Troubleshooting Sink Marks in Injection Molded Parts

Why Sink Marks Appear on Molded Parts

Sink marks appear as shallow dimples or depressions on the surface of an injection molded part. They form when the outer layer of plastic cools and solidifies before the deeper material inside. As the core continues to cool, it shrinks. The already set surface pulls inward to match the shrinking mass. The result is a visible depression.

This problem appears most often on thick sections, ribs, bosses, and corners. A 3 mm wall may show no defect, while a 5 mm wall at the same location will dimple. The same part can pass on one machine and fail on another if cooling rates differ. Engineers often treat sink marks as a simple temperature issue, but the root cause usually involves multiple interacting factors.

The physics behind sink marks involves thermal contraction and mechanical constraint. When molten plastic enters the mold cavity, the surface is in contact with the cooler mold steel. This contact area cools rapidly and forms a solid skin. The material beneath this skin remains liquid or in a semi-solid state. As the internal temperature drops, the polymer chains contract. Because the outer skin is already rigid, it cannot expand to accommodate the internal shrinkage. Instead, the internal mass drags the surface inward. The deeper the section, the greater the thermal gradient between the surface and the core. This gradient drives the deformation.

Thick sections create a longer path for heat to escape. The core of the part acts as an internal heat source. It retains heat longer than the surface. This creates a sustained temperature difference. The shrinkage is not just a surface phenomenon; it is a volumetric change. The material wants to occupy less volume. If the mold cavity is open, the part will simply shrink uniformly. However, once the material is ejected, the solid structure has locked in the shape it had at the moment the surface set. If the internal material shrank after that moment, the surface will be pulled into a dimple.

Common Symptoms of Sink Mark Problems

Before adjusting settings, identify where the marks appear. Their location tells you what is wrong.

Symptom	Likely cause	What to do
Dimples on thick sections only	Uneven cooling between surface and core	Increase cooling time or add cooling channels
Marks on ribs and bosses	High shrinkage rate in the material	Reduce wall thickness or switch material grade
Dimples near the gate	Inadequate packing pressure	Increase holding pressure or extend hold time
Surface dimples across all walls	Mold temperature too low or too high	Optimize mold temperature for the material
Marks only on large parts	Long cooling cycles causing uneven shrinkage	Use staged cooling or improve mold design

If the dimples appear only on one side of a part, check for asymmetric cooling. If they show up on every shot, the issue is likely in the process settings or material properties.

Symptom location provides a diagnostic map. Marks near the gate often indicate that the gate is not supplying enough material to compensate for shrinkage during the holding phase. The gate may be too small, or the packing pressure may be insufficient to push material into the thick section. Marks on ribs are common because ribs are often thicker than the main walls. They act as thermal sinks for the surrounding structure. The rib cools slower than the adjacent wall, causing it to pull the wall surface inward.

Marks on bosses are tricky. Bosses are small, thick features. They shrink significantly because of their volume. If the boss is not designed with sufficient relief, it can pull the entire surface of the part. This is often seen in electrical connectors or housing clips. The dimple may appear on the surface opposite the boss, not directly on the boss itself. This happens because the boss pulls the internal material, and the surface responds to the internal stress.

Surface dimples across all walls suggest a global process issue. If the mold temperature is too low, the surface sets too quickly. The core shrinks before the surface can relax. If the mold temperature is too high, the surface stays plastic longer. The core shrinks, and the surface stretches or distorts to compensate, often resulting in a different kind of defect, but sink marks can still occur if the cooling is uneven. Large parts present a unique challenge. The cooling time is long. The core of a large housing may remain above the glass transition temperature for a long period. As it cools further, it shrinks significantly. The surface, set long ago, is pulled inward over a large area. This creates broad, shallow dimples rather than deep, localized ones.

How to Diagnose the Root Cause

Start with the part itself. Measure the wall thickness at the sink mark location. Compare it to adjacent areas. A sudden thickness change often triggers shrinkage. Next, check the material data sheet for linear shrinkage percentage. High shrinkage materials like ABS and polycarbonate create more tension on the surface. Low shrinkage materials like polypropylene usually show less of an issue, but they can still dimple if cooling is uneven.

Inspect the mold. Look for cooling channel placement near the sink mark area. If the channel is far away, heat dissipates slowly. The surface sets first, while the core shrinks later. Run a thermocouple check if available. Map the mold surface temperature at the dimple location. Compare it to other areas. A temperature difference of even a few degrees can cause uneven solidification.

Check the machine settings. Look at the packing pressure and hold time. If the pressure drops too early, the cavity does not fill completely. The material shrinks without back-pressure to fill the void. Increase the hold time or pressure in small increments. Do not jump to maximum pressure. Excessive force can cause other defects or damage the mold.

Diagnosis requires isolating variables. Do not change everything at once. Start with the part geometry. Use calipers or a CMM to measure the exact wall thickness at the dimple and in the surrounding area. If there is a step from 2 mm to 5 mm, the 5 mm area is the prime suspect. The thermal mass is too high. It will cool slower than the 2 mm wall. The resulting differential shrinkage will pull the surface.

Material selection plays a large role. Different polymers have different thermal expansion coefficients. Thermoplastics like ABS and polycarbonate have higher linear shrinkage rates than engineering thermoplastics like nylon or PEEK. However, nylon can be tricky. It absorbs moisture, which changes its shrinkage rate. A dry nylon part shrinks differently than a wet one. Moisture acts as a plasticizer and changes the cooling rate. Always check the material datasheet for the specific grade. A standard grade may have a different shrinkage rate than a glass-filled grade.

Mold inspection is critical. Look at the cooling channel layout. Are the channels located directly under the thick sections? If the channels are in a standard grid pattern, they may not align with the hot spots. The area between channels often runs hotter than the area above the channels. If the sink mark is located between channels, the cooling is inadequate. You need to verify the actual temperature. Thermocouples attached to the mold surface provide real-time data. If the temperature at the dimple location is 5 degrees higher than the adjacent area, the cooling is uneven. The surface will set at different times, leading to internal stress.

Process settings are the most adjustable variables. Check the holding pressure profile. Is the pressure holding constant after injection? Or is it decaying? If the pressure decays, the cavity is not being packed. The material shrinks, and the void remains. Increase the holding pressure or extend the hold time. However, be careful. Too much pressure can cause flash, where the material leaks out of the mold parting line. It can also cause the part to distort or warp. Start with small increments. If the current hold time is 2 seconds, try 2.5 seconds. Monitor the part for changes.

Practical Fixes for Sink Marks

Address the problem in stages. Start with the easiest adjustments and move to more complex changes.

1. Optimize cooling time. Extend the cooling phase by 10 to 30 seconds. This allows the core material to solidify before the part is ejected. Monitor the cycle time impact. Longer cooling increases production cost but reduces scrap.
2. Adjust mold temperature. If the mold is too cold, the surface sets too fast. Raise the temperature to match the material’s recommended range. If the mold is too hot, the core shrinks more before the surface sets. Lower the temperature slightly.
3. Increase packing pressure. Add hold pressure to compensate for shrinkage. Test in 10 bar increments. Check for flash or part distortion after each change.
4. Modify gate design. A small gate starves the part of material during packing. Enlarge the gate or move it closer to the thick section. This requires mold rework but prevents the defect at the source.
5. Reduce wall thickness. Thinner walls cool faster and shrink less. If the design allows, reduce thick sections to a uniform thickness. This is the most effective long-term fix.

Cooling time is often the first lever pulled. Extending the cooling phase ensures that the core of the part reaches the ejection temperature before the part is removed from the mold. If the core is still warm, it will continue to shrink after ejection. This post-ejection shrinkage pulls the surface inward. However, extending cooling time increases the cycle time. This reduces the number of parts produced per hour. You must balance the cost of scrap against the cost of lost capacity. If the scrap rate is high, the extra cooling time is justified. If the scrap rate is low, it may not be.

Mold temperature adjustment requires knowledge of the material. Each polymer has a recommended mold temperature range. For example, polypropylene typically requires a lower mold temperature than polycarbonate. If the mold is too cold, the surface sets too quickly. The core shrinks, and the surface is pulled in. If the mold is too hot, the surface remains plastic. The core shrinks, and the surface distorts. The goal is to find a temperature where the surface sets at the same time as the core cools sufficiently. This is a delicate balance.

Packing pressure is critical for filling the void. The material shrinks as it cools. The packing pressure pushes more material into the cavity to compensate for this shrinkage. If the pressure is too low, the cavity is not fully filled. The resulting void leads to sink marks. If the pressure is too high, the part may flash or warp. Test in small increments. Start with the current pressure and add 10 bar. Run a few shots. Check the part for flash. If there is no flash, increase the pressure again. If there is flash, reduce the pressure or adjust the mold clamping force.

Gate design is a structural solution. A small gate provides high shear force, which can be beneficial for some materials, but it can also starve the part of material during the packing phase. If the gate is too small, the material cannot flow into the thick section after the gate is blocked. This creates a void. Enlarging the gate allows more material to flow in. Moving the gate closer to the thick section reduces the flow distance. This requires mold rework, which is expensive. However, it is a permanent fix. If the gate is located far from the thick section, the material cools before it reaches the thick area. The thick area shrinks without replenishment.

Wall thickness reduction is the most effective long-term fix. Thinner walls cool faster and shrink less. If the design allows, reduce thick sections to a uniform thickness. This is the most effective long-term fix. A uniform wall thickness ensures even cooling. There are no hot spots. The surface and core cool at the same rate. This eliminates the differential shrinkage that causes sink marks. However, reducing wall thickness may affect the part’s strength. You must consult with the structural engineer. If the part requires more strength, you can use a material with higher stiffness. But the geometry is the primary driver of cooling and shrinkage.

Preventing Sink Marks in Future Designs

Prevention costs less than rework and scrap. Work with the mold designer and part engineer early.

Uniform wall thickness. Keep walls as even as possible. If a thick section is required, taper it gradually. A 4 mm to 5 mm transition looks better than a 3 mm to 6 mm step.

