Injection Molding Shock Marks Near the Gate: Causes and Solutions
In the molding of rigid plastic parts, dense corrugated marks sometimes appear on the surface near the gate. These marks are typically centered around the gate area and are often referred to as shock marks or flow shock patterns.
This defect usually occurs when the melt viscosity is too high and the molten plastic fills the cavity in a hesitant or stagnant flow pattern. When the leading edge of the melt contacts the cavity surface, it cools, condenses, and shrinks quickly. The following melt then pushes forward and expands the partially cooled material, allowing the flow to continue. This repeated alternation between cooling, shrinkage, and forward expansion creates visible surface corrugation in the flow direction.
Main Causes of Shock Marks
- Melt viscosity is too high, resulting in unstable cavity filling.
- Barrel temperature or nozzle temperature is too low, reducing melt flowability.
- Mold temperature is too low, causing the melt front to freeze too quickly.
- Injection pressure or injection speed is insufficient for fast and uniform filling.
- Runner or gate dimensions are too small, creating excessive flow resistance.
- Mold venting is poor, affecting smooth melt flow.
- Part wall thickness is too thin, increasing filling difficulty.
Solutions
- Increase the barrel temperature, especially the nozzle temperature, to improve melt flow.
- Increase mold temperature to reduce premature cooling at the cavity surface.
- Raise injection pressure and injection speed so the cavity fills more quickly and smoothly.
- Optimize runner and gate size to reduce excessive resistance during filling.
- Improve mold venting and provide a sufficiently large cold slug well.
- Avoid designing plastic parts with excessively thin wall sections whenever possible.
Conclusion
Shock marks near the gate are mainly caused by unstable melt flow combined with rapid surface cooling during injection molding. By improving melt temperature, increasing filling speed and pressure, optimizing gate and runner design, and ensuring proper venting, manufacturers can significantly reduce this type of surface defect and improve part appearance.