n marine applications, high technology plastics are used in creative ways that stretch their typical abilities. Increasingly, the non-flammable, high-temperature, high-strength qualities of these plastics mandate that they be made by expensive processes such as compression molding and filament winding. However, these processes are not suited for smaller parts used in industries that require rigorous series of tests. These parts must also withstand an extended service life without failure.

Craftech Industries, Inc. has been molding polyetherimide (PEI) since it was first commercially developed in the 1980’s. For the past number of years we have worked with a major defense customer to manufacture injection molded, glass-filled PEI paneling retention nuts used in the U.S. submarine fleet. On the surface, these parts look like many other Craftech projects but, to meet the stringent requirements for submarines, the fabrication behind the finished part is much different.

While PEI is not the most expensive or high temperature material on the market, it is used extensively in demanding applications as it exhibits extremely low outgassing and can tolerate temperatures from -40°F/-40°C to 350°F/176.7°C without losing elasticity. Additionally, PEI is microwave clear, a requirement for the submarine application.

For this application, Craftech decided not to machine the parts from extruded rod due to geometric considerations and a high price per unit. Additionally, if extruded rod was used the company would not be able to control the fiber flow in a desired and consistent way. Because of this, Craftech turned to injection molding processes for this application.

Injection molding is a common manufacturing choice for PEI parts as it is a quick and cheap process but is not generally high precision for high temperature plastics. This meant that Craftech had to approach the processes differently.

To make the nuts out of a brittle material that still retains strength when torqued required a special way of feeding material into the cavities of the mold fed by a hot half. Material was fed, curtain-style, around the top of the nut maximizing hoop strength. By doing this the company was able to vastly exceed the strength requirement of 3,500 pounds for the nuts, maintaining test numbers of 6,000 pounds or above. To provide some context, a grade two, mild steel nut fails at 5,750 pounds.

This job was unique for Craftech in another way. Since running an unwinding mold requires very exact tolerances that must line up with half a thousandth of an inch or less, we could no longer depend on the traditional ways of unwinding molds. Since the nuts required a secondary process after ejection from the molding machine, the company developed what became its first native software program for running mold and machine on the molding machine’s platform. Since then, Craftech has continued to rely on their own home-grown automation, as the coming shortage of workers and overseas competition will eventually mandate this.