Stock material or miscellaneous articles – Sheet including cover or casing – Including elements cooperating to form cells
Reexamination Certificate
1998-12-14
2001-10-09
Jones, Deborah (Department: 1772)
Stock material or miscellaneous articles
Sheet including cover or casing
Including elements cooperating to form cells
C428S116000, C052S793100, C264S630000, C156S197000
Reexamination Certificate
active
06299958
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the manufacture of novel honeycomb structures from plasticized batches of inorganic or organic powders.
Ceramic and metallic honeycomb structures are widely used for applications such as catalyst substrates, honeycomb heaters, and the like, and the fabrication of such structures from plasticized batches of inorganic powders is well known. U.S. Pat. No. 3,320,044 to Cole describes a method of constructing ceramic honeycombs from sheets or ribbons of plasticized ceramic material, while U.S. Pat. Nos. 3,790,654 and 3,905,743 to Bagley describe direct extrusion methods and apparatus for such manufacture.
The more widely used extrusion methods for forming honeycomb structures commonly involve compounding a batch comprising inorganic powders together with added plasticizer, vehicle and binder components to achieve a plastic consistency. The plasticized batch is thereafter forced through an extrusion die to form a honeycomb shape which is then solidified by drying, heat-curing, reaction sintering or similar processing.
Dies for honeycomb extrusion typically comprise a die body incorporating a plurality of feedholes on an inlet face which extend through the body to convey the plasticized batch material to a discharge section on an opposing die outlet face. The discharge section incorporates a criss-crossing array of discharge slots, cut into the outlet face to connect with the feedholes within the die body, these slots reforming the batch material supplied by the feedholes into the interconnecting cell wall structure of the desired cellular honeycomb structure.
As the uses for such honeycomb structures have increased, so also has the need for providing more finely structured honeycombs. A fundamental limitation of the extrusion approach, however, is the fact that neither the feedholes nor the discharge slots in conventional extrusion dies may be multiplied without limit. Real limits on the cell density (the number of cells per unit honeycomb cross-section) and cell wall thicknesses obtainable by extrusion through honeycomb extrusion dies are imposed by available die machining methods. Also limiting are the finite strength and stiffness of available die fabrication materials. Die extrusion pressure increases with increasing cell density due to higher shear rates from thinner slits and due to increased die friction drag area. Thus it remains difficult to provide ceramic honeycombs of very fine dimensions for various specialty applications where conventional honeycomb dimensions are too large.
Also of interest for a variety of fluid processing applications are honeycombs offering channels of other than straight cylindrical or polygonal cross-sectional shape. U.S. Pat. Nos. 5,393,587 and 5,633,066, for example, disclose honeycomb designs offering curved or twisting flow paths through a channeled structure, for purposes such as controlling flow rates, enhancing fluid-wall contact, and the like. However, the cell and cell wall dimensions of these honeycombs remain relatively large, and maintaining precise control over the size, shape and direction of the channels forming the flowpaths is difficult.
It is therefore one object of the present invention to provide novel processes for the manufacture of cellular honeycomb structures from plasticized powder batch materials that can offer products of much finer cell structure and thinner cell walls than can be produced through the direct extrusion of honeycombs from conventional plasticized powder batch materials.
It is another object of the invention to provide novel designs for honeycomb structures, and methods for making them, including curved, conical, or other functionally graded honeycomb shapes offering new capabilities for the controlled conveyance and/or treatment of fluid streams arising within catalytic treatment or other chemical processing environments.
Other objects and advantages of the invention will become apparent from the following description thereof.
SUMMARY OF THE INVENTION
The present invention provides a novel honeycomb forming process comprising the re-shaping, by molding, extrusion, drawing, or the like, of honeycomb structures produced by other, more conventional methods. The forming process of the invention utilizes the controlled rheology and incompressibility of selected plastic filler materials to produce new and previously unattainable cell and cell wall configurations in the final honeycomb products.
Inventive products which may be produced by honeycomb reforming in accordance with the invention include cellular honeycomb structures of very high cell density and very low wall thickness. Also provided are honeycombs with functionally graded (e.g., curved, twisted, or tapering) cell shapes, these being available in both conventional and high-cell-density forms.
In a first aspect, then, the invention includes a method of producing a cellular structure by a reforming procedure. That method comprises first selecting a honeycomb body formed of a plasticized powder batch material. The honeycomb will have a cellular structure which includes a plurality of axial channels having predetermined cross-sections.
The parallel channels of the honeycomb are next filled with a selected filler of appropriate plasticity to form a filled composite. Generally, the plasticity required of the filler material is a plastic deformation behavior compatible with that of the plasticized powder batch material forming the honeycomb. Compatibility must be exhibited in at least one temperature range within which the honeycomb can be reshaped.
The filled composite thus provided is then reshaped to modify the channel size, shape, and/or direction within the honeycomb. Reshaping can be accomplished by methods such as drawing, extruding, compressing, bending, or twisting of the filled honeycomb, and will be carried out at a temperature in the range where the viscoplastic properties of the honeycomb and filler are compatible. The preferred reshaping methods are those which will reduce the cross-sections of a plurality of the channels in at least one cross-sectional dimension, thus providing a cell structure which is finer in both cell size and cell wall thickness in that dimension than the starting honeycomb.
Particularly useful reformed shapes include frustum shapes wherein the honeycomb has been reformed to impart a taper from a relatively large part cross-section to a relatively small part cross-section. In accordance with the invention, a proportional tapering of all honeycomb design elements, including channel size, channel shape, and channel wall cross-section, is secured in the reshaped part. Thus both channel size and channel wall thickness decrease along the axis of the frustum in near proportion to the reduction in part dimensions.
Other useful reformed shapes include honeycomb structures of extremely high cell density and slight channel wall thicknesses. Cell densities of 1600 cells/in
2
and greater, preferably at least 2000 cells/in
2
, with cell wall thicknesses below 0.004 inches, are readily attained through this procedure.
After reshaping has been completed, the plastic filler is removed from the reconfigured channels and the reformed honeycomb body is solidified. Typically, solidification involves drying and/or firing to sinter or reaction-sinter the particles present in the original powder batch into an integral honeycomb structure.
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“Microfabrication of Ceramics by Co-extrusion”, Van Hoy et al.,J.Am.Ceram. Soc., 81[1] 152-158 (1998).
“The Mechanics of Extru
St. Julien Dell J.
Wight, Jr. John F.
Wu Shy-Hsien
Zaun Kenneth E.
Boss Wendy
Corning Incorporated
Jones Deborah
Sterre Kees van der
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