Abstract:
A process and system for purifying molten metal utilize a filter holder which has a spring-loaded clasp for releasably engaging a filter component. The filter holder is particularly well-suited to engaging and releasing a fabric filter having a frame surrounding its perimeter. The process involves releasing the filter component from the filter holder by inserting the filter component into a tapered recess of a mold composite. Another mold composite is mated over the filter component and forms a molten metal flow path, across which the filter is disposed. Then, a feed stream of molten metal is introduced to the flow path and through the filter. An advantage of the filter holder is that the spring tension within the filter holder can be adjusted to allow secure travel before insertion into the tapered recess and easy release of the filter component after insertion is complete.

Description:
FIELD OF INVENTION 
     The present invention relates to the filtration of ferrous and nonferrous molten metal. More particularly, the invention relates to a system and method for purifying molten metal using a filter holder, especially adapted for engaging and releasing a fabric filter surrounded by a supporting frame. 
     BACKGROUND OF THE INVENTION 
     Molten metal filtration is the process used to obtain high quality metals suitable for casting. By removing undesirable impurities from the molten metal, the filtration process improves the products of the casting operation. Both mechanical and physical properties of casting products are enhanced by filtration. 
     In general, molten metal raw feed contains undesired impurities from sources such as particles of refractory from the lining of a vessel that contains the molten metal, alumina as a byproduct of deoxidization and reoxidation, fragments of slag or other insoluble impurities. Upon solidification of the cast product, these impurities adversely affect product properties such as surface finish, ease of drawing and forming, ease of welding, and strength. Therefore, a primary objective of the foundry industry is to remove impurities from molten metal raw feed by filtration. Filtration of the molten metal occurs prior to the casting operation and after the melting operation. 
     The melting operation involves melting metal so that it may be used in the casting operation. The metal is melted in a furnace wherein the constituent components are added in the form of unmelted scrap and/or refined virgin metal, deoxidizing agents in various forms (solid and gaseous or a combination of both) and alloying elements. Gases and low density solids tend to migrate to the surface of the melt where they either effervesce or float in combination with partially and completely solidified oxides commonly known as slag and dross. The higher density impurities in the melt tend to remain in the liquid phase of the metal, or melt, as the fluid flow convection currents are generated within that melt by the heat applied by the furnace. 
     During the melting operation, the furnace functions as a holding vessel for the metal while it is being melted. The furnace may also be used to refine the metal depending on what type of metal is being processed. Metal is refined when gases as well as low density metals migrate to the surface. The molten metal is transferred to another vessel, such as a ladle, to be transported to the molding operation. An alternative method would be to provide a direct flow path from the furnace to the casting operation. In both instances, prior to the casting/molding operations, the molten metal is routinely filtered. 
     The filtering system requires an efficient process to prevent solidification of the metal. Moreover, the filter medium must be suitable to withstand high melting temperatures and chemical reactions. Furthermore, the filter component must maintain its structural integrity. Lastly, the filter medium must be capable of either entrapping or preventing the flow of impure solids, liquids, and semi-liquids, all of which are non-metallic or intermetallic, either by chemically reacting with such impurities and/or by mechanically preventing the flow of such impurities through the filter medium, while still permitting and facilitating the flow of the molten metal through the filter. 
     Ceramic foam filters are commonly used in filtration operations by foundries. Ceramic filters reduce the number of castings that must be scrapped due to the presence of impurities and therefore improve casting cleanliness. Generally, ceramic filters are incorporated into the casting operation upstream of the mold cavity. The ceramic filters remove dross, slag and other impurities found in molten metal prior to the casting operation. The most common type of ceramic filters comprise hard-fired cellular ceramic structures and rigid reticulated ceramic foam. The ceramic filters, due to their mass, often chill the first molten metal that reaches the filter. Therefore, they require coarse openings to ensure reliable passage of the molten metal. Cellular extruded ceramic filters therefore rely on the formation of a filter cake on the upstream side to remove smaller inclusions that would tend to pass through the openings of the filter. Thus, ceramic foam filters are dependent on the formation of the filter cake to remove smaller inclusions. Due to the massive size and filtering problems that are inherent with the ceramic filters, fabric filters present a new opportunity for the casting operation. 
     At the present time, there is no commercially available filtration system which allows a continual replacement of a filter component which utilizes a fabric filter medium. A filter component having a fabric filter medium that is compact and effective during filtration would be desirable. It would be even more desirable to utilize such a filter in an automated, continuous filtration system. 
     SUMMARY OF THE INVENTION 
     In view of its purposes, the present invention provides a process for purifying molten metal comprising first engaging a filter component with a filter holder, wherein the filter holder has a spring-loaded clasp for releasably engaging the filter component. Then, the filter component is released from the filter holder by inserting the filter component into a tapered recess of a first mold composite. A second mold composite is mated with the filter component and with the first mold composite, to form a molten metal flow path, across which the filter component is disposed. A feed stream of the molten metal is introduced to an inlet conduit forming a down sprue in fluid flow communication with the molten metal flow path, such that the molten metal flows through the filter component. 
