STRAIGHT-ACTING MOLD-CLAMPING DEVICE IN AN INJECTION MOLDING MACHINE

Abstract: Straight-acting mold-clamping device for an injection molding machine uses a servomotor (10) for high-speed drive of a moving platen (1) in opening and closing. During mold closure, the servometer speed is lowered when the moving platen reaches a safe limit point for high-speed displacement. When, subsequently, a moving and stationary platen contact position is reached, the servomotor is stopped and the moving platen is driven by induction motor (12) transmitted via a reduction gear (14), clutch (15) and timing gear (16). This drive is accompanied by reduction of a count which is in an error register in a servo circuit (25) and corresponds to requisite displacement from the platen contact position to a mold clamping position. When this count becomes zero, an induction motor brake (13) is applied.

Description: The present invention relates to a direct-pressure-type mold-clamping apparatus for an injection-molding machine, which is capable of effecting mold-closing and mold-opening at high speed and of producing a required mold-clamping force. An injection-molding machine can be furnished either with a toggle-type mold-clamping mechanism, in which a drive source and a movable platen are connected by means of a toggle link, or with a direct-pressure-type mold-clamping mechanism, in which the drive source and the movable platen are connected directly, so as to open or close and clamp dies that are attached to the movable platen and the stationary platen. WO-A-80/03161 (published 05.06.86), which corresponds with EP-A-201610, for instance, shows a toggle-link mechanism, while US 2484712 shows a direct-pressure machine. In the mold clamping operation, the dies must be clamped with a great force, and thus a requisite for the drive source of a mold-clamping mechanism is the capability of producing this large clamping force. A further requirement for the process of injection molding is that the time required for the manufacture of each molding, i.e. the cycle time, should be as short as possible. Another requisite for the drive source, therefore, is that it be able to drive the dies at high speed while they are being opened or closed. In the toggle-link version the link acts so as to help the dies to be opened or closed at high speed, and to facilitate the production of a large clamping force. Thus, the toggle-type mold-clamping mechanism fulfils both of the aforesaid requirements. The direct-pressure-type mold-clamping mechanism, on the other hand, has the advantage over the toggle-type mold-clamping mechanism that it does not require mold thickness adjustment for compensating for a change in die thickness. It is difficult for the direct-pressure-type mold-clamping mechanism, however, to satisfy both the aforementioned requirements. In the case of a direct-pressure-type mold-clamping mechanism using a motor as its drive source, for example, both those requirements can be fulfilled only if the motor used has a very large capacity, and therefore is expensive. In consequence, the manufacturing cost of the injection-molding machine inevitably increases. Accordingly, an object of the present invention is to provide a motor-operated, direct-pressure-type mold-clamping apparatus, capable of performing accurate, high-speed mold closing and opening operations and of producing the required mold clamping force, without the use of any special large-capacity motor. According to the present invention there is provided a direct-pressure-type mold-clamping apparatus for an injection-molding machine having a fixed platen and a movable platen each to be fitted with a die, the apparatus comprising: conversion means including a rotating member and a rectilinear motion member movable in one with the movable platen, the conversion means serving to convert a rotary motion of the rotating member into a rectilinear motion of the rectilinear motion member; a first motor for opening and closing the mold; second motor for clamping the mold; transmission means for transmitting the rotation of the first motor to the rotating member; a speed reducer for reducing the rotating speed and enhancing the rotatory force of the second motor and transmitting the rotary force, thus enhanced, to the transmission means; clutch means for releasably coupling the speed reducer and the transmission means; position-detecting means for detecting the position of the movable platen; and means for ensuring that the clutch means is actuated at a predetermined position of the movable platen corresponding to touching of dies fitted to the platens; characterized in that : the first motor is a servomotor and the second motor is an induction motor; the position-detecting means is coupled to the servomotor; the apparatus further includes a braking means for braking the induction motor; a numerical control means coupled to the detecting means for controlling the servomotor, the induction motor, the clutch means and the braking means; and a servo circuit for driving the servomotor, including an error register for receiving a pulse signal from the position-detecting means and a distributed pulse signal from the numerical control means; and the numerical control means is adapted: to supply a torque limit command which causes the servomotor to be substantially inoperative; to actuate the clutch means when the movable platen, driven by the servomotor reaches the said predetermined position, so that the movable platen is driven substantially only by the induction motor from the said predetermined position to a