Abstract:
A mechanical apparatus and method for the casting of metal components is disclosed. The apparatus includes a gooseneck having dual plungers for drawing molten metal from a crucible of hot metal and for forcing the drawn molten metal through the system, a hot runner assembly having a thermal valve developed in time and positioned adjacent the mold cavity, and a machine nozzle positioned between the gooseneck and the hot runner assembly. The dual plunger is fitted with a shot plunger and a shutoff plunger which work in conjunction to allow for molten metal to be drawn into the gooseneck but to stop its passage into the gooseneck when the metal is forced through the system into the die. Both temperature and flow rate are carefully monitored and controlled. Two embodiments of the gooseneck are provided in which the positions of the shot plunger and the shutoff plunger are altered and in which the molten metal is drawn into the gooseneck at different locations.

Description:
GOVERNMENT CONTRACT INFORMATION 
       [0001]    This invention was made with United States Government support awarded by the following program, agency and contract: NIST Advanced Technology Program, the United States Department of Commerce, Contract No. 70NANBOH3053. The United States has certain rights in this invention. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates generally to the casting of magnesium components. More particularly, the present invention relates to a method and apparatus for die-casting magnesium components from molten magnesium using a hot runner system in which both temperature and flow rate are controlled. 
       BACKGROUND OF THE INVENTION 
       [0003]    Magnesium is an attractive material for application in motor vehicles because it is both a strong and lightweight material. The use of magnesium in motor vehicles is not new. Race driver Tommy Milton won the Indianapolis 500 in 1921 driving a car with magnesium pistons. A few years after that magnesium pistons entered mainstream automotive production. By the late 1930&#39;s over 4 million magnesium pistons had been produced. Even in the early days of car production, the weight-to-strength ratio of magnesium, compared with other commonly-used materials, was well-known. 
         [0004]    Considering the recent increase in fuel prices driven largely by increased global demand, more attention is being given to any practical and economically viable step that can be taken to reduce vehicle weight without compromising strength and safety. Accordingly, magnesium is increasingly becoming an attractive alternative to steel, aluminum and polymers, given its ability to simultaneously meet crash-energy absorbing requirements while reducing the weight of vehicle components. Having a density of 1.8 kg/L, magnesium is 36% lighter per unit volume than aluminum (density=2.70 kg/L) and is 78% lighter per unit volume than steel (density=7.70 kg/L). Magnesium alloys also hold a competitive weight advantage over polymerized materials, being 20% lighter than most conventional glass reinforced polymer composites. 
         [0005]    Beyond pistons, numerous other vehicle components are good candidates for being formed from magnesium, such as inner door panels, dashboard supports and instrument panel support beams. In the near-term it is anticipated that components made from magnesium for high volume use in the motor vehicle might also include powertrain, suspension and chassis components. 
         [0006]    The fact that the surface “skin” of die-cast magnesium has better mechanical properties over the bulk than more commonly used materials, thinner (ribbed) and lighter die-castings of magnesium enables products to meet their functional requirements. Such components can have sufficiently high strength per unit area to compete with more common and heavier aluminum and plastic components. Furthermore, magnesium has considerable manufacturing advantages over other die-cast metals, such as aluminum, being able to be cast closer to near net-shape thereby reducing the amount of material and associated costs. Particularly, components can be routinely cast at 1.0 mm to 1.5 mm wall thickness and 1 to 2 degree draft angles, which are typically ½ that of aluminum. The extensive fluid flow characteristics of magnesium offers a single, large casting to replace a plurality of steel fabrications. Magnesium also has a lower latent heat and reduced tendency for die pick-up and erosion. This allows a reduced die-casting machine cycle time (˜25% higher productivity) and 2 to 4 times longer die life (from 150-200,000 to 300-700,000 shots) compared with that of aluminum casting. 
         [0007]    However, the use of magnesium in automotive components is burdened with certain drawbacks. While magnesium is abundant as a natural element, it is not available at a level to support automotive volumes. This situation causes hesitation among engineers to design and incorporate magnesium components. On the occasion when the magnesium is selected as the material of choice, designers fail to integrate die-casting design with manufacturing feasibility in which the mechanical properties, filling parameters, and solidification profiles are integrated to predict casting porosity and property distribution. 
