Abstract:
An ejection mechanism for an injection molding machine comprising a combination of hydraulic booster cylinders for initiating an ejection cycle of an ejection apparatus and an electromechanical drive for completing the ejection cycle and retracting the ejection apparatus after completion of the ejection the molded parts.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates injection molding machinery. More specifically, the present invention relates to an ejector assembly for ejecting molded parts from a mold. 
       BACKGROUND OF THE INVENTION 
       [0002]    Some examples of known molding systems are: (i) the HyPET™ Molding System, (ii) the Quadloc™ Molding System, (iii) the Hylectric™ Molding System, and (iv) the HyMet™ Molding System, all manufactured by Husky Injection Molding Systems Ltd. 
         [0003]    U.S. Pat. No. 5,122,051 (Inventor: Joyner; Published: 1992-06-16) discloses an injection molding machine with an article-ejection apparatus where linear and rotary ejection means can be used individually, or in sequence. More specifically, this patent discloses a part ejection apparatus for ejecting molded parts from a mold carried by a moveable platen in an injection molding machine. A plurality of linearly operating hydraulic cylinders move an ejector bar that is carried along guide rods supported by a moveable platen so that the ejector bar moves toward and away from a mold member that is carried by the moveable platen. Suitable connections can be provided between the ejector bar and the ejection mechanism of the mold to drive ejector pins carried by the mold for separating the molded part from the mold surface. Additionally, a rotary motor is also carried by the moveable platen and has its axis coincident with the longitudinal axis of the moveable platen for connection of an output shaft of the motor with a drive mechanism carried by a mold. The drive mechanism is suitable for rotating cores that form internal threads on the molded article. The linear and rotary ejection apparatus can be used individually, or they can be used in appropriate sequence in a core-type mold where separation of the cores does not simultaneously effect separation of the part from the mold. 
         [0004]    Japanese Patent Application Number 4-168018 (Inventor: Watanabe et al; Published: 16 Jun., 1992) discloses an ejection mechanism in an ejector device for an injection molding machine. More specifically, this patent application discloses an ejector device with an attachment frame fixed to the rear surface of the moving platen in an injection molding machine. A servo motor and a speed reducer are connected to the fixed attachment frame on which a slide plate with an ejector pin projects from its front surface so as to permit movement to the front and rear. The output shaft of the speed reducer and the slide plate are connected by a crank mechanism in which the first link is longer than the second link. One end of the first link is attached firmly to the output shaft of the speed reducer. One end of the second link is attached to the rear surface of the slide plate so as to be freely rotatable. The other ends of the first link and the second link are rotatably joined together. A control means is provided which controls the rotation of the servo motor so that the first link moves reciprocally over a prescribed angular range, which is less than 45 degrees with respect to the line of the injection axis. 
         [0005]    U.S. Pat. No. 5,736,079 (Inventor: Kamiguchi et al; Published: 1998-04-07) discloses controlling an ejector for an injection molding machine in which an ejector mechanism is driven by a servo motor. An ejector pin in the ejector mechanism is made to perform a motion such that the ejector pin reaches a predetermined protrusion limit position beyond a position where the removal of a molded product from a cavity or core is completed after making a check for positioning. A plurality of cycles of reciprocating motion of short amplitude is performed such that the ejector pin: (i) neither retracts beyond a position where the removal of the molded product from the cavity or core is started, (ii) nor protrudes to the protrusion limit position without requiring a check for positioning. 
         [0006]    U.S. Pat. No. 5,824,350 (Inventor: Wietrzynski; Published: 1998-10-20) discloses a plastic molding tool with an accessory and an ejector or core pin unit that includes a date marking unit at a pin end that faces the plastic material during a molding operation. The date marking unit is used for marking the date of manufacture on components. More specifically, this patent discloses a mold for molding or injection-molding polymer compounds. At least one mold ancillary unit, in particular an ejector device, preferably an ejector pin, and/or a core pin device is distinguished by the fact that the mold ancillary device has, at least over a region, a marker unit in the region facing the polymer compound during the molding or injection-molding of the article. 
