In the old days of pressure-controlled injection molding machines, the first stage pressure was used to completely fill the mold slowly while a lower pressure (approx. 40-60%) was used to compensate for shrinkage while waiting for the gate to freeze.In the newer machines, if the mold is completely filled during first stage, then the old 40-60% rule of thumb is often applicable.

If a scientific or decoupled type of process is being used, where the mold is not completely filled during first stage, then there are many factors which play into the final second stage packing pressure. Essentially, the only rule of thumb is to select a pressure which produces an acceptable part. The first stage injection pressure should have no bearing on the second stage packing pressure. I have seen thick parts using second stage packing pressures as low as 25% of the pressure at transfer, and I have seen thin-walled parts use as much as 125%.

The fundamental underlying basis of good injection molding is to separate mold filling from mold packing. This means you should first establish a good fill, and then establish your part packing.

-Andy