Patent Publication Number: US-2013251843-A1

Title: Valve gate system

Description:
RELATED APPLICATIONS 
     This application is a division of copending U.S. patent application Ser. No. 13/332,809, filed Dec. 21, 2011, which is in turn a division of U.S. patent application Ser. No. 12/750,228, filed Mar. 30, 2010, now issued U.S. Pat. No. 8,113,818. The disclosures and drawings of those applications are fully incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to the field of injection molding, particularly the design of valve gate systems. Referring to  FIGS. 1A and 1B , valve gated injection molding systems according to the prior art such as the one, indicated generally at  100 , use a reciprocating piston  108  connected to a shutoff pin  112  inside a melt-flow bore  114  to control the flow of molten polymer. The melt flow bore  114 , which is the passageway in which the molten polymer travels, is sealed from an unheated clamp plate  102  and parts interior to it by a manifold seal  116 . Often, a small chamber  118  separates the manifold seal  116  and the cylinder  110 . 
     The piston  108  is pneumatically moved along its axis  120  by selectively supplying a fluid, preferably air or an inert gas, into the cylinder  110  with the fluid acting on the piston  108 . A close gate supply port (not shown) is disposed to be rearward of a rear pressure surface  122  of the piston  108 , such that the close gate supply port is in fluid communication with the rear pressure surface  122  of the piston  108 . As the piston  108  moves, the associated shutoff pin  112  moves with it, closing the gate  105  and shutting off the flow of molten polymer that flows down the melt-flow bore  114  and into the mold  107 . 
     The piston  108  travels along its axis  120  inside the cylinder  110 . Cylinder  110  rests inside the clamp plate  102  with the forward end  109  of the cylinder  110  directly contacting a hot manifold  104 . A rear end  111  of the shutoff pin  112  is affixed to the piston  108  and passes through the cylinder  110  through a hole at its forward end. The shutoff pin  112  extends through the hot manifold  104  and into a manifold plate  106 . The hot manifold  104  is heated to keep the polymer in its molten state. The forward end  113  of the shutoff pin  112  terminates at a nozzle seal  115  at a forward end of the manifold plate  106  and controls the flow of the molten polymer into the mold. Thus, the gate  105  is formed by the forward end  113  of the shutoff pin  112  and the nozzle seal  115 . The gate  105  is closed when the piston  108  is in the forward position and open when the piston  108  is in the rear position. If the gate  105  is open, molten polymer passes through the gate  105  and into the mold cavity  132 . 
     Referring to  FIG. 1B , showing a prior art assembly  100  with the piston in the rear position, the piston  108  has a forward pressure surface  124  and a piston stop surface  126  which contacts a cylinder stop surface  128  on the forward wall  117  of the cylinder  110  when the piston is in the forward position. To raise the piston  108 , the fluid enters through an open gate supply port  130  that is in fluid communication with the forward pressure surface  124 . 
     As shown in  FIGS. 1A and 1B , the piston stop surface  126  is radially inward of the forward pressure surface  124 . Since the cylinder  110  is in direct contact with the heated hot manifold  104 , thermal energy from the hot manifold  104  passes into the cylinder  110  and, when the piston  108  is in the forward position, then into the front end of the piston  108 , creating a temperature gradient between the forward portions of the assembly  100  and the rear portions of the assembly. This heat transfer into the cylinder  108  is undesirable because the piston uses O-rings  119  to maintain the pneumatic pressure on the piston  108  and stop leakage of fluid from one side of the piston to the other. Heat from the hot manifold  104  causes the O-rings  119  to degrade. This causes increased maintenance time and lost production. 
     Accordingly, it is an object of the present invention to provide a system that reduces the heat transferred from the hot manifold  104  to the piston  108 , thereby reducing maintenance on the system and increasing the run time. 
     Further, it is an object of the present invention to increase the ease of the maintenance of the injection molding systems by providing a system wherein the piston and the shutoff pin may be removed without removing the clamp plate from the hot manifold. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, a valve gate assembly for use with a hot manifold is formed around an axis and comprises a piston that is actuable within a cylinder between a forward position and a rear position along the axis. The piston has a cylindrical wall with a forward end and piston stop surface extending radially inwardly from the forward end of the cylindrical wall and terminating in a radially inward end. The piston has a forward pressure surface extending radially inwardly from the inward end of the piston stop surface such that a fluid acting on the forward pressure surface urges the piston in a rearward direction. 
