Investigate of Parameter Setting in Plastic Injection Molding
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Transcript of Investigate of Parameter Setting in Plastic Injection Molding
KOLEJ UNIVERSITI TEKNIKAL KEBANGSAAN MALAYSIA
Investigate of Parameter Setting in Plastic Injection Molding
Thesis submitted in accordance with the requirements of the
Kolej Universiti Teknikal Kebangsaan Malaysia for
Bachelor of Manufacturing Engineering (Honours) (Manufacturing Process)
Sullyfaizura Mohd Rawi
Faculty of Manufacturing Engineering
June 2006
ABSTRAK
Pengacuaan suntikan merupakan proses yang paling meluas digunakan untuk
menghasilkan pelbagai kompenan plastik pada tahap kualiti yang tinggj. Projek ini
adalah menyiasat mengenai penetapan parameter dalam mesin pengacuan suntikan.
Mesin pengacuan suntikan &burg adalah i n d e n t yang digunakan dalam projes ini
untuk mneghasilkan "Dog bone specimen. sebagai produk. Pelbagai parameter di
dalam proses pengacuaan suntikan seperti tekanan penyuntikan, tekanan pegangan,
suhu pencairan bahan, suhu acuan, isipadu dos, and program pengapit dan parameter
proses lain seperti masapenyejukan turut dilihat. Di samping itu, projek ini juga
adalah untuk mendapatkan parameter yang optimum menght jumlah experiment
yang dijalankan berdasarkan factorial 3k yang digunakan dalam experiment
rekabentuk. Selain itu, pengujian bagi melihat keadaan mekanikal specimen yang
dihasilkan menggunakan mesin Universal Testing Machine 0 untuk menguji
kekuatan tegangan pada specimen. bacaan maximum bagi ujian kekuatan tegangan
diambil berdasarkan pengujian yang telah dijalankan.
CHAPTER 1
INTRODUCTION
1.1 BACKGROUND
Injection molding is a practical technique used in manufacturing for mass producing
plastics parts quickly and inexpensively. As plastic parts have become more popular
and critical in modem engineering applications, demand for quality has increased.
The principle of injection molding is very simple. Injection molding is process in
which a plastic material is heated until it becomes soft enough to force into a closed
mold at which point the material cools to solidify and from a specifio product. The
action that takes place is much like the filling of jelly donut. A hypodermic style
cylinder and nozzle inject the heated plastic into an opening creates in closer
container(mo1d). The material is allowed to harden, a finished part is ejected and the
cycle is represents as often as necessary to produce the total number of pieces
required.
This project conduct to investigate and optimize of setting parameter in plastic
injection molding. For the injection molding process, the parameters include ram
speed, injection pressure, barrel and nozzle temperature, mold temperature, mold
clamp force, dwell time, cooling time, and material properties. However, for this
study only five parameters (temperature of melt, injection pressure, holding pressure,
dosage volume and clamping program) were varied while the rest were held constant.
3k factorial design, a formal method of the Design of Experiments (DOE) was
applied to test these parameters in an efficient manner, using the least amount of
experiments and therefore saving resources and time. The material will be use in this
project polypropylene. We choose this material because this material have a
different properties and parameter for other material. So, finally we will find also the
accurate result for polypropylene and we can conclude of the quality of product. We
also will be do the tensile test of the product produced and analysis the result of the
test using the UTM. Processing conditions have very strong influences on properties
and performance of parts and products. Changes in processing conditions can lead to
improvements or degradation of accuracy, shape, surface finish, fracture resistance
and many other part properties and characteristics. One of the major activities of
manufacturing engineering is the assessment of the effects of changing process
parameter values on part characteristics. The primary use of process models is to
predict these effects. Often process models are inadequate for this task, usually
because the process is very complex or because accurate material behavior
descriptions at processing conditions are unavailable. So the defect of from the result
will produce should be analyze and try to improve the quality of the product.
Outcome of the defect will be define to produce the better parameter.
