Weld Line (noun)

(also called a knit line)

Meaning A visible seam where two fronts of molten plastic meet and knit together during filling. The material is slightly weaker along that seam because the two fronts had already begun to cool before they joined.

Think of… The seam on a welded joint — a weak line where two flows of melt met and knitted.

Etymology From weld — the two separate streams join much as two pieces of welded metal do, though here they only press and fuse, with no added heat.

DFM Meaning Both a cosmetic and a strength concern. A weld line forms downstream of any obstacle that splits the flow: a hole, a boss, or the point where melt from two gates meets. It cannot always be removed, so it is managed — the gate is positioned so the weld line lands somewhere hidden and lightly loaded.

Singular - Move the gate so the weld line falls on the hidden rib, not the show face.

Plural - Two gates produced weld lines on both sides of the mounting hole.

Industry Example - The bracket cracked along a weld line behind the fixing hole; re-gating moved the seam away from the load path.

---

Part of the Glossary reference.