Page 118 - Injection Molding Advanced Troubleshooting Guide

104 12 Cavity Balance

In the early 1990s John Beaumont discovered the true root cause of imbalance on
molds with multiple cavities [1]. His experiments led him to the understanding
that as melt streams are split while travelling through a runner system the high
shear layers of material will be consistently positioned in a mold; this is due to
laminar flow of plastic within the runners (see Figure 12.4). The Beaumont Effect
on mold filling has been solved by John Beaumont’s company Beaumont Inc. They
have developed a patented technology called the MeltFlipper . The basic premise
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of the MeltFlipper technology is to cause a rotation of the melt stream as it travels
through the runner system, which in turn allows an equal distribution of high- and
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low-shear material to all cavities. For more on the MeltFlipper contact Beaumont
Inc. [2]. The definitive explanation of shear imbalance can be found in John Beau-
mont’s book “Runner and Gating Design Handbook” published by Hanser.

Figure 12.3 Traditional eight-cavity runner that results in an unbalanced fill pattern

Figure 12.4 Eight-cavity runner system, notice laminar flow of dark tracer flow

Many experimental runner designs have been tried to balance out shear imbal-
ance. If the mold has room a spoke layout (Figure 12.5) is an excellent choice for
preventing shear imbalance. See Figure 12.6 for an example of an unsuccessful
attempt to eliminate the shear variation on an eight-cavity mold.