Abstract:
A mold clamping apparatus for an injection molding machine includes a stationary platen fixedly holding the stationary mold, a movable platen fixedly holding the movable mold and disposed opposite to the stationary platen, tie bars for guiding the movable platen for movement toward and away from the stationary platen, a movable platen driving mechanism for moving the movable platen along the tie bars to close and open the mold, a movable platen fixing mechanism for fixing the movable platen to the tie bars at a set position immediately before a position where the movable mold is joined to the stationary mold, and a clamping force applying mechanism including sealed hydraulic cylinder actuators capable of converting energy of a hydraulic fluid into an intensified mold clamping force and of applying the intensified mold clamping force to the stationary platens. In one embodiment, a rear plate located behind the movable platen carries a hydraulically driven ram support and clamping ram, and the ram support has a piston portion that is carried in a cylinder bore in the rear plate. The ram hydraulically clamps the mold closed after the movable platen is electrically driven to bring the mold halves together.

Description:
This is a Divisional of National Application No. 09/144,341 filed Aug. 31, 1998, now issued as U.S. Pat. No. 6,270,333 on Aug, 7, 2001 the entire contents of which is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a mold clamping mechanism for an injection molding machine and, more specifically, to improvements in a mold clamping apparatus for injection molding machine, provided with a means for intensifying mold clamping force. 
     2. Description of the Related Art 
     In a generally known mold clamping mechanism, a movable platen is moved to clamp a mold between a stationary platen and the movable platen. The mold clamping mechanism is capable of keeping the mold closed by a high clamping force against the pressure of a molten polymer injected into the mold. Straight hydraulic mold clamping mechanisms have prevalently been used for clamping a mold. Straight hydraulic mold clamping mechanisms include those of a boost ram system employing a hydraulic cylinder actuator having a cylinder, and a ram internally provided with a boost ram and axially slidably fitted in the cylinder, those of a side cylinder system employing a plurality of side cylinders in addition to a clamping cylinder and those of a pressure intensifying cylinder system employing a clamping hydraulic cylinder actuator and a pressure intensifying hydraulic cylinder actuator connected in series to the clamping cylinder. These known straight hydraulic mold clamping mechanisms need a clamping hydraulic cylinder actuator having a large capacity and a complicated hydraulic circuit to open and close the mold quickly and to provide a sufficient mold clamping force, and a tank for storing the hydraulic fluid for operating the hydraulic cylinder actuator. The straight hydraulic mold clamping mechanisms of the boost ram system and the side cylinder system, in particular, need a large high pressure pump to exert a high clamping force on the mold, and the straight hydraulic mold clamping mechanisms of the pressure intensifying system has a relatively great longitudinal size. 
     A mold clamping mechanism disclosed in JPB2 No. 17851/1989 is provided with a screw mechanism and a cylinder actuator in combination. This mold clamping mechanism is provided with a first doubleacting cylinder actuator having a doubleacting ram, and first and second pressure chambers, the volumes of which vary equally as the doubleacting ram moves, and a second doubleacting cylinder actuator having a pressure intensifying chamber. The first doubleacting cylinder actuator operates for opening and closing a mold with the first and the second pressure chamber communicating with each other. When clamping the mold, the first and the second pressure chamber are disconnected from each other, and the pressure in the pressure intensifying chamber is increased by the screw mechanism to drive the second doubleacting cylinder actuator. A hydraulic fluid is transferred from the first or the second pressure chamber to the second or the first pressure chamber by a pump to advance or retract the doubleacting ram when opening or closing the mold. Therefore, any additional tank for reserving the hydraulic fluid is not necessary. 
     A known electricdriven mold clamping mechanism is provided with a screw feed device which is driven by an arc servomotor to move a movable platen. In this electricdriven mold clamping mechanism, it is unavoidable that the ac servomotor must be of a large capacity and the screw feed device must be of a large size to exert a sufficiently high mold clamping force on the mold. 
