Abstract:
A new clamping machine is disclosed, which can perform a straight-hydraulic mold clamping and a tie-bar drawing-out operation in a reduced working space, and enables a shortened mold exchanging time, by effecting retraction of a movable die plate and a tie-bar drawing-out operation simultaneously. In order to draw out tie bars  18  from a fixed die plate  10  only when molds are exchanged, annular pistons  36 , which construct mold clamping cylinders in the fixed die plate, are provided. Abutting each piston, a pair of half nuts  48 A,  48 B are arranged and driven to engage with an engagement portion  18 A provided at one end of each tie bar on the side of the fixed die plate, usually, other than during the drawing out operation. A relative position of each tie bar to the half nuts is set to be able to engage with the half nuts when a right end face of the tie bar contacts with the stopper plate  46 . When pressure for clamping molds is increased during such engagement, the annular pistons are driven to transmit a force, for performing an initial operation, to the tie bars.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2004-179025, filed in Japan on Jun. 17, 2004. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a clamping machine, such as die-cast machines or injection molding machines (hereinafter, referred to as molding machines), and particularly to a clamping machine which effects a drawing-out operation of tie bars on exchanging molds using a driving mechanism of movable die plates. 
     2. Prior Art 
     In clamping machines, such as die-cast machines, tie bars may affect both new and used molds when they are exchanged. In particular, in the case of die-cast machines, one or more core cylinders are often attached to a mold in the horizontal direction. In such a case, tie bars should be drawn out from fixed die plates, prior to exchange of molds, such that the tie bars will have no influence on the core cylinder. 
     In conventional clamping machines, special hydraulic cylinders are provided for drawing out the tie bars. However, the hydraulic cylinder of this type has a relatively long cylinder stroke, and thus tie bars are to be moved behind to a distance. Therefore, a predetermined space accommodating the movement should be provided behind the machine. In the case of large-sized die-cast machines, in addition to a long stroke of the special hydraulic cylinder, heavy tie bars themselves are also responsible for lengthening their moving time. Moreover, since the tie bars should be drawn into fixed fie die plates again after new molds are attached to the device, it should take a considerably long time to carry out the whole cycle for exchanging molds. 
     In conventional clamping machines, devices which are driven by a toggle link mechanism have been widely used. In the case of die-cast machines provided with such a toggle mechanism, however, a link housing is arranged for adjusting the mold thickness to be carried out after exchanging new and used molds. Thus, the entire length of these machines should be considerably long. Accordingly, while the clamping machines of such a toggle link type have a merit in that both of the make-and-break (opening-and-closing) operation and the clamping operation of the molds can be performed by a toggle link mechanism, another type of devices, which include a driving mechanism for the opening-and-closing operation and another driving mechanism for the clamping operation in place of the toggle link mechanism, have emerged recently. 
     In this case, a clamping cylinder for the clamping operation is arranged on the side of the fixed die plate or of the movable die plate. Since the clamping cylinder cannot accommodate adjustment of the mold thickness alone, various mechanisms have been proposed for driving the clamping operation related to adjustment of the mold thickness. 
     However, we have not known so far any clamping devices in which efficiency of the above-described drawing-out operation of tie bars and simplicity of the mechanisms are well considered. 
     Among clamping devices including a clamping cylinder arranged on the side of the fixed die plate, an example of the prior art, which enables tie bars to be drawn out with ease using a make-and-break operation of the movable die plate will be explained with reference to Japanese Patent No. 1996-5060 (TOKUKOUHEI No. 8-5060). 
       FIGS. 7 and 8  illustrate a clamping machine disclosed in Japanese Patent No. 1996-5060, respectively.  FIG. 7  is a front view of a clamping machine, showing a state in which tie bars are drawn out from a fixed die plate, and  FIG. 8  is a view of the clamping machine in  FIG. 7 , showing a state immediately prior to closing molds. 
