Abstract:
An injection apparatus for a motor-driven injection molding machine comprised a barrel unit for heating and plasticating polymer material, a screw disposed in the barrel unit rotatably and movably in the axial direction, a first rotating shaft having one end connected to a rear end of the screw, a charging motor for driving the first rotating shaft for rotation, a ball screw mechanism consisting of a second rotating shaft having a threaded rod portion, and a threaded nut member engaging with the threaded rod portion, an injection carriage having a front plate for mounting the barrel unit thereon, and a rear plate for supporting the second rotating shaft at the rear end thereof, an injection motor for driving the ball screw mechanism, and an intermediate mount plate, disposed movably between the front plate and the rear plate, for linking the first rotating shaft to the second rotating shaft coaxially with a predetermined distance spaced apart between the rear end of the first rotating shaft and the front end of the second rotating shaft. Back pressure acting on the screw in a charging process can accurately be controlled without requiring a difficult control operation for controlling the injection motor and the charging motor for synchronous operation.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an injection apparatus for a motor-driven injection molding machine. In particular, it relates to an improvement that facilitates the control of back pressure during charging process. 
     2. Description of the Related Art 
     FIG. 7 shows an injection apparatus of a conventional motor-driven injection molding machine. The reference numeral  1  denotes an injection apparatus. A barrel unit  3  which is fed with plastic material is mounted on an injection carriage  2 . The barrel unit  3  includes a heated barrel having an internal elongated cylindrical bore. A screw  4  is rotatably and axially movably disposed in the cylindrical bore of the heated barrel  3 . 
     A ball screw  5  extends coaxially and integrally from a rear end portion of the screw  4 . A spline shaft  6  is integrally connected to the rear end of the ball screw  5 . 
     An injection motor  7  which is for use in the injection process and drives the ball screw  5  is installed in the injection carriage  2 . A charging motor  8  which is for use in the charging process and drives the spline shaft  6  is as well installed in the rear end of the injection carriage  2 . A rotor  9  of the injection motor  7  is designed to serve as a ball screw nut that engages with the ball screw  5 . And a rotor  10  of the charging motor  8  is designed to serve as a spline nut which the spline shaft  6  is slidably fitted therethrough. A numerical control unit  11  controls the operation of the injection motor  7  and charging motor  8  so that the screw  4  moves forwardly in the injection process, or rotates in the charging process. 
     During the charging process, the spline shaft  6  permits the transmission of the rotation of the charging motor  8  to the screw  4  integral with the spline shaft  6  and ball screw  5 . The screw  4  rotates and plasticizes the material fed into the barrel  3 . The melt material accumulates in front of the screw  4 , forcing back the screw  4  rearwardly. 
     During the injection process, the combination of the ball screw  5  and ball nut  9  converts the rotation of the injection motor  7  to the liner motion which causes the screw  4  to move forwardly(the leftward direction in the figure), whereby the screw  4  injects the molten material accumulated in front of the screw  4  into a mold cavity. The numerical control unit  11  controls the injection motor  7  to rotate and controls the charging motor  8  to keep from rotating, so as to enable the screw  4  to move forwardly. 
     The movement of the screw  4  in the charging process consists of the rotation for plastication and the retreat motion caused by the pressurized molten material accumulated in front of the screw  4 . The rotation of the screw  4  simultaneous with retreat is properly achieved, in conjunction with the injection motor  7  to control of the back pressure to be applied on the screw  4 . 
     However, in this prior-art injection apparatus, the ball screw  5  is made integral with the spline shaft  6  and it is so arranged that the charging motor  8  rotates the spline nut  10  fitted onto the spline shaft  6 . Therefore, while the charging motor  8  rotates the spline shaft  6  to cause the screw  4  to rotate, the ball screw  5  rotates inevitably. This inevitable rotation of the ball screw  5  brings the screw  4  to move rearwardly, regardless of the amount of the molten material forced forwardly by the rotating screw  4 . 
