JPH03213323A - Pressure controlling method in injection molding machine - Google Patents

Abstract

PURPOSE:To accurately perform zero correction for a drift and to enable precise molding by previously calculating the conversion coefficient in the ideal conditions and correcting the controlling amount related to the injection pressure on the basis of the conversion efficient and the correction amount calculated from the measured value before starting injection at a time for starting injection. CONSTITUTION:The intrinsic pressure Po corresponding to the injection pressure in the ideal conditions is previously calculated from the controlling element Co in a servomotor 2a. Further the conversion coefficient k=Po/Co is calculated from the controlling element Co and this intrinsic pressure Po. The obtained conversion coefficient (k) is stored in a CPU 22. On the other hand, in the injection stages of the respective molding cycles, the positioning control for a screw 3 is performed before starting injection and the screw 3 is set at the orientation. At this time, actually generated torque T and the detection pressure Pn corresponding to the injection pressure obtained from a load cell 13 are fetched into the CPU 22 and correction amount S=Pn -k.T is calculated. Correction is performed for the controlling amount related to the injection pressure by this correction amount S at a time for starting injection. Thereby the quality of the molding in the respective molding cycles is made uniform and stabilization in long-run molding can be achieved.

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 injection pressure with a pressure sensor and performing closed loop control.

[Conventional technology]

Conventionally, when controlling the pressure of an injection molding machine, especially when 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 based on the detected pressure. Closed-loop control (feedback control) of injection pressure was performed.

Closed-loop 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 rose-drop loop control like this, even if the detected value of the pressure sensor is used as is, the detected value contains error components based on various factors other than the normal injection pressure, so accurate pressure control cannot be achieved. becomes difficult. Therefore, conventionally, the detected value of the pressure sensor (initial detected value) before the start of injection has been regarded as an error component (drift component), and zero correction for drift has been 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 a 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. 11, and the drift component of the pressure detection signal (pressure P) as shown in FIG. Like, pressure sensor (load cell)
Pressure detection as shown in FIG. 13 is based on the assembly force change due to the expansion (contraction) of the outer frame 92 or the expansion (contraction) of the pressure sensor 91 when the outer frame 91 is mechanically assembled to the outer frame 92. Drift component of the signal (pressure P), and residual pressure component 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 after the end of the weighing process, or insufficient pressure relief, and transmission. Contains mechanical error components based on frictional forces in the mechanism.

In this way, since the actual initial detection value includes the residual pressure component as an error component, the true drift component is not detected. After all, with the conventional method, 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 existing in the conventional technology.

[Means to solve the problem]

In the pressure control method for an injection molding machine according to the present invention, the injection pressure generated by the screw 3 pressurized by the actuator 2 is detected by the pressure sensor 4, and the injection pressure is controlled in a closed loop based on the detected pressure. , in advance, the control element CO related to the actuator 2
, the true pressure P corresponding to the injection pressure under ideal conditions.

Then, from the control element Co and the true pressure PO, find the conversion coefficient k = P o / Co, and before the start of injection, calculate the detected pressure corresponding to the actual control element Cn and the injection pressure detected from the pressure sensor 4. From Pn, correction amount 5=
The present invention is characterized in that Pn-k·Cn is determined, and the control amount related to the injection pressure is corrected using the correction amount S at the start of injection. In this case, the conversion coefficient under ideal conditions may be determined by calculation, or may be determined from actual values obtained with the screw 3 fixed and the actuator 2 operated. Note that when the servo motor 2a is used as the actuator 2, the control element Co
As (Cn), the torque T of the servo motor 2a or the current value Im of the servo motor 2a can be used. Furthermore, when the hydraulic cylinder 2b is used as the actuator 2, the current value Is of the servo valve 5 connected to the hydraulic cylinder 2b can be used as the control element Go (Cn).

[For production]

According to the pressure control method for an injection molding machine according to the present invention, first, from the control element CO related to the actuator 2 and the true pressure Po corresponding to the injection pressure at this control element Co, a conversion coefficient k = P o / Co is required. The conversion coefficient can be calculated in advance from mechanical conditions such as the transmission path, or it can be calculated from actual values sampled while the screw 3 is fixed and the actuator 2 is operated. This is a coefficient that does not include an error component (drift component).

