Injection molding process-the correct setting method of holding pressure

In injection molding, before the cavity is filled, the injection pressure (read from the pressure gauge) is very low, because it only needs to overcome the resistance of the nozzle, runner, and melt flow in the cavity. This situation continues until the cavity is 100% filled.

The pressure control of the injection molding machine is only the upper limit control of the pressure. If the pressure of a certain injection section is set to 90 bar, it only means that the pressure of this section cannot exceed 90 bar, but the pressure below 90 bar is determined by the melting resistance. This pressure can be read from the pressure gauge during the injection. Therefore, before the cavity is filled, the injection molding machine cannot be controlled by pressure, but can only be controlled by speed.


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

The stability of injection molded parts is closely related to the stability of injection speed. The full closed-loop injection is aimed at achieving a specified injection speed for each injection. For the pressure setting of the injection section, please use the system pressure, which is the highest pressure such as 140 bar or 160 bar. There is enough pressure to ensure effective speed control during the injection.

Since the cavity is not full, high-pressure settings will not be the cause of burrs.

After the cavity is filled, it does not mean that the injection process is over, and it needs to be squeezed. In fact, the most difference 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 the melt and the continuous injection of the screw. It can also be regarded as overfilled. The melt equivalent to a few percent of the cavity volume is squeezed in after the cavity is filled, causing the pressure to suddenly Rise.

The squeeze section is actually the last section of injection. Only the squeeze section needs to control the injection pressure and set an upper limit to prevent the generation of burrs. At the end of squeezing, switch to holding pressure.

The role of pressure holding

The function of holding pressure is to maintain a pressure when it melts and cools or solidifies and shrinks and continues to inject the melt to fill the shrinking space, reducing or avoiding the generation of dents.

The set pressure of the pressure holding section cannot exceed the set pressure of the extrusion section, otherwise, burrs may be generated in the pressure holding section. The multi-stage holding pressure decreases every stage, and the ideal decrease is a linear gradual decrease. It is a non-class decrease and needs to meet the actual needs of gradual contraction.

Due to the slow shrinkage, the forward speed of the screw is also slow, and a speed of 2% is sufficient. The energy-saving of the injection molding machine is mainly to reduce the flow rate of the pump to about 3% when maintaining the pressure. The flow rate is always 100% of the fixed pump ratio, which saves 97%.

The longer the holding time (large wall thickness), the more power consumption will be saved. From this, it can be seen that when maintaining pressure, only the pressure is controlled but the flow cannot be controlled, because the shrinkage rate is not controlled by the injection molding machine. This is the opposite of the pre-injection period.