Injection mold processing plant Do you really understand the often mentioned injection and pressure maintaining

Speed dominated injection

Before the mold cavity is filled, the injection pressure (read from the pressure gauge) is very low, as long as the resistance of nozzle, runner and melting flow in the mold cavity is overcome. This continues until the cavity is 100% full.

The pressure control of the injection molding machine is only the upper limit control of the pressure. If the pressure of an 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 injection. Therefore, before the cavity is filled, the injection molding machine cannot be controlled by pressure, but only by speed.

The pressure comes from the resistance, but 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. The purpose of full closed loop injection is to achieve the specified injection speed for each injection section. For the pressure setting of the injection section, please use the system pressure, that is, a very high pressure of 140 bar or 160 bar. There is enough pressure to ensure that the injection speed can be effectively controlled.

Because the cavity is not full, a high pressure setting will not be the cause of burrs.

The filling of the cavity does not mean the end of the injection process, but also extrusion. In fact, the difference between injection and other molding methods is the extremely high injection pressure. This pressure is typically between 1000 and 2000 bar. You should know that at the bottom of the deepest abyss on the earth, the water pressure is less than 2000 bar.

Extrusion is generated by the melting compression amount and the continuous injection of the screw. It can also be seen as overfilling. The melting, which is equivalent to several percentage points of the cavity volume, is squeezed in after the cavity is filled, making the pressure rise sharply.

The extrusion section is actually the injection section at the end. Only the extrusion section needs to control the injection pressure and set an upper limit to prevent burrs. When the extrusion is over, turn to pressure maintaining.

Pressure maintaining function

The function of pressure maintaining is to maintain a pressure when it is cooled by melting or shrinks by curing, and continue to inject melting to fill the shrinkage space, so as to reduce or avoid the generation of dents.

The set pressure of the pressure retaining section cannot exceed the set pressure of the extrusion section, otherwise burrs may be generated in the pressure retaining section. The multi segment pressure maintaining pressure decreases with each segment, and the ideal decline is linear gradual decline, non class decline, which needs to meet the actual needs of gradual contraction.

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

The longer the pressure holding time (extra large wall thickness), the more power consumption will be saved. It can be seen from this that only the pressure can be controlled but the flow can not be controlled during pressure maintaining, because the shrinkage is not controlled by the injection molding machine. This is just the opposite of the pre injection segment.

As for the pressure holding time, when the production weight no longer rises, the pressure holding can be stopped. The next step is plasticizing. It starts at the same time as the cooldown time.

If the mold cavity is filled, it can be said that the pressure holding point is the screw position for separate injection (including extrusion) and pressure holding. The operator needs to set the pressure holding point, which is to find out the screw position when the mold cavity is filled. There are three methods as follows:

1. Theoretical calculation

For example, d=screw diameter, s=screw stroke, w=total weight of plastic parts (including cold runner weight), ρ= Room temperature density of plastics

Then: w=(π d2)/(400) × s / 10 × zero point eight five × ρ (0.85 is a coefficient).

So s=(4000w)/(0.85 ρ πd ²) = (1497.9w) / ( ρ d ²)

Pressure holding point h=p - s, p=plasticized screw position

2. When the mold cavity is filled, the forward speed of the screw will suddenly drop

This can be observed from the (mechanical) pointer of the screw position of the injection molding machine during injection and pressure maintaining, and read the screw position at the moment of slowing down from the paper ruler behind the pointer, and set this position as the pressure maintaining point value.

3. Observe the system pressure gauge during injection and pressure maintaining

When the mold cavity is filled, the oil pressure rises sharply. At this time, read the screw position displayed by the pointer or the computer, and set this position as the pressure maintaining point value. This method is only suitable for thick wall injection molding with slow injection speed.

Pressure and speed control selection

Operation of injection molding machine

In terms of operation, injection and pressure maintaining have the following similarities.

1. Except for the injection servo valve, the injection direction valve is open during injection and pressure maintaining, and will not be closed (of course, it will not turn).

2. On the computer screen, speed and pressure control are available for injection and pressure maintaining.

3. Although the segmentation of injection is very accurate by screw position, it can also be distinguished by time (called time injection), which is the same as the segmentation of pressure maintaining.

Therefore, some operators do not use the pressure maintaining section but use the rear section or several sections of injection for pressure maintaining.

Note: If injection 3 and injection 4 select time injection, this limits that injection 1 and injection 2 (extrusion section) can only use time injection, rather than using more precise location injection to distinguish.

Computer injection molding machines have the following two functions:

1. In the position+time mode, the upper limit of injection time can be specified. When the injection upper limit time is up, the screw has not reached the pressure holding point, and the process will still turn to pressure holding. This will occur when the runner with cavity is blocked during multi cavity injection molding. When the pressure maintaining section is not used, this mode will not be used, and only time injection can be used.

2. There are two alarm areas outside the range before and after the pressure holding point. The area where the screw is not in place is called the under injection area, and the area where the screw passes the pressure holding point is called the overflow area. Both have the opportunity to generate waste products and notify the operator to handle them. This function is also unavailable without pressure maintaining section.

If the computer can provide the linear gradual pressure drop of the pressure maintaining section, this function will not be used without the pressure maintaining section. Closed loop control will be adopted for precision and thin-wall injection molding. The closed-loop control only allows the injection section to be controlled by the speed, with the upper pressure limit control of the extrusion section, while the pressure holding section is controlled by the pressure.