Ultrasonic inserting is the process of embedding a metal component into a thermoplastic part. This process replaces the costly, time-consuming, conventional method of injection molding plastic around the metal component. In ultrasonic insertion, a hole slightly smaller than the insert to be embedded is either molded or drilled into the plastic part. The hole provides a degree of interference (usually 0.015 - 0.020 inch) and guides the insert into place. The metal insert is usually designed with exterior knurls, undercuts or threads to resist loads imposed on the finished assembly. (Fig. 1)
The functional characteristics or requirements of an application determine the insert and hole design. A sufficient volume of plastic must be displaced to fill the exterior undercuts, knurls and/or threads of the insert to lock it in place and produce the strength required for the operation.
A typical threaded insert, designed primarily for pullout strength should have multiple undercuts to provide maximum resistance (Fig. 2). Where maximum torque strength is required, insert should have long axial knurls (Fig. 3). Combimation inserts with both undercuts and knurls are designed to provide pullout and torque strength.