1.2 PROBLEM STATEMENT
Nowadays, quite a variety of different technique are employed in the fonning
polymeric material. Injection molding is the most common method for method for
fonning plastic polymer. Injection molding is the most widely used molding process
for thermoplastics. Injection molding is economical only for large production
quantities. Thus, the product of produced using injection molding have are
troubleshooting. Most of the defect of the product have a come h m not proper
parameter setting in plastic injection molding. Beside that, in this project also have to
optimize the parameter of the injection and determine the accurate value of the
parameter. Before this, the parameter is manually setting and don't have the
accurate value. The other side, the problem is to minimize of the defect of the
injection molding. A through understanding of the molding process will be help
determine the causes.
1.2 OBJECTIVE OF PROJECT
Objective of this project is :
1. To optimize of the parameter in plastic injection molding.
2. To determine the maximum tensile strength value of polypropylene.
1.4 SCOPE OF THE PROJECT
This project to investigate the parameter setting in plastic injection molding, so for
the started to optimize the parameter, the sequence of the process is:
i. Material selection for polypropylene. Pure polypropylene have been
used in to investigate the parameter setting in injection molding
ii. Produce the specimen using Plastic injection Molding. The
parameter should be setting based on the parameter selection.
iii. Tensile test for the produced specimen using Universal Testing
Machine (UTM) to find maximum tensile strength of value for each
specimen based on different parameter setting.
iv. Analyze the result and find the optimum result for this investigation
for parameter setting of Plastic Injection Molding.
CHAPTER2
LITERATURE REVIEW
2.1 INTRODUCTION OF PLASTIC INJECTION MOLDING
Injection molding, the polyiner analogue of die casting for metal is the widely used
technique for fabricating thermoplastic material. It also is perhaps the most common
and versatile method of forming plastic into plastics. Injection molding process is a
process in which polymer is heated to highly plastic state and forced to flow under
high pressure into mold cavity, where it solidifies. The molded part called is
molding, is then removed fiom the cavity. The process produces discrete components
that are almost always net shape. Complex and intricate shapes are possible with
injection molding the limitation being the ability to fabricate a mold whose cavity is
the same geometry as the part. In additional, the mold must provided for part
removal. Injection molding is the most widely used molding process for
thermoplastic. Some thermoset and elastomers are injection molded., with
modification in equipment and the operating parameters are allow for cross-linking
of these material. In this project, I use polypropylene as material to produce the
product based on the optimum parameter will be setting in plastic injection molding.
Injection molding is cyclical process, where each cycle produces a part or parts. The
cycle of the machine starts with of closing of the mold, the machine then fills the
molds, the part solidifies, the mold opens, the part is extracted and the mold close
again. Generally injection molding have four stage starting the filling, packing,
cooling and finally ejection. According A. Tolga Bozdana, Omer Eyerci'ogSlu are
said injection molding process is a cyclic process. Four significant stages of the
process are filling, packing, cooling and ejection. The first stage is the "filling
stage" in which the mould cavity is filled with hot polymer melt at injection
temperature. After the cavity is filled, in the "packing stage", additional polymer
melt is packed into the cavity at a higher pressure to compensate the expected
shrinkage as the polymer solidifies. Next, the mould is cooled until the part is
sufficiently rigid to be ejected, and this stage is the "cooling stage". The last one is
the "ejection stage" in which the mould is opened and the part is ejected, after
which the mould is closed again to begin the next cycle. For thermoplastic materials,
the injection molding machine converts granular or pelleted raw plastic material into
final molded parts via melting, injection, packing, cooling and ejection cycle.
2.2 ELEMENT OF PLASTIC INJECTION MOLDING
The injection molding machine itself consists of the clamp unit, the injection unit, the
control unit and a hopper. The hopper becomes modified to include a loader, dryer,
and the same cases an additive feeder.
onveyor r l
I Loaded
Dryer
AF
Figure 2.1 Element Of Plastic Injection Molding Bernie A. Olmsted and Martin E.
