As the plastic flows through the injection moulding machine and the different parts of the mould, the pressure acting on the leading edge of the plastic flow is lost due to the effects of drag and friction. In addition, as the plastic contacts the mould wall, it begins to cool, increasing the viscosity of the plastic, requiring additional pressure to push the plastic forward.
The plastic skin layer formed in the mould wall reduces the cross-sectional area of the plastic flow, resulting in a pressure drop. The pressure at the set injection speed for pushing the screw at the set injection speed is maximally limited.
The pressure required to advance the screw at a set injection speed should never exceed the maximum pressure available.
For example, considering that the maximum available hydraulic pressure for an injection moulding machine is 2200 psi, the required screw speed is 5 inches/second. In order for the screw to advance at 5 inches per second, if it requires 2400 psi, the machine will not be able to provide such pressure so that the screw will not travel at 5 inches per second.
In this case, the process is limited by pressure. In the process of process development, understanding the pressure loss on each part helps determine the overall pressure loss and where a large pressure drop occurs. The mould can then be modified to reduce the pressure drop for better continuous flow. It is important to ensure that the maximum pressure is not reached.