Method and apparatus for producing ceramic honeycomb filter

A method for producing a ceramic honeycomb filter having a ceramic honeycomb structure having plugs in predetermined cells: comprising using an apparatus having a reservoir having an inlet for a plugging material slurry and an upper opening, a porous plate with pluralities of openings covering the upper opening of the reservoir, and a holding member fixed to an upper end of the reservoir; keeping a lower surface of the sealing film attached to a lower end surface of the ceramic honeycomb structure apart from an upper surface of the porous plate by a distance D of more than 0 mm and 2.0 mm or less; supplying a predetermined volume of the plugging material slurry into the reservoir to introduce it into the predetermined cells of the ceramic honeycomb structure; rotating the ceramic honeycomb structure after sealing of the ceramic honeycomb structure is released; and lifting the ceramic honeycomb structure after the rotation starts.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No. PCT/JP2017/035585 filed Sep. 29, 2017, claiming priority based on Japanese Patent Application No. 2016-193213, filed Sep. 30, 2016.

FIELD OF THE INVENTION

The present invention relates to a method and an apparatus for producing a ceramic honeycomb filter by continuously forming plugs with high efficiency by introducing a plugging material slurry into end portions of a ceramic honeycomb structure.

BACKGROUND OF THE INVENTION

To remove carbon-based particulate matter from an exhaust gas discharged from diesel engines, ceramic honeycomb filters having ceramic honeycomb structures with flow paths alternately sealed at both ends has been used. As shown inFIGS. 12(a) and (b), a ceramic honeycomb filter1comprises a ceramic honeycomb structure10composed of pluralities of cells13partitioned by porous cell walls12, and a peripheral wall11; and plugs14a,14balternately sealing both end portions15a,15bof cells13in a checkerboard pattern. The exhaust gas containing particulate matter flows into cells13aopen on an inlet-side end surface16a, passes through the cell walls12, and then flows out of adjacent cells13bopen on an outlet-side end surface16b, during which particulate matter in the exhaust gas is captured by fine pores (not shown) in the cell walls12.

To form plugs14a,14bin both end portions15a,15bof cells13of the ceramic honeycomb structure10, for example, the following method has been conventionally used. That is, sealing films6made of a resin, etc. are attached to both end surfaces16a,16bof the ceramic honeycomb structure10[FIG. 13(a)], and provided with penetrating pores6a,6balternately at positions corresponding to open cells in a checkerboard pattern by laser beams [FIG. 13(b)], such that only one end portion of each cell13is sealed by the sealing films6.

The end surface16aof the ceramic honeycomb structure10is immersed in a plugging material slurry40containing ceramic powder and a dispersing medium in a vessel90[FIG. 13(c)], and pushed downward to a predetermined depth. The plugging material slurry40is introduced into the predetermined cells13through the penetrating pores6aof the sealing film6, to form plugs14aof the predetermined length in the end portions15aof the cells13at the end surface16a[FIG. 13(d)].

The ceramic honeycomb structure10provided with the plugs14ain one-side end portions15ais taken out of the vessel90[FIG. 13(e)]. With the end surface16bimmersed upside down in the plugging material slurry40, the ceramic honeycomb structure10is pushed downward to a predetermined depth, to form plugs14bin a checkerboard pattern in the other-side end portions15bof the cells13[FIG. 13(f)]. Finally, the ceramic honeycomb structure10provided with the plugs14a,14bin the end portions15a,15bis taken out of the vessel90[FIG. 13(g)].

The plugs14a,14bare dried and sintered, to obtain the ceramic honeycomb filter1having the plugs14a,14bin a checkerboard pattern in the end portions15a,15bof the cells13.

In this conventional method, when the ceramic honeycomb structure10having a plugging material slurry40charged into its end portions is lifted from the vessel90, the plugging material slurry40falls from the end portions, resulting in large unevenness in length of the resultant plugs. A plugging material slurry40may drop off from pluralities of cells13, resulting in a certain area of cells13with no plugs. When the plugs in the ceramic honeycomb structure10are largely uneven in length, the cell walls filtering an exhaust gas also have largely uneven surface areas, providing a filter having unstable particulate-matter-capturing performance and pressure loss performance, and thus resulting in a defective ceramic honeycomb filter, which should be discarded.

