Abstract:
An apparatus for dispensing a molten material from a reservoir of molten material includes a dispensing chamber in communication with the reservoir and a first valve adapted to regulate communication of the dispensing chamber with the reservoir. A riser communicates with the dispensing chamber for dispensing the molten material, and a second valve is adapted to regulate communication of the riser with the dispensing chamber. Also disclosed is a method for reducing the inclusion of oxides in a casting of a molten metal.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a dispensing apparatus for dispensing a molten material and to a method for dispensing a molten material into a mold by means of such an apparatus. More particularly, the present invention is directed toward an apparatus for dispensing a molten metal that reduces the inclusion of oxides in a casting of the metal.  
         BACKGROUND OF THE INVENTION  
         [0002]    The transfer of liquid metal, in particular liquid aluminum, into molds to make castings is usually carried out by simply pouring under gravity. There are a number of severe disadvantages to this technique, in particular, the entrainment of air and oxides as the metal falls in a relatively uncontrolled way.  
           [0003]    Counter-gravity is usually employed to avoid this problem. However, when making a series of castings using a counter-gravity system and a riser tube to supply metal to a mold, it has been found that if the metal is allowed to fall back down the riser tube during the process, oxides are immediately generated on the internal walls of the tube and subsequently carried into the next casting. The surface oxide exhibits the consistency of tissue paper and is easily folded into the melt, creating a folded film defect. In fact, the introduction of unwanted oxides into metal castings, especially in those applications using alloys having minimal or no silicon, is such a severe problem that often only the first casting is of an acceptable quality. All subsequent castings are unacceptable due to high oxide content.  
           [0004]    To overcome the worst features of this method of mold filling, the so-called Low Pressure (LP) Casting Process was developed. In this technique the metal is held in a large bath or crucible, usually of at least 200-kg capacity of liquid metal, which is contained within a pressurizable enclosure known as a pressure vessel. The pressurization of this vessel with a low pressure (typically a small fraction such as 0.1 to 0.3 atmosphere) of air or other gas forces the liquid up a riser tube and into the mold cavity which is mounted above the pressure vessel.  
           [0005]    The LP Casting Process suffers from the refilling of the internal crucible or bath. The metal has to be introduced into the vessel via a small door, through which a kind of funnel is inserted to guide the liquid metal from a refilling ladle through the door opening and into the pressure vessel. The fall into the funnel, the turbulent flow through the funnel and the final fall into the residual melt all re-introduce air and oxides to the liquid metal, the very contaminants that the process seeks to avoid.  
           [0006]    Additional control problems occur in the filling of the mold because of the large size of the casting unit. First, the large volume of gas above the melt is of course highly compressible, and thus gives rather “soft” or “spongy” control over the rate of filling. Second, the problem is compounded because of the large mass of metal in the furnace, which needs to be accelerated by the application of the gas pressure. The problem is akin to attempting to accelerate (and subsequently decelerate) a battering ram weighing 200 kg or more by pulling on a few weak elastic bands.  
           [0007]    The so-called Cosworth Process was designed to avoid this problem by the provision of melting and holding furnaces for the liquid metal, usually aluminum, which were joined at a common level, so that the metal flowed from one to the other in a tranquil manner. The liquid is finally transferred into the mold cavity by uphill transfer, using an electromagnetic (EM) pump which is permanently immersed in the melt, and which takes its metal from beneath the liquid surface, and moves it up a riser tube into the mold cavity without moving parts.  
           [0008]    The control over the rate of flow of the metal is improved because the working volume in the pump and its delivery pipe is only a few kg. However, the driving force is merely the linkage of lines of magnetic flux, resembling the elastic bands in the mechanical analogy, so that control is not as precise as might first be thought.  
           [0009]    Although there are many advantages to the Cosworth solution, the EM pump is not without its problems:  
           [0010]    (i) It is expensive in capital and running costs. The high maintenance costs mainly arise as a result of the special castable grade of refractory for the submerged sections of the pump. These require regular replacement by a skilled person. In addition, they are subject to occasional catastrophic failure giving the various types of EM pumps a poor reputation for reliability. The disappointing trustworthiness is compounded by their extreme complexity and delicacy.  
           [0011]    (ii) The relatively narrow passageways in the pump are prone to blockage. This can occur gradually by accretion, or suddenly by a single piece of foreign material.  
