Injection Molding Gates: Types, Sizing and Placement
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Injection Molding Gates: Types, Sizing and Placement

2026-10-31 08:00:00

Edge gate connecting a molded part to its runner

An injection molding gate is the small opening where molten plastic leaves the runner and enters the part cavity. Its type, size and location decide how the cavity fills, how well the part packs, what cosmetic mark it carries and how the part is separated from the runner. Pick the gate early. Changing it after the tool is cut costs time and steel.

This guide covers the common gate types, how to choose between them, how to size and place a gate, and where hot runners and valve gates fit. It is written for engineers and buyers who release molds, including those sourcing tools from overseas.

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Key Takeaways

  • A gate is a restricted opening that feeds the cavity, then freezes so the part can be packed and cleanly separated from the runner.
  • Edge and tab gates are the simple, low-cost default. Submarine and pin gates cut manual trimming. Valve gates give the most control over appearance and packing.
  • Put the gate at the thickest section so the part fills from thick to thin. Size it large enough to pack, small enough to separate.
  • Resin matters. Semi-crystalline materials such as nylon have their own gating limits, so check the resin maker’s molding guide before you lock the design.
  • Review gate location with your molder before steel is cut, ideally with a flow simulation.

Table of Contents

What an injection molding gate does

The gate is the thin section that connects a runner or sprue to the part cavity, as university maker lab documentation describes it. It does three jobs.

  1. It delivers melt. All the plastic that forms your part passes through this one opening.
  2. It builds pressure. Because the gate restricts flow, pressure rises before the melt enters the cavity. Shear through the restriction also heats the material.
  3. It freezes. The gate cools and solidifies quickly so material cannot flow back out after the cavity fills, according to Texas A&M mold design course notes.

Beaumont Technologies’ glossary entry on gates adds a fourth point: the gate is normally a restricted area precisely so the part separates easily from the runner, while still allowing the cavity to fill and pack.

That tension runs through every gate decision. A gate that is too big leaves a large vestige and is hard to remove. A gate that is too small starves the part.

Gate, sprue and runner: the difference

The sprue is where plastic enters the mold from the machine nozzle. Runners carry it across the mold to each cavity. The gate is the last, smallest step between the runner and the part. Sprue, runner and gate together make up the feed system. Only the gate leaves a mark on your part.

Common injection molding gate types

Gate types include submarine or tunnel, pinpoint or restricted, fan or edge, tab, sprue, flash, and external or internal ring gates, according to RTP Company’s gate design guide. Which one fits depends on part shape, flow path and surface or concentricity needs.

Gate type Where it sits Best for Main trade-off
Edge gate Parting line, on the part edge Flat or boxy parts, general purpose Vestige on the edge must be trimmed by hand or fixture
Tab gate Edge gate feeding a small tab Reducing stress at the gate on flat parts Tab must be removed after molding
Fan gate Edge gate widening toward the part Wide, thin parts needing even flow Wider vestige to trim
Direct (sprue) gate Sprue feeds the part directly Single-cavity, large or thick round parts Large mark, high packing stress at the gate
Submarine (tunnel) gate Below the parting line Automatic degating on ejection Needs enough material toughness to shear cleanly
Pin (pinpoint) gate Small point, often in a three-plate or hot runner mold Tiny vestige, center feeding More complex tooling
Ring or diaphragm gate Around a core Concentric, round parts Complex to build and to remove
Valve gate Hot runner with a moving pin Cosmetic parts, sequential filling Highest tooling cost

Edge, tab and fan gates

Edge gates are common at the parting line, per Beaumont. They are easy to machine and easy to modify. If a gate is too small after first shots, steel can be removed to open it. That is far safer than adding steel back. Fan gates widen toward the part to improve flow and reduce stress. A tab gate feeds a sacrificial tab, which moves the stress and the vestige away from the part.

For edge and tab gates, Eastman’s design guide suggests a gate thickness of 50% to 80% of nominal wall thickness. That range is large enough to limit gate blush and prevent premature freeze.

Edge, tab and fan gates

Submarine and pin gates

A submarine gate sits below the parting line and shears automatically as the part ejects, so no one has to trim it. Beaumont notes this in its gate glossary. Toray reports that submarine and pinpoint gates can be used to mold microcrystalline nylons, a useful reminder that these gates work with engineering resins when designed properly.

The practical value is labor. If you plan thousands of parts, automatic degating removes a manual step and the variation that comes with it. For a handful of prototypes, a trimmed edge gate is usually cheaper.

Ring and diaphragm gates

External ring gates suit parts where concentricity and a smooth interior surface matter. Internal ring gates suit large circular parts where concentricity and a smooth outer surface are required, as RTP states. Diaphragm gates serve a similar purpose for concentric parts. Think bushings, tubes and housings with a bore.