To suppress such unevenness in length of the plugs, it is contemplated to slide a ceramic honeycomb structure10having a plugging material slurry40charged into its end portions horizontally when taken out of the vessel90. However, this method not only requires an at least partially detachable vertical wall in the vessel90, but also fails to conduct the formation of plugs continuously.

As a method for suppressing the unevenness in length of the plugs, JP 2008-55347 A discloses, as shown inFIG. 14, a plugging method comprising (a) using an apparatus comprising a vessel100composed of a vertically movable side wall101and a bottom102; a device110for holding a ceramic honeycomb structure, which comprises a pressing member104attached to a vertically movable arm103, and a gripping member105, and a chuck member120for sealing a clearance between an opening of the vessel100and the gripping member105of the gripping device110; (b) charging a plugging material slurry into the vessel100; (c) setting the ceramic honeycomb structure130gripped by the gripping device110in the plugging material slurry in the vessel100; (d) sealing the clearance of the vessel100with the chuck member120; (e) lifting the bottom of the vessel100to introduce the plugging material slurry under pressure into cell end portions of the ceramic honeycomb structure, thereby forming plugs; (f) rotating the bottom102of the vessel100to separate the plugging material slurry filled in the cell end portions from a small amount of the plugging material slurry remaining on the bottom102of the vessel100, thereby preventing the plugging material slurry in the cell end portions from being pulled back into the vessel100; (g) moving the side wall101downward; and then (h) sliding the ceramic honeycomb structure130along the bottom102to take it out of the vessel100.

In the method of JP 2008-55347 A, however, because the bottom102of the vessel100is rotated while the clearance of the vessel100is sealed with the chuck member120, air does not intrude into the plugging material slurry, failing to easily separate the plugging material slurry even with a shear force. Although the rotation of the bottom102gives a shear force to the plugging material slurry, the shear force disappears while the side wall101is moved downward, failing to obtain the effect of rotating the ceramic honeycomb structure130. It has been found that when the ceramic honeycomb structure130slides along the bottom102, the plugging material slurry in the cell end portions is physically separated from that remaining on the bottom102, making the rotation of the bottom102unnecessary.

Furthermore, because the method of JP 2008-55347 A is a batch-type process having many steps, it takes too much time. Particularly, because a batch of a plugging material slurry should be supplied to the vessel100after lifting the once-lowered side wall101, and because a subsequent step should wait until a surface of the plugging material slurry in the vessel100becomes flat, large time loss is inevitable. In addition, if the next batch of a plugging material slurry were supplied with some plugging material slurry remaining on the side wall101and the bottom102of the vessel100, the amount of the plugging material slurry in the vessel100would not be constant, resulting in large unevenness in length of plugs formed in the cells. The supplying of the next batch of a plugging material slurry after removing the plugging material slurry remaining on the side wall101and the bottom102of the vessel100would need additional time.

As a method for forming plugs having uniform length in open end portions of cells, JP 2009-6629 A discloses, as shown inFIG. 15, an apparatus for plugging a honeycomb structure, comprising a plugging material slurry reservoir201, a plate202with pluralities of openings203disposed on the reservoir201, an inlet204for supplying a plugging material slurry to the reservoir201, a valve205disposed in the inlet204, and a piston206for supplying the plugging material slurry under pressure through pluralities of openings203of the plate202. Because the plugging apparatus has pluralities of openings203, the plugging material slurry with high viscosity can pass through the openings203under pressure by the piston206, thereby forming a plugging material slurry layer with uniform thickness on an upper surface of the plate202. Therefore, a leveling process is not necessary after the plugging material slurry is supplied. By disposing a honeycomb structure210, to which a sheet having openings corresponding to predetermined cells is attached, on the plugging material slurry layer on an upper surface of the plate202, and then lifting the piston206toward the plate202, the plugging material slurry is introduced under pressure into the predetermined cells to form plugs. Because the plate202has a flat surface, the honeycomb structure210can be easily horizontally moved on the plate202to be taken out of the plugging apparatus.

