Injection molding machine for controlling measurement of an in-line screw

A method for controlling measurement in a motor-driven in-line screw type injection molding machine using servo-motors as a measurement driving source and an injection driving source. Control is made so that all of a measured value of back pressure controlled by the measurement servo-motor, a measured value of an astern speed of a screw controlled by the injection servo-motor, and a measured rotational speed of the screw turn zero concurrently at timing when the screw reaches a measurement completion position. Thus, suck-back can be made dispensable.

FIELD OF THE INVENTION

The present invention relates to a technique for controlling measurement in a motor-driven in-line screw type injection molding machine using servo-motors as a measurement driving source and an injection driving source.

BACKGROUND ART

In a motor-driven in-line screw type injection molding machine in the background art, a measuring stroke is often controlled as follows. That is, a measurement servo-motor is controlled by rotational velocity feedback control so that the rotational velocity of a screw coincides with a set value, while an injection servo-motor is controlled by pressure feedback control so that the back pressure applied onto the screw (resistant pressure against astern movement of the screw) coincides with a set value. When such rotational velocity feedback control is performed on the measurement servo-motor while such pressure feedback control is performed on the injection servo-motor, the back pressure applied to the screw is improved to coincide with the set value. However, assume that setting is done to make the back pressure zero at the end of measurement. When control is performed in this setting to make the back pressure zero at the end of measurement, there must be a variation in the position where the screw stops its astern movement, due to the injection servo-motor subjected to the feedback control giving preference to pressure.

Therefore, the present inventor has proposed a measurement control method in Japanese Patent Application No. 2003-199230. In the measurement control method, measuring operation is carried out as follows. That is, speed feedback control is performed on an injection servo-motor so that the astern speed of a screw follows an astern speed setting pattern, while pressure feedback control is performed on a measurement servo-motor so that the back pressure applied to the screw follows a back pressure setting pattern (in other words, feedback control is performed to control the measuring rotational velocity to follow the back pressure setting pattern). Thus, the position where measurement is completed is made to coincide with a set position.

In the previously proposed measurement control method, feedback control is applied to a rotational velocity command to be supplied to the measurement motor so that the back pressure follows the back pressure setting pattern. Accordingly, in the beginning of measurement, shortage of raw resin leads to failure in expected increase of the back pressure. As a result, the measuring operation may be unstable. In addition, the measuring operation may be unstable in some operating condition of a raw resin supply system or in some dry condition of the resin. This is because the feed rate of the resin fed by the screw is not always increased in spite of increase in rotational velocity of the measurement motor, with the result that the back pressure does not increase to its expected value.

Therefore, the following configuration is also conceivable. That is, in a period between the start of a measuring stroke and the middle of the measuring stroke, driving of the measurement servo-motor is controlled by open control in which the set rotation number is constant, while driving of the injection servo-motor is controlled by back pressure feedback control in which the astern speed of the screw is controlled to make a measured back pressure value coincide with a set back pressure value. On and after the middle of the measuring stroke in which the astern speed of the screw is stabilized (that is, the back pressure is stabilized), the driving of the measurement servo-motor is controlled by back pressure feedback control in which the rotational velocity of the measurement servo-motor is controlled to follow a back pressure setting pattern. Thus, the back pressure can be controlled stably by the measurement servo-motor. On the other hand, the injection servo-motor is controlled by astern speed control in which the position where the screw stops its astern movement is made to coincide with the position where the measurement is completed. When control is performed thus, the position where the measurement is completed can be controlled to coincide with its set position by the injection servo-motor, while back pressure can be controlled stably by feedback control using the measurement motor on and after the middle of the measuring stroke.

When control is performed thus, the following problem remains in spite of various advantages. That is, setting is done so that the position where the astern speed is zero in a deceleration setting pattern for the injection servo-motor in the ending of the measuring stroke coincides with the predetermined position where the measuring stroke is completed. Further, the driving of the measurement servo-motor is controlled by back pressure feedback control following a back pressure decompression setting pattern calculated into a value proportional to the deceleration setting pattern. In spite of such control, there is a problem that it cannot be guaranteed that all of a measured value of the astern speed of the screw controlled by the injection servo-motor, a measured value of the back pressure controlled by the measurement servo-motor and a measured rotational velocity of the screw turn zero concurrently at timing when the screw reaches a position where measurement is completed. The background art has showed no consideration for the control to make all of the measured value of the astern speed of the screw, the measured value of the back pressure and the measured rotational velocity of the screw zero concurrently. That is, in the background art, the deceleration setting pattern for the astern speed of the screw is not proper but is apt to be set to be steep. Accordingly, the deceleration in the rotational velocity control upon the measurement servo-motor controlling the back pressure cannot follow the control in which the astern speed of the screw approaches zero rapidly. Even when the astern speed of the screw is close to zero, the measurement servo-motor has a certain degree of rotational velocity (resin is fed at a certain rate). Thus, the back pressure is adversely increased near the zero point of the astern speed of the screw. As a result, a known suck-back operation has to be carried out after the completion of the measuring operation.

