Abstract:
A disintegrative member, such as a salt core, is provided with a vent opening extending through its thickness from an outer surface to an inner surface in order to allow gases to pass radially inwardly through the body of the salt core and away from a region proximate the salt core&#39;s outer surface. The escape of these gases through the vent opening decreases the likelihood that porosity will be formed as a result of those gases being trapped in a region proximate the outer surface of the salt core because the molten metal solidifies more slowly in this area because of the thermal insulative qualities of the disintegrative member.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is generally related to a disintegrative core for use in die casting and, more particularly, to a particular structure of a salt core which reduces porosity in a cast metallic object in the vicinity of a cavity formed around the disintegrative core. 
     2. Description of the Prior Art 
     Those skilled in the art of die casting are familiar with the use of disintegrative cores, such as salt cores, that are used to define the shape of a cavity within a cast metallic component. 
     U.S. Pat. No. 3,764,575, which issued to Anderko et al. on Oct. 9, 1973, describes a salt core containing synthetic resin and water-glass as binders. In the manufacture of a casting having a cavity, wherein a water soluble salt core is suspended in a mold, molten material is poured into the mold about the core, the molten material is allowed to harden to the desired casting, and the core is washed away with water to leave a hollow casting. 
     U.S. Pat. No. 3,801,334, which issued to Dewey on Apr. 2, 1974, describes salt casting mixtures. Alkaline metal nitrates and nitrates are useful as core material for making disposable cores, mandrels and other forms for use in making hollow plastic articles. 
     U.S. Pat. No. 3,963,818, which issued to Sakoda et al. on Jun. 15, 1976, describes a water soluble core for pressure die casting and process for making the same. The process includes pre-drying a granular water soluble salt having grain size of less than about 1,000 microns so that the moisture content thereof becomes less than 1%, molding under compression the granular water soluble salt into a desired shape and volume at a pressure of between 1.5 to 4 tons per square centimeter, and if necessary, sintering the molded salt at a temperature of between about 100 degrees to 300 degrees centigrade. 
     U.S. Pat. No. 3,964,534, which issued to Rabinowitz on Jun. 22, 1976, describes a casting method with a vacuum bonded dry sand core. A method for preparing sand cored parts is disclosed which is applicable to certain types of molding techniques where the mold cavity is at least slightly porous, such as a shell molding, green sand molding, and most importantly, the cavityless method of molding. 
     U.S. Pat. No. 4,361,181, which issued to Wischnack et al. on Nov. 30, 1982, describes a casting core and process for the production thereof. A casting core is intended for the creation of difficultly accessible cavities in castings of aluminum or of one of its alloys, produced from a water soluble salt as base substance and burnt sugar as a binding agent, and a process for the production of such a casting core wherein the base substance is mixed with burnt sugar in aqueous or organic solution, pressed in molds, and baked at elevated temperature. 
     U.S. Pat. No. 4,446,906, which issued to Ackerman et al. on May 8, 1984, describes a method of making a cast aluminum based engine block. A method of making a die cast aluminum based engine block with a closed deck is disclosed. A core assembly having at least one water soluble alkaline metal salt core member is stationed on a bore die of a die casting assembly for the block. 
     U.S. Pat. No. 4,875,517, which issued to Donahue et al. on Oct. 24, 1989, discloses a method of producing salt cores for use in die casting. A pattern, identically proportional in configuration to the salt core to be produced, is initially formed from an evaporable foam material. The evaporable foam pattern is positioned in a mold and surrounded with an unbonded flowable material, such as sand. The pattern is contacted with a molten salt and the high temperature of the salt will vaporize the pattern, with the vapor being captured within the interstices of the sand while the molten salt will fill the void created by vaporization of the foam to provide a salt core identical in configuration to the pattern. 
     U.S. Pat. No. 5,165,464, which issued to Donahue et al. on Nov. 24, 1992, discloses a method of casting hypereutectic aluminum-silicon alloys using a salt core. A method of high pressure casting of hypereutectic aluminum-silicon alloys using a salt core to form wear resistant articles, such as engine blocks, is described. To produce an engine block, one or more solid salt cores are positioned within a metal mold with the space between the cores and the mold defining a die cavity. 
     U.S. Pat. No. 5,273,098, which issued to Hyndman et al. on Dec. 28, 1993, describes removable cores for metal castings. A method for the manufacture of salt cores is described. 