second position, in which a set mold-clamping force is produced; to effect pulse distribution corresponding to movement of the movable platen from the first predetermined position to the second position; and to determine the point at which the movable platen reaches the second predetermined position, and to actuate the braking means to lock the movable platen, by ascertaining that an error amount in the said error register is reduced to zero by virtue of rotation of the servomotor accompanying the rotation of the induction motor. Preferably, the numerical control means is adapted to stop the delivery of the torque limit command and de-energize the clutch in a mold-opening mode, so as to cause the servomotor to drive the movable platen. Thus, in embodiments of the present invention, in a mold-clamping mode the output torque of the induction motor enhanced by the speed reducer is converted into a driving force for driving the movable platen by the transmission means and the conversion means, and the movable platen is moved by the driving force so as to produce a large mold-clamping force, whereas, in a mold closing or opening mode, the transmission means and the induction motor are disconnected from each other, and the movable platen is moved by means of the servomotor only. Thus, a large mold-clamping force can be produced in the mold-clamping mode, while the movable platen can be moved accurately and at high speed in the mold closing or opening mode, and further no large-capacity motor is required. US 2484712, mentioned above, discloses a two-motor clamping apparatus functioning in a manner analogous to that just described. This apparatus uses electric motors and includes limit switches for detecting the travel of the mold carriage past certain points. In particular a forward limit switch is used to cause the insertion of a resistance into the main motor circuit, slowing down the main motor when the mold halves nearly touch. A contact switch is further provided for actuating the clutch and switching in the supplementary mold-closing motor. This mold-closing motor is actuated for a given time in order to close the mold. In contrast the present invention uses a servomotor for mold closing and an induction motor for clamping, and a numerical control unit for controlling the action of these motors. Lockup of the mold is determined by the zero value of the content of the error register associated with the servomotor, which although de-energized during clamping is rotated by the induction motor. Moreover, the highly controllable servomotor for mold opening and closing operations is combined with the induction motor for producing the mold-clamping force, which can easily provide a high output and is relatively low in price. Therefore, an injection-molding machine can be obtained which is economical and high in control accuracy, and has a short cycle time. Furthermore, the drive source for the movable platen need not be a servomotor with a specially great output torque. In limiting the torque of the servomotor for die protection during the mold-closing operation, therefore, an optimum torque limit operation can be achieved even though the resolution of the torque limit value setting is low. Further, since arrival to the set clamping-force-producing position is determined by monitoring the error register value, the clamping force can be accurately controlled. Fig. 1 is a schematic view showing a direct-pressure type mold clamping apparatus according to an embodiment of the present invention; Fig. 2 is a block diagram showing the servo circuit of Fig. 1; and Fig. 3 is a flow chart for illustrating the mold closing and clamping operations of the mold clamping apparatus. In Fig. 1, numerals 1, 2 and 3 denote a movable platen, a stationary platen, and a rear platen, respectively. The stationary platen 2 and the rear platen 3 are fixed to a base (not shown) of an injection-molding machine. The movable platen 1 is supported, by means of four tie bars (two of which are denoted by numeral 4) supported at either end by the platens 2 and 3, so as to be movable from side to side, as illustrated, along the tie bars 4. The movable plate, 1 and the stationary platen 2 are fitted individually with dies (not shown). A hollow cylindrical nut mounting member 5, extending parallel to the tie bars 4, is fixed to the central portion of the rear-platen-side end face of the movable platen 1. A ball nut 7 is fixed to the distal end of the mounting member 5 so as to be coaxial with the member 5. The mounting member 5 is formed with a through hole extending along the axis thereof. This through hole communicates with a tapped hole of the ball nut 7, and contains a ball screw 6, in engagement with the nut 7, for axial movement. A shaft 8 integral with the ball screw 6 extends outward through the rear platen 3, and is supported by the platen 3 with the aid of a thrust bearing and a radial bearing, not shown. A timing gear 9 is fixed to the outer end of the ball screw shaft 8. A servomotor 10 with an absolute position detector PC is fixed to the rear platen 3, and a timing gear 11 is fixed to the output shaft of the servomotor 10 which extends outward through the rear platen 3. An induction motor 12 with a braking system 13 is fixed to the base of the injection-molding machine. The output shaft of the motor 12 is coupled to the input shaft of a speed reducer 14, whose output shaft is coupled to the input shaft of a clutch 15. The output shaft of the clutch 15 is supported by the rear platen 3, and extends outward through the platen. A timing gear 16 is fixed