SUMMARY OF THE INVENTION

The present invention was developed in consideration of the foregoing problem. It is an object of the present invention to make all of a measured value of the astern speed of a screw, a measured value of the back pressure and a measured rotational velocity of the screw (measurement servo-motor) zero concurrently at timing when the screw reaches a position where measurement is completed, so that suck-back can be made dispensable.

In order to attain the foregoing object, according to one aspect of the invention, in a method for controlling measurement in an in-line screw type injection molding machine using servo-motors as a measurement driving source and an injection driving source:

in a period between a measurement start position and a measurement control changeover position on a measuring stroke, driving of the measurement servo-motor is controlled by open control with a constant set rotational velocity of a screw, while driving of the injection servo-motor is controlled by back pressure feedback control in which an astern speed of the screw is controlled to follow a set value of back pressure set in advance;

in the period between the measurement control changeover position and the measurement completion position, driving of the injection servo-motor is controlled by open control following an astern speed setting pattern having a region of a constant astern speed calculated from a measured astern speed in a position short of the measurement control changeover position, while driving of the measurement servo-motor is controlled initially by back pressure feedback control in which the rotational velocity of the screw is controlled to follow the set back pressure value set in advance, and next controlled by back pressure feedback control in which the rotational velocity of the screw is controlled to follow a back pressure decompression setting pattern calculated into a value proportional to a deceleration pattern in the astern speed setting pattern for the open control;

the deceleration pattern between a position where the astern speed is constant and a position where the astern speed is zero in the astern speed setting pattern between the measurement control changeover position and the measurement completion position is set so that the position where the astern speed is zero coincides with only the measurement completion position, and a position where a rotation number is zero in a deceleration pattern of a measured rotational speed of the screw with respect to a time axis coincides with only timing when the screw reaches the measurement completion position; and

control is made so that all of a measured value of the back pressure controlled by the measurement servo-motor, a measured value of the astern speed of the screw controlled by the injection servo-motor, and a measured rotational speed of the screw turn zero concurrently at timing when the screw reaches the measurement completion position.

According to the invention, control is made so that all the measured value of the astern speed of the screw, the measured value of the back pressure, and the measured rotational speed of the screw (measurement servo-motor) turn zero concurrently at timing when the screw reaches the measurement completion position. Accordingly, the residual pressure of molten resin staying in a heating cylinder till the next injection becomes zero. Thus, there is no fear that drooling or cobwebbing occurs when a mold is opened and a molded item is released from the mold. It is therefore unnecessary to perform suck-back after measurement is completed. In addition, since it is unnecessary to perform suck-back and any injection operation can be always started in a constant measurement completion position, the injection rate is stabilized, and the cushion thickness is also stabilized. Furthermore, the back pressure and the screw (measurement servo-motor) rotation turn zero as soon as the screw stops its astern movement. Thus, the variation in weight among molded items can be made as low as possible in cooperation with the stabilized density of the measured molten resin, the stabilized injection rate and the stabilized cushion thickness.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will be described below with reference to the drawings.FIG. 1is a block diagram showing the configuration of a measurement control system in a motor-driven in-line screw type injection molding machine according to an embodiment of the present invention (hereinafter referred to as “this embodiment”).

InFIG. 1, the reference numeral1represents a system controller for administering control all over the injection molding machine. Here, only the configuration of a measurement control system is depicted in the system controller1in order to simplify the illustration.