     From this point of view, a prior art mold clamping mechanism disclosed in JPA No. 246806/1994 is provided with an improved servomotor as a driving source and an improved screw feed mechanism. This prior art mold clamping mechanism is provided with a pressure intensifying mechanism which pressures a hydraulic fluid contained in a sealed bag with a threaded rod having one end linked to an internally threaded nut rotated by a servomotor to produce a high mold clamping force. This pressure intensifying mechanism, however, is not reliable in surely holding the mold clamping force due to the leakage of the hydraulic fluid from the sealed bag. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a simple, compact mold clamping apparatus having a high degree of freedom of design. 
     Another object of the present invention is to provide a mold clamping apparatus capable of surely and properly holding a mold clamping force when increasing pressure, of preventing the leakage of a hydraulic fluid and of smoothly supplying the hydraulic fluid. 
     According to a forst aspecg of the present invention, a mold clamping apparatus for an injection molding machine, for closing and opening a mold including a stationary mold and a movable mold and for clamping the closed mold, said mold clamping apparatus comprises: a stationary platen fixedly holding the stationary mold; 
     a movable platen fixedly holding the movable mold and disposed opposite to the stationary platen; tie bars for connecting the movable platen to the stationary platen in a manner that the movable platen moves toward and away from the stationary platen; movable platen driving means for moving the movable platen along the tie bars to close and open the mold; a movable platen fixing means for fixing the movable platen to the tie bars at a set position immediately before a position where the movable mold is closed; and 
     a clamping force applying means having sealed hydraulic cylinder actuators capable of converting energy of a hydraulic fluid into an intensified mold clamping force and of applying the intensified mold clamping force to the stationary platen. 
     According to a second aspect of the present invention, a mold clamping apparatus for an injection molding machine, for closing and opening a mold including a stationary mold and a movable mold and for clamping the closed mold, said mold clamping apparatus comprises: a stationary platen fixedly holding the stationary mold; 
     a movable platen fixedly holding the movable mold and disposed opposite to the stationary platen; tie bars for connecting the movable platen to the stationary platen in a manner that the movable platen moves toward and away from the stationary platen; 
     a movable platen driving means for moving the movable platen along the tie bars to close and open the mold; a rear mount plate disposed behind the movable platen and connected to the tie bars; 
     a mold clamping ram extended between the rear mount plate and the movable platen to exert a mold clamping force on the movable platen; and a clamping force applying means having a sealed hydraulic cylinder actuator capable of converting energy of a hydraulic fluid into an intensified mold clamping force and of applying the intensified mold clamping force to the mold clamping ram. 
     According to a third aspect of the present invention, a mold clamping apparatus for an injection molding machine, for closing and opening a mold including a stationary mold and a movable mold and for clamping the closed mold, said mold clamping apparatus comprises: a stationary platen fixedly holding the stationary mold; 
     a movable platen fixedly holding the movable mold and disposed opposite to the stationary platen; tie bars for connecting the movable platen in a manner that the movable platen moves toward and away from the stationary platen; an intermediate mount plate disposed behind the movable platen and movable along the tie bars; a rear mount plate fixedly disposed behind the intermediate mount plate; toggle mechanisms interconnecting the intermediate mount plate and the rear mount plate to move the intermediate mount plate forward and backward along the tie bars; a mold clamping ram extended between the intermediate plate and the movable platen to exert a mold clamping force on the movable platen; and a clamping force applying means having a sealed hydraulic cylinder actuator capable of converting energy of a hydraulic fluid into an intensified mold clamping force and of applying the intensified mold clamping force to the mold clamping ram. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following description taken in connection with the accompanying drawings, in which: 
     FIG. 1 is a schematic sectional view of a mold clamping apparatus in a first embodiment according to the present invention for an injection molding machine; 
     FIG. 2 is a schematic view of a driving mechanism included in a movable platen driving unit included in the mold clamping mechanism shown in FIG. 1; 
     FIG. 3 is a schematic sectional view of a mold clamping apparatus having another type of the movable platen driving unit; 
     FIG. 4 is an enlarged typical view of a pressure controller included in a clamp force applying unit included in the mold clamping apparatus shown in FIG. 1; 
     FIG. 5 is an enlarged typical view of a modification of the pressure controller included in the clamp force applying unit, shown in FIG. 4; 
     FIG. 6 is a schematic sectional view of a mold clamping apparatus in a second embodiment according to the present invention for an injection molding machine; 
     FIG. 7 is an enlarged diagrammatic view of a pressure controller included in a clamping force applying unit included in the mold clamping mechanism shown in FIG. 6; 
     FIG. 8 is an enlarged sectional view of a pressure controller in a modification of the pressure controller shown in FIG. 7; 
     FIG. 9 is a schematic sectional view of a mold clamping apparatus in a third embodiment according to the present invention for an injection molding machine; 
     FIGS.  10 ( a ) and  10 ( b ) are schematic sectional views of a mold clamping apparatus in a fourth embodiment according to the present invention for an injection molding machine, in a mold opening state and in a mold clamping state, respectively; and 
     FIGS.  11 ( a ) and  11 ( b ) are schematic sectional views of a mold clamping apparatus in a fifth embodiment according to the present invention for an injection molding machine, in a mold opening state and in a mold clamping state, respectively. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     Referring to FIG. 1 showing a mold clamping apparatus in a first embodiment according to the present invention, a stationary mold  10  of an injection mold is attached to a stationary platen  12  fixed to a base  11 . A movable mold  16  of the mold is attached to a movable platen  18  disposed opposite to the stationary platen  12  on the base  11 . The movable platen  18  is guided for forward and backward movement by tie bars  14  extended from the stationary platen  12 . A movable platen driving unit  20  drives the movable platen  18  for forward movement to close the mold and for backward movement to open the mold. A movable platen fixing unit  22  fixes the movable platen  18  to the tie bars  14  at a predetermined position immediately before a position where the movable mold  16  is joined to the stationary mold  10  to close the mold. A hydraulic intensifying unit  24  intensifies clamping force to be exerted on the movable mold  16  after the movable mold  16  has been moved to the stationary mold  10  together. 
     Movable Platen Driving Mechanism  20   
     Two threaded rods  32  (FIG. 2) extend through the movable platen  18 , and each has one end supported for rotation in a bearing  27  on the stationary platen  12 . Internally threaded nuts  26  are fixed to the movable platen  18  with threaded rods  32  extending through them to form ball screw mechanisms, respectively. The other end of each threaded rod  32  is supported in a bearing on a mount plate  28 . As shown in FIG. 2, the two threaded rods  32  are driven by a servomotor  34 . A driven pulley is mounted on the output shaft of the servomotor  34 , driven pulleys  30  are mounted respectively on the end of the threaded rods  32 , and a synchronous belt  36  is wound around the drive pulley and the driven pulleys  30  to transmit the output torque of the servomotor  34  to the threaded rods  32 . The rotation of each threaded rod  32  is converted into a straight-line motion by the ballscrew mechanism to move the movable platen  18  forward and backward on the base  11  along the bars  14 . 
     The ball screw mechanisms for the movable platen driving mechanism  20  may be substituted by ball screw mechanisms shown in FIG.  3 . The ball screw mechanisms involves a threaded rod  32 , which has one end supported on the movable platen  18 . A nut  35  having a threaded bore is supported for rotation on a mount plate  33  disposed behind the movable platen  18 . The nut  35  is restrained from axial movement and linked to the threaded rod  32 . A driven pulley  37  is fixedly combined with the nut  35 . The driven pulley  37  may be driven for rotation by a servomotor provided on the mount plate  33 . A drive pulley similar to the one shown in FIG. 2 is mounted on the output shaft of the servomotor  34  and a synchronous belt wound around the drive pulley and the driven pulley  37  to rotate the nut  35 . The nut  35  drive the threaded rod  34  to move in the axial direction so that the movable platen  18  moves forward and backward along the tie bars  14 . 
     Movable Platen Fixing Unit  22   
     Each tie bar  14  has a treaded section  14   a  in a predetermined length. Half nuts  42  are supported on the back surface of the movable platen  18 , and is engaged with the threaded sections  14   a  of the tie bars  14 . The half nuts  42  are adapted to be operated by cylinder actuators  40 . The cylinder actuators  40  tighten the half nuts  42  to fasten the movable platen  18  fixedly to the tie bars  14  and loosen the half nuts  42  to release the movable platen  18  from the tie bars  14 . 
     Clamping force applying Unit  24   
     Referring to FIGS. 1 and 4, the clamping force applying unit  24  comprises, as basic components, sealed hydraulic cylinder actuators  50  connected respectively to the tie bars  14 , a pressure controller  52 , and a hydraulic circuit including a nonleakage valve  66 . Each of the sealed hydraulic cylinder actuators  50  has a cylinder provided with a cylinder bore  54 , a piston  14   b  fitted in the cylinder bore so as to divide the bore  54  into a first pressure chamber  54   a  and a second pressure chamber  54   b,  and piston rods connected to the piston  14   b  so as to extend on the opposite sides of the piston  14   b.  The piston rod extending through the first pressure chamber  54   a  is connected to the tie bar  14 . 
     The pressure controller  52  has a body  55  provided with a cylindrical chamber in which a pressure applying member  62  is disposed movably. The pressure applying member  62  divides the cylindrical chamber into a pressure chamber  52   a  and a back pressure chamber  52   b.  The pressure chamber  52   a  is communicated with the first pressure chamber  54   a  through a hydraulic line  58   a  and the back pressure chamber  52   b  is communicated with the second pressure chamber through a hydraulic line  58   b.  The pressure controller  52  controls pressures in the first pressure chamber  54   a  and the second pressure chamber  54   b  which exert on the opposite sides of the piston  14   b.  The sectional area of the piston  14   b  which is greater than that of the pressure applying member  62  is large enough to produce a mold clamping force. Therefore, the piston  14   b  converts a hydraulic pressure into a desired intensified mechanical force to clamp the mold. This mechanical force is transmitted to pull the tie bar  14  and serves as the mold clamping force. 
     The pressure applying member  62  consists of a first member  62   a  and a second member  62   b.  The first member  62   a  is an induction member serving as a secondary conductive member driven by a linear motor  60 . The first member  62   a  and second member  62   b  are disposed in an axial arrangement, and the first member  62   a  is capable to take an independent movement. 
     The body  55  of the pressure controller  52  is provided with a passage  64  having one end opening into the back pressure chamber  52   b  and the other end opening into the atmosphere. 
     In the case where the pressure applying member  62  is a single member, when the pressure applying member  62  is moved for pressure intensifying action to the left, a negative pressure produced in the back pressure chamber  52   b,  whereby a negative pressure is produced in the second pressure chamber  54   b  connected to the back pressure chamber  52   b  by the line  58   b.  Therefore, air and external dust may possibly be sucked into the second pressure chamber  54   b  of the sealed hydraulic cylinder actuator  50 . 
     Since the pressure applying member  62  consists of the two members  62   a  and  62   b,  and the first member  62   a  driven by linear motor  60  is acting independently, any negative pressure is produced in a space behind the second member  62   b.  Moreover, even if the second member  62   b  is moved to the left due to the hydraulic liquid introduced to the back pressure chamber  52   b  from the second pressure chamber  54   b,  the back pressure chamber  52   b  is brought open into the atmosphere through the passage  64 . Any negative pressure is not produced in the back pressure chamber  52   b  and second pressure chamber  52   b,  thereby preventing dust from being sucked into the second pressure chamber  54   b.    