Davis, (200 1)
Clamp Mold
Injection Unit
I I Control Unit Plastic
Temp controller Grinder - -
The injection unit heats, melts, pumps and injects the plastic into the mold then
mold "closed" . the control unit monitors and as the name implies, control the
functioning of the injection unit and the clamp unit. The mold is mounted within the
clamp unit and this unit opens the mold allow plastic parts to be ejected and holds the
mold closed when melted plastic is being injected. The mold is purchased from mold
maker, whose capabilities may include computer- aided-design (CAD) and
computer-numerically-controlled (CNC) milling machine that help automate the
manufactures of complex mold. The mold consists of two halves, the core half (or
male part shape) and the cavity half (or female part shape). Because the core is made
to be a little cavity represents the part. This area between the core and the cooled and
ejected fiom the mold to become the plastic part. In order to solidify the plastic part
in the mold so that it can be removed, it is usually necessary to cool the mold. The
cooling is accomplish by circulating cool water through cooling channel that are
machined into the mold itself. The water is cooled by chiller, which can either be fi-ee
standing unit nearly the press or by a part of the temperature controller system that
may be serve several molds in several injection molding machines. Temperature
controller may takes the from chiller as describe in the parameter of the plastic
injection molding.
2.3 CATEGORIZING THE PARAMETER
There are so many parameter to control, they can be detailed within the confines of
the major categories. Parameter is important to create the great result and good
product. Philip Mit Chell are said many parameter affect the injection molding
process. A practical approach to understanding these parameter is appropriate, and
those parameter that have the greatest effect on the quality and cost effectiveness of
the molded product are targeted. Figure 2.2 shown that the parameter involve one or
more of four basic categories: temperature, pressure, time and distance.
Figure 2.2 : Categories Of Parameter (Douglas M. Bryce, 1997)
23.1 TEMPERATURE
2.3.1.1 TEMPERATURE OF THE MATERIAL
Temperature of the material. The primary temperature of concern is the
temperature to which to which the plastic material must heated before it is injected
into a mold. All material have range of temperature within which they are most
efficiently injected while still maintaining maximum physical properties. Philip Mit
Chell(1996) also said melt temperature or temperature of material is that
temperature at which the plastic material is maintained throughout the flow path.
This path begin when the material is transferred h m the machine hopper into the
heating cylinder of the injection unit. It is then augured through the heating cylinder
and into the machine nozzle. From the material is injected into the mold where it
travels along a runner system, through gates and into cavity. Control the melt
temperature is essential all along that path. For amorphous materials(those that
soften-when not melt heat is applied) this range is rather broad: with aystalline(those
that actually melt when is applied) it is fairly narrow. With both types of material,
however there is a temperature point at which point at which the flow the easiest and
still maintains proper physical properties. This is called the ideal melting point and
must be attained through educated guesses and trial-and-error. While this seem
primitive it only required as a fine-tuning adjustment once a specific production run
is initiated and is finalized specification for specific product. The guessing process
actually begins by setting the temperature of heating cylinder such that the material
being injected is a temperature recommended for that generic material. The plastic
temperature is measured as it leaves the heating cylinder to make sure it is within the
proper range and then adjusted up or down depending on cycle time, required
pressures, mold temperature and variety of other parameters. These adjustment are
made are made during a pilot run of the process and until acceptable parts are
produced. When parts meet specification a setup sheet is created listing the values
for all parameter of concern. The sof€ening(or melting) of plastic is achieved by
applying heat to the plastic material causing the individual molecules to go into
motion. To a point the more heat that is applied the faster the molecules move. So,
more heat is applied to degrade of the plastic material. The heater bands, which
resemble hinged bracelets are assembled to control the temperature. There are three
basic temperature zones for the heating cylinder; rear, center, and h n t .
2.3.2 PRESSURE
Pressure is required for variety of reasons in injection molding process. The areas of
the injection machine require pressure and pressure control: the injection unit and the
clamp unit. We will focus on injection pressure, holding pressure and clamping
pressure. The two closely related in that the clamp unit must develop enough
pressure overcome the pressure developed by the by injection unit during the
molding process.
23.2.1 INJECTION PRESSURE
Injection pressure is a primary pressure used for the injection molding process. It
can defined as the amount of pressure required to produce the initial filling of the
mold cavity. The cavity image is the opening in the mold that will be filled with
plastic to form the product being molded. Initial filling represents approximately
95% of the total filling of the cavity image. This is applied to molten plastic.
