Stock material or miscellaneous articles – Structurally defined web or sheet – Honeycomb-like
Reexamination Certificate
1999-01-22
2001-06-19
Lorin, Francis J. (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Honeycomb-like
C428S118000
Reexamination Certificate
active
06248421
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ceramic honeycomb structure used as a catalyst carrier in a catalytic converter for cleaning exhaust gases from internal combustion engines and to a die for extruding the honeycomb structure.
2. Description of the Related Art
A ceramic honeycomb structure such as of cordierite has heretofore been used as a catalyst carrier in a catalytic converter for cleaning gases emitted from internal combustion engines such as automobile engines (see, for example, Japanese Examined Patent Publication (Kokoku) No. 61-47135). In recent years, it has been demanded to further decrease the weight and pressure loss of a ceramic honeycomb structure which serves as a catalyst carrier for automotive use, and it has been attempted to decrease the thickness of the cell walls to meet the demand. However, a decrease in the cell wall thickness results in a decrease in the strength of the honeycomb structure which may then be cracked since it can no longer withstand the load when it is being incorporated in a converter. Therefore, it was not possible to decrease the thickness of the cell walls to less than 100 &mgr;m. To prevent a drop in the strength, it can be contrived to either maintain the strength by increasing the density of cordierite that constitutes the honeycomb structure or reinforce the strength relying upon the structure. In order to decrease the thickness by using the same cordierite starting materials as the one traditionally used without changing the coating property of the catalyst, however, it becomes necessary to reinforce the strength relying upon the structure.
As technology for reinforcing the ceramic honeycomb structure, Japanese Unexamined Patent Publication (Kokai) No. 49-88908 discloses a structure for enhancing the strength by forming the outer circumferential skin layer A thicker relative to the cell walls B, as shown in FIG.
1
. However, an increase in the thickness of the outer circumferential skin layer A results in a decrease in the moldability of the ceramic honeycomb structure and in the resistance against thermal shock causing the probability of developing cracks due to heat at the time of cleaning exhaust gases. According to a honeycomb structure disclosed in Japanese Unexamined Patent Publication (Kokai) No. 51-155741, the strength is increased by increasing the thickness of the cell walls B′ over a range of ±22 degrees in the diagonal directions, as shown in FIG.
2
. According to this structure, however, thick portions are formed over increased areas, and the cell walls become thick at the central portions where the exhaust gases flow in large quantities, offsetting the merit of decreasing the thickness of the cell walls.
As described above, according to the conventional structures, it is difficult to maintain the strength of the structure while decreasing the thickness of the cell walls to smaller than 100 &mgr;m without decreasing the moldability or the thermal shock resistance. The object of the present invention is therefore to provide a ceramic honeycomb structure which compensates for a decrease in the strength derived by reduction in thickness of the cell walls, sufficiently withstands the load at the time of being incorporated in a catalytic converter, and exhibits favorable moldability and thermal shock resistance. The invention further provides an extrusion die used for producing the ceramic honeycomb structure.
SUMMARY OF THE INVENTION
(1) The ceramic honeycomb structure of the present invention comprises a number of cells extending in the axial direction formed by the partitioning walls inside the outer circumferential skin layer of nearly a cylindrical shape. In the outer circumferential portion close to the outer circumferential skin layer, at least one a reinforcing layer of nearly a cylindrical shape is provided extending from one end surface toward the other end surface. The reinforcing layer, the outer circumferential skin layer and the partitioning walls are made of the same material and made into a unitary body. The thickness of the partitioning walls is not larger than 100 &mgr;m from at least the central portion to the vicinity of the reinforcing layer.
According to the above-mentioned constitution, the reinforcing layer of nearly a cylindrical shape provided inside of the outer circumferential skin layer helps increase in the strength in the circumferential direction to compensate for a decrease in the strength caused by a decrease in the thickness of the partitioning walls which are the cell walls. Furthermore, without reduction of the moldability or thermal shock resistance as in the prior art construction of making the outer circumferential skin layer thick, sufficient merit of decreasing the thickness of the partitioning walls is obtained by making the thickness of the partitioning walls not larger than 100 &mgr;m in the portions where exhaust gases flow in large amounts inside the reinforcing layer. It is thus possible to realize a light weight and low pressure-loss ceramic honeycomb structure which maintains a strength large enough to withstand the load at the time when it is being incorporated in a catalytic converter.
(2) A plural number of reinforcing layers having different diameters are concentrically arranged inside the outer circumferential skin layer. In general, the strength increases with an increase in the number of the reinforcing layers. Therefore, the number of the reinforcing layers and the thickness thereof are suitably selected to provide a required strength.
(3) The partitioning walls along the outer circumferential portion are formed with a larger thickness than the partitioning walls in the other portions. A decrease in the thickness of the partitioning walls causes a defect in the molding such as collapse of cells, which, however, is prevented by increasing the thickness of the partitioning walls in the outer circumferential portion. Besides, since an exhaust gas flows in relatively small amount in the outer circumferential portion, the pressure loss is low.
(4) Preferably, when either the reinforcing layer exists in a single number or in a plural number, the reinforcing layer closest to the outer circumferential skin layer is arranged within a one-cell pitch from the outer circumferential skin layer. This highly contributes to increasing the strength of the ceramic honeycomb structure in the circumferential direction.
(5) It is desired that the reinforcing layer is so formed as will not to pass through the lattice points of the outermost circumference cells within ranges of from −4 degrees to +4 degrees with respect to the radial lines extending from almost a center point of the ceramic honeycomb structure having a circular shape in cross section, toward the directions of diagonal lines of the polygonal cells. Since the reinforcing layer does not pass through the lattice points of the cells within the above-mentioned ranges where cells tend to be collapsed during the molding, the cell walls are supported by the reinforcing layer and the cells are prevented from being collapsed.
(6) More preferably, the reinforcing layer is so formed as will not to pass through the lattice points of the cells two cell-pitches from the outermost circumference, in order to further enhance the effect of preventing the collapse of cells.
(7) An extrusion die, for extrusion-molding the ceramic honeycomb structure of the above-mentioned constitution, comprises an extrusion portion having extrusion grooves in the form of a honeycomb shape and a material-feeding portion provided on the inlet side of the extrusion portion and having many round holes communicated with said extrusion grooves to feed a material, wherein one or more ring-like slits are formed in the outer peripheral portion of the extrusion portion. The thus constituted extrusion die makes it possible to extrusion-mold ceramic honeycomb structures having the constitutions described in (1) to (6) above.
(8) The extrusion grooves along the outer circumferential edge o
Andou Yosiyasu
Koike Kazuhiko
Nakanishi Tomohiko
Lorin Francis J.
Nippon Soken Inc.
Pillsbury & Winthrop LLP
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