to the distal end of the output shaft of the clutch 15. A timing belt 17 is passed between and around the timing gears 9, 11 and 16. Symbols SW1 to SW3 designate switches, which are turned on and off in response to commands from a numerical control unit (hereinafter referred to as NC unit) 20 (mentioned later), thereby connecting and disconnecting power supply from a power source 18 to the induction motor 12, the braking system 13, and the clutch 15. The NC unit 20 includes a microprocessor (hereinafter referred to as NCCPU) 21 for numerical control and a microprocessor (hereinafter referred to as PMCCPU) 22 for programmable machine controller. The NCCPU 21 is connected with a ROM 23, which stores a control program for generally controlling the injection-molding machine, and a RAM 24 for tentatively storing various results of arithmetic operations. The NCCPU 10 is also connected, through a servo-interface 26, with servomotors (not shown) for various axes for injection, screw rotation, ejector operation, etc., and a servo-circuit for controlling the drive of the servomotor 10 of the aforementioned mold clamping mechanism (among which only the servo-circuit for the servomotor 10 is denoted by numeral 25). On the other hand, the PMCCPU 22 is connected with a ROM 27, which stores a sequence control program or the like for the injection-molding machine or the like, and a RAM 28 for tentatively storing the results of arithmetic operations or the like. Moreover, a bus-arbiter controller (hereinafter referred to as BAC) 29, which is interposed between the two CPUs 21 and 22, is connected with a nonvolatile common RAM 30 storing operation control programs for the injection-molding machine and other programs, an input circuit 31, an output circuit 32, and an operator panel controller (hereinafter referred to as OPC) 33. The OPC 33 is connected with a manual input control panel with a CRT display (hereinafter referred to as MDI/CRT) 34. The switches SW1 to SW3 and the servo circuit 25 are connected to the output circuit 32. In the servo circuit 25, an error register 40 is periodically supplied with a move command, constituted by a pulse train and providing a moving amount of the movable platen for a pulse distribution period, from the NCCPU 22 through the servo-interface 26, and is supplied with a pulse train from the position detector PC with rotation of the servomotor 10, so as to store an error amount indicative of the difference therebetween. The error amount is converted into an analog voltage, as a speed command value, by means of a digital-to-analog converter (hereinafter referred to as D/A converter) 42. Further, a speed feedback is performed to improve a response characteristic of the servo circuit. That is, a voltage obtained by converting a signal from the encoder PC and corresponding to the actual servomotor speed is subtracted from the speed command value, and this subtraction result, i.e., the difference between the commanded speed and the actual speed, is amplified in a compensator 42, so as to obtain a torque command or a voltage corresponding to a value of current that should flow through the armature of the servomotor 10. Furthermore, a torque limit circuit 43 is provided, which receives the torque command to limit the output torque of the servomotor. In order to improve the response with respect to the output of the torque limit circuit 43, the difference between the torque command and a feedback voltage signal supplied from a current detector 48 and corresponding to the armature current of the servomotor 10 is amplified by a compensator 44. Moreover, the output of the compensator 44 is amplified by a power amplifier 45 and is then applied to the servomotor 10. Reference numeral 47 designates a D/A converter for converting the torque limit command value from the NC unit 20 into an analog signal which is applied to the torque limit circuit 43. In the meantime, the NCCPU 21 is arranged to read out the error value of the error register 40 through the servo-interface 26. Referring now to Fig. 3, the operation of the mold clamping apparatus with the aforementioned construction will be described. First, mold clamping control parameters, such as a mold opening end position, die protection start position, die touch position, mold clamping force position, mold opening acceleration position, mold opening deceleration position, and die moving speed, are previously set by means of the CRT/MDI 34 using, e.g., macro-variables. At the start of mold closing, the PMCCPU 22 causes the output circuit 32 to turn off the switch SW3, thereby disengaging the clutch 15 (Step S1). On the other hand, the NCCPU 21 performs pulse distribution in accordance with an NC program, to drive the servomotor 10 in a mold closing direction through the medium of the servo-interface 26 and the servo-circuit 25. As the motor rotates, the ball screw 6 rotates, driven by the timing gear 11, the timing belt 17, and the timing gear 9. The movable platen 1 is advanced by means of the ball nut (7), in engagement with the ball screw 6, and the nut mounting member 5 movable in one with the nut (Step S2). In the meantime, the timing gear 16 on the side of the induction motor 12 keeps on racing. When the movable platen 1 reaches the die protection start position, the rotating speed of the servomotor 10 is reduced to a die protection speed, and the movable platen 1 is further advanced. Thereafter, when pulse distribution up to the set die touch position is completed with the movement of the movable platen 1, the NCCPU 21 writes a pulse distribution end signal in