     A nonleakage valve  66  is provided in a hydraulic line communicating the first pressure chambers  54   a  and second pressure chamber  54   b.  The nonleakage valve  66  is controlled by control signals for the selective connection and disconnection of the pressure chambers  54   a  and  54   b.    
     A sealing member  72 , such as an O ring, is placed in an annular groove in the circumference of the piston  14   b  to ensure the liquid-tight contact between the piston  14   b  and the cylinder of the sealed hydraulic cylinder actuator  50 . If the hydraulic fluid in the first pressure chamber  52   a  leaks into the second pressure chamber  52   b  through the sealing member  72 , the nonleakage valve  66  is opened to allow the hydraulic fluid to flow from the second pressure chamber  54   b  into the first pressure chamber  54   a.    
     The hydraulic circuit shown in FIG. 4 must be replenished with the hydraulic fluid if the hydraulic fluid leaks through the sealing member  72  fitted on the piston  14   b.  A reservoir tank  68  containing the hydraulic fluid is installed which supply the hydraulic fluid to the hydraulic circuit through a supply passage  68   a  when necessary. The hydraulic fluid is supplied into the pressure chamber  52   a  and the line  58   a  immediately before the pressure intensifying action commences. 
     The pressure Pr of the hydraulic fluid supplied into the first pressure chamber  54   a  of the sealed hydraulic cylinder actuator  50  is magnified in proportion to the sectional area of the piston  14   b  to produce a mold clamping force Pf exerted on the tie bar  14 . The pressure Pr of the hydraulic fluid (or the mold clamping force Pf) is measured by a pressure sensor  70 . The pressure sensor  70  gives a pressure signal expressing the measured pressure Pr to a linear motor controller, and then the linear motor controller controls the linear motor  60  so that the pressure of the hydraulic fluid is maintained at a set pressure. 
     In operations for closing the mold, the threaded rods  32  are driven for rotation by the servomotor  34  to advance the movable platen  18  toward the stationary platen  12  so that the movable mold  16  moves quickly to the stationary mold  10 . When the movable mold  16  is on the point of stopping against the stationary mold  10 , the cylinder actuators  40  of the movable plate fixing unit  22  tighten the movable platen  18  at the set position. As soon as the movable platen  18  is fixed, the linear motor  60  of the pressure controller  52  is actuated to move the pressure applying member  62   a  for pressure intensifying action. The pressure applying member  62   a  increases the pressure in the first pressure chamber  54   a  in which the pistons  14   b  moves forward together with the tie bars  14 , pulling the tie bars  14  to clamp the movable mold  16  and the stationary mold  10  firmly together. 
     Thus, the clamping force applying unit  24  is capable of always exerting the predetermined mold clamping force on the tie bar  14  and of smoothly replenishing the hydraulic circuit with the hydraulic fluid in case the hydraulic fluid leaks. 
     According to the mold clamping apparatus of the embodiment, quick and smooth closing operation of the mold is easily achieved and the mold is maintained stably in a clamped state while injection phase. 
     FIG. 5 shows a pressure controller  52  in a modification of the pressure controller  52  of the pressure intensifying unit  24  shown in FIG.  4 . As shown in FIG. 5, the pressure controller  52  employs a servomotor  80  instead of the built-in linear motor  60  employed in the pressure controller  52  shown in FIG.  4 . 
