ABSTRACT
This chapter discusses fastening-site design for self-threading screws and provides suggestions for rectifying problem sites. It deals with the subject of tapping in plastics. Proper design of the fastening site plays a major part in the successful use of self-threading screws. Most fastening areas consist of protruding posts called bosses, which have taper-cored pilot holes. Hole size is determined by examining a combination of screw performance requirements, such as torque and pullout, which result from the interplay of three major factors: the size of the fastener, the type of fastener, and the plastic material used. A well-designed fastening site should accommodate an engagement length two to three times the diameter of the screw. Screws can, of course, be installed to whatever depth is required. One of the most common problems when using self-threading screws in plastics is stripping during assembly. The relatively low strength of many plastics can result in low torque values.