Because the honeycomb structure210horizontally moves on the plate202, the apparatus of JP 2009-6629 A does not have such a problem that plugs are pulled back when the honeycomb structure210is taken out. However, when the plugging material slurry is introduced under pressure by the piston206into the honeycomb structure210disposed on the plate202, the plugging material slurry leaks from a lower edge of an outer periphery of the honeycomb structure210. JP 2009-6629 A does not describe any mechanism of preventing the leakage of a plugging material slurry. Because the honeycomb structure210provided with plugs is horizontally moved on the plate202to be taken out of the plugging apparatus, it is not easy to install a mechanism for preventing the leakage of the plugging material slurry on the plate202. The leakage of a plugging material slurry leads to large unevenness in length of plugs in the resultant honeycomb structures210, resulting in defective ceramic honeycomb filters, which should be discarded.

As shown inFIG. 16, JP 2004-25098 A discloses a method for producing a ceramic honeycomb filter, comprising the steps of (a) pressing a lower end surface of a ceramic honeycomb structure301having flow paths303partitioned by pluralities of cells304on a bottom surface of a vessel309containing a plugging material slurry308, thereby introducing the plugging material slurry308under pressure into predetermined cells304, to form plugs302; (b) rotationally lifting the ceramic honeycomb structure301by a slight distance, to form an air layer310between the plugs302and the bottom surface of the vessel309to separate them; and then (c) taking the ceramic honeycomb structure301provided with the plugs302out of the vessel309. In the method of JP 2004-25098 A, however, because the ceramic honeycomb structure301is rotationally lifted from a state where the lower end surface of the ceramic honeycomb structure301is in close contact with the bottom surface of the vessel309, the plugging material slurry308left between the ceramic honeycomb structure301and the vessel309is decompressed, causing the plugs302in the cells304to be pulled back, resulting in unevenness in their lengths.

As shown inFIG. 17, JP 2008-55796 A discloses a method for plugging a ceramic honeycomb structure486by using an apparatus having an annular upper housing463communicating with an air inlet, an annular lower housing464connected to the upper housing463, a movable housing465disposed in the annular upper housing463and the annular lower housing464, which is inflatable by supplying an compressed air, an elastic body467contained in the upper housing463, which is inflatable by supplying an compressed air, and a second elastic body466connected to a vessel; the method comprising gripping a ceramic honeycomb structure486with the movable housing465and the elastic body467inflated by an compressed air; and immersing the ceramic honeycomb structure486in slurry stored in the vessel. However, JP 2008-55796 A fails to teach or suggest a process of taking the ceramic honeycomb structure486provided with plugs out of the vessel. When the ceramic honeycomb structure486provided with plugs is lifted from the vessel without rotation, part of the plugged slurry is pulled back, resulting in unevenness in the length.

OBJECT OF THE INVENTION

Accordingly, an object of the present invention is to provide a method and an apparatus for continuously producing a ceramic honeycomb filter, which can effectively make smaller the unevenness in length of plugs formed by introducing a plugging material slurry under pressure into predetermined cells of a ceramic honeycomb structure, not only in one ceramic honeycomb structure but also among individual ceramic honeycomb structures.

SUMMARY OF THE INVENTION

As a result of intensive research on why plugs formed by introducing a plugging material slurry into predetermined cells of a ceramic honeycomb structure have uneven lengths, the inventor has found that (a) when the ceramic honeycomb structure is lifted from a plugging material slurry in a vessel, at least part of the plugs in cells is pulled back, resulting in the uneven length of plugs; and that (b) to prevent the plugs in the cells from being pulled back, the ceramic honeycomb structure should be rotated before lifted from the plugging material slurry in the vessel, to sufficiently fluidize the plugging material slurry between the ceramic honeycomb structure and a porous plate, and to make it easy for air to enter from the surroundings. The present invention has been completed based on such finding.

Thus, the method of the present invention for producing a ceramic honeycomb filter having a ceramic honeycomb structure having pluralities of longitudinal cells partitioned by porous cell walls, each of the longitudinal cells extending from one end to the other end, and plugs formed in predetermined cells;

uses an apparatus comprising (a) a plugging material slurry reservoir having an inlet through which a plugging material slurry is supplied and an upper opening through which the plugging material slurry exits, (b) a porous plate with pluralities of openings covering the upper opening of the reservoir, and (c) a holding member fixed to an upper end of the reservoir for holding the ceramic honeycomb structure to which a sealing film is attached, the holding member having an elastic member on the inner peripheral side, the elastic member being inflatable to come into close contact with an outer peripheral surface of the ceramic honeycomb structure during the formation of the plugs; the method comprising the steps of

