Long reliant on thermoset carbon-fiber materials for making very strong composite structural parts for aircraft, aerospace OEMs are now embracing another class of carbon-fiber materials as technological advances promise automated manufacture of new non-thermoset parts at high volume, low cost, and lighter weight.
While thermoplastic carbon-fiber composite materials "have been around a long time,” only recently could aerospace manufacturers consider their widespread use in making aircraft parts, including primary structural components, said Stephane Dion, v-p engineering at Collins Aerospace’s Advanced Structures unit.
Thermoplastic carbon-fiber composites potentially offer aerospace OEMs several advantages over thermoset composites, but until recently manufacturers could not make parts out of thermoplastic composites at high rates and at low cost, he said.
In the past five years, OEMs have begun to look beyond making parts from thermoset materials as the state of carbon-fiber composite part manufacturing science developed, first to use resin infusion and resin transfer molding (RTM) techniques to make aircraft parts, and then to employ thermoplastic composites.
GKN Aerospace has invested heavily in developing its resin-infusion and RTM technology for the manufacture of large aircraft structural components affordably and at high rates. GKN now makes a 17-meter-long, single-piece composite wing spar using resin infusion manufacturing, according to Max Brown, v-p of technology for GKN Aerospace’s Horizon 3 advanced-technologies initiative.
OEMs’ heavy composite-manufacturing investments in the past few years have also included spending strategically on developing capabilities to allow high-volume manufacturing of thermoplastic parts, according to Dion.
The most notable difference between thermoset and thermoplastic materials lies in the fact that thermoset materials must be kept in cold storage before being shaped into parts, and once shaped, a thermoset part must undergo curing for many hours in an autoclave. The processes require a great deal of energy and time, and so production costs of thermoset parts tend to remain high.
Curing alters the molecular structure of a thermoset composite irreversibly, giving the part its strength. However, at the current stage of technological development, curing also renders the material in the part unsuitable for re-use in a primary structural component.
However, thermoplastic materials don’t require cold storage or baking when made into parts, according to Dion. They can be stamped into the final shape of a simple part—every bracket for the fuselage frames in the Airbus A350 is a thermoplastic composite part—or into an intermediate stage of a more complex component.
Thermoplastic materials can be welded together in various ways, allowing complex, highly shaped parts to be made from simple sub-structures. Today induction welding is mainly used, which only allows flat, constant-thickness parts to be made from sub-parts, according to Dion. However, Collins is developing vibration and friction welding techniques for joining thermoplastic parts, which once certified it expects will eventually allow it to produce “truly advanced complex structures,” he said.
The ability to weld together thermoplastic materials to make complex structures allows manufacturers to do away with the metal screws, fasteners, and hinges required by thermoset parts for joining and folding, thereby creating a weight-reduction benefit of about 10 percent, Brown estimates.
Still, thermoplastic composites bond better to metals than do thermoset composites, according to Brown. While industrial R&D aimed at developing practical applications for that thermoplastic property remains “at an early-maturity technology readiness level,” it might eventually let aerospace engineers design components that contain hybrid thermoplastic-and-metal integrated structures.
One potential application could, for instance, be a one-piece, lightweight airliner passenger seat containing all of the metal-based circuitry needed for the interface used by the passenger to select and control his or her inflight entertainment options, seat lighting, overhead fan, electronically controlled seat recline, window shade opacity, and other functions.
Unlike thermoset materials, which need curing to produce the stiffness, strength, and shape required from the parts into which they get made, the molecular structures of thermoplastic composite materials don’t change when made into parts, according to Dion.
As a result, thermoplastic materials are far more fracture-resistant upon impact than thermoset materials while offering similar, if not stronger, structural toughness and strength. “So you can design [parts] to much thinner gauges,” said Dion, meaning thermoplastic parts weigh less than any thermoset parts they replace, even apart from the additional weight reductions resulting from the fact thermoplastic parts don’t require metal screws or fasteners.
Recycling thermoplastic parts should also prove a simpler process than recycling thermoset parts. At the current state of technology (and for some time to come), the irreversible changes in molecular structure produced by curing thermoset materials prevent the use of recycled material to make new parts of equivalent strength.
Recycling thermoset parts involves grinding up the carbon fibers in the material into small lengths and burning the fiber-and-resin mixture before reprocessing it. The material obtained for reprocessing is structurally weaker than the thermoset material from which the recycled part got made, so recycling thermoset parts into new ones typically turn “a secondary structure into a tertiary one,” said Brown.
On the other hand, because the molecular structures of thermoplastic parts do not change in the parts-manufacturing and parts-joining processes, they can simply be melted down 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 to choose from in designing and manufacturing parts. “A pretty wide range of resins” is available into which one-dimensional carbon fiber filaments or two-dimensional weaves can be embedded, producing different material properties, said Dion. “The most exciting resins are the low-melt resins,” which melt at relatively low temperatures and so can 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 for thermoplastics in comparison with thermoset materials. Typically, they cost more than thermosets, and aircraft manufacturers must consider that fact in their cost/benefit design calculations, said Brown.
Partly for that reason, GKN Aerospace and others will continue to focus most on thermoset materials when manufacturing large structural parts for aircraft. They already use thermoplastic materials widely in making 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—particularly in the burgeoning eVTOL UAM market, concluded Dion.