A close-up of a thermoset slit tape spool showing the slit tape mated to the liner and transverse wound across a core. The edges of this spool are tapered to match the specifications of specific AFP equipment; different winding patterns and edge profiles are possible to custom-tailor spool density and weight to end-use needs.

Recent advancements in composite production and processing are making thermoplastics a viable option in a wider array of aerospace applications.

Traditionally, aerospace manufacturers have turned to composites for their significant weight reductions and cost savings compared with conventional aerospace materials, such as aluminum. Thermoset composites have been the composite of choice in recent years for their perceived high quality, commercialized pricing, more mature manufacturing process and well-established supply chain. They found application in diverse areas, including primary structural applications. Thermoplastics, in contrast, have been considered a costlier option and were mainly used in small semi-structural applications, such as clips and brackets.

These perceptions are now changing, and thermoplastics are increasingly being considered for new applications in primary structural components, such as stringers or stiffeners, wing boxes and fuselage panels.

Thermoplastics vs. Thermosets

Wide parent rolls of prepreg material are precision slit into slit tapes in widths ranging from 1/8 to 1+ inch. The tapes are then spooled for use by AFP and ATL fabrication processes.

Thermoplastic and thermoset resins have similar performance characteristics, and both can be combined with carbon, glass or other fibers to form prepreg systems. The prepregs are then fashioned into finished components. The main physical difference between the two materials involves their resin systems. An example of an aerospace-grade thermoset resin is an epoxy resin, while a comparable thermoplastic polymer can be a polyether ketone family resin.

There are many types of thermoplastic materials with different thermal, chemical and mechanical properties, but the most promising for aerospace applications are PEEK (polyetherether-ketone), PEKK (polyetherketoneketone) and PPS (polyphenylene sulfide). All three are semicrystalline thermoplastics with superior chemical and heat resistance and the ability to withstand high mechanical load. PEEK and PEKK in particular are well suited for the creation of large structural components.

The two materials differ dramatically in processing temperature, handling and storage requirements. Thermosets are processed at much lower temperatures, ranging from room temperature to ~300°F. They are typically “set” and cured in an autoclave. The curing cycle can take as long as 12 hours. However, prior to curing, thermosets must be refrigerated to prevent resin advancement and maintain their mechanical properties. Typically, they have a limited shelf life of about 12 months from prepreg production to part completion. In addition, once cured, they cannot be remelted, remolded or recycled.

Producing thermoplastic prepregs and completed thermoplastic parts is generally considered more difficult. They require high processing temperatures of ~600°F or more. However, no autoclave curing is normally needed. The main advantage over thermosets is that prior to curing, they have an unlimited shelf life at room temperature and do not require refrigeration. Also, recycling is possible with thermoplastics. They can be remelted and reformed post cured, offering flexibility and sustainability advantages.

Facilities that produce aerospace-grade thermoplastics and thermosets must strictly adhere to the AS9100 standards for quality and safety required by aerospace manufacturers and suppliers. These facilities typically feature Controlled Contamination Areas and clean rooms designed to protect materials from foreign object debris (FOD). Contamination by FOD can result in production issues, material consolidation problems and eventual part rejection.

Processing Improvements

Because thermoplastic prepreg material is not tacky at room temperatures, thermoplastic slit tape spools can require different configurations than thermoset slit tape spools.

The latest manufacturing and processing advances have improved thermoplastic quality and made the material more cost-competitive with thermosets, especially when the total cost of production is considered. The advances affect every stage of thermoplastic manufacturing, including prepreg production, converting and formatting, and component production. For example, prepreg makers are using new innovative resin systems to improve both consistency in quality and scalability. Companies that format thermoplastic prepreg have made dramatic strides in process innovations. These include improvements for both the precision and the variety of output formats that support new processes. In addition, component producers have further improved the quality and efficiency of their processing methods.

The processes used to turn thermoplastic prepreg materials into finished components have undergone major modifications and improvements. The following four processes show the greatest potential for continuing innovation:

  • Automated fiber placement/automated tape laying (AFP/ATL)

  • Continuous compression molding

  • Compression molding

  • Automated press/thermoforming.