Abstract:
A method of immersion casting objects from molten metal, by crystallizing the metal outwardly from a heat-absorbing forming element such that upon removal from the molten metal bath, the solidified object has an internal surface defined by the shape of the forming element, and an outer surface that features random crystallization and a high degree of texture. The method can be facilitated by the interaction of the forming element and molten metal with molten salt provided as a layer on the molten metal. When the object is cast from a high purity metal such as aluminum or copper, the exposed crystal structure is especially random and highly reflective and can be enhanced by electro-chemical brightening.

Description:
BACKGROUND 
       [0001]    The present invention relates to the casting of decorative metal items, such goblets, vases, or other such vessels, and plaques or other flat substrates. Although such decorative items have been well known for years, if not centuries, artisans continually strive to find new ways of creating interesting shapes, surfaces, and manufacturing techniques that both capture the imagination yet can be achieved in a cost-effective manner. 
       SUMMARY 
       [0002]    The present invention provides a method of casting such metal items or objects from molten metal, by crystallizing the metal outwardly from a forming element such that upon removal from the molten metal bath, the solidified object has an internal surface defined by the shape of the forming element, and an outer surface that features random crystallization and a high degree of texture. 
         [0003]    When the object is cast from a high purity metal such as aluminum or copper, the exposed crystal structure is especially random and highly reflective. The brightness of the reflectivity can be enhanced by electro-chemical brightening. 
         [0004]    In one general aspect, the invention is directed to a method of manufacturing a shaped metal object, comprising immersing a preform with a shape corresponding to the shape of the metal object into the molten metal bath; maintaining the immersed preform at a temperature lower than the temperature of the molten metal, whereby the molten metal crystallizes outwardly from the preform with increasing thickness of solid metal in a raw shape complementary to the preform; withdrawing the raw shaped solid metal and preform from the molten metal bath together, whereby the shaped solid exhibits a randomly crystallized outer surface; separating the shaped solid from the preform; and preferably treating the randomly crystallized outer surface with a brightening agent to produce a finished shaped metal object. 
         [0005]    In another aspect, the invention is directed to a method and associated system of manufacturing a vessel comprising the steps of providing a molten metal bath; immersing a cooled forming element into the metal bath whereby the molten metal crystallizes outwardly from the forming element, thereby creating a solid metal, raw vessel around the forming element; withdrawing the raw vessel and forming element from the molten bath, whereby the raw vessel has developed a randomly crystallized outer surface; and separating the forming element from the raw vessel. As a preferred final processing, the inner surface of the raw vessel can be smoothed and the outer surface can be further brightened and/or sealed. 
         [0006]    The interface between the forming element and the outwardly solidifying metal preferably includes an agent that facilitates removal of the forming element and solidified object together from the molten bath, while also facilitating the subsequent separation of the metal object from the perform. 
         [0007]    This agent is preferably in the form of a supernatant layer of molten salt over the molten metal. Upon immersion of the forming element into the layer of salt a layer of solid salt solidifies on the forming element. Upon further immersion of the forming element into the molten metal the solidified salt is converted to molten form at the molten metal interface while the salt at the interface with the forming element remains solid. The solid salt at the interface with the forming element shrinks toward and thereby adheres to the forming element and the solidified molten metal at the interface with the molten salt shrinks toward the forming element and thereby adheres to the molten salt. This occurs as the molten metal and salt begin to cool and solidify at the interface. After withdrawing the metal object and forming element together from the molten metal bath, the salt layer can readily be fractured and the forming element slid out of the metal object. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0008]    An embodiment will be described in detail below with reference to the accompanying drawing, in which: 
           [0009]      FIG. 1  is a schematic of one set up for immersion casting of a goblet or the like; and 
           [0010]      FIG. 2  is a stylized representation of the finished goblet as observed from above. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    With reference to  FIG. 1 , the system set up  10  of one non-limiting example will be described for the casting of a tubular object such as a high purity aluminum vessel. A high-conductivity, e.g., copper, forming element in the shape of a rod or water cooled tube  12  is immersed into a molten metal bath  14  contained in a crucible  16  or furnace on which there is a supernatant layer  18  of molten salt. Upon immersion of the high-conductivity forming element  12  into the bath a layer of solid salt  20  solidifies at the interface with the forming element. The salt can exist in two phases, as a solid at the interface on the forming element and as molten material at the interface between the salt and the molten metal. The salt and molten metal solidify as heat is withdrawn through the forming element  12 . However, depending on the salt compound, the salt can remain molten as the metal solidifies, as indicated at  22 . 
         [0012]    The salt  20  stays in place during this process because it shrinks onto the forming element  12 . The aluminum crystals grow from the inside to the outside of the casting  22 . For a forming element  12  in the shape of a uniform cylinder, the molten metal solidifies radially outwardly from the axis and downwardly to an extent sufficient to form a base  22 ′ for the vessel. 
         [0013]    After a predetermined or monitored dwell time, sufficient aluminum has crystallized  22  on the immersed forming element  12  to establish the dimensions of the vessel. The forming element  12  with attached metal vessel  22  is extracted from the crucible  16 . After cooling, the casting  22  is removed by slight tapping of the forming element. The salt layer  20  fractures and the casting is simply pulled off the forming element. If further efforts are required to remove the casting from the forming element, water is sprayed on the casting to dissolve the salt. 
         [0014]    With further reference to the schematic representation of  FIG. 2 , the raw casting  24  has an inner sidewall diameter  26  and associated inner surface texture established by the interface with the salt. This surface in not normally visible in the finished vessel, but can optionally be smoothed with a rotary sander or the like. The raw vessel is cleaned before final processing. 
         [0015]    The raw casting  24  has an unusual, highly textured outer surface  28 . This is shown schematically as an irregular or random pattern of peaks and valleys of crystalline structure, of randomly varying height and depth. The bottom of the casting and top surface  30  can be ground smooth, with optional tapering at the top for a goblet or the like. 
         [0016]    For enhancing the decorative appeal, the outer surface  28  can be finished with the following steps: (a) electrochemical brightening in a standard electro-chemical brightening bath containing, e.g., phosphoric acid; (b) electrolytic anodizing in a solution of sulfuric acid; (c) dyeing in a solution of ferric ammonium oxalate; and (d) sealing in a high temperature water bath of at least 200-212 degrees F. with additions of nickel salts. 
         [0017]    In a working example of a 99.99% pure molten aluminum bath, a solid cooper forming element with associated heat sink, and a sodium chloride salt, the following table shows the relationship of densities and melt temperatures: 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Al 
                 Cu 
                 NaCl 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Spec. Gravity 
                 2.70 
                 8.92 
                 2.16 
               
               
                   
                 Melting Point (C.) 
                 660 
                 1083 
                 801 
               
               
                   
                   
               
             
          
         
       
     
         [0018]    In general, the metal material has a relatively lower melt temperature and a relatively higher density than the salt, and the forming element has a higher melt temperature than the metal and the salt. However, regardless of the melt temperature of the salt, the density of the salt must be lower than the density of the metal to assure that the salt layer floats on the molten metal. The temperature of the molten metal should be high enough for the salt to be molten at the interface with the molten bath as the forming element enters the bath, but low enough that the salt solidifies at the interface with the forming element as the forming element enters the bath and the molten metal solidifies outwardly from the forming element during the dwell time of the forming element as the forming element provides a heat sink. 
         [0019]    The use of dual molten salt layers for casting a metal or other sheet between them is described in my U.S. Pat. No. 2,754,550, issued Jul. 17, 1956 for “Method for the Casting of Sheets of a Fusible Material”, the disclosure of which is hereby incorporated by reference. Although the immersion casting method of the present invention is different and not readily derivable from my prior patent, many examples of the salts and metals listed therein are usable in the present invention. Barium chloride alone or mixed with sodium chloride is another good candidate for the salt layer of the present invention, especially for the casting of a steel object. Other options include casting of tin, with a sodium chloride layer and copper rod; casting of silver, with a silicon oxide layer and an iron rod. Graphite can also serve as a suitable rod. 
         [0020]    With further reference to  FIG. 1 , depending on the size and complexity of the metal object to be cast, the forming element  12  can be a solid rod for immersion into the bath, with a large, integrated disc  32  or the like at the upper end of the rod providing a relatively large heat sink from the rod. A drive mechanism with associated struts  34  or the like and controller (not shown) raise and lower the integrated rod and disc together as shown at  36 , into and out of the molten bath  14 , according to the desired shape of the metal object  22 . 
         [0021]    For example, a goblet having a cylindrical sidewall can be formed by simply immersing a cylindrical forming element into the molten metal bath, leaving the forming element in position within the bath for a preselected time, and then removing the forming element with solidified object. 
         [0022]    For a vase, the forming element can also be a cylindrical rod for which the dwell time along the axial dimension of the rod is varied to produce a varying outer diameter of the solidified metal. Alternatively, the forming element can have a non-uniform diameter and remain fully immersed in the bath for a specified period of time, which will produce a vessel having a varying inside diameter and varying outside diameter. 
         [0023]    It can be appreciated that a wide variety of forming elements  12  can be used for implementing the inventive concept. The forming element can be a thin plate defining opposite planar faces.  FIG. 1  can represent this as well, wherein the one edge of a plate-shaped forming element  12  is visible, with the length of the plate  12  extending well into the plane of the drawing sheet, and the relative thickness of the casting  22  less than suggested in the figure. The forming element can be immersed in a molten bath to produce a solid metal layer all around the forming element, but with the predominant feature being solidification of two relatively large metal plates which exhibit an unusual, highly textured crystalline surface. Upon removal from the molten bath, the forming element can be separated from what in essence is a metal sleeve. The connecting web (thickness edges) between the planar faces can be cut and ground to produce two beautiful plates that can be further processed into engraved plaques or frames for mounting any form of graphic or the like. 
         [0024]    In a more complex but efficient implementation for high production volume, the forming element  12  can be actively cooled with internal cooling flow paths  38   a,    38   b  connected to a cooling manifold  32 . The cooling manifold  32  is connected to an inlet  40  from a source of cooling fluid, and an outlet  42  for return flow to be re-cooled. The manifold has internal piping or baffling  44   a,    44   b  for providing the cooling flow  38   a,    38   b  within the forming element tube  12 . The manifold  32  and forming element  12  are vertically displaceable  36  with respect to the crucible  16 . 
         [0025]    To achieve the highly textured outer surface, the object  22  must be withdrawn from the molten bath  14  while the metal material adjacent the side and bottom walls  46 ,  48  of the crucible remains molten. This assures that the outward crystallization is not inhibited by the sidewalls of the crucible. With freedom for uninhibited outward growth, the crystal structure at the outer surface of the raw, solidified metal object exhibits a preferred peak-to-valley roughness in the range of about 0.125 to 0.250 inch.