     U.S. Pat. No. 5,303,761, which issued to Flessner et al. on Apr. 19, 1994, describes a die casting using casting salt cores. A process of providing a disposable core for use in die casting processes is described. A salt material is molten and cast into a core of a desired configuration under exacting conditions. The fluidity of the molten salt is controlled enabling casting the salt material into a core by die casting methods. 
     U.S. Pat. No. 5,632,326, which issued to Gough on May 27, 1997, describes a mold and method for the casting of metals and refractory compositions for use therein. A mold for metal casting contains a bonded refractory composition comprising hollow alumina-containing microspheres in which the alumina content is at least 40% by weight. 
     U.S. Pat. No. 5,803,151, which issued to Carden on Sep. 8, 1998, describes a soluble core method of manufacturing metal cast products. An improved soluble core for die casting metals or metal matrix compositions is formed of a mixture of salt and up to 20% weight of ceramic material blended together to produce a homogenous mixture and compacted under pressure to produce a soluble core having little or no porosity. 
     U.S. Pat. No. 6,458,297, which issued to Moschini on Oct. 1, 2002, describes a method for producing pressure die cast or injection molded articles using salt cores. A method for producing pressure die cast or injection molded articles having a unit for producing salt grains, a press for forming salt filler cores by compressing the salt grains, a pressure die casting machine for making the said articles by injecting a material in the liquid state into a mold having at least one salt filler core, and a salt removal and washing unit designed to remove the salt filler cores that have been trapped within the articles is described. 
     U.S. Pat. No. 6,755,238, which issued to Hirokawa on Jun. 29, 2004, describes a disintegrative core for high pressure casting. It discloses a method for manufacturing a disintegrative core for use in high pressure casting. The disintegrative core can be applied where a light metal such as an aluminum alloy or magnesium alloy is subjected to high pressure casting, such as die casting or squeeze casting and is manufactured from a water soluble salt which is high in latent heat and ranges, in melting point, from 280 to 520 degrees centigrade. The salt, alone or in combination with a fine hard powder, is melted and solidified in a core mold. Alternatively, the melt is produced into a fine powder which is then molded into a core mold. The method can be applied for the manufacture of complex shapes of cores. 
     The patents described above are hereby expressly incorporated by reference in the description of the present invention. 
     SUMMARY OF THE INVENTION 
     A method for producing a cast metallic object having a cavity formed therein, in accordance with a preferred embodiment of the present invention, comprises the steps of providing a disintegrative member having an outer surface portion which is shaped to conform with a desired shape of the cavity. The disintegrative member has an internal surface. The method further comprises the step of providing a vent opening extending between the outer surface of the disintegrative member and the inner surface. 
     A preferred embodiment of the present invention further comprises the step of providing an internal opening within the disintegrative member which is shaped to receive an extension of a die used in a die casting operation. The internal surface can be a surface of the internal opening within the disintegrative member which is shaped to receive the extension of the die. 
     In a preferred embodiment of the present invention, the extension is a mandrel and the internal opening has a hexagonal cross section. The disintegrative member is made of salt and the cast metallic object is an engine block. The cavity is a cylinder in the engine block. The outer surface portion of the disintegrative member is generally cylindrical and the vent opening is a hole extending through the outer surface portion and through the internal surface of the disintegrative member. 
     In a particularly preferred embodiment of the present invention, the method further comprises the steps of attaching the disintegrative member to a die which is shaped to define the cast metallic object, injecting molten metal, such as aluminum, into the die and around the disintegrative member, and dissolving the disintegrative member after the molten metal solidifies to form the cast metallic object and the cast metallic object is removed from the die. 
     The disintegrative member, in a preferred embodiment of the present invention, has an outer surface portion which is shaped to conform with the desired shape of the cavity. It also has the internal surface and a vent opening, or hole, extending between the outer surface and the internal surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be more fully and completely understood from a reading of the description of the preferred embodiment in conjunction with the drawings, in which: 
         FIG. 1  is a simplified representation of a disintegrative member used to form cavities in cast metallic objects; 
         FIGS. 2 and 3  are two views of a die used to form a cast metallic object; 
         FIG. 4  shows the die of  FIG. 2  with a salt core attached thereto; 
         FIG. 5  is a simplified representation of a disintegrative member with a vent opening of the present invention; 
         FIG. 6  is an isometric view of an engine block with salt cores disposed within its cylinder openings; 
         FIG. 7  is similar to  FIG. 6 , but with the salt cores removed from the cylinder bores; 
         FIG. 8  is an isometric view of a salt core made in accordance with a preferred embodiment of the present invention; and 
         FIG. 9  is an alternative view of the core illustrated in  FIG. 8 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Throughout the description of the preferred embodiment of the present invention, like components will be identified by like reference numerals. 
