Techniques on plastic mold runner design

This article talks about  some basic rules for runner design of plastic toolings, which, if you follow, will help you succeed with an injection mold design.  To provide part-to-part uniformity in a multicavity mold, the runner system should enable each cavity to fill at the same rate.

Designing runners involves several compromises:

  • The runner system should be compact to minimize the amount of rework.
  • The runners must deliver melt that has retained maximum heat to completely pack out the cavity.
  • At the same time, runners must cool quickly after delivering the melt to aid ejection.

If the flow distances from the sprue to the various cavities are equal, the runner system is balanced. Since balanced runner systems contribute to melt uniformity among cavities, dimensional tolerances are easier to maintain. Unbalanced systems can cause cavity to cavity variations. (Figure 1)

Figure 1

Techniques on plastic mold runner design

Occasionally, balancing the runner system and minimizing runner length conflict. In most cases it is better to accept the additional rework and use a more balanced system. Increased injection pressure can be a disadvantage in balanced systems with their longer runners and more turns.

Pressure can be reduced by increasing runner sizes, thereby converting a pressure penalty to a regrind penalty. In the “H” pattern shown in Figure 15, the “H” crossbar is the primary runner fed by the sprue. It should be 1.6mm (0.0625″) larger in diameter than the runner it feeds. These runners in turn should be 1.6mm (0.0625″) larger than the branch runners feeding the gates.

When possible full round runners are preferred. They provide minimum surface area per unit volume, and so have minimum pressure drops and reduced heat losses. For most parts the branch runner should be at least as thick as the heaviest section of the part, not less than 2.3mm (0.090″) and need not be more than 9.5mm (0.375″). (Figure 2)

Figure 2

Techniques on plastic mold runner design

Trapezoidal runners are acceptable if the depth-to-width ratio is about 2:3, for example 4.8mm deep by 6.3mm wide (0.188″ deep by 0:25″ wide). Trapezoidal runners are often used in three-plate molds.

This article only scratches the surface of the science behind runner desgin regarding plastic injection mold techniques. If you would like to learn more about this topic, please contact


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