     In  FIGS. 7 and 8 ,  201  is a fixed die plate which is fixed to one end of a base (not shown), and to which a fixed mold  202  is attached.  203  is a movable die plate which is mounted to the base movably in both front and back directions, and to which a movable mold  204  is attached.  205  are tie bars, and one end of each tie bar is fixed to the movable die plate  203  with a nut  207 . The other end of each tie bar  205  includes a screw portion  206  which mates or engages with half nuts  208 . On the rear side of a mold attachment surface of fixed die plate  201  a half nut attachment plate  210  is attached, and four half nuts  208  are provided corresponding to each tie bar  205  across the attachment plate  210 . Each half nut  208  is opened and closed, for example, by an effect of a hydraulic cylinder  209  or the like, and is constructed to mate with the corresponding screw portion  206  of the tie bar. The half nut attachment plate  210  is attached to the fixed die plate  201  so as to enable the horizontal movement of the plate  210  in the same direction as the moving direction of movable die plate  203  using a linear guide  211  at a lower surface of the fixed die plate  201  and a linear guide  212  (a bush or the like) at a side surface thereof. 
     The linear guide  212  is composed of a guide bar  213  fixed to the half nut attachment plate  210 , a bush  214  fixed to the fixed die plate  201 , and a compression spring  215  attached between the guide bar  213  and the bush  214 . The compression spring applies a force to the half nut attachment plate  210  such that it is always pulled toward the fixed die plate  201 . 
       216  is a mold thickness adjusting device to adjust the tie bar screw  206  and the corresponding screws of half nuts  208  to always take positions for enabling their mating even when the thickness of the mold is changed. A pulse motor  220 , to which a screw  218  is connected directly, is attached rotatably to a rack  217 , and the rack  217  is connected to a bottom face of the movable die plate  203 .  221  is a body holding a nut  219 , and a rod  222  projecting toward the fixed dies plate  201  is fixed to the body  221 . 
       230  is a cylinder for opening and closing molds, in which a cylinder body is attached to the fixed die plate  201 , while a distal end of the cylinder rod is attached to the movable die plate  203 , so that the movable die plate  203  can be moved in both front and back directions.  231  is a cylinder for increasing clamping pressure, which is incorporated in the fixed die plate  201 .  232  is a ram which is moved in the right direction in  FIG. 6  by applying pressurized oil into an oil chamber  234  when the clamping pressure is increased, and pushes the half nut attachment plate  210  at its distal end  233 . In this case, the half nuts  208  mate with the corresponding screw  206  of the die bar, thereby to generate a clamping force.  235  is also an oil chamber. 
     Next, the operation of this clamping machine is explained. 
     When pressurized oil is supplied to the oil chamber on the rod side of the mold opening-and-closing cylinder  230 , the movable die plate  203  advances toward the fixed die plate  201  to perform a mold closing operation.  FIG. 8  illustrates a state immediately prior to closing molds when a distal end of the rod  222  of mold thickness adjusting device  216  contacts with the half nut attachment plate  210 . The mold closing operation further continues from this state, and the distal end of the rod  222  pushes the half nut attachment plate  210  so that the attachment plate  210  is moved in the horizontal direction following the guide of linear guides  211 ,  212 . When the fixed mold  202  and the movable mold  204  mate with each other completely, the mold closing operation is completed. Then, the horizontal movement of the half nut attachment plate  210  is stopped. 
     In  FIG. 8 , threads of the tie bar screw  206  and those of the half nuts  208  are shown to have substantially the same positions, so that the half nuts  208  cannot be closed because both of the thread portions meet on closing the half nuts in such a state. A distance that the half nut attachment plate  210  moves from a time when the rod  222  contacts with the half nut attachment plate  210  to a time when the mold closing is completed may translate to a distance to be corrected such that the tie bar screw  206  and the screws of half nuts  208  may mate properly with each other, and is determined automatically by a length L 1  of molds. For example, a distance L 0  from a mold attachment surface of the movable die plate  203  to a start point of cutting the tie bar screw  206  is predetermined, and thus the positions of peaks (or threads) and valleys of the screw can also be known in advance. 
