In thermal insertion, a metal insert is placed in a cored or drilled hole which is slightly smaller than the insert. This hole provides a certain degree of interference and also serves to guide the insert into place. The heat from the thermal tip travels through the insert to the interface of the metal and plastic. Heat generated by the thermal tip causes the plastic to melt, and as the thermal tip advances, the insert is imbedded into the component. The molten plastic flows into the serrations, flutes, or undercuts of the insert, and when the heat terminates, the plastic resolidifies and the insert is securely encapsulated in place.
Inserts that can be installed by thermal insertion include a variety of bushings, terminals, ferrules, hubs, pivots, retainers, feed-through fittings, fasteners, hinge plates, binding posts, handle-locating pins and decorative attachments.
Typically, the plastic component is fixtured and the insert is driven in place by the thermal tip (see illustration). The functional characteristics or requirements of an application actually determine the insert and hold configuration. In all cases, a sufficient volume of plastic must be displaced to fill the undercuts, flutes, knurls, threads and/or contoured areas of the insert. Care should be exercised in selecting the proper inserts. Inserts are designed for maximum pull-out strength requirements while inserts with vertical grooves or knurls are usually recommended for high torque retention. Regarding the hold configuration or insert selection, the recommendations provided by the insert manufacturer should always be observed. To prevent a “jack-out” condition, the top of the seated insert should be flush or slightly above the surface of the part. Rigid fixturing should be placed directly under the insert.To maintain an accurate depth of insertion, the total distance the thermal tip travels should be limited either mechanically by a positive stop, electrically by a lower-limit linear encoder, or both.