Normally pressure is depend
2.3.2.2 HOLDING PRESSURE
Holding pressure is applied at the very end of the primary injection stoke and used
for the final 5% filling of the cavity image. This pressure have to completes the final
mold filling and maintains pressure against the plastic that was injected so that it can
modify while staying dense and packed. It is called holding pressure because the
holds pressure against the cooling plastic in the cavity image while that plastic
solidifies. This helps to ensure a dense part, molded with uniform pressure and
controlled shrinkage. Holding pressure are usually in the range of 50% of the
primary injection pressure. Herman F. Mark (2003) also said the secondary pressure
is half (or less) of the initial injection pressure.
2.3.2.3 CLAMP PRESSURE
Clamp pressure can be defined as the amount of pressure required to hold the mold
closed against injection pressure. The clamp unit of molding machine can be
mechanically or hydraulically activated and this pressure is applied against the mold
that forms the plastic product. The clamp force or clamp pressure must be equal the
injection force. The degree of the applied must be at least equal to the amount of the
pressure applied by the injection unit. If the lOOOOpsi injection pressure is used then
at least lOOOOpsi clamp pressure must be used. In fact, a precautionary measure of
additional equal to approximately 10% should be used to ensure that the clamp stays
closed in the event that in injection pressure drifts upward slightly. If the clamp
pressure is too low, the mold will blow open during injection. Flash occurs and the
cavity image will not fill with plastic. If clamp pressure is too great, the mold may
collapse from the total force being applied.
Pressure and temperature distribution within the mould be as uniform as possible but
is impossible to achieve with injection molding. The pressure will drop when the
mould is being filled due to flow resistance. Temperature difference also occur as the
fill takes a finite time (even through very short). The objective is to achieve the most
uniform state possible in the fill the process. The flow resistance during mould fill is
a crucial factor here. A low flow resistance ensures a faster mould fill and a
reduction in local pressure difference.
23.3 TIME
During the injection molding process, many internal activities take places. Some
occur while others are active(paralle1) and some must wait until others are
completed. The most important activity at this point is the overall cycle time.
2.3.3.1 INJECTION TIME
Injection Time is related to injection rate (ccJsec.) and injection rate should be high
enough to avoid freezing of melt during filling phase. Higher injection rate does not
affect the thermally stable commodity polymers like PP, PS ect. The higher injection
rate can be limited on account of sensitivity of polymer to shearing while passing
through narrow passages (especially for engineering polymers). Freezing time is
proportional to cube of minimum wall thickness. Generally injection time is also
proportional to square of wall thickness.
233.2 COOLING TIME
Cooling Time it can be observed that the largest portion of cycle time is woling time
which is proportional to square of maximum wall thickness and also efficiency of
woling set up. Therefore for faster production, wall thickness has to be low and
efficiency of cooling system in mould as high as possible.
23.4 DISTANCE
The final parameter is that of distance. Although it's the last item on the list of
parameter priorities, control of distance is critical to producing highquality products
at reasonable cost. This is primarily due to the fact that excessive distance requires
excessive time as started earlier time is money. Because distance is so closely related
to time the various functions involving requires are basically the same as those
related to time plus a few other.
While selecting injection molding machine the following specifications are required
to be evaluated.
Table 2.1 Specification Are Required In Plastic Injection Molding (Edward S.
Wilks,, 2001)
INJECTION UNIT I Maximum swept volume cc 1 max. shot weight
I @-
Maximum metering stroke in rnm.
i Maximum injection speed g/s or cds i I
1 I Maximum injection pressure Kg/cm2
Maximum Injection Power .Kgcm/sec.
Plasticizing rate g/s or Kglhr.
1 CLAMPING UNIT I
Clearance between Tie bars and platen size mm x I
To understand
calculation of max. shot weight for a
material.
dependence of quality of melt for
consistency of molding.
how it ensues melt to spread through out
when in fluid condition- before it fieezes.
Its relationship with freezing time.
how it overcomes resistance to flow
during filling and; pressure phase on
account of flow ratio and; viscosity.
how it takes care of difficulty in filing for
thinner walled and high flow ratio parts.
how it influences cycle time.
To understand
how it accommodates mould. I
Maximum daylight mm and mould open strokc
mm
Minimum mould height
i Clamping force
I Torque Kgm and rpm I I DlWE POWER I Power supply frequency 60 or 50 Hz.
Pump-motor rating. Kw
NO LOAD CYCLE TIME
its significance for ejection of deep parts.
its relationship with mould open stroke
and daylight.
its dependence on cavity pressure and
method to compute cavity pressure.
