Pressing ahead with sustainable composites

For years now, engineers have been struggling with a paradox. To make vehicles lighter, and hence more efficient, you often need to adopt materials that are more energy-intensive to process, and sometimes harder to recycle. That’s particularly true of composites. Using traditional techniques, carbon fibre production is around 14 times more energy-intensive than steel production.

Part of the problem is that thermosetting composite parts such as body panels, seat frames or wheels generally need to be cured by the application of vacuum, heat and pressure in an autoclave. Maintaining the heat and pressure required in an autoclave is expensive – circa £500 per cycle and running into the thousands for a part like a Formula 1 tub that requires around eight curing cycles.

For applications where light weight is critical but the extremely high strength-to-weight ratio of traditional carbon fibre isn’t paramount, there is a more affordable and environmentally sustainable option in the form of compression moulding. Instead of an autoclave, this uses a heated press to apply heat and pressure to cure the part, slashing the cycle times. The lower costs and shorter cycle times of compression moulding make it a more attractive proposition for mass production. And when it comes to environmental impact, medium-to-high volume applications are where the real differences can be made.

Unfortunately, compression moulding has its own issues. A conventional hydraulic press requires pressure to be applied continuously throughout the process, so it’s still a relatively high-energy process. What’s more, there are limitations to how precisely the curing process can be controlled, which makes it less well-suited to resins with a high bio content.

Retrac Composites believes it has a solution in the form of a new electric servo-driven press. Combined with the company’s own specification of control software, it’s said to deliver parts to a higher quality at a faster rate than when using conventional composite materials, but it also makes it possible to achieve the required quality using natural fibres and resins with a higher bio content.

The most significant development is the design of the press itself. Controlled using four or more servo-driven screws, it’s said to offer a much finer degree of control

The Swindon-based company has been a key supplier to more than half of the Formula One World Championship-winning constructors over the past 25 years. New CEO Dan Walmsley, who was brought in to head up an employee buy-out at the beginning of 2021, was formerly the motorsport director at McLaren Automotive. Prior to that he led Strakka Racing to victory in the LMP2 class at the Le Mans 24 Hours. Now he’s steering Retrac towards a more sustainable future.

“Sustainable bio resins tend to have a narrower performance window than traditional epoxy resins,” he explains. “Our technique makes it possible to use resins with a much higher bio content and still achieve the same quality results. We can use almost twice as much bio content as would normally be possible with compression moulding.”

Retrac has developed its own range of materials, including both sustainable and recycled composites. But the most significant development is the design of the press itself. Controlled using four or more servo-driven screws, it’s said to offer a much finer degree of control, which enables the use of less forgiving chemistries. A series of sensors across the mould feed information to a closed-loop control system that ensures the entire part cures correctly. What’s more, because the servos are only active when each individual screw is adjusted, it requires significantly less energy to operate.

“The minutiae of the pressure is extremely important to manage the viscosity and the behaviour of the resin during wet pressing,” Walmsley points out. “It’s the very last tenths of a millimetre when closing the press where it really counts.”

Using a 60-tonne, servo press with a built-in vacuum system, Retrac says it’s possible to produce a part in around four minutes. An autoclave cycle for a similar part is likely to be three to six hours. Even factoring in an autoclave’s ability to cure multiple parts at once, the company says it speeds up production by approximately a factor of 10, while using considerably less energy.

“Volume attainability is largely determined by the length of time it takes to manufacture a component,” comments Tim Scott, CCO of Retrac. “We’re looking at a cycle time of four-to-six minutes for a typical component, so you’ve got bio composites being produced at a volume rate.”

Although bio materials are not generally suitable for highly-loaded structural parts with the current techniques, there are plenty of applications where they can be just as useful as conventional composites, he says: “I think structural composites with bio materials will come. In some cases it’s already possible to hybridise the design with a mixture of, say, aluminium and bio composites used in different areas. But there are other areas where the strength-to-weight ratio of conventional carbon fibre isn’t strictly necessary. We see motorsport teams making tools out of carbon fibre, for instance. They do it because it makes them lighter to transport around the world, but they don’t actually need the strength.”

Other examples include bodywork and interior systems in both heavy good vehicles and light commercial vehicles, where payload capacity and energy efficiency are both key targets. Non-structural parts in high performance road cars are another potential area. Here, traditional carbon fibre, with its distinctive weave, is often valued for its aesthetic finish and its high-performance image as much as its physical properties, but that may not always be the case.

“People are paying more attention to the sustainability of the materials used in cars, so it may be that having lots of visual carbon fibre on display one day becomes the automotive equivalent of walking around in a mink coat. It may not be socially acceptable, even in applications where the cost of traditional carbon fibre is not an issue,” says Scott.

Retrac’s process typically uses low-cost chopped fibres, which provide a random pattern with an attractive silvery black finish. The resulting material can be lacquered or painted, but no specific post-processing is required beyond the trimming and inspection. This, the company points out, further reduces the processing costs.

Volume is another important aspect of the process. Although one of its key selling points is the low cost of the materials and the operation, the press requires metal tooling, which comes with a significant up-front investment. Retrac estimates that the break-even point typically sits at around 1,500 units – roughly speaking, that’s a little more than Morgan’s annual production levels, but comfortably below the likes of Aston Martin and McLaren in a normal year. At present, Retrac is looking to scale the business by manufacturing in-house, but the company says there’s also the potential to commercialise the press and its control technology for third parties. “We see it as an opportunity both on a component level and a system level,” comments Walmsley. “The world has to go through a lightweighting revolution to meet the ambitious Net Zero targets that have been laid out.”

Composites have a clear role to play in that weight reduction strategy. But there’s also a growing awareness among engineers and consumers that these efficiency benefits cannot come at the expense of increased energy usage elsewhere in the product’s lifecycle. This sort of holistic approach – considering materials, production techniques and end of life recycling – will surely become a necessity.