Real Time Closed Loop Cavity Pressure Compensation for Polymer Viscosity Drift
Closed loop cavity pressure regulation corrects melt viscosity drift by dynamically modulating switchover point and packing profile to hold volumetric shrinkage constant.
Melt Flow Rate Variation
Meaning
Material property variation that describes the fluctuation in flow behavior of a polymer melt between different resin batches or within the same lot. High melt flow rate variation indicates an inconsistent molecular weight distribution in the resin supply. This fluctuation makes it difficult for injection moulders to maintain consistent cavity filling and packing.
Moulders must monitor this variable because it directly impacts the dimensional stability of the finished parts.
Viscosity Instability
When the melt flow rate varies, the viscosity of the polymer changes, altering the pressure required to fill the mould. A batch with a higher flow rate will fill the cavity faster and at lower pressures, potentially causing flash. Conversely, a batch with a lower flow rate has higher viscosity, which can lead to incomplete filling or short shots.
These shifts force the operator to constantly adjust machine parameters to maintain part weight. This continuous manual adjustment increases the scrap rate and extends cycle times.
Process Adjustment
Using advanced cavity pressure sensors is the most effective way to compensate for melt flow rate variation. These sensors detect pressure changes and adjust the injection speed or switchover point in real time. This automated compensation maintains part quality even when the resin flow properties shift.
Without these systems, the moulder is dependent on continuous quality inspection.
Sourcing Strategy
Virgin resins from top-tier suppliers typically exhibit minimal variation, while low-cost or recycled resins present much wider fluctuations. Sourcing managers must weigh the lower purchase price of recycled polymer against the increased sorting and rejection costs that occur when using unstable materials. In high-precision applications like medical moulding, the permissible variation is restricted to a very narrow range, and every batch must be certified by the supplier.
Moulders often run incoming melt flow index tests to verify these certificates before dumping resin into the silos. This pre-production check prevents bad batches from contaminating the system and disrupting production schedules.