To understand
torque requirement for -viscosity of- melt.
its influence on speeds.
to match the application - usage of
machine.
Conventional 1 Proportional and
Cartridge valve Hydraulic
Controls
Electrical I solid state 1
microprocessor controls
Open loop or closed loop controls
[t indicates the time for non processing
?art of the cycle time
2.4 UNDERSTANDING DEFECT
The complexity the injection molding process and the inter-dependence of many
variable involved, means that any molding defect may have several different causes.
Of which more than one may be present at any given time. Consequently a remedy
that curves one fault may engender another. The conclusion is that injection molding
trouble shooting is a job for the expert. Provided these limitations are understood, the
trouble shooting chart will provide a usehl guide for the problem solving.
Table 2.2 Trouble shooting of plastic injection molding ( Bernie A. Olmsted and
Martin E. Davis, 2001)
Problem
Short Shots
Sink marks
Possible Cause
Insufficient feed
Insufficient pressure
Melt Temperature too low
Injection time too short
Nozzle cold on the start-up
Mold cold
Feed system too small
Air trapped in mold
Plasticizing capacity inadequate
Unbalanced cavity in multi-cavity
mold
Excessively thin region
Melt temperature
Suggested remedy
Increase
Increase
Lengthen cycle
Increase temperature gradually
Increase screw speed and back
pressure
Increase
Fit nozzle heater
Reduce coolant flow
Fit mold temperature controller
Enlarge sprue or runner or gate
Add or clean vents
Increase cycle time
Use a larger machine
Adjust runner or gate size
Redesign part
Reduce barrel temperatures
Voids
Surfaces defects
near gates
Flash
Insufficient material injected
Insufficient dwell time
Premature gate freezing
Sharp variations in wall thickness
Wrong gate location
Part ejected too hot
Cavity pressure too low
Volatiles from overheated material
Consideration on granules
Premature freezing of flow path to
thick
Mold too cold
Mold too hot
Excessive injection pressure
Excessive melt temperature
Mold parting face faulty
Insufficient clamp force
Increase feed
Raised barrel temperature
Increase mold temperature
Enlarge gates
Increase
Enlarge gate
Increase
Redesign part
Relocate
Increase cooling time
Use nucleated grade
Increase
Raise barrel temperature
Increase mold temperature
Enlarge gate
Reduce heating
pre-dry
Improve storage
Increase pressure
Increase mold temperature
Use nucleated grade
Enlarge gates
Increase mold temperature
Increase pressure
Increase injection speed
Cold mold near gates
Reduce pressure
Reduce runner
Reduce heating
Repair mold
Increase
Flows marks
Weld lines
Bad surface finish
Brittleness
Warping
Foreign matter on the mold parting
face
Flow restriction in one or more
cavities of multi-cavity mold
Melt temperature too low
Incorrect gate location
Incorrect gate type
Injection pressure too low
Inadequate venting
old cavity soiled
Mold temperature too low
Flow length too great
Excessive use of mold lubricant
Melt temperature
Mold too cold
Melt degraded by excessive
heating
Material contaminated
Incorrect part design
Excessive use of regrind
Melt temperature too low
Incorrect part design
Over packing near gate
Sharp variations in wall thickness
Flow length too great
Use a larger machine
Clean mold
Identify and remove
Increase heating
Relocate
Adjust
Increase
Vent cavity
Clean mold
Increase
Relocate gate
Increase number of gates
Mold lubricants not recommended
Increase heating
Increase mold temperature
Decrease heating
Clean hopper and barrel
Redesign part
Reduce proportion of regrind
Increase heating
Redesign part
Reduce shot volume
Reduce injection pressure
Reduce injection time
Reduce heating
Check runner and gates sizes
Redesign part
Relocate gates
r
Wrapping
Silver streaks
Nozzle drool
Bum marks
Part sticking
Unbalanced multiple gates
Part ejected too hot
Inadequate or badly located
ejectors
Temperature variations between
the mold halves
Melt temperature too low
Melt too cold
Condensation on mold
Entrapped volatiles
Excessive nozzle temperature
Excessive melt temperature
Incorrect filling pattern
Molding too hot
Insufficient draft on side walls
Excessive injection pressure
Cavity finish poor
Cores misaligned by injection
pressure
Increase number of gates
Relocate gates
Balances feed system
Increase cooling time
Use nuclected grade
Modify mold
Adjust cooling circuits
Modify mold
Increase heating
Increase mold temperature
Dry mold
Increase mold temperature
Predry material
Improve storage
Vent mold
Reduce heating
Reduce heating
Purge barrel
Reduce gate
Improve venting
Increase cooling
Increase draft angle
Decrease
Polish mold
Redesign part
Relocate gate
2.4.1 PARAMETER EFFECT
Table 2.3 Parameter Change versus property Effect (Douglas M. Bryce, (1 997)
Table 2.3 shows some of the property values that can be adjusted by plus or minor
minus change in some of the more common molding parameter. There are some
examples. But the notice how some properties are changed in the same way by
different parameters. For instance "less shrinkage" can be attained by either
increasing injection pressure or increasing mold temperature, and "less degradation"
can be achieved by lowering back pressure as well as lowering melt temperature.
