Abstract:
Apparatus for the gravity-cast, bottom-fill, “lost foam” casting of metal castings, including a fugitive, pyrolizable pattern (for forming a casting cavity), and a hollow sprue (for conducting melt to the casting cavity) embedded in a bed of loose sand. The sprue is free from pyrolyzable foam and conducts melt from above the pattern to a gating system supplying melt to the pattern. The sprue is constructed so as to cause the melt to approach the gating system from beneath and keep any pyrolysis products from entering the sprue.

Description:
TECHNICAL FIELD 
     This invention relates to apparatus for the gravity-cast, bottom-filled, “lost foam” casting of metal, and more particularly to sprues therefor that reduce porosity and inclusions in the casting. 
     BACKGROUND OF THE INVENTION 
     The so-called “lost foam” casting process is a well-known technique for producing metal castings wherein a fugitive, pyrolizable, polymeric, foam pattern is covered with a thin, gas-permeable, ceramic coating, and embedded in an unbounded sand mold to form a mold cavity within the sand. Molten metal (e.g., iron or aluminum) is then introduced into the mold cavity to pyrolize the foam pattern, and displace it with molten metal. Gaseous and liquid pyrolysis products escape through the gas-permeable, ceramic coating into the interstices between the unbonded sand particles. The most popular polymeric foam pattern comprises expanded polystyrene foam (EPS) having densities varying from 1.2 to 1.6 pounds per cubic foot. Other pyrolizable, polymeric foams such as polymethylmethacrylate (PMMA), and copolymers are also known. The molten metal may be either gravity cast (i.e., melt is poured from an overhead ladle or furnace) or countergravity cast (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 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 one or more gates located above the pattern, or (2) bottom-fill the mold cavity by pouring the melt into the flow channel of an elongated 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 the bottom through one or more gates located beneath the pattern. After cooling, the metal left in the sprue and the gate(s) are cut from the casting and recycled. FIGS. 1 through 5 (to be discussed hereinafter) depict various known sprue arrangements for bottom filling lost foam molds. Castings produced by these arrangements suffer from (1) undesirable porosity, (2) folds formed by trapped liquid pyrolysis products (hereafter liquid-induced folds) and/or (3) oxide inclusions in the finished casting resulting from the presence of pyrolizable foam in the sprue&#39;s flow channel. In this regard, when foam in the sprue&#39;s flow channel pyrolizes, gaseous pyrolysis products bubble back up through the melt in the flow channel where they cause considerable turbulence over and above that caused by pouring alone. This extra turbulence causes air, as well as gaseous and liquid pyrolysis products to become entrained in the melt, and carried forward into the mold cavity with the melt, where they cause liquid-induced folds, porosity and oxide inclusions which weaken the casting. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to reduce the formation of pores, liquid-induced folds and oxide inclusions in bottom-filled, gravity cast, lost foam castings by eliminating pyrolizable foam from the flow channel of the sprue that supplies molten metal to the mold. More specifically, the present invention contemplates apparatus for the bottom-fill, gravity, lost-foam casting of a casting which apparatus comprises: a bed of loose sand forming a mold having a molding cavity therein for shaping molten metal into the casting; a flask containing the bed of sand; a fugitive pattern embedded in the sand and shaping the mold cavity, which pattern has the shape of the casting to be cast and comprises a polymeric foam pyrolizable by the molten metal; a fugitive body attached to the pattern and forming a gating system in the sand for supplying molten metal to the mold cavity, which body has an underside and is comprised of a pyrolizable foam; a downwardly-facing inlet to the gating system for admitting molten metal upwardly into the gating system into contact with the underside of the body; a hollow sprue embedded in the sand for conducting molten metal to the inlet, which sprue is free of pyrolizable foam and made from a material that is not pyrolizable by the molten metal; a mouth at one end of the sprue higher than the pattern for admitting molten metal into the sprue; and an upwardly-facing outlet at the other end of the sprue underlying the gating system and engaging the inlet for directing molten metal from the sprue upwardly into the gating system. Preferably, the sprue is made from a porous, gas-permeable ceramic. Most preferably, the porous ceramic sprue is made from ceramic fibers or particles (e.g., alumina, alumina silicate, silicon carbide, fiberglass, bonded sand, bonded glass spheres, bonded hollow ceramic spheres, and ceramic aggregates). 
