New Twist in Thermoplastic Composites Makes High-Strength, Net-Shape Fasteners

A Swiss manufacturer of aerospace, automotive, and medical parts has developed a novel compression molding process that delivers net-shape thermoplastic composites with strength per unit weight reportedly competitive with those of machined steel, aluminum, and titanium. Icotec AG says its Composite Flow Molding (CFM) process puts up to 62% by volume of carbon fiber into a thermoplastic, yielding high-strength and abrasion-resistant screws, bolts, inserts, studs, anchor nuts, and other fasteners. CFM composites are also being used to make medical implants and small (up to 0.5-in.-long) structural bearings and other parts. The favored matrix polymer is PEEK, supplied by Victrex PLC in the U.K. and in the U.S.

CFM composites offer specific strength (i.e., per unit weight) exceeding those of competing metals, says Bettina Hale, Icotec market manager. The absolute strength values of 62% carbon-reinforced PEEK are equally impressive: Flexural strength is about seven times higher than those of aluminum and 30% glass-reinforced nylon 6 or PEEK.

 

Pultruded and molded

The CFM process begins with a pultruded PEEK/carbon fiber rod made by Faigle Kunststofftechnik in Weissensberg, Germany. This rod supplier employs a proprietary method of incorporating high levels of carbon-fiber tow into PEEK with virtually no damage to the integrity of the reinforcements.

Icotec then cuts the PEEK rod into a blank whose volume is precisely equal to the volume of the final part. The blank is transferred by robot to a heating chamber where the PEEK melts and wets-out the continuous fibers, minimizing voids. The blank is then transferred to a compression mold cavity where it is pressed at precise speed, temperature, and pressure to form a part with a predictable fiber orientation to enhance part strength.

The illustrated cross-section of a CFM screw shows fiber in the helical (thread) sections oriented in ways that increase thread twist strength or torsional resistance, allowing screws to resist breakage even when fastening metal parts. Orientation at the screw core, however, is unidirectional, which enhances overall screw strength. In the intermediate regions, fiber is oriented in a way said to anchor the torque-resistant screw threads securely to the stabilized core. 

Victrex USA technical-service manager Rich Liebfried says the PEEK matrix adds more dimensions to CFM part performance. “PEEK wets out fiber fully and processes well in this compression molding process.” PEEK also enhances the chemical and thermal resistance of parts, offering a continuous-use temperature of 500 F. Low outgassing makes PEEK suitable for corrosion-resistant fasteners in aerospace applications.

In marine applications like rig fittings, the vibration-resistance of CFM fasteners has considerable appeal. The company also recently developed a line of internally lubricated plastic fasteners for dynamic applications in which parts are subjected to high levels of vibration. In those uses, metal fasteners often lose their preload tension and are inclined to loosen and fail.

Icotec is working to broaden the capability of CFM processing with additional resins such as nylon 6 and PBT. The process also permits fibers other than carbon to be used, notably ceramic, glass, and tantalum. Glass is favored in applications calling for high thermal or electrical insulation. Carbon fiber offers higher strength but can block accurate reading of x-ray or MRI images when used in bone implants. Icotec’s response is a radio-translucent tantalum-reinforced formulation.