on the other hand, in order to remove such defect as the welding mark and to produce plastic parts with high lucency, dielectric such as high temperature and high pressure steam, high pressure hot water or thermal conducting oil should be applied to the molds to raise surface temperature molds. The most ideal status is that the surface temperature of molds can uniformly remain at the heat distortion temperature of plastic parts and meanwhile the speed of cooling on the section of the product should be kept as uniform as possible.