         [0008]    The raw material cost of magnesium relative to other commonly used materials is also an impediment to mass implementation in the automotive industry. Current techniques for casting parts from magnesium make expanding the use of magnesium into a broader array of products less attractive. Presently, all large die-castings are produced in high pressure, cold-chamber machines where the metal is injected from one central location. This approach results in inferior material properties and waste material. 
         [0009]    Accordingly, in order to make the use of magnesium in the production of vehicle components more attractive to manufacturers, a new approach to product casting is needed. This new approach is the focus of the present invention. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention represents advancement in the die casting process of magnesium and similar metals. The primary objective of the present invention is to provide a multi-point injection hot runner system for introducing molten magnesium into production die cavities at a controlled temperature and flow rate. The method and apparatus of the present invention provides an approach that minimizes wastage while maximizing manufacturing repeatability thus providing a cost-effective and practical answer to the problems ordinarily associated with known approaches to the formation of articles from magnesium. 
         [0011]    The present invention accomplishes these and other objectives by providing a self-contained, self-enclosed system which maximizes control over heat and molten metal flow while minimizing contamination. The system utilizes a gooseneck having dual plungers that draws molten metal from a crucible and directs the molten metal to a hot runner assembly via a machine nozzle. The dual plunger comprises a shot plunger and a shutoff plunger. The shot plunger draws the molten metal from the crucible and drives it through the system. The shutoff plunger works in concert with the shot plunger to regulate flow of molten metal both into and out of the gooseneck. The molten metal exits the hot runner assembly through a hot runner tip into a mold cavity. The hot runner assembly is provided to gate directly on or very near the part surface. 
         [0012]    Each of the machine nozzle, the hot runner assembly, and the hot runner tip is heated by adjacent heating elements which may be coil heaters, tubular heaters or band heaters or a variety of such heating elements. By providing such an array of heaters the temperature of the molten metal can be readily and accurately maintained. 
         [0013]    Flow of the molten metal is regulated by use of the gooseneck which incorporates the shutoff plunger and the shot plunger. The shutoff plunger and the shot plunger are selectively positioned so as to draw molten metal from a crucible into which the plunger is at least partially submerged. Once the gooseneck is filled with molten metal the molten metal is forced under pressure by movement of the shot plunger out of the gooseneck and into the machine nozzle. A preferred and accurate pressure is maintained by the amount of force applied by the piston upon the molten metal. This pressure is maintained evenly throughout the system such that the molten metal moves at a constant, regulated flow out of the gooseneck and through the machine nozzle, the hot runner assembly, the hot runner tip, and into the cavity. 
         [0014]    To maintain this constant pressure or zero pressure difference by avoiding the return of molten metal back into the gooseneck when the piston extracts or moves to apply pressure to the molten metal, the shutoff plunger is moved to prevent such an outflow. During the extraction step a thermal valve (“TV”) is formed at the tip of the hot runner assembly, thus preventing flow of molten metal from the mold cavity and back into the hot runner tip. The formation of the blockage at the tip of the thermal valve is accomplished by a balance of both temperature regulation and tip opening geometry. With this arrangement the molten metal is retained in and completely fills entire feeding system. This is necessary because magnesium molten metal needs to be present in the machine nozzle at all times, before and after each shot. 
         [0015]    Flow of the molten metal is regulated by use of the dual plunger which incorporates an internal reciprocating plunger to selectively draw molten metal from a crucible into which the dual plunger gooseneck is at least partially submerged. As briefly noted above, the shutoff plunger works in concert with the shot plunger to allow the selective entry and exit of fluid into and out from the gooseneck. By being moved to a fluid flow passing position to fill the gooseneck, the shutoff plunger is open to allow the passage of fluid from the crucible while the shot plunger draws metal into the gooseneck. Once the filling of the gooseneck is complete, the shutoff plunger is moved to a fluid-closed position. In this position the molten metal is allowed to pass thereby under pressure of the shot plunger. A preferred and accurate pressure is maintained by the amount of force applied by the shot plunger upon the molten metal. This pressure is maintained accordingly throughout the system such that the molten metal moves at a constant, regulated flow out of the gooseneck and through the machine nozzle, the hot runner assembly, out of the hot runner tip and into the cavity. With this arrangement the molten metal is retained in the entire feeding system. This is necessary because molten magnesium metal is expected to be filled 100% in the machine nozzle at all times, before and after each shot. 
