In plastic injection molding, a sink mark is a surface defect that appears as a depression, dimple, or dent on the surface of a molded part. It typically occurs in thicker sections of the part or near features like ribs, bosses, or other areas where there is a significant variation in wall thickness. Sink marks are caused by uneven cooling and shrinkage of the plastic material during the molding process.
Causes of Sink Marks
1. Material Shrinkage:
As the molten plastic cools and solidifies, it shrinks. If the outer surface cools and hardens faster than the inner material, the inner material continues to shrink, pulling the surface inward and creating a sink mark.
2. Insufficient Packing Pressure or Time:
During the holding/packing phase, additional material is pushed into the mold to compensate for shrinkage. If the packing pressure or time is too low, the material shrinks excessively, leading to sink marks.
3. Thick Sections:
hicker areas of the part cool more slowly, increasing the likelihood of shrinkage and sink marks.
4. High Melt Temperature:
Higher melt temperatures can increase the material’s shrinkage as it cools, contributing to sink marks.
5. Improper Gate Location or Size:
If the gate (the entry point of molten plastic into the mold) is too small or poorly located, it may restrict material flow, preventing adequate packing and causing sink marks.
6. Material Properties:
Some materials, especially semi-crystalline plastics (e.g., polypropylene, nylon), have higher shrinkage rates and are more prone to sink marks.
How to Identify Sink Marks
Sink marks are visible as shallow depressions on the surface of the part.
They are most common in areas with thicker walls or near structural features like ribs, bosses, or mounting points.
How to Prevent Sink Marks
1. Optimize Part Design:
Maintain uniform wall thickness throughout the part.
Use ribs or coring to reduce material volume in thick sections.
2. Adjust Process Parameters:
Increase holding/packing pressure and time to ensure adequate material packing.
Optimize cooling time to allow uniform cooling of the part.
Reduce melt temperature to minimize shrinkage.
3. Improve Mold Design:
Position gates near thick sections to ensure proper material flow and packing.
Optimize cooling channels to ensure even cooling, especially in thick areas.
4. Select Appropriate Materials:
Use materials with lower shrinkage rates, such as amorphous plastics (e.g., ABS, PC).
For crystalline plastics, consider using filled or reinforced materials to reduce shrinkage.
Corrective Actions for Existing Sink Marks
Increase holding pressure and time.
Reduce melt temperature.
Extend cooling time.
Modify part design to reduce wall thickness or add coring.
Adjust gate size or location.
By addressing these factors, sink marks can be minimized or eliminated, resulting in higher-quality injection-molded parts.