         [0007]    PCT Patent Application Number WO 02/40246 (Inventor: Weinmann et al: Published: 23 May 2002) discloses ejection of injection molded components from a molding tool that utilizes a load stored in a spring at the end of an ejection cycle to assist with molding release in the following cycle. More specifically, this patent application discloses a method and an ejection device for the ejection of injection-molded pieces from injection molds. The ejection device includes an electric motor drive for the ejector pin. The electric motor drive, at least in the last section of the return travel thereof, tensions a spring for energy storage. The spring tension force is subsequently used to begin the ejection movement by supporting the breaking free of the injection molded pieces. By means of a particularly advantageous embodiment, the spring force is applied in combination with a cam drive to augment the cam drive maximum in the region of the dead point with the equivalent maximum of the tensile spring force. A slide plate on which ejector pins are mounted is displaced by means of an electric motor using a lever drive. Two particular features are: (i) a support frame open at the rear for the cam movement, and (ii) preferably four guide columns on which the slide plate runs. The slide plate movement is thus more stable and, above all, takes account of the usually unequal releasing force for the various ejector pins. 
         [0008]    U.S. Pat. No. 6,811,391 (Inventor: Klaus et al; Published: 2004-11-02) discloses an electrically operated ejector mechanism for ejecting molded parts from a mold. The ejector mechanism uses an electric, reversible servo motor with an output shaft connected to a cam. More specifically, this patent discloses a molded part ejection system that includes a drive mechanism having a reversible servo motor. The drive mechanism for the ejection system includes a cam-and-follower arrangement whereby a circular cam member is driven by the servo motor through a drive shaft that is: (i) connected with the cam member and (ii) offset from the center of the circular cam track. A cam follower is connected with an ejector drive rod. The cam follower rides in the cam track to cause linear movement of the ejector drive rod as the cam follower follows the circular cam through its non-circular path of motion. Rotation of the servo motor in one direction of rotation operates the part ejection system, while rotation of the servo motor in the opposite direction of rotation provides power to another portion of the machine during another portion of a molding machine operating cycle, such as a core-pull system. The servo motor drive shaft includes a pair of one-way clutches that are each operable in a different direction of rotation of the motor drive shaft. In one direction of rotation, the motor actuates a part ejection mechanism. In the other direction of rotation, the motor can provide power to operate a different system of the molding machine. A single motor is permitted to perform two functions at different times during the operating cycle of an injection molding machine. 
         [0009]    German Patent Number 10,060,128 (Inventor: Becker et al: Published: 2005-05-12) discloses an ejection mechanism for injection molded components. The mechanism has an ejector plate moved by a crank drive between active and non-active positions. The mechanism includes a spring compressed at the end of the return stroke. More specifically, this patent discloses a device for ejecting injection molded parts from an injection mold held by a platen in an injection molding machine. The device includes an ejection plate to which at least one ejector pin is fixed or can be fixed. The pin is positioned in a moveable tool half of the injection mold in an axially displaceable manner. The ejector-receiving element can be moved from a retraction position into an ejection position via linear guiding mechanisms, parallel to the longitudinal axis of the at least one ejector pin, by means of a motor-driven crank mechanism that includes a crankshaft, a crank and a connecting rod. A spring force storage device is provided in which the spring force thereof acts in the direction of the ejection position. The connecting rod is embodied as an arched component. 
