Theory and practice
Theory is summed up from practice. The same is true of injection molding process.
When the injection molding machine was used in the 1950 s, we filled the mold cavity with injection pressure. Later, it developed into secondary injection, which is the predecessor of "pressure holding".
Injection and pressure keeping are separated, because the roles are different and the emphasis on speed and pressure is different. Especially in the application of high-quality injection molding, thin-wall injection molding and T-shaped mold, it is necessary to distinguish the two.
Effect of injection
The function of injection is to fill the cavity and apply pressure to transfer the shape of the cavity and core, even the smallest place, to the melt.
speed advantage
Before the mold cavity is full, the injection pressure (read from the pressure gauge) is very low because only the resistance of melt flow in the nozzle, runner and mold cavity needs to be overcome. This situation continues until the cavity is full 100%.
It is also clear that the pressure control of injection molding machine is only the upper pressure limit control. For example, if the pressure of the injection part is set to 90 bar, this only means that the pressure in this part cannot exceed 90 bar, but which pressure is lower than 90 bar is determined by the melting resistance. This pressure can be read from the pressure gauge during injection.
Therefore, before filling the cavity, the injection molding machine cannot be controlled by pressure, but only by speed. Pressure comes from resistance, which is beyond the control of injection molding machine.
The stability of injection molded products is closely related to the stability of injection speed. All closed-loop injections are aimed at achieving the specified injection speed in each injection.
For the pressure setting of the injection part, please use the system pressure, that is, the highest pressure, such as 140 bar or 160 bar. There is enough pressure to ensure that the injection speed is controllable. Because the mold cavity is not full, high pressure setting will not be the cause of burr. After the cavity is full, it does not mean that the injection process is over and extrusion is needed.
squeeze
In fact, the biggest 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. It is the extrusion of such a large injection pressure after the cavity is filled that the information as small as CD and as big as the luster of "mirror" can be transferred to the melt.
Extrusion is produced by compression of melt and continuous injection of screw, which can also be regarded as overfilling. The melt equivalent to several percentage points of the cavity volume is squeezed in after the cavity is full, which makes the pressure rise sharply. The extrusion section is actually the last injection section, and only the extrusion section needs to control the injection pressure and set the upper limit to prevent burrs. After the extrusion, keep the pressure. This screw position is called "pressure holding point" and is set on the computer screen.
Pressure maintaining function
The function of pressure keeping is to keep a pressure when the melt cools/solidifies, and continue to inject the melt to fill the shrinkage space, so as to reduce or avoid the occurrence of dents.
The set pressure of the holding section cannot exceed the set pressure of the extrusion section, otherwise the holding section may produce burrs.
Multi-stage pressure keeping is gradually decreasing, and the ideal decreasing is linear and stepless decreasing, so as to meet the practical needs of gradual contraction.
Because the shrinkage is slow, the screw advances slowly, for example, 2% speed is enough. The energy saving of injection molding machine is mainly to reduce the flow rate of pump to 3% when maintaining pressure, which is 97% more energy saving than that of fixed pump with flow rate of 100% forever. The longer the holding time (ultra-thick wall), the more power is saved.
Thus, it is only necessary to control the pressure but not the flow rate, because the shrinkage rate is not controlled by the injection molding machine. This is the opposite of the first stage of injection.
Regarding the length of holding time, when the production weight no longer rises or the dent is acceptable, the holding can be stopped. The next step is plasticization. It starts at the same time as the "cooling time".
Judge the hole filling.
It can be said that the "pressure holding point" is the position where the screw is injected (including extrusion) and held at pressure respectively. The operator should set the pressure maintaining point, that is, find out the screw position when the mold cavity is full. There are three ways.
1. Theoretical calculation.
Such as d = screw diameter, mm
S = screw stroke, mm
W = total weight of plastic parts (including cold runner weight), g
ρ = room temperature density of plastic, g/cc
W can be expressed like this
W = (π d2)/(400 )× s/10× 0.85× ρ, where 0.85 is the coefficient. Therefore, s = (4000 w)/(0.85 ρ π d2) = (1497.9 w)/(ρ d2).
And pressure holding point, mm
h = p - s
P = screw position after plasticization, mm
2. When the cavity is full, 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 when the injection pressure is maintained, and the screw position at the moment of deceleration can be read from the paper ruler behind the pointer. Set this position to the value of Pressure Hold Point.
3. Observe the system pressure gauge during injection and pressure maintenance. When the mold cavity is full, the oil pressure rises sharply. At this time, the observer shouted out, and another observer read the position of the pointer or the screw displayed by the computer. Set this position to the value of Pressure Hold Point. This method is only suitable for thick-walled injection molding with slow injection speed.
Pressure control or speed control?
Based on the above points, the control of injection, extrusion and pressure holding is listed as follows. The injection part is separated by the screw position, which is the most common, but it is guaranteed.
The department of public information can only be separated by time.
Injection molding machine operation
From the operational point of view, injection and pressure holding have the following similarities.
1. Unless an injection servo valve is used, the injection direction valve is open during injection and pressure maintenance, and will not be closed during this period (of course not.
Will turn).
2. On the general computer screen, both injection and pressure holding are controlled by speed and pressure.
3. The segmentation of injection, although the screw position is the most accurate, can also be distinguished by time, (called timed injection) and protection.
The pressure section is the same.
Therefore, some operators do not use the pressure maintaining section at all, but use the latter section or the latter sections of injection for pressure maintaining. In the case of simple injection molding, such as two-stage injection molding and two-stage pressure holding, it is possible, as shown below, but there will be problems in the concept of optimization in higher-end applications.
Note that if injection 3 and injection 4 choose timed injection, this also limits that injection 1 and injection 2 (extrusion section) can only use timed injection, and cannot be distinguished by more accurate position injection.
Computer injection molding machine has the following two functions.
1. In "position+time" mode, you can specify the upper limit of injection time. After reaching the injection upper limit time, the screw has not reached the pressure holding point,
The process will still turn to maintaining pressure. This happens when there are cavities in multi-cavity injection molding. When the pressure maintaining section is not used, this mode is not used, and only timed injection can be used.
2. There are two out-of-range alarm areas before and after the "pressure maintaining point". The area where the screw is not in place is called under-injection area, and the screw has passed the pressure maintaining point.
It's called the flash zone. Both of them have the opportunity to produce waste products and inform the operator to deal with them. This function cannot be used without pressure maintaining section.
If the computer can provide a linear gradual pressure drop of the pressure holding part, this function will not be used without using the pressure holding part.
Precision and thin-walled injection molding will adopt closed-loop control. Closed-loop control only allows speed control in the injection section, upper pressure limit control in the extrusion section and pressure control in the pressure maintaining section.
The injection molding machines/computers in Europe and America are not equipped with pressure control at the injection front end, but only at the extrusion section. This avoids the possibility of maintaining pressure by injection.
conservation of energy
Even if the injection molding machine is equipped with energy-saving variable pump or frequency converter, the energy-saving effect will be reduced if the flow rate in the pressure maintaining section is set higher (> 3%).
abstract
There are many parameters for multistage injection and pressure maintaining. Without theoretical basis, it is a long process to blindly set and find optimization parameters. Injection is speed-oriented, extrusion and pressure-oriented theory, which can halve the set parameters. These theories are of course necessary for closed-loop injection. Understand the extrusion theory and you will know how to solve the burr problem. Understanding the importance of pressure holding point can stabilize injection molding. If you know the theory of holding pressure, you will know how to solve the dent problem. Of course, you can also make energy-saving settings.