Abstract:
A casting machine furnace apparatus that includes a furnace adapted to receive molten metal is described herein. The furnace includes an outer wall structure, a cover adapted to seal the furnace, a source of fluid, and a casting apparatus in fluid communication with the molten metal. The fluid is supplied into the furnace for applying fluid pressure on the molten metal. The application of fluid pressure on the molten metal causes the molten metal to supply the casting apparatus. The outer wall structure of the furnace is provided with a plurality of exhaust ports where the ports are provided in the outer wall structure at predetermined locations. The ports are selectively controllable between a first closed position, where the exhaust ports do not allow air to be exhausted from the furnace, and a second opened position, where the exhaust ports enable air to be exhausted from the furnace.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates in general to a casting machine and in particular to an improved method and apparatus of venting a fluid from a lined pressure furnace of such a casting machine. 
     Pressure pouring of molten metal from a furnace to fill a mold cavity has been used for several decades despite a number of problems. At room temperature, the metal is solid and becomes fluid when melted with sufficient heat. It is known to use a low pressure countergravity casting apparatus to cast molten metal into a mold. One example of such an apparatus is described in U.S. Pat. No. 5,215,141. Basically, in a low pressure countergravity casting apparatus, molten metal is supplied to a machine furnace under pressure. The molten metal is first received into a furnace of the machine furnace. The molten metal in the furnace is then transported to a mold though a feed tube. The machine furnace includes a supply conduit for introducing a gas under pressure into the machine furnace. As the gas is introduced into the machine furnace, the molten metal in the machine furnace is forced through a submerged feed tube, or evacuation conduit, into the mold. The evacuation conduit is commonly referred to as a stalk tube. The mold receives the molten metal through holes in the bottom of the mold. The molten metal in the mold cooling and hardening produces a cast article. A controller is used to adjust the pressure at which the gas is being introduced into the machine furnace. Thus, it can be seen that the machine furnace, the casting apparatus, and the mold are in fluid communication. 
     One potential problem during a casting operation is that the lined pressure furnace can suffer from excess gases flowing up though a bath of molten metal when the furnace is depressurized. In normal pressure casting operations, gases are introduced into the porous lining of the furnace during the pressurization cycle, which often lasts more than sixty seconds. Upon rapid depressurization, gases that have infiltrated the porous lining of the furnace seek the easiest way or path of least resistance out of the lining. Much of the trapped gas finds its way out of the porous lining below the surface of the melt and can contaminate the melt with oxygen as the gas rises to the surface (as shown by the phantom arrows X 3  in  FIG. 3 ). As the gas rises in the melt, the gas tends to effervesce or bubble thereby creating a greater interaction with, and thus contamination of, the melt. The porous lining as well as the riser tubes can also become contaminated with oxide formation. The rising gases can also stir sediment from the bottom of the furnace thereby adding contamination to the melt. When immersion heaters are used in this type of furnace, the gas bubbles that come into contact with the surface of the heaters form insulating oxides on the surface of the heaters, thereby reducing their effectiveness. Bubbles that are small rise slowly and some are in suspension near the riser tubes long enough to be forced into the next casting when the next cycle begins, thereby degrading the quality of the castings. Thus, it would be desirable to provide an improved method and apparatus for a pressure lined furnace of a casting machine which is operative to reduce the contamination of the melt by providing an easier path for the gases to escape from the lining of the furnace. 
     SUMMARY OF THE INVENTION 
     This invention relates to a casting machine furnace apparatus that includes a furnace adapted to receive molten metal. The furnace includes an outer wall structure, a cover adapted to seal the furnace, a source of fluid, and a casting apparatus in fluid communication with the molten metal. The fluid is supplied into the furnace for applying fluid pressure on the molten metal. The application of fluid pressure on the molten metal causes the molten metal to supply the casting apparatus. The outer wall structure of the furnace is provided with a plurality of exhaust ports where the ports are provided in the outer wall structure at predetermined locations. The ports are selectively controllable between a first closed position, where the exhaust ports do not allow air to be exhausted from the furnace, and a second opened position, where the exhaust ports enable air to be exhausted from the furnace. 
     Other advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial sectional elevation view of a portion of a prior art casting machine furnace apparatus. 
