How to Design the Gating System in Injection Molds: A Practical Guide

The overall mold structure strongly affects how easy a part is to set up and run in production. The design of the gating system determines whether the melt can flow smoothly and fill the cavities completely. When the mold structure and the gating system work well together, the mold is stable, efficient, and easier to use on the shop floor.

This guide covers the four key principles for choosing gate location, how to select the right gate type for your part, how to size the sprue, runner, and gate properly — plus a real-world case study from a Dongguan factory that illustrates what happens when gating goes wrong.

4 Key Principles for Gate Location Selection

1. Gate at the thickest wall section

Gate location should be at the thicker wall sections so the molten plastic flows from thick to thin areas. This helps the cavity fill more completely and reduces problems like sink marks and weld lines.

2. Keep flow paths short and balanced

Try to keep the flow path as short and as even as possible. For multi-cavity molds, the runner lengths to each cavity should be roughly balanced to reduce uneven filling between cavities.

3. Avoid visible surfaces and weak areas

Avoid placing the gate on visible cosmetic surfaces or in strength-critical areas that carry the main load or are prone to stress concentration. For example, don't place the gate near screw bosses or at the root of thin ribs. Use the gate location to move weld lines into less critical, hidden areas of the part.

How to Choose the Gate Type

Common gate types include sprue gates, pin gates, edge gates, submarine gates, and fan gates. You need to select the gate type based on part geometry, material flow characteristics, cosmetic requirements, and how easy it is to remove or trim the gate after molding.

• Small parts with visible surfaces where automatic de-gating is preferred — engineers often choose submarine gates or pin gates.
• Large parts or structural components where surface appearance is less critical — edge gates are widely used to ensure stable cavity filling.
• Thin-wall parts with large surface areas — fan gates work well because they spread the molten plastic evenly across the cavity, helping reduce flow marks and short shots.
• Hot runner molds — the main options are valve gates and open gates. You should consider gate vestige, flow balance in the hot runner system, and overall mold cost.

Interactive Gate Type Selector

Answer 4 quick questions about your part, and the selector below recommends the most appropriate gate type — plus a backup option — based on the 4 principles above and the six gate types we run every day on our dental and medical gas equipment OEM programs.

How to Design Runner and Gate Dimensions

How to Design the Sprue

The sprue should be tapered — wider at the top and narrower toward the parting line — to allow easy ejection from the mold.

• Taper angle: typically 2° to 6°
• Inner wall surface finish: Ra ≤ 0.4 μm to reduce friction and help the sprue release cleanly
• Inlet diameter: should match the nozzle tip diameter of the injection molding machine
• Outlet diameter: should be slightly smaller than the inlet

How to Design the Runner

Runner cross-sections should be circular or as close to circular as possible, since this shape gives the best flow-to-surface-area ratio and minimizes heat loss.

As a general guideline, runner diameter is typically between 4 mm and 10 mm, depending on part weight, material flow properties, and the flow length ratio. The goal is to allow smooth filling without wasting excess material.

For multi-cavity molds, runners must be designed using a balanced layout to ensure that the fill time difference between cavities is no more than 5%.

How to Size the Gate

Start with a smaller gate size based on experience values. After the first trial, adjust the gate by enlarging or polishing to improve filling, surface quality, and packing performance. This is especially important for multi-cavity molds, where gate balance directly affects part consistency.

However, do not make the gate too large. Oversized gates can cause:

• Residual stress concentration
• Visible sink marks on the surface
• Warpage and dimensional instability

Processing and Maintenance

To make machining and later adjustments easier, try to place the sprue, runners, and gates either on the parting line or on removable inserts. This allows you to modify dimensions, change gate types, or polish surfaces without reworking the entire mold.

Cold slug wells should have enough volume to capture the cold material at the front of the melt stream. It is also good practice to provide venting near the ends of the flow path so that trapped air can escape, reducing the risk of short shots or burn marks on the parts.

Case Study: ABS Trophy Base in Dongguan

A factory in Dongguan, Guangdong was producing an ABS trophy base with a wall thickness of about 20 mm.

In the original mold design, the main sprue was very thick, but the gate was extremely small. This created very high flow resistance, and the parts showed severe sink marks and obvious flow lines, making stable mass production impossible.

During troubleshooting, we found that the claim of "the old mold had no problem" was a misunderstanding. The mold base was reused from an old tool, but the mold core was completely new. In reality, this was a new mold, and the gate design itself was not reasonable.

After increasing the gate size according to our recommendations, the sink marks were reduced to an acceptable level and the project returned to normal production.

Summary of Key Gating System Points

Design Factor	Common Issue	Correct Approach
Sprue vs. gate	Oversized sprue combined with an extremely small gate, causing contradiction in flow	Match the runner cross-section to the gate cross-section
Thick-wall parts	Gate too small, leading to incomplete filling	Enlarge the gate cross-section according to wall thickness
Gating method	Incorrect choice of pin gate, submarine gate, or direct gate	Select the gate type based on part geometry, material, and cosmetic requirements
Rib / boss structure	Trapped air cannot escape, causing burn marks	Use insert construction and provide venting channels