On the other hand, before starting injection, from the actual control element Cn and the detected pressure Pn corresponding to the injection pressure obtained from the pressure sensor 4,
Correction amount 5=Pn-k·Cn is determined. In this case, r
PnJ is the pressure component that includes all actual error components, and “k-
Cn'' does not include a drift component, but is a pressure component that includes pressure components such as residual pressure.

As a result, the correction amount S becomes only the true drift component from which the influence of the pressure component is removed, and an accurate correction amount S can be obtained.

〔Example〕

Hereinafter, preferred embodiments of the present invention will be described in detail based on the drawings.

First, a schematic configuration of an injection molding machine that can implement the pressure control method according to the present invention will be described with reference to FIG.

The figure shows only the control system related to the injection process. In the figure, 10 is a heating cylinder of an injection device in an injection molding machine, with an injection nozzle 11 at the front end and a material supply hopper 1 at the rear.
2, and a screw 3 inside thereof so as to be rotatable and movable forward and backward.

The rear end of the screw 3 is connected to the nut portion 1 of the pole screw mechanism 14 via a load cell 13 (-pressure sensor 4 for boat fishing).
Connect to the 4a side. Further, the rotating shaft 15 of the servo motor 2a is coupled to the screw portion 14b side of the mechanism 14.

Note that the servo motor 23 is equipped with a pulse generator 16 that detects the number of rotations.

On the other hand, the control system includes a driver 21 that drives and controls the servo motor 2a, and a CPU (central processing unit) that executes various processes.
22.

The load cell 13 is connected to the CPU 22 via an A/D converter (analog-digital converter) 23, and is also connected to the inverting input section of the comparator 24.
The detected pressure Pn (Pi) of the load cell 13 is applied to the CPU 22 and the comparator 24. Also, servo motor 2
a and the pulse generator 16 are connected to the driver 2■. On the other hand, the driver 21 is connected to the CPU 22 via the A/D converter 25, and provides the CPU 22 with control elements Cn for the servo motor 21. Moreover, the CPU 22 is D
/A converter (digital-to-analog converter) 26.
27 to the non-inverting input portion of the comparator 24 and the non-inverting input portion of the comparator 28, respectively, and applies the correction amount S to the comparator 24 and the pressure command value Ps to the comparator 28. The output part of the comparator 24 is connected to the inverting input part of the comparator 28 , and the output part of the comparator 28 is further connected to the driver 21 via a PID compensation circuit 29 .

The above constitutes a closed loop control system for injection pressure including a correction system.

Next, a pressure control method according to the present invention will be explained.

First, in advance, from the control element CO in the servo motor 2a, the true pressure P corresponding to the injection pressure under ideal conditions is determined.
Find O.

Also, the control element CO and the resulting intrinsic pressure P.

From the equation below, find the conversion coefficient k.

Conversion coefficient k = P o / G.

Next, regarding a specific method of calculating the conversion coefficient, a case of a drive system constituted by the servo motor 2a and the ball screw mechanism 14 shown in FIG. 1 will be exemplified.

The control element co of the servo motor 21 includes various elements such as the torque T1 of the servo motor 2a and a current value of 1 m, but in this embodiment, a case where the torque T is used will be described.

First, torque T can be obtained from the following equation.

However, 2πηN) ID = Screw diameter PO: True pressure corresponding to injection pressure (injection resin pressure) μ: Friction coefficient W Weight of moving parts (screw, etc.) ■L: Screw movement speed η 1 Mechanical efficiency N,: Motor rotation speed When calculating PO from this formula, PO=-T+b where k=2ηNM/(D/2)"VLb--μw/
(D/2)”π.

Here, the relationship between the screw moving speed (■) when the moving speed is zero and the motor rotation speed N, 1 is as follows: VL=(LB/Z) NM where LB・Pornenried Z: Gear ratio. As a result, the conversion coefficient is k=2ηZ/(D/2)'L.

becomes.