         FIG. 2  is a sectional view of a portion of a first embodiment of a casting machine furnace apparatus according to the present invention, the casting machine furnace apparatus being shown during a pressurization cycle with the venting mechanism being shown in a closed position. 
         FIG. 3  is a sectional view of a portion of the first embodiment of the casting machine furnace apparatus according to the present invention, the casting machine furnace apparatus being shown during a depressurization cycle with the venting mechanism being shown in an open position. 
         FIG. 4  is a schematic view of an exhaust system of the casting machine furnace according to the present invention. 
         FIG. 5  is a sectional view of a portion of a second embodiment of a casting machine furnace apparatus according to the present invention, the casting machine furnace apparatus being shown during a depressurization cycle with the venting mechanism being shown in an open position. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIG. 1 , there is illustrated a portion of a prior art casting machine furnace apparatus, indicated generally at  10 . The casting machine furnace apparatus  10  is illustrated as being a low pressure countergravity casting apparatus. Although this invention will be described and illustrated in conjunction with the particular casting machine furnace apparatus  10  disclosed herein, it will be appreciated that the invention may be used in conjunction with any other suitable types of casting machine furnace apparatuses. The general structure and operation of the prior art casting machine furnace apparatus  10  is conventional in the art. Therefore, only those portions of the prior art casting machine furnace apparatus  10  that are necessary for a full understanding of this invention will be explained and illustrated in detail. As shown in  FIG. 1 , the illustrated prior art casting machine furnace apparatus  10  includes a casting machine furnace  12  in fluid communication with a supply furnace  16  which supplies the casting machine furnace  12  with molten metal  15  through a passageway  14 . The passageway  14  may include one or more suitable heating coils  17  proximate thereto, which are operative to generally prevent the molten metal  15  from cooling excessively as it passes through the passageway  14 . The molten metal  15  is supplied to the supply furnace  16  by a holding furnace  20 . 
     The machine furnace  12  preferably supplies the molten metal  15  to a casting apparatus (partially shown at  12 A) thereof through a stalk tube  21  to produce a molded part (not shown). However, the machine furnace  12  can supply the molten metal  15  to any other suitable device or location. An example of a casting apparatus  12 A which can be supplied with the molten metal  15  is disclosed in U.S. Pat. No. 5,215,141 to Kuhn et al., and U.S. Pat. No. 6,627,146 to McKibben et al., the disclosures of which are incorporated herein by reference. Thus, it can be seen that in the illustrated embodiment, the molten metal  15  generally flows in a “downstream” direction from the holding furnace  20  through the supply furnace  16  to the casting machine furnace  12  and to the casting apparatus  12 A. 
     The illustrated casting machine furnace  12  includes a furnace  22  having an outer wall  24 . An intermediate insulation layer  28  covers an inner surface of the outer wall  24 . The insulation layer  28  is preferably made of a material that does not transfer heat well. An inner liner  32  is positioned adjacent an inner surface of the insulation layer  28 . The inner liner  32  is preferably made of a more refractory material that does transfer heat well. Typically, the outer wall  24  is made of steel, the insulation layer  28  is made of an Alumina Silica material, and the inner liner  32  is made of a silicon carbide material. Alternatively, the insulation layer  28  and/or the inner liner  32  can be made from other suitable materials. 
     The casting machine furnace  12  further includes a cover  36  made of a suitable type of material, preferably an insulating type of material. A typical material for the cover  36  is 4140 steel. The casting machine furnace  12  is provided with a fluid inlet  40  to allow a suitable fluid  42  to be selectively added to the casting machine furnace  12 . The fluid inlet  40  can be provided in the cover  36  as shown, or can be provided in the cover  36  at any suitable location or locations. Preferably, the fluid  42  is a gas that does not interfere with the physical or chemical properties of the molten metal  15  in the casting machine furnace apparatus  10 . A suitable fluid  42  that can be used is nitrogen gas or very dry air. In prior art  FIG. 1 , a dotted line A is provided and is used to illustrate the associated levels of the molten metal  15  and the gas  42  in the casting machine furnace  12 . 
     The illustrated casting machine furnace  12  preferably includes one or more heating elements  44  (two of such heating elements  44  being illustrated in prior art  FIG. 1 ). As shown in prior art  FIG. 1 , at least a portion of each of the heating elements  44  preferably extends into the molten metal  15  in the casting machine furnace  12 . It can be appreciated that any suitable heating apparatus can be used with the casting machine furnace  12 , such as a glow bar heater (not shown in  FIG. 1 ). The glow bar heater is preferably covered with a protection tube in order to protect the heater surface from contact with the molten metal  15 . 
     The holding furnace  20  is a suitably shaped vessel designed to hold the molten metal  15 . The illustrated holding furnace  20  includes a pump  48 . The pump  48  is provided to pump the molten metal  15  from the holding furnace  20  to the supply furnace  16 . Any suitable pump  48  can be used for this purpose. One pump  48  that can be used is a Lindberg Varco  100  pump, manufactured by Lindberg/MPH of Riverside, Mich. The pump  48  is operative to move the molten metal  15  from the holding furnace  20  to the supply furnace  16  through a conduit  52 . 
     The illustrated conduit  52  is a generally L-shaped pipe and includes a first generally vertical portion  56  in fluid communication with a second downwardly extending portion  60 . Preferably, the conduit  52  is a ceramic lined discharge elbow and is available from Lindberg/MPH of Riverside, Mich. The downwardly extending portion  60  is operatively joined to a tube  62 . Preferably, the tube  62  is a silicon carbide ceramic tube. Alternatively, the tube can be made from other suitable materials. 
     The conduit  52  includes a fluid inlet  64  provided therein to allow a suitable fluid  68  to be added to the conduit  52 . Preferably, the fluid  68  is a gas that does not interfere with the physical or chemical properties of the molten metal  15 . A suitable fluid that can be used is nitrogen gas. 
     The illustrated supply furnace  16  includes the outer wall  24  covered by the intermediate insulation layer  28 . The insulation layer  28  is covered by and supports the inner liner  32 . The tube  62  extends through the outer wall  24 , the insulation layer  28 , and the inner liner  32  of the supply furnace  16  to allow the molten metal  15  to be supplied from the holding furnace  20  to the supply furnace  16 . In prior art  FIG. 1 , a dotted line B is provided and is used to illustrate the associated levels of the molten metal  15  and the gas  42  in the supply furnace  16 . The illustrated supply furnace  16  further includes a cover  72  made of a suitable type of material, preferably an insulating type of material. In the preferred embodiment, the casting machine furnace  12  and the supply furnace  16  include common components, namely the outer wall  24 , the insulation layer  28 , and the inner liner  32 . Alternatively, the construction of the casting machine furnace  12  and the supply furnace  16  can be other than illustrated if so desired. 
     The inner liner  32  of the supply furnace  16  is operative to define a receptacle  76 . The receptacle  76  includes a first or upper opening  80  and a second or lower opening  84 . The top opening  80  is defined by a side wall  94  of the receptacle  76 . The bottom opening  84  is formed in an end wall  98  of the receptacle  76 . The cover  72  covers the top opening  80 . The supply furnace  16  includes a stopper moving device  86  that selectively allows and prevents molten metal  15  from flowing from the supply furnace  16  to the passageway  14  and the machine furnace  12 . The construction and operation of the casting machine furnace apparatus  10  thus far described is conventional in the art. 
     Referring now to  FIGS. 2 and 3  and using like reference numbers to indicate corresponding parts, there is illustrated a cross-sectional view of a portion of a machine furnace  12  portion of a first embodiment of a casting machine furnace apparatus  10 A according to the present invention. Particularly, a portion of an outer wall structure  25  of the casting machine furnace apparatus  10 A is shown, including the plurality of layers that form the outer wall structure  25  according to the present invention. In this embodiment, the inner liner  32  is preferably made from an insulating refractory material. The inner liner layer  32  is preferably a high-density refractory material that can be cast in place or can be a pre-cast element. An intermediate insulation layer  28  is provided between the inner liner layer  32  and the outer wall  24 . The intermediate insulation layer  28  is also preferably made from a refractory material. However, it is preferred that the intermediate insulation layer  28  is made from a respectively lower density material than the material of the inner liner layer  32 . It is also preferable that the intermediate layer  28  be cast in place. The inner liner  32 , the intermediate insulation layer  28  or both can be made from other suitable materials. For example, the intermediate insulation layers can be made from ceramic or brick, if so desired. It is further preferred that both the inner liner  32  and the intermediate insulation layer  28  are somewhat permeable to the fluid  42 , thereby allowing some of the fluid  42  to pass through the layers  28  and  32 . 
     In the illustrated embodiment, the outer wall structure  25  of the present invention further includes a refractory paper layer  100  and an “opened” metal grid layer  102  positioned between the intermediate insulating layer  28  and the outer wall  24  of the furnace  22 . The refractory paper layer  100  is commercially available under the Trademark “FIBERFRAX” from the Unifrax Corporation of Niagara Falls, N.Y. As with the inner liner refractory layer  32  and the insulating refractory layer  28 , the refractory paper layer  100  is preferably permeable to the fluid  42  that is used to pressurize the machine furnace  12 . More preferably, the refractory paper layer  100  is more highly permeable to the fluid  42  than both the inner liner refractory layer  32  and the insulating refractory layer  28 . Similarly, the metal grid layer  102  will also allow the fluid  42  to pass through it due to the nature of the openness created by its woven or lattice like grid structure. The “openings” of the grid of the grid layer  102  can be any suitable size, the purpose of which will be described below. However, if the grid structure of the grid layer  102  is formed having relatively large openings, it is preferred that an intermediate or secondary grid layer  105  having smaller grid openings be positioned between the refractory paper layer  100  and the metal grid layer  102 , as shown in the lower portion of the embodiment illustrated in  FIGS. 2 and 3 . 
     The relatively smaller grid openings of the intermediate grid layer  105  assist in supporting the paper layer  100  to prevent the paper layer  100  from being pressed or forced through the relatively larger openings of the metal grid layer  102 . Using the refractory paper layer  100  and the metal grid layer  102  between the insulating refractory layer  28  and the outer wall  24  allows the fluid  42  to pass more easily through the inner liner refractory layer  32  and the insulating refractory layer  28  so as to be vented from the casting machine furnace apparatus  10 A according to the present invention as will be discussed below. This is because the paper layer  100  and metal grid layer  102  are more permeable than the refractory layers  28  and  32 . Thus, as will be discussed below, the fluid  42  will naturally move towards and through the less dense refractory paper layer  100  and the metal grid layer  102 . In the preferred embodiment, the insulating refractory layer  28  and inner liner layer  32  are cast in place. Thus, as described above, the refractory paper layer  100  is used to prevent the material used to form the insulating refractory layer  28  from penetrating through the openings of the metal grid layer  102  during the casting-in-place operation. Therefore, it can be appreciated that if the insulating liner layer is bricked (or similarly formed) as opposed to being cast in place, the refractory paper layer  100  can be omitted since the brick would not typically pass through the openings of the metal grid layer  102 . Thus, a chamber containing the metal grid layer  102  and possibly an air space could be formed between the outer surface of the insulating refractory layer  28  and the inner surface  24 A of the outer wall  24 . Alternatively, the chamber between the insulating refractory layer  28  and the inner surface  24 A of the outer wall  24  could contain no metal grid layer  102 . Although the formation of the chamber has been described with respect to an insulating layer that is not cast in place, it can be appreciated that a air space chamber could also be formed with an embodiment having a cast in place insulating layer. The chamber could be formed using any suitable methods. 
     Formed through the outer wall  24  of the outer wall structure  25  of the furnace  22  is at least one passage or opening  106  (two of such passages  106  are shown in  FIGS. 2 and 3 ). The passage  106  preferably extends from an inner surface  24 A of the outer wall  24  to an outer surface  24 B of the outer wall  24  of the furnace  22 . It is further preferred that an exhaust port  104  is provided on the outer surface  24 B of the outer wall  24  of the furnace  22  at each respective passage  106 . In the illustrated embodiment, the port  104  is formed as a separate part and is secured to the furnace  22  adjacent the passage  106  by suitable means, such as for example welding. Alternatively, the port  104  could be formed integrally with the furnace  22  if so desired. The port  104  has a bore  108  formed therethrough such that the passage  106  is in fluid communication with the bore  108  of the port  104 . The purpose of the exhaust port  104  will be explained in greater detail below. In the illustrated embodiment, the bore  108  of the port  104  is provided with internal threads  108 A. 
     It is preferred that a plurality of such ports  104  are formed on the outer wall  24  of the furnace  22 , the number of the ports  104  corresponding to the number of the passages  106 . It is further preferred that the plurality of the passages  106  and the ports  104  are spaced about the perimeter of the furnace  22  in both predetermined lateral and vertical positions on the outer surface  24 B of the outer wall  24  of the furnace  22  and/or on the cover of the furnace  12 B. Preferably, the passages  106  and the ports  104  are spaced on the selected components of the furnace  12 B in predetermined lateral and vertical positions in both the molten metal area defined below line A and the gas area defined above line A. Each port  104  is also preferably connected via airtight plumbing to a valve or valves, as will be discussed below in connection with  FIG. 4 . The valves can be operated to control the amount of pressure within the furnace  12 B during the casting cycle as well as between cycles, as will be described in greater detail below. In the illustrated embodiment, the furnace  12 B further includes one or more glow bar heaters  107  (only one of such heaters  107  shown in  FIGS. 2 and 3 ). The glow bar heater  107  is preferably covered with a protection tube  107 A in order to protect the heater surface from contact with the molten metal  15 . Alternatively, the structure of the furnace  12 B and/or the structure of the outer wall structure  25  of the furnace  12 B can be other than illustrated if so desired. 
     Referring now to  FIGS. 2 and 3  and also prior art  FIG. 1  for those components not illustrated in  FIGS. 2 and 3 , the operation of the first embodiment of the casting machine furnace apparatus  10 A according to the present invention will be described. In the illustrated embodiment, molten metal  15  is supplied to the casting machine furnace  12 B from the supply furnace  16 . It can be appreciated that although a particular delivery method has been described herein, any suitable method of delivering molten metal into the furnace  12 B can be used. The supply line is preferably insulated to prevent heat loss from the molten metal  15  being supplied by the supply furnace  16  to the casting machine furnace  12 B. The molten metal  15  is preferably maintained at a generally consistent level in the furnace  12 B, as indicated by the dotted line A. There is preferably an enclosed fluid space  42  between the molten metal  15  and the cover  36 . The fluid inlet  40  communicates with the casting machine furnace  12  to supply the fluid  42  to the casting machine furnace  12 . The machine furnace  12  preferably supplies the molten metal  15  to a casting apparatus (partially shown at  12 A in  FIG. 1 ) thereof through a stalk tube  21  to produce a molded part (not shown). However, the machine furnace  12  can supply the molten metal  15  to any other suitable casting device or location. 
     The molten metal  15  is supplied to the casting apparatus  12 A as described herein to produce a cast article (not shown) in the casting apparatus cavity. The cast article is preferably a vehicle component. However, it can be appreciated that the cast article can be any desired article that can be formed using this casting method, such as a vehicle wheel, household goods, vehicle workpieces and the like. It should be understood that the cast article is preferably about the same shape and about the same contour as the casting apparatus cavity. Also, it can be appreciated that it is preferred that the casting apparatus cavity is preferably an airtight cavity, and that the molten metal  15  that enters the casting apparatus  12 A is contained within the casting apparatus cavity. However, the casting apparatus cavity is not required to be airtight. 
     To supply the molten metal  15  from the casting machine furnace  12 B into the casting apparatus  12 A, a controlled amount of the fluid  42  is supplied through the fluid inlet  40 , which in turn causes the molten metal  15  to move upwardly through the stalk tube  21 , and into the casting apparatus  12 A. The fluid  42  is preferably supplied under pressure, thereby causing the pressure within the furnace  12  to achieve a first pressurization level, P 1 . It should be understood that by selectively controlling the amount of pressure in the furnace  12 , the rate at which molten metal  15  is supplied to the casting apparatus  12 A is selectively controlled. This pressurization sequence is generally conventional in the art and the first pressurization level, P 1  is generally maintained until the desired amount of molten metal  15  is received within the casting apparatus  12 A. Additionally, in this embodiment of the invention, to maintain the first pressurization level, P 1 , the valve or valves connected to the ports  104  are preferably closed during this portion of the casting cycle. 
     Once the desired amount of molten metal  15  is received within the casting apparatus  12 A, the pressure above the melt (i.e., above the dotted line A) is preferably maintained until the casting(s) is solidified. During the period in which the first pressurization, P 1  is maintained, it is possible that the fluid  42  can permeate the porous lining layers  28  and  32  of the machine casting furnace  12 B (for discussion purposes, such fluid  42  which permeates the layers  28  and  32  is indicated by the “crooked” arrows X 1  shown in  FIG. 2 , and the fluid  42  acting upon the metal  15  is shown by straight arrows X). Upon completion of the casting cycle the casting machine furnace  12 B is typically depressurized so that the casting machine furnace  12 B can be replenished with additional molten metal  15  prior to the commencement of another casting cycle. Using conventional methods, upon the release of the fluid pressure above the molten metal, the fluid trapped in the porous lining layers  28  and  32  of the furnace can escape into the molten metal  15 , pass through the melt, and then rise to the surface of the melt. When this process occurs, it can leave contaminants in the molten metal  15  as well as on other components of the furnace, such as the inner surface of the inner liner  32 , on the stalk tube  21  and on the immersion heater  44  protection tubes. However, in the preferred embodiment according to the present invention, the depressurization is controlled by the valves, described above, to control and reduce the amount of fluid  42  that reenters the molten metal  15  to prevent or reduce the contaminants from occurring in the molten metal  15  as well as on the other components of the furnace. 
     As illustrated in  FIG. 2 , the arrows X 1  indicate the permeation of the fluid  42  into the porous refractory layers, (inner liner  32  and insulation layer  28 ) of the outer wall structure  25  of the furnace  12 B, as well as the refractory paper layer  102  and metal grid layer  104  of the outer wall structure of the furnace  12 B. Due to the valves being closed during the first pressurization stage of the casting cycle, there is no path that would allow the fluid  42  to pass out of the layers that form the outer wall structure  25 . Thus, the fluid that has permeated those layers will remain in such layers until the furnace  12 B is depressurized. As discussed above, due to the refractory paper layer  100  and the metal grid layer  102  (and metal grid layer  105  if used), being more permeable than the inner liner refractory layer  32  and the insulating refractory layer  28 , there will be a tendency for the permeated fluid  42  to migrate towards the refractory paper layer  100  and the metal grid layer  102 . This will, in effect, saturate those layers with the fluid  42 . While the pressure P 1  is maintained, the fluid  42  remains in the outer wall structure  25 . 
     Referring now to  FIG. 3 , there is illustrated the casting machine furnace  12  according to the first embodiment of the present invention during an initial depressurization stage. During the initial depressurization stage, one or more of the valves connected to the plurality of ports  104  on the furnace  22  are selectively opened to reduce the pressure (first pressurization level P 1 ) within the furnace  12 B. Selectively opening the valves causes a controlled release of the pressure of the fluid  42  that has accumulated in the refractory paper layer  100  and the metal grid layer  102 , as well as the pressure of the fluid  42  that has accumulated in the liner refractory layer  32  and the insulating refractory layer  28 . The laws of physics dictate that fluid has a tendency to move from an area of relatively higher pressure to one of a relatively lower pressure. Thus, when the valves are opened, the fluid  42  within the outer wall structure  25  will flow through the passage  106  and the port  104 , the plumbing lines connected to the port  104 , and be externally vented or exhausted to the atmosphere. Using this process of the present invention, a substantial portion if not the entire amount of the fluid  42  that is contained within the outer wall structure  25  will be vented from the furnace  12 B rather than being reintroduced into the molten metal  15  (as indicated by the phantom arrows X 3 ). It can be appreciated that the fluid  42  can be exhausted to a storage structure rather than being released to the atmosphere if so desired. The flow of the fluid  42  vented from the furnace  12 B according to the present invention is illustrated for discussion purposes in  FIG. 3  by the crooked arrows X 2 . 
