Die for molding ceramic honeycomb structure

A die comprising molding grooves arranged in a lattice pattern and moldable-material-supplying holes communicating with the molding grooves for molding a ceramic honeycomb structure, the molding grooves having width of 0.05-0.5 mm, the moldable-material-supplying holes being arranged in every intersecting portions of the molding grooves, or in every other intersecting portions of the molding grooves in a checkerboard pattern, and an average distance between the centers of the intersecting portions of the molding grooves, at which the moldable-material-supplying holes are arranged, and the center axes of the moldable-material-supplying holes being 10-100 μm.

FIELD OF THE INVENTION

The present invention relates to a die for molding a ceramic honeycomb structure, which comprises molding grooves arranged in a lattice pattern, and moldable-material-supplying holes communicating with the molding grooves and arranged in every or every other intersecting portions of the molding grooves.

BACKGROUND OF THE INVENTION

A ceramic honeycomb structure is produced, for instance, by extruding a cordierite-based ceramic, moldable material through a die for molding a ceramic honeycomb structure (hereinafter referred to as “molding die” below) in a direction from moldable-material-supplying holes to molding grooves, to form a ceramic honeycomb molding, and drying and sintering it. As shown inFIGS. 4(a) and4(b), a molding die31comprises molding grooves12arranged in a lattice pattern, and moldable-material-supplying holes13communicating with the molding grooves.

As shown inFIGS. 5(a),5(b) and5(c), supply holes13are arranged in the molding die31such that they overlap the intersecting portions33of molding grooves. A ceramic, moldable material introduced into the molding die31through the supply holes13is formed to a honeycomb shape by the molding grooves12. The supply holes13are communicating with every intersecting portions33of the molding grooves12arranged in a lattice pattern, or with every other intersecting portions33in a checkerboard pattern [see inFIGS. 5(a),5(b) and5(c)].

In order that the ceramic honeycomb structures for cleaning exhaust gases from automobiles have larger opening areas of exhaust gas paths, and that their temperatures are more quickly elevated to activation temperatures when carrying catalysts, their cell walls are as thin as 0.05-0.5 mm. Accordingly, molding grooves12in the molding die31have smaller width12w[seeFIG. 5(c)]. On the other hand, to prevent the deformation and bending of moldings, the depth12dof the molding grooves12should be 10 times or more the width12w.

As the molding grooves12have smaller width12wand larger depth12d, a moldable material (shown by the arrows of thick dotted lines) supplied from the holes13receives larger resistance while passing through the molding grooves12. Accordingly, the molding die is bent (exaggerated by phantom lines), and large stress is applied to overlapping portions14bof the moldable-material-supplying holes13and the molding grooves12.

JP 2006-142579 A discloses a molding die comprising pluralities of cell blocks for defining molding grooves, their pitches expanding stepwise from a center portion to a peripheral portion. JP 2006-142579 A describes that the use of this molding die suppresses cell pitch unevenness between the center portion and the peripheral portion, which is caused by deformation due to uneven heating during drying, providing ceramic honeycomb structures with uniform cell pitches.

JP 2006-88556 A discloses a die comprising molding grooves arranged in a lattice pattern, and moldable-material-supplying holes staggeringly arranged in intersecting portions of the molding grooves for communication therewith for molding a ceramic honeycomb structure, which meets the conditions of A/L=1-5, and A/D=0.05-0.3, wherein inFIG. 5(c), A represents the shortest distance between a side surface13aof a moldable-material-supplying hole13communicating with one groove12and a side surface14a(side surface of cell block14) of another molding groove12adjacent to the above molding groove12, L represents the longitudinal length of an overlapping portion14bof the molding groove12and the moldable-material-supplying hole13, and D represents length obtained by subtracting L from the depth of the molding groove. JP 2006-88556 A describes that this molding die has such high strength that it is not broken during die machining and the extrusion of moldings, providing ceramic honeycomb structures with improved sintering strength.

