TWI Frequently asked questions
Material selection should always be integral to the design process. The initial stages of design involve four key factors for success.
- Creativity
- e.g. using plastics for a new concept or a variation of an existing design
- Complexity
- e.g. think about method of manufacture and performance of the new design
- Compromise
- e.g. balance conflicting requirements of the plastic such as durability and cost
- Choice
- e.g. availability in colour or FDA (Food and Drugs Administration) approved grades
- Case Study:
- Plastic Car Bumpers
Changing from a chrome plated steel to a plastic car bumper required creativity in the designers thinking, and comparison between the physical, chemical and thermal properties of steel and plastics. Manufacturing plastic bumpersrequires completely different equipment and procedures involving the establishment of optimum injection moulding conditions and subsequent vibration welding to produce the box-beam bumper used on most modern cars.
For a steel fabricator who was accustomed to hot forming of steel sheet to produce bumpers, a decision to refocus the business to manufacture thermoplastic bumpers raises numerous technical and business issues (such as adopting newdesign approaches, the need to purchase plastics welding equipment etc.) which must be addressed.
Plastic bumpers are usually made from either a PC/PBT (polycarbonate/polybutylene terephthalate) alloy called Xenoy TM produced by GE Plastics or EPDM modified PP (ethylene propylene diene monomer modified polypropylene) produced by a number of PP manufacturers. (Borealis, Montell, BASF). The compromise between thephysical/chemical/mechanical/thermal properties of PC/PBT and PP is a balance the car manufacturer must make. In general, it is thought that PP bumpers are cheaper but PC/PBT bumpers are more durable. Both are recyclable, but PC/PBTretains its impact properties better than PP. Both come as body coloured bumpers.
To select either of the two materials, car manufacturers would conduct a material evaluation exercise to select the right combination of material properties to match their requirements. The properties of plastic materials can be found by contacting the plastics material manufacturers (contact The British Plastics Federation for details or Society of Plastics Engineers in USA). There are also a number of software packages widely available, including CAMPUS, which is produced by a consortium of plastics producers as a series of data for each grade of plastic manufactured.
