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.
Jay Shoemaker has held several positions within Moldflow, including Design Engineer, Customer Services Engineer, and Senior Technical Instructor and Designer, his current role. Jay's primary responsibility is to develop technical training material and deliver training classes for Moldflow Plastics Insight (MPI) products. Learn more.