         [0016]    The present invention teaches an arrangement of the dual plunger system which includes a shot plunger reciprocatingly provided in a shot plunger cylinder and a shutoff plunger reciprocatingly provided in a shutoff plunger cylinder. The shot plunger cylinder and the shutoff plunger cylinder as part of the gooseneck are substantially parallel to one another. The shot plunger is movable between a molten metal drawing position and a molten metal injecting position. The shutoff plunger is movable between a molten metal halting position and a molten metal passing position. In operation, the shutoff plunger is moved to its molten metal passing position while the shot plunger moves to its molten metal drawing position whereby molten metal is drawn into the shot plunger cylinder and the gooseneck. Thereafter the shutoff plunger is moved to its molten metal halting position and the shot plunger is moved to its molten metal shot injecting position whereby molten metal is forced by the shot plunger for injection into the mold cavity. 
         [0017]    By providing a mechanical apparatus and a method according to the present invention several advantages are achieved. First, the quality and consistency of die castings is improved. Second, reductions in cycle time are achieved. Third, less waste and less recycling of material is achieved. Fourth, the present invention reduces the level of maintenance required as compared with known systems. 
         [0018]    Other advantages and features of the invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    For a more complete understanding of this invention, reference should now be made to the embodiment illustrated in greater detail in the accompanying drawings and described below by way of examples of the invention wherein: 
           [0020]      FIG. 1  illustrates a diagrammatic view of a casting apparatus utilizing the gooseneck with dual plungers according to the present invention; 
           [0021]      FIG. 2  illustrates a sectional view of a hot runner assembly in position relative to a die according to the present invention; 
           [0022]      FIG. 3  illustrates a sectional view of the hot runner body of  FIG. 2  illustrating an alternate arrangement for heating; 
           [0023]      FIG. 4  illustrates a sectional view of the hot runner tip according to the present invention in relation to a portion of a cast part; 
           [0024]      FIG. 5  illustrates a perspective and partially sectioned view of a machine nozzle according to the present invention; 
           [0025]      FIG. 6  illustrates a sectional view of a first preferred embodiment of a gooseneck according to the present invention, illustrating the gooseneck filling mode; 
           [0026]      FIG. 7  illustrates the same view of  FIG. 6  but shows the gooseneck in its shot mode; 
           [0027]      FIG. 8  illustrates a side view of the shutoff plunger of the preferred embodiment of the present invention; 
           [0028]      FIG. 9  illustrates an end view of the shutoff plunger shown in  FIG. 8 ; 
           [0029]      FIG. 10  illustrates a sectional view of a modified version of the first preferred embodiment of the present invention; 
           [0030]      FIG. 11  illustrates a sectional view of a second preferred embodiment of a gooseneck according to the present invention in its filling mode; 
           [0031]      FIG. 12  illustrates the same view of  FIG. 11  but shows the gooseneck in its metal injection mode; and 
           [0032]      FIG. 13  illustrates a sectional view of a modified version of the second preferred embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0033]    In the following figures, the same reference numerals will be used to refer to the same components. In the following description, various operating parameters and components are described for one constructed embodiment. These specific parameters and components are included as examples and are not meant to be limiting. 
         [0034]    With reference to  FIG. 1 , a diagrammatic view of the hot chamber apparatus of the present invention is illustrated, being generally illustrated as  10 . The apparatus  10  is entirely self-enclosed, preventing atmospheric exposure of the liquid melt. It is to be understood that while the present invention is directed at the formation of components from molten magnesium alloy, other metals including zinc may be used. 
