JPH01226319A - Pressure controlling apparatus of motor-driven injection molding machine - Google Patents

Abstract

PURPOSE:To make it possible to change response output characteristics of a servomotor for injection, by adding successively the amt. of increase and decrease in pressure at every treating cycle from dwell switching time and switching from the preceding torque limit value to the present one within the time const. CONSTITUTION:E.g., to increase or decrease continuously and stepwise from the max. injection pressure Ps at the time when injection is finished to a determined pressure P1, at first, a determined time const. T1 is divided by a treating cycle tau to obtain a cycle number n of the treating cycle between the time const. T1 and, then, a pressure difference between the max. injection pressure Ps and a determined pressure P1 at the first step of dwelling is divided by a treating cycle number n during the time const. T1 is passed to calculate the amt. of increase and decrease P to be added at every one cycle of CPU11 for NC. The amt. of increase and decrease P is added to the present determined pressure and the value is output to RAM17 as the value of a command pressure, i.e., a torque limit value. Pressure control is performed by means of servo CPU12 based on said torque limit value. In this instance, by setting and changing variably the const. T1, working up and down characteristics of the pressure of the servomotor 1 can be freely controlled.

Description

[Detailed description of the invention] Industrial applications The present invention relates to a pressure control device for an electric injection molding machine.

``In conventional injection molding machines, when the pressure holding process begins after the injection process is completed, the pressure holding process is divided into several stages and the pressure at each stage is controlled.

In electric injection molding machines, a torque limit is applied to the injection shaft, that is, the motor that moves the screw in the direction of the screw axis, to control the pressure at each stage of the pressure holding process. The pressure in this pressure holding process is 1$lJ
TLL has an open-loop control method that controls the holding pressure by sequentially changing the torque limit value as time passes from the start of holding pressure, and an open-loop control method that sequentially changes the torque limit value and controls the pressure actually applied to the resin. There is a zero-zero loop control system that detects and performs feedback control, and each of them is well known.

On the other hand, when pressure control is performed using a closed-loop control method for boat fishing, the actual response pressure will overshoot or undershoot with respect to the command pressure, and there are means to eliminate this overshoot and undershoot. As such, so-called PfD control is performed in which proportional, differential, and integral parameters of feedback gain are adjusted.

However, even if you adjust the parameters using PIDfl and 1ltll and adjust the loop gain, the response may saturate depending on the actuator's capabilities and characteristics.
D control may not be able to handle the situation.

The same goes for pressure control in injection molding machines; depending on the capabilities and characteristics of the actuator that applies pressure to the resin, that is, the servo motor for injection, overshoot and undershoot can occur regardless of oven loop control or closed loop control. It was difficult to find a way to eliminate it. For example, the only option available was to replace the injection servo motor.

Problems to be Solved by the Invention When controlling pressure during the pressure holding process of an injection molding machine, whether it is open-loop control or closed-loop control, pressure control is performed by changing the torque limit value, and the response of the servo system is Because the torque limit is changed quickly, the response output may overshoot or undershoot due to the inertia of the motor or screw. The only way to eliminate this problem was to replace the injection servo motor, as described above.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a pressure control device that can change the response output characteristics of an injection servo motor without replacing the injection servo motor.

Means for Solving the Problems Figure 1 is a block diagram of the means adopted by the present invention to solve the above problems. a setting means a for setting the value, pressure holding time, and time constant when switching the pressure from the previous stage to the current stage, and the torque limit value and pressure holding time of each stage set by the setting means a.

The storage means for storing the time constant includes a cycle number calculation means C for calculating the number of cycles by dividing the time constant stored in the storage means by the processing cycle when switching the pressure from the previous stage to the current stage; An increase/decrease lifting means d divides the difference between the torque limit values of the stages by the number of cycles determined by the cycle number calculation means C to obtain an increase/decrease amount, and sequentially sends the increase/decrease amount to a processing station WJfji from the time of pressure holding switching. The above-mentioned problem was solved by providing a pressure switching means e that adds up the torque limit value and switches from the previous stage to the current stage torque limit value within the above-mentioned time constant.

Operation When the setting means a sets and stores the number of stages of the pressure holding process, the torque limit value for holding pressure at each stage, and the pressure holding time 1 time constant in the storage means and drives the injection molding machine. When the injection process is completed and the pressure holding process begins, the pressure holding operation from the first stage to the setting stage is started.