         [0010]    Japanese patent 07214610 (Inventor: Hiroshi, published 1995-08-15) teaches an ejector device of an injection molding machine. The purpose of the machine is to shorten a molding cycle by a method a large ejection force is produced in an initial ejection stage, and after the release of a molded piece from a mold, the molded piece is ejected at a high speed. A flange is provided on a movable platen through guide rods. An ejector bar is movably thrust through by the guide rods. A driving means for driving the ejector bar is provided. The drive means is composed of a large-diameter hydraulic cylinder having a short stroke and small-diameter hydraulic cylinders having a long stroke. A top surface of a piston rod of the large-diameter cylinder separately abuts on a head of an adjuster bolt. A top of a piston rod of the small-diameter cylinder is connected to the ejector bar. The large-diameter cylinder and the small-diameter cylinders are each connected to the pipes in parallel to be connected to an oil pressure source and a tank through a three-position solenoid operated valve. 
         [0011]    PCT patent application WO/02057062A1 (Inventor: Zelleröhr, published 2002-07-25) teaches The invention relates to an ejector device for moulding machines such as injection moulding machines, diecasting machines or the like. The ejector device comprises an ejector tool ( 40 ) which is fixed to an ejector plate ( 30 ). The ejector plate ( 30 ) is displaced by a drive from a spindle nut ( 60 ) and a threaded spindle ( 54 ) in the direction of a mould mounting plate ( 10 ) for ejection of a moulded part from a mould. In order to provide as high a force as possible at the beginning of the ejection process, an additional mechanical drive with another spindle nut ( 50 ) and a corresponding threaded section ( 52 ) is provided. The thread pitch of the threaded section ( 52 ) is lower than that of the threaded section ( 54 ). The second spindle nut ( 50 ) can be rotated counter to the mould mounting plate but is secured against axial displacement. If the second threaded nut is rotated, it displaces the threaded spindle and thereby the ejector plate in the direction of the mould mounting plate with a relatively large amount of force. 
         [0012]    In our earlier filed application U.S. Ser. No. 11/627,413 (Inventors: Schad et al.: filed 2007-01-26), there is provided an actuator of a molding system. The actuator includes: (i) an ejector plate, (ii) an ejector rod fixedly connected to the ejector plate, (iii) connecting links pivotally coupled to the ejector plate to transmit substantially balanced applied forces to the ejector plate, (iv) cranks pivotally connected to a respective link of the connecting links, (v) crank shafts fixedly connected to respective cranks, (vi) a drive shaft, (vii) an electric motor configured to rotate the drive shaft, and (viii) a belt coupling the drive shaft to the crank shafts, in response to the drive shaft being rotated by the electric motor. The belt moves so as to rotate the crank shafts. In response to the crank shafts being rotated by the belt, the cranks rotate so as to move the connecting links. In response to the connecting links being pushed by the cranks, the ejector plate receives the substantially balanced applied forces from the connecting links. In response to the ejector plate receiving the substantially balanced applied forces, the ejector rod move through a moveable platen of the molding system and a moveable mold portion supported by the moveable platen, the ejector rods pushes a molded article molded and retained in the moveable mold portion. 
       SUMMARY OF THE INVENTION 
       [0013]    According to a first aspect of the present invention, there is provided an ejector assembly for a molding machine, operable to move an ejector plate between a retracted position and an extended position to detach a molded article from a mold, the ejector assembly having an electromechanical assembly operable to move the ejector plate between the retracted position and the extracted position. At least one booster is operable to move the ejector plate from the retracted position at least a portion of the distance towards the extended position, the at least one booster providing additional force to detach the molded article from the mold. 
         [0014]    According to a second aspect of the present invention, there is provided an ejector assembly, comprising a hollow electric motor; a ball screw drive; at least one hydraulic piston; and a part ejector assembly. The hollow electric motor is configured to drive the ball screw drive and the ball screw drive is configured to move the ejector assembly laterally. The at least one hydraulic piston is configured to initiate lateral movement of the ejector assembly toward an ejection position. 
         [0015]    According to a third aspect of the invention, there is provided an ejector assembly for a molding machine, operable to move an ejector plate between a first position and a second position, having an electromechanical assembly, operable to urge the ejector plate towards one of the first position and the second position with a first quantity of force. A fluid power assembly is provided, operable to urge the ejector plate a portion of the distance towards the first position with a second quantity of force. 
         [0016]    The invention further provides an ejector mechanism for a molding machine that is operable to move an ejector assembly between a first position and a second position that includes an electromechanical assembly operable to urge the ejector assembly towards one of the first position and the second position with a first quantity of force and a fluid power assembly operable to urge the ejector assembly at least a portion of the distance towards the first position with a second quantity of force. The present invention provides a fast, but compact, system for ejecting parts from a mold. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    Exemplary embodiments of the present invention will now be described with reference to the accompanying drawings in which: 
           [0018]      FIG. 1  is a perspective view of a molding system according to a first exemplary embodiment; 
           [0019]      FIG. 2  is a perspective view of an ejector assembly mounted to the molding system of  FIG. 1 ; 
           [0020]      FIG. 3  is a side view of the ejector assembly shown in  FIG. 2 ; 
           [0021]      FIG. 4  is a side cross-sectional view of the ejector assembly shown in  FIG. 3 , with an ejector plate in the retracted position. 
           [0022]      FIG. 5  is a side sectional view of the ejector assembly shown in  FIG. 3  with the ejector plate in a slightly forward position; 
           [0023]      FIG. 6  is a side sectional view of the ejector assembly shown in  FIG. 3  with the ejector plate in the fully forward position; and 
           [0024]      FIG. 7  is a side sectional view of a booster for the ejector assembly shown in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]      FIG. 1  is the perspective view of a molding system  20  (preferably an injection molding system hereafter referred to as the “system  20 ”) according to the first exemplary embodiment. The system  20  is used to mold one more molded articles (not shown). The system  20  includes components that are known to persons skilled in the art and these known components will not be described here; these known components are described, by way of example, in the following references: (i) Injection Molding Handbook by Osswald/Turng/Gramann ISBN: 3-446-21669-2; publisher: Hanser, and (ii) Injection Molding Handbook by Rosato and Rosato ISBN: 0-412-99381-3; publisher: Chapman &amp; Hill. 
         [0026]    The system  20  includes (amongst other things): (i) an injection-type extruder  22  (hereafter referred to as the “extruder  22 ”), (ii) a hopper  24 , (iii) a control cabinet  26 , (iv) a human-machine interface, hereafter referred to as the “HMI  28 ”, (v) a stationary platen  30 , (vi) a moveable platen  32 , and (vii) an ejector assembly  34  (described in greater detail below).  FIG. 1  depicts an approximate location of the ejector assembly  34  relative to the system  20 . The extruder  22  has a barrel and a reciprocating screw disposed in the barrel. Alternatively, the extruder  22  could be a two stage shooting pot configuration. The hopper  24  is coupled to a feed throat of the extruder  22  so as to deliver pellets of moldable material to the extruder  22 . The extruder  22  is configured to: (i) process the pellets into an injectable molding material, and (ii) inject the injectable material into a mold that is held closed by the platens  30 ,  32  after the platens  30 ,  32  have been stroked together. The control cabinet  26  houses control equipment that is used to control the system  20 . The HMI  28  is coupled to the control equipment, and the HMI  28  is used to assist an operator in monitoring and controlling operations of the system  20 . 
         [0027]    The stationary platen  30  is configured to support a stationary mold portion of a mold (not shown). The moveable platen  32  is configured to: (i) support a moveable mold portion of the mold, and (ii) move relative to the stationary platen  30  so that the mold portions of the mold (neither shown) may be separated from each other or closed together. A platen stroke actuator  36  (hereafter referred to as the “actuator  36 ”) is coupled to the platens  30 ,  32 . Preferably, there are two platen stroke actuators, each of which are mounted, respectively, at opposite diagonal corners of the platens  30 ,  32 . The actuator  36  is used to stroke the moveable platen  32  relative to the stationary platen  30 . The stationary platen  30  supports four clamp actuators  38  that are each positioned in respective corners of the stationary platen  30 . Four tie bars  40  each extend from their respective clamp actuators  38  toward respective corners of the moveable platen  32 . The tie bars  40  are lockable relative to the moveable platen  32  by usage of respective tie-bar locks  41  that are each supported in respective corners of the moveable platen  32 . 
