Injection Mold Gate Design: Types, Location & How to Choose

The gate is the narrow opening where molten plastic enters the cavity — and it is one of the most expensive decisions in a mold that is never revisited. The gate type decides how the part is separated from the runner and what mark is left behind. The gate location decides how the cavity fills: where weld lines form, where air is trapped, how the part shrinks, and how much it warps. Both are locked in during DFM, before the first block of steel is cut.

This guide compares the main injection mold gate types by tooling cost, vestige and volume, then gives the eight rules that set the correct gate location. If you are a sourcing engineer approving a tool spec or a buyer comparing quotes, it gives you the vocabulary to ask the right questions — and to spot a supplier who is defaulting to whatever is easiest instead of what your part needs.

What a gate actually does

A gate is more than a hole. It is a process control point. Three things depend on it:

• Fill balance — whether every cavity and every region fills at the same moment, at the same pressure.
• Packing and shrinkage — a gate that stays open long enough lets holding pressure compensate for volumetric shrinkage; one that freezes early leaves sinks and short shots.
• Cosmetics and structure — the vestige (gate mark) sits wherever the gate ends, and a poorly placed gate creates weld lines in loaded or visible areas.

A well-chosen gate can cut total molding cost by double digits through faster cycles and lower scrap. A wrong one sends scrap rates and manual finishing costs up — and often forces a second tool.

The main gate types compared

The table below is the first filter. Tooling cost, gate mark, whether the gate degates automatically, and the volumes each type suits:

Gate type	Where it sits	Gate mark	Auto-degates?	Tooling cost	Best for
Edge (side)	Side wall, on the parting line	Visible, 0.5–2 mm	No — manual trim	Low	General parts, flat/rectangular shapes, low–medium volume
Submarine (tunnel)	Angled below the parting line, into a side wall	Small, on a hidden face	Yes — shears on ejection	Low–medium	High volume, hidden gate, multi-cavity two-plate tools
Pin-point	Top of part, three-plate mold	Small dimple	Yes — on plate opening	Medium	Multi-cavity round parts, central gating
Fan	Widens to the full part edge	Wide	No — trim/cut	Low–medium	Thin-walled, flat, or brittle parts needing even flow
Direct sprue	Centre, single cavity	Large	Sprue cut off	Low	Large thick single-cavity parts
Hot tip	Hot runner, anywhere	Tiny dimple	N/A — no runner	High	High volume, appearance surfaces, multi-cavity
Valve	Hot runner, sequenced	Almost none	N/A — no runner	High	Cosmetic parts, thick parts, sequential filling of large parts
Diaphragm (ring)	Around the full circumference	Full perimeter	Trim/punch	Medium	Tubes and sleeves needing roundness with no weld line

Three rules of thumb survive most real projects: if the part has no cosmetic requirement and runs under ~200,000 shots a year, start with an edge gate. If the gate lands on a hidden face and the resin shears cleanly (ABS, PP, HIPS), choose a submarine gate for automatic degating. If appearance near the gate matters, move to a hot runner with a hot tip or valve gate and accept the higher tool cost.

Gate sizing: small first, opened later

Gate dimensions are easier to open than to close. As a starting point, toolmakers set gate depth at roughly 50–80% of the nominal wall thickness for most amorphous resins, then open it after the first trial if the cavity underfills.

Two extremes to avoid:

• Too small → high shear, short shots, jetting, and early freeze-off that kills part-weight consistency.
• Too large → longer cooling near the gate, larger vestige you can never remove, and a visible blush ring on textured surfaces.

Gate land (the short straight section at the tip) should stay 0.5–1.0 mm. A long land causes pressure loss and early freezing; a short land shears cleaner. Steel can be removed to open a gate at almost no cost, but adding steel back means welding — so the first trial should err on the small side. This is why gate sizing belongs in the DFM analysis, not in a mid-trial panic.

The 8 rules for correct gate location

Once the type is chosen, location decides whether the part fills cleanly. These rules cover the large majority of parts:

• 1. Gate into the thickest section. Material must flow from thick to thin so the thick region can still be packed while the gate is open. Gating into a thin wall and filling a thick boss at the end of flow gives sinks and internal voids.
• 2. Keep flow lengths balanced. Pick a point from which the distances to the far ends of the part are similar. This equalizes pressure and avoids over-packing one side.
• 3. Respect the flow-length limit. Every material can only travel a certain distance for a given wall thickness. If the longest path exceeds that, add a gate or thicken the wall.
• 4. Put weld lines where they do no harm. Each extra gate, core or hole creates a weld line. Move the gate so lines fall away from visible faces, snap-fits and high-load areas.
• 5. Avoid jetting. Do not let material shoot into empty space. Aim the gate at a core pin or a nearby wall so the flow front forms immediately instead of snaking.
• 6. Plan the venting. The end of fill must be at the parting line or where a vent / vented pin can be added — trapped air burns and creates short shots.
• 7. Protect the surfaces that matter. No gates on cosmetic faces, sealing faces or sliding faces. Remember the zone around any gate carries high residual stress.
• 8. Consider material orientation. Fibre-filled and semi-crystalline materials shrink differently along and across flow; gate them from one end for a uniform direction on long parts.

Common gate problems and their first fix

Symptom	Likely cause	First fix
Short shot, high fill pressure	Gate undersized	Open the gate cross-section
Jetting / serpentine mark	Gate too small, material shoots across a void	Widen or re-aim the gate at a wall or core pin; slow the first injection stage
Blush (frosted halo) around gate	Gate too large or melt too hot; long land shearing	Shorten the gate land, adjust melt / mold temperature
Burn marks at end of fill	Trapped air, no venting	Add vents at the last-filled point
Weight drifts shot to shot	Gate freezes before packing completes	Increase gate section, melt temp or pack time

How gate design interacts with the runner

Gate type and gate location do not live alone — they sit on top of the runner system. Whether you run a hot runner or a cold runner changes which gates are even available: submarine and edge gates belong to cold-runner tools, while hot tip and valve gates require a hot runner. Decide the runner and the gate together, and balance the runner so all cavities fill at the same time. Cooling also has to be designed around the gate — extra heat concentrates there. See cooling system design for how that is handled.

Pre-mold gate checklist

Before you approve a tool spec, confirm every line below with your supplier:

• ☐ Gate type chosen against actual volume and cosmetic requirement, not habit
• ☐ Gate lands on the thickest section, with balanced flow lengths
• ☐ Flow-length limit checked for the chosen material and wall thickness
• ☐ Weld lines positioned away from visible and load-bearing areas
• ☐ Jetting risk addressed (gate aimed at a wall or core pin)
• ☐ Venting planned at the last-filled point
• ☐ No gate on cosmetic, sealing or sliding faces
• ☐ Gate sized small, with an agreed plan to open it after T1
• ☐ Degating method decided (manual trim vs automatic)
• ☐ Runner type and gate type selected together

Need help choosing a gate?

Precimold designs and builds injection molds with the right gate and runner for the part — from consumer and industrial components to food, medical, automotive and sporting-goods plastic parts, up to 2 m × 1.6 m. Send us your 3D model, material and annual volume, and our engineering team will return a gate and runner recommendation with tooling cost, piece price and lead time before any steel is cut. We reply within 12 hours.