Meaning
Threaded metal components designed for post-moulding installation into thermoplastic bosses define this hardware category. A heat-set insert uses thermal energy to melt a thin layer of surrounding plastic, allowing the knurled exterior to sink into the pre-formed hole. Once the resin re-solidifies, the metal is mechanically locked into the part.
Thermal Bonding
Precise control of the insertion temperature ensures a clean melt without degrading the polymer chains. Cold-pressing the same hardware would create internal stresses that lead to cracks.
Melt Flow
Surface geometry on the outer diameter of the brass or stainless steel body dictates the final grip within the plastic. Helical or diamond knurls provide paths for the molten resin to flow, filling every void to create a high-surface-area interface. If the hole is too small, excess melt is forced out of the top, creating an unsightly flash that interferes with the mating part.
Conversely, a hole that is too large prevents the knurls from engaging deeply enough to resist torque.
Load Retention
Resistance to axial pull-out and rotational torque depends on the depth of the knurl and the grade of the host resin. Amorphous resins like polycarbonate provide excellent dimensional stability around the insert, while crystalline resins like nylon may require specific knurl patterns to handle moisture-related expansion. The resulting joint provides the durability of a metal thread with the light weight of a plastic housing.
