Abstract:
A pressurized-fluid-operated actuator has multiple piston surfaces for providing increased output force. The actuator includes a stationary cylinder that contains a movable cylinder having multiple inner chambers separated by stationary inner pistons. The inner chambers are in fluid communication with extension and retraction ports provided in the stationary cylinder wall. Introduction of pressurized fluid into one port causes the fluid pressure to act on “n” piston surface(s) to retract the movable cylinder. Introduction of pressurized fluid into the other port causes the fluid pressure to act on “n+1” piston surfaces to provide an increased output force without increasing the diameters of the cylinders and without increasing the pressure of the fluid.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a linear actuator having a plurality of piston surfaces for providing a higher output force upon application of a pressurized fluid to a given diameter actuator. More particularly, the present invention relates to a pressurized-fluid actuator of compact size and that includes a movable cylinder having one or more axially spaced piston surfaces for applying an actuating force for movement of the cylinder in a retraction direction, and at least one additional piston surface for applying additional actuating force for moving the cylinder in an extension direction. 
     2. Description of the Related Art 
     Linear actuators incorporating pistons that move within cylinders upon application of a pressurized fluid are well known in the art and are used for many different purposes. Generally, the output force provided by such actuators can be increased either by increasing the pressure of the fluid supplied to operate the actuator, or by increasing the diameter of the piston to increase the surface area of the piston. However, some applications require the actuator to be contained within a very limited space (so that the diameter of the piston cannot be increased). In such applications, the pressure necessary to provide the required actuating force may exceed practical limits. Accordingly, an alternative design is needed that will provide the required actuating force in a limited space at a reasonable pressure. 
     A number of actuator constructions have been devised in an effort to respond to the shortcoming described above. Although the prior art discloses various devices for providing increased output force from a pressurized-fluid operated actuator, the devices typically either involve a complex mechanism or otherwise fail to reduce the size of the actuator sufficiently to enable its use in a confined space. 
     SUMMARY OF THE INVENTION 
     Briefly stated, in accordance with one aspect of the present invention an actuator is provided that includes a stationary cylinder of tubular form that defines an inner cylindrical surface. A movable cylinder is slidably carried within the stationary cylinder and has a rod affixed thereto, the rod extending from the movable cylinder in an axial direction relative to the stationary cylinder. The movable cylinder divides the stationary cylinder into an upper chamber and a lower chamber. The movable cylinder also includes an inner cylindrical space. 
     A fixed piston extends transversely across the inner cylindrical space of the movable cylinder, thereby dividing the inner cylindrical space into a first inner chamber and a second inner chamber. A first fluid conduit is in communication with the upper chamber of the stationary cylinder and with the first inner chamber of the movable cylinder. Supplying a pressurized fluid to the first fluid conduit thus initiates movement of the movable cylinder and rod in a forward direction relative to the stationary cylinder, thereby providing a rod extension stroke. A second fluid conduit is in fluid communication with the second inner chamber for moving the movable cylinder and rod in a reverse direction relative to the stationary cylinder when pressurized fluid is introduced, thereby providing a rod retraction stroke. 
     In accordance with another aspect of the present invention, the actuator includes a stationary cylinder containing a movable cylinder that is divided into multiple inner chambers by fixed (stationary) pistons. The inner chambers are in fluid communication with extension and retraction ports provided in the cylinder wall. Introduction of pressurized fluid into the retraction port causes the fluid pressure to act on “n” piston surface(s) to retract the movable cylinder and rod, while introduction of pressurized fluid into the extension port causes the fluid pressure to act on “n+1” axially-spaced piston surfaces to extend the movable cylinder and rod. Accordingly, the actuator provides an increased extension force without increasing the overall diameter of the actuator and without increasing the pressure of the fluid. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary, side elevational view, partially in cross-section, of a portion of a mold assembly in an injection molding machine incorporating a linear actuator in accordance with the present invention operatively associated with a valve gate for controlling the flow of plasticated material to a mold cavity, wherein the movable cylinder is in a fully retracted position so that the valve pin is in the open position to allow flow of plastic melt into the mold cavity. 
