Abstract:
An injection molding method comprises the steps of melting molten resin within a heating cylinder, advancing an injection screw within the heating cylinder to inject the molten resin into a mold, retracting the injection screw to a predetermined position before completion of the injection, further advancing the injection screw under pressure control.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method of controlling a filling process for injection molding machines, and a control device therefor. 
     An explanation will be given to a molding operation of an injection molding machine. In particular, an explanation will be given to the case of a motor driven injection molding machine to center on motions of an injection device. 
     (A) A servo-motor for driving rotation of a screw rotates the screw whereby resin falling on a rear portion of the screw from a hopper is melted and a given amount thereof is fed to a tip end of a heating cylinder. At this time, the screw retreats while being subjected to pressure of molten resin accumulating at the tip end of the heating cylinder. 
     Connected directly to the rear end of the screw is a drive shaft. 
     The drive shaft is rotatably supported on a pressure plate through bearings. The drive shaft is driven through a timing belt by a servo-motor for driving rotation of the screw. The pressure plate is driven through a ball screw by a servo-motor for injection to advance and retreat along guide bars. The foregoing pressure of molten resin is detected by a load cell in a manner described later. A detected value of the load cell is fed back by a feed-back control loop for pressures. 
     (B) Then driving of the servo-motor for injection causes the pressure plate to advance to fill molten resin into a metal mold with the screw tip end as a piston. 
     (C) At the end of the filling process, the molten resin fills a cavity of the metal mold. At that time, the advancing motion of the screw causes conversion of velocity control into pressure control. Such conversion of velocity control into pressure control is called a V-P conversion. 
     (D) Thereafter, the resin in the cavity of the metal mold becomes cold under a set pressure. Resin pressure is controlled in feed-back control loop like the above-mentioned pressure control. 
     In the injection device, when the process (D) is terminated, it goes back to the process (A) and shifts to the succeeding molding cycle. Meanwhile, in a mold clamping device, concurrently with the process (A), the metal mold is opened to permit an ejector mechanism to discharge a molding product having been cooled and solidified, and then the metal mold is closed to shift to the process (B). 
     With reference to FIG. 1, an explanation will be given hereinbelow to the molding motion of a motor driven injection molding machine to center on motions of an injection device. The injection device performs filling of molten resin by converting rotating motion of a servo-motor into linear motion with the use of a ball screw and a nut. In FIG. 1, rotation of the servo-motor  10  for injection is transmitted to a ball screw  11 . A nut  12  adapted to advance and retreat upon rotation of the ball screw  11  is fixed to a pressure plate  13 . The pressure plate  13  is movable along a plurality of guide bars  14  (only two being shown) fixed to a base frame (not shown). Advancing and retreating movements are transmitted to a screw  18  through a load cell  15 , a bearing  16 , and a drive shaft  17 . The drive shaft  17  is also rotatingly driven through a timing belt  20  by a servo-motor  19  for driving rotation of the screw. 
     Rotating driving of the servo-motor  19  causes the screw  18  to retreat in a heating cylinder  21  while rotating whereby molten resin is accumulated at the tip end of the heating cylinder  21 . And rotating driving of the servo-motor  10  causes advancement of the screw  18  to thereby fill the metal mold with the accumulated, molten resin and pressurize the resin for molding. At this time, forces, which push the resin, are detected as reaction forces by the load cell  15 . 
     A detected value from the load cell  15  is amplified by a load cell amplifier  22  to be input into a controller  23 . Mounted on the pressure plate  13  is a position detector  24  for detection of amounts of movements of the screw  18 . A detected value from the position detector  24  is amplified by an amplifier  25  to be input into the controller  23 . 
     In accordance with setting established by an operator, the controller  23  outputs to servo-amplifiers  26 ,  27  current (torque) commands depending upon the respective processes. The servo-amplifiers  26 ,  27  control drive currents of the servo-motors  10 ,  19  to control output torque of the motors. 