(1) sealing a clearance between the ceramic honeycomb structure and the holding member with the elastic member inflated, while a lower surface of the sealing film attached to a lower end surface of the ceramic honeycomb structure is apart from an upper surface of the porous plate by a distance D of more than 0 mm and 2.0 mm or less;

(2) supplying a predetermined volume of a plugging material slurry into the reservoir through the inlet to introduce it into the predetermined cells of the ceramic honeycomb structure;

(3) rotating the ceramic honeycomb structure after the sealing of the ceramic honeycomb structure is released; and

(4) lifting the ceramic honeycomb structure after the rotation starts.

The apparatus of the present invention for producing the above ceramic honeycomb filter comprises

(a) a plugging material slurry reservoir having an inlet for supplying the plugging material slurry and an upper opening for ejecting the plugging material slurry;

(b) a porous plate with pluralities of openings covering the upper opening of the reservoir;

(c) a holding member fixed to an upper end of the reservoir for holding the ceramic honeycomb structure to which a sealing film is attached, the holding member having an elastic member on the inner peripheral side, the elastic member being inflatable to come into close contact with an outer peripheral surface of the ceramic honeycomb structure during the formation of the plugs;

(d) a device for inflating the elastic member to seal a clearance between the ceramic honeycomb structure and the holding member, while a lower surface of the sealing film attached to a lower end surface of the ceramic honeycomb structure is apart from an upper surface of the porous plate by a distance D of more than 0 mm and 2.0 mm or less;

(e) a device for supplying a predetermined volume of the plugging material slurry into the reservoir through the inlet to introduce it into a predetermined cells of the ceramic honeycomb structure;

(f) a device for rotating the ceramic honeycomb structure after the sealing of the ceramic honeycomb structure is released; and

(g) a device for lifting the ceramic honeycomb structure after the rotation starts.

The openings of the porous plate preferably have inner diameters of 0.5-1.5 mm.

The porous plate is preferably a metal net. The metal net preferably has a mesh size of 0.5-1.5 mm.

The ceramic honeycomb structure is preferably rotated in one direction by a predetermined angle, and then lifted while rotating in an opposite direction.

The holding member preferably has a fixing member having a through-hole communicating with the elastic member, so that air is introduced under pressure into a closed space between the fixing member and the elastic member via the through-hole, to inflate the elastic member to come into close contact with the outer peripheral surface of the ceramic honeycomb structure.

A second elastic member with which the elastic member inflated is brought into close contact is preferably disposed on a lower portion of the outer peripheral surface of the ceramic honeycomb structure.

The apparatus of the present invention for producing a ceramic honeycomb filter preferably further comprises a baffle plate for preventing a flow of the plugging material slurry supplied through the inlet from coming into direct contact with the porous plate between the inlet and the upper opening in the reservoir. The baffle plate preferably has pluralities of pores with an opening area ratio of 10-60%.

Effects of the Invention

According to the present invention, by inflating an elastic member to seal a clearance between a ceramic honeycomb structure and a holding member, while a lower surface of a sealing film attached to a lower end surface of a ceramic honeycomb structure is apart from an upper surface of a porous plate by a distance D of more than 0 mm and 2.0 mm or less, introducing a plugging material slurry into predetermined cells of the ceramic honeycomb structure, rotating the ceramic honeycomb structure, and lifting it after the rotation starts, the plugging material slurry between the lower end surface of the ceramic honeycomb structure and the porous plate is sufficiently fluidized, and air easily enters from the surroundings, due to the rotation of the ceramic honeycomb structure with a gap of the distance D. As a result, when the ceramic honeycomb structure starts to be lifted from the holding member, plugs in the cells would not be pulled back. Accordingly, the length unevenness of the plugs formed in the cells can be reduced not only in one ceramic honeycomb structure but also among the individual ceramic honeycomb structures.

The method and apparatus of the present invention can continuously form the plugs in the ceramic honeycomb structures with good efficiency by introducing a predetermined volume of a plugging material slurry into predetermined cells of the ceramic honeycomb structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention will be specifically explained below without intention of restricting the present invention thereto. Proper modifications and improvements can be made based on the usual knowledge of those skilled in the art within the scope of the present invention. It should be noted that explanations of each embodiment will be applicable to any other embodiments unless otherwise mentioned.