     FIG. 2 is an enlarged view of the portion of FIG. 1 contained within the circle  2 . 
     FIG. 3 is a cross-sectional view of the actuator in accordance with the present invention, taken along the line  3 — 3  of FIG.  1 . 
     FIG. 4 is a cross-sectional view of the actuator in accordance with the present invention, taken along the line  4 — 4  of FIG.  1 . 
     FIG. 5 is a partial side elevational view similar to that of FIG. 1, showing the movable cylinder of the actuator in an intermediate position between fully retracted and fully extended positions. 
     FIG. 6 is a partial side elevational view similar to that of FIGS. 1 and 5 showing the movable cylinder of the actuator in a fully extended position. 
     FIG. 7 is a fragmentary, side elevational view, partially in cross-section, of a portion of a mold assembly, showing an alternate embodiment of an actuator in accordance with the present invention capable of providing increased actuating force, wherein the movable cylinder of the actuator is in a fully retracted position. 
     FIG. 8 is a partial side elevational view of the same alternate embodiment shown in FIG. 7, wherein the movable cylinder of the actuator in a fully extended position. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to FIGS. 1 and 2 of the drawings, there is shown a valve actuator  10  in accordance with the present invention. Operatively associated with the actuator  10  is a flow nozzle  12  for receiving a flow of molten plastic material from an injection unit (not shown) supplied via a molding material passageway  14 . The nozzle  12  conveys and controls the supply of plastic melt to a mold cavity  16  that is defined by respective opposed, suitably-shaped recesses formed in a first mold segment  18  and a cooperating second mold segment  20 . As will be appreciated by those skilled in the art, the first mold segment  18  is maintained in a stationary condition. The second mold segment  20  is supported for movement relative to the first mold segment  18  to define the closed mold cavity  16  when the mold segments  18 ,  20  are in contact, and to allow removal of a molded part by separating the mold segments  18 ,  20  to open the mold cavity  16 . 
     During operation, the plastic melt from the injection unit is caused to flow through the molding material passageway  14  and into the nozzle  12 . The nozzle  12  includes a discharge outlet or gate  22  that communicates directly with the mold cavity  16 . Flow of the plastic melt through the nozzle  12  is controlled by a valve pin  26  that is movable toward and away from the gate  22  to close and open selectively the gate  22  at appropriate times during a molding cycle. As shown in FIG. 1, the valve pin  26  is in the retracted or open position, which will allow flow of plastic melt through the nozzle  12  and into the mold cavity  16 . 
     The valve pin  26  is an end portion of an elongated rod  28  that has its opposite end connected to the actuator  10 . More specifically, the actuator  10  includes two concentric cylinders, a movable cylinder  30  that connects to the rod  28 , and a stationary cylinder  32  that slidably receives the movable cylinder  30 . An upper end wall  34  and a lower end wall  36  serve to close the movable cylinder  30 , so that it functions as a piston within the stationary cylinder  32 , as will be more fully described later. An end cap  38  closes one end of the stationary cylinder  32  to define an upper chamber  40  between the end cap  38  and the upper end wall  34  of the movable cylinder  30 . Preferably, the actuator  10  is contained within an appropriately sized bore  24  in a mold plate  44 , and is held in place by appropriate fasteners, such as bolts  42  through the end cap  38 . The mold plate  44  is suitably secured relative to the nozzle  12  in mold segment  18  so that the rod  28  and associated valve pin  26  are properly oriented relative to the valve seat  46  in the nozzle  12 . As shown, the valve seat  46  includes a tapered passageway that diverges from the gate  22  to a cylindrical bore  48  that interconnects with the passageway  14 . 
     Preferably, the stationary cylinder  32  has a stepped diameter to be received by the similarly stepped bore  24  in mold plate  44 . This stepped design facilitates an economical assembly, using a fluid-tight seal between the stationary cylinder  32  and the bore  24 , so that the lower portion of the bore  24  and the stationary cylinder  32  work together to contain the movable cylinder  30 . This minimizes the overall length of the stationary cylinder  32  since the lower end wall  36  of the moving cylinder  30  seals against the lower portion of the bore  24  in the mold plate  44 . Alternatively, the bore  24  in the mold plate  44  could be machined with a constant diameter to receive a stationary cylinder  32  having a straight diameter and extending to the bottom of the bore  24  to fully contain the movable cylinder  30 . In either case, a lower chamber  41  is defined by the space between the lower end wall  36  and the bottom wall  25  of the bore  24  in plate  44 . A bore  45  in the plate  44  is sized to allow passage of the rod  28  with sufficient clearance to provide a vent to ambient atmosphere for the lower chamber  41 . 
     Within the stationary cylinder  32  a support post  50  rigidly connected to the end cap  38 , as by a bolt  52 , extends downwardly and passes through the upper end wall  34  and into the interior of the movable cylinder  30 . The support post  50  terminates at a transversely-extending fixed piston  54 . The fixed piston  54  is a disk-shaped member that has a peripheral edge  56  that is spaced inwardly of the inner surface of the cylinder side wall  58  of the stationary cylinder  32 , so that the piston  54  is contained within the movable cylinder  30 . The peripheral edge  56  preferably includes an annular recess  60  to receive a peripheral sealing ring  62 . 
     The movable cylinder  30  is a hollow, generally cylindrical structure that is received within the stationary cylinder  32  and bore  24  for axial, sliding movement along the inner surfaces thereof. The annular lower end wall  36  of the cylinder  30  extends transversely inside the bore  24  just below the stationary cylinder  32 . As shown, the lower end wall  36  can comprise two portions fastened together by bolts  64  to facilitate manufacture, as well as attachment of the rod  28  to the movable cylinder  30 . The lower end wall  36  includes a flanged portion  66  with an outer peripheral recess  68  to receive a first outer sealing ring  70  on the movable cylinder  30 , which is slidable along and that sealingly engages the inner surface of the bore  24 . As shown, the lower end wall  36  with flanged portion  66  is positioned between the fixed piston  54  and the bottom wall  25  of the bore  24 . 
     Extending axially from the periphery of the lower end wall  36  adjacent and along the inner surface of the cylinder side wall  58  and toward the end cap  38  is a tubular side wall  72  of the movable cylinder  30 . A second outer sealing ring  74  and a third outer sealing ring  76  are each carried in annular recesses  78 ,  80 , respectively, on the outer periphery of the side wall  72  of the movable cylinder  30 , in axially spaced relationship with the first outer sealing ring  70  and in axially spaced relationship with each other. Each of the second and third sealing rings  74 ,  76  are slidable along and sealingly engage the inner surface of the cylinder side wall  58 . The end  82  of the rod  28  opposite from the valve pin  26  is securely received within the flanged portion  66  of the lower end wall  36 , so that both the movable cylinder  30  and the valve pin  26  move together. 
     Spaced axially along the side wall  72  from the lower end wall  36  and on the opposite side of the fixed piston  54  from the lower end wall  36  is the upper end wall  34  of the movable cylinder  30 , extending across the interior space defined by the side wall  72 . As shown most clearly in FIG. 2, the inner surface of the side wall  72  includes a radial step  84  against which the upper end wall  34  rests, and an annular retaining ring  86  is received in an inner peripheral groove  88  formed in the inner surface of the side wall  72  to retain the upper end wall  34  in position relative to the side wall  72 . Additionally, the upper end wall  34  preferably includes an outer peripheral recess  89  to receive a sealing ring  90 , as well as an inner annular recess  92  to receive a sealing ring  94 , to facilitate a fluid tight seal of the upper end wall  34  with the side wall  72  and support post  50 , respectively. 
