Sink marks are small craters or depressions that develop in thicker areas of the injection molded prototype when shrinkage occurs in the inner portions of the finished product. The effect is somewhat similar to sinkholes in topography, but caused by shrinkage rather than erosion.
Sink marks are often caused when the cooling time or the cooling mechanism is insufficient for the plastic to fully cool and cure while in the mold. They can also be caused by inadequate pressure in the cavity, or by an excessive temperature at the gate. All else being equal, thick sections of the injection molded part take longer to cool than thin ones and so are more likely to be where sink marks are located.
In order to avoid the sink mark, mold temperatures should be lowered, holding pressure increased, and holding time prolonged to allow for more adequate cooling and curing. Reducing the thickness of the thickest wall sections will also ensure faster cooling and help reduce the likelihood of sink marks.
The following photos show the sink mark on thick solid plastic wall:
