Patent Publication Number: US-2021170658-A1

Title: Mold clamping device of injection molding machine and method of adjusting mold thickness of mold clamping device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Japan Application No. 2019-221872, filed on Dec. 9, 2019. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
     BACKGROUND 
     Technical Field 
     The disclosure relates to a mold clamping device of an injection molding machine and a method of adjusting a mold thickness of the mold clamping device, and particularly to a hybrid-type mold clamping device and a method of adjusting a mold thickness of the mold clamping device in which a mold opening/closing operation is electrically performed and a mold clamping operation is hydraulically performed. 
     Description of Related Art 
     Conventionally, in mold clamping devices of an injection molding machine, there is one having a configuration of being divided into a mold opening/closing drive device that performs a mold opening/closing operation and a mold clamping cylinder device that performs a mold clamping operation in order to reduce a time for a mold opening operation and a mold closing operation and generate a large mold clamping force in the mold clamping operation. 
     Japanese patent No. 3892580B2 discloses a mold clamping device of an injection molding machine which includes a fixed platen which is fixed on a machine base and to which a fixed mold is attached, a support platen coupled to the fixed platen by tie bars and supported to be slightly slidable with respect to the machine base, a movable platen to which a movable mold is attached and which moves between the fixed platen and the support platen, and a mold clamping shaft fixed to a rear surface of the movable platen. The mold clamping device further includes a mold opening/closing drive device constituted by an electric actuator such as a servomotor and a ball screw mechanism, and a mold clamping cylinder device constituted by a mold clamping cylinder, a mold clamping ram, a half nut, and the like. The mold clamping ram hydraulically presses the mold clamping shaft to perform mold clamping. 
     In such a so-called hybrid-type mold clamping device, immediately after power is supplied or when the mold is replaced, it is necessary to set the origin of the device for mold thickness adjustment. 
     An injection molding machine disclosed in U.S. Pat. No. 6,468,449B1 stores a position of a movable platen as an origin and sets the origin of the movable platen in a mold closed state in which the movable mold attached to a movable platen is moved forward and is brought into contact with a fixed mold attached to a fixed platen. Thereafter, the injection molding machine moves a mold clamping ram rearward to a rearward end position (machine limit position), stores the rearward end position of the mold clamping ram as an origin, and thus sets the origin of a mold clamping cylinder. 
     However, in order to actually perform a mold clamping operation for injecting and filling a molding material into the mold, the origin of the movable platen is set, the origin of the mold clamping cylinder is set thereafter, and it is further necessary to move the mold clamping ram forward from the rearward end position and search for an engagement portion between a convex part provided on a mold clamping shaft and a half nut using a detector such as an optical sensor to perform the mold clamping. 
     Conventionally, since it takes about 3 seconds to set the origin of the movable platen, about 7 to 12 seconds to set the origin of the mold clamping cylinder, and about 17 to 21 seconds to detect an engagement position of the half nut engaged with the convex part of the mold clamping shaft, it takes a long time for mold thickness adjustment before the mold clamping operation, and there have been many requests for reducing the time from workers who use such a mold clamping device. 
     Therefore, the disclosure provides a mold clamping device of an injection molding machine and a method of adjusting a mold thickness of the mold clamping device in which a time for mold thickness adjustment when the mold is replaced can be greatly reduced. 
     Further, the disclosure provides a mold clamping device of an injection molding machine and a method of adjusting a mold thickness of the mold clamping device in which an accurate engagement position of a half nut can be calculated even with a shortest time of mold thickness adjustment. 
     SUMMARY 
     The disclosure provides a mold clamping device of an injection molding machine in which a mold clamping shaft integrated with a mold opening/closing nut, the mold opening/closing nut is adapted to be moved by rotating a mold opening/closing screw shaft to perform opening and closing of a movable platen, a mold clamping ram incorporated in a support platen pressing the mold clamping shaft, the mold clamping ram hydraulically performing mold clamping of the movable platen, wherein the mold clamping shaft includes annular convex parts at a constant pitch on an outer circumference thereof, a half nut device incorporating a half nut which engages with the annular convex parts is coaxially attached to a distal end of the mold clamping ram, and the mold clamping shaft, the half nut device, and the mold clamping ram are disposed on one shaft, and the mold clamping device further includes a ram position detecting member which detects a position of the mold clamping ram, an encoder which detects a position of the movable platen, a ram position control member which hydraulically controls the position of the mold clamping ram, a storage unit which stores a difference value between mold closing positions of the movable platen before and after mold replacement, and a control unit which performs control of the mold clamping device, in which the control unit calculates the difference value when mold thickness adjustment is performed, calculates a movement position of the mold clamping ram on the basis of a current position of the mold clamping ram and the difference value, moves the mold clamping ram to the movement position, and then causes the half nut to engage with the mold clamping shaft to perform the mold clamping of the movable platen. 
     Also, the disclosure provides a method of adjusting a mold thickness of a mold clamping device in which opening and closing of a movable platen is performed by rotating a mold opening/closing screw shaft to move a mold opening/closing nut and moving a mold clamping shaft integrated with the mold opening/closing nut, and mold clamping of the movable platen is hydraulically performed by a mold clamping ram incorporated in a support platen pressing the mold clamping shaft, and the method of adjusting a mold thickness of a mold clamping device includes a difference value calculation step of calculating a difference value of mold closing positions of the movable platen before and after mold replacement, a movement position calculation step of calculating a movement position of the mold clamping ram on the basis of a current position of the mold clamping ram and the difference value, and a mold clamping step of performing mold clamping of the movable platen by engaging a half nut coaxially attached to a distal end of the mold clamping ram with the mold clamping shaft after the mold clamping ram is moved to the movement position. 
     According to the disclosure, since a difference value of mold closing positions of the movable platen is calculated and the mold clamping ram is moved on the basis of the difference value when the mold thickness adjustment is performed, a step of moving the mold clamping ram to a rearward limit each time the mold thickness is adjusted can be omitted, and a time for the mold thickness adjustment can be greatly reduced. 
     Further, according to the disclosure, since a difference value of mold closing positions of the movable platen is calculated and a mold thickness difference between the mold before mold replacement and the mold after the mold replacement is automatically calculated when the mold thickness adjustment is performed, work of manually inputting a mold thickness by an operator can be reduced, and an input error by the operator can be prevented. 
     The mold clamping device of an injection molding machine of the disclosure may further include a photoelectric detector provided in the half nut, in which, when a first mold thickness adjustment is performed after power is supplied to the mold clamping device, the control unit may move the mold clamping ram from a rearward limit, stop the mold clamping ram when the photoelectric detector detects that the annular convex parts have reached a position facing teeth of the half nut, and perform the mold clamping, and when the mold thickness adjustment is performed for a second time and subsequent times after power is supplied to the mold clamping device, the control unit may move the mold clamping ram to the movement position calculated on the basis of the difference value and then perform the mold clamping. 
     In the method of adjusting a mold thickness of a mold clamping device according to the disclosure, when a mold thickness is adjusted for a first time after power is supplied to the mold clamping device, the mold clamping ram may be moved from a rearward limit, the mold clamping ram may be stopped when it is detected that annular convex parts have reached a position facing teeth of the half nut, and the mold clamping may be performed, and when a mold thickness is adjusted for a second time and subsequent times after power is supplied to the mold clamping device, the mold clamping ram may be moved to the movement position calculated in the movement position calculation step and then the mold clamping may be performed. 
     According to the disclosure, at the time of the first mold thickness adjustment after power is supplied, since an engagement position of the annular convex parts engaged with the teeth of the half nut is detected by the photoelectric detector, and the mold clamping ram is moved with the engagement position as a reference when the mold is replaced thereafter, the engagement position of the half nut can be determined with higher accuracy than when only a difference value of mold thicknesses is detected to operate the mold clamping ram. 
     The mold clamping device of an injection molding machine of the disclosure may offset the movement position by a pitch of the annular convex parts when the movement position calculated by adding the current position of the mold clamping ram to the difference value does not fall within a range from a rearward limit position of the mold clamping ram to a forward limit position of the mold clamping ram. 
