The 2010s were a transformative decade for composites as the industry’s focus shifted from production of small, non-structural parts made with woven fabrics to larger, structural parts made with unidirectional reinforcements. The impetus for change came from the automotive and aerospace industries. They appreciated composite materials’ unique properties – especially their light weight – and wanted to incorporate more composites into their products. But there was a stumbling block to widespread adoption; the speed of thermoset composite production wasn’t up to large-scale applications.
“They said, ‘We need to have fast manufacturing solutions,’ and that’s where thermoplastic composites come in,” says Sebastiaan Wijskamp, technical director of the ThermoPlastic Composites Research Center (TPRC) in The Netherlands. Founded 10 years ago, TPRC is a consortium of industrial and academic members whose goal is to enable a wider use of thermoplastic composites by eliminating technological barriers.
One reason for the high speed of thermoplastic composite production is its use of unidirectional tape and automated fiber placement (AFP) equipment. “You can build up a large piece, kind of like using a 3D printer, but with carbon reinforced tapes,” says Wijskamp. “If you do that well, you shouldn’t have to do an autoclave step to melt and consolidate the part under pressure. If we can stop that autoclaving step, we should be able to make big parts in a very short time.”
There are several other benefits, too. Using AFP, manufacturers can tailor parts by placing unidirectional fibers to reinforce areas that need strength and eliminating unnecessary material where it’s not required for structure. In addition, thermoplastic composite parts can be quickly joined together through processes like fusion bonding or heat welding, speeding manufacture and enabling faster repair of composite components.
Another benefit to thermoplastic composites is their recyclability. Production scraps and no-longer-needed parts can be kept out of landfills and reused. “The nice thing is that you only have to shred the composite pieces and melt them,” says Wijskamp. “There is no pyrolysis or burning of the matrix or any chemical process involved. So it’s an energy efficient and very clean way of recycling.”
The development of new materials and equipment for thermoplastic composites is one indicator of the market’s growing interest in them. One example is the VICTREX™ AE250 product family, which includes polyaryletherketones (PAEK) and carbon-fiber based unidirectional tapes that enable faster manufacturing of thermoplastic composites while reducing their weight.
“Developing a new polymer is a big investment, and polymer manufacturers don’t usually undertake such an action before they see market potential,” says Wijskamp. “You also see it with machine builders. Ten to 15 years ago there were only some very dedicated, small companies building machines for thermoplastic composite processing. Now you see all major machine builders going into this market.”
Ongoing Research to Encourage Adoption
While thermoplastic composites offer many benefits and have come a long way in the past decade, there’s still work to be done to advance mainstream use. TPRC has been a leader in Europe’s thermoplastic composite research. Consortium members share what TPRC researchers discover in a pre-competitive environment and apply it to their own manufacturing processes.
Ongoing research at TPRC includes many aspects of thermoplastic composite production, including stamping and overmolding, fusion of thermoplastic parts and long-term performance. Researchers develop software modeling tools that can help characterize the materials, fine-tune processes and improve tool production.
Sustainability is a major focus at TPRC. As the use of thermoplastic composites increases, the industry must be prepared with solutions for production scrap and for the recycling of end-of-life products, Wijskamp says. For one project, TPRC supported GKN Aerospace in the production of compression-molded access panels manufactured from recycled thermoplastic waste material. The panels were installed and tested in Bell Helicopter’s V-280 Valor.
End Users Seek an Array of Properties
As thermoplastic composite manufacturers turn their attention to bringing larger parts to market, equipment manufacturers like Coriolis Composites are feeling the impact. In the past, the company has only built high-performance AFP machines that use ¼-inch wide tapes for the production of small, intricate thermoplastic parts. Now customers want something different. Coriolis is being asked to design and build wing and fuselage machines that use wider tapes to speed up the thermoplastic composite manufacturing process, according to Burak Uzman, the company’s general manager.
There is a tradeoff for that speed, however. Wider tapes are less steerable and do not afford the same flexibility for design optimization as narrower tapes do, Uzman says. Designing for wider tapes foregoes opportunities to have a lighter weight structure.
In addition to a move toward wider tapes, Uzman currently sees a lot of interest in in-situ consolidation of thermoplastic composite components. The challenge there is ensuring the material forms strong bonds between layers. “In thermosets we take the bond between the different layers for granted, because thermosets spend hours in the autoclave and you put the pressure on them. That gives the resin chemistry plenty of time to form those linkages,” he says. But with in-situ consolidated thermoplastic composites, getting the required interlaminate properties depends on how the AFP machine processes the laminate, and, even more important, how well the engineers that program the machine understand the processing parameters like resin chemistry.
Customers want to try thermoplastic composites for a variety of reasons, according to Uzman. In structural applications, end users want the interlaminar shear and tension strength of thermoplastic composites, which makes them a good choice for parts that try to flex under load, such as I-beams, spars, fuselage frames and some wing skins. Other customers are more interested in thermoplastics’ damage-resistance properties. They want to protect aircraft surfaces from damage when a tool is dropped, when items get shoved into cargo bays or when a passenger in high heels walks on the laminate.
“Automotive is interested in black sheet metal – they want composites that behave like steel when they form them. So we do a lot of work in the simulation of 3D shapes in the forming process,” Uzman adds. These customers are experimenting with a variety of thermoplastic manufacturing techniques. “In the automotive world, the material system has not been locked in place yet. Some favor the infusion process with epoxies, some favor thermoplastics and lamination forming, and there is some effort in the infusion of thermoplastics with low viscosity thermoplastic resins.”
Striking the Right Balance
Consulting firm Forward Engineering assists international automotive OEMs and parts manufacturers that are interested in lightweight structures and composite materials. Forward Engineering has worked primarily with thermoset composites in the past, but today its projects are split evenly between thermoset and thermoplastic composites. The company’s clients are looking at the potential of thermoplastic composite in high-volume, low-cycle-time, automated production of structural parts.
“OEM’s have a broad portfolio of materials to choose from, including lightweight metals that they are very familiar and comfortable with,” says Adam Halsband, Forward Engineering’s managing director. “For thermoplastic composites to be attractive, they must strike the right balance of mass, cost and performance.” OEMs consider design, material and manufacturing to determine the feasibility of using any product, and thermoplastic composites manufacturers have some remaining challenges to overcome there.
One is the lack of good data. To properly design structural parts, engineers need to understand thermoplastic composites’ properties so they can input that information into their modeling and simulation programs. (Halsband refers to this data as “material cards.”) Forward Engineering has been working with leading material suppliers to accelerate the development of these material cards.
The availability of thermoplastic composites in the necessary quantities and at a competitive price is another problem. “For manufacturing, there needs to be a capable supply base,” Halsband says. Advances in automated manufacturing processes and in the high-volume production of continuous and discontinuous fiber organosheets should help address both availability and cost concerns.