Determining Cavitation Count and Cycle Time in Multi Cavity Injection Molds
Determining cavitation count and cycle time requires balancing thermal cooling physics against clamping limits, runner shear, and capital amortisation curves.
Pressure Drop Calculations
Meaning
Flow resistance encountered as a melt travels from the nozzle to the cavity determines the required injection pressure. Pressure drop calculations govern the selection of the injection machine and the sizing of the resin delivery system. They determine if the available injection pressure is enough to fill the part completely.
Flow Resistance
Performing pressure drop calculations requires knowledge of the melt viscosity and the geometry of the flow path. Longer runners or thinner gates increase the resistance and lead to higher pressure requirements. A sudden change in the cross-section of a channel also causes an immediate drop in pressure.
Machine Selection
Results from pressure drop calculations help in choosing a press with sufficient injection pressure capacity. If the calculated drop exceeds the machine limit, the part will suffer from short shots where the cavity is not filled. Engineers often increase runner diameters to lower the total pressure needed.
Thermal Impact
Temperature changes in the melt heavily influence the outcome of pressure drop calculations. Cooler resin is more viscous and requires more force to push through the tool. Proper mold heating and melt temperature control are essential to keep the actual pressure drop within the predicted range.