By Design: Molded threads, Part 2

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 buttress threads (Figure 1).

Of these three types, the American Standard or machine screw is the most common. Unfortunately, the American Standard threads were designed with sharp corners at their roots and crests. These sharp corners become stress concentrators in a molded plastic thread. A better thread profile for plastic materials would be the British Whitworth thread, which provides radiuses at the roots and crests of the threads (for a drawing of each thread, see Part 1 of this series at immnet.com/articles/2006/October/3007). These small radiuses help to distribute the stress on the thread over a broader area and that significantly increases the threaded part’s strength. These radiuses must, however, be kept small so as not to interfere with mating thread engagement.

An ideal thread profile for a molded plastic thread would be the buttress thread shown in Figure 2. This thread is actually a modification of the metal Acme thread. The advantages of this thread are its resistance to shearing off due to its thicker thread cross section and the wider spacing between threads that allow more room for radiusing the roots and crests of the thread. A good example of this type of thread can be found on blowmolded plastic bottles and threaded bottle caps. Specifications and tolerances for blowmolded bottle threads are issued by SPI’s Plastic Bottle Institute (www.plasticsindustry.org).

The disadvantage of modified buttress threads is that it is not possible to get many turns of thread in a given length, due to the thread’s wide profile. However, the advantages of this thread profile are such that it should be specified wherever possible.

Thread location considerations

Threads are very useful mechanical design features, but they do complicate the molding process. The inside thread shown in Figure 3 represents undercuts. In relatively soft materials these undercuts can sometimes be stripped from the mold. In the majority of instances, the threaded core pin must be rotated and withdrawn from the molded part before the part can be ejected from the mold.

Moldmakers have evolved many different ways of rotating threaded cores. A hydraulic-cylinder-activated gear and rack is the most frequently used mechanism. Inside threads require a more costly and maintenance-prone mold. These threads may also increase the cost of molding a part. With more than one full turn of a thread, additional cycle time will be required to unscrew the threaded core and return it to its original starting position.

Outside threads may or may not require an unscrewing mechanism. If the thread can be located on the mold’s parting line, as shown in Figure 4, the part can be molded without an unscrewing mechanism. Locating the thread on the parting line eliminates the added cost of an unscrewing mechanism in the mold and the longer molding cycle required to unscrew the threaded core.

Every effort must be made to locate outside threads on the mold’s parting line. In some cases this may require a complete redesign of the part. In most instances the redesign is a good investment.

One remaining problem with the part shown in Figure 4 is that the threaded projection is the thickest section in the part. This thicker section will require more plastic material and a longer molding cycle. The threaded projection will stay hot longer and shrink more than the rest of the part. This will make it difficult to maintain precision dimensions on the threaded projection. The thread could be cored out by molding a hole the length of the threaded projection. Coring out the threaded projection would reduce the cycle time and the amount of plastic material being used, while improving the dimensional reproducibility of the thread.

Regrettably, coring out the threaded projection would require a side-acting-core-pulling mechanism. This increases the tool cost and complicates the mold. But maintaining a uniform wall thickness is highly desirable. Whether or not a more complex mold is justified can only be determined on a case-by-case basis.