Plastics Design Principles: Acceptable Runner / Cavity Ratio

By Jay Shoemaker

Related links: Moldflow Plastics Design Guide | Moldflow Communicator™ | Moldflow Design Guide: A Resource for Plastics Engineers

The ratio of the volume of the feed system to the total volume of the cavities should be as low as possible. This is to reduce the material being wasted in the runners and to reduce the amount of regrind. In Figure 14, the runners cannot be made much smaller and still maintain a balanced fill and acceptable packing. In this example, the ratio of runner to cavity volume is 85%, which is very high. Ideally, the volume of the runners should be 20% of the part volume or less.

Figure 14: Acceptable Runner / Cavity Ratio

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