the common RAM 30. When the PMCCPU 22 reads this pulse distribution end signal from the common RAM 30, it causes the output circuit 32 to turn on the switch SW3, thereby engaging the clutch 15, and delivers a torque limit of zero so that the output torque of the servomotor 10 is reduced to zero (Step S3). Then, the NCCPU 21 starts pulse distribution up to a set mold clamping force producing position, i.e., a lockup position, and to the servo-circuit 25. Meanwhile, the PMCCPU 22 causes the output circuit 32 to turn on the switch SW1, thereby driving the induction motor 12 in the mold closing direction (Step S4). The output of the induction motor 12 is reduced in rotating speed and enhanced in torque by the speed reducer 14, and is then transmitted through the clutch 11 to the timing gear 16, thereby driving this gear. Accordingly, the timing gear 9 fixed to the shaft 8 of the ball screw 6 is driven in the mold closing direction by the timing gear 16 driven by means of the induction motor 12 through the medium of the timing belt 11. Thus, the ball screw 6 is rotated in the mold closing direction with a great torque. As a result, the movable platen 1, which is movable in one with the ball nut 7 in engagement with the ball screw 6, is subjected to a great driving force in the mold closing direction, thus clamping the dies while causing the tie bar 4 to extend. On the other hand, the error register 40 of the servo circuit 25 is supplied with distributed pulses up to a lockup position, and the value of the error register 40 is converted into an analog voltage by the D/A converter 41. Further, a voltage supplied from the F/V converter 46 and indicative of the actual servomotor speed is subtracted from the error register value, to be supplied to the torque limit circuit 43 through the compensator 42. However, since the torque limit value is set to "0", the output from the torque limit circuit 43 remains "0" and hence the servomotor 10 is substantially de-energized. But, the timing gear 11 fixed to the motor shaft of the servomotor 10 is rotated by means of the timing belt 17 driven by the induction motor 12 and thus the motor shaft of the servomotor 10 is rotated. With this motor rotation, the encoder PC generates feedback pulses to reduce the error value of the error register 40. In this manner, the error value of the error register 40 decreases as the movable platen 1 is moved forwardly. Thereafter, a lockup operation is completed and the error value of the error register 40 is brought to zero. Under these conditions, when an in-position signal is supplied from the servo circuit 25 to the NCCPU 21 through the servo-interface 26, the NCCPU 21 writes a lockup end signal into the common RAM 30. When the PMCCPU 22 reads this lockup end signal, it causes the output circuit 32 to turn on the switch SW2, thereby working the braking system 13 to prevent the induction motor 12 from rotating. As a result, the movable platen 1 is held in the lockup position (set mold clamping force producing position) by the agency of the speed reducer 14, clutch 15, timing gear 16, timing belt 17, timing gear 9, ball screw 6, and ball nut 7. At the same time, the PMCCPU 22 causes the output circuit 32 to turn on the switch SW1, thereby stopping the drive of the induction motor 12 (Step S5). Thus, the mold closing and clamping operations are finished. Thereafter, molten molding resin is injected into the dies and cooled, whereupon mold opening is performed. At the start of the mold opening, the PMCCPU 22 terminates the delivery of the torque limit value of "0" and causes the output circuit 32 to turn off the switches SW2 and SW3, thereby removing the braking action of the braking system 13 and disengaging the clutch 15. Meanwhile, the NCCPU 21 drives the servomotor 10 in accordance with the NC program, through the medium of the servo-interface 26 and the servo-circuit 25, thereby causing the movable platen 1 to retreat at a mold release speed. Thereafter, the mold opening end position is reached after acceleration and deceleration, the drive of the servomotor 10 is stopped. In the mold closing and opening modes, as described above, the movable platen 1 is driven by means of the servomotor 10 whose position and speed are controlled by the NC unit 20. Accordingly, the mold closing and opening operations can be performed accurately and at high speed. In the mold clamping mode, on the other hand, the movable platen 1 is driven with the great driving force by the output of the induction motor 12 which is enhanced by means of the speed reducer 14, so that a great mold clamping force can be obtained. Thus, the function to produce the mold clamping force and the function to execute the mold closing and opening operations are allotted to the induction motor and the servomotor, respectively, so that an economical mold clamping apparatus can be obtained without requiring the use of any large-capacity motors for these individual motors. Moreover, the servomotor for the mold opening and closing operations requires an output torque of only about 2 to 3 tons. Thus, if the resolution of torque limit value setting is 1/100, the output torque of the servomotor can be controlled in blocks of about 20 to 30 kg, so that die protection can be achieved most suitably. When performing the mold opening and closing operations and the mold clamping operation by means of one motor, on the other hand, the motor used must produce the maximum output torque of about 100 tons. Thus, the motor output torque cannot be reduced to about 1 ton or less, and the die protection is difficult.

