Abstract:
A method for making parts in conjunction with a mold having a negative shape of the part. The mold is coated with a release agent and thereafter the part material is deposited on the release agent in the mold by gas dynamic cold spray. Thereafter, the release agent is separated from both the mold and the part either thermally, chemically, or mechanically.

Description:
GOVERNMENT INTEREST 
     The invention described herein may be manufactured, used, and licensed by or for the United States Government. 
    
    
     BACKGROUND 
     I. Field 
     The present invention relates generally to molding processes and, more particularly, to a mold release process for use with cold spraying. 
     II. Description 
     There are many different types of additive manufacturing processes, such as thermal spray, cold spray, laser forming, casting, and the like in which the manufactured parts are produced using a mold or die. For example, in one known method, molten material is deposited onto or into a mold and, upon cooling, solidifies to form the desired part. The mold must then be removed from the part. 
     There are many known different ways for separating the part from the mold. For example, in some known methods, the mold with the formed part is immersed in a chemical bath which dissolves the mold but not the part. In other cases, the mold is mechanically machined away from the formed part. In both cases, however, the mold or die is destroyed during the process of separating it from the formed part. 
     A still further known method of casting parts is known as the “lost wax process.” In the lost wax process, the mold is comprised of a low melting material, i.e. the wax that is formed into a negative image of the desired finished part. That wax negative image is then immersed into a ceramic slurry which coats the entire mold surface, except for sprue openings, with the ceramic slurry which is then allowed to dry and harden. That process is repeated until there is a sufficient buildup of hardened ceramic coating surrounding the surface of the wax negative image. 
     The coated mold is then placed into an oven which heats the mold and the wax to a temperature sufficient to melt the wax which is then removed from the mold leaving the image of the desired part. The part material is then introduced into the mold cavity and, upon cooling, the part is formed. 
     The process of cold spraying, also known as gas dynamic cold spray, also presents unique problems in molding parts. In a cold spray process, the part material in powder form is directed at high speed against the mold surface. Due to the high velocity of the part material at impact against the mold surface, the part material fuses together and solidifies by a cold forming solid state operation. After sufficient material has been deposited on the mold surface, the mold must be removed in order to obtain the desired manufactured part. 
     Previously, during the cold spray process, the part material adheres and bonds to the mold surface. Consequently, in order to extract the part from the mold, it has been previously necessary to either mechanically machine away the mold from the finished part, or chemically dissolve the mold. In either case, however, the mold cannot be reused since it is destroyed during the removal process. This, in turn, increases the overall cost of molding parts using the cold spray process. 
     SUMMARY 
     The present invention provides a method for making parts using the cold spray process which overcomes the above mentioned disadvantages of the known methods. 
     In brief, in the method of the present invention, a mold is first formed so that the mold has a negative shape of the desired finished part. The mold may be constructed of any suitable material, such as metal, synthetics, and the like. 
     The mold surface is then coated with a release agent. Preferably, the release agent comprises a metal which is itself deposited on the mold surface using the cold spray process. This release agent, furthermore, forms a thin layer between the mold surface and the finished part and thus duplicates the shape of the mold surface. 
     Thereafter, part material is deposited on the release agent on the mold surface by utilizing a cold spray process. The overall temperature of the mold during the cold spray operation is relatively low, typically less than 300° Fahrenheit. 
     It is important that the release agent be compatible with both the mold as well as the part material. Specifically, the release agent must be compatible with the mold so that the release agent will adhere to the mold to permit the subsequent cold spray process of the part material. Similarly, the release agent must not only adhere to the part material, but must be of sufficient toughness to avoid cracking or other deformations during the cold spray operation of the part material against the release agent. 
     After a sufficient amount of part material has been deposited on the release agent in the mold, the release agent must be removed from both the mold and the finished part. Different methods may be used to achieve this separation. 
     First, the release agent may be selected such that it has a lower melting temperature than both the material forming the mold and also the melting temperature of the part material. Consequently, following formation of the part by using the cold spray process, the mold, release agent, and finished part are heated in any appropriate fashion, such as an oven, to a temperature above the melting material of the release agent, but less than the melting temperature of the mold material and part material. Once the release agent melts, the finished part may be easily removed from the mold while retaining the mold intact for subsequent molding operations. 
     Alternatively, in order to separate the finished part from the mold while retaining the mold intact for subsequent use, a chemical solvent is applied to the part, mold, and the release agent. This solvent is selected so that it dissolves the release agent, but not the mold or the part material. Consequently, upon immersion of the mold, release agent, and finished part into a bath of the solvent, after time the solvent dissolves the release agent thus separating the finished part from the mold. 
     A still further method to separate the finished part from the mold while retaining the mold intact is to select a release agent that forms only a weak mechanical bond with both the mold as well as the part material. This bond must be sufficient to adhere to both the mold and the part material to allow the formation of the part using the cold spray process, but sufficiently weak so that the release agent may be mechanically separated, e.g. by pressing, the release agent from the finished part and mold following the cold spray operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A better understanding of the present invention will be had upon reference to the following detailed description when read in conjunction with the accompanying drawings, wherein like reference characters refer to like parts throughout the several views, and in which: 
         FIG. 1  is a side diagrammatic view illustrating the coating of the mold with the release agent; 
         FIG. 2  is a view similar to  FIG. 1 , but illustrating the mold, release agent, and the finished part on the mold; and 
         FIG. 3  is a view similar to  FIGS. 1 and 2 , but illustrating the separation of the finished part from the mold. 
     