Assuming that "μW" is sufficiently smaller than "(D/2)'πPoJ", Po=kT.

Therefore, the conversion coefficient can be obtained from the following equation.

k=Po/T 2ηz/(D/2)''LB Such a conversion coefficient is determined by operating the servo motor 2a with the screw 3 fixed, the torque T at this time, and the injection pressure obtained from the load cell 13. It may be calculated from the true pressure PO corresponding to .Furthermore, it is also possible to calculate in the same way using the current value of 1 m.

The obtained conversion coefficients are then stored in the CPU 22.

On the other hand, in the injection process in each molding cycle, positioning control for the screw 3 is performed before injection starts to set the screw 3 at a fixed position. At this time, the actually generated torque T and the detected pressure Pn corresponding to the injection pressure obtained from the load cell 13 are taken into the CPU 22, and the CPU 22 calculates the correction amount S using the following equation.

Correction amount 5 = -T to Pn- Then, at the start of injection, the control amount related to the injection pressure is corrected by the correction amount S. That is, by applying the detection pressure Pi corresponding to the correction amount S and the injection pressure obtained from the load cell 13 to the comparator 24, the comparator 2
The corrected detected pressure Pf=Pi−S is obtained from the output section of No. 4.

Further, this detected pressure Pf is applied to a comparator 28 to obtain a deviation value from the pressure command value Ps.

Therefore, the deviation value is supplied to the driver 21, and normal feedback control of the injection pressure is performed.

Note that FIG. 2 shows the relationship between control elements and true pressure (detected pressure) under ideal conditions and before correction. As is clear from the figure, the detected pressure Pn may be larger or smaller than the true pressure Po, and the correction amount S can be a net value or a liability.

FIG. 3 shows actual characteristic curves of torque, injection speed, and injection pressure, which are the molding conditions of the injection molding machine based on the servo motor 2a. In the characteristic curve, the section indicated by E indicates the positioning control section, in which the positioning control is performed and the detected pressure Pn and torque T are sampled. Point D indicates the start of injection, and zero correction for drift is performed.

On the other hand, Fig. 4 shows the drift component of the zero point over time, the characteristics after zero correction by the correction Its, and the pressure command value P.
This shows the relationship between s.

Next, another modification example will be explained.

Although FIG. 1 shows the case where the detected pressure Pi is corrected based on the correction amount S, the pressure command value Ps may also be corrected. Fifth
The figure shows a case where the pressure command value Ps is corrected by the correction ts. In this case, the pressure command value Ps is determined by the comparator 30.
and the correction Its to obtain the corrected pressure command value Pm, and the comparator 28 adds the corrected pressure command value Pm
and the deviation value of the detected pressure Pf is obtained. In FIG. 5, the same parts as in FIG. 1 are given the same reference numerals, and detailed explanation thereof will be omitted.

Further, in both FIGS. 1 and 5, correction is performed using hardware using an analog signal, but correction processing may also be performed using software as shown in FIGS. 6 and 7. In this case, correction to the detected pressure Pi is performed by calculating rpi-SJ in the CPU 22 as shown in FIG.
Detected pressure Pf after correction output from U22 (=Pi-
8) to the comparator 2 via the D/A converter 31.
Granted to 8. On the other hand, the correction to the pressure command value Ps is
As shown in FIG. 7, rPs+sJ is arithmetic processed in the CPU 22, and the corrected pressure command value Pm (-Ps+S) output from the CPU 22 is applied to the comparator 28 via the D/A converter 32. As shown in FIG. 8, the servo function can also be corrected by software servo executed by software using the CPU 22, and the control signal SV is supplied from the CPIJ 22 to the driver 21 via the D/A converter 33. Ru. In addition, the 6th
Figures 7 and 8), other omitted parts are shown in Figure 1.
It is constructed and functions similarly to the figure.

Furthermore, although the servo motor 2a is used as an example, the same implementation is possible even when a hydraulic cylinder 2b is used as the actuator 2, as shown in FIG. In this case, the hydraulic cylinder 2b is used as the control element Co (Cn).
The current value Is of the servo valve 5 connected to can be used. In addition, in FIG. 9, 13 is the hydraulic cylinder 2.
A load cell connected to detect the oil pressure in b is shown. In this case, since the oil pressure is directly detected, the detected value is converted into injection pressure through a predetermined conversion formula. 52 indicates a screw position detection sensor. Other parts that are the same as those in FIG. 1 are given the same reference numerals.

