Patent Publication Number: US-6708752-B2

Title: Injector for molten metal supply system

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a division of U.S. Ser. No. 09/957,846 filed Sep. 21, 2001 now U.S. Pat. No. 6,505,674, which claims the benefit of U.S. Provisional Application Serial No. 60/284,952 filed Apr. 19, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a molten metal injector and, more particularly, a molten metal injector for use with a molten metal supply system and method of operating the same. 
     2. Description of the Prior Art 
     The metal working process known as extrusion involves pressing metal stock (ingot or billet) through a die opening having a predetermined configuration in order to form a shape having a longer length and a substantially constant cross-section. For example, in the extrusion of aluminum alloys, the aluminum stock is preheated to the proper extrusion temperature. The aluminum stock is then placed into a heated cylinder. The cylinder utilized in the extrusion process has a die opening at one end of the desired shape and a reciprocal piston or ram having approximately the same cross-sectional dimensions as the bore of the cylinder. This piston or ram moves against the aluminum stock to compress the aluminum stock. The opening in the die is the path of least resistance for the aluminum stock under pressure. The aluminum stock deforms and flows through the die opening to produce an extruded product having the same cross-sectional shape as the die opening. 
     Referring to FIG. 1, the foregoing described extrusion process is identified by reference numeral  10 , and typically consists of several discreet and discontinuous operations including: melting  20 , casting  30 , homogenizing  40 , optionally sawing  50 , reheating  60 , and, finally, extrusion  70 . The aluminum stock is cast at an elevated temperature and typically cooled to room temperature. Because the aluminum stock is cast, there is a certain amount of inhomogeneity in the structure and the aluminum stock is heated to homogenize the cast metal. Following the homogenization step, the aluminum stock is cooled to room temperature. After cooling, the homogenized aluminum stock is reheated in a furnace to an elevated temperature called the preheat temperature. Those skilled in the art will appreciate that the preheat temperature is generally the same for each billet that is to be extruded in a series of billets and is based on experience. After the aluminum stock has reached the preheat temperature, it is ready to be placed in an extrusion press and extruded. 
     All of the foregoing steps relate to practices that are well known to those skilled in the art of casting and extruding. Each of the foregoing steps is related to metallurgical control of the metal to be extruded. These steps are very cost intensive, with energy costs incurring each time the metal stock is reheated from room temperature. There are also in-process recovery costs associated with the need to trim the metal stock, labor costs associated with process inventory, and capital and operational costs for the extrusion equipment. 
     Attempts have been made in the prior art to design an extrusion apparatus that will operate directly with molten metal. U.S. Pat. No. 3,328,994 to Lindemann discloses one such example. The Lindemann patent discloses an apparatus for extruding metal through an extrusion nozzle to form a solid rod. The apparatus includes a container for containing a supply of molten metal and an extrusion die (i.e., extrusion nozzle) located at the outlet of the container. A conduit leads from a bottom opening of the container to the extrusion nozzle. A heated chamber is located in the conduit leading from the bottom opening of the container to the extrusion nozzle and is used to heat the molten metal passing to the extrusion nozzle. A cooling chamber surrounds the extrusion nozzle to cool and solidify the molten metal as it passes therethrough. The container is pressurized to force the molten metal contained in the container through the outlet conduit, heated chamber and, ultimately, the extrusion nozzle. 
     U.S. Pat. No. 4,075,881 to Kreidler discloses a method and device for making rods, tubes, and profiled articles directly from molten metal by extrusion through use of a forming tool and die. The molten metal is charged into a receiving compartment of the device in successive batches that are cooled so as to be transformed into a thermal-plastic condition. The successive batches build up layer by layer to form a bar or other similar article. 
     U.S. Pat. Nos. 4,774,997 and 4,718,476 both to Eibe disclose an apparatus and method for continuous extrusion casting of molten metal. In the apparatus disclosed by the Eibe patents, molten metal is contained in a pressure vessel that may be pressurized with air or an inert gas such as argon. When the pressure vessel is pressurized, the molten metal contained therein is forced through an extrusion die assembly. The extrusion die assembly includes a mold that is in fluid communication with a downstream sizing die. Spray nozzles are positioned to spray water on the outside of the mold to cool and solidify the molten metal passing therethrough. The cooled and solidified metal is then forced through the sizing die. Upon exiting the sizing die, the extruded metal in the form of a metal strip is passed between a pair of pinch rolls and further cooled before being wound on a coiler. 
