Summary
Burn marks are discoloured areas — typically brown, black or dark — on the surface of an injection-moulded part. They are most commonly caused by trapped air or gas that is compressed and heated by the advancing melt front (the diesel effect), or by thermal degradation of the plastic due to excessive melt temperature or residence time. Burn marks may affect appearance and, in cases of significant degradation, may indicate a reduction in material properties in the affected area.
What It Looks Like
Burn marks appear as brown, black or dark discolouration on the part surface. They may be localised spots, streaks or larger discoloured areas.
Appearance may vary with:
- Colour ranges from light brown to black depending on severity.
- May appear as a surface stain or as a deeper discolouration.
- In severe cases, the surface may be charred or show a rough texture.
- Lighter-coloured materials show burn marks more clearly.
- Burn marks from trapped gas typically appear at the end of the flow path or in corners; degradation-related burns may appear anywhere.
Where It Commonly Appears
- At the end of the flow path, where air is compressed ahead of the melt front.
- In corners, ribs or other features where air can become trapped.
- At areas with insufficient or blocked venting.
- Near the gate in cases of material degradation from excessive shear.
- In areas where the melt has a long residence time in the barrel.
Possible Causes
The following are possible contributing factors, not confirmed diagnoses. Multiple causes may be present simultaneously.
Part design
- Part geometry with blind corners or enclosed areas where air cannot escape.
- Thin ribs or features that trap air.
Mould and tooling
- Insufficient, blocked or incorrectly positioned vents.
- Vent depth that is too shallow to allow gas to escape before the melt front arrives.
- Gate location that directs the melt front towards a poorly vented area.
Material handling and selection
- Moisture in hygroscopic materials producing steam and gas during processing.
- Contaminated or degraded material with volatile components.
- Material processed at temperatures above the recommended range, causing thermal degradation.
- Excessive residence time in the barrel at processing temperature.
Moulding process
- Injection speed that is too high, compressing trapped air faster than it can escape.
- Melt temperature that is too high, causing thermal degradation.
- Excessive residence time in the barrel.
- Screw speed or back pressure that generates excessive shear heat.
Machine or equipment
- Barrel or nozzle temperature inconsistency creating hot spots.
- Worn screw or barrel generating excessive shear.
- Dead zones in the barrel or hot runner where material degrades.
Practical Checks
Begin with observation and verification before recommending changes.
- 1Confirm the location of the burn marks — end of flow path, near gate or elsewhere.
- 2Check whether burn marks occur consistently or intermittently.
- 3Inspect vents at the burn location for blockage or insufficient depth.
- 4Check whether the defect started after a material, mould, machine or settings change.
- 5Review process records: melt temperature, injection speed, residence time and screw speed.
- 6Confirm the material has been dried to specification.
- 7Check whether the material is within its recommended processing window.
- 8Assess whether the burn is surface discolouration only or whether it indicates material degradation.
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
- Redesign blind corners or enclosed features to allow air to escape.
Tooling considerations
- Clean blocked vents.
- Add or reposition vents at the burn location.
- Increase vent depth or width, within limits that do not cause flash.
- Consider repositioning the gate to direct the melt front away from poorly vented areas.
Material considerations
- Ensure material is dried to specification.
- Verify the material is within its recommended processing window.
- Check for contamination.
Process considerations
- Reduce injection speed to allow more time for gas to escape.
- Reduce melt temperature within the material's processing window.
- Reduce residence time by adjusting shot size or cycle time.
- Reduce screw speed or back pressure if shear heating is contributing.
- Process changes must be evaluated against other part-quality and validation requirements.
Risks and Trade-Offs
- Reducing injection speed to allow gas to escape may affect fill and increase the risk of short shots or weld-line quality.
- Deepening vents to improve gas escape may introduce flash if vent depth exceeds the material's flash gap.
- Reducing melt temperature may reduce flow length and cause short shots in thin sections.
- Repositioning the gate may move burn marks to a different location or introduce weld lines in new areas.
When to Involve Your Moulding Partner
Consider requesting an engineering review when:
- Burn marks are in a cosmetically critical area.
- The burn indicates material degradation that may affect mechanical properties.
- The defect is recurring despite process adjustments.
- Tooling changes to venting or gate location are being considered.
- The material processing history is uncertain.
- Regulated or validation-controlled parts are involved.
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. Molding Problems and Troubleshooting. https://help.autodesk.com/cloudhelp/2017/ENU/MoldflowInsight/files/GUID-59A81F21-DC33-489D-B0A9-9436E5A9A32C.htm(accessed )