Creating Formed Threads in Long Fiber Composite Bolts

Fiber composite hardware for aerospace applications and more specifically composite bolts have those qualities sought in engineered composite products; corrosion resistance, high strength to weight, toughness and stiffness. Historically, short chopped fibers (0.2 – 0.4mm) have been used for molded composite hardware. The combination of typical engineering polymers such as PA6 (Polyamide 6, nylon 6), PA66, PP (Polypropylene) and PPS (polyphenylene sulfide) are often used with glass, carbon and other fibers. The fiber content has a strong effect on properties and fiber contents from 20% to 60% are used.

Short fibers, however, cannot match the mechanical property advantages of long fibers. Long fibers are typically in the 10mm – 20mm range or larger. When these long fibers are combined with the engineering thermoplastics, mechanical properties across the board see a 25% to 150% increase. The largest beneficiary of these improvements is notched impact strength which sees a 150% increase.

Composite bolts should see a great benefit from the combination of long fibers with an engineering thermoplastic such as PPS. The thread roots, to benefit from the continuous nature of the long fiber, must maintain the fiber along the root notch. However, if the threads are cut into the bolt, the fibers at the thread root are also cut and thereby lose their effectiveness in toughness and strength. A better method is molding the thread into the bolt using a compression molding process. This process will maintain the integrity of the long fibers at the thread root, but extrusion compression molding has its difficulties and short coming relative to production of threaded bolts.