         [0028]    Referring now to  FIGS. 2 and 3 , ejector assembly  34  is shown in greater detail, and depicts the location of the ejector assembly  34  relative to the movable platen  32 . The ejector assembly  34  is used to move, displace or linearly translate an ejector plate  42 . The ejector plate  42  is fixedly connected to a set of ejector pins  44  (that is, one or more ejector pins  44 ), which extend through apertures  46  defined in the movable platen  32 . For the purposes of clarity, some of the ejector pins  44  have been removed from  FIGS. 2 and 3 . The ejector pins  44  further extend through apertures in the adjacent mold half (none shown), and are operable to dislodge molded articles via the movement of ejector plate  42  in a manner known to those of skill in the art. 
         [0029]    Referring additionally to  FIG. 4 , ejector assembly  34  further includes an electro-mechanical assembly operable to linearly translate ejector plate  42 . The electro-mechanical assembly includes a motor  50  that is mounted to a housing  52 . Preferably, motor  50  is reversible hollow electric motor, but other types of motors will occur to those of skill in the art. When motor  50  is engaged, motor  50 , a rotor  54  drives an output shaft  56 , the output shaft  56  being supported in housing  52  by bearings  64 . In the presently-illustrated embodiment, the output shaft  56  is an annular shaft, coaxially mounted within the hollow cavity of rotor  54 , but other transmission arrangements between output shaft  56  and motor  50  are within the scope of the art. For example, output shaft  56  could be belt or gear driven by motor  50  (although such a transmission would typically require more packaging than the currently-illustrated embodiment). Alternatively, depending on the sizing of the shaft and the motor, a gear reducer could be used (none shown). 
         [0030]    Motor  50  reversibly drives a ball screw. A ball nut  58  is fixedly mounted on the distal end of output shaft  56  (i.e., away from motor  50 ) by drive pins  60  so that engaging motor  50  rotates ball nut  58 . Ball nut  58  and output shaft  56  define a common diameter bore  62  (best seen in  FIG. 6 ). A screw shaft  66 , having threads complementary to ball nut  58 , extends through bore  62 . Balls (omitted for the purposes of clarity) are packed between ball nut  58  and screw shaft  66 . When motor  50  engages, ball nut  58  translates the screw shaft  66  via the balls. The range of travel of screw shaft  66  is delimited by a bottom  70  on motor  50  in the first, inward direction, and normally, by the movable platen  32  ( FIGS. 1 and 2 ) in the second, outward direction. (If motor  50  is engaged while the ejector assembly  34  is dismounted from the movable platen  32 , then engaging motor  50  in the second direction will cause screw shaft  66  to exit bore  62 . 
         [0031]    Housing  52  is spaced apart from and mounted to the movable platen  32  ( FIGS. 1 and 2 ) by parallel guide rods  68 . In the presently-illustrated embodiment, four guide rods  68  are radially distributed around housing  52  (for the purposes of clarity, only two guide rods  68  are shown). A first end  72  for each of guide rods  68  extend through a bore  74  provided within housing  52 , and is retained in place by a flange portion  76  on one side of housing  52  and a nut  78  on the other side of housing  52 . A second end  79  of each guide rod is fixedly mounted to the movable platen  32  ( FIGS. 1 and 2 ). 
