Abstract:
An apparatus and method to delay the application of pressure on a molten metal front to equalize a pressure gradient present at the molten metal front during pressurized lost foam casting processes, or other pressurized casting processes. A pressure equalization member is placed over a pouring cup to divert the direct application of pressure to molten metal present in the pouring cup. The pressure equalization member allows for increased pressurization rates in such processes, facilitating interdendritic feeding while reducing microporosity and metal penetration defects.

Description:
BACKGROUND OF THE INVENTION 
     In lost foam or evaporable foam casting, a pattern is produced from a polymeric foam material, such as polystyrene, and has a configuration identical to the metal article to be cast. A porous ceramic coating is applied to the outer surface of the pattern. One or more patterns may be located within an outer mold or flask, where a polymeric foam gating system connects the pattern to a sprue. The space between the pattern or and the flask is filled with a finely divided inert material, such as sand, and the material also fills the internal cavities within the pattern. 
     In the lost foam casting process, molten metal is fed from a pouring cup into the sprue and the heat of the molten metal decomposes the polymeric foam material of the gating system, as well as the pattern. The molten metal occupies the void created by the decomposition of the foam material and the products of the foam decomposition pass through the porous ceramic coating on the pattern and become trapped within the interstices of the sand. In practice, when casting large objects such as engine blocks, and when utilizing 5 to 50 atmospheres of pressure, it takes approximately 30–60 seconds for the molten metal to decompose and occupy the polymeric foam pattern. When the molten metal solidifies, the resulting cast article has a configuration identical to the polymeric foam pattern. 
     It is recognized in the art that the application of external pressure to the molten metal before solidification is completed aids in the interdendritic feeding of the molten metal during solidification and prevents both the precipitation of hydrogen porosity and the formation of microporosity in the final product. This pressurization is commonly accomplished by placing the flask and its content, including the pouring cup, into an outer pressure vessel. 
     However, in the current state of the art, pressurization of the flask contents is closely controlled. This is done because too rapid of a pressurization rate leads to failure of the ceramic coating. When the ceramic coating fails, metal penetration defects appear in the cast product. The failure results from the occurrence of a large  pressure gradient across the molten metal front that appears upon the sudden application of pressure. For example, when pressure is suddenly applied to the vessel, the pressure in the liquid metal rises rapidly. The velocity at which the pressure rises in the metal is at the speed of sound in liquid metal. At the same time, it is recognized that the pressure rise in the finely divided inert material is dependent upon the size and shape of the material utilized. Generally, the rise in pressure in the liquid metal is much faster than the rise in pressure in the finely divided inert material. This disparity in rate of pressure application causes a pressure gradient to appear across the molten metal front. If the pressure differential of the pressure gradient exceeds a threshold value when the molten metal front reaches the ceramic coating, then the ceramic coating will fail. When the ceramic coating fails, metal penetration of the coating results, and the casting finish takes on the surface of the sand. 
     Conversely, if the pressurization rate of the vessel is slow, the benefits of applying the pressure may not be realized in all portions of the casting. Thus, the casting would not see the benefits of reduction in gas porosity due to hydrogen precipitation, improved delivery during the initial stages of solidification and improved interdendritic feeding during the last stages of solidification. 
     Another drawback of traditional pressure application methods is that the proper application of pressure requires a very narrow processing window. This results in limitation on pressurization rates, limitations on the size and distribution of the finely divided inert material, and often results in unacceptable cast products that are scrapped. Furthermore, the narrow process window creates restrictions on manufacturing processes that create inefficiencies. 
     SUMMARY OF THE INVENTION 
     As aforementioned, the application of external pressure in lost foam casting processes aids in the interdendritic feeding of the molten metal during solidification. However, traditional methods require a very narrow process window limiting the pressurization rates and creating restrictions on manufacturing processes which in turn create inefficiency in the industrial processes in which lost foam casting methods are applied. 
     An increase in the pressurization rate exacerbates metal penetration defects because the applied pressure reaches the molten metal front first through the molten metal, and then later through the finely divided inert material, ceramic coating and foam pattern. This creates a pressure gradient across the molten metal front resulting in significant metal penetration defects, especially with rapid pressurization rates. Therefore, in order to allow more rapid pressurization rates, the molten metal front must be stabilized throughout pressurization. 
     The present invention surprisingly found that the incorporation of a pressure equalization member in association with the pouring cup efficiently equalizes the pressure at the molten metal front and allows for increased pressurization rates without metal penetration defects, as occurred in the past. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The drawing illustrates the best mode presently contemplated of carrying out the invention. 
       In the drawing: 
         FIG. 1  is a vertical section of an apparatus that can be used to carry out the method of the invention.  
     
    
    
     DETAILED DESCRIPTION 
     The drawing illustrates an apparatus that can be used in carrying out the casting method of the invention. The apparatus includes a pressure vessel  2  with an open end enclosed by a hinged lid  4 . In practice, the lid  4  may be hinged to the pressure vessel  2  and moved between an open and closed position by mechanical equipment (not shown). When casting relatively large objects, such as engine blocks or internal combustion engines, the pressure vessel may be of substantial size, for example, about 36 inches in diameter and 42 inches high. 
     In the casting process, a metal, generally cylindrical, open top mold or flask  6  is positioned in pressure vessel  2  and one or more ceramic coated patterns  8  formed of a polymeric foam material, such as polystyrene, are located in the flask  6 . Each pattern  8  has a configuration corresponding to the article to be cast. Patterns  8  are connected through a gating  10  to a sprue  12 . The gating  10  and sprue  12  are also formed of polymeric foam material. A pouring cup  18 , preferably formed of non-porous ceramic  fiber material, is located at the upper end of the flask  6  and communicates with the sprue  12 . 
     Surrounding the pattern  8  and gating  10  in the flask  6  is a finely divided inert material  14 , such as silica sand or other similar material. The finely divided inert material  14  also fills any voids or cavities present in the pattern  8 . 
     As a feature of the invention, a cup-shaped pressure equalization member  16  is placed within the lid  4  of the pressure vessel  2 . The base  20  of the pressure equalization member  16  is attached to the undersurface  22  of the lid  4 . The surface area of the pressure equalization member  16  is greater than the surface area of the pouring cup  18 . The shape of the pressure equalization member  16  is directly related to the shape of the pouring cup  18  and therefore may vary with the shape of the pouring cup. In the illustrated embodiment, the pressure equalization member is constructed of non-porous, high temperature ceramic material, but may be constructed of other comparable materials. 
     In a method of the invention, the pouring cup  18  is attached to the sprue  12  and the finely divided inert material  14  fills the flask  6  to a level approximately halfway to the top of the pouring cup  18 . A gas pressure line  26  is mounted within an opening in lid  4  and connects a suitable source of compressed air or an inert gas, such as nitrogen or argon, to the interior of the pressure vessel  2 . A pressure gauge  28  may be mounted in line  26 . Alternately, pressure line  26  may be connected through the side wall of the pressure vessel  2 . 
     In carrying out of the process of the invention, after the inert material  14  fills the flask  6  to the desired level, the inert material is compacted by conventional methods using vertical compaction. With the lid  2  in the open position and the pouring cup  18  connected to the sprue  12 , a molten metal  30 , which may be an aluminum silicon alloy, a magnesium alloy, stainless steel or the like, is subsequently poured into the pouring cup  18  and the lid  4  is moved to the closed and sealed position. 
     As the lid  4  is pivoted to the closed and sealed position, the pressure equalization member  16  is correspondingly moved over the pouring cup  18 . The side wall  23  of the pressure equalization member  16  surrounds, and is spaced outwardly of, the pouring cup  18  and a lower edge  34  of the side wall  23  is embedded in the inert  material  14 . As the pressure equalization member  16  is larger than the pouring cup  18 , an annular, pressurized compartment head section  32  is formed between the sub-member  16  and the pouring cup  18 . After the pressure vessel  2  is sealed, pressure, preferably in the range of 5 to 50 atmospheres, is applied to the contents of the flask  6  through the pressure line  26 . When pressure is applied, the pressure must travel down into the finely divided inert material  14  and back up into the pressurized compartment head section  32  in order to reach the molten metal  30 . 
     The molten metal  30  poured into the pouring cup  18  is generally at a temperature greater than 1250° Fahrenheit and the heat of the molten metal will melt, vaporize and decompose in various fractions the polymeric structures  8 ,  10  and  12  present in the flask  6 . The molten metal  30  first decomposes the sprue  12  and gating  10 , with the resulting decomposition products passing through the porous ceramic coating present on such structures and into the interstices of the finely divided inert material  14 . As the molten metal occupies the void created by the vaporization of the polymeric foam material, the molten metal front progressively moves through the gating  10  and into the pattern  8 . Similarly, as the molten metal front moves into the pattern  8 , the polymeric material of the pattern melts, vaporizes and decomposes. The byproducts of decomposition pass through the porous ceramic coating on the pattern and into the interstices of the surrounding finely divided inert material  14 . In this manner, the molten metal  30  fills the void created by decomposition of the pattern  8  to produce a casting identical in configuration to the pattern  8 . In practice, when casting engine blocks and utilizing  10  ATM of pressure, it takes approximately 30–60 seconds for the molten metal  30  to decompose the pattern  8 . 
