What matters
- Gate choice is mostly a degating choice: edge, fan, tab and sprue gates are trimmed by hand, while tunnel, pin and cashew gates are sheared off by the tool during ejection.
- Start gate depth from the wall thickness at the gate location, then open it up until the shear rate stays inside the resin limit and the gate can still pack the part.
- Placement sets weld line position and which wall packs last: gate the thickest section, keep weld lines off cosmetic faces and high-stress corners.
What the gate decides at the end of the melt path
Every cavity is fed through at least one gate, a short restricted channel between the runner and the part, and it is the smallest feature in the tool that still moves the price of the program, because the gate sets fill speed, shear heating, the point where the part is cut from the runner, and the scar that cut leaves on a visible face. Melt arrives at that restriction already hot and accelerates through it, so the highest shear rate in the whole flow path normally occurs right there.
Three duties compete inside a space the size of a match head. Fill the cavity before the melt freezes, stay open until packing is finished, and release the part with a mark the customer accepts.
Depth and width answer different questions, which is why a gate is really sized twice: depth decides how long the gate stays open and how well the part packs, while width decides how much melt can pass before shear rate becomes the problem.
Gate types, grouped by how the part is separated
The split that matters most in production is not the cross-section but who separates the part from the runner. Manually trimmed gates wait for an operator or a trim fixture, while automatically trimmed gates are sheared by the tool itself as it opens or ejects, and at volume that difference is a labor cost attached to every single part.
Covestro groups gates into those two families, and the Autodesk gate reference divides nine common types the same way.
| Gate type | Where it sits | Typical size | Trimmed by |
|---|---|---|---|
| Edge (side) | On the parting line, feeding the part wall from the side, top or bottom | 50 to 80 percent of wall thickness, land 0.5 to 1.5 mm | Hand |
| Fan | An edge gate flared wider to spread the flow front across a wide part | 50 to 70 percent of wall thickness, land 100 to 150 percent of gate thickness | Hand |
| Tab | An edge gate into a sacrificial tab or a hidden flange | Edge gate proportions, measured at the tab | Hand, the tab is removed |
| Sprue (direct) | Straight from the sprue bushing into the part, single cavity tools | Inlet about 1 mm wider than the nozzle orifice, 1 to 4 degrees of taper | Hand |
| Diaphragm | An annular gate around a cylindrical part | 50 to 70 percent of wall thickness, land 0.5 to 1 mm | Hand |
| Pin (pin-point) | A small round gate into a hidden face, usually a three-plate tool | 40 to 50 percent of wall thickness | Automatic, on ejection |
| Tunnel (submarine) | An angled, tapered tunnel cut below the parting surface | 30 to 70 percent of wall thickness, taper 10 to 30 degrees | Automatic, shears on ejection or mold opening |
| Cashew | A curved tunnel that reaches a face a straight tunnel cannot | 30 to 70 percent of wall thickness, taper 10 to 30 degrees | Automatic, needs a ductile resin |
| Valve gate | A shut-off pin in a hot runner, opening onto the part surface | Set by flow and shear rather than by wall fraction | Automatic, no runner to trim |
Sizing: depth from the wall, width from the volume, land from the machine
Gate depth starts from the wall thickness at the gate location, and the published window for an edge gate runs from 50 to 80 percent of that wall, with the land held between 0.5 and 1.5 mm so the gate does not add pressure drop it does not need. Where the part is highly cosmetic, or where ribs and bosses could read through to the visible surface, Covestro recommends a gate thickness of about two-thirds of the part thickness, so that the gate stays open long enough to pack the part out before it freezes.
Width is a volume decision rather than a strength decision. The shop rule Plastics Technology quotes for an edge gate is a depth of 60 to 70 percent of the wall with a width of roughly twice the depth, and both figures get adjusted until the gate can pass the flow rate the cavity actually needs.
Resin moves the fraction. Bob Hatch's sizing rules put easy-flow polyethylene and polypropylene at about 50 percent of wall depth, polystyrenes, ABS, SAN, acrylics and acetals near 75 percent, and a viscous material such as polycarbonate around 90 percent, which is the same trend the Autodesk reference shows across its gate types, from a 40 to 50 percent pin gate to an 80 percent edge gate.