     As best seen in FIG. 5, the volume between the lower end wall  36  and the fixed piston  54  defines a first inner chamber  96  within the cylinder  30 , and the annular volume between the upper end wall  34  and the fixed piston  54  defines a second inner chamber  98  within the cylinder  30 . The side wall  72  includes radially-extending openings  100  that provide fluid communication with the second inner chamber  98 , as will be more fully explained later. 
     A first port  102  and second port  104  in the mold plate  44  open into the bore  24 , each of the ports  102 ,  104  being adapted to be alternately in communication with either a source of pressurized fluid (not shown), such as pressurized gas or pressurized hydraulic fluid, or with a lower pressure fluid reservoir (not shown). The connections between the ports  102 ,  104  and the respective pressurized fluid source and lower pressure fluid reservoir can be effected through a suitable reversible flow control valve (not shown) of a type that is well known to those skilled in the art. The first port  102  connects with an annular chamber  106  surrounding the lower portion of the stationary cylinder side wall  58  and adjacent the lower end wall  36  of the movable cylinder  30 . The annular chamber  106  communicates with both the first inner chamber  96  and the upper chamber  40  of the stationary cylinder  32  via a passage  108  in the lower end wall  36 . More specifically, the passage  108  opens into the first inner chamber  96 , which communicates with the upper chamber  40  via a second passage  110  and radial openings  118  in the support post  50  of the fixed piston  54 , thus enabling fluid communication of the first port  102  with both the first inner chamber  96  and upper chamber  40 . 
     The second port  104  terminates at a channel  112  that connects with radial openings  114  through the side wall  58  of the stationary cylinder  32 . The radial openings  114  communicate with an annular chamber  116  around the upper portion of the movable cylinder  30 . The annular chamber  116  connects with the second inner chamber  98  through the radially-extending openings  100  in the side wall  72  just below the radial step  84 , thereby enabling fluid communication between the second port  104  and the second inner chamber  98  of the movable cylinder  30 . 
     The actuator  10  is shown in FIGS. 1 and 2 with the movable cylinder  30 , rod  28 , and valve pin  26  each in their retracted positions, relative to the stationary cylinder  32  and to the valve seat  46 . In operation, to cause the valve pin  26  and cylinder  30  to move from their retracted positions, pressurized fluid is introduced through the first port  102 , while the second port  104  is in fluid communication with a lower pressure fluid reservoir, or the like. The introduction of pressurized fluid at the first port  102  causes the pressurized fluid to enter into and to flow through the annular channel  106 , then through the first passage  108  and into the first inner chamber  96  within the cylinder  30 . Simultaneously, a portion of the pressurized fluid flows from the first inner chamber  96  into and through the second passage  110  and radial openings  118  to enter into the upper chamber  40 . Consequently, each of the upper chamber  40  and the first inner chamber  96  are at an elevated pressure, relative to the lower chamber  41 , which is vented to the atmosphere through the bore  45  around rod  28 , and relative to the second inner chamber  98 . The second inner chamber  98  is in fluid communication with the lower pressure fluid reservoir through the openings  100 , annular chamber  116 , radial openings  114 , annular channel  112 , and second port  104 . 
     The resultant pressure differentials acting against each of the upper end wall  34  and lower end wall  36  cause the cylinder  30  to move toward the bottom wall  25  of the bore  24 , which causes the valve pin  26  to move toward the valve seat  46 ; i.e., from the position shown in FIG.  1  through an intermediate position, such as that shown in FIG.  5 . For the time during which the cylinder  30  and valve pin  26  are moving in this manner, the volume of each of the lower chamber  41  and the second inner chamber  98  is decreasing, while the volume of each of the first inner chamber  96  and upper chamber  40  is increasing. At the same time, any fluid within the second inner chamber  98  passes through the openings  100 , into annular chamber  116 , through radial openings  114 , into annular channel  112 , and finally out through the second port  104 , which is at a lower pressure than the first port  102 . The continued application of greater fluid pressure at the first port  102  will cause the cylinder  30  to travel to the end of its extension stroke, as shown in FIG.  6 . Upon completion of the extension stroke, the upper end wall  34  makes contact with a forward cushion  120  attached to the fixed piston  54  and the end of the valve pin  26  is held tightly against the valve seat  46  to block flow through the gate  22 . To avoid a pressure build-up that would act against the lower end wall  36 , air contained within the lower chamber  41  is exhausted through the bore  45 . 
     In order to move the actuator  10  from the extended position shown in FIG. 6, thereby opening the gate  22  and allowing the flow of molten plastic material into the mold cavity  16 , the first port  102  must be disconnected from the source of pressurized fluid and is placed in communication with a lower pressure reservoir, or the like. The second port  104  is then connected with the source of pressurized fluid, and pressurized fluid enters the annular channel  112  through the second port  104 . From the annular channel  112  the pressurized fluid flows through the radial openings  114  in the side wall  58  and into the second inner chamber  98  within the cylinder  30 . The entry of pressurized fluid into the second inner chamber  98  applies a force against the inner surface the upper end wall  34 , causing the cylinder  30  and the valve pin  26  to retract into the stationary cylinder  32  until it reaches the position shown in FIG. 1, where the lower end wall  36  makes contact with a retract cushion  122  attached to the fixed piston  54 . As a result, the valve pin  26  retracts into the nozzle  12  and away from the gate  22  to allow molding material to flow through the nozzle  12  and into the mold cavity  16 . The partial vacuum that would otherwise be generated within the lower chamber  41  is relieved by allowing ambient air to enter the lower chamber  41  through the bore  25  around the rod  28 . 
     FIGS. 7 and 8 show an alternate embodiment of a valve actuator  125  in accordance with the present invention that employs additional surfaces to further enhance the actuation force. The actuator  125  has an outer housing  126  and end cap  138  that are received in a mold plate  143 . A stationary cylinder  132  is received within the interior space defined by the housing  126 , the end cap  138  and a bore  124  in adjacent mold plate  144  that matches the inside diameter of the housing  126 . Preferably, bolts  142  passthrough the end cap  138  and housing  126  to engage the underlying mold plate  144  to fix the actuator  125  in the mold assembly. Alternatively, the stationary cylinder  132  could be received within mating bores in the mold plates  143 ,  144  or within a single mold plate, as in the previously described embodiment, eliminating the need for the outer housing  126 . A movable cylinder  130  is slidably received within the stationary cylinder  132 . An elongated rod  128  has one end  182  securely received within a lower end wall  136  of the movable cylinder  130 , so that both the movable cylinder  130  and the rod  128  move together, with the opposite end of the rod  128  functioning as a valve pin, as previously described. The end cap  138  closes one end of the stationary cylinder  132  to define an upper chamber  140  between an upper wall  134  of the movable cylinder  130  and the end cap  138 . A bottom wall  127  of the bore  124  closes the opposite end of the stationary cylinder  132  to fully contain the movable cylinder  130 . 
     The actuator  125  includes a first port  202  and a second port  204 , each of which is alternately adapted to be in communication with a source of pressurized fluid (not shown), such as pressurized gas or pressurized hydraulic fluid, and with a lower pressure fluid reservoir (not shown), as described previously. The first port  202  extends through the outer housing  126  and terminates at a channel  206  that connects with a first passage  208  contained in a tubular side wall  158  of the cylinder  132 . The first passage  208  opens at one end  209  into the upper chamber  140 , as well as an intermediate point  210  and an opposite end  211  to enable fluid communication between the first port  202  and the interior of the movable cylinder  130  for purposes that will be hereinafter explained. The second port  204  extends through the outer housing  126  and terminates at a channel  212  that connects with a second passage  214  contained in the wall  158  of the cylinder  132 . The second passage  214  opens at one end  215  into an annular channel  216 , as well as at an opening  217  to a second annular channel  218  to enable fluid communication between the second port  204  and the interior of the movable cylinder  130 . 