[1] Production of Ceramic Honeycomb Structure

A ceramic honeycomb structure used for a ceramic honeycomb filter is formed by mixing and blending at least ceramic material powder, an organic binder and water, and if necessary a pore-forming material, a lubricant, etc., to prepare a moldable material; extrusion-molding it to a honeycomb shape having, for example, an outer diameter of 150 mm or more, a wall thickness of 0.2-0.5 mm and a cell density of 100-400 cells/inch by using an extrusion-molding die; drying the resultant ceramic honeycomb green body in a hot-air furnace, a microwave-drying apparatus, etc.; roughly cutting it to a predetermined length with a band saw; and then sintering it. Both ends of the sintered ceramic honeycomb structure is ground to a predetermined length with a diamond wheel. Materials preferable for the ceramic honeycomb structure include cordierite, alumina, silica, silicon nitride, silicon carbide, aluminum titanate, LAS, etc. Among them, a ceramic comprising cordierite as a main crystal phase is most preferable, because it is inexpensive and has excellent heat resistance and chemical stability.

[2] Attachment and Perforation of Sealing Film

Sealing films6,6are attached to both end surfaces16a,16bof the ceramic honeycomb structure10shown inFIG. 1(a)[FIG. 1(b)], and provided with penetrating pores6a,6bcorresponding to the cells13in a checkerboard pattern, to have cells13aopen on the end surface16aand cells13bopen on the end surface16b[FIG. 1(c)]. The penetrating pores6a,6bare preferably formed by laser beams because of accuracy and high speed, but may be formed by any methods capable of perforating the sealing films6,6, for example, by perforating the films with a metal rod having a sharp tip or by pushing a hot metal rod against the films. With the penetrating pores6a,6bformed, the cells13acommunicated with penetrating pores6aof the sealing film6at one end surface16aare sealed with the sealing film6at the other end surface16b, and the cells13bcommunicated with penetrating pores6bof the sealing film6at the other end surface16bare sealed with the sealing film6at one end surface16a.

The sealing film is preferably a rubber film based on an acrylic rubber, a styrene-butadiene rubber, etc., or a resin film made of polypropylene, etc. to which an acrylic adhesive is attached.

The plugging material slurry contains at least ceramic material powder and water, and may contain an organic binder, an inorganic binder, etc., if necessary.

(1) Ceramic Material Powder

The ceramic material powder is preferably made of the same material as that of the ceramic honeycomb structure, for integration after sintering. Therefore, it may be made of cordierite, alumina, silica, silicon nitride, silicon carbide, aluminum titanate, LAS, etc. When the ceramic honeycomb structure is made of cordierite, the ceramic material powder is preferably cordierite-producing material powder which forms cordierite after sintering, or sintered cordierite powder for reduced shrinkage during sintering plugs.

The particle size distribution of the ceramic material powder may have either one peak or two or more peaks. To make it easy to introduce a plugging material slurry into the cells of the ceramic honeycomb structure, the average particle size of the ceramic material powder is preferably 30-150 μm.

(2) Viscosity of Plugging Material Slurry

The viscosity of the plugging material slurry is preferably 10-25 Pa·s. When the viscosity of the plugging material slurry is as low as less than 10 Pa·s, just-formed plugs cannot fully remain in the cells while the ceramic honeycomb structure is lifted from the reservoir, failing to form the plugs of a desired length, resulting in large unevenness in length of the plugs. On the other hand, when the viscosity of the plugging material slurry is as high as more than 25 Pa·s, it is difficult to introduce the plugging material slurry into the cells, resulting in large unevenness in length of the plugs not only in one ceramic honeycomb structure but also among individual ceramic honeycomb structures. The viscosity of the plugging material slurry is preferably 12-22 Pa·s.

FIGS. 2(a)-2(d)show an example of apparatuses for introducing a plugging material slurry into end portions15a,15bof predetermined cells13open on both end surfaces16a,16bof the ceramic honeycomb structure10. This apparatus comprises a plugging material slurry reservoir20, a porous plate24covering an upper opening of the reservoir20, and a holding member30fixed to an upper end of the reservoir20. The plugging material slurry reservoir20, the porous plate24and the holding member30may be fixed to each other by bolts, clamps, etc., and may be provided with sealing members such as rubber packing (not shown) between them.

(1) Plugging Material Slurry Reservoir

The plugging material slurry reservoir20shown inFIGS. 2(a) and 2(b)comprises a thin cylindrical reservoir body21having an upper opening23, and an inlet22located in a central bottom portion of the reservoir body21. The inlet22is connected to a pump (not shown), and supplied with a predetermined volume of a plugging material slurry40. The reservoir body21needs to have a volume enough for a predetermined volume of the supplied slurry40to have sufficient surface smoothness. When the reservoir body21has an insufficient volume, there is a density (pressure) distribution in the plugging material slurry40supplied to the reservoir body21, resulting in uneven lengths of the resultant plugs.

To supply the plugging material slurry40in a volume necessary for forming plugs of a desired length, a Mohno pump is preferably used as the pump. The Mohno pump supplies a liquid by rotating a spiral shaft in a pipe. Because a constant volume of a fluid is supplied by the rotation of the spiral shaft, the Mohno pump can always supply a predetermined volume of a plugging material slurry. Therefore, a plugging material slurry having a volume corresponding to the desired length of the plugs can surely be introduced into the cells.

As shown inFIG. 2(d), the porous plate24covering the upper opening23of the plugging material slurry reservoir20comprises a porous portion24athrough which the plugging material slurry40passes, and a ring portion24bfixing an outer periphery of the porous portion24a. Because the plugging material slurry40passes through the porous plate24, the porous plate24should have enough mechanical strength, wear resistance and corrosion resistance. Therefore, the porous plate24is preferably made of a metal, particularly, stainless steel.

When the porous portion24ais a plate having pluralities of openings, it may be integrated with the ring portion24b. The porous portion24aof the porous plate24may be a metal net, because of easy separation of a plugging material slurry layer40aformed between the porous portion24aand a lower end surface of the ceramic honeycomb structure10(seeFIG. 9), and a large opening ratio and inexpensiveness. When the porous portion24ais a metal net, an outer periphery of the metal net is preferably welded to the ring portion24b.

The inner diameters of openings of the porous plate24are preferably about 0.5-1.5 mm, more preferably about 0.7-1.3 mm. When the opening is in a square shape, the inner diameter is defined as its diagonal length. Therefore, when the porous portion24ais a metal net, the inner diameters of openings of the porous portion24aare 1.4 times the mesh size of the metal net. An opening ratio of the porous plate (ratio of a total opening area to a whole area of the porous portion24a) is preferably 30% or more to make it easy to introduce the plugging material slurry40into the cells, and preferably 80% or less to make it easy to separate the plugging material slurry layer40a. The opening ratio of the porous plate is more preferably 40-70%.

When the porous portion24ais a metal net, the metal net24apreferably has a mesh size of 0.5-1.5 mm (inner diameter: 1.4 times). When the metal net24ahas a mesh size of less than 0.5 mm (inner diameter: less than 0.7 mm), it may be clogged with ceramic material powder in the plugging material slurry40. On the other hand, when the metal net24ahas a mesh size of more than 1.5 mm (inner diameter: more than 2.1 mm), the plugging material slurry layer40abetween a lower surface6eof the sealing film6attached to the lower end surface of the ceramic honeycomb structure10and an upper surface of the porous plate24is hardly separated by the rotation of the ceramic honeycomb structure10. The metal net24amore preferably has a mesh size of 0.7-1.3 mm. In addition, metal wires constituting the metal net24apreferably have a diameter of 0.1-0.8 mm. When the wire diameter is less than 0.1 mm, the metal net24ais easily deformed to have undulation by a small force, failing to secure the distance D between itself and the lower surface6eof the film6. On the other hand, the wire diameter of more than 0.8 mm is economically meaningless, because the metal net24acan withstand sufficient tension without having such a large wire diameter. The metal wire constituting the metal net24amore preferably has a diameter of 0.2-0.7 mm. Incidentally, two or more metal net24amay be laminated, and their openings may have the same or different sizes.