         [0035]    The hot chamber  10  includes a casting die  12 . The casting die  12  includes a cover half  14  and an ejector half  16 , a hot runner assembly  18  partially recessed within the cover half  14  of the casting die  12 , a gooseneck  19 , a shot plunger  20  operatively associated with the gooseneck  19 , a shutoff plunger  21  also operatively associated with the gooseneck  19 , and a machine nozzle  22  fitted between the hot runner assembly  18  and the gooseneck  19 . A substantial portion of the gooseneck  19  is submerged within a crucible  24  of molten metal. 
         [0036]    Referring now to  FIG. 2 , a sectional and detailed view of the hot runner assembly  18  is illustrated. As noted above, the hot runner assembly  18  is partially recessed within the cover half  14  of the casting die  12 . The hot runner assembly  18  consists of a hot runner body  26  having a long axis along which a molten metal passage  28  is formed. The hot runner body  26  includes a molten metal input end  30  and a molten metal output end  32 . The molten metal input end  30  includes an outer cone  34  which can be inserted into a receiving end of the machine nozzle  22  as shown in  FIG. 5  and as discussed in relation thereto. 
         [0037]    With reference still to  FIG. 2 , the molten metal output end  32  includes a cavity  36  defined therein into which a hot runner tip  38  is partially positioned. The outward end of the hot runner tip  38  terminates at a part line  39  formed between the cover half  14  and ejector half  16  of the casting die  12 . The hot runner tip  38  includes an end  41  that is open to the mold cavity. 
         [0038]    The hot runner tip  38  is provided to establish thermal valving in the apparatus  10  whereby a thermal plug (shown in  FIG. 4  and discussed in relation thereto) is formed at the orifice outlet of the hot runner body  26 . The opening of the hot runner tip  38  may be of a variety of possible sizes, although an orifice size of about 8 mm provides an effective configuration. The objective of the hot runner tip  38  is to prevent the flow of molten magnesium downwards into the gooseneck  19  during each complete casting cycle because of the ability of the thermal plug formed adjacent the die cavity by the hot runner tip  38  to retain the pressure difference in the hot runner assembly  18  and the gooseneck  19 . 
         [0039]    The hot runner body  26  is positioned in a hot runner body cavity  40  which is recessed within the cover half  14  of the casting die  12 . The hot runner body cavity  40  is held in place by a support ring  42  which may be fastened to the cover half  14  of the casting die  12  by conventional means such as by mechanical fasteners  44  and  44 ′. 
         [0040]    It is important in the operation of the apparatus  10  that the molten metal be maintained at high temperatures at all stages between the crucible  24  and the die  12 . Accordingly, a series of insulators and heaters are provided to maintain the needed temperatures. To this end the hot runner assembly  18  includes both insulators and heaters. A hot runner body insulator ring  46  is fitted between the hot runner body  26  and the support ring  42 . A thermal valve insulator ring  49  is fitted between the hot runner tip  38  and the cover half  14  of the casting die  12 . The hot runner body insulator ring  46  and the thermal valve insulator ring  49  are formed from known insulating material. 
         [0041]    To keep the hot runner assembly  18  as uniform a temperature as possible external heaters are applied. As illustrated in  FIG. 2 , a pair of spaced-apart band heaters  48  and  50  is fitted to the hot runner body  26 . The band heaters  48  and  50  are electrically powered and controlled in a known manner. 
         [0042]    In addition or as an alternative to the use of band heaters as illustrated in  FIG. 2 , coil or tubular heaters may also be used to create and maintain the desired level of heat in the hot runner assembly  18 . An example of such an alternative is illustrated in  FIG. 3  where a coil heater  52  is fitted to the hot runner body  26  in lieu of the band heater  48 . As a further modification, a hot runner tip band heater  54  is shown in  FIG. 3  externally positioned on the hot runner tip  38 . Other variations may be possible provided the objective of establishing and regulating the desired levels of heat with respect to the hot runner body  26  is achieved. Accordingly, the application of heat using bands and coils as shown is intended as being illustrative and not limiting. 
         [0043]    Referring now to the hot runner tip  38 , this component is illustrated in sectional view in  FIG. 4  and is shown in relation to a portion of a cast part “P”. The cast part P is illustrated as having been removed from the mold cavity and thus separated from the hot runner tip  38 . A molten metal passage  58  is defined along the long axis of the hot runner tip  38 . The hot runner tip  38  may be threadably attached to the hot runner body  26  or may be attached by other mechanical means. 
         [0044]    The hot runner tip heater  54  is provided to keep the hot runner tip  38  at a pre-selected temperature such that the metal at the end  41  may flow freely into the mold cavity during the plunger shot but will form a solid blockage once the shot is completed. Accordingly, there is a temperature differential between the end  41  and the hot runner tip  38 . This temperature differential means that the area of the opening of the hot runner tip  38  into the mold cavity will be cooler than the rest of the hot runner tip  38 , thus allowing the molten metal in the immediate area of the tip to cool and become solidified locally in the area of the tip. This arrangement prevents molten metal from leaking from the cavity and back into the hot runner tip  38  at the end of the shot. 
         [0045]    The temperature differential is dependent upon the metal being used to make the cast component. By way of example, magnesium alloy (for example, AZ91) becomes solid at 470° C. and is fully molten at temperatures over 595° C. Accordingly, the temperature of the hot runner tip  38  must be such that the metal therein is molten to allow it to flow. Conversely, the temperature at the end  41  of the hot runner tip  38  that is open to the mold cavity must be cooler than that of the rest of the hot runner tip  38  and specifically must approach, but not necessarily meet, the temperature of 470° C. at which magnesium alloy is solid. Of course, the temperature of the thermal valve  38  may be adjusted up or down depending on the metal alloy being used. 
         [0046]    As illustrated in  FIG. 4 , a thermal valve “TV” of an ideal size and configuration has been formed within the hot runner tip  38 . The thermal valve TV prevents the back-flow of molten metal into the hot runner tip  38  after the completion of the shot. 
         [0047]    The machine nozzle  22  is illustrated in  FIG. 5 . A quarter of the machine nozzle  22  has been removed for illustrative purposes. The machine nozzle  22  includes a machine nozzle body  60  having a molten metal passage  62  defined along its long axis. The machine nozzle  22  also includes a molten metal input end  64  which has an outer cone  68  to mate with the gooseneck  19 . The machine nozzle  22  also has a molten metal output end  66  defined as a conical cavity  70  which mates with outer cone  34  of the molten metal input end  30  of the hot runner assembly  18 . 
         [0048]    As noted above, it is important to establish and maintain desired temperatures at all points between the crucible  24  and the die  12 . Accordingly, the machine nozzle  22  is also provided with a heating element. Two forms of heating elements are illustrated in  FIG. 5 . The first form is heating element  72  which is a coil-type heating system. The second form is heating element  73  which is a band heater. The coil, band, or tubular form of heating elements may be used, alone or in combination. 
         [0049]    Delivery of the molten metal from the crucible  24  to the machine nozzle  22  is accomplished by the gooseneck which is presented herein in two embodiments. The first embodiment of the gooseneck of the present invention, generally illustrated as  19 , is illustrated in  FIGS. 6 and 7  with a variation of this embodiment illustrated in  FIG. 10 . A second embodiment of the gooseneck of the present invention, generally illustrated as  110 , is illustrated in  FIGS. 11 and 12  with a variation of this embodiment shown in  FIG. 13 . In either embodiment, the body of the gooseneck may be made of a superalloy steel. 
         [0050]    Referring to  FIGS. 6 and 7 , the gooseneck  19  includes a plunger body  74 . The plunger body  74  includes a shot plunger cylinder  76  and a molten metal passageway  78  through which the molten metal flows out of the plunger body  74 . The shot plunger cylinder  76  and the molten metal passageway  78  are substantially parallel to one another, with the diameter of the shot plunger cylinder  76  being larger than the diameter of the molten metal passageway  78 . 
