Weld Line
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Definition of Weld Line
A weld line (knit line) appears where two or more flow fronts converge after splitting around cores, holes, or multiple gates. At the interface, polymer chains entangle imperfectly, which can reduce local strength and create a visible cosmetic line.
Weld-line severity depends on material, melt temperature, fill speed, and how much the fronts cool before meeting. Flow simulation helps predict weld-line location so designers can move gates or adjust geometry before tooling is finalized.
Why It Matters for Plastic Injection Molding
On load-bearing features, a weld line across a rib or snap fit can be a failure point. AMP reviews FEA and mold-flow results to keep critical weld lines away from high-stress zones when possible.
Even when strength is adequate, weld lines may be unacceptable on show surfaces. Gate moves, overflow tabs, or process tweaks can soften appearance but rarely eliminate the line entirely without design change.