     Also, in the method of adjusting a mold thickness of a mold clamping device of the disclosure, the movement position calculation step may calculate a position that is offset from the movement position by a pitch of the annular convex parts as a new movement position when the movement position calculated by adding the current position of the mold clamping ram to the difference value does not fall within a range from a rearward limit position of the mold clamping ram to a forward limit position of the mold clamping ram. 
     According to the disclosure, when the movement position of the mold clamping ram calculated on the basis of the difference value of the mold thicknesses is outside a stroke range of the mold clamping ram, a new movement position that is offset from the original movement position by a pitch of the annular convex parts is calculated, and thereby the mold clamping ram can be moved to an appropriate engagement position with the half nut. 
     According to the disclosure, it is possible to provide a mold clamping device of an injection molding machine and a method of adjusting a mold thickness of the mold clamping device in which a mold thickness difference of molds before and after replacement is automatically calculated and the mold clamping ram is moved on the basis of the mold thickness difference when the mold is replaced, and thereby a time for mold thickness adjustment can be greatly reduced, and furthermore, an engagement position of the annular convex parts engaged with the teeth of the half nut is detected by the photoelectric detector, and thereby an accurate engagement position of the half nut can be calculated even with a shortest time of mold thickness adjustment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view illustrating a mold clamping device  100  according to an embodiment of the disclosure. 
         FIG. 2  is a longitudinal sectional view illustrating a main part of the mold clamping device  100  of  FIG. 1  on a rear side with respect to a movable platen  4 . 
         FIG. 3  is a cross-sectional view illustrating a structure of a half nut device  20  and its surroundings along line Y-Y of  FIG. 1 . 
         FIG. 4A  is an enlarged cross-sectional view illustrating the half nut device  20  of  FIG. 3 .  FIG. 4B  is an enlarged view of the half nut device  20  of  FIG. 3  along line X-X of  FIG. 4A . 
         FIG. 5  is a schematic view illustrating a ram position detecting member  70  and a ram position control member  90  according to the embodiment of the disclosure. 
         FIG. 6  is a flowchart showing a first mold thickness adjustment step of the mold clamping device  100  according to the embodiment of the disclosure after power is supplied. 
         FIG. 7A  is a schematic view illustrating a positional relationship between annular convex parts  31   a,  half nuts  21 , and a detection optical axis B of a photoelectric detector  80  in a process in which mold thickness adjustment is performed in the mold clamping device  100  when the annular convex parts  31   a  do not block the detection optical axis B. 
         FIG. 7B  is a schematic view illustrating a positional relationship between the annular convex parts  31   a,  the half nuts  21 , and the detection optical axis B of the photoelectric detector  80  in a process in which mold thickness adjustment is performed in the mold clamping device  100  when the annular convex parts  31   a  block the detection optical axis B. 
         FIG. 8  is a flowchart showing a mold thickness adjustment step at the time of mold replacement performed by the mold clamping device  100  according to the embodiment of the disclosure. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments of the disclosure will be described with reference to the drawings. In the present specification, a moving direction of a mold clamping ram  51  is referred to as a front-rear direction, a direction in which the mold clamping ram  51  moves to a fixed platen  2  is forward, and an opposite direction thereof is rearward. Also, a direction from the mold clamping ram  51  to the fixed platen  2  is referred to as a positive direction, and an opposite direction thereof is referred to as a negative direction. 
     As illustrated in  FIG. 1 , the mold clamping device  100  includes the fixed platen  2  fixed to a machine base  1  and a support platen  3  slidable on the machine base  1 . A movable platen  4  is movably supported along four tie bars  5  crossing between the fixed platen  2  and the support platen  3 . A fixed mold  6  is attached to the fixed platen  2 , and a movable mold  7  is attached to the movable platen  4 . The movable platen  4  moves forward with respect to the fixed platen  2  to perform mold closing and moves rearward to perform mold opening. After the mold closing, the movable platen  4  is further pressed against the fixed platen  2  to perform mold clamping. 
     When a servomotor  11  fixed to the support platen  3  rotates a mold opening/closing screw shaft  41  to be described below, a mold clamping shaft  31  moves the movable platen  4  forward toward the fixed platen  2  to perform the mold closing. Then, the mold clamping ram  51  incorporated in the support platen  3  presses the movable platen  4  to the fixed platen  2  side via a half nut device  20  attached to a forward end of the mold clamping ram  51 , and thereby the mold clamping is performed. As illustrated in  FIG. 3 , the mold clamping shaft  31 , the half nut device  20 , and the mold clamping ram  51  are coaxially disposed. 
     Next, the movable platen  4  moves rearward to perform the mold opening. At the time of the mold clamping, the tie bars  5  extend and the support platen  3  moves rearward by a mold clamping allowance W corresponding to a mold clamping force as in conventional cases. A ram position detecting member  70  to be described below is provided in the support platen  3 , and a photoelectric detector  80  is provided in the half nut device  20 . 
     As illustrated in  FIG. 2 , a back-surface support member  40  having an extending part that extends in a front-rear direction in a cylindrical shape is formed is fixed to a rear surface of the support platen  3  by a flange part  40   f  thereof. On an inner side of the back-surface support member  40 , an inner cylinder surface  40   h  is formed through the extending part in the front-rear direction. A pair of bearings  43  are housed in a rear end member  46  that closes a rear end of the back-surface support member  40 , and the mold opening/closing screw shaft  41  is rotatably supported by the bearings  43 . A ball screw is formed on an outermost circumference of the mold opening/closing screw shaft  41 , and a pulley  13  is fixed to a shaft head at a rear end thereof. The servomotor  11  is fixed to a motor base attached to the rear end member  46 . A pulley  12  is fixed to an output shaft of the servomotor  11 . A timing belt  14  is stretched between the pulley  12  and the pulley  13 . An encoder  111  is incorporated in the servomotor  11 , and thereby a position in the front-rear direction of the movable platen  4  is detected, and a mold opening/closing operation is controlled. 
     The mold clamping shaft  31  is fixed to a rear surface of the movable platen  4  so that axial centers thereof coincide with each other, and annular convex parts  31   a  each having a triangular cross section are formed at a constant pitch on an outer circumference of the mold clamping shaft  31 . A hollow hole is formed along the axial center of the mold clamping shaft  31 , and a mold opening/closing nut  42  is fixed to a rear end thereof. The mold opening/closing nut  42  is a ball nut and is screwed onto the mold opening/closing screw shaft  41 . In this way, a mold opening/closing mechanism is configured such that the mold opening/closing screw shaft  41  is coaxially and compactly housed in the hollow hole of the mold clamping shaft  31 , and the mold clamping shaft  31  moves as the mold opening/closing screw shaft  41  rotates. 
     Also, a rear end member  33  is integrally attached to a rear end of the mold clamping shaft  31 , and a slide bearing bush  34  is fitted therein. The rear end member  33  is housed to be movable in the inner cylinder surface  40   h  of the back-surface support member  40 . In this way, when the mold is opened and closed, the mold clamping shaft  31  is configured to be guided by the inner cylinder surface  40   h  of the back-surface support member  40  and move. A predetermined amount of a lubricating oil is stored in a vacant chamber  32  surrounded by the inner cylinder surface  40   h  of the back-surface support member  40 , the rear end member  46 , the rear end member  33  of the mold clamping shaft  31 , and the mold opening/closing nut  42 . An air breather  44  is attached to a through hole opening in the back-surface support member  40  on a side above the vacant chamber  32  and configured such that air in the vacant chamber  32  comes in and out of the air breather  44  when a volume of the vacant chamber  32  changes according to opening and closing of the mold. In this way, an oil bath lubrication method is employed so that frictional resistance is reduced when the slide bearing bush  34  of the mold clamping shaft  31  slides. 
