
Noticeable irregularities could be seen in the dispensed preforms after a short period
of time. The shape of the preform at the instant of cropping was regular and of even
cross section, for both the ring and the strip specimens. However, after only a few
seconds of recovery, irregularities began to appear in the form of a lobing effect on
the ring preforms and a bulging effect on the sheet samples.
(i) Filling and Packing the Meter Cavity
This phenomenon was investigated, for both dispenser insert types, with a standard
dispense motion sequence and the fill pressure was varied from low (stall at approx.
7000kPa) to maximum pressure (23637kPa). The size and shape of the preforms
were noted.
(ii) Dispensed Preform Size (Rings)
Rough measurements were made, with a 300mm rule, of the dispensed preform size
(diameter) after they had been cropped from the dispenser. The initial measurement
was made as soon as possible after crop and further measurements were made at
103
30, 60, 120, 180 and 240s (±10s) and a final size measurement was made at 24hrs
(±4hrs).
4.4.2 Mouldings (Product)
A number of tests were carried out on the mouldings that were produced by the
FORM system and by conventional moulding methods. It has already been stated that
moulding procedures and conditions were as near identical as possible to enable a
fair comparison.
4.4.2.1 Product Examination/Inspection
All of the mouldings produced (FORM and conventional), were inspected visually after
demoulding. In cases where flash was present it was, generally, trimmed unless the
ring was produced as part of a flash free moulding trial in which case it was left for
examination.
4.4.2.1.1 Rings
In order so set a stringent test for acceptance of rings for further testing, SS
6442:1984: Limits of surface imperfections and elastomeric toroidal sealing rings ('0'
rings)25 was used as a baseline. Limits for flow marks, non-fills, foreign materials and
indentations are not tight at two in any 25mm of circumference. All instances of non
fills and foreign materials were instantly discarded. The remaining rings were rejected,
in this study, if more that two of the other faults were visible on the whole
circumference. Limits for offset (or mismatch), backrind and combined flash (after
trimming where necessary) were in accordance with the standard.
The conventionally produced rings were examined closely for join marks and a
number of rings (conventional and FORM) were cut into eight equal sections and the
densities measured as a test of evenness.
104
4.4.2.1.2 Sheet
Sheet specimens were treated in a similar fashion to the rings previously described.
The surfaces needed to be smooth and free from all imperfections. All sheets with
non-fills, foreign materials and indentations noticeable on the major surfaces were
rejected and discarded. Flash was trimmed from both the conventionally moulded and
FORM system sheets. The FORM system sheet mould was not intended for the
production of flash free mouldings. The design of the plunge mould, however,
prevented any instance of offset.
4.4.2.2 Mould Shrinkage
Mould shrinkage is generally defined as the difference between the dimensions of the
moulding and those of the mould cavity at room temperature26•27
.
4.4.2.2.1 Rings
To determine the mould shrinkage of the a-rings, measurements were taken of the
rings across their inner diameter and across the ring on the line of the diameter to
provide ring cross section diameter measurements. Rings were also carefully cut, in
line with their diameter and the cross section measured in different directions.
Measurements were made with a travelling microscope for whole ring diameters and
some of the cross section diameters. The cross section diameters were generally
measured with a Shadomaster shadowgraph and checked with the travelling
microscope and/or a dial gauge.
In order to ensure that the true diameter of the rings was being measured a card with
concentric circles 1mm apart and diametric lines 45 0 apart was used as a guide. The
rings were easily centralised and measurements taken.
(i) FORM system
The FORM system ring diameters were also checked by carefully cutting a ring in line
with its diameter and placing the split ring into the mould ensuring the ring was tight to
the inner diameter of the cavity and measuring the gap. The ring circumference and
105
diameter were then calculated. Component measurements were compared to
measurements taken from the mould cavity.
(ii) James Walker (Estimated)
The shrinkage of the conventionally moulded rings produced by James Walker was
estimated from measurements taken of samples (as above) and comparing them with
the stated dimensions of the mould used for their manufacture.
4.4.2.2.2 Sheet
The sheet mould and sheets were measured with a Vernier calliper. Great care was
taken not to compress the sheet with the calliper during measurement, spot check
measurements were made with a travelling microscope. The sheet thickness was
measured with a dial gauge. The average was taken of at least three measurements
for each dimension. The mould cavity and sheet measurements were compared.
4.4.2.3 Swelling in Good Solvent
To investigate the integrity and highlight any molecular anisotropy present, samples of
the mouldings were placed in a good solvent for swelling. The solvents chosen for the
swelling test were all SlR grade obtained from Fisher Scientific. The swelling regimes
are shown in Table 4.15. The tests were carried out broadly in accordance with
standards2B•
Material Solvent NR (NR), SBR1 (SBR), Methanol SBR2 (SBR), NBR (NBR) Toluene PB80 (NBR) and EOl (H-NBR) FR58 (FKM) Acetone
Table 4-15 Material and Solvent Regimest
t The regimes as stated in the table were complete however some extra tests were carried out in the process, i.e. the FR58 was also immersed toluene and methanol in some the tests).
106
4.4.2.3.1 Ring Shape
(i) Cross Sectional Area
A segment of each ring was cut and marked for identification. A thin slice was then
taken from each segment and the freshly exposed surface on each side marked. It is
assumed that these two faces have the same shape. The segment was placed in
solvent and the slice placed on a shadowgraph and photographed. When the
segment was removed from the solvent, a thin slice was taken from the end that had
been marked and placed in a Petri dish with a drop of solvent to keep it saturated and
the dish placed on the Shadowgraph so it could be photographed. The area of the
cross sections of each of at least three different segments from three different rings
was then determined.
(ii) Circularity of Cross Section
The largest and smallest diameters of the cross section of each of the ring segments
was measured before and after immersion in the solvent from the shadowgraph
photographs.
4.4.2.3.2 Volume
The volume of both ring and sheet specimens was determined before and after
immersion in solvent.
4.4.2.3.2.1 Sheet Samples
The sheet specimens 38·1 x 12.7mm (±O·1mm) were prepared for immersion from
sheet nominally 2mm thick. The sheets were assigned reference directions, A and 8
(Figure 4-6). The conventionally moulded sheet reference direction 'A' was parallel
with the direction of milling and therefore any mill grain. The FORM system sheet
reference direction '8' was parallel with the direction of extrusion from the preform.
Five samples were cut from each sheet. Their dimensions were measured with a
Vernier calliper, periodic measurements were made with a travelling microscope. All
the samples were immersed in toluene except those cut from sheet moulded from
107
FR58 which was immersed in acetone. The dimensions were measured after 1,7, 14
and 21 days immersion in the solvent. Volume was calculated from the linear
dimensions.
4.4.2.3.2.2 O-ring Samples
The change in volume due to the action of solvent on sections of the moulded O-ring
was measured simply by displacement of water in a measuring cylinder. The sample
was placed in the cylinder, before and after swelling and water from a burette to a
graduation (e.g. 20ml) which was recorded after making sure all air bubbles were
eliminated. The amount of water added from the burette was also recorded the
difference between the two measurements was taken as the volume of the sample B ..
I-r- r "I
'1 DDDDD 122·5
D D D 152·
D 9
D
I~ 1200 ~I-~ \.. .,)
90·3
All DIMS IN MM
Figure 4-6 Schematic of Moulded Sheet Showing the Direction of Sample Cutting
4.4.2.4 Compression Set
Compression set tests were carried out on segments cut from O-rings produced both
conventionally and with the FORM system. The test was carried out in accordance
with SS 903: Part A629. However, the standard test pieces, could not be used.
Sections of O-ring (5 off) 25mm (±1 mm) in length were cut, and their height (vertical
diameter) measured with a dial gauge. They were then clamped in the apparatus
(Figure 4-7) with spacers limiting the compression, nominally, to 75% of their original
height. The samples were placed in an oven at 100°C (± 3°C) for 24hrs~25'
108
On removal from the oven the sample heights were measured and recorded after a
recovery of 30sec, 30min and again after a period of some months.
r-_Compressed 0-Ring Segment
Plates
Figure 4-7 Schematic of Compression Set Test Apparatus
4.4.2.5 Tensile Testing of Dumbbells Cut from Sheet
Tensile tests were carried out on SS 903:Part A2 Type 2 dumbbell30 specimens (5 off)
cut from the 2 mm thick sheet in each direction, parallel and normal to the reference
directions A and S (Figure 4-6).
A Hounsfield Test Equipment H10KM Universal Testing machine and 500l laser
Extensometer31 were used with a 1000N load cell. Samples clamped, in spring loaded
jaws, between the fixed base and driven crosshead and were extended at a rate of
500mm/min. The extension was measured by the laser from two reflective markers
attached to the sample gauge length. The width and thickness of each sample were
entered into the controller to enable output of tensile stress for predetermined
extensions (100, 200, 200, 400 and 500%) and break, the extension at break was
also recorded and mean and standard deviation calculated.
109
4.5 References
' ASTM D 2934 - 89: Standard Practice for Rubber Seals - Compatibility with Service Fluids, Annual Book of ASTM Standards,09.02, American Society for Testing and Materials (1989)
2G. F. Bloomfield (revised by G. M. Bristow) in Rubber Technology and Manufacture 2"" ed. Ch4 pp80 -81 (C. M. Blow and C. Hepburn Eds.), (Butterworth Scientific, London, 1982)
'W. Hofmann, Rubber Technology Handbook, CH2 pp16 - 18, (Carl Hanser Verlag, Munich, 1989)
'G. J. van der Bie, J. M. Rellage and C. Vervloet (revised by L. H. Krol) in Rubber Technology and Manufacture 2nd ed. Ch4 p92 (C. M. Blow and C. Hepburn Eds.), (Butterworth Scientific, London, 1982)
5ASTM D 1765 - 96a: Standard Classification System for Carbon Blacks used in Rubber Products, Annual Book of ASTM Standards, 09:01, American Society for Testing and Materials (1997)
BE. Schmidt, Ind. Engng. Chem., 43, 679 (1951)
'B. 8. Boonstra, Polymer, 20, 691 (1979)
BASTM D 3765 - 96 Standard Test Method for Carbon Black- CTAB (Cetyletrimethylammonium Bromide) Surface Area, Annual Book of ASTM Standards, 09:01, American Society for Testing and Materials (1997)
9ASTM D 2414 - 96a Standard Test Method for Carbon Black - n-Dibutyl Phthalate Absorption Number, Annual Book of ASTM Standards, 09:01, American Society for Testing and Materials (1997)
lOp. K. Freakley, Rubber Processing and Organisation, Ch3 p51, (Plenum Press, New York, 1985)
"A. King, Plastics Rubb. 1nl. 14 (1), 23 (1989)
12S. N. Ghafouri and P. K. Freakley, Polym. Test, 11, 101 (1992)
"P. K. Freakley, Rubber Processing and Organisation, Ch2 pp22-23, (Plenum Press, New York, 1985)
"P. Khunkamchoo, Ph.D. Thesis, Loughborough University of Technology (1993)
'SWallace Test Equipment - Precision Cure Analyser Manual D14a, H. W. Wallace Ltd. (1982)
lBp. K. Freakley, Rubber Processing and Organisation, Ch2 p37, (Plenum Press, New York, 1985)
17DSC10 Differentail Scanning Calorimiter Operator's Manual, TA Instruments (1994)
lBp. K. Freakley, Rubber Processing and Organisation, Ch2 p30, (Plenum Press, New York, 1985)
19M. E. Brown, Introduction to Thermal Analysis Techniques and Application Ch4 pp25 - 38, (Chapman Hall,1988)
20G. Kampf in Characterization of Plastics by Physical Methods - Experimental Techniques and Practical Application Ch4 pp179 - 191, (Hanser Publishers, Munich, 1986)
"D. C. Ginnings and G. T. Furukawa, J. Am. Chem. Soc., 75, 522 (1953)
22D. A. Ditmars et al., J. Res. Nal. Bur. Stand. 87 (2), 159 (1982)
23H. Y. Afeeiy, J. F. Liebman and S. E. Stein in NIST Chemistry WebBook [http://webbook.riiSl.govj, NIST Standard Reference Database Number 69, (W.G. Mallard and P.J. Linstrom Eds), (National Institute of Standards and Technology, Gaithersburg, USA 1998)
110
-----------------
24ASTM D 297 - 81 Standard Test Methods for Rubber products - Chemical Analysis: Part A 15, Annual Book of ASTM Standards, 09:01 , American Society for Testing and Materials (1985)
25BS 6442: 1984 British Standard Specification for Limits of Surface Imperfections on Elastomeric Toroidal Sealing Rings ('O'-rings), British Standards Institution (1984)
26A. W. Fogiel, H. K. Frensdorff and J. D. MacLachlan, Rubber Chem. Technol., 49, 35 (1976)
27 J. G. Sommer, Rubber Chem. Technol., 51, 368 (1978)
2BBS 903:Part A16:1987, British Standard Methods of Testing Vulcanised Rubber,Part A6, Determination of the Effect of liquids, British Standards Institution (1987): ASTM D 471 - 79 (reapproved 1991) Standard Test Methods for Rubber Property - Effect of liquids: Annual Book of ASTM Standards, 09:01, American Society for Testing and Materials (1991)
29BS 903:Part A6:1989, British Standard Methods of Testing Vulcanised Rubber,Part A6, Determination of Compression Set after Constant Strain, British Standards Institution (1989)
30BS 903:Part A2:1989, British Standard Methods of Testing Vulcanised Rubber,Part A6, Determination of Tensile Stress-Strain Properties, British Standards Institution (1989)
31Houndsfield Test Equipment H10KM Operating Instructions, Rev. A - 1/11/88, Houndsfield Test Equipment Ltd., Surrey (1988): Houndsfield Test Equipment 500L Laser Extensometer Operating Instructions, Rev. A - 26\02\90, Houndsfield Test Equipment Ltd., Surrey (1990)
111
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Chapter Five
5. Finite Element Modelling (FEA)
Modelling the FORM system using FEA was undertaken in a number of stages. The
early stages were to aid in the design of the dispenser and the later stages to predict
the behaviour of the material as it passes through and out of the system. The work
was roughly divided in two areas, heat transfer and flow modelling. The latter,
comprising the greater proportion of the work, was itself sub·divided into two
discemible section, (a) filling the meter cavity, and (b) dispensing from the meter
cavity.
The Engineering Mechanics Research Corporation (EMRC) NISA (Numerical
Integrated elements for System Analysis) 11 suite of finite element programs had
been chosen because the NISA 11 standard package was already available within
the department and there was also a certain amount of user experience that could
be drawn upon. It was also desirable that the package could be run on an IBM PC
compatible platform to ensure that the modelling would be accessible to industry, off
the shelf, if one of the results of the project were to be an FEA based design tool for
the end user. Another important feature of suited FEA software such as NISA 11 is
compatibility between modules.
5.1 Model Construction
Model construction was similar for all models. Three main stages are required to
solve a problem. These are pre-processing, analysis and post-processing. NISA 11
requires a structured ASCII text file input in order to perform an analysis. If the
112
format is known to the user then any text editor is suitable. The required information
consists of such elements as: (i), the type of analysis (e.g. heat transfer, fluid, 20,
3D etc.); (ii), material properties (e.g. density, viscosity, thermal conductivity etc.);
(iii), discretisation or meshing (co-ordinates of nodes, the element boundaries and
their connectivity); (iv), the boundary conditions (e.g. applied forces and fluxes,
attachment points and constraints etc.); and, (v), the initial conditions (e.g.
temperature, velocity etc. at the start of the problem). The NISA 11 specific structure
is shown in an abridged NISA 11 input file in Appendix C.
To help the user, the process of entering and formatting the input information is a
sophisticated graphical user interface (GUI) DISPLAY Ill. The GUI has a
considerable drawing capability and the option to enter the information by means of
typed input and/or pull down menus as well as analysis interface options such as
dedicated forms and menus for the entry of NISA 11 specific data for the creation of
NISA 11 input files (<filename>.NIS files). Display III is also capable of reading,
displaying and manipulating the binary output or results files (<filename>26.DAT
[model data) and <filename>27.DAT files [results data)).
5.1.1 Geometric Modelling
For analysis the system geometry (boundary and the interior) needs to be modelled
or defined mathematically. DISPLAY III allows items to be defined in space
graphically. Complex geometry was built up from simple geometric entities, locations
in space (called Grids), straight or curved lines (called Lines), surfaces (called
Patches) and solids (called Hyperpatches). The lower order entities, such as grids,
can be used to construct higher order entities such as lines (e.g. at least two grids
can be connected to form a line) or patches (e.g. at least three grids can be
connected to form a surface). Conversely, lower order entities can be derived from
higher order entities (e.g. the surface, above, formed by three grids could be used to
'extract' the three lines that form the edges of the surface). Other functions were
also used. Circles, ellipses, etc. can be formed as parametric items and entities can
be copied, mirrored, translated, rotated, etc. This means that, in most cases, there
113
are numerous routes to defining any, even highly complex and intricate, geometric
form.
Before these tools were used to define the geometry of problems, the systems were
studied to determine if they were regular or it there were any axes of symmetry.
Many of the problems posed by the proposed FORM system geometries are simple,
regular shapes with an axis of rotation. Discoidal, cylindrical and annular shapes
constituted much of the geometry for the proposed, and ultimate, FORM system
flow paths. These were modelled as 20 problems rather than 3D problems, thus
making the geometric definition much simpler and vastly reducing the run time
required for the analyses. More complex geometries with planar, mirror symmetry
were halved along the plane of symmetry. This was done under the assumption that
the behaviour of the system would be balanced and the section modelled would be
representative of, and exhibit the same characteristics as, the section omitted.
Figure 5-1 shows how the geometry of a squat cylinder, representing a transfer pot,
can be simplified to the shaded rectangle representing half the cross section. This is
the same information required to generate a solid of revolution thus the whole is
actually defined (or definable) in space.
5.1.2 Meshing (Finite Element Modelling)
Once the geometry had been defined to give the problem some bounds in space,
the process of meshing (or discretisation) was to divide the problems into, and
define the finite elements on which, the calculation of the solution depends.
A mesh is created on the geometric entities Patch, for 20 surface and 20
axisymmetric problems, and Hyperpatch for 3D solid problems. This was achieved
using the 'Automesh' mode for heat transfer models with simple geometry and
parametric meshing, a semiautomatic mode, for fluid flow models, using the FEM
(Finite Element Meshing) menu, FEG (Finite Element Generation) option. In most
the patch or hyperpatch and the number of divisions in the x and y or x, y, and z
directions of the entity local co-ordinate system were selected, respectively.
114
100
le 1
~~",.- .•• -.- •.• I_._._.- .. -.-.--.-
Figure 5-1 Diagram Showing how model geometry can be simplified. The shaded rectangle shows is the geometry that needs to modelled for am axisymmetric solid model.
Great care needed to be taken when creating a mesh, because NISA 11 elements
have a directional component which is derived from a directional component of the
geometric entities from which they are created. For example, four grids (1, 2, 3, and
4) were used to create two lines (L 1 and L2) and a patch (P1) was created from the
two lines. In Figure 5-2 (a) the local Cartesian co-ordinate positive directions
(labelled a. and P), for a patch created from line L 1 to line L2, are shown and in
Figure 5-2 (b) the patch was created from L2 to L 1. The lines were both created in
the same direction, 1 to 2 and 3 to 4, respectively, and hence the a. directions
coincide with each other. The directional components of the elements in a mesh
need to coincide or the results and predictions may be unreliable. This is particularly
important for 3D flow modelling, where incorrect elemental directions can influence
the NISA 11 results.
