Often when specifying a particular grade for use in a redesigned or upgraded product, it comes down to the question of "What did you use last time?" Needless to say, this may not be the best choice according to Mark Lewis, general manager at Perrite (Johor, Malaysia), a division of Vita Thermoplastic Compounds Malaysia.
Speaking at the recent 3rd International Conference for South & Southeast Asia on Innovations in High Performance Engineering Plastics in Singapore, Lewis added that resin selection often takes a back seat among the 100 other things that the designer needs to address. Cost can also be the starting point, while the compliance and safety department can be over-influential, and there are also existing relationships with suppliers that may influence the final decision.
Lewis cites the example of the humble electrical socket back plate, which is typically molded from polyamide, PBT or PC. The back plate needs to be physically tough for installation, but just once, notes Lewis. And while thermal resilience is also required, "two hours at 100 deg C is not particularly demanding," he adds. In terms of flame retardance, V2 or V-0 materials are typically used in the back socket but in actual fact, this level is not demanded. "The BS1363 glow wire flammability index is sufficient," says Lewis.
The material must be injection moldable, but tight tolerances are not required and while back plates are typically colored black, "there is no need for any particular coloration because the component is not visible," according to Lewis. Perrite has shown that impact copolymer PP filled with a 10% loading of talc is more than capable of meeting system requirements, meeting glow wire tests and also being RoHS and Reach-compliant.
Perrite has worked on other projects to reduce raw material costs, in one case substituting a glass-filled PP for an impact-modified, 15% glass-fiber-reinforced PA66 grade in a home appliance application. "A holistic approach needs to be taken and in this case, we had to modify a TPU component that was also part of the assembly, reduce the flexural modulus of the PP, and modify the design of the component itself to eliminate a failure point," says Lewis. "We need to look at all aspects as opposed to the material in isolation."
"Another important issue when comparing alternative materials is to consider that not all resin suppliers quote the same properties in their literature," says Lewis. "Some quote the best properties achievable, some quote the lowest, conservative values, while some will quote typical values," notes Lewis.‑[email protected]