     According to one embodiment, the sprue is L-shaped having a central flow channel through which the melt flows, a vertical leg that receives gravity-poured molten aluminum from an overhead ladle or furnace, and a horizontal leg extending from the vertical leg to beneath the gating system. The mouth that receives the poured melt is atop the vertical leg and the outlet that engages the inlet to the gate is atop the horizontal leg. 
     Most preferably, the sprue has a J-shaped flow channel having: a first leg that receives molten metal gravity-poured into the sprue and flows it downwardly adjacent the pattern; a second leg, shorter than the first leg, for flowing the molten metal upwardly toward the inlet to the gating system; and a transition section joining the first and second legs for changing the direction of flow of the molten metal between the first leg and the second leg. Preferably, the cross-sectional area of the flow channel transverse the second leg is greater than the cross-sectional area of the transition section between the legs to slow down the rate of advance of the melt front toward the gating system. 
     The present invention prevents any pyrolysis products from becoming entrained in the melt in the sprue, and insures that any pyrolysis products that are formed are pushed into the gating system and/or molding cavity ahead of the advancing melt front. 
    
    
     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 side sectional view of a sand-filled, lost foam casting flask having a pattern, and prior art sprue therefor, embedded therein; 
     FIG. 2 is a plan view of a sand-filled, lost foam casting flask having another prior art pattern, and sprue arrangement embedded therein; 
     FIG. 3 is a sectional view in the direction  3 — 3  of FIG. 2; 
     FIG. 4 is a side sectional view of a sand-filled, lost foam casting flask having still another prior art pattern and sprue arrangement embedded therein; 
     FIG. 5 is view in the direction  5 — 5  of FIG. 4; 
     FIG. 6 is partially broken-away, partially sectioned view of a lost foam casting flask having a pattern and sprue arrangement according to one embodiment of the present invention, suspended therein; 
     FIG. 7 is a view taken in the direction  7 — 7  of FIG. 6; 
     FIG. 8 is view (sans sand/flask) taken in the direction  8 — 8  of FIG. 6; and 
     FIG. 9 is a partially broken-away, partially sectioned view of a lost foam casting flask having a pattern and sprue arrangement according to a preferred 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  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 an article of manufacture (hereafter “casting”), here shown to be a head for an internal combustion engine. While a single head could be cast in a single pouring of melt, in actual commercial practice, two heads are formed at the same time in a single pouring. In this regard, it is common practice to attach two discrete patterns  8  to a gating system (not shown), which is common to both patterns  8 , and which serves to distribute melt to both the mold cavities  10  during pouring. Such a common gating system will be discussed in more detail in conjunction with FIG. 7. A riser  28  atop the gating system receives additional melt and supplies it back to the gating system to make up for shrinkage during cooling/solidification. If only one article is cast per pour, a simpler gating system may be employed, e.g., one or more gate(s) admitting melt directly into the bottom of the mold cavity  10 . 
     Molten metal is supplied to the gating system from a sprue  12  which is made from the same pyrolizable foam as the pattern  8 , and is coated with a thin gas-permeable ceramic layer  13 . The sprue  12  has: (1) a mouth  24  at one end, (2) a hollow portion  14  extending from the mouth  24  to a level below the pattern  8 , and (3) a solid foam portion  16  extending from the lower end  21  of the hollow portion  14  to the inlet  26  to the gating system. The hollow portion  14  comprises a foam wall  18  defining an internal flow channel  20 . A metal fill cup  22  positioned in the mouth  24  of the sprue  12  receives melt from an overhead ladle or furnace (not shown), and directs it into the flow channel  20 . Alternatively, it is known to use a similar sprue arrangement, but wherein the hollow portion  14  is replaced with solid foam. In either case, the heat from the molten metal pyrolizes the foam that makes up both the hollow and solid portions of the sprue  12 . The pyrolysis gases bubble-up through the melt in the sprue causing turbulence in the melt. The turbulence results in air, and some of the pyrolysis liquid and gaseous pyrolysis products, becoming entrained in, and/or reacting with, the melt, which causes liquid-induced folds, pores, and nonmetallic inclusions, to form in, and weaken, the casting. 
