Abstract:
A valve for controlling the flow of a high pressure fluid is provided. The valve provides a reliable way of controlling the flow of high pressure fluids in applications requiring multiple cycles where equipment may be exposed to environmentally harsh conditions, such as high temperatures and pressures.

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 09/710,756, filed Nov. 10, 2000, by Kim et al., titled “Valve for Injection Molding,” which claims priority to U.S. Provisional Application Serial No. 60/242,866, filed Oct. 24, 2000, by Kim et al., titled “Valve for Injection Molding,” the disclosures of which application are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed to a valve for controlling the flow of a high pressure fluid and, in particular, a valve for use in high cycle polymer operations. 
     BACKGROUND OF THE INVENTION 
     The flow control of high pressure fluids is important in a variety of industries, including manufacturing, chemical production, refrigeration, plastics molding, automotive and medical applications. In the plastics molding industry, for example, fluids are often transferred at temperatures of several hundred degrees F and at pressures of several thousand psi. 
     In several methods of polymer production, described in International Publication No. WO 98/31521 (Pierick et. al.), International Publication No. WO 99/32544 (Anderson et. al.), and International Publication No. WO 98/08667 (Burnham, et. al.), each of which is hereby incorporated by reference herein, a gas blowing agent is mixed with a molten polymer to produce a mixture of gas and polymer. These processes may be used to produce, for example, injection molded, blow molded or extruded polymeric materials. Typically, a high pressure gas is injected and mixed with a molten polymer prior to the polymer being molded or extruded. Certain processes, e.g. injection molding, involve producing product cyclically, e.g., with many parts being made sequentially in the same mold. In such cases it may be preferred that the high pressure gas can be isolated from the polymer stream during the time the polymer is being transferred to a mold. 
     High pressure fluid valves used in cyclic operations are typically subjected to strenuous mechanically harsh conditions. It is one goal of the invention to provide a robust high-pressure valve for such use. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention provides a valve comprising a valve seat, a valve pin having a drive end and a sealing end, the valve pin mounted in the valve such that the sealing end is capable of mating with the valve seat, and a packing washer supporting the valve pin, the packing washer having a first end and a second end. An internal diameter of the packing washer varies between the first end and the second end. 
     In another aspect, the invention provides a valve comprising a valve seat positioned in a valve body, a valve pin positioned in the valve body, the valve pin having a sealing end and a drive end. The sealing end is capable of mating with the valve seat to prevent flow of fluid through the valve. A piston stem is positioned in a housing, the piston stem having a first end and a second end and capable of transferring axial force to the valve pin. The piston stem is uncoupled from the valve pin. 
     In another aspect, the invention provides a valve comprising a valve seat positioned in a valve body, a valve pin positioned in the valve body. The valve pin has a sealing end and a drive end. The sealing end is configured to mate with the valve seat to prevent flow of fluid through the valve. An energized seal surrounds a portion of the valve pin, and prevents flow of fluid past the seal. 
     In another aspect, the invention provides a valve comprising a valve seat, a valve pin capable of forming a fluid-tight seal with the valve seat, and a valve pin guide adjacent the valve seat. The valve pin guide has an internal diameter that is substantially the same as the outer diameter of the valve pin. 
     In another aspect, the invention provides a valve comprising a valve seat mounted in a valve body, and a valve pin having a sealing end and a drive end. The valve pin is slidably mounted in the valve body and axially movable between a first position wherein the sealing end is mated with the valve seat and a second position wherein fluid can flow through the valve. A valve guide is positioned adjacent to the valve seat, and an energized seal surrounds a portion of the valve pin. A piston stem moves the valve pin from the second position to the first position, and the piston stem is capable of transmitting axial force to the valve pin while permitting the axis of the piston stem to move independently of the axis of the valve pin. 
     In another aspect, the invention provides a system comprising any of the above valves in communication with a polymeric foam processing apparatus such as extrusion, injection molding, or blow molding apparatus. 
    
    
     Other advantages, novel features, and objects of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings, which are schematic and which are not intended to be drawn to scale. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates in cross section a prior art gas injection valve. 
     FIG. 2 illustrates in cross section one embodiment of a fluid injection valve according to the invention. 
     FIG. 3 illustrates in cross section an enlarged view of a seal of the fluid injection valve illustrated in FIG.  2 . 
     FIG. 4 illustrates in cross section, an enlarged view of the seal area of an embodiment of the invention. 
