Abstract:
A method and apparatus is disclosed for forming a metal casting sprue for a lost foam casting process, the sprue formed in sand for receiving molten metal and directing the molten metal to a foam pattern in a mold cavity, wherein the sprue formation is facilitated by an insert having a plurality of apertures formed therein, the apertures facilitating an application of a coating to sand surrounding the insert prior to removal of the insert from the sand, and wherein the sprue facilitates a minimization of production costs and an optimization of material properties of the resultant casting.

Description:
FIELD OF THE INVENTION 
     The invention relates to lost foam casting for producing metal castings and more particularly to a method and apparatus for forming a sprue in sand for receiving molten metal and directing the molten metal to a foam pattern in a mold cavity, wherein the sprue formation is facilitated by an insert having a plurality of apertures formed therein, the apertures permitting an application of a coating to sand surrounding the insert prior to removal of the insert from the sand. 
     BACKGROUND OF THE INVENTION 
     A so-called “lost-foam” casting process is a well-known technique for producing metal castings. A fugitive, pyrolizable, polymeric, foam pattern (including casting, gating, runners, and sprue) is covered with a thin (typically in the range of 0.25–0.5 mm), gas-permeable refractory coating/skin such as mica, silica, alumina, or alumina-silicate, for example. The pattern is embedded in compacted, unbonded sand to form a mold cavity within the sand. Molten metal is then introduced into the mold cavity to melt, pyrolyze, and displace the pattern with molten metal. 
     Gaseous and liquid decomposition/pyrolysis products escape through the gas-permeable, refractory skin and into the interstices between the unbonded sand particles. Typical fugitive polymeric foam patterns comprise expanded polystyrene foam (EPS) for aluminum castings and copolymers of polymethylmethacrylate (PMMA) and EPS for iron and steel castings, for example. 
     The polymeric foam pattern is made by injecting pre-expanded polymer beads into a pattern mold to impart the desired shape to the pattern. For example, raw EPS beads (typically 0.2 to 0.5 mm in diameter) containing a blowing/expanding agent (e.g. n-pentane) are: (1) first, pre-expanded at a temperature above the softening temperature of polystyrene and the boiling point of the blowing agent; and (2) molded into the desired configuration in a steam-heated pattern mold which further expands the beads to fill the pattern mold. Complex patterns and pattern assemblies can be made by molding several individual mold segments, and then joining the mold segments by gluing, for example, to form the pattern or pattern assembly. 
     The filling of a “lost-foam” casting with molten metal is typically achieved with a gravity-cast or a countergravity-cast method. In a gravity-cast lost-foam process, an overhead ladle or furnace pours metal into a pouring basin and sprue which is in communication with the casting pattern. The metallostatic head in the basin and sprue is the force driving the metal into the casting pattern. In a countergravity-cast lost-foam process, an applied pressure drives the molten metal into the pattern. This pressure can be applied in the furnace vessel, which sits below the pattern, or in the pattern flask itself. 
     There are three categories of gating systems in gravity-cast lost-foam process which are based on the orientation of the metal front as it enters the casting pattern. These categories are top-fill, bottom-fill and side-fill gating. A top-fill gating system has the sprue and runners located above the casting pattern. This causes molten metal to flow downward against the casting foam pattern. A bottom-fill gating system has runners which are located below the casting pattern. The metal flows downward though the vertical sprue, but flows upward against the foam casting pattern. A side-fill gating has a plurality of runners along the length of a sprue and casting pattern. The vertical sprue may be flanked by two or more patterns for making multiple castings with a single pour. Typically, a side gated foam pattern has a complex metal front of varying orientations. 
     Bottom-fill casting is often preferred in lost foam castings. The advantage of bottom gating is a reduction in gas bubbles that make their way into the casting causing voids or porosity defects. Coatings employed during lost foam casting often cannot absorb all the foam decomposition products as quickly as they are produced. If the molten metal is above the foam, as in top-fill casting, the gas bubbles move upward through the molten metal and collect at a top surface thereof. These gas bubbles lead to subsurface void defects in the casting. If the molten metal is below the foam, as in bottom-fill casting, the gas merely collects and slows the molten metal front movement. Thus, defects are minimized when using the bottom-fill configuration. 
