The presence of voids in the sprue is very common, but the need to troubleshoot this is unique to sprue-gated parts. Voids in a sprue gate are a little different than a typical sink or void at the gate.

There are three ways to approach this… Cooling, Pressure, and Temperature.

The first way is to improve cooling at the sprue with additional cooling in the sprue area or mold cavity at the gate region. Beryllium copper inserts or a sprue made from an alternative material would improve the cooling in this region.

The second way to reduce this would be to increase the pressure applied during second stage time. As this pressure is increased, the time required to seal the gate will actually increase. Since the increased pressure causes a higher tendency for the polymer to flow back through the sprue.

The third way to improve the situation would be to increase the mold temperature… assuming sinks in the region are acceptable. Since higher mold temperatures increase the tendency to for sinks over voids.

Such a situation is difficult to correct… and typically requires much trial and error to determine a correct process. Again, as with any other process, you really need to document the process thoroughly once the issue is resolved.

-Andy