Fixing Sink Marks in Thin-Walled Injection Molding Parts

What causes sink marks in thin walls

Sink marks show up as shallow dimples on the surface of a part. They appear directly behind thicker areas, such as bosses, ribs, or gate regions. The defect forms because the resin in those thick zones stays liquid longer than the material in the thin walls.

Consider a housing with a 1.5 mm wall and a 4 mm circular boss. As the melt flows into the cavity, the thin wall cools rapidly against the cold mold steel. It solidifies almost immediately. The boss, however, acts as a heat sink. It retains thermal energy and remains in a viscous state for a longer period.

When the thin wall cools and solidifies, it locks in place. The thicker section continues to shrink as it loses heat. This internal shrinkage pulls the already-set surface inward. The result is a visible depression. The force is not uniform; it is concentrated where the thermal mass is highest.

Thin-walled parts are more vulnerable to this behavior. The walls cool fast, but they lock onto the mold surface quickly. If the mold does not remove heat evenly, the resin underneath the thick areas remains mobile. The shrinkage force becomes large enough to distort the finished surface.

A common misconception is that sink marks are caused solely by high injection pressure. While pressure contributes to the initial fill, the primary driver is thermal imbalance. A low-pressure process with poor cooling can produce severe sink marks, just as a high-pressure process with excellent cooling might produce none. The core issue is the time difference between solidification of the thin wall and the thick feature.

Common symptoms of the defect

Before adjusting the machine or redesigning the tool, identify exactly where the issue appears. Different symptoms point to different root causes.

• Dimples directly behind ribs or bosses.
• Shallow depressions on flat, thin panels.
• Sink marks that appear only at the end of long flow paths.
• Surface dimples that become deeper at low production temperatures.
• Dimples near the gate where resin enters the cavity.

If the dimple is small and shallow, the issue may be minor. If it is deep enough to affect assembly or appearance, the cooling imbalance is significant.

Pay attention to the location relative to the flow. A dimple at the end of a long flow path often indicates that the resin cooled and solidified before the packing stage could replenish the volume. A dimple directly behind a feature indicates a local thermal mass issue.

Check the pattern across the mold. If only one cavity shows the defect, inspect that specific cavity for cooling blockages, chip damage in the mold steel, or air leaks in the coolant lines. If all cavities show the defect, the problem is likely process-related or systemic to the mold design.

Troubleshooting table for sink marks

Use this table to match the symptom with a likely cause and a practical fix. The goal is to isolate the problem before changing multiple variables at once.

Symptom	Likely cause	What to do
Dimple behind a boss	Boss cools slower than surrounding wall	Reduce boss height or add a vent hole
Shallow dimple on flat panel	Uneven mold temperature across surface	Balance cooling channel flow rates
Deep sink at flow end	Resin shrinks after wall sets	Increase injection pressure or slow cooling
Dimple near the gate	High shear heating keeps resin liquid	Reduce injection speed or lower melt temperature
Mark appears only in winter	Mold surface temperature drops unevenly	Increase mold temperature or insulate cooling

Cooling strategy for thin sections

Cooling is the primary lever for controlling shrinkage. The mold must remove heat at the same rate across the entire cavity. If one area stays hot, the resin there remains fluid longer.

For thin-walled parts, the mold surface temperature matters more than the barrel temperature. Thin walls solidify quickly, but the resin underneath thick features can stay soft for seconds or minutes. Cooling channels should be sized to remove heat efficiently without creating hot spots.

Check the temperature at the gate and at the farthest point of the part. If the difference exceeds a few degrees, the cooling is unbalanced. Adjust the coolant flow rate or the temperature of the cooling medium. In many cases, increasing the flow rate to the cooler side of the mold balances the shrinkage forces.

Inspect the cooling layout during setup. Ensure that the coolant lines are not blocked by scale or debris. A blocked line acts as an insulator, keeping the mold steel hot. This creates a local hot spot where the resin lingers.