     However, disadvantages do exist when using bottom-fill, gravity cast systems. The vertical sprue required in bottom-fill casting is typically formed using a foam pattern having the shape and configuration of the desired final shape and configuration of the sprue. Thus, the molten metal must still travel through a long section of thick foam to reach the casting area. Undesirable gases are created, but are unlikely to be carried into the casting area. However, oxide films are created which travel with the molten metal into the casting resulting in a reduction of a fatigue life of the metal. Thus, it is desirable to reduce or eliminate the foam used in the sprue to optimize the material properties of the casting. 
     Commonly owned U.S. Pat. No. 6,619,373 B1 is incorporated herein by reference to provide additional background and provide an example of other attempts at providing a solution for the above-mentioned disadvantages. 
     It would be desirable to develop a method and apparatus for forming a sprue for receiving molten metal and directing the molten metal to a foam pattern in a mold cavity for a lost foam casting process used in producing metal castings, wherein the sprue facilitates a minimization of production costs and an optimization of material properties of the resultant casting. 
     SUMMARY OF THE INVENTION 
     Consistent and consonant with the present invention, a method and apparatus for forming a sprue for receiving molten metal and directing the molten metal to a foam pattern in a mold cavity for a lost foam casting process used in producing metal castings, wherein the sprue facilitates an optimization of material properties of the resultant casting, has surprisingly been discovered. 
     In one embodiment, the insert for forming a sprue comprises an elongate hollow stem member having a first end and a second end, the first end of the stem member adapted to abut a lost foam casting pattern, at least a portion of the stem member is formed by a porous material to facilitate passage of a fluid therethrough; and means for expelling a fluid disposed in the stem member, the means for expelling a fluid adapted to be connected to a source of resin, wherein the resin is expelled by the means for expelling a fluid and passes through the porous material of the stem member. 
     In another embodiment, the insert for forming a sprue comprises an elongate hollow stem member having a first end and a second end, the first end of the stem member adapted to abut a lost foam casting pattern, at least a portion of the stem member is formed by a porous material to facilitate passage of a fluid therethrough; a perforated tube member disposed in the stem member, the tube member adapted to be connected to a source of resin, wherein the resin is caused to be expelled from the tube member and passes through the porous material of the stem member; and an inlet member formed by a porous material and disposed on the second end of the stem member, the inlet member adapted to receive said tube member. 
     The invention also provides methods of forming a sprue. 
     In one embodiment, the method of forming a sprue comprises the steps of providing a formed and coated lost foam casting pattern; providing an insert for forming a sprue comprising an elongate hollow stem member with a first end and a second end, at least a portion of said stem member formed by a porous material to facilitate passage of a fluid therethrough, the insert further comprising means for expelling a fluid disposed in the stem member, the means for expelling a fluid connected to a source of resin; providing a lost foam casting flask; positioning the lost foam casting pattern and the insert in the casting flask, wherein the insert abuts the lost foam casting pattern to cooperate with the lost foam casting pattern to define a flow path for molten metal through the casting flask; providing unbonded sand and at least partially filling the casting flask with the sand and compacting the sand around the lost foam casting pattern and the insert; causing the resin to flow through the means for expelling a fluid into the insert, through the porous material, and into the sand compacted around the insert; curing the resin to create a bonded sand layer; and withdrawing the insert from the sand leaving a sprue formed in the sand, the sprue lined by the bonded sand layer. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which: 
         FIG. 1  is a perspective view of a typical foam casting pattern for a gating system, with the remainder of the casting pattern removed for clarity; 
         FIG. 2  is a partial perspective view showing a lost foam casting flask in section with the flask housing a sprue forming insert and including the foam gating illustrated in  FIG. 1 , according to an embodiment of the invention; 
         FIG. 3  is a perspective view of the flask, the insert, and the foam gating illustrated in  FIG. 2  with the insert and the foam gating embedded in sand within the flask, the flask and the sand shown in section; 
         FIG. 4  is an enlarged perspective view of an inlet portion of the insert illustrated in  FIGS. 2 and 3 ; and 
         FIG. 5  is a partial perspective view of the flask and foam gating illustrated in  FIGS. 2–4  after removal of the insert and insertion of a basin into the sprue, and prior to pouring of the molten metal into the sprue. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following detailed description and appended drawings describe and illustrate various exemplary embodiments of the invention. The description and drawings serve to enable one skilled in the art to make and use the invention, and are not intended to limit the scope of the invention in any manner. In respect of the methods disclosed and illustrated, the steps presented are exemplary in nature, and thus, the order of the steps is not necessary or critical. 