However, the molding dies described in JP 2006-142579 A and JP 2006-88556 A cannot sufficiently reduce stress applied to the moldable-material-supplying holes13and their overlapping portions14bwith the molding grooves12during extrusion. Particularly when dies having narrower molding grooves are used to mold ceramic honeycomb structures having thinner cell walls, stress is likely concentrated in the overlapping portions14b, resulting in cracking between adjacent moldable-material-supplying holes13,13. Accordingly, dies suitable for molding ceramic honeycomb structures having thin cell walls are desired.

OBJECT OF THE INVENTION

Accordingly, an object of the present invention is to provide a molding die receiving reduced stress in overlapping portions of moldable-material-supplying holes and molding grooves during extrusion, so that cracking unlikely occurs in the overlapping portions even when the molding grooves are narrow, and that cracking if any does not easily propagate, to ensure its use for a long period of time.

DISCLOSURE OF THE INVENTION

As a result of intensive research in view of the above object, the inventor has found that with the centers of the intersecting portions of molding grooves deviating from the center axes of the moldable-material-supplying holes arranged in the intersecting portions, stress applied to overlapping portions of the moldable-material-supplying holes and the molding grooves can be reduced during extrusion. The present invention has been completed based on such finding.

Thus, the die of the present invention for molding a ceramic honeycomb structure comprises molding grooves arranged in a lattice pattern, and moldable-material-supplying holes communicating with the molding grooves, the molding grooves having width of 0.05-0.5 mm, the moldable-material-supplying holes being arranged in every intersecting portions of the molding grooves, or in every other intersecting portions of the molding grooves in a checkerboard pattern, and an average distance between the centers of the intersecting portions of the molding grooves, at which the moldable-material-supplying holes are arranged, and the center axes of the moldable-material-supplying holes being 10-100 μm.

The center axes of moldable-material-supplying holes are preferably arranged along each molding groove on both sides of its centerline.

The center axes of moldable-material-supplying holes are preferably arranged along each molding groove on the same side of its centerline.

The center axes of moldable-material-supplying holes are preferably arranged staggeringly (alternately on both sides) along a centerline of each molding groove.

DESCRIPTION OF THE BEST MODE OF THE INVENTION

In a die comprising molding grooves each having a width of 0.05-0.5 mm for molding a ceramic honeycomb structure, moldable-material-supplying holes are arranged such that their center axes are not aligned with the centers of the intersecting portions of molding grooves in which the moldable-material-supplying holes are arranged, thereby preventing adjacent moldable-material-supplying holes from having a constant interval. An average distance between the centers of the intersecting portions of molding grooves and the center axes of the moldable-material-supplying holes is 10-100 μm. With the moldable-material-supplying holes thus arranged, dispersed stress is applied to overlapping portions of the moldable-material-supplying holes and the molding grooves during extrusion, so that cracking is less likely between adjacent moldable-material-supplying holes. Even when cracking occurs, it does not easily propagate. As a result, the molding die can be used for a long period of time. The average distance between the centers of the intersecting portions of molding grooves and the center axes of the moldable-material-supplying holes is measured on 10 arbitrarily selected moldable-material-supplying holes.

When the above average distance is less than 10 μm, the moldable-material-supplying holes have substantially constant intervals, so that high stress is applied to overlapping portions of the moldable-material-supplying holes and the molding grooves during extrusion, making it likely that cracking occurs between adjacent moldable-material-supplying holes. On the other hand, when the above average distance is more than 100 μm, a ceramic, moldable material does not easily spread to the molding grooves uniformly during extrusion, resulting in the likelihood of forming bent or deformed moldings. The above average distance is preferably 20-90 μm.

To arrange the moldable-material-supplying holes such that the centers of the intersecting portions of molding grooves are not aligned with the center axes of moldable-material-supplying holes, the positions of the molding grooves and/or the moldable-material-supplying holes may be adjusted by conventional die-producing technologies. The center axes of moldable-material-supplying holes may be dislocated from the centers of the intersecting portions of molding grooves along the longitudinal and/or transverse directions of the molding grooves.

