Injection Molding Process - Correct Setting of Holding Pressure

When the mold cavity is not filled, the injection pressure (read from the pressure gauge) is very low, because the resistance to melt flow in the nozzle, runner and mold cavity can be overcome. This continues until the cavity is filled with 100%.

The pressure control of the Injection Molding Machine is only the upper pressure control. If the pressure of an injection section is set to 90 bar, it only means that the pressure in this section cannot exceed 90 bar, but which pressure below 90 bar is determined by the melting resistance. This pressure can be read from the pressure gauge at the time of injection. Therefore, before the cavity is filled, the injection molding machine can not be controlled by pressure, but can only be controlled by speed.

Pressure comes from resistance, but resistance is not controlled by injection molding machines.

The stability of the injection product is closely related to the stability of the injection speed. Full closed-loop injection is aimed at achieving the specified injection rate for each injection. For the pressure setting of the injection section, please use the system pressure, which is the maximum pressure such as 140 bar or 160 bar. There is enough pressure to ensure that the injection can effectively control the speed.

Because the mold cavity is not full, the high pressure setting will not be the reason for the hairy edge.

Filling the mold cavity does not mean that the injection process is over, but also extrusion. In fact, the most different between injection and other molding methods is the extremely high injection pressure. This pressure is usually between 1000 and 2000 bar. You know, at the bottom of the deepest abyss on Earth, the water pressure is less than 2000 bar.

Extrusion is produced by the compression of melting and the continuous injection of screw. It can also be seen as super full filling. A few percentage points of melting equivalent to the volume of mold cavity is squeezed in after the cavity is filled, so that the pressure rises sharply.

The extrusion section is actually the last stage of injection, only the extrusion section needs to control the injection pressure, set a upper limit to prevent the emergence of hair edges. When the extrusion is over, turn to hold the pressure.

The effect of holding pressure

The function of holding pressure is to maintain a pressure when it melts cooling or curing shrinkage, continue to inject melting to fill the shrinkage space, and reduce or avoid the occurrence of indentation.

The setting pressure of the pressure holding section should not exceed the set pressure of the extrusion section, otherwise the wool edge may be produced in the pressure holding section. The pressure holding pressure of multiple stages decreases in each stage, while the ideal decrease is linear gradual decline and non-class decline, so it is necessary to meet the actual needs of gradual contraction.

Because of the slow shrinkage, the speed of the screw is also slow, 2% of the speed is enough. The energy saving of injection molding machine is mainly to reduce the flow rate of pump to about 3% when holding pressure, and the flow rate is always 100% quantitative pump ratio, which saves 97%.

The longer the voltage holding time (the wall thickness is oversized), the more power consumption will be saved. From this, we can only control the pressure and can not control the flow rate, because the shrinkage is not controlled by the injection molding machine. This is the opposite of the previous part of the injection.