115
t(: -'
(a) (b)
Figure 5-2 Two NISA 11 Patches, showing the directional component that is derived from the method and order of creation in DISPLAY Ill.
Mesh generation was not carried out with the auto-meshing routine (Automesh) in
DISPLAY III/NISA 11 for flow modelling because it had proved unreliable in the past
for NISA II/FLUID'. The 'stiffness type' of element had to be defined for both
methods of mesh generation. This consisted of assigning two variables NKTP and
NORDR, the former specifying the element type and the latter specifying the
element shape and the number and position of associated nodes (e.g. NKTP = 3
NORDR = 1 is an 'axisymmetric solid' quadrilateral element with 4 nodes).
The validity of a generated mesh was checked with routines that check the size,
shape and connectivity of the elements and nodes. Duplicated and erroneous nodes
and unconnected elements would be highlighted so that they could be altered.
From experience and information in the literature, a set of practical rules for
meshing2.3
,4 was developed early during the modelling:
(i) A finer mesh and therefore an increased number (or density) of nodes
should be located where the model is constrained, loads are concentrated
and heat fluxes applied.
116
(ii) Nodes should be located where displacements and temperatures are
constrained and/or concentrated (e.g. the narrowing of flow through a tube of
decreasing diameter).
(iii) Nodes should be located where springs and masses and their thermal
analogues are present.
(iv) Nodes should be located along lines and on surfaces where the
pressures, shear stresses, heat fluxes and surface convection are present.
(v) Nodes should be located along lines of symmetry.
(vi) Nodes should be located at interfaces between different materials.
(vii) The element aspect ratio (ratio of the largest to the smallest element) of
the entire model should be no more than five and element density should
vary gradually rather than being abrupt.
(viii) Symmetric configurations should have symmetric meshes.
(ix) Elemental and nodal density should be increased in areas of the model
where high gradients or directional changes are expected.
(x) Where possible, in areas of the model where gradients are low, meshes
should be uniform.
(xi) An element type that is suitable to a particular analysis should be chosen,
some elements and solution methods are unstable and do not converge to a
result.
117
5.1.3 Boundary and Initial Conditions
The analysis required the specification of boundary and initial conditions such as
applied temperatures and velocities and the initial temperature of the material. (e.g.
if the dispenser body were held at ao°c and the piston was pushing the material
through the system at 20mm/s and the new material entering the system were at
room temperature 20°C). These were entered through the FEM (Finite Element
Mesh) menu 'Boundary Conditions' option.
5.1.4 Units
NISA 11 is entirely independent of units of the physical quantities specified in the
input data. The only requirement is that the units be self-consistent. The SI system
of units is based on the fundamental dimensions: length, (L), measured in meters,
(m); mass, (M), measured in kilograms (kg); and, time (t) measured in seconds, (s).
On some occasions, the SI recommended decimal sub-multiples of length,
millimetres (mm) and mass, grams (g) were used.
5.1.5 Post-Processing
The results of the analysis stage were converted into a more manageable (or man
readable) form with DISPLAY Ill. Graphs, charts and contour plots were produced
and some taken as hard copy. The output from the analysis is in the form of binary
results files «filename>26.DAT and <filename>27.DAT) that require decoding.
ASCII text data can be saved in an output file (<filename>.out) if so desired.
Typically the results calculated for each node and in, say, heat transfer analysis the
temperatures of each node at each iteration would have been recorded.
5.2 Static Heat Transfer
The very early work was carried out before the acquisition of the NISA 11/3D-FLUID
module. The NISAII/HEAT heat transfer module was used to estimate the warm-up
time, (i.e. the time required to heat the given volume of material to the proposed
working temperature of aO°C). This was modelled in order to simulate different
11a
- - -----------
geometries of the dispenser storage 'reservoir' or buffer stock of material if it had,
say, been left over night and allowed to cool. The reservoir geometries considered
are given in Appendix O.
5.2.1 Conditions and Assumptions
To enable fair comparison of each of the suggested geometries several
assumptions were made and wherever possible the settings for the FEA heat
transfer model were identical to those of all the other models. The assumptions and
standard conditions were as follows:
(i) A set of standard material values for mass density, thermal conductivity
and specific heat capacity were used in all models, these values were 0.0014
g/mm3, 0·00017 W/mm.K and 1·1643 J/g.K, respectively.
(ii) All models were treated as 20 axisymmetric (Le. volumes of revolution)
and the heat generation element type NKTP = 103 and quadrilateral elements
NOROR = 2 with a nodes per element were used.
(iii) As a termination condition all designs would be required to reach the
same temperature of ao°c to within 2°C throughout the whole volume.
(iv) The whole volume of the reservoir would be considered to be filled with
material at room temp (20°C) with no voids.
(v) A constant temperature of ao°c was applied to all external (edge) nodes.
(vi) There would be no heat flow across the axis of symmetry.
(vii) Element meshes were all created by the NISA 11 'Automesh' function.
5.3 Heat Transfer with Incremental Flow (Pseudo-flow)
The heat transfer modelling was developed to include elements of movement and
time in an attempt to simulate the flow of the material through the dispenser. The
119
flow path was considered as cylindrical. Although some of the runners in the design
are not actually cylindrical, the cross-section was approximated as a tube with the
same volume as the runner.
At the start of the flow path, the modelled cylindrical 'plug' of rubber is heated at
80°C around its circumference and at one end. This simulates heating at the wall
(boundary) of the flow path and the piston. The elements which form the other end
of the plug are restricted so that no heat flow across this face can occur. The heat is
then applied for a fixed period of time (15 seconds), representing the residence time
of the volume of rubber in that section of tube. The period of 15 seconds was
chosen as this was the design target interval between dispensing each shot.
The FE model was run for the required time step and the nodal temperature data
from the run was saved and mapped onto the next section of the flow path as the
initial starting conditions. This second, and subsequent, iterations were the same in
every respect except the piston end was not heated. This process was repeated
until the 'apparent volume' traversed by the plug of material equalled the total
volume of the length of flow under consideration.
At the point where the diameter of the flow narrows and divides, the nodal
temperatures of the previous run were transposed onto the new geometry of
reduced diameter tube in such a way that the warmer outer temperatures were
maintained at the extremities and the cooler inner temperatures at the inner part of
the new geometry. This was considered to be a reasonable approximation.
5.4 Flow Modelling with NISAlFLUID
Modelling of the flow using the NISA 11/3D-FLUID module was undertaken in two
sections, the flow through the dispenser representing the situation of flow when the
meter cavity is being filled and flow during dispense. The flow during fill is driven by
the 'stuffer' piston whereas the dispense flow is driven by the relative motion of the
120
-- - ---_._----
inner and outer parts of the dispenser insert (meter valve). These were considered
to be two distinct and separate phases, because the action of the meter valve
physically isolates the material in the meter cavity from that in the rest of the system.
5.4.1 Dispenser Fill Modelling.
A 3D model of the internal geometry of the flow path was created and meshed as
previously described. Where possible, a central core of simply shaped (rectangular)
geometric entities (and hence elements and nodes) ran the length of the flow path.
This was good modelling practice and aided the modelling of more complex
geometric features such as constrictions, expansions, changes of direction (bends),
divisions and combinations.
Two families of models were created. One was based on the dispenser geometry
required for producing O-ring preforms and the other for the two cavity, geometry
required for sheet or strip preforms. The model input requirements were very similar
for both sets of models. The most commonly used functions, commands, analysis
and input data used in the modelling are given Table 5-1.
The models created, although referred to here by the dispenser from which the
geometry was derived, only represent the dispenser in that they are actually models
of the material (virtual rubber) in the respective dispenser.
5.4.1.1 Ring Dispenser Geometry
Two geometries were modelled for the ring dispenser. These are given below. The
two designs are very similar because the type and design of the feed mechanism
and the upper part of the dispenser and dimensions of the annular meter cavity (set
out in the initial design brief) had been fixed at the time they were created.
5.4.1.1.1 Ring Dispenser Geometry I
Geometry I (Figure 5-3), the first of the considered designs, had the simplest
geometry from the point of view of both modelling and manufacturing. The 'stuffer'
piston will cause flow from A in the negative x direction (the 'entry' plane is in y-z for
121
the purposes of NISA 11 flow modelling), through a 90° bend at B in the x-y plane
(flow in the negative y direction). At C, the flow is divided into four equidistant
diverging runner channels (in the x-z plane), at right angles to the preceding flow, to
enter the annular meter cavity at D. The flow 'exit' (NISA 11) is at the bottom of the
meter cavity in the x-z plane. A shaded representation of the flow geometry is shown
in Figure 5-3 and the hyperpatch wire-frame geometry that was meshed and used in
the modelling is shown in Figure 5-6 (a).
5.4.1.1.2 Ring Dispenser Geometry 11
The flow in the second geometry considered for dispensing ring preforms (Figure 5-
4) is similar to that of geometry I from A' through to B' and differs at C' where the
flow divides in to six equidistant runners that are at 45° to the flow in the axis y.
Distance B'- C' is less than B - C as the overall height is the same. The points,
labelled D', are where the flow enters the meter cavity. The exit plane (E') is, again,
in the x-y plane the at the bottom of the meter cavity. The wire-frame representation
of the modelled half is shown in Figure 5-6 (b).
5.4.1.2 Sheet Dispenser Geometry
The upper geometry of the sheet dispenser (Figure 5-5 and Figure 5-6 (c)) before
C" is similar to that previously described. The flow division and cavity arrangement
after C" was, however, considerably different. Four runners (two on each side) feed
two separate metering cavities through the points at D" with the exit plane at E" in
the x-z plane as before.
122
NISA 1113D-FLUID Command LabelNariable Description
Executive Command Block ANALvsis FLUHT Fluid and heat transfer analvses DIMEnsion AX Axisymmetric solid problem
3D Three-dimensional Problem FILEname <file_name> Output binary data files name (6 characters of less in
DOS as designation from SAVEfile will be appended (e.a. file26.DAn
SAVEfile 26,27 Data file designations: 26 - model and analysis data, 27 - analvsis results data
INITialc (ondition) U, V, W, UVW and T Specific initial conditions for velocities U, V and W in [VALUE/S] directions x, y and z respectively and temperature T
condition reouires value STDS ON/OFF Steady state analysis. The FLCNtl analysis data card
needs to be implemented for both ON and OFF conditions for iteration control in the former and time-step control in the latter
BOUNDary ON/OFF Automatic computation of domain boundary. U=V=W=O and U=V=O for 3D and axisymmetric problems (i.e. extremities of the wireframe are assumed to be walls with no-slip boundary condition)
VDISip ON/OFF Viscous dissipation or heat generated due to viscous [Conversion factor] effects is accounted for in the analysis (conversion
factor onlv if inconsistent units are used) Model Data Block
ELTYpe NKTP [value] Element type (3 - AXIS., 4 . 3D) NORDR [value] Nodal ord~r 1 or 2 (4 or 8 nodes -AXIS. and 8 or 20
nodes - 3D NODEs Nodal values string Nodal ID and co-ordinate definitions ELEMents Element values string Element and material ID, nodal and elemental
connectivitv oarameters MATFluid DENS [value] Material property data for fluid generally values for
VISC [value] density, viscosity, thermal conductivity and specific CON~ r\valu~] heat capacity are required SPEC value
NONNewtonian POWER [value] Non-Newtonian power law fluid, the value required is the non-Newtonian or power law index n
Analysis Data Block FLCNtI NLSTP [value(,;SO)] Fluid load case control. NLSTP requires a value for
ITMAX [value] the number of time steps and ITMAX specified the maximum no. 01 iterations(in STDS) or iterations per time step if transient analvsis used.
BCDVAR U, V, Wand T [value] Variable Nodal Boundary conditions these generally take precedence over globally set boundary conditions
BCDR U, V, Wand T [node ID] Global boundary condition release (i.e. exit plane nodes need to be released when BOUNDary - ON)
ICDS U, V, Wand T [node ID Specified nodal initial condition a node ID and value] boundary condition value required
PRINTcntl U, v, W, T, P, SXX, Selects the data required to PRINT to file value SYV, SZZ, SXY, SXZ, and SYZ [value)
required 0 for all -1 for none.
Data Terminator Block ENDdata I Input Data Terminator
Table 5-1 NISAl3D-FLUID Input, Analysis and Data Commands used in the Modelling of the Form System
123
5.4.2 Dispense Modelling
Modelling of material flow through the meter cavities was carried out using a
different set of FEA models to those described above. The dispense stroke was
treated as a distinct and separate action to that of filling. The geometry also looks
somewhat different to that previously pictured this is because the dispense is a
transient situation, the inner and outer parts of the dispenser moving relative to one
another creating the narrow opening through which the material is forced.
124
~ If!f:-
r z L,
Figure 5-3 Ring Dispenser Flow Path Geometry I
125
[' ,
Figure 5-4 Ring Dispenser Flow Path Geometry 11
126
Figure 5-5 Sheet Dispenser Flow Path Geometry
127
(a)
(b)
(c)
Figure 5-6 Wire Frame Diagrams of the Actual FEA Model Geometry: (a), Ring Dispenser Geometry I; (b), Ring Dispenser Geometry 11; and, (c), Sheet Dispenser Geometry. The plane x-y is a Plane of symmetry in all cases therefore only half the actual geometry needs to be modelled for FEA.
128
5.4.2.1 Ring Dispense Modelling
The annular geometry of the meter cavity for the ring preform allowed very simple
axisymmetric modelling of the dispense flow. The lower narrow section, or tail,
represents the slit that is created when the dispense stroke is triggered .
.,,----A.rea Modelled
........ _____ Dispenser Inner
Figure 5-7 Scrap Section of Geometry Modelled in Axisymmetric Meter Cavity Dispense (the arrows show relative motion of dispenser inner and outer)
5.4.2.1.1 Ring Dispenser Alternative Geometries.
A number of alternative geometries were modelled in a "what if?" manner. These
were divided conveniently into two sets, in the same way as the dispenser design
and other geometries. The conditions for all models were standardised to enable fair
comparison. The material parameters and volume flow rate were consistent and the
nominal preform diameter was set to be consistent with that of the FORM system
design. Details of the standard material parameters and flow rate are reported in
Table 5-2.
129
Parameter Value
Flow rate 6·71 x1 0·sm3/s
Equivalent to a constant speed of 10mm/s movement in standard dispenser meter cavity
Initial Temperature(INIT T) 60°C Density (DENS) 1200 kClfm3
Specific Heat Capacity (SPEC) 1500 Thermal conductivity (COND) 0·17 W/m/K Viscosity (VISC) at 1 s·' (K) 100000 Pa Non-Newtonian Power Law Index, n
0·25 (NONN)
Table 5-2 Model Parameters for "What if?" Dispense FEA.
Alternative Cross Sections for the Annular Preform Cavity.
A couple of more radical geometries were modelled to gain an idea of the effect of
cavity shape on flow. It is assumed that it would be possible to produce the meter
cavities of the geometries devised. Examples are given in Figure 5-8.
y Figure 5-8 Example NISA 11 Meshes of Alternative Annular Dispenser Geometry
5.4.2.2 Sheet Dispense Modelling
The flow of material through the strip dispenser meter cavities could not be
modelled as simply as the annular cavity (Le. using axisymmetric elements) because
a solid of revolution cannot be formed. Only a single cavity was modelled because
of symmetry. The commands used for the model are described in Table 5-1. This
was, essentially, a 3D flow problem similar to those previously described.
130
5.5 FLUID - STATIC Interface
NISA 11 static packages were used in conjunction with the output of NISA/3D-FLUID
in an attempt to model the shape change of the preform after it has left the
dispenser. The normal stresses in the exit plane of the 3D-Fluid 20 axisymmetric
model were saved at every time step iteration, extracted from the output file with the
FLUTL (NISA/3D-FLUID Utilities) package and mapped manually onto the simple
rectangular meshes used as the starting geometry for resolution of the stresses.
Figure 5-9 shows the logic of this two stage modelling process. The stresses are
integrated over a defined 'surface' (which is an in edge in 20 axisymmetric
modelling), the 'surface' (edge) normals are computed to be positive when pointing
outwards. These values were then applied manually, as boundary conditions, to
elemental faces using loadcase type L 1. Two types of model geometries were used:
(i), normal stresses were mapped onto an axisymmetric solid; and (ii), normal
stresses were mapped onto an axisymmetric shell.
The STATIC modelling was conducted in much the same way as that previously
described for 3D-fluid. Table 5-3 gives the commonly used commands and analysis
data for STATIC modelling.
Stage 1
FLUID/STATIC Interface
Stage 2 *--------
-+--------
-+-------- ---------.
___ --3D Fluid Model (stress determination)
Flow Exit Plane
STATIC Model (stress resolution)
t Stresses mapped according to time step (i.e. most recently logged values applied
t, closer to the interface).
Figure 5-9 Logic of the FLUID to STATIC modelling. Above the FLUID/STATIC Interface Represents the NISAIU3D-Fluid model, below the stresses are mapped on to a separate model for resolution.
131
· - ------------
NISA 11I3D-FLUID LabelNariable Description Command
Executive Command Block ANALysis STATIC Static analysis BLANk common n=50000 Assign size of dynamic memory (used as swap file
durino processino) FILEname <file_name> Output binary data files name (6 characters of less in
DOS as designation from SAVEfile will be appended (e.g. file26.DAn
SAVEfile 26,27 Data file designations: 26 - model and analysis data, 27 - analvsis results data
GEOM properties ON/OFF, Computes and saves geometric properties (volume. LlST/NOLlST mass etc.)
Model Data Block ELTYpe NKTP [value] Element type (3 - AXIS., 4 - 3D)
NORDR [value] Nodal Order 1 or 2 (4 or 8 nodes -AXIS. and 8 or 20 nodes - 3D)
NODEs Nodal values strino Nodal ID and co-ordinate definitions ELEMents Element values string Element and material ID, nodal and elemental
connectivity ~arameters MATErial EX, EY, EZ [values] Elastic moduli (force/area)
NUXY, NUXZ NUYZ [values] Poisson's Ratio DENS [value] Density
Analysis Data Block LDCASE KSTR [value] Element Stress calculation L1 Data [values1 Element ID, Face and property value PRINTcntl AVNDstresses[value] Averaged nodal stresses
DISPlacements[valuel Nodal displacements Data Terminator Block
ENDdata Input Data Terminator
Table 5-3 NISAlSTATIC Input, Analysis and Data Commands used in the FLUID/STATIC Interface Models,
5.6 References
'Wilde & Partners, Stockport, Private communication (1995)
20. W. Nicholson and N. Nelson, Rubber Chem. Technol., 63, 368 (1990)
3NISA 11 User's Manual, CH3 NISA Capabilities, Engineering Mechanics Research Corporation, USA, (1994)
4NISA 3D FLUID User's Manual, Appendix 0 Modelling Hints, Engineering Mechanics Research Corporation, USA, (1994)
132
Chapter Six
6. Results and Discussion
This chapter presents and discusses the results of the experimental work described
in Chapter 4 and the modelling described in Chapter 5. The first section of this
chapter deals with factors that affect the production and processibility of the uncured
rubber compound, It reports the results of the tests that were employed to determine
the material quantities required for: (i) the finite element modelling, and, (ii) the
production of the moulded samples used for the physical testing for the evaluation of
the FORM system. These are covered in, respectively, the second and third
sections.
6.1 Mixing and Material Characterisation
6.1.1 The Factors Affecting Processibility
The main factors affecting the properties of polymer compounds are molecular
weight, or molecular weight distribution, molecular structure and the type of filler.
