CENTRAL INSTITUTE OF PLASTICS ENGINEERING & TECHNOLGOY GUINDY, CHENNAI - 600 032 MATERIAL SELECTION CRITERIA FOR SPECIFIC END USE Dr.T.O. VARGHEESE CIPET, CHENNAI
How do we decide which Plastics, if any,is best for a particular application?Material Selection is not as difficult as it mightappear but it does require an awareness of thegeneral behaviour of plastics as a group , aswell as a familiarity with the specialcharacteristics of individual plastics.
Need Recognition Functional Specification Concept Generation and EvaluationPart Design Process Design•Layout/drawing •Manufacturing Method Selection•Constrains •System layout•Analysis •Integration of system•Material Choice •Manufacturing procedure Prototype creation & Verification Production Implementation
First Step – Important To define clearly the purpose & function of the proposed product and to identify service requirements.Second StepAssess the suitability of a range of candidatematerials.
Most Important characteristics requiringconsideration for most Engineering components.• Mechanical Properties –Strength, Stiffness, Fatigue, Toughness andthe influence of high or low temperature.
1.Corrosion susceptibility and degradation2. Wear resistance & Frictional property3. Special Property requirement for example:Thermal, Electrical, Optical, Magnetic &Damping capacity.• Moulding and or other method of Fabrication.• Total cost (material & Manufacturing).
Fatigue:Plastics are susceptible to brittle crack growthfractures as a result of cyclic stresses. Plastics arealso prone to thermal softening if the cyclic stressor 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 plasticsinclude Nylon 66, LDPE, LLDPE, EVA andPolyurethane structural foam. At sub-zerotemperatures it is necessary to consider plasticssuch as ABS, Polycarbonate and EVA.
Degradation:Physical or Chemical attack:Plastics are best corrosion resistance material.Plastics are susceptible to chemical attack anddegradation. Degradation of plastics is alsocaused by heat, stress and radiation. GenerallyCrystalline plastics offer better environmentalresistance than Amorphous. Nylon 66, PEEK &PPS. Noknown solvent at room temperature forPP, PE, PPS & PEEK.
Weathering:This generally occurs as a result of the combinedeffect 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 insituation where there is sliding contact e. g. gears,piston rings, seals cams etc. The advantage ofplastics are low rates of wear in the absence ofconventional lubricants, low cof. The ability toabsorb shock and vibration with the ability tooperate with low noise and power consumption.The plastics with the best resistance to wear areUltra High Molecular Weight Polyethylene (used inHip joint replacement) and PTFE lubricatedPolyamide (Nylon), Acetal & PBT
The plastics with the best resistance to wear are UltraHigh Molecular Weight Polyethylene (used in Hip jointreplacement) and PTFE lubricated Polyamide (Nylon),Acetal & PBT. It is NOT recommended to use the sameplastics for both mating surfaces in applications such asgear wheels.
Special Properties:Thermal Properties:Properties of Plastics are Temperature dependent.Glass Transition Temp. below which the materialbehaves like Glass. For example Polystyrene andAcrylic are below their Tg at room temp. Thematerial is in Glassy state. Same PE is above itsTg and hence very Flexible.
Special Properties:Electrical Properties:PTFE & PE are among the best insulatingmaterials available.Insulators should haveResistivity>104Ω.Local breakdown may occur due to tracking andPTFE, Acetal, Acrylic & PP/PE copolymers offervery good resistance.Plastics can be made conductive for specialapplications. (Microwave Oven).