Abstract:
A sprue for use in investment casting may comprise a pin and a shell enclosing a part of the pin. A method for creating a sprue may comprise partially enclosing a pin in a shell.

Description:
FIELD 
     The disclosed method and apparatus pertain to the manufacturing of items by investment casting. 
     BACKGROUND 
     The lost wax investment casting process may be used to manufacture a variety of items, including jewelry, ornaments, figurines, dental components and industrial parts. In the lost wax investment casting process, a wax or plastic pattern of an item may be created by, for example, injecting wax or plastic into a rubber or metal mold, or by hand carving. These patterns  11  may then be attached to a sprue  10  to create a pattern or casting “tree”  13 , such as that illustrated in  FIGS. 1A and 1B . The sprue  10  is typically a constant diameter cylinder formed of solid wax or plastic by extrusion or injection molding. The sprue  10  may be mounted in a base  12  that may hold the sprue  10  in a substantially stable orientation. The base  12  may comprise a cone  14  for holding the sprue  10 , and lip  16  for sealing the base  12  to a flask (not shown). 
     The wax or plastic patterns  11  may be affixed to the sprue  10  using a variety of methods and tools, such as with softened wax or plastic or with adhesive. The finished tree  13  may then be placed in a container called a flask (not shown). The base  12  may serve as a base or lid for the flask. Powdered investment material and water may be mixed. The investment mixture may be poured into the flask, submerging the tree  13  of patterns  11 . After the investment mixture solidifies, the flask may be heated to cure the investment material to strengthen it into an investment mold into which molten metal may be poured. The base  12  may be removed after the investment mold has set and before heating. During the flask “curing” process, the patterns  11  and sprue  10  may be melted and/or burned out of the investment mold, leaving mold cavities of the pattern shape and channels into which molten metal may flow. The patterns may also be removed by chemical dissolution. Once the curing and burnout process is complete, the flask may be placed in a casting machine. Molten metal may then be poured into the investment mold. The cavity created in the investment mold by the cone  14  of the base  12  may serve to funnel the molten metal into the channels and pattern cavities formed in the investment mold by the tree. After the metal solidifies, the investment mold may be removed, and the cast objects may be cut from the tree and finished. 
     Using a solid wax or plastic sprue may result in increased wax or plastic material costs, in increased burnout emissions and byproducts, in trees that bend or twist as patterns are attached to the tree, in relatively large sprues that leave correspondingly large channels to be filled by molten metal, and in excessively wide channels that increase turbulence of molten metal as it flows into the flask. For example, if the sprue bends or twists in the flask, some patterns may be pushed closer to one side of the flask, resulting in an investment mold that is weaker in that area. Greater mold non-uniformity may increase the risk that the investment mold will break during curing and use. There exists a need for a method and apparatus for reducing or eliminating one or more of those disadvantages. 
     SUMMARY 
     A sprue for use in investment casting may comprise a pin and a shell enclosing a part of the pin. A method for creating a sprue may comprise partially enclosing a pin in a shell. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  provides a perspective view of an embodiment of a prior art sprue with patterns attached. 
         FIG. 1B  provides a side cross-sectional view of the sprue, patterns and base of  FIG. 1A . 
         FIG. 2A  provides a perspective view of an embodiment of a sprue comprising a pin and a shell. 
         FIG. 2B  provides a side cross-sectional view of the sprue of  FIG. 2 . 
         FIG. 3  provides a perspective view of an embodiment of a pin for a sprue. 
         FIG. 4  provides a perspective view of the pin of  FIG. 3  at least partially enclosed by a shell. 
         FIG. 5  provides an exploded view of an embodiment of a mold for at least partially enclosing a pin in a shell. 
         FIG. 6  provides a side cross-sectional view of a tree positioned inside a flask. 
         FIG. 7  provides a side cross-sectional view of the tree of  FIG. 6  set in investment material, and having the pin removed from the shell and from the base. 