The center axes of the moldable-material-supplying holes are preferably arranged along one molding groove on both sides of its centerline as shown inFIG. 6(a). With such arrangement, stress applied to overlapping portions of the moldable-material-supplying holes and the molding grooves is more dispersed, thereby suppressing cracking from occurring between the adjacent moldable-material-supplying holes. It is particularly preferable that as shown inFIG. 6(d), the center axes of the moldable-material-supplying holes are arranged staggeringly along a centerline of each molding groove.

When the center axes of moldable-material-supplying holes are arranged along one molding groove on the same side of its centerline as shown inFIGS. 6(b) and6(c), too, dispersed stress is applied to overlapping portions of the moldable-material-supplying holes and the molding grooves, suppressing cracking between adjacent moldable-material-supplying holes.

The embodiments of the present invention will be explained below.

Embodiment 1 is directed to a die11having a post-sintering diameter of 120 mm for molding a cordierite-based ceramic honeycomb structure. This molding die11can be produced, for instance, by pre-hardening a die material having a composition comprising 0.10-0.25% by mass of C, 1% by mass or less of Si, 2% by mass or less of Mn, 1-2.5% by mass of Cr, 1% by mass or less as (Mo+½W) of Mo and/or W, 0.03-0.15% by mass of V, 0.1-1% by mass of Cu, 0.05% by mass or less of S, and 2% by mass or less of Ni, the balance being Fe and inevitable impurities to HRC of 29-33, and then machining the die material to form moldable-material-supplying holes13and molding grooves12. Known die materials may be used, and for instance, alloyed tool steel such as JIS SK1313D61, martensitic stainless steel such as JIS SUS420J2 are preferable.

The molding grooves12consist of a large number of longitudinal grooves arranged with a width12wof 0.26 mm and a pitch12pof 1.56 mm, and a large number of transverse grooves arranged with the same width and pitch perpendicularly to the longitudinal grooves, as shown inFIGS. 1(a),1(b) and1(c). The moldable-material-supplying holes13each having a diameter13dof 1.2 mm and a depth of 20 mm are arranged in the intersecting portions33of the molding grooves12in a checkerboard pattern. The center axis13cof each moldable-material-supplying hole13is arranged along a lateral centerline X of each molding groove, such that it is separate from a center12cof each intersection33of the molding grooves12by distance Z on the same side along the centerline Y of each vertical molding groove. The separation distances Z of the moldable-material-supplying holes13are not constant, within a range of 10 μm to 100 μm when averaged on 10 points. In each molding groove12, a groove bottom is connected to side surfaces via continuously curved surfaces.

Using an image measurement system “Quick Vision” available from Mitutoyo Corporation, the distance Z can be determined by taking an image of the molding die11from the side of the molding grooves12, determining the intersection centers12cof the molding grooves12and the center axes13cof the moldable-material-supplying holes13, and averaging distances therebetween on 10 points. The intersection centers12cof the molding grooves12are determined by the image detection of four corners of the intersecting portions of the molding grooves12, and the center axes13cof the moldable-material-supplying holes13are determined by the image detection of part (four portions) of the outlines of the moldable-material-supplying holes13appearing in the molding grooves12.

FIG. 2(a) schematically shows the propagation of cracking (CRK) between adjacent moldable-material-supplying holes13,13, when a moldable material is extruded from the moldable-material-supplying holes13to the molding grooves12in the molding die11of Embodiment 1. Because the center axes13cof the moldable-material-supplying holes13are deviated from the centers12cof the intersecting portions33of the molding grooves12in the same direction along the X-axis centerlines of the molding grooves in the molding die11, adjacent moldable-material-supplying holes13,13in the molding grooves have long intervals13win a Y-axis direction, reducing stress applied to intersecting portions13bof the bottoms12aof the molding grooves12and the moldable-material-supplying holes13. Even when repeated extrusion causes metal fatigue in overlapping portions14b, resulting in cracking (CRK) between the moldable-material-supplying holes13,13, the cracking (CRK) does not easily propagate, making it possible to use the die for a long period of time.