     FIG. 5 illustrates in cross section, an enlarged view of the seal area of another embodiment of the invention. 
     FIG. 6 illustrates in cross section, a view of an embodiment of a polymeric processing system according to the invention. 
    
    
     DETAILED DESCRIPTION 
     An example of a typical prior art valve used to control the flow of a high pressure fluid, such as compressed nitrogen, into a polymer mix, is illustrated in FIG.  1 . This description is not meant to imply that other, similar arrangements are not present in the prior art. The valve includes a valve body  10  having a valve seat  20  supported in the valve body. Valve pin  30  is slidably mounted in the valve body  10  so that the sealing end  32  of the valve pin can mate with valve seat  20  and prevent the flow of gas through the valve body and out of gas exit  40  that may lead to a polymeric foam processing system. The diameter of the valve pin is uniform until the conical taper at the sealing end, and the internal diameter of the valve body is uniform throughout. Typically, the difference between these two diameters is large enough, at least 0.03 inch, to allow an adequate flow of compressed gas between the valve pin  30  and the interior of the valve body  10  when the valve is opened. The valve seat may be composed of 316 SS and the valve pin of 17-4 hardened steel. Although, in all embodiments described herein, the valve of the invention is described as being able to prevent the flow of fluid in one position, while allowing fluid to flow in another position, the valve can easily be modified by those of ordinary skill in the art with the benefit of the present disclosure to inhibit (although not completely prevent) the flow of fluid in the first position while allowing fluid to flow in the second position. The valve can easily be constructed to allow for control of fluid flow between and among essentially any fluid flow levels from complete prevention of fluid flow to free fluid flow in the valve&#39;s completely “open” position. Accordingly, in all embodiments, the valve can allow a first level of fluid flow in a first position and a second level of fluid flow (different from the first level of flow) in a second position. Either of the first or second levels of fluid flow can define complete prevention of fluid flow. 
     Valve pin  30  is supported in the valve body by upper washer  50 , lower washer  60  and fluoropolymer seal  70 . Upper washer  50  is adjustable via nut  55  and serves to compress seal  70  between upper washer  50  and lower washer  60 . The internal diameter of upper washer  50  is consistent and is designed to close tolerances to precisely guide valve pin  30  as the valve pin slides to open and close the valve. For example, the internal diameter of upper washer  50  can be less than 0.0005 inch greater than the diameter of valve pin  30 . Thus, upper washer  50  may be used to align and direct valve pin  30 . 
     Valve pin  30  is swaged to piston stem  80  at junction  81  so that any movement to piston stem  80  is translated to valve pin  30 . Thus, if piston stem  80  moves downward (referring to FIG.  1 ), valve pin  30  also moves downward, to a position where sealing end  32  mates with valve seat  20  to form a pressure tight seal. If piston stem  80  moves upward, valve pin  30  is pulled away from valve seat  20  and gas may flow through inlet  12 , through valve body cavity  14  between the wall of the cavity and the valve pin, through a gate between pin sealing end  32  and valve seat  20 , and through exit  40  that is in communication with a polymeric foam processing system. Thus, by controlling the movement of piston  110  by supplying compressed air to either chamber  112  or chamber  114 , the movement of piston stem  80 , and therefore valve pin  30 , is controlled. When the valve is to be closed, compressed air is directed to chamber  112 . When the valve is to be opened, compressed air is fed to chamber  114 . 
     When high pressure gas is injected through inlet  12  into the valve body, it is important for the system to be sealed to prevent the escape of high pressure gas. This can be facilitated to some extent via PTFE seal  70  placed around valve pin  30  to form a seal between the inner wall of valve body  10  and valve pin  30 . As the PTFE seal may start to “cold flow” or “creep,” upper washer  50  can be tightened and thereby compressed against adjustable seal  70 , to expand the seal in order to minimize leakage past the seal. When the seal has flowed to such an extent that it is no longer capable of sealing, it can be replaced. PTFE is a material that can provide a seal and allow the reciprocation of valve pin  30  between the open and closed positions. Reciprocation may typically involve a travel distance of 0.125 inch or greater. 
     The valve illustrated in FIG. 1 may work adequately in a high pressure polymeric foam processing system, however, it also may fail after about 20,000 cycles, defining a limited lifetime. Not only does seal  70  require repeated tightening and replacement, but an elliptical hole can become worn into valve seat  20 , thus allowing gas to escape past the seat through exit  40  when the valve is in the closed position. When used with a polymeric foam processing system, this is disadvantageous as high pressure gas is allowed to pass through the processing system when downstream gates are opened. Frequent replacement of the valve seat may be required with the system of FIG.  1 . 
