Weaving the future

There is a global trend to lightweight parts across all industries. But while the materials have existed for decades, the manufacturing processes are only just coming of age.

Automated fibre placement (AFP) really is an amazing process to witness. It’s akin to watching a classical artist sculpting with molten plastic. And, it’s an additive process, with layer upon layer of reinforced polymers placed onto a mould surface.

Low-tack polymers such as prepreg or dry-fibre tapes can be laid at speeds of up to 1m/s and heated to hundreds of degrees with an infrared lamp or laser, onto a mould. The mould tooling can be constructed from a variety of materials including invar, steel, aluminium, reinforced silicone rubber, nickel or even carbon fibre.

The UK’s National Composites Centre (NCC) in Bristol is a state-of-the-art facility that brings together companies and academic institutions to develop composite materials and the corresponding manufacturing processes.

One of the member companies, Coriolis Composites, has built a huge robotic arm enclosure called the ‘Automated Fibre-Placement Cell’ that uses a magnificent arm that gracefully and speedily lays down polymer structures to make them ready for other processes such as stiffening or autoclaving. A number of these AFP machines are in place around the UK and France, giving a composites production capability that is faster, more efficient and more repeatable.

High-quality, low-volume parts such as Bombardier’s C-series aircraft aft-fuselage are produced using AFP techniques within minutes, resulting in lightweight parts with a relatively short cycle time. Other examples include Daher-Socata’s A350 main landing gear doors and Safran-Aircelle’s A320 nacelle parts. However, it’s not just aerospace parts that AFP is suitable.

The tooling headache

Tooling, to date, has however been a big headache for OEMs. Though lower operational costs are certain due to reduced fuel costs, it is the upfront investment in the complex tool that still puts many industries off.

High-volume automotive parts are also being produced using AFP but critically they are opting for a thermoplastic, as opposed the thermoset materials, to avoid the need to cure it.

Recently an unnamed car manufacturer began work with an ATP process, to produce a vibration damping systems as steel and aluminium didn’t work for them and short fibre reinforced nylon was too restrictive.

Coriolis saw the opportunity and presented the automated fibre placement process, which was received with ‘cautious interest’. Upon demonstrating the technology, a prototype was produced in just 35s.

Impressed with the result, optimised lay-ups are being looked at to produce short-fibre thermoplastic anti-vibration systems for vehicle engines. This would result in complex part cost reductions, vehicle weight reductions and much lower tooling costs.

Weaving the future

As part of the ‘Clean Skies’ initiative, Rolls Royce has challenged industry partners to lightweight its engine pipework. The performance criteria set was strict, with 50% weight reductions and 15 minute survivability in temperatures of 11,000C. That’s quite a challenge for a non-metal.

Sigma Precision Components, immediately pursued a composite solution. Skilled and well established in rigid pipe assemblies and precision machined components, it moved beyond its capabilities in heated pressing and out-of-autoclave curing, to provide innovative braided products.

Finding inspiration from the textile industry, it is able to braid many different fibres including carbon, glass and aramid, or a mixture of all three – utilising triaxial braiding methods if needed. Though it is unclear exactly how the thermoplastic and fibres form a complete matrix – i.e. how the part is ‘wet out’ – one theory is that the fibres are coated in a thermoplastic prior to braiding, and then heated to get the required uniform consistency throughout.

The main advantages of braiding are again cycle time and dramatically reduced upfront tooling investments. For example, a 250mm long, 130mm diameter tube can be braided in 5s, which is comparable to High Pressure Resin Transfer Moulding (HP-RTM) and more significantly, comparable to metal alloys.

Steve Barbour, managing director at Sigma says: “With thermoplastic braided composites, we can achieve with a few tools, what would have required hundreds of tools, using thermoset materials.”

Tooling is based often on a huge range of parameters including the end item tolerance, the desired or required surface condition, the method of cure and, if you’re using glass, the glass transition temperature of the material being moulded. Tooling also needs to consider the moulding method, the matrix type and numerous other considerations. It’s a lot to think about, so depending on which materials are used thermoplastic braided composites offer rapid and highly-automated production capability without the need for copious tooling requirements.

However, the benefits of the process are cumulative. There is very little wastage of materials and a higher-level of performance can be factored into designs, such as progressive failure modes for more durable requirements. In addition, it also creates a natural mechanism to evenly distribute load throughout the structure, meaning that braids are stronger, tougher, more flexible compared to woven fibres.

The next ‘next generation’

Sigma is also taking the flexibility of braided composites to the next stage, working with other leading OEMs to develop ‘smart’ composite parts that incorporate sensors for health monitoring and system feedback while in service. Tuned to the environments they experience in service (such as temperatures, pressures and torque forces), smart composite parts, including torque shafts and aircraft wing actuators, are already being developed with prototypes due to be produced by the end of 2017.

Due to the production speeds obtained, Sigma estimates its capacity for high-volume production of composite parts to be in the scale of 100,000 parts per year. Pipes, torque shafts, magnet retention sleeves, structural parts and pressure vessels are all likely to be on the menu.

A Commercial Opportunity

According to a recent Automotive Council assessment, there could well be a composites supply chain market to the tune of £2bn by 2030. That’s quite a chunk of change for those composite firms looking for motivation to invest in fast and efficient methods of manufacturing.

“The use of metal in the automotive industry can’t go on,” says Roger Ford director at Suffolk based Integrated Materials Technology. “Second generation composites are here and a huge opportunity awaits that will dwarf the existing composites industry. The automotive sector will be the engine for growth.”

But, it won’t be easy. The gold standards for production of parts in the automotive industry are cycle times of less than 120s. There are also quality and cost requirements to be met, which is why the introduction of composites in high volumes has been so slow.

However, the mindset of decision makers is softening and many feel it is time to break preconceptions by providing industry with hard and fast proof that composites are just as just as strong, just as durable and just as cost-effective as pressed metal.

“Today’s high cost and low productivity of thermosets have sparked a renewed interest in thermoplastic composites,” says Ford. “[The problem is] people always approach thermoplastics in the same way that they have done for thermosets, but thermoplastic composite parts have to be looked at and produced in an entirely different way.”

One company has set its sights on meeting and surpassing the 120s challenge. Global producer of specialty chemicals and materials Arkema has been driven by what it sees as the ever-increasing demand for high throughput of composite parts.

Sebastien Taillemite, composites business manager, says: “Arkema will be offering high volume, thermoplastic composites production using high-pressure resin transfer moulding, for up to 200,000 parts per year, per tool, within two to five years.

“Thermoplastic pultrusion will be going to commercial capability within one year.”

Its pultrusion method involves impregnating fibres with a thermoplastic ‘resin’ in a bath or by direct injection. However, a lack of available resins to carry out this process is currently railroading further developments.

“Our reactive resins and pultrusion process can hit the two-minute cycle time, but with the lack of resins available, we don’t expect this for one or two years.”

It is also refining its current thermo-stamping capabilities to reduce scrap material, which affects the final part cost and initial cost of the continuous fibre reinforced thermoplastic ‘organo-sheets’.

Recyclability benefit

There is also another key to cracking this market opportunity: recyclability. The End of Life Directive targets 85% of a vehicle by weight should be reused or recycled at the end of its life. Thermoset composite have long been criticised over the inability to recycle them, with plans to use the high end exotic material as road aggregate if a suitable method of reuse can’t be used. Conversely, thermoplastic composites can be safely and effectively recycled numerous times by reheating and remoulding, a key attribute for widespread uptake in the automotive industry.

Author
Dan Carmel

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