the method and apparatus of the invention is used to heat the surface of the mold insert or cavity by high frequency current . there are holes near the heated surface in the mold and the coils can be installed into the holes . the coils surround the heated surface and are conducted with high frequency current . due to the directional change of the current , the blocks that are surrounded by the coils will be heated by the hysteresis losses and the eddy - current losses . the surface of the mold insert or cavity will be heated rapidly . there are cooling holes set near the heated surface or beside the coil - pipe . the cooling liquid or air can flow in the holes to carry out extra energy and the temperature of the mold will be decreased . the position of the cooling holes , the flow speed and temperature deviation of the liquid and air will influence the temperature of the mold . the method and apparatus will improve the quality of the thermal - plastic products and elevate the number of the cavity .