JPH0544898B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a pressure control method for an injection molding machine for detecting pressure with a pressure sensor and performing feedback control.

[Conventional technology]

Conventionally, when controlling the pressure of an injection molding machine, for example, controlling the injection pressure, the injection pressure is detected by a pressure sensor (load cell, etc.) interposed between the screw and the actuator (drive device), and injection is performed based on the detected pressure. Feedback control of pressure was performed. Feedback control has the advantage of preventing fluctuations in molding conditions and achieving accurate operation even in the presence of disturbances such as temperature that are difficult to control, and is widely used in precision molding.

By the way, when performing such feedback control, even if the detected value of the pressure sensor is used as is, accurate pressure control is difficult because the detected value contains error components based on various factors other than the normal pressure. becomes. Therefore, conventionally, the detected value (initial detected value) of the pressure sensor immediately before the start of injection (metering) was regarded as an error component (drift component), and zero correction for drift was performed based on this initial detected value.

[Problem to be solved by the invention]

However, the actual error component in the above-mentioned initial detection value is the drift component of the pressure detection signal (pressure P) caused by the pressure sensor itself due to the change in temperature Th, as shown in FIG. A drift component of the pressure detection signal (pressure P) as shown in FIG. 9 based on a change in assembly force due to expansion (contraction) of the assembly part or expansion (contraction) of the pressure sensor in the case of mechanical assembly; Furthermore, after the end of the metering process, there are residual pressure components caused by the movement of the resin toward the front of the screw due to residual pressure in the compression zone or supply zone of the screw, or insufficient pressure relief, and mechanical error components caused by frictional force in the transmission mechanism. etc.

In this way, since the actual initial detection value includes a pressure component such as residual pressure as an error component, a true drift component is not detected. In the end, with conventional control methods, it is difficult to perform accurate zero correction for drift, and there are limits to precision molding.

An object of the present invention is to provide a pressure control method for an injection molding machine that solves the problems of the conventional techniques.

[Means to solve the problem]

The pressure control method for an injection molding machine according to the present invention detects the pressure acting on the screw 2 with the pressure sensor 3 and performs feedback control on the pressure, and in the decompression process after the end of the metering process or the injection process. , or by forcibly retracting the screw 2 after the end of the metering process or the injection process, the pressure is reduced until it reaches a preset pressure setting value Ps near zero (including zero, and the same applies to others), and The pressure is the pressure set value Ps
From the point when the distance is reached, the screw 2 is moved back by the set distance Ls. The pressure is detected after the screw 2 moves back by a set distance Ls, and the control amount related to pressure control is corrected based on the detected pressure value Pc.

[Effect]

According to the pressure control method for an injection molding machine according to the present invention, by retracting the screw 2 after the end of the metering process or the injection process, the pressure is first reduced until it reaches a preset pressure setting value Ps near zero. .
Normally, the retraction distance of the screw 2 at which the pressure decreases to zero varies depending on the back pressure setting, the amount of resin to be measured, the elastic modulus of the resin, etc., so it is practically impossible to set the pressure to zero based on the retraction distance of the screw 2 from the beginning. It is possible. Furthermore, since it is difficult to find a complete pressure zero point, the pressure setting value Ps can be set near zero and this point can be regarded as the zero point.

On the other hand, from the time when the pressure reaches the pressure set value Ps, the screw 2 is moved backward by a preset distance Ls. As a result, the pressurizing action of the screw 2 on the pressure sensor 3 is released,
The pressure sensor 3 is no longer affected by back pressure or rotation of the screw 2. Note that the distance setting value Ls is the minimum distance at which the pressure is completely relieved, and can be determined in advance by calculation or experiment.

After the screw 2 reaches the distance setting value Ls, the pressure component acting on the pressure sensor 3 is eliminated, so
If pressure detection is performed thereafter, the detected pressure value Pc will be only an error component (drift component) that does not include pressure components such as residual pressure. Therefore, if the control amount related to pressure control is corrected based on this pressure value Pc, accurate correction can be performed.

〔Example〕

Below, preferred embodiments according to the present invention are listed,
This will be explained in detail based on the drawings.

First, a schematic configuration of an injection molding machine capable of implementing the pressure control method according to the present invention will be described with reference to FIG. Note that FIG. 3 shows only a system for correcting injection pressure.