     Illustrated in  FIG. 4  is a schematic diagram of a portion of the casting apparatus  10 A according to the present invention. Particularly, only the furnace portion  12 B is illustrated. Also schematically illustrated in  FIG. 4  is a plurality of exhaust lines  112 ,  116 ,  120  extending, respectively, from the plurality of ports  104 A,  104 B, and  104 C provided on the exterior of the furnace  12 B. As described above, the ports  104 A,  104 B, and  104 C are in fluid communication with the interior of the furnace  12 B via the passages  106 . Also as stated above, the ports  104 A,  104 B, and  104 C can be formed at selected positions around the exterior of the furnace  12 B at selected vertically and horizontally spaced apart locations on each side of the furnace  12 B. 
     As is known in the art, a pressure supply member  200  is connected with at least one fluid inlet port  40  at or near the cover  36  or some other portion of the furnace  12 B to supply the pressurized fluid  42  to the furnace  12 B. In the embodiment illustrated in  FIG. 4 , three of such ports  40  are shown. As is also known, the ports  40  can also act as fluid outlets to vent the fluid  42  from the cover  36 . Each of the ports  40  are connected via a set of exhaust lines  124  to a controlled depressurization exhaust member  126 . In the illustrated embodiment, each of the three ports  40  are also connected via the exhaust lines  124  to an emergency dump member  128 . As described above, using the furnace  12 B described with respect to  FIG. 1 , the pressure supply member  200  will control the supply of fluid  42  into the furnace  42 . Under pressurization, the furnace will supply the casting apparatus  12 A with molten metal. Upon depressurization, with conventional furnace apparatuses, the controlled depressurization exhaust member  126  would allow the fluid to be released to atmosphere. Since the ports  40  are only formed on the cover with conventional furnace apparatuses, the only exit path for the fluid  42  that has permeated the refractory layers  25  will be through the melt, thereby causing contamination thereof, and out the ports  40 . 
     In the preferred embodiment, in addition to the prior art ports  40 , each side of the furnace  12 B has multiple ports formed thereon in the illustrated embodiment. It is preferred that a first set of ports  104 A is formed at or near a lower or bottom portion of the furnace  12 B. It is further preferred that a second set of ports  104 B is formed at or near an upper or top portion of the side of the furnace  12 B. It is further preferred that a third set of ports  104 C is operatively connected to each of the fiber cones  122  of the furnace  12 B. However, it can be appreciated that the ports  104 A and  104 B can be formed at any position on the sides of the furnace  12 B. It can also be appreciated that any suitable number of ports  104 A and  104 B can be positioned on each side. In the illustrated embodiment, the furnace  12 B includes a total of 32 first or lower ports  104 A, a total of 18 second or upper ports  104 B, and a total of 5 third or cone ports  104 C. Each of the first ports  104 A is preferably connected to an exhaust member  114  via a first set of exhaust lines  112 . The exhaust lines  112 , intersecting with a second set of exhaust lines  116 , are used to transport the vented fluid  42  from the furnace  12 B to the atmosphere. The second set of exhaust lines  116  are preferably connected to the second ports  104 B that are formed at an upper portion of the furnace  12 B. The second set of exhaust lines  116  is also used to transport the vented fluid  42  from the furnace  12 B and to the atmosphere. A third set of exhaust lines  120  are preferably connected to the third ports  104 C that are formed at the cones  122  of the furnace  12 B. The third set of exhaust lines  120  is also used to transport the vented fluid  42  from the furnace  12 B and to the atmosphere via an exhaust port  118 . Alternatively, the structure of the ports  104 A,  104 B and  104 C and/or the exhaust lines  112 ,  116 ,  120  and  124  can be other than illustrated if so desired. 
     According to the present invention, the timing of the opening of the valves to release the fluid  42  under pressure in the outer wall structure  25 A of the furnace  12  is selectively and controllably adjustable in anticipation of the release of the fluid pressure above the molten metal  15 , as well as the decay time of the fluid  42  under pressure in the outer wall structure  25 A of the furnace. Thus, the initial depressurization stage will commence to release the fluid pressure from the outer wall structure  25 A. The desired amount of fluid  42  to be released can be controlled as a factor of time, or of pressure within the furnace  12 B, or of the pressure of the fluid  42  within the outer wall structure  25 A (which includes the inner liner layer  32 , insulating layer  28 , refractory paper layer  100  and metal grid layer  102 ). During the initial depressurization stage, the fluid  42  will travel through the first set of exhaust lines  112  to the exhaust member  114 . Once the desired amount of venting of the fluid  42  that has permeated the outer wall structure layers  25 A is achieved, (or a given decay time of the fluid  42  is met), a secondary depressurization stage can commence. 
