Abstract:
Apparatus for the gravity-cast, “lost-foam” casting of metal castings, including a fugitive, pyrolizable foam pattern forming a casting cavity in a bed of loose sand, and a hollow sprue for supplying melt to the casting cavity, wherein the sprue consists essentially of the same metal as is being cast. A high-temperature, porous vent is provided adjacent the discharge end of the metal sprue to expel air from the sprue that would otherwise be trapped therein.

Description:
TECHNICAL FIELD 
     This invention relates to apparatus for the gravity-cast, bottom-filled, “lost-foam” casting of metal, and more particularly to readily recyclable sprues therefor. 
     BACKGROUND OF THE INVENTION 
     The so-called “lost-foam” casting process is a well known method for producing metal castings wherein a fugitive, pyrolizable, polymeric, foam pattern is covered with a thin, gas-permeable, ceramic coating, and embedded in an unbonded sand mold to form a mold cavity within the sand. Molten metal (e.g. iron or aluminum inter alia) is then introduced into the mold to pyrolize, and displace the pattern with molten metal. Gaseous and liquid pyrolysis products escape through the gas-permeable, ceramic coating into the interstices between the unbonded sand particles. Typical fugitive polymeric foam patterns comprise expanded polystyrene foam (EPS), polymethylmethacrylate (PMMA), and certain copolymers. The molten metal may be either gravity-cast (i.e. melt is poured from an overhead ladle or furnace), or countergravity-cast (i.e. melt is forced, e.g. by vacuum or low pressure, upwardly into the mold from an underlying vessel). 
     In gravity-cast lost-foam processes, the hydraulic head of the melt is the driving force for filling the mold cavity with melt. Gravity-cast lost-foam processes are known that (1) top-fill the mold cavity by pouring the melt into a basin overlying the pattern so that the melt enters the mold cavity through a gating system comprising one or more gates located above the pattern, or (2) bottom-fill the mold cavity by pouring the melt into a vertical sprue that lies adjacent the pattern and extends from above the mold cavity to the bottom of the mold cavity for filling the mold cavity from beneath through a gating system having one or more gates located beneath the pattern. Heretofore, the sprues have been formed (1) from porous-ceramic-coated fugitive foams like that used for the patterns, or (2) from porous ceramic shells like those described in copending US Patent application U.S. Ser. No. 10/132,878 filed Apr. 25, 2002, and assigned to the assignee of the present invention. After cooling, the metal left in the sprue (hereafter “sprue-metal”) and gating system are cut from the casting and recycled. In either case, the sprue-metal is covered with a layer of ceramic that must be removed from the sprue-metal before the sprue-metal can be remelted and reused. 
     SUMMARY OF THE INVENTION 
     The present invention eliminates the need to have to remove a ceramic layer from the surface of gravity-cast, lost-foam sprue-metal before reusing the sprue-metal. The present invention involves apparatus for the gravity, bottom-fill, lost-foam, casting of molten metal into a desired shape which apparatus comprises: (1) a bed of loose sand forming a mold having a molding cavity therein that conforms to the shape of the casting; (2) a flask containing the bed of sand; (3) a pyrolizable, fugitive, polymeric pattern embedded in the sand and shaping the molding cavity; (4) a fugitive body attached to the pattern and forming a gating system in the sand for supplying molten metal to the molding cavity, (5) an inlet to the gating system for admitting molten metal into the gating system; and (6) a hollow sprue embedded in the sand for supplying molten metal to the inlet. In accordance with the present invention, sprue consists essentially of the metal being cast so that the sprue-metal can be recycled at the end of pouring without first having to remove a ceramic outer layer therefrom. Preferably, the metal being cast and the metal that comprises the sprue will contain the same alloyants in about the same concentrations. Most preferably, the metal being cast and the metal that comprises the sprue will contain the same alloyants, but in sufficiently different concentrations that the metal that comprises the sprue has a higher melting point than the pouring temperature of the metal being cast to retard melting of the sprue during pouring of the molten metal. 