           [0012]    (iii) Occasional voltage fluctuations cause troublesome overflows when the system is operating with the metal at the standby (bias) level.  
           [0013]    (iv) At low metallostatic heads, the application of full power to the pump to accelerate the metal as quickly as possible sometimes results in a constriction of flow inside the pump as a result of the electrical pinch effect at high current density. If the pinch completely interrupts the channel of liquid metal current arcing will occur, causing damage, and temporarily stalling the flow. The pump has difficulty in recovering from the condition during that particular casting, with the consequence that the casting is filled at too low a speed, and is thus defective.  
           [0014]    A number of attempts have been made to emulate the Cosworth Process using pneumatic dosing devices which are certainly capable of raising the liquid into the mold cavity. However, in general these attempts are impaired by the problem of turbulence during the filling of the pressurizable vessel, and by the large volume of the apparatus, thus suffering the twin problems of large mass to be accelerated and large compressible gas volume to effect this action.  
           [0015]    One of the first inventions to answer these criticisms effectively is described in British Patent 1,171,295 applied for Nov. 25, 1965 by Reynolds and Coldrick. That invention provides a small pressure vessel that is lowered into a source of liquid metal. An opening at its base allows metal to enter. When levels inside and out are practically equalized, the base opening is closed. The small internal gas space above the enclosed liquid metal is now pressurized, forcing the metal up a riser tube and into the mold cavity. After the casting has solidified, the pressure in the pump can be allowed to fall back to atmospheric, allowing the metal to drain back down the riser tube. The base opening can be re-opened to refill the vessel, which is then ready for the next casting. The compact pneumatic pump has been proven to work well in service.  
           [0016]    The only major problem in service when pumping liquid aluminum has been found to be the creation of oxides in the riser tube. These are created each time the melt rises and falls. Thus the riser tube may not only become blocked, but oxides which break free are carried into the casting and impair its quality, possibly resulting in the scrapping of the casting. As mentioned, this is a particular problem with low silicon melts.  
           [0017]    In U.S. Pat. No. 6,103,182, the disclosure of which is incorporated herein by reference, an apparatus for dispensing liquid metal is disclosed in which the metal is held between castings in a dispensing riser tube at a “stand-by” level that is close to, or actually at, the top of the riser tube. This inhibits the formation of oxides in the tube and greatly reduces the presence of oxides in the final castings. While this apparatus solved the oxide problem, it is relatively complex and expensive to produce, calling for multiple chambers and seals to be placed within the apparatus. In addition, a problem occurs in that the relatively limited diameter of the riser tube allows the molten metal held therein to cool much more rapidly than does the molten metal in the pressure vessel.  
           [0018]    Thus, an apparatus is needed for dispensing low silicon containing melts into a mold that inhibits the contamination of the castings with oxides, that is mechanically relatively simple, that keeps the melt in the riser tube hot, and that is easy and inexpensive to operate and produce.  
         SUMMARY OF THE INVENTION  
         [0019]    The invention provides, in a first aspect, an apparatus for dispensing a molten material from a reservoir. The apparatus includes a dispensing chamber arranged to receive the molten material from the reservoir, a pressure variation means whereby the dispensing chamber can be pressurized, a first valve adapted to regulate communication of the dispensing chamber with the reservoir, a riser communicating with the dispensing chamber, and a second valve adapted to regulate communication of the dispensing chamber with the riser.  
           [0020]    In a second aspect, the invention provides an apparatus for continuously dispensing a molten material from a reservoir. The apparatus includes two dispensing chambers arranged to receive the molten material from the reservoir, a first set of valves adapted to regulate communication of each of the dispensing chambers with the reservoir, at least one riser communicating with the two dispensing chambers for dispensing the molten material, and a second set of valves adapted to regulate communication of the riser with the dispensing chambers, such that the molten material can be maintained in the riser at a level above the level of the molten material in the chambers.  