Molecular weight is the Single most important factor in determining the viscosity of a
polymer (and hence its processibility)l,2. Polymers of lower molecular weights have
a lower viscosity and are, therefore, processed and formed more easily. Polymers
with higher molecular weight will, all things being equal, have superior strength
properties. Unfortunately, all things are not equal and polymers with higher
molecular weights, in general, exhibit greater deviations from Newtonian flow
behaviour. This can, at least qualitatively, be explained by the argument that the
average number of entanglements per chain is greater, and the probability of there
133
being an entanglement that will break down under a lower average stress is greater.
Molecular orientation will also tend to be higher, after processing, when the
molecular weight is high. This orientation may enhance strength in the direction of
the flow (parallel to the molecular orientation) but this is likely to be at the expense
of properties in the direction normal to the flow (orientation).
The amount and type of filler significantly affects the processing behaviour of a
compound. Particle size3.4 and structure (bulkiness) have an effect on both the
processibility and the ultimate properties. Horns has described, for a range of both
processing (rheological unvulcanised behaviour) and ultimate vulcanisate (cured)
properties, the effects of decreasing particle size (CATB6) and increasing structure
(DBPA\ General trends showing the effect on properties related to these indicators
are, in general terms: (a), the smaller the particle size the more reinforcement and
the poorer the processability; and (b), the higher the structure the stiffer (harder
processing) and less 'nervy' the unvulcanised compound, for the same volume
fraction of filler. There are also other measurese•9
•1o
, all broadly based upon some
measure of particle/aggregate size. The fillers used in the trial compounds produced
in-house were a highly reinforcing N330 grade and a moderately reinforcing N660
grade (as classified in ASTM D 1765-96a11) respectively.
6.1.2 Mixing
The ASTM NBR compound12 was mixed using a two-stage method because
batches that were mixed using a single-stage mixing cycle needed an abnormally
high mixer power consumption and ran very close to the safety dump temperature,
In one case the batch dumped prematurely and showed signs of premature scorch.
This was indicated by visible lumps of cured material and a tendency for the batch
to crumble. There was also failure to completely incorporate the carbon black,
indicated by low final batch weight and a visible coating of unincorporated carbon
black. Confirmation that single-stage batches of NBR, which were not prematurely
dumped and on visual inspection of the mixed compound seemed acceptable, were
scorched was obtained from curemeter tests. All batches of the four rubber
134
compounds that were mixed in the Francis Shaw K1 Intermix using the recipes and
mixing cycles given in Chapter 4 were satisfactory.
6.1.3 Compound Rheology (Negretti TMS Biconical-Rotor Rheometer)
The Negretti TMS rheometer13,14 was used to determine the relation between the
shear stress (t) and the shear strain rate (y) (hereafter referred to simply as strain
rate or by its symbol y) and the apparent viscosity (11a) for each of the trial
compounds. It is assumed that the behaviour of each compound can be well
described by the power-law relationship,
(6-1 )
where the constants K and n are, respectively, the consistency constant (or
apparent viscosity (11a), K '" 11a at y = 1 S·1) and the non-Newtonian (or power-law)
index. Since the relationship between shear stress and strain rate is not direct
proportionality (as it is for Newtonian fluids) the concept of an 'apparent viscosity'
(11a) is often used to enable a quantitative comparison between compounds. At a
specified strain rate, 11a is defined as
11a = 't/y. (6-2)
Apparent viscosity is also temperature dependent and the relationship can be
described by,
T\ _ T\ e·b(T, • Tr.,l '18 - ,,8 ref (6-3)
(or
K - K e·b(T, • Tr • ., t- ref , (6-3a))
where 118 ral is the apparent viscosity (or Kral the consistency coefficient) at the
reference temperature, Tral, and 11a is the apparent viscosity (or Kt the consistency
coefficient) at the desired temperature Tt and b is a constant.
135
Strain Rate Apparent Viscosity (Pa.s) at 1000 e (5·') NR SBRl SBR2 NBR EOl PB80 FR58 0.1 330000 900000 520000 570000 922850 1223090 965500 0.4 172500 425000 250000 282500 361350 475640 262980 1 86000 205000 142000 158470 173960 210550 119530 4 30500 67500 49500 60590 58470 61790 37050 10 14700 32100 24700 29800 28840 30710 18750 40 4680 10100 8750 10230 9420 11220 7050 100 2010 3990 3600 3540 - - 3980
Table 6-1 Apparent viscosity for each compound for each strain rate measured
Table 6-1 shows the apparent viscosity of each of the trial compounds for each of
the measured strain rates at the reference temperature (100°C). It should be noted
that the temperature quoted is the rheometer test starting set-point temperature and
that there will be a certain amount of shear heating, caused by viscous dissipation in
the material during the test. Table 6-1 allows a direct comparison of the materials
under similar flow conditions. For example, compound SBR1 has a lower value of
apparent viscosity than compound FR58 at a strain rate of 0.1s-\ but at 0·4s-1 the
position is dramatically reversed. This indicates that flow is easier for SBR1 in the
former condition and that, therefore, it would be processed more easily than FR58
and that the converse is true in the latter condition. The trend of a reduction of
apparent viscosity with an increase in strain rate (and to an extent temperature) is
clear for all of the trial compounds. The NR compound, obviously, has the lowest
apparent viscosity in all flow conditions and the SBR1 compound has one of the
highest, as expected. No value is quoted for either EOl or PB80 at the highest
strain rate (100s·1) because the measured shear stress values were far too low,
indicating that the condition was too extreme and melt fracture (or possibly wall slip)
had occurred rendering the measurement unrealistic.
Another, perhaps more useful, method of analysis is a plot of log (shear stress) vs.
log (strain rate) (Figure 6-1 and Figure 6-2) which gives a straight line of slope n 15,
the non-Newtonian (power-law) index. In Figure 6-1 the log (shear stress) vs. log
(strain rate) curves show the aforementioned comparative trends which highlight the
differences in the behaviour of the compounds under conditions of flow. Figure 6-2
shows a log (shear stress) vs. log (strain rate) plot for a range of test starting
temperatures for the compound SBR2. This plot also shows the best-fit trendline
136
(and its equation) for the 100°C curve the gradient of which is calculated giving the
power-law index, n. Appendix E gives the log (shear stress) vs. log (strain rate) plots
for all of the trial compounds.
·1 -0.5
Log ~ vs. Log y for all Compounds at 100·C 3
0.5
o 0.5 1.5
Log y (Strain Rate) 2
~NA
--SBAl SBA2
- NBR - EO!. - PBaO ~FA58
Figure 6-1 Log (shear stress) vs. Log (stra in rate) plot of all compounds at 100·C.
A plot of In (shear stress) vs. temperature (at constant a strain rate of 1 S·l) yields the
temperature dependence of the viscosity index, b1S Figure 6-3 shows the plot of In
(shear stress) vs. temperature for each of the measured strain rates for the
compound SBR2. The best-fit trendline is shown for the curve corresponding to the
test conducted at a strain rate of 1 S·l. Its gradient is index b. Plots of In (shear
stress) vs. temperature for all of the trial compounds are given in Appendix F.
137
Log 1: vs. Log .y for SBR2
3
2.5
en .. ! y = 0.2762x + 2.0867 r8O
•
C u; ~ --- -- 90· C .. '" 1.5
1OO·C J: !!!.
--- 120·C .. Cl
.3
0.5
I
-1 ·0.5 0 0.5 1.5 2
Log y (Slraln Rale)
Figure 6-2 Example log (shear stress) vs. log (strain rate) plot of Negrettl TMS blconlcal rotor rheometer data. The data shown are for SBR2 compound.
en .. '" ~ u; ~ .. '" J: !!!. .. .5
:I 5
4
3
2
o 80
-+ 85
In 1: vs. Temperature (·C) for SBR2
: : - ... -•
Y = ·0,Q11 x + 6.0854
--~-------+-------+
90 95 100 105 110
Temperalure (·C)
! ~
-+- 0,1 • ....... 0.4
1 -- 4 ....... 10 -- 40 -+- 100
115 120
Figure 6-3 Example In (shear stress) vs. temperature plot of Negretti TMS biconlcal rotor rheometer data (for SBR2).
138
-- ---------------
The constants that were determined from the shear stress measurements made
using the TMS rheometer for use in describing the behaviour of the materials in FEA
and lumped parameter the modelling of the flow in the FORM system dispenser are
summarised in Table 6-2.
Material (consistency constant) (or 1'1. at 1s" and 1'1.,0' at 1s"
n non-Newtonian (or power-law) index
temperature dependent viscosity index
Table 6-2 Constants determined from the Negretti TMS Rheometer shear stress measurements and used to describe the behaviour of the compounds in the in the FEA and lumped parameter modelling
6.1.4 Physical Constants
6.1.4.1 Specific Heat Capacity
The differential scanning calorimeter (DSC) was used to measure the specific heat
capacity of the trial rubber compounds'6,17. The values are given in Table 6-3
Compound Specific Heat Capacity (Cp)
JlkgoC NR 1627 SBR1 1563 SBR2 1347 NBR 2017 EOl 1663 PB80 1231 FR58 1070
STD' 1500
Table 6-3 Specific heat capacities measured for the trial compounds
*The values in the tables given under the label STD are not measured values but values selected to be representative of a typical compression moulding rubber compound. These values were used as a reference throughout the modelling phases of the current work.
139
6.1.4.2 Density Measurement
The densities of each of the trial compounds were determined by carefully
measuring the weight in air and water (in accordance with ASTM D 297 - 81 18) and
are given in Table 6-4.
Compound Density (p) ko/m3
NR 1049 SBR1 1171 SBR2 1112 NBR 1218 EOl 1206 PB80 1317 FR58 1845
STD" I 1200
Table 6-4 Densities of the uncured trial compounds
6.1.5 Scorch and Cure (Vulcanisation) Time.
Samples of all of the trial compounds were tested in the Wallace-Shawbury
Precision Cure Analyser to determine their scorch and cure times at 160°C. The
results are given in Table 6-5.
Compound Scorch Time at Cure Time 160°C (secs) at 160°C(min)
NR (20 phr CB) 120 8 SBR1 (60 phr CB) 90 10 SBR2(40 phr CB) 180 15 NBR(ASTM) 30 7.5 PB80 90 8 EOl - 15 FR58/90 60 16·5
Table 6-5 Scorch and cure times for all trial compounds at 160°C
Further cure tests were carried out at 185°C on the commercially produced
compounds PB80, EOl and FRS8 (Table 6-6) to see how this compared with the
recommended processing conditions (Table 6-7). It should be remembered that the
EOl and FRS8 undergo considerable post-cure and, according to Hofmann19, it is
only in this post-cure that the cross-linking reaction is completed and the good
vulcanisate properties are obtained in the fluorocarbon rubber (FRS8).
140
Compound Scorch Time Cure Time at 185·C (secs) at 185·C(min)
PB80 30 4 EOl - 6 FR58/90 - 7
Table 6-6 Scorch and cure times for the industrially produced trial compounds at 18SoC, the temperature recommended for vulcanisation
Compound Cure Time Post Cure Min. at 185·C (min) Time (hrs)/Temp. (OC)
PB80 4 -EOl 4 6/150 FR58/90 6 121230
Table 6-7 Cure for the industrially produced trial compounds as recommended by the manufacturer
Moulding temperature offsets were applied to the PCA set temperatures when
moulding samples with both the FORM system and conventional sheet moulding.
The author has assumed that the conventionally produced O-rings were made with
a mould surface temperature close to the 185°C stated. The FORM system is
consistently about 6-8°C lower in temperature at the surface of the mould than the
temperature measured by the thermocouple in the platen. Similarly the conventional
hydraulic press used in the current work is 10°C below that of the set-point. The set
points were, therefore, adjusted by adding the offset to the machine set-point to take
this factor into account and obtain the correct cure temperature at the mould
surface.
6.2 Finite Element Modelling
The conditioning history of the material prior to cross-linking plays a key, if not
dominant, role in determining the degree of molecular orientation in the vulcanisate.
This orientation will occur in the FORM system in the preforming dispenser unit and
during mould closure.
The flow and heat histories were investigated using finite element analysis (FEA),
which has been developed into a very powerful resource for design and diagnostics
in many engineering contexts20• Much of this modelling was carried out before the
dispenser design had been finalised, and metal cutting started. The results had a
141
significant influence on the design of the FORM system prototype produced for the
current investigation.
FEA relies on various physical phenomena for chemical, thermal, electromagnetic,
solid mechanics, fluid mechanics and dynamics being expressed in terms of partial
differential equations. These can be solved numerically and thus FEA can be used
to approximate temperatures, stresses, pressure, velocity and magnetic field etc.
The fundamental information for the application of FEA to rubber products has been
known for more than quarter of a century but, excepting the case of tyre
manufacture, has only been applied to rubber technology over the last five to ten
years. Nichelson and Nelson21 suggest the following reasons for why this might be
so.
(i) rubber is nearly incompressible, developing high stresses in regions of
confinement.
(ii) rubber undergoes large strains, indeed its compliance is one of its major
attractions.
(iii) rubber is often bonded to much stiffer materials.
(iv) rubber components are often small and thin.
(v) failure often occurs at the interface with a stiff material.
(vi) rubber material properties expressed as a strain energy function are very
difficult to characterise experimentally.
The finite element modelling was carried out using an 'off the shelf' application to
encourage its use by making it readily accessible and easily reproducible 'on the
desk-top' in an industrial environment22. The finite element analysis software that
was chosen to conduct the work was the NISA 11 family of applications from EMRC.
6.2.1 Heat Transfer Modelling
The early modelling was simply concerned with direct heat transfer. Several
concepts for material feed mechanisms were considered, and two were shortlisted
for further consideration.
142
6.2.1.1 Reservoir Geometry
The design concepts were narrowed down to two (i) the injector and (ii) a transfer
pot. The geometry is defined in Appendix O. The results of the static heat transfer
analysis are summarised in Table 6-8. The geometries with smaller volumes warm
up quickest. Results show that it can take some considerable time to heat up a
mass of rubber from room temperature to 80°C by simple conduction. Some of
these times are clearly excessive and could not be tolerated in a production
environment. A significant reduction in the warm-up time can be achieved. without
excessive loss of reservoir capacity. by increasing the contact surface area of the
rubber with the reservoir by adding a heated core.
The early work was carried out using a mesh constructed of 8-noded (2nd order) 20
axisymmetric elements (NISA element type NKTP = 103). This seemed to cause
some temperature instability at some of the corner nodes on some of the elements
(Figure 6-4 (a)). The problem only occurs when a radical temperature change is
required and can be lessened by ramping the applied temperature rise (Figure 6-4
(b)). Figure 6-5 shows a model in which 4-noded elements were used. The smooth
contours indicate the absence of instability.
On further investigation it was discovered in literature that Oamjanic and Owen23
and Barrett et 81.24 have shown. that when FEA is applied specifically to heat
transfer problems. a mesh of 4-node elements gives better results and exhibits
fewer oscillations than an identical mesh of 8-node elements. 4-node elements were
used subsequently and the work repeated. However. the re-calculated results only
showed an error in the order of a few percent by the end of each run.
Reservoir Shape and Volume (mO) Warm up time (min.) size (mm)
Pot - 0200 x 50 1·571x10·o 157 Pot- 0115 0·519x10·o 130 Pot - 0115 x 50 with 0·484x10·o 92 030 core Injector - 050 x 130 0·158x10·o 68 stroke Injector - 050 x 130 0·153x10·o 35 stroke with 010 core
Table 6-8 Warm-up times for the five proposed reservoir geometries.
143
6.2.1.2 Static Heat Transfer (Pseudo-Flow Simulation)
A draft design for the dispenser was used as the basis for heat transfer modelling.
This early design consisted of a large diameter (50mm) main bore with a flow
division into four smaller diameter bores (30mm) distributing the flow to the meter
cavity. Volume flow rate was used to determine the residence time of the material in
the system. The results of this calculation were considered to be lower than was
desirable so a similar calculation was conducted on a similar geometry (in terms of
flow path length) but representing bores that were smaller in diameter (main bore
30mm and sub-division bore 20mm).
Flow Rate Temperature QC (m3/15s) Dispenser Geometry (bore diameter mm)
50130 30/20 0·lx10·· 23 21
0·01x10" 54 43
Table 6-9 Direct heat transfer modelling of two dispenser runner configurations at two volume flow rates volume flow rates
The predicted temperature for the two schemes modelled is given in Table 6-9.
There does not appear to be significant difference in the two systems. However it is
worth noting that the residence time of the material in the system with the smaller
bores is less than half that of the system with larger bores for a similar temperature
gain. There is also a very noticeable step in the rate
narrow bores in each system. The narrower
144
of temperature rise in the
bores give a greater
""""""'" """""" .. 11 ." " ... ,. ... "." ll ." "." n.lt " ... a ... "." s,.., " ... ..... " .• ... n " ." G." 11.13 .... .. ... , ... " " , tl
"." 'It.'' .... "' ... a .14 "' ... 1',43 " ...
(a) (b)
Figure 6-4 Model of the 0S0x130mm injector (a) early in the sequence (S minutes) shows the nodal instabilities clearly (the temperature ranges from 1S.43· - 81.30· C); (b) the same model much later (nearly 70 minutes) shows a much lessened, but still present effect (the temperature ranges from 78.46· - 80.0· C)
,£W_ oo .•
" .• ".n 19." " . ., "." ?f ,1I
n ...
"It.,, ", .n
"'." ... ., ..... "'." "' .•
Figure 6-S Example of a 2D-axisymmetric model of the 0 200mm pot geometry, constructed with 4-noded elements, after a period of about 160 minutes. The smooth contour bands indicate that the instabilities (Figure 6-4) are absent (the temperature, in this case, ranges from 70.00· - 80.00· C)
145
temperature rise for a given volume of material because heat transfer takes place
through a greater surface area. The fact that the residence time of the material in
the system is much less does not seem to be too detrimental. These results only
take direct heat transfer into account. Viscous heating has not been considered.
6.2.2 Fluid Flow Modelling
6.2.2.1 Dispenser Flow (To Fill the Meter Cavity)
More sophisticated models of fluid flow through the dispenser were created using
the NISA 11/3D-FLUID module. The aim, in the first instance, was to predict the
temperature and pressure history of the material in the dispenser and later to predict
stresses and preform shape change. The methodology and data produced are
intended to help further development of the FORM system. They could possibly be
the basis of a set of design rules or be incorporated in a design package to aid the
design of dispenser inserts and moulds.
6.2.2.1.1 Initial Design (Flow Geometry I of Chapter 5)
This modelling was undertaken concurrently with the design. By the time that this
model was created, the feed mechanism (a reciprocating stutter piston) and the
meter cavity geometry had been settled. The interconnecting flow path however was
still under consideration. Previous heat transfer modelling had been used as the
basis of the design decision to change the bore diameter from the initially proposed
50mm to 30mm in order to obtain a better heating rate.
A set of material parameters, labelled STD and stated elsewhere (pp 141-142) in
this chapter, were contrived, purely for the purposes of modelling. They represent
realistic values for a typical compression moulding rubber compound. The STD
values were used in the modelling of the first generation dispenser design. Figure 6-
6 shows a plot of the predicted total pressure drop across the entire flow path and
also the terminal temperature. The initial starting temperature for the material was
20°C and the body temperature of the simulated dispenser was set at 60°C. The
volumetric flow rates used in the modelling are given in Table 6-10.
