As the plastic flows through the injection molding machine and different parts of the mold, there is a loss of pressure on the plastic flow front due to drag and friction. In addition, as the plastic contacts the mold wall, it begins to cool, increasing the viscosity of the plastic and requiring additional pressure to push the plastic forward.
The plastic skin formed on the mold wall reduces the cross-sectional area of the plastic flow, resulting in a pressure drop. There is a maximum limit to the pressure available on the injection molding machine to push the screw at a set injection speed. The pressure required to push the screw forward at a set injection speed should never exceed the maximum pressure available.
For example, consider that the maximum available hydraulic pressure on the injection molding machine is 2200psi and the required screw speed is 5 inches/second. In order for the screw to advance at 5 inches/second, it would require 2400psi, however the machine will not be able to provide this pressure and the screw will not travel at 5 inches/second. In this case, the process is limited by pressure.