     In the above-mentioned injection device, pressure control is conventionally carried out after V-P conversion. Pressure control is slow in response speed. As a result, this is responsible for dispersion in weight of molding products and over-filling among molding products, for which rapid depressurization from the filling pressure to the dwelling pressure is required. 
     FIG. 2 shows an example of varying waveforms of filling velocity and of resin pressure before and after the V-P conversion. As apparent from FIG. 2, with the prior art, rapid depressurization is difficult due to slow response speed since it is switched to pressure control after the V-P conversion. For example, with molding products for DVD (Digital Video Disc) and connectors, a dwelling process is very short. In this case, the manner of depressurization after the V-P conversion affects a molding product much. On the other hand, with molding products, which require high injection velocities, pressure changes very fast, so that pressure cannot be controlled by the response of pressure control. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an injection molding method, which can achieve stability in quality of molding products by retreating a screw immediately before conversion from velocity control into pressure control to obtain a required pressure waveform. 
     It is another object of the invention to provide a control system suited for the above-mentioned method. 
     An injection molding method according to the present invention comprises the steps of melting molten resin within a heating cylinder, advancing an injection screw within the heating cylinder to inject the molten resin into a mold, retracting the injection screw to a predetermined position before completion of the injection, further advancing the injection screw under pressure control. 
     An injection molding machine control system according to the present invention comprises a velocity control system which comprises a position detecting unit for detecting a position of an injections screw, a position setting unit for providing a set value of a position of the injection screw, and a first feedback circuit for outputting a first speed command value to an injection motor based upon a difference between a detected value from the position detecting unit and the set value from the position setting unit. The system also comprises a pressure control system which comprises a pressure detecting unit for detecting a resin filing pressure, and a pressure setting unit for providing a set value of a filling pressure and a dwelling pressure, and a second feedback circuit for outputting a second speed command value to the injection motor based upon a difference between a detected value from the pressure detecting unit and the set value from the pressure setting unit. The system further comprises a switching unit for switching between an output of the first feedback circuit and second feed back circuit to provide a selected speed command value to the injection motor. The velocity control system is configured to perform a control action for returning the injection screw to a set at a predetermined velocity when the injection screw has advanced to a predetermined position during a filing process, and the switching unit switches to the second feedback circuit thereafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view showing a motor driven injection molding machine with an injection device disposed centrally thereof. 
     FIG. 2 is a view showing an example of varying waveforms of filling velocity and of resin pressure before and after a V-P conversion. 
     FIG. 3 is a block diagram showing a constitution of a drive control system of an injection motor according to the present invention. 
     FIG. 4 is a view showing an example of varying waveforms of filling velocity and of resin pressure before and after a V-P conversion according to the present invention. 
     FIG. 5 is a view showing a waveform of screw position-filling velocity in the filling process in multi-stage control. 
     FIG. 6 is a view showing waveforms of time-filling velocity and time-pressure, time-pressure setting and time-actual pressure in the filling process based on multi-stage control. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 3, an explanation will be given to a control device for a motor driven injection molding machine, according to an embodiment of the present invention. The control device according to the present invention is applicable to a motor driven injection molding machine illustrated in FIG.  1 . Therefore, an explanation will be given to the case, in which the servo-motor  10  for injection is controlled. The control device comprises a velocity control system  31  for carrying out velocity control, and a pressure control system  35  for carrying out pressure control. Conversion between these controls, that is, a V-P conversion is carried out by means of switches SW 1 , SW 2  in a manner described later. 
     The velocity control system  31  serves to implement filling velocity control before the V-P conversion. The velocity control system  31  comprises a first feed-back system for outputting a speed command value to the servo-motor  10  on the basis of a difference between a detected value S fb  from a position detecting unit for detecting a screw position and a set value S rf  from a position setting unit  31 - 1  for providing a set value of a screw position. In addition, the position detecting unit comprises a speed sensor  31 - 2  for detecting a rotating speed of the servo-motor  10 , and a calculating unit  31 - 3  for integrating a detected value V fb  from the speed sensor  31 - 2  to calculate a screw position S fb . The position detecting unit may be realized by the position detector  34  illustrated in FIG. 1. A signal indicative of the screw position S fb  calculated in the calculating unit  31 - 3  is output to a computing unit  31 - 4  (first computing unit) to provide for computation of a difference between it and the set value S rf . 