146
Volume Flow Rate (Q) Piston Speed Volume Flow Rate(Q) Piston Speed (m3/s) 10.6 (mm/s) (m3/s) 10.6 (mm/s)
141 200 42·4 60 124 175
, 38·9 55
10·6 150 35·3 50 84·8 120 31·8 45 70·7 100 28·3 40 63·6 90 21·2 30 60·1 85 14·1 20 56·5 80 7·07 10 53 75 3·53 5
49·5 70 0·71 1 45·9 65
Table 6-10 Volume flow rates and the corresponding piston speeds calculated for a 30mm diameter bore
As can be seen from Figure 6-6, for the STD input parameters temperature rises to
nearly 600 e at the lowest flow rates. At the highest flow rates a temperature of 80 0 e, a rise of some 60 0 e, is attained. The pressure drop across the system is an
indication of the work needed to force the material through the system.
Pressure drop and temperature are the quantities calculated by the finite element
model but throughout the rest of this chapter the pressure drop has been converted
to the force required to make the material flow through the system, because this is a
more useful engineering quantity from the design point of view. Full pressure drop
and temperature rise details are given in Appendix G. Figure 6-7 shows the FE
model output for the three starting temperatures, 20°, 60° and 1000 e. The
relationship,
P = F/A (6-4)
was used to calculate the force required from the pressure drop output from the
model. The bore was assumed to be 30mm in diameter and the maximum force
(47908N) was calculated from the maximum hydraulic pump delivery (23637kPa) in
a cylinder 50·8mm in diameter.
147
.." cC' c ~
CD en , en
" ~ CD 11/ 11/ C ~ CD Cl. ~
0
" .. ::J Cl. .... CD ~
3 ~
co ::J !!!. .... CD
~ 3 ~ " CD (Xl ~ ..
Cl. -.. ~
::J .. .. .. ~
Cl. .... c ~
CD
0' ~
Cl. or " CD ::J 11/ CD ~
::!! 0 ~ cc
CD 0 3 CD .... -<
120,OE+6
100,OE+6
80,OE+6
60,OE+6
40,OE+6
20,OE+6
70
60
-I CD
50 -5 CD -+-Total Pressure Ql c Drop (across
40 ;; system)
30
20
10
Temperature (temp. rise + initial temp.)
OOO,OE+O +---+---+----+----+----+----+---+----+----/-----+ 0
o 20 40 60 80 100 120 140 160 180 200
Piston Speed (mmls) Proportional to Volume Flow Rate (a)
-." 0_,
~'" VII: -i~ C~ 9O,OE+3 en 3.:.. !!l.." ~. ~ 80,OE+3 AI " -~ .,,~
AI ~ iil'" 3 !:, 70,OE+3 ~; ~ ~c.
~ iilO' ~ ~ E 60,OE+3 Max, Force :::!> AI 0 - Available :e ..
>- -+- 20· C AI .. (47908 N)
" 50,OE+3 ~ ;;:: 60· C 0 en 0 en -~ - ." ~100·C .,. :r .,
40,OE+3 CD ~ ~
c. ':; iij' C' ., ." ~
~ -::I ., 30,OE+3 .. u
~ ~
0 ~ u. C' AI 20,OE+3 .. ~ c. a ~ 10,OE+3 ::I ;:;: ~
~
iD OOO,OE+O 3 ~ 0 20 40 60 80 100 120 140 160 180 200 ::I -3 Piston Speed (mmls) 0 Proportional to Volume Flow Rate (0) c. !!.
This flow path design (flow geometry I of Chapter 5) was rejected because the force
required for flow through the system starting at 20°C, which is akin to, say, starting
up the machine at the beginning of a shift after it has been left over night, is too
high. At 60°C, the proposed working temperature, operation would be possible at
the highest flow rates for a material with the same characteristics as STD. However,
even moderate flow rates come close to requiring the maximum possible force. This
leaves little spare capacity. A compound with the same rheological characteristics
as STD would be a typical moulding compound with a typical viscosity so that there
would be little scope to process high viscosity compounds.
6.2.2.1.2 Modified Dispenser (Flow Geometry 11 of Chapter 5)
The above FE modelling exercise was repeated on the next generation deSign, flow
geometry 11. From experience, the results of the FE modelling are of the expected
order, but to provide further verification the theory of flow of viscous polymeric fluids
through channels of simple cross-section described by Brydson25 and by
Fredrickson and Bird26 was also used to calculate the pressure drop across the
system. For the purposes of this modelling two simple cross-sections are
considered, namely, a cylindrical tube and an annulus. There are some non
cylindrical channels in the dispenser design and these were approximated by the
equivalent cylindrical pipes for this modelling exercise. For a pipe
,Jr3n+llnr 4Qln
]
2 Ll~JlJl"R3J M = --=---:----=
R (6-5)
Here R is the radius of tube, Q is volumetric flow rate, L is the length of the pipe, K
is the consistency constant and n is the non-Newtonian or power-law index. For an
annulus
M= Qn2KL rlmJ(R,+Ro)(Ro-R,)21J1J-n (Ro - R,) L 2(2n+ I)
(6-6)
where Ri and Ra are the inner and outer wall radii, respectively.
150
-." c: _ .
3'" 'Cc: co iil Q.",
~c» Dl41 3 co coQ. CD ir ... :!: 3 g 0 .. Q. o !!.=." cE~ .. n _co ~; .. .c a!:. ::I ' "'ll --co 0 3 ' 'C~ co 0 iil :e -.. !:; n co Cl .... So" N~ 00 _. :e "'''' oco ·0
.. 3 5.!. .... 0< 8= .0'
0'< :::!I ::I
;: co co 3 co ::I -.. ::I Q.
g E ~ >-..
;;:::
.s ~ ::I
! ., ~ o LL
70,OE+3
60,OE+3
50,OE+3
40,OE+3
30,OE+3
20,OE+3
10,OE+3
Max. Force Available (47908 N)
__ STD(20)C
--- STD(60)C STD(100)C
_ ..... ....-~ ..... _ ............ .-...--...... ---:-~==:===:===J--STD(20)M -+- STD(60)M -+- STD(l OO)M
OOO ,oE+o +I------+�------+�------~I~----_rI------~I------+1------+I------~I~-----rl ----~I o W ~ M ~ 100 lW 1~ lM 1~ 200
Piston Speed (mm/s) Proportional to Volume Flow Rate (a)
-----
Since the consistency constant K is temperature dependent, the calculation was
divided into sections each representing a portion of the flow. The parameters are
assumed to be constant for each section of flow. This technique is called 'lumped
parameter modelling'. The required force predictions based on the results of both
the finite element and the lumped parameter modelling (based on Equations 6-5 and
6-6) for the STD 'compound' are shown in Figure 6-8. The notation adopted for the
figures is as follows: CAPS (e.g. STD) indicates the set of material properties used
for the predictions, (number) (e.g. (20)) indicates the model start temperature and M
or C represent Model result (FEA) and Calculated result (lumped parameter
modelling). The results show that it would be possible to process this compound
(STD), to some extent, under all of the starting conditions. The higher flow rates
under the 20°C starting condition, however, could not be processed as the force
required exceeds the maximum that can be delivered. This was not considered a
problem because it would be very rare, if at all, that processing of rubber
compounds would take place in the FORM system at such low temperatures.
The predicted temperatures for the same flow conditions are given in Figure 6-9.
NISAl3D-FLUID outputs the result of its viscous dissipation calculation directly as
fluid temperature. To obtain the temperature from the lumped parameter modelling it
is necessary to assume that the volume average temperature rise ~T is related to
the work done by the material and is directly proportional to pressure drop ~p27.28.
This relationship can be represented as
~T = ~P/pCp, (6-7)
where p is the density of the material and Cp the specific heat. The temperature of
the material after flow through the system, then, is the initial temperature plus ~T.
The plot of terminal temperature vs. flow rate (piston speed) is given in Figure 6-9.
As expected, because viscosity decreases with increasing temperature, the
temperature rises for lower starting temperatures are greater than those for higher
temperatures.
152
0I"T1 :l _. 0.'" I: - ~
I: '" 3",
'l "0 ' ",,,, Q.~ • • • • "0'" • • • • • • • • 01 ~ • • • • • • ~ 3 • 01 _ .
3 i: "'- lOO --'" '" ~ 3 3"0 0'" Q.ji\
'" - 80 =1: -+- STO(20)C :S' CD
"'''' E --STD(60)C !O'
STD(100)C '" ~ 1! ar~ = 60 --*- STD(20)M ::1,0 i! -. ~ ..
__ STD(60)M :l 01 Q.
'" n E -~ ..
....-. STD(100)M ~ '" 0 0-
c.n 3 ., W "0 '" 40 "'= ~ 0
~~ !:;'" '" '" .. 0 o 3 20 --", t.>-~-< ~= 0"0 o~
01 ~ 0 :l _. Q.n 0 20 40 60 60 l OO 120 140 160 160 200 -~'" 00. Piston Speed (mmls) 0C' n'< Proportional to Volume Flow Rate (Q)
:!I :l ~
'" '" co 3 '" :l -
-"T1 " -, 3'" "," CD ~ c.CD
en ", , AI ... ~ 0 AI ::0 3 CD CDJ:I
-" CD _, ~ ~
3 CD c.
0 ... c.o CD ~ =n :j' ~ g ",< , !" E
", Q)
ijj' 1ii () >.
'" ::l
"' -", g CD 'C CD Q) C. ~ ... :;
~ 0 er ~ Q)
(J1 !!!. .=. .l>- Q) - ~
~ 0 ~ IL
n 0 3 ", 0
" ::l C.
"' n AI n " ii' -CD c. .,. '< "T1 m :I> AI ::l C.
70,OE+3
60,OE+3 -+-STD(60)M ___ NR(60)M
50,OE+3 (47908N) -*- SBR2(60)M ~~a:X:'F~o~r!ce~A~~va:il:a~bl:e~~::;:~;;;;;:;::,~::~~~~======~~~~~~~~~~ SBR1(60)M
.:::::;~~~=~;;;;~;;;~;~~~ -lIE-NBR(60)M
:;~~;~;;;~;~~~~~~~::::::::: -+- EOL(60)M
-!- PB80(60)M
;::~~~;:~::::::: - FR58(60)M ~::::~~====tir====r===i --STO(60)C ::: -+- NR(60)C
~~~~~~~~~::::=:::-:==:=====:::-:===::===l SBRl (60)C SBR2(60)C
-*- NBR(60)C ___ ___ E-O-L(60)C
~:::~- -+- PB80(60)C
40,OE+3
30,OE+3
20,OE+3
FR58(60)C
OOO,OE+O .J----~--~---~---~--~---~---~---~--~----o ~ ~ M M 100 1~ 1~ lM lM 200
Piston Speed (mmls) Proportional to Volume Flow Rate (0)
"t1"t1 m-' >'g '" ~ 105 ::> CD c.cn , - ... : ~ 1: ... : 3-i
100 ::::===: -+-- STD(60)C "'CD CD ~ C.3 __ NR(60)C '" _. '" ::> " SBR1(60)C ~!!.
95 ~~ "'-3 CD --SBR2(60)C CD 3 ----", • __ NBR(60)C ~ CD • 3 iil 90 .-.-- -+-- E-O-L(60)C oc c.~
~ -+- PB80(60)C CD CD I-> : ="' ;e - FR58(60)C -'- 85 ::> 0 e '~ tp ~
~ = - STD(60)M E -+-- NR(60)M - 8. ~ 80 ~ E SBR1 (60)M " ./ ~ n ....
SBR2(60)M (Jl 0 (Jl 3
'" 75 --NBR(60)M 0 I: EOL(60)M ::> c.
--PB80(60)M "' 0' FR58(60)M ~
:::!O 0 :e 65
I.Q .. 0 3 CD 60 --< 0 20 40 60 80 100 120 140 160 180 200 = n Piston Speed (mmls) '" Proportional to Volume Flow Rate (0 ) c:; I: ~ -CD C. C' '<
On the basis of the results of this modelling, the design of the dispenser and primary
(a-ring) meter cavity was fixed as flow geometry 11.
Later the same models were employed to predict the behaviour of all the trial
compounds (Figures 6-10 and 6-11)). Again calculations were carried out for the
three starting temperatures but for the sake of brevity only those for the 60°C
starting point are reported now. Further plots are given in Appendix H. The
temperature rise in the material as it flows across the system is important, as one of
the purposes of the dispenser unit is to pre-warm the material to a working
temperature of at least 60°C. The previously described heat transfer modelling
shows that direct heat transfer takes a considerable time. The more rapid the rise in
temperature the higher the throughput attainable. The temperature rises indicated
by the modelling are encouraging in this respect because elevations in temperature
of 5° to over 20° are obtained at the lowest flow rate. It was noted that the finite
element predictions retum consistently higher values than those calculated with the
Equations 6-5 and 6-6.
6.2.2.2 Dispense Flow (Metering and Preforming)
Modelling of flow for the dispense cycle was treated as a topic that is distinct and
separate from flow in the feed and runner system. The FORM system dispense
motion physically shuts off the runner system from the meter cavity and at the same
time creates the opening through which the material is forced to exit the dispenser.
This creates two independent flow systems. Figures 6-12 and 6-13 show the forces
required to dispense a preform for the a-ring and the sheet dispenser units,
respectively. The sheet dispenser requires less force for its operation and all the trial
compounds could be processed although PB80 requires the maximum force at the
very highest flow rates.
156
""TT '" _. Q.'" c: 'tI' m CO ",en 0':"
200,OE+3
tIIN ':T'tI 0_ c: 0
1 BO,OE+3 --Q. O 0' .... COO' III , O'n -CO CO , -CO O..Q O'c:
160,OE+3
140,OE+3 (149713 N)
__ STD fD ~.
Q.CO -.Q. tII _
"0 CO < '" CO : iil Q.c: O'tII '«
~ -0 :r -
(J1 CO c: -..j 33
III CO n:!! :r0 :i' ~
~ ____ NR ~ 120,OE+3 Cl> SBR1 " ~ 0 __ SBR2 u.
100,OE+3 " --lIf-- NBR Cl> ~ __ EOl ·S 0' BO,OE+3 --+-PBBO Cl> a:
- FR5B 60,OE+3
CO iil -!" 40,OE+3
~ 0 0 20,OE+3 3
" 0 c:
'" OOO,OE+O +----I------1I-------+----+----+----+----+----+------j
Q. tII OOO,OE+O 30,OE-6 60,OE-6 90,OE-6 120,OE-6 150,OE-6 1BO,OE-6 210,OE-6 240,OE-6 270,OE-6 CO >< n Volume Flow Rate (m3/s) CO
" --':T CO Z m ;0
n" 0-· 3'l! -gc; co> ::I ' CI. ... ..... .. :l! ::1"0 0-C 0 0:: .,.0 ID Cl III ID .,., CD~ S' 5 . .,.a ID Cl.
~cr 0' nCl. : c;;' ",,, ID ID CI.::1 _. .. ::I ID -< ::I"c ID iil ~CD .. .. " < ~2. '" c ID 3 ~ ID
::!I
~ i 0' , III .. ::I" ID ~
" a 0' ~ ~
~ .. ~ o IL .., e -S .,. ~
160.0E+3
140.0E+3
120.0E+3
--+-STD 100.0E+3 --NR
-+-SBR1
----SBR2 80.0E+3 ---NBR
---EOl
-t-PB80 60.0E+3 -FR58
40.0E+3
20.0E+3
OOO.OE+O +----_+_---_+---~f__---+_---_t_---_+----t_---_+_---___i
OOO.OE+O 30.0E-6 600E-6 90.0E-6 1200E-6 150.0E-6 180.0E-6 210.0E-6 240.0E-6 270.0E-6
Volume Flow Rate (m'/s,
The temperatures that are achieved by the flow from the O-ring and sheet meter
cavities are plotted in figures 6-14 and 6-15. The temperature rise in the metered
dispense phase of the FORM process is much more significant than in the filling
phase and, from a production point of view, this is an asset. The material is
significantly warmed by working in the preforming stage. This brings the temperature
of the preform much closer to the vulcanisation temperature and could reduce 'in
mould' cure time. The fact that this large step rise in temperature does not occur
until just before the material is expelled from the machine means that the chance of
the material curing in the dispenser and preventing machine operation is low.
Comparison of the two charts shows that higher temperatures are predicted for the
material in the O-ring dispenser than for that in the sheet dispenser. This result is
not unreasonable as the flow path geometry in the sheet dispenser is considerably
shorter than that in the O-ring dispenser. Hence the pressure drop, force required
and temperature, which are all related, would be lower.
159
Z"T1 -cC' Ch e >", =CO
--'" 5' , .... :;::::;:.a:a. co -t co co (;'3 3'" co co " '" -!!'. 3 e
'" 0 co c. .. ~= ::D) " -, cc" '~ G'
C' ~ '< ~ -::r " co -ca - ~
'" ., !: c. n E
~ 0 .,
Cl 3 l-Q ..,
0 e
" c. III
5' -::r co 0 , ~,
" cc c. iir .., co :s III co '" .., '" co c. 0' -co c. C' '<
115
110
105
100
95
90
85
-+- STD --- NR
SBR1
""*'" SBR2 -lI!- NBR
--EOL
-+-PB80 --FR58
80 +------+------~------+_----_+------~------~----_+------~----~
OOO,OE+O 30,OE-6 60,OE-6 90,OE-6 120,OE-6 150,OE-6 180,OE-6 210,OE-6 240,OE-6 270,OE-6
Volume Flow Rate (m'/s)
Z"T1 1ii cS' »5; =CD :::!!'" ::I':' ;:::;:UI CD-I !tCD
~.g CD CD
aD1 32' o ~ Q.CD
~~ S' 2!. IQ::I • CD
Q.
IT '< ... ::r CD
<;' ~ n o 3
" o C ::I Q. UI
S· ... ::r CD UI ::r CD ~ Q.
or " CD ::I ., CD ~
" <; Q.
~ Q.
IT '<
90
88
86
84
U 82 ~
~ ::I
f 80
" Q.
E ~ 78
76
74
72
70+-------+-------+-------+-------;-------;-------1-------1--------r------~
OOO.OE+O 30.0E-6 600E-6 90.0E-6 1200E-6 1500E-6 180.0E-6 210.0E-6 240.0E-6 270.0E-6
Volume Flow Rate (m3/s)
-+-STD
----NR __ SBR1
--SBR2 --NBR --EOL
-+-PB80 -FR58
6.2.2.2.1 What if? Modelling of Other Possible Meter Cavity Configurations
Two other O-ring meter cavities were modelled on a "What if?" basis. It was
assumed that it would be possible to manufacture the proposed geometry. The
same initial conditions were used for these models as were used to model the 0-
ring meter cavity. Only the STD material parameters were used.
20mm
12mm
18mm
5mm
Opening 1 mm wide
(a) (b)
Figure 6-16 Dimensions of the "What if?" O-ring meter cavity cross-sections. The centre of the 1mm opening falls on a 97mm radius.
Configurations designated 0001 and 0002 are shown in Figures 6-16 (a) and (b).
The geometries are considerably different but they have been designed as flow
channels 'funnelling' down to give a similar opening (the direction of flow is top to
bottom in the figure). 0001 has a wider flow path and narrows later whereas 0002
narrows immediately and then becomes a 1 mm wide parallel channel. It should be
remembered that the figure represents a cross-section of an annular cavity. The
characteristics of the flow in each case have considerably different pressure
temperature-force relationships (Figure 6-17).
162
o:!! Occ OC: ... c;;
'" en :::I , 0. ...
160,OE+3 75
Maximum Force 0'" 0"" og Nn c: CD .. ~ _. CD
(149713 N) 73 140,OE+3
71 ".c ccc: - ::::; ' 120,OE+3 :rCD CD 0. 69 C/)'" -i:::l 00. -3 CD
'" 3 -.., ~ CD -.~
'" '" --.., c: '" ~ ~ CD
'" ~ ~ 3 CD
'" m CD n (,) ;-:r
~ CD 0. ...
