Meaning
Mould filling stages represent the portion of the injection moulding cycle where the polymer melt is injected to fill the cavity space. Reaching the volumetric fill point occurs just before the machine switches from velocity control to pressure control. This transition is essential for preventing the part from flashing while ensuring complete cavity replication.
Cavity Pressure
Hydraulic pressure rises rapidly as the melt reaches the extremities of the mould cavity during the injection phase. The transition to the packing phase must be timed to coincide with the volumetric fill to avoid a pressure spike that could damage the tool. This pressure curve is monitored by sensors in the mould to ensure consistency.
Transition Point
Changing from injection speed to holding pressure is a critical step in establishing a stable moulding process. If the switchover occurs too early, the cavity is underfilled, resulting in a short shot. If the switchover is too late, the cavity is overfilled, which can cause part deformation and sticking in the mould.
Modern moulding machines use cavity pressure sensors or screw position to trigger this transition, which reduces the sensitivity of the process to variations in the viscosity of the material.
Shrinkage Control
Holding pressure is applied after the volumetric fill stage to feed additional material into the cavity as the polymer cools and shrinks. This packing phase is necessary to prevent sink marks and internal voids in thick sections of the part. The duration of the hold time depends on the gate freeze-off time of the specific mould.
