Thermoplastics meet car parts
A new technique could be used to mass produce lighter thermoplastic fibre composite car parts that are cheaper, safer and recyclable.
Derived from the conventional resin transfer moulding (RTM) method for making thermoset fibre composites, researchers at the Fraunhofer Institute for Chemical Technology (ICT) in Pfinztal, Germany, have developed a technique called thermoplastic resin transfer moulding (T-RTM). It is said to enable the production of stronger, lighter materials, suitable for large-scale use in vehicle construction (see image).
At the moment, many car makers use a thermoset matrix for such composite materials, but the approach is said to face a number of drawbacks – although the material has good technical performance and lightweight potential, it is difficult to efficiently mass produce and is non-recyclable. Furthermore, its brittle nature means that it can break into dangerous splinters in a collision.
Researchers at ICT say that the T-RTM method makes the use of thermoplastics more viable. The process involves inserting a pre-heated textile structure into a temperature-controlled moulding, ensuring the fibres are aligned with anticipated stress and injecting the moulding chamber with an
activated monomer melt that contains a catalyst and activator system.
‘Normally thermoplastic materials melt and solidify and remelt again,’ explains Dr Stefan Tröster, a spokesperson for the ICT. ‘You get some fibres in it, but the amount is limited by the viscosity of the honey-like polyamide. The RTM process we introduced means that the composite has a much higher fibre-volume. The more fibres you have in the matrix, the better your mechanical performance. In the end, it’s a thermoplast, but, with the behaviour of the fibres, it is like a thermoset.’
Components made using T-RTM are allegedly produced much quicker than thermosets due to the quicker chemical reaction, and can take as little as three minutes to set, compared to the 20 minutes required for thermoset parts. With this increased turnaround time, researchers claim it will be possible to produce up to 100,000 parts a year. With the cost of thermoplastic matrix material, and its processing, said to be up to 50% lower than the equivalent costs for thermoset structures, the research could have an impact on the automotive industry.
Of particular interest to car manufacturers is that not only can T-RTM components be up to 40% lighter than other composites, but they have higher crashworthiness than thermoset plastics, according to the ICT team. ‘They are less brittle than thermosets so they have a much better crash performance,’ Tröster says. ‘A thermoplastic material is more ductile and absorbs energy differently to a thermoset. If the fibres break or pull out, they disseminate a lot of energy and that is the main factor behind their high crash performance.’
The researchers also suggest that the T-RTM’s parts are fully recyclable (compared to thermoset parts). They can be shredded, melted down and reused to produce high-quality parts.
Dr Rod Martin, CEO of the Materials Engineering Research Laboratory in Hertfordshire, UK, and a specialist on fracture mechanics and automotive composites, says, ‘Their technique for T-RTM of thermoplastic parts is a step forward for the rapid manufacture of thermoplastic composites. I guess at present this will be for medium range of production’.
However, he notes, ‘It is not just about the material, but the geometry of the fibre. A laminated thermoplastic material will delaminate, as will a thermoset, and a braided RTM thermoset will not.’ The team at ICT says it has achieved optimum fibre placement for crashworthiness.
Martin also suggests the need for more consideration on recyclability. ‘There are still challenges to recycling them efficiently,’ he adds. ‘It takes energy and if they are contaminated with oil they end up going into pretty low- grade recycled products.’
Nevertheless, Tröster is confident that the T-RTM research will dovetail with other industry innovations, and the wider automotive manufacturing industry is already adapting the injection moulding processes necessary to implement T-RTM.
‘Hundreds of thousand of companies in Europe are working with injection moulding – chemical companies, clients, manufacturers. It’s another step towards increased productivity because it is standardised plastic processing, and it’s very cheap and well established,’ says Tröster. ‘The trend is towards working with lightweight materials.’