     The presence of the sub-member  16  creates an equilibrium between the application of the pressure to the molten metal  30  through the finely divided inert material  14 , and the application of pressure applied to the molten metal  30 . Without the presence of the pressure equalization member  16 , pressure would be directly applied to the molten metal  30  present in the pouring cup  18 . Concomitantly, with or without the sub-member  16 , pressure is applied to the molten metal front from a second direction, through the finely divided inert material  14 , the porous ceramic coating, and pattern  8 . The finely divided inert material  14  creates a tortious path to  the molten metal front, creating a lag time in the application of pressure to the molten metal front from this second direction. Therefore, a differential pressure gradient occurs across the molten metal front during pressurization when the pressure application through the first path is not regulated. As previously described, such differential pressure is undesirable as it results in metal penetration defects. With the presence of the sub-member  16 , the differential pressure is avoided as the first path to the molten metal front is regulated. 
     The differential pressure gradient is avoided with the presence of the sub-member  16  because the pressure equalization member  16  forces the pressure that would be directly applied to the molten metal  30  to travel through the tortious pathway through the inert material  14  and around the lower edge of the member  16 , and thus creates a delay in application of pressure to the molten metal  30 . In order for the pressure to reach the molten metal, the pressure must travel both: (a) through the inert material  14  and across the ceramic coating on pattern  8 ; and (b) down through the inert material  14 , up into the pressurized compartment head section  32  and thereinto the molten metal  30 . In this manner, the application of pressure to the molten metal  30  occurs at relatively the same time, resulting in the significant reduction of pressure differential at the molten metal front. Therefore, the presence of the permeable sub-member advantageously allows for an increased pressurization rate while avoiding the development of a differential pressure at the molten metal front that would result in metal penetration defects. 
     The invention has a further advantage in the fact that the pressure equalization member is reusable. Thus, a new pressure equalization member need not be inserted after every casting. 
     In another embodiment, the pressure equalization member  16  may be constructed of a selectively permeable material, the permeability being equivalent to the permeability of the finely divided inert material  14 . The selectively permeable material may be a porous high temperature ceramic material, or the like. In this embodiment, the molten metal front is stabilized as the pressure traveling through the pressure equalization member  16  is forced to travel at the same rate as the pressure traveling through the finely divided inert material  14 .  
     In still another embodiment of the invention, the pressure equalization member  16  is constructed of impermeable material, while the pouring cup  18  is constructed of a selectively permeable material, the permeability of the material matching the permeability of the finely divided inert material  14 . The selectively permeable material may be a porous high temperature ceramic material, or the like. In this embodiment, the sub-member  16  would fit directly onto the pouring cup  18  and close off the upper end of the cup, and the molten metal front would be stabilized as the pressure application to the molten metal  30  would travel through the selected permeability pouring cup  18  at the same rate as it travels through the finely divided inert material  14  and subsequently through the ceramic coating. 
     It is further understood that the present invention is applicable to lost foam casting processes which (1) apply pressure after the polymeric pattern  8  is fully decomposed, and to processes which (2) incorporate large volume pouring cups in order to apply pressure before the polymeric foam pattern  8  is fully ablated. In both circumstances, the presence of the sub-member  16 , in any of the embodiments described above, operates to equalize pressure at the molten metal front when the molten metal front reaches the porous ceramic coating of the polymeric foam pattern. 
     The above description describes what is believed to be the preferred embodiments of the present invention. It is contemplated that numerous other pressure equalization members may be utilized in accordance with this invention. For instance, the pressure equalization member may be constructed to effectively seal the pouring cup, with the pressure equalization member being constructed of a permeable high temperature ceramic material having the same permeability as the inert material used to fill the flask. In this manner the pressure equalization member would be of a shape other than cup-like, such as disc-like. However, those skilled in the art will recognize that other and further changes and modifications may be made thereto without departing from the spirit and scope of the invention, and is intended to claim all such changes and modifications which fall within the true scope of the invention.