     Referring to FIG. 5, a pressure applying member  62  has a threaded section  82 , and two piston sections  82   a  and  82   b  formed at the opposite ends of the threaded section  82 . The piston sections  82   a  and  82   b  are fitted in a pressure chamber  52   a  and a back pressure chamber  52   b,  respectively. An internally threaded nut  84  is mounted on and linked to the threaded section  82  of the pressure applying member  62  and is driven for rotation by a servomotor  80  to move the pressure applying member  62 . The internally threaded nut  84  is inserted in and coupled with a rotating member  86 , and a driven pulley  86   a  is mounted on the rotating member  86 . A drive pulley  80   a  is mounted on the output shaft of the servomotor  80  and a synchronous belt  88  is wound around the drive pulley  80   a  and the driven pulley  86   a  to drive the rotating member  86  for rotation together with the internally threaded nut  84 . In FIG. 5, indicated at  64   a  is an open chamber for preventing a negative pressure from generating which is led to be communicated with the atmosphere by means of an opening  64 . 
     The function of the pressure applying member  62  is entirely the same as that of the driving mechanism shown in FIG.  4 . 
     Second Embodiment 
     In a mold clamping mechanism in a second embodiment according to the present invention shown in FIG.  6  and FIG. 7, each of sealed hydraulic cylinder apparatus  50  has an open chamber  54   b  opening into the atmosphere in place of the pressure chamber  54   b  in the first embodiment. 
     The mold clamping apparatus in the second embodiment has a clamping force applying unit  24  comprising sealed hydraulic cylinder actuators  50 , a pressure controller  52 , and a hydraulic circuit including a nonleakage valve  66  and a directional control valve  76 . Each of the sealed hydraulic chamber actuators  50  has a cylinder provided with a cylinder bore  54 , and a piston  14   b  fitted in the cylinder bore  54  of the cylinder so as to divide the cylinder bore  54  into a pressure chamber  54   a  and the open chamber  54   b  opening into the atmosphere. A line  58   c  provided with a nonleakage valve  66  and a directional control valve  76  is connected to the pressure chamber  54   a.  The line  58   c  can be connected through the directional control valve  76  to a pressure chamber  52   a  formed in a cylinder included in the pressure controller  52  and to a reservoir tank  68 . 
     A pressure applying member  62  is fitted in the first pressure chamber  52   a  of the pressure controller  52 . The pressure applying member  62  is driven to move axially by a linear motor  60 . 
     The operation of the clamping force applying unit  24  will be described in connection with the operation for clamping the mold. 
     When clamping the mold, the directional control valve  76  and the nonleakage valve  66  are set in an open position for communicating the pressure chamber  52   a  of the pressure controller  52  with the pressure chamber  54   a  of the sealed hydraulic cylinder actuator  50 . The pressure applying member  62  of the pressure controller  52  is moved axially to the left to increase the hydraulic pressure in the pressure chamber  54   a.  The increased hydraulic pressure exerts to the piston  14   b  having a large pressure receiving area. The piston  14   b  converts the hydraulic pressure into a high mechanical clamping force applying to the tie bar  14 . 
     When holding the mold in a clamping state, the directional control valve  76  and the nonleakage valve  66  are set in an closed position for disconnecting the pressure chamber  52   a  from the pressure chambers  54   a  to maintain the hydraulic pressure acting on the piston  14   b.    
     When releasing the clamping force, the directional control valve  76  is kept in the closed position and the nonleakage valve  66  is set in the open position to communicate the pressure chamber  52   a,    54   a  with the reservoir tank  68 . Consequently, the pressure in the pressure chamber  54   a  is relieved and the hydraulic fluid is supplied into the pressure chamber  52   a.    
     FIG. 8 is an enlarged sectional view of a modified pressure controller  52  shown in FIG.  7 . The pressure controller  52  shown in FIG. 8 employs a servomotor  80  instead of the linear motor  60  shown in FIG.  7 . 
     Referring to FIG. 8, a piston used as a pressure applying member  62  is fitted in a pressure chamber  52   a  formed in a cylinder of the pressure controller  52 . A threaded rod  83  is connected to one end of the pressure applying member  62  facing the back pressure chamber  52   c.  An internally threaded nut  84  is linked to the threaded rod  83  and is interlocked with the drive shaft of the servomotor  80  by a gear train  90 . 
