MULTI-ELEMENT CERAMIC FILTER FOR MOLTEN METAL

Abstract: A filter for molten metal prior to casting in a mold has a plurality of ceramic elements in contact with each other, each element (11) having numerous apertures, e.g. slots (13) separated by ribs (12), and the contacting portions (14) of the elements are arranged, so that the apertures (13) of adjacent elements are staggered so that the metal flowing out of one aperture passes through the space (16) between adjacent elements before flowing through aperture of the next element; thus fine filtration is achieved. The distance (H) between the ribs of different elements is preferably less through the widths (W) of the aperture, e.g. H < 0.8 mm. The elements may be flat plates rectangular in shape (Figs 1-3) or circular; or frusto-conical or tubular; in the latter cases they nest one inside each other. The apertures may be rows of circular holes (Fig 21-22). Successive elements may be identical or non-­ identical as regards their arrangement and size of apertures. Elements can be made by injection molding and sinter­ ing of a ceramic, e.g. silica, and a binder; preferably they are microporous.

Description: This invention relates to ceramic pouring filters for use with molten metal, for instance, in the casting of molten metals, such as nickel base high temperature alloys used in casting of parts of turbine blades, iron, aluminum or stainless steel. It is customary to pass the molten metal through a pouring filter before it enters the mold cavity. The commercially available filters for such uses are of various types. Cellular ceramic filters, which have a honeycomb type of construction made up relatively deep cells of square cross-section, are described in the article by Day in Modern Casting , April, 1984, pages 16-18. Among the other types of commercial filters are ball filters, bonded particles, bonded or fused foam filters and fibrous filters. A ball filter is essentially a relatively deep box filled with balls, usually of about 2 cm diameter, of alumina or similar ceramic material. A bonded particle filter is made by bonding together grains of a ceramic material, which may be fabricated from crystalline aluminum oxide or silicon carbide. One type of foam filter is a porous foam of phosphate-bonded alumina or zircon. A foam filter may be prepared by a method which involves impregnating an organic cellular foam (such as polyurethane foam) with a dispersion of ceramic ingredients, followed by firing, during which the organic foam is burned away. DE-A-2848005 (and its corresponding GB-A,B-2108427. etc) discloses ceramic filters for filtering molten metal, consisting of a celluar ceramic plate with a plurality of interconnecting voids surrounded by a web of fused ceramic; the filter is useful for removing particles from molten aluminium alloy to allow a non-turbulent flow down the sprue. Fibrous filters may be of woven fiberglass or of non-woven ceramic fibers. US-A-2835007 discloses a filter or screen in the form of a flat plate having a plurality of perforations, and composed of several layers of mica. According to the present invention we provide a filter for filtering molten metal, comprising a plurality of shaped refractory ceramic filter elements, each element having a plurality of apertures for the passage of flowing molten metal therethrough, and the elements are so arranged that the apertures of adjacent elements are staggered so that at least the major part of each aperture opens against a solid portion of the filter area of the next element, the filter areas containing the plurality of apertures being spaced from each other by a distance not greater than the width of the apertures so that the molten metal is able to flow freely in the spaces between the successive elements, whereby the flowing streams of metal follow a tortuous path through the elements of the filter. The apertured ceramic elements are closely spaced, with their apertures in staggered relationship. The filter is strong, economical to manufacture, readily inspectable to allow detection of hidden internal defects present in other filters, and provides controlled removal of fine particles of undesired impurities at good flow rates. The invention is illustrated in the accompanying drawing in which: Fig. 1 is a cross-sectional view of a stack of filter plates of the invention. Fig. 2 is a three-dimensional view of a filter plate of the stack. Fig. 3 is a cross-sectional view of a stack of plates having asymmetric positioning means. Fig. 4 is a plan view of a portion of a filter plate having curved slots. Figs. 5 and 6 are cross-sectional views of portions of stacks of (differently) modified filter plates. Fig. 7 is a cross-sectional view, taken in a plane running along an upper slot, of a portion of a stack comprising another embodiment. Fig. 8 is a plan