Dynamic Cavity Pressure Integral Correlation with Part Dimensions in Injection Moulding
Dynamic cavity pressure curve integration over hold time governs volumetric polymer density and predicts final part dimensions before tool ejection occurs.
Holding Time
Meaning
Injection moulding cycles include a discrete duration where pressure is maintained on the polymer melt after the cavity has been filled. This holding time represents the period during which the machine screw remains forward to exert pressure, preventing the compressed material from flowing back out of the gate. It ensures that the cavity remains packed with polymer as it cools and shrinks, thereby establishing the final part weight and density.
Gate Solidification
Solidification of the gate occurs when the polymer in the narrow entry channel cools to its freezing temperature, isolating the cavity from the runner system. If holding time is shorter than the gate freeze time, molten polymer escapes from the cavity, resulting in high shrinkage and internal voids. Moulders determine the precise freezing point by weighing parts from successive runs where holding pressure duration is incremented step by step until the weight stabilizes.
This weight-stabilization study represents the standard industrial method for establishing the minimum required holding pressure duration for any given runner and gate geometry.
Dimensional Stability
Inadequate holding time causes unpredictable shrinkage and warpage due to the loss of packing pressure. When pressure is released too early, the relaxation of polymer chains leads to high internal stress and localized sinks in thick-walled sections. Molded dimensions can drift outside acceptable tolerance bands if the pack and hold stages are cut short.
Cycle Optimization
Cycle times must balance part quality against production efficiency, as excessive holding time wastes energy and increases cycle duration without providing any technical benefit. Once the gate is frozen, further pressure application has no effect on the material inside the cavity. Optimizing this stage involves setting the holding time just past the gate freeze point to maximize throughput while maintaining part weight consistency.