Which type of composite material is right for you?

Make no mistake, composites are on the increase. The perception that the material is an exotic choice for engineers with big budgets is becoming more dated by the day. Composites are now filtering down from industries such as motorsport and aerospace, and finding more applications, in more industry sectors.

That said, composites are still regarded as a premium material. They are not cheap, but implemented correctly, and effectively, the material can be cost effective.
Two distinct advanced matrix systems are available to make a composite; a thermoset resin popularly a polyester or epoxy, or a thermoplastic such as a polypropylene.

Thermoset composites have been successfully – and increasingly – applied over the last 15 years in many industries including Formula One and the airliner industry. Thermoplastic composites on the other hand are a much newer development. Though long fibre thermoplastics and glass filled polyamides are examples of successful application, some of the newer innovations in the material are currently seeking out more significant uptake.

Materials innovator DuPont is putting significant effort and technical development in to its thermoplastic composite materials in the hope it can answer many of the volume production reservations that the automotive industry has expressed.

"Our success has been built on replacing metal and other materials with our plastics," says Mark Young, marketing director at DuPont. "And now we want to include thermoset composites within that. Thermoplastic composites are progressively penetrating in to applications occupied by thermosets, as well as higher volume applications traditionally reliant on metals."

Thermoset and thermoplastic composites both have high expectations in providing the solution to industry's weight problem. However, part of that solution must also address major concerns around processability. Autoclaves cause a bottleneck in the production of thermoset composites, and that is an instant turn off for many, even those producing at moderate volumes. While Resin Transfer Moulding (RTM) goes someway to address the need for 'out-of-autoclave' thermoset composites, it is neither quick nor easily automated.

While thermoplastic composites are newer to market, they offer some fairly distinct advantages that should be considered. The supply of the material is increasingly coming to market in preformed sheets, where a fibre has already been infused by a polypropylene, for example.

DuPont intends to target the automotive market with these sheets to allow carmakers to stamp form the sheets in much the same way as they do currently with sheet metal. It might even be possible to use existing metal tooling, with only slight modification needed for pre-heating the thermoplastic sheets.

This approach sits well with many as it will allow a cost effective route toward composite uptake and deliver an easily implemented volume production process without the need to buy new machinery, and in particular avoid the expense of an autoclave oven. However, while this might provide a bridge to encourage wider uptake, exploiting the true potential of composite materials - and the possible weight reduction - would still be a way off.

"Stamp forming of thermoplastic composite sheets is no doubt attractive to those people and industries that have been stamp forming metals, and want to continue to do so with composites," says Chris Hare, a technology project manager at NetComposites. "But it is not really making use of the possibilities of composites, and it leaves a lot of room for optimisation. Composites allow much more flexibility in design and there is a lot of potential for variation in properties given fibres, matrix systems, lay-up and processing. So, while stamp forming will be good for some applications , it is certainly not the ultimate solution for everything."

Correct and effective application of composites to essential to getting the full benefit. And that needs clever design early on. This usually means a reduction in part count, a philosophy users of thermoset composites already have well established.

Given the expense of thermoset composites, users are always keen to get the most value from the material and that means large one-piece parts, hence so many composite monocoque chassis. And, from a production point of view, due to the immature nature of thermoplastic composites, the material has perhaps more restrictions in geometry and surface finish than the more establish thermosets.

"We are looking at using thermoplastic composites as body panels and one of the routes to getting a Class A surface finish is actually not to use woven materials but to instead use stitched or non-woven materials," says Hare. "This avoids the crimped effect and helps avoid the build up of any pockets of resin. Many thermoset processes use post-moulding finishes on the surface to get a better surface finish, but that is expensive."

How can you decide?
Both thermoset and thermoplastic composites are going to increasingly get specified by engineers. However, a major concern remains around the quality and security of the supply chain. Many automotive OEMs have gone so far as to set up and implement their own. And this throws up even more questions of testing and standardisation. With so many combinations of fibres, fibre length, weave pattern, matrix system, and manufacture, many are concerned about correct specification. So there is still plenty of work to be done to convince the masses.

Justin Cunningham

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