Troubleshooting Sinking Marks in Polycarbonate Parts

Why Polycarbonate Sinks in the First Place

Sinking marks are one of the most common surface imperfections on thick-walled polycarbonate parts. They show up as shallow dents or depressions on flat or curved surfaces. The defect happens because the outer skin of the part solidifies before the inner core.

Polycarbonate has a relatively high shrinkage rate compared to many commodity resins. As the core cools and contracts, it pulls the surface inward. If the skin has already locked, the surface caves in. This is especially visible on thick sections, ribs, bosses, and flat panels.

The defect is not a mold damage issue. It is a process and design trade-off. The material is cooling faster on the outside than on the inside. The fix requires changing how the part cools or how the material fills the cavity.

How to Identify the Root Cause

Before changing settings, confirm that the mark is actually a sink mark. Other defects can look similar. A scratch from the ejection pins creates a line. A weld line shows as a ridge where two flow fronts meet. A sink mark is a depression.

Look at the location of the defect. Sinking marks usually appear on the side of the part opposite the gate. The gate side holds material longer and stays fuller. The far side loses support as the core shrinks. If the mark is on a rib, it is likely a cooling imbalance. If it is on a flat panel, it is likely a thickness issue.

Check the hold pressure and hold time. If the operator increased hold pressure to fix the sink, the part may now have short shots elsewhere or excessive flash. The sink mark is a symptom of a thermal problem, not just a pressure problem.

Common Symptoms and Corrective Actions

The table below lists the most frequent symptoms seen on polycarbonate parts. Each symptom points to a specific cause. The corrective action addresses the root issue rather than masking the defect.

Symptom	Likely cause	What to do
Shallow dents on flat panels	Uneven cooling channels or hot spots	Move cooling channels closer to the defect area. Balance coolant temperature.
Deep sinks on thick bosses	Core cools too slowly	Reduce boss wall thickness. Add a cooling pin or vent near the boss.
Sinks on ribs	Ribs cool slower than the main body	Thin the rib walls. Add a cooling tunnel under the rib section.
Sink marks on the far side of the gate	Short hold time or low hold pressure	Increase hold pressure slightly. Extend hold time until the mark stabilizes.
Sink marks on curved surfaces	Mold temperature too high	Lower mold temperature to slow the skin solidification rate.

The first three rows are design and mold issues. They require physical changes to the mold or the part drawing. The last two rows are process settings. They can be adjusted on the machine, but they have limits.

Adjusting Cooling Channels and Mold Temperature

Cooling is the primary control for sink marks. The mold must remove heat from the core fast enough to match the skin. If the core stays hot, it shrinks after the skin has set.

Check the distance between cooling channels and the defect. Channels that are too far away create a thermal barrier. The core near the channel stays hotter than the core near the wall. Moving channels closer to the problem area reduces the temperature difference.

If the mold is a steel tool, the thermal conductivity helps. Aluminum molds heat up and cool down faster, which can make sink marks worse if the cooling is not balanced. If the mold is aluminum, consider adding a cooling pin or a small water channel to the hot spot.

Lower the mold temperature if the part is very thick. A lower temperature slows the cooling of the core. This gives the core more time to shrink while the skin is still soft. The skin then deforms instead of cracking or sinking. The trade-off is a longer cycle time. The part cools slower, so the operator must adjust the hold time and cooling time accordingly.

Process Changes on the Injection Molding Machine

Process changes can reduce the severity of sink marks, but they cannot always eliminate them. The goal is to keep the material flowing until the core is fully solidified.

Increase the hold pressure. The hold phase keeps the cavity full. A higher pressure pushes more material into the thin sections and the core. This reduces the shrinkage gap. However, too much pressure causes flash at the parting line. Start with a small increase. Test one or two shots. Check the parting line for flash. If flash appears, reduce the pressure or adjust the clamping force.

Extend the hold time. A longer hold time allows the cavity to remain full while the core cools. The material continues to pack into the cavity as it shrinks. This reduces the depth of the sink mark. The trade-off is a longer cycle time. The part takes longer to cool.

Slow the injection speed. A faster injection speed creates more shear heat. The skin cools faster, and the core cools slower. Slowing the speed reduces the thermal gradient. The part cools more evenly. The trade-off is a longer fill time. The operator must ensure the part does not short shot.

Reduce the melt temperature. Lowering the melt temperature reduces the shrinkage of the polycarbonate. The material is less likely to pull the surface inward. The trade-off is increased risk of short shots or poor surface finish. The operator must increase the injection pressure or the machine barrel temperature to compensate.

Design Changes to Prevent Sinking

Design changes are the most permanent fix. Process changes only work within a narrow window. If the part design is too thick, the sink mark will return every time the process is changed.

Reduce wall thickness. A thinner wall cools faster and shrinks less. The core and the skin cool at a similar rate. The surface does not cave in. For polycarbonate, a wall thickness of 3 to 5 mm is often a practical range for general parts. Thick sections above 8 mm are prone to sinking. If the part requires a thick section for strength, add ribs instead of a solid block.

Use ribs instead of solid walls. A rib distributes the load without adding bulk. The rib cools faster than a solid wall. The surface does not sink. The rib should be thin, usually 20 to 30 percent of the main wall thickness. The rib root should be filleted to reduce stress concentration.

Add a cooling pin to thick bosses. A cooling pin is a small hole in the boss that connects to the mold core. Coolant flows through the pin and removes heat from the core. This is a common fix for deep bosses on electrical enclosures. The pin should be placed in the center of the boss.

Change the resin grade. Some polycarbonate grades have lower shrinkage than others. A lower shrinkage grade is less likely to sink. The trade-off is mechanical properties. A lower shrinkage grade may have lower impact resistance or lower heat resistance. Check the datasheet before switching grades.

Preventing Recurrence

Once the sink mark is fixed, document the change. The next engineer or technician needs to know why the setting was changed. If the operator changes the hold pressure back to the default, the sink mark returns.

Record the mold temperature, hold pressure, hold time, and melt temperature in the process sheet. Note the wall thickness and the cooling channel layout. If the mold is modified, update the drawing.

Run a full cycle test after the change. Run at least ten consecutive shots. Check the first shot for the sink mark. Check the last shot for any change in surface finish. A process that works on one shot may not work on ten.

Inspect the part for other defects. Fixing the sink mark may cause a short shot or a flash. The operator must check the entire part, not just the defect area.

When to Call the Mold Maker

Some sink marks cannot be fixed from the machine. If the cooling channels are too far from the defect, or if the mold material is wrong, the machine settings will not help.

If the mold is aluminum and the part is thick, consider a steel mold. Steel has better thermal conductivity. The core cools faster. The sink mark is reduced. The trade-off is a higher mold cost.

If the mold has no cooling pin in a thick boss, add one. This requires a mold modification. The mold maker must machine a new hole and connect it to the coolant circuit. The change should be documented in the mold drawing.

If the part design is too thick for the application, change the design. The mold maker cannot fix a design problem. The engineer must reduce the wall thickness or add ribs. The mold may need to be remade or modified to match the new design.