     The function and effect of the pressure controller  52  employing the servomotor  80  for driving the pressure applying member  62  are the same as those of the pressure controller  52  employing the linear motor  60 . 
     Third Embodiment 
     A mold clamping apparatus in a third embodiment according to the present invention for an injection molding machine, shown in FIG. 9 is the same in constitution as the mold clamping apparatus shown in FIG. 1 except only a clamping force applying unit. Therefore, parts shown in FIG. 1 like or corresponding to those shown in FIG. 1 are designated by the same reference characters and the description thereof will be omitted. 
     Referring to FIG. 9, an elastic member  92  for returning the piston  14   b  is placed in the chamber  54   b  of the cylinder of the sealed hydraulic cylinder actuator  50  to force the piston  14   b  toward a pressure chamber  54   a  through which the tie bar  14  extends. A line  58   d  connected to the pressure chamber  54   a  and provided with the nonleakage valve  66  is connected to the pressure chamber  52   a  of the pressure controller  52 . 
     When the nonleakage valve  66  is not actuated by solenoid, the nonleakage valve retains the pressure in the pressure chamber  54   a  communicates the pressure chamber  52   a  with the reservoir tank  68 . The reservoir tank  68  replenishes the hydraulic circuit with the hydraulic fluid immediately before starting a claming operation. When the nonleakage valve  66  is actuated by the solenoid, the nonleakage valve  66  shuts off a line  68   a  connected to the reservoir tank  68  and communicate the pressure chamber  54   a  with the pressure chamber  52   a.    
     A pressure applying member  62  is driven for axial movement by a linear motor  60 . The pressure applying member  62  applies a increased hydraulic pressure to the piston  14   b  to convert the hydraulic pressure into a clamping force which is large enough for pulling the tie bars  14  to clamp the mold. 
     In this mold clamping mechanism, the pressure controller  52  is provided with the linear motor  60  and the reservoir chamber  54   a  is sealed with sealing members  72 . 
     It is noted that the servomotor  80  shown in FIG. 8 is applicable to the driving means for the pressure applying member  62  instead of the linear motor  60 . 
     Fourth Embodiment 
     FIGS.  10 ( a ) and  10 ( b ) show a mold clamping apparatus in a fourth embodiment according to the present invention for an injection molding machine in a mold opening state and in a mold clamping state, respectively. 
     The mold clamping apparatus in the fourth embodiment comprises, as basic components, a stationary platen  12  holding a stationary mold  10 , a movable platen  18  for holding a movable mold  16  which is disposed opposite to the stationary platen  18  and is guided for longitudinal movement toward and away from the stationary plate  12  by tie bars  14 , a movable platen driving unit  20  for moving the movable platen forward and backward along the tie bars  14 , and a mold clamping ram  44  extended between the movable platen  18  and a rear mount plate  38  disposed behind the movable platen  18 . 
     The rear mount plate  38  can be fixed to a base at an appropriate longitudinal position. The longitudinal position of the rear plate  38  along the tie bars  14  is adjustable according to the thickness of a mold to be used on the injection molding machine by means of position adjusting mechanisms  45  interlocked with the tie bars  14 . A ram support member  46  supports the mold clamping ram  44  axially movably therein and has a piston section  46   a  at one end thereof. A cylinder bore  54  which constitutes a hydraulic cylinder actuator  50  is confined in the rear mount plate  38 . The piston section  46   a  of the ram support member  46  is fitted in the cylinder bore  51  which is sealed by a sealing bag  47 . 
     A Clamping force applying unit  24  includes the hydraulic cylinder actuator  50 , the piston section of the mold clamping ram  44  and pressure controller  52 . The pressure controller  52  may be of a construction similar to that of any one of the pressure intensifying units  24  employed in the first to the third embodiment. 