         FIG. 8  provides a side cross-sectional view of the channels formed in the investment material after burnout of the tree of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION 
     With reference to the embodiment of  FIGS. 2A and 2B , a sprue  20  may comprise a pin  22  and a shell  24 . The pin  22  may be inserted into the cone  34  of a base  36  to mount the sprue  20  to the base  36  in a relatively stable orientation. The base  36  may be substantially rigid or relatively flexible, or a combination of rigid and flexible parts. For example, the cone  34  may be relatively rigid and the remainder of the base may be relatively flexible to allow sealable mounting to a flask (not shown). 
     With reference to the embodiment of  FIG. 3 , a pin  22  may comprise a pin shaft  26 , a pin shoulder  28  and a support section  30 . Preferably the pin shaft  26  may be tapered. The pin shoulder  28  may be of a different diameter or cross-sectional area than the pin shaft  26 , and may taper or curve to meet the pin shaft  26 . In some embodiments, the pin shoulder  28  may have a major diameter that is the same as the major diameter of the support section  30 , or may have a greater major diameter than that of the support section  30  to better seat the pin  22  in the base (for example, as may be seen in the embodiment of  FIG. 2B ). Preferably, the pin shoulder  28  may be substantially smooth. Alternatively, the pin shoulder  28  may be provided with threads (as in the embodiment of  FIG. 3 ). In other embodiments, the pin  22  may comprise a pin shaft  26  and support section  30  without shoulder section  28 , such that the pin shaft tapers directly to the support section  30 . The support section  30  may have a varying or substantially constant cross-sectional area, and may be configured for insertion into the cone  34  of a base  36  (as in  FIGS. 2A and 2B ). 
     The support section  30  may comprise a base end  32  having a variety of cross-sectional shapes, for example, of round, square, hexagonal, triangular or other suitable cross-sectional shape. Non-round cross-sectional shapes may allow a mold technician to more easily remove the pin  22  from the shell  24  using a turning force such as by using a wrench. Alternatively, the base end  32  may be provided with one or more slots or other apertures (not shown) configured to receive tools or keys, such as a screwdriver, socket wrench or Torx™ wrench. Preferably, little force should be required to remove the pin  22  from the shell  24 . Thus, in the embodiment of  FIG. 3 , the shoulder  28  may be threaded and the support section  30  may include a base end  32  having a hexagonal cross-sectional area, thus allowing a mold technician to use a wrench to unscrew the pin  22  from the shell for removal. For such embodiments, the cone  34  of the base  36  may be provided with an opening configured to receive the base end  32  of the pin  22 . After the investment mold has set, the base  36  may be removed from the flask (not shown), and the pin  22  turned to unscrew it from the shell  24 . In yet another embodiment, if the pin  22  comprises metal or other readily heat conductive material, the pin  22  may be heated to partially melt the shell  24  and allow easier removal of the pin  22 . In other embodiments, the pin  22  may be released from the shell and investment mold during the pattern burn out process as the investment mold is heated in an oven. The pin  22  may comprise any suitably rigid material, such as metal, alloy, ceramic, metal/ceramic material, stiff plastic, wood or glass. In yet other embodiments, the pin  22  may be permanently mounted to the base  36 , or may be formed with the base  36  as an integrated structure. 
     In other embodiments, the pin  22  may be of constant diameter, or may include one or more curves or taper angles. The pin  22  may be hollow, or may be perforated to better retain a shell  24 . By using a relatively rigid pin  22 , a sprue may be made much thinner than prior art sprues of solid wax or plastic, yet still remain sufficiently rigid for pattern attachment. A thinner sprue may result in smaller channels in the investment mold, which may correspondingly reduce the amount of molten metal that fills the channels after casting and must be recovered for reuse. This may yield a better ratio of castings to sprue weight per mold. 
       FIG. 4  illustrates the pin of  FIG. 3  having a shell  24  mounted thereto. As may be seen in  FIG. 4 , the shell  24  may cover the pin shoulder  28 , and may form a ridge end  25  with respect to the support section  30  of the pin  22 . The ridge end  25  may help seat the pin  22  in a base. The shell  24  may have a substantially constant wall thickness, may have a tapering wall thickness or may have any other wall thickness variation suitable for the investment casting process. The shell wall thickness may depend on the size, shape, weight and placement of pattern on the sprue. Preferably, the shell  24  comprises wax or plastic or other organic material suitable for mounting patterns. In other embodiments, the shell may completely enclose the pin. 