     Meanwhile, with respect to the positions of peaks and valleys of the half nuts when the molds are closed and when the half nut attachment plate  210  is not moved horizontally, since the thickness L 1  of the molds, a length L 2  from a mold attachment surface of the fixed die plate  201  to a half nut attachment surface of the half nut attachment plate  210  are predetermined, so that the positions of peaks and valleys of the half nuts  208  can also be known in advance with calculation of the sum of (L 1 +L 2 ). Accordingly, comparing the positions of threads (peaks) of the screw in the tie bar  205  to the positions of valleys of the half nuts  208 , a displacement from a proper mating position can be known. This is a value to be adjusted and determined as a moving amount of the half nut attachment plate  210 . Namely, knowing only the mold thickness L 1 , the corresponding amount of adjustment can be known automatically. 
     The above-described amount of adjustment, i.e., the displacement amount of the attachment plate  210  is determined by a distance L 3  from the mold attachment surface  203  of movable die plate  203  to the distal end of rod  221  of the mold thickness adjusting device  216 . In this case, a pulse signal corresponding to the amount of adjustment is transmitted to a pulse motor  220  to rotate a screw  218 . Thus, the rod  222  is moved to set a position of the attachment plate  210 . Accordingly, in the mold closing operation immediately after attachment of molds, the distance L 3  is set at an original point. When a completely mold-closed state is reached, the mold thickness L 1  is detected by an automatic reading apparatus (not shown), calculating the amount of adjustment, projecting the rod  222  by driving the pulse motor  220 , and moving the half nuts by the calculated distance. In this stage, the projected rod  222  may not be moved again as long as the same molds are used, so that it could effect opening-and-closing operations repeatedly while remaining in a fixed state. After the molds are closed and half nuts  208  are moved by the calculated distance, the half nuts  208  are closed together by an effect of the hydraulic cylinder  209 . 
     Next, when supplying pressurized oil into the oil chamber  234  of oil hydraulic cylinder  231 , the ram  232  is moved toward the half nut attachment plate  210  and pushes the plate at its distal end  232 . Thus, the attachment plate  210  and the half nuts  208  are urged in the right direction, with the half nuts  208  meshing with the tie bar screw  206 . In this way, a force for clamping the molds is generated. 
     When the molds are opened after the molding operation, the pressure in the oil chamber  234  of hydraulic cylinder  231  is lowered, and the half nuts  208  are opened by the effect of the hydraulic cylinder  209 . Thereafter, when supplying pressurized oil on the side of the head of mold opening-and-closing cylinder  230 , the movable die plate  203  is urged in the left direction to move the molds. At the same time, the half nut attachment plate  210  is moved toward the fixed die plate  201  by extension of the compression spring  215  and returns to its original position. In this case, besides the extension force of compression spring  215 , the restoring force may be augmented positively by supplying pressurized oil into the oil chamber  235  of hydraulic cylinder  231 . The tie bar screw  206  and half nuts  208  may be replaced by a plurality of circumferential grooves. 
     However, in the clamping machine disclosed in Japanese Patent No. 1996-5060, while adjustment of the mold thickness is facilitated as described above, the device has the following four problems. 
     (1) Since one end of each tie bar  205  is fixed securely to the movable die plate  203 , the tie bar  205  should be drawn out from the fixed die plate  201  every time the die plate  203  is moved for a mold opening-and-closing operation during a molding cycle, the energy consumed is considerably large. 
     (2) The tie bars  205 , when the molds are opened, are supported in a cantilever state by the movable die plate  203 . Therefore, the tie bars may tend to bend in the operation. 
     (3) While the half nuts  208  for mating with the screw portion  206  of each tie bar  205  is provided on the side of the fixed die plate  201 , the mold thickness adjusting device  216  is located on the side of the movable die plate  203 . Displacement by a predetermined distance of a lower portion of the half nut attachment plate  210  using the rod  222  against a bias force of the spring  215  requires a complicated mechanism. 