These example demonstrate that the basic molding parameters do work closely
together, and that changing a parameter in one area may affect a value of some
property in another area. By understanding this relationship, it is possible to
minimize the number of adjustments required when it is necessary to make a
correction due to an unexpected change in some variable of the process.
Parameter
Injection pressure (+)
Injection pressure (-)
Back pressure (+)
Back pressure (-)
Melt temperature (+)
Melt temperature (-)
Mold temperature (+)
Mold temperature (-)
2.5 PROPERTIES OF THE MATERIAL
Property Effect
Less shrinkage, higher gloss, less wrap, harder to eject
More shrinkage, less gloss, more wrap, easier to eject
Higher density, more degradation, fewer voids
Lower density, less degradation, more voids
Faster flow, more degradation, more brittle, flashing
Slower flow, less degradation, less brittle, less flashing
Longer cycle, higher gloss, less wrap, less shrinkage
Faster cycle, lower gloss, greater warp, higher
shrinkage
2.5.1 THERMOPLASTIC
Thermoplastic materials that used in high volume, widely recognized applications
and are known as wmmodity thermoplastics. Some resin manufacturer have objected
to the commodity designation because that term can imply that the material are
interchangeable from supplier to supplier without different in properties. Some
differences can be seen, but within a product classification, they are not great. All of
the commodity thermoplastic that will be considered are made by addition
polymerization method. Polymer materials(molecu1ar viewpoint), this method
requires that the monomer have carbon-carbon double bond and all the monomer
meet that requirement. The differences between the monomers used make these
commodity thermoplastics are in the hctional groups attached to the carbons.
Although functional group substitution can be made at four locations on a carbon-
carbon double bond, only one site is used for substitution in all major types of
wmmodity thermoplastics which will be considered.
Where --X can be among other:
----H polyethylene ---CH3 p o l ~ ~ r o ~ ~ l e n e ---CI polyvinylchloride - polystyrene
Figure 2.3 General Representation Of Commodity Thermoplastics (Hans-Geog Elias,
2002).
Therefore, all these commodity thermoplastics monomer and polymer can be
represents by general formula given in figure 2.3 where X represents a functional
group of type. Note that in one hydrogen is wnsidered are also attached to the to the
carbon-carbon double bond. The differences between the wmmodity thermoplastic
arise, therefore from the difference caused by the substitution of one functional group
on the carbon-carbon double bond. One of most important effects is steric that is the
consequences of differences in the size of the functional groups. When is structured
groups are small (such as hydrogen, then little steric hindrance the polymers are
relatively fiee to rotate, bend, and pack together. The carbon double bond, with the
results of restricted polymer motion, less ability to pack densely, and changes in
mechanical, physicals, and chemical properties.
2.5.2 POLYPROPYLENE
Polypropylene is one of those rather versatile polymers out there. It serves double
duty, both as a plastic and as a fiber. As a plastic it is used to make things like
dishwasher-safe food containers. It can do this because it doesn't melt below 160 OC,
or 320 OF. Polyethylene, a more common plastic, will anneal at around 100 OC, which
means that polyethylene dishes will warp in the dishwasher. As a fiber,
polypropylene is used to make indoor-outdoor carpeting, the kind that you always
find around swimming pools and miniature golf courses. It works well for outdoor
carpet because it is easy to make colored polypropylene, and because polypropylene
doesn't absorb water, like nylon does. Structurally, it is a vinyl polymer, and is
similar to polyethylene, only that on every other carbon atom in the backbone chain
has a methyl group attached to it. Polypropylene can be made fiom the monomer
propylene by Ziegler-Natta polymerization and by metallocene catalysis
polymerization.
Figure 2.4 Structure Of Polypropylene (Clive Maier and Teresa, 1998)
Polypropylene is an extremely versatile plastic and is available in many grades and
also as copolymer(ethylene/propylene). References from R J Crawford for the book
Plastic Engineering. He said It has the lowest density of all thermoplastic(in the
order of 900kg/m3 and this combined with strength, stifhess and excellent fatigue
and chemical resistance make it attractive in many situations. These include crates,
small machine parts, car components (fan, fascia panels etc), chair shells, cabinets
for TV, tool handle etc. its excellent fatigue resistance is utilized in the molding of
integral hinges ( e.g accelerators pedals and forceps/ tweezers). Polypropylene is also
available in fibre from (for ropes, carpet backing ) and as a film (for packaging).
Table 2.4 Polypropylene At A Glance (Clive Maier and Teresa, 1998),
A. Brent Strong are said In order to obtain regular arrangement of atoms required to
make isotatics PP a catalyst is used to force this arrangement during the
polymerizations of the polymer. Such catalyst are called stereoregular. The Ziegler-
Natta catalysts used to produce HDPE is of this type.m other types of stereoregular
catalyst have been developed that are increasingly used to produce PP, in part
because the ability to control the shape and length of the polymer is even better with
the new catalyst. Therefore commercial grades of PP are made using Ziegler-Natta or
some other stereoregular catalyst.
uses:
Monomer:
Polymerization:
Morphology:
Melting temperature:
Glass transition temperature:
Thermoplastics, fibers, thermoplastic
elastomers
Propylene
Zieglar-Natta polymerization,
metallocene catalysis polymerization
highly crystalline (isotactic), highly
amorphous (atactic)
174 OC (1 00% isotactic)
-17 OC
It is not surprising that PP and PE especially HDPE have similar properties and
compete for many of the same application. However, PP and PE differ in some
important respects and these differences have led to preferences for one or the other
in various application. PP is stiffer than PE so in application requiring flexibility
(such as wire coating) one of the PE material would be used. On the other hand if
greater stifiess is needed PP is preferred resin. This is especially true if the
application also requires abrasion resistance or hardness, such as for gears, toys,
automotive battery cases and seats for stacking chairs. The resistance to environment
factors is similar for PP and PE. PP is somewhat more susceptible to W and
oxidative degradation than is PE but is more resistant to stress cracking than PE.
Hence, cross linking of PP for improve ESCR is not practiced commercially,
partially because the electron beam radiation degrades the PP. PP has a higher glass
transition points and higher melting points than PE. This means that processing
temperature are generally higher, but it also means that service temperature are
higher. Sterilizable medical devices, dishwasher-safe food container and appliance
parts are often of PP for this reason. A very important property differences that has
led to many application for PP is its superior resistance to cracking h m mechanical
stresses. PE material will readily blush and craze when subjected to bending, but PP
will not. Application requiring this polymer include carpets, ropes, strapping tape
and molded item incorporating integral hinges.
The superiors stifiess of PP over PE and low price of PP compared to the
engineering plastics have led to its use in some structural applications. If additional
sti&ess or strength is needed, reinforcement can added to PP. for instance the
addition of 30% shirt fiberglass reinforcement can double the tensile strength and
impact resistance of PP. impact modifier can be added to PP to further improve
impact strength especially for low temperature application where PP is less impact
resistant than HDPE. EPDM the copolymer of PP, PE, and dime monomer, has
improved impact properties and much greater elongation than either PP and PE.
Filler (such calcium carbonate or talc are often added to PP up to about 30%
concentration by weight. The filled plastic has improved stiffness, lower mold
shrinkage and lower cost. Many molded automotive parts have been converted h m
thermoset materials to filled PP because PP can be molded into very complex shapes
using fast molding cycles and still retain the dimensional stability that was
previously provided by the thermoset materials.