Reference numeral 10 denotes an injection device in an injection molding machine, and a heating cylinder 11 is provided at the front part. The heating cylinder 11 has an injection nozzle 12 at its front end, a material supply hopper 13 at its rear, and a screw 2 inside thereof so as to be rotatable and movable forward and backward. The screw 2 is controlled to rotate and advance and retreat by a screw drive mechanism 14 provided at the rear of the injection device 10.

The screw drive mechanism 14 includes an injection table 15 that rotatably holds the rear end of the screw 2.
is rotated by a metering motor (servo motor) 16 fixed to the injection table 15 and a gear transmission mechanism 17. Further, the injection table 15 is supported so as to be slidable in the horizontal direction by a guide rail 19 provided on the machine body 18, and is moved forward and backward by a ball screw mechanism 20 and an injection motor (servo motor) 21 provided at the rear end. In addition, the metering motor 16 and the injection motor 2
1 have pulse generators 16a and 21a, respectively.
Equipped with. It also includes a metering motor driver 25 and an injection motor driver 26 connected to the central controller 22, and the metering motor 16 and the injection motor 21 are driven and controlled by the respective drivers 25 and 26, respectively.

On the other hand, the injection table 15 has a built-in load cell 31 (generally a pressure sensor 3) to detect the pressure acting on the screw 2. Further, 30 is a processing section, which is usually built in the central controller 22.
The processing section 30 includes a CPU (central processing unit) 32 that executes various processes, and a load cell 31.
is connected to the CPU 32 via an A/D converter (analog-to-digital converter) 33 and also to a non-inverting input part of a comparator 34, and applies the detected pressure of the load cell 31 to the CPU 32 and the comparator 34. Also, CPU32
is connected to the inverting input part of the comparator 34 and the non-inverting input part of the comparator 35, and gives the correction amount S to the comparator 34 and the pressure command value Po to the comparator 35, respectively. Furthermore, the output part of the comparator 34 is connected to the inverting input part of the comparator 35, and the output part of the comparator 35 is connected to the injection motor driver 26 via the PID compensation circuit 36. The above constitutes a feedback control including a correction system.

Next, the first part about the pressure control method according to the present invention will be explained.
The explanation will be made according to the characteristic diagram shown in the figure, the flowchart shown in FIG. 2, and FIG. 3.

This example shows a case where correction is performed after the end of the metering process, and also shows a case where a decompression process is included during the molding cycle.

Note that the central controller 22 has a pressure setting value.
Ps and distance setting value Ls are set in advance (see [Operation] above).

First, upon completion of the metering process, the injection motor 26
is controlled to rotate in the opposite direction, and the screw 2 is retreated to perform pressure relief (decompression operation) (section A: step 50,
51, 52). On the other hand, the central controller 22 monitors the pressure detection value from the load cell 31, and when the pressure detection value reaches the pressure set value Ps, monitors the retraction distance of the screw 2 from this point on (section B: steps 53, 54). . This retraction distance can be monitored by the pulse generator 21a. Then, when the distance has retreated by the set distance Ls, the pressure value Pc is sampled (section C: steps 55 and 56). In addition,
The pressure value Pc is obtained by importing the pressure value detected from the load cell 31 into the CPU 32 at predetermined time intervals and averaging a plurality of pressure values (steps 57, 58).
Since the pressurizing force acting on the load cell 31 after the screw 2 moves back by the distance setting value Ls is originally zero, if the pressure value Pc at this time is a value other than zero, this pressure value Pc will be equal to the drift. A correction amount S based on this pressure value Pc is applied from the CPU 32 to the comparator 34 (steps 59, 60, 61). In addition, when the retraction distance of the screw 2 in the decompression process remains, the screw 2 is retracted until the end.
Finish the decompression process (section D:
Steps 62, 63). Through the above process, the correction amount S can be determined in the decompression step where the influence of pressure is the least.

On the other hand, the correction amount S and the pressure value Pi obtained from the load cell 31 are compared in a comparator 34,
A corrected pressure value Pf=Pi-S is obtained from the output of the comparator 34.

Further, the pressure value Pf is applied to a comparator 35 to obtain a deviation value from the pressure command value Po.

Therefore, the deviation value is supplied to the injection motor driver 26 via the PID compensation circuit 36, and normal feedback control of the pressure is performed using the corrected pressure value.

Next, another modification example will be explained.

FIG. 3 shows a case where the pressure value Pi detected from the load cell 31 is corrected based on the correction amount S.
The pressure command value Po may be corrected. FIG. 4 shows a case where the pressure command value Po is corrected by the correction amount S.
In this case, the adder 37 adds the pressure command value Po and the correction amount S to obtain the corrected pressure command value Pm, and the comparator 35 obtains the deviation value between the pressure command value Pm and the detected pressure Pi. In addition, the fourth
In the figure, other omitted parts are constructed and function similarly to those in FIG. 3.

In addition, although Figures 1 and 2 show the case where the decompression process included in the molding cycle is used, as shown in Figure 5, when the decompression process is not included in the molding cycle, the weighing process After the completion of , the screw 2 is forcibly retreated to reduce the pressure to a preset pressure setting value Ps near zero, and from the time when the pressure setting value Ss is reached, the screw 2 is retreated by a preset distance Ls,
After this, the pressure value Pc may be detected.

In this case as well, the method is basically the same as shown in FIGS. 1 and 2.

Furthermore, FIGS. 6 and 7 show the case where the correction is made after the injection process is completed. FIG. 6 shows a case where the molding cycle includes a decompression step after the injection step, which corresponds to the metering step shown in FIG. In this case, the pressure control method itself is carried out in accordance with the flowchart shown in FIG. 2 in the same manner as after the metering process is completed. Furthermore, FIG. 7 shows the case without the decompression step, and corresponds to the measuring step shown in FIG. 5. Also in this case,
The pressure control method itself can be carried out in the same manner as after the measuring process shown in the figure.

Although various embodiments have been described above, the present invention is not limited to these embodiments.
For example, the control method and detection method for retracting the screw until the pressure applied to the pressure sensor becomes zero does not exclude other methods. Further, the present invention can be similarly applied to an injection molding machine driven by hydraulic pressure. Furthermore, the pressure sensor includes any sensor other than a load cell. Other details,
The method and the like can be arbitrarily changed without departing from the gist of the present invention.

〔Effect of the invention〕

As described above, in the pressure control method for an injection molding machine according to the present invention, when the pressure acting on the screw is detected by the pressure sensor and feedback control is performed on the pressure, the The pressure is detected after the screw is moved back to the position where the pressing force of the screw becomes zero, and the control amount related to pressure control is corrected based on the detected pressure value, so the true Accurate zero correction for drift can be performed using the correction amount based on the drift component, thereby achieving uniform molding quality in each molding cycle, prevention of molding defects, and stabilization of long-run molding. .

[Brief explanation of the drawing]

Figure 1: Characteristic diagram of screw pressure, screw rotation speed and screw speed with respect to time according to the pressure control method according to the present invention, Figure 2: Flowchart showing the same pressure control method corresponding to the characteristic diagram of Figure 1. Figure 3: Block system diagram of an injection molding machine that can implement the same pressure control method, Figure 4: Block system showing part of the control system of an injection molding machine that can implement the pressure control method according to another embodiment of the present invention. Figure, Figure 5, Figure 6
7: Characteristic diagrams of screw pressure, screw rotation speed, and screw speed with respect to time according to a pressure control method according to another embodiment of the present invention, FIGS. 8 and 9
Figure: Background diagram showing the influence of drift on pressure sensors. In the drawing, 2: screw, 3: pressure sensor.

Claims (1)

[Scope of Claims] 1. In a pressure control method for an injection molding machine in which the pressure acting on the screw is detected by a pressure sensor and feedback control is performed on the pressure, the pressure applied to the screw relative to the pressure sensor is A pressure control method for an injection molding machine, which comprises detecting pressure after retracting the screw to a position where the pressure becomes zero, and correcting a control amount related to pressure control based on the detected pressure value. 2. In the decompression step after the end of the metering step or the injection step, the pressure is detected after the screw has retreated by a preset distance from the point when the preset pressure near zero is reached. A pressure control method for an injection molding machine according to claim 1. 3. After the metering process or injection process is completed, the screw is forcibly retreated to reduce the pressure until it reaches a preset pressure setting value near zero, and the screw is set in advance from the time when the pressure reaches the pressure setting value. 2. The method for controlling pressure in an injection molding machine according to claim 1, wherein the pressure is detected after retracting by a set distance value.