What does it mean to hold pressure?
The holding pressure means that the screw does not immediately retreat after injection, and pressure is still applied to the front melt. In the pressure holding stage, the plastic of the cavity can be reduced in size due to cooling shrinkage. If the gate is not frozen, the screw advances slowly under the pressure of the holding pressure, so that the plastic continues to be injected into the cavity for feeding.
The role of injection
The effect of the injection is to fill the cavity and apply pressure, and transfer the shape of the cavity and the core, even the most subtle, to the melt.
Speed dominate
The injection pressure (read from the pressure gauge) is very low before the cavity is filled, because the resistance to the melt flow in the nozzle, runner and cavity is overcome, and this continues until the cavity fills 100. %until.
It is also clear that the pressure control of the injection molding machine is only the upper pressure control. For example, the pressure of an injection section is set to 90 bar, which means that the pressure of this section cannot exceed 90 bar, but which pressure is below 90 bar is determined by the melting resistance. of. This pressure can be read from the pressure gauge at the time of injection.
Therefore, before the cavity is filled, the injection molding machine cannot be controlled by pressure, but can only be controlled by speed. The pressure comes from resistance, and the resistance is not controlled by the injection molding machine.
The stability of the injected product is closely related to the stability of the injection speed. Full-closed injections are designed to achieve a specified injection rate for each injection.
Pressure holding effect
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.
From this, it can be known that only the pressure is controlled and the flow rate cannot be controlled during the pressure keeping, since 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.
It can be said that the pressure holding point is the screw position of separate injection (including extrusion) and pressure holding. The operator has to set the pressure holding point, which is to find the screw position when the cavity is filled. There are the following three methods.
1, theoretical calculations.
For example, d = screw diameter, s = screw stroke, w = total weight of plastic parts (including cold runner weight), gρ = plastic room temperature density, g / ccw can be expressed as w = (πd2) / (400) × s / 10 × 0.85 × ρ where 0.85 is a coefficient. Therefore s = (4000w) / (0.85ρπd2) = (1497.9w) / (ρd2) and the pressure holding point, h = p - s , p = plasticized screw position
2. When the cavity is filled, the speed of the screw advance will suddenly drop.
This can be observed from the (mechanical) position of the screw position of the injection molding machine during injection and holding, and the position of the screw that is slowed down for a moment is read from the paper ruler behind the pointer. Set this position to the hold point value.
3. Observe the system pressure gauge during injection and pressure keeping.
When the cavity is filled, the hydraulic pressure rises. At this point the observer shouted out and the other observer read the pointer or the screw position displayed by the computer. Set this position to the hold point value. This method is only suitable for thick-wall injection molding with slow injection speed.
Injection molding machine operation
From the operational level, injection and holding pressure have the following similarities.
1. Unless the injection servo valve is used, the injection directional valve is opened during injection and holding pressure and will not close during the process (and of course will not turn).
2. On the general computer screen, both injection and pressure control have speed and pressure control.
3. The segmentation of the injection, although most accurately distinguished by the position of the screw, can also be distinguished by time (referred to as time injection) as the segment of the pressure.
to sum up
The above is the related knowledge of injection and pressure keeping compiled by Xiaobian, I hope to help everyone.