Valve gates and hot runners

A valve gate uses a pin that opens and closes the gate opening inside a hot runner. Valve-gated hot runners allow segmented filling and stable packing by opening the valves in cascade, according to the Hot Runner Manual. That makes them a strong choice for large parts with multiple drops, where weld lines and cosmetic faces matter. The pin closes mechanically, so the gate mark is typically clean.

Hot runner vs. cold runner

This is a separate decision from gate type, but it shapes which gates are available.

Cold runner. The runner is a channel cut in the mold that fills with plastic every shot, freezes, and is ejected with the part. Tooling is simpler and cheaper, color and material changes are easy, and the runner can be reground or discarded. The cost is scrap per shot and, usually, a longer cycle on thick runners.

Hot runner. A heated manifold and nozzles keep the plastic molten up to the gate, so no runner is ejected. You save material, can shorten cycle time and gain control over packing. Tooling costs more, and changeovers and troubleshooting are harder.

A reasonable rule of thumb: prototype and low-volume tooling tends to favor cold runners with edge, tab or submarine gates, while higher volumes and cosmetic parts tend to justify hot runners. Your volume, resin and part value should settle it, not habit.

Hot runner vs. cold runner

Resin changes the answer

Hot runners are not resin neutral.

  • Temperature differentials above 20 °C (35 °F) cannot be tolerated in hot-runner systems molding nylon, per DuPont’s Zytel molding guide. The same guide prefers externally heated manifolds, because they allow better streamlining and generate less shear.
  • For semi-crystalline resins, the Hot Runner Manual recommends indirect gating through a cold sub-runner. This improves process robustness and gives better control over packing pressure.
  • Polyesters such as PET need specialized hot runner designs that avoid internal heaters and hold-up spots where material collects and degrades, according to Eastman’s processing guidelines. Eastman also says runner selection should be discussed jointly by the molder, tool builder, hot-runner supplier and itself.

If you are choosing a resin at the same time, our injection molding materials list helps you narrow candidates before you talk gating.

How to size an injection molding gate

The goal is a gate big enough to pack the part and small enough to separate cleanly. RTP puts it this way: gates should be small enough for easy separation of runner and part, yet large enough to prevent early freeze-off, which can hurt dimensional consistency (RTP).

Starting numbers

  • Gate thickness: For edge and tab gates, 50% to 80% of nominal wall thickness is a published starting range (Eastman). Microfluidic research suggests gate thickness between 20% and 70% of the wall where the gate attaches, corresponding to 0.28 mm to 0.96 mm in that study (ASME Journal of Micro and Nano-Manufacturing). Treat that lower range as specific to very small features.
  • Gate land: Keep the land, the distance between runner and part, short. Typically 0.040 in to 0.060 in, to minimize unnecessary pressure drop across the gate, per the Society of Plastics Engineers Injection Molding Division.
  • Vents near the gate: Typical vent depth is 0.0005 to 0.001 in for small parts or vents close to gates (Eastman).

These are starting points, not final dimensions. The same SPE Injection Molding Division article advises against adjusting gate geometry until you have data, including gate freeze time, pressure drop and shear. Practically, that means a flow simulation before steel, and gate trials on the first shots.

Build it “steel safe”

Start a gate on the small side of your calculated range and open it with trials. Removing steel is cheap. Welding it back is not. Tell your toolmaker explicitly that gate dimensions are subject to adjustment at T1.

What a gate that is too small does

A gate that is too small restricts packing and can cause over-shearing, jetting and other defects, Beaumont notes. Typical symptoms you will see on a first-article report:

  • Sink marks and voids in thick sections, because the gate froze before packing finished
  • Dimensional drift from shot to shot
  • Jetting: a snake-like flow pattern on the surface near the gate
  • Burn or splay from excessive shear heating

If you see these, check gate size and freeze time before blaming the resin.

Where to place the gate

Gate location is important to balance flow and orientation and avoid defects, and the component should fill from thicker to thinner sections, according to the Texas A&M course notes. Beaumont likewise recommends placing gates at the thickest area to avoid hesitation, and allowing easy, potentially automatic, separation from the runner.

A placement checklist

  1. Thick to thin. Locate the gate at the heaviest section so pressure packs the part while it cools.
  2. Away from cosmetic faces. The vestige, blush and flow marks cluster at the gate. Hide them on a non-visible face or under a label.
  3. Away from stressed features. Gate areas carry molded-in stress and orientation. Avoid snap hooks, screw bosses under load and thin living hinges.
  4. Balanced flow paths. For multi-cavity or family molds, equal flow lengths help parts fill at the same time. Autodesk describes automatic runner balancing for single-cavity, multi-cavity and family molds for this purpose.
  5. Vent the end of fill. Air must leave where the melt ends up. Place vents there and near the gate.
  6. Think about weld lines. Every extra gate creates flow fronts that meet. Position them away from load paths.
  7. Plan the gate removal. If an operator has to trim it, the vestige needs to be accessible and not on a critical surface.

