How to Reduce Sink Marks and Internal Shrinkage in Injection Molded Parts
Shrinkage in injection molded parts is a common defect that includes both surface sink marks and internal voids. These problems are usually caused by insufficient filling of thicker sections during cooling.
In many cases, simply increasing injection pressure, enlarging the gate, or extending injection time does not completely solve the problem, especially for thick-walled plastic parts.
Why Shrinkage Occurs
Shrinkage happens when the outer layer of the plastic part cools and solidifies faster than the inner section. As the internal material continues to cool and contract, it creates a vacuum effect inside the part.
This internal shrinkage pulls the surface inward, causing visible sink marks or internal voids. The problem is more severe in materials with high shrinkage rates or slower surface hardening.
Among commonly used materials, PC and PP are particularly difficult when it comes to controlling shrinkage. PC cools quickly, while PP tends to develop both internal voids and surface sink marks more easily.
Methods to Reduce Shrinkage
1. Reduce Cooling Time and Eject Earlier
One effective method is to shorten the cooling time and eject the molded part while it is still relatively hot.
When the outer layer remains warm and less hardened, the temperature difference between the surface and the center is smaller. This promotes more uniform overall shrinkage and reduces localized sink marks and internal voids.
2. Increase Surface Cooling After Ejection
After early ejection, the part can be cooled externally to help maintain sufficient surface hardness. This makes the surface less likely to collapse under internal vacuum pressure.
For severe shrinkage problems, stronger cooling methods such as chilled water can quickly harden the surface layer and reduce visible sink marks. However, internal voids may still remain inside the part.
3. Extend Holding Time
Extending the packing or holding stage instead of only extending cooling time can improve both surface sink marks and internal shrinkage. This allows more molten material to enter the cavity before the gate freezes.
4. Optimize Mold Temperature
If the mold temperature is too low, shrinkage may become more severe. In most cases, the mold should be cooled using standard machine water rather than extremely cold water.
For some materials such as PC, increasing mold temperature can improve shrinkage performance. For example, a mold temperature around 100°C may significantly reduce internal voids and sink marks in PC parts.
However, if the goal is specifically to reduce visible sink marks on the surface, lower mold temperature may sometimes be more effective because it hardens the surface faster.
Using Gas-Assisted Injection Molding
For severe sink mark problems in thick-wall or partial-thickness parts, gas-assisted injection molding can be an effective solution.
Gas-assisted injection molding introduces high-pressure gas into the thick section of the plastic part, creating a hollow channel inside the part. The gas maintains internal pressure during cooling, which helps eliminate sink marks and reduce internal shrinkage.
Compared with traditional injection molding, gas-assisted molding offers several advantages:
- Reduced sink marks and shrinkage defects
- Lower internal stress
- Improved dimensional stability
- Lighter product weight
- Better surface appearance
- Higher part strength in thick-wall applications
Conclusion
Shrinkage defects are closely related to wall thickness, cooling speed, mold temperature, and material selection. For difficult materials such as PC and PP, controlling sink marks requires more than simply increasing pressure or injection time.
By optimizing cooling strategy, extending holding time, adjusting mold temperature, and using gas-assisted injection molding where necessary, manufacturers can greatly reduce both surface sink marks and internal shrinkage in molded parts.