         [0051]    The molten metal passageway  78  includes an inlet end  80  and an outlet end  82 . The inlet end  80  is in fluid communication with the shot plunger cylinder  76  by way of a molten metal channel  84 . The outlet end  82  terminates at a plunger molten metal outlet port  86 . The plunger molten metal outlet port  86  is preferably of a conical configuration as illustrated so as to mate snugly with the outer cone  68  of the molten metal input end  64  of the machine nozzle  22 . 
         [0052]    The shot plunger  20  having a pair of spaced apart sacrificial rings  89 ,  89 ′ is reciprocatingly provided within the shot plunger cylinder  76 . The shot plunger  20  is selectively driven by a plunger drive shaft  90 . The plunger drive shaft  90  is operatively associated with a plunger drive mechanism (not shown). The sacrificial rings  89 ,  89 ′ are provided to take up wear endured as the shot plunger  20  reciprocates within the shot plunger cylinder  78  during normal operations, thus saving the shot plunger  20  from wear. After a given number of cycles the gooseneck  19  is disassembled and the worn sacrificial rings  89 ,  89 ′ are replaced by a new set. 
         [0053]    The shot plunger cylinder  76  includes a molten metal passageway  92  which is fluidly connected with a shutoff plunger cylinder  94 . The shutoff plunger cylinder  94  is generally parallel with both the shot plunger cylinder  76  and the molten metal passageway  78 . The shutoff plunger cylinder  94  includes a molten metal inlet  96  which is in fluid communication with the crucible  24  of molten metal (shown in  FIG. 1 ). 
         [0054]    A shutoff plunger  21  is reciprocatingly provided within the shutoff plunger cylinder  94 . The shutoff plunger  21  is selectively driven by a shutoff plunger drive shaft  100 . The shutoff plunger  21  has an upper set of sacrificial shutoff rings  102 ,  102 ′, and  102 ″ and a lower set of sacrificial shutoff rings  104 ,  104 ′, and  104 ″. Like the sacrificial rings  89  and  89 ′ fitted to the shot plunger  20 , the sacrificial rings  102 ,  102 ′,  102 ″,  104 ,  104 ′, and  104 ″ are provided to suffer wear instead of the shutoff plunger  21 . They may also be replaced along with the sacrificial rings  89  and  89 ′ after a predetermined number of cycles. The shutoff plunger  21  is operatively associated with a shutoff plunger drive mechanism (not shown). 
         [0055]    In  FIG. 6  the gooseneck  19  is illustrated in its filling position in which the shutoff plunger  21  having been moved upward as indicated by the arrow to its fluid passing position such that the upper set of sacrificial shutoff rings  102 ,  102 ′, and  102 ″ is above the molten metal inlet  96  and the lower set of sacrificial shutoff rings  104 ,  104 ′, and  104 ″ is below the molten metal passageway  92 . Once in this position, the shot plunger  20  is drawn upward as illustrated by the arrow. This movement creates suction within the shot plunger cylinder  76 , the suction effecting the movement of molten metal (not shown) from the crucible  24 , through the molten metal inlet  96 , through the molten metal passageway  92 , and into the shot plunger cylinder  76 . The shot plunger  20  continues its upward movement until the requisite amount of molten metal has been drawn into the shot plunger cylinder  76 . Because of the formation of the thermal valve TV the flow of molten metal back into the system is prevented as there is no pressure differential within the system between the hot runner assembly and the shot plunger cylinder  76 . 
         [0056]    In  FIG. 7  the shutoff plunger  21  has been moved as illustrated by the arrow to its fluid flow blocking position such that the upper set of sacrificial shutoff rings  102 ,  102 ′, and  102 ″ is below the molten metal inlet  96 . Once in this position the shot plunger  20  is moved downward as illustrated by the arrow. This movement creates pressure within the shot plunger cylinder  76 , forcing the molten metal out of the shot plunger cylinder  76 , through the molten metal passageway  78 , and out of the outlet end  82  of the gooseneck  19  and into the machine nozzle  22  (not shown). 
