Over-engineering offers opportunities in plastics

The fact that many metal parts are over-engineered gives designers an opportunity to re-design components successfully using plastics Delegates at Eureka’s ‘Designing in Plastics’ Design Day – held on 29 March at the Kaetsu Centre in Cambridge – heard that plastics usually have inferior mechanical properties to metals. But where they failed in stiffness, strength and temperature resistance, they compensated through their low density, toughness and design flexibility. “Use the design freedom that plastics can bring,” said plastics consultant John Hockey. “Plastic may be more expensive than the material you are replacing, but by combining functions you can actually save money.” He cited a number of case studies, including a tool changer. The original version comprised 160 aluminium castings, weighing a total of 15kg. It took four hours to assemble. The re-designed part was made from 22 identical, snap-fit acetal mouldings that took 20 minutes to assemble. Total weight was reduced to 5kg. While plastics can replace metals, it will never be a direct replacement, he stressed. “Recognise that plastic is not metal,” said Hockey. “Don’t start with a metal design and just make it in plastic or you’ll come a cropper.” Craig Norrey, technical programmes manager at polymer supplier DuPont, also urged delegates to recognise key differences between plastics and metals. “Plastics are more challenging design materials,” he said. “Converting the conservative ‘metal bashers’ is a challenge.” He added that while plastics could often be proved to have a lower system cost, it was important to factor in the total cost of switching over from metals. “If you replace metal with plastic, somebody has to sign that off – and it needs to be tested,” he told delegates. “The cost of testing is not usually included when you start calculating the cost of a new part – so this needs to be added to the overall cost of replacement.” One example where it worked was re-designing an air intake manifold in plastic instead of aluminium. The component was designed to meet actual requirements, rather than simply mimicking the performance of the aluminium original. “Part vibration was the key design challenge, due to the lower stiffness of the plastic part,” said Norrey. “This was controlled by bracing ribs.” The final component was made in two parts that are vibration welded together. This helped to dictate the choice of glass-filled PA6 – which is slightly cheaper than PA66, and easier to weld. The final weight saving was around 70%, while cost savings were 60%. An added bonus was a 1-2% increase in engine performance. “This was a bonus we were not expecting,” said Norrey. “It was not due to the weight saving, but the fact that the internal pipes are far smoother than they were in the aluminium part.” Frank views A common theme that emerged at the event was the need to get designers together early. This was in order to avoid situations where a product is designed and then thrown ‘over the wall’ for the housing design. Peter Frank, of new product design consultancy Product Innovation, warned that plastics had to be used intelligently within a design. “A plastics housing should be an integral part of the design and not an afterthought,” he said. “Components should help each other.” He argued that designers of all parts of the component should be brought together as early as possible. Often, he said, housings are simply designed to fit around a product: “It shouldn’t be that way.” He pointed to a product he had been asked to redesign – a portable carbon monoxide alarm, for use by fire-fighters – for half the cost of the original. In addition to better aesthetics – in terms of curved appearance and the use of coloured buttons – there were several important functional improvements. For example, the buttons push directly against the switches on the internal PCB. The PCB itself is held in place automatically when the two halves of the housing are snapped together. No screws are used, which drives cost down further. “Adding functionality costs you nothing, other than half an hour’s thought,” said Frank. “It can help you stay ahead of competitors.” The final cost was a quarter of the original. Alternative technique While most plastics parts are designed to be produced by one of the standard production methods – particularly injection moulding – delegates heard of an alternative production method that has been used to make bespoke parts in relatively high volumes. Stuart Jackson, regional manager for the UK and Ireland at EOS, said that laser sintering has begun to move from a prototyping technique into true manufacturing production. Where once it would be used to make tens of product, it can now make thousands of parts. He pointed to the example of hearing aids. As each person’s ear cavity is a unique shape, this is an ideal application for laser sintering. Manufacturer Phonak takes an electronic file – from a model of the person’s ear cavity – and produces a bespoke hearing aid housing. Laser sintering was also used to make the housing – and other parts – for a centrifuge. The advantage was that tooling costs were removed and the finished parts were of higher quality – despite a reduction in production costs. “Phonak has made 50,000 laser-sintered hearing aids so far – that’s 1400 per week,” said Jackson.