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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.

injection mold design tutorial

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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

  1. When gating multiple cavities, always place the gates symetrically in relation to the sprue. This is critical for injection molding plastic parts.
  2. 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.
  3. Parts that are cup-shaped, such as capacitors, should be gated near the base. This will help avoid gas buildup and burning.
  4. Cosmetic parts or those that cannot show gate marks should be tunnel or submarine gated.
  5. Always gate into the part with the greatest wall thickness.

 injection molding plastic gates and runners  injection molding plastic gates and runners2

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.

 gates and runners  gates and runners

The gate on the left is correct, it is filled from the end. The gates on the right are incorrect and would result in poor fill and quality.

   Submarine gates


Another type of gate is a tunnel, or submarine gate. These are quite common and have many variations. Smiley gates and banana gates are just two examples.

When injection molding plastic parts, often you must not see the gate vestige. It is a clean and efficient way to make a gate, but also requires a bit of expertise. It is much easier to machine a tab gate, for example, but any moldmaker worth his pay should be able to EDM or mill a tunnel gate.

It is also important that the sub-gate have a smooth finish in order for the gate to eject. Injection molding plastic parts is hard enough without sticking gates! This is sometimes overlooked and the result is flaking on the face of the mold during the molding operation. The ceramic stones available for polishing, or the diamond impregnated stones work great for this.

There are many ways to locate the tunnel gate, and CNC EDM's have made the process much easier. In the past, when all the gates were machined on a milling machine, it was a common problem to properly locate the gate, as well as cut it, especially in hardened steel.injection mold design tutorial

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