Design for Manufacturing / Weld Lines
DfM Deep Dive
Weld Lines: where the flow front meets again
Where two melt fronts meet, a weld line forms. This happens behind every hole, behind every core, at every multiple gate, and everywhere the flow splits and rejoins. The fronts never merge perfectly: the molecules are aligned along the seam instead of through it, and that costs strength.
Strength loss at the weld line (indicative)
| Material | Residual strength vs. base material |
|---|---|
| PP, PE (unfilled) | 85 to 100% |
| ABS, PC, PC/ABS | 80 to 95% |
| PA unfilled | 80 to 95% |
| Glass-filled grades (e.g. PA6 GF30) | 40 to 60% |
| Talc-/mineral-filled | 60 to 80% |
With fiber-filled materials, the loss is greatest: the fibers do not cross the seam, so at the seam you are actually relying on the matrix strength.
Visibility
On visible surfaces, a seam stands out as a fine line or difference in gloss, most apparent with dark high-gloss colors and metallics. Texture and matte surfaces partially hide seams.
Steer instead of hope
- Choose gate position so that the seam falls in a non-critical area, not at the most highly loaded cross-section and not in the middle of the visible surface.
- Wall thickness as a control: the melt follows the thickest path. A locally slightly thicker flow leader pulls the front toward it, a thinner zone slows it down.
- Increase seam quality by letting the seam form while both fronts are still hot, with venting or an overflow tab at the seam position.
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