MOLD CLAMPING DEVICE OF AN INJECTION MOLDING MACHINE

Abstract: Device comprises several mass-produced drive motors (M1,M2) a main shaft constituted by a screw (2), a ball nut (3) engaging this screw, a follower timing gear pulley (4) rotating integrally with the ball nut, several driver timing gear pulleys (8,9) provided on the output shafts of respective drive motors and a timing belt (7) passing around the driver and follower timing gear pulleys. Each drive motor has a control circuit. The rotational position of one drive motor is detected and a single torque command based on the detection output is supplied to both the control circuits, which in response cause the drive motors to produce the same torque.

Description: The present invention relates to a mold clamping apparatus for use in an injection molding machine, and more particularly to a spindle driving device in the mold clamping apparatus. In injection molding machines, there have been known a mold clamping apparatus of a type comprising a drive source consisting of an electric motor (hereinafter referred to as a motor), and a power transmission system for converting a turning force generated by the drive source into a driving force for axially driving a spindle. A movable platen is arranged to be movable in unison with the spindle and is thereby axially driven by the drive source through the power transmission system so as to perform mold-opening, mold-closing and mold-clamping. In a mold clamping apparatus of this kind, the drive source may consist of a single general-purpose motor capable of generating a required mold clamping force in the case of a small or medium apparatus where the required mold clamping force varies from 5 tons to 10 tons. On the other hand, in a large mold clamping apparatus requiring a larger mold clamping force, however, the drive source must be of a specially ordered high-output type. For this reason, a considerable cost increase occurs and further operational response of the motor is deteriorated since inertia of the rotor becomes large. There is disclosed in JP-A 60 247 946 a mold clamping apparatus according to the preamble of the attached claim 1, wherein a first motor drives a movable die initially and then stops to be replaced in the driving function by a second motor which operates at low speed and high torque. Thus two driving systems are provided, one for high speed and the other for low speed and high torque. According to the present invention there is provided a mold clamping apparatus comprising a movable platen, a spindle operatively connected to drive the movable platen, a plurality of general-purpose motors for driving the spindle, and coupling means coupling the motors to the spindle, characterised in that the mold clamping apparatus is adapted for use in an injection molding machine controlled by a host control unit and comprises: detecting means for detecting at least one of a rotational position and a rotational speed of one of the motors, which are servomotors, and for producing an output signal indicative of said detected position and/or speed; a control circuit for generating a torque command signal in accordance with a difference between said output signal from said detecting means indicative of the detected position or speed, and a command signal from the host control unit; and driving circuits for driving corresponding respective ones of the servomotors in accordance with said torque command signal supplied from said control circuit, said driving circuits corresponding in number to that of said plurality of servomotors and being arranged to drive the servomotors simultaneously so that they are driven in synchronism with one another to provide a combined output equivalent to that of a single higher output motor. Embodiments of the present invention may provide a mold clamping apparatus in an injection molding machine, which is low in cost and has an improved response. Thus, since the spindle of the mold clamping apparatus is adapted to be driven by means of the plurality of motors, a spindle driving device of the mold clamping apparatus may be low in cost. Moreover, since a rotor of each motor may be prevented from becoming large in diameter size, the sum of the rotor inertias of the plural motors can be less than that of a single special motor, and hence the operational response of the motors in particular the acceleration/deceleration characteristics thereof, can be improved. Brief Description of the Drawings Fig. 1 is a perspective view showing a spindle driving device of a mold clamping apparatus, according to an embodiment of the present invention; Fig. 2 is a perspective view showing, by way of example, a conventional spindle driving device; and Fig. 3 is a circuit diagram showing a control unit for driving the motors of Fig. 1. Referring to Fig. 2, a conventional spindle driving device for use in a mold clamping apparatus of an injection molding machine will be explained. In Fig. 2, a ball screw 2 as a spindle is rotatably supported by a rear platen 1, and a ball nut 3 is threadedly engaged with an outer portion of the ball screw 2 projecting rearwardly of the rear platen 1. A timing pulley (hereinafter referred to as a pulley) 4 is coupled to the ball nut 3 in a manner rotatable integrally with the same nut. Further, the pulley 4 is operatively coupled to a servomotor 5 through a timing belt 7 which is wound around the pulley 4 and a second pulley 6 fixed to the output shaft of the motor 5. The ball screw 2 has its inner end coupled to a movable platen (not shown) through a crosshead (not shown) of the mold clamping apparatus (in case that the apparatus has a toggle mechanism) or coupled directly thereto (in the case of the direct-acting type clamping apparatus). On the occasion that a single motor is employed as the drive source for mold clamping in the aforementioned type of mold clamping apparatus of the injection molding machine, there occur the above-mentioned drawbacks such as increased cost of the spindle driving device, irrespective of whether the single motor consists of a servomotor or not. In the following, a spindle driving device of a mold clamping apparatus for use in an injection molding machine, according to an embodiment of the present invention, will be explained with reference to Fig 1. Basically, this spindle driving device is constructed in the same manner as that shown in Fig. 2, parts common to Figs. 1 and 2 being shown by like reference numerals or symbols. Symbols M1 and M2 designate spindle driving motors which each consist of an AC servomotor consisting of a general-purpose permanent-magnet synchronous motor, respectively. These motors are fixed integrally to the rear platen 1 by means of an appropriate means, not shown, and have their output shafts to which drive pulleys 8 and 9 are fixed, respectively. Further, a driven pulley 4 is fixed to a ball nut 3 which is threadedly engaged with a ball screw 2 as the single spindle of the mold clamping apparatus. The motors M1, M2 are operatively coupled to the ball nut 3 through the pulleys 4, 8 and 9 and a timing belt 7 wound around these pulleys. Furthermore, in order to produce, from the two general-purpose motors M1 and M2, the combined output equivalent to that of a single high-output type motor, the mold clamping device according to this embodiment is arranged to drive these motors so that the motors M1, M2 are driven in synchronism with each other to produce their output torque acting in the same rotational direction. More specifically, a control unit is provided, which has a control circuit responsive to a position/speed signal associated with one of the motors, the motor M1, for instance, and two motor driving circuits for these motors M1 and M2, the control unit being arranged to deliver the same drive current command from its control circuit to both of the motor drive circuits. Next, with reference to Fig. 3, a further explanation as to the control unit will be given. Basically, this control uint is constructed in the same manner as a conventional PWM control unit known as a servomotor driving control unit, but differs from the conventional one in that two transistor inverters as power amplifiers are provided, these inverters being connected in parallel with each other. In Fig. 3, reference symbol E designates a three-phase power source, numeral 10 denotes a rectifier, and 11 a transistor inverter, as a power amplifier, which generates an output for driving the servomotor M1. An ordinary PWM control unit has this transistor inverter 11 alone by which a single servomotor is driven. Contrary to this, in the present embodiment, a further transistor inverter 11' is connected in parallel with the transistor inverter 11 and is arranged to drive a corresponding servomotor M2. Reference numeral 12 denotes a transistor PWM control circuit, and symbol P designates the position detector, such as a pulse encoder, for detecting the position and speed of a rotor of the servomotor M1. The transistor PWM control circuit 12 is arranged to compare a present speed S, detected by the position detector P, with a speed command V0 from a host control unit such as an NC, to turn on-off transistors TA to TF and TA' to TF' of the transistor inverters 11 and 11', so as to control currents flowing through the U-, V-, and W-phase windings of the servomotors M1 and M2, thereby controlling the rotating speed of the servomotors M1 and M2. Namely, in response to PWM signals PA and PB delivered from the transistor PWM control circuit 12, the transistors TA and TA' and the transistors TB and TB' of the transistor inverters 11 and 11' are simultaneously turned on or off, whereby the currents flowing through the respective U-phase windings of the servomotors M1 and M2 are controlled. Likewise, in response to PWM signals PC and PD, the transistors TC, TC' and the transistors TD, TD' are simultaneously turned on or off, whereby the currents flowing through the respective V-phase windings of the servomotors M1, M2 are controlled. In response to the PWM singals PE and PF, moreover, the transistors TE, TE' and the transistors TF, TF' are simultaneously turned on or off, whereby the currents flowing through the respective W-phase windings of the servomotors M1, M2 are controlled. As a result, the servomotors M1 and M2 are driven in synchronism with each other