     The prior-art injection apparatus provided with the charging motor  8  having such a structure that the spline shaft  6  passes through the rotor  10  involves following drawbacks to control the back pressure. The rotation of the rotor  10  prevents the spline shaft  6  from slipping axially through the rotor  10 . That needs to apply a appropriate back pressure to the screw  4  to cause it to retreat at the velocity corresponding to the feed rate of the molten material. For that reason, it is difficult to control the charging motor  8  and injection motor  7  with the rotation of the former being synchronized with that of the latter. That fails in the precise control of the back pressure applied to the screw  4 . As a result, The screw  4  is forced to retreat only by the pressure of the molten material. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an injection apparatus for a motor-driven injection molding machine which enable to remove therefrom the above-described drawbacks that make it difficult to control the charging motor and injection motor synchronously, and to achieve the precise control of the back pressure applied to the screw. 
     According to a first aspect of the present invention, an injection apparatus for a motor-driven injection molding machine having a barrel unit for heating and plasticating polymer material and a screw disposed in the barrel unit rotatably and movably in the axial direction comprises a first rotating shaft having one end connected to a rear end of the screw, a charging motor for driving the first rotating shaft for rotation, a ball screw mechanism consisting of a second rotating shaft having a threaded rod portion, and a threaded nut member engaging with the threaded rod portion, an injection carriage having a front plate for mounting the barrel unit thereon, and a rear plate for supporting the second rotating shaft at the rear end thereof, an injection motor for driving the ball screw mechanism, and an intermediate mount plate, disposed movably between the front plate and the rear plate, for linking the first rotating shaft to the second rotating shaft coaxially with a predetermined distance spaced apart between the rear end of the first rotating shaft and the front end of the second rotating shaft. 
     According to a second aspect of the present invention, an injection apparatus for a motor-driven injection molding machine having a barrel unit for heating and plasticating polymer material, and a screw disposed in the barrel unit rotatably and movably in the axial direction comprises a first rotating shaft having one end connected to a rear end of the screw, a ball screw mechanism consisting of a second rotating shaft having a threaded rod portion, and a threaded nut member engaging with the threaded rod portion, an injection carriage having a front plate for mounting the barrel unit thereon, and a rear plate for supporting the second rotating shaft at the rear end thereof, an intermediate mount plate, disposed movably between the front plate and the rear plate, for linking the first rotating shaft to the second rotating shaft coaxially with a predetermined distance spaced apart between the rear end of the first rotating shaft and the front end of the second rotating shaft, a charging motor for driving the first rotating shaft for rotation, and mounted on the intermediate mount plate, and an injection motor for driving the threaded nut member of the ball screw mechanism for rotation, and mounted on the rear plate of the injection carriage. 
     According to a third aspect of the present invention, an injection apparatus for a motor-driven injection molding machine having a barrel unit for heating and plasticating polymer material, and a screw disposed in the barrel unit rotatably and movably in the axial direction, comprises a first rotating shaft having one end connected to a rear end of the screw, a second rotating shaft having a threaded rod portion, a threaded nut member engaging with the threaded rod portion, an injection carriage having a front plate for mounting the barrel unit thereon, and a rear plate with the threaded nut member fixed thereon, an intermediate mount plate, disposed movably between the front plate and the rear plate, for linking the first rotating shaft to the second rotating shaft coaxially with a predetermined distance spaced apart between the rear end of the first rotating shaft and the front end of the second rotating shaft, a charging motor for driving the first rotating shaft for rotation, and mounted on the intermediate mount plate, and an injection motor for driving the second rotating shaft for rotation, and mounted on the intermediate mount plate. 
     According to a fourth aspect of the present invention, an injection apparatus for a motor-driven injection molding machine having a barrel unit for heating and plasticating polymer material, and a motor-driven screw disposed in the barrel unit rotatably and movably in the axial direction comprises a first rotating shaft having one end connected to a rear end of the screw, a second rotating shaft having a threaded rod portion, an injection carriage having a front plate for mounting the barrel unit thereon, and a rear plate for supporting the second rotating shaft at the rear end thereof, an injection motor for driving the second rotating shaft for rotation, and mounted on the rear plate of the injection carriage, an intermediate mount plate disposed movably between the front plate and the rear plate, a charging motor for driving the first rotating shaft for rotation, and mounted on the intermediate mount plate, and a threaded nut member fixed on the intermediate mount plate and engaging with the threaded rod portion of the first rotating shaft with a predetermined distance spaced apart coaxially between the rear end of the first rotating shaft and the front end of the second rotating shaft. 
