Mold Concerns
| Balanced Flow |
Runners for multi-cavity molds require special attention. Runners for family molds, molds producing different parts of an assembly in the same shot, should be designed so that all parts finish filling at the same time. This reduces over-packing and/or flash formation in the cavities that fill first, leading to less shrinkage variation and fewer part-quality problems. Consider computerized mold-filling analysis to adjust gate locations and/or runner section lengths and diameters to achieve balanced flow to each cavity. The same computer techniques balance flow within multi-gated parts. Molds producing multiples of the same part should also provide balanced flow to the ends of each cavity. Naturally balanced runners provide an equal flow distance from the press nozzle to the gate on each cavity.
Artificially balanced runners provide balanced filling and can greatly reduce runner volume. Artificially balanced designs usually adjust runner-segment diameters to compensate for differences in runner flow length. For instance, in ladder runners, the most common artificially balanced runner design, a primary runner feeds two rows of cavities through equal-length secondary runners. The diameters of these secondary runners are made progressively smaller for the cavities with shortest runner flow distance.
