Runners must be designed to fill the cavity rapidly, routine design must provide for easily ejection and easy removal (de-gating) from the molded part. For multi cavity system, balanced runner layout is practical for the best uniformity and part quality. Runners may be balanced either naturally or artificially. Runner balancing may be achieved by changing the runner size and length, Changing the gate dimensions may seem to give a reasonably balanced fill but this will affect the gate freeze-off time, which is detrimental to part uniformity.
Smaller runner sizes are preferred to larger ones in order to minimize scrap volume and generate viscous (frictional) heating. Viscous heating generated in the runner system is an effective way to raise the melt temperature as opposed to using high cylinder temperatures, which may cause material degradation.
The cross-sectional area of the runner should not be smaller than that of the sprue, to permit rapid, unaltered flow of material to the gating areas.Each time a runner is branched, the diameter of the branch runner should be smaller than that of the main runner. This is because less material flows through die branch runners and it is economically desirable to minimize the amount of material in the runner system.
The depth of a trapezoidal runner is approximately equal to its width with a 5—15°taper of draft angle on each side wall.
The minimum recommended runner diameter for most materials is 1.5 mm (0.06 in).For most materials, the runner surface and sprue should be polished in the line of material flow. Cavities and cores should be polished in the line of draw unless an alternative finish has been specified, It is desirable to have multiple sprue pullers and ejection locations in extended runner systems.
The selection of a cold runner diameter should be based on standard machine tool cutter sizes.When designing hot runner systems, it is advisable to consult the suppliers for availability and recommendations for the correct manifold and drops.