Speed leading injection
When the mold cavity is not filled, the injection pressure (read out from the pressure gauge) is very low, as long as it can overcome the resistance of the melt flow in the nozzle, the runner and the mold cavity. This continues until the cavity is filled with 100%.
The pressure control of injection molding machine is the upper limit of the pressure control, such as a section of the injection pressure setting is 90 bar, the only pressure must not exceed 90 bar, but below 90 bar which is the pressure is determined by melt resistance. This pressure can be read out from the pressure gauge at the time of the injection. Therefore, the mold cavity is not filled, the injection molding machine and can not be controlled by pressure, and can only use speed to control.
Pressure from resistance, but the resistance is not controlled by the injection molding machine.
The stability of injection product is closely related to the stability of injection speed. All closed loop injection is to achieve the specified injection speed as the purpose of each injection. Pressure settings for the injection section, please use the system pressure, such as the maximum pressure of 140 bar or 160 bar. There is sufficient pressure to ensure that the injection can be controlled effectively.
Because the cavity is not full, high pressure settings will not be caused.
Cavity filling does not mean the end of the injection process, but also squeezing. In fact, injection and other molding methods are the most different is the high injection pressure. This pressure is usually between 1000 and 2000 bar. To know the most deep in the earth pit bottom, the water pressure is also less than 2000 bar.
Extrusion is by the amount of melt injection and compression to produce, can see a super full filling, a few percentage points equal to the mold cavity volume of the melt in the cavity is filled and then squeeze in, so that the pressure rise.
The extrusion is actually the last paragraph only need to control the injection pressure injection, extrusion, set an upper limit, in order to prevent burr. Extrusion is over, then go to the holding pressure.
The function of holding pressure
The pressure is when the melt cooling or curing shrinkage, maintain a pressure, continue to inject melt to fill the contraction of the space, reduce or avoid the dent.
Set the pressure set pressure not keeping pressure over the extrusion section, otherwise the flash may produce in the packing section. Multi section of the pressure is a decline in the pressure of each segment, and the ideal decline is a linear decline, the decline in non class, need to meet the actual needs of the gradual contraction.
Due to the slow contraction, the forward speed of the screw is also slow, 2% of the speed is enough. Injection molding machine energy saving, mainly in the pressure of the pump will be reduced to about 3% of the flow rate, flow rate is always 100% of the quantitative pump ratio, saving 97%.
The holding time is long (large wall thickness), save more power consumption. From now on, we can know that when the pressure is controlled, the flow rate can not be controlled, and the shrinkage rate is not controlled by the injection machine. This is just the opposite of the injection.
About how long the holding time, when the weight of production is no longer rising, the pressure can be stopped. The next step is to plastic. It starts at the same time as the cooling time.
To determine the cavity fill, can be said that the point is to separate the pressure of injection (including extrusion) and the pressure of the screw position. The operator to set the pressure point, is to find the cavity filled with the screw position, there are three methods:
1, theoretical calculation
Such as the d= s= screw diameter, screw stroke, w= plastic parts (including the total weight of the cold runner weight), room temperature density = plastic
There is w= (n D2) / (400) / 10 * s * 0.85 * P (of which 0.85 is a coefficient).
So s = (4000W) / (0.85 PI D - P) = (1497.9w) / (D -)
And the holding pressure point H = s - p=, P after the end of the screw position
2, when the mold cavity filled, the speed of the screw forward will suddenly down
This can be seen from the position of screw injection molding machine (Mechanical) pointer out was observed in the injection and holding pressure, slow down the moment screw position and read from the paper ruler behind the position of the pointer, set pressure value.
3, in the injection and holding pressure, observe the system pressure gauge
When the cavity is filled, hydraulic pressure rise. When the read pointer or the computer display the screw position, this position is set to the pressure point. This method is only suitable for thick wall injection with slow injection speed.
Selection of pressure and speed control
Injection molding machine operation
From the operation level, injection and holding pressure have the same.
1, in addition to the use of injection servo valve, injection direction valve in the injection and holding pressure is open, which will not be closed (of course, will not turn).
2, on the computer screen, and the pressure of injection and pressure control.
3, the injection of the segmentation, although the screw position to distinguish the most accurate, but also can be used to distinguish between time (known as time injection), and the same section of the same.
Therefore, some operators do not use the pressure section and the use of the injection of the paragraph or after a few paragraphs to do pressure.
Note: for example, three and four of the injection time injection, which limits the injection and injection of two (extrusion) can only use the time injection, and can not be used with a more precise location of the injection to distinguish.
Computer injection molding machine will have the following two functions:

1, in the position + time mode, can specify the injection time limit. Injection time to the upper limit of the screw has not yet to the point of pressure, the process will still go to keep pressure. This is in the multi cavity injection mold cavity flow channel blockage will appear. When the pressure is not used, it can not use this mode, can only use the time injection

2, before and after the pressure point outside the scope of the two areas, the screw is not in place, known as the less area, the screw had a point of pressure is known as the overflow area. Both have the opportunity to produce waste, to inform the operator to deal with. Do not use the pressure section is not on this function.
If the computer can provide a linear gradually reduce the pressure of the pressure, do not use this function can not be used. Closed loop control is used in precision and thin wall injection molding. Closed loop control only allows the injection speed control, with the pressure of the upper limit of the pressure control section, and the pressure control section is used to control the pressure.