Optimizing Holding Pressure Time to Reduce Overall Cycle Duration

Holding pressure is applied after the initial cavity filling to pack more material into the part and compensate for shrinkage as the material cools and solidifies. This phase is crucial for achieving correct part dimensions and minimizing sink marks or voids. However, the duration of the holding pressure phase directly impacts the overall cycle time. Traditionally, long hold times were used to ensure complete compensation for shrinkage. Modern understanding and techniques allow for significant optimization, reducing this time without sacrificing part quality.

The key is to apply holding pressure only for as long as it is effective. As the material in the gate area freezes, it cuts off the flow of additional material from the injection unit. Continuing to apply holding pressure beyond this point ('gate freeze') is futile and simply adds dead time to the cycle. By determining the precise moment of gate freeze, either through empirical testing or advanced cavity pressure sensors, the holding time can be minimized to just the necessary duration.

Part geometry and material properties dictate the optimal holding strategy. Thick-walled parts typically require longer hold times as their cores take longer to solidify and shrink. Thin-walled parts may need very little or no hold time at all. Materials with high shrinkage rates or those prone to sink marks might benefit from a higher holding pressure for a shorter time, rather than a lower pressure for a long time. Tailoring the holding pressure profile (starting high and dropping off) can also optimize this phase for speed and quality.

Switching from a time-based to a pressure-based or position-based end-of-hold criterion is a powerful optimization technique. Instead of holding for a fixed number of seconds, the machine switches to the next phase (cooling) once the screw position stops changing significantly or the cavity pressure reaches a certain threshold. This dynamic approach ensures that holding is neither too short nor unnecessarily long, directly contributing to a shorter, more efficient cycle time tailored to the specific requirements of each unique part and material combination.