     A mold clamping cylinder hole  50  is formed in the support platen  3 , and the mold clamping ram  51  is housed in the mold clamping cylinder hole  50  to be movable forward and rearward. A front surface support member  30  closes the mold clamping cylinder hole  50  to be liquid-sealed and supports the mold clamping ram  51  in a liquid-sealed state. A mold opening oil chamber  91  and a mold clamping oil chamber  92  are formed in front of and behind a piston part  51   a  of the mold clamping ram  51 . A pressure oil is supplied to the mold opening oil chamber  91  and the mold clamping oil chamber  92  by a ram position control member  90  serving as a hydraulic control unit to be described below. 
     A control unit  15  in  FIG. 1  performs control of the entire mold clamping device  100  and may be configured by, for example, a computer or a microcomputer having an arithmetic processing unit such as a central processing unit (CPU) and a memory. The control unit  15  performs control of the entire operation on the basis of a program stored in a storage unit  16  or an input signal from the outside. 
     The storage unit  16  may include, for example, a read-only memory (ROM), a random-access memory (RAM), a hard disk, or the like, and stores programs of various types and data. 
     As illustrated in  FIGS. 3, 4A, and 4B , the half nut device  20  is attached such that a center thereof coincides with a shaft center of the mold clamping shaft  31  and includes a pair of half nuts  21 . The half nuts  21  are provided on both sides of the mold clamping shaft  31  and are guided by a guide rail  23 . Air cylinders  22  and  22  are provided so that the half nuts  21  advance and retreat with respect to the mold clamping shaft  31 , and rods thereof are coupled to the half nuts  21  and  21 . A sensor  22   b  which detects that each of the half nuts  21  has advanced and engaged with the mold clamping shaft  31  and a sensor  22   a  which detects that each of the half nuts  21  has retreated and released the engagement with the mold clamping shaft  31  are attached to the air cylinders  22 . Further, in order to help understanding, the half nut  21  illustrated on an upper side on the paper surface indicates a state in which it has retreated, and the half nut  21  illustrated on a lower side on the paper surface indicates a state in which it has advanced. As a matter of course, the half nuts  21  advance and retreat with respect to the mold clamping shaft  31  at the same time before and after a mold clamping operation. When they are implemented, the half nuts are preferably disposed to open and close laterally. This is because the air cylinders  22  disposed to face both side surfaces of the mold clamping device are convenient for maintenance. 
     Teeth  21   a  that engage with the annular convex parts  31   a  of the mold clamping shaft  31  described above are formed on an end surface of the half nut  21  facing the mold clamping shaft  31 . The teeth  21   a  are formed over substantially a semi-circumference to be in contact and engage with the annular convex parts  31   a  in an area as large as possible. When the mold clamping force is significantly large, it is configured to be formed with three or more teeth. By doing as such, the mold clamping force is applied to the entire surface of engaged teeth faces of the annular convex parts  31   a  via the teeth  21   a,  and thereby wear of the half nut  21  and the mold clamping shaft  31  is made small. Also, since one mold clamping ram  51  presses one mold clamping shaft  31  coaxially in the mold clamping device  100  via one set of the half nuts  21  to perform the mold clamping, the mold clamping force is accurately and evenly applied to the movable platen  4 . 
     The mold clamping device  100  further includes the photoelectric detector  80 , the ram position detecting member  70 , and the ram position control member  90 . 
     The photoelectric detector  80  detects that the annular convex parts  31   a  have approached to a predetermined position and is constituted by a light emitting element  83  that outputs light, a light receiving element  84  that detects light from the light emitting element  83 , and a bracket  82 . As illustrated in  FIGS. 4A, 7A, and 7B , the light emitting element  83  and the light receiving element  84  are respectively disposed at front ends of circumferential edges of the teeth  21   a  of the half nuts  21  and  21 . 
     The photoelectric detector  80  outputs light from the light emitting element  83  to the light receiving element  84  while the mold clamping ram  51  moves forward in mold thickness adjustment as will be described below and detects a position of the annular convex parts  31   a  of the mold clamping shaft  31 . Specifically, when the mold clamping ram  51  moves forward with the half nuts  21  opened, positions of the annular convex parts  31   a  can be detected when corners of the annular convex parts  31   a  cross a detection optical axis B. 
