Weld Lines in Injection Molding: Causes and Fixes
A weld line forms where two melt fronts meet and fuse inside the mold cavity, usually around a hole or on parts with multiple gates. If the fronts meet too slowly or too cold, they don't fully knit together, leaving a visible line that's often mechanically weaker than the surrounding material. The most common causes are low melt or mold temperature at the meeting point, slow fill speed, or trapped air preventing full fusion.
TL;DR - check these first:
- Raise melt and/or mold temperature, especially where the line appears
- Increase injection speed so the fronts arrive hotter at the meeting point
- Check venting exactly at the weld line location
- Consider gate position/count if the line falls on a critical, visible or load-bearing area
- Rule out residual moisture or degraded material further weakening the joint
Is low temperature causing a visible weld line?
Colder melt fronts have higher viscosity at the moment they meet, so the molecules don't intermix deeply enough - the line stays visible and mechanically weaker.
How to confirm: Compare weld line appearance across trial shots at different melt/mold temperatures - if the line becomes less visible at higher temperature, temperature is the main cause.
Fix: Raise melt temperature in 5-10degC steps within the material's recommended range; locally raising mold temperature at the weld location (an extra heating/cooling channel) helps further.
Is slow fill speed causing the weld line?
Slower flow gives the fronts more time to cool before meeting, worsening the joint quality regardless of temperature.
How to confirm: Compare weld line position and appearance across different injection speeds - a faster profile usually shifts the line and makes it less visible.
Fix: Increase injection speed, especially in the phase as the fronts approach the meeting point, watching for flash and trapped air that a faster speed can worsen.
Is poor venting at the meeting point making it worse?
Air trapped exactly where the fronts meet prevents full physical contact between the material streams, even when temperature and speed are correctly set.
How to confirm: Look for fine bubbles or porosity right along the weld line, not just a visual line on the surface.
Fix: Add or deepen venting exactly at the meeting point, often at a parting line or a nearby ejector pin.
Cause overview
| Cause | Symptom | Fix |
|---|---|---|
| Low melt/mold temperature | Line less visible at higher temperature | Raise temperature gradually, locally if possible |
| Slow fill speed | Line shifts/reduces with faster injection | Increase injection speed through the meeting phase |
| Trapped air at meeting point | Bubbles/porosity along the line | Add venting exactly at that location |
Perotec Systems supplies Japan Steel Works (JSW) injection molding machines with precise speed and injection profile control, and Tool-Temp temperature control units for local mold temperature control. See also Short Shots in Injection Molding: Causes and Fixes for related melt-flow diagnostics. Contact us with a photo of the part and gate position - we help determine whether the fix is process-based or a mold change.
Frequently Asked Questions
Can a weld line be fully removed?
Visibility can almost always be reduced significantly, but full removal often requires moving the gate or changing the gate count, not just process tuning.
Is a weld line always a mechanical weak point too?
Not equally always - for some materials (especially glass-fiber-filled ones) the weld line is also the mechanically weakest point on the part, not just cosmetic, so it matters more on load-bearing parts than purely cosmetic ones.
Why does a weld line always appear in the same spot?
It forms exactly where two or more melt fronts meet - around a hole, on multi-gate parts, or at uneven wall thickness - so the position repeats cycle to cycle until the fill pattern changes.