How to Design Injection Molding Plastic Gates and Runners

How To Design Injection Molding Plastic Gates and Runners

Injection molding plastic parts requires a thorough understanding of gates and runners. The correct design of runners and gates in injection molds is critical to producing a quality plastic part, yet this is often done by guess-work.

Here are some definite injection mold design rules to follow that will ensure your success. Of course, there are almost endless applications for gates and runners, plus the wide array of materials can dramatically influence the design. A good tutorial, as mentioned below, can help with the molding of plastic parts by using the correct gate and runner.

You can learn everything you need to know about injection mold design with this tutorial. It covers everything from the most basic aspects to the detailed analysis of mold flow and cooling. Injection molding plastic parts can become quite frustrating, if the gates and runners are wrong!

The section on gates and runners is throrough and exhaustive. Every gate and runner you can imagine are explained and clarified, so you know what to do, how to do it, and why you are doing it! This can make all the difference for injection molding plastic parts.

Basic rules for gate and runner design

When gating multiple cavities, always place the gates symetrically in relation to the sprue. This is critical for injection molding plastic parts.
Round parts, such as gears, should be gated centrally. Use a diaphragm gate or three plate mold. This will ensure that the parts are balanced and round.
Parts that are cup-shaped, such as capacitors, should be gated near the base. This will help avoid gas buildup and burning.
Cosmetic parts or those that cannot show gate marks should be tunnel or submarine gated.
Always gate into the part with the greatest wall thickness.


The gating on the left is correct, it is balanced. The gates on the right are incorrect, the result would be uneven fills and poor quality.


The gate on...