Z 100,OE+3 67 ~ -.. 3 0001(N) u
~ ..,
0 CD __ 0002(N) u. 80,OE+3 65 iil .., - -+-0001 (OC) .. c: .: C;; ____ 0002 (0C) :::I ~
er 63,9 .. 60,OE+3 a:
61 0 ~
:::!! 40,OE+3
0 :I; 59
:::I -:r CD
20,OE+3 57
~ :r '" - OOO,OE+O +----t----f----+-----1C-----t-----t----+-----t---+ 55 ::;; ~
OOO,OE+O 30,OE-6 60,OE-6 90,OE-6 120,OE-6 150,OE·6 180,OE-6 210,OE-6 240,OE-6 270,OE-6
3 CD
Volume Flow Rate (m3/s) -CD ~
n '" < ;:;: iij' ..
6.2.3 Prediction of Preform Shape Change Using Fluid Flow and Static Finite Element Modelling in Combination.
Two methods were used to try and predict the shape change of the preform as it is
released from the constraint of the meter cavity. In the first, an axisymmeteric solid
cylinder was created onto which the normal stresses were mapped. The dimensions
of the aperture were assumed for the size of cylinder and the length calculated from
volume. The stresses were progressively released with a time step function to
simulate the rate of extrusion from the dispenser. In the second a similar method
was applied to axisymmetric shell elements.
(i) Axisymmetric Solid Model
Figure 6-18 shows the element mesh before and after the resolution of the applied
stresses. The time step function is clear from the resulting predicted deformation but
the result is quite clearly incorrect. The expected result would show a swelling and
shortening of the preform. Zero deformation was expected at the top of the model
because these nodes were fixed in space representing the point of attachment
before the preform is cropped from the dispenser.
\ \\\ \\\\
Figure 6-18 Axisymmeteric solid finite element mesh and the deformation predicted.
164
(ii) Shell elements
The results from the axisymmetric shell element modelling were more inconclusive
than those reported above
It is not known whether the models created or the method used are fundamentally
flawed or the analysis package is simply incapable of being used for modelling in
this manner. It could be that it is not currently possible to predict the deformation of
the fluid but only entities external to the fluid, other analyses using the results
obtained from the flow module in the static modelling are possible. For example, the
deformation of a pipe due to the pressure in the flow in that pipe can be resolved but
this is not usually performed on the flow rubber.
6.3 Experimental Work with the FORM System
Once the FORM system prototype had been built and was installed in the laboratory
at Loughborough work began on determining, developing and optimising the
operating procedures and evaluating the system performance. Mouldings were
produced both with the FORM system and by conventional compression moulding.
A series of tests were carried out to investigate the anisotropy, molecular orientation
and general performance of the parts and the results compared.
6.3.1 Preforming with the FORM Dispenser
The dispensing operation is a central part of the FORM concept. It is necessary to
shape the material, to meter the quantity and to manage the flow for the production
of isotropic or near-isotropic mouldings.
6.3.1.1 Filling the Meter Cavity and Preforming
An automated feed mechanism was designed to allow swift and easy charging of
the runner system and meter cavity. It consists of magazines that can be pre-Ioaded
with cut strip or milled sheet, slotted into position over the feed pocket and then
forced into the feed pocket by means of a pneumatic cylinder. However, the rubber
jammed in the mouth of the feed pocket, when the stuffer ram was actuated,
165
material failed to be forced into the runner system. The automatic feed mechanism
was abandoned and filling the meter cavity had to be achieved by manually feeding
the cut milled sheet into the feed pocket and actuating the stuffer ram under push
button-control.
This was a setback in terms of eliminating the inconsistency that manual input often
brings to any production or processing operation. The preforms that were produced
initially varied considerably both in terms of dimension and in terms of shot weight.
To investigate the cause, the length of the dispense stroke was set to a fixed value
and the displacement obseNed with the dispenser empty and in a variety of filled
conditions with a number of compounds. The dispense stroke was measured with a
rule (±O·5mm) and on the machine computer control VDU which was set up to
display the current Moire-fringe encoder reading during a dispense cycle (the
obseNed accuracy was no more 5 divisions which equates to ±O.027mm). The
dispense stroke remained consistent throughout. Therefore the only other possible
source of origin for the obseNed variation had to lie within the steps that constitute
the filling procedure. Two methods, previously described, of filling the meter cavity
consistently were determined and, with great care, repeatability was attained.
6.3.1.2 Preform Consistency (Shot-to-Shot Repeatability)
The consistency of the preforms produced by the two cavity filling methods was
remarkably high as indicated by Tables 6-11 - 6-13. Although it has to be mentioned
that achieving this level of accuracy was painstaking and laborious. It does, however
give great confidence in the capability of the system to meter highly accurate
preformed charges repeatably if a suitable automated feed mechanism, such as a
screw pump, can be found.
The O-ring dispenser provides better accuracy than the sheet dispenser, standard
deviations for both dispenser units are well below two. The sheet samples measured
166
~---------
Shot No. Recorded weight (g) 1 50·4 2 49·9 3 49·7 4 50·8 5 49·3 6 50·4 7 50·4 8 49·8 9 51·4 10 49·3
Mean SO Coel. of Maximum Minimum Range Variation Weioht weioht
50·14 0·667 1·33 51·4 49·3 2·1
Table 6-11 Shot-to-shot accuracy and repeatability for NR. The weights of 10 individual shots are given shots are given. The dispenser was setup to deliver a preform of a target weight of 50g
Material Mean SO Coel. of Maximum Weight Minimum Range Variation weight
NR 44·60 0·71 1·60 45·50 43·30 2·2 SBR1 45·98 0·90 1·96 47·60 44·60 3 SBR2 43·62 0·86 1·97 45·20 42·30 2·9 NBR 40·92 1·88 4·60 43·20 37 6·2 PB80 52·27 0·94 1·80 53·40 50·40 3 EOL 48·34 0·93 1·92 49·50 46·40 3·1 FR58 71·94 1-11 1·54 74·60 70·60 4
Table 6-12 Results of the experiment to determine the accuracy and repeatability of the O-ring dispenser.
Material Mean SO Coel. of Maximum Weight Minimum Range Variation weight
NR 36·09 1·32 3·66 38·70 34·10 4·6 SBR1 36·22 1·21 3·33 38·50 34·50 4 SBR2 34·80 1·53 4·41 37·20 32·50 4·7 NBR 28·84 1·54 5·35 30·90 26·80 4·1 PB80 39·11 1·28 3·27 41·60 37·80 3·8 EOL 34·52 1·58 4·57 36·50 32·00 4·5 FR5B 57·08 1·21 2·12 58·90 54·50 4·4
.. Table 6-13 Results of the experiment to determine the accuracy and repeatablhty of the sheet dispenser.
for this experiment were produced from two cavities. If the cavities are treated
individually then the standard deviations fall slightly. There is a slight but consistent
difference in the delivery from the two cavities which amounts to no more than about
3g, depending on the compound, for the above test.
167
The cause of this difference is not known but the explanation favoured by the author
is an imbalance in the flow in the dispenser runner system. The cavity sizes were
measured for an obvious difference in size but none was found. This could have
implications for the development of the FORM system for mUlti-cavity applications.
6.3.1.3 Preform Size Range (Weight)
The possible operating envelope was determined for two dispenser inserts. This is a
measure of minimum and maximum possible dispense; it is an indicator of the
versatility of the machine and highlights one of its limitations. The system could be
used to produce a range of products that have similar dimensions but different
volumes, for instance, a range of O-rings with the same diameter but different
gauge.
I Minimum DisDense((])i Maximum Dispense (Q) O-rirm Disoenser NR I 6·3 I 120·5 SBR I 6·8 I 135·70 Sheet Disnenser NR I 1·5 I 89·3 SBR I 2·4 I 84·2
Table 6-14 Maximum and minimum weights for dispensed preforms
6.3.1.4 Dispensed Preform Temperature
The temperature of the preforms was measured at dispense. This is the only direct
method of comparison and verification of the finite element modelling. It would have
been desirable to make a measurement of pressure drop directly from the flow of
the compound rather than inferring the pressure drop from the material and the
dispenser geometry but this was not possible with this equipment.
The predicted temperature rise has been replotted in Figures 6-19 and 6-20, as the
measured temperature of the material verses dispense time. The results for the in
house and commercial compounds have been separated for clarity. As can be seen
the measured temperatures are all lower than those predicted by the FE modelling.
The reason for this could be simply that there are heat losses from natural
convection that occur as soon as the material of the preform is released from
168
constraint. There is a noticeable tendency for the results to converge at the lower
dispense times (higher flow rates).
169
3!! CD CD .. I: .. iil I: ~'" CD , Q. .... ~ ... ... oc ~ _. 0" ,"0 ~ CD _. :s :s ..
CD CD Q.Q.
;n' " -ga; :sO' .. ~ ~ 3 -- U
CD 0
3 -Cl) "0 ...
CD :s ~
15 .. -I: ... iil
Cl) c. .. E ~
0 -...I Cl) ... I-e -". E CD ... S· ~ , ". ... 0 Il. I: .. CD n 0 3 "0 0 I: :s Q. .. -;; m "0 ~
CD Q. ,;-c: 0 :s .. .. :s Q.
115
110
105
100
95
90
85
80+-----~------~------r_----_+------;_----~~----_+------+_----~------~
o 2 4 6 8 10 12 14 16 18 20
Preform Dispense Time (s)
--NR
--SBRl ___ SBR2
--M-- NBR
-.- NR(ACT)
-+- NBR(ACT)
....... SBR1(ACT)
--a- SBR2(ACT)
'C"T1 ~ _. (I)'" Cl. I: _. ~ g.(I)
er er» "N "0 .. C " _. CI. ..
3"5: (I) " .. .. .. (I) I: Cl. ~ (I)'C CI.~ -(I) 0'0' 03 , ... :::::!.CD " 3 "''C Cl. (I) _. ~ .. .. "ge" iil ::: .. :"' 0 ... ...
::r (I) n o 3 3 (I) Cl !: .:c 'C Cl Cl. I: n CD Cl. n o 3 'C o I:
" Cl. .. " m
115
110
105
cr o -Cl) ... :::s l$ 100 ... Cl) Co E Cl) I-
E ... ~ ... Co
95
90
85
~==. ===-: -----------------..-
80+------;------~----~r_----_r------+_----_r------+_----_+------~----~
o 2 4 6 8 10 12 14 16 18 20
Preform Dispense Time (s)
-+-EOL
--PB80
--.- FR58
-PB80(ACT)
-FR58(ACT)
--><- EOL(ACT)
6.3.1.5 Preforming - Observations
6.3.1.5.1 Curtaining
The 0·05 - O·OBmm gap clearance between the inner and outer parts of the
dispenser at the bottom of the meter cavity proved small enough to prevent material
escaping during the process of filling the meter cavity. At the normal operating
temperature of 60°C only the NR and SBR compounds exuded material through the
gap when filling was taking place. This effect could only be observed if the stuffer
was being used at or near maximum hydraulic pressure. The curtain has a very
similar appearance to extrusions from narrow annular dies29•3o which crinkle as a
result of post-extrusion swelling and a subsequent drawing process. The curtain can
prevent the preform from falling into the mould cavity after it is cropped from the
dispenser. The problem is simply resolved by reducing the fill pressure.
6.3.1.5.2 Preform Shape
The shape of the preforms was unexpected. The author and co-workers had
assumed the preform would be reasonably regular in dimension and section. This
was not the case. The preforms from both the O-ring and sheet dispensers were
more elongated than expected in the direction of their extrusion, in some cases
extremely so. Sections of preforms were taken and examined; some examples are
shown in Figure 6-21. The extent of this elongation (Le. the shape of the preform)
seemed to be governed more by the pressure of fill than dispense rate. The shape
of the preform section could be modified by altering the fill regime without
significantly affecting the volume of the dispense. The FORM system was designed
with a certain amount of elongation in mind. The extrusion direction during dispense
and the mould closure direction are the same so that orientation in that direction will
be disrupted on mould closure. The shape of some of the preforms was
unacceptable. In some cases moulding was prevented because when the two
halves of the mould approached, the preform would splay or fold and fall outside the
cavity and produce a short-shot.
172
(a) (b) (c) (d)
Figure 6-21 Sections of C-ring preforms showing substantial elongation in the direction of dispense (top-to-bottom) and predominant tear-drop shape. (a) an extreme tear-drop shape in PBSO, (b) FR5S, (c) NR (meter cavity filled under high pressure) and (d) NR (meter cavity fill pressure modified).
6.3.1.5.3 Lobing
Immediately after cropping the preforms were regular in shape. Subsequently,
however, they were observed over a period of minutes to deform with bulges or
lobes appearing (Figures 6-22 and 6-23) , indicating considerable anisotropy in the
preforms.
50mm
Figure 6-22 The bulging or lobing effect seen in the dispensed sheet preforms. The bulges are in registration with the meter cavity feed runners indicating that there is a memory effect that is geometry related. The material pictured is PBSO.
173
100mm
Figure 6-23 Photograph of an NR O-ring preform showing the lobes that appear shortly after it is freed from the constraint of the meter cavity. The lobes occur on the top edge of the preform in registration with the six runners that feed the meter cavity.
Figure 6-24 An O-ring preform after a period of several months. Recovery is probably complete and the shape changed form a circle at the time of dispense to an almost regular hexagon with the lobes prominant on the corners.
It is suspected that this deformation is a molecular recovery effect. The long-chain
molecules have been oriented and extended in flow and they start to relax and
recover when flow stops. The recovery process continues for many days or even
weeks although visible deformation has ceased (Figure 6-24) .
174
The extent of this effect is governed by the pressure used to fill and pack the meter
cavity. Figure 6-25 shows a sheet preform that was produced using the same
conditions as those for Figure 6-22, except that the meter cavity was filled using a
lower pressure. It is clear from these photographs that the rubber has a memory of
its processing history.
50mm
Figure 6-25 A PBSO sheet preform dispensed after the meter cavity was filled under a moderate pressure. The memory effect correlating to the form of the runner system is still evident but the preform is essentially flat.
This memory effect is time-dependent31 and can be illustrated by Figure 6-26. In this
dispense, the memory of the runner system has almost disappeared. This preform
was the first dispensed after the dispenser had been left standing overnight. The
extended period of time that the material experienced in the meter cavity allowed
molecular recovery and relaxation to take place. The recovery would also be
enhanced by the elevated temperature of the dispenser increasing the ability of the
long chain molecule to move (macro-Brownian motion)32.
175
100n1ll1
Figure 6-26 A regular preform produced after the material experienced an extended period of relaxation in the meter cavity.
6.3.1.5.3.1 Lobing and Molecular Orientation in O-ring Preforms
There are some striking effects that can be seen in the lobes that occur on the 0-
ring preforms and these can be related to the conditions under which the preform
was produced. The first, and most common, condition is the complete lobe (Figure
6-27 (a)), the second has for the purposes of this work been termed 'suckback'
(Figure 6-27 (b)) which is a partial lobe and the third and final condition is the 'ideal'
or desired condition where there is no lobe (Figure 6-27 (c)).
176
- --- -- -----------
20mm
(a)
(b)
20mm
(c)
Figure 6-27 Three different effects that can be seen in the lobes that give an indication of the molecular orientation in the O-ring preform.
177
These three effects can be explained in terms of the molecular orientation that
occurs during the flow of material from the runner system into the meter cavity and
the molecular recovery that occurs when flow ceases. Long-chain molecules are
long and thin when they are extended and when they recover they reduce in length
and increase in diameter or thickness. During flow the molecules get extended
parallel with the direction of flow. The lobe occurs when meter cavity filling is carried
out at the higher pressures. The orientation of the molecules in the lobe is not in the
direction of expected flow, i.e. along the rubber and circumferentially around the
annular cavity (Figure 6-27). If they were, then the most likely result would be the
that the suckback condition would occur as molecules highly elongated parallel to
the direction of flow shorten in length as they recover. The small lumps to either side
of the 'drawn-in' portion of the ring could be caused by the recovery of the molecules
that are just turning the corner as they increase in diameter during their recovery.
The lobe, then, is formed by the recovery of molecules that are oriented in the
direction normal to the flow. This situation occurs at the junction between the runner
and the meter cavity just after the meter cavity has been filled and flow from the
runner has nowhere to go. The desired condition occurs when the meter cavity is
filled optimally and the recovery of the molecules in the direction of flow draws in just
enough material to create the near perfect preform. This perfect fill condition is rare
and the author does not know if it would be possible to devise a method of
determining when this point is reached.
178
6.3.1.5.4 Preform Shrinkage
Preform shrinkage is another effect that can be attributed to the orientation.
Circumferential orientation in the annular meter cavity induced by flow during the
course of filling the meter cavity produces dramatic preform shrinkage was noticed.
In an attempt to measure the shrinkage, it was found the there was also
considerable ('extrudate') swelling with the preforms being considerably larger,
initially, than the dispenser aperture. The swell and shrinkage were measured
roughly with a 300mm rule at a range of times from dispense to approximately 240s.
A high degree of accuracy is not claimed for the results given in Tables 6-15 and 6-
16.
Material Diameter in mm at Time (±10s 0 30 60 120 180 240
NR 225 205 195 195 190 190 SBRl 215 200 202 197 197 197 SBR2 212 202 197 195 193 192 NBRl 215 203 197 197 197 197 PB80 210 200 200 197 195 195 EOl 215 201 200 198 197 197 FR58 212 200 200 199 198 198
Table 6-15 Rough measurement of dispense diameter vs. time after dispense for maximum pressu re fill.
Material Diameter in mm at Time (±10s 0 30 60 120 180 240
NR 205 203 203 197 197 197 SBRl 200 197 195 195 192 192 SBR2 205 202 197 195 195 195 NBRl 200 200 197 197 195 195 PB80 207 202 200 200 198 197 EOl 205 203 203 200 200 197 FR58 202 200 197 197 197 196
Table 6-16 Rough measurement of dispense diameter vs. time after dispense for low pressure fill.
Although the experiment was not rigorous, there is a trend. The initial size of the
preform can be varied by some 10% depending on the material and pressure used
to fill the meter cavity.
179
6.3.2 Elimination of Lobes and Preform Shrinkage with a Modified Dispenser
The lobing (Figures 6-23 and 6-24) in the preforms produced by the system as it
was originally designed (Figure 3-4 and a three-dimensional representation of the
flow path is shown in Figure 5-4) indicated that the preforms contained significant
anisotropy immediately prior to mould closure and that the orientation would be
cured-in because, at the time of initial mould closure, the preform was still relatively
regular (Le. recovery and relaxation had not taken place to a significant extent). The
phased closure (close mould to initially form and warm part - open mould slightly to
allow recovery - close mould to form and cure part) in the direction of preforming
seems to go along way to alleviate a substantial amount of the molecular orientation
in the moulding as illustrated by results given later in this chapter. The relaxation
and recovery that caused the preformed rings to shrink from their initial diameter
and the elongated shape of the preforms (Figure 6-21) were considered to be
problems that would be significant in the production environment and for mould
design (size).
Scrap section and plan view of the flow spUtter plate: Divergent flow occurs in the direction of the arrows. The flow of material divides in the down tube and recombines after the petaloid pedestal before entering the meter cavity.
Figure 6-28 Configuration of the modified dispenser unit. The feed flow path changed considerably.
In the case of a low-stress deformation round a bend followed by the high-shear
deformation of passing through a narrow die shortly thereafter, the low-stress
deformation will be remembered at the die exit causing the extrudate to curl even
180
though the material has been through the high-shear deformation31. The time-scale
for which the material has memory is determined by its viscosity and modulus in
steady flow. Clearly if the time-scale of the material can be accounted for in the
process, then it would be possible to produce preforms that would not suffer from
these orientation effects. Increasing the residence time of the material in the meter
cavity to exceed the natural time-scale of the material would be a resolution (Figure
6-26) but this would have a drastic effect on production rate.