     In view of the foregoing, an object of the present invention is to provide an injector that is configured to operate directly with molten metal and may be used as part of a molten metal supply system for supplying molten metal to downstream metalworking or forming processes. A further object of the present invention is to provide an injector having the benefit of greatly reduced wear between its moving parts and the ability to generate relatively high working pressures with correspondingly small amounts of stored energy. 
     SUMMARY OF THE INVENTION 
     The foregoing objects are accomplished with an injector for a molten metal supply system and method of operating the same in accordance with the present invention. The injector includes an injector housing configured to contain molten metal. A molten metal supply source is in fluid communication with the housing. A piston is reciprocally operable within the housing. The piston is movable through a return stroke allowing molten metal to be received into the housing from the molten metal supply source, and a displacement stroke for displacing the molten metal from the housing to a downstream process. The piston has a pistonhead for displacing the molten metal from the housing. A gas supply source is in fluid communication with the housing through a gas control valve. The injector is operable such that during the return stroke of the piston a space is formed between the pistonhead and the molten metal and the gas control valve is operable to fill the space with gas from the gas supply source. The injector is further operable such that during the displacement stroke of the piston the gas control valve is operable to prevent venting of gas from the gas filled space such that the gas in the gas filled space is compressed between the pistonhead and molten metal received into the housing and displaces the molten metal from the housing ahead of the pistonhead. 
     The piston may include a piston rod having a first end and a second end. The first end may be connected to the pistonhead and the second end may connected to an actuator for driving the piston through the return stroke and the displacement stroke. The second end of the piston may be connected to the actuator by a self-aligning coupling. An annular pressure seal may be located about the piston rod to provide a substantially gas tight seal between the piston rod and the housing. A cooling water jacket may be positioned about the housing substantially coincident with the pressure seal for cooling the pressure seal. The first end of the piston rod may be connected to the pistonhead by a thermal insulation barrier. The piston rod may define a central bore that is in fluid communication with a cooling water inlet and outlet for supplying cooling water to the central bore in the piston rod. 
     The housing and piston rod may be made of high temperature resistant metal alloy. The pistonhead may be made of high temperature resistant metal alloy, refractory material, or graphite. The housing may include a refractory material liner or a graphite liner. The molten metal supply source may be a supply of molten aluminum, magnesium, copper, bronze, iron, and alloys thereof. The gas supply source may consist of helium, nitrogen, argon, compressed air, or carbon dioxide. 
     The injector may further include a floating thermal insulation barrier located between the pistonhead and the molten metal received into the housing. The floating barrier preferably remains substantially in contact with the molten metal throughout the return and displacement strokes of the piston. The injector may further include an injection port connected to the housing for injecting the molten metal displaced from the housing to the downstream process. The molten metal supply source may be in fluid communication with the housing through a check valve, which may be located in the injection port. A second check valve may be located in the injection port and configured to allow the displacement of molten metal from the housing. 
     The injector of the present invention may be configured to operate with a liquid medium rather than a gas medium. The injector, according to a second embodiment of the present invention, also includes an injector housing configured to contain molten metal. A molten metal supply source is in fluid communication with the housing. A liquid chamber is positioned above and in fluid communication with the housing. The liquid chamber contains a liquid chemically resistive to the molten metal contained in the molten metal supply source. A piston is reciprocally operate within the housing. The piston is movable through a return stroke allowing molten metal to be received into the housing from the molten metal supply source, and a displacement stroke for displacing the molten metal from the housing. The piston has a pistonhead for displacing the molten metal from the housing. The liquid chamber is in fluid communication with the housing such that during the return and displacement strokes of the piston, liquid from the liquid chamber is located about the pistonhead and between the molten metal received into the housing and the liquid chamber. 
     The liquid in the liquid chamber is preferably a viscous liquid such as boron oxide. The liquid chamber may be positioned directly on top of the housing and the piston may be reciprocally operable such that during the return stroke of the piston, the pistonhead retracts at least partially upward into the liquid chamber. The pistonhead may define a circumferentially extending recess, with the recess filled with liquid from the liquid chamber during the return and displacement strokes. 
     The present invention is further directed to a method of operating an injector for a molten metal supply system that may include the steps of: providing an injector having an injector housing configured to contain molten metal and a piston reciprocally operable within the housing, with the piston movable through a return stroke and a displacement stroke, with the piston having a pistonhead located within the housing, and with the housing in fluid communication with a molten metal supply source and a gas supply source; receiving molten metal from the molten metal supply source into the housing during the return stroke of the piston, with the pistonhead defining a space with the molten metal flowing into the housing; filling the space with gas from the gas supply source during the return stroke of the piston; and compressing the gas in the gas filled space between the pistonhead and the molten metal received into the housing during the displacement stroke of the piston to displace the molten metal from the housing to a downstream process in advance of the compressed gas. 
