Why does sink mark often occur in the molding with thicker section and how to improve it?

Sink marks and voids are caused by localized shrinkage of the material at thick sections without sufficient compensation when the part is cooling. After the material on the outside has cooled and solidified, the core material starts to cool. Its shrinkage pulls the surface of the main wall inward, causing a sink mark. If the skin is rigid enough, as in engineering resins, deformation of the skin may be replaced by formation of a void in the core.

Sink marks can be eliminated by decreasing the barrel temperature. Increase the amount of pressure or the time applied. Upon initial startup the mold should be filled incrementally starting with intentional short shots (if the mold design allows) and progressively increasing pressure shot-by-shot until the mold is filled and packed properly. If injection pressure and time are adequate the shrinkage should be uniform and consistent resulting in parts without sink marks.