       FIG. 1  is a simplified schematic representation of an exemplary disintegrative member  10 . In a typical application of the present invention, the disintegrative member  10  is a salt core. The disintegrative member  10  is shaped to conform with a desired shape of a cavity which is intended to be formed in a cast metallic object. The disintegrative member  10  has an internal surface  16 . As will be described below, the internal surface  16  in one embodiment of the present invention is a surface of an internal opening  20  that is formed within the disintegrative member  10  and shaped to receive an extension of a die, such as a mandrel of a die casting mold. 
     With continued reference to  FIG. 1 , the internal opening  20  can be shaped to have a hexagonal cross section, the disintegrative member  10  can be made of salt, and the cast metallic object can be an engine block with the cavity being a cylinder in the engine block. 
     With continued reference to  FIG. 1 , the exemplary disintegrative member  10  is shown with two protrusions,  24  and  26 , extending from its outer surface portion  14 . These protrusions are intended to form openings in the wall of the cavity, such as air intake passages through the wall of a cylinder within an engine block. 
       FIG. 2  is a highly simplified exemplary representation of two portions,  30  and  32 , of a die which are usable in a die casting machine. The internal cavity  36  of the die is shaped to define the desired surfaces of the cast metallic object. One portion  32  of the die is provided with a mandrel  40  that is shaped to hold a disintegrative member in place during the die casting operation.  FIG. 3  shows a bottom view of the second portion  32  of the die which illustrates the hexagonal shape of the mandrel  40 , or extension. 
       FIG. 4  shows the disintegrative member  10  associated with the mandrel  40 , or extension, of the second portion  32  of the die. The internal opening  20  of the disintegrative member  10 , discussed above in conjunction with  FIG. 1 , is shaped to receive the mandrel  40  within it and retain the disintegrative member  10  in place prior to and during the injection of molten metal into the cavity  36  of the die. The presence of the disintegrative member  10  within the cavity  16  of the die allows a cavity to be formed within the cast metallic object. After the cast metallic object is removed from the die, the disintegrative member  10  can be disintegrated, or dissolved, to remove it from the cast metallic object, leaving a precisely shaped cavity within the metallic object. Those skilled in the art of die casting are familiar with the use of disintegrative members and are also familiar with the many different applications in which these disintegrative members  10  can be used. Cylinders of an engine block are only one of numerous examples where salt cores are shaped to define internal cavities within the cast metallic object and are then later dissolved and removed from the cast metallic object, leaving precisely shaped and dimensioned cavities in the cast metallic object. 
       FIG. 5  is an enlarged representation of the disintegrative member  10  and the portion of the mandrel  40  which is disposed within the internal opening  20  of the disintegrative member  10 . For purposes of illustration, the space  50  surrounding the mandrel  40  within the internal opening  20  is shown in an enlarged representation in  FIG. 5 . In reality, the internal opening  20  is shaped to provide a sliding fit between the internal surface  16  of the internal opening  20  and the outer surface  52  of the mandrel. That space  50  is exaggerated in  FIG. 5  for purposes of facilitating a description of the beneficial operation resulting from the present invention. 
     With continued reference to  FIG. 5 , a vent opening  60  is illustrated through a portion of the wall of the disintegrative member  10  between the outer surface portion  14  and the internal surface  16  of the internal opening  20 . As a result, when molten metal surges against the outer surface of the disintegrative member  10 , the vent opening  60  provides a path through which gases can flow and move away from the material of the cast metallic object. It prevents entrapment of those gases in the metallic object. The flow of these gases, and some molten metal, is represented by the arrows G in  FIG. 5 . 