     The method may further include the step of venting the compressed gas in the gas filled space to atmospheric pressure approximately when the piston reaches the end of the displacement stroke. In addition, the method may further include the steps of: moving the piston through a partial return stroke in the housing after the step of compressing the gas in the gas filled space to partially relieve the pressure in the compressed gas filled space; venting the gas in the gas filled space to atmospheric pressure with the piston located at about the end of the partial return stroke in the housing; and returning the piston substantially to the end of the displacement stroke position in the housing. 
     When the injector is configured to operate with a liquid medium, the method according to the present invention may include the steps of: providing an injector having an injector housing configured to contain molten metal and a piston positioned to extend at least partially into the housing and reciprocally operate within the housing, with the piston movable through a return stroke and a displacement stroke, and with the piston having a pistonhead, with the housing in fluid communication with a molten metal supply source, and with the housing in fluid communication with a liquid chamber located above the housing and containing a liquid chemically resistive to the molten metal contained in the molten metal supply source; receiving molten metal from the molten metal supply source into the housing during the return stroke of the piston; supplying liquid from the liquid chamber around the pistonhead and between the molten metal received into the housing and the liquid chamber; and moving the piston through the displacement stroke to displace the molten metal from the housing to a downstream process. The liquid chamber is preferably in fluid communication with the housing such that during the return and displacement strokes of the piston, liquid from the liquid chamber is located around the pistonhead and between the molten metal received into the housing and the liquid chamber. 
     Further details and advantages of the present invention will become apparent from the following detailed description read in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a prior art extrusion process; 
     FIG. 2 is a cross-sectional view of an injector according to a first embodiment of the present invention showing the injector in fluid communication with a molten metal supply source and an outlet manifold; 
     FIG. 3 is a cross-sectional view of the injector of FIG. 2 showing the injector at the beginning of a displacement stroke; 
     FIG. 4 is a cross-sectional view of the injector of FIG. 2 showing the injector at the beginning of a return stroke; 
     FIG. 5 is a cross-sectional view of the injector according to a second embodiment of the present invention also showing the injector in fluid communication with a molten metal supply source and an outlet manifold; 
     FIG. 6 is a graph of piston position versus time for one operating cycle of the injector of FIGS. 2-4; and 
     FIG. 7 is an alternative gas supply and venting arrangement for the injector of FIGS.  2 - 4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 2-4 show a molten metal injector  100  for use with a molten metal supply system according to a first embodiment of the present invention. The injector  100  includes a housing  102  that is used to contain molten metal prior to injection to a downstream apparatus or process, such as a metalworking or metal forming apparatus or process. A piston  104  extends downward into the housing  102  and is reciprocally operable within the housing  102 . The housing  102  and piston  104  are preferably cylindrically shaped. The piston  104  includes a piston rod  106  and a pistonhead  108  connected to the piston rod  106 . The piston rod  106  has a first end  10  and second end  112 . The pistonhead  108  is connected to the first end  110  of the piston rod  106 . The second end  112  of the piston rod  106  is coupled to a hydraulic actuator or ram  114  for driving the piston  104  through its reciprocal movement. The second end  112  of the piston rod  106  is coupled to the hydraulic actuator  114  by a self-aligning coupling  116 . The pistonhead  108  preferably remains located entirely within the housing  108  throughout the reciprocal movement of the piston  104 . The pistonhead  108  may be formed integrally with the piston rod  106 , or separately therefrom as shown FIGS. 2-4. 
     The first end  110  of the piston rod  106  is connected to the pistonhead  108  by a thermal insulation barrier  118 , which may be made of zinconia or a similar material. An annular pressure seal  120  is positioned about the piston rod  106  and includes a portion  121  extending within the housing  102 . The annular pressure seal  120  provides a substantially gas tight seal between the piston rod  106  and housing  102 . 
     Due to the high temperatures of the molten metal with which the injector  100  is used, the injector  100  is preferably cooled with a cooling medium, such as water. For example, the piston rod  106  may define a central bore  122 . The central bore  122  is in fluid communication with a cooling water source (not shown) through an inlet conduit  124  and an outlet conduit  126 , which pass cooling water through the interior of the piston rod  106 . Similarly, the annular pressure seal  120  may be cooled by a cooling water jacket  128  that extends around the housing  102  and is located substantially coincident with the pressure seal  120 . 