     The present invention also provides a system for purifying molten metal comprising a first mold composite having a tapered recess, a filter component, and a filter holder comprising a spring-loaded clasp for engaging the filter component and releasing the filter component into the tapered recess. The system also includes a second mold composite mated with the first mold composite to form a molten metal flow path across which the filter component is disposed and an inlet conduit forming a down sprue in fluid flow communication with the molten metal flow path for introducing molten metal into the molten metal flow path. 
     Another aspect of the present invention is an apparatus for use with a molten metal filtration system comprising a filter component comprising a fabric medium for filtering molten metal and a frame for supporting the fabric medium, and a filter holder having a spring-loaded clasp for releasably engaging the frame. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is best understood from the following detailed description when read in connection with the accompanying drawings. 
     FIG. 1 is a top plan view of the first and second mold composites mated together. The filter component, which is positioned between the mold composites, is shown in hidden view (i.e., dashed lines). 
     FIG. 2 is a sectional view of the second mold composites taken along is line  2 — 2  of FIG. 1 
     FIG. 3 is a sectional view of the assembled first and second mold composites taken along line  3 — 3  of FIG.  1 . 
     FIG. 4 is a top plan view of the first mold composite with the filter component inserted therein. 
     FIG. 5 is a sectional view showing the tapered recess of the first mold composite taken along line  5 — 5  of FIG.  4 . 
     FIG. 6 is a partial sectional view of the first composite and the filter holder, which engages the filter component. 
     FIG. 7 is a front plan view of the filter holder, shown in FIG.  6  and rotated 90°, mounted onto the core mask. 
     FIG. 8 is a sectional view of the filter holder and the spring-loaded clasp. 
     FIG. 9 is a sectional view of the clasp of the filter holder. 
     FIG. 10 is a top plan view of the top plate of the filter holder. 
     FIG. 11 is a bottom plan view of the base plate of the filter holder. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings in detail, wherein like reference numerals represent like elements throughout the several figures, FIG. 1 shows a top plan view of an imprint defined by a first mold composite  15 A and a second mold composite  15 B. The imprint is formed by individually compressing each mold composite against a patterned print whose shape corresponds to that of the desired imprint. Mold composites are typically sand and a resin, and patterned prints can be any metal or plastic, although metals are preferred due to their relative hardness compared with plastics. Upon mating in a known way, the first mold composite  15 A and second mold composite  15 B define a molten metal flow path. The dashed lines within the mold cavity represent a filter component  17 . The filter component  17  comprises a filter medium  18  and a filter frame  19 . The inner most dashed line represents the outer periphery of filter medium  18  while the outer periphery of filter frame  19  is displayed by the outermost dashed line. Sand traps  20  are further indentations in the mold composites extending outward from line  2 — 2  and are formed by protrusions in the patterned print. Sand traps  20  are used to trap sand that is displaced during the molten metal filtration process, in a known way. 
     FIG. 2 shows a sectional view of second mold composite  15 B along line  2 — 2 . The sectional view of filter component  17  comprising filter medium  18  and filter frame  19  is also shown. The filter component  17  is located at the center of the mold composite cavity  16 , which can be formed by compressing a mold composite against a patterned print, as discussed above, and fits snugly in a tapered recess  14 . An inlet conduit  22  defines a down sprue  24  and is positioned above the mold composite cavity  16  and the filter component  17 . The molten metal enters the inlet conduit  22  to commence the filtration process. Thus, down sprue  24  and mold composite cavity  16  form part of the molten metal flow path. The down sprue  24  defined by inlet conduit  22  directs the molten metal into the molten metal cavity  16 . After completing the passage through filter medium  18  the molten metal exits the cavity through the outlet  30 . As shown in FIG. 2 only, a casting passage  51  directs purified molten metal to a casting area  53 . Casting passage  51  is preferably formed in one of the mold composites. In addition, one or both of the mold composites may define one or more casting areas, although only one casting area is shown. 