     (4) The ram  232 , which is a piston of the cylinder  231  for increasing the clamping pressure, presses the half nut attachment plate  210  at its right end on increasing the clamping pressure, and each tie bar  205  is moved in the right direction by the force transmitted thereto through the screw portion  206  mating with the half nuts  208 , thus applying a clamping force to the movable die plate  203 . However, when opening the molds after a product is molded, application of pressure to the ram  232  is stopped, and it is not involved in the initial mold-opening operation. Namely, including the initial mold-opening operation, the whole cycle for opening the molds is performed by the mold opening-and-closing cylinder  230 . Since a considerably large force is required for the initial mold opening operation, a hydraulic cylinder which can generate a large power sufficient for moving the movable die plate  203  should be used as the cylinder  230 . 
     We found that the above-mentioned problems can be solved by providing a construction such that one end portion of each tie bar is always connected with a fixed die plate, except of the operation for drawing out the tie bar on exchanging molds, in a straight-hydraulic mold clamping machine which applies a clamping force to molds using a clamping cylinder comprising annular pistons arranged on the side of the fixed die plate. 
     Accordingly, it is an object of the present invention to provide a new clamping machine which can perform a straight-hydraulic mold clamping and a tie-bar drawing-out operation in a reduced working space, and enables a shortened mold exchanging time, without using a drawing-out cylinder exclusively used for drawing out tie bars, by effecting retraction of a movable die plate and a tie-bar drawing-out operation simultaneously. 
     In order to accomplish the above-mentioned object of the present invention, the clamping system according to the present invention is provided mold opening and closing means for driving said movable die plate to advance to and retract from said fixed die plate; tie bars each extending through said movable die plate and said fixed die plate for guiding said movable die plate, and having a first engagement portion in which a groove is formed at a rear end on the side of said movable die plate and a second engagement portion in which a groove is formed at a front end on the side of said fixed die plate; a mold clamping cylinder including annular pistons in which said tie bars are fitted slidably and concentrically, first oil chambers each having a larger pressure receiving surface disposed on one side of each annular piston and generating a clamping force to clamp said fixed mold and movable mold subjected to a mold closing operation by an effect of pressurized oil supplied to the surface, and second oil chambers each having a smaller pressure receiving surface on the other side of each annular piston and generating an initial releasing force on an initial mold opening operation by an effect of pressurized oil supplied to the surface, and being incorporated in said fixed die plate; first connecting means disposed on an opposite side of a mold attachment surface of said fixed die plate, having half nuts for engaging with said first connecting portions on the side of said fixed die plate, for connecting distal ends of said tie bars to said die plate detachably; second connecting means disposed on an opposite side of a mold attachment surface of said movable die plate, having half nuts for engaging with said second connecting portions on the side of said movable die plate, for connecting rear ends of said tie bars to said movable die plate detachably; and force transmitting means for connecting said annular pistons of the mold clamping cylinder and said first connecting means so that these are moved together, and transmitting, in cooperation with said first connecting means, an initial releasing force for pushing said tie bars toward said movable die plate and a mold clamping force for drawing said tie bars toward said fixed die plate, to said tie bars in corresponding directions, respectively. 
     According to the present invention, since a drawing-out tie-bar operation is effected by a driving means which effects also mold opening-and-closing operations of a movable die plate, there is no need to provide a hydraulic cylinder exclusively used for drawing out tie bars, and the operational cost can be saved due to simplification of the mechanism. 
     A usual operation is carried out, with tie bars being connected to a fixed die plate, and the drawing-out operation of tie bars is effected only when molds are exchanged. The energy can be saved, and the tie-bar drawing-out operation can be started only by disengaging a distal end of each tie bar from each half nut, which is a first engaging means on the side of the fixed die plate. Thus, the time required for starting a mold exchanging operation can be greatly reduced. 
     In a drawing-in operation of tie bars after exchanging molds, an axial position of each tie bar can be set only by contacting a distal end of the tie bar with a stopper. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a clamping system according to the present invention. 
         FIG. 2  is a view showing details of a mechanism of a connecting means shown in  FIG. 1 . 
         FIGS. 3A-3D  are schematic diagrams of a clamping system, illustrating initial four states (designated by numerals  1  to  4 ) of one cycle including the mold operating and closing operations and mold clamping operation of a mold clamping system according to the present invention. 