At the start of pressure holding in each stage, the period number calculation means C reads the time constant of the current stage from the previous stage of the stage stored in the storage means, and divides this time constant by the processing period to calculate the number of periods. ,
The increase/decrease calculation means d calculates the difference between the set torque limit value of the previous stage (in the first stage, the torque limit value during the injection process (usually the maximum torque limit value)) and the set torque limit value of the current stage by the number of cycles. The pressure switching means e adds this increase lliJm to the current torque limit value for each processing cycle, outputs it as the torque limit value for the processing cycle, and applies the torque limit.

This process is performed for the number of cycles obtained above, that is, the torque limit value is continuously switched from the torque limit value of the current stage to the set torque limit value of the current stage in 9 steps during the set time constant, and when the set torque limit value is reached, that value is held. When the set pressure holding time of the current stage ends, the pressure holding time of the next stage is continuously switched in two steps by the same process as described above.

Example An embodiment of the present invention will be described below.

In FIG. 2 showing the main parts of an electric injection molding machine according to an embodiment, the reference numeral 1° indicates a driving source that drives the screw 2 of the injection molding machine in the direction of the screw axis to perform injection and pressure holding operations. The servo motor 1 is a servo motor, and a pulse coder 3 is attached to the motor shaft of the servo motor 1. Note that this electric injection molding machine has reliable servo motors such as a screw rotating shaft and a clamp shaft like a normal one, but since these are not directly related to the present invention, a description thereof will be omitted.

Reference numeral 10 denotes a control device for the electric injection molding machine, and the control device 10 has a numerical control system (hereinafter referred to as "numerical control") that controls the operation of the entire injection molding machine and the pulse distribution process of each axis servo motor.
Central processing unit (hereinafter referred to as CPU) 1
1 and the position of each axis servo [-ta] based on the command value of the NC CPU 11.

Servo C for servo control that controls speed, torque control, etc.
It is equipped with PU12.

The NC CPU II has a ROM that stores a control program for managing the entire injection molding machine, a sequence program for controlling sequence operations of the injection molding machine, a program for controlling the servo CPU 12, etc.
13. RA that stores the set pressure of each holding pressure stage and the corresponding holding pressure time 1 time constant, as well as various setting values, parameter values, etc., as shown in table TB in Fig. 3.
A manual data input Vt 15 (hereinafter referred to as CRT/MDI) with a CRT for setting various setting values and parameter data in the RAM 14 is connected via a bus 16.

On the other hand, the servo CPU 12 receives command values such as torque limit values and various data during pulse distribution to each axis in each distribution period outputted from the NG CPL 111, as well as information that the NC CPL 111 rotates the bus 21 when the power is turned on. Control the servo CPU 12 to be transferred! Speed control that controls the speed of the servo motor 1 based on the RAM 17 used for temporary storage of the program for the servo motor 1, the pulse distribution m commanded by the NC CPU 11, and the feedback signal from the pulse coder 3 attached to the servo motor 1. A circuit 18 and a silk control circuit 1 to a silk control circuit 19, which similarly control the torque of the servo motor 1, are connected via a bus 20. That is, in this embodiment, the servo CPU 12. Speed control circuit 18. The torque control circuit 19 and the like constitute a servo means, a so-called software servo. In addition, servo C
The PU 12 and the NC CPU 11 are connected to buses 20, 21.
16.

Next, an outline of the operating principle of pressure control in this embodiment will be explained.

FIG. 5 shows the table TB (third table) stored in the RAM 14.
FIG. 3 is a diagram illustrating the relationship between the set pressure Pi set for each pressure holding stage and the pressure holding time Ti in the case (see figure), taking as an example the case where four stages of pressure holding stages are set. That is, the maximum injection pressure p
After the injection is completed at s, hold the pressure at the set pressure P1 for 11 seconds,
After 11 seconds have elapsed and the first stage of pressure holding is completed, the pressure is held at the set pressure P2 for T2 seconds, and thereafter four stages of pressure holding are carried out in the same manner.

However, if the NC CPU 11 outputs these set pressures as a direct command pressure, that is, a torque limit value, in the form shown in FIG. Overshoot and undershoot occur due to the inertia of No. 2, and the pressure applied to the resin fluctuates.

Therefore, as shown in FIG.
Configure so that Ti can be set.

This time constant ΔTi actually indicates the acceleration/deceleration control time from the end of the pressure holding process in the previous stage to the transition to the pressure holding process in the current stage, and for example, the maximum When transitioning from the injection pressure ps to the set pressure P1 of the first holding pressure stage, a time constant ΔT1 is provided immediately after the end of injection, as shown in FIG. The pressure torque limit value is continuously corrected in nine steps to perform pressure acceleration/deceleration control to eliminate pressure overshoot and undershoot when changing the set pressure.

Figure 7 shows - As an example, the maximum injection pressure PS at the end of injection
This figure shows an example of acceleration/deceleration control when the pressure is shifted from to the set pressure P1 of the first stage of holding pressure.

Acceleration/deceleration control is performed by increasing the pressure within the set time constant 611.
Continuous one-step increase/decrease from s to Pl (in this case, decrease)
First, divide the set time constant ΔT1 by the processing cycle to find the number n of processing cycles between the time constants 611, and then calculate the maximum injection pressure ps and the first holding pressure stage. The pressure difference from the set pressure P1 is divided by the number n of processing cycles during the elapse of the time constant ΔT1, and the increase/decrease ΔP to be added for each processing cycle of the NC CPU 11 is calculated.
Every processing cycle, the current set pressure (maximum injection pressure Ps at the beginning in this example) is increased or decreased by ΔP and outputted to the RAM 17 as a command pressure value, that is, a torque limit value, and the servo is controlled based on the torque limit value. The pressure control is performed by the CPU 12.

Therefore, from the state of maximum injection pressure ps, the time constant Δ■1
The number of processing cycles n during the elapse of (4 times in Figure 7)
When the increase/decrease ωΔP is sequentially added to the command pressure, that is, the torque limit value, the command pressure value becomes equal to the set pressure P1 of the first stage of holding pressure, and at this time, the time constant Δ
T1 will end. Thereafter, in the first stage of pressure holding, this command pressure value, that is, the set pressure P1 is held, and the first stage of pressure holding processing is executed for T1 seconds as a whole.

The same applies to the case of switching from one stage of holding pressure to two stages of holding pressure, the case of switching from two stages of holding pressure to three stages of holding pressure, etc.

By changing the time constant ΔTi in various settings, the amount of change in the torque limit value that becomes the command pressure value, that is, the rise in the pressure of the servo motor 1, is determined in relation to the pressure difference between each pressure holding stage. Fall characteristics can be freely controlled.

For example, by setting the time constant ΔTi large, the rise and fall characteristics of the servo motor 1 are made slow, and by setting the time constant ΔTi small, the rise and fall characteristics of the servo motor 1 are made sharp. be able to.

Hereinafter, the pressure control operation of this embodiment will be described in detail according to the processing flowchart 17 of the NC CPU 11 in the pressure holding operation shown in FIGS. 4(a) to 4(b).

First, as shown in Figure 3, the number of holding pressure stages ■, the holding pressure of each stage,
That is, the pressure holding time T1 of each stage of torque limit value P1 to PL
~T1. The time constants ΔT1 to ΔTl, which are the acceleration/deceleration control time when switching the pressure from the previous stage to the current stage, are CRT/MD I 15
, and store it in the table TB of the RAM 14. Note that after setting various other setting values, the injection molding machine is driven. Every time the injection process ends and the pressure holding process begins, N
The C CPU 11 executes the pressure holding process shown in FIGS. 4(a) to 4(b) as a task process every processing cycle. In this embodiment, this processing cycle is used as the pulse distribution cycle to the servo motor, which is the basic unit of the control device 10.

When the injection process is finished and the pressure holding process starts, the NC CPU1
1, it is determined whether or not the value of a correction number counter C indicating the number of corrections of the torque limit value in each holding pressure stage performed in each processing period (pulse distribution period) is 0 (step S1).

If the value of the correction number counter C is O, the correction number counter C remains in the state reset by the initialization when the power is turned on, or in the state reset by the correction end processing in the previous pressure holding stage. Because there is, C for NC
Next, the PL 111 obtains the time constant data ΔH+ of the holding pressure i+1 stage indicated by the holding pressure stage number counter i, that is, the next stage, from the table TB shown in FIG. 3 stored in the RAM 14.
+1 is read (step 82). Currently, the holding pressure stage number counter i is also in the state reset by initialization, that is, in the state of 0, so in this cycle, i+1=1
Therefore, the time constant data ΔT1 of the first stage of holding pressure is read.

Next, the time constant data ΔT1 is divided by the pulse distribution cycle τ of the NC CPU 11, and the number of pulse distribution cycles executed during the time constant ΔT1, which is the acceleration/deceleration control time to move to the next holding pressure set pressure. Select n (step 33).

Next, it is determined whether the value of the holding pressure stage number counter 1 is O (step S4), but since it is currently the first cycle of i=0, the process moves to step S5 and the current The current set pressure is stored in the register R (Pl) that stores the set pressure of
That is, the maximum injection pressure ps is set.

Next, in step S6, the holding pressure i is determined from the table T8.
+1st stage, that is, the set pressure data of the 1st stage of holding pressure Pi+1
Register R (P2) that reads and stores the set pressure for the next stage.
Set to .

Next, calculate the pressure difference between them by subtracting the current set pressure set in register R (Pl) from the next stage set pressure set in register R (P2), and further calculate this pressure difference. The increase/decrease portion ΔP of the torque limit value for each pulse distribution cycle is calculated by dividing by the number n of pulse distribution cycles executed during the time constant ΔT1 determined in step S3 (step 87).

Next, the process moves to step S8, and the value of the increase/decrease portion ΔP for each pulse placement period is added to the value of the register R (Pi) that stores the current set pressure, that is, the maximum injection pressure ps, and this value is The command pressure in the first cycle is output to the RAM 17 as a torque limit value, and the torque limit value is rewritten (step 89). therefore,
The servo CPLJ 12 is operated via the torque control circuit 9 until the pulse distribution of the next cycle of the NG CPLJ 12 is executed.
The torque of the servo motor 1, that is, the holding pressure of the screw 2 is controlled based on the rewritten torque limit value.

Next, in step 510, it is determined whether the value of the correction number counter C indicating the number of corrections of the torque limit value is 0 or not.
), the value of the correction counter C has not been updated in the previous steps and remains in the O state, so the process moves to step 811 and the value of the holding pressure 1+1 stage, that is, the holding pressure The pressure holding time T1 of the first stage is read, and this value is set in the timer TE for monitoring the pressure holding time, and the timer TE is started (step 512).

Next, 1 is added to the value of the correction number counter C indicating the number of times the torque limit value has been corrected (step 513), and the processing in this cycle is ended.

In the next distribution cycle, it is determined in step S1 whether the value of the correction number counter C indicating the number of corrections to the torque limit value is 0 or not, but it has already been updated to 1 in step 813 of the previous cycle. Therefore, the process moves to step 814, and it is determined whether or not the set time of the timer TE that monitors the holding pressure R interval has expired.If it has not finished, the process proceeds to step 8.
15, it is checked whether the value of the correction number counter C indicating the number of corrections of the torque limit value has reached the number n of pulse distribution executed during the time constant ΔTi.

If the value of the correction number counter C has not reached n, the process moves to step S8, and the increase/decrease result ΔP is added again to the value of the register R (Pl) that has stored the set pressure for that cycle in the first cycle. Correct the torque limit value of
This value is output to the RAM 17 as the command pressure in the second cycle, that is, the torque limit value, and the torque limit value is rewritten (step 89).

Next, it is determined whether the value of the correction number counter C is O or not (step 510). Since G=1 at present, the processes from step 311 to step 812 are not executed.

Next, the value of the correction number counter C is updated by adding 1 (step 513), and the processing for this cycle is ended.

In this way, the NC CPLIII checks in each cycle whether the value of the correction number counter C has reached the number n of pulse distribution cycles executed during the time constant ΔTi (step 515), and the torque Set the limit value as the value added to the torque limit value of the previous cycle by the increase/decrease portion ΔP (step S8), output this value to the RAM 17, rewrite the torque limit value, and sequentially change the torque limit value, that is, the holding pressure. Increase or decrease (step S9). Then, 1 is added to the value of the correction number counter C to update the counter value.

In this way, when the value of the correction number counter C reaches the number n of pulse distribution cycles executed during the time constant ΔTi (step 513), this state is detected in step S15 of the next cycle. At this time, the NG CPU II has finished all the rewriting processing of the torque limit value that should be executed during the time constant ΔTi, and the torque limit value at this time is the set pressure of the next stage, that is, the holding pressure of the i+1 stage. It becomes equal to Pi+1, and the time of the time constant Δ7i has also ended. That is,
The NC CPU 11 adjusts n within the time constant ΔT1 from the set pressure at the end of injection, that is, from the maximum injection pressure ps.
The torque limit value is corrected by the amount of increase/decrease ωΔP in each cycle, and the set pressure P1 of the next stage, that is, the holding pressure stage 1 is reached. And step S1
When it is confirmed in step 5 that the value of the correction number counter C has reached the number n of pulse distribution executed during the time constant ΔTi, the NC CPU 11 ends the processing for this cycle.

In subsequent cycles, steps S1 to S514 to S15 are performed, and in step S14 the timer TE is
until it detects that the NC CPU has timed up.
11 is the step S1-step 514- for each cycle.
Only the process of step S15 will be repeatedly executed. During this time, the torque limit value is not modified, so the set pressure P1 of the first stage of holding pressure is maintained.

While the processing for each cycle is executed in this manner, it is determined in step 814 that the timer TE has completed the set time, that is, the pressure holding process for 11 seconds at the set pressure P1 of the first stage of holding pressure has ended. When confirmed, NC CPU1
1 is updated by adding 1 to the value of the holding pressure stage number counter i, and the value of the correction number counter C is reset to O (step 81
6 to step 517). Next, it is determined whether the value of the holding pressure stage counter i has reached the set holding pressure stage number I (I=4 when set as shown in FIGS. 5 and 6) (step 818). , the value of the holding pressure stage counter i is the set holding pressure stage number I
If the value has not been reached, the process moves to step S2.

The NC CPU 11 that has proceeded to step S2 reads the time constant ΔTi+1 of the holding pressure i+1st stage from the table TB as shown in FIG. Note that since i=1 at present, the time constant ΔT2 for the second stage of holding pressure is read. Next, it is determined whether the value of the holding pressure stage number counter i is 0 or not (step S4), but since i = 1 at present, setting of holding pressure 1+1 stage, that is, holding pressure 2 stage is set from table TB. pressure P
Register R(
P2) is set to 1- (step 86).

Hereinafter, the current holding pressure (register R(P
When switching from the torque limit value stored in register R (P2) to the next stage holding pressure (torque limit value stored in register R (P2)), increase or decrease 1Δ for each cycle of the torque limit value
Calculate P (step S7), add the obtained increase/decrease ΔP to the value of the register R (PI) that stores the current set pressure, that is, the set pressure of the first stage of holding pressure, and output the result to the RAM 17 (step S9); Rewrite the torque limit value. Also, since the value of the modification number counter C was reset to O in the previous modification completion process, the value of the modification number counter C was
After the determination process, the holding pressure time T of holding pressure i+1 stage (=2 stage)
Read i+1 (T2) (step 511), set this value to timer TE, and start it (step 51).
2) Add 1 to the value of the correction number counter C and update it (
Step 513).

Hereinafter, in the same manner as in the case of switching from the maximum injection pressure ps to the first holding pressure stage, the increase/decrease ΔP is added to the value of the register R (Pl) that stores the current set pressure for each cycle, and the increase/decrease ΔP is added to the value of the register R Update the value of (Pl) and store this value in RAM.
In step S9), the torque limit value is rewritten.

While repeating the process for each cycle in this way, when the value of the correction number counter C reaches the number n of pulse distribution cycles executed during the time constant ΔTi, this is detected in step S15 of the next cycle, and the time constant All the torque limit value rewriting processing to be executed during ΔTi is completed, and the current torque limit value becomes equal to the set pressure P2 of the holding pressure of the next stage, that is, the second stage.

Hereinafter, the NG CPU II repeats and executes only the processes of step S1 - step 514 - step S15 every cycle, but during this time, the torque limit value is not corrected, so the set pressure P2 of the second stage of holding pressure is Retained.

Then, when it is recognized that the timer TE to which the set pressure holding piece interval (T2) of this stage has been set has timed up (
Step 814), the NG CPU 11 ends this stage of pressure holding processing.

Next, the CPLJ11 for NC is set to step S in the same manner as last time.
16, the value of the holding pressure stage number counter i is updated by adding 1, and the value of the correction number counter C is reset to O by the correction end processing (Step 517), and the value of the holding pressure stage number counter l is set to the set holding pressure. It is determined whether or not the number of stages ■ has been reached (step 818); if the set number of pressure holding stages has not been reached, the same processing as described above from step 82 onwards is carried out to carry out the pressure holding processing of the next stage. Then, when the value of the holding pressure stage number counter i reaches the set holding pressure stage number I, all the set pressure holding stages 1 (1 = 1 to
The pressure holding process for I) is completed.