view of a portion of a filter plate having connected ribs. Fig. 9 is a plan view of a portion of a circular filter plate. Fig. 10 is an exploded three-dimensional view of a conical filter and pouring cup. Fig. 11 is a cross-sectional view of a conical filter element. Fig. 12 is a three dimensional view of a pyramidal filter element. Fig. 13 is a cross sectional view of a filter comprising nesting tubes. Fig. 14 is cross-sectional view of the bottom portion of a pouring cup having a retainer for a stack of filter plates. Figs. 15 and 16 are cross-sectional views of a portion of stack of filter plates, illustrating spacing arrangements. Figs. 18 and 19 are cross sectional views of portions of filter retainers, holding filter stacks. Fig. 20 is a cross-sectional view of a portion of a stack of filter elements in which the slots are wider than the ribs. Fib. 21 is a plan view of a portion of a stack of plates with round holes. Fig. 22 is a cross-sectional view of the stack shown in Fig. 21. The filter in Fig. 1 is composed of a series or slotted ceramic plates. A stack of 3 such plates is illustrated, but it will be understood that a stack may be made up of simply two plates and that more than three plates may be used. Each plate 11 (see Fig 2) has a slotted portion composed of alternating ribs 12 and slots 13 and a slighthly thicker peripheral portion 14 . Accordingly, when the peripheral portions of adjacent plates are in face-to-face contact (as shown) there is a shallow space 16 between the ribs of one plate and those of the adjacent plate. The plates are positioned with the slots of one plate staggered with respect to those of the next plate so that the metal flows in a tortuous path as shown by the arrows P in Figs 5, 6 and 20: first through a slot, then through a restriction (the shallow space 16) and then through a slot of the plate below it. The plates may be kept in this staggered relationship by any suitable means; thus the plates 11 may have depressions 17, and raised areas 18 which fit into these depressions, so as to fix the relative positions of adjacent plates. The height H of the restricted passage connecting the outlets of the upper slots with the inlets of the slots in lower plates (i.e., the shallow space 16 ) is preferably less than the width W of the slots, so that the arrangement acts as a fine filter, to block passage of particles small enough to pass through the slots. It will be seen that almost every slot (except for slots adjacent to the thicker peripheral portion) has two outlets through the shallow space 16 . This makes possible fine filtration without significant reduction in the open area available for flow of the molten metal through the filter. For instance, even when the height H of the shallow space is only one-half the width W of the slots the cross-sectional area available for flow of the metal through the shallow space is substantially the same as that for simple flow through the slots. In contrast, if the width of the slots were halved to accomplish similar filtration while keeping the widths of the ribs unchanged (to avoid weakening the structure), the cross-sectional area for flow would be cut in half. In the embodiment shown in Figs. 1 and 2 the contacting faces 19 , 21 of the peripheral portions of the plates 11 are smooth and flat so that the plates are kept in the desired alignment and there is no significant flow of metal out of the main path defined by the central zone containing the ribs and slots. Also, in that embodiment, the three illustrated plates are of identical construction, the middle plate being simply turned end-to -end with respect to the top arid bottom plates. In that embodiment the slots and ribs are asymmetrically arranged (i.e., the slots at the extreme left and right sides of the top plate are at different distances from the corresponding edges of the plate.) It will be understood that one may employ instead plates whose slot arrangement is symmetrical (as shown in Fig 3 for instance) with asymmetrical position-fixing means, so that when identical plates are stacked the desired staggered relationship of slots is obtained. It will also be understood that the plates of a stack need not be identical so long as they give the staggered relationship of the openings. Variations may be made in the configurations of the apertures (e.g. slots 13 ) which pass through the places and of the shallow spaces 16 into which those apertures lead. For instance, the apertures may be curved, instead of straight, slots 22 (Fig. 4 ). They may be tapered (Fig. 5 ). The downstream faces of the ribs 12 may be curved (Fig 6) to provide shallow spaces of varying effective diameter and filtering capacity, or they may have spaced extensions 23 as shown in Fig. 7 (which is a view taken from a plane running lengthwise of the slots and being within