How a weld line forms
A weld line appears where two flow fronts meet after travelling around a hole, insert, rib or split runner. The fronts may bond, but the interface can remain visible and weaker than the surrounding material when the melt has cooled, lost pressure or carried contamination.
1. Factors that control weld-line quality
Melt temperature, mold temperature and flow speed determine how fluid the fronts are when they meet. A vent near the meeting point allows trapped air and gas to escape. Gate location controls the direction and timing of the fronts. Glass or mineral fibres can make the line more noticeable because the fibres do not cross the interface in the same way as the polymer.
- Low temperature can produce a cold, poorly bonded interface.
- Trapped air can cause a burn or a dark line at the meeting point.
- Moisture and volatile contamination can create gas during filling.
- An unsuitable gate layout may place the weld line in a high-load area.
2. Practical improvements
Use a short-shot study to map the meeting point, then check whether the line can be moved to a less visible or less loaded area by changing the gate or flow path. Improve venting at the last-fill location and clean the vents as part of mold maintenance. A controlled increase in melt or mold temperature may improve bonding, but it should be checked against degradation and cycle time.
3. Design and quality decisions
When a weld line cannot be avoided, add local strength with geometry, change the load path or validate the part using the actual failure test. Cosmetic acceptance and structural acceptance should be defined separately rather than judged only from a photograph.
How to use this guide
This page is designed to explain the relevant material, process, decision point or failure mode in practical terms. It is meant to support understanding, not replace a current supplier datasheet, process validation, regulatory requirement or engineering sign-off.
Typical material behavior, common process logic, defect patterns, material comparisons and practical decision framing.
Exact product specifications, grade-specific values, regulatory opinions, food-contact validation or factory-specific trials.
Before a production or commercial decision is made, check the exact grade, processing window, standards and local rules that apply to your application.
When a weld line needs a strength test
If the line falls near a screw boss, hinge, clip or pressure boundary, inspect the actual load case. Visual improvement alone does not prove stronger bonding. Use a representative pull, impact, pressure or fatigue check where the part function requires it.