System and method for purifying molten metal

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.

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.

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 15A and a second
 mold composite 15B. 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 15A and second mold
 composite 15B 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 15B 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 15A
 and 15B 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 15A and 15B 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 15A
 and 15B. 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 15A 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 15A.
 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.RTM.
 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 15A
 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
 15B will be mated with the first mold composite 15A. 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.sub.1 near line 28 to height h.sub.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.sub.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.sub.2 of the tapered recess 14 of mold
 composites 15A and 15B. 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 15A 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.TM. 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 1/32.sup.nd or 1/64.sup.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 15A, the filter holder 34 inserts the filter component 17
 into the tapered recess 14 of the first mold composite 15A, 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.sub.2 of the tapered
 recess 14 of the first mold composite 15A. This arrangement causes a
 frictional engagement between first mold composite 15A and the frame 19.
 The frictional engagement between the first mold composite 15A 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 15A 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 15B 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.times.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.times.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.