The function of holding pressure is to maintain a pressure when it is melted/solidified and shrink, and continue to inject molten to fill the space of shrinkage, reducing or avoiding the generation of dents.
The set pressure of the holding section cannot exceed the set pressure of the pressing section, otherwise the burr may be generated in the holding section. The multi-stage holding pressure is reduced in each segment, and the ideal descent is linear gradual decline, non-class decline, to meet the actual needs of gradual shrinkage.
Due to the slow contraction, the advance speed of the screw is also slow, such as 2% speed is sufficient. The energy saving of the injection molding machine is mainly to reduce the flow rate of the pump to 3% when the pressure is maintained, and save 97% compared with the fixed pump ratio where the flow is always 100%. The longer the holding time (the wall thickness is too large), the more power is saved.
It is only necessary to control the pressure during the pressure keeping and not to control the flow rate, because the shrinkage rate is not controlled by the injection molding machine, which is exactly the opposite of the injection stage.
Regarding how long the holding time is, when the production weight no longer rises, or the dent is acceptable, the pressure holding can be stopped. The next step is plasticization. It starts at the same time as the cooling time, and it is judged that the cavity is filled.