Views: 0 Author: Site Editor Publish Time: 2026-05-29 Origin: Site
A practical guide to choosing the right gate type, avoiding common defects, and optimizing your plastic part quality.
If you've ever dealt with injection molding defects like jetting, burn marks, or sink marks, chances are the root cause traces back to one small but critical feature: the gate.
The gate is the tiny opening that allows molten plastic to enter the mold cavity. It might be small, but it controls everything – part appearance, dimensional stability, strength, and cycle time.
In this guide, we'll walk through the most common gate types – from workhorse edge gates to automated pinpoint gates – and show you exactly how to design them for success.
Before we dive into specific gate types, internalize these three principles:
Position first – Always place the gate at the thickest section of the part. Never aim it directly at thin cores or delicate inserts.
Freeze before backflow – The gate should freeze (solidify) immediately after cavity filling to prevent material from flowing back out.
Easy removal – Match the gate type to your production volume. Manual trimming works for prototypes; automatic degating is mandatory for high-volume runs.
What it looks like: A rectangular opening on the parting plane, feeding plastic from the edge of the part.
Best for: Flat panels, housings, boxes, and almost all thermoplastics (ABS, PP, PC, etc.).
Parameter | Recommended Value | Why |
|---|---|---|
Gate width (W) | 1.5 – 5.0 mm | Wider = smoother fill |
Gate thickness (H) | 0.5 – 1.5 mm (≈0.5–0.8× part wall) | Too thin = jetting |
Gate length (L) | 0.5 – 1.0 mm | Shorter = less pressure drop |
Draft angle | 10° – 20° | Helps automatic degating |
Problem | Cause | Solution |
|---|---|---|
Jetting (snake flow) | Gate too thin or aimed at core | Increase H or add a fan transition |
Flow marks | Gate too narrow | Increase W or switch to fan gate |
What it looks like: An edge gate that gradually widens and thins as it approaches the cavity.
Best for: Large thin-walled parts, optical lenses (PMMA, PC), and applications requiring low residual stress.
Entry width = 1.5–2× the runner width
Spread angle = 30° – 60° (larger angle = flatter flow front)
Gate thickness at cavity = 0.25–0.5 × part wall thickness
Why use it: The fan shape delivers a wide, slow flow front that minimizes weld lines and internal stress – critical for transparent parts.
What it looks like: The gate is hidden below the parting plane or behind an ejector pin. It breaks off automatically during ejection.
Best for: High-volume production, cosmetic surfaces (no gate mark), fully automated molding.
Type | Angle | Diameter | Best for |
|---|---|---|---|
Ejector pin submarine | 30°–45° | Φ0.8–1.5 mm | Parts requiring clean gate vestige |
Tunnel submarine | 20°–30° | Φ1.0–2.0 mm | General-purpose automatic degating |
High wear – Glass-filled materials will erode the gate quickly → use hardened steel (SKD11) with nitriding.
High pressure loss – Only use with good-flow materials like PP, PA, POM.
Angle too low (<20°) – The gate won't break; it will bend instead.
What it looks like: An extremely small circular gate (0.5–1.5 mm diameter), typically used with three-plate molds or hot runners.
Best for: Small precision parts (gears, connectors), multi-cavity molds, and applications requiring automatic gate removal.
Parameter | Recommended | Note |
|---|---|---|
Gate diameter (d) | 0.5 – 1.2 mm | 0.3 mm possible with high pressure |
Taper angle | 6° – 10° | Helps pull the gate cleanly |
Gate length | 0.5 – 0.8 mm | Longer = cold slug risk |
Puller pin recess | 0.5 mm deep | Ensures clean break |
Three-plate mold required – The runner system must be pulled off by a separate stripper plate.
Symmetrical layout – All cavities must have equal flow length from the gate.
Avoid with – PA and POM (they tend to string or clog the small orifice).
Rule of thumb: Never use a pinpoint gate next to thin, tall cores – the high-velocity jet will bend them.
Application | Recommended Gate | Why |
|---|---|---|
Large flat panel (500×500 mm) | Multi-point edge or fan gate | Prevents warpage |
Gear / round part | Diaphragm or ring gate | Concentric fill, no weld line |
Long thin rod (L/D > 20) | Pinpoint gate (at end) | Reduces bending stress |
30% glass-filled nylon | Fan gate (extra thick) | Minimizes fiber orientation |
Cosmetic surface (no mark) | Submarine (behind ejector) or valve gate | Invisible vestige |
Thick-thin transition | Tab (ear) gate | Slow down before entering cavity |
Material | Recommended | Avoid | Special Note |
|---|---|---|---|
PP / PE | Edge, submarine | None | Small gates work well, but watch for stringing |
ABS | Edge, fan | Very small pinpoints | Thickness ≥ 0.8× part wall |
PC | Fan, film | Pinpoint, submarine | Must use large gate to prevent stress cracking |
PMMA | Fan, large edge | Pinpoint | Smooth, rounded transitions only |
PA6/66 | Submarine, edge | Pinpoint (stringing) | Always include a cold slug well |
POM | Edge, fan | Submarine (wear) | Minimum gate diameter = 1.0 mm |
Glass-filled | Fan, tab | Pinpoint, submarine | Increase gate thickness by 50% |
Defect | Most Likely Cause | Fix |
|---|---|---|
Burn marks / gas near gate | Gate too small → high shear heating | Increase gate cross-section or slow injection |
High gate vestige | Poor puller geometry | Deepen puller recess to 0.8 mm |
Jetting (snake flow) | Abrupt thickness change | Switch to fan or tab gate |
Ejector pin push-through / crack | Submarine gate poorly positioned | Move gate onto ejector pin tip |
Uneven multi-cavity fill | Unbalanced gate sizes | Scale diameters by flow length ratio |
Here's the golden rule of gate sizing:
Always start with a conservative (small) gate and gradually increase size during mold trials.
Why? Because making a gate larger is a simple milling operation. Making it smaller requires welding and re-machining – expensive and risky.
Start small. Add 0.1 mm at a time. Stop when the part fills completely without defects.
Edge gates – Use them for 80% of projects. Simple, reliable, easy to adjust.
Submarine gates – Best for high-volume, automatic degating. Avoid with glass-filled materials unless using hardened steel.
Pinpoint gates – Require three-plate molds. Perfect for small precision parts, but never near thin cores.
Difficult materials (PC, PMMA) – Always use large fan gates with smooth transitions to prevent cracking or silver streaks.