     A signal indicative of such difference (referred hereinbelow to as a first difference signal) is input to a compensator  31 - 5  (first compensator). The compensator  31 - 5  outputs a signal indicative of a speed command value for the servo-motor  10  on the basis of the first difference signal thus input. While the filling velocity control is implemented, that is, when the switch SW 1  (first switch) is made ON, a difference between the speed command value and the detected value V fb  from the speed sensor  31 - 2  is calculated in a computing unit  31 - 6  (third computing unit). Such difference signal is given to a motor drive  34  via a speed limiter  32  and a compensator  33 . The motor drive  34  controls the servomotor  10  on the basis of an input from the compensator  33 . 
     The pressure control system  35  functions to implement pressure control after the V-P conversion. The pressure control system  35  comprises a second feed-back system for outputting a speed command value to the servo-motor  10  on the basis of a difference between a detected value P fb  from a pressure detector  35 - 1  for detecting resin filling pressure and a set value P rf  from a pressure setting unit  35 - 2  for providing a set value of filling pressure. The pressure detector  35 - 1  can be realized by the load cell  15  illustrated in FIG.  1 . In the second feed-back system, a computing unit  35 - 3  (second computing unit) calculates a difference between a detected value P fb  from the pressure detector  35 - 1  for detecting resin filling pressure and a set value P rf  from the pressure setting unit  35 - 2 . A signal (referred below to as a second difference signal) indicative of the difference is input into a compensator (second compensator)  35 - 4 . The compensator  35 - 4  outputs a speed command value to the servomotor  10  on the basis of the second difference signal. While the pressure control is implemented, that is, when the switch SW 2  is made ON, a difference between the speed command value and the detected value from the speed sensor  31 - 2  is calculated in a computing unit  31 - 6 . The difference signal is given to the motor drive  34  via the speed limiter  32  and the compensator  33 . The motor drive  34  controls the servo-motor  10  on the basis of an input from the compensator  33 . 
     The embodiment has a feature in that with the above-described control device, a necessary pressure waveform can be produced by returning the screw to a set position at a set velocity to effect depressurization when the screw has advanced to a predetermined position in the filling process of injection molding. Concretely, control for returning the screw to the set position to effect depressurization is performed by using the velocity control system  31  to control the servo-motor  10  while the switch SW 1  is made ON (the switch SW 2  is made OFF). Subsequently, control after the screw has been returned to the set position is performed by using the pressure control system  35  to control the servo-motor  10  while the switch SW 2  is made ON (the switch SW 1  is made OFF). 
     FIG. 4 shows an example of varying waveforms of filling velocity and of resin pressure before and after the V-P conversion. As apparent from FIG. 4, whether or not the screw has advanced to the predetermined position is detected. A desired pressure waveform is obtained by returning the screw to the set position at the set velocity to effect depressurization control immediately before the V-P conversion when the screw has advanced to the predetermined position. 
     In addition, FIGS. 2 and 4, respectively, show a state, in which the filling velocity is of one stage, for simplicity. Actually, with the embodiment, the filling process is carried out, as shown in FIGS. 5 and 6, in multi-stage control, in which the filling velocity is switched in accordance with a position of the screw. FIG. 5 shows a waveform for screw position-filling velocity, and FIG. 6 shows a waveform for time-filling velocity, time-pressure setting and time-actual waveform. In this case, depressurization is performed by returning the screw to a separately set position at a separately set velocity after it is detected that the screw has reached a final stage position. 
     In addition, the present invention is applicable to not only motor driven type injection molding machines but also hydraulic type injection molding machines. 
     As described previously, the present invention enables rapid depressurization since when the screw has advanced to a predetermined filling position (set value), the screw is returned to a set position at a set velocity whereby the screw can move in response to velocity control. As a result, a necessary pressure waveform can be optionally set to thereby make molding products stable in quality.