     The cylinder has a cylindrical first wall that is formed to be parallel to the axis and that is axially displaceable along the wall. The assembly has a forward opening to be positioned adjacent to the hot manifold and communicates with a hot manifold pin hole. 
     The cylinder also has a cylinder stop surface that is disposed to contact the piston stop surface when the piston is in the forward position. The cylinder stop surface is radially outward of at least a portion of the forward pressure surface of the piston and a shutoff pin is substantially aligned with the axis. The shutoff pin has an elongated shaft with a front end that is forward of the cylinder and a rear end that is affixed to the piston. The shaft of the shutoff pin is passable through the forward opening and the hot manifold pin hole to a gate of the mold. The front end of the shutoff pin is operable to open and close the gate. 
     Preferably, the cylinder has a chamber that is forward of the cylindrical wall, an annular channel in the chamber, and a support member (such as a shoulder) that is forward of the channel. More preferably, a debris washer having a central hole contacts the support member and a retaining ring is disposed in the channel rearward of the debris washer. 
     In another embodiment, a valve gate assembly for use with a hot manifold is formed around an axis and comprises a piston that is actuable between a forward position and a rear position within a cylinder. The piston has a cylindrical wall with a forward end and piston stop surface extending radially inwardly from the forward end of the cylindrical wall and terminating in a radially inward end. The piston has a forward pressure surface extending radially inwardly from the inward end of the piston stop surface such that a fluid acting on the forward pressure surface urges the piston in a rearward direction. 
     The cylinder has a cylindrical first wall that is formed to be parallel to the axis and that is axially displaceable along the first wall. The assembly has a forward opening disposed to be adjacent to the hot manifold and communicates with a hot manifold pin hole. The cylinder also has a cylinder stop surface that is disposed to contact the piston stop surface when the piston is in the forward position. An open gate supply port is in fluid communication with the forward pressure surface and is forward of the cylinder stop surface. 
     According to another aspect of the invention, a chamber for collecting debris is forward of the forward end of a first cylindrical wall of the cylinder and the cylinder also has a purging supply port in fluid communication with the chamber for purging it with fluid and an exhaust port for purging fluid out of the chamber. Preferably, the purging supply port is forward of the cylinder stop surface. 
     In another aspect of the invention, a method of in situ purging of a shutoff valve of an injection molding apparatus comprises the steps of providing a chamber to be axially rearward of a manifold shutoff pin hole that opens into the chamber. A forward surface of the piston defines a rearward surface of the chamber. 
     The method further includes the steps of accumulating debris in the chamber and positioning a piston, which is actuable between a forward position and a rear position along an axis inside a cylinder, to a purging position. The debris typically results from polymer flowing from a polymer melt flow bore through the shutoff pin hole into the chamber. The method further comprises the steps of flowing a purging fluid from a purging supply port into a chamber and exhausting the purging fluid and debris out of an exhaust port that is in fluid communication with the chamber. Preferably, the piston is in the forward position when purging the fluid flows into the chamber. 
     According to another aspect of the invention, a valve gate assembly for injection molding comprises a clamp plate having a first bore with an axis and a first diameter and a second bore that is adjacent to and forward of the first bore. The second bore is aligned with the axis and has a second diameter that is larger than the first diameter. The apparatus further comprises a cylinder with a third diameter that is larger than the first diameter but is smaller than the second diameter. The cylinder is insertable into the second bore and a piston having a rear piston stop surface and a gate shutoff pin aligned with the axis and affixed to the piston is insertable into the cylinder through the first bore. A cover plate is disposed to be rearward of the rear piston stop surface and has a cover plate stop surface that is disposed to contact the rear piston stop surface with the piston in the rear position. 
     According to another aspect of the invention, a method of adjusting the stroke of a piston of a valve gate assembly comprises the step of providing a valve gate assembly having a cylinder and a piston actuable along an axis within the cylinder between a forward position and a rear position. The method further includes the step of providing a plurality of cover plates with each cover plate having at least one leg. Each leg has a predetermined length and a cover plate stop surface that contacts a piston stop surface when the piston is in the rear position. Finally, the method comprises the step of adjusting the stroke of the piston by replacing a first cover plate having a leg of a first predetermined length with a second cover plate having a leg of a second, different predetermined length. The legs from one cover plate may be modified by grinding or cutting to achieve the desired length. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further aspects of the invention and their advantages can be discerned in the following detailed description, in which like characters denote like parts and in which: 
         FIG. 1A  is an axial sectional view of an injection molding apparatus according to the prior art showing a clamp plate, a cylinder, a hot manifold, a manifold plate, a shutoff pin, a nozzle seal, and a piston in the forward position; 
         FIG. 1B  is a view of the injection molding apparatus shown in  FIG. 1A , but showing a clamp plate, a cylinder, a hot manifold, a manifold seal, a manifold plate, a shutoff pin, and a piston in the rear position; 
         FIG. 2A  is an axial sectional view according to another aspect of the invention showing a clamp plate, a hot manifold, a cylinder, shutoff pin, a manifold seal, a cover plate, a nozzle seal, and a piston in the forward position; 
         FIG. 2B  is an axial sectional view according to another aspect of the invention showing a clamp plate, a hot manifold, a cylinder, shutoff pin, a manifold seal, a cover plate, a nozzle seal, and a piston in the rear position; 
         FIG. 3  is an axial sectional view according to another aspect of the invention showing a clamp plate, a hot manifold, a manifold seal, a cylinder, shutoff pin, a cover plate, and a piston in the forward position; 
         FIG. 4  is a flow diagram showing a method for in situ purging of an injection molding apparatus; 
         FIG. 5  is an axial sectional view of an injection molding apparatus according to another aspect of the invention showing a clamp plate, a cylinder, a hot manifold, a shutoff pin, and a piston in the rear position; 
         FIG. 6  is an axial exploded view of the injection molding apparatus and a set wrench; 
         FIG. 7  is an isometric view of the cover plate showing the forward section thereof; 
         FIG. 8  is an isometric view of the piston showing the forward section thereof; 
         FIG. 9  is an isometric view of the piston showing the rearward section thereof; 
         FIG. 10  is an isometric view of the cylinder showing the forward section thereof; 
         FIG. 11  is an isometric view of the cylinder showing the rearward section thereof; 
         FIG. 12  is an isometric view of the set wrench showing the forward section thereof; and 
         FIG. 13  is a flow diagram showing a method for adjusting the stroke of a piston of a valve gate assembly. 
     
    
    
     DETAILED DESCRIPTION 
     The invention has overcome the limitations of the prior art by moving the point at which the piston contacts the stop surface of the cylinder rearward and away from the hottest portions of the cylinder. This reduces the amount of heat transferred to the piston and the O-rings and extends the time between maintenance shutdowns. Additionally, by moving the piston rearward from the front of the cylinder, the invention has a chamber that is capable of collecting debris resulting from any molten polymer seeping past the manifold seal. The polymer may be one or a combination of any number of polymers including, but not limited to, polyethylene, polypropylene, thermoplastic elastomers, polyethylene terephthalate, fluoropolymers, polyesters, vinyl polymers, silicone, etc. Thus, the polymer that seeps past the manifold seal may form debris in the form of a continuous mass or, as in the case of polyethylene terephthalate, a plurality of granules (dust). 
     Referring to  FIG. 2A ,  FIG. 2B , and  FIG. 6 , a valve gate assembly, indicated generally at  200 , is formed around an axis  202  and comprises a piston  204  that is actuable between a forward position ( FIG. 2A ) and a rear position ( FIG. 2B ) within a cylinder  210  (see also  FIGS. 10 and 11 ). The piston  204  has a first wall  205  having a forward end  207 , piston stop surface  206 , and a forward pressure surface  208 . The piston stop surface  206  extends radially inwardly from the forward end  207  and terminates at an inward end  209 . The forward pressure surface  208  extends radially inwardly from the inward end  209  of the piston stop surface  206  (see  FIGS. 8 and 9 ). 
     The cylinder  210  has a first cylindrical wall  212  formed to be parallel to the axis  202  and the piston  204  is axially displaceable along the first wall  212 . A forward opening  216  of the cylinder  210  is positioned to be adjacent to the hot manifold and communicates with a hot manifold pin hole  214 . The first wall  212  of the cylinder  210  and the first wall  205  of the piston  204  may be circumferential around the axis  202 , but the cylinder may have any additional walls that are necessary to give the cylinder the desired cross sectional shape. Thus, the cross section of the cylinder may be circular, triangular, rectangular, or any other shape where the piston is capable of reciprocating between the forward and backward positions within the cylinder. 