     The inventors have found several reasons that may contribute to the premature failure of the prior art valve of FIG.  1 . For example, as the adjustable seal  70  flows and is repeatedly tightened, the thickness of the seal begins to decrease. At this point, the upper washer  50  may contact the interior surface of valve body  10  and become misaligned, as shelf  16  is typically not exactly perpendicular to the axis of valve pin  30 . This means that the bore of upper washer  50  may no longer be properly aligned with the axial movement of valve pin  30 , and because valve pin  30  is carefully fit into upper washer  50  at very low tolerances, any change in this angle forces the valve pin away from center so that conical valve tip  32  is no longer axially aligned with valve seat  20 . As conical valve tip  20  is cycled repeatedly off center, it begins to unevenly wear an edge of valve seat  20  where the valve pin first comes into contact with the valve seat upon closure. In addition, because valve pin  30  is swaged to piston stem  80 , any change in the angle of direction of movement of piston stem  80  urges a similar change in the motion of valve stem  30 , resulting in additional forces that can result in misalignment between valve pin  30  and valve seat  20 . Thus, the overall rigidity of the valve components, previously thought to provide a repeatable, long-lasting system capable of maintaining alignment under high pressure and high temperature conditions, actually contributes to the valve pin misalignment and the resulting failure of the valve itself. 
     The present invention provides a valve that has been shown to provide consistent sealing of high pressure fluid through more than 20,000 cycles of an injection molding system without the need to adjust seals or change valve seats. Preferred valves of the invention can provide consistent sealing through 100,000, 500,000, 1 million, 2 million, or greater than 2.5 million cycles. One embodiment of the invention is provided in FIG.  2 . FIG. 2 provides an illustration of an embodiment of a fluid control valve  200  that can control the flow of fluids, including gases and supercritical fluids, and provide consistent repeatable cycling at temperatures of at least about 650° F. and pressures greater than 6,000 psi. Valve  200  includes a valve pin  230 , a valve seat  220  shaped to receive a sealing end  232  of the pin, a piston stem  280 , a piston  310  operatively linked to the stem and moveable within a cylinder  311 , and a ball  320  positioned to transfer a force from stem  280  to pin  230 . Valve pin  230 , which can be cylindrical, can be composed of a temperature robust material, e.g., a metal or alloy such as tungsten carbide, and has a sealing end  232  designed to mate with valve seat  220  and a drive end  234  opposite to sealing end  232 . Sealing end  232  may be any shape that can form a fluid type seal when in contact with seat  220  and may be conical, as shown in FIG. 2, or, for example, can be hemispherical. In operation, sealing end  232  slides into a valve guide  245 , which may be a section of chamber  214  that is narrower than is the section above it. 
     Valve guide  245  may be designed to close tolerances, with very little clearance between valve pin  230  and the walls of valve guide  245 . The guide may be an integral part of valve body  210  or, alternatively, may be a separate insert. Preferably, the inner diameter of valve guide  245  is substantially the same as the diameter of valve pin  230 . Valve guide  245  may extend up to the full length of interior surface  216 , but it is preferred that the valve guide be as short as possible while still constraining valve pin  230  when valve pin  230  is fully withdrawn from valve seat  220 . In this way, resistance to fluid flow between inlet  212  and outlet  240  is minimized, as is friction between valve pin  230  and valve guide  245 . In this manner, sealing end  232  is maintained in proper alignment with valve seat  220  due to the close tolerances between valve guide  245  and valve pin  230 . 
     In order to ease the passage of high pressure fluid through inlet  212  to chamber  214  and out of exit  240  when the valve pin is in the upper, or open position, especially when valve guide  245  is present, fluid channels may be provided within or proximate the valve guide. One or more channels may be formed in the interior wall of valve guide  245 , for example, by using EDM techniques known to those skilled in the art, or, alternatively, one or more shapes, such as flats or concavities, may be machined into the surface of valve pin  230  in the area of end  232 . For example, flat  238  may be machined into valve pin  230  to provide a fluid pathway between valve pin  230  and valve guide  245 . Preferably, two or more flats are symmetrically opposed to each other. 