InFIG. 1, the reference numeral2represents a measuring stroke control portion for administering host control of measurement operation. The measuring stroke control portion2supplies a command output directly to an amplifier4through a control changeover switch24and a serial port17so as to perform open control to control the driving of an injection servo-motor3serving as a driving source for driving a screw forward/backward. Alternatively, the measuring stroke control portion2performs feedback control upon the injection servo-motor3through an injection servo-motor feedback control system7, the control changeover switch24, the serial port17and the amplifier4so as to control the driving of the injection servo-motor3. In addition, the measuring stroke control portion2supplies a command output directly to an amplifier6through a control changeover switch25and a serial port23so as to perform open control to control the driving of a measurement servo-motor5serving as a driving source for rotating the screw. Alternatively, the measuring stroke control portion2performs feedback control upon the measurement servo-motor5through a measurement servo-motor feedback control system8, the control changeover switch25, the serial port23and the amplifier6so as to control the driving of the measurement servo-motor5. Incidentally, the measuring stroke control portion2has a function of recognizing the position or the astern speed of the screw based on an output of a not-shown encoder provided additionally to the injection servo-motor3or recognizing the rotational velocity of the screw based on an output of a not-shown encoder provided additionally to the measurement servo-motor5, a function of storing various set values or calculating and setting the set values based on computing results, and so on. In addition, the measuring stroke control portion2also has a function of changing over the control changeover switches24and25synchronously. In the control changeover switch24, the output of the feedback control system is selected till a measurement control changeover position S3which will be described later, while the output of the open control system is selected on and after the measurement control changeover position S3. In the control changeover switch25, the output of the open control system is selected till the measurement control changeover position S3which will be described later, while the output of the feedback control system is selected on and after the measurement control changeover position S3.

A set back pressure value (back pressure command value following a predetermined back pressure setting pattern) from the measuring stroke control portion2and a measured back pressure value from a back pressure measuring portion13are supplied to a deviation detection portion14of the injection servo-motor feedback control system7. The back pressure measuring portion13measures the measured back pressure value on the basis of an output of a pressure sensor9received through an amplifier10and a serial port12. The pressure sensor9measures pressure applied to the screw. A deviationeobtained in the deviation detection portion14is output to a PID computing portion15. The PID computing portion15performs a computing process for performing a feedback process based on PID (Proportional-Integral-Differential) operation using the input deviationeso as to calculate a manipulated variable for making the measured back pressure value coincide with the set back pressure value. The calculated manipulated variable is output to an output computing portion16. In the output computing portion16, a control output value is calculated using the input manipulated variable. The calculated control output value is supplied to the injection servo-motor3through the control changeover switch24, the serial port17and the amplifier4. Based on the control output, back pressure feedback control is performed as follows. That is, the astern speed of the screw is controlled by the injection servo-motor3so that the measured back pressure value coincides with the back pressure setting pattern in a region of the measuring stroke as will be described later.

A set back pressure value (back pressure command value following a predetermined back pressure setting pattern or a set back pressure value based on a back pressure setting pattern obtained by calculation in the measuring stroke control portion2as will be described later) from the measuring stroke control portion2and a measured back pressure value from a back pressure measuring portion19are supplied to a deviation detection portion20of the measurement servo-motor feedback control system8. The back pressure measuring portion19measures the measured back pressure value on the basis of an output of a pressure sensor10received through an amplifier11and a serial port18. The pressure sensor10measures pressure applied to the screw. A deviationeobtained in the deviation detection portion20is output to a PID computing portion21. The PID computing portion21performs a computing process for performing a feedback process based on PID operation using the input deviationeso as to calculate a manipulated variable for making the measured back pressure value coincide with the set back pressure value. The calculated manipulated variable is output to an output computing portion22. In the output computing portion22, a control output value is calculated using the input manipulated variable. The calculated control output value is supplied to the measurement servo-motor5through the control changeover switch25, the serial port23and the amplifier6. Based on the control output, back pressure feedback control is performed as follows. That is, the rotational velocity of the screw is controlled by the measurement servo-motor5so that the measured back pressure value coincides with the back pressure setting pattern in a region of the measuring stroke as will be described later.

Next, the measuring operation in this embodiment will be described with reference toFIGS. 2 and 3showing the condition of the measuring stroke in this embodiment.FIG. 2shows the transition in relationship among the screw rotational velocity, the back pressure and the screw astern speed along a position axis.FIG. 3shows the transition in relationship among the screw rotational velocity, the back pressure and the screw astern speed along a time axis. Incidentally, here, a molded item (product) weighing 3.5 g is molded by way of example.

