Meaning
Injection moulding sequences transition from the high-rate velocity-controlled filling stage to the pressure-controlled packing stage at a specific point in the cycle. A precise machine switchover prevents cavity overpressurization and the resulting part flash by stopping the primary injection stroke before the cavity is completely full. This transition relies on screw position, hydraulic pressure, or cavity pressure signals.
Transition Trigger
Process technicians select the most reliable variable to initiate the shift in control modes. Screw position is the most common trigger because it is simple to measure and adjust on the controller. When screw position alone is insufficient, cavity pressure sensors provide a more accurate signal that compensates for material variation.
Defect Prevention
Improper timing of the transition causes significant defects like short shots or sink marks in the molded parts. An early machine switchover results in an unfilled cavity because the packing pressure cannot push the melt to the extremities. Conversely, a late transition overfills the mould, causing flash and damage to the tooling.
Continuous monitoring of the switchover point maintains part consistency across runs.
Process Control
Modern control loops adjust the switchover point dynamically based on real-time feedback from the machine. These systems analyze the injection pressure curve to detect changes in resin viscosity. This automated adjustment stabilizes the moulding process.