     FIG. 3 displays a cross sectional view of the assembled mold composites  15 A and  15 B and filter component  17  taken along line  3 — 3  of FIG.  1 . The line  28  represents the mating line where the first and second mold composites are joined in a known manner. Mold composites  15 A and  15 B are mated with one another to provide a closed molten metal flow path in a conventional manner. In FIG. 3, sand traps  20  are shown to emerge from the ends of the tapered recess  14 . The sand trap  20  contains any sand flow during the filtration process. To ensure that the integrity of the frame  19  is maintained during the filtration process, there must be a snug fit at the intersection of the top and bottom of frame  19  with mold composites  15 A and  15 B. This close tolerance can be achieved by closely matching the height of frame  19  with the height of the patterned print at a region corresponding to intersection region  55 , where the mold will intersect the frame. Preferably, for reasons discussed below, the it height of the patterned print at these regions is slightly less than (e.g., on the order a few thousandths of an inch less than) the height of frame  19 . For example, in one embodiment, the height of the patterned print in regions corresponding to intersection region  55  is about 140 thousandths of an inch and the height of frame  19  is about  155  thousandths of an inch. As shown again in FIG. 3, the inlet conduit  22  forms down sprue  24  disposed above the mold composite cavity  16 . 
     A top plan view of the imprint formed in the first mold composite  15 A with the filter component  17  engaged therein is shown in FIG.  4 . The filter component  17  comprising the filter medium  18  and filter frame  19  is securely inserted into the tapered recess of the first mold composite  15 A. The filter medium  18  is preferably a fabric filter composed of refractory filaments or yarn comprising alumina, fiberglass, silica or a combination thereof. One such fabric filter is sold under the trademark SILTEMP® by AMETEK, Inc. of Paoli, Pa. The filter medium  18  is firmly supported by a frame  19  around its periphery. The frame is preferably composed of commercially available chipboard, cardboard or a mixture thereof. The filter medium  18  can be attached to frame  19  by any conventional means, such as gluing. 
     A sectional view of the tapered recess formed in first mold composite  15 A and the filter component  17  taken along line  5 — 5  of FIG. 4 is represented in FIG.  5 . Line  28  depicts the location in which the second mold composite  15 B will be mated with the first mold composite  15 A. The filter component  17  is inserted within the tapered recess  14  of the first mold composite. As shown most clearly by FIG. 5, the tapering of tapered recess  14  refers to its change in height from height h 1  near line  28  to height h 2  near region  55 . Preferably, the change in height is slight, for example about 30 to 90 thousandths of an inch (and in one exemplary embodiment 60 thousandths), and the angle of tapering is also slight. The height h 2  need only be sufficient to allow an easy insertion of the filter component  17 . Sand trap  20  is a further indentation in the mold composite. In a preferred embodiment as discussed above, the thickness of the frame  19  is slightly greater than height h 2  of the tapered recess  14  of mold composites  15 A and  15 B. This allows the filter component  17  to maintain a secure fit with the mold components upon insertion and during the filtration process. 
     FIG. 6 depicts a partial sectional view of the first mold composite  15 A as the filter holder  34  is inserting filter component  17  into tapered recess  14 . The filter holder  34  is located within a recessed area of a core mask  32 . The core mask  32  is part of a conventional mold making machine (not shown), such as a DISAMATIC™ mold making machine commercially available from Georg Fischer Disa of Switzerland. The mounting plate  40  of the filter holder  34  is mounted onto the core mask  32  by at least one mounting screw  42 . In a preferred embodiment, the filter holder  34  is composed of aluminum, although any metal or hard plastic is suitable. The filter component  17  is positioned between a top plate  47  and a bottom plate  49  of the filter holder  34  (also shown in FIG.  7 ), which forms a spring loaded clasp  45  for engaging the filter component  17  and for releasing the filter component  17  into the tapered recess  14 . Spring-loaded clasp  45  comprises the top plate  47 , the bottom plate  49 , screws  36 , and a spring  37 . Although only one screw  36  is apparent in this sectional view, more than one screw can be used. 
     As shown in FIGS. 6 and 7, top plate  47  has a first portion  57  adapted to contact bottom plate  49  and a second portion  58  defining, with the bottom plate, a clasp recess  35  adapted to receive the filter component  17 . The delineation between first portion  57  and second portion  58  define the width w of the clasp recess  35 . Preferably, width w is essentially equivalent to the width of the frame. Screw  36 , which has a head with a bearing surface, extends through top plate  47  and bottom plate  49  and engages a nut  38  abutting against bottom plate  49 . One or more washers (not shown) may be used in conjunction with this arrangement. A spring  37  extends between the bearing surface of the head of the screw  36  and top plate  47  to exert a force on the top plate towards bottom plate  49 . In a known manner, the force can be adjusted by adjusting screw  36 . 
     FIG. 7 depicts a front plan view of the filter holder  34  and the core mask  32 . The mounting plate  40  of the filter holder  34  is securely mounted onto the core mask by the mounting plate screws  42 . Although two mounting plate screws  42  are shown in this diagram, one screw or more than two screws are also acceptable. The top plate  47  and bottom plate  49  are joined with two spring loaded screws  36  and adjoining nuts  38 , as discussed above. According to the invention, the filter component  17  is placed in the clasp recess  35  between the top  47  and bottom  49  plates. 