On the other hand, the conversion coefficient when using the hydraulic cylinder 2b can be calculated by the following equation.

First, the valve opening degree relative to the output (pressure) of the servo valve 5 is known for each servo valve as a pressure gain characteristic shown in FIG. 1O.

Therefore, since the true pressure PO corresponding to the injection pressure under ideal conditions can be calculated as Po = -Is, the conversion coefficient is k = Po
/CoPo/! It becomes s.

Although the present invention has been described above with reference to various embodiments, the present invention is not limited to these embodiments, and the detailed structure, method, etc. can be arbitrarily changed without departing from the gist of the present invention.

〔Effect of the invention〕

As described above, the method for controlling the pressure of an injection molding machine according to the present invention uses a pressure sensor to detect the injection pressure generated by the screw pressurized by the actuator, and performs closed-loop control of the injection pressure based on the detected pressure. In this case, the conversion coefficient k under ideal conditions is determined in advance.
and corrected Is from the actual measurement value to the conversion coefficient before starting injection.
In addition, since the correction is made based on this correction jlS at the start of injection, it is possible to perform accurate zero correction for drift with the correction amount based on the true drift component that is not affected by residual pressure, etc. , uniform molding quality in each molding cycle, prevention of molding defects, and stabilization of long-run molding can be achieved.

[Brief explanation of drawings]

Fig. 1.2 A block system diagram of an injection molding machine that can implement the pressure control method according to the present invention. Fig. 2. Diagram of the relationship between control elements and true pressure (detected pressure). Fig. 3. Molding conditions in the injection molding machine. Figure 4: Characteristic diagram showing correction amounts and set values over time; Figure 5: Block system diagram of the same injection molding machine according to a modified example of the present invention; Figures 6 and 7; Figure 8: A block system diagram showing a part of the control system of the injection molding machine according to another modification of the present invention. Figure 9: A part of the control system of the injection molding machine according to another modification of the present invention. Fig. 1O: Characteristic diagram showing the relationship between the valve opening degree and pressure of the servo valve in the same injection molding machine, Fig. 11, Fig. 12, Fig. 13: Showing the influence of drift on the pressure sensor. Foreground explanatory diagram. In the drawing, 2: Actuator: Servo motor 2b: Hydraulic cylinder: Screw: : Servo valve

Claims (1)

[Scope of Claims] [1] In a pressure control method for an injection molding machine, the injection pressure generated by a screw pressurized by an actuator is detected by a pressure sensor, and the injection pressure is controlled in a closed loop based on the detected pressure. , in advance, calculate the true pressure Po corresponding to the injection pressure under ideal conditions from the control element Co related to the actuator, and from the control element Co and the true pressure Po, calculate the conversion coefficient k=Po/Co, and before the start of injection. , the actual control element Cn
and the detected pressure Pn corresponding to the injection pressure detected by the pressure sensor, a correction amount S=Pn-k・Cn is obtained, and the control amount related to the injection pressure is corrected by the correction amount S at the start of injection. Pressure control method for injection molding machine. [2] The pressure control method for an injection molding machine according to claim 1, wherein the conversion coefficient k is determined by calculation. [3] The method for controlling pressure in an injection molding machine according to claim 1, wherein the conversion coefficient k is determined from an actual value obtained with the screw fixed and the actuator operated. [4] The method for controlling pressure in an injection molding machine according to claim 1, wherein the actuator uses a servo motor, and the torque of the servo motor is used as the control element. [5] The method for controlling pressure in an injection molding machine according to claim 1, wherein the actuator uses a servo motor, and a current value of the servo motor is used as the control element. [6] The method for controlling pressure in an injection molding machine according to claim 1, wherein a hydraulic cylinder is used as the actuator, and a current value of a servo valve connected to the hydraulic cylinder is used as the control element.