     According to the present inventions, it is not necessary to perform difficult motor control based on the synchronization between the charging motor and the injection motor as in the prior apparatus and it is possible to control the charging motor and/or the injection motor independently, and to perform the proper charging operation by apply appropriate back pressure to the screw with a high precision. 
    
    
     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 partly sectional, schematic front view of an injection apparatus in a first embodiment according to the present invention for an motor-driven injection molding machine; 
     FIG. 2 is an end view taken in the direction of the arrow Z in FIG. 1; 
     FIG. 3 is a partly sectional, schematic front view of an injection apparatus in a second embodiment according to the present invention for an motor-driven injection molding machine; 
     FIG. 4 is a partly sectional, schematic front view of an injection apparatus in a third embodiment according to the present invention for an motor-driven injection molding machine; 
     FIG. 5 is a partly sectional, schematic front view of an injection apparatus in a fourth embodiment according to the present invention for an motor-driven injection molding machine; 
     FIG. 6 is a partly sectional, schematic front view of an injection apparatus in a fifth embodiment according to the present invention for an motor-driven injection molding machine; and 
     FIG. 7 is a partly sectional, schematic front view of a conventional injection unit for an motor-driven injection molding machine. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     A first embodiment of the present invention will be explained with reference to FIGS. 1 and 2. The reference numeral  20  denotes an injection apparatus and  21  denotes an injection carriage. The injection carriage  21  includes a bottom plate  26 , a front plate  23  disposed at the front end of the bottom plate  26 , and a rear plate  25  disposed to the rear end of the bottom plate  26 . The barrel unit  22  is mounted on the front plate  23 , and an injection motor  24  is fixed onto the rear plate  25 . The barrel unit  22  is provided with a heated barrel which has a internal elongated cylindrical bore. A screw  27  is disposed in the bore of the barrel unit  22  so as to be able to rotate and to move in the axial direction. A first rotating shaft  34  is connected to the rear end portion (a right end part as viewed in FIG. 1) of the screw  27 . The first rotating shaft  34  is supported rotatably by bearings on an intermediate mounting plate  29  disposed movably between the front plate  23  and the rear plate  25 . A charging motor  28  drives the first rotating shaft  34  for rotation through a power transmitting mechanism  33  composed of a belt drive transmission. 
     As shown in FIG. 2, the charging motor  28  is linked to a first pulley  30  for synchronous belt drive in order to transmit the drive power therefrom to the first rotating shaft  34 . A second pulley  31  for synchronous belt drive is attached to the first rotating shaft  34 . A synchronous belt  32  connect the first pulley  30  and second pulley  31 , whereby it is arranged to transmit the drive power of the charging motor  28  to the first rotating shaft  34 . 
     A second rotating shaft  36  is linked to the intermediate mount plate  29  in alignment with the first rotating shaft  34  with a predetermined distance spaced apart from the rear end of the first rotating shaft  34 , so that the second rotating shaft  36  is free from interference due to the rotation of the first rotating shaft  34 . The second rotating shaft  36  has a threaded rod portion  35  which is adapted to serve as a threaded rod of a ball screw mechanism with a front end thereof abutting against a rear end surface of the intermediate mount plate  29 . 
     A rotor  37  of the injection motor  24  formed integrally with a threaded nut member  38 , which is adapted to serve as a threaded nut of the ball screw mechanism, is linked in engagement with the threaded rod portion  35  of the second rotating shaft  36 . The rotor  37  of the injection motor  24  causes to rotate the threaded nut member  38  to move the second rotating shaft  36  in the axial direction. In an injection process, the second rotating shaft  36  is capable to apply the trust force to the intermediate mounting plate  29  to make the screw  27  move forward (to the left as viewed in FIG.  1 ). 
     In this embodiment, a load cell  39  for measuring the thrust force exerted to the intermediate mount plate  29  is disposed between the end surface of the intermediate mount plate  29  and the front end of the second rotating shaft  36  to detect back pressures applied to the screw  27  during a charging process. A rotary encoder  40  for measuring the angle of revolution of the rotor  37  is associated with the outer circumference of the rotor  37  for the purpose of the position control of the screw  27  during the injection process and the charging process. 
     Guide bars  41  extending through the intermediate mount plate  29  are placed parallel to the axis of the screw  27  between the front plate  23  and the rear plate  25  of the injection carriage  21 . The movement of the intermediate mount plate  29  during the charging process and the injection process is guided by the guide bars  41 . The guide bars  41  need not necessarily be extended between the front plate  23  and the rear plate  25  of the injection carriage  21 . A substitute for the guide bars  41  may be spanned between two support legs placed on the bottom plate  26  of the injection carriage  21 , provided that the guide bars  41  extending through the intermediate mount plate  29  run parallel to the axis of the screw  27 . 
     Operation of the injection unit thus constructed will be described hereinafter. 
     In the injection process, electrical power is applied to the injection motor  24 , and the rotor  37  rotates together with the threaded nut member  38 . The rotational power of the injection motor  24  is converted to a thrust force by the threaded nut member  38  engaging with the threaded rod portion  35  of the second rotating shaft  36  to drive the second rotating shaft  36  for axial movement. The thrust force is transmitted to the intermediate mount plate  29  and the first rotating shaft  34  to move the screw  27  in the axial direction. Consequently, the screw  27  is forced to move to the left direction in FIG. 1, and injects the molten material accumulated in front of the screw  27  into a mold cavity (not shown). 
     In the charging process, electrical power is applied to the charging motor  28  to drive the first rotating shaft  34  through the power transmitting mechanism  33  and the screw  27  connected to the first rotating shaft  34  is rotated. Resin pellets fed through a inlet, not shown, into the barrel unit  22  is plasticated by heater element ( not shown) and the shearing action of the rotating screw  27 . The rotation of the screw  27  causes the molten material to flow forward, and the molten material is accumulate and charged in front of the screw  27 . The screw  27  retreats (to the right as viewed in FIG. 1) accompanying with rotation, due to the pressure of the molten material accumulated in front of the screw  27 . 
     In the charging process, the first rotating shaft  34  rotates and retreats together with the intermediate mount plate  29  while the screw  27  moves rearward. However, the first rotating shaft  34  and the second rotating shaft  36  are linked through the intermediate mount plate  29  so that the rear end of the first rotating shaft  34  is spaced from the front end of the second rotating shaft  36 , which is free from interference due to the rotation of the first rotating shaft  34 . That provides a smooth retreat motion of the screw  27  with appropriate back pressure applied thereto, while the injection motor  24 , which leaves the threaded nut member  38  rotating freely, permits the second rotating shaft  36  to move rearward. Consequently, during the charging process, unlike the conventional injection apparatus as shown in FIG. 7, in which the charging motor and the injection motor are necessarily driven synchronously with complicated back pressure control, the appropriate back pressure can be applied to the screw  27  by braking the molten material accumulated in front of the screw  27  with only the injection motor  24  alone being controlled. 
     According to the embodiment, in the case of transmitting the power of the charging motor  28  to the first rotating shaft  34  by using the synchronous belt  32  and the pulleys  30 ,  31  in the charging process, it is possible to achieve the transmission with an appropriate reduction ratio between the pulley  30  and the pulley  31  which decreases the revolution rate of the screw  27 . As a result, it is possible to decrease the motor torque and the current supplied to the charging motor  28 , and to enable to use an amplifying element with smaller capacity. 
     Second Embodiment 
     An injection apparatus  50  in a second embodiment according to the present invention will be described with reference to FIG. 3, in which parts like or corresponding to those of the foregoing embodiment are designated by the same reference characters. In this embodiment, the injection motor mounted on the intermediate mount plate drives the second rotating shaft directly and the threaded nut is fixed on the rear plate. 