     The injector  100 , according to the present invention, is preferably suitable for use with molten metals having a low melting point such as aluminum, magnesium, copper, bronze, alloys including the foregoing metals, and other similar metals. The present invention further envisions that the injector  100  may be used with ferrous-containing metals as well, alone or in combination with the above-listed metals. Accordingly, the housing  102 , piston rod  106 , and pistonhead  108  are made of high temperature resistant metal alloys that are suitable for use with molten aluminum and molten aluminum alloys, and the other metals and metal alloys identified hereinabove. The pistonhead  108  may also be made of refractory material or graphite. The housing  102  has a liner  130  on the interior surface. The liner  130  may be made of refractory material, graphite, or other materials suitable for use with molten aluminum, molten aluminum alloys, or any of the other metals or metal alloys identified previously. 
     The piston  104  is generally movable through a return stroke in which molten metal is received into the housing  102 , and a displacement stroke for displacing the molten metal received from the housing  102 . FIG. 3 shows the piston  104  at a point just before it begins a displacement stroke (or at the end of a return stroke) to displace molten metal from the housing  102 . FIG. 4, conversely, shows the piston  104  at the end of a displacement stroke (or at the beginning of a return stroke). A molten metal supply source  132 , as shown in FIG. 2, is provided to maintain a steady supply of molten metal  134  to the housing  102 . The molten metal supply source  132  may contain any of the metals or metal alloys discussed previously. The molten metal supply source  132  is in fluid communication with the housing  102  through a first valve  136 , which is preferably a check valve for preventing backflow of molten metal  134  to the molten metal supply source  132  during the displacement stroke of the piston  104 . Thus, the first check valve  136  permits inflow of molten metal  134  to the housing  102  during the return stroke of the piston  104 . 
     The first check valve  136  is located in an injection port  138  connected to the housing  102  as shown in FIG.  2 . The injection port  138  may be fixedly connected to the lower end of the housing  102  by any means customary in the art, or formed integrally with the housing. The injection port  138  is connected to an outlet manifold  140  used, for example, to distribute the molten metal  134  displaced from the housing  102  to a downstream process. A second check valve  142  is located in the injection port  138 . The second check valve  142  is similar to the first check valve  136 , but is now configured to provide an exit conduit for the molten metal  134  received into the housing  102  to be displaced from the housing  102  to a downstream process. 
     A pressurized gas supply source  144  is in fluid communication with the housing  102  through a gas control valve  146 . The gas supply source  144  is provided to pressurize a space that is formed between the pistonhead  108  and the molten metal  134  flowing into the housing  102  during the return stroke of the piston  104 , as discussed more fully hereinafter. The space between the pistonhead  108  and molten metal  134  is formed during the reciprocal movement of the piston  104  within the housing  102  and is identified in FIG. 3 with reference numeral  148 . In order for gas from the gas supply source  144  to flow to the space  148  formed between the pistonhead  108  and molten metal  134 , the pistonhead  108  has a slightly smaller outer diameter than the inner diameter of the housing  102 . Accordingly, there is very little to no wear between the pistonhead  108  and housing  102  during operation of the injector  100 . The gas control valve  146  is configured to pressurize the space  148  formed between the pistonhead  108  and molten metal  134  as well as vent the space  148  to atmospheric pressure at the end of each displacement stroke of the piston  104 . For example, the gas control valve  146  may be a three-way, controlled solenoid valve. Alternatively, the single gas control valve  146  may be replaced by two separate valves, such as a vent valve and a gas supply valve, as discussed herein in connection with FIG.  7 . Either configuration is acceptable. A pressure transducer  149  is used to monitor the pressure in the space  148  during operation of the injector  100 . 
     The gas supply source  144  may be a source of inert gas such as helium, nitrogen, or argon, a compressed air source, or carbon dioxide. A floating thermal insulation barrier  150  is located in the space  148  to separate the pistonhead  108  from direct contact with the molten metal  134  received in the housing  102  during the reciprocal movement of the piston  104 . The insulation barrier  150  floats within the housing  102  during operation of the injector  100 , but generally remains in contact with the molten metal  134  received into the housing  102 . The insulation barrier  150  may be made of, for example, graphite or an equivalent material suitable for use with molten aluminum or aluminum alloys. 