           [0021]    The invention provides, in a third aspect, a method of reducing the inclusion of oxides in a casting of a molten metal, including the steps of  
           [0022]    (i) providing a reservoir of molten metal, a dispensing chamber communicating with the reservoir and a riser communicating with the dispensing chamber;  
           [0023]    (ii) flowing the molten metal from the reservoir into the dispensing chamber;  
           [0024]    (iii) flowing the molten metal from the dispensing chamber into the riser;  
           [0025]    (iv) discharging the molten metal from the riser;  
           [0026]    (v) terminating the step of discharging;  
           [0027]    (vi) holding the molten metal in the riser at a predetermined level above the level in the dispensing chamber; and  
           [0028]    (vii) heating the riser adjacent the predetermined level. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]    The present invention will be described in detail with several preferred embodiments and illustrated, merely by way of example, in the accompanying drawings.  
         [0030]    [0030]FIG. 1 is a cross-sectional view of a prior art apparatus for dispensing molten metal;  
         [0031]    [0031]FIG. 2 is a cross-sectional view of an apparatus according to a first embodiment of the present invention;  
         [0032]    [0032]FIG. 3 is a cross-sectional view of an apparatus according to a second embodiment of the present invention;  
         [0033]    [0033]FIG. 4 is a cross-sectional view of an apparatus according to a third embodiment of the present invention;  
         [0034]    [0034]FIG. 5 is a cross-sectional view of an apparatus according to a fourth embodiment of the present invention;  
         [0035]    [0035]FIG. 6 is an enlarged side elevational view, partially in cross section and broken away, of a first type of valve suitable for use in the present invention; and  
         [0036]    [0036]FIG. 7 is an enlarged side elevational view, partially broken away, of a second type of valve suitable for use in the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0037]    With reference to FIG. 1, a prior art molten metal pump is shown as comprising a dispensing chamber  10  surrounded by and adapted to receive liquid metal or melt from an intermediate chamber  11 . The intermediate chamber  11  is immersed in and adapted to receive liquid metal from a reservoir  12  of liquid metal.  
         [0038]    Molten metal passes from the reservoir  12  to the intermediate chamber  11  and from the intermediate chamber  11  to the dispensing chamber  10  through intermediate chamber valve  13  and dispensing chamber valve  14  respectively. The intermediate chamber valve  13  is closable by means of a stopper-rod  15  operatively associated with a bellows  16 . Similarly, dispensing chamber valve  14  is closable by means of a stopper-rod  17  operatively associated with a bellows  18 . A riser tube  19  extends from the dispensing chamber  10  to a conventional mold (not shown). The riser tube is sealed relative to the chamber by means of a gas-tight seal  20 .  
         [0039]    The pressure in the two chambers is changed as required by the application of a vacuum through a first gas valve  21  and/or the admission of a pressurizing gas through a second gas valve  22 . The pressure is indicated by means of a pressure gauge  23 . A pair of heat shields  24  minimizes heat loss from the two chambers  10  and  11 .  
         [0040]    When the pump is lowered into the reservoir  12  of molten metal, the liquid metal enters both the chambers  10  and  11  as regulated by valves  13  and  14 . The closing of the intermediate chamber valve  13  and the introduction of pressurized gas via the second gas valve  22  pressurizes both chambers, with the result that metal is forced up the riser tube  19  and into a mold to make a casting. The dispensing chamber valve  14  is then closed, sealing and isolating the dispensing chamber  10  so that the molten metal is kept at a level at or near the top of the riser and the intermediate chamber is refilled. The pump is now ready to repeat its cycle once a new mold is placed in position on the casting station.  
         [0041]    The present invention retains all of the advantages of the prior art while being simpler to construct and easier to operate. It also has several additional benefits. With reference to FIG. 2, and in accordance with a first embodiment of the present invention, a molten metal pump is provided comprising a dispensing chamber  100  immersed in and adapted to receive molten material from a reservoir  102  through a first valve  104 . A riser  106  extends from the dispensing chamber  100  to a conventional mold (not shown) and is adapted to receive melt from the dispensing chamber  100  through a second valve or riser valve  108 . A first gas valve  142  allows for the introduction of pressurized gas from a gas reservoir  146  or the application of a vacuum in the dispensing chamber  100  while a second gas valve  144  is a vent that allows the dispensing chamber  100  to equalize to atmospheric pressure. Other conventional valve arrangements are contemplated that accomplish the same objectives.  
         [0042]    In this embodiment, the riser  106  is disposed inside the dispensing chamber  100  and extends through a top surface  112  of the dispensing chamber. The riser  106  can be sealed relative to the dispensing chamber  100  at a point where it passes through the top surface  112  of the dispensing chamber by means of a gas-tight seal  114  (which may be, for example, a heat-insulating, ceramic-fiber-packed gland).  
         [0043]    Preferably, a heater  110  encloses a part of the riser  106  that extends above the top surface  112  of the dispensing chamber  100 . The heater  110  heats the riser  106  and prevents the molten material within the riser from cooling and solidifying as well as discouraging oxide formation. The heater  110  can be any type of heating mechanism capable of maintaining sufficient heat in the riser  106 . For example, the entire pump apparatus can be situated in a furnace (not shown), with the furnace acting as a heater for the riser. Alternately, a conventional gas, electric resistance, inductance or other conventional type of heater can be used.  
         [0044]    A layer of insulation  148  can be disposed around the outside of the heater  110  to improve the heating performance and to conserve energy. This insulation can comprise ceramic fiber or any other type of material known to provide insulating properties.  
         [0045]    A pressure-monitoring device  136  such as a pressure gauge can be connected to the dispensing chamber. This can be used to monitor the pressure in the dispensing chamber  100  as dictated by the application of a vacuum and/or the admission of a pressurizing gas through first gas valve  142 . The pressure reading can be measured and correlated to the height of the molten material in the riser.  
         [0046]    The first valve  104  can be constructed in a variety of ways. For example, with reference to FIG. 6, automatic, or passive, closing can be effected by the use of a ball  116  of a refractory material of density higher than that of the liquid metal, which is located in a countersunk, conical valve seat  118  forming the entrance of the valve  104 . A stopper rod  124  is used to prevent the ball  116  from becoming so far displaced from its conical valve seat  118  that it would not seat correctly subsequently. In a passive sealing system, the stopper rod  124  is fixed in place and acts merely to prevent the ball from lifting so high that it would be in danger of becoming permanently displaced from its conical seating  118 . One drawback of such a passive sealing system is that it hinders the draining of the pump when the pump is lifted from the reservoir.  
         [0047]    With continued reference to FIG. 2, the second valve  108  can be an active sealing system of suitable design such as a hemisphere  120  that engages the base of the riser tube  106  to form a seal. The hemispherical stop valve  120  is supported and actuated with a one or more rods  122  acting together and positioned on either side of the riser  106 . However, both the passive sealing device of FIG. 6, namely the non-return ball valve, and the active sealing system of FIG. 2, namely the hemispherical rod-operated valve described above, are subject to leakage if a piece of debris prevents the proper seating of the ball or hemisphere.  
         [0048]    Therefore, it should be appreciated that a variety of other known valve types can also be used for both the first and second valve  104  and  108 . For example, as depicted in detail in FIG. 7, an active closing mechanism could be used in which a valve  164  is closed solely by means of a movable stopper rod  174 . An end  182  of the stopper rod  174  may be hemispherically shaped to provide a better fit in a conical valve seat  168 . In this embodiment, the stopper rod is vertically movable such that it can be raised and lowered to alternately seal and unseal against the conical valve seat  168  of a chamber  150 .  
         [0049]    With continued reference to FIG. 2, operatively associated with a movable stopper rod is a conventional manipulation and sealing assembly  128 . In an active sealing mechanism as described above, this assembly can take various forms but must be able to permit vertical movement of the rod as well provide a gas-tight seal relative to the dispensing chamber  100 . Preferably, the assembly  128  also allows rotation of the stopper rod  174  about its longitudinal axis. The closure force can be adjusted to reduce the incidence of leaks, such as employing a partial rotation of the rod after closing to assist the effectiveness of the seal.  
         [0050]    The active closing valve of FIG. 7 contrasts with the hemispherical stop valve  120  depicted in FIG. 2, which suffers from being a rather loose engineering structure that cannot transfer an effective twisting action, since any attempt to do so simply causes one or more rods used to move it to wind around the riser tube. The further advantage of the active sealing mechanism over the passive sealing valve shown in FIG. 6 is that the active seal allows the pump to be drained quickly if necessary.  
