Sink Marks

Sink Marks are depressions or shrinkage marks that occur on the surface of a molded part due to variations in cooling rates.


Sink marks are a common defect encountered in the injection molding process, characterized by depressions or shrinkage marks on the surface of molded parts. These marks can affect both the aesthetics and structural integrity of the final product. Understanding the causes and preventive measures for sink marks is crucial for manufacturers striving to produce high-quality molded parts. In this article, we delve into the intricacies of sink marks, exploring their root causes and providing insights into minimizing their occurrence.

Causes of Sink Marks

Sink marks primarily result from variations in cooling rates during the solidification phase of the molded part. Several factors can contribute to the formation of sink marks:

  1. Part Geometry: Thick sections in the design of the part tend to cool more slowly than thinner sections, leading to differential cooling rates and potential sink marks.
  2. Material Selection: Certain polymers, such as those with high filler content or low melt flow rates, are more prone to sink marks due to their higher viscosity and slower cooling characteristics.
  3. Injection Parameters: Inadequate packing pressure or insufficient cooling time can result in insufficient material flow and premature solidification, leading to sink marks.


To minimize the occurrence of sink marks and enhance the overall quality of molded parts, manufacturers can implement the following preventive measures:

  1. Design Optimization: Careful part design can help alleviate sink marks. Consider incorporating uniform wall thickness, avoiding abrupt changes in part geometry, and adding ribs or gussets to provide additional support.
  2. Gate Placement: Proper gate placement helps ensure adequate material flow and packing. Positioning gates near thicker sections of the part promotes even filling and reduces the likelihood of sink marks.
  3. Cooling System Optimization: Optimizing the cooling system within the mold helps maintain consistent cooling rates across the part. This can be achieved by incorporating cooling channels in critical areas, adjusting cooling time, or using mold temperature controllers.
  4. Material Selection: Choosing materials with lower viscosity or additives that promote better flow and faster cooling can mitigate the risk of sink marks.
  5. Process Monitoring and Optimization: Regular monitoring of process parameters, such as injection speed, packing pressure, and cooling time, allows for fine-tuning of the injection molding process to minimize sink marks.


Sink marks can be a common challenge in injection molding, affecting the quality and appearance of molded parts. By understanding the underlying causes and implementing preventive measures, manufacturers can effectively minimize the occurrence of sink marks. Through careful part design, optimized gate placement, cooling system enhancements, appropriate material selection, and process optimization, manufacturers can produce high-quality molded parts with reduced sink marks, ensuring customer satisfaction and operational efficiency in the injection molding process.