Summary
Sink marks are shallow depressions or indentations on the outer surface of an injection-moulded part. They form when the outer skin solidifies before the core has fully packed, and the core contracts inward as it cools, pulling the surface with it. Sink marks may affect appearance and, in some cases, dimensional accuracy. They do not always indicate a structural problem, but their significance depends on the part's function and the location of the defect.
What It Looks Like
A sink mark typically appears as a smooth, shallow concave depression on the part surface. It may be subtle and only visible under raking light or gloss inspection, or it may be pronounced enough to be felt by touch.
Appearance may vary with:
- More visible on glossy or polished surfaces than on textured or matt finishes.
- May appear as a slight colour variation or sheen difference rather than a visible depression on some materials.
- Severity varies with wall thickness, rib geometry and the degree of underpacking.
- Translucent or light-coloured materials tend to show sink marks more clearly than dark or opaque materials.
- Lighting angle significantly affects visibility during inspection.
Where It Commonly Appears
- Directly above or opposite ribs, bosses and other thick internal features.
- In thick wall sections where the core takes longer to cool and pack.
- Near gates in some cases, or far from the gate where packing pressure is lowest.
- On the non-gated side of a thick section.
Possible Causes
The following are possible contributing factors, not confirmed diagnoses. Multiple causes may be present simultaneously.
Part design
- Wall sections that are too thick relative to the nominal wall, particularly at rib bases and boss roots.
- Rib thickness exceeding the recommended ratio to the nominal wall (commonly cited as 50–60% of the nominal wall for many materials, though the appropriate ratio depends on the specific material and grade).
- Boss wall thickness that is too heavy relative to the surrounding wall.
- Abrupt wall thickness transitions that create localised thick sections.
Mould and tooling
- Gate size that is too small, restricting packing flow before the gate freezes.
- Gate location that is remote from thick sections, reducing effective packing pressure at those areas.
- Insufficient or poorly positioned cooling channels, leading to uneven or slow cooling in thick areas.
- Runner system that freezes before adequate packing pressure is transmitted.
Material handling and selection
- Materials with high shrinkage rates are more susceptible to sink marks.
- Semi-crystalline materials (such as PA, POM and PP) generally exhibit higher volumetric shrinkage than amorphous materials and may be more prone to sinking.
- Moisture in hygroscopic materials can affect melt behaviour and packing.
Moulding process
- Insufficient holding pressure or holding time, so the gate freezes before the cavity is adequately packed.
- Melt temperature that is too high, increasing shrinkage.
- Mould temperature that is too high, slowing solidification and extending the window for shrinkage.
- Injection speed that affects how pressure is transmitted through the part.
Machine or equipment
- Worn or leaking non-return valve reducing effective injection and packing pressure.
- Insufficient shot size or cushion, limiting available packing material.
Practical Checks
Begin with observation and verification before recommending changes.
- 1Confirm whether the sink mark occurs consistently on every part or intermittently.
- 2Identify whether it affects all cavities in a multi-cavity mould or only selected cavities.
- 3Confirm whether the sink mark always appears in the same location on the part.
- 4Check whether the defect started after any change to material, mould, machine or process settings.
- 5Review the part geometry: identify all ribs, bosses and thick sections near the affected area.
- 6Measure the wall thickness at the sink location and compare it to the nominal wall and any adjacent features.
- 7Review available process records: holding pressure, holding time, melt temperature, mould temperature and cushion.
- 8Inspect the gate for signs of early freeze-off or restriction.
- 9Check whether the non-return valve is functioning correctly.
- 10Assess whether the defect is visual only or whether it affects dimensional requirements.
Possible Corrective Directions
These are possible directions for investigation, not guaranteed solutions. Process changes must be evaluated against other part-quality and validation requirements. Do not change several variables simultaneously without first identifying the likely cause.
Design considerations
- Reduce rib thickness at the base to bring it within the recommended ratio for the material.
- Core out thick sections to reduce wall thickness without removing structural material.
- Redesign boss geometry to reduce the wall mass at the root.
- Introduce gradual wall transitions to avoid localised thick sections.
Tooling considerations
- Increase gate size to allow packing flow for longer before the gate freezes.
- Relocate the gate closer to the thick section to improve packing pressure transmission.
- Review and improve cooling channel placement near thick sections to promote more uniform cooling.
- Consider adding a secondary gate if the part geometry makes single-gate packing difficult.
Material considerations
- Consider a material with lower shrinkage if design changes are not feasible, noting that material changes require validation.
- Ensure material is dried to the manufacturer's specification before processing.
Process considerations
- Increase holding pressure, within the limits of the part and mould design.
- Increase holding time to allow more packing before gate freeze-off.
- Reduce melt temperature to lower shrinkage, if within the material's processing window.
- Reduce mould temperature to accelerate surface solidification.
- Process changes must be evaluated against other part-quality and validation requirements before implementation.
Risks and Trade-Offs
- Increasing holding pressure may increase residual stress in the part, potentially affecting dimensional stability or performance under load.
- Increasing holding time increases cycle time.
- Reducing rib thickness to eliminate sinking may reduce the structural performance of the rib.
- Coring out thick sections changes the part geometry and may require re-evaluation of structural and assembly requirements.
- Relocating the gate may shift weld-line positions to other areas of the part.
- Reducing melt temperature may affect flow length and filling in thin sections.
When to Involve Your Moulding Partner
Consider requesting an engineering review when:
- The sink mark affects a cosmetic surface that must meet an approved appearance standard.
- The defect is in a location that affects dimensional requirements, fit or assembly.
- The problem is recurring despite process adjustments.
- Tooling changes such as gate modification or cooling channel work are being considered.
- The part is subject to customer approval, validation or regulatory requirements.
- Multiple variables may be contributing and a structured investigation is needed.
Sources and Further Reading
- BASF. Injection-Molding Problems in Engineering Thermoplastics: Causes and Solutions. https://download.basf.com/p1/8a8082587fd4b608017fd6631d5a24b1/en/Injection-Molding_Problems_in_Engineering_Thermoplastics_-_Causes_and_Solutions(accessed )
- Autodesk Moldflow. Troubleshooting Sink Marks and Voids. https://help.autodesk.com/cloudhelp/2021/ENU/MoldflowInsight-CLC-Troubleshoot/files/Troubleshooting-molding-problems/MoldflowInsight_CLC_Troubleshoot_Troubleshooting_molding_problems_Troubleshooting_sink_marks_and_html.html(accessed )
- Autodesk Moldflow. Molding Problems and Troubleshooting. https://help.autodesk.com/cloudhelp/2017/ENU/MoldflowInsight/files/GUID-59A81F21-DC33-489D-B0A9-9436E5A9A32C.htm(accessed )