     As illustrated in  FIG. 5 , the ram position detecting member  70  is attached between the support platen  3  and the mold clamping ram  51 . The ram position detecting member  70  is constituted by a rotary encoder  75  fixed to a back surface of the support platen  3  by a bracket  76 , a pinion  74  fixed to a detection shaft of the rotary encoder  75 , a connecting member  71  fixed to a front end surface of the mold clamping ram  51 , an operating rod  72  connected to the connecting member  71  and attached to be movable back and forth through the support platen  3 , and a rack  73  that is coaxially fixed to a distal end of the operating rod  72  and engages with the pinion  74 . A position in the front-rear direction of the mold clamping ram  51  that moves forward and rearward with respect to the support platen  3 , that is, a position in the front-rear direction of the half nut device  20  is detected by the ram position detecting member  70  using the rotary encoder  75  as a position in which the rack  73  moves forward and rearward. Further, reference sign  77  is a sliding bearing of the operating rod  72 , and reference sign  78  is a bearing of the rack  73 . 
     The ram position control member  90  is a member that moves the mold clamping ram  51  with respect to the support platen  3  using a hydraulic force and is connected to hydraulic lines c and d communicating with the mold opening oil chamber  91  and the mold clamping oil chamber  92  in the front and rear of the piston part  51   a  of the mold clamping ram  51 . The ram position control member  90  is a hydraulic control unit including at least a directional control valve  95 , a check valve  94  provided in the hydraulic line c, a check valve  93  provided in the hydraulic line d, a hydraulic pump P that supplies a hydraulic operating oil to the directional control valve  95 , and a hydraulic tank T. Here, the hydraulic pump P is a pump capable of variably controlling a discharge pressure and a flow rate. Also, the directional control valve  95  is a four-port three-position switching valve of double solenoids a and b, and an intermediate position thereof is a control valve in which only a P port from the hydraulic pump P is blocked. Also, the check valves  93  and  94  are valves in which a check valve opens due to a pilot pressure from the outside, and a pilot piping is made so that hydraulic oils of the hydraulic lines c and d of the two check valves are supplied to each other as an external pilot pressure for the other check valve. 
     Control of moving a position of the mold clamping ram  51  with respect to the support platen by the ram position control member  90  configured as described above is performed as follows. When the mold clamping ram  51  is moved rearward, control of moving the mold clamping ram  51  rearward is performed by exciting the solenoid b of the directional control valve  95  and supplying the hydraulic oil to the mold opening oil chamber  91 . At this time, the check valves  93  and  94  in the middle of the hydraulic lines hold the position of the mold clamping ram by storing a predetermined pressure oil in the mold opening oil chamber  91  and the mold clamping oil chamber  92 . On the other hand, when the mold clamping ram  51  is moved forward, the control of moving the mold clamping ram  51  forward is performed by exciting the solenoid a of the directional control valve  95  and supplying the hydraulic oil to the mold clamping oil chamber  92 . 
     Although a mold clamping operation and a depressurization operation are also controlled by the ram position control member  90 , since a hydraulic valve required for controlling such an operation, for example, a valve for preventing a depressurization shock, is not changed from that of conventional cases, illustration and explanation are omitted. 
     Flow of First Mold Thickness Adjustment Step after Power is Supplied 
     A flow of a first mold thickness adjustment step performed for the first time after power is supplied to the mold clamping device  100  is different from a flow of a mold thickness adjustment step performed when the mold is replaced thereafter. Here, the flow of the first mold thickness adjustment step after power is supplied will be described with reference to  FIG. 6 . 
     When the mold thickness adjustment is performed for the first time after power is supplied, in step  101 , a mold is attached to the fixed platen  2  and the movable platen  4 , and the servomotor  11  is driven to perform a mold closing operation. Specifically, the control unit  15  drives the servomotor  11  fixed to the support platen  3  to rotate the mold opening/closing screw shaft  41  and move the mold clamping shaft  31  forward, and then the movable platen  4  moves forward toward the fixed platen  2  to perform the mold closing. In step  102 , the control unit  15  excites the solenoid b of the directional control valve  95  to open the mold clamping oil chamber  92  to the hydraulic tank T side, and then a low-pressure hydraulic oil is supplied to the mold opening oil chamber  91  to move the mold clamping ram  51  rearward to a rearward limit position. When the mold clamping ram  51  is moved to the rearward limit position, in step  102 , the half nuts  21  are opened by the air cylinders  22 . 
     When the mold closing operation is completed, in step  103 , the control unit  15  detects a current position O P1  of the movable platen  4  in the front-rear direction using the encoder  111  incorporated in the servomotor  11  and stores the position in the storage unit  16  as an original position O PL . 
     In step  104 , the control unit  15  excites the solenoid a of the directional control valve  95  to supply a low-pressure and low flow rate hydraulic oil from the pump P to the mold clamping oil chamber  92  and opens the mold opening oil chamber  91  to the tank T side to move the mold clamping ram  51  forward slowly from the rearward limit position with a weak thrust. 
     In step  105 , the control unit  15  detects a position of the annular convex parts  31   a  using the photoelectric detector  80 . When the photoelectric detector  80  detects that the annular convex parts  31   a  have reached a position facing the teeth  21   a  of the half nuts  21 , the processing proceeds to step  106 , and the control unit  15  returns the directional control valve  95  to the intermediate position and stops the mold clamping ram  51  at that position. Otherwise, the processing returns to step  104 . 
     Following step  106 , the control unit  15  closes the half nuts  21  using the air cylinders  22  in step  107 . In steps  108 , the control unit  15  performs a mold clamping operation. After the mold clamping operation, in step  109 , the position of the mold clamping ram  51  in the front-rear direction is detected by the rotary encoder  75  and is stored in the storage unit  16  as a current position O KL  of the mold clamping ram  51 . 
     Flow of Mold Thickness Adjustment Step When Mold is Replaced 
     After the first mold thickness adjustment after power is supplied is completed, the mold may be replaced in order to perform injection molding on another mold, in which case it is necessary to perform the mold thickness adjustment again. A flow of the mold thickness adjustment step in that case will be described below with reference to  FIG. 8 . 
     At the time of mold replacement, a mold to be replaced is attached to the fixed platen  2  and the movable platen  4 , and the control unit  15  drives the servomotor  11  to perform a mold closing operation in step  201 . The control unit  15  drives the servomotor  11  fixed to the support platen  3  to rotate the mold opening/closing screw shaft  41 , and thereby the mold clamping shaft  31  moves forward, and the movable platen  4  moves forward toward the fixed platen  2  to perform the mold closing. 
     When the mold closing operation is completed, a current position O PT  of the movable platen  4  in the front-rear direction is detected by the encoder  111  incorporated in the servomotor  11 . In step  202 , the control unit  15  calculates D P , which is a difference value between the original position O PL  stored in the storage unit  16  and the current position O PT  of the movable platen  4 , using Math. 1 and stores it in the storage unit  16  (difference value calculation step). 
         D   P   =O   PT   −O   PL   (1)
 
     D P  is a difference in positions of the movable platen  4  in the front-rear direction caused when molds having different mold thicknesses are attached. 
     In step  203 , the control unit  15  updates the current position O PT  of the movable platen  4  to an original position O PL  of the mold opening and closing as a new original position and stores it in the storage unit  16 . 
     Next, in step  204 , the control unit  15  calculates a position to which the mold clamping ram  51  moves. Specifically, a movement position O KT1  of the mold clamping ram  51  is calculated from the current position O KL  of the mold clamping ram  51  and the difference value D P  using Math. 2 (movement position calculation step). 
         O   KT1   =O   KL   +D   P   (2)
 
     In step  205 , the control unit  15  determines whether or not the calculated movement position O KT1  of the mold clamping ram  51  is between a rearward limit position O KB  and a forward limit position O KF  of the mold clamping ram  51 . The rearward limit position O KB  and the forward limit position O KF  are stored in the storage unit  16  in advance, and the control unit  15  reads them for comparison with the movement position O KT1 . 