so as to produce the same torque. A detailed description of the transistor PWM control circuit 12, which is a conventional one, is omitted herein. In operation, when the speed command value V0 from the host control unit such as an NC, is applied to the transistor PWM control circuit 12, the circuit 12 compares the speed command value V0 with the present speed S detected by the position detector P. In accordance with the difference between the value V0 and the speed S, the control circuit 12 delivers the PWM signals PA to PF, and causes the transistor inverters 11 and 11' to drive the servomotors M1 and M2, so as to control the motor speeds to a command speed. At this time, the transistors associated with the corresponding phases of the transistor inverters 11, 11', serving to control the currents flowing throgh the windings of the individual phases of the servomotors M1 and M2, are turned on or off in accordance with the same PWM signal, so that one and the same current flows synchronously through the windings of each phase of the servomtors M1 and M2. In consequence, the servomotors M1, M2 rotate in synchronism with each other, with the same output torque, to drive the timing belt 7. In the meantime, since the two timing gear pulleys 8 and 9 are in engagement with the timing belt 7, respectively, no phase dislocation occurs between these timing gear pulleys. In connecting the motors M1, M2 with the ball screw 2 as the spindle, a gear train may be employed in place of the timing belt. Further, more than two or three motors may be employed as distinct from the above-mentioned embodiment having two motors. In this case, it is only required to provide additional one or more transistor inverters which correspond in number to additional one or more servomotors, and connected in parallel with the transistor inverters 11, 11', and hence neither additional position detector P such as a pulse encoder nor additional transistor PWM control unit is required. Although an AC servomotor consisting of a permanent-magnetic synchronous motor is employed in the above-mentioned embodiment, a DC servomotor may be used. In this case, one or more additional power amplifiers alone should be provided in the servomotor control unit, while other control circuits and position detectors may be commonly employed.

Claims: A mold clamping apparatus comprising a movable platen, a spindle (2) operatively connected to drive the movable platen, a plurality of general-purpose motors (M1,M2) for driving the spindle (2), and coupling means (3,4,7,8,9) coupling the motors (M1,M2) to the spindle (2), characterised in that the mold clamping apparatus is adapted for use in an injection molding machine controlled by a host control unit and comprises: detecting means (P) for detecting at least one of a rotational position and a rotational speed of one of the motors (M1,M2), which are servomotors, and for producing an output signal (S) indicative of said detected position and/or speed; a control circuit (12) for generating a torque command signal in accordance with a difference between said output signal (S) from said detecting means (P) indicative of the detected position or speed, and a command signal (VO) from the host control unit; and driving circuits (11,11') for driving corresponding respective ones of the servomotors (M1,M2) in accordance with said torque command signal supplied from said control circuit (12), said driving circuits (11,11') corresponding in number to that of said plurality of servomotors (M1,M2) and being arranged to drive the servomotors (M1,M2) simultaneously so that they are driven in synchronism with one another to provide a combined output equivalent to that of a single higher output motor. A mold clamping apparatus according to claim 1, wherein the spindle (2) is a ball screw and said coupling means comprise a ball nut (3) threadedly engaged therewith. A mold clamping apparatus according to claim 2, wherein said coupling means further comprise a driven timing gear pulley (4) arranged to be rotatable integrally with said ball nut (3), a plurality of driving timing gear pulleys (8,9) fixed to respective output shafts of said motors (M1,M2), and a timing belt (7) wound between said driven timing gear pulley (4) and said plurality of driving timing gear pulleys (8,9). A mold clamping apparatus according to any preceding claim, wherein each of said servomotors (M1,M2) has windings (U,V,W) of respective phases and said driving circuits (11,11') have switching elements (TA to TF, TA' to TF') which respectively control electric currents flowing through the windings (U,V,W) of the respective phases of said servomotors (M1,M2), and wherein said control circuit (12) performs on and off control of said switching elements (TA to TF, TA' to TF') in accordance with the difference between the output signal(S) and the command signal (VO), so that the same electric current flows through the respective windings (U,V,W) which are associated with the same phase when the mold clamping apparatus is in operation.

Inventors: INABA, YOSHIHARU, TOKUNAGA, SHIGEO
Applicants: FANUC LTD.
Application year: 1987
Main classification: B29C45
IPC codes: B29C45