Flow characteristics are determined by melt temperature, pressure, and shear rate. A polymer flows more easily as injection velocity is increased. However, injection velocity that is too high can create excessive shear and result in problems such as splay and jetting. The way in which plastics flow during fill is also affected by their viscosity, or resistance to flow. Plastic pressure, another variable, increases sharply during fill.
Plastic pressure is the primary variable of concern during the pack phase.
The screw maintains pressure in the melt, compensating for shrinkage, which can cause sinks and voids. After the mold is packed, the plastic is held in the mold until it is partially solidified and the gate freezes. One way to optimize this phase is to decrease the hold time until the part weight changes. At that point, the gate is no longer sealed and resin backflows out of the mold. If hold continues after the gate seals, cycle time increases, using more time and energy to produce the part.