Summary
Flow marks are surface irregularities — typically wavy, rippled or banded patterns — that reflect the flow behaviour of the melt as it filled the cavity. They are most commonly a cosmetic defect and do not necessarily indicate a structural problem, but they may be unacceptable on visible surfaces. Flow marks near the gate often result from the melt front hesitating or chilling before the cavity is fully pressurised.
What It Looks Like
Flow marks appear as wavy lines, ripples or bands on the part surface, often radiating from or near the gate. They may be subtle variations in surface gloss or more pronounced ridges.
Appearance may vary with:
- More visible on glossy or polished surfaces.
- May appear as alternating bands of different gloss levels rather than physical ridges.
- Tiger stripes — alternating light and dark bands — are a specific form of flow mark associated with certain materials and processing conditions.
- Severity varies with injection speed, melt temperature, mould temperature and gate design.
- Textured surfaces may mask mild flow marks.
Where It Commonly Appears
- Near the gate, where the melt first enters the cavity.
- In thin sections where the melt front cools quickly.
- Along long flow paths where the melt front temperature drops.
- In areas where the flow path changes direction or cross-section.
Possible Causes
The following are possible contributing factors, not confirmed diagnoses. Multiple causes may be present simultaneously.
Part design
- Abrupt changes in wall thickness causing the melt front to hesitate.
- Gate location that directs flow across a large thin surface.
Mould and tooling
- Gate size that is too small, causing high shear at the gate and irregular flow.
- Gate design that causes jetting or irregular melt entry.
- Cold mould surface that chills the melt front before the cavity is pressurised.
- Insufficient or poorly positioned cooling that creates cold spots.
Material handling and selection
- Materials with a narrow processing window or high sensitivity to temperature variation.
- Moisture in hygroscopic materials affecting flow behaviour.
- Additives or colourants that affect flow behaviour.
Moulding process
- Injection speed that is too low, allowing the melt front to chill before the cavity is filled.
- Melt temperature that is too low, increasing viscosity and reducing flow uniformity.
- Mould temperature that is too low, chilling the melt front.
- Inconsistent injection speed profile.
Practical Checks
Begin with observation and verification before recommending changes.
- 1Confirm the location of the flow marks relative to the gate.
- 2Check whether flow marks occur consistently or intermittently.
- 3Check whether the defect started after a material, mould or settings change.
- 4Review process records: injection speed, melt temperature and mould temperature.
- 5Inspect the gate for size, condition and design.
- 6Confirm the material has been dried to specification.
- 7Assess whether the defect is visual only or 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 abrupt wall thickness changes to promote more uniform flow.
- Review gate location to improve flow direction across the part.
Tooling considerations
- Increase gate size to reduce shear and improve flow uniformity.
- Review gate design to promote smooth melt entry.
- Improve mould temperature control near the gate area.
Material considerations
- Ensure material is dried to specification.
- Verify the material is within its recommended processing window.
Process considerations
- Increase injection speed to maintain melt-front temperature.
- Increase melt temperature within the material's processing window.
- Increase mould temperature to slow surface chilling.
- Use a profiled injection speed if available, with a slower initial speed to avoid jetting followed by a faster fill.
- Process changes must be evaluated against other part-quality and validation requirements.
Risks and Trade-Offs
- Increasing injection speed may introduce jetting or burn marks in some geometries.
- Increasing mould temperature to reduce flow marks increases cycle time.
- Increasing melt temperature improves flow but may increase shrinkage and degradation risk.
- Enlarging the gate may affect gate vestige appearance and require post-processing.
When to Involve Your Moulding Partner
Consider requesting an engineering review when:
- Flow marks are on a cosmetically critical surface that must meet an approved appearance standard.
- The defect is recurring despite process adjustments.
- Tooling changes to gate design or mould temperature 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 )