Cygnet Texkimp is a custom machine builder specialising in fibre processing technologies for the composites markets, including prepreg, coating, laminating, slitting and filament winding. Our company is well known for its carbon fibre processing machines based on thermoset principles – indeed we are one of the largest suppliers of thermoset prepreg technologies to the global aerospace industry.

Six years ago, we began developing an alternative to this technology using thermoplastics. Our thermoplastic manufacturing line can process a range of polymers including nylon, carbon and glass, through to PEEK. The technology has been developed to manufacture thermoplastic prepreg sheet that can be slit into tapes and used to form structures by filament winding or other tape-laying processes. This thermoplastic sheet complements other machines in our portfolio, including filament winding, 3D winding, slitting and automation (loading and unloading).

Enabling more aerospace manufacturers to explore thermoplastics is an exciting area of our work, but there are significant challenges that need to be overcome along the way.

Most of the composites used in aerospace today are based on thermoset technology, and it’s important to point out that this is still state-of-the-art technology with ongoing development to further enhance both materials and processes. However, the most advanced aircraft in the world use composite structures based on thermoset technologies first developed decades ago.

A thermoset is a resin or glue that is set during a chemical process to permanently change its structure. Thermoset composites are created by combining the thermoset with a fibre or mix of fibres such as carbon or glass. Often the fibre and resin are combined in a pre-impregnated sheet so that resin content can be precisely controlled throughout the end structure.

It’s a material whirl

In contrast, the plastics we see in everyday life are, almost without exception, thermoplastics – plastics that can be melted and re-melted repeatedly. They are relatively easy to handle, come with a range of useful properties, are quick to process and easy to recycle. So why does the wider composites industry, including aerospace, still opt for a system that the rest of the world left behind in the 1970s?

Firstly, the epoxy resins used in the manufacture of thermoset composites are well known, and in aerospace this really matters. After 50 years of development, the safety, consistency and reliability of thermoset technologies has been comprehensively proven. Once combined with the fibre and cured, thermosets create extremely solid laminates. Importantly, an epoxy resin system can be made to wet-out the fibre very effectively, which means all of the fibres are coated and held together with a minimum amount of resin to make a really strong and rigid composite part. This rigidity is not always desirable, but at least it’s understood.

However, epoxy resin systems used in the production of thermoset composites have some undeniable disadvantages that are becoming increasingly relevant – notably speed of manufacture, instability and environmental impact.

It’s not practical to make a large number of thermoset composite parts using existing techniques. Indeed, manufacturing using conventional thermoset technology is a laborious and involved process when compared to more widely used production techniques. This is down to the typically lengthy processes needed to manufacture thermoset parts, which involve curing and possibly post curing phases lasting several hours.

Stability of the resin system is also a concern for manufacturers. As soon as it’s mixed, the resin becomes chemically active and the countdown to its ‘out-life’ (the length of time the active resin remains useful) begins. Managing the resin means significant investment in additional processing equipment is essential, including mixing rooms, freezers, autoclaves and ovens to control the speed of this chemical reaction.

Finally, the environmental impact of thermoset technologies is a growing consideration. As well as the toxicity of the chemicals used in the production of thermosets, the chemically-fixed composite part cannot yet be easily recycled.

There is now an increasing drive in all transport sectors to reduce weight and therefore improve energy efficiency. Hundreds of projects worldwide are attempting to harness the benefits of composite construction in order to reduce component weight at a sensible cost and workable rate of production.

A proposed solution to these challenges – one that would make composites much more attractive in a much wider range of applications - is to use thermoplastics as a matrix instead of an epoxy resin. This is by no means a new methodology – chopped short fibres have been used to modify the properties of polymers for many years in injection mouldings – but it does offer some important advantages for aerospace and other markets.

For example, significant improvements in impact or heat resistance can be achieved more easily by choosing a suitable polymer at the outset to balance the needs of performance versus cost.

Thermoplastics offer a significantly faster and more cost-effective route to composite manufacturing than thermosets because there is no chemical reaction to be managed. Effectively, the polymer is simply melted and set to create prepreg material, for example, which can then be remelted and set to form parts in a multitude of ways.

Cost and space savings can also be made because a full-scale thermoplastic processing line has a considerably smaller footprint than a conventional thermoset machine. There is no need for resin mixing equipment, coating machinery or paper rewinds, for example, in the manufacture of thermoplastic composites. They don’t require ovens or autoclaves to process them, special handling equipment to manipulate them on and off the manufacturing line, or freezers to store and transport them.

Another interesting point from a machine building perspective is that thermoplastic processing lines are heated electrically, which is a cleaner alternative to conventional thermoset lines using oil heaters and water coolers.

When speed meets demand

All of this means that the process of manufacturing thermoplastic prepregs is significantly faster than that of thermosets and therefore potentially attractive in many different sectors. But herein lies a challenge: the current rate of carbon fibre production across the world isn’t great enough to sustain widespread take-up of thermoplastic processing techniques, which means manufacturers would quickly run out of carbon. It’s difficult to specify a material that can’t be reliably sourced, and this in turn stifles growth. With a carbon fibre production plant taking two to three years to get up and running, this is a challenge that will take considerable investment to address.

As I described previously, certification of thermoplastics is also a challenge, and a major reason why thermosets still dominate in aerospace. Introducing new technology into this market – particularly for the manufacture of structural parts - is a challenge that cannot be overlooked. However, the market for thermoplastics in non-structural parts, such as seats and their ancillaries, is expected to gather pace more quickly once production techniques have been refined.

In terms of their sustainability, thermoplastics offer exciting future-proofing benefits. For example, they can be welded, which is hugely significant because it means that they can be joined quickly and easily. Although some mechanical joining of components may still be required to satisfy regulatory requirements these may be moulded into the component.

They are also more recyclable, which gives them a growing advantage over thermosets as composites start to enter the mainstream. While the recycling of composites is also in its infancy, increased volume of materials will push innovation and investment in that area.

In the meantime, our objective is to work in collaboration with more aerospace manufacturers to develop thermoplastic technology in ways that support their specific goals and aspirations.

https://cygnet-texkimp.com