Self tapping screws

Self-tapping screws are used for assembling parts. Fewer parts need to be assembled compared to bolt-and-nut connections, with lower fastener and equipment costs. No nuts are required, so one smooth surface is obtained. Also the aesthetics of self-tapping screws are better than if bolts are used. Recyclability is good.

In self-tapping screw assemblies, mating plastic threads are formed directly on the part when the screw is tightened into the assembly. They have limited durability and repeated assembly is possible to a certain extent.

A self tapping screw assembly typically includes a through clearance boss and a boss with a pilot hole. The holes can be molded-in or drilled through. The diameter of the pilot hole is greater than the root diameter of the screw, but smaller than the outside diameter of the screw.

Self tapping screws can be distinguished as either thread cutting screws or thread forming screws.

Thread cutting screws cut the thread during assembly. That means that every time the screw is assembled some material will be cut away. For that reason this type of screw is not recommended for repeated assembly and disassembly. In general, thread cutting screws are used for polymers with a low elongation at break and no ability to deform plastically. The relatively low hoop stress level associated with their use makes them suitable for use with glassy amorphous materials subject to crazing. As this type of screw generates small “chips” during the cutting process, space for the chips must be provided when blind holes are used. The chips can be a nuisance when through pilot holes are used.

Thread forming screws do not cut but deform the thermoplastic. Close to the screw the stresses can be high. Thread forming screws are generally used with lower modulus plastics, since ductility or cold flow is a prerequisite for their use. Thread forming screws can be used for repeated assembly and disassembly. In general, thread forming screws have higher drive and strip torque values than thread cutting screws.