Consider the coolant temperature. Lowering the coolant temperature increases the cooling rate, but it can also increase the risk of warpage if the part cools unevenly. For thin-walled parts, a slightly higher mold temperature can sometimes help the resin pack more uniformly before solidification. The key is to match the cooling rate to the part thickness.

Adjusting process parameters

Changing the machine settings can reduce sink marks without touching the mold. However, each change affects other properties. A faster injection speed may reduce the time the thick section stays liquid, but it can increase shear heating.

1. Increase the injection pressure in the final packing stage. This forces more resin into the thick areas, reducing the volume that will shrink.
2. Slow down the cooling rate. This keeps the thick section fluid longer, allowing the resin to flow into the void before the thin wall sets.
3. Lower the melt temperature. Cooler resin has lower viscosity and shrinks less during solidification.
4. Reduce the holding time. This minimizes the amount of extra resin packed into the thick zones.

Adjust one parameter at a time. If you change both pressure and temperature simultaneously, you will not know which change improved the result.

Start with the packing pressure. If the dimple is deep, the part is likely short of material. Increasing the pack pressure forces more resin into the thick sections. Monitor the clamping force of the machine. If you exceed the machine’s rated tonnage, the mold may open, causing flashes or other defects.

Next, evaluate the cooling time. If the part is well-packed but still shows sink marks, the cooling is too fast. The resin in the thick section is solidifying before the shrinkage force can be relieved. Extending the cooling time allows the part to shrink uniformly. However, this increases cycle time, so find the balance between quality and productivity.

Design changes that prevent the defect

Sometimes process adjustments cannot fully eliminate the problem. In those cases, the mold design needs revision.

Reduce the wall thickness. Thinner walls cool faster and lock in place sooner. The resin in thick areas has less time to shrink and pull the surface. A uniform wall thickness also simplifies the cooling design.

Add ribs instead of thickening the wall. A rib with a small cross-section adds stiffness without the large volume of a solid wall. The rib cools faster than a thick wall of the same height. This reduces the shrinkage force behind the feature.

Move the gate closer to the thick area. The gate is usually the last part to cool. If it is far from a boss, the boss may solidify before the gate. This creates a sink mark. Moving the gate or adding a secondary gate can help.

Review the part geometry for thermal breaks. Sharp corners or sudden transitions from thin to thick sections create stress concentrations and uneven cooling. Filleting these corners allows the resin to flow more smoothly and reduces the thermal gradient.

Consider the material selection. Some resins shrink more than others. If the current material has a high shrinkage rate, switching to a lower shrinkage grade might resolve the issue. However, this must be done with an understanding of the mechanical properties required for the part.

Preventing future defects

Once the current issue is fixed, document the settings and design changes. This prevents the problem from returning when the mold is moved to a new machine or when the resin lot changes.

Run a design review before tooling. Check the wall thickness profile in the CAD file. Look for sudden changes from thin to thick sections. These transitions are the most likely places for sink marks to appear.

Validate the cooling layout during mold construction. The cooling channels should follow the part geometry. A straight line of cooling channels across a curved boss will not remove heat evenly. The channels should be offset or shaped to match the hot resin volume.

Perform a first article inspection with a known resin lot. Record the sink mark depth if any dimples appear. This baseline helps you detect changes during production.

Maintain a log of process parameters. If the machine is moved to a different location or the resin lot changes, the baseline helps you quickly identify and correct drift.

When to escalate the issue

Some sink marks are too deep to fix with minor adjustments. If the dimple exceeds the acceptable tolerance for the part, the mold may need a hot runner change or a rework of the cooling lines.

In rare cases, the resin itself may be the problem. Some materials have high shrinkage rates. Switching to a lower shrinkage resin can reduce the defect, but it may change the part’s mechanical properties.

Always check the part drawing first. If the drawing allows for a certain amount of sink, the part may be within spec. If the drawing requires a flat surface, the mold and process must be adjusted to meet that requirement.