(3) Holding Member

As shown inFIGS. 2(a)-2(c), the holding member30comprises an annular fixing member34made of a metal (for example, stainless steel), and an annular, inflatable elastic member33disposed on its inner peripheral side. A cavity31is defined inside the annular elastic member33.

As shown inFIGS. 3(a) and 3(b)in detail, the elastic member33comprises an inflatable portion33aand both end portions33b,33b. Both end portions33b,33bpartially inserted into grooves34a,34aof the fixing member34are fixed to the fixing member34by screws37,37via annular pressing plates36a,36bmade of stainless steel. A closed space35is formed between the inflatable portion33aand an inner peripheral surface of the fixing member34.

By injecting air into the closed space35between the elastic member33and the fixing member34through a hole38, the inflatable portion33aof the elastic member33is inflated. While the cavity31defined by the inner peripheral surface of the not-inflated inflatable portion33aof the elastic member33has a larger diameter than the outer diameter of the ceramic honeycomb structure10[FIG. 3(a)], the inner peripheral surface of the inflated inflatable portion33aof the elastic member33comes into close contact with an outer peripheral surface of the ceramic honeycomb structure10[FIG. 3(c)]. The difference between the diameter of the cavity31and the outer diameter of the ceramic honeycomb structure10is preferably about 5-20 mm.

Materials for the elastic member33are not particularly limited as long as they are inflatable, but their preferred practical examples are natural rubber, butadiene rubber, butyl rubber, nitrile rubber, ethylene propylene rubber, chloroprene rubber, acrylic rubber, urethane rubber, silicone rubber, fluorine rubber, etc. Among them, silicone rubber is most preferable.

Though direct contact is permissible, the elastic member33is preferably brought into contact with the ceramic honeycomb structure10via a second elastic member18attached to an outer peripheral surface of the ceramic honeycomb structure10, as shown inFIG. 4. The second elastic member18attached to the outer peripheral surface of the ceramic honeycomb structure10is preferably softer than the elastic member33. As long as the second elastic member18is softer than the elastic member33, materials for the second elastic member18may be the same as or different from those of the elastic member33, but are preferably flexible, elastic materials such as foamed urethane rubber. The second elastic member18is preferably an annular, flexible rubber belt for easy attachment. When the inflated elastic member33is brought into contact with the outer peripheral surface of the ceramic honeycomb structure10via the second elastic member18, (a) a peripheral wall of the ceramic honeycomb structure10is not damaged, and (b) the elastic member33is brought into well close contact with the second elastic member18, resulting in complete sealing between the holding member30and the ceramic honeycomb structure10.

(4) Another Example of Plugging-Material-Slurry-Introducing Apparatuses

FIG. 5shows another example of plugging-material-slurry-introducing apparatuses, which is used in the present invention. The same reference numerals are assigned to the same members as in the above example. This apparatus comprises a plugging material slurry reservoir20, a porous plate24covering an upper opening of the reservoir20, a holding member30fixed to an upper end of the reservoir20, a pipe50connected to an inlet22of the plugging material slurry reservoir20, a pump52disposed in a portion of the pipe50, a plugging material slurry tank53connected to an upstream end of the pipe50, and a blade54for stirring the plugging material slurry40in the tank53.

As shown inFIG. 6, the plugging material slurry reservoir20comprises a thin cylindrical reservoir body21having a funnel-shaped bottom portion21a, an inlet22located in a center portion of the funnel-shaped bottom portion21a, an upper opening23of the reservoir body21, and a baffle plate25supported by pluralities of legs26between the inlet22and the upper opening23. The reservoir body21should have a volume enough for a predetermined volume of a plugging material slurry40supplied to have sufficient surface smoothness.

The baffle plate25may be a nonporous plate, but preferably has pluralities of pores to make a flow of the plugging material slurry40more uniform. The opening ratio of the pores is preferably 10-60%. The baffle plate25between the inlet22and the upper opening23can disperse a flow of the plugging material slurry40supplied to the reservoir body21through the inlet22, thereby making the plugging material slurry40more uniform in the reservoir body21.

As in the above example, the pump52is preferably a Mohno pump supplying a constant volume of a plugging material slurry40by rotating a spiral shaft in a pipe.