         [0057]    The shutoff plunger  21  is illustrated in  FIG. 8 . With reference thereto, the threaded attachment of the shutoff plunger drive shaft  100  is shown. It is to be understood that while each of the upper set of sacrificial shutoff rings  102 ,  102 ′, and  102 ″ and the lower set of sacrificial shutoff rings  104 ,  104 ′, and  104 ″ is composed of three spaced apart rings, a greater or lesser number of rings may be employed. 
         [0058]    An end view of the shutoff plunger  21  the sacrificial shutoff ring  104 ″ is illustrated in  FIG. 9 . A series of molten metal passageways  106  are formed in the lower set of rings  104  (as well as the upper set of rings  102 ). The molten metal passageways  106  are provided to allow some molten metal to flow within the shutoff plunger cylinder  94  thereby providing partial equalization of pressure throughout the gooseneck  19  to prevent extreme system pressure differentials which might result in slowed reciprocation of the shutoff plunger  21 . 
         [0059]    As noted above with reference to  FIG. 6 , a pair of sacrificial rings  89  and  89 ′ is provided to endure the operational wear instead of the shot plunger  20 . This wear is the result of the metal-to-metal contact between the sacrificial rings  89  and  89 ′ and the wall of the shot plunger cylinder  76 . Similarly, the sacrificial shutoff plunger rings  102 ,  102 ′,  102 ″,  104 ,  104 ′, and  104 ″ are provided to prevent wear on the shutoff plunger  21 . An alternative approach to the use of the sacrificial rings on either the shot plunger  20  or on the shutoff plunger  21  is illustrated in  FIG. 10  where a gooseneck  19 ′ is illustrated. The gooseneck  19 ′ includes a plunger body  74 ′, a shot plunger cylinder  76 ′, a shot plunger  20 ′, a shutoff plunger cylinder  94 ′, and a shutoff plunger  21 ′. With the exception of the design and construction of the plunger body  74 ′, the shot plunger cylinder  76 ′, the shot plunger  20 ′, the shutoff plunger cylinder  94 ′, and the shutoff plunger  21 ′, the gooseneck  19 ′ includes elements that are preferably identical in design and function to those of the gooseneck  19  discussed above and shown in  FIGS. 6 and 7 . Accordingly, only the differences will be discussed. 
         [0060]    The plunger body  74 ′ is configured so as to eliminate the need of having to change sacrificial rings. Accordingly, the shot plunger  20 ′ and the shutoff plunger  21 ′ are provided without sacrificial rings. This is accomplished by use of a shot plunger ceramic liner  105  provided to line the shot plunger cylinder  76 ′. Similarly, a shutoff plunger ceramic liner  107  is provided to line the shutoff plunger cylinder  94 ′. The ceramic liners  105  and  107  are sleeves that are shrink-fitted within the plunger body  74 ′. The ceramic liners  105  and  107  may be composed of a variety of ceramic materials, but preferably are composed of a silicon nitride material such as SN-240 manufactured by Kyocera. Other ceramic materials may be used in the alternative. By using ceramic liners in the gooseneck  19 ′ the metal-to-metal wear of the arrangement of the gooseneck  19  is eliminated. 
         [0061]    An alternate embodiment of the dual plunger design of the present invention presented above is illustrated in  FIGS. 11 and 12 . According to this embodiment, the gooseneck  110  includes a plunger body  114 . The plunger body  114  includes a molten metal passageway  116  through which molten metal flows out of the plunger body  114 . Adjacent the molten metal passageway  116  is a shutoff plunger cylinder  118  housing therein a reciprocating shutoff plunger  120 . The shutoff plunger  120  includes an upper set of sacrificial rings  122 ,  122 ′, and  122 ″ and a lower set of sacrificial rings  124 ,  124 ′, and  124 ″. The shutoff plunger  120  is selectively driven by a shutoff plunger drive shaft  126 . The shutoff plunger  120  is operatively associated with a shutoff plunger drive mechanism (not shown). 