     The cylinder  210  has a cylinder stop surface  218  disposed to contact the piston stop surface  206  with the piston  204  in the forward position, the cylinder stop surface  218  being radially outward of at least a portion of the forward pressure surface  208 . The cylinder stop surface  218  preferably comprises a frustoconical shoulder  217  that projects radially inwardly from the first wall  212  of the cylinder  210 . 
     Prior art  FIGS. 1A and 1B  show that the cylinder stop surface  128  contacts the cylinder close to the hot manifold  104 . In contrast,  FIG. 2A  shows that the cylinder  210  contacts the cylinder stop surface  218  at a point that is farther axially rearward of the hot manifold  221  than in  FIGS. 1A and 1B . This additional distance rearward allows thermal energy that is conducted from the hot manifold  221  into the cylinder  210  to be dissipated along a greater axial distance. Thus, less thermal energy enters the piston  204 , the life of the O-rings  254  is extended, and less maintenance time is required. 
     Additionally, a shutoff pin  220  is substantially aligned with the axis  202  and has an elongated shaft  222  with a front end  211  forward of the cylinder  210  and a rear end  224  that is affixed to the piston  204 . Preferably, the rear end is removably affixed to the piston by inserting the front end  211  through a piston hole  234  with a diameter larger than the diameter of the shaft  222  of the shutoff pin  220 . The rear end  224  of the shutoff pin  220  is receivable into the piston  204  but is not capable of passing through the piston pin hole  234 . The rear end  224  operates with the second positioning member  232  to affix the shutoff pin  220  to the piston  204 . 
     The shaft  222  is also passable through the forward opening  216 , is received into the hot manifold pin hole  214 , passes through the hot manifold  221  into the melt flow bore  227 , and operates to open and close the gate  213 . The gate  213  may be formed by the front end  211  of the shaft  222  and a nozzle seal  215 , or can be formed by the front end  211  of the pin  220  and the steel of the mold cavity  260  (not shown). Once the gate  213  is open, molten polymer passes through the gate  213  and into the mold cavity  260 . 
     The cylinder  210  is affixed by compression to the hot manifold  221 , which has a preferably screw-threaded manifold seal  223 . The manifold seal  223  preferably has a plurality of protrusions (not shown) extending from the wall of an inside bore  225  thereof to reduce the amount of molten polymer seeping from the melt flow bore  227 . 
     In a preferred embodiment, the piston  204  has a threaded first bore  226 , a threaded second bore  228 , a first externally-threaded positioning member  230  that is receivable into the first bore  226 , and a second externally-threaded positioning member  232  that is receivable into the second bore  228 . More preferably, an O-ring  262  seals the threads of the first and second positioning members  230 ,  232  and prevents leakage of fluid through the threads between the two sides of the piston  204 . 
     More preferably, the first and second bores  226 ,  228  are oppositely threaded so the shutoff pin  220  may be adjusted forwardly or rearwardly along the axis  202 . This is advantageous because the dimensions of the shutoff assembly  200  must be precisely adjusted to prevent damage to the shutoff pin  220  or the nozzle seal  215  and because the length of the shutoff pin  220  will vary according to its temperature. To position the positioning members  230 ,  232 , the first positioning member  230  is adjusted to the desired position in the second bore  228 . The second positioning member  232  is then inserted into the first bore  226 . A hollow set wrench  604  (see  FIGS. 6 and 12 ) having a first fitting  1202  is placed over a complementary second fitting  606  on the piston  204  to prevent rotation of the piston  204  as the second positioning member  232  is tightened against the first positioning member  230 . 
     The new design has the additional advantage of creating a large chamber  236  that is forward of the cylindrical wall  212  of the cylinder  210 . Typically, a small amount of molten polymer from the melt flow bore  227  seeps by the manifold seal  223 . If too much polymer leaks by, it can damage the assembly  200  or affect the ability of the piston  204  to move all the way into the forward position and stop the flow of the molten polymer. The larger chamber  236  acts as a reservoir for debris and, in turn, increases the time between maintenance shutdowns. 
     Preferably, the chamber  236  has an annular channel  238  and a support member or shoulder  240 . The chamber  236  has a first diameter at this approximate axial location, and the annular channel  238  has a second diameter that is larger than the first diameter. The chamber  236  preferably has a debris washer  242  affixed within the chamber  236  across the axis  202  to span the chamber. Debris washer  242  acts as a barrier to any debris contacting the cylinder stop surface  218 . The debris washer  242  has a washer hole  246  that is substantially aligned with the axis  202  such that the shutoff pin  220  can pass through the washer hole  246 . The debris washer  242  may be affixed in the chamber  236  with a retaining ring  244  in the channel rearward of the debris washer  242  and typically rests on the support member or shoulder  240  that is forward of the channel  238 . 