     The movable cylinder  130  is a hollow, generally cylindrical structure that is received within the stationary cylinder  132  for axial, sliding movement along the inner surface thereof. The movable cylinder  130  includes an annular lower wall  136  that extends transversely to seal against the inside the cylinder  132 . The lower end wall  136  includes an outer peripheral recess  168  to receive a first outer sealing ring  170  that is slidable along and that sealingly engages the inner surface of the cylinder  132 . Extending axially from the periphery of the lower end wall  136  adjacent and along the inner surface of the wall  158  of the cylinder  132  and toward the end cap  138  is a tubular side wall  172 . The annular upper end wall  134  extends across the interior of the end of the movable cylinder  130  adjacent the end cap  138 . The upper end wall  134  is held in place by retaining rings  186  in grooves  184  in the side wall  172 . Four outer sealing rings  174  are each carried in annular recesses  176  on the outer periphery of the side wall  172  in axially spaced relationship with the first outer sealing ring  170  and in axially spaced relationship with each other. Each of the outer sealing rings  174  is slidable along and sealingly engages the inner surface of the side wall  158  of the stationary cylinder  132 . 
     Attached to the end cap  138  by a bolt  152  is a support post  150  that passes through the upper end wall  134  of the cylinder  130  and extends into the interior of the cylinder  130  to terminate at a transversely-extending, first fixed piston  154 . A second fixed piston  155  is attached to the support post  150  by suitable means, such as retaining rings  156 , at a position intermediate the end cap  138  and the first fixed piston  154 . The fixed pistons  154 ,  155  are disk-shaped members that are sized to be received within the tubular side wall  172 . The peripheral edge of each of the fixed pistons  154 ,  155  includes an annular recess  160  to receive a peripheral sealing ring  162 . Spaced axially along the side wall  172  from the lower end wall  136  and between the fixed pistons  154 ,  155  is an intermediate wall  135  that extends across the interior of the movable cylinder  130  and is attached to the side wall  172  by retaining rings  186 . The intermediate wall  135  and upper end wall  134  can include interior and peripheral recess  188 ,  192  to receive an annular sealing rings  190 ,  194 , as shown. 
     The volume between the lower end wall  136  and the first fixed piston  154  defines a first chamber  195  within the movable cylinder  130 . The annular volume between the first fixed piston  154  and the intermediate wall  135  defines a second chamber  196 . The annular volume between the intermediate wall  135  and the second fixed piston  155  defines a third chamber  197 , and the annular volume between the upper end wall  134  and the second fixed piston  155  defines a fourth chamber  198  within the movable cylinder  130 . The side wall  172  includes radially extending openings  146  to allow fluid communication between the first inner chamber  195  and the first passage  208  through the end opening  211 . Similar openings  149  provide a passage between the second inner chamber  196  and the annular channel  218 , openings  148  provide a passage between the third inner chamber  197  and the annular channel  219 , and openings  147  provide a passage between the fourth inner chamber  198  and the annular channel  216 . 
     With the actuator  125  constructed as described, the upper chamber  140 , the first chamber  195  and the third chamber  197  are in continuous fluid communication with the first passage  208 , and thereby with the first port  202  via annular channel  206 . Similarly, the second chamber  196  and fourth chamber  198  are in continuous communication with the second passage  214  and thereby with the second port  204  via channel  212 . 
