Sink marks are one of the most frustrating cosmetic and dimensional defects in injection molding. A part can look fully filled, the machine can appear stable, and the operator may keep increasing holding pressure, yet the depression at the rib root or thick section still comes back.
The reason is simple but often missed: sink marks are usually not a filling-stage problem. They appear after cooling, when the thick section continues to shrink but can no longer receive enough molten material from the gate or runner system.
Why Sink Marks Are Not Only a Packing Pressure Problem
A common first reaction is to increase packing pressure or extend holding time. That can help when the feeding path is still open. But when the thin wall has already frozen and blocked the melt channel, extra pressure cannot reach the area that needs compensation.
This is why sink marks often appear near rib roots, bosses, thick bases, and wall-thickness transitions. Thin regions solidify first. Thick regions keep shrinking longer. Once the path between them freezes, the thick section cannot be fed, and the surface pulls inward.
Start With Gate Position and Feeding Path
Gate location is one of the strongest mold-side controls for sink marks. The gate should support the last area that needs compensation, especially when thick ribs or structural bases are involved.
Runner size also matters. If the runner is too restrictive, packing pressure is lost before it reaches the cavity. A larger and better-balanced runner can improve compensation efficiency, especially for thick-wall or reinforced parts.
Use a Three-Stage Packing Profile Instead of One Long High Pressure Step
A single high packing pressure can create new problems. It may overpack easy-flow regions, increase internal stress, and produce flash while the sink mark area remains underfed. A staged packing profile is usually more controllable.
Stage 1: High Pressure to Complete Cavity Filling
The first stage uses higher pressure, often around 80% to 90% of the practical pressure setting, to complete filling and bring the part close to full volumetric condition.
Stage 2: Medium Pressure for Controlled Compensation
The second stage reduces pressure to a medium level, often around 50% to 60%, so the part can be compensated without forcing excessive material into already packed areas.
Stage 3: Low Pressure Until Gate Freeze
The third stage uses lower pressure, often around 25% to 30%, and holds until the gate freezes.
Find Gate Freeze Time With a Part Weight Curve
A simple method is to mold parts while increasing holding time in small steps, such as 0.3 seconds, and weigh each part. When the part weight no longer increases, the gate is effectively frozen.
Control Mold Temperature Difference Between Thick and Thin Areas
A useful process principle is to keep the thick-wall area slightly warmer than the thin-wall area, commonly by about 5 to 10 degrees Celsius where the mold and resin system allow it. This delays solidification in the heavy section just enough to preserve the feeding path during packing.
Use Material Changes When Mold and Process Adjustments Are Not Enough
Adding short glass fiber can reduce shrinkage because the fibers restrict polymer contraction. For semi-crystalline polymers, nucleating agents can also help by making crystallization faster and more uniform.
How to Verify That Sink Marks Are Truly Solved
1. Cut the Sink Mark Area and Inspect the Section
Cut through the problem area and inspect for voids or internal shrinkage.
2. Measure 20 Consecutive Part Weights
Stable compensation should produce low weight variation. A tight weight window indicates that the gate freeze time and packing profile are under control.
3. Use a Contact Area or Transfer Test
For local sink areas, a marking or contact test can show whether the surface is actually supported and packed.
Conclusion
Injection molding sink marks are not simply a sign of low pressure or operator error. They are usually the visible result of blocked feeding, uneven wall thickness, cooling imbalance, and uncontrolled shrinkage after the cavity has been filled.
The strongest fixes start with the feeding path: gate location, runner efficiency, packing profile, gate freeze time, and mold temperature zoning.