     FIGS. 2 and 3 depict another known lost foam mold and sprue arrangement. A lost foam mold  30  comprises a metal flask  32  filled with loose sand  34  packed around a fugitive, EPS foam pattern  36  that forms a mold cavity  38  in the sand  34 . The pattern  36  is coated with a thin ceramic layer  35  as is well known in the art. The pattern  36  is filled from the side, and includes a narrow section of foam  40  that forms a gate  41  in the sand for supplying melt to the mold cavity  38 . The gate-forming, narrow section  40  is attached to an EPS foam pad  42  that forms a chamber  44  in the sand  34  that receives the melt before it is supplied to the mold cavity  38 . A hollow sprue  46  sits atop the pad  42  and comprises a porous, gas-permeable, ceramic fiber shell commercially available to the lost foam foundry industry under the trade name PYROTEK CF 300™. A thin (e.g., {fraction (1/16)} inch), fusible, aluminum (e.g., 356A) wafer  48  separates the bottom end of the sprue  46  from the foam pad  42 . The aluminum wafer  48  reduces the turbulence in the melt poured into the sprue by allowing some of the melt to accumulate in the sprue before the wafer  48  melts. When the aluminum wafer  48  melts, the melt flows into the chamber  44  and thence into the mold cavity  38 . The molten metal pyrolyzes the foam in the pad  42  and the pyrolisis gases bubble-up through the column of melt in the sprue  46  causing turbulence therein which results in air, liquid pyrolysis products and some of the pyrolysis gases becoming entrained in, and reacting with, the melt. Pores, folds and nonmetallic inclusions are thereby formed in and weaken the casting. It is also known to substitute a porous ceramic filter for the aluminum wafer  48  with similar results. 
     FIGS. 4 and 5 depict still another known lost foam mold and sprue arrangement. A lost foam mold  50  comprises a metal flask  52  filled with loose sand  54  packed around a fugitive, EPS foam pattern  56  that forms a mold cavity  58  in the sand  54 . The pattern  56  is coated with a thin gas-permeable ceramic layer  57 . The pattern  56  is filled from the bottom by means of a horizontal runner  60  that connects the bottom of the mold cavity  58  with the outlet  62  of a hollow sprue  64 . The runner  60  is formed in the sand  54  by a slab  66  of pyrolizeable EPS foam. The hollow sprue  64  sits atop the slab  66 , and comprises a porous, gas-permeable, non-pyrolizable, commercially available shell made from ceramic fibers (PYROTEK supra). The molten metal poured into the sprue  64  pyrolyses the foam in the slab  66 , and the pyrolysis gases therefrom bubble-up through the melt in the sprue  64  causing turbulence therein which results in air, liquid pyrolysis products, and some of the pyrolysis gases becoming entrained in, and reacting with, the melt. Pores, folds and nonmetallic inclusions are thereby formed in, and weaken, the casting. 