     The actuator  125  is shown in FIG. 7 with the movable cylinder  130  and rod  128  (and the associated valve pin) each in their retracted positions, relative to the stationary cylinder  132 . In operation, to cause the valve pin and movable cylinder  130  to move from their retracted positions, pressurized fluid is introduced through the first port  202 , while the second port  204  is in fluid communication with a lower pressure fluid reservoir, or the like. The introduction of pressurized fluid at the first port  202  causes the pressurized fluid to enter into and to flow through the annular channel  206  and into the first passage  208 . The fluid then flows simultaneously through (a) opening  209  into the upper chamber  140 , (b) opening  210  to channel  219 , through the openings  148  and into the third chamber  197 , and (c) opening  211 , through openings  146  and into the first chamber  195 . Consequently, each of the upper chamber  140 , the third chamber  197  and the first chamber  195  are at an elevated pressure relative to the second and fourth chambers  196 ,  198 . The resultant pressure differentials acting against each of the upper end wall  134 , intermediate wall  135  and lower end wall  136  cause the movable cylinder  130  to move toward the bottom wall  127 , i.e., from the positions shown in FIG. 7 to the positions shown in FIG.  8 . 
     As the movable cylinder  130  moves from the retracted position to the extended position, the volume of each of the fourth chamber  198  and the second chamber  196  is decreasing, while the volume of each of the first chamber  195 , third chamber  197  and upper chamber  140  is increasing. At the same time, any fluid within the forth chamber  198  and second chamber  196  passes through the channels  218 ,  216 , through the passage  214  and out the second port  204 , which is at a lower pressure than is the first port  202 . The continued application of greater fluid pressure at the first port  202  will cause the movable cylinder  130  to travel to the end of its extension stroke, as shown in FIG. 8, at which position the lower end wall  136  is in abutment with a forward cushion  164  attached to the bottom wall  127 , and the valve pin will be against the valve seat to block flow through the mold gate. To avoid the resistance that would otherwise act against the lower end wall  136 , air contained between the lower end wall  136  and the bottom wall  127  is exhausted through a bore  145  in the mold plate  144  through which the rod  128  also passes. 
     In order to open the valve and allow the flow of molten plastic material into the mold cavity, the first port  202  is disconnected from the source of pressurized fluid and is placed in communication with a lower pressure reservoir, or the like. The second port  204  is then connected with the source of pressurized fluid, and pressurized fluid enters the channel  212 , flows into the passage  214 , and then into the annular channels  216 ,  218  via the openings  215 ,  217 . From the annular channels  216 ,  218 , the pressurized fluid flows through the openings  147 ,  149  in the side wall  172  and into the fourth chamber  198  and second chamber  196 . The increased pressure against the inner surface of the upper end wall  143  and intermediate wall  135  causes the movable cylinder  130  to retract into the stationary cylinder  132 . As a result, the valve pin retracts away from the gate to allow molding material to flow through the nozzle. The reduced air pressure that would otherwise be generated between the lower end wall  136  and the bottom wall  127  is relieved by admitting ambient air through the bore  145  surrounding the rod  128 . 
     It will therefore be apparent that an actuator in accordance with the present invention provides a greater output force within the same cylinder diameter, thereby allowing such an actuator to be utilized in confined spaces that would preclude larger diameter cylinders if higher actuation forces were needed. If a similar space limitation existed but a higher actuation force than would be available using a single piston were needed with only a relatively low fluid pressure source available, the actuator in accordance with the present invention would provide an increased actuation force at that lower fluid pressure. In addition, as illustrated by the alternate embodiment, the actuation force can be further multiplied, as desired, by repeating the described arrangement of the fixed pistons and intermediate movable cylinder walls, thereby increasing the number of actuating surfaces. 
     The foregoing discussion and the illustrated embodiments of the invention have been in the context of the use of the actuator in a plastics injection molding machine for controlling the flow of molten plastic material from an injection unit to a mold cavity, to provide increased actuation forces where space is limited or where available fluid pressures are low. It will be apparent to those skilled in the art that various changes and modification can be made without departing from the concepts of the present invention. It is therefore intended to encompass within the appended claims all such changes and modification that fall within the scope of the present invention.