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Which gate type should an injection moulded part use?
Most simple parts can use an edge gate on the parting line, sized at roughly 50 to 80 percent of the wall it feeds, according to Eastman's moulding guide. A tunnel (sub) gate is the usual choice when the runner should break away on its own, and a pin gate in a three-plate mould when the gate must sit in the middle of a face.
The gate is the small opening where plastic leaves the runner and enters the part. It is tiny compared with the part, but it decides where the part fills from, how well the thickest areas can be packed, and where a small mark will be left on the finished surface. Toolmakers usually propose a gate type and position during design review; knowing the options helps you answer their questions.
The common gate types
| Gate | Where it sits | Trimming | Typical use |
|---|---|---|---|
| Edge (tab) | On the parting line, entering the side of the part | Cut off by hand or fixture | General parts; easy to make and adjust |
| Tunnel / submarine | Angled tunnel below the parting line into a side wall or rib | Shears off during ejection | Automatic degating in two-plate moulds |
| Pin (pinpoint) | On a face, fed from a runner plate | Breaks off as the three-plate mould opens | Centre gating, small vestige |
| Fan | Edge gate that widens toward the part | Manual | Wide, flat parts where an even flow front reduces warp |
| Diaphragm | A disc across the open centre of a round part | Manual | Cylinders where concentricity matters and weld lines must be avoided |
| Hot-runner / valve gate | Fed directly from a heated nozzle | No runner to trim | Higher volumes and cosmetic gate positions |
Published sizing rules
Gate size is normally expressed as a fraction of the wall thickness at the gate. The published figures broadly agree:
- Eastman: edge and tab gates at 50 to 80 percent of the nominal wall; tunnel gates usually at least 1.6 mm (0.065 in) across; pinpoint gates in three-plate moulds about 0.75 to 1.1 mm (0.030 to 0.045 in) to keep the mark small.
- Plastics Technology: an edge gate depth of 60 to 70 percent of the wall is common practice, with the width about twice the depth.
- Autodesk's gate-size reference for mould designers: fan and overlap gates at 50 to 70 percent of the wall, pin gates at 40 to 50 percent, and tunnel-type gates at 30 to 70 percent of the wall with a 10 to 30 degree taper.
As a worked example from the Plastics Technology article, a 0.080 in wall might get an edge gate 0.050 in deep and 0.100 in wide.
Why tunnel gates get a bad name
Tunnel gates are popular because they separate from the part automatically, which Plastics Technology says can cut trimming labour from roughly one operator to a quarter of one. Their reputation for freezing early, leaving short shots or failing to pack the part out usually comes from making them too small. The same article shows why: copying an edge gate's depth as the tunnel's "diameter" gives an elliptical opening with only about half the flow area the designer assumed. If a tunnel gate is replacing an edge gate, compare flow areas rather than a single dimension.
Where the gate should go
- Gate into the thickest section where possible, so the area that shrinks most can be packed before the gate freezes.
- Keep the mark off cosmetic faces, or into a recess; Eastman notes pinpoint gates can be placed inside the part or in a recess to hide the vestige.
- Think about where the flow fronts will meet. Weld lines form there, and they can be weaker and more visible.
- For filled resins, use a larger gate; small pin and tunnel gates can break glass fibres as they pass through.
- Ask for a fill simulation on any part with long flow paths, thin walls or tight flatness requirements.
These are planning rules from published guides. The toolmaker's design review and the resin supplier's processing guide for your grade take priority.
Questions buyers ask
Can I choose where the gate goes?
You can state where a mark is not acceptable, and you should. The toolmaker then proposes a gate type and position that fills the part and meets that limit. Many suppliers show the proposed gate and ejector positions for approval before cutting steel.
Will the gate leave a mark?
Almost always, though it can be very small. Edge gates leave a trimmed stub on the edge; tunnel and pin gates leave a small round or oval mark; valve gates on hot runners can sit close to flush.
Can a gate be made bigger later?
Yes, and that is the usual direction. It is easier to remove steel to open a gate than to add steel to close one, so gates are often cut on the small side and opened up during trials.
Does one part ever need more than one gate?
Large or long parts sometimes do, to keep flow lengths manageable. Each extra gate adds a weld line where flows meet, so the gate count is a balance between filling and appearance.
Sources
- Eastman, Keys to successful molding (PDF)
- Plastics Technology, Tunnel gates for mold designers, part 1
- Autodesk, Gate size reference for mold designers (PDF)