The dispenser was modified by redesigning the O-ring dispenser insert, the main
body (outer and inner) remained unchanged. The two-fold rationale was to manage
the flow and hence the molecular orientation in the meter cavity: (i) to increase the
residence time of the material in the system between entering the meter cavity and
the last major disturbance in the flow and, (ii), to create predominantly radial rather
than circumferential molecular orientation in the meter cavity. The six runners were
eliminated and replaced by an extended flow channel and a much shorter four-way
flow splitter which opens the flow out into a 'disk-like' flow path in order to fill the
meter cavity around the its whole circumference. The direction of material flow
would then be radially, from the centre of the 'disk' into the annular cavity rather than
circumferentially around the cavity because the material would not have to flow
around the meter cavity during filling. The direction of flow is indicated in the inset of
Figure 2-28.
Comparison of dispenser insert features: Original
• long narrow 6-way flow division material recombination in meter cavity
• Iow residence time of material in dispenser
• meter cavity filled at 6 points equispaced on circumference
• circumferential molecular orientation
Modified
• short, smooth and wiser 4-way flow division and material recombination before meter cavity
• increased internal volume and therefore increased material residence time in dispenser
• continuous circumferential feed to meter cavity during filling
• radial molecular orientation • 'disk-like 'flow path'
181
----_._--
6.3.2.1 Preforming with the Modified Dispenser
O-ring preforms were produced with the modified dispenser and examined in a
similar way to that described above. Essentially, the filling and metering phases of
the dispenser were not changed. Only the flow path between the main feed bore
and the meter cavity was changed. The geometry of the meter cavity itself was not
changed.
Preform consistency (shot-to-shot), preform size-range (weight) and dispense
temperature were all similar to those reported above, indicating that these measures
are governed primarily by the geometry of the meter cavity and its action rather than
the flow history of filling it. They also provide further evidence of the metering
capability of the of the dispenser.
6.3.2.1.1 Preform Shape
The preforms produced with the modified dispenser do not show the pronounced
elongation in the direction of dispense that was exhibited by the those produced with
the initial dispenser design. Figure 6-29 shows examples of preform sections. When
they are compared to the examples in Figure 6-21 it is clear that they are much
closer to the desired O-ring shape.
(a) (b) (c)
Figure 6-29 Section of preforms produced with the modified dispenser. (a) FR58 preform showing "squarish" section but this is a considerable improvement, (b), NR with an almost
182
round section and (c) PBSO showing the most marked improvement in shape; this a massive improvement on the extremely elongated tear-drop shape that was produced with this compound previously.
(a) (b)
Figure 6-30 Ring preforms produced with the dispenser of modified geometry. (a) a PBSO preform shortly after «1 hr) preforming, this preform is (b) a PBSO preform some months after preforming.
Figure 6-30 shows the shape of complete rings produced with the modified
dispenser. Figure 6-30 (a) shows a preform shortly after dispense and this could be
described as 'O-ring like'. From the point of view of limiting 'in-mould' flow this is
near-ideal as it is close to the desired dimensions before forming in the mould has
taken place. The effect of molecular orientation has not been eliminated completely,
however, Figure 6-30(b), shows a similar PB80 preform after a period of months has
elapsed. The polymer's memory of the processing history has now become evident.
A slight squaring of the preform is noticeable, which is an effect related to the
memory of the flow between the four petaloid pedestals of the 'splitter' plate in the
dispenser.
183
6.3.2.1.2 Preform Shrinkage
The reduction in diameter of the preforms produced with the modified dispenser is
considerably less marked than those made with the initial dispenser design. A
similar set of rough measurements was undertaken and the are results reported in
Material Diameter in mm at Time (± 1 Os 0 30 60 120 180 240
NR 227 225 223 223 223 220 SBRl 217 217 215 213 213 213 SBR2 225 222 218 215 215 215 NBRl 220 220 217 215 215 215 PB80 210 212 205 205 205 205 EOl 218 214 213 210 210 210 FR58 215 213 210 210 210 210
Table 6-17 Rough measurement of dispense diameter vs. time after dispense for maximum pressure fill in the modified dispenser unit.
Material Diameter in mm at Time (±10s 0 30 60 120 180 240
NR 205 203 200 200 200 200 SBRl 202 200 198 198 195 195 SBR2 207 205 205 200 200 200 NBR1 200 200 200 198 196 195 PB80 205 203 203 200 200 200 EOl 207 205 203 202 202 200 FR58 200 200 200 198 198 197
Table 6-18 Rough measurement of dispense diameter vs. time after dispense for low pressure fill in the modified dispenser unit.
Tables 6-17 and 6-18. A similar amount of swell is obtained and the preforms are
larger than the diameter of the aperture through which they are produced. The size
of the preform is also affected by the fill pressure in a similar manner, as those
produced with a high meter-cavity fill pressure are larger than those produced with
lower pressures.
The molecular orientation in the preform will be radial rather than circumferential.
Therefore the molecular relaxation and recovery will tend to pull the top and bottom
(viewed in section) of the preform towards each other, as the molecules shorten,
rather than causing the significant reduction in diameter seen previously. Orientation
effects in the preforms produced with the modified dispenser design are
184
considerably less noticeable than in the preforms produced with the original
dispenser design.
6.3.3 Moulding - Observations, Problems and Defects
Positive land ,---Mould cavity
Metal-ta-metal ,--------O=contact point
___.....:==",."'--Spew cavity
Figure 6-31 Schematic cross-section of the O-ring mould showing the O-ring cavity, the metalto-metal contact points: (i) at the positive land adjacent to the mould cavity and (ii), the large metal contact area outboard of the spew cavity
Figure 6-31 shows a schematic cross-section of the a-ring mould. A complete ring
preform, is dispensed directly into the cavity and the part is formed on closure of the
mould. A phased closure sequence, opening and closing the mould (akin to the
industry standard practice of 'bumping-off,33) is employed to expel trapped air and
allow molecular recovery for (i) the production of near-isotropic parts and, (ii) the
production of flash free parts.
Several problems with moulding were encountered during the course of the
moulding trials. Many of these problems are encountered in conventional moulding
systems34.35.36.37. Blisters (air trapped in the compound) can be attributed directly to
the piston feed mechanism chosen for the FORM system prototype. The blisters
185
were more prevalent in the compounds of higher viscosity, the FR58 especially. It is
thought that the problem of trapping air in the rubber compound would be resolved
in a production system that employed a different feed mechanism. Short shots were
(a) (b)
Figure 6-32 The results of an elongated preform folding over in the mould. Both samples shown are FR58 (a) shows the effect in the moulded part and (b) shows the catastrophic failure that can occur after immersion in acetone.
not typical and occurred in the case of setting up the machine or determining the
programme (recipe) to be used. In rare cases the preform would hang up on the
dispenser or a preform with a pronounced curtain would stand proud of the mould
and not be pushed correctly into the mould on closure. The final problem of folding
is specifically related to the elongated preform shape produced by the dispenser.
The preform folds as the mould closes and if it is partially scorched it may not knit
together in the mould properly during cure. The effect can be seen if Figure 6-32.
6.3.3.1 Flash Free Moulding
The flash-free moulding mechanism employed in the FORM has been developed
with the goal of manufacturing parts without flash and eliminating the need to for any
subsequent processing (deflashing). It differs significantly from previously proposed
methods for injection38,39 and transfer40 moulding which even-out pressure across
186
the by mould with and in-built flexibility. In addition the mould surfaces are
manufactured with a certain roughness in order to allow air to escape at the surface
but not through the rubber. The FORM system approach relies on mould design and
phased closure. Even pressure across the entire mould surface is also an important
factor in the FORM process, but it is achieved by having a controlled compound
charge and highly parallel rigid mould. The phased closure and mould design
however, are perhaps, the most important components of this process. The mould
has raised lands adjacent to the cavity to trap escaping material in a highly strained
state on initial mould closure. The mould is then opened by a small amount for a few
seconds. The highly strained material recovers during this open period. The
molecules recover into the product and into the main body of the flash. They tear
apart and the flash separates from the component as the recovery proceeds. In
subsequent closure of the mould for final curing, metal-to-metal contact is achieved
between the lands and the opposite half of the mould in the void created where the
rubber has tom and separated.
6.3.3.1.1 Phased Closure
The phased closure used to produce flash-free components is governed by the time
of the initial mould closure and the distance and duration of the first mould opening.
The duration of the first mould closure is the least critical part of the sequence. The
only criterion is that cure should not set in to any great extent. Generally in this study
initial mould closure took no more than 1 s. The mould open distances were set at
0·038, 0·05, 0·1, 0·16, 0·26 and 0·4mm and the mould open time was set between 1
and 8s in 1s steps. The results are given in Table 6-19. The numbers quoted in the
table represent the minimum duration of mould opening to obtain a flash-less a-ring.
Where a second number is given in parentheses this indicates the duration of the
upper limit of the flash-free moulding window.
187
Mould O~ en Time (5)
Mould Open Distance 0·038 0·05 0·1 0·16 0·26 0·4 (mm)
NR - 5 3 3 3 3 SBR1 - - - 4 4(7) 4(7)-SBR2 - - 4 3 3 3 NBR - - - 6 6 5 PB80 - - 5 4 4 4 EOl - - 5 5(7) 4(8) 4(7) FR58 - - 7 5(7) 5(8) 5(8)
Table 6-19 Mould open distance and duration required to obtain components completely free of (separated from) flash. The numbers in parentheses indicate the upper limit of the flash-free moulding window.
At the smallest gap openings (0·038 and O·OSmm) between the moulds flash-free
components are not produced. With the single exception of the NR compound, this
is thought to be because the rubber is still partially confined in the mould cavity and
between the lands. Recovery of the strained material between the lands is not
possible because of this physical constraint. As the duration and the gap opening
increase flash-free components are produced by natural molecular recovery in the
highly strained rubber.
The upper limit of the flash-free moulding window, noticed in some cases, is caused
by the material in the lower half of the hot mould having time to flow under gravity
and spreading onto the land. Material is then trapped and cured between lands on
subsequent mould closure. It was possible to produce flash-free components with all
of the compounds in the trial. The more elastic, lower viscosity and more easily
processible compounds form flash-free parts at lower openings and opening
durations. The explanation of this phenomenon probably lies in the ease the
molecules in the compound have in recovery due to Brownian motion. Figure 6-33
shows a photograph of the flash separated from the component in the mould.
188
Flash separated from 0-ring
Metal surface of mould
O-ring (in cavity)
Walls of plunge cavity
Figure 6-33 Photograph of an O-ring and flash in the plunge mould immediately after opening. The flash can clearly be seen to have separated from the O-ring. The land adjacent to the cavity where metal-to-metal contact was achieved after the phased closure (breathe cycle) is visible.
6.3.4 Product Testing
To assess the capability of the FORM process in producing isotropic mouldings, and
to provide a comparison with conventional compression moulding, a number of tests
were carried out on the a-rings and sheet that were produced with the FORM
system and by conventional methods. Swelling due to the action of solvent and
mould shrinkage were measured for both sheet and ring samples. In addition a
visual examination was made. The solvent swelling tests were perhaps the most
useful in showing the anisotropy that occurs in moulded parts. The test is simple if
not necessarily quick. The area and shape (of the cross-section) of each sample
were measured and the volume calculated before and after immersion in a range of
solvents. Tensile tests were carried out on the sheet samples and compression set
tests on the a-ring samples.
6.3.4.1 Examination of Moulded Product
The first tests for both the a-rings and sheet that were moulded for the system trial
was an initial visual inspection (based on BS 6442:198441) to make sure there were
189
no obvious defects. a-rings and sheet with obvious defects were rejected
immediately. The conventionally compression moulded sheet and commercially
produced, conventionally moulded a-rings were also inspected. The conventionally
moulded a-rings were inspected for join marks and other manufacturing defects.
None were found.
6.3.4.2 Physical Testing of O-Rings
The results of the physical testing of the moulded a-ring samples, both FORM and
conventional, are given in the following sections. Although rings were moulded from
all trial compounds with the FORM system, only three compounds, PB80, EOl and
FR58, were used to mould a-rings conventionally. These results are reported now.
The results appear under the headings Conventional, FORM I and FORM 11. These
represent those conventionally moulded, FORM moulded with the original dispenser
and FORM moulded with the modified dispenser, respectively.
6.3.4.2.1 Swelling in solvent
(i) Shape change (circularity)
The measure of shape used in this test is the degree of non-circularity, which is
defined as the ratio of the difference between the maximum and minimum
measured diameters to the minimum diameter. The larger the value the greater the
degree of non-circularity (or less round) the a-ring. This acts both as a measure of
the quality of initial standard of manufacture showing the shape of the section
(roundness) before swelling and also as an indicator of shape change, hence
anisotropy and orientation, by the action of the solvent.
190
Material Shape - Non-circularity of O-ring section (% of Minimum Diameter)
Conventional FORM I I FORM 11 Prior to swellinQ
PB80 1·25 1·3 1·25 EOl 2·55 3·8 2·68 FR58 3·86 1·25 1·64
After swelling in methanol PB80 4·28 2·5 3·3 EOl 2·81 3·17 2·06 FR58 2·19 2·34 2·23
After swellinQ in toluene PB80 4·84 2·24 2·36 EOl 8·1 2·13 3·0 FR58 3·75 1·21 1·65
Table 6-20 The degree of non-circularity of the O-ring cross-section before and after immersion in toluene or methanol.
(b)
Figure 6-34 Shape change due to the action of solvent in SBR1 O-ring moulded in a conventional two plate mould. The resolution of uneven stress can be seen in (a) and distinct anisotropy caused orientation due to escape flows at the mould split line cured-in. Highly strained molecules swell less in the direction they are elongated causing the dimples in the top and bottom of the swollen sample in (b)
191
The conventionally produced rings show a greater degree of non-circularity in most
cases. This is probably because the there was a slight amount of offset (or mould
mismatch) in the conventionally moulded rings that was not present in the rings
produced by the FORM process. The mismatch was not outside the standard limits
but is bound to show up in the result. (Examples of this mismatch can be seen
clearly in the sections of rings shown in appendix I). The FORM system rings are
similar which is not unexpected as both sets were produced with the same mould
set. There is a definite trend towards increasing non-circularity for all samples with
swelling.
Figure 6-35 Example of FORM system a-ring where despite the fact that considerable swelling has taken place the over all shape has remained constant indicating that the part is almost isotropic.
Using the area of the cross-section of the swollen O-ring sample as a measure
swelling it is clear from Table 6-21 that the FORM system rings swell significantly
less than those produced conventionally. Moreover, in most cases the FORM 11
rings swell less than the FORM I rings, but the difference between these two is less
marked. Figures 6-33 and 6-34 show sections of rings before and after swelling in
solvent.
192
(ii) Change in size (area of section)
Material Area of a-ring cross-section (% size increase)
Conventional FORM I I FORM 11 After swellina in methanol
PB80 14 8 4 EOl 13 2 6 FR58 12 8 4
After swellinq in toluene PB80 90 60 32 EOl 63 60 64
FR58" - 4 6 After swelling in acetone
FR58 78 I 53 I 47
Table 6-21 Change in size of the O-ring cross-section due to the action of solvent.
(iii) Change in volume
Material Change in Volume (% of oriqinal volume)
Conventional I FORM I FORM 11 After swellinq in methanol
PB80 110 105 102 EOl 104 104 104 FR58 101 101 101
After swelling in toluene PB80 200 196 192 EOl 256 242 235 FR58 125 121 119
After swellina in acetone FR58 I 272 I 266 261
Table 6-22 Change in volume due to the action of solvent on sample of O-rings
The full picture of the amount of swell due to solvent cannot be gained without a
measure of the total volume, but the area measurements given above are useful for
highlighting anisotropy. The volume change due to swelling shows that swelling is
less in the rings produced by the FORM system. This could be due to different cure
conditions but every effort has been made to ensure that the moulding conditions for
all samples were constant. The reason for the difference preferred by the author is
that the molecular structure in the FORM rings is one of molecules that are coiled
and relaxed before cure takes place. This coiling of the molecules provides a greater
number of available cross-link sites in a given volume and a more even 'tighter'
network of cross-links is obtained.
• FR58 (a flourocarbon elastomer) has good resistance to swelling in toluene so the size changes are very small. There was no measurable increase in size for the FORM 11 samples.
193
6.3.4.2.2 Compression Set
Apart from the fact that the standard test specimen could not be used, the
compression set test was carried out in accordance with SS 903: Part A642• The
results in Table 6-23 show similar results to those of the swell tests. The more
isotropic structure of the mouldings produced by the FORM process show greater
compression set resistance for similar reasons.
Material Compression Set (%)
Conventional FORM I FORM 11 PB80 20 18 18 EOl 15 15 15 FR58 14 20 12
Table 6-23 Compression set in a-ring samples
6.3.4.2.3 Mould shrinkage
Material Mould Shrinkage (Diameter (mm)l% shrinkage)
Conventional FORM I FORM 11 Mould dia. 198mm Mould dia. 199·53mm Mould dia. 199·53mm
PB80 194·95/1·54 197·04/1·25 197·311·12 EOl 191·57/3·25 193·4213·1 195·04/2·25 FR58 190·24/3·92 191·41/4·07 191·95/3·8
Table 6-24 Mould shrinkage results for FORM and conventional a-rings manufactured from the commercial compounds.
The shrinkage in the conventional and FORM I O-rings is comparable. The likely
molecular orientation in these two sets of rings is in the same direction,
circumferentially around the ring. The extrusion of the chord used for the
conventional rings will orient the molecules in the direction of flow along the chord
and the flow required to fill the meter cavity is also in the same direction. The
recovery of the molecules in these rings will have the effect to reduce the size
(diameter) of the ring. In the FORM II rings the radial molecular alignment shows
less shrinkage in the diameter. The results of the mould shrinkage measurements
are in good agreement with those of previous studies in literature43,44,45,46
194
6.3.4.3 Physical Testing of Sheet
Tests were carried out on the sheet samples manufactured conventionally and with
the FORM system. Molecular orientation as a result of processing (Le. milling 'grain')
prior to moulding is a well known phenomenon. Therefore all the tensile tests on the
sheets were conducted in two mutually perpendicular directions.
6.3.4.3.1 Mould shrinkage
The mould shrinkage of the sheet sample produced for the trial was measured. The
dimensions of the moulds were carefully measured with Vernier callipers. The
average dimensions are given in Table 6-25. The reduction in linear dimension,
Mould Length(mm) Width(mm) Thickness (mm)
Conventional 122·5 120·0 1·93 FORM 152·9 90·3 1·97
Table 6-25 Dimension of the moulds used for the production of sample sheet for the trial
expressed as a percentage change of the original mould dimension for the FORM
and conventional products, is given in Tables 6-26 and 6-27, respectively. The ratio
of the change in the two major directions is also given as a measure anisotropy of
shrinkage. All of the sheets measured in the test shrank in the two major directions
Table 6-26 Mould shrinkage and anisotropy of mould shrinkage for the sheet sample produced with the FORM process
195
Material Chanae in linear dimensianl%) lenath Width Thickness Anisatropv
NR 2.68 1.23 -1.59 2.18 SBR1 2.24 1.98 -2.97 1.13 SBR2 2.12 2.11 -0.98 1.00 NBR 2.78 1.23 -2.74 2.26 PB80 1.25 0.95 -1.27 1.32 EOl 3.92 2.75 -3.34 1.43 FR58 3.21 2.38 -1.75 1.35
Table 6-27 Mould shrinkage and anisotropy of mould shrinkage for the sheet sample produced by conventional compression moulding
(length and width) and expanded in the direction of mould closure (thickness); this is
indicated by the negative numbers in the thickness column. The shrinkage cannot
be due only to the thermal expansion of the material because, if it were, the sheet
would not get thicker in the direction of mould closure, it would shrink. This is due to
molecular orientation in the flow directions as closure of the mould forces the charge
to fill the extremities of the mould. Molecules are extended in this flow and their
recovery after demoulding is responsible for the reduction in the two major directions
and the increase in thickness.
