Sink marks and voids both result from localized shrinkage of the material at thick sections without sufficient compensation.
Sink marks appear as depressions on the surface of a molded part. These depressions are typically very small; however they are often quite visible, because they reflect light in different directions to the part. The visibility of sink marks is a function of the color of the part as well as its surface texture so depth is only one criterion. Although sink marks do not affect part strength or function, they are perceived to be severe quality defects.
Sink marks are caused mainly by thermal contraction (shrinkage) during 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, deformation of the skin may be replaced by formation of a void in the core.
Voids are caused when the outer skin of the part is stiff enough to resist the shrinkage forces thus preventing a surface depression. Instead, the material core will shrink, creating voids inside the part.
Optimize packing profile. As sink marks occur during packing, the most effective way to reduce or eliminate them is to control the packing pressure correctly. To determine the effects of packing on sink marks, use a simulation package such as Autodesk Simulation Moldflow Insight.