Table 2.5 Properties of the Polypropylene (R J Crawford, 1998),
2.6 MEASURING MECHANICAL PROPERTIES OF MATERIAL
Name of plastic
Polypropylene
2.6.1 TENSILE TEST
Method for determining behavior of materials under axial stretch loading. Data h m
Polymer repeat unit --(--C---C--)n--
! C
test are used to determine elastic limit, elongation, modulus of elasticity, proportional
limit, reduction in area, tensile strength, yield point, yield strength and other tensile
properties. Tensile tests at elevated temperatures provide creep data Procedures for
tensile tests of metals are given in ASTM E-8. Methods for tensile tests of plastics
are outlined in ASTM D-638,
Properties comparisons
Resists stress cracking Strongerand stiffer than HDPE Resistant to water and solvent Lowcost
A tensile test, also known as tension test, is probably the most fundamental type of
mechanical test can perform on material. Tensile tests are simple, relatively
inexpensive, and l l l y standardized. By pulling on something, very quickly
determine how the material will react to forces being applied in tension. As the
material is being pulled, and then will find its strength along with how much it will
elongate.
Typical uses
Containers with integral hinges Microwave containers Utility fibers (woven bags, ropes, w t s )
Brand Names
Marlex (Philips) Polyfort (schulman) Pro-fax (montell) Vistalon (exxon) Vrestolen Oluls)
"Moh'd Sarni" Ashhab said tensile test is a type of mechanical test that is performed
on a material. The tensile test is the most widely used mechanical property test. The
intent is to measure inherent material behavior. Tensile tests are simple, relatively
inexpensive, and l l l y standardized. By applying a variable tension load (usually
increasing) to a metallic specimen can learn a lot of properties about the material
such as the maximum load it can with stand and elongation-load relationship.
In the tensile test, an axial load is applied to a cylindrical tension specimen which has
an initial length Lo and initial diameter do. The test specimen is really longer but the
length under test in which change in length is measured is the central long section
(refer to Figure 2.6 below). The tensile test is performed with a universal testing
machine (UTM).
Figure 2.4: Test specimen under axial tension load (James S. Preraro ,2000)
2.6.1 UNIVERSAL TESTING MACHINE (UTM)
A universal testing machine (UTM) is used to determine modulus of elasticity
(Young's modulus), tensile strength at yield or break, and stress-strain curves. The
test specimen is loaded between the grips of the UTM. A mechanical (contact)
extensometer is used to measure strain up to a speed of 20000 mmlmin. The
specimen is stretched at a constant speed while load and strain are recorded. The
speed of testing is specified by the relevant standard, or client request. This test
provides high strain rate stress-strain data for the simulation of impact situations. It is
usually combined with stress-strain data spanning several decades of strain rates to
create a rate dependant model of the material.
A UTM should comply with the major and applicable standards, but additional,
special features will increase the purchase price for things that might be unnecessary
for a particular application. Some of the most relevant features to consider when
selecting a UTM test system are discussed below.
Load-Frame Capacity And Dimensions
The selection of load b e capacity is based on the maximum force required to
cause the material being tested to fracture. Specifications for UTM load frame
capacity and dimensions are vital to the equipment-selection process.
Dimensional specifications must take into consideration clearances between columns
and vertical clearance to adequately handle the products being tested. Some
materials, such as elastomers and soft polymers will elongate substantially, and
sufficient vertical travel must be available to allow the material to stretch as far as
necessary without running out of travel space. Also, consideration should be given to
any special grips, fixtures and environmental chambers that could require additional
space in both directions.
Frame Stiffiess
In some instances, h e stiffiess is a feature that can be overrated. The stiffiess of
the test frame could be an important factor where only crosshead motion is being
used instead of a separate extensometer or deflection-measuring device. Most
applications that comply with international tensile-testing standards call for the use
of an extensometer or deflection-measuring devices.
There are many machine components that can affect the frame stiffiess including
screw diameter, ball-nut fit, crosshead stiffiess, screw-bearing fit and frame stiffiess
In addition, compliance of the specimen itself, pull rods and the specimen-gripping