Aircraft manufacturers are going to be very busy over the next two decades, and the thermoplastic composites industry is positioning itself to take full advantage of the opportunity.
By 2039, there will be 48,400 commercial aircraft flying throughout the world – 22,500 more than there were in 2019, according to Boeing’s Commercial Market Outlook for 2020-2039. At the same time, airline companies are accelerating the replacement cycle for their older aircraft to improve the efficiency and sustainability of their fleets. Thermoplastic composites can help aerospace manufacturers meet that burgeoning demand.
Thermoplastics offer many advantages for this industry. Lightweight carbon fiber-reinforced thermoplastic (CFRTP) parts provide excellent strength and stiffness; corrosion, chemical and fatigue resistance; and durability. They often perform better than equivalent metal parts.
They’re also a sustainable material. Thermoplastic parts weigh less than corresponding metal parts, enabling airlines to reduce their fuel and carbon emissions. In addition, thermoplastic composites are recyclable, so manufacturers can melt down and reuse the materials from production scrap and end-of-life parts.
One drawback to more widespread adoption of thermoplastic aircraft parts has been production speed. Until the past decade or so, the layout, consolidation and parts formation processes used for thermoplastics were similar to those used for thermosets. That included autoclave processing, which can take hours.
Developments in both materials and manufacturing have opened avenues to faster production. Using automated equipment and out-of-autoclave processing, manufacturers are demonstrating that they can turn out higher quality thermoplastic parts at faster speeds, making them a cost-effective option for aircraft production.
Designers are also getting more comfortable with thermoplastics. In Europe especially, predictive modeling and software has increased engineers’ confidence in the material. “Once the engineering teams at OEMs and Tier 1s understand how to design and implement the material, and have confidence in the manufacturing processes, you’re able to really overcome that mental wall that says, ‘This is new. I don’t know how to design and implement thermoplastics in production programs,’” says Evan Young, head of engineering R&D at Qarbon Aerospace.
Upgrades in unidirectional tape (UDT) are one example of how the materials used in thermoplastics manufacturing have changed. “The quality of the material has improved in terms of consistency of fiber, polymer distribution and elimination of voids within the prepreg,” says David Leach, business development director at ATC Manufacturing. Material consistency is essential to rapid, large-scale production and automation.
There have been advances in fiber reinforcement as well. Aerospace designers have primarily used continuous fiber thermoplastics to achieve the strength and performance predictability required for aircraft parts. But continuous fiber materials have some drawbacks when it comes to fabricating complex parts. “You are generally forming these parts very quickly, and getting the fibers and plies to move around in an extremely short time – often a matter of seconds – is a challenge,” says Leach. “If you have a discontinuous form, you can actually make much more complex parts, because you’re allowing some movement to occur in the fiber direction.” But parts designers have had concerns about the performance of discontinuous fiber materials.
Now there’s another option. In conjunction with NASA, the University of Delaware’s Center for Composite Materials recently developed a form of discontinuous fiber material that has properties equivalent to continuous fiber thermoplastics. Using TuFF (tailored universal feedback for forming) materials, manufacturers could produce aerospace-quality parts at the same production rate as automotive parts, according to the center. (TuFF received an Excellence in Composites “Infinite Possibility for Growth” award at CAMX 2019.)
New resins are speeding up processing as well. Thermoplastic composite manufacturers have been incorporating resins in the polyaryletherketone (PAEK) family into their products since the 1980s. These high-performance polymers are challenging to work with, however, because of more demanding processing conditions, such as high temperature. A few years ago, Victrex introduced low melt PAEK (LMPAEK™) to speed things up.
“LMPAEK was developed to find the balance of these high-performance polymer composite materials with the processability and suitability for automated production systems. These materials require less overall energy to lay up, which can contribute to significantly faster processing speeds,” says James Myers, head of research and development for aerospace and composite applications at Victrex.
Low melt PAEK has similar properties for mechanical strength, chemical resistance and other characteristics as polymers in the PAEK family, but its melting point is about 40 degrees Celsius lower. Although that may not seem significant, it makes a huge difference in manufacturing processes like stamp forming, injection molding and automated fiber placement (AFP), says Gilles Larroque, Victrex’s global strategic marketing manager.
Victrex recently worked with aerospace tooling and automation manufacturer Electroimpact on a LMPAEK demonstrator project. “Using LMPAEK with an automated fiber placement process, we’ve been able to reach 100 meters per minute in lay-up speeds. That’s close to four times faster than a similar PAEK UDT,” says Larroque.
In October 2020, Victrex and French aircraft manufacturer and equipment supplier Daher announced that they had produced a 176-ply, 1.2-inch-thick thermoplastic structural aircraft panel using AFP and LMPAEK UDT. This thickness has not been obtainable previously, according to Victrex. The company says the 47 x 23-inch panel meets aerospace industry standards for porosity, crystallinity, consolidation and ply bonding.
“What we see here is our unique UDT properties contributing to the fast AFP process, developed and demonstrated by partners Coriolis Composites and Electroimpact. These manufacturing rates enable all the exciting possibilities of metal replacement with thermoplastic composites on a far-reaching scale in aircraft design,” commented Tim Herr, director of Aerospace at Victrex, in a company press release.
Larroque says the parts made with AFP and LMPAEK could be used for large primary structures. The material would also be appropriate for secondary structural parts like brackets and system attachments, which carry high loads.
Both the LMPAEK resin and the parts made from them must go through the aircraft industry certification process before they can be used for large-scale production.
Reduced Manufacturing Time
Thermoplastic composite companies are using new equipment to reduce the time and steps in production. Continuous compression molding (CCM), for example, enables manufacturers to make a large quantity of laminates very cost effectively. Traditional presses have limitations on the size of the parts produced. “The required pressure, temperature, size and cost of equipment go up exponentially as you go to larger and larger laminates,” says Leach.