On the other hand,FIG. 2(b) schematically shows the propagation of cracking (CRK) between adjacent moldable-material-supplying holes13,13, when a moldable material is similarly extruded from a conventional molding die31(seeFIG. 5). Because the centers12cof the intersecting portions33of the molding grooves12are aligned with the center axes13cof the moldable-material-supplying holes13in the moldings die31, resulting in constant intervals13wbetween the adjacent moldable-material-supplying holes13,13, stress concentrated in the intersecting portions13bof the bottoms12aof the molding grooves12and the moldable-material-supplying holes13is not dispersed. Repeated extrusion causes metal fatigue in the overlapping portions of14b, resulting in more cracking (CRK) between the moldable-material-supplying holes13,13than in Embodiment 1, if any. As a result, cracks easily propagate, making it difficult to use the die for a long period of time.

Embodiment 2 is directed to a molding die21having a post-sintering diameter of 100 mm for forming a cordierite-based ceramic honeycomb structure. This molding die21comprises moldable-material-supplying holes13arranged at intersecting portions33of molding grooves12in a lattice pattern as shown inFIG. 3(a), and can be produced by the die material shown in Embodiment 1.

The molding grooves12consist of a large number of longitudinal grooves arranged with a width12wof 0.22 mm and a pitch12pof 1.25 mm, and a large number of transverse grooves arranged with the same width and pitch perpendicularly to the longitudinal grooves. The moldable-material-supplying holes13each having a diameter13dof 1.0 mm and a depth of 22 mm are arranged in the intersecting portions33of the molding grooves12. The molding grooves12are formed after the moldable-material-supplying holes13are formed. The center axis13cof each moldable-material-supplying hole13is arranged along a centerline of each X-axis molding groove, such that it is separate from a center12cof each intersection33of the molding grooves12by distance Z staggeringly along the centerline of each Y-axis molding groove. The separation distances Z of the moldable-material-supplying holes13are not constant, within a range of 10 μm to 100 μm when averaged on 10 points. In each molding groove12, a groove bottom is connected to side surfaces via continuously curved surfaces.

FIG. 3(a) schematically shows the propagation of cracking (CRK) between adjacent moldable-material-supplying holes13,13, when a moldable material is extruded from the moldable-material-supplying holes13to the molding grooves12in the molding die21of Embodiment 2. Because the center axes13cof the moldable-material-supplying holes13are deviated from the centers12cof the intersecting portions33of the molding grooves12staggeringly along the centerlines of the X-axis molding grooves in the molding die21, adjacent moldable-material-supplying holes13,13in the molding grooves have long intervals23win a Y-axis direction, reducing stress applied to intersecting portions13bof the bottoms12aof the molding grooves12and the moldable-material-supplying holes13. Even when repeated extrusion causes metal fatigue in overlapping portions14b, resulting in cracking (CRK) between the moldable-material-supplying holes13,13, the cracking (CRK) does not easily propagate, making it possible to use the die for a long period of time.

The present invention will be explained in more detail referring to Examples below without intention of restricting it thereto.

A test die material block was produced by pre-hardening a die material having a composition comprising 0.20% by mass of C, 0.44% by mass of Si, 1.95% by mass of Mn, 1.25% by mass of Cr, 0.50% by mass of Mo, 0.04% by mass of V, 0.30% by mass of Cu, and 0.015% by mass of S, the balance being Fe and inevitable impurities, to HRC of 31.3 before forming moldable-material-supplying holes13and molding grooves12.

This die material block was mounted to a machining center (not shown) to form moldable-material-supplying holes13each having a diameter13dof 1.2 mm and depth of 20 mm with a pitch of 3.12 mm by a cemented carbide drill staggeringly as shown inFIGS. 1(a),1(b) and1(c). In this case, moldable-material-supplying holes13were formed such that they were arranged on the same side of the centerlines of molding grooves within a range of 10 μm to 100 μm from the centers of the intersecting portions of molding grooves to be formed later. With the die material having the moldable-material-supplying holes13mounted to a groove-forming machine, a large number of longitudinal grooves each having a depth12dof 4 mm and a pitch12pof 1.56 mm were formed by an electrodeposited diamond grinder having a width of 0.26 mm, and then transverse grooves were formed like the longitudinal grooves, to produce a test die11A.

In the molding die11A, moldable-material-supplying holes13were arranged at intersecting portions33of the lattice-patterned grooves12in a staggering manner. The center axes13cof the moldable-material-supplying holes13were arranged, such that they were separate from the centers12cof the intersecting portions33of molding grooves12on the same side of the centerlines Y of the longitudinal molding grooves by distance Z along the centerlines X of transverse molding grooves. The distance Z was 105 μm or less, and its average was 11 μm when measured on 10 arbitrary points.

The molding dies11A of Examples 2-6 were produced in the same manner as in Example 1, except for forming moldable-material-supplying holes13with their arrangement and the average distance Z relative to each centerline of grooves as shown in Table 1.

Comparative Example 1

The test die31A of Comparative Example 1 was produced in the same manner as in Example 1, except for forming moldable-material-supplying holes13such that their center axes13cwere aligned at the centers12cof intersecting portions33of the grooves12.

Comparative Example 2

The test die31B of Comparative Example 2 was produced in the same manner as in Example 1, except for forming moldable-material-supplying holes13with the average distance Z of 9 μM.

Comparative Example 3

The test die31C of Comparative Example 3 was produced in the same manner as in Example 3, except for forming moldable-material-supplying holes13with the average distance Z of 110 μm.

Durability Test of Dies

Using each of the test dies11A of Examples 1-6 and the test dies31A,31B,31C of Comparative Examples 1-3, a cordierite-based ceramic, moldable material was repeatedly extruded for a durability test. The cordierite-based ceramic, moldable material was produced by mixing kaolin powder, talc powder, silica powder, alumina powder, etc. to prepare cordierite-forming material powder comprising 50% by mass of SiO2, 35% by mass of Al2O3and 13% by mass of MgO, sufficiently dry-mixing 100 parts by mass of the cordierite-forming material with 7 parts by mass in total of methylcellulose and hydroxypropyl methylcellulose as a molding aid and a proper amount of graphite as a pore-forming material, and then sufficiently blending the resultant mixture with a predetermined amount of water.

The durability of each die was evaluated by counting the number of molding operations until the molding die became unusable because of cracking between moldable-material-supplying holes as a result of repeated extrusion, and by observing by the naked eye the deformation of a honeycomb molding obtained by the 100-th extrusion. The number of molding operations until the molding die became unusable is expressed by a relative value, assuming that the number of molding operations was 1 for the die of Comparative Example 1. The deformation of moldings was evaluated according to the following standard. The results are shown in Table 1.Good The molding had no bending and deformation.Fair The molding was usable as a ceramic honeycomb structure despite bending and deformation.Poor The molding was not usable as a ceramic honeycomb structure because of bending and deformation.

As shown in Table 1, the molding dies11A of Examples 1-6, in which the average of distances Z between the centers12cof the intersecting portions33of the molding grooves12and the center axes13cof the moldable-material-supplying holes13was 10-100 μm, had the numbers of molding operations 1.20 times to 1.48 times as much as that of Comparative Example 1, with smaller deformation in moldings.

On the other hand, the molding die31A of Comparative Example 1, in which the centers12cof molding grooves12were aligned with the center axes13cof the moldable-material-supplying holes13, had a smaller number of molding operations than those of the molding dies11A of Examples 1-6. The molding die31B of Comparative Example 2, in which the average of distances Z between the centers12cof molding grooves12and the center axes13cof moldable-material-supplying holes13was less than 10 μm, had a smaller number of molding operations than those of the molding dies11A of Examples 1-6. The molding die31C of Comparative Example 3, in which the average of distances Z between the centers12cof the intersecting portions33of molding grooves12and the center axes13cof moldable-material-supplying holes13was more than 100 μm, had larger deformation in the moldings, because a moldable material extruded from the supply holes did not easily spread uniformly to molding grooves.

EFFECTS OF THE INVENTION

Because the molding die of the present invention can conduct extrusion with reduced stress applied to overlapping portions of the moldable-material-supplying holes and the molding grooves, less cracking occurs in the overlapping portions, and cracking if any does not easily propagate, so that it can be used for a long period of time. Accordingly, narrow grooves suitable for molding ceramic honeycomb structures having thin cell walls can be formed.