     More preferably, the support member  240  is a ledge that is radially inward of the channel  244 . The diameter of the debris washer  242  is larger than the diameter of the shoulder  240  but is smaller than the second diameter. Thus, the debris washer  242  is radially expandable and contractable within the channel  238 . This has the benefit of allowing the debris washer  242  to move as the assembly  200  heats up. 
     In order to move the piston  204  to the forward position, a close gate supply port  248  is disposed to be rearward of a rear pressure surface  250  of the piston  204 , such that the close gate supply port  248  is in fluid communication with the rear pressure surface  250  when the piston  204  is in the rear position. Fluid (such as air) is introduced through the close gate supply port  248  and acts upon pressure surface  250  to urge the piston  204  forward (in  FIG. 2A , downward). 
     To move the piston  204  to the rear position ( FIG. 2B ), the fluid under pressure is supplied through an open gate supply port  252  in fluid communication with the chamber  236  and the forward pressure surface  208  of the piston  204 . The fluid acts on pressure surface  208  to urge the piston  204  rearward (upward in  FIG. 2B ) until the piston  204  contacts a cover plate  258  having one or more legs  264 . See also  FIGS. 6 and 7 . The cover plate  258  is attached to the clamp plate with one or more bolts  602  (see  FIG. 6 ) and sealed with an O-ring  266 . The legs  264  have a cover plate stop surface  256  which contacts the piston  204  and prevents further rearward movement of the piston  204 . 
     In an another embodiment and still referring to  FIGS. 2A &amp; 2B , an open gate supply port  252  is forward of the cylinder stop surface  218  and is in fluid communication with the forward pressure surface  208 . As above, the cylinder stop surface  218  preferably comprises an annular shoulder  217  that projects radially inwardly from the first wall  212 . The portion of the chamber  236  that is forward of the debris washer collects any debris that leaks past the manifold seal  223 . 
     In another embodiment and referring to  FIG. 3 , a valve gate assembly, indicated generally at  300 , comprises a piston  302  that is actuable between a forward position and a rear position within a cylinder  310 . The piston  302  has a first cylindrical wall  305  having a forward end  307 , a piston stop surface  304 , and a forward pressure surface  306 . The piston stop surface  304  extends radially inwardly from the forward end  307  and terminates at an inward end  303 . A forward pressure surface  306  extends radially inwardly from the inward end  303  of the piston stop surface  304 . A fluid acts on the forward pressure surface  306  to urge the piston  302  in a rearward direction. 
     The cylinder  310  has a first cylindrical wall  312  formed to be parallel to the axis  308  and the piston  302  is axially displaceable along the first wall  312 . A forward opening  316  in the cylinder  310  communicates with a hot manifold pin hole  314 . 
     The cylinder  310  further comprises a cylinder stop surface  318  that is disposed to contact the piston stop surface  304  with the piston  302  in the forward position, which is the position shown in  FIG. 3 . The cylinder  310  has an expanded chamber  320  that is forward of the inward end  303  of the piston stop surface  304 . 
     Additionally, the cylinder  310  has a purging supply port  322  for purging the chamber  320  which is in fluid communication with the chamber  320 . A plurality of exhaust ports  324  for exhausting the purging fluid out of the chamber  320  are in fluid communication with the chamber  320 . The purging supply port  322  is preferably forward of the cylinder stop surface  318  and rearward of a debris washer  330 . The purging supply port  322  may be the open gate supply port or a separate, dedicated line or port in fluid communication with the chamber  320 . 
     As above, the chamber  320  has an annular channel  326  and a support member or shoulder  328  forward of the annular channel  326 . The chamber  320  has a first diameter and the channel  326  has a second diameter that is larger than the first diameter. The debris washer  330  contacts the support member  328  and a retaining ring  332  is disposed in the channel  326  rearward of the debris washer  330 . The debris washer  330  has a washer hole  334  that is substantially aligned with the axis  308  and receives the shutoff pin  220 . 