         FIGS. 4A-4D  are schematic diagrams of a clamping system, illustrating four states (designated by numerals  5  to  8 ) of one cycle including the mold operating and closing operations and mold clamping operation of a mold clamping system according to the present invention. 
         FIGS. 5A-5C  are diagrams illustrating three states (designated by numeral  9  to  11 ) of one cycle including the mold operating and closing operations and mold clamping operation of a mold clamping system according to the present invention. 
         FIGS. 6A-6B  are schematic diagrams of a mold clamping system, illustrating two states (designated by numerals  12 ,  13 ) of tie bars being drawn out. 
         FIG. 7  is a front view of a mold clamping system, illustrating a state of tie bars having been drawn out from a fixed die plate in a conventional mold clamping system. 
         FIG. 8  is a view showing a state immediately before closing molds in the mold clamping system of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention will be described with reference to  FIGS. 1 to 7 . 
       FIG. 1  is a plan view showing a clamping machine CLM according to an embodiment of the present invention. On a base frame BF, a fixed plate  10  and a movable plate  12  are arranged. A fixed mold  14  is attached to a mold attachment surface of the fixed die plate  10  which is standing on and fixed to the base frame BF. Facing the fixed die plate  10 , a movable mold  16  is attached to a mold attachment surface of the movable die plate  12  which is slidable in a direction of the machinery central axis (M/C) or in opening and closing directions. 
     As illustrated in  FIG. 1 , since components shown above the machinery central axis M/C and corresponding components below the axis M/C are symmetrical, only the components shown above the axis M/C will be explained hereinafter. 
     Reference numeral  18  designates a tie bar. On the left side end of tie bar  18  an engagement portion  18 B is provided, at which the tie bar  18  engages with a pair of half nuts  32 A,  32 B disposed on the side of movable die plate  12 . The tie bar  18  extends through the movable die plate  12 , and is slidable in the through hole. Further, on the right side end of tie bar  18  an engagement portion  18 A is provided, at which the tie bar  18  engages with a pair of half nuts  48 A,  48 B disposed on the side of fixed die plate  10 . Reference numeral  44  designates a hydraulic cylinder which open and close the half nuts  48 A,  48 B in a vertical direction to the machinery central axis M/C. 
     Reference numeral  20 , as shown in the drawing, is a tie bar support guide for preventing the tie bar  18  from being in a cantilever position when it is drawn out from the fixed die plate  10  on exchanging molds. The tie bar support guide  20  is fixed horizontally to the movable die plate  12 , and the tie bar  18  slides in the tie bar support guide. Reference numeral  34  is a hydraulic cylinder for driving a push pin (not shown) which separates a molded article remaining attached to the movable mold  16  after opening molds. 
     Reference numeral  26  designates a ball screw shaft which is driven to rotate by a servo motor  24 . In the clamping system of this embodiment, an electric mold opening-and-closing mechanism is provided as a driving mechanism for driving the movable die plate  12 . A ball nut  28  is fixed onto a support plate  22  which is attached to the movable die plate  12 . The ball nut  28  engages with the ball screw shaft  26 . The servo motor  24  is attached to a support frame (not shown) which is fixedly attached to the base frame BF. 
     Accordingly, with rotations of the ball screw shaft  26  driven by the servo motor  24  in both normal and reverse directions, the movable die plate  12  moves forward and backward in the machinery central axis direction, respectively, using the tie bar  18  as a guide through the ball nut  28 . 
     Reference symbol TCL designates a central axis of the tie bar  18 . 
     Next, clamping cylinder  40  is described. In this embodiment, a direct-hydraulic type cylinder  40  of a straight-hydraulic type is provided as a mechanism for generating a clamping force after closing molds. Reference numeral  36  is a piston of the clamping cylinder  40  disposed in the fixed die plate  10 . Namely, the annular piston  36  has an aperture H which is formed concentrically with the central axis TCL of tie bar  18 , and the tie bar  18  is inserted slidably in the aperture H. Reference numerals  38 ,  39  designate oil chambers of the clamping cylinder provided on both sides of the piston  36 . 
