Method of improving the removal of investment casting shells

Abstract

A method of improving the removal of an investment casting shell surrounding a metallic article. The shell is made from the deposit of layers refractory slurry and stucco onto a pattern. The shell contains, after firing, an amount of an alkali metal oxide or an alkaline earth metal oxide sufficient to reduce the strength of the shell. By this invention, after firing of the shell and pouring of the molten metallic part into the shell, the shell is hydrated to further reduce the strength of the shell, to facilitate its removal from the metallic part.

1 . A method of improving the removal of an investment casting shell surrounding a metallic article, said shell being made from the deposit of layers of refractory slurry and stucco onto a pattern, and containing, after firing, an amount of an alkali metal oxide or an alkaline earth metal oxide sufficient to reduce the strength of the shell, which comprises the steps of: 
 firing the shell, pouring the molten, metallic part into the shell, and then hydrating the shell, whereby the strength of the shell is further reduced to facilitate its removal from the metallic part.    
   
   
       2 . The method of  claim 1  in which said alkaline earth metal oxide is present.  
   
   
       3 . The method of  claim 2  in which the alkaline earth metal oxide comprises calcium oxide.  
   
   
       4 . The method of  claim 1  in which said hydrating of the shell is performed by a water spray or dip.  
   
   
       5 . The method of  claim 1  in which said hydrating of the shell is performed by exposure to air of at least about fifty percent relative humidity.  
   
   
       6 . The method of  claim 1  in which said alkali metal oxide or alkaline earth metal oxide is formed in situ in the shell by heat decomposition of an alkali metal carbonate or alkaline earth metal carbonate.  
   
   
       7 . The method of  claim 6  in which said alkaline earth metal carbonate is present in at least one layer used to form said shell.  
   
   
       8 . The method of  claim 6  in which said alkaline earth metal carbonate is present in at least one layer of stucco used to form said shell.  
   
   
       9 . The method of  claim 1  in which an amount of 1 to 40 weight percent of said alkali metal oxide or said alkaline earth metal oxide is present, based on the weight of the total shell.  
   
   
       10 . The method of  claim 31  in which an amount of about 8 to 25 weight percent of said alkali earth metal or alkaline earth metal is present, based on the weight of the total shell.  
   
   
       11 . The method of  claim 1  in which at least one layer of said shell contains from about 25 to 75 weight percent of said alkali metal oxide or alkaline earth metal oxide.  
   
   
       12 . The method of  claim 1  in which said hydrating of the shell takes place with humid air at an air temperature of at least about 50° C.  
   
   
       13 . The method of  claim 1  in which said alkaline earth metal oxide is formed in situ in the shell by heat decomposition of calcium carbonate.  
   
   
       14 . The method of  claim 1  in which said alkali metal oxide or alkaline earth metal oxide has a first average grain size, and is present in at least one layer of said stucco, said stucco also containing a finely divided refractory material having a second average grain size that is no more than 50% greater or less than the first average grain size.  
   
   
       15 . The method of  claim 14  in which said refractory material comprises fused silica.  
   
   
       16 . The method of  claim 14  in which said refractory material comprises aluminosilicate.  
   
   
       17 . The method of  claim 14  in which said refractory material comprises zircon.  
   
   
       18 . The method of  claim 14  in which said first average grain size is 30-50 mesh.  
   
   
       19 . The method of  claim 1  in which the shell is fired at a temperature above about 1100° F. and less than about 1800° F.  
   
   
       20 . A method of improving the removal of an investment casting shell surrounding a metallic article, said shell being made from the deposit of layers of refractory slurry and stucco onto a pattern, and containing, after firing, an amount of calcium oxide or magnesium oxide sufficient to reduce the strength of the shell, which comprises the steps of: 
 firing the shell at a temperature above about 1100° F. and less than about 1800° F., pouring the molten, metallic article into the shell, and then hydrating the shell, whereby the strength of the shell is further reduced to facilitate its removal from the metallic part.    
   
   
       21 . The method of  claim 20  in which said hydrating of the shell is performed by exposure to air of at least about 50% relative humidity.  
   
   
       22 . The method of  claim 20  in which calcium oxide is present in said shell.  
   
   
       23 . The method of  claim 20  in which said calcium oxide or magnesium oxide is formed in situ in the shell by heat decomposition of a calcium or magnesium salt.  
   
   
       24 . The method of  claim 1  in which a calcium or magnesium salt is present in at least one layer of a refractory stucco used to form said shell.  
   
   
       25 . The method of  claim 20  in which said magnesium or calcium oxide has a first average grain size, and is present in at least one layer of said stucco, said stucco also containing a finely divided refractory material having a second average grain size that is no more than about 50% greater or less than the first average grain size.  
   
   
       26 . The method of  claim 25  in which said refractory material is fused silica and the first average grain size is 30-50 mesh.  
   
   
       27 . A method of improving the removal of an investment casting shell surrounding a metallic part, said shell being made from the deposit of layers of refractory slurry and stucco onto a pattern, and containing, after firing, an amount of calcium oxide sufficient to reduce the strength of the shell, which comprises the steps of: 
 firing the shell, pouring the molten metallic part into the shell, and then hydrating the shell, said calcium oxide having a first average grain size and being present in at least one layer of said stucco, said stucco also containing a finely divided refractory material having a second average grain size that is no more than about 50% greater or less than the first average grain size, whereby the strength of the shell is further reduced to facilitate its removal from the metallic part, and then hydrating the shell, whereby the strength of the shell is further reduced to facilitate its removal from the metallic part.    
   
   
       28 . The method of  claim 27  in which said refractory material comprises fused silica and the first average grain size is 30-50 mesh.  
   
   
       29 . The method of  claim 27  in which the shell is fired at a temperature above about 1100° F.  
   
   
       30 . The method of  claim 1  in which said alkali metal oxide or alkaline earth metal oxide is formed in situ in the shell by heat decomposition of alkali earth metal or alkaline earth metal is selected from the group consisting of: calcium carbonate, calcium sulfate, calcium magnesium carbonate, magnesium carbonate, magnesium sulfate, strontium carbonate, sodium carbonate, potassium acetate, and mixtures thereof.  
   
   
       31 . The method of  claim 1  in which said alkali metal oxide or alkaline earth metal oxide is formed in situ in the shell by heat decomposition of an alkali earth metal or alkaline earth metal, respectively.  
   
   
       32 . The method of  claim 31  in which an amount of about 5 to 40 weight percent of said alkali earth metal or alkaline earth metal is present, based on the weight of the total shell.  
   
   
       33 . The method of  claim 14  in which said refractory material is selected from the group consisting of: fused silica; quartz; alumina; aluminosilicate; zircon; and zirconia.