an upper slot and passing through a rib of the lower plate). The ribs may be joined by spaced integral reinforcing connectors 24, as in Fig. 8 . The plates may be circular rather than rectangular (Fig. 9 ). The apertured elements need not be flat plates. For instance, they may be of slotted frusto-conical configuration (Figs. 10,11), with one cone 26 nesting within the other, with the slots 27 of the cones in staggered relationship and with the cones being so supported (for instance in a conventional ceramic pouring cup 28 on top of a mold, not shown) that the desired shallow space is maintained between the outlets of the slots of the upper cone and the inlets of the slots of the lower cone. For instance, a cone may have shallow spacing bosses 29 as indicated in Fig. 11 . Any suitable means may be provided for keeping the desired staggered relationship; for instance, an integral locating lug 31 may be positioned on the rim 32 of one cup so as to fit into a corresponding locating recess in the rim of the adjacent cup. Instead of being conical the filter elements may be pyramidal or frusto-pyramidal (see, e.g., Fig. 12 ). They also may be tubular, of round or angular cross-section, as Fig. 13 in which one slotted tube 33 of rectangular cross-section is mounted within a similar but larger slotted tube,34, being spaced therefrom a predetermined distance to establish the desired restricted passage between apertures, the slots (not shown) being in the predetermined staggered relationship; such tubular filters may be used in the known in-line filter systems which comprise replaceable cartridges of parallel filter tubes mounted in a refractory-lined box containing molten metal with the metal passing inwardly through the tube walls and then being fed from the tubes to the casting zone. Successive filter elements may have different kinds of apertures. Thus, the filter may comprise a filter element (e.g., a plate) having long slots positioned just above, or below, an adjacent filter element having round holes. The filter may comprise elements of decreasing aperture width, e.g., the top elements may have the widest apertures, with lower elements having successively narrower apertures; for example, the filter may be made up of slotted plates, the slot widths being successively (starting at the top) about 3 mm, 2mm and 1,5 mm, with each restricted passage having a height equal to about one-half the width of the apertures in the plate overlying that passage. The filters of this invention may be positioned at locations within the mold, e.g., in the gating system of the mold, such as in the positions shown on page 18 of the Day Article mentioned above (e.g., at the base of a sprue, in a horizontally-parted runner system,in the sprue of a vertically parted mold, or after the reaction changer for an in-the-mold nodulizing treatment). They may also be positioned outside the mold, e.g., at the base of a ceramic pouring cup 36 on top of the mold, the pouring cup (Fig. 14 ) being modified by providing at its base a flanged retainer 37 which provides a slot into which, for instance, the stack of flat filter plates may be slid. Various other means for spacing and positioning the apertured filter elements may be employed. For instance, the spacing means may comprise separate thin strips or annular disks 41 (Fig. 15 ) disposed between the apertured plates 11' such spacers may, for instance, be of ceramic, or of metal which may have a ceramic protective coating at zones where the spacing means could come into contact with the molten metal. The aperture plates may have extensions 42 (Fig. 16 ) of reduced thickness which may be engaged by separate spacing elements 43 . The spacing means 44 , 46 may be part of the retainer 37 which supports the apertured plates 47,48,as in Figs. 18,19. The filter elements described above can be conveniently and economically prepared by conventional techniques used for injection molding of ceramic elements, e.g. by preparing a mixture of fine particles of ceramic material with a thermoplastic organic binder (such as a thermoplastic polymer) and then heating the mixture and injecting the resulting plastic mass into a cooled mold, followed by heating to remove (e.g., destroy) the binder and sinter the ceramic particles. The ceramic material is a refractory ceramic capable of withstanding the molten metal being filtered, such as silica, alumina, aluminosilicate, zirconia, etc.. Such injection molded ceramic is usually microporous (e.g. pores constitute about 10-40%, such as about 25%,of its volume) and relatively brittle (e.g. its modulus of rupture is in the range of about 7x10⁶ - 21x10⁶ Pa (1000 to 3000 psi); such as about 11x10⁶ - 14x10⁶ Pa (1500-2000 psi); its porosity enhances its thermal shock resistance. It is within the scope of the invention, however to make the filter elements of stronger refractory ceramic material such as high fired alumina or zirconia having a modulus of rupture of the order of 36x10⁸ Pa (50,000 psi.) To provide strength for the filter elements, the ceramic structures between apertures (e.g., the ribs) are preferably relatively wide, above about 0.7 mm. such as about 1 to 4 mm. The slots or other apertures generally have widths equal to, more preferably less than, the widths of the ribs (or other intervening ceramic structures). However, the slots (or other apertures) may be somewhat larger than the widths of the ribs (or other intervening ceramic structures), as illustrated in Fig. 20 showing ribs 51 and slots 52 . Here, the effective height of the shallow space is the distance H'. The heights of the shallow spaces between the elements are usually in the range of about 0.2mm to 4 mm. Preferably, as previously indicated, they are less than the widths of the apertures, such as less than about 0.8 mm., e.g., about 0.5 mm. The filter elements preferably have a thickness of at least about 1 mm. Suitable dimensions for the plate shown in Fig. 2 are a square of 50 mm by 50 mm, whose solid peripheral portion 14 has a width of about 10 mm and a thickness of about 4 mm, the height of the recessed-portion 16 being about 0.5 mm, the ribs being about 2 mm wide and the slots having widths of about 1 mm. The apertures need not be elongated in cross-section (e.g. slots). They may be round holes 53 and 54 for instance, as in the plates 56, 57 shown in Figs. 21, 22. In these figures the spacing between holes is less than the hole diameter (width) and the effective filtering height of the shallow space is more than its actual height (as described in connection with Fig. 20). Such structures may be made conviently by the conventional pressing of a blend of ceramic and binder, particularly where the distance D between holes is greater than about 4 mm and is at least about half the hole diameter.

Claims: A filter for filtering molten metal, comprising a plurality of shaped refractory ceramic filter elements, each element (11, 26, 33, 56) having a plurality of apertures (13, 22, 53) for the passage of flowing molten metal therethrough, and the elements are so arranged that the apertures of adjacent elements are staggered so that at least the major part of each aperture opens against a solid portion of the filter area of the next element, the filter areas containing the plurality of apertures being spaced from each other by a distance (H) not substantially greater than the width (W) of the apertures so that the molten metal is able to flow freely in the spaces between the successive elements, whereby the flowing streams of metal follow a tortuous path (P) through the elements of the filter. A filter as claimed in Claim 1, wherein each filter element (11) comprises a plate having an apertured filter area surrounded by a non-apertured area (14) which contacts the non-apertured area of the adjacent plate. A filter as claimed in Claim 1 or 2, wherein the apertures are slots (13, 22) in said plates (11), which maybe rectangular (11) or circular (14). A filter as claimed in Claims 1 or 3, wherein the elements (26) are frustro-conical (26) or pyramidal in shape and nested one within the other. A filter as claimed in Claim 1, wherein the elements are tubular and angular (33, 34) or round in cross-section and successive elements are of different cross-sectional size and are arranged one within the other. A filter as claimed in any preceding Claim, which is composed of elements having apertures of successively decreased width. A filter as claimed in any preceding Claim which was made by pressing or injection molding, followed by sintering of a mixture of a refractory ceramic material, and a binder; and may be microporous. A filter as claimed in any preceding Claim, wherein the distance (H) between adjacent filter areas is less than the width (W) of the next apertures of the adjacent element, or of the minimum width of a non-uniform cross-section aperture. A filter as claimed in any preceding Claim, wherein the width of an aperture (52) is greater than the width of the intervening portion (51) of the filter area of each element. A filter as claimed in any of Claims 1 to 8, wherein the apertures are slots having a width (W) of 1 to 4 mm and the distance (H) between the filter areas of adjacent elements is less than 0.8 mm, and the distance between adjacent slots (13) is 1 to 4 mm and preferably is greater than the width of the apertures. A method of filtering a molten metal, which comprises passing the molten metal through a filter as claimed in any preceding claim, before the metal enters a mold cavity.

Inventors: URAM, STUART ZAME
Applicants: CERTECH INCORPORATED
Application year: 1985
Main classification: B01D39
IPC codes: B01D29, C22B9, C04B38, B01D39