     Opposite of end  232  of valve pin  230  is drive end  234  which may include a broadened contact area as illustrated in FIG. 2, the surface of which may be either flat, as illustrated in FIG. 2 or, alternatively, concave or convex. Shown just above drive end  234  is ball  320  which may be made out of any suitable material, including tungsten carbide or hardened steel. Ball  320  serves as a point source to transfer force from piston stem  280  to the drive end of valve pin  230 . Piston stem  280  may be driven by any device capable of providing force, including a pneumatic piston, as illustrated in FIG. 2, a hydraulic piston, or an electrically powered actuator. Ball  320  may be contained between piston stem  280  and valve pin  230  without being affixed to either of the components with which it makes contact. In this manner, when piston stem  280  is withdrawn, ball  320  may move freely upward until retained by a stop, such as shoulder  292 . In alternative embodiments, the ball, or another point source, may form an integral part of either piston stem  280  or valve pin  230 , without being affixed to the other. It is preferable, however, that force be transferred from piston stem  280  to valve pin  230  via a point source and that piston stem  280  and valve pin  230  not be fixed together. In this manner, if piston stem  280  should become out of axial alignment with valve pin  230 , or if a point source on either one of the components should move off center, the driving force may still be cleanly transferred from piston stem  280  to valve pin  230  without applying undue torque to valve pin  230  that might cause premature wear toward one side of valve seat  220 . The implementation of a stop, such as shoulder  292 , provides for a preferred amount of minimal travel for valve pin  230  while not requiring precise, limited movement of piston stem  280 . The stop may be positioned to limit the travel of the valve pin directly or can be positioned to limit the travel of the ball  320  which, in turn, limits the travel of valve pin  230 . 
     Valve pin  230 , in conjunction with lower washer  260 , energized seal  270 , and packing washer  250 , forms a fluid-tight seal to prevent flow of fluid into cavity  291 , which may be fluidly connected to the environment outside of the valve. Bottom washer  260  may be of any size and shape appropriate for retaining seal  270  inside chamber  214 . A gland  290  is used to hold packing washer  250  in place. 
     Energized seal  270  can be a nonadjustable seal that can form a fluid tight seal without external forces being applied to the top or bottom of the seal. Energized seal  270  can be, for instance, a “V” seal providing a sealing surface for both the interior and exterior of the seal. The energized seal should allow for the reciprocating movement of valve pin  230  without allowing the passage of significant quantities of high pressure fluid. Energized seal  270 , shown in cross-section in FIG. 3, may contain a spring  420  that provides an expanding force that serves to both push the outer edge of energized seal  270  into contact with interior surface  216  and also push the inner edge of energized seal  270  into contact with the wall of valve pin  230 . Spring  420  may be any resilient substance that, in compression, provides an outward force. Preferably, spring  420  is a toroidal spring that forms a circle within the cylindrical seal. The contacting portion  410  of the energized seal that is in contact with either the surface of the valve pin  230  or the inner wall of valve body  210  can be made out of any material capable of forming a fluid-tight seal under the high temperature and high pressure conditions encountered in the process with which the seal is used. Preferably, energized seal  270  is comprised of heat resistant elastomer, and more preferably is polymer/PTFE. In one embodiment, a seal from Parker GNP, referred to as type HS-11-008-S-106, has been shown to provide acceptable results after 2.7 million cycles. 
     Packing washer  250  may be a flanged washer, as shown in FIG. 2, and may serve to retain energized seal  270  as well as to center and align valve pin  230 . Preferably, the inner bore of packing washer  250 , through which valve pin  230  passes, is of different diameter at the upper end than at the lower end. For example, the lower end of packing washer  250  may have an inner diameter very close to the diameter of the valve pin, and the upper end of packing washer  250  may have an inner diameter several thousandths of an inch wider. This configuration still allows the packing washer to help center valve pin  230 . The variation in diameter, however, provides enough freedom of movement that any misalignment between packing washer  250  and valve pin  230  will not result in a force that pushes valve pin  230  out of alignment with valve seat  220 . This design helps to resolve the problem encountered with the valve of FIG. 1 where any misalignment of the upper washer  50  tended to force the valve pin offline and to wear an elliptical hole in valve seat  20 . In a most preferred embodiment, the inner diameter of packing washer  250  is tapered at about 1.2° (exaggerated in FIGS. 2,  4  and  5 ), and more preferably, the inner diameter is smaller at the bottom of the washer and larger at the top, although a reverse taper could function similarly. 
     Valve  200  can also include a compression spring  236  providing an upward force to remove valve pin  230  from valve seat  220  when piston stem  280  is retracted, especially when stem  280  and pin  230  are not affixed to each other, i.e. not operatively linked to cause retraction of pin  230  when stem  280  is retracted. The compression spring may be any type of spring capable of providing a force to open the valve and may be made of, for example, metal, polymer or resilient elastomer that is robust enough to withstand the environmental rigors of the application with which it is being used. Preferably, the reciprocal movement of valve pin  230  is limited to less than about 0.1 inch and more preferably is in the range of 0.01 to 0.1 inch. In a most preferred embodiment, valve pin  230  moves approximately 0.050 inch between the open and closed positions, the length of travel being controlled by shoulder  292 . 
     In another embodiment, illustrated in FIG. 4, additional seals may be utilized to further isolate the high pressure fluid entering through inlet  212  from cavity  291  and from the environment external to the valve. In addition to lower washer  260  and energized seal  270 , additional seals either upstream or downstream from the initial seal may be added to make a more robust valve. The seals may be placed back to back or, alternatively, as shown in FIG. 4, a spacer such as intermediate washer  360  may be positioned between initial seal  270  and secondary seal  370 . Secondary seal  370  may be identical to initial seal  270  or may be of a different design and different material. Appropriate materials may include polytetrafluoroethylene (PTFE) and synthetic rubber, such as that sold under the trademark VITON® (DuPont Dow Elastomers, L.L.C.). 
     FIG. 5 illustrates another embodiment in which a secondary seal is used to further isolate the high pressure fluid from the environment. In addition to lower washer  260  and initial seal  270 , a secondary seal, O-ring  340 , may be placed between the primary seal  270  and upper washer  350 . The O-ring may be placed directly behind initial seal  270 , or an intermediate washer or other spacer may be placed between the two seals. The O-ring may be composed of any material capable of withstanding the pressure, temperature and wear conditions encountered by the seal. For example,  0  ring  340  may be composed of PTFE or synthetic rubber, such as that sold under the trademark VITON® (DuPont Dow Elastomers, L.L.C.). Of course, tertiary and additional seals may also be incorporated if found to be useful in a specific machine, or process. 
     Referring back to FIG. 2, a chamber  312  defined within cylinder  311  above the pistons may be connected to a source of high pressure air, for example, air at 60 psi, in order to provide adequate force to piston stem  280  in order to close valve  200 . Compression spring  316  may provide additional force to aid in the closure of the valve as high pressure gas entering inlet  212  can, in some arrangements, tend to push valve pin  230  upward. In order to open the valve, a similar amount of air pressure may be transmitted to chamber  312 , and the force supplied in an upward direction from the compressed air, in combination with the force provided by the high pressure fluid entering the system at inlet  212 , is great enough to overcome the force provided by spring  316 , and therefore is able to quickly open the valve. 
     The valve may be operated at high pressures and temperatures, and materials of construction should be chosen appropriately. A temperature difference of 300, 500 or more than 600° F. between various parts of the valve may be encountered in some applications, including polymer processing applications. For instance, the valve may reach temperatures surpassing 600° F. near an injector body but be considerably cooler, for example, 140° F. near the air actuator. Parts subject to the conditions near the injector body should be made of suitable heat resistant material. This high temperature operation may preclude the use of polymeric valve seats and components, and metals or ceramics may be preferable. In addition, if the valve is used to control the flow of supercritical fluids (SCF), the fluid may act as a solvent and distort the polymer over time. Polymers may also be prone to particulate contamination, swelling, or distortion, due to heat. Metals are much less prone to these problems. In addition, it is preferred that the valve pin, seat and valve guide are of metal because the seat in preferred embodiments should be able to endure millions of impact cycles at high temperature, and the valve pin preferably is able to reciprocate continuously without galling. The choice of durable, heat resistant materials for the guide, valve pin and seat allow these components to be in close proximity to the polymer melt stream. Thus a valve comprising a seat, a guide and a valve pin of temperature resistant metal may be preferred in a high temperature application. The drive components, such as the piston and piston stem, may be placed farther from the heat source and thus may not be subjected to the same extreme conditions. 
     Valve seat  220  and valve pin  230  may be made of the same or different materials, however, to prevent galling, it is preferred that valve pin  230  be of a harder material than is valve seat  220 . It is preferred that valve seat  220  is made of a hard material, however, so that metal particles are not embedded into the seat during operation. It is preferred that the valve seat and valve pin be made of resilient, heat-resistant materials such as hardened steel, alloys, ceramics or, possibly, in some applications, high-temperature polymers. For example, valve seat  220  may be made of H1150 17-4PH hardened steel having a Rockwell C hardness of about 33-36, while valve pin  230  may be made of tungsten carbide having a Rockwell C hardness of about 66 or greater. 