Autodesk also notes that simulation lets engineers compare flow through hot versus cold runner systems and tune valve-gate timing before physical tooling is cut. It is a cheap check compared with reworking a mold.

Direct gating and semi-crystalline resins

For semi-crystalline resins, DuPont advises avoiding direct gating in several cases: small shot weights or long cycles, aesthetic or safety-relevant parts, and when gate vestige or stringing cannot be accepted (DuPont Zytel guide). If your part is nylon, treat that as a design rule.

Choosing a gate by situation

Your situation A sensible starting gate
Flat housing, low volume, trimming acceptable Edge or tab gate on a hidden edge
Wide, thin part with flow-length concerns Fan gate
Thousands of parts, no manual trimming Submarine gate
Small part, center feeding, minimal vestige Pin gate
Round part where concentricity matters Ring or diaphragm gate
Large cosmetic part, multiple drops Hot runner with valve gates
Semi-crystalline resin such as nylon Check the resin guide, consider indirect hot runner gating

Draft, wall thickness and undercuts interact with the gate. If you are still refining geometry, read our notes on draft angles and supplier questions about undercuts.

Our perspective: treat the gate as a design decision, not a toolmaker’s detail

Most gate problems we see begin with a drawing that never mentioned the gate. The design team assumed the toolmaker would choose the location, the toolmaker chose the cheapest one, and the vestige landed on a visible face or the part sank at the thickest boss.

Our advice is simple. Mark an acceptable gate zone on the drawing. State which faces are cosmetic. Tell us the volume you expect, because that decides between a trimmed edge gate and an automatic one. And agree up front that gate size is adjustable at the first trial.

For prototype programs, we favor the least complicated gate that meets function. For production, we want the gate conversation to happen alongside resin selection, not after it. If you are sourcing tools from China, our guide to injection molding gates when sourcing molds lists what to confirm in writing before tooling starts. Our engineers review gate location as part of the quote, and project managers keep you informed through T1.

— Nas

Reducing rework before you cut steel

A short routine saves most gate-related rework.

  1. Send a 3D model with notes. Mark cosmetic surfaces, critical dimensions and preferred gate zones.
  2. Agree the gate type and the degating method. Hand trim, fixture, or automatic.
  3. Request a flow analysis for complex parts. Look at fill balance, weld lines and pressure at the gate.
  4. Confirm the resin’s guidance. Check the supplier molding guide for gating and hot runner limits.
  5. Plan T1 adjustments. Leave steel for opening the gate and vents.
  6. Document the final gate dimensions. Record them in the tool file so repeat orders match.

If you want the larger process view, see our step-by-step injection molding overview and the injection molding workflow.

Get your gate reviewed with the quote

We make injection-molded prototypes and low-volume production parts for engineering teams, with no minimum order quantity and ISO-certified manufacturing. Upload your CAD through our instant quote system and a project manager will confirm specifications with you, including gate location and type, before the order is placed. Start with our injection molding service.

If you only need a handful of parts to validate fit, CNC machining or vacuum casting can skip the gate question altogether until the design settles.

FAQ

What is the most common type of injection molding gate?

The edge gate is the most common general-purpose choice. It sits at the parting line, is easy to machine and is easy to adjust after first shots. Its main drawback is that the vestige on the part edge has to be trimmed.

What is a valve gate in injection molding?

A valve gate is a hot runner gate with a pin that opens and closes the opening mechanically. Valve-gated systems can open valves in cascade, which allows segmented filling and stable packing. They suit large or cosmetic parts, at a higher tooling cost.

How big should an injection molding gate be?

For edge and tab gates, a published starting range is 50% to 80% of nominal wall thickness. The gate land should be short, typically 0.040 in to 0.060 in. Confirm with simulation and first-shot trials, and begin on the small side so steel can be removed.

Is a hot runner better than a cold runner?

Neither is better in general. Hot runners save material and give more packing control but cost more and are harder to run. Cold runners are cheaper and simpler, which suits prototypes and low volumes. Resin, volume and part appearance should decide.

Where should the gate go on a plastic part?

At the thickest section, so the part fills from thick to thin and packs well. Keep it off cosmetic faces and away from stressed features. Make sure the vestige can be reached for trimming if the gate is not self-degating.

Sources

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