    
    
     DETAILED DESCRIPTION 
     With reference first to  FIG. 1 , a portion of an exemplary mold  10  is shown having a mold surface  12 . The mold surface  12  is a negative image of the desired finished part. Furthermore, the mold  10  may be constructed of any suitable material, such as metal, composites, and the like. 
     Still referring to  FIG. 1 , a release agent  14  is applied to the mold surface  12  so that the release agent  14  forms a thin coating on the mold surface  12 . Since the release agent  14  constitutes only a thin coating, the actual shape of the mold surface  12  is duplicated by the release agent  14 . 
     The release agent  14  is selected so that it is compatible with the mold  10 . That compatibility requires that the release agent  14  adhere to the mold  10  during a cold spray operation. 
     The release agent  14  may be applied to the mold surface  12  of the mold  10  by any conventional method. However, in embodiments of the invention, the coating of the release agent  14  is applied to the mold surface  12  using a cold spray operation illustrated diagrammatically by a cold spray high-speed nozzle  16  in  FIG. 1 . 
     With reference now to  FIG. 2 , in order to form a finished part  18 , the part material is deposited over the coating of the release agent  14  by the cold spray process so that the deposited part material  18  also assumes the shape of the mold surface  12 . During the cold spray operation, the mold temperature  14  may reach temperatures as high as 300° Fahrenheit due to the high-speed impact of the part material against the mold surface  12  covered with the release agent  14 . 
     It is important that the release agent  14  be compatible with the part material  18 . Specifically, the release agent  14  must be of sufficient toughness and durability that it resists cracking or deformation from the part material during the cold spray operation. In addition, the part material must also adhere to the release agent  14 . 
     Following the cold spray process to form the finished part  18 , the finished part  18  must be removed from the mold  10  without destruction of the mold  10  during that removal process. This can be achieved in any of several ways. 
     First, the release agent  14  may be thermally removed from both the part  18  and the mold  10 . In order to accomplish this, the material for the release agent  14  is selected such that it has a melting temperature lower than the melting temperature of both the mold  10  and the material of the finished part  18 , but sufficiently high to resist deformation or destruction during the cold spray process. For example, assuming that the mold  10  is constructed of a material with a high melting temperature, e.g. steel, and the finished part  18  is made of copper, zinc could be chosen as the material for the release agent  14 . Zinc is compatible with the both the mold material and the finished part material, i.e. it will adhere to both, and is also sufficiently tough that it will withstand deformation or cracking during the cold spray process of the copper against the layer of the release agent  14 . 
     Just as importantly, zinc has a relatively low melting temperature of approximately 787° Fahrenheit. Thus, the melting temperature of zinc is much less than steel and less than the melting temperature of copper. However, the melting temperature of approximately 787° Fahrenheit for zinc is still well above the temperatures generated by the cold spray process. 
     After the zinc has been sprayed on the mold  10  to form the release agent layer  14  and copper sprayed onto the release agent layer  14  to form the finished part, the mold, finished part, and release agent are heated to a temperature above the melting temperature of zinc, but less than the melting temperature of both the mold  10  and the finished part  18 . Consequently, as shown in  FIG. 3 , the release agent  14  will melt and flow away from both the mold  10  and finished part  18 . After the release agent  14  has been completely melted from between the finished part  18  and the mold  10 , the finished part  18  can be easily retrieved. Since the mold remains undamaged during the entire molding operation, the mold  10  may also be reused in subsequent molding operations. 
     An alternative method of separating the mold  10  from the finished part  18  following the cold spray operation of the finished part material onto the mold  10  is to chemically dissolve the release agent  14  with a solvent. In this method of separating the finished part  18  from the mold  10 , the material for the release agent  14  is selected so that it dissolves when immersed in a particular chemical solvent but both the mold  10  and the finished part  18  are unaffected, i.e. do not dissolve, when immersed into the solvent. 
     For example, assuming that it is desired to produce a tantalum part on a ceramic mold, aluminum could be selected as the release agent. The aluminum is then applied to the mold surface on the ceramic mold using the cold spray process. Tantalum is then sprayed on the aluminum layer of the aluminum release agent on the mold surface. 
     The resulting mold, release agent  14 , and finished part  18  are then immersed in a bath of solvent containing sodium hydroxide. Sodium hydroxide will attack and dissolve the aluminum, but does not dissolve either tantalum or ceramic. Consequently, upon the complete dissolution of the aluminum release agent, the finished part  18  is completely separated from the mold  10  and the mold  10  can be used to manufacture subsequent parts. 
     A still further method for separating the finished part  18  from the mold  10  is to select the material for the release agent  14  that forms only a weak mechanical bond between the mold  10  and the finished part material during the cold spray process. For example, aluminum can be sprayed against the mold surface  12  to form the release agent  14  and tantalum can be sprayed onto the aluminum release agent  14  to form the finished part. Aluminum, however, forms only a weak mechanical bond with both the mold  10  as well as the finished part  18  so that the finished part  18  may be mechanically removed from the mold  10  without damaging the mold  10 . Any conventional mechanism may be used to apply the mechanical force, such as pressing, to separate the finished part  18  from the mold  10 . 
     From the foregoing, it can be seen that the present invention provides a simple yet highly effective method for forming finished parts using the cold spray operation and which avoids destruction of the mold in order to separate the finished part from the mold. Having described our invention, however, many modifications thereto will become apparent to those skilled in the art to which it pertains without deviation from the spirit of the invention as defined by the scope of the appended claims.