Thread Design

Thread are used in plastic for the purpose of providing a secure anchorage or locking device for a mating plastic injected part. There are two types of threads are used in plastic injected part design: internal and external thread. External or internal screw threads can be molded. An internal thread is on the inside of the plastic injected parts which molded-in by a threaded core. An external thread is on the outside of a plastic injected part which molded by threaded pipe or threaded hold or by two half-molds.

Threads in plastic injected parts are obtained by four methods: molded-in; trapped; threaded metal inserts and thread inserts are pressed or cemented into plastic injected parts after injection molding.

Threads formed by thread plug or tube should be taken to eject the plastic injected parts from the mold. Usually, mechanical unscrewing or collapsible cores can be used in injection molding. There are different types of threads used in the plastic industry. It includes the V sharp, square, acme, buttress, round profile thread, unified screw thread. Fig 1 shows profiles of different types of threads which usually used in plastic injected parts.

Fig 1a shows a developed unified screw thread which is used frequently. The feature is that the tip or crest of the thread is flat, the root of the thread has a radius and does not leave a sharp edge. It can be quick and easy assembly of the plastic injected parts which are desired, and for all work where conditions do not use of fine-pitch threads. 

A square thread (Fig 1b) is used where the highest strength is desired such as in pipe fittings. The thread unscrewing is not as easy as unified screw thread during molding.

The acme thread (Fig 1c) is widely used in applications requiring strength because this type of thread is much easier to mold or unscrew than the square thread.

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Fig 1 Profiles of different types of threads