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Helping the melt flow
A runner system directs the melt flow from the sprue to the mold cavities. Additional
pressure is required to push the melt through the runner system. Shear (frictional) heat
generated within the melt while the material is flowing through the runner raises the melt
temperature, also facilitating the flow.
Runner size considerations
Although properly sizing a runner to a given part and mold design has a tremendous pay-off, it is
often overlooked since the basic principles are not widely understood.
Pros and cons of large runners
While large runners facilitate the flow of material at relatively low pressure requirements, they
require a longer cooling time, more material consumption and scrap, and more clamping force.
Pros and cons of small runners
Designing the smallest adequate runner system will maximize efficiency in both raw material use
and energy consumption in molding. At the same time, however, runner size reduction is
constrained by the molding machine’s injection pressure capability.
Optimal runner size
C-MOLD Runner Balancing is an excellent software tool for computing the optimal runner size that
conveys a balanced filling pattern with a reasonable pressure drop.
Payoffs of good runner design
A runner system that has been designed correctly will:
l   Achieve the optimal number of cavities
l   Deliver melt to the cavities
l   Balance filling of multiple cavities
l   Balance filling of multi-gate cavities
l   Minimize scrap
l   Eject easily
l   Maximize efficiency in energy consumption
l   Control the filling/packing/cycle time.