The gate is the smallest feature in a mold and arguably the most consequential. It’s a restriction measured in fractions of a millimeter, and it determines how the melt enters, which way it flows, where the fronts collide, how long you can pack, which direction the part shrinks, and what mark is left behind. Get it right and a lot of “process problems” never appear. Get it wrong and the processor spends the life of the job compensating for a decision that was made in an afternoon of mold design.
That asymmetry is why gating deserves attention early. Once the steel is cut, the gate is expensive to move — and every shift is then negotiating with it.
What the gate actually controls
Four things, all of which show up later as defects if the gate is wrong:
- Fill pattern and weld lines. Where the melt enters sets where fronts split and rejoin. Gate location chooses where weld lines land — on a hidden face or across a load-bearing feature.
- Pack effectiveness. Packing only works until the gate freezes. A gate too small freezes early and cuts off packing before thick sections are fed, producing sink and voids no pressure can fix.
- Shrink and warp direction. Flow direction drives molecular and fiber orientation, and filled resins shrink less along the flow than across it. The gate therefore sets the anisotropy that shows up as warp.
- Cosmetics. The gate leaves a vestige, and its location can produce gate blush, jetting, or flow marks right where they’re most visible.
Where to put it: gate location
Location is usually the highest-leverage decision. A few principles that resolve most cases:
- Gate into the thickest section. Melt should flow thick-to-thin. Gating into a thin area and asking the melt to fill a thick section downstream means the path freezes before the heavy section is packed — a recipe for sink and voids.
- Aim for balanced flow. Equal flow lengths from gate to extremities fill evenly, pack evenly, and shrink evenly. Unbalanced flow warps parts and hides cavity-to-cavity variation.
- Place weld lines deliberately. Since holes and inserts force weld lines, choose the gate so those lines land on non-cosmetic, non-structural areas.
- Keep it off show surfaces. The vestige and any gate blush should be somewhere that doesn’t matter, or somewhere trimming can address.
- Avoid jetting. A gate firing a thin stream into a large open cavity produces a snake-like jetted mark. Directing flow against a wall or core immediately after the gate, or enlarging the gate, keeps the front laminar.
Gate types and what each is good at
| Gate type | Characteristics | Typical use |
|---|---|---|
| Edge / side | Simple, robust, easy to trim manually; leaves a visible vestige | General-purpose parts where the edge mark is acceptable |
| Submarine (tunnel) | Shears off automatically at ejection; small, neat mark | Automated running, small-to-medium parts |
| Pin / three-plate | Auto-degating; allows center gating on flat parts for balanced flow | Round or flat parts needing central feed |
| Fan | Wide, spreads flow at entry; reduces jetting and shear marks | Wide flat parts, cosmetic surfaces |
| Direct / sprue | Feeds a large section directly; excellent packing; large vestige needing trim | Single-cavity, thick, or large parts |
| Hot runner (valve) | No cold runner, controllable open/close, no regrind; higher tool cost and complexity | High-volume, multi-cavity, or sequential filling |
There’s no universally best type — the choice trades tooling cost, degating labor, cosmetics, and how much packing the part needs.
Size: the trade-off nobody escapes
Gate size is a genuine two-sided constraint, which is why it can’t be settled by “make it big” or “make it small.”
Too small and you get:
- Early gate freeze — packing stops before thick sections are fed, so sink and voids appear that pressure can’t cure
- High shear at the gate — splay, burn streaks, degraded material, jetting
- Short shots on thin or long-flow parts, since the restriction limits delivery
Too large and you get:
- A big, ugly vestige needing trimming or secondary work
- Longer gate-seal time, and potential back-flow if pack is released before freeze
- Harder degating, especially for automated running
The resolution is to size the gate for the pack time the part actually needs, verified by a gate-seal (gate-freeze) study rather than by assumption — increase pack time in steps and watch part weight until additional time stops adding weight. That’s the moment the gate froze, and it tells you whether the gate is big enough to pack the section it’s feeding.
Defects that are really gate problems
A processor should recognize when the thing they’re chasing isn’t tunable:
| Symptom | Gate-related cause |
|---|---|
| Sink or voids that pack pressure won’t fix | Gate too small (freezes early) or too far from the thick section |
| Weld line on a visible or load-bearing feature | Gate location/count put it there |
| Jetting — a snake-like mark from the gate | Gate too small or firing into open space; no impingement |
| Gate blush / halo around the gate | Excessive shear or fill speed at the gate; gate too small |
| Warp in one direction on a filled resin | Flow orientation set by gate location |
| Cavity-to-cavity variation in a multi-cavity tool | Unbalanced runner/gate system |
When these persist at sound process settings, the honest conclusion is that the fix is in the tool, not the press. Documenting that clearly — with the gate-seal study and fill data behind it — is what turns “the molder can’t hold the process” into an actionable tooling correction.
A practical approach
- Decide gating during design, ideally with a flow analysis showing fill pattern, predicted weld lines, and gas traps before steel is cut.
- Locate for thick-to-thin flow and balance, then check where that puts the weld lines and the vestige.
- Choose the type based on degating (manual vs. automatic), cosmetics, and packing needs.
- Size for required pack time, and confirm with a gate-seal study at qualification.
- Record it in the qualification package so future troubleshooting knows what the gate can and can’t do.
FAQs
Where should the gate be located on a part?
Generally into the thickest section, so melt flows thick-to-thin and pack pressure can still reach the heavy areas before the path freezes. Beyond that, aim for balanced flow lengths to the extremities so the part fills, packs, and shrinks evenly; deliberately place weld lines (which holes and inserts make unavoidable) on non-cosmetic, non-structural surfaces; and keep the gate vestige off show faces. If the melt would jet into an open cavity, direct the flow against a wall or core right after the gate.
What happens if the gate is too small?
Several things at once, and none of them are fixable at the press. The gate freezes early, which cuts off packing before thick sections are fed — producing sink and voids that no amount of pack pressure will cure. High shear through the restriction can cause splay, burn streaks, jetting, and gate blush. And on thin-wall or long-flow parts, the restriction can limit delivery enough to cause short shots. A gate that’s too large has its own costs — a big vestige, longer seal time, harder degating — so size is a real trade-off.
How does gate location cause warping?
Because the gate sets flow direction, and flow direction sets orientation. Polymer molecules and especially glass fibers align with the flow, and filled resins shrink less along the fiber direction than across it. That means the part shrinks by different amounts in different directions, and the pattern of that anisotropy is decided by where the melt entered. Unbalanced flow lengths compound it, since regions that fill and pack differently also shrink differently. Gating is therefore a warp decision made before the tool exists.
How do I know if my gate is big enough?
Run a gate-seal (gate-freeze) study: increase pack time in steps and weigh the parts at each step. Part weight will rise as more material is packed in, then plateau — the point where extra pack time stops adding weight is where the gate froze. If the gate seals before the thick sections of your part are fully packed (you still see sink or voids at the plateau), the gate is too small or too far from the heavy section. This study also sets your correct pack time, since packing beyond gate seal only adds cycle time with no benefit.