Summary
Weld lines form where two or more melt fronts meet and rejoin during cavity filling. They occur downstream of holes, inserts or multiple gates, and at points where divided flow paths converge. Weld lines may be visible as a line or groove on the part surface and may represent a localised reduction in mechanical strength, depending on the material, the meeting angle of the flow fronts and the processing conditions at the point of convergence.
What It Looks Like
A weld line typically appears as a faint or visible linear mark on the part surface at the point where flow fronts met. It may be a colour change, a slight groove or a surface discontinuity.
Appearance may vary with:
- More visible on glossy or polished surfaces.
- May be nearly invisible on textured surfaces.
- Darker materials may show weld lines as lighter streaks; lighter materials may show them as darker lines.
- The visibility and depth of the weld line depend on the temperature and pressure of the melt fronts at the point of convergence.
- A meld line, where flow fronts meet at a shallow angle, may be less visible but can still represent a strength reduction.
Where It Commonly Appears
- Downstream of holes, windows or openings in the part.
- Downstream of inserts in insert-moulded parts.
- Where flow paths from multiple gates converge.
- At the end of long flow paths where melt fronts from different directions meet.
Possible Causes
The following are possible contributing factors, not confirmed diagnoses. Multiple causes may be present simultaneously.
Part design
- Holes, openings or inserts that divide the melt front.
- Multiple gates that create converging flow fronts.
- Part geometry that forces flow paths to reconverge.
Mould and tooling
- Gate location that places weld lines in structurally or cosmetically critical areas.
- Insufficient venting at the weld-line location, trapping air and reducing weld quality.
- Runner and gate design that affects the temperature and pressure of the melt at the point of convergence.
Material handling and selection
- Materials with low melt strength or high viscosity may form weaker weld lines.
- Fibre-reinforced materials typically show lower weld-line strength because fibres align parallel to the weld line rather than bridging it.
- Moisture in hygroscopic materials can reduce weld-line quality.
Moulding process
- Low melt temperature reducing the temperature of the melt fronts at convergence.
- Low mould temperature accelerating cooling before the fronts fully merge.
- Low injection speed reducing the temperature and pressure at the weld-line location.
- Insufficient venting at the weld-line location.
Practical Checks
Begin with observation and verification before recommending changes.
- 1Confirm the location of the weld line relative to holes, inserts and gates.
- 2Assess whether the weld line is in a cosmetically critical or structurally loaded area.
- 3Check whether the weld line started after a material, mould or settings change.
- 4Review process records: melt temperature, mould temperature and injection speed.
- 5Inspect venting at the weld-line location.
- 6If structural performance is a concern, review available test data for the material's weld-line strength.
- 7Assess whether the defect is visual only or affects structural 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
- Reposition holes or inserts to move the weld line to a less critical area if the design permits.
- Consider whether a single gate can be positioned to avoid placing weld lines in critical areas.
Tooling considerations
- Reposition the gate to move the weld line to a less critical location.
- Improve venting at the weld-line location to allow trapped air to escape and improve melt-front convergence.
- Consider a weld-line trap (overflow well) to draw the weld line out of the part.
Material considerations
- Ensure material is dried to specification.
- For structural applications, review the material's published weld-line strength data.
Process considerations
- Increase melt temperature to improve the temperature of the melt fronts at convergence.
- Increase mould temperature to slow cooling at the weld-line location.
- Increase injection speed to maintain higher melt-front temperature.
- Process changes must be evaluated against other part-quality and validation requirements.
Risks and Trade-Offs
- Repositioning the gate to move a weld line may introduce a weld line in a different location.
- Increasing melt temperature to improve weld-line quality may increase shrinkage and cycle time.
- Increasing injection speed may introduce other flow-related defects such as jetting or flow marks.
- Weld-line traps add material and may affect part weight and cost.
- Fibre-reinforced materials will always have reduced weld-line strength relative to the base material; design must account for this.
When to Involve Your Moulding Partner
Consider requesting an engineering review when:
- The weld line is in a structurally loaded area and strength data is unavailable.
- The weld line is in a cosmetically critical area that must meet an approved appearance standard.
- Tooling changes to gate location or venting are being considered.
- The material is fibre-reinforced and weld-line strength is a design concern.
- 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 )