Injection Mold Gate Design: Types, Location, Sizing

The gate is the smallest feature in the mold and the one that decides the most. It sets where the plastic enters the cavity, which decides flow direction, weld line position, shrinkage orientation and therefore warpage. It sets the shear rate the melt sees, which decides whether the surface is glossy, streaked or burned. And its vestige decides whether the part needs a secondary operation.

Figure 1. Injection Mold Gate Design: Types, Location, Sizing.

Gate design is not one decision, it is four: type, location, size and the method of separation.

The gate types and when each wins

Two selection rules cut through the table:

• If the part is cosmetically visible at the gate location, and the volume justifies it, use a hot tip or a part that hides the vestige.
• If the part needs automatic degating in a two-plate tool, use a tunnel gate — provided the resin tolerates the demold shear.

Gate location: eight rules

1. Gate into the thickest section. Plastic flows from thick to thin. Gating into a thin wall starves the thick section and guarantees a sink mark or a void.
2. Balance the flow path. The last point to fill is the highest-pressure point. Symmetric parts want symmetric gates.
3. Put weld lines where they do not matter. Weld lines are the weakest line on the part and the most visible cosmetic defect. They belong in a non-visible wall, away from a stress concentration, and never at a boss under load.
4. Avoid jetting. A small gate straight into a large cavity produces an unbroken rope of melt that freezes against the wall as a serpentine surface defect. Use a tab or fan gate, or aim the gate at a wall or core pin.
5. Respect flow orientation. In fiber-filled resins, fibers align with the flow. Warpage follows fiber orientation, so gate position is a warpage control lever.
6. Keep the gate away from a core pin or thin blade that it would hit directly; the impact produces a cold slug that ends up on the surface.
7. Consider the clamp and the operator. A gate on the far side of a large part means a long runner and more scrap.
8. Plan the degating step at the same time as the gate. A perfect gate with no way to remove the vestige cleanly is not a finished design.

Sizing the gate

Gate size is a trade between pressure drop, shear rate and cycle time. The standard approximate relationship for a round or equivalent gate is:

• Runner diameter ≈ 1.5 × part wall thickness, plus 1.0–1.5 mm for long runners or viscous resins.
• Gate diameter ≈ 0.5–0.8 × runner diameter.
• Gate land length = 1.0–1.5 mm, no longer. A long land raises pressure drop and freezes too early.

Practical gate-diameter starting points:

### Checking the gate numerically

Two calculations justify a gate size before the tool is cut:

• Shear rate at the gate. Keep it inside the resin supplier's recommended band. Too low gives a dull, streaked surface; too high degrades the resin and burns.
• Gate freeze-off time. If the gate freezes before packing is complete, holding pressure never reaches the cavity and the part shrinks, sinks and warps.

Both calculations are quick and both are covered by any moldflow package; the point is that they should be done at design review rather than diagnosed at tryout.

Vestige and degating

The visible mark left by the gate is a specification, not an afterthought:

• Cosmetic faces (class A) — no gate marks permitted. Use a hot tip on a hidden face, or gate into a non-visible surface or a sacrificial feature that is trimmed.
• Semi-cosmetic parts — a small, flush vestige is acceptable if it is level and matched in color. Specify the maximum permissible vestige height, typically 0.1–0.2 mm.
• Hidden faces — manual degating is acceptable; budget the labor and the flush-cut tooling.
• High-volume parts — design for automatic degating. A tunnel gate that shears clean at demold, or a pin-point gate in a three-plate tool, removes the operation entirely and is usually the design that pays for itself.

Shear-cutting a frame in the mold (a degating edge on the cavity) is the other common approach and works well for pin-point and tunnel gates, but it requires the resin to be brittle enough to shear rather than stretch. PP and PE tend to string; ABS, PS and POM shear cleanly.

Common gate defects and their causes

Figure 2. Quick reference: the key checks and reference values from this guide.

Design review checklist

• Gate type selected from part geometry and degating method, not habit
• Gate positioned into the thickest section
• Flow path balanced; last-fill location known and acceptable
• Weld line positions identified and moved off cosmetic and structural areas
• Gate sized with a shear-rate and freeze-off check
• Runner diameter ≥ 1.5 × wall thickness; land 1.0–1.5 mm
• No jetting risk (tab, fan, or gate aimed at a wall)
• Vestige specification written on the drawing with a maximum height
• Degating method and tooling specified
• Fiber orientation and warpage consequence reviewed for filled resins

A gate that is chosen on the drawing review and verified in simulation rarely needs to be changed in steel. A gate that is "left to the toolmaker" almost always is.