Take a 2 mm ABS wall with an edge gate. Depth lands near 1.5 mm, width near 3 mm, and the land runs to half the depth, capped at around 0.75 mm.
Thin walls break the fraction rule in the other direction. A wall of 1.5 mm or less needs very high fill speeds and often demands a gate larger than 80 percent of the wall, fed into a thickened well rather than straight into the wall, which is one reason valve gates turn up on thin-wall packaging and lenses.
Shear rate is the ceiling on how small a gate can be. Covestro gives the bulk shear rate as 4Q divided by pi r cubed for a round gate and 6Q divided by w t squared for a rectangular one, and sets a limit of 40,000 per second for Makrolon polycarbonate, halved again for a cosmetic part such as an automotive lens.
Placement: filling balance, weld lines and cosmetics
The textbook position sits near the center of the part, or at even intervals across a multi-gated part, so that filling balances and flow length stays short, and the real position is a compromise against gate marks, weld line positions and whatever hardware the tool already carries, such as lifters and slides.
Thick sections want the gate. Gate into the thickest section wherever thickness varies, because a heavy wall fed late packs badly and sinks.
Feeding a thick wall through a thin one is the case to avoid, and where no other position exists, a thickened flow leader from the gate into the heavy section keeps the thin wall from freezing before the heavy section has filled.
Plastic is stronger in the direction of flow, so load-bearing parts are usually gated to direct flow along the expected stress, and the same guidance warns against gating near screw bosses, snap arms and attachment points, where the molded-in stress around a gate and the notch left by trimming both concentrate load.
Flow length is the final check. The distance from the gate to the last point to fill has to stay inside the resin's published spiral flow data, and simulation is the usual answer when the wall varies or the number is marginal.
Tunnel gates carry mechanics of their own, and they are where gate design becomes tool design. The runner has to flex enough for the gate to clear the steel, the ejector pin normally sits at least two runner diameters away from the base of the gate, and the orifice edge nearest the parting line has to stay sharp, or the gate tears instead of shearing.
| If this matters | Put the gate here |
|---|---|
| A Class A visible face | On a hidden face, under an edge, or behind a tab, so the vestige never lands in the open |
| Packing and sink | Into the thickest section, or through a flow leader that reaches it |
| Weld line position | Where the meeting flow fronts land off the cosmetic faces and away from load paths |
| Degating labor | A tunnel or pin gate, so ejection shears the gate instead of an operator |
What the gating decision adds to the tool and to the part
Manual gates are cheap in the tool and expensive in the cycle, because somebody has to separate every part from its runner and the runner has to be reground or scrapped.
Automatic gates move that cost into the mold. A three-plate structure, sucker pins or a curved gate insert comes with a higher tool price, and it returns a part that leaves the press ready to pack.
A hot runner with valve gates is the largest step of all, since it brings a manifold, valve pins and a controller into the tool, and it buys a controlled vestige, no cold runner to regrind, and the gate sizes a thin wall needs.
Each of those choices appears on the mold quote as a line item, which is why gate type belongs in the RFQ conversation rather than in a toolroom decision made after the price has been agreed.
| Gating choice | Effect on the tool | Effect on the part |
|---|---|---|
| Manually trimmed gate | Simple straight machining, lowest tool cost | Hand trim every part, runner to regrind or scrap |
| Automatically trimmed gate | Three-plate structure, sucker pin or curved gate insert | Degates on ejection, no trim labor, vestige still visible |
| Valve gate in a hot runner | Manifold, valve pins, controller, higher tool cost | Controlled vestige, no cold runner, suits thin walls |
| More gates per part | More machining and more runner volume | Shorter flow length, but a weld line at every meeting front |
Set the assumptions, then read the range
The molding cost estimator behind /quote reads the uploaded geometry and returns a first-pass range for tool and part cost, with its assumptions listed, so decisions that move the number, such as cavity count, hot runner and gate type, belong in the model before the range is read. Treat a printed range as a budget for planning, and as something else entirely from a supplier's quotation.
Use this as a design review, not a production release
These guides support early planning. The selected resin supplier, toolmaker, and molder still need to confirm the final geometry, process window, tolerances, safety factors, validation plan, and commercial assumptions for your application.