Injection Mold Gate Design Guide

The gate is the smallest cross-section in an injection mold's runner system — but it has an outsized effect on whether a part passes first-shot trials. Get it wrong, and you're looking at sink marks, jetting, cracking, or worse, a mold rework. This guide covers the gate's core functions, the four most common gate types, placement principles, sizing formulas, and troubleshooting common gate-related defects.

1. The Four Core Functions of a Gate

A gate is the short channel connecting the runner to the mold cavity. It handles four jobs at once:

Fill control: Gate cross-section determines how fast the melt enters the cavity. Too large, and you get jetting; too small, and the melt may not reach the far end of the cavity.

Packing and shrinkage compensation: During the packing stage, melt continues flowing through the gate to compensate for volumetric shrinkage as the part cools. If the gate freezes too early, this pathway closes off, and sink marks appear near the gate.

Degating: When the mold opens, the gate should break cleanly or be easy to trim by hand, separating the part from the runner with minimal secondary processing.

Backflow prevention: Once the gate freezes, it blocks melt from flowing back into the runner, keeping part density and dimensions consistent.

2. Four Common Gate Types Compared

Type	Structure	Pros	Cons	Best For
Direct (Sprue) Gate	Melt flows straight from the nozzle through the sprue into the cavity; works with a simple two-plate mold	Low pressure loss, good packing, simple tooling	High residual stress, hard to trim, leaves a visible scar	Large deep-cavity parts, tubs, boxes, housings
Edge (Side) Gate	Located on the parting line, feeding from the side of the part; rectangular cross-section, roughly 3:1 width-to-depth ratio	Simple to machine, precise dimensions, works with multi-cavity tools	Leaves a mark on the side of the part, may cause weld lines	Small-to-medium parts where the cosmetic surface isn't on the parting line
Pin-point Gate	Very small diameter (0.5–1.5mm), 5°–10° taper, requires a three-plate mold	Minimal gate mark, breaks off automatically, works well for balanced multi-cavity layouts	Complex tooling, higher cost, higher pressure loss	Small precision parts with high cosmetic requirements
Submarine Gate	Feed point tucked below the parting line, 30°–45° angle, 0.25–1.5mm orifice	Cuts itself on mold opening, suited to automation, no visible gate mark on the cosmetic surface	More complex to machine, not ideal for brittle materials	High-volume automated production, cosmetic parts

Selection logic: start with the part's cosmetic requirements, then decide on mold structure (two-plate vs. three-plate), and finally match it to your automation needs.

3. Six Rules for Gate Placement

Balance the flow: Position the gate so melt reaches all extremities of the cavity at roughly the same time. Unbalanced flow causes trapped air, flash, and dimensional variation.

Favor venting: Choose a flow direction that helps air escape the cavity, avoiding voids and burn marks.

Avoid weak sections: Don't gate into thin walls or slender features — feed from a thicker section instead, or you risk short shots and premature freezing.

Minimize weld lines: Keep the gate away from cosmetic surfaces and load-bearing areas. With multiple gates, aim to minimize the number of weld lines they create.

Reduce warpage: Place gates symmetrically so shrinkage is even. Asymmetric gating creates uneven stress and warps the part.

Think about appearance: Put the gate on a non-cosmetic or hidden surface. For parts with strict cosmetic requirements, a pin-point or submarine gate is usually the better choice.

Gate placement directly shapes the flow pattern: a well-chosen location produces laminar, balanced filling with minimal trapped air, while a poor one leads to turbulence, air entrapment, and uneven filling — all of which show up as quality issues in the finished part.

4. Gate Sizing Formulas

· Gate thickness (a): a = (1/3 to 2/3) × t, where t is the part's wall thickness

· Gate width (b): b ≈ 3a

· Gate length (L): L < 1.5mm — shorter is better, to minimize pressure loss

Cross-sections are typically rectangular (edge gates) or round (pin-point gates). The standard approach is to start small: size the gate conservatively using the formulas above, then increase it incrementally during trial molding based on fill, sink, and flash — this avoids the rework that comes from oversizing the gate up front.

5. Common Defects and Fixes

Defect	Cause	Fix
Sink marks	Insufficient packing or the gate freezing too early	Enlarge the gate cross-section, extend packing time, slow the cooling rate
Residual stress at the gate	Abrupt cross-section change or excessive packing pressure, causing stress concentration and cracking	Smooth the transition, reduce packing pressure, optimize gate geometry
Jetting (snake marks)	Gate too small or poorly positioned, causing high-velocity melt to jet into the cavity	Enlarge the gate, switch to a fan gate, adjust injection speed
Flow marks (cold slugs)	Cold material entering the cavity	Add a cold slug well, raise mold temperature, increase injection speed

Planning for a cold slug well and smooth cross-section transitions at the design stage — rather than fixing them during trial molding — saves significant rework time down the line.

6. Summary

Good injection mold gate design follows a clear logic: start by defining the gate's function (fill, pack, degate, prevent backflow), select a gate type based on cosmetic requirements, mold structure, and automation needs, apply the six placement rules to choose a location, use the sizing formulas for a starting point, and refine through trial molding while designing out common defects from the start. Get this sequence right, and you'll significantly improve your odds of a successful first shot — and cut the time from design to production.

Need Help With Injection Mold Gate Design or Mold Sampling?

If you're choosing a gate type for a new part, evaluating mold structure, or want to validate a design before cutting steel, send us your 3D files — SG Prototype offers one-stop CNC machining, injection molding, and rapid prototyping services, with DFM feasibility analysis and quotes within 24 hours. Send us your CAD files for a free manufacturability review.