         [0032]    The ejector plate  42  is movably located between housing  52  and movable platen  32 , and during operation of ejector assembly  34 , translates along guide rods  68 . A first planar surface  82 A on ejector plate  42  faces towards the movable platen  32  ( FIGS. 1 and 2 ), and a second planar surface  82 B faces towards housing  52 . Each guide rod  68  extends through a guide bore  84  defined by ejector plate  42 . Preferably, each guide bore  84  includes bearings to ensure smooth movement of ejector plate  42  along the guide rods  68 . Pin mounts  80  are provided to locate and fasten ejector pins  44  to ejector plate  42 . 
         [0033]    A second end  88  of screw shaft  66  is fixedly mounted to the second surface  82 B of ejector plate  42  via a bolt  90 , thereby coupling the linear motion of screw shaft  66  to ejector plate  42 . Locator pins  94  assist positioning ejector plate  42  on screw shaft  66 . Motor  50  acts as the primary motivator to translate ejector  80  between a retracted position (proximate housing  52 ) and an extended position (proximate the movable platen  32 ) for parts ejection, and then return it back to the retracted position. 
         [0034]    Radially spaced around the peripheral regions of housing  52  is a plurality of boosters, each operable to act as a secondary motivator in moving ejector plate  42  from the retracted position to the extended position. In the presently-illustrated embodiment, the boosters are hydraulic pistons  86 . Generally speaking, hydraulic pistons  86  generate a significantly larger force acting on ejector plate  42  than motor  50 , albeit only for a portion of the travel distance of the ejector plate from the retracted position ( FIG. 4 ) to the extended position ( FIG. 6 ). Also generally speaking, hydraulic pistons  86  do not assist ejector plate  42  in returning to the retracted position from the extended position. 
         [0035]    Referring additionally to  FIG. 7 , each hydraulic piston  86  has a piston rod  92 , slidably located within a booster cylinder  96 , the piston rod being operably extensible towards the ejector plate  42 . The piston rods  92  are aligned parallel with the longitudinal axis of screw shaft  66 . The hydraulic pistons  86  initiate movement of the ejector plate  42  by driving piston rods  92  into engagement with the second surface  82 B of ejector plate  42  when the ejector plate is in the fully withdrawn position as will be more fully described hereinafter. In the presently-illustrated embodiment, four hydraulic pistons  86  are provided (for the purposes of clarity, only two are shown) but a different number could be used. 
         [0036]    In the currently-illustrated embodiment, each booster cylinder  96  defines a first cylindrical chamber  98  and a second, narrower cylindrical chamber  100  that is in communication with first cylindrical chamber  98 . The booster cylinders  96  can be mounted to housing  52 , or integrally formed as part of housing  52 . Second cylindrical chambers  100  are defined within end caps  102 . The piston rods  92  have a larger diameter portion  104 , sized for a fluid-tight fit within first cylindrical chamber  98 , and a narrower diameter portion  106 , sized for a fluid-tight fit within second cylindrical chamber  100 . The larger diameter portion  104  of piston rod  92  subdivides first cylindrical chamber  98  into portions  116 A and  116 B. When assembling the boosters  86 , the piston rods  92  are positioned within the first cylindrical chambers  98 , and then, the second cylindrical chambers  100  slid over the narrow diameter portions  106  of the piston rods  92 . The end caps  102  are secured to housing  52  via fasteners  110 . A shoulder  108  on piston rod  92  prevents the larger diameter portion  104  from exiting first cylindrical chamber  98  into the second cylindrical chamber  100 . 
         [0037]    Piston rods  92  move between from a retracted position, where the base of piston rod  92  abuts a cylinder base  114  and an extended position, where shoulder  108  abuts the end cap  102  (i.e., towards ejector plate  42 ) by hydraulically pressurizing portion  116 A of each first cylindrical chamber  98 . For the purposes of clarity, the hydraulic ports and lines have been omitted from the illustration. Seals  120  are provided to prevent leakage of hydraulic fluid. Bearings  124  help piston rods  92  from moving between the retracted and extended positions. Hydraulic pistons  86  can be configured as single-action or dual action. Single-action hydraulic pistons  86  move piston rods  92  to the extended position by hydraulically pressurizing portion  116 A of each first cylindrical chamber  98 , but move the piston rods to the retracted position by the return movement of ejector plate  42 . Dual action piston hydraulic pistons  86  move the piston rods  92  to the extended position by hydraulically pressurizing portion  116 A of each first cylindrical chamber  98 , and return the piston rods to the retracted position by hydraulically pressurizing the portion  116 B of each first cylindrical chamber  98 . 
