Meaning
Two-stage filling methodology divides the injection moulding process into separate velocity-controlled and pressure-controlled phases to reduce part variation. Implementing decoupled molding ensures that the screw fills the cavity to approximately ninety-five percent of its volume under speed control before switching to pressure control for packing. This separation prevents variations in viscosity from affecting the fill rate, resulting in a more stable moulding process.
The technique requires a fast-acting switchover signal, typically based on screw position or cavity pressure.
Phase Separation
High-speed filling occurs during the first stage, driven solely by the injection speed of the screw. When using decoupled molding, the machine ignores the pressure limit during this initial phase, allowing the machine to maintain a constant flow rate. This approach minimizes the effect of material viscosity shifts caused by temperature changes or lot-to-lot resin variations.
Once the screw reaches the transfer position, the machine switches to the second stage. This second stage applies a lower pressure to pack and hold the polymer melt.
Cavity Dynamics
Pressure inside the cavity rises rapidly once the screw switches from filling to holding. This pressure profile is more consistent because the fill stage ended at a precise volume. If the switchover occurs too early, the part will have short shots, while a late switchover can cause flash or tool damage.
By separating the stages, the peak pressure is controlled, which reduces the internal stress in the finished part. This control is essential for producing high-precision components.
Quality Benefit
Parts produced with this technique exhibit consistent weights and dimensions over long production runs. Scrap rates decrease because the process adjusts for natural variations in raw materials. Moulders can run more cavitation with fewer defects.
This process stability directly improves the efficiency of the moulding operation.
