Over the past decade, the global commercial aerospace industry has enjoyed a period of considerable growth, during which passenger traffic has increased. To meet the demand for air travel and to replace aging fleets, airliners have placed orders for substantial numbers of aircraft. This has resulted in a growing backlog of aircraft on order as aircraft manufacturers struggle to keep pace with demand.

Aircraft manufacturers already use thermoplastic composites in small parts like brackets and clips, and in limited cases bigger components like the wing leading edge of the Airbus A380. They are exploring their use in larger structures like wing torsion boxes and fuselage panels. Source: Dutch Thermoplastic Components. (Click image to enlarge)As of July 31, Airbus had a backlog of 7,198 aircraft to manufacture, while Boeing’s backlog stood at 5,745 aircraft, according to Forecast International. At 2018 production rates, it would take Airbus 9 years and Boeing 7.1 years to deliver on these existing orders, not counting new orders during that time.

To meet demand, manufacturers are searching to improve production rates so that current and future aircraft models are not hampered by manufacturing bottlenecks. One promising avenue improves cycle times for composite part production.

Composite materials – consisting of a polymer matrix reinforced with fibers – are popular in modern aircraft for their high strength-to-weight ratios compared to metals, enabling lighter airplanes that burn less fuel. The most common composite material on new aircraft like the Boeing 787 and Airbus A350 XWB is comprised of a thermoset plastic and carbon fiber reinforcement. These thermoset composites require a curing reaction to fully polymerize, adding extra time to production cycles.

[Discover carbon/graphite-filled thermoplastics and thermoplastic resins on Engineering360.]

Thermoplastic composites

Thermoplastics are an alternative to thermoset polymers in composites and hold promise for increasing production rates. Unlike composites incorporating thermoset polymers, thermoplastic composites do not require a curing step after consolidation, in which the composite is formed by applying heat and pressure to multiple prepreg layers to form a solid laminate. Thermoplastic composites simply need to be heated up past the melting point of the thermoplastic matrix, consolidated and cooled, unlike thermoset composites which require a curing time for polymer crosslinks to form in the molecular structure.

Teijin’s Tenax TPUD, unidirectional, carbon fiber-reinforced, thermoplastic, pre-impregnated tape. Source: Teijin (Click image to enlarge)

Thermoplastic composites are commonly packaged as rolls of tape comprised of unidirectionally aligned carbon fiber that is pre-impregnated with thermoplastic resin. Thermoplastic composites are also available in the form of woven fabric tapes or reinforced thermoplastic laminates.

Thermoplastic composites for aerospace applications use high-performance thermoplastic resins, including polyetheretherketone (PEEK), polyetherketoneketone, polyaryletherketone, polyetherimide and polyphenylene sulfide. Aerospace thermoplastic composites typically have percentages of carbon fiber around 50-60% by volume. The ratio of carbon fiber to thermoplastic resin is tailored to achieve the desired mechanical properties and compatibility with the manufacturing process. A higher ratio of fiber to resin is desirable for maximizing mechanical performance, but may be more suitable for manufacturing processes with longer cycle times and higher pressures. Lower ratios of fiber to resin may accommodate faster process cycles and lower pressures.

Properties of typical thermoplastics used as the resin matrices of thermoplastic composites for aerospace applications. Data source: Advanced Thermoplastic Composites Manufacturing (Click image to enlarge)

Manufacturing processes

In a typical thermoplastic composite manufacturing process, thermoplastic tape is laid up, heated and formed by processing machines into final thermoplastic composite components. Tape laying is often performed by automated tape laying (ATL) machines, which mechanically place thermoplastic tape on flat or shaped molds to create multi-layered laminates. The placement of tape by ATL machines is precisely controlled by computer to create layers with specific alternating carbon fiber orientations (e.g., 0°/90°/+45°/-45°/90°/0°). The orientations of the carbon fibers are matched with the directions of the application’s expected major loads, giving the final composite excellent mechanical strength. The laminates are then formed under heat and pressure to consolidate the material into the final part through stamp forming, press molding or continuous compression molding.

Tri-Mack’s thermoplastic composite manufacturing process starts with Tri-Mack’s Fiberforge Relay 1000, which automates the layup of thermoplastic tape. The custom laminate is then heated in an oven and shuttled to a 500 ton press for thermoforming into a complex shape in a mold. Sources: Tri-Mack, Thermoplastic Composites Research Center (Click image to enlarge)

In-situ, out-of-autoclave manufacturing processes have potential to provide the fastest production rates. In one example, an ATL machine lays premade thermoplastic composite tape, a laser heats the substrate and tape and a roller applies pressure to form and consolidate the composite, all in one step outside of an oven.

A laser-assisted ATL machine accomplishes in-situ consolidation of thermoplastic composite by heating the tape just before it is pressed on to the part by a roller in one step. Sources: Airborne, Thermoplastic Composites Research Center (Click image to enlarge)

Thermoplastic composite parts can be joined together and with other materials via a variety of assembly techniques, including bonding with adhesives, co-consolidation and mechanical fastening. Thermoplastic composites can also be welded together with processes like inductance, resistance or ultrasonic welding.

[Discover plastic welding equipment on Engineering360.]

Advantages

The biggest benefit of thermoplastic composites over thermoset composites may be a faster production cycle, but thermoplastic composites have other advantages as well. These include high toughness, low flammability as measured by low fire, smoke and toxicity and Ohio State University heat release ratings, less degradation of mechanical properties in hot and wet conditions, unlimited shelf life in room temperature storage and full recyclability.

Thermoplastic composite components can also be joined by fusion bonding, or welding. Cured thermoset composite parts cannot be welded and must be joined by mechanical fasteners, which require stress-concentrating holes, or by adhesives, which introduces challenges in non-destructive testing (NDT). However, a new technique permits thermoset composite part welding by incorporating a surface layer of thermoplastic as the top ply of the thermoset composite, which can be welded to a like surface.

[Discover NDT equipment on Engineering360.]

Stelia Aerospace, along with partners, developed a dynamic induction welding process that joins unidirectional carbon fiber/polyetherketoneketone (CF/PEKK) stringers to CF/PEKK skin, while adjusting for variations in thickness of the parts, to create a thermoplastic composite fuselage demonstrator for the Arches Box TP project. Source: Composite Integrity (Click image to enlarge)

The downsides of thermoplastic composites compared to thermoset composites include higher processing temperatures, handling challenges due to higher viscosities and higher raw material costs.

Thermoplastic and thermoset composites have a number of advantages in common over metals, including higher stiffness and strength-to-weight ratio, great fatigue resistance and the ability to tailor designs to applications. They also have excellent corrosion resistance and withstand contact with moisture and chemicals, including solvents, fuels and hydraulic fluids.

As aircraft manufacturers look for ways to accelerate production to fill order backlogs, thermoplastic composites represent a promising material technology with the potential to improve cycle times. Companies are investing in thermoplastic composite manufacturing processes to mature the technology, reduce process costs and increase production rates as they explore broader deployment of the material.

Stay tuned to Engineering360 for part 2 of this series on recent thermoplastic composite manufacturing advances.