     Referring to  FIG. 4 , a method, indicated generally at ( 400 ), of in situ purging of a gate valve system  300  of an injection molding apparatus comprises the steps of providing ( 402 ) a chamber  320  to be axially rearward of a hot manifold pin hole  314 , the hot manifold pin hole  314  opening into the chamber  320  and a forward pressure surface  306  of a piston  302  defining a rearward surface of the chamber  320 . 
     The method further comprises accumulating ( 404 ) debris in the chamber  320 , positioning ( 406 ) the piston  302  in a purging position, and flowing ( 410 ) a purging fluid from a purging supply port  322  into the chamber  320 . The method further comprises exhausting ( 412 ) the fluid and debris out of at least one exhaust port  324  that is in fluid communication with the chamber  320 . 
     The method preferably comprises the step of securing ( 408 ) the piston  302  in the purging position. This step is done so that none of the purging fluid pressure or force is relieved by displacing the piston  302  upward within the cylinder  310 . The purging position of the piston  302  may be the forward position, the rear position, and any position in between. Preferably, the purging position is the piston  302  in the forward position because it minimizes the residence time of the purging fluid and keeps the debris from settling in between a piston stop surface  304  and the cylinder stop surface  318 . The piston  302  may be secured ( 408 ) into the purging position by activating ( 414 ) a close gate supply port  248  that is in fluid communication with a back pressure surface  250  of the piston  302 . 
     More preferably, the step of flowing ( 410 ) the purging fluid from a purging supply port comprises the substep of activating ( 416 ) an open gate supply port  322 . Thus, the close gate supply port  248  and the open gate supply port  322  may be active at the same time, such that the fluid entering through the open gate supply port  322  flows into the chamber  320  and out the exhaust port or ports  324 . 
     Most preferably, the step of flowing ( 410 ) purging fluid further comprises the substep of flowing ( 418 ) the purging fluid forward through the debris washer hole  334  in a debris washer  330  that is forward of the purging supply port  322  and rearward of the exhaust port  324 . This ensures that the debris moves generally forward from the rear of the chamber  320  to the exhaust port  324 . 
     Referring to  FIG. 5 , a valve gate assembly, indicated generally at  500 , comprises a clamp plate  502  having a first bore  504  having an axis  506  and a first diameter  508 . The clamp plate  502  further comprises a second bore  510  adjacent to and forward of the first bore  504 . The second bore  510  is aligned with the axis  506  and has a second diameter  512  that is larger than the first diameter  508 . 
     The assembly  500  further comprises a cylinder  514  having a third diameter  516  that is larger than the first diameter  508  but is smaller than the second diameter  512 . The cylinder  514  is insertable into the second bore  510  and a piston  518  having a rear piston stop surface  520  is insertable into the cylinder  514  through the first bore  504 . 
     A cover plate  522  is disposed to be rearward of the rear piston stop surface  520  and has a cover plate stop surface  524  disposed to contact the rear piston stop surface  520  with the piston  518  in the rear position. This is advantageous because the piston  204  and the shut off pin  220  can easily be removed from the assembly  500  by removing the cover plate  522 . In contrast, the prior art of  FIG. 1  requires that the clamp plate  102  be separated from the hot manifold  104  to remove the piston, significantly increasing maintenance time. 
     Referring to  FIGS. 7 and 13 , a method ( 1300 ) for adjusting the forward and backward distance, or stroke, of the piston  204  comprises the steps of providing ( 1302 ) the valve gate assembly  200  and providing ( 1304 ) a plurality of the cover plates  258 , with each cover plate  258  having at least one leg  264 . Each leg  264  has at least one predetermined length  702  and a cover plate stop surface  256  that contacts the rear piston stop surface  520  with the piston  204  in the rear position. The stroke of the piston  204  may be adjusted ( 1306 ) by replacing ( 1308 ) the cover plate  258  with a different cover plate having legs  264  of a second, different predetermined length  704 . Alternatively, the legs from one cover plate  258  may be modified by grinding or cutting to achieve the desired length. The second length  704  may be longer or shorter than the first predetermined length  702 . 
     In summary, the described embodiments of the invention are an improvement over the prior art because the cylinder stop surface is moved rearward, reducing the heat transferred to the piston and the O-rings on the piston. Moving the stop also has the added benefit of creating a large chamber in which polymer or other debris may accumulate. Finally, the piston and/or the shutoff pin may be removed from the assembly without having to disassemble the assembly. 
     While illustrated embodiments of the present invention have been described and illustrated in the appended drawings, the present invention is not limited thereto but only by the scope and spirit of the appended claims.