     According to a most preferred embodiment, the sprue includes a vent adjacent its outlet end for venting air that would otherwise be trapped in the sprue during pouring of the molten metal. The vent will preferably comprise a high temperature, porous material (e.g. ceramic or metal). The vent material may take the form of one or more porous plugs in an aperture(s) through the sidewall of the sprue, one or more porous patches covering such aperture(s), a porous sleeve surrounding/covering such aperture(s), or a porous sleeve-coupling/collar that joins the outlet/discharge end of the metal sprue to the inlet to the gating system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood when considered in the light of the following detailed description of certain specific embodiments thereof which is provide hereafter in conjunction with the several figures in which: 
         FIG. 1  is a partially-sectioned, side view of a sand-filled, lost-foam casting flask having a pattern, and prior art sprue therefor, embedded therein; 
         FIG. 2  is a view in the direction  2 — 2  of  FIG. 1 ; 
         FIG. 3  is a sectioned, side view of a sand-filled, lost-foam casting flask having another prior art pattern and sprue arrangement embedded therein; 
         FIG. 4  is a partially-sectioned, side view of a sand-filled, lost-foam casting flask according to one embodiment of the present invention; and 
         FIG. 5  is a partially-sectioned, side view of a sand-filled, lost-foam casting flask according to another embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE INVENTION 
       FIG. 1  depicts a known, lost-foam mold  2  comprising a metal flask  4  filled with loose sand  6  packed around a fugitive, EPS foam pattern  8  that forms a mold cavity  10  in the sand  6 . The pattern  8  conforms to the shape of the article being cast, and is coated with a thin, gas-permeable, ceramic layer as is well known in the art. The mold cavity  10  receives and shapes molten metal supplied thereto into a casting, here shown to be the head of an internal combustion engine. While a single head could be cast in a single pouring of melt, in actual commercial practice, several heads are formed at the same time in a single pouring. In this regard and as shown in  FIG. 2 , it is common practice to attach two or more discrete patterns  8 ,  10  to a piece of fugitive foam  12  that forms a gating system  14  in the sand  6  that is common to the molding cavities  16 ,  18  formed by the patterns  10 ,  8  respectively. The gating system  14  simultaneously dispenses melt to the adjacent mold cavities  16 ,  18  as the melt progressively rises in the gating system  14  and spills over into each of the mold cavities  16 ,  18  via a plurality of gates A-L. A fugitive foam crown  20  atop the gating system  14  forms a riser in the sand  6  that receives additional melt and supplies it back to the gating system  14  to make up for shrinkage during cooling/solidification of the melt. If only one article is cast per pour, a simpler gating system may be employed, e.g. one or more gate(s) that admit(s) melt directly into the mold cavity. 
     Referring again to  FIG. 1 , molten metal is supplied to the gating system  14  from a hollow sprue  22  which is made from the same pyrolizable foam as the pattern  8 , and is likewise coated with a thin gas-permeable ceramic layer  25  like that coating the pattern  8 . The sprue  22  has: (1) a mouth  24  at one end for receiving molten metal, (2) a hollow portion  26  extending from the mouth  24  to a level below the pattern  8 , and (3) a discharge/outlet end  30  for discharging the molten metal from the sprue. The discharge end  30  engages a piece of solid foam  28  that extends from the discharge end  30  of the sprue  22  to the inlet  32  to the gating system, and has a thin porous ceramic coating  29  thereon. The hollow portion  26  comprises a fugitive foam wall  34  defining an internal flow channel  36 . A metal fill cup  38 , positioned in the mouth  24  of the sprue  22 , receives melt from an overhead ladle or furnace (not shown), and directs it into the flow channel  36 . It is also known to use a similar sprue arrangement, but wherein the hollow portion  26  is replaced with solid foam. After pouring and solidification, the metal left in the sprue (i.e. the “sprue-metal”) and in the gating system is cut away from the casting, cleaned to remove the ceramic coating left by the sprue, and recycled back to the furnace where it is remelted and reused. 
       FIG. 3  depicts another known lost-foam mold and sprue arrangement. A lost-foam mold  40  comprises a metal flask  42  filled with loose sand  44  packed around a fugitive, EPS foam pattern  46  that forms a mold cavity  48  in the sand  44 . The pattern  46  is coated with a thin, gas-permeable ceramic layer  50 . The mold cavity  48  is filled from the bottom by means of a horizontal runner  52  that connects the bottom of the mold cavity  48  with the outlet  54  of a hollow sprue  56 . The runner  52  is formed in the sand  44  by a slab  58  of ceramic-coated pyrolizable EPS foam. The hollow sprue  56  sits atop the slab  58 , and comprises a porous, gas-permeable, non-pyrolizable, ceramic shell made, for example, from ceramic fibers commercially available to the lost-foam foundry industry under the trade name ™ PYROTEK CF 300. After pouring and solidification, the metal in the sprue  56  and runner  52  is cut away from the casting, cleaned to remove the ceramic shell and coating thereon, and recycled back to a furnace where it is remelted and reused. 
       FIG. 4  depicts one embodiment of the present invention and is similar to the structure shown in  FIG. 1  except for the composition of the sprue  60 . In this embodiment, a hollow, sprue  60  consists essentially of the same metal as is being cast (e.g. aluminum). A mouth  66  at the upper end of the sprue  60  holds a pouring cup  68  for receiving molten metal from an overhead ladle or furnace (not shown). An internal flow channel  70  directs the molten metal to beneath the pattern  62 , and thence into the gating system that feeds the molten metal into the molding cavity  64  formed by the foam pattern  62  via channel  61  formed in the sand underlying the pattern by the fugitive foam  76 . One or more high temperature, porous plugs  74  fill aperture(s)  75  through the metal wall of the sprue  60  adjacent the discharge end  72  thereof where it meets the solid foam  76 . Alternatively, high temperature, porous patches (not shown) are affixed (e.g. glued) over the apertures  75  in lieu of use of the plugs  74 . In still another variation, a porous sleeve (not shown) may surround the discharge end of the sprue so as to cover the aperture(s). The high temperature porous plug(s)/patches/sleeve may comprise any of a variety of materials that serve to vent air from the sprue that would otherwise be trapped in the sprue  60  during the pouring of the molten metal. By “high-temperature” material is meant a material that will resist melting by, the molten metal being poured until after the air has been expelled from the sprue. Hence, the plug(s) may comprise porous metals, glass or ceramics in such forms as sintered products, screens, fibrous batts, inter alia. 
       FIG. 5  depicts another embodiment of the invention wherein the sprue  78  is generally J-shaped, is foam-free, has a first vertical leg  80  for receiving molten metal from an overhead ladle or furnace, and a second vertical leg  82 , shorter than the first leg  80 , for directing the flow of molten metal upwardly into the inlet  84  to the gating system which is formed by the fugitive foam projection  86 . The second, shorter vertical leg  82  insures that the melt approaches the EPS projection  86  from beneath so as to prevent the pyrolysis gases from flowing into the first vertical leg  80  (see U.S. Ser. No. 10/132,878 supra). The first and second vertical legs are joined by a transition/connector section  88  that is preferably curved at both ends  90  and  92  to provide a smooth, non-turbulent flow in the sprue. The cross-sectional area of the flow channel  94  in the second vertical leg  82  is greater than the cross sectional area of the flow channel  96  in the transition/connector section  88  so as to slow the rate at which the melt front advances upwardly in the second vertical leg  82 . A foam crown  98  forms a riser in the sand above the pattern for back-feeding melt into the gating system as the casting cools/solidifies. The outlet end  100  of the sprue  78  is coupled to the projection  86  by means of a porous sleeve-coupling, or collar  102  that serves to vent air from the sprue  78  that would otherwise be trapped in the sprue  78  when molten metal is poured into the sprue. Like the plug(s)/patches of the embodiment shown in  FIG. 4 , the porous venting collar  102  comprises a high-temperature porous material. 
     Sprues made in accordance with the present invention will consist essentially of the same metal as is being cast. Hence if aluminum is the metal being cast, the sprue will also be made from aluminum. Preferably, the sprue will comprise the same aluminum alloy as is being cast and will have a wall thickness of about 0.15 mm to about 0.35 mm to insure that the sprue does not melt before pouring is complete. Alternatively, the sprue alloy may comprise the same alloyants as the metal being cast, but in different concentrations adjusted to provide the sprue with a higher melting point than the pouring temperature of the metal being cast which allows the sprue to have thinner walls than would be possible with a lower melting alloy. Other alloyants may be present in the metal that comprises the sprue so long as, after recycling, the presence of such other alloyants will not degrade the properties of the metal being cast. When the composition of the sprue alloy does not exactly match the composition of the casting alloy and the sprue alloy is recycled back to the furnace providing the casting alloy, the composition of the casting alloy in the furnace will periodically be adjusted to keep it within the specifications of required for the casting alloy. 
     While the invention has been described in terms of certain specific embodiments thereof it is not intended to be limited thereto, but rather only to the extent set forth hereafter in the claims which follow.