Page 408 - Injection Molding Advanced Troubleshooting Guide

41.3 Sink Troubleshooting 407

Figure 41.5 Ball test on wall thickness

41.3.2.6„ Mold: Cavity Balance
Cavity balance problems can lead to situations where some cavities are packed out
and others have sink. If a multicavity mold is displaying varying levels of sink by
cavity the cavity balance should be examined.
See Chapter 12 on cavity balance for more information.

41.3.2.7„ Mold: Gate Size, Location, and Quantity
If gate size is inappropriately small the gate may freeze before the part wall is
packed out. There are many rules of thumb for gate sizing but they are rough esti-
mates. There is a shear impact as material travels through the gate and the smaller
the gate the more shear occurs; this can sometimes lead to a better packed part
with a smaller gate.
Do not jump to the conclusion that the gate should be increased in size. There will
be times that increasing the gate size only leads to increased cycle time without
improving the sink on the part. Especially avoid changing gate size on a multi-
cavity mold because this will create imbalances in shear rate, pack pressure, and
gate seal time. Find the root cause rather than trying to balance cavity fill time by
adjusting gate sizes.
If the gate is located too far from a part detail it will be impossible to pack the detail
out. Verify flow lengths and distance from gates to make sure that end of cavity
details can be packed out. In some cases it may be necessary to add additional
gates to provide adequate pack pressure to all regions of a part. Also note that
there will be cases where a rib or boss located close to a gate will actually freeze
while the mold is filling and lead to problems with sink.