         [0051]    For apparatus suitable for dispensing liquid aluminum and aluminum-based alloys, the dispensing chamber  100 , valves  104 ,  108  and riser  106  can all be bought at modest cost from existing suppliers of crucibles, thermocouples and tubes, in commonly available materials such as clay/graphite, clay/SiC, or clay/fused silica refractories. Additional suitable materials include silicon carbide-based or silicon nitride-based materials or related ceramics such as sialon, and particularly fused silica-based refractories that have been converted to a mixture of corundum and aluminum. Some of these materials are designed to be especially damage-tolerant at temperature, becoming tough as their glassy phase bond partially softens. At operating temperature, such materials are designed to deform, rather than to fail in a brittle manner.  
         [0052]    For apparatus suitable for dispensing liquid magnesium and magnesium-based alloys, the dispensing chamber  100 , valves  104 ,  108  and riser  106  can all be fabricated from iron, mild steel or ferritic stainless steel. Thus, the material and the fabrication costs are relatively low and the material is resistant to brittle failure at temperature, so that the device itself is robust. The pressurizing gas can be dry air or dry carbon dioxide, both inexpensive gases, but rendered inert by the admixture of up to about 5 percent by volume of sulfur hexafluoride (or other more environmentally benign gas).  
         [0053]    For dispensing higher-temperature liquid metals, the materials of the apparatus will become progressively more expensive. Such materials as SiC, SiN and SiAlONs (ceramics based on silicon/aluminum oxy-nitride) and possibly various oxide based ceramics may become necessary. A substantially inert pressurizing gas such as argon will also be required for such service.  
         [0054]    The operation of the pump of FIG. 2 will now be described. When the dispensing chamber  100  is lowered into the reservoir  102  of molten metal, liquid metal enters both the dispensing chamber  100  and the riser  106  via open valves  104 ,  108 . The metal level in both the dispensing chamber  100  and the riser  106  is equalized by allowing the gas in the chambers to vent to atmosphere via the second gas valve  144  and the riser tube  106 .  
         [0055]    The closing of valve  104  and the introduction of pressurized gas via the first gas valve  142  pressurizes the dispensing chamber  100 , with the result that metal is forced up the riser tube  106  and into a mold (not shown) to make a casting. The valve  108  is then closed, sealing and isolating the riser  106  so that the molten metal is kept at a level at or near the top of the riser. Vent  144  and valve  104  are then opened to allow the depressurization of the dispensing chamber  100  and its refilling. The pressurized gas can be collected and reused to conserve the amount of gas needed for the process. The refilling phase can, of course, be speeded up by closing second gas valve  144 , and applying a modest partial vacuum via the first gas valve  142 . In this way the cycle time of the pump can be greatly increased. In addition, the technique of using the vacuum to aid the filling of the dispensing chamber  100  can be useful if the general liquid level in the reservoir  102  is low, allowing the dispensing chamber  100  to fill to a predetermined level that is higher than the level of the material in the reservoir  102 .  
         [0056]    When the dispensing chamber  100  is refilled, valve  104  can be closed. The pump is now ready to repeat its cycle once a new mold is placed in position on the casting station. The pressure in the dispensing chamber  100  is subsequently raised to that in the riser  106  and the valve  108  can then be opened. Continuing transfer of pressurized gas into the dispensing chamber  100  will then displace liquid metal, forcing it up and out of the riser  106 . By continuing this process, a continuous cycle of refilling the dispensing chamber  100  and dispensing material from the riser  106  is performed, with material always remaining at a stand-by level in the riser at or near its top.  
         [0057]    With reference now to FIG. 3, a second preferred embodiment is shown in which a molten metal pump is provided comprising a dispensing chamber  200  immersed in and adapted to receive molten material from a reservoir  202  through a first valve  204 . A riser  206  extends from the dispensing chamber  200  to a conventional mold (not shown) and is adapted to receive melt from the dispensing chamber  200  through a second valve or riser valve  208 . A heater  210  is positioned around the portion of the riser  206  that extends out of the dispensing chamber  200 . A first gas valve  242  allows for the introduction of pressurized gas or the application of a vacuum to the dispensing chamber  200  while a second gas valve  244  is a vent that allows the dispensing chamber  200  to equalize to atmospheric pressure.  