     In one aspect, the valve of the present invention may be used with a polymeric foam processing apparatus such as a blow molding, injection molding or extrusion molding machine. In one embodiment, illustrated in FIG. 6, valve  200  is used in conjunction with injection molding system  500 . The injection molding system includes a barrel  532  and a screw  538  contained in the barrel for moving and mixing polymer melt. Also provided is a drive motor  540  for driving the screw and a heating units  542  for maintaining the barrel at an elevated temperature. Polymeric material is provided in hopper  544  and is fed into the extruder  514  through orifice  546 . A source  590  of blowing agent, which can be a supercritical fluid (SCF), is supplied to the extruder and the fluid flow is regulated by valve  200  which serves to selectively transport blowing agent from source  590  to blowing agent port  516 , which communicates with polymer processing space  534 . SCF may be received into blowing agent receiving section  562  and is mixed with polymer in mixing section  560 . The mixture of polymer and blowing agent then passes through a nucleating pathway  567  and through outlet  570  into mold  580 . Valve  200  may be synchronized with the action of gate  564  and screw  538  so that the supply of blowing agent is isolated from the extruder  514  when gate  564  is open. Valve  200  may be opened when it is desirable to add blowing agent to the polymer mix. Preferably, valve  200  is proximate polymer processing space  534 , so that when the valve is closed, substantially all of the high pressure fluid in communication with the polymer has already entered the mixing section  560  of the extruder. 
     In such a configuration, valve  200  may be opened (see FIG. 2) to allow fluid, such as SCF, to pass through inlet  212  into cavity  214  and through outlet  240  in order to allow the fluid to mix with polymer. When the polymer is to be injected into a mold, a force, for example, high pressure air, is supplied to cavity  312 , supplying a downward force on piston  310  and piston stem  280 . The downward force is transferred to ball  320  which then provides a point source to drive end  234  of valve pin  230 . Valve pin  230  is in turn driven downwardly until sealing end  232  is contacted with seat  220  in order to provide a fluid-tight seal. Once the fluid-tight seal is made, and the supply of high pressure fluid is cut off from exit  240 , and polymer in the plastics production system may be injected into a mold cavity without the addition of, or interference from, the fluid. 
     When the molding cycle is complete and additional gas is desired to be mixed with new polymer material, a signal is sent to an actuator, and high pressure air, for example at 60 psi, is provided to chamber  314  within cylinder  311 , below the piston, which is isolated by the piston from chamber  312 . The high pressure air in chamber  314  provides an upward force to piston  310  which in turn retracts attached piston stem  280  upwardly. With piston stem  280  retracted, spring  236  can provide an adequate force, optionally in combination with that provided by any high pressure fluid in communication with the valve, to move valve pin  230  and ball  320  upwardly until ball  320  is retained by shoulder  292  in gland  290 . The amount of travel of pin  230  is controlled to be less than that which would be required for the pin to be fully extracted from valve pin guide  245 . In this manner, the valve pin  230  remains in proper center alignment for seating with valve seat  220  during the next injection cycle. Once an adequate supply of high pressure fluid has passed through exit  240 , the air source may be transferred from chamber  314  back to chamber  312  to reinitiate the closing process. 
     One feature of preferred embodiments of the invention is that a valve can include a piston that can move reciprocally to drive a valve pin into and out of a valve seat, where the piston is decoupled from the valve pin. That is, the piston is not operatively linked to the valve pin in a way that components connecting the piston with the valve pin affect alignment of the valve pin. Specifically, the piston drives a piston stem which, in turn, drives the valve pin (optionally via an auxiliary object such as a ball between the stem and pin), while the axes of the stem and pin are free to move relative to each other. One benefit of this is that the piston and stem do not affect alignment of the valve pin relative to the valve seat. This feature, in combination with a preferred, internally-tapered packing washer, allows maximal freedom of axial movement of the valve pin. The valve pin then can be guided, solely, by the valve pin guide, which will not come out of alignment relative to the valve seat, and the valve pin thus is assured of remaining in alignment with the valve seat. 
     Those skilled in the art would readily appreciate that all parameters listed herein are meant to be exemplary and that actual parameters would depend upon the specific systems with which the invention is used. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described.