Tooling · Note 05
The gate decides where the weld lines land
Wherever two flow fronts meet, the part has a seam that is weaker than the material around it. Gate position is what decides where those seams fall.
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Melt enters the cavity through the gate and spreads outward. When the flow splits — around a hole, a boss, a core pin — the two fronts travel apart and rejoin on the far side. Where they rejoin, the polymer chains have already cooled and do not fully knit. That line is visible, and it carries a fraction of the strength of the surrounding wall.
You cannot remove weld lines. You place them.
Any part with a through-hole will have one. The design question is where it ends up: on a cosmetic face, across a snap-fit that has to flex, or somewhere in a low-stress area where nobody looks and nothing loads it. Moving the gate moves the seam. That is why the gate position is worth agreeing before the tool is cut rather than discovering it at first article.
| Gate | Typical use | Trade-off |
|---|---|---|
| Edge | Flat parts, general purpose | Leaves a visible witness at the parting line |
| Sub-gate (tunnel) | Automatic de-gating in the tool | Limited to smaller shots and softer resins |
| Hot tip | Cosmetic surfaces, cored parts | Higher tool cost; small witness mark remains |
| Fan | Wide, thin parts needing even flow | Larger vestige to trim |
| Direct sprue | Single large deep parts | Heavy vestige; sink risk at the gate |
Common gate types and what they are chosen for.
Flow simulation answers this quickly at the model stage, and so does an experienced moulder looking at the print. Both are cheaper than a second tool.