     Abutting a right end face of the annular piston  36 , a pair of half nuts  48 A,  48 B are disposed, which engage with the engaging portions  18  A of the tie bars  18 , respectively. Each of the half nuts  48 A,  48 B is connected to a piston rod of a hydraulic cylinder  44 . Engagement and disengagement of the half nuts  48 A,  48 B to the tie bar  18  is performed by moving these half nuts  48 A,  48 B forward and backward in a vertical direction to the central axis TCL. 
     In  FIG. 1 , reference numeral  30  denotes an encoder, which is a detector for discriminating and detecting a rotational amount and a rotational direction of the ball screw shaft  26 , and a detected signal is fed back to a controller  54  to generate a signal related to a current position of the die plate  12  in the direction of machinery central axis M/C. Further, controller  60  receives the detected signal as a feedback information and performs a positional control and a speed control of the movable die plate  12 . Reference numeral  55  is a sequencer which controls operations of the half nuts  32 A,  32 B, and half nuts  48 A,  48 B in accordance with an order predetermined in a series of molding cycles including mold opening-and-closing operations and clamping operations. 
     Reference numerals  56 ,  57  designate electromagnetic directional control valves, respectively. Among these valves, the directional control valve  56  is connected to the cylinder  32  for opening and closing the half nuts  32 A,  32 B on the side of movable die plate  12 , respectively. On the other hand, the directional control valve  57  is connected to the cylinder  44  for opening and closing the half nuts  48 A,  48 B on the side of the fixed die plate  10 , respectively. Further, reference numeral  58  is an electromagnetic directional control valve for controlling flow directions of pressurized oil which is supplied to the clamping cylinder  60 . 
       FIG. 2  illustrates details of a construction for coupling the half nuts  48 A,  48   b , which engage the engaging portion  18 A of tie bar  18  on the side of fixed die plate  10  of  FIG. 1 , with the clamping cylinder  40  integrally, and for transmitting a clamping force and a releasing force on an initial operation for opening molds to the tie bar in the respective directions. 
     Namely, in  FIG. 2 , a plate  52 A is fastened to an end face of piston  36  using a bolt. A plate  52 B is arranged in parallel to the plate  52 A with a space therebetween. These plates  52 A,  52 B are fixed to a plate  52 C to maintain a space such that the half nut  48 B can contacts slidably with the opposite surfaces of plates  52 A,  52 B, respectively. Further, the whole structure including these plates  52 A,  52 B,  52 C is attached movably to the fixed die plate  10  via a guide  42 A on a rack  42  which is secured horizontally to the die plate  10 , thereby to move in the axis TCL of tie bar  18  along the guide  42 A. In this case, similar to plate  46  shown in  FIG. 1 , after exchanging molds and when the tie bar  18  is inserted into the movable die plate  10 , the plate  52 B contacts with an end face of tie bar  18  and serves as a stopper for restricting a position of tie bar  18  in the axial direction. 
     Next, in  FIG. 2 , the operation of clamping cylinder  40  is described. In this case, it is assumed that an end face of the engagement portion  18 A of tie bar  18  is in contact with the plate  52  B, with the half nuts  48 A,  48 B engaging with the engagement portion  18 A. 
     When the movable mold  16  is contacted with the fixed mold  14  and the clamping pressure is further increased, an pressurized oil is supplied to the oil chamber  39 . Oil pressure in the oil chamber  39  applies to an end face of the annular piston  36  to generate a force for moving the annular piston  3  in the right direction. The force is then transmitted, as a clamping force, to the half nuts  48 A,  48 B through the plate  52 A, so that the tie bar  18  is drawn and extends into the fixed tie plate. 
     On the other hand, in order to release the movable mold  16  from the fixed mold  14  after steady clamping, an initial operation for applying a relatively great force to the movable die plate  12  via the tie bar  18 . When molds are opened, pressurized oil is first supplied to oil chamber  38 , and a releasing force (opening force) is generated by the effect of the pressurized oil on the end face of annular piston  36 , thereby moving the annular piton  36  in the left direction. This force is transmitted to the half nuts  48 A,  48 B interposed between the plates  52 B,  52 C, then to the tie bar  18 , so that the tie bar  18  is urged toward the movable die plate  12 . In this state, comparing the end face on the side of oil chamber  38  and that on the side of oil chamber  40 , the latter has a larger area for receiving a pressure. Therefore, it has a larger clamping force, and a difference is given between the clamping force and the releasing force. 
     In the engagement portions  18 A,  18 B of tie bar  18  and the corresponding engagement portions of half nuts  32 A,  32 B and  48 A,  48   b , grooves having a sawtooth-like or square-wave shape are formed. 
       FIGS. 3 to 5  are schematic plan views of a clamping system, illustrating representative eleven operations (designated by numerals  1  to  13 ) in one cycle of the mold opening and closing operations and mold clamping operation according to the present invention shown in  FIG. 1 . As shown throughout the eleven drawings, the half nuts  48 A,  48 B on the side of fixed die plate  10  are always kept to engage with the engagement portions  18 A of the tie bars, respectively. 
     In  FIGS. 3A-3D , among operations  1  to  4 , the die plate  12  starts to advance from a position shown in operation  1 . In this operation, each ball screw  26  is rotated at a high speed by the servo motor  24 , so that a mold closing operation, in which the movable die plate  12  is transferred at a high speed, is performed. When the movable die plate  12  reaches a predetermined position shown in operation  2 , the rotational speed of servo motor  24  is lowered, and the operation is switched to a low speed mold closing operation. This switching of the mold closing operation from a high speed to a low speed is conducted by the controller  60  in  FIG. 1 . 
     Next, the low speed mold closing operation proceeds, and immediately before the movable mold  16  contacts with the fixed mold  14  as shown in operation  3 , the state is changed from a low pressure clamping state to a mold contact state. In this state, the servo motor  24  is stopped, and the mold closing movement of movable die plate  12  is ended. 
     Operation  4  illustrates a mold thickness adjustment which is conducted first after exchanging molds. In this operation  4 , the engagement portions  18 B of tie bars  18  and the half nuts  32 A,  32 B remain disengaged before the mold thickness adjustment. In the mold thickness adjustment, after contacting both molds together, the position of each tie bar  18  is finely adjusted to match grooves of the engagement portion  18  with phases of the half nuts  32 A,  32 B, respectively, by slightly moving annular piston  36  of each clamping cylinder  40 . This operation is performed with a monitor using a sensor (not shown). The sensor detects a relative position(s) between the engagement portion  18 B of tie bar  18  and the half nuts  32 A,  32 B, and disposed in the vicinity of engagement portion  18 B, for example. After confirmation of the mold thickness adjustment, the sequencer  55  switches the magnetic directional control valve  56  to close the half nuts  32 A,  32 B. Thereafter, the engaged state between the half nuts  32 A,  32 B and the engagement portion  18 B remain until opening the molds. 
     Next,  FIGS. 4A-4D  and  FIGS. 5A-5C  show a series of operations of a molding cycle from clamping to removal of a molded article, respectively. 
     In operation  5  of  FIGS. 5A-5C , in order to carry out a mold clamping as stated above, pressurized oil is supplied to oil chamber  39  of each clamping cylinder  40 , and the annular piston  36  moves in a direction designated by an arrow to elevate the pressure and keep the mold clamping state. Then, injection filling of a molding material into a cavity defined by the molds is conducted. After a molded article is solidified in the cavity, the supply of pressurized oil into the oil chamber is stopped, and pressure release in the clamping cylinder  40  is conducted. In operation  6 , in order to remove the molded article, pressurized oil is supplied to each oil chamber  38  to carry out an initial operation for opening the molds. As stated above, the annular piston  36  moves in a direction designated by an arrow, and a releasing force is transmitted to the tie bars  18  to pull the movable die plate  12  in the left direction. In this way, the movable mold  16 , which cohered to the fixed mold due to the mold clamping, is separated from the fixed mold. Then, until the movable mold  16  is separated completely from the fixed mold, the high pressure mold opening operation is continued. 