         [0038]    Referring now to  FIGS. 4-6 , an ejection sequence for ejector assembly  34  is shown. In  FIG. 4 , motor  50  is turned off and piston rods  92  are in their fully retracted position, i.e., fully withdrawn into hydraulic pistons  86 . The ejector plate  42  is also in the retracted position, so that its second planar surface  82 B abuts against the distal end faces  118  of piston rods  92 . 
         [0039]    In  FIG. 5 , the hydraulic pistons  86  have been activated, and the movement of the ejector plate  42  towards the extended position has been initiated. Portion  116 A of each first cylindrical chamber  98  is pressurized, moving the piston rods  92  to the extended position. When the booster cylinders are charged, the piston rods  92  are extended and push against the ejector plate  42 . The pressure on the piston rods  92  is sufficient to overcome all the inertia created by the ejector assembly  34 , including the ejector plate  42 , ball nut  58  and the screw shaft  66 . The motor  50  may also be engaged, but need not be during this initial phase as the initial driving force provided by the hydraulic pistons  86  is significantly greater that the force provided by the motor  50  through the output shaft  56  and ball nut  58 . Piston rods  92  initiate accelerated lateral motion of the ejector plate  42  to a position beyond the end faces  118  of the piston rods  92 . Movement of the ejector plate  42  creates lateral movement of the screw shaft  66  and accelerates rotation of ball nut  58  and rotor  54 . If motor  50  was not engaged simultaneously with hydraulic boosters  56 , then it is now energized so that rotor  54  rotates to rotate output shaft  56 . The rotation of output shaft  56  rotates ball nut  58  to move screw shaft  66  laterally, driving ejector plate  42  towards its extended position. 
         [0040]    In  FIG. 6 , the ejector plate  42  is in its fully extended position. The ejector pins  44  will have removed the molded articles from the mold cores (none shown). When the screw shaft  66  reaches the end of its stroke with the ejector plate  42  in its fully extended position the motor  50  is turned off. 
         [0041]    After the ejection cycle is completed the motor  50  is energized in reverse to rotate the output shaft  56  and ball nut  58  in the reverse direction and thereby retract the screw shaft  66 , ejector plate  42  and ejector pins  44  to the retracted position. If hydraulic pistons  86  are single-action hydraulic pistons  86 , the piston rods  92  are retracted by the return movement of ejector plate  42  when no significant hydraulic charge exists in cylindrical chamber  116 A. Alternatively, with double action hydraulic pistons  86 , then cylindrical chamber  116 B may be pressurized to retract the piston rods  92 . Once the screw shaft  66  and piston rods  92  are returned to the fully retracted position shown in  FIG. 4 , the ejector assembly  34  is ready for its next ejection cycle. 
         [0042]    When movement of the ejector plate  42  has been initiated by the action of the piston rods  92 , the screw shaft  66  is also in motion. When the ejector plate  42  is in motion, the motor  50  does not need to overcome the inertia of a stopped system and can readily continue motion of the ejector assembly to eject molded parts. This combination of initiating motion by hydraulic means and maintaining motion by electromechanical means enables the assembly to be more compact. As the hydraulic pistons only move the plate a short distance the booster cylinders are short, compact and require a minimal amount of hydraulic fluid. As the motor  50  does not have to overcome the inertia of a stopped ejector assembly  34  a much smaller electric motor is needed. 
         [0043]    While the present invention has been described with respect to what is presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.