Claims: A direct-pressure-type mold-clamping apparatus for an injection-molding machine having a fixed platen (2) and a movable platen (1) each to be fitted with a die, the apparatus comprising: conversion means including a rotating member (6) and a rectilinear motion member (7) movable in one with the movable platen, the conversion means serving to convert a rotary motion of the rotating member (6) into a rectilinear motion of the rectilinear motion member (7); a first motor (10) for opening and closing the mold; second motor (12) for clamping the mold; transmission means (9,11,17) for transmitting the rotation of the first motor (10) to the rotating member (6); a speed reducer (14) for reducing the rotating speed and enhancing the rotatory force of the second motor (12) and transmitting the rotary force, thus enhanced, to the transmission means; clutch means (15) for releasably coupling the speed reducer and the transmission means; position-detecting means (PC) for detecting the position of the movable platen (1); and means for ensuring that the clutch means (15) is actuated at a predetermined position of the movable platen (1) corresponding to touching of dies fitted to the platens (1,2); characterized in that: the first motor (10) is a servomotor and the second motor (12) is an induction motor; the position-detecting means (PC) is coupled to the servomotor (10); the apparatus further includes a braking means (13) for braking the induction motor (12); a numerical control means (20) coupled to the detecting means (PC) for controlling the servomotor (10), the induction motor (12), the clutch means (15) and the braking means (13); and a servo circuit (25) for driving the servomotor, including an error register (40) for receiving a pulse signal from the position-detecting means (PC) and a distributed pulse signal from the numerical control means (20); and the numerical control means (20) is adapted: to supply a torque limit command which causes the servomotor (10) to be substantially inoperative; to actuate the clutch means (15) when the movable platen (1), driven by the servomotor (10) reaches the said predetermined position, so that the movable platen (1) is driven substantially only by the induction motor (12) from the said predetermined position to a second position, in which a set mold-clamping force is produced; to effect pulse distribution corresponding to movement of the movable platen (1) from the first predetermined position to the second position; and to determine the point at which the movable platen (1) reaches the second predetermined position, and to actuate the braking means (13) to lock the movable platen, by ascertaining that an error amount in the said error register (40) is reduced to zero by virtue of rotation of the servomotor (10) accompanying the rotation of the induction motor (12). A direct-pressure-type mold-clamping apparatus according to claim 1, wherein the numerical control means (20) is adapted to actuate the servomotor (10), after completion of mold clamping, to move the movable platen (1) from the second predetermined position to a third predetermined position in which mold opening ends. A direct-pressure-type mold-clamping apparatus according to claim 1 or 2, wherein the rectilinear motion member (7) comprises a ball nut and the rotating member (6) comprises a ball screw mating with the ball nut (7). A direct-pressure-type mold-clamping apparatus according to any preceding claim, wherein the transmission means includes timing gears (16,11,9) fixed respectively to the output shaft of the speed reducer (14), the output shaft of the servomotor (10) and the rotating member (6), and a timing belt (17) passed between and around the timing gears.

Inventors: KAMIGUCHI, MASAO
Applicants: FANUC LTD.
Application year: 1987
Main classification: B29C45
IPC codes: B29C45, B22D17