a technique for permanently joining thermoplastic parts in a single stroke converts kinetic energy into thermal energy . in this technique , one of the thermoplastic parts includes a linear shaft while the other includes a cavity having the same general shape but being slightly smaller in its cross - section . the thermoplastic parts are permanently joined when the shaft is thrust into the cavity at a velocity that exceeds a predetermined minimum value which is a function of the thermoplastic materials used and the relative dimensions of the shaft and cavity . an interference fit between the shaft and cavity causes frictional heat to be generated which melts the contacting surfaces and thus welds the parts together after a brief cooling period .