Long dependent on carbon fiber thermoset materials for manufacturing high-strength composite structural parts for aircraft, aerospace OEMs are now embracing another class of carbon fiber materials as technological advancements promise automated manufacturing of new non-thermoset parts. high volume, low cost and lighter weight.
While carbon fiber thermoplastic composite materials have “been around for a long time”, it’s only recently that aerospace manufacturers have been able to consider their widespread use in manufacturing aircraft parts, including primary structural components. said Stephane Dion, vice president of engineering for Collins Aerospace’s Advanced Structures unit.
Carbon-fiber thermoplastic composites potentially offer aerospace OEMs several advantages over thermoset composites, but until recently, manufacturers could not manufacture thermoplastic composite parts at high speeds and at low cost, it said. he declares.
Over the past five years, OEMs have begun to look beyond manufacturing parts from thermoset materials as the state of the science of manufacturing carbon fiber composite parts has developed. first to use resin infusion and resin transfer molding (RTM) techniques to manufacture aircraft parts, then to use thermoplastic composites.
GKN Aerospace has invested heavily in the development of its Resin Infusion and RTM technology for manufacturing large aircraft structural components at an affordable price and premium rates. GKN is now manufacturing a 17-meter-long one-piece composite wing spar using resin infusion manufacturing, according to Max Brown, vice president of technology for GKN Aerospace’s Horizon 3 advanced technology initiative.
According to Dion, large investments by OEMs in composite manufacturing in recent years have also included strategic spending to develop capabilities to enable high-volume manufacturing of thermoplastic parts.
The most notable difference between thermoset and thermoplastic materials is that thermoset materials must be kept cold before being shaped into parts, and once shaped, a thermoset part must be cured for several hours in an autoclave. . The processes require a lot of energy and time, and therefore the production costs of thermoset parts tend to remain high.
Curing irreversibly changes the molecular structure of a thermoset composite, giving the part its strength. However, at the current stage of technology development, hardening also renders the part material unsuitable for reuse in a primary structural component.
However, thermoplastic materials do not require cold storage or curing when made into parts, according to Dion. They can be stamped into the final shape of a simple part – each support for the Airbus A350 fuselage frames is a thermoplastic composite part – or into an intermediate stage of a more complex component.
Thermoplastic materials can be welded together in a variety of ways, allowing complex, highly streamlined parts to be made from simple substructures. Today, induction welding is mostly used, which only makes it possible to produce flat parts of constant thickness from sub-parts, according to Dion. However, Collins is developing vibration and friction welding techniques to join thermoplastic parts, which, once certified, should eventually allow him to produce “really advanced complex structures,” he said.
The ability to weld thermoplastic materials together to create complex structures allows manufacturers to eliminate the metal screws, fasteners, and hinges required by thermoset parts for assembly and bending, creating a weight reduction benefit of approximately 10 %, says Brown.
Still, thermoplastic composites bond to metals better than thermoset composites, according to Brown. While industrial R&D aimed at developing practical applications for this thermoplastic property remains “at an early technological readiness level”, it could eventually allow aerospace engineers to design components containing hybrid thermoplastic and metal integrated structures.
A potential application could, for example, be a lightweight one-piece airliner passenger seat containing all of the metal-based circuitry necessary for the interface used by the passenger to select and control their in-flight entertainment options, lighting of the seat, the ceiling fan, electronically controlled seat tilt, opacity of the blinds and other functions.
Unlike thermoset materials, which must cure to produce the required stiffness, strength and shape of the parts they are made from, the molecular structures of thermoplastic composite materials do not change when they are formed into parts, according to Dion.
As a result, thermoplastic materials are much more resistant to fracture on impact than thermoset materials while providing similar, if not stronger, structural toughness and strength. “So you can conceive [parts] to much thinner gauges,” Dion said, which means the thermoplastic parts weigh less than all the thermoset parts they replace, even outside of the additional weight reductions that result from the thermoplastic parts not requiring screws or metal fasteners.
Recycling thermoplastic parts should also be a simpler process than recycling thermoset parts. In the current state of technology (and for some time to come), the irreversible changes in molecular structure produced by the hardening of thermoset materials prevent the use of recycled materials to manufacture new parts of equivalent strength.
Recycling thermoset parts involves grinding the carbon fibers of the material into short lengths and burning the fiber and resin mixture before reprocessing. The material obtained for reprocessing is structurally weaker than the thermoset material from which the recycled part was made, so recycling thermoset parts into new ones generally transforms “a secondary structure into a tertiary structure,” Brown said. .
On the other hand, because the molecular structures of thermoplastic parts do not change in the part manufacturing and assembly processes, they can simply be melted into liquid form and reprocessed into parts as strong as the originals, according to Dion.
Aircraft designers can choose from a wide selection of different thermoplastic materials available for designing and manufacturing parts. “A fairly wide range of resins” are available into which one-dimensional carbon fiber filaments or two-dimensional weaves can be embedded, producing different material properties, Dion said. “The resins of most interest are the low-melting point resins,” which melt at relatively low temperatures and can therefore be shaped and formed at lower temperatures.
Different classes of thermoplastics also offer different stiffness properties (high, medium, and low) and overall quality, according to Dion. The highest quality resins cost the most and affordability represents the Achilles heel of thermoplastics compared to thermoset materials. Typically, they cost more than thermosets, and aircraft manufacturers need to factor that fact into their cost-benefit design calculations, Brown said.
Partly for this reason, GKN Aerospace and others will continue to focus primarily on thermoset materials when manufacturing large structural parts for aircraft. They already make extensive use of thermoplastic materials to manufacture smaller structural parts such as empennages, rudders and spoilers. Soon, however, when high-volume, low-cost manufacturing of lightweight thermoplastic parts becomes routine, manufacturers will use them much more widely, especially in the booming eVTOL UAM market, Dion concluded.