A set measurement completion position S2is defined by a predetermined distance from a measurement start position S1(or defined by an absolute value). A measurement control changeover position S3is defined by a predetermined distance L1from the set measurement completion position S2. The measuring stroke control portion2changes over its control in the measurement control changeover position S3as a boundary. First, between the measurement start position S1and the measurement control changeover position S3, the measuring stroke control portion2controls the driving of the measurement servo-motor5by open control using a predetermined set screw rotational velocity31(here, for example, 350 rpm). Thus, the measurement servo-motor5is controlled so that a measured screw rotational velocity32substantially coincides with the set screw rotational velocity31. In addition, between the measurement start position S1and the measurement control changeover position S3, the measuring stroke control portion2controls the driving of the injection servo-motor3by back pressure feedback control using a predetermined set back pressure value32(here, for example, 180 kg/cm2). Thus, the screw astern speed of the injection servo-motor3is controlled so that a measured back pressure value34detected from the output of the pressure sensor9coincides with the set back pressure value33. As a result, the measured screw astern speed35is stabilized gradually while increasing. The measured screw astern speed35has been stabilized in a predetermined region before the set measurement completion position S2.

Before the screw reaches the measurement control changeover position S3, between a position short of the measurement control changeover position S3by a predetermined distance L2and the measurement control changeover position S3, the measuring stroke control portion2calculates and sets screw astern speed setting patterns36and37on and after the measurement control changeover position based on an average value of measured screw astern speeds35sampled a plurality of times. The screw astern speed setting patterns36and37serve as set values for performing open control upon the injection servo-motor3on and after the measurement control changeover position S3. The initial setting pattern36set from the measurement control changeover position S3serves to generate a region where the astern speed is constant and equal to the average value of the measured screw astern speeds35in the aforementioned stable region. Here, for example, the initial setting pattern36is set to be 20 mm/sec. The screw astern speed setting pattern37set in succession to the setting pattern36and until the set measurement completion position S2is determined in accordance with a deceleration/decompression start position S4set by an operator. This is an optimum deceleration pattern of the screw astern speed depending on a distance L3(here, for example, 2.5 mm) optimized based on the set measurement completion position S2. This screw astern speed setting pattern (set deceleration pattern)37is calculated as setting pattern (set deceleration pattern)37along the position axis, based on a setting pattern (set deceleration pattern)37′ along the time axis. The setting pattern (set deceleration pattern)37′ is obtained by linearly connecting the value of the screw astern speed at the deceleration/decompression start timing shown on the time axis inFIG. 3, and the zero value at the timing when the astern movement of the screw is completed. The setting pattern (set deceleration pattern)37′ can be obtained from the value of the astern speed in the setting pattern36and the time (here, for example, 0.25 sec) defined by the aforementioned distance L3. Before the screw reaches the measurement control changeover position S3, the measuring stroke control portion2has set a back pressure deceleration setting pattern38based on the screw astern speed setting pattern (set deceleration pattern)37′ so that the back pressure deceleration setting pattern38has a value calculated in proportion to the screw astern speed setting pattern (set deceleration pattern)37′. This back pressure deceleration setting pattern38is calculated as a back pressure deceleration setting pattern38along the position axis, based on a back pressure deceleration setting pattern38′ obtained by proportional operation from the screw astern speed setting pattern (set deceleration pattern)37′ shown along the time axis inFIG. 3.

Next, description will be made about the operation on and after the measurement control changeover position S3. Between the measurement control changeover position S3and the position where the measurement is completed, the measuring stroke control portion2controls the driving of the injection servo-motor3by open control following the setting pattern36and the setting pattern (set deceleration pattern)37of the screw astern speed. Thus, the measured screw astern speed35follows the setting patterns36and37so that the screw stops its astern movement in a position corresponding to the set measurement completion position S2. Between the measurement control changeover position S3and the position where the measurement is completed, the measuring stroke control portion2controls the driving of the measurement servo-motor5initially by back pressure feedback control in which the screw rotational velocity is controlled to follow the predetermined set back pressure value33. On and after the deceleration/decompression start position S4, the measuring stroke control portion2controls the driving of the measurement servo-motor5by back pressure feedback control in which the screw rotational velocity is controlled to follow the back pressure deceleration setting pattern38. Thus, the measured back pressure value34is controlled to follow the set back pressure value33and the back pressure deceleration setting pattern38. Further, the measured screw rotational velocity32based on the control of the measurement servo-motor5subjected to the back pressure feedback control decreases due to the decompression of the back pressure and reaches zero in the set measurement completion position S2.

InFIG. 3, dashes are put on those corresponding to the reference numerals31-38inFIG. 2. In the example shown inFIGS. 2 and 3, the aforementioned distance L3is optimized so that all of a measured screw astern speed35′, a measured screw rotational velocity32′ and a measured back pressure value34′ turn zero concurrently at the timing when the astern movement of the screw is completed (the timing when the screw stops its astern movement).

