Ceramic honeycomb structure and its production method, and honeycomb-molding die

1. A method for producing a ceramic honeycomb structure comprising (i) a ceramic honeycomb body having pluralities of longitudinal flow paths partitioned by square-lattice-cross-sectioned cell walls and (ii) an outer peripheral wall formed on an outer periphery of said ceramic honeycomb body, comprising the steps of

extrusion-molding a moldable ceramic material to form a ceramic honeycomb green body integrally comprising square-lattice-cross-sectioned cell walls defining pluralities of longitudinal flow paths, and an outer peripheral cell wall having an outer peripheral surface shape reflecting the square lattice shapes of said cell walls for constituting an outermost periphery of said ceramic honeycomb body;

drying and sintering said ceramic honeycomb green body to obtain said ceramic honeycomb body; and

applying a coating material to an outer peripheral surface of said outer peripheral cell wall of said ceramic honeycomb body, and heat-treating said coating material to form said outer peripheral wall, thereby producing said ceramic honeycomb structure;

a die used in said extrusion-molding step comprising a die body comprising supply holes for supplying said moldable ceramic material, and square-lattice-patterned slits formed on the opposite surface side to a surface apertured for having said supply holes, such that said square-lattice-patterned slits are communicating with said supply holes to extrusion-mold said moldable ceramic material supplied through said supply holes to a honeycomb shape, and a guide ring arranged on the square-lattice-patterned slits surface side for regulating an outer peripheral surface shape of said outer peripheral cell wall of said ceramic honeycomb green body;

the square-lattice-patterned slits surface side having a cell-walls-forming region for forming said square-lattice-cross-sectioned cell walls, and an outer peripheral region positioned outside said cell-walls-forming region via a step H, such that said cell-walls-forming region constitutes an elevated surface;

said cell-walls-forming region having an outer peripheral shape reflecting the square lattice shapes of said slits;

an outer periphery of said cell-walls-forming region being in accordance with inside surfaces of slits defining said outer peripheral cell wall;

said guide ring comprising a surface in contact with said outer peripheral region of the square-lattice-patterned slits surface side, a non-contact surface providing a clearance L with said outer peripheral region to supply said moldable ceramic material for forming said outer peripheral cell wall, and an inner peripheral surface having a shape along said outer peripheral shape of said cell-walls-forming region;

said clearance L being constant and smaller than said step H;

said square-lattice-patterned slits communicating with said supply holes in said outer peripheral region having a constant length;

a ratio T/t of a minimum thickness T of said outer peripheral cell wall to a thickness t of said cell walls meeting 1<T/t≤5; and

the outer peripheral cell wall having thicknesses measured in arbitrary directions perpendicular to the cell walls, and said minimum thickness T being a minimum of the thicknesses of the outer peripheral cell wall measured in arbitrary directions perpendicular to the cell walls,

wherein the moldable ceramic material for forming the outer peripheral cell wall does not receive a force toward a center of the honeycomb green body.

2. The method for producing a ceramic honeycomb structure according to claim 1 , wherein the ratio T/t meets 3≤T/t≤5.

3. The method for producing a ceramic honeycomb structure according to claim 1 , wherein said coating material comprises ceramic aggregate particles and an inorganic binder as main components.

4. The method for producing a ceramic honeycomb structure according to claim 3 , wherein said ceramic aggregate particles are made of at least one selected from the group consisting of cordierite, silica, alumina, mullite, silicon carbide, silicon nitride and aluminum titanate.

5. The method for producing a ceramic honeycomb structure according to claim 1 , wherein said thickness t of said cell wall is less than 0.15 mm.

6. The method for producing a ceramic honeycomb structure according to claim 1 , wherein said inside surfaces of said slits defining the outer peripheral cell wall are parallel to an extrusion direction.

7. A method for producing a ceramic honeycomb structure comprising (i) a ceramic honeycomb body having pluralities of longitudinal flow paths partitioned by square-lattice-cross-sectioned cell walls and (ii) an outer peripheral wall formed on an outer periphery of said ceramic honeycomb body, comprising the steps of

extrusion-molding a moldable ceramic material to form a ceramic honeycomb green body integrally comprising square-lattice-cross-sectioned cell walls defining pluralities of longitudinal flow paths, and an outer peripheral cell wall having an outer peripheral surface shape reflecting the square lattice shapes of said cell walls for constituting an outermost periphery of said ceramic honeycomb body;

drying and sintering said ceramic honeycomb green body to obtain said ceramic honeycomb body; and

applying a coating material to an outer peripheral surface of said outer peripheral cell wall of said ceramic honeycomb body, and heat-treating said coating material to form said outer peripheral wall, thereby producing said ceramic honeycomb structure;

a die used in said extrusion-molding step comprising a die body comprising supply holes for supplying said moldable ceramic material, and square-lattice-patterned slits formed on the opposite surface side to a surface apertured for having said supply holes, such that said square-lattice-patterned slits are communicating with said supply holes to extrusion-mold said moldable ceramic material supplied through said supply holes to a honeycomb shape, and a guide ring arranged on the square-lattice-patterned slits surface side for regulating an outer peripheral surface shape of said outer peripheral cell wall of said ceramic honeycomb green body;

the square-lattice-patterned slits surface side having a cell-walls-forming region for forming said square-lattice-cross-sectioned cell walls, and an outer peripheral region positioned outside said cell-walls-forming region via a step H, such that said cell-walls-forming region constitutes an elevated surface;

said cell-walls-forming region having an outer peripheral shape reflecting the square lattice shapes of said slits;

an outer periphery of said cell-walls-forming region being in accordance with inside surfaces of slits defining said outer peripheral cell wall;

said guide ring comprising a surface in contact with said outer peripheral region of the square-lattice-patterned slits surface side, a non-contact surface providing a clearance L with said outer peripheral region to supply said moldable ceramic material for forming said outer peripheral cell wall, and an inner peripheral surface having a shape along said outer peripheral shape of said cell-walls-forming region;

said clearance L being constant and smaller than said step H;

said square-lattice-patterned slits communicating with said supply holes in said outer peripheral region having a constant length;

a ratio T/t of a minimum thickness T of said outer peripheral cell wall to a thickness t of said cell walls meeting 1<T/t≤5; and

the outer peripheral cell wall having thicknesses measured in arbitrary directions perpendicular to the cell walls, and said minimum thickness T being a minimum of the thicknesses of the outer peripheral cell wall measured in arbitrary directions perpendicular to the cell walls,

wherein said ceramic honeycomb structure consists of (i) said ceramic honeycomb body and (ii) said outer peripheral wall, and wherein said ceramic honeycomb structure is a cylindrical ceramic honeycomb structure.