     FIG. 5 shows a second embodiment of the molten metal injector of the present invention and designated with reference numeral  200 . The injector  200  shown in FIG. 5 is substantially similar to the injector  100  discussed previously, with the injector  200  now configured to operate with a liquid medium rather than a gas medium. The injector  200  also includes an injector housing  202  and a piston  204  positioned to extend downward into the housing  202  and reciprocally operate within the housing  202 . The piston  204  includes a piston rod  206  and a pistonhead  208 . The pistonhead  208  may be formed separately from and fixed to the piston rod  206  by any means customary in the art, or formed integrally with the piston rod  206 . The piston rod  206  includes a first end  210  and a second end  212 . The pistonhead  208  is connected to the first end  210  of the piston rod  206 . The second end  212  of the piston rod  206  is connected to a hydraulic actuator or ram  214  for driving the piston  204  through its reciprocal motion within the housing  202 . The piston rod  206  is connected to the hydraulic actuator  214  by a self-aligning coupling  216 . The injector  200  is also preferably suitable for use with molten aluminum and aluminum alloys, and the other metals discussed previously in connection with the injector  100 . Accordingly, the housing  202 , piston rod  206 , and pistonhead  208  may be made of any of the materials discussed previously in connection with the housing  102 , piston rod  106 , and pistonhead  108  of the injector  100 . The pistonhead  208  may also be made of refractory material or graphite. 
     The injector  200  differs from the injector  100  in that the injector  200  is specifically adapted to use a liquid medium as a viscous liquid source and pressurizing medium. Accordingly, the injector  200  includes a liquid chamber  224  positioned on top of and in fluid communication with the housing  202 . The liquid chamber  224  is filled with a liquid medium  226 . The liquid medium  226  is preferably a highly viscous liquid such as a molten salt. A suitable viscous liquid for the liquid medium is boron oxide. As with the injector  100 , the piston  204  is configured to reciprocally operate within the housing  202  and move through a return stroke in which molten metal is received into the housing  202 , and displacement stroke for displacing the molten metal received into the housing  202  from the housing  202  to a downstream process. However, the piston  204  is further configured to retract upward into the liquid chamber  224 . A liner  230  is provided on the inner surface of the housing  202  and may be made of any of the materials discussed previously in connection with the liner  130 . 
     A molten metal supply source  232  is provided to maintain a steady supply of molten metal  234  to the housing  202 . The molten metal supply source  232  may contain any of the metals or metal alloys discussed previously in connection with the injector  100 . The molten metal supply source  232  is in fluid communication with the housing  202  through a first valve  236 , which is preferably a check valve for preventing backflow of molten metal  234  to the molten metal supply source  232  during the displacement stroke of the piston  204 . Thus, the first check valve  236  permits inflow of molten metal  234  to the housing  202  during the return stroke of the piston  204 . The first check valve  236  is located in an injection port  238  connected to the housing  202 . The injection port  238  is connected to an outlet manifold  240  in a similar manner to the injector  100  discussed previously. A second check valve  242  is located in the injection port  238 . The second check valve  242  is similar to the first check valve  236 , but configured to provide an exit conduit for the molten metal  234  received into the housing  202  to be displaced from the housing  202 . 
     The pistonhead  208  may be cylindrically shaped and received in a cylindrically shaped housing  202 . The pistonhead  208  further defines a circumferentially extending recess  248 . The recess  248  is located such that as the piston  204  is retracted upward into the liquid chamber  224 , the liquid medium  226  from the liquid chamber  224  fills the recess  248 . The recess  248  remains filled with the liquid medium  226  throughout the return and displacement strokes of the piston  204 . However, with each return stroke of the piston  204  upward into the liquid chamber  224 , a “fresh” supply of the liquid medium  226  fills the recess  248 . In order for liquid medium  226  from the liquid chamber  224  to remain in the recess  248 , the pistonhead  208  has a slightly smaller outer diameter than the inner diameter of the housing  202 . Accordingly, there is very little to no wear between the pistonhead  208  and housing  202  during operation of the injector  200 , and the highly viscous liquid medium  226  prevents the molten metal  234  received into the housing  202  from flowing upward into the liquid chamber  224 . 
     The end portion of the pistonhead  208  defining the recess  248  may be dispensed with entirely such that during the return and displacement strokes of the piston  204 , a layer or column of the liquid medium  226  is present between the pistonhead  208  and the molten metal  234  received into the housing  202  and is used to force the molten metal  234  from the housing  202  ahead of the piston  204 . 
     Because of the large volume of liquid medium  226  contained in the liquid chamber  224 , the injector  200  generally does not require internal cooling as was the case with the injector  100  discussed previously. Additionally, because the injector  200  operates with a liquid medium the gas sealing arrangement (i.e., annular pressure seal  120 ) found in the injector  100  is not required. Thus, the cooling water jacket  128  discussed previously in connection with the injector  100  is also not required. As stated previously, a suitable liquid for the liquid chamber  224  is a molten salt such as boron oxide, particularly when the molten metal  234  contained in the molten metal supply source  232  is an aluminum-based alloy. The liquid medium  226  contained in the liquid chamber  224  may be any liquid that is chemically inert or resistive (i.e., substantially non-reactive) to the molten metal  234  contained in the molten metal supply source  232 . 
