Composites ready for mass production of auto industry

Composites and the automotive industry are yet to answer the volume question. So will thermoplastics allow lightweight structures be produced in their millions quickly and at the right price? James Bakewell finds out.

Toyota's Mirai could be a game changer. The four seat sedan is the first hydrogen fuel cell powered vehicle to be produced in commercial numbers. It can travel just short of 500km on a single tank of hydrogen, can be refuelled in less than five minutes and the most harmful compound emitted by its powerplant is water.

Less likely to grab the headlines – but of significant interest to automotive structural engineers – is the fact that the Mirai is claimed to be the first mass produced vehicle to feature a carbon fibre reinforced thermoplastic (CFRTP) structural component.

The chief executive officer of market research company Lucintel, Dr Sanjay Mazumdar, says: "This is a significant development for mass produced vehicles. It demonstrates that suitable processing technology can be developed for the consistent manufacture of CFRTP parts in high volumes. Such moves by original equipment manufacturers (OEMs) will accelerate the use of CFRTP in more vehicle platforms."

The CFRTP component is the undercover of the Mirai's fuel cell stack and forms part of the floor of the vehicle. It is interesting to note that while Toyota is happy to make over 5,000 of its patents relating to hydrogen fuel cells available on a royalty-free basis, it refuses to even discuss the technology behind the production of the CFRTP stack frame.

However, what is known is that the stack frame is fabricated using a press moulding process developed by Toyota and carbon fibre manufacturer Toray, which the two companies claim is suitable for high-volume production. Like all carbon fibre reinforced polymer (CFRP) parts, it will be light and strong.

Furthermore, the two companies will have capitalised on the advantages that thermoplastics present over the thermoset matrices more commonly used in the automotive industry for the production of structural composite parts.

The need to lighten
Automotive OEMs are working hard to reduce the weight of their vehicles in the face of strict regulations on fuel economy and carbon dioxide emissions. For the production of lightweight structural components, they are increasingly looking to CFRPs. According to chemical giant BASF, the market for composites in automotive body and chassis applications will be worth €2 billion by 2025.

However, thermoset based CFRPs are expensive and the processes used to convert them into production ready components have traditionally been too slow for use in high-volume manufacture. Carmakers and composites producers are now looking to develop materials and processes that would solve these short comings.

First, existing production processes are being optimised, as is the case with the resin transfer moulding (RTM) process employed by BMW to fabricate the chassis of its i3 and i8 – currently the only mass-market vehicle to feature the extensive use of CFRP. Second, new material systems are being introduced, such as epoxies with drastically reduced curing times.

Despite these advances, the epoxy used by BMW still takes five minutes to cure at 100°C, and 10 minutes to de-mould – a long time for carmakers more used to being able stamp a metal component in a matter of seconds.

The road to commercialisation
Japanese chemical and pharmaceutical company Teijin and its partner General Motors are working on materials and processes that could enable the press-forming of continuous-fibre reinforced thermoplastic components in cycle times of under a minute.

Known simply as Sereebo, the materials technology has been blanketed in secrecy since its announcement in March 2011. Under development at the Teijin Composites Application Center (TCAC) in the USA, the Sereebo range comprises three intermediate materials.

The first, called U Series, is a unidirectional material offering high directional strength. The second, I Series, is isotropic offering a balance of shape, ease of moulding and multi-directional strength. The third, P Series, is a long carbon fibre reinforced thermoplastic (LFT) pellet, and is suitable for the injection moulding of components.

Dr Nick Weatherby, technical director of independent polymer composite specialist EPL Composite Solutions, says: "Thermoplastics possess a combination of recyclability, and can exhibit unique properties with regard to impact and resisting strain [fatigue]. They really are the future."

In contrast to thermosets, whose cure reaction cannot be reversed, thermoplastics harden when cooled yet retain their plasticity. It means they will remelt and can be reshaped by heating above their original processing temperature, and so can be more easily reused and recycled, unlike thermosets.

Furthermore, once melted thermoplastics harden quickly at relatively low temperatures, meaning that reinforced thermoplastic parts could be produced rapidly in short cycle times – especially using compression moulding processes.

