Gas dwell, upon the completion of filling in the mold cavity and on the premises that the gas pressure is preserved, the product should be put aside until it gradually cools to compensate shrinkage of the product due to the cooling of materials. At this stage, the gas applies uniform pressure on the plastics from inside to outside so that outer surface of the product always clings to the inner wall of mold cavity, thereby leaving no depression on the outer surface of parts with thick walls and improving quality of the product.