     In the fourth embodiment, a pressure controller  52  is built in the rear mount plate  38 . A pressure chamber  52   a  included in the pressure controller  52  is connected to a pressure chamber  54   a  in which a hydraulic pressure is built up by a line  58   e.    
     In operation for closing the mold, the threaded rods  32  are driven for rotation to advance the movable platen  18  together with the mold clamping ram  44  toward the stationary platen  12  so that the movable mold moves quickly to the stationary mold  10 , when the movable mold  16  is on the point of stopping against the stationary mold  10 , the pressure controller  52  is actuated to move the pressure applying member  62   a  for pressure intensifying action. The pressure applying member  62   a  build up the hydraulic pressure in the pressure chamber  54   a  so that the piston section  46   a  applies the clamping force to the mold clamping ram  44  to clamp the movable mold  16  and stationary mold firmly together. 
     Fifth Embodiment 
     FIGS.  11 ( a ) and  11 ( b ) show a mold clamping apparatus in a fifth embodiment according to the present invention for an injection molding machine, in a mold opening state and in a mold clamping state, respectively. 
     The mold clamping apparatus in the fifth embodiment comprises, as basic components, a stationary platen  12  for holding a stationary mold  10 , a movable platen  18  for holding a movable mold  16  which is disposed opposite to the stationary platen  12  and is guided for longitudinal movement toward and away from the stationary platen  12  by tie bars  14 , an intermediate mount plate  48  disposed behind the movable platen  18 , a mold clamping ram  44  extended between the movable platen  18  and the intermediate mount plate  48 , a pair of toggle mechanisms  94  for moving the movable platen forward and backward along the tie bars  14 . 
     The mold clamping arm  44  has a piston section  44   a  at a rear end thereof. A cylinder bore which constitutes a sealed hydraulic cylinder actuator  50  is confined in the intermediate mount plate  48 . The piston section  44   a  of the mold clamping ram  44  is fitted in the cylinder bore which is divided into a pressure chamber  54   a  and a back pressure chamber  54   b.    
     The toggle mechanisms  94  are mounted between the intermediate mount plate  48  and a rear mount plate  38 . The rear mount plate  38  can be fixed to a base at an appropriate longitudinal position. The longitudinal position of the rear plate  38  along the tie bars  14  is adjustable according to the thickness of a mold to be used on the injection molding machine by means of position adjusting mechanisms  45  interlocked with the tie bars  14 . A toggle mechanism driving unit involves a motor  95  mounted on the rear mount plate  38 . The motor  95  drives a ball screw  96  which is rotatably supported in the rear mount plate  38 . A nut  97  engaged with the ball screw  96  is connected to toggle link members of the toggle mechanisms  94 . 
     A Clamping force applying unit  24  includes the sealed hydraulic cylinder actuator  50 , the piston section  44   a  of the mold clamping ram  44  and pressure controller. The pressure controller may be of a construction similar to that of any one of the pressure controller  52  employed in the first to the third embodiment. 
     In the fifth embodiment, the pressure chamber  54   a  and the back pressure chamber  54   b  are communicated with the pressure controller, now shown, by lines  58   a  and  58   b  formed in the intermediate mount plate  48 . 
     In operation for closing the mold, the mechanisms  94  are driven for stretch to advance the movable platen  18  together with the mold clamping ram  44  toward the stationary platen  12  so that the movable mold moves quickly to the stationary mold  10 , when the movable mold  16  is on the point of stopping against the stationary mold  10 , the pressure controller (not shown) build up the hydraulic pressure in the pressure chamber  54   a  so that the piston section  46   a  applies the clamping force to the mold clamping ram  44  to clamp the movable mold  16  and stationary mold firmly together. 
     Although the invention has been described in its preferred embodiments with a certain degree of particularity, obviously many changes and variations are possible therein. It is therefore to be understood that the present invention may be practiced otherwise than as specifically described herein without departing from the scope and spirit thereof.