     FIGS. 6-8 depict one embodiment of the present invention and has a hollow, foam-free, L-shaped sprue  66  made from a material that is not pyrolizable by the molten metal. The sprue  66  has a vertical leg  65 , a horizontal leg  67 , a mouth  68  at the upper end of the vertical leg  65  for receiving molten metal from an overhead ladle or furnace (not shown), and an internal flow channel  70  for directing melt to an upwardly facing outlet  76  in the horizontal leg  67  at the other/exit end of the sprue  66 . As best shown in FIG. 7, horizontal leg  67  of the sprue  66  supplies melt to a fugitive body of foam  78  that is attached to two discrete patterns  72 ,  74  for forming corresponding mold cavities in the sand. The foam body  78  forms a gating system in the sand that simultaneously dispenses melt to the mold cavities formed by the patterns via a plurality of gates  80 - 102  so that two castings are formed in a single pouring. Molten metal is introduced into the bottom of the gating system through a downwardly facing inlet  103  formed in the sand by the foam projection  104  on the underside of the body  78 . The end  105  projection  104  is necked-down and nests within the upwardly facing outlet  76  at the exit end of the sprue  66 . A bead of glue  106  secures the projection to the outlet  76 . Molten metal poured into the mouth  68  of the sprue  66  travels down through the flow channel  70 , and then upwardly out of the outlet  76  where it contacts end  105  of the projection  104 . The heat from the melt pyrolizes the projection  104  leaving the inlet  103  open for melt to pass through into the gating system formed by the fugitive foam body  78 . The melt displaces the foam body  78  and progressively rises in the gating system and spills over into each of the mold cavities formed by the patterns  72 ,  74  via the several gates  80 - 102 . As there is no pyrolizable foam anywhere in the flow channel, no pyrolysis gases can form therein and bubble back up through the channel causing excessive turbulence. Rather, pyrolysis begins when the heat from melt begins to dissociate the projection  104 . However, the pyrolysis liquids and gases resulting from dissociation of the projection  104 , and the body  78 , all move ahead of the melt front that advances upwardly into the gating system formed by the foam body  78 . These gases pass through the permeable ceramic coating that was left by the patterns  72 ,  74  and now holds the sand in place, and into the sand forming the mold cavities. Hence, the pyrolysis gases that form from the foam body  78  do not bubble back into the flow channel  70 . A foam crown  108  atop the body  78  forms a riser in the sand for receiving melt near the end of the pour and feeding it back into the gating system as the melt shrinks during cooling and solidification. 
     FIG. 9 depicts a preferred embodiment of the invention wherein the sprue  110  is generally J-shaped having a first vertical leg  112  for receiving molten metal from an overhead ladle or furnace, and a second vertical leg  114 , shorter than the first leg  112 , for directing the flow of molten metal upwardly into the inlet  116  to the gating system formed by the projection  118  at the bottom of a foam body (not shown) that forms the gating system. The second, shorter vertical leg  14  insures that the melt approaches the EPS projection  118  from beneath such that the pyrolysis gases that form are trapped in the leg  114  between the rising melt front and the foam that has not yet pyrolyzed. These gases can only escape through the walls of the sprue or upwardly into the gating system and thence into the surrounding sand. The first and second vertical legs are joined by a transition member or connector section  120  that is preferably curved at both ends  122  and  124  to provide a smooth, non-turbulent flow transition around the bends between the vertical legs  112 ,  114  and the connector section  120 . Most preferably, the cross-sectional area of the flow channel  126  in the second vertical leg  114  will be greater than the cross sectional area of the flow channel  128  in the transition/connector section  120  so as to slow the rate at which the melt front advances upwardly in the second vertical leg and the gating system. A foam crown  130  forms a riser in the sand for feeding melt into the gating system as the casting cools/solidifies. 
     The non-pyrolizable material that forms the sprue will preferably comprise a thermally insulating ceramic that, most preferably, is also gas-permeable. The sprue may be made from sintered ceramic particles (e.g., silicon carbide, alumina silicate, alumina, SiO 2 , etc. supra), or most preferably, from slip-cast or slurry-cast ceramic fibers that are bonded together and have a porosity of about 30% to about 80%. The sprues may also be injection molded. Gas permeability is desirable as it provides an escape route through the sprue&#39;s walls for gases that might otherwise be trapped in the melt as it flows through the sprue into the mold cavity. Thermally insulating the melt from the sand permits casting articles using lower temperature melts, which results in considerable energy savings and slower pyrolysis rates for less gas entrainment. For example, it has been found that by using porous, foam-free sprues made from ceramic fibers, the pouring temperature of an A356 aluminum alloy can be reduced from 1440° F. to 1325° F. with no loss in properties. 
     Castings made according to the most preferred embodiment of the invention (i.e., the J-shaped sprue) have consistently demonstrated porosities of 0.04% or less 0.04% and pore sizes of 163 μm (max), in contrast to porosities of 0.15% and pore sizes of 296 μm (max) for castings poured using a sprue arrangement like that shown in FIG. 1, but with the hollow foam portion  14  of sprue  12  replaced with a hollow ceramic fiber sprue. 
     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.