     Referring to FIGS. 2-4 and  6 , operation of the injector  100  will now be discussed. Referring first to FIGS. 3 and 6, FIG. 3 shows the injector  100  at a point just prior to the piston  104  beginning a displacement (i.e., downward) stroke in the housing  102 . The space  148  between the piston head  108  and the molten metal  134  is substantially filled with gas from the gas supply source  144 , which was supplied through the gas control valve  146 . The gas control valve  146  is a three-way valve operable to supply gas from the gas supply source  144  to the space  148  (i.e., pressurize), vent the space  148  to atmospheric pressure, and to close off the gas filled space  148  when necessary during the reciprocal movement of the piston  104  in the housing  102 . The gas control valve  146  is controlled by a control unit  160  such as personal computer (PC) or programmable logic controller (PLC), which is used to automate the injection cycle of the injector  100 . The control unit  160  is further connected to the hydraulic actuator  114  to control the movement of the piston  104  and, hence, the injection rate of the injector  100 . The pressure transducer  149  is used to provide input signals to the control unit  160 . 
     In FIG. 3, the piston  104  is in a return stroke position within the housing  102  just before beginning its displacement stroke and the gas control valve  146  is in a closed position, which prevents the gas in the gas filled space  148  from discharging to atmospheric pressure. The location of the piston  104  within the housing  102  in FIG. 3 is represented by point D in FIG.  6 . The control unit  160  is used to activate the hydraulic actuator  114  to cause the piston  104  to begin moving through its displacement stroke. As the piston  104  moves downward (i.e., a displacement stroke) in the housing  102 , the gas in the gas filled space  148  is compressed in situ between the pistonhead  108  and the molten metal  134  received in the housing  102 , substantially reducing its volume and increasing the pressure in the gas filled space  148 . The pressure transducer  149  monitors the pressure in the gas filled space  148  and provides this information as a process value input to the control unit  160 . When the pressure in the gas filled space  148  reaches a “critical” level, the molten metal  134  received in the housing  102  begins to flow into the injection port  138  and out of the housing  102  through the second check valve  142 . The critical pressure level will be dependent upon the downstream process to which the molten metal  134  is being delivered. For example, the downstream process may be a metal extrusion process or a metal rolling process. These processes will provide different amounts of return or “back pressure” to the injector  100 . The injector  100  must overcome this back pressure before the molten metal  134  will begin to flow out of the housing  102 . The amount of back pressure experienced at the injector  100  will also vary from one downstream extrusion process to another. Thus, the critical pressure at which the molten metal  134  will begin to flow from the housing  102  is process dependent and its determination is within the skill of those skilled in the art. The pressure in the gas filled space  148  is monitored by the pressure transducer  149 , which is used to identify the critical pressure at which the molten metal  134  begins to flow from the housing  102 . The pressure transducer  149  provides this information as an input signal (i.e., process value input) to the control unit  160 . 
     At approximately this point in the displacement movement of the piston  104  (i.e., when the molten metal  134  begins to flow from the housing  102 ), the control unit  160  is used to control the downward movement of the hydraulic actuator  114 , which controls the downward movement (i.e., speed) of the piston  104 , and, thus, the flow rate at which the molten metal  134  is displaced from the housing  102  through the injection port  138 . For example, the control unit  160  may be used to speed up or slow down the downward movement of the hydraulic actuator  114  depending on the molten metal flow rate desired at the downstream process. Thus, the control of the hydraulic actuator  114  provides the ability to control the molten metal flow rate out of the injector  100 . The insulation barrier  150  and compressed gas filled space  148  separate the end of the pistonhead  108  from direct contact with the molten metal  134  throughout the displacement stroke of the piston  104 . In particular, the molten metal  134  is displaced from the housing  102  in advance of the floating insulation barrier  150 , the compressed gas filled space  148 , and the pistonhead  108 . Eventually, the piston  104  reaches the end of the downstroke or displacement stroke, which is represented by point E in FIG.  6 . At the end of the displacement stroke of the piston  104 , the gas filled space  148  is tightly compressed and may generate extremely high pressures on the order of greater than 20,000 psi. 