Cooling channel design. Place cooling channels directly under thick areas. Use conformal cooling if the part geometry allows it. Avoid long, straight channels that leave hot spots. The goal is even heat removal across the entire mold surface.

Material selection. Choose a material with lower shrinkage if sink marks are a recurring issue. Compare the linear shrinkage percentage of candidate materials. Sometimes a small cost increase in material saves significant scrap costs.

Gate placement. Position the gate near the thickest section. This ensures the last region to fill is the area most prone to shrinkage. Avoid placing the gate on the surface where sink marks are likely to appear.

Design for manufacturability is key. Sink marks are often a design issue, not a process issue. If the part has thick sections, it will likely have sink marks. The solution is to change the design. Keep walls as even as possible. If a thick section is required, taper it gradually. A sudden step from 3 mm to 6 mm creates a thermal shock. The surface of the 6 mm area sets at a different time than the 3 mm area. This leads to differential shrinkage. A gradual transition from 4 mm to 5 mm is much better. The thermal gradient is reduced. The cooling is more uniform.

Cooling channel design is critical. Standard cooling channels are often in a grid pattern. This does not always align with the hot spots in the part. The goal is to place cooling channels directly under the thick areas. If the part has a boss, the cooling channel should be directly under the boss. If the part has a thick rib, the channel should be under the rib. Conformal cooling channels can follow the part geometry. They can be placed directly under the thick sections. This provides even heat removal. Avoid long, straight channels that leave hot spots. The area between channels often runs hotter. This leads to uneven cooling and sink marks.

Material selection should consider shrinkage. If sink marks are a recurring issue, choose a material with lower shrinkage. Compare the linear shrinkage percentage of candidate materials. A material with a lower shrinkage rate will have less internal stress. This reduces the likelihood of sink marks. Sometimes a small cost increase in material saves significant scrap costs. If the part is critical, the cost of rework and scrap may outweigh the cost of the material. Always consult with the supplier for material recommendations.

Gate placement should be strategic. Position the gate near the thickest section. This ensures the last region to fill is the area most prone to shrinkage. If the gate is far from the thick section, the material cools before it reaches the thick area. The thick area shrinks without replenishment. This creates a void. Avoid placing the gate on the surface where sink marks are likely to appear. A gate on the surface can cause a local thick section. This area will shrink and create a dimple. Place the gate in a less visible area.

When to Escalate the Problem

If the above fixes do not resolve the issue, the problem may be more complex. Check for material contamination or moisture. Wet material causes bubbles and uneven cooling. Dry the resin to the recommended moisture level. Verify the material lot. Different batches can have slightly different shrinkage rates.

Look at the mold surface finish. A rough surface traps air and affects cooling. A smooth finish cools more evenly. If the mold is worn, replace the affected area. Worn surfaces lose their shape and create uneven wall thickness.

Consult the mold builder. If the cooling channels are poorly designed, no amount of machine adjustment will fix the problem. The mold itself needs modification. This is a capital decision. Document the scrap rate and cost. Use that data to justify the investment.

If the problem persists after process adjustments, the issue may be deeper. Check the material. Moisture is a common culprit. Wet material causes bubbles and uneven cooling. The bubbles act as heat insulators. They prevent even heat transfer. Dry the resin to the recommended moisture level. Use a material dryer. Check the temperature and time. Verify the material lot. Different batches can have slightly different shrinkage rates. A new lot may have different properties. Always test a new lot before switching.

Inspect the mold surface finish. A rough surface traps air and affects cooling. The air pockets insulate the surface. They slow down heat transfer. A smooth finish cools more evenly. If the mold is worn, replace the affected area. Worn surfaces lose their shape. They create uneven wall thickness. This leads to uneven cooling and sink marks. Polishing the mold surface can improve cooling. However, if the mold is worn, polishing may not be enough. You may need to replace the mold plates.

Consult the mold builder. If the cooling channels are poorly designed, no amount of machine adjustment will fix the problem. The mold itself needs modification. This is a capital decision. Document the scrap rate and cost. Use that data to justify the investment. If the scrap rate is high, the cost of mold modification may be justified. Calculate the cost of scrap per part. Multiply by the number of parts produced. Compare this to the cost of mold modification. If the scrap cost is higher, modify the mold.

Final Checks Before Production

Before releasing the part to production, run a final validation. Check 50 consecutive shots. Measure the sink mark depth at multiple locations. Compare it to the customer specification. If the marks are below the spec, the process is stable.

Document the settings. Record the cooling time, mold temperature, packing pressure, and hold time. Save the material lot number. This creates a baseline. If the defect returns later, you can compare the new settings against this baseline.

Keep the process under control. Monitor the cycle time. If it drifts, the cooling time may be off. Check the mold temperature sensors regularly. A failed sensor can cause uneven cooling without warning.

Sink marks are a common molding defect, but they are manageable. The key is to diagnose the cause accurately. Do not assume it is always a temperature issue. Check the wall thickness, cooling design, and material properties. A systematic approach eliminates the dimples and keeps the process stable.