As shown inFIG. 7, when the ceramic honeycomb structure10gripped by a handling device60is set in this plugging-material-slurry-introducing apparatus, such that the lower surface6eof the sealing film6attached to a lower end surface of the ceramic honeycomb structure10is apart from an upper surface of the porous plate24by a distance D, there is a slight clearance [(5-20 mm)/2] between the inner surface of the elastic member33of the holding member30and the outer peripheral surface of the ceramic honeycomb structure10(the outer peripheral surface of the second elastic member18in the depicted example). This clearance is sealed by the inflated elastic member33.

[5] Formation of Plugs

Using the plugging-material-slurry-introducing apparatus shown inFIGS. 2 and 3with a metal net as the porous plate24, a method for forming plugs in the ceramic honeycomb structure10will be explained in detail referring toFIG. 8. In the example shown inFIG. 8, second elastic members18,18are attached to both upper and lower end portions of the outer periphery of the ceramic honeycomb structure10. The difference between the diameter of the cavity31and the outer diameter of the second elastic member18attached to the ceramic honeycomb structure10is preferably about 5-20 mm.

(1) Setting of Ceramic Honeycomb Structure

The ceramic honeycomb structure10having sealing films6,6open on upper and lower end surfaces16a,16band second elastic members18,18attached to both upper and lower end portions is gripped by a handling device (for example, a robot hand)60[FIG. 8(a)], and then a lower end of the ceramic honeycomb structure10is set in the holding member30, such that a lower surface6eof the sealing film6attached to the lower end surface16aof the ceramic honeycomb structure10is apart from the upper surface of the porous plate24by a distance D [FIG. 8(b)].

Before the ceramic honeycomb structure10is set, the plugging material slurry40is charged into the plugging material slurry reservoir20so as to slightly cover the upper surface of the porous plate24. This surface height of the plugging material slurry40is adjusted to be the same as that of a plugging material slurry left on the metal net24when the ceramic honeycomb structure10provided with plugs is lifted. In a case where this adjustment is difficult, an adjusting ceramic honeycomb structure to which a sealing film6is attached is first set while the lower surface6eof the sealing film6is apart from the upper surface of the metal net24by the distance D, a predetermined volume of the plugging material slurry40is supplied to the plugging material slurry reservoir20through the inlet22to introduce the plugging material slurry40into the predetermined cells13aof the adjusting ceramic honeycomb structure, and the adjusting ceramic honeycomb structure is then rotationally lifted to determine the surface height of the plugging material slurry left on the metal net24, which corresponds to the distance D.

By injecting air into the closed space35between the elastic member33and the inner peripheral surface of the fixing member34through injection holes38, the elastic member33is inflated to come into close contact with the second elastic member18attached to the outer peripheral surface of the ceramic honeycomb structure10[FIG. 8(c)]. With the elastic member33inflated, the ceramic honeycomb structure10is held by the holding member30at such a position the lower surface6eof the sealing film6attached to the lower end surface16aof the adjusting ceramic honeycomb structure10is apart from the upper surface of the metal net24by the distance D of more than 0 mm and 2.0 mm or less.

The distance D is set such that the plugging material slurry layer40abetween the lower surface6eof the sealing film6attached to the lower end surface of the adjusting ceramic honeycomb structure10and the upper surface of the metal net24(defined by a flat surface in contact with the upper surface of the metal net) is sufficiently fluidized by the rotation of the ceramic honeycomb structure10. Specifically, the distance D is more than 0 mm and 2.0 mm or less. When the distance D is 0 mm (in close contact with the metal net24), the plugging material slurry layer40ahardly exists, resulting in insufficient fluidization even when the ceramic honeycomb structure10is rotated. To suitably fluidize the plugging material slurry layer40a, the distance D is preferably 0.1 mm or more. On the other hand, when the distance D is more than 2.0 mm, the plugging material slurry layer40ais too thick, resulting in insufficient fluidization even when the ceramic honeycomb structure10is rotated. The distance D is preferably 1.0 mm or less, more preferably 0.7 mm or less.

The distance D can be measured before setting the holding member30to the plugging material slurry reservoir20, for example, by determining a coordinate position of the handling device (robot hand)60providing the distance D in advance; placing the ceramic honeycomb structure10to which the sealing film6is attached on the metal net24at the above coordinate position by the handling device60; and inserting a feeler gauge into the gap between the ceramic honeycomb structure10and the metal