     After the piston  104  reaches the end of the displacement stroke (point E in FIG.  6 ), the piston  104  optionally moves upward in the housing  102  through a short reset or return stroke. The control unit  160  through the hydraulic actuator  114  actuates the piston  104  to move upward in the housing  102 . The piston  104  moves upward a short “reset” distance in the housing  102  to a position represented by point A in FIG.  6 . The optional reset movement or stroke of the piston  104  is shown as a broken line in FIG.  6 . By moving upward a short distance within the housing  102 , the volume of the compressed gas filled space  148  increases thereby reducing the gas pressure in the gas filled space  148 . As stated previously, the injector  100  of the present invention is capable of generating high pressures in the gas filled space  148  on the order of greater than 20,000 psi. Accordingly, the short reset stroke of the piston  104  in the housing  102  may be utilized as a safety feature to partially relieve the pressure in the gas filled space  148  prior to venting the gas filled space  148  to atmospheric pressure through the gas control valve  146 . This feature protects the housing  102 , annular pressure seal  120 , and gas control valve  146  from damage when the gas filled space  148  is vented. Additionally, as will be appreciated by those skilled in the art, the volume of gas compressed in the gas filled space  148  is relatively small, so even though relatively high pressures are generated in the gas filled space  148  the amount of stored energy present in the compressed gas filled space  148  is low. 
     At point A, the gas control valve  146  is operated by the control unit  160  to an open or vent position to allow the gas in the gas filled space  148  to vent to atmospheric pressure. As shown in FIG. 6, the piston  104  only retracts a short reset stroke in the housing  102  until the gas control valve  146  is operated to the vent position. Thereafter, the piston  104  is operated (by the control unit  160  through the hydraulic actuator  114 ) to move downward to again reach the displacement stroke position (as shown in FIG.  4 ), which is identified by point B in FIG.  6 . If the reset stroke is not followed, the gas filled space  148  is vented to atmospheric pressure at point E and the piston  104  may begin a return stroke within the housing  102 , which will also begin at point B in FIG.  6 . 
     At point B, the gas control valve  146  is operated by the control unit  160  from the vent position to a closed position and the piston  104  begins the return or upstroke in the housing  102 , which again forms the space  148  between the pistonhead  108  and the molten metal  134 . The piston  104  is moved through the return stroke by the hydraulic actuator  114  after the hydraulic actuator  114  is signaled by the control unit  160  to begin moving the piston  104  upward in the housing  102 . However, the space  148  is now substantially at sub-atmospheric (i.e., vacuum) pressure, which causes molten metal  134  from the molten metal supply source  132  to enter the housing  102  through the first check valve  136 . The piston  104  continues to move upward in the housing  102  until it reaches point C in FIG.  6 . Point C is a preselected position that preferably corresponds with the point at which the housing  102  is entirely filled with molten metal  134  from the molten metal supply source  132 . At point C, the gas control valve  146  is operated by the control unit  160  to a position placing the housing  102  in fluid communication with the gas supply source  144 , which pressurizes the “vacuum” space  148  with gas, such as argon or nitrogen, forming a new gas filled space (i.e., gas charge)  148 . The piston  104  continues to move upward in the housing  102  as the gas filled space  148  is pressurized. 
     At point D during the return stroke of the piston  104  within the housing  102 , the gas control valve  146  is operated by the control unit  160  to a closed position, which prevents further charging of gas to the gas filled space  148  formed between the pistonhead  108  and molten metal  134 , as well as preventing the discharge of gas to atmospheric pressure. The control unit  160  further signals the hydraulic actuator  114  to stop moving the piston  104  upward in the housing  102 . As stated, the return stroke position of the piston  104  is represented by point D in FIG. 6, and may coincide with the full return stroke position of the piston  104  (i.e., the maximum possible upward movement of the piston  104 ) within the housing  102  but not necessarily. When the piston  104  reaches the return stroke position (i.e., the position of the piston  104  shown in FIG.  3 ), the piston  104  may be moved downward through another displacement stroke and the cycle illustrated in FIG. 6 begins over again. The second check valve  142  located in the injection port  138  permits displacement of the molten metal  134  from the housing  102  to the outlet manifold  140  and a selected downstream process or apparatus during the downward movement of the piston  104 . The control unit  160  is used to automate the injection cycle of the injector  100  by controlling the operation (i.e., sequencing) of the gas control valve  146  and the movement of the piston  104  within the housing  102  through control of the hydraulic actuator  114 . The pressure transducer  149  provides the necessary pressure process value inputs to the control unit  160 . 
