Plastics Design Principles: Thermal Shutoff for Runners

By Jay Shoemaker

The runners should be sized so they allow the parts to fill and pack out without controlling the cycle time. In Figure 13, the freeze time of the part is about 3.4 seconds. The runners have freeze times that are at least that of the part. Notice, however, that the cooling time of the sprue is about 10 times that of the part. This would suggest the sprue is too large and should be made smaller if possible. The largest cooling time in a runner should preferably be at most 2 to 3 times that of the part, but this is often difficult to do. In the case of the runner in Figure 13, if the runners were made smaller while maintaining a balanced runner system, the smallest runner, which currently has a cooling time of 4.7 seconds, would quickly become much smaller than the part. As a general rule, if there are no critical dimensions or sink mark quality criteria, the cooling time of the runners can be as low as about 80% of the cooling time of the part. When dimensions are more critical, the cooling time for the runners should be greater than the part.

Figure 13: Thermal shutoff for runners

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