     In order to fully understand the benefit provided by the present invention, it is necessary to understand one of the problems that can be encountered in die casting processes, particularly in die casting processes which use disintegrative members. It has been observed that porosity within the structure of the cast metallic object can occur in certain regions, particularly those regions proximate the outer surface of the disintegrative member, during the die casting process. Metallographic analysis has shown that the porosity, which is typically exposed by subsequent machining of the cavity, is different from normal porosity that typically affects engine block integrity. The porosity experienced in the vicinity of the disintegrative member is a layered porosity and is associated with the hot metal immediately proximate to the disintegrative member, such as the salt core. This hot metal is inhibited from cooling in a normal manner because the salt core is an efficient insulator. As a result, the disintegrative member, or salt core, does not allow the same type of heat transfer through its structure that occurs through the metallic structure of the die. As a result, engine blocks or other large structures, that are made with large salt cores, experience a disadvantageous situation because the large disintegrative members change the temperature gradient profile through the cast metallic object during the solidification of the molten metal. As a result, a plane of porosity occurs at the interface between the solidification front of the molten metal and the outer surface of the disintegrative member. Rapid heat extraction through the metal dies and the delayed solidification front near the salt core surface contribute to this problem. The interface next to the outer surface of the disintegrative member forms adjacent to the salt core and is generally exposed at a later time during the machining of the cylinder bore or cavity formed by the salt core. Trapped cavity gases normally do not have a path for escaping from the molten metal as it solidifies. These cavity gases reside at the interface near the outer surface of the salt core. Examination of these cast metallic objects suggest that a thermal gradient phenomena results during the solidification event and the evidence indicates that certain portions of the molten metal remain in “hot spots” and cool at a slower rate because of the insulating characteristic of the disintegrative member, such as the salt core. 
     The intent of the present invention is to provide a way to allow these gases to escape from these “hot spots” that result from the low thermal conductivity of the material used to make the disintegrative cores. Since a small amount of space  50  is available between the internal surface  16  of the internal opening  20  and the outer surface  52  of the mandrel  40 , a hole, such as the vent opening  60 , formed in a location such as that represented in  FIG. 5  can allow gases G to escape from the molten and solidifying metal to the space  50 . It is also likely that a quantity of molten metal will pass from the region proximate the outer surface portion  14  and flow through the vent opening  60  into the space  50  proximate the internal surface  16 . However, that small amount of metal can be removed by machining after the salt core  10  is removed from the solidified cast metal object. 
     With continued reference to  FIGS. 1–5 , it should be understood that the disintegrative member  10  need not have a large number of vent openings  60  formed through its wall structure. Instead, the number and location of the vent openings  60  can be limited to those areas that have previously been identified as being regions where porosity has formed or is likely to form as a result of the overall conditions of the die cavity, the quantities of molten metal used to form the cast metallic object, and the geometries associated with the disintegrative members  10 . After a region of porosity is identified, a hole  60  can be formed in that specific region to allow the escape of gases through the vent opening  60  and into the space  50  within the internal opening  20 . 
     It should be understood that  FIGS. 1–5  are simplified representations which are illustrated in a basic manner for the purpose of describing the underlying philosophy of the present invention. In the figures that will be described below, a particularly preferred embodiment of the present invention will be disclosed in relation to an engine block of a V-6 engine that is die cast with the use of six disintegrative members that define the shape and size of six cylinders of the engine. 
       FIG. 6  is an isometric view of a V-6 engine  70  in which the cylinder bores are made through the use of the disintegrative members  10 . The cylinder bores,  71 – 76 , are each shown in  FIG. 6  with the disintegrative member  10  remaining in place subsequent to the die casting operation through which the engine  70  was formed. As is known to those skilled in the art, the disintegrative members  10  are subsequently dissolved, through the use of water, and the shape of the disintegrative member  10  and its various protrusions result in the shape of the cylinder  71 – 76  and its various intake ports and exhaust ports. The internal surface  16  is identified in conjunction with cylinders bores  73  and  76  in  FIG. 6 . In the embodiment of the disintegrative member  10  shown in  FIG. 6 , the internal surface  16  is generally hexagonal and is shaped to receive the mandrel  40  which is described above in conjunction with  FIGS. 2 ,  3 ,  4 , and  5 . The other portions of the engine  70  illustrated in  FIG. 6  are not directly related to the use of the disintegrative member  10  and will not be described in detail herein. 
       FIG. 7  shows the engine  70  after the disintegrative members have been removed by dissolving them with water. Each of the cylinder bores is provided with several intake ports and an exhaust port. With reference to cylinder bore  76  in  FIG. 7 , an intake port  80  is visible along with intake ports  82 . It should be understood that in the particular engine block shown in  FIG. 7 , other intake ports are also provided in each cylinder, but are not readily visible in  FIG. 7 . In addition, each cylinder is provided with an exhaust port  86 . The exhaust ports  86  of cylinders  71 – 73  are visible in  FIG. 7  where they extend into the exhaust manifold which is located centrally between the two banks of cylinders. 
