Plastics Design Principles: Flow Balancing

By Jay Shoemaker

All the flow paths within a mold should fill at the same time and with equal pressure. For multi-cavity molds, this means each cavity should fill at the same time. Within parts, the same holds true; the extremities of the part should fill at the same time also.

There are two types of balanced runner systems: naturally balanced, sometimes called geometrically balanced runners, and artificially balanced runners. In a naturally balanced runner system, the flow length from the sprue to each of the parts is the same for all cavities, as shown in Figure 2: Naturally balanced runner system. Generally this type of runner system has a larger processing window than artificially balanced runners.

Figure 2: Naturally balanced runner system

The artificially balanced runner system achieves its balance by changing the size of the runners. This can be a very useful technique for balancing runners, as there is generally less runner volume required than a naturally balanced runner. However, due to the changes in the runner diameter, the processing window is generally smaller than a naturally balanced runner. Injection time is generally the main limiting factor. As a result, in situations that have high runner pressure drops plus low part pressure drops, tight tolerances, thin sections in the part, and potential sink mark issues, artificially balanced runners may have a very small molding window or may not be practical. The greater the runner length ratio between the longest and shortest path the greater the potential problem. Figure 3: Artificially balanced runners shows an example of an artificially balanced runner.

Figure 3: Artificially balanced runners

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