     Referring to FIG. 3, an injection carriage  51  of the injection apparatus includes a bottom plate  26 , a front plate  23  disposed at the front end of the bottom plate  26 , and a rear plate  57  disposed to the rear end of the bottom plate  26 . The barrel unit  22  is mounted on the front plate  23 , and is provided with a heated barrel in which a screw  27  is disposed in the bore of the barrel unit  22  so as to be able to rotate and to move in the axial direction. A first rotating shaft  54  is connected to the rear end portion (a right end part as viewed in FIG. 3) of the screw  27 . The first rotating shaft  54  is supported rotatably by bearings on an intermediate mount plate  53  disposed movably between the front plate  23  and the rear plate  57 . The charging motor  28 , which is mounted on the intermediate mount plate  53 , drives the first rotating shaft  54  for rotation through a power transmitting mechanism  33  composed of a belt drive transmission which has the same mechanism as shown as FIG.  2 . 
     A second rotating shaft  55  is driven for rotation by an injection motor  52  which is mounted on the intermediate mount plate  53 . The second rotating shaft  55  is linked by the intermediate mount plate  53  in alignment with the first rotating shaft  54  with a predetermined distance spaced apart from the rear end of the first rotating shaft  54 , so that the second rotating shaft  55  is free from interference due to the rotation of the first rotating shaft  54 . The second rotating shaft  55  has a threaded rod portion  56  which is adapted to serve as a threaded rod of a ball screw mechanism. 
     A threaded nut member  58 , which is adapted to serve as a threaded nut of the ball screw mechanism, is fixed on the rear plate  57  and linked in engagement with the threaded rod portion  56  of the second rotating shaft  55 . The injection motor  52  rotates the second rotating shaft  55  to move it in the axial direction, so that the second rotating shaft  55  is capable to apply the trust force to the intermediate mount plate  53  to make the screw  27  move forward (to the left as viewed in FIG.  3 ). 
     In this embodiment, a load cell  39  for measuring the thrust force exerted to the intermediate mounting plate  53  is disposed at rear end of the second rotating shaft  55  to detect back pressures applied to the screw  27  during a charging process. A rotary encoder  40  for measuring the angle of revolution of the injection motor  52  is associated with the rotor thereof for the purpose of the position control of the screw  27  during the injection process and the charging process. Guide bars  41  extending through the intermediate mount plate  29  are placed parallel to the axis of the screw  27  between the front plate  28  and the rear plate  57  of the injection carriage  51 . The movement of the intermediate mount plate  53  during the charging process and the injection process is guided by the guide bars  41  in the same manner as the first embodiment. 
     The operation of the injection unit thus constructed will be described hereinafter. 
     In the injection process, electrical power is applied to the injection motor  52  to rotate the second rotating shaft  55 . The rotational power of the injection motor  52  is converted to a thrust force by the threaded nut member  58  engaging with the threaded rod portion  56  of the second rotating shaft  55  to drive it for axial movement. The thrust force is transmitted to the intermediate mount plate  53  and the first rotating shaft  54  to move the screw  27  in the axial direction. Consequently, the screw  27  is forced to move to the left direction in FIG. 3, and injects the molten material accumulated in front of the screw  27  into a mold cavity (not shown). 
     In the charging process, electrical power is applied to the charging motor  28  to drive the first rotating shaft  54  through the power transmitting mechanism  33  and the screw  27  connected to the first rotating shaft  54  is rotated. Resin pellets fed through a inlet, not shown, into the barrel unit  22  is plasticated by heater element (not shown) and the shearing action of the rotating screw  27 . The rotation of the screw  27  causes the molten material to flow forward, and the molten material is accumulated and charged in front of the screw  27 . The screw  27  retreats (to the right as viewed in FIG. 3) accompanying with rotation, due to the pressure of the molten material accumulated in front of the screw  27 . 
     During the charging process, the first rotating shaft  54  rotates and retreats together with the intermediate plate  53  while the screw  27  moves rearward. However, the first rotating shaft  54  are linked to the second rotating shaft  55  through the intermediate mount plate  53  so that the second rotating shaft  55  is free from interference due to the rotation of the first rotating shaft  54 . That provides a smooth retreat motion of the screw  27  with appropriate back pressure applied thereto, while the injection motor  52 , which leaves the second rotating shaft  55  rotating freely, permits the second rotating shaft  55  to move rearward. Consequently, during the charging process, unlike the conventional injection apparatus as shown in FIG. 7, in which the charging motor and the injection motor are necessarily driven synchronously with complicated back pressure control, the appropriate back pressure can be applied to the screw  27  by braking the molten material accumulated in front of the screw  27  with only the injection motor  52  alone being controlled. 
     Third Embodiment 
     An injection unit  60  in a third embodiment according to the present invention will be described hereinafter with reference to FIG.  4 . All the elements common to all embodiments have the same reference numerals. In this embodiment, unlike the second embodiment, the charging motor mounted on the intermediate mount plate drives the first rotating shaft directly without the power transmission mechanism as shown in FIG.  2 . 
     Referring to FIG. 4, the barrel unit  22  is mounted on a front plate  62  of an injection carriage  61 , and is provided with a heated barrel in which a screw  27  is disposed in the bore of the barrel unit  22  so as to be able to rotate and to move in the axial direction. A first rotating shaft  68  is connected to the rear end portion of the screw  27 . The first rotating shaft  68  is supported rotatably by bearings on an intermediate mount plate  66  which is disposed movably between the front plate  62  and a rear plate  63 . A charging motor  64  which drives first rotating shaft  68  is mounted on the front end of the intermediate mount plate  66 , and an injection motor  65  is mounted on the rear end of the intermediate mount plate  66 . 
     A second rotating shaft  72  driven for rotation by the injection motor  65  is linked to the intermediate mount plate  66  in alignment with the first rotating shaft  68  with a predetermined distance spaced apart from the rear end of the first rotating shaft  68 , so that the second rotating shaft  72  is free from interference due to the rotation of the first rotating shaft  68 . The second rotating shaft  72  has a threaded rod portion  71  which is adapted to serve as a treaded rod of a ball screw mechanism. A threaded nut member  69 , which is adapted to serve as a threaded nut of the ball screw mechanism, is fixed on the rear plate  63  and linked in engagement with the threaded rod portion  71  of the second rotating shaft  72 . 
     In the injection process, electrical power is applied to the injection motor  65  to rotate the second rotating shaft  72 . The rotational power of the injection motor  65  is converted to a thrust force by the threaded nut member  69  engaging with the threaded rod portion  71  of the second rotating shaft  72  to drive it for axial movement. The thrust force is transmitted to the intermediate mount plate  66  and the first rotating shaft  68  to move the screw  27  in the axial direction. Consequently, the screw  27  is forced to move to the left direction in FIG. 4, and injects the molten material accumulated in front of the screw  27  into a mold cavity (not shown). 
     In the charging process, electrical power is applied to the charging motor  64  to drive the first rotating shaft  68  and the screw  27 . Resin pellets fed into the barrel unit  22  is plasticated, and the molten material is accumulated and charged in front of the screw  27 . The screw  27  retreats (to the right as viewed in FIG. 4) accompanying with rotation, due to the pressure of the molten material accumulated in front of the screw  27 . 
     During the charging process, the first rotating shaft  68  rotates and retreats together with the intermediate mount plate  66  while the screw  27  moves rearward. However, the first rotating shaft  68  are linked to the second rotating shaft  72  through the intermediate mount plate  66  so that the second rotating shaft  72  is free from interference due to the rotation of the first rotating shaft  68 . That provides a smooth retreat motion of the screw  27  with appropriate back pressure applied thereto, while the injection motor  65 , which leaves the second rotating shaft  72  rotating freely, permits the second rotating shaft  55  to move rearward. Consequently, unlike the conventional injection apparatus as shown in FIG. 7, the charging motor  64  and the injection motor  65  are need not to be driven synchronously with complicated back pressure control. And the appropriate back pressure can be applied to the screw  27  by braking the molten material accumulated in front of the screw  27  with only the injection motor  65  alone being controlled. 
     In this embodiment, rotary encoders  73 ,  74  measures the angle of rotation of the charging motor  64  and the injection motor  65 , respectively. A load cell  75  measures the thrust force exerting on the intermediate mount plate  66 . In FIG. 4, guide bars for guiding the intermediate mounting plate  66  fixedly holding the charging motor  64  and the injection motor  65  are omitted for drawing. 
     Fourth Embodiment 
     An injection unit  80  in a fourth embodiment according to the present invention will be described hereinafter with reference to FIG.  5 . All the elements common to all embodiments have the same reference numerals. In this embodiment, unlike the first embodiment, the charging motor mounted on the intermediate mount plate drives the first rotating shaft directly without the power transmission mechanism as shown in FIG.  2 . 
     An injection carriage  81  includes a bottom plate  26 , a front plate  82  disposed at the front end of the bottom plate  26 , and a rear plate  83  disposed to the rear end of the bottom plate  26 . The barrel unit  22  is mounted on the front plate  82 , and is provided with a heated barrel in which a screw  27  is disposed in the bore of the barrel unit  22  so as to be able to rotate and to move in the axial direction. A first rotating shaft  87  is connected to the rear end portion of the screw  27 . The first rotating shaft  87  is supported rotatably by bearings on an intermediate mount plate  85  disposed movably between the front plate  82  and the rear plate  83 . The charging motor  84 , which is mounted on the intermediate mount plate  85 , drives the first rotating shaft  87  for rotation directly. An injection motor  88  is mounted on the rear plate  83  of the injection carriage  81 . 
     A second rotating shaft  92 , which is driven by the injection motor  88 , is linked to the intermediate plate  85  in alignment with the first rotating shaft  87  with a predetermined distance spaced apart from the rear end of the first rotating shaft  87 , so that the second rotating shaft  92  is free from interference due to the rotation of the first rotating shaft  87 . The second rotating shaft  92  has a threaded rod portion  91  which is adapted to serve as a threaded rod of a ball screw mechanism with a front end thereof abutting against a load cell  95  attached to a rear end surface of the intermediate mounting plate  85 . 
     A rotor  89  of the injection motor  88  formed integrally with a threaded nut member  90 , which is adapted to serve as a threaded nut of the ball screw mechanism, is linked in engagement with the threaded rod portion  91  of the second rotating shaft  92 . The injection motor  88  drives the threaded nut member  90  to move the second rotating shaft  92 , which can apply the trust force to the intermediate mount plate  85 . 
     In this embodiment, rotary encoders  93 ,  94  measures the angle of rotation of the charging motor  84  and the injection motor  88 , respectively. The load cell  95  measures the thrust force exerting on the intermediate mount plate  85 . In FIG. 5, guide bars for guiding the intermediate mount plate  85  fixedly holding the charging motor  84  are omitted for drawing. 
     In the injection process, electrical power is applied to the injection motor  88 , and the rotor  89  rotates together with the threaded nut member  90 . The rotational power of the injection motor  88  is converted to a thrust force by the threaded nut member  89  engaging with the threaded rod portion  91  of the second rotating shaft  92  to drive it for axial movement. The thrust force is transmitted to the intermediate mount plate  85  and the first rotating shaft  87  to move the screw  27  in the axial direction. Consequently, the screw  27  is forced to move to the left direction in FIG. 5, and injects the molten material accumulated in front of the screw  27  into a mold cavity (not shown). 
     In the charging process, electrical power is applied to the charging motor  84  to drive the first rotating shaft  87 , and the screw  27  connected to the first rotating shaft  34  is rotated. Resin pellets fed into the barrel unit  22  is plasticated by heater element (not shown) and the shearing action of the rotating screw  27 , and the molten material is accumulate and charged in front of the screw  27 . 
     The screw  27  retreats (to the right as viewed in FIG. 5) accompanying with rotation, due to the pressure of the molten material accumulated in front of the screw  27 . The first rotating shaft  87  rotates and retreats together with the intermediate mounting plate  85 , which forces the second rotating shaft  92  to move rearward (to the right as viewed in FIG. 5 ). The threaded nut  90 , which is free to rotate, allows the screw  27  to retreat to the initial position for the injection process. 