       FIG. 1  depicts a coated lost foam casting pattern  10 . Only a sprue  12  and gates  14  of the pattern  10  are shown, as the remainder of the casting pattern  10  has been omitted for clarity. The remainder of the pattern  10  can be any lost foam casting pattern such as that depicted in U.S. Pat. No. 6,619,373, for example, which has been incorporated herein by reference. A thin, gas permeable refractory material is used to coat the pattern  10  such as mica, silica, alumina, or alumina-silicate, for example, as is well known in the art. The pattern  10  can be produced by any conventional method such as by injecting pre-expanded polymer beads into a pattern mold (not shown) to impart the desired shape to the pattern  10 . For example, raw EPS beads containing a blowing and expanding agent such as n-pentane, for example, are pre-expanded at a temperature above the softening temperature of polystyrene and the boiling point of the blowing agent. The pre-expanded beads are then molded into the pattern  10  in a heated pattern mold (not shown) which further expands the beads to fill the pattern mold. Typically, the pattern  10  is produced from an expanded polystyrene foam (EPS) for an aluminum casting and a copolymer of polymethylmethacrylate (PMMA) and EPS for an iron and a steel casting, for example. 
     A first end of a horizontal coated foam runner  16  is disposed on an inlet portion  18  of a first end of the sprue  12 . A second end of the runner  16  is spaced horizontally from the inlet portion  18 . An uncoated foam protuberance  20  is formed adjacent the second end of the runner  16 . A riser  23  is disposed at a second end of the sprue  12 . 
       FIG. 2  shows a lost foam casting flask  22  which houses the pattern  10  illustrated in  FIG. 1  and a sprue forming insert  24 . The insert  24  includes an elongate hollow stem member  26 , a frustoconical transition member  28 , an inlet member  30 , and a cover  32 . A first end of the stem member  26  is adapted to receive the protuberance  20  therein. The insert  24  extends upwardly from the protuberance  20  substantially parallel to the pattern  10  to a point vertically above the pattern  10 . 
     The transition member  28  is disposed on a second end of the stem member  26  and flares radially outwardly and upwardly therefrom. The inlet member  30  is disposed on and is in communication with the transition member  28 . The cover  32  is disposed on the inlet member  30 . A first aperture  34  and a second aperture  36  are formed in the cover  32 , as clearly illustrated in  FIG. 4 . The first aperture  34  is adapted to receive a perforated hollow tube  38  therein. By perforated, it is meant that a plurality of apertures  40  is formed in a wall forming the tube  38 , or the wall forming the tube  38  is formed by a structure facilitating the distribution of a coating therethrough in a desired manner, for example. The tube  38  extends through the first aperture  34  and substantially the entire length of the insert  24 . The apertures  40  are formed in a portion of the tube  38  disposed within the insert  24 . 
