The "H" (branching) and radial (star) systems are considered to be naturally balanced. The naturally balanced runner provides equal distance and runner size from the sprue to all the cavities, so that each cavity fills under the same conditions.
When high quality and tight tolerances are required the cavities must be uniform. Family moulds are not considered suitable. Nevertheless, it might be necessary for economical reasons to mold different parts in one mold. The cavity with the largest component should be placed nearest to the sprue.
The maximum number of cavities in a mold depends on the total cavity volume including runners in relation to the maximum barrel capacity and clamping force of the injection molding machine.
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
dmain= dbranch x N1/3
At all runner intersections there should be a cold slug well. The cold slug well helps the flow of material through the runner system by stopping colder, higher viscosity material moving at the forefront of the molten mass entering into the cavity. The length of the well is usually equal to or greater than the runner diameter and this is achieved by extending the length of the primary runner at the intersection with the secondary runner (see figure below).
Runnerless molds / Hot runners
Recommended design of cold slug well or overflow.