Assembly Considerations

Thread profiles of the types used in metal typically have sharp edges and corners that can reduce the part's mechanical performance and create molding problems in plastic designs. The sharp inside corners formed at the root of standard threads act as stress concentrators and can greatly diminish the ability of the plastic threads to bear loads and withstand impacts.

Rounding the thread's crests and roots lessens these effects. Crests can be flattened to increase the thread cross section to better resist stripping and shear forces. Figure 1 shows some modified thread profiles commonly used in plastic applications.

Very fine molded-in threads should be avoided in plastics as they are difficult to mold and can cause cross threading if extreme care is not taken during thread engagement and tightening. A thread pitch of 32 threads per inch is considered to be the maximum limit for plastic, molded-in threads.

Stop threads short at the ends to avoid making thin feathered edges that are flimsy and prone to cross-threading and damaged. Figure 2 shows the preferred ways to begin and end molded-in threads.

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