         [0058]    In this embodiment, the first valve  204  and the second valve  208  are both of the type depicted in FIG. 6 or  7  and described above. Preferably, both of the valves  204 ,  208  are active closing valves as depicted in FIG. 7 without the use of a ball  116 . In this regard, the riser  206  is provided with an upwardly facing conical seating for the riser valve  208  such that a second stopper rod  226  extends down from the top of the dispensing chamber  200  and sits evenly on the riser opening. When the second valve  208  is an active closing valve, an end  234  of the second stopper rod  226  is rounded to provide a seal. As noted, this type of valve arrangement allows for a better seal around the riser tube  206  opening than the arrangement depicted in FIG. 2. The operation of the embodiment of FIG. 3 is identical to the embodiment of FIG. 2.  
         [0059]    In a third preferred embodiment, and with reference to FIG. 4, a riser  306  is located external to a dispensing chamber  300  located in a reservoir  302  of melt. Preferably, the riser  306  is J-shaped and is attached to a bottom surface  340  of the dispensing chamber  300 . This embodiment maintains all the advantageous features of the previous embodiments. In addition, it provides the added benefit of eliminating the necessity of a gas-tight seal between the riser  306  and the top surface  312  of the dispensing chamber, as required in the first described embodiment depicted in FIG. 2. This is a difficult feature to manufacture, since it needs to hold the riser tube firmly without fracturing it, while also needing to be gas-tight and insulate the heat of the riser from the top surface of the dispensing chamber. Sealing the connection point of the riser  306  on the bottom surface  340  of the dispensing chamber is more easily done. This is because such a seal does not need to be made gas-tight, but only must present a seal against the leakage of liquid metal, which has a viscosity approximately two orders of magnitude greater than a typical pressurizing gas.  
         [0060]    In addition, the placing of the riser  306  externally, some distance from the dispensing chamber  300  allows more room for a riser heater  310  as well as easily allowing positioning of a casting station (not shown) that does not obstruct access to the top surface  312 .  
         [0061]    As noted above, the heater  310  is positioned around the riser  306 . The heater  310  will extend along a height of the riser  306  necessary to prevent the melt within the riser from cooling to a point where it becomes difficult to dispense. Thus, in this embodiment, the heater  310  may extend from some point above the level of the reservoir  302  to a point just below the top of the riser  306 . An insulating layer  348  can surround the riser  306  radially outward of the heater  310 . Gas valving  342 ,  344  and melt valving  304  and  308  is also provided. The operation of this embodiment is similar to that described for FIG. 2.  
         [0062]    In a fourth embodiment illustrated in FIG. 5, at least a first and a second dispensing chamber  400 ,  450  are connected to the same riser  406 . Components of the second dispensing chamber  450  are identical with corresponding structures within the first dispensing chamber  400 . Thus, only the first chamber will be discussed in detail herein, it being understood that the second dispensing chamber  450  has the identical structure. With this setup, melt can be supplied continuously through a riser  406 . The two (or more) pumps are coordinated so that one is a half (or an appropriate fraction) of a cycle behind the other. In this way, one pump will be dispensing the melt through the riser  406  while the other pump is refilling the dispensing chamber  400 ,  450  thus ensuring a continuous flow of melt from the riser. Alternately, the two pumps can be synchronized such that both pumps will dispense melt from the respective dispensing chambers  400 ,  450  through the riser  406  at the same time. In this arrangement, the amount of melt dispensed by the riser  406  during each cycle of operation will be twice that which would be dispensed if only one pump were connected to the riser. In either case, a larger mold can be filled more quickly.  
         [0063]    The operation of the pump of FIG. 5 will now be described. When the dispensing chambers  400  and  450  are lowered into a reservoir  402  of molten metal, liquid metal enters both the dispensing chambers  400 ,  450  and the riser  406  via open valves  404 ,  408 ,  454 ,  458 . The metal level in both the dispensing chambers  400 ,  450  and the riser  406  is equalized by allowing the gas in the chambers to vent to atmosphere via gas valves  444 ,  494  and the riser tube.  
         [0064]    The closing of valves  404 ,  454  and the introduction of pressurized gas via gas valves  442 ,  492  pressurizes the dispensing chambers  400 ,  450 , with the result that metal is forced up the riser tube  406  and into a mold (not shown) to make a casting. The valves  408 ,  458  are then closed, sealing and isolating the riser  406  so that the molten metal is kept at a level at or near the top of the riser. Vents  444 ,  494  and valves  404 ,  454  are then opened to allow the depressurization of the dispensing chambers  400 ,  450  and their refilling. The pressurized gas can be collected and reused to conserve the amount of gas needed for the process. The refilling phase can, of course, be speeded up by closing vents  444 ,  494 , and applying a modest partial vacuum via valves  442 ,  492 . In this way the cycle time of the pump can be greatly increased. In addition, the technique of using the vacuum to aid the filling of the dispensing chambers  400 ,  450  can be useful if the general melt level in the reservoir  402  is low, allowing the dispensing chambers  400 ,  450  to fill to a predetermined level that is higher than the level of the material in the reservoir.  
