PURPOSE: To permanently fuse a thermoplastic material within a short cooling time by bringing a shaft and a cavity to a contact state and further pushing the shaft into the cavity at a speed sufficient to bring about the melting of the thermoplastic material of a thermoplastic part in at least one region and cooling the melted thermoplastic material to weld the same.

CONSTITUTION: A boss 11 and a shaft 12 are formed in the structure of a part 16. A cavity 13 is provided in a slightly larger dimension so as to receive the shaft 12 for the purpose of mutually bonding the parts 10, 12 in the boss 11. The inlet 14 of the cavity 13 is inclined in order to easily introduce a pair of shafts 21 in a manufacturing process. The cavity 13 is tapered so that the diameter of 0.097 inch in the leading end thereof is reduced to the diameter of 0.092 inch of the bottom thereof. The shaft 21 is pushed at speed generating the surface melting of the thermoplastic material in the cavity 13. After a short cooling time (1 sec or less), the shaft 12 is welded to the boss 11. The shaft 21 and the boss 11 have a round cross section and constituted of an ABS material. When the desired dimensions of the shaft and the cavity are used, welding is generated at a speed of 50 inch per sec.