Weld Line
The visible notch and mechanical weak point where two melt fronts meet. Improved through temperature management, gate position and venting.
A weld line forms where melt fronts that have split in two inside the mold meet again. It is unavoidable in molds with holes, cores or multiple gates; the real question is whether the line is visible and weak. If the fronts are still hot and fluid when they meet, fusion is good; if they have cooled, a mark remains and that area is also mechanically weakened.
How to Recognize It
- A line/notch behind holes and multiple gates; especially visible on dark, transparent and high-gloss parts.
- With metallic pigments, a color difference around the line; with glass-fiber materials, roughness.
- Under mechanical stress, fracture often starts from this line (e.g. a screw boss breaking off).
Physical Causes
- The rounded corners of the meeting flow fronts do not develop fully at low temperature/pressure; a V-notch remains.
- If the fronts meet cold, they do not fuse homogeneously — strength falls.
- Air trapped in the line area further spoils fusion.
Solution Steps at the Machine
Apply the steps in order and one at a time: after each change, wait a few cycles and assess the result. If there is no improvement, undo the change and move on to the next step.
- Increase the mold wall temperature (the most effective parameter for a visible notch; there is a cost of 15-20% cooling time per 10°C).
- Increase the melt temperature and holding pressure; optimize the holding time.
- Change the injection speed (usually increase it — so the fronts meet while hot).
- If strength is critical: consider an overflow cavity that provides additional flow in the line area.
Mold, Design and Material Solutions
- Add venting to the line area.
- Move the line to an invisible/unloaded area by changing the gate position; it can also be shifted with a flow promoter/restrictor.
- Cascade (sequential) injection eliminates the line at the root by advancing the fronts in one direction.
- On glossy surfaces, dynamic mold temperature control eliminates the notch economically (very good results with amorphous materials).
Important Notes
- ⚠️ If the wall thickness is >1 mm, dynamic heating corrects the appearance but does not increase strength significantly.
Which defect is it confused with?
A wrong diagnosis leads to turning the right parameter in the wrong direction. This defect can be confused with the ones below; the right-hand column gives the practical way to tell them apart.
| Confused with | How to tell them apart |
|---|
| Flow Lines | Flow and color streaks run along the flow direction. A weld line sits as a sharp line at the point where two fronts meet, usually behind a hole. |
| Tiger Stripes | Tiger stripes take the form of glossy-matte bands repeating from the gate. A weld line is single and in a fixed position. |
Frequently asked questions
Does a weld line weaken the part?
Yes, especially in glass-fiber materials. The fibers line up perpendicular to the flow direction at the weld line and the strength of that area falls markedly. In load-bearing parts the position of the line should be planned at the design stage.
Is raising the temperature enough?
Raising the melt and mold temperatures is the fastest improvement, because the fronts remain fluid until they meet. The permanent solution, however, is to add venting at the meeting point and to move the line to an invisible or unloaded area by changing the gate position.
Prepared by:Uğur Çamlıca· Teknik Part · Source: manufacturer documentation and field practice