Inspired thinking

A new kind of injection moulding process developed at Warwick University could mean electronic circuits being embedded directly within car bumpers and roofs.

The technology, called IN-SPIRE, could allow a bumper to act as a reversing sensor. It also has the potential to turn roofs into photovoltaic cells to charge car batteries.

Developed by the Warwick Manufacturing Group in collaboration with German injection moulding equipment manufacturer Battenfeld, the team originally designed the technique for painting plastic car parts inside the injection moulding process.

‘We were looking at ways of avoiding the paint shop,’ said the project’s principal researcher Gordon Smith. ‘The coating of a vehicle is said to be 20 per cent of its overall cost. This fact, combined with the demand for environmentally-friendly processes, encouraged us to look at ways to improve the coatings and the techniques used to apply them.’

The process produces a thermoset coating on a thermoplastic moulding within a single injection cycle. The properties of the surface are similar to those that would be produced by conventional thermoset painting processes.

The patented technique begins when a powder coat is explosively sprayed into the mould cavity using a pressurised powder feed unit. By this gas blast, the powder evenly and randomly coats the cavity walls. The plastic substrate is then injected using traditional injection moulding, equipment, tooling and processes.

A heated tool softens the thermoset material and forms a film on the tool’s surface. The heat of the injected thermoplastic substrate cures the thermoset material before cooling. The cycle time depends on the cure reaction of the thermoset coating material, the whole process adding only a couple of seconds to typical injection moulding.

After seeing the success of this technique the researchers realised they could add layers into the process. First they tried it with multiple thin coats of paint — clear, colour and metallic. ‘Following on from that, we realised we could build up different conductivity,’ said Smith. ‘We could have a semiconducting film, an insulating paint and then a conductive paint on top so that we could build up function on the surface.’

The researchers demonstrated that polar particles such as pigment components can be manipulated and manoeuvred during the injection moulding process with techniques such as dye diffusion, which is commonly used in photo lab printers. In that application, colour dies are heated so they vaporise and permeate glossy paper before returning to solid form. The result is unpixelated photographs.

Warwick’s team is using a similar technique to develop areas of conductive pathways on an injection-moulded part. ‘If the injection moulding machine is charged with different materials, you can develop individual circuits depending on which injection gun you fire,’ said Smith.

Researchers are still deliberating on how to produce the exact design required for the electronic circuitry. The next challenge will be deciding how to commercialise a technology with such a wealth of possibilities.

Smith said the technology could go beyond the automotive industry. He envisages plastic drink bottles with moving displays created as an integral part of the bottle. It could also replace RFID tags hidden inside products for tracking. The whole plastic product could effectively be turned into a giant, impossible-to-remove-tag.

The process will, according to its developers, be ready for use in production cars within six months.

Smith has been researching the technology for 20 years and, while it seems like it might be coming to commercial fruition, he does not plan on an early retirement.

‘An academic is always looking for the next challenge,’ he said.