     When the movement position O KT1  is between the rearward limit position O KB  and the forward limit position O KF  in step  205 , in step  208 , the control unit  15  moves the mold clamping ram  51  to the movement position O KT1  of the mold clamping ram  51  calculated by Math. 2 to perform the mold clamping operation. 
     Specifically, the control unit  15  excites the solenoids a and b of the directional control valve  95  to supply a low-pressure hydraulic oil to the mold clamping oil chamber  92  or the mold opening oil chamber  91  and move the mold clamping ram  51  forward or rearward with respect to the movement position O KT1 . Thereafter, the mold clamping ram  51  is moved to the movement position O KT1  by the ram position detecting member  70 , the mold clamping ram  51  is stopped, and the half nuts  21  are opened to perform the mold clamping operation (mold clamping step). 
     When the movement position O KT1  is positioned on a rear side with respect to the rearward limit position O KB  in step  205 , in step  206 , the control unit  15  offsets the movement position O KT1  forward by a pitch size SN of the annular convex parts  31   a.  Specifically, the control unit  15  reads the pitch size SN of the annular convex parts  31   a  stored in the storage unit  16  in advance, and offsets the movement position O KT1  of the mold clamping ram  51  in the positive direction by one pitch interval of the annular convex parts  31   a  to be between the rearward limit position O KB  and the forward limit position O KF  as described in Math. 3. The control unit  15  sets the position as a new movement position O KT2  of the mold clamping ram  51  (movement position calculation step). The processing proceeds to step  208  following step  206 . Here, the control unit  15  moves the mold clamping ram  51  to the movement position O KT2  of the mold clamping ram  51  and performs the mold clamping operation. 
         O   KT2   =O   KT1   +S   N   (3)
 
     When the movement position O KT1  is positioned on a front side with respect to the forward limit position O KF  in step  205 , in step  207 , the control unit  15  offsets the movement position O KT1  in the negative direction by the pitch size S N  of the annular convex parts  31   a.  Specifically, the control unit  15  reads the pitch size S N  of the annular convex parts  31   a  stored in the storage unit  16 , and offsets the movement position O KT1  of the mold clamping ram  51  in the negative direction by one pitch interval of the annular convex parts  31   a  so that the movement position O KT1  of the mold clamping ram  51  falls between the rearward limit position O KB  and the forward limit position O KF  as described in Math. 4. The control unit  15  sets the position as a new movement position O KT3  of the mold clamping ram  51  (movement position calculation step). The processing proceeds to step  208  following step  207 . Here, the control unit  15  moves the mold clamping ram  51  to the movement position O KT3  of the mold clamping ram  51  and performs the mold clamping operation. 
         O   KT3   =O   KT1   −S   N   (4)
 
     After the mold clamping operation in step  208 , in step  209 , the control unit  15  stores the position of the mold clamping ram  51  detected by the rotary encoder  75  in the storage unit  16  as the current position O KL  of the mold clamping ram  51 . 
     As described above, in the mold thickness adjustment at the time of mold replacement, since a mold thickness difference of the molds between before and after the replacement is automatically calculated using a difference in the mold closing position of the movable platen  4  before and after the mold replacement, and the mold clamping ram  51  moves according to the mold thickness difference, a step of moving the mold clamping ram  51  rearward to the rearward limit and detecting the annular convex parts  31   a  of the mold clamping shaft  31  each time the mold is replaced can be omitted, and a time for the mold thickness adjustment can be reduced. 
     Also, at the time of the first mold thickness adjustment after power is supplied in the mold clamping device  100 , since an engagement position of the annular convex parts  31   a  engaged with the teeth  21   a  of the half nuts  21  is detected by the photoelectric detector  80 , and the mold clamping ram  51  is moved with the engagement position as a reference when the mold is replaced thereafter, the engagement position of the half nuts  21  can be determined with higher accuracy than with operation in which only a mold thickness difference is detected. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed embodiments without departing from the scope or spirit of the disclosure. In view of the foregoing, it is intended that the disclosure covers modifications and variations provided that they fall within the scope of the following claims and their equivalents.