     The shell  24  may be applied to the pin  22  in a variety of ways. As shown in the embodiment of  FIG. 5 , a mold may be used to enclose a pin  22  in a shell  24 . A first mold block  40  may comprise a recess  42  configured to receive the pin  22 . The recess  42  may be configured to snugly receive the support section  30  of the pin  22  such that when the pin  22  is placed in the recess  42  and a second mold block  46 , also having a recess (not shown) to receive the pin  22 , is clamped to the first mold block  40 , the shaft  26  and shoulder  28  of the pin  22  do not contact either the first mold block  42  or the second mold block  46 . The first mold block  40  and second mold block  46  may be held together using any suitable device, such as a jig, clamp, weighted object, rubber bands, and the like. An injection port  48  may allow melted wax or plastic to flow into the mold and around the pin shaft  26  and pin shoulder  28 . After the wax or plastic hardens over the pin  22  to form a sprue  44 , the two mold blocks  40  and  46  may be separated, and the sprue  44  may be removed from the mold. Of course, the mold may comprise any suitable material, such as metal or ceramic, and comprise one or more blocks or parts, depending, e.g., on pin and shell design and material. Preferably, the recess  42  is configured to form a smooth shell surface that will leave a relatively smooth channel in the investment mold. Smoother channels in the investment mold tend to reduce the turbulence of molten metal as it is poured into the investment mold. Reducing turbulence may reduce common casting imperfections, such as porosity and inclusions. 
     A pin may be enclosed by a shell in other ways, as well, such as by dipping a pin in melted wax or plastic that is then allowed to harden on the pin. Alternatively, melted wax or plastic may be applied with a brush to the pin and allowed to harden. If wax or plastic is provided in a thin sheet, the wax or plastic sheet may be wrapped around the pin and smoothed with a hot knife to create a shell. 
     As may be seen in  FIG. 6 , a sprue  60  may be mounted in the cone  62  of a base  64  by inserting the support section  63  of the pin  65  into the cone. The ridge end  66  of the shell  68  may butt against the cone  62 , and preferably any gaps between the cone  62  and shell  68  may be sealed with wax or plastic. Patterns  70  with sprue runners or “gates”  72  may be attached to the sprue  60  by known methods to form a tree. For example, if the shell  68  is made of wax, then the sprue runner  72  ends may be dipped into softened or sticky wax or melted with a hot gun, and then held against the sprue  60  until the connection hardens. Alternatively, sprue runners  72  may be attached to the sprue  60  by using globules of softened wax. After the gated patterns  72  are attached to the sprue  60  to form a tree, a flask  74  may be sealingly mounted to the base  64 . In the embodiment of  FIG. 6 , the base  64  comprises flexible rubber. Lip  76  formed in the base  64  may compress against the outer circumference of the flask  74  to substantially prevent investment material  78  from leaking out of the flask  74 . Investment material  78  may be poured into the flask through an open top  80 , and may flow around the tree. After the flask  74  has been filled with investment material  78  and any trapped air bubbles released, the investment material  78  may be allowed to harden. 
     After the investment material has hardened, the base  90  and pin  65  may be removed from the flask  92 , as shown in  FIG. 7 . In  FIG. 7 , the base  90  and pin  65  are shown as removed from the flask  92 , leaving the sprue shell  94  embedded in the investment mold. Removal of the pin  65  leaves a void  96 . The flask  92  and investment material  102  may then be placed in an oven (not shown) for heating. The oven heat melts and/or burns out the shell  94 , as well as the patterns  98  and sprue runners  100 , resulting, as may be seen in  FIG. 8 , in an investment mold having channels  101  for receiving molten metal. By using a pin and shell, much less sprue material must be melted or burned out, resulting in fewer gas emissions and other byproducts. By using a pin and shell, the sprue volume may be minimized with respect to the number of patterns that must be mounted, thus reducing the total amount of molten metal required for casting production. After the investment material has cured, it may be turned upside-down so that molten metal may be poured into the cavities left by the tree for casting as described above. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition, or matter, means, methods and steps described in the specification. As one will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods or steps.