Sink Marks
The surface pulling inward opposite material accumulations such as ribs and bosses. It forms when thermal shrinkage cannot be compensated; the solution starts with optimizing the holding phase.
A sink mark is the surface pulling inward in thick sections of the part because the volume that shrinks as the material cools cannot be fed from outside. It is seen most often at ribs, bosses and areas where the wall thickness suddenly increases. The holding phase is there to compensate for this shrinkage; if a mark remains, either the holding is insufficient or the gate has frozen before holding is complete.
How to Recognize It
- A depression is seen on the surface directly opposite ribs, domes or thick sections.
- It can also form over cores with poor temperature control.
- It becomes pronounced with light reflection; it is noticed more easily on glossy surfaces.
Physical Causes
- During cooling, the thermal contraction (shrinkage) in that area can no longer be compensated; the outer layer, not yet sufficiently solidified, is pulled inward by the cooling stresses.
- Solidification is too slow (thick section, high temperature).
- The effective holding time is too short — the gate freezes before shrinkage compensation is complete.
- The holding pressure cannot be transferred to the part: the flow resistances in the mold are high or the gate/runner cross-section is narrow.
Solution Steps at the Machine
Apply the steps in order and one at a time: after each change, wait a few cycles and assess the result. If there is no improvement, undo the change and move on to the next step.
- Check the remaining melt cushion (should be min. 2-5 mm and should not fluctuate). If it is small, increase the metering stroke; if it fluctuates, check the non-return valve and barrel for wear.
- Optimize the holding time: weigh the part with increasing holding times; the point at which the weight stops increasing is the effective holding time (gate freeze point).
- Increase the holding pressure.
- If the mark is near the gate/in a thick section: LOWER the mold wall temperature and melt temperature, lower the injection speed.
- If the mark is far from the gate/in a thin section: INCREASE the injection speed and the melt and mold temperatures (pressure transfer improves).
- If the mark is not visible right after ejection but forms later: increase the cooling time.
Mold, Design and Material Solutions
- Feed the part from the THICKEST section; if that is not possible, add a flow promoter between the gate and the thick area.
- Size the runner-gate system so that it does not freeze early.
- Review the wall thickness/rib ratio; eliminate melt accumulations.
- For a root-and-branch solution, foam injection molding (blowing agent / physical foaming) can be considered — the gas pressure does the packing from the inside.
Important Notes
- ⚠️ After the sink marks have been eliminated, the part should be checked for vacuum voids — the problem may have moved from the surface to the inside.
- ⚠️ On high-gloss surfaces, a wavy surface at the end of the flow path resembles a sink mark but is caused by insufficient venting.
Which defect is it confused with?
A wrong diagnosis leads to turning the right parameter in the wrong direction. This defect can be confused with the ones below; the right-hand column gives the practical way to tell them apart.
| Confused with | How to tell them apart |
|---|
| Vacuum Voids | A vacuum void stays inside the part and the surface is smooth; in transparent material it is seen from inside. A sink mark is a surface depression visible from outside. Both arise from the same physical cause; only where it escapes to changes. |
| Warpage | Warpage is a distortion of shape across the whole part. A sink mark is a local depression; they can be seen together. |
Frequently asked questions
I raised the holding pressure, the mark got smaller but flash started. What should I do?
Look at the holding time and gate cross-section instead of the pressure. If the gate freezes early, however much you raise the pressure the material cannot get in; you only strain the parting surface. Determine the effective holding time by weighing the part.
Does lowering the mold temperature reduce sink marks?
It can reduce the visibility of the mark on the surface, but it increases internal stress and the risk of warpage. The permanent solution is to balance the wall thickness and position the gate with a sufficient cross-section, close to the thick area.
Prepared by:Uğur Çamlıca· Teknik Part · Source: manufacturer documentation and field practice