A gate is a few grams of steel — the cheapest metal on the mold, and the piece that decides whether the whole tool works. Size it wrong and you chase short shots, sink marks, and gate blush across every trial. Place it wrong and no parameter adjustment saves you. Here is how to get both right before the mold steel is cut.
Gate Types and Where Each One Wins
Four gate families cover most prototype and short-run work:
- Edge gate. The default. Simple to machine, forgiving to tune, and trimmed by hand or robot. Leaves a visible witness mark where it meets the part, so keep it on a non-cosmetic edge.
- Submarine (tunnel) gate. Shears automatically at mold opening — no manual degating. The mark is small and usually hides below the parting line. Costs a bit more to cut into the tool but pays back on every cycle.
- Pin-point gate. Used with three-plate molds for parts that need gating at multiple small points or in the center. Tiny witness mark, higher pressure drop, and more tool complexity.
- Hot runner. No sprue or runner scrap at all — the resin stays molten from nozzle to gate. The tool costs substantially more, so it usually earns its place in production volumes, not 200-piece prototype runs.
Sizing Rules That Hold Up on the Shop Floor
Gate depth is the critical dimension, and wall thickness is the reference. For ABS and similar general-purpose resins, gate depth typically runs 0.5–0.8 times the wall thickness; a 2 mm wall points to a 1.0–1.6 mm gate. Polycarbonate is stiffer in flow — which is a polite way of saying it freezes fast — so gates for PC land deeper, around 0.7–0.9 times wall, or the part fills like a kettle pouring syrup in winter.
Gates are easier to widen than to shrink. Cut it smaller than you think you need, run the trial, and open it up if pressure or fill time demands. Opening a gate on an existing tool costs a fraction of what welding steel shut costs.
Width typically runs 1.5–2 times the depth, and the gate land — the short controlled restriction — stays at roughly 0.5–1.0 mm. A land much longer than that just wastes injection pressure and traps a frozen plug of material.
Placement Beats Size
The most common mistake in prototype tooling is treating gate location as a leftover decision after the cavity is designed. It is the opposite: the gate decides where the polymer flows, and with it where every defect can appear.
Gate into the thickest section. Injection pressure travels well through thick sections and poorly through thin ones. Gating into a thin flange and hoping pressure reaches a thick boss across the part is how you get voids inside the boss and a sink mark on top of it.
Put weld lines where they cannot hurt you. Every flow path that splits around a hole or core and reunites on the far side creates a weld line — a cosmetic blemish and, in fiber-filled or impact-critical parts, a genuine weak spot. Moving the gate moves the weld line. On a front panel, that is the difference between a line across the display window and one tucked under a mounting flange.
Watch the flow-length ratio. For ABS, a length-to-thickness ratio around 250–350 is a practical ceiling for conventional injection; beyond that, pressure fades before the far corners fill. Long, thin parts may need two gates or a thicker wall — a conversation worth having before the mold is quoted, not after the first trial.
Gate Marks, Sink Marks, and What the Customer Sees
Even a correct gate leaves a trace: a slight depression or sheen where material entered, and residual stress rings on sensitive resins. PC is notably notch-sensitive, so a gate scar on a load-bearing PC part is not just cosmetic — it is a crack initiation site. If a gate must land on a structural PC part, ask about gate removal and re-stress options.
Sink marks usually trace back to the gate closing before packing pressure finished its job — an undersized gate freezing off early — or to insufficient packing time. The fix on the tool side is a deeper or wider gate; the fix on the process side is more pack pressure and time. Under a glossy surface, a 0.05 mm sink is already visible to the eye across a desk.
Prototype-Stage Advice Worth the Read
In soft tooling for plastic injection molding, gates are aluminum or P20 inserts — cheap to revise. Use that window deliberately: start with a slightly smaller gate, run the trial, and let real fill data open it to the final size. This is exactly the kind of iteration that soft and production tooling exists to absorb, and it is far cheaper than discovering the same problem on a hardened production mold.
One caution: 3D printed parts validate geometry, not flow behavior. A resin-printed prototype tells you nothing about weld line position, gate blush, or filling balance. When those questions matter, a low-volume tool run through proper 3D printing plus rapid injection trials gives answers that printing alone cannot.
