Meaning
Injection moulding control parameters identify the exact volume of polymer required to transition from the primary high pressure velocity packing stage to the secondary constant pressure holding stage. A switchover threshold defines this transition by setting a specific screw position, cavity pressure, or time interval to signal the hydraulic system. Precise calibration of this setpoint prevents overpacking or short shots within the mould cavity.
Pressure Transition
Maintaining consistent density across every molded component depends upon the accuracy of this handover from speed-controlled injection to pressure-controlled packing. Small variations in melt viscosity or ambient temperature trigger shifts in the optimal point for this handoff. Moulders monitor the peak cavity pressure to ensure that the machine reaches the programmed limit before the material begins to freeze in the gate.
Accurate execution avoids high internal stress levels in the part that often cause structural failure.
Tooling Variance
Mould geometry dictates where the switchover threshold sits relative to the total shot size. A complex part with thin walls and long flow paths necessitates an earlier switchover to avoid flash formation at the parting line. Tools with multiple cavities require a higher injection velocity to ensure the melt fills the furthest flow front before the threshold command initiates the packing cycle.
Variations in gate design or cooling channel efficiency create a permanent shift in the required sensor readings.
Material Performance
Virgin resin properties differ from regrind percentages because the viscosity changes as polymer chains undergo shear during the processing cycle. Frequent recalibration of the threshold accounts for these physical shifts in the material batch. High levels of regrind lower the melt viscosity and require an adjustment of the screw position to prevent part weight drift.
Consistent performance during long production runs relies on the ability of the control system to hold the switchover point within strict physical limits.
