By Design: Molded threads, Part 2

November 30, 2006

In this recurring column, Glenn Beall of Glenn Beall Plastics Ltd. (Libertyville, IL) shares his special perspective on issues important to design engineers and the molding industry.


It can get complicated, but there is a right way to mold threads.

Threads are an important part of mechanical design. They are found in products as diverse as plastic bottle caps and plumbing fittings. Their widespread use became practical when the United States established thread standards in 1864. These standards allowed interchangeability, which supported the rapid growth of the machine age.

Prior to the wide acceptance of plastic materials in the 1940s, metal was the primary material for mechanical products. Between the 1860s and 1940s the machine tool industry perfected and refined thread design. Technical information on how to design and use threads was widely published. The most comprehensive information was, and still is, Machinery’s Handbook published by Industrial Press Inc. This handbook contains more information on the design, production, and testing of threads than anyone needs or wants to know. Curiously, the word plastic does not appear in the 400 or so pages the handbook devotes to threads.

Regrettably, the plastics industry has not produced comparable technical information on the design of injection molded threads. The logic behind this apparent oversight is that by the 1940s, when plastic was gaining acceptance, metal threads were already well established. These early plastics engineers simply designed threads that duplicated the metal threads. In many instances the plastic threads performed satisfactorily, but others failed in unexpected ways. Back in those days the industry did not have the high-performance engineering materials that we have today. In addition, the molding procedures were not as good or reliable as those now being used.

Why threads failed

A significant number of these thread failures were the result of improper part design. The designs being used at that time were intended for metal parts. These thread design details had evolved over many years of trial and error to be ideal for metal threads. Some of these metal details were unsuitable for plastic threads, but no one knew any better at the time.

The plastics industry has subsequently learned how to design good-quality injection molded threads. Unfortunately, that information is not widely disseminated. Many engineers are continuing to make the mistake of designing plastic components according to the specifications developed for metal threads.

Once the standards for metal threads were established, they remained basically unchanged. One reason for this was that whether the threads were chased on a lathe or individually cut with taps and dies, they required special tools. It was difficult to produce these special tools for a different thread pitch or profile. The important advantages of interchangeability also encouraged the use of standardized threads. Interchangeability was the primary reason why early plastic parts duplicated metal thread designs.

Common plastic thread profiles

All types and sizes of threads can be injection molded. However, some thread profiles are preferred over others. The most frequently specified thread profiles for plastics are the American Standard 60º sharp thread, tapered pipe thread, and

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