6.3.4.3.2 Tensile Testing
The tensile tests carried out on the sheet samples moulded by both the FORM
system and conventional means demonstrate the prevalence of anisotropy in
compression moulding. Tensile tests conducted in the two mutually perpendicular
directions chosen show differences in strength and elongation at break. A small but
noticeable trend can be seen in the results given in Table 6-28 and typical examples
of the stress strain curves obtained are given in Figure 6-36.
196
26
24
22
20
18 ~
ca 16 Q.
:E 14 -(/) (/) 12 I!! -10 (J)
8
6
4
2
0 0 40 80 120 160 200 240 280 320 360 400 440 480 520 560
Strain (%)
Figure 6-36 Typical stress-strain curves for selected compounds
NA - SBA1
SBA2 - NBA - EQL
- PB80
Overall the tests , the elongation at break is lower in the direction labelled parallel (/1)
(i.e. with the milling 'grain' or in the case of FORM the parallel with the longest side
of the rectangular mould) . A closer look at some of the individual results, for
example NR, shows that there is significant difference of 10% between the
elongation at break in the two measured directions perpendicular Cl ) and parallel
(11). A value of 10% is typical of this set of data indicating that there is significant
molecular anisotropy. The range for the ratio 1. : II for this data se is from +20% to
-10%. These results indicate that the molecular orientation, due to an extension of
the polymer molecules in processing, is generally in the direction 1/. The molecules
are already partially extended in the II direction before the test and the tensile load
is applied. Therefore the amount of further extension possible, before the molecules
reach maximum extension and then rupture, is less than might be expected. This is
true for all of the conventionally moulded samples and about half of those moulded
using the form system.
Globally the strength at break data does not show any significant trend in either of
the measured directions but, as above, individual compounds show significant
197
differences in the two measured directions indicating anisotropic orientation of the
molecules in the components.
Direction of test in Modulus at 300% Strength at break Extension at Break relation to direction elongation (MPalSD) (%/SD) of moulding (MPalSD)
NR FORM I! 6·2210·08 25·7/1·2 570/11 FORM 1. 5·43/0·49 29·211·8 629/23 Conv.!1 4·3410·17 15·35/2·76 533/40 Conv.1. 4·56/0·33 18·53/2·78 570/30
SBR1 FORM I! 20·7/1·5 26·0/1·5 390/41 FORM1. 19·6710·56 25·410·8 39419 Conv.!1 19-43/0·63 25·3/0·8 389121 Conv.1. 20·010·5 26·1/1·4 399133
SBR2 FORM I! - 7·70/0·51 271132 FORM1. - 8·49/1·13 265137 Conv.!1 - 7·6110-46 212124 Conv.1. - 7·39/0·17 258116
NBR FORM I! 21·1/0·7 24·212·28 389/53 FORM1. 21·6/0·6 24·710·5 380121 Conv.!1 21·610·9 22·6/2·4 320159 Conv.1. 21·8/0·4 24·3/0·8 359126
PB80 FORM I! - 16·31/0·23 352117 FORM1. 15·05/0·36 15·40/0·44 315/19 Conv.!1 16·10/0·85 15·87/0·84 293/55 Conv.1. 15-46/0-30 15-63/0-54 330/44
EOl FORM I! - 26-8/0-9 223111 FORM1. - 25·1/0·9 236114 Conv.!1 - 26·311·2 237119 Conv.1. - 24·9/1-6 256123
FR58 FORM I! - 16·34/1·56 267/34 FORM1. - 17-21/0-95 253157 Conv.!1 - 16·85/1·15 245/65 Conv.1. - 17-36/1·32 276/54
Table 6-28 Results of tensile test carried out on dumbbells cut from moulded sheet. The directions FORM 11 and 1. represent the direction parallel to and normal to B (Figure 4-6) respectively and similarly Cony. 11 and 1. represent the directions parallel to and normal to A (Figure 4-6). A is the direction of the milling grain. Mean modulus at 300% extension and strength at break are given together with extension at break. Standard deviations are for all measurements are also given.
These results do not support the idea that the FORM sheets are isotropic but they
do highlight the fact that there is a significant amount of anisotropy in most moulded
components. The effect of mill grain and molecular orientation due to in mould flow
does seem to be evident in the moulded sheets, especially the conventionally
moulded sheet. However the effects are very small. On examination of the
processes the similarity of the results could have been expected because the two
198
sheet moulding processes are similar. A small charge of polymer is placed in the
centre of the mould cavity and, on mould closure, is forced to flow to the cavity's
extremities. It is difficult to imagine how the FORM sheet dispenser could be
modified to prevent this need for in mould flow and permit the production of isotropic
sheet.
6.3.4.4 Summary of Preform Production and Physical Testing
The tests on the O-ring samples show that the FORM system is capable of
producing rings that are substantially isotropic. The formation of a complete ring
preform that is near the size and shape of the mould cavity limits flow to the
absolute minimum and phased closure (moulding 'breathe') allows rapid recovery of
molecular orientation in a semi-constrained state. These effects and elevated
temperature all combine to reduce molecular orientation. The O-rings produced
using the FORM process compare favourably with similar rings produced
conventionally. Post-demoulding distortion is the same or less and when immersed
in solvent the FORM system rings distort less even at high swell ratios.
The results of the FORM system sheet production are, unfortunately, not so
encouraging. The behaviour of the sheet on demoulding is comparable to
conventionally produced sheet. The effects of orientation are measurable. This is
undoubtedly due to the fact that the material has to undergo considerable flow in the
mould as closure forces the material to the extremities of the cavity. The orientation
due to flow is too severe for recovery to be aided by a 'breathe' cycle and the
material has to undergo similar amounts of flow in both the conventional and FORM
sheet moulding processes.
6.4 References
'w. Hofmann in Rubber Technology Handbook, Ch3 (3.2.4) pp50-S2 (Carl Hanser Verlag, Munich, 1989)
2F. N. Cogswell, Polymer Melt Rheology: A Guide for Industrial Practice, Ch4 (4.2) pp77-81 (George Goodwin Ltd., London, 1981)
199
3E. Schmidt, Ind. Engng. Chem., 43, 679 (1951)
4B. B. Boonstra, Polymer, 20, 691 (1979)
'J. B. Horn, Rubb. Plasl. Age, 50, 457 (1969)
BASTM D 3765 - 96 Standard Test Method for Carbon Black- CTAB (Cetyletrimethylammonium Bromide) Surface Area, Annual Book of ASTM Standards, 09:01, American Society for Testing and Materials (1997)
7 ASTM D 2414 - 96a Standard Test Method for Carbon Black - n-Dibutyl Phthalate Absorption Number, Annual Book of ASTM Standards, 09:01, American Society for Testing and Materials (1997)
8 ASTM D 3265 - 96 Standard Test Method for Carbon Black - Tint Strength, Annual Book of ASTM Standards, 09:01, American Society for Testing and Materials (1997)
9 ASTM D 3493 - 96 Standard Test Method for Carbon Black - n-Dibutyl Phthalate Absorption Number of Compressed Sample, Annual Book of ASTM Standards, 09:01, American Society for Testing and Materials (1997)
10ASTM D 1510 - 96b Standard Test Method for Carbon Black - Iodine Absorption Number, Annual Book of ASTM Standards, 09:01, American Society for Testing and Materials (1997)
11 ASTM D 1765 - 96a: Standard Classification System for Carbon Blacks used in Rubber Products, Annual Book of ASTM Standards, 09:01, American Society for Testing and Materials (1997)
12ASTM D2934 - 89: Standard Practice for Rubber Seals-Compatibility with Service Fluids, American Society for Testing and Materials, (1989)
13A. King, Plastics Rubb. 1nl. 14 (1), 23 (1989)
14S. N. Ghafouri and P. K. Freakley, Polym. Test, 11, 101 (1992)
"p. K. Freakley, Rubber Processing and Organisation, Ch2 pp17-18, (Plenum Press, New York, 1985)
1BM. E. Brown in Introduction to Thermal Analysis Techniques and Application Ch4 pp25 - 38, (Chapman Hall,1988)
17G. Kampf in Characterization of Plastics by Physical Methods - Experimental Techniques and Practical Application Ch4 pp179 - 191, (Hanser Publishers, Munich, 1986)
18ASTM D 297 - 81 Standard Test Methods for Rubber products - Chemical Analysis: Part A 15, Annual Book of ASTM Standards, 09:01, American Society for Testing and Materials (1985)
"'W. Hofmann in Rubber Technology Handbook, Ch3 (3.3.15) p123 (Carl Hanser Verlag, Munich, 1989)
'OAnon, Machine Design, 65,12,27 (1993)
21 D. W. Nicholson and N. Nelson, Rubber Chem. Technol., 63, 368 (1990)
22p Dvorak, Machine Design, 65, 5,102 (1993)
23F. Damjanic and D. R. J. Owen, Nuclear Eng. Des. 69,109 (1982)
24K. E. Barrett, D. M. Butterfield, J. H. Tabor and S. Ellis, Int J. Mech. Eng. Edu. 18,59 (1989)
200
25J. A. Brydson, Flow Properties of Polymer Melts 2nd ed., Ch2 pp 18-28 and 212-213. (George Goodwin Ltd., London 1981)
26A. G. Fredrickson and R. B. Bird, Ind. Eng. Chem. 50, 347 (1958)
27F. N. Cogswell, Polymer Melt Rheology: A Guide for Industrial Practice, p137 (George Goodwin Ltd., London, 1981)
2BA. W. Birley, B. Haworth and J. Batchelor, Physics of Plastics: Processing, Properties and Materials Engineering, Ch 3 p59 (Carl Hanser Verlag, Munich, 1991) 29F. N. Cogswell, Polymer Melt Rheology: A Guide For Industrial Practice, p105 (George Goodwin Ltd., London, 1981)
30 J. S. Schaul, M. S. Hannon and K. F. Wissburn, Trans. Soc. Rheol., 19, 351 (1975)
31F. N. Cogswell, Polymer Melt Rheology: A Guide For Industrial Practice, p49 George Goodwin Ltd., London, 1981)
32W. Hofmann, Rubber Technology Handbook, p222 (Carl Hanser Verlag, Munich, 1989)
33 J. Menough,Rubber World, 173, 1,67 (1983)
34M. A. Wheelans, Rubber Chem. Technol., 51,1023 (1978)
35J. G. Sommer, Rubber Chem. Technol., 51,738 (1978)
36J. G. Sommer, Rubber Chem. Technol., 58,662 (1985)
37M. A. Wheelans in Injection Moulding of Rubber, Ch5 pp188-195 (Butterworth & Co. Ltd., London, 1974)
38H. F. Jurgeleit, Rubber Age, 90,763 (1962)
39H. F. Jurgeleit, British Patent, 1022084 (1964)
4oH. G. Gilette, RubberWorld,157,1, 67 (1967)
41 BS 6442: 1984 British Standard Specification for Limits of Surface Imperfections on Elastomeric Toroidal Sealing Rings ('O'-rings), British Standards Institution (1984)
42BS 903:Part A6:1989, British Standard Methods of Testing Vulcanised Rubber,Part A6, Determination of Compression Set after Constant Strain, British Standards Institution (1989)
43J. R. Beatty, Rubber Chem. Technol., 51,1044 (1978)
44K. Nakashima, H. Fukuta and M. Mineki, J Appl Pol. Sci., 17, 769 (1973)
45A.w. Fogiel, H. K. Frensdorff and J. D. MacLachlan, Rubber Chem. Technol., 49, 34 (1976)
46J. D. MacLachlan and A.w. Fogiel, Rubber Chem. Technol., 49, 43 (1976)
201
Chapter Seven
7. Conclusions
From this work on the design, development, modelling, and evaluation of the novel
computer-controlled compression-moulding, or FORM, system it is clear that the
moulding of rubber components is not simple. The treatment of the material prior to
forming and curing has a significant effect on the orientation of the long-chain
molecules in the mouldings. The effects of this molecular orientation are
phenomena that designers, toolmakers and moulders have worked around and had
to cope with for many years.
Analysis of extruded chord and preforms etc. can be used as a window into the
molecular orientation induced in premoulding processes. It is well known in polymer
processing that a screw can put a curl or wave into the extrudate. In this study
particular attention was paid to the preform recovery behaviour and shape. Long
term observation showed that recovery occurred over several months.
Preforms produced with the first generation O-ring dispenser showed considerable
anisotropy. In the case of the O-rings this anisotropy was largely undetectable in the
moulded product. O-ring preforms showed considerable shrinkage (molecular
recovery), in the order of a 10% reduction in diameter, in the first 240 seconds after
production. The second-generation dispenser gave a more reliable shrinkage of no
more than 5%.
The preforms produced with the initial dispenser were elongated, lozenge or tear
drop in cross-section and having a shape factor of 3 typically but 7.5 in extreme
202
cases. Those produced with the modified dispenser have a shape factor of 1.25
typically, with some examples being very close to 1. In the latter dispenser the
molecular orientation is managed so that elongation during, and recovery after
dispense occur in the same direction. The fact that these preforms are very close to
the shape of the mould cavity also means that very little flow takes place in the
mould during shaping.
The preforming O-ring dispenser, as initially designed, produced preforms that
showed considerable anisotropy, due to their flow history. The internal flow path was
successfully re-designed to account for the memory the polymer has of its flow
history. The modified dispenser delivers substantially isotropic preformed rings.
The novel compression moulding system was used to produce O-rings and sheet
and comparison with conventionally moulded O-rings and sheet was undertaken. A
range of tests to determine anisotropy were carried out on both the FORM and
conventionally moulded O-rings and sheet.
In comparison FORM system O-rings show better integrity than conventionally
moulded samples. The FORM system O-rings exhibit less mould shrinkage and
compression set in standard tests. In solvent swelling tests measurements of
change in shape, cross-sectional area and volume were made. The new moulding
system shows less change in shape and lower swelling as measured by area and
volume.
Mould shrinkage and tensile testing were used as measurements of the anisotropy
of the sheet samples produced by both manufacturing methods. In both cases there
was no obvious difference between the two methods, however the results do serve
to highlight the significant level of anisotropy that can occur in moulded rubber
products with typically a 10% difference in the extension at break in the 2 mutually
perpendicular directions measured.
The O-rings produced with the novel compression moulding system show
considerably less anisotropy than the conventionally moulded O-rings. The sheet
203
samples produced with the novel moulding system are of comparable anisotropy
with the conventionally moulded sheet.
Finite element models of the flow geometry were created to predict the temperature
rise and pressure drop and therefore calculate the force required, firstly to enable
flow through the specified geometry and secondly to dispense a metered preform.
The methods and techniques developed for this modelling can be used for the
further development of the system or applied to flow in other rubber processing
applications. At the moment the models give an over-estimate of up to 20% of the
temperature rise measured for dispensed preforms.
The combination of a computer control system and the preforming dispenser has
enabled the automation of the, still relatively labour intensive, compression moulding
process and production rates for compression moulding with this system could be
brought in to line with those of injection moulding.
A method of producing flash-free compression moulded a-rings was developed. A
combination of mould design and the precise control of the closure profile to form
the part utilise the inherent viscoelastic properties of uncured rubber compounds
and enable separation of flash from the component in the mould.
7.1 Suggestions for Further Work
The FORM system, as is stands now, can produce isotropic flash-free a-rings.
However, there are areas in which it could be could be improved. The development
of a new feed mechanism to replace the current reciprocating piston would enable
better, more precise control over the, seemingly critical, fill pressure. Further system
development could be to redesign the sheet moulding dispenser, and possibly the
mould set, to produce near-isotropic sheet. The design and production of preforming
dispenser inserts and mould sets could be extended to other products or component
shapes to enhance the product range.
204
The development of the finite element modelling could include such factors as
convection to atmosphere to see if the modelling can recreate reality more faithfully.
The prediction of the change of shape of a dispensed preform should remain a goal
of the modelling associated with the FORM system. If this tool could be developed it
would be a most powerful asset and enable rapid design of dispenser and mould
sets.
205
Appendix A
Piston and Feed pocke
> <
L ______ ~L-__________ J
Press Dispenser
This schematic shows the dispenser and press configuration. The dispenser is
mobile and can traverse between the platens, produce a preform and retract before
the press/mould closure sequence is initiated.
Appendix B
r-------------------
! ______________________ J
Above a schematic of the mould used for producing flash-free O-rings is given ..
above. Below are a series of pictures, representing the section enclosed by the
dashed line, showing the different stages of flash"free moulding,
Here the mould halves are approaching initial closure. The preform which is near
shape and size, rests in the cavity.
On initial closure (above) flash is formed and molecules get extended and trapped
between the lands due to high flow rates. -' ',.'
. .,
The mould is then opened again and molecular recovery allowed. The molecules in
the flash between the lands recover and tear apart separating the flash from the
ring.
Subsequent mould closure allows metal to metal contact.between. the lands for
curing. ., ":.
Appendix C
**EXECUTIVE data deck for 3D-FLUID [EMRC NISA] ANALYSIS: FLUHT Analysis type. DIMENSION : AX Axisymmeteric FILE : sq15 SAVE:26,27,54,55 STDS : ON,0.3,0.3,0.3 NONNEWTONIAN : 0.288 VDISIP : ON,0.5E-04 *ELTYPE
I, 3, 1 *NODES
Out put data file types
Non-Newtonian index Viscous disipation factor
Nodal co-ordinates 1, , " 2, , , ,
9.89500E+01, O.OOOOOE+OO, O.OOOOOE+OO, 0 9.93667E+01, O.OOOOOE+OO, O.OOOOOE+OO, 0
3, , I , 9. 97833E+01, O. OOOOOE+OO, 0. OOOOOE+OO, 0
227, , , , 1. 02450E+02, 3.80000E+01, O.OOOOOE+OO, ° 228, I I I 1.04200E+02, 3.80000E+01, O.OOOOOE+OO, ° 229, I , I 1.05950E+02, 3.80000E+01, O.OOOOOE+OO, ° 230, , , I 1.07700E+02, 3.80000E+01, O.OOOOOE+OO, 0 231, , , , 1.09450E+02, 3.80000E+01, O.OOOOOE+OO, 0
* ELEMENTS Elemental Connectivities 1, 1, 1, 1, ° 1, 2, 9, 8, 2, 1, 1, 1, ° 2, 3, 10, 9,
216, 217, 224, 223, 175, 1, 1, 1, 0 218, 219, 226, 225, 176, 1, 1, 1, 0 219, 220, 227, 226, 177, 1, 1, 1, 0 220, 221, 228, 227, 178, 1, 1, 1, 0 221, 222, 229, 228, 179, 1, 1, 1, 0 222, 223, 230, 229, 180, 1, 1, 1, ° 223, 224, 231, 230,
*MATFLUID Material data DENS, 1,0, 1.40000E-03, VISC, 1,0, 8.53400E+01, COND, 1,0, 1. 70000E-04, SPEC, 1, 0, 1.16430E+OO, *FLCNTL, ID: 1 10,50,1,1.0,0.001,0.lE+09,1.0,1.0,0.0,250.0 * BCDVAR
** BCDVAR SET : 1 1,U 1,V 7,U 7,V 8,U
229, V
O.OOOOOE+OO, , , O.OOOOOE+OO", O.OOOOOE+OO, , , 0. OOOOOE+OO, , , O.OOOOOE+OO",
,-1.50000E+01",
0, 0, 0, 0, 0,
0,
Bountary conditions Velocities in u and v directions
o
° ° o o
o
*rCDS
229,T 230,U 230,V 230,T 23l,U 23l,V 231,T
, 8. OOOOOE+Ol, , , , O.OOOOOE+OO" , ,-1.50000E+Ol",
8. OOOOOE+Ol, , , O. OOOOOE+OO, , , O. OOOOOE+OO, , , 8. OOOOOE+Ol",
** rCDS SET = 1 1,T 8.00000E+Ol 2,T 8.00000E+Ol 3,T 8.00000E+Ol 4,T 8.00000E+Ol 5,T 8.00000E+Ol 6,T 8.00000E+Ol
2l4,T 8.00000E+Ol 2l5,T 8.00000E+Ol 2l6,T 8.00000E+Ol 2l7,T 8.00000E+Ol 2l8,T 8.00000E+Ol 2l9,T 8.00000E+Ol 220,T 8.00000E+Ol 221,T 8.00000E+Ol 222,T 8.00000E+Ol 223,T 8.00000E+Ol 224,T 8.00000E+Ol 225,T 8.00000E+Ol 226,T 8.00000E+Ol 227,T 8.00000E+Ol 228,T 8.00000E+Ol 229,T 8.00000E+Ol 230,T 8.00000E+Ol 231,T 8.00000E+Ol
*ENDDATA
0, 0 0, 0 0, 0 0, 0 0, 0 0, 0 0, 0
Initial conditions ( temperature)
Appendix D
All dimensions in mm. A - A represents Axis used in FEA Model. Hatched area represents usable volume.
