Composite parts have replaced many metallic components because they offer similar performance but at lower weight. Most composites consist of thermoset resins, such as epoxy, impregnated into a reinforcing material such as glass or carbon fibre. During production, resin molecules cross-link and set permanently. This curing process may require hours at elevated temperatures and pressures inside an industrial oven, an autoclave.
Now, epoxies are being replaced in favour of thermoplastic materials which can be moulded when warm but do not cure permanently. Instead, these polymers provide the really useful property of having a glass transition temperature substantially below their melting point, which means, in layman’s terms that, as the temperature rises, they become malleable before they melt.
As technology firm Cygnet Texkimp describes it, “the polymer is simply melted and set to create prepreg material, for example, which can be remelted and set to form parts.” Prepreg refers to the semi-processed material of resin mixed into reinforcement. To simplify part forming in complex shapes, and to optimise the amount of product consumed, thermoset composites are often supplied pre-impregnated within a woven reinforcing fibre mat (the prepreg), which is then cut, shaped and heated – often under pressure – to cure.
Switching to thermoplastic resins from thermoset offers a number of benefits, including out-of-autoclave processing, as forming requires only heat for melting and cool for setting (and this could be done in multiple cycles). Nor do they need to be kept frozen – as some thermoset prepregs are – to ward off the curing reaction.
Thermoplastics permeate our daily life in the form of drinks bottles (PE), toys (ABS) and signs (PMMA) – as pictured above – so material recycling schemes already exist, offering a greener alternative to thermoset composites, whose recyclability remains difficult.
“With the aerospace industry’s increased focus on sustainability, lightweighting and cost, as well as a need for faster methods of manufacturing, we expect to continue to see a significant move to thermoplastic composites by OEMs over the next 5-10 years,” said Gareth Deering, commercial sales manager at plastics firm Denroy, at last year’s Farnborough Air Show.
At the event, it announced its involvement in a three-year, £3.8m Innovate UK-funded R&D programme on thermoplastics with Spirit Aerosystems and the Northern Ireland Technology Centre at Queen’s University, Belfast. That research investigates producing parts for aircraft wings using a hybrid injection moulding process, adding short and long-strand carbon fibre reinforcements. Sample parts will be tested in harsh conditions.
PROCESSING
Injection moulding is just one of the processing methods available to thermoplastics; others include overmoulding (when integrated into injection moulding), automatic tape layup, thermoforming and plastic welding (creating seamless joints), according to resin supplier Toray Advanced Composites. Under the Cetex branding, it offers thermoplastic resins including PEI, PPS, PEEK, PC, PMMA, PET, PP, HDPE in a range of products including UD (unidirectional) tape, woven prepreg, laminates and cast parts. It also published a white paper on the subject available via www.is.gd/ranimi.
The company announced in March that it has won accreditation for a new NADCAP aerospace supply chain audit standard, ‘Non-metallic materials manufacturing – thermoplastic prepreg manufacturing AC7124/6’. At that time, it claimed to be the ‘first and only advanced composite company’ to have received that recognition. The audit standard covers three types of processes: liquid polymer application (in which the resin is in a kind of slurry), molten polymer application, in which the resin is applied as a melted liquid at high temperature, and solid polymer application, in which a film or powder is applied and then processed at high temperature.
Automatic tape layup processes have seen some recent successes when twinned with thermoplastic resins. For example, in January, UK firm Victrex announced it had received aerospace approval for thermoplastic UD tape Victrex AE 250-AS4, which incorporates Hexcel’s HexTow carbon fibre in a Victrex proprietary thermoplastic polymer, LMPAEK (polyarlyetherketone), whose melting temperature is 40°C less than PEEK at 305°C, making it easier to process.
For aerospace programmes, it was approved for inclusion in an allowed materials database by the US-based National Center for Advanced Materials Performance (NCAMP). Other reinforcement tapes listed there include the Teijin Tenax-E TPWF/TPCL PEEK and Toray T700 UD tape. Victrex said the qualification involved was a two-year project.
In 2020, Victrex announced that had made a 32mm-thick panel of the stuff using an automatic fibre placement method, to a thickness said never to be achieved outside of an autoclave. The 120cm by 60cm panel consisted of 176 plies of material, which was processed at 280°C, followed by an oven cycle. The process was developed by Coriolis Composites, for Daher.
Another Victrex partner, Electroimpact, announced a few years ago that it had developed a unidirectional tape layup process that increased speeds to 4,000 inches per minute. The layup was heated by a variable-spot size laser. With the process, “...for the first time that we know of, thermoplastics are able to achieve thermoset lay-up speeds,” comments Michael Assadi, Electroimpact chief engineer.
Also developing thermoplastic production processes at an industrial scale is Cygnet Texkimp, which announced last year that it had built a thermoplastic lab line (pictured, left), which, at 5m by 1m, is half the size of the full-scale line first developed in 2020. The company claims it is the first commercially-available thermoplastic composite line that takes standard polymer pellets to create thermoplastic prepregs. The line is capable of producing product up to 600mm wide and as thin as 0.1mm. Compatible resins include PP and PEEK.
Now available for customer trials in a new innovation centre (see box), Cygnet Texkimp’s direct melt impregnation thermoplastic composite technology is said to be able to produce ‘strong, lightweight, durable, recyclable car parts and building materials’ at high volumes.
BOX: Composites prove-out space
Northwich, Cheshire-based machine builder and composites technology company Cygnet Texkimp has created an open-access innovation centre as a development, prototyping and testing space for the advanced materials and composites industry. The 15,000ft² R&D facility will house machines from across the company’s entire product range, including filament winding, prepreg processing, slitting and spooling, automation and recycling equipment. Organisations can reserve time in the centre to carry out trials to optimise and validate their process design, evaluate materials, and gather evidence to prove their business case or justify investment.
In addition to its thermoplastic processing line, which produces prepregs, it also offers:
- Multi-roll stack, a high-speed, short-footprint, vertically stacked prepreg manufacturing line
- High-precision slitter-spooler-rewinder to process UD (unidirectional) prepreg slit tapes
- 9-axis robotic filament winding system with a range of fibre feed and resin dosing systems capable of high tension and thermoplastic winding
- Multi-axis and 3D winders providing high-rate deposition for wound parts of varying geometry
- Automated filament winding cell showcasing Cygnet Texkimp’s work in high-rate manufacturing of composite components
- Composites reclaiming & recycling solutions including those powered by DEECOM
- Spread tape line for low crimp fabrics
- High-temperature consolidation line
- Automation demonstration equipment
- Automated guided vehicles (AGV)
- Fibre unrolling creels
