The runner system should be designed so that all of the parts finish filling at the same time.
Each part should be analyzed before you design the runners for a family mold. When you have confirmed that each cavity will fill, you can design the runner system to create balanced fill paths in each cavity. Unbalanced runners can result in molding problems such as hesitation, underflow and overpacking.
The following diagram shows an unbalanced family mold. The runners are all the same length ad diameter, but because the cavities are of different sizes, the flow will be unbalanced. The smaller part (bottom left) will fill first, resulting in overpacking, and the largest part will fill last.
The following diagram shows a balanced family mold. The smallest cavity has the thinnest runner, restricting plastic flow into it. This means that the four cavities will all fill at the same time, reducing the possibility of molding problems. You can use a Runner Balance Analysis to balance the runners.
