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Overview
Runner design dictates part quality and productivity. The design rules that follow provide
general guidelines for runner design.
Runner size
Runner cross section
The cross-sectional area of a runner should not be smaller than that of the sprue, to permit rapid,
unaltered flow of the material to the gating area.
Cold runner diameter
The selection of a cold runner diameter should be based on standard machine tool cutter sizes. The
minimum recommended runner diameter for most materials is 1.5 mm (0.06 inches). Typical runner
diameters for various unfilled generic materials are shown in Typical runner diameters for unfilled
generic materials.
See Determining runner dimensions for more information on this topic.
Trapezoidal runners
The depth of a trapezoidal runner is approximately equal to its width, with a 5º to 15º draft (taper)
on each side wall.
Scrap
Use a small runner size to minimize scrap volume.
Raising melt temperature
Smaller runner size is preferred over larger runner size in order to generate viscous (frictional)
heating. Viscous heating generated in the runner system is an effective way to raise the melt
temperature instead of using a high barrel temperature, which is likely to cause material
degradation.
Branched runners
Each time a runner is branched, the diameter of the branch runners should be smaller than the main
runner, because less material flows through the branches and it is economically desirable to use
minimum material in the runners.
Where N is the number of branches, the relationship between the main runner diameter (dmain) and
the branch runner diameter (dbranch) is

Runner intersections
All runner intersections should have a cold solid slug well to help the flow of material through the
runner system and into the cavity. the diagram below shows that the length of the well is usually
equal to the runner diameter.

FIGURE 1. The cold slug well is typically an extension of the runner at an intersection with another runner.
Hot runners
When designing Hot runner systems, it is necessary to consult suppliers for availability of the proper
manifold and drops.
Easy ejection
Runner design must provide for easy ejection and easy removal from the molded part, with proper
cross-sectional and draft angle. For most materials, the runner surface must be polished to facilitate
flow and part ejection. Extended runner systems should have multiple sprue pullers and ejection
locations.