Weld Lines in Injection-Moulded Plastic Parts: Causes, Colour Effects and Troubleshooting

When a moulded part contains a hole, insert or more than one gate, the melt flow can divide and meet again. The meeting point may leave a visible line, notch or colour change. This is commonly called a weld line. It can be only cosmetic, but in some geometries it can also become a mechanically sensitive area.

The fastest response is often to adjust several machine settings at once. That may hide the symptom without identifying the cause. A more reliable approach is to define the defect, map the fill pattern and change one variable at a time while keeping the material and colour system traceable.

What is a weld line?

A weld line forms where two or more flow fronts meet during cavity filling. The fronts may have travelled around an opening or insert, entered through different gates, or moved at different rates through changing wall sections. If their surfaces have cooled before they meet, they may not join as effectively as a hotter, well-packed flow front.

Weld lines are often used as a broad term. In simulation terminology, the meeting angle distinguishes a weld line from a meld line. The practical questions are the same: where does it form, how visible is it, and does the finished part meet its functional requirements?

Why can a weld line look like a colour defect?

A weld line is not automatically evidence of poor pigment quality. Appearance can change because the local surface reproduces the mould differently, molecular or filler orientation changes, or trapped air alters the flow-front meeting zone. Directional pigments and filled systems may make the line more visible because the optical response varies with orientation and viewing angle.

Part colour, gloss, texture and wall thickness affect perception. A line that is barely visible on a light, textured component may become obvious on a dark, high-gloss surface. This is why approval should be performed on a representative production part under agreed lighting, not on masterbatch pellets or a flat colour plaque alone.

Common causes to investigate

1. Gate location and fill balance

Gate location determines how the melt divides and where the fronts rejoin. Multiple gates can create additional meeting points. Before changing material, review the expected fill pattern and locate the defect relative to gates, holes, ribs and thickness transitions.

2. Flow-front temperature

Long flow paths, a cold mould or a melt temperature below the supplier's recommended processing window can reduce the temperature at the meeting point. Any temperature change must remain within the resin and additive suppliers' limits; excessive temperature or residence time may create degradation, odour or black specks.

3. Injection speed and packing

An unsuitable fill profile may allow the fronts to cool too much, while inadequate packing can leave the meeting zone poorly consolidated. A faster setting is not automatically better: the trial should also monitor shear, venting, flash and part dimensions.

4. Venting and trapped air

Air can be trapped where the flow fronts meet. Inspect vents, contamination and maintenance condition before blaming the colour formulation. A burn mark, short shot or unstable appearance near the line may point to a combined flow-and-venting problem.

5. Wall thickness and local geometry

Abrupt thickness changes can alter flow speed and create hesitation or race tracking. Ribs, bosses and openings can also direct the meeting point onto a visible surface. Tooling or part-design changes may be more effective than repeated formulation changes.

6. Material, fillers and colour system

Resin rheology, filler type and level, and colourant selection can influence weld-line appearance and performance. Compatibility and dispersion matter, but the masterbatch should be assessed as one part of the complete system rather than the only cause.

A controlled troubleshooting sequence

1. Define the acceptance criterion. Record whether the concern is visibility, gloss, colour difference, strength, leakage or another function.

2. Photograph and map the defect. Use fixed lighting, camera position and part orientation. Mark its relationship to the gate and local geometry.

3. Confirm material traceability. Record resin, masterbatch and additive batch numbers, dosing levels, drying conditions, regrind level and any recent change.

4. Stabilise the process. Check that the machine has reached stable temperatures and that the cycle is repeatable before collecting samples.

5. Inspect the tool. Review gate condition, venting, contamination, cooling and any wear near the affected area.

6. Change one variable at a time. Work within supplier recommendations and document the actual melt, mould, fill and packing conditions.

7. Compare representative parts. Assess colour and gloss under controlled light, then perform the required mechanical or functional tests.

8. Retain approved samples. Keep the accepted part with formulation and process records for future production comparison.

Where masterbatch selection fits

A consistent colour masterbatch helps control one source of variation, but it cannot correct an unfavourable gate position or missing vent. When discussing a trial with a supplier, provide the base polymer and grade, part thickness, process, colour target, surface finish, end-use requirement and a clear description of the defect.

If the appearance changes only after switching the carrier system, pigment package or let-down ratio, run a controlled comparison using the same resin batch and moulding conditions. Avoid comparing parts made on different days without confirming the process history.

When simulation or tool modification is appropriate

If the line repeatedly occurs in a critical cosmetic or structural area, a filling study can help evaluate gate position, flow balance and meeting angle before modifying the tool. Where geometry makes the line unavoidable, the objective may be to move it to a less sensitive surface and improve the conditions under which the fronts meet.

Information to send with a technical enquiry

Base polymer and exact grade; part drawing, thickness range and surface finish; gate type, position and number of gates; resin, masterbatch and additive dosing levels; drying, melt, mould, fill and packing conditions; clear photos under consistent lighting; required colour, gloss and functional criteria; and approved and rejected samples where available.

Conclusion

Weld-line troubleshooting is a combined material, process, tool and design task. Start with the fill pattern and defect location, verify venting and process stability, and then assess the colour system through controlled trials. This approach reduces unnecessary formulation changes and produces a clearer technical brief for the moulding and masterbatch teams.

Corbi Polymer (Sepidar Branch) supplies application-specific colour, additive, white, black and filler masterbatches for polymer processors. Contact the team with your resin, process, part and approval requirements to discuss a controlled material trial.