200 mm dia. Cylindrical Pot.
50 mm dia. Ram Injector.
1------'30-----... 115 mm dia. Cylindrical Pot.
;
i
, .... /' ...
. . ~ > '
sP
SO mm dia. Ram Injector with Heated Core.
: • .J • . ,
115 mm dia. Cylindrical Pot with Heated Core.·
U· - ... . , : "
"'2. 5 :::j~~s;~;;~;;~~~~=-::-=-=-=-~-i5.'.. •. r . ,.
'" I------,.l,,~-..-...ll
Appendix E
Log Shear Stress Vs. Log Strain Rate for SSRl 3
2.5
2 Y =< 0.2077x + 2.2645
1.5
... : ~ '.
1
0.5
.... :, -' .. ~ .. :.-.... -- .
-I -0.5
o 0.5 1
log Y (Strain Rate) 1.5 2
--ao·C • 90·C
-""'lOQ'C ~120·C
j
j
J
j
j
j
j
3
2.5
1.5
1
0.5
-1 -0.5 o
Log 't VS. Log yfor SBR2
y = 0.2762x + 2.0S67
. ~.'. . if ~ ~ . ,,-
.-,'
0,15 i ~ .. r. ;. . ,. _ .1 Log y (Strain Rate)
--SO°C ---90°C --100°C' --120°C
1.5 2
1.5
1
0.5
-1
-0.5
o
Log S ...... S .... Vs. Log St", .. Rate 10, .. S. a
, ..
." . ~ '., . ,.,' .
0.5
Log r (Sfrlli/l Rate) . 1
1.5 ,', - .
. '" • i
--ao·c ---90·C -'-100·e ~120·C
I
~
3
2.5
2
1.5
1
0.5
-1 -0.5
o
Log Shear Stress "S. Log Strain Rate for EOL
Y = 0.2295)( + 2.226
-.~ -.. ';:.~ .~.
0.5
log Y (Strain Rate) 1 1.5
2
--eoGe ---90Ge ........ lOOGC
-H- 120oe
2.5
1.5
1
0.5,.
-1 -0.5
o
Log Shear Stress Vs. Log Strain Rate for PBSO 3
y" 0.1982)( + 2.3107
.! . .
. -
0.5
Log y (Strain Rate) 1
1.5 2
--BO·C ---90·C ........ 100·C
~120·C
Log Shear Stress Vs. Log Strain Rate for FR5S-90 3
2.5 -. -- -= & .,.
:u
2
Cl> .c e .. 1.5 Cl> 0
..J Y = 0.2057x + 2.1134
, . -,.
. "-. .
0.5
. -1
-0.5 o _
0.5
Log Y (Strain Rate) 1
1.5
.. ... ---
--90°C ---100·C ......... 120·C
j 2 j
j j j j j
j j j j j j j j
j j j j
j j
j
j j
!
I ... !
0.5
-1
-0.5
o
1.5
log s"-"'"'SS vs. log SI""" ..... "" HR c_ ... <.5
1 .. _ .
". '.~,
0.5
LQg Y (Strain ~ate) 1
1.5
--aooc ---90·C --'-100·C ~1<OoC
2
j
J
Appendix F
6
In Shear Stress vs. Temperature ("C) for HR Compound
y = -O.0127x + 5.7352
2
1
O+----------r---------+--------~------~--~--------~--------+---~----~--------~ 90 95 100
Temperature ('C) .
105 110 115 120
80 85
__ 0.1
...-004
....-1
-M-4 ....... 10 __ 40·
-+-100
1
1
1
In Shear Stress vs. Temperature (·C) for SBR1
7
~ .- . :."... ~. 1
y = -0.0069x + 6.0193
2
.' ~- .
80 85 90 95· 100 . -- 105 110
Temperature ("C)
115 120
__ 0.1 ___ 0.4
........ 1
-*""4 ___ 10
-+-40
-1-100 -Linear (1)
.
f • (.
In 't vs. TemperatureeC) for SBR2
7 r
6~~====~~------~~==============~ ~
__ 0.1 5 . -en en
. i __ 0.4
l!! -4 I-
-: . .....-1
UJ - ---4 .. lIS Cl)
--10 .c: 3
y = -0.011x + 6~0854· UJ I-- --40
~
.. ~100
c - 2 --
1 . -' .: .. : • ' • .(>!"-'~"" ."
'<'. , " ' 0+_------~1r---~--+---~~~,--~--~r-------r-1------+-1-------r------~ '. - ' .. '- :'. -. .
80 85 90 95" . 100: , 105 . 110 115 120
Temperature ee}
In Shear Stress vs. Temperature (GC) for NBR· .- -" . . .... p..' ... -
7
__ 0.1
______ - __ . ~..,.....~....:...: ___ .:....~ ____ ~ __ =:::::::::::::::::..._ ;=:.4 ""'" __ 10
__ 40
y ~-0.0247x + 7.6269 -1-100 -Linear (1)
2
1
O+--------+--------+-----~~~~~--+--------+----~~+-------~--------~ 110 115 120
80 85 90 ... 95 . .. 100
Temperature (OC)
105
In Shear Stress vs. Temperature ("C) for EOL
7
6~====~=---~-----------
14 y = -0.0107x + 6.2878
I ~3L---------------~----~--------,· .~~ .. -,-. ~--:--------------------------+ .5
2
.. ';" .. 1
,'" H. : . .!.. _~~ ::._;:'.::. •• ._
O+---------~----~--~--~~~-+~------~~------~----------~--------~---------;· 80 85 90 95 100 105 110 115 120
. Temperature (OC)
__ 0.1
-4-0.4
-+-1 __ 4
__ 10 ___ 40
-+-100
-j .. m .If!
!!!. .. .5
In Shear Stress vs. Temperature (OC) for PBSO
6
5~=====-----------~===== 4
" , .. y = -0.0293x + 7.3n2
3
,',' ..
2 , .
1
.... '-' o+-______ ~----~--~---+------~----~+-~--~----~-+-------r------~----~ 86 87 BB 89 90
80 81 82 83 84 , 85
Temperature ('C)
__ 0.1
"""'0.4 -.-1
-*"'4 ......-10 __ 40
-+-100
In Shear Stress vs. Temperature (OC) for FR5S·90
- ;
_ y '" .().0339X +8.2811 -
... , .......
2
__ 0.1
....... 0.4
-k-1 __ 4
.....-10 __ 40
-+-100 _Linearl1)
0+-__ --------~~-----------r--------~~+-----~-----+------------~------------4 1 , . .. ~
105 .-
110 115
100 .". - .... Temperature ("Cl 95
120
90
Appendix G
TEIf>ERATURE
67.U
66.32
65.53
64.73
63.94
63.1.4
92.35
6:!..!56
60.76
59.97
TEHPERATURE
74.33
72 . 24
7j,.:1.4
70.:!.!!
59.35
Sample plots of the flow of SBRlin the dispenser meter cavity, showing temperature at increasing flow rates.
TEtf'ERATURE
90.47
89.29
88.12
86.95
85.77
74.60
65.42
62.25
61.07
59.90
TEHPERATIJRE
142.2
132.6
123.0
103.4
94.%
92.92
90.%
84.06
75.36
69.75
80 ~ LUMPED PARAMETER CALCULATION --+ STD(20)
• NR(20)
SBR1(20)
70 .. x SBR2(20) -+ +
-+ • NBR(20)
+ + • E-O-L(20) -+ + • • .. + + • ... .. ..- • .. PB80(20) 60 + + • .+ • FR58(20) • •
+ • • • - STD(80) • • --" • • NR(80) 6 +
50 • • ,. SBR1(80) - + - -.. - SBR2(60) .. ., -a: --- • NBR(60) .. - -- -~ 40 7< • E-O-L(60)
" .. - ~ 0 PB80(60) to • • ~ • • --e -.. r -:1 x • FR58(60) 0. ___ '- 1 -1 • i:J • ~ 30 • x • STD(loo)
.~ ... ..¥ -¥ III X • • • I- _a-- - --- )()( • - NR(lOO)
0 x X K • • ~
,- • • SBRl (100) • • .. 0 • • • • " °A - • • ,. • SBR2(loo)
20 • • 0 • • . " • • • • ! • NBR(loo) . fo' ..... ...-.- . • .- 0 • If t " . J .. -- • ! •... • • • t • x E-O-L(loo)
I • • • t • • • • .. .. .. ----. • -t .. PB80(lOO) •• • , , f.-::I---'--- • • .- • • .. i - d 10 "'d - o FR58(loo) ---"i .... 0 0 0 f - .:: - • 0 0 0 • 0 0
• • • • • • 0 • .... 0
0 20 40 60 80 100 120 140 160 180 200 Piston Speed (mm/s)
Proportional to Volume Flow Rate (Q)
~
ca ~ ~ ., co c: ., Co co i5 co co 0 ~
u <I: Co 0 ~
0 ., ~
:::J co co ., ~
Cl.
LUMPED PARAMETER CALCULATION
1,60E+08
1,20E+08
1,00E+08
8,00E+07
•
6,00E+07 I • • • -- • - 0 • • • . -. • • --- .. • 0-M· iI - • -11 • • J'!. .. •
It - -4,00E+07 - - -
---
• - • .. - " - x..... • x
~
2,00E+07 ----
O ,OOE+OO +----+-----~----~------~------_+------_+------_+------~------~------~
200 180 160 140 120 100 80 Piston Speed (mmls)
Proportional to Volume Flow Rate (a)
60 40 20 o
-+-- STO(20)
• NR(20) SBR1(20)
x SBR2(20)
--+- NBR(20)
---E-O-L(20) -+- PB80(20)
I- FR58(20)
STO(80)
• NR(60) SBR1 (60)
SBR2(60)
• NBR(60)
• E-O-L(60)
• PB80(60) FR58(60)
-- STO(l 00)
- NR(l 00)
-+-- SBR1(100)
--+- SBR2(100)
-..- NBR(l 00)
--- E-O-L(l 00)
---- PB80(l 00) . - FR58(100)
90 NISA" DATA STO(20) •
• NR(20)
SBR1(20) 80 .. • SBR2(20)
+- +-• NBR(20) -+ • + +- • E-O-L(20) + .. • + -+ • 70 + + +
+ • + PB80(20) + +- • -. • - - FRS8(20) • • + • • • • - STD(80) 60 • • -+ • • NR(80) 6 ..
SBRl (80) 0 +--.. er - -- SBR2(80) .. 50 a: • • NBR(80) • .. + • ---- • -. E-O-L(80) • ~
• .-:::I - s / -i • • • PB80(80) la 40 • -f • ~
I' __ ,,-f -t 0 ! . FRS8(80) ..
Cl. -1- --1--'-- .'
., E o ~1 " · STD(l00) .. .'
! . t' 1I I- 0 -t • , " - NR(l00) .. ...... • 30 0 • ~
,. SBR1 (100) ~ ~ SI I • 0 J I .. i " , • • SBR2(100) , 1 ~ .- I .. ~ • • • • NBR(l00) o , • • • • • • • • • 20 • ..... t • • • ~ . ...- t t x E-D-L(l00) .. - f-=1-I~ • -It • ..
1 • PB80(l00) • • ~-- • FR58(l00) 10 . J~ - ~
• • • • • • • • • • • - • • • • • ~ . • •
0 0 20 40 60 80 100 120 140 160 180 200
Piston Speed (mmls) Proportional 10 Volume Flow Rate (0)
1,80E+08 NISA 11 DATA
-- --- .. - - -~---.. ------ .. • •
• • - - - • --f'---1--.-. • • .. , , • • 2,OOE+07 • • . ... • -. - • ., - - • • • •
O,OOE+OO +-------+_------~------_r------_+------_+------~--------r_------+_------+_----~ 200 180 160 140 120 100
Piston Speed (mmls) NISAII DATA
80 60 40 20 o
-+- STD(20)
• NR(20) SBR1(20)
• SBR2(20) . • NBR(20)
--E·O·L(20)
-+- PBBO(20)
- - FR58(20)
STD(60)
• NR(60) SBR1(60)
SBR2(60)
• NBR(60)
E·O·L(60)
• PB60(60)
FR58(60)
- - STO(1 (0)
- NR(100)
I-+-SBR1(100)
... SBR2(1 (0)
1-'-NBR(l00)
• E·O· L( 1 (0)
• PBBO(l00)
• FR58( 1 (0)
Appendix H
120,OE+3
100,OE+3
g E 80,OE+3 ! .. ~
Cl> 40,OE+3 ~ o
u..
20,OE+3
LUMPED PARAMETER CALCULATION
, 11
__ STD(20)C
-e- NR(20)
SBR1 (20) __ SBR2(20)
-lIf- NBR(20)
-+- E-O-L(20)
-+- PB80(20)
- FR58(20)
- STD(60)C
--NR(60)C
SBR1 (60)C
SBR2(60)C __ NBR(60)C
-- E-O-L(60)C -+- PB80(60)C
FR58(60)C
- STD(100)C
- NR(100) __ SBR1 (1 00)
-e- SBR2(1 00) __ NBR(100)
-- E-O-L(1 00)
OOO,OE+O +----+------1-----+----+----+----+----+----+------,1-------1 -lIf- PB80(100) o 20 40 60 80 100 120 140 160 180 200 -+- FR58(1 00)
Piston Speed (mmls) Proportional to Volume Flow Rate (Q)
-----120,OE+3 NISA 11 DATA
-+- STD(20) __ NR(20)
SBR1(20) ___ SBR2(20)
100,OE+3 __ NBR(20)
-- E-O-L(20) -+- PB80(20)
g - FRS8(20)
~
E 80,OE+3 - STD(60)
., - -+- NR(60) .. >-.. SBR1(60)
;: SBR2(60) 0 60,OE+3 - NBR(60)
" ., E-O-L(60) ~
':; Cl'
-+- PB80(60) ., ~ ~
FRS8(60) ., 40,OE+3 - .. u .- - STD(100) ~
~ : ~ 0 .. • u. , :: • - NR(100)
~ '-'-- '-"-~~~--,. "J\,....{. - - -+- SBR1 (100) __ SBR2(100)
20,OE+3 -.- NBR(1 00)
--- E-O-L(1 00) __ PB80(1 00)
OOO,OE+O -+- FRS8(1 00)
0 SO 100 1S0 200 2S0 Piston Speed (mmls)
Proportional to Volume Flow Rate (Q)
Appendix I
The following pictures are thin sections of O-rings that have placed on a Shadograph
and then photographed. The primary reason was to investigate the shape change due
to anisotropy immersion in a good solvent.
The first two sides each show three pictures each of an O-ring section made from a
different compound. The first side shows conventionally moulded O-rings and the
second side shows sections of rings in the same three compounds moulded with the
new compression moulding system. Anisotropy can clearly be seen in the
conventionally moulded sample for FR58 (lowest on the page). The second set of
photos are all considerable more regular.
The area growth was measured by counting the graduations and the degree of
circularity was measured by taking two diameters on each picture, the longest and
shortest giving that ratio.
The following pictures are each show a before and after (Le. one not swollen and one
swollen) for the compound PS 80 in both toluene and Methanol. In this way they can
be used to show shape change. It was the shape change here that was the most
important factor under investigation. The shape change due to swelling, if any, will
highlight molecular anisotropy and orientation.
James Walker Samples. (Top Down: PBSO, Elast-o-Lion and Fr5S/90)
. L
:"
',' . , ..
..... ' .
'" ~.,.
.:. ··';1(:'·~,0·;· , .... "
:'.'t;~>~:
. 'i .. ...,.. ~·l~; ~1/~i',~ •. .. JfiJ,....,'!. . . ·:~+~J:(.~~f~~}rrfr
" "
:-', · •. ,f,'~·
. ' .' Form System Samples.
(Top Down: PB80, Elast-o-Lion and Fr58/90)
,t, ~.
i ....... ! . : . : !.~
'(,0: :'::'
• I" ., i I. q ! t
::::~I'" ! :: i: .;'r. I" ••• '.
\,::: ,: :,L •. 'i' t t"
I ; .•.... -~ .. ...,-I I t· I
: ,-'
;~?:'. . j', ,i .••.
. . ,~ ,
• j. :::±C:O'-'-'"
.-:\
;~3:;';~;. , " :.".j
, .. ~
. "
.:
James Walker PB80 - Before and After (Solvent: Methanol)
• '~'~ •• <
.', .
';- .
.. ,. . '," t',<'J.- - '. L,!
"
------------------------
::rt."-." ...
#f .• !::. :: I : . ...... .... . -...... ,. ., ..... . i,'
: ~ j; : : .. ··1", .'. .... + ••. - .. . . .. .. ~
",'
, ~: ',~ '"'i, •. '~",.",.,"
',~. ": ;".: '.' " ~ ..
"
•. ', .. j :-'.,:;- ,~":;: - ".'
,',",'.',.;.' J , •. :';' ":'~:;'f~f!cf~""#: : : !rH <, ••
James Walker PB80 - Before and After (Solvent: Toluene)
.\.\f.:.:-.i-:"i' .\
: '". f""
,"
• tr ) t:. . >~.:Jr':> c
,i ..... -, r- •• '
fI 1
".
, . . . . ~. ~i :.
".'
Form System PB80 - Before and After (Solvent: Methanol)
: <!~t~t;\;~.:t~~ ',:
,".'-
. " "
'I '; ::.l:..;A :, ~:.'
..' ;( .. , .~ .'
: ~.)~.
, . .,.~. ''' ..
'.:
. : .;.":
.1. • • . ' ..
/;t~r.{~};,:, ':':
: }/...:;!}..~-5~;
-- ::
>~~~t1~v. '\.';' r : _ . , .
----------------
Form System PBBO - Before and After (Solvent: Toluene)
-'!". ;;" ':,{~~:~·~.~;;1~~~~~.: .:,~ .. ~.~.~:': .~ . • -',0