     The spring loaded screw  36  coupled with the nut  38  join the top plate  47  and bottom plate  49  are also shown in FIG.  8 . This configuration forms the clasp  45  of the filter holder  34  apparatus. As shown in FIG. 8, spring  37  extends between and bears against the bearing surface of the head of the screw  36  and a bearing surface of top plate  47  formed below the top surface of top plate  47 . Alternatively, the spring may bear against the bearing surface of the head of the screw  36  and the top surface of top plate  47  (as shown in FIG.  6 ). The relative diameter of the screw  36  and the diameter of the opening through which the screw extends are dictated by the particular needs of the application. To avoid lateral movement of the filter component  17 , the difference in size between the diameter of the screw  36  and the diameter of the opening can be designed to below, such as about {fraction (1/32)} nd   or {fraction (1/64)} th   of an inch. 
     The clasp is more apparent in the cross sectional view of FIG.  9 . The clasp  45  defines clasp recess  35  between the top plate  47  and bottom plate  49 . FIG. 9 clearly depicts the chamfered edges  60  and  61  of the top plate  47  and bottom plate  49  of the filter holder  34 . In a preferred embodiment, the edges  60  and  61 , which are adjacent clasp recess  35 , of both top plate  47  and bottom plate  49  are chamfered as shown. However, the system would also function with at least one edge chamfered or with no chamfered edge. Chamfered edges facilitate the placement of the filter component into the clasp recess. The mounting plate  40  is used to mount the filter holder  34  onto the core mask  32 . 
     FIG. 10 represents a top view of the filter holder  34 . In the preferred embodiment, mounting plate  40  is positioned against a core mask  32 . The openings for the mounting plate screws  42  are used to mount the mounting plate  40 . Two spring loaded screws  36  entering through the top plate  47  and exiting the bottom plate  49  are used, as discussed above. However, one spring loaded screw  36  can also be used to form the clasp  45  of the filter holder  34 . The dashed lines within the top plate  47  depict delineation between first portions  57  and second portion  58  of top plate  47 , which define, along with bottom plate  49 , the clasp recess  35 . As shown, these dashed lines are curved at their edges to form chamfered edges  62  around the screws  36 . Chamfered edges  62 , which extend perpendicular to the width w of clasp recess  35 , serve the same function as chamfered edges  60  and  61 , which extend along the width w of the clasp recess. 
     FIG. 11 shows the mounting plate  40  and bottom plate  49  integrally formed therewith. A linear configuration forming openings for tightening and loosening nuts  38  is shown, although other configurations (or no configuration) need be present. 
     This process for purifying molten metal using a filter holder of the present invention is accomplished in a series of steps. First, the filter component  17  is engaged with the filter holder  34 , typically by being manually inserted into the clasp recess  35 . Upon formation of the first mold composite  15 A, the filter holder  34  inserts the filter component  17  into the tapered recess  14  of the first mold composite  15 A, such as by causing core mask  32  to move into place, as is automatically accomplished in automatic mold making machines. As mentioned above, the thickness of the filter component  17  is greater than height h 2  of the tapered recess  14  of the first mold composite  15 A. This arrangement causes a frictional engagement between first mold composite  15 A and the frame  19 . The frictional engagement between the first mold composite  15 A and the filter component  17  exert a force far greater than the spring force of the clasp  45  and any frictional force between clasp  45  and the frame  19 . This results in the release of the filter component  17  by the clasp  45 . Subsequently, core mask  32  and thus the filter holder  34  retract from the first mold composite  15 A and the second mold composite mates with the first mold composite defining the molten metal flow path. Mating the two mold composites includes first aligning the tapered recess of second mold composite  15 B with the filter component, in a known manner. The filter holder  34  is then manually (or automatically) fed again with a new filter component to continue the process. 
     The filter medium  18  within the filter component  17  is available in a variety of dimensions and mesh sizes. The dimensions of the filter component  17  are dictated by the mold making machine with which the component is used and the mesh sizes are a function of the impurities present in the metal, the type of metal, and other system parameters, such as flow rate. The selection of the suitable mesh size is well known to those skilled in the art. Also, the dimensions of the filter holder  34  can altered to accommodate the different sizes of the filter component  17  by adjusting at least one of the top plate  47  or bottom plate  49 . The dimensions and mesh size of the fabric filter medium  18  can be adjusted to achieve the desired flow rates for both ferrous and nonferrous metals (e.g., white iron, gray iron, malleable iron, compacted graphite iron, ductile iron, carbon steel, stainless steel, brass, bronze, aluminum). In one embodiment, white iron is filtered using a fabric filter medium  18  with dimensions of 2×2 inches at a flow rate of 4.20 lbs./sec. in a DISAMATIC mold making machine. The mesh size of the fabric filter medium is 1.0 mm×1.0 mm. 
     Although illustrated and described above with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention.