Advanced thermoplastic composites-and their emergence as prominent materials for future aerospace applications–are sparking a flurry of activity from aerospace manufacturers, designers, component producers and formatters.

A Web Industries technician processes thermoplastic slit tape.

Research and development efforts are picking up, and more trials involving thermoplastic composites are under way. New companies are entering the market, earning qualification from aerospace manufacturers and fortifying the existing supply chains. Innovative fabrication methods using thermoplastics are being developed, refined and brought online. The signs all point to much greater use of thermoplastic materials in next-generation commercial aircraft and related applications.

The increasing activity is explained by several factors, including advances in the production, formatting and fabrication of thermoplastic materials. These materials offer clear advantages over thermosets and metals such as aluminum in certain aerospace applications. They also mesh nicely with emerging trends in aerospace manufacturing, including the faster pace of aircraft assembly and production and the development of advanced commercial aircraft designs.

Thermoplastics Properties

Aerospace-grade thermoplastic composites such as carbon-reinforced polyetheretherketone (PEEK) and polyetherketoneketone (PEKK) show the most promise for modern aerospace applications. Thermoplastic prepregs are produced on master rolls and can be converted into slit tapes, chopped fibers or other formats. These formatted products are optimized to allow for highly efficient and increasingly streamlined final part production.

Aerospace-grade thermoplastics offer lightweight, high-temperature performance and exhibit a high degree of toughness and impact resistance. Other key characteristics include:

  • Low moisture absorption

  • Excellent wear and abrasion resistance

  • Superior flame/smoke toxicity performance

  • Low emissions of volatile chemicals

  • Low coefficient of thermal cycling expansion

Although thermosets have more fully developed supply chains and a longer history as aerospace components, recent technological and processing developments are improving the competitive picture for thermoplastic materials. Fabricators and precision formatters, for example, are increasing the accuracy of their slitting and converting processes to produce a wider variety of advanced components.

A Web Industries technician selects a roll of thermoplastic slit tape.

Thermoplastics and thermosets have relatively similar performance characteristics, but they differ significantly in processing and handling requirements. While thermoplastics require higher temperatures to process than thermosets, they can be stored at room temperature and have a virtually unlimited shelf life. In contrast, thermosets must be transported and stored frozen as well as thawed prior to processing in order to retain their mechanical properties and prevent resin advancement. The defined shelf life of thermoset materials, the time needed for thawing and freezing, as well as the need to track total frozen time and thawed time, all add up to additional costs not associated with thermoplastics.

Thermoplastic materials also have the advantage of being recyclable. Unlike thermosets, which undergo an irreversible chemical reaction when processed and cannot be re-melted, thermoplastics can be reprocessed after use, allowing the thermoplastic resin and integrated fibers to be recycled or repurposed for appropriate applications.

Next-Generation Designs

A Web Industries technician holds a roll of thermoplastic prepreg.

Advanced thermoplastic composites give designers of next-generation aircraft an expanded toolbox of materials to work with. Thermoplastics allow for more creativity and flexibility in aircraft design. They can be formed into complex contours and shapes, allowing aircraft to be molded, rather than machined. Thermoplastic composites are currently used in semi-structural components such as clips and brackets, but are under trial and consideration for primary structural components such as fuselage panels, wing boxes and stringers.

Next-generation aerospace designs will require faster manufacturing and assembly. The speed of thermoplastic part fabrication is a good match for the projected needs of aircraft manufacturers and designers. Consider the case of a fuselage section produced using automated fiber placement. If the part is made with thermoset materials, after layup the part has to be pressurized and cured using an autoclave. In contrast, thermoplastic materials can be consolidated with advanced thermo processing which can reduce fabrication time involved in aircraft production.

The use of thermoplastic materials also aids efforts to reduce the overall component count in aircraft. The materials potentially allow adjacent parts to be welded together without changing dimensional tolerances, eliminating the need for numerous small fasteners.

Thermoplastic slit tape (left) is used in precision fiber placement manufacturing processes while chopped materials (right) are used in discontinuous compression molding applications.

Satellite Applications

In addition to commercial aircraft, thermoplastic materials also find use in demanding satellite applications. In one case, a formatted thermoplastic slit tape with extremely smooth edges and beyond industry standard dimensional tolerances was provided to a satellite manufacturer for use in their proprietary process. The component's light weight is ideal for the launch vehicle and the payload. Once in space, it maintains its stiffness when deployed. Due to a low coefficient of thermal expansion, the material remains dimensionally stable as the satellite experiences thermal cycling.

Precise Formatting

The main fabrication methods for producing thermoplastic components include automated fiber placement/automated tape laying (AFP/ATL), continuous compression molding, discontinuous compression molding and automated press/thermoforming. Each method is designed for certain types of aerospace components and requires prepreg materials custom-formatted for that purpose. Continuous compression molding, for example, requires oriented material on wound spools or pads, while AFP/ATL makes use of slit tape. Precision formatters can supply converted materials to exacting levels of accuracy, saving time and costs for aerospace manufacturers.

Even as the technologies for processing and fabricating thermoplastic composites evolve and improve, research and development on thermosets and aerospace metals continues. They are also undergoing modifications and improvements. No single material will be dominant in future aerospace applications. But given their many advantages, it's likely that advanced thermoplastics will play an increasingly important role in the coming years.

This article was written by Jim Powers, Business Development Manager, and Mark Richardson, Thermoplastic Composite Program Manager, Web Industries, Inc. (Marlborough, MA). For more information, visit here.