         [0065]    When the dispensing chambers  400 ,  450  are refilled, valves  404 ,  454  can be closed. The pump is now ready to repeat its cycle once a new mold is placed in position on the casting station. The pressure in the dispensing chambers  400 ,  450  is subsequently raised to that in the riser  406  and the valves  408 ,  458  can then be opened. Continuing transfer of pressurized gas into the dispensing chambers  400 ,  450  will then displace liquid metal, forcing it up and out of the riser  406 . By continuing this process, a faster rate of refilling the dispensing chambers  400 ,  450  and dispensing material from the riser  406  can be performed. To operate the pumps continuously, the pumps could be working in sequence while allowing material to always remain at a stand-by level in the riser at or near its top.  
         [0066]    The pump as described in the previous embodiments is compact in size and mechanically relatively simple, thus entailing a low capital outlay. In addition, by pressurizing only a relatively small dispensing chamber rather than an entire reservoir, there is a reduced demand for gas, allowing inert gas to be used economically. This enhances casting quality while extending pump life and allows for more precise control over flow and pressure.  
         [0067]    In addition, the present invention is simpler and less expensive to produce than the two-chamber pump disclosed in U.S. Pat. No. 6,103,182. Also, the operation of the pump is quicker in that only a single chamber needs to be filled. In contrast, the material in the previous pump needed to pass through an additional valve and fill a second chamber. The pump according to the present invention is more versatile in that the riser can be made external to the dispensing chamber. Not only does this reduce the chance of leakage by eliminating the gas seal around the top of the riser tube, it also allows greater room for the heater and insulation around the top of the riser and allows access to the top of the dispensing chamber. Finally, the present invention allows the possibility of connecting two or more pumps to a common riser, thus increasing the amount of metal that can be dispensed per unit time from a single riser.  
         [0068]    When compared to an EM pump, the maintenance of the melt at the stand-by level is much safer and more reliable. When using an EM pump, the molten material can be maintained at a high level even during the re-charging of the furnace, but only so long as there is no loss of electrical power. The maintenance of the material at a high level in the riser depends on an active power system. In addition, the provision of electrical power to drive the pump in this “stalled” mode creates significant stirring of the liquid metal in the internal volume of the pump. In addition, there is also the possibility with EM pumps of software faults or main voltage fluctuations, which can cause the melt to overflow unpredictably from the casting station and pose a serious threat to the safety of operating personnel. Also, with an EM pump, oxides can accumulate at the top of the riser tube when the pump is used this way for long periods. It is thought that these oxides are created by air entrainment through the permeable ceramic, or through the joints between the ceramic components of the pump, due to the recirculating action of the liquid.  
         [0069]    The present invention, on the other hand, is unique in that the molten material can be held at the top of the riser indefinitely in all circumstances such as the recharging of the furnace with additional metal, even when all services to the pump (electricity, gas, compressed air) are cut off. In addition, because the mechanism holding the material in the riser requires no power, the melt sits passively with no deleterious stirring induced in the pump.  
         [0070]    The present invention combines the advantages of the EM pump with the simplicity of a pneumatic delivery system, without the disadvantages of either, thereby providing a compact pneumatic pump which has the capability to retain the melt at a high level, just below the top of the riser tube, at all times during the sequential production of castings, thus minimizing the creation of oxides.  
         [0071]    Such apparatus may be used in dispensing molten metal, for example aluminum-based or magnesium-based alloys, into molds for manufacturing castings. The apparatus finds particular usefulness in dispensing molten aluminum alloys designed for wrought applications that have either no silicon or have only low levels of silicon, which are particularly prone to oxide formation.  
         [0072]    The invention has been described with reference to various preferred embodiments. Modifications and alterations will occur to others upon a reading and understanding of the specification. The invention is intended to include all such modifications and alterations insofar as they come within the scope of the appended claims and the equivalents thereof.