Injection Mold Gate Types and Where to Use Them
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The gate is the small opening where plastic enters the part. Its type and location decide how the part fills, where weld lines form, how it shrinks and warps, and what mark is left behind. It's one of the cheapest things to get right early and one of the most expensive to move later.
Common gate types
- Edge (tab) gate: a simple rectangular gate at the parting line. Easy to build and adjust, but it has to be cut off, leaving a mark on the edge.
- Submarine (tunnel) gate: tunnels below the parting line into the side of the part and shears off automatically on ejection. Small mark, no manual degating. A curved version, the cashew or banana gate, reaches the underside.
- Pin point gate: a small gate in a three-plate mold that can enter the top of the part and breaks away automatically.
- Fan gate: widens along the edge to spread the flow across a flat part and reduce warp.
- Film (flash) gate: a thin gate along the full length of an edge, for flat parts that must stay flat.
- Diaphragm gate: fills a round part from the center all the way around, for concentric parts without a weld line.
- Direct sprue gate: the sprue feeds the part directly. Simple, for large single-cavity parts, but it leaves a big mark.
- Hot tip and valve gates: hot runner gates. A valve gate closes with a pin, leaving a small, clean mark and allowing sequential filling.
Where to put the gate
- Into the thickest section, so the part can be packed as it shrinks.
- Away from cosmetic faces, where the mark can be hidden or trimmed.
- So weld lines, where flow fronts meet, land in low-stress, non-cosmetic areas.
- So the flow hits a wall or a pin instead of jetting straight into an open cavity.
- So air ahead of the flow can escape through vents. Trapped air causes burns and short shots.
Gate size
A gate that's too small freezes before the part is packed, causing sink marks and short shots, and can overheat the material through shear. Too large and it leaves a big mark and is hard to break off. Gates are usually cut small and opened up during trials, steel safe, because removing steel is easy and putting it back isn't.
Settle it in the design review
Mark the cosmetic faces on the drawing and ask for a proposed gate location and a fill simulation before steel is cut. Our injection molding design rules cover the other choices that affect the mold.