In order to make all the measured screw astern speed35′, the measured screw rotational velocity32′ and the measured back pressure value34′ zero concurrently on the time axis, it is essential to confirm that the zero position of the rotation number in the deceleration pattern of the measured screw rotational velocity32′ in view of the time axis coincides with only the timing when the astern movement of the screw is completed. The present inventor has discovered that it can be guaranteed in such a manner that the measured back pressure value34′ turns zero at the time when the astern movement of the screw is completed. The aforementioned distance L3is optimized by trial shots with the distance L3being changed appropriately till it is confirmed that the zero position of the rotation number in the deceleration pattern of the measured screw rotational velocity32′ in view of the time axis coincides with only the timing when the astern movement of the screw is completed.

Next, description will be described about a problem when the aforementioned distance L3has not been optimized, with reference toFIGS. 4 and 5.FIG. 4shows the transition in relationship among the screw rotational velocity, the back pressure and the screw astern speed along the position axis.FIG. 5shows the transition in relationship among the screw rotational velocity, the back pressure and the screw astern speed along the time axis. Incidentally, also here, a molded item (product) weighing 3.5 g is molded by way of example.FIG. 4corresponds toFIG. 2, andFIG. 5corresponds toFIG. 3.

In the example shown inFIG. 4, the deceleration/decompression start position S4to be set short of the set measurement completion position S2by the distance L3is set closer to the set measurement completion position S2than inFIG. 2. Here, the distance L3is set to be about 0.31 mm. In accordance with this distance L3, the screw astern speed setting pattern (set deceleration pattern)37and the back pressure deceleration setting pattern38are obtained by calculation in the same manner as in the previous example. Thus, the obtained setting patterns37and38are set automatically. The driving of the injection servo-motor3is controlled by open control following the screw astern speed setting pattern (set deceleration pattern)37. As a result, the measured screw astern speed35is decelerated suddenly following the setting pattern37so that the screw stops its astern movement in a position corresponding to the set measurement completion position S2. On the other hand, the driving of the measurement servo-motor5is controlled by back pressure feedback control in which the screw rotational velocity is controlled following the back pressure deceleration setting pattern38. The measured back pressure value34rises slightly short of the set measurement completion position S2. Thus, the measured back pressure value34has a positive value in the set measurement completion position S2even when the measured screw rotational velocity32turns zero in the set measurement completion position S2.

This will be described with reference to the transition on the time axis inFIG. 5. The measured screw astern speed35′ is decelerated suddenly at the start timing of deceleration/decompression. Even when the measured screw astern speed35′ approaches zero, the measured screw rotational speed32′ still has a certain value such that the measured back pressure value34′ rises slightly before the timing when the astern movement of the screw is completed. This is because resin is fed due to the rotation of the screw when the screw is stopping its astern movement. That is, the deceleration of the measured screw rotational velocity32′ resulting from the back pressure feedback control in which the screw rotational velocity is controlled with respect to the measured screw astern speed35′ cannot follow the deceleration of the measured screw astern velocity35′. Thus, the measured screw rotational velocity32′ turns zero after the timing when the astern movement of the screw is completed. As a result, the back pressure is applied even when the screw stops rotating. It is therefore necessary to perform suck-back after the measurement is completed.

In contrast, in this embodiment, as described previously, control is made so that all the measured value of the screw astern speed, the measure value of the back pressure and the measured rotational velocity of the screw (measurement servo-motor) turn zero concurrently at the timing when the screw reaches the measurement completion position. Accordingly, the residual pressure of molten resin staying in the heating cylinder till the next injection becomes zero. Thus, there is no fear that drooling or cobwebbing occurs when a mold is opened and a molded item is released from the mold. It is therefore unnecessary to perform suck-back after measurement is completed. In addition, since it is unnecessary to perform suck-back and any injection operation can be always started in a constant measurement completion position, the injection rate is stabilized, and the cushion thickness is also stabilized. Furthermore, the back pressure and the screw (measurement servo-motor) rotation turn zero as soon as the screw stops its astern movement. Thus, the variation in weight among molded items can be made as low as possible in cooperation with the stabilized density of the measured molten resin, the stabilized injection rate and the stabilized cushion thickness (the variation range R of measured weight values was 0.07 g with respect to a molded item (product) weighing 3.5 g in the conditions ofFIG. 5while the variation R was reduced to 0.03 g with respect to a molded item (product) weighing 3.5 g in the conditions ofFIGS. 2 and 3).