In partnership with Cytec, EPL is working on UK-ECOPROCESS, a Composites Innovation Cluster project funded by the UK Government's Advanced Manufacturing Supply Chain Initiative (AMSCI), to develop methods for the production of low-cost, near-net shaped thermoplastic prepregs – reinforced with glass, carbon or aramid fibres – and suitable for a wide variety of recyclable components such as roofs, bonnets, boot lids and wheel arches.

Defying convention
Using conventional methods for the production of thermoplastic prepregs, thermoplastic fibres are woven with reinforcing fibres to create a fabric or tape. The methods being developed through UK-ECOPROCESS avoid the need for this step, reducing cost.

The base materials employed in the process are chopped rovings made from a thermoplastic and the chosen reinforcement.

Dr Weatherby says: "We send the rovings, which can be intermingled, co-mingled, co-extruded or a mixture of separate rovings, straight through a very specialised chopper system and then we use a robot to spray them onto a tool in an oriented way."

The resulting preform is then subjected to heat and pressure in a high-speed press to produce a part. The properties of the component can be optimised by adjusting the chopped length and orientation of the fibres.
According to Dr Weatherby: "This allows us to tune the materials as if they were woven, but of course, they are not."

The project partners are able lay down up to 2.5kg of chopped material a minute, and their hybrid chopper is able to process around 100 tonnes of glass based rovings without the need for maintenance — a significant improvement over conventional breaker or shearer choppers. This means that the process is capable of producing large amounts of thermoplastic composites in a cost-effective manner.

The disadvantage?
Of course, thermoplastics do have their drawbacks, such as high viscosity – which can limit volume fraction – and high melt temperatures; typically between 180 and 250°C.

These high melt temperatures make the capital equipment needed to process thermoplastics expensive, although as Dr Weatherby points out: "This is less of an issue for automotive and industrial OEMs because they tend not to concern themselves too much with the cost of capital equipment. It's more about the volume part price."

However, the high viscosity of many thermoplastics prevents them from wetting out fibre preforms in a consistent fashion, precluding them from use in the RTM processes favoured by many OEMs for the production of structural parts.

As a result, Volkswagen (VW), KraussMaffei and BASF have developed a low-viscosity reactive system for use with a thermoplastic resin transfer moulding (T-RTM) process.

To demonstrate the process, the partners have produced a continuous fibre reinforced polyamide (PA) B-pillar reinforcement, a part currently made by VW using high-strength steel. The composite component is 36% lighter than its metal counterpart.

The reactive system comprises a caprolactam, an activator or a catalyst, and additional additives supplied by BASF in the form of two ready mixed components. Above 70°C, the caprolactam melts and possesses a water-like viscosity, lower than that of many epoxies. This enables the monomer to efficiently wet the reinforcement at an injection temperature of around 100°C. The system then polymerises to a PA6 in an isothermically heated mould in approximately three minutes.

The injection and curing of the demonstrator B-pillar reinforcement took less than five minutes, and the project partners are confident they can reduce this time further.

Furthermore, BASF has launched a package of materials and services that it claims would enable the mass production of reinforced structural and semi-structural thermoplastic components. Called Ultracom, the package comprises of continuous fibre reinforced semi-finished products, specially adapted compounds for over-moulding products and complementing engineering support.

The semi-finished products – laminates based on woven fabrics and unidirectional (UD) tapes impregnated with Ultramid PA or Ultradur polybutylene terephthalate (PBT) – have been developed in partnership with TenCate Advanced Composites and Owens Corning.

The overmoulding materials – based on Ultramid and Ultradur compounds – have been developed specifically for use with these laminates. By using them in combination with the laminates and tapes, it is possible to mould complicated parts that are highly reinforced in precisely defined locations while incorporating other structures on their surfaces, such as ribs.

The company is currently developing applications for its T-RTM process and is working with a variety of partners. The company is also working with carbon fibre manufacturer SGL to develop thermoplastic compatible coatings for carbon fibres that increase the strength of the bond between the fibre and the resin.

Much like hydrogen fuel cell powerplants, the increasing use of thermoplastics could have a profound effect on the automotive industry. But, it remains to be seen exactly how the two technologies will evolve. Time will tell.

Justin Cunningham

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