As aircraft manufacturers strive to fulfill large order backlogs for commercial airplanes, they are exploring new materials for many aircraft components and structures. One option is thermoplastic composites, an alternative to widely-used thermoset composites with similar strength-to-weight advantages compared to metals but faster processing cycle times than thermosets.

[Read part 1 of this series, Thermoplastic composites for aerospace applications, for an overview of thermoplastic composite material properties, manufacturing processes and advantages compared to thermoset composites.]

The Airbus A380’s wing leading edge is made of thermoplastic composite comprised of polyphenylene sulfide (PPS) reinforced with glass fibers. Source: Roger Green / CC BY-SA 2.0

Thermoplastic composites received attention starting in the late 1960s. Into the 1980s, they were used, in particular, in military applications. Interest in them, however, subsided as defense spending declined in the 1990s. At that time, thermoset composite development was picking up, eventually leading to their extensive use in the Boeing 787, Airbus A350 XWB and other aircraft.

In recent years, there has been renewed interest in thermoplastic composites in the aerospace industry for their potential production cycle speed improvements over thermoset composites.

Up until recently, thermoplastic composites on airplanes have been used for smaller parts like clips or brackets. That’s starting to change, with thermoplastic composites being substituted for traditional materials in components like the horizontal tailplane of the Leonardo AW169 helicopter, the rudder and elevators on the empennage of the Gulfstream G650, and the wing leading edge of the Airbus A380.

New applications

Process improvements are opening the door to additional applications for thermoplastic composites.

Herone’s hollow profiles – made of carbon fiber/polyetheretherketone (CF/PEEK) – can have functional elements integrated through overmolding, including injection-molded CF/PEEK gears and metallic load transfer elements. Source: Carbon Composites e.V.German manufacturer Herone has developed a process to form parts using hollow tubes called “organoTubes” made of braided thermoplastic composite tapes. The organoTubes are heated to consolidate them into a variety of profiles and cross-sections. The process can form complex parts with integrated functional elements like driveshafts with gears, fitting on pipes and load transfer element on struts. In addition, polymer solutions provider Victrex and parts manufacturer Tri-Mack have developed a hybrid molding process that overmolds polyetheretherketone (PEEK) over low-melt temperature polyaryletherketone (PAEK) to form fused hybrid parts.

In an effort to modernize ancillary aircraft engine piping, the EU Clean Sky program’s COMpipe project sought to develop composite “dressings” for aero-engines, including pipes and fittings for drains, and scavenge and vent lines as lightweight replacements for heavy, traditional metallic components. The project assessed manufacturing processes and demonstrated the capability of thermoplastic composites to meet the demanding performance and environmental requirements for pipe assemblies in aero-engines, although further development and testing is necessary to develop a viable product, with specific challenges regarding leaking end fittings following thermal shock and cycling. Participants in the project, which wrapped up in 2015, included Rolls-Royce Plc., Sigma Precision Components Ltd. and TWI Ltd.

In another effort to achieve wider deployment of thermoplastic composite technology, the first private consortium focused on advancing thermoplastic composite materials and process technologies — IRG CosiMo (Industry Research Group: Composites for Sustainable Mobility) — launched in August 2018. The group’s founding members — Solvay, Premium AEROTEC and Faurecia Clean Mobility — seek to enable high-volume thermoplastic composite production for the aerospace and automotive markets.

Larger structures

To realize the full potential cycle time improvements and weight savings of thermoplastic composites, the lightweight material will need to replace thermoset composites and metals in larger primary aircraft structures like the fuselage and wing boxes.

The skin and L stringers of GKN Fokker’s thermoplastic composite fuselage panel are formed by co-consolidation. Circumferential stringers are welded on later to complete the orthogrid. Source: GKN Fokker

For this purpose, one of the efforts underway involves a partnership between GKN Fokker and Gulfstream Aerospace that began in 2017. The pair is advancing thermoplastic composite fuselage technology developed by GKN Fokker through the Thermoplastic Affordable Primary Aircraft Structure (TAPAS) consortium, a collaboration between Airbus and several Dutch companies and research institutes. GKN Fokker and Gulfstream displayed their first thermoplastic composite fuselage panel demonstrator at JEC World in 2019 and are continuing work to improve its technology readiness level for eventual introduction on production aircraft.

Thermoplastic composite fuselages are expected to offer cost and weight savings compared to traditional aluminum and composite fuselage shells. With no rivets to install and no reinforced fastener holes to fashion, labor costs are reduced and overall vehicle weight drops.

Preparing for new demand

ATC’s second continuous compression molding line for thermoplastic composite laminates. Source: ATCATC (Advanced Thermoplastic Composites Manufacturing), a pioneer of thermoplastic technology in the aerospace industry founded in 2004, is gearing up for a boom in thermoplastic composites. The company recently added a second state-of-the-art continuous compression molding line for the production of fiber-reinforced thermoplastic laminates. It also has plans drawn up to add another 52,500 ft² of manufacturing space onto its existing 67,000 ft² facility in Post Falls, Idaho, as well as nearly ten acres of land available for additional expansion.