     During the above high pressure mold opening operation, the servo motor  30  is kept in a state to enable a torque-free rotation. With retraction of the movable die plate  12 , each ball nut  28  rotates the ball screw shaft  26 , and the servo motor  30  rotates with the rotation of the ball screw shaft  26  without resistance. In this way, the movable mold  16  is retracted by the initial operation, and molds are opened by a predetermined amount. 
     Thereafter, in operation  7 , a mold opening operation due to an electric mold opening-and-closing mechanism, which is driven by the servo motor  30 , is ready to start. In this case, the sequencer  55  shown in  FIG. 1  switches the electromagnetic directional control valve  58  to stop the supply of pressurized oil to the oil chamber  38  of clamping cylinder  40 , while switching the electromagnetic directional control valve  56  to open the half nuts  32 A,  32 B on the side of movable die plate  12  and disengage the tie bars  18  from the movable die plate  12 . In this state, the fixed die plate  10  and the tie bars  18  are kept to connect with each other by the half nuts  48 A,  48 B. 
     Next, in operation  8 , the servo motor  30  starts, and the operation is changed to a low speed mold opening operation. Since distal ends of the tie bars are fixed to the fixed die plate  10 , the movable die plate  12  is retracted, sliding around the tie bars  18 . When the movable die plate  12  is retracted to a position shown in the operation  8 , the rotational speed of servo motor  30  is switched to a high speed, and the operation is changed to a high speed mold opening operation. In  FIGS. 5A-5C , when the movable die plate  12  comes near a retracted position shown in operation  9 , the operation is changed to a low speed mold opening operation again, and is stopped a predetermined position shown in operation  10 . Thereafter, as shown in operation  11 , a molded article remaining in the movable mold  16  is pushed out using a push pin by the effect of cylinder  34  and separated from the movable mold  16 . Then, until an instruction to start a next molding cycle is given from the sequencer  55 , cavities in both of the molds are subjected to spray and air blow processes. In subsequent cycles, since the mold thickness adjusting operation in step  4  is not required, the other operations  1  to  3  and  5  to  11  will be repeated. 
     Next, in  FIGS. 6A-6B , operations  12 ,  13  are different from the molding cycle comprising operations  1  to  11 , and show operations for exchanging used molds to new ones. 
     Operation  12  shows a position of the movable die plate  12  before starting to exchange molds. In order to pull out the tie bars  18  from the fixed die plate  10  by moving the movable die plate  12 , the sequencer  55  shown in  FIG. 1  switches the electromagnetic directional control valves  56 ,  57  to change the engagement state between the half nuts  32 A,  32 B and  48 A,  48 B and the tie bars  18  to a state (described below) different from that of the molding cycle. Namely, the half nuts  48 A,  48 B on the side of fixed die plate  10  are opened to disengage from the engagement portions  18 A, respectively. Along with this operation, the half nuts  32 A,  32 B on the side of movable die plate  12  are closed to engage with the engagement portions  18 B. Thereafter, as shown in operation  13 , the movable die plate  12  is retracted together with the tie bar  18  due to the ball screw shaft  26  driven by the servo motor  30 . With the retraction of movable die plate  12 , the tie bars  18  are drawn out from the fixed die plate. As shown in the state  13 , during the retraction to a position where mold exchanging is carried out, since the weight of each tie bar is supported by the support guide  20 , flexure of the tie bars can be prevented. 
     In  FIG. 1 , when a mold exchanging work is completed, the movable die plate  12  advances, and is inserted in the fixed die plate  10  until the distal end of each tie bar  18  contacts with the stopper  46 . Then, the half nuts  48 A,  48 B mate with the engagement portions  18 A, and the half nuts  32 A,  32 B disengage from the engagement portions  18 B, respectively. Thereafter, from the state shown in operation  1 , a series of operations including mold closing, mold thickness adjustment, mold clamping, molding a molded article, and mold opening will be repeated.