Triangular cell metal filters

#The present invention relates to cellular ceramic structures useful as filters for the casting of metal parts. The filters function to remove debris from a molten metal stream as it is cast into a mold, so that castings free of inclusions affecting the strength, homogeneity, performance, durability and/or appearance of the finished cast parts may be produced.

#One type of molten metal filter for this use consists of a ceramic honeycomb body comprising a multiplicity of parallel through-channels through which a molten metal is to be cast. Examples of molten metal filters of this type are disclosed in U.S. Pat. Nos. 4,681,624, 5,281,462 and 5,104,540. In general, filters made in accordance with those patents are made by extrusion of a plasticized ceramic or mineral batch through a honeycomb die to form a green honeycomb shape, followed by firing of the green honeycomb to produce a strong, refractory channeled body having good filtration efficiency. As noted in U.S. Pat. No. 4,681,624, the honeycomb may in principle comprise any structure which has substantially parallel open channels, or longitudinally-extending cells, therethrough defined by relatively thin separator walls and in which the transverse cross-sectional configuration of the channels or cells is of any rounded (i.e., circular or elliptical) or polygonal (i.e., triangular, rectangular, hexagonal) shape. Ceramic compositions which may be used included zircon and zircon-containing compositions, as well as compositions of alumina, mullite, zirconia, spinel, cordierite, other aluminosilicates, aluminates, and titanates such as aluminum titanate.

#At present, current extruded honeycomb molten metal filters of approximately 100 or 200 square-channel cell density, i.e., filters incorporating 100 or 200 square cpsi (cells per square inch) of filter frontal surface area, manufactured by honeycomb extrusion as described in the foregoing patents, are commercially employed. In general, these are considered have filtration efficiencies very similar to those of pressed ceramic filters of similar cell density. The latter filters are made from fired pressings of ceramic precursor materials rather than from extruded honeycombs of those materials, and they typically incorporate round rather than the conventional square channel or cell openings.

#The invention may be further understood by reference to the Drawing, wherein:

#FIG. 1 is an extruded honeycomb metal casting filter in accordance with the prior art; and

#FIG. 2 is an extruded honeycomb metal casting filter of triangular-cell cross section provided in accordance with the invention.

#Referring more particularly to the drawing, FIG. 1 shows a structure typical of prior art filters, these generally comprising channels or cells 12 of square (or round) cross-section traversing a ceramic filter body 8 of approximately 2 cm or less thickness. The cell density of such filters varies, but typically falls in the range of 14-17 cells/cm.sup.2 (90-110 cells/in.sup.2) of filter frontal area. The thicknesses of the cell walls 14 also vary. However, in the case of extruded ceramic filters of square-celled design, cell wall thicknesses are generally in the range of 0.25-0.75 mm (0.010-0.030 in).

#FIG. 2 of the drawing shows a typical design for a triangular-cell filter provided in accordance with the invention. In that Figure, filter 10 comprises a plurality of triangular cells consisting of triangular channels 12 separated by cell walls 14.

#While not intending to be bound by theory, the improvements in filtration efficiency obtained in accordance with the invention are presently considered to result from a combination of at least two factors. First, the particular ranges of cell size and cell wall thickness adhered to in the practice of the invention are such as to achieve controlled reductions in the hydraulic diameter of the cells which do not incur significant reductions in the open frontal area (OFA) of the filters. Secondly, the triangular cross-sectional geometry of the cells appears to be more effective in trapping and removing certain classes of debris inclusions from molten metal casting streams than the square or round cross-sectional cell geometries of prior art designs.