Manufacturers are relying more on composite materials for business aircraft construction thanks to a drop in manufacturing costs and better automation. Largely because of improved curing processes, OEMs are gradually eliminating expensive tooling and slashing the overall number of parts needed for a given aerostructure.
An increased reliance on automation is certainly one way to improve composite airframe manufacturing because it ensures repeatability, industry insiders say. “Advances in automation as well as low-temperature curing materials are beginning to bring fabrication to more cost-acceptable levels,” said Frank Simmons, a structures staff scientist for Gulfstream.
Composite engineers are attempting to simplify and shorten the curing phase or even get rid of the autoclave altogether. Autoclaves, which are large ovens with high-pressure capability, are expensive to purchase and operate, as are autoclave-capable tools. Moreover, an autoclave can create production bottlenecks if something breaks.
In response, the industry has been developing systems that allow curing without high pressures or temperatures. According to Tim Shumate, marketing manager for Cytec Engineered Materials, the new processes require relatively low temperatures of 180 to 250 degrees F (80 to 120 degrees C), local vacuum pressure and a short post-cure phase.
Post curing involves an additional temperature exposure. “To get a complete cross link in a thermoset, sometimes you need to take the resin to a higher temperature in an oven,” Shumate said. This often can be done “free standing” because the cured structure holds its shape and does not need separate tooling. A two-phase process, with a low-temperature cure and higher-temperature free-standing cure, will require less expensive tools than a single-phase, high-temperature cure process. Thermosets are polymer materials that cure, through the addition of energy, to a stronger form.
According to Shumate, a goal of aircraft makers is to develop a process involving advanced fiber-placement techniques whereby the material cools as it lays on the tool within the fiber-placement machine. Some studies have shown positive results when thermoset resins are cured in place using pressure and temperature applied by the fiber placement-machine. Post curing would probably be necessary, but it would not be much of a disadvantage. However, Shumate said he believes that the porosity of the finished material could be an issue with such a process.
There is probably even more potential in manufacturing thermoplastic (as opposed to thermoset) resin-based systems, according to Shumate. “Thermoplastic materials do not cure like thermosets,” he said. “Thermoplastics are heated to a temperature that allows them to flow. So, if you can develop a fiber-placement machine that applies temperature and pressure at the correct levels, you could make a part out of thermoplastic composites and totally avoid the autoclave and oven. This would be a game changer.” Such R&D is occurring today, but the technology is still years away from being perfected, he said.
How close has Califonia composite aircraft builder Spectrum Aeronautical and its partner, Rocky Mountain Composites, gotten to making breakthroughs in manufacturing? For the construction of the S-33 Independence and S-40 Freedom business jets, Spectrum touts a technology called Fibex. The company only says the process is very similar to fiber placement but that it uses no autoclave.
A majority of aerospace composites are thermosets. Is this because they outperform thermoplastics? Maybe not. “Roughly speaking, the preference for thermoset is simply because they were more ready for use when composites hit the market,” Shumate said. Actually, thermoplastics do have some advantages. For example, storing them is easier. In addition, it can be argued that thermoplastics are stronger.
But thermosets (like epoxy) were adopted first and are well established. They are easier to handle when using a hand lay-up process, which was the predominant practice of manufacturers in the 1980s and early 1990s. But now that automated processes are being developed for thermoplastics, they are gaining popularity. “Change is slow in the industry, but one day thermoplastics may suddenly catch on,” Shumate said. Dutch-based Stork Aerospace is offering floor beams made of fiber-reinforced thermoplastic.
EADS Socata, the manufacturer of the TBM 850 turboprop single, has launched a research and development project on another thermoset process that may reduce the part count in an airframe. The idea is to use a liquid-resin infusion (LRI) process. “At the beginning of the LRI process, the resin is outside the mold,” Socata project manager Pierrick Venisse explained. “The latter holds dry fibers under a vacuum bag and the pressure differential allows the resin to be sucked into it.” The part is then cured without an autoclave. Compared to the more usual use of “prepregs” (pre-impregnated fibers), LRI enables more complex and integrated parts in one operation.
LRI is used for parts that are smaller or less complex than what Socata is aiming at. The airframer now wants to manufacture structure panels with numerous functions integrated. “They will integrate frames, stringers and window and door contours,” Venisse said.
As a result, the part count can be drastically reduced. This will translate into lower manufacturing costs–notably because the process is less labor-intensive. In addition, cycles will be shortened.
Socata R&D people believe LRI best suits their company. “Fiber placement involves major investments–notably for big autoclaves–that can be substantiated with larger fuselages and/or higher production rates,” Venisse said.