         [0062]    Adjacent the shutoff plunger cylinder  118  is a plunger cylinder  128 . A shot plunger  129  selectively driven by a plunger drive shaft  132  is reciprocatingly provided within the plunger cylinder  128 . The plunger drive shaft  132  is operatively associated with a plunger drive mechanism (not shown). The plunger  129  includes a set of sacrificial rings  130 ,  130 ′, and  130 ″. 
         [0063]    A molten metal fluid passageway  134  is formed between the molten metal passageway  116  and the shutoff plunger cylinder  118 . Another molten metal fluid passageway  136  is formed between the shutoff plunger cylinder  118  and the plunger cylinder  128 . A molten metal inlet  138  is formed at the lower end of the shutoff plunger cylinder  118  and is open to the crucible  24  of molten metal (shown in  FIG. 1 ). 
         [0064]    In  FIG. 11 , as illustrated by the arrow, the shutoff plunger  120  has been moved to its fluid flow passing position such that the lower set of rings  124 ,  124 ′, and  124 ″ are positioned above the molten metal fluid passageway  136 . Once the shutoff plunger  120  is so positioned, the plunger  129  is moved upward as indicated by the arrow thereby creating suction within the plunger cylinder  128 . Molten metal travels from the crucible  24  into the plunger cylinder  128  via the molten metal inlet  138  defined at the base of the gooseneck  110 . The plunger  129  continues its upward travel in the direction of the arrow until the plunger cylinder  128  is filled with molten metal. 
         [0065]    In  FIG. 12  the shutoff plunger  120  has been moved in the direction of the arrow to its fluid halting position such that the lower set of rings  124 ,  124 ′, and  124 ″ is below the molten metal fluid passageway  136  and the upper set of rings  122 ,  122 ′, and  122 ″ is above the molten metal fluid passageway  134 . Once the shutoff plunger  120  is in its fluid halting position, the shot plunger  129  is moved downward as indicated by the arrow. This movement of the shot plunger  129  forces the molten metal through the molten metal passageway  136 , around and by the shutoff plunger  120 , into and through the molten metal passageway  116  and out of the gooseneck  110  to the machine nozzle  22  (shown in  FIG. 1 ). 
         [0066]    As an alternative to the embodiment shown in  FIGS. 11 and 12 , the sacrificial rings on the shot plunger  129  and the shutoff plunger  120  may be eliminated in favor of ceramic linings as discussed above in relation to  FIG. 10 . This alternative is shown in  FIG. 13 . A gooseneck  110 ′ is shown and includes a plunger body  114 ′, a shot plunger cylinder  128 ′, a shot plunger  129 ′, a shutoff plunger cylinder  118 ′, and a shutoff plunger  120 ′. With the exception of the design and construction of the plunger body  114 ′, the shot plunger cylinder  128 ′, the shot plunger  129 , the shutoff plunger cylinder  128 ′, and the shutoff plunger  120 ′, the gooseneck  110 ′ includes elements that are preferably identical in design and function to those of the gooseneck  110  discussed above and shown in  FIGS. 11 and 12 . The differences are discussed hereafter. 
         [0067]    According to the embodiment shown in  FIG. 13 , the plunger body  114 ′ eliminates the need for the sacrificial rings shown in  FIGS. 11 and 12 . A shot plunger ceramic liner  140  is provided in the shot plunger cylinder  128 ′. A shutoff plunger ceramic liner  142  is provided to line the shutoff plunger cylinder  118 ′. Like the shutoff liners  105  and  107  of the gooseneck  19 ′ discussed above, the liners  140  and  142  are sleeves that are shrink-fitted within the gooseneck  110 ′ 
         [0068]    The arrangements shown of the goosenecks  19  and  110  illustrated in their respective figures and in their variations provided a positive method for assuring that a constant flow of molten metal at a constant pressure can be maintained in the hot chamber  10  at all times. This arrangement assures that no back flow of molten metal out of the system and back into the crucible  24  can occur. 
         [0069]    The foregoing discussion discloses and describes an exemplary embodiment of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the true spirit and fair scope of the invention as defined by the following claims.