When the plastic melt is filled into the mold cavity and cooled rapidly, the cooling rate of the surface of the product is much faster than that of the inner layer, and the surface layer cools quickly and solidifies.

When the gate of the mold is closed, the internal cooling shrinkage position can no longer be refilled.

The inside of the product will be in a tensile state due to shrinkage, while the surface layer will be in the opposite state of compressive stress, which will remain in the product after the mold is opened and will not be able to eliminate the surface, which is called excessive residual stress.