     While the screw  27  moves rearward, the first rotating shaft  87  and the second rotating shaft  92  are linked through the intermediate mount plate  85  so that the rear end of the first rotating shaft  85  is spaced from the front end of the second rotating shaft  92 . That provides a smooth retreat motion of the screw  27  with appropriate back pressure applied thereto during the charging process. Unlike the conventional injection apparatus as shown in FIG. 7, in which the charging motor and the injection motor are necessarily driven synchronously with complicated back pressure control, the appropriate back pressure can be applied to the screw  27  by braking the molten material accumulated in front of the screw  27  with only the injection motor  88  alone being controlled. 
     Fifth Embodiment 
     An injection unit  100  in a fifth embodiment according to the present invention will be described hereinafter with reference to FIG.  6 . All the elements common to all embodiments have the same reference numerals. 
     Referring to FIG. 6, the barrel unit  22  is mounted on a front plate  102  of an injection carriage  101 , and is provided with a heated barrel in which a screw  27  is disposed in the bore of the barrel unit  22  so as to be able to rotate and to move in the axial direction. A first rotating shaft  107  is connected to the rear end portion of the screw  27 . The first rotating shaft  107  is supported rotatably by bearings on an intermediate mount plate  105  which is disposed movably between the front plate  102  and a rear plate  103 . A charging motor  106  which drives first rotating shaft  107  is mounted on the front end of the intermediate mount plate  105 , and an injection motor  104  is mounted on the rear end of the rear plate  103 . 
     A second rotating shaft  110 , which is driven for rotation by the injection motor  104 , is supported rotatably by bearings on the rear plate  103 . The second rotating shaft  110  has a threaded rod portion  111  which is adapted to serve as a threaded rod of a ball screw mechanism. A threaded nut member  108 , which is adapted to serve as a threaded nut of the ball screw mechanism, is fixed on a load cell  109  which is attached to intermediate mount plate  105 . The threaded nut member  108  is linked in engagement with the threaded rod portion  111  of the second rotating shaft  110  in alignment with the first rotating shaft  107  with a predetermined distance spaced apart from the rear end of the first rotating shaft  107 . 
     In this embodiment, the load cell  109  disposed between the intermediate mount plate  105  and the threaded nut member  108  measures the thrust force exerting on the intermediate mount plate  105 . In FIG. 6, guide bars for guiding the intermediate mount plate  105  fixedly holding the charging motor  64  and the injection motor  106  are omitted for drawing. 
     In the injection process, electrical power is applied to the injection motor  104  to rotate the second rotating shaft  110 . The rotational power of the injection motor  104  is converted to a thrust force by the threaded nut member  108  engaging with the threaded rod portion  111  of the second rotating shaft  110  to drive it for axial movement. The thrust force is transmitted to the intermediate mount plate  105  to move the screw  27  in the axial direction. Consequently, the screw  27  is forced to move to the left direction in FIG. 6, and injects the molten material accumulated in front of the screw  27  into a mold cavity (not shown). 
     In the charging process, electrical power is applied to the charging motor  106  to drive the first rotating shaft  107  and the screw  27 . Resin pellets fed into the barrel unit  22  is plasticated, and the molten material is accumulated and charged in front of the screw  27 . The screw  27  retreats (to the right as viewed in FIG. 6) accompanying with rotation, due to the pressure of the molten material accumulated in front of the screw  27 . The first rotating shaft  107  rotates and retreats together with the intermediate mount plate  105 . The threaded nut member  108  forces the second rotating shaft  110  to rotate and move rearward (to the right as viewed in FIG. 6 ). The second rotating shaft  110 , which is free to rotate, allows the screw  27  to retreat to the initial position for the injection process. 
     The charging motor  106  dose not suffer any operational interference from the second rotating shaft  110  and the injection motor  104 . Unlike the conventional injection apparatus as shown in FIG. 7, in which the charging motor and the injection motor are necessarily driven synchronously with complicated back pressure control, the appropriate back pressure can be applied to the screw  27  by braking the molten material accumulated in front of the screw  27  with only controlling the injection motor  104 . 
     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.