     The secondary depressurization stage preferably includes the same or additional valves connected to the second ports  104 B on the outer wall  24  of the furnace  22  being opened, valves (not shown) formed on or near the cover  36  being opened, or any other suitable depressurization strategy. During the secondary depressurization stage, in the preferred embodiment, the fluid  42  will travel through the second set of exhaust lines  116  to the exhaust member  114 . An example of another depressurization strategy includes, after the initial depressurization stage, ports  104 B on an upper portion of the furnace  12 B being vented, followed by ports  104 C on the cover  36  being vented. The ports  104 C can be provided in associated fiber cones  120  of the furnace  12 B. The ports  104 C are preferably connected via a third set of exhaust lines  124  to the controlled depressurization exhaust member  126  and the emergency dump exhaust member  128 . Once the initial depressurization stage and any subsequent depressurization stages have been completed, the cover  36  could be removed. Removal of the cover  36  only after some of the fluid  42  has been vented from the furnace  12  will minimize or eliminate the amount of fluid  42  that effervesces into the molten metal  15 . This in turn will reduce the contamination effect the fluid  42  has on the molten metal  15  contained in the furnace  12 B. 
     Pressure casting furnaces typically have one large servo valve and one large control valve thorough which the fluids used to pressurize the furnace  12  flow. When the casting cycle is complete, the servo valve  200  goes to zero pressure and the large control valve  126  goes to exhaust. During an emergency, it may be necessary to rapidly exhaust the furnace. The casting apparatus  12 A according to the present invention includes, but does not require, a second control valve  114  much smaller in size. The pressurization cycle will be similar to the prior art pressurization cycles, except during depressurization. During depressurization, the servo valve  200  will go to zero and the fluid will be exhausted though the small control valve  126  with an adjustable exhaust orifice. By using the smaller valve  114  with an adjustable orifice for exhausting at a controlled rate, the pressure decline above the melt will be better matched to the declining pressure in the outer wall structure  25 . In this embodiment, during an emergency, both valves can go to exhaust. 
     Turning now to  FIG. 5  and using like reference numbers to indicate corresponding parts, there is illustrated a portion of a second embodiment of a casting machine furnace apparatus  10 B having a machine furnace  12 C according to the present invention. In this embodiment, the furnace  12 C includes an outer wall structure  25 A having an inner liner  32 , an outer wall  24 , and an intermediate insulation layer  28 . As shown therein, the insulation layer  28  is provided with a plurality of pockets or openings  28 A formed therein (only two of such pockets  28 A being shown). Thus, in this embodiment, the outer wall structure  25 B does not include the layers  100  and  102 / 105  of the first embodiment shown in  FIGS. 2 and 3 . In operation, the pockets  28 A function similar to the layers  100  and  102 / 105  in  FIGS. 2 and 3  and permit the fluid  42  to vent through the associated passages  106  and ports  104 A and  104 B. 
     One advantage of the present invention is that the furnace includes an outer wall structure that is effective to provide an easier path for the fluid to escape or vent from the furnace to reduce or prevent contamination of the melt. Also, the present invention reduces oxide build-up on the immersion heaters and/or glow bar heaters thereby simplifying cleaning and extending the life span and efficiency of the heaters. Also, the present invention reduces oxide build-up on the inside surfaces of the furnace, i.e., inner liner, thereby simplifying cleaning and extending the life span of the furnace lining. In addition, the present invention reduces the contamination of the melt and oxide formation on both the heaters and the furnace lining from the residual moisture contained in the lining. 
     The principle and mode of operation of this invention have been described in its preferred embodiments. However, it should be noted that this invention may be practiced otherwise than as specifically illustrated and described without departing from its scope.