Thread cutting and thread forming self tapping screws for plastics

Self-tapping screws for plastics,  form or tap its threads when screwed into the plastic material with either pre-drilled or moulded pilot holes.

Standard screws and self-tapping screws for metal such as type A and type AB are not suitable for plastic because of their wide flank angle and shallow threads. They are used on sheet metal and thick plastic sheets like UPVC frames. The two types of self-tapping screws available to use on most common plastics are thread forming and thread cutting screws.

Thread forming
Thread forming screws for plastic doesn’t have a pointed tip and would need a pilot hole.  Screw thread deforms the plastic material to flow around the screw threads.  Hence the overlapping amount (thread engagement) and thread profile angle are crucial.

Thread forming screws creates high internal stresses because the material is deformed rather than removed. As a result, these screws can only be utilised in soft polymers with lower flexural modulus.

Thread cutting
Thread cutting screws for plastics, also known as a thread tapping screws, has a sharp cutting edge that cuts plastic when they are screwed into the pilot hole, forming a threaded joint. This produce reduced internal stresses compared to thread forming screws, making them suitable for stiffer plastic material with higher flexural modulus.
It is important to have the correct pilot hole diameter and depth. Hole depth has to be deeper than the screw engagement length to allow space for the removed material to move. For fibre or glass-filled plastics, these provide high thread engagement, high clamp loads & reduced stress. The biggest drawback of the thread cutting screw is thread stripping during disassembly.