In this way, when all the pressure holding processes are completed, the NC C
PLJ11 resets the value of the holding pressure stage number counter i to O (step 519). Note that since the value of the correction number counter C has already been reset to 0 in step 817,
After all the pressure holding process processing is completed in step S19, the pressure holding stage number counter i and the number of corrections counter C are initialized, and a series of molding cycles such as total M and injection are completed, and the next holding process is started. During the pressure step, the above-mentioned process for pressure control is repeatedly executed based on the set pressure Pi for each stage of holding pressure and the holding time ■i2 time constant ΔTi set in the table TB of FIG.

For example, as shown in Figure 5, when the number of holding pressure stages is set to 4, the pressure is controlled by acceleration/deceleration at the transition from the end of the injection process to the first stage of holding pressure, and from each stage to the next stage. The pressure is switched at a set time constant, and the holding pressure is controlled as shown in FIG.

Incidentally, in the above embodiment, acceleration/deceleration control is not performed at the time of transition from the pressure holding process to the metering process, that is, at the time of transition from the holding pressure at the final stage of holding pressure to the back pressure at the metering process, but if necessary,
The acceleration/deceleration control may be performed using the above-described method using a set time constant even when transitioning to back pressure.

Although the above embodiment shows an example of oven loop control, the same applies to closed loop control, in which the torque limit command value at the time of pressure switching is set in stages for each pulse distribution period, as described above. The acceleration/deceleration control may be performed by increasing or decreasing the value continuously.

Effects of the Invention The present invention sets the time required for pressure switching as a time constant when switching pressure in the pressure holding process, accelerates and decelerates pressure switching using this set time constant, and controls the rise and fall of pressure. Since the pressure is set arbitrarily to 11, overshoot and undershoot caused by pressure switching can be reliably prevented. In addition, since the pressure characteristics of the pressure holding process, that is, the pressure curve, can be arbitrarily obtained by changing the time constant and the holding pressure of each stage, high-quality molded products can be manufactured by changing this pressure curve. Can be done. In other words, the quality of the molded product can be controlled using this pressure curve (time constant) (the pressure curve also serves as data for quality control).

[Brief explanation of the drawing]

Fig. 1 is a block diagram of the means adopted by the present invention to solve the problems of the prior art, Fig. 2 is a block diagram showing the main part of an embodiment of the present invention, and Fig. 3 is a block diagram of the main part of an embodiment of the present invention. table T
4(a) to 4(b) are flowcharts showing the processing operation of the NG CPU of the same embodiment, and FIGS. 5 to 7 are the principle of operation of the same embodiment. FIG. 1... Servo motor, 2... Screw, 3...
Pulse coder, 10...Control device, 11-N (JI
ICPLI, 12...Servo CPU, 13...RO
M, 14. '17...RAM, 15...Manual data input with CRT display, 16.20.2
1... Bus, 18... Speed control circuit, 19... Torque control circuit. Figure ro: Figure 6

Claims (1)

[Claims] In the pressure control device of an electric injection molding machine, the holding pressure of each stage is controlled by the holding pressure time set for each stage of the holding pressure process and the torque limit value of the servo motor that drives the screw in the axial direction. Torque limit value for each stage of pressure process,
a setting means for setting a pressure holding time and a time constant when switching pressure from the previous stage to the current stage; a storage means for storing the torque limit value, pressure holding time, and time constant of each stage set by the setting means;
When switching the pressure from the previous stage to the current stage, a cycle number calculation means that divides the time constant stored in the storage means by the processing cycle to calculate the number of cycles, and calculates the number of cycles by calculating the difference between the torque limit values of the previous stage and the current stage. an increase/decrease calculation means that calculates an increase/decrease by dividing by the number of cycles determined by the means; and an increase/decrease calculation means that calculates the increase/decrease by dividing the increase/decrease by the number of cycles determined by the means, and adds the above increase/decrease sequentially for each processing cycle from the time of pressure holding switching to calculate the torque limit value from the previous stage to the current stage within the above time constant. A pressure control device for an electric injection molding machine equipped with a pressure switching means for switching to.