[Discover molding machines on Engineering360.]

Demand for thermoplastic composites could be driven by new aircraft like Boeing’s New Mid-market Airplane (NMA), a twin-aisle aircraft designed to fill a gap in Boeing’s addressed market between the 787-8 and 737 MAX 10. The company is considering a decision to launch the NMA in 2020 for entry into service in 2025. NMA would likely incorporate extensive use of weight-saving composites in its design, and possibly more widespread utilization of thermoplastic composites.

In January 2019, Boeing qualified a new thermoplastic composite tape for use in primary structural parts in its aircraft. Teijin Limited expects to begin commercial shipments of its Tenax carbon fiber thermoplastic unidirectional pre-impregnated tape (Tenax TPUD) to Boeing-approved parts manufacturers within two years.

Manufacturing automation

Advances in manufacturing processes will continue to bring down the costs and increase production rates of thermoplastic composite parts, improving the viability of their use in aerospace and other applications like automotive, marine, energy and healthcare.

GA-ASI and Composite Automation’s dual robot, tool-less thermoplastic composite automated fiber placement machine performs in-situ consolidation. Source: Composite Automation LLCGeneral Atomics Aeronautical Systems Inc. (GA-ASI) is working with Composite Automation LLC to develop a thermoplastic composite fabrication process that eliminates conventional tooling and molds to perform in-situ thermoplastic consolidation. The process uses automated tape laying equipment from Mikrosam and two six-axis robots from KUKA. A tape-placing robot with a laser heater consolidates thermoplastic composite atop a support structure held by the second robot. The two robots move with coordinated motion through 3D space, adjusting tape placement in real time according to feedback from a vision flaw-detection system developed by Trilion Quality Systems that compares the growing form to CAD models.

[Discover flaw detection instruments and systems on Engineering360.]

Composite automation specialist Airborne is developing automated composite manufacturing cells to fabricate small to medium size aerospace parts like ribs, stringers, skin panels and couplings. The company’s composites automation facility in The Hague, Netherlands, is equipped with a large, dual industrial robot cell, a prototype continuous fiber placement machine, and end effectors for automation technologies including tape-laying, fiber steering, pick and place and automated bonding.

High-speed laminate production

At the inspection and trimming station of SABIC and Airborne’s Digital Composite’s Manufacturing Line, the top side of thermoplastic composite laminates are inspected and trimmed, then flipped over by a robot to repeat the process on the bottom side. Source: SABICAirborne teamed up with chemical and polymer manufacturer SABIC to create the Digital Composites Manufacturing Line (DCML), an automated thermoplastic composite laminate production system. The automated system transforms the tape conversion process — in which layers of thermoplastic composite tape are consolidated into multi-ply laminates — from a slow, costly one to a high-speed, automated affair. DCML, which uses technology from industrial conglomerate Siemens AG and robot manufacturer KUKA, can produce up to four, 15-ply laminates per minute and 1.5 million laminates per year including full digital inspection.

DCML accomplishes this with a conveyor system consisting of trays passing under feeders that drop ply layers with the correct fiber orientations according to a layup schedule. Robots place the ply stack on metal plates that heat up and consolidate the laminate via contact heating. The piece then moves on to an inspection and trimming station before packaging. Aimed initially at the consumer electronics industry, the companies see other markets as prime targets for the technology including aerospace, automotive and healthcare.

3D printing

The German Aerospace Center’s (DLR) AddCompS™ project enables 3D printing of thermoplastic composites. Source: DLRAdditive manufacturing has the potential to bring greater cost efficiencies to thermoplastic composite fabrication processes. Combining composite production and 3D printing, the German Aerospace Center’s (DLR) AddCompS™ project advances automated tape laying (ATL) and automated fiber placement (AFP) technology to enable in-situ fiber impregnation and additive extrusion of thermoplastic composites to create semi-finished parts.

Developed by the DLR’s Institute of Composite Structures and Adaptive Systems, AddCompS™ uses a robotic tooling head to extrude consolidated thermoplastic reinforced with fiber. The system impregnates continuous 24K carbon fiber rovings with thermoplastic resin inside the extrusion unit and extrudes the consolidated material to form layers of composite in three-dimensional shapes with controlled fiber orientations. The tooling head is small enough to be mounted on most robotic systems and the technology is compatible with standard thermoplastics like polylactic acid (PLA), acrylonitrile butadiene styrene (ABS) and polyamide (PA) as well as high-performance thermoplastics like polyetherimide (PEI) and polyetheretherketone (PEEK).

Discover products on Engineering360

• Polymer and plastic composites
• Composite fillers and reinforcements
• Composites and composite materials
• Thermoplastics and thermoplastic resins
• Thermosets and thermoset resins

Resources

Read more about thermoplastic composite advances at CompositesWorld:

• Thermoplastic composites: Poised to step forward
• Re-dressing aeroengines with composite pipes
• Injection-forming for high-performance, unitized thermoplastic structures
• High-speed, high-rate thermoplastic composites manufacturing
• I want to say two words to you: "Thermoplastic tapes"