Abstract:
A gravity pour vacuum casting method and apparatus providing uniform set-up provisions for gas permeable molds regardless of mold size, shape, or mold type employed in accordance with the present invention. Apparatus includes a sprue-forming device that when combined with a disposable foundry patter and subsequently invested together as a unit forms a gas permeable mold in accordance with the present invention, and further includes a vacuum chamber configured to match corresponding counterparts of said sprue-forming device.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is entitled to the benefit of Provisional Patent Application Ser. No. 60/159,966, filed Oct. 18, 1999. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSERED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     REFERENCE TO A MICROFICHE APPENDIX 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the art of investment casting and more specifically to a method and apparatus for combining a conforming gas permeable mold within a correspondingly suitable vacuum chamber in a manner that subjects said mold to vacuum pressure while molten metal is introduced by gravity means from outside the vacuum chamber. 
     2. Description of the Related Art 
     The basic principles and methodogoly of investment or “lost wax” casting have remained basically the same during the 6000 years it has been practiced. Investing a disposable pattern in a mold, eliminating the disposable pattern by burning it out, and introducing molten metal into the resultant mold cavity has never changed other than the quality of materials and equipment technology employed in the process. However, there have been many improvements made in the art during the time it has been known. Among the most important developments in modern times include the discovery that vacuum pressures applied to a gas permeable mold while molten metal is added will greatly enhance the fill out probabilities of complex shapes which otherwise depend entirely on sprue weight, elaborate conduits, venting gates, and hydrostatic pressures to completely fill out a complex mold. 
     Major changes in recent years have advanced vacuum assisted casting to the point where it is now standard practice to produce very complex articles, including highly detailed finished parts that require no additional machining, expensive assembly procedures, or further finishing of the cast article beyond sprue removal. Prior art examples include a wide variety of vacuum casting methods and equipment, which have proven to be successful in their intended applications. However, of the various processes heretofore used or proposed for use, few have received commercial consideration for high production industrial applications involving the precision casting of large and complex articles. The inherent fragility of large gas permeable molds such as ceramic shell molds hinder the progress towards utilizing vacuum casting for a broader range of casting applications. The method and apparatus comprising the present invention provide the combined benefits and advantages of ceramic shell molds with an ability to more economically produce larger, and more detailed precision castings requiring only minimum rework for manufacturing a finished product. 
     Therefore it is the primary objective of the present invention to reinforce and strengthen ceramic shell molds in a manner that makes them self-supporting, and to provide a uniform means for said molds to be mounted and sealed within a vacuum chamber regardless of mold size, shape, or mold type used, up to the size limitations of the particular vacuum chamber used in the process. The invention further permits molten metal to be introduced by gravity means from outside the vacuum chamber while said mold remains subjected to vacuum pressure. 
     It is another object of the invention to provide a method and apparatus that more efficiently drains melted pattern material from the mold during the pattern elimination process resulting in a cleaner mold at the beginning of the burnout phase, consequently reducing exposure to smoke and pollution in the workplace and to reduce undesirable emissions into the atmosphere. 
     It is still another object of the invention to provide a method and apparatus that furnishes a means for handling sprued patterns and finished molds safely to lessen the chance of damage while investing, drying, and setting up the mold for casting. Other related objectives and advantages are: 
     (a) To provide an improved method and apparatus that increases yield of the metal being cast by reducing the amount of metal that must be cut off the cast article in the form of sprues, gates, and risers. 
     (b) To provide an improved method and apparatus that allows thinner walled castings to be cast resulting in lighter finished articles and consequently less costly articles. 
     (c) To provide an improved method and apparatus that is applicable to the casting of a wide variety of different metals, including both ferrous and nonferrous metals. 
     (d) To provide an improved method and apparatus that permits the casting of articles of varying sizes and shapes one after the other in any desired order on a single production line thereby speeding up production and reducing costs. 
     (e) To provide an improved method and apparatus that can be used to sprue and cast any number of articles of any given configuration within a single mold up to the size limitations of the vacuum casting unit being used. 
     (f) To provide an improved method and apparatus that is capable of use with a variety of different casting designs involving different types of casting molds such as poured investment molds, ceramic shell molds, multiple mold sections, inserts, disintegrable cores, and the like. 
     (g) To provide an improved method and apparatus that can be used to cast different mold types one after the other in any desired order. 
     (h) To provide an improved method and apparatus that requires fewer production man-hours to be expended due to the high quality and precise detail of the cast articles. 
     (i) To provide an improved method and apparatus that is adaptable to high production output by combining any desired number of vacuum casting chambers in successive order such as a carousel arrangement or the like. 
