It is important for the feed system in multi-cavity molds to be balanced so that the plastic melt fills each seperate mold cavity simultaneously.
Non-uniform fills can result in some cavities producing good parts and other cavities producing inferior parts due to short shots, overpacking or flash.
Before designing a multi-cavity mold, you should analyze each cavity without the runner system. When you know how each cavity will fill, you can design the runner system to create balanced fill paths. The following diagram shows a conventional way of filling a multi-cavity part, which may cause molding problems because the flow path for the outer parts is much longer than the flow path for the inner parts. In this example, the runners must be balanced.
In the following diagram, the runners in this naturally balanced runner system all have the same flow length which means that the polymer will reach the gate of each part simultaneously.
Another method of balancing flow paths is to use artificial runner balancing, where the runners have different diameters to promote flow to the more distant cavities. If you have a conventional runner layout, you can artificially balance the runners using this method.