     As will be appreciated by those skilled in the art, the single gas control valve  146  will require appropriate sequential and separate actuation of the gas supply (i.e., pressurization) and vent functions of the gas control valve  146 . The embodiment of the gas control valve  146  discussed previously in which the gas supply (i.e., pressurization) and vent functions are performed by two individual valves would require sequential activation of the valves. The embodiment of the present invention wherein the gas control valve  146  is replaced by two separate valves is shown in FIG.  7 . In FIG. 7, the gas supply and vent functions are performed by two individual valves  162 ,  164  that operate, respectively, as gas supply and vent valves. 
     The injector  200  shown in FIG. 5 operates in an analogous manner to the injector  100  discussed hereinabove. However, because the injector  200  operates with a liquid medium rather than a gas medium the gas control valve  146  is not required and the piston  104  does not move through the “reset” stroke described previously. The liquid chamber  224  provides a steady supply of liquid medium  224  to the piston  204  and housing  202 , which acts to pressurize the injector  200 . The liquid medium  224  may also provide certain cooling benefits to the injector  204 . 
     In FIG. 5, the piston  204  is shown at a substantially full displacement or downstroke position, which delivers the molten metal  234  received in the housing  202  to the outlet manifold  240 . As the piston  204  moves upward in the housing  202  from the position shown in FIG. 5, sub-atmospheric (i.e., vacuum) pressure is generated within the housing  202 , which causes molten metal  234  from the molten metal supply source  232  to enter the housing  202  through the first check valve  236 . As the piston  204  continues to move upward, molten metal  234  from the molten metal supply source  232  fills in behind the pistonhead  208 . However, the highly viscous nature of the liquid medium  226  present in the recess  248  and above in the housing  202  prevents the molten metal  234  from flowing upward into the liquid chamber  224 . The liquid medium  226  present in the recess  248  and above in the housing  202  provides a “viscous sealing” effect that prevents the upward flow of the molten metal  234  and, further, enables the pistonhead  208  to develop high pressures in the housing  202  during its displacement stroke as discussed hereinafter. 
     The piston  202  continues its upward movement until the pistonhead  208  reaches the liquid chamber  224 . The piston  204  is preferably configured to move upward such that the recess  248  formed in the pistonhead  208  is in substantial fluid communication with the liquid medium  226  in the liquid chamber  224 . The liquid medium  226  filling the recess  248  is replaced by a “fresh” supply of the liquid medium  226 . Alternatively, the piston  204  may be retracted entirely upward into the liquid chamber  224  so that a layer or column of the liquid medium  226  separates the end of the piston  204  from contact with the molten metal  234  received into the housing  202 . This situation is analogous to the “gas filled space” of the injector  100  discussed previously. 
     At this point, the housing  202  is preferably completely filled with another charge of the molten metal  234  and the recess  248  is filled with a fresh supply of the liquid medium  226 . The piston  204  then begins a displacement stroke to displace the molten metal  234  from the housing  202 . During the displacement stroke, the first check valve  236  prevents back flow of the molten metal  234  to the molten metal supply source  232  in a similar manner to the first check valve  136  in the injector  100 . The liquid medium  226  present in the recess  248  and above in the housing  202  provides a viscous sealing effect between the molten metal  234  being displaced from the housing  202  and the liquid medium  226  present in the liquid chamber  224 . In addition, the liquid medium  226  present in the recess  248  and above in the housing  202  is compressed during the downstroke of the piston  202  generating high pressures within the housing  202  that force the molten metal  234  received into the housing  202  from the housing  202 . Because the liquid medium  226  is substantially incompressible, the injector  200  reaches the “critical” pressure discussed previously in connection with the injector  100  very quickly. As the molten metal  234  begins to flow from the housing  202 , the hydraulic actuator  214  may be used to control the molten metal flow rate at which the molten metal  234  is delivered to the downstream process. 
     The second check valve  242  in the injection port  238  permits displacement of the molten metal  234  from the housing  202  to the outlet manifold  240  during the downstroke of the piston  204 . The entire process described hereinabove for the injection cycle of the injector  200  is controlled by a control unit  260  (PC/PLC), which controls the operation and movement of the hydraulic actuator  214  in a similar manner to the injector  100 . 
     The present invention provides a molten metal injector that may be used to deliver molten metal to a downstream metalworking or forming process or apparatus. The present invention provides the benefits of greatly reduced wear between the piston and housing of the injector and the ability to generate relatively high working pressures with correspondingly small amounts of stored energy. While preferred embodiments of the present invention were described herein, various modifications and alterations of the present invention may be made without departing from the spirit and scope of the present invention. The scope of the present invention is defined in the appended claims and equivalents thereto.