material selection criteria

1. CENTRAL INSTITUTE OF PLASTICS ENGINEERING & TECHNOLGOY
GUINDY, CHENNAI - 600 032
MATERIAL SELECTION
CRITERIA FOR
SPECIFIC END USE
Dr.T.O. VARGHEESE
CIPET, CHENNAI

2. 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.

3. Need Recognition
Functional Specification
Concept Generation and Evaluation
Part Design Process Design
•Layout/drawing •Manufacturing Method Selection
•Constrains •System layout
•Analysis •Integration of system
•Material Choice •Manufacturing procedure
Prototype creation & Verification
Production Implementation

4. First Step – Important
To define clearly the purpose & function of
the proposed product and to identify service
requirements.
Second Step
Assess the suitability of a range of candidate
materials.

5. Most Important characteristics requiring
consideration for most Engineering components.
• Mechanical Properties –
Strength, Stiffness, Fatigue, Toughness and
the influence of high or low temperature.

6. 1.Corrosion susceptibility and degradation
2. Wear resistance & Frictional property
3. Special Property requirement for example:
Thermal, Electrical, Optical, Magnetic &
Damping capacity.
• Moulding and or other method of Fabrication.
• Total cost (material & Manufacturing).

7. Density Tensile Flexural %
Material (Kg/m3) Strength Modulus Elongation at
(MN/m²) (GN/m²) 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

8. Density Tensile Flexural
Material (Kg/m3) Strength Modulus % Elongation
(MN/m²) (GN/m²) 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
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

9. 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).

10. 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.

11. 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.

12. 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.

13. 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

14. 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.

15. Typical wear rates for different plastics.
Material Coefficient of Friction Relative Wear
Static Dynamic Rate
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 -

16. 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.

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

29. Conveyor

30. Hair dryer housing

31. Gears

32. Compact disk

33. Water bottles