       FIG. 8  is an isometric view of a disintegrative member  10  made in accordance with a preferred embodiment of the present invention. The disintegrative member  10  has an outer surface portion  52  which, as described above, is shaped to conform with the desired shape of the cavity to be shaped in the cast metal object. In other words, the outer surface portion  52  is generally cylindrical and shaped to form the cylindrical portion of a cylinder, such as one of the cylinders  71 – 76  described above in conjunction with  FIGS. 6 and 7 . Several protrusions are shown extending radially outwardly from the outer surface portion  52 . For example, the protrusions identified by reference numerals  90  and  92  are shaped to result in the formation of intake openings through the wall of the cylinder of the cast metallic object, such as the engine  70  described above in conjunction with  FIG. 7 . It should be noted that the protrusions  90  and  92  are generally analogous to the exemplary protrusions  24  and  26  described above in conjunction with  FIGS. 1 and 5 . Another protrusion  96  extends from the outer surface portion  52  of the disintegrative member  10  and is shaped to result in the formation of an exhaust cavity  86  which was described above in conjunction with  FIG. 7 . At the end of the disintegration member  10 , a protrusion  98  is shaped to result in an opening through which a connecting rod extends for connection between a piston and a crankshaft of the engine. The vent opening  60  is illustrated at a location which is below the protrusion  96  for the exhaust port and between that protrusion and the closed end  100  of the disintegrative member  10 . As described above in conjunction with  FIG. 5 , the purpose of the vent opening  60  is to allow gases to pass radially inwardly toward the internal surface  16  of the disintegrative member  10 , such as a salt core, in order to vent those gases away from the region proximate the outer surface portion  52  of the disintegrative member  10 . 
     With continued reference to  FIG. 8 , one particular embodiment of the present invention utilizes a single vent opening  60  for these purposes. However, it should be understood that other applications of the present invention could use a plurality of vent openings  60  to prevent porosity at other locations close to the outer surface portion  52  of the disintegrative member  10 . In a possible application of the present invention, regions of porosity can be identified after occurrence and detection and, subsequently, the salt core can be provided with a vent opening  60  in the particular region identified as being a problem area with regard to the formation of porosity that results because of the insulative characteristics of the disintegrative member  10 . 
       FIG. 9  is an isometric view of the disintegrative member  10  showing the open end of the salt core. The internal surface  16 , which is generally hexagonal in cross section, is shown along with a bottom rib  110  formed in the bottom of the internal opening of the disintegrative member  10 . An alignment protrusion  114  is formed in one surface of the hexagonally shaped opening and is used as an indexing aid to make sure that the disintegrative member  10  is properly aligned in association with the mandrel  40  which is shaped to be received within the opening and in close proximity to the internal surfaces  16 . The vent opening  60  is not visible in  FIG. 9 , but is located behind the protrusion  96  that forms the exhaust port in the cast metallic object. In addition to the protrusions,  90  and  92 , which form the intake ports of the cylinders, protrusions  106  are provided to form intake ports. 
     With reference to  FIGS. 1–9 , it can be seen that the method for producing a cast metallic object having a cavity formed therein, in accordance with a preferred embodiment of the present invention, comprises the steps of providing a disintegrative member  10  having an outer surface portion  52  which is shaped to conform with a desired shape of the cavity, such as a cylinder  71 – 76 . The disintegrative member  10  has an internal surface  16 . The method of the present invention further provides a vent opening  60  extending between the outer surface  52  and the internal surface  16 . An internal opening  20  is provided within the disintegrative member  10  and this opening  20  is shaped to receive an extension of a die, such as a mandrel  40 . The internal opening  20  has a generally hexagonal cross section in a preferred embodiment of the present invention and the disintegrative member  10  is made of salt which is dissolvable through the use of water after the cast metallic object solidifies. The cast metallic object can be an engine block  70  and the cavity formed by the disintegrative member  10  can be a cylinder of the engine block. The outer surface portion  52  of the disintegrative member  10  is generally cylindrical and the vent opening  60  can be a hole extending through the outer surface portion  52  and through the internal surface  16 . By providing the vent opening  60 , gases are allowed to escape inwardly into the central opening  20  of the disintegrative member  10  and porosity in the region proximate the outer surface portion  52  can avoided. 
     Although the present invention has been described with considerable specificity and illustrated to show a particularly preferred embodiment in conjunction with an engine block, it should be understood that alternative embodiments are also within its scope.