Meaning
Hardware designed to displace material and carve its own mating thread in a pre-drilled or moulded hole represents the fastening method. In polymer assembly, a thread-forming screw creates a strong mechanical connection without the need for brass inserts or separate nuts. This method lowers assembly costs and reduces the number of components in the bill of materials.
Installation Torque
Drive torque must be high enough to form the thread but low enough to avoid stripping the plastic. The difference between the driving torque and the stripping torque is the process window for the assembly. If the plastic is too hard or the pilot hole is too small, the torque will spike, which can fracture the screw or split the boss, especially when using filled polymers that have low elongation at break.
Moulding Detail
Design of the pilot hole is critical for a successful fastening operation. A molded hole should have a slight taper to facilitate mold release, but the entry diameter must match the screw specifications. If the mold core pin is worn, the hole will be too small, increasing the installation stress and causing the plastic around the hole to crack.
Stress Field
Fastener insertion creates high residual stress. These stresses can lead to delayed cracking of the boss if the material is susceptible to stress cracking.
