MATERIAL SELECTION CRITERIA FOR

SPECIFIC END USE

Dr.T.O. VARGHEESECIPET, CHENNAI

CENTRAL INSTITUTE OF PLASTICS ENGINEERING & TECHNOLGOYCENTRAL INSTITUTE OF PLASTICS ENGINEERING & TECHNOLGOY

GUINDY, CHENNAI - 600 032GUINDY, CHENNAI - 600 032

How do we decide which Plastics, if any, is best for a particular application?

Material Selection is not as difficult as it might appear but it does require an awareness of the general behaviour of plastics as a group , as well as a familiarity with the special characteristics of individual plastics.

Need Recognition

Functional Specification

Concept Generation and Evaluation

Part Design•Layout/drawing•Constrains•Analysis•Material Choice

Process Design•Manufacturing Method Selection•System layout•Integration of system•Manufacturing procedure

Prototype creation & Verification

Production Implementation

Second Step

Assess the suitability of a range of candidate materials.

First Step – ImportantTo define clearly the purpose & function of the proposed product and to identify service

requirements.

Most Important characteristics requiring consideration for most Engineering components.

1. Mechanical Properties –

Strength, Stiffness, Fatigue, Toughness and the influence of high or low temperature.

2.Corrosion susceptibility and degradation

3. Wear resistance & Frictional property

4. Special Property requirement for example:

Thermal, Electrical, Optical, Magnetic & Damping capacity.

5. Moulding and or other method of Fabrication.

6. Total cost (material & Manufacturing).

MaterialDensity (Kg/m3)

Tensile Strength (MN/m²)

Flexural Modulus (GN/m²)

% Elongation at

Break

ABS (High Impact) 1040 38 2.2 8

Acetal (Homopolymer) 1420 68 2.8 40

Acetal (Copolymer) 1410 70 2.6 65

Acrylic 1180 70 2.9 2

Cellulose Acetate 1280 30 1.7 30

CAB 1190 25 1.3 60

Epoxy 1200 70 3.0 3

Modified PPO 1060 45 2.3 70

Nylon 66 1140 70 2.8 60

Nylon 66 (30% Glass Filled) 1380 115 5.1 4

PEEK 1300 62 3.8 4

PEEK (30% carbon) 1400 240 14 1.6

PET 1360 75 3 70

PET (30% Glass) 1630 180 12 3

Polystyrene 1050 40 3.0 1.5

Low Density Polyethylene 920 10 0.2 400

High Density Polyethylene 950 32 1.2 150

PTFE (Teflon) 2100 25 0.5 200

SAN 1080 72 3.6 2

UPVC (Rigid) 1400 50 3.0 80

PPVC (Soft) 1300 14 0.007 300

Polyester (DMC) 1800 40 9.0 2

Polyester (SMC) 1800 70 11.0 3

MaterialDensity (Kg/m3)

Tensile Strength (MN/m²)

Flexural Modulus (GN/m²)

% Elongation at Break

Phenolic (Mineral Filled) 1690 53 8.0 0.8

Polyamide-imide 1400 185 4.5 1.2

Polycarbonate 1150 65 2.8 100

Polyethersulphone 1370 84 2.6 60

Polypropylene 905 33 1.5 150

Polysulphone 1240 70 2.6 80

Fatigue:

Plastics are susceptible to brittle crack growth fractures as a result of cyclic stresses. Plastics are also prone to thermal softening if the cyclic stress or rate is high. The best plastics are Polypropylene (PP), Ethylene-Propylene copolymer and PVDF. (Self Hinge application).

Toughness:

By toughness we mean the resistance to fracture. At room temperature the unreinforced plastics include Nylon 66, LDPE, LLDPE, EVA and Polyurethane structural foam. At sub-zero temperatures it is necessary to consider plastics such as ABS, Polycarbonate and EVA.

Degradation:

Physical or Chemical attack:

Plastics are best corrosion resistance material. Plastics are susceptible to chemical attack and degradation. Degradation of plastics is also caused by heat, stress and radiation. Generally Crystalline plastics offer better environmental resistance than Amorphous. Nylon 66, PEEK & PPS. Noknown solvent at room temperature for PP, PE, PPS & PEEK.

Weathering:

This generally occurs as a result of the combined effect of water absorption and exposure to Ultra-Violate radiation.

Oxidation:

This is caused by contact with oxidising acids, exposure to UV.

Wear Resistance and Frictional Properties:

The use of plastics in bearing application and in situation where there is sliding contact e. g. gears, piston rings, seals cams etc. The advantage of plastics are low rates of wear in the absence of conventional lubricants, low cof. The ability to absorb shock and vibration with the ability to operate with low noise and power consumption. The plastics with the best resistance to wear are Ultra High Molecular Weight Polyethylene (used in Hip joint replacement) and PTFE lubricated Polyamide (Nylon), Acetal & PBT

The plastics with the best resistance to wear are Ultra High Molecular Weight Polyethylene (used in Hip joint replacement) and PTFE lubricated Polyamide (Nylon), Acetal & PBT. It is NOT recommended to use the same plastics for both mating surfaces in applications such as gear wheels.

Material Coefficient of Friction Relative Wear RateStatic Dynamic

Nylon 0.2 0.28 33

Nylon/Glass 0.24 0.31 13

Nylon/Carbon 0.1 0.11 1

Polycarbonate 0.31 0.38 420

Polycarbonate/Glass 0.18 0.20 5

Polybutylene Terephthalate (PBT) 0.19 0.25 35

PBT/glass 0.11 0.12 2

Polyphenylene Sulphide (PPS) 0.3 0.24 90

PPS/glass 0.15 0.17 19

PPS/carbon 0.16 0.15 13

Acetal 0.2 0.21 -

PTFE 0.04 0.05 -

Typical wear rates for different plastics.

Special Properties:

Thermal Properties:

Properties of Plastics are Temperature dependent. Glass Transition Temp. below which the material behaves like Glass. For example Polystyrene and Acrylic are below their Tg at room temp. The material is in Glassy state. Same PE is above its Tg and hence very Flexible.

Special Properties:

Electrical Properties:PTFE & PE are among the best insulating materials available.Insulators should have Resistivity>104.

Local breakdown may occur due to tracking and PTFE, Acetal, Acrylic & PP/PE copolymers offer very good resistance.

Plastics can be made conductive for special applications. (Microwave Oven).

ConveyorConveyor

Hair dryer housingHair dryer housing

GearsGears

Compact diskCompact disk

Water bottlesWater bottles