     An inlet  42  is disposed in the second aperture  36  and extends axially outwardly therefrom. The inlet  42  includes an adapter  44  adapted to be connected to a source of a catalyst (not shown). In the embodiment shown, the stem member  26 , the transition member  28 , and the inlet member  30  of the insert  24  are formed by a porous material which permits a fluid to pass therethrough. Any conventional porous material such as a screen, a perforated sheet material, or other porous material, for example, can be used to form the stem member  26 , the transition member  28 , and the inlet member  30 . 
     In  FIG. 3 , the casting flask  22  is shown with unbonded sand  46  compacted around and embedding the pattern  10  and the insert  24 . Pouring and compacting of the sand  46  in the casting flask  22  is well known in the art. The cover  32  is exposed from the sand  46  facilitating access to the tube  38  and the inlet  42 . 
     To assemble the structure shown in  FIG. 3 , the pattern  10  is provided using known forming and coating methods. The protuberance  20  is uncoated. It is understood that a coating can be applied to the entire pattern  10 , including the protuberance  20 , and then the coating removed from the protuberance  20 , or the protuberance  20  can be masked or otherwise protected from being coated during the coating process used to coat the remainder of the pattern  10 . The insert  24  is provided as shown and described herein. The pattern  10  is assembled with the insert  24  in the configuration shown in  FIGS. 2 and 3 . The casting flask  22  is provided and the assembled pattern  10  and insert  24  are placed in the casting flask  22 . It is understood that the pattern  10  and the insert  24  can be assembled in the casting flask  22 , if desired. 
     Unbonded sand  46  is provided and placed in the casting flask  22  to surround the pattern  10  and the insert  24 . The sand  46  is compacted to maintain the configuration of the pattern  10  and the insert  24 . It is understood that additional patterns  10  and additional inserts  24  can be positioned in the casting flask  22  as desired to facilitate forming of multiple castings and provide for efficient pouring of the multiple castings. 
     The tube  38  is then inserted in to the first aperture  34  to extend into the insert  24  as shown. A source of resin (not shown) is then connected to the tube  38 . Any conventional resin can be used which maintains the sand wall shape under the heat and pressure of the molten metal. The resin is caused to flow into the tube  38  and is expelled from the apertures  40  of the tube  38 . The resin passes through the porous material which forms the insert  24  and penetrates the sand  46  surrounding the insert  24 , as generally depicted by the arrows ‘A’. The source of catalyst is connected to the inlet  42  and caused to flow into the insert, through the porous material, and into contact with the resin sprayed into the sand  46 . The catalyst causes the resin to cure and harden to create a bonded sand layer  50  surrounding the insert  24 . The insert  24  is withdrawn from the sand  46  in the casting flask  22 , leaving a sprue  48  formed in the sand  46 , as shown in  FIG. 5 . It is understood that the resin can be cured or bonded by other means such as heat, for example, without departing from the scope and spirit of the invention. 
     A pouring basin  52  is provided and inserted into the sprue  48  formed by the inlet member  30 . Molten metal (not shown) is provided from an overhead ladle or furnace, for example, and poured into the basin  52 . The molten metal is directed downwardly through the sprue  48  and into contact with the uncoated protuberance  20 . The heat from the molten metal pyrolizes the protuberance  20 , thus permitting the molten metal to advance to the horizontal runner  16 , the heat from the molten metal then pyrolizing the foam therein. The molten metal is then introduced into the sprue  12 , gates  14 , and the remainder of the pattern  10 , to melt, pyrolyze, and displace the pattern  10 . The riser  23  receives the molten metal therein and supplies the molten metal back to the gates  14  to account for shrinkage during cooling and solidification of the casting after pouring of the molten metal. 
     Gaseous and liquid decomposition/pyrolysis products escape through the gas-permeable, refractory material used to coat the pattern  10  and into the interstices between the unbonded sand  46  particles. The amount of pyrolized gases caused to be directed back into the molten metal is minimized, thereby minimizing turbulence created in the molten metal and defects resulting therefrom in the casting. 
     From the foregoing description, one ordinarily skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications to the invention to adapt it to various usages and conditions.