What Causes Sink Marks in Injection Molding and How to Reduce Them

During plastic part production, sink marks are one of the most common molding defects. As the injected plastic cools inside the mold, the material undergoes volumetric shrinkage. The outer surface cools and hardens first, while the inner material continues to cool more slowly. If shrinkage inside the part is not properly compensated, internal voids or bubble-like shrinkage zones may form, and the surface above these areas can be pulled inward, creating the visible defect known as a sink mark.

Materials with higher shrinkage rates are generally more likely to develop sink marks. When molding conditions are adjusted to reduce sink marks, the process is usually changed in a direction that reduces shrinkage. This often includes lowering mold temperature, lowering barrel temperature where appropriate, and increasing injection and packing pressure. However, these adjustments should be made carefully, since excessive pressure or overly aggressive settings may create residual internal stress in the molded part.

In some cases, sink marks are not highly visible and do not affect part function. When appearance requirements allow, the mold surface may be given a textured finish such as grain, matte, or etched patterns to help reduce the visual impact of minor sink marks. For certain materials such as HIPS, lowering the mold temperature may also help reduce the visibility of the defect on the finished surface. However, once a polished part shows obvious sink marks, it is usually difficult to correct the appearance without changing the mold design or molding process.

Materials with high mold shrinkage, such as PE and PP, are especially prone to sink marks. Even relatively small ribs or localized thick sections may create visible surface depressions if wall thickness and packing conditions are not properly controlled.