     (j) To provide an improved method and apparatus that is reasonably simple and safe enough for use in a home workshop or teaching facility as well as a commercial foundry and is versatile enough for smaller units to be transported and used at any convenient site such as demonstration workshops or the like. 
     (k) To provide an improved apparatus that has a long operating life and minimal maintenance requirements. 
     (l) To provide an improved apparatus that accommodates any size, type, or shape of gas permeable mold in random or any desired order up to the maximum size of the vacuum casting unit used. 
     (m) To provide a method and apparatus suitable for space manufacturing projects utilizing the natural vacuum outside the earth&#39;s atmosphere. Further objectives and advantages of my invention will become apparent from a consideration of the drawings and ensuing description. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention discloses a vacuum casting unit and a gas permeable mold, both being equipped with uniform mounting, sealing, and filling features configured to register with their corresponding counterparts. When a ready to cast gas permeable mold is set-up in an activated vacuum casting unit, molten metal is introduced into said mold by gravity means from outside the vacuum chamber while the mold is subjected to vacuum pressure. 
     The vacuum casting unit consists of a vacuum tank with a means for connecting and controlling a vacuum device, and a top closure that when joined together form a vacuum chamber configured to accept a correspondingly configured gas permeable mold. 
     Features of the top closure include a flat sealing surface with an orifice, through which a sprue-forming device, in accordance with this invention, projects and extends a gas permeable mold&#39;s sprue, and through which molten metal is introduced into the vacuumized gas permeable mold&#39;s cavity by gravity means. The top closure further includes adjustable hanger devices that connect the gas permeable mold to the top closure and which provides the means to adjust the gas permeable mold&#39;s sealing properties from outside the vacuum chamber by lowering or raising the mold up to form a tighter abutment between the sealing surfaces. 
     The invention discloses a sprue-forming device that when combined with a disposable foundry pattern and subsequently invested together as a unit forms a gas permeable mold in accordance with this invention. Said sprue-forming device furnishes uniform sprue, mounting, and sealing features for all sizes, shapes, and mold types pertinent to this invention. The preferred embodiment of said sprue-forming device forms a structural framework that surrounds the disposable foundry pattern and carries all the weight of the mold through every step of the foundry process. Said sprue-forming device is comprised of a sprue former ring, a base plate, and other structural elements such as tie wire and connecting rods or other suitable equivalents depending upon the amount of support required for a particular application. A sprue former ring is required to be sprued and invested as an integral part of all molds utilized in this process. The sprue-forming device may be structured to the most appropriate form conditional to the disposable foundry pattern and mold type being cast. 
     The sprue former ring is comprised of a sprue former flask a sealing surface, and mounting device all axially arranged and permanently fixed together. Said sprue former flask is a hollow metal cylinder, flanged at one end to form the sprue former ring&#39;s sealing surface. Set-up of a sprue former ring to a disposable foundry pattern involves attaching a wax, or it&#39;s equivalent, sprue pattern to the disposable foundry pattern. Said wax sprue pattern is long enough to extend concentrically all the way through the sprue former flask where it is invested in place. After burning out the mold, a sprue is left through the investment that now lines the sprue former flask and through which, molten metal gravity flows into the mold cavity during casting. The exterior surface of the sprue former flask and its associated sealing surface must be kept clean and free of investment before setting the mold up in the vacuum chamber in order for the sprue former flask to slip freely through the top closure&#39;s center hole and to allow tight contact between sealing surfaces. 
     The aforementioned base plate, when used as part of a sprue-forming device, is positioned at the opposite end from the sprue former ring and held in place by connecting rods secured by winding tie wire around each juncture of a connecting rod with a base plate anchor bar and sprue former ring. The base plate includes a handle mount and a concentrically located drain port. A handle may be threaded into said handle mount to provide for convenient handling of unwieldy molds. The drain port serves two functions. First, it provides alignment for a wax coated sprue pipe when the disposable foundry pattern is sprued, and secondly, during the pattern elimination phase it provides a conduit for melting pattern material to flow out of the mold cavity. Alignment of the sprue pipe with the base plate is assured by threading an alignment plug through the drain port. The alignment plug is a long bolt that is threaded for part of its length and smoothed to a smaller diameter for the remainder of its length. The outside diameter of the smooth part of the alignment plug allows that portion of the alignment plug to slip fit into the inside of the wax coated sprue pipe holding it firmly in place until investing is complete. After investing is complete, and when the investment is thoroughly dry, the alignment plug is removed to facilitate pattern elimination. After burnout and before setting the gas permeable mold up in the vacuum chamber a drain plug must be threaded into the drain port to seal the mold cavity. 
     While the invention is described in connection with certain described embodiments and applications, it will be understood that it is not intended to limit the invention to these particular descriptions or uses. On the contrary, it is intended to cover all alternatives, modifications, and equivalent arrangements as may be included within the scope of this invention. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG.  1 —Is an isometric view of a typical vacuum casting apparatus containing a ready to cast gas permeable mold in accordance with the present invention. 
     FIG.  2 —Shows a typical embodiment, isometric view, of a large disposable foundry pattern sprued to a sprue-forming device and ready to invest in accordance with the present invention. 
     FIG.  3 —Shows a simplified cross sectional view through a vacuum casting apparatus containing a large gas permeable mold being cast in accordance with the present invention. 
     FIG.  4 —Is an isometric view of an alternative embodiment of sprued and ready to invest small disposable foundry pattern in accordance with the present invention. 
     FIG.  5 —Is an enlarged cross sectional view showing base plate to disposable foundry pattern connections and handle installation ready for investing in accordance with the present invention. 
     FIG.  6 —Is an enlarged cross sectional view showing drain plug installation in drain port of a burned out and ready to cast gas permeable mold in accordance with the present invention. 
     FIG.  7 —Is an isometric view showing a typical flask mold ready for investing in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred embodiment of the present invention is illustrated in FIG. 1 (isometric view) and FIG. 3 (cross sectional view) showing a loaded and sealed vacuum casting unit  46  which when vacuumized is ready to accept molten metal into gas permeable mold  10  through mold sprue  86 . Vacuumization is accomplished by activating a vacuum device (not shown) connected to vacuum pump connecting port  70 . 
     A ready to cast gas permeable mold  10 , in accordance with the present invention, is conjoined with a top closure  48  (FIG. 3) by inserting sprue former flask  22  through center hole  50  and seating sealing surface  24  against sealing surface  62  by physical abutment and latching the two parts together by attaching mounting devices  26  to adjustable hanger devices  56 . Sealing surface  24  must fit flat and tight against sealing surface  62  to ensure proper vacuum pressure. A compressible seal  54  may be placed between the two sealing surfaces should either sealing surface become warped or damaged from repeated use or rough handling. 
     Top closure  48  along with it&#39;s conjoined gas permeable mold  10  is installed as a unit into vacuum tank  64  and secured with latching devices  60  which squeezes compressible seal  74  thereby resulting in a tight seal between top closure and vacuum tank. Air is evacuated from the vacuum chamber  66  when vacuum device (not shown) is activated. Further sealing, if needed, is accomplished by turning adjustable hanger devices  56  upward to raise the ready to cast gas permeable mold  10  and it&#39;s sealing surface  24  up tighter against sealing surface  62  inside the vacuum chamber  66 . Set up is complete and ready to cast when vacuum gauge  68  indicates negative pressures exist in vacuum chamber  66 . Complete vacuum is not achievable at this point in the operation due to the vacuum drawing outside air into the mold cavity  80  through the exposed mold sprue  86  and thus through the gas permeable mold wall  84 , which stabilizes low pressure in the vacuum chamber. Once no more air leaks are detected, molten metal can then be gravity poured into mold sprue  86 . As molten metal  78  rises in mold cavity  80  (FIG.  3 ), any trapped air  82  is evacuated from mold cavity  80  through mold wall  84  of gas permeable mold  10 . At the same time, vacuum pressure holds the molten metal tight against the inside wall of the mold cavity where it is held in place until solidification of the metal is complete. Vacuum pressure is maintained for a brief time after pouring to allow the molten metal to completely solidify in place thereby lessening potential shrinkage problems in the cast article (not shown). Accidental spillage of molten metal may occur while pouring due to misalignment of the crucible (not Shown) or because of over filling the sprue till run over occurs. Such waste metal flows into spill guard  52  where it is contained, thereby preventing molten metal from flowing into the work area and endangering those in the workplace. 
     The cast mold is removed from vacuum chamber  66  by opening vacuum release valve  72  and relieving vacuum pressure, then releasing latching devices  60  in order to lift top closure  48  along with the cast mold clear of vacuum tank  64  using lifting devices  58  if needed. The cast mold is then separated from top closure by disengaging adjustable hanger devices  56  from mounting devices  26 . Compressible seal  54 , when used, is removed from sprue-forming device  12  and saved for future use. 
     The cast article (not shown) is retrieved from its mold by breaking away the ceramic shell investment  94  coating the cast article and the sprue-forming device. The investment is broken up and chipped off the sprue-forming device whereby all metal parts are sandblasted clean and reused. The cast article is further processed according to conventional foundry practices. 
     FIG. 2 illustrates a preferred method of sprueing a large disposable foundry pattern  14  requiring maximum structural strength and support. Metal components comprising sprue-forming device  12  form a cage that surrounds a disposable foundry pattern. Said cage supports and carries all the weight of the gas permeable mold before and after casting. Molds requiring maximum structural strength are supported by conjoining base plate  28  to sprue former ring  20  with connecting rods  42 . Said base plate forms a platform when suspended from sprue former ring  20  by connecting said base plate to said sprue former ring with connecting rods  42  and firmly securing all joints with tie wire  44  or suitable equivalent. Said base plate is comprised of a drain port  30 , a handle mount  32  (FIGS. 5, and  6 ,), and base plate anchor bars  38 . Said base plate anchor bars along with sprue former ring  20  provide solid attachment points for connecting rods  42 . 
     Molds deemed strong enough to require minimal support need only have tie wire  92  (FIG.  4 ), or suitable equivalent, looped loosely around (but not touching) the disposable foundry pattern  98  and secured to sprue former ring  20 . Said tie wire, when invested as an integral part of a gas permeable mold, performs the same function for a gas permeable mold as the aforementioned cage. Investing is done according to conventional foundry methods. 
     Disposable foundry pattern configuration dictates mold requirements such as whether or not a mold is considered more suitable for a large mold set-up as FIG. 2 illustrates, a small mold set-up as shown in FIG. 4, a flask type pourable investment mold as shown in FIG. 7, or whatever mold or mold type may be used in accordance with the present invention. A sprue former ring  20  (FIGS. 2,  3 ,  4 , and  7 ) is required to be invested as an integral part of all gas permeable molds applicable to the present invention. Said sprue former ring incorporates a sprue former flask  22 , a sealing surface  24 , and a mounting device  26 . Sprue former ring  20 , when invested as an integral part of a gas permeable mold, provides the means for said mold to be mounted and sealed within the confines of a vacuum casting unit. 
     The present invention discloses a long sprue pipe  16  (FIG. 2) and a short sprue pipe  90  (FIGS.  4  and  7 ). Said sprue pipes provide rigid support for a central sprue to which disposable foundry patterns are attached in order to create a gas permeable mold. Said sprue pipes may be any desired length but need not be any longer than is necessary to accommodate a particular disposable foundry pattern. Coating a sprue pipe with wax, or suitable equivalent, and shaping a wax sprue pattern  76  is the first step towards setting up a typical embodiment of a sprue-forming device in accordance with the present invention. The wax coated sprue pipe is conjoined with sprue former ring  20  by concentrically arranging the smaller diameter sprue pipe inside the larger diameter sprue former flask  22  (FIG. 3) and filling the separating space with a suitable mold material, such as pourable investment  88  (FIGS. 2,  3 ,  4 , and  7 ). When positioning the wax coated sprue pipe inside the sprue former ring and prior to pouring investment, said sprue pipe may be adjusted lengthwise either way in the sprue former ring to allow more or less of the sprue pipe to be used for the actual sprue. Too long a sprue pipe for a particular disposable pattern is unwieldy and may impede the investing process, which is best avoided by simply selecting a shorter pipe. The two parts become firmly joined together once the pourable investment, or it&#39;s equivalent, has completely set. 
     Once a sprue former ring  20  and an appropriate sprue pipe  16  or  90  has been joined together with pourable investment  88  it is ready to attach a disposable foundry pattern using the usual accepted foundry methods for attachment. Only a single sprue attachment is usually all that is required for the present invention, however, for strength purposes it may be practical to add additional gates to fragile details that may be at risk for damage during the investing process. At this time, in the process, it must be determined which embodiment of the sprue-forming device is most appropriate for the particular disposable foundry pattern being cast and set-up accordingly. Investing, drying, and burnout of the mold is done according to conventional foundry practices. 
     The sprue pipe is a convenient handling device during the sprueing, investing, and drying process but must be removed after investing is complete and before burnout. The sprue pipe is removed by applying sufficient heat to the sprue pipe to soften it&#39;s wax coating thereby allowing the sprue pipe to be easily withdrawn from the mold sprue. 
     A large gas permeable mold set-up requiring all components of the previously described sprue-forming device  12  includes a drain port  30  (FIGS. 5, and  6 ) as an integral part of base plate  28 . Said drain port performs a dual function in the course of utilizing said sprue-forming device in accordance with the present invention. In one function, alignment plug  36  (FIG. 5) is threaded through drain port  30  and inserted into said sprue pipe. The outside diameter of the alignment plug is the same as the inside diameter of the sprue pipe thereby allowing the two parts to slip fit together where they hold fast, in proper alignment, until connecting rods  42  can be positioned and used to securely join the base plate  28  to the sprue former ring  20  along with it&#39;s conjoined sprue pipe and attached disposable foundry pattern(s). The alignment plug  36  is left in place throughout the investing and mold drying process to provide greater rigidity to the assembly of parts comprising sprue-forming device  12  (FIGS. 2, and  5 ). Said alignment plug is removed after investing, and before burnout to leave the drain port open in order for melted pattern material to drain freely from mold cavity  80  during burnout. A drain plug  34  (FIG. 6) must be threaded into said drain port  30  before the mold can be cast. 
     An alternative feature for sprue pipes involves drilling a plurality of drain holes  18  (FIG. 5) in sprue pipe to allow melted pattern material to flow inward into the sprue pipe and then out through drain port  30 . Providing melted mold material a place to drain reduces the chance of mold cracking due to mold material expansion during pattern elimination. 
     Base plate  28  provides a convenient means for handling whenever set-up as part of the sprue-forming device. A handle mount  32  (FIGS. 5, and  6 ) allows a handle  40  (FIGS. 2, and  5 ) to be connected to said base plate by threading said handle into said handle mount. The handle and the sprue-forming device on the other end of the mold provides easy handling for two people to handle a large mold and also allows a mold to be laid across two saw-horses or similar support that allows the mold to be rolled to aid in investing. Said handle is removed before placing mold in oven (not shown) for disposable pattern elimination and burnout. Mold requirements not using drain port provisions (FIG. 4) in accordance with the present invention are invested and burned out in accordance with conventional foundry practices. 
     An alternative embodiment (FIG. 7) shows a typical flask type investment mold set-up with sprue former ring  20  along with short sprue pipe  90 , and with sprued disposable foundry pattern  106  in place and ready to introduce pourable investment (not shown) into flask  100 . Generally accepted methods of investing a flask type investment mold requires a sprued disposable foundry pattern be mounted sprue down to a flat plate  102  or other suitable sprue device. A flask is placed over the pattern and sealed down to said flat plate with wax  96  or other suitable equivalent. Said wax seals the flask down to the flat plate and prevents leakage of the pourable investment as it is poured into the flask. Vent holes  104  equalize and distribute vacuum pressure more evenly to all sides of the flask and throughout the mold when the mold is subjected to vacuum pressure and ready to cast. Tape or other suitable equivalent (not shown) is used to seal the vent holes until pourable investment has thoroughly set. 
     Securing one end of tie wires  108  or suitable equivalent, to various locations around the sprue former ring  20  and allowing long ends of said wires to dangle loosely down into the space separating the flask from the pattern provides sufficient bonding of the sprue former ring to the pourable investment (not shown) once said investment is thoroughly set. Investing and burn out of said flask type mold is done according to conventional foundry practices. 
     SEQUENCE LISTING 
       10  Gas permeable mold 
       12  Sprue-forming device 
       14  Disposable foundry pattern 
       16  Long sprue pipe 
       18  Drain holes 
       20  Sprue former ring 
       22  Sprue former flask 
       24  Sealing surface 
       26  Mounting device 
       28  Base plate 
       30  Drain port 
       32  Handle mount 
       34  Drain plug 
       36  Alignment plug 
       38  Base plate anchor bar 
       40  Handle 
       42  Connecting rod 
       44  Tie wire 
       46  Vacuum casting unit 
       48  Top closure 
       50  Center hole 
       52  Spill guard 
       54  Compressible seal 
       56  Adjustable hanger device 
       58  Lifting device 
       60  Latching device 
       62  Sealing surface 
       64  Vacuum tank 
       66  Vacuum chamber 
       68  Vacuum gauge 
       70  Vacuum pump connecting port 
       74  Compressible seal 
       76  Wax sprue pattern 
       78  Molten metal 
       80  Mold cavity 
       82  Trapped air 
       84  Mold wall 
       86  Mold sprue 
       88  Pourable investment 
       90  Short sprue pipe 
       92  Tie wire 
       94  Ceramic shell investment 
       96  Wax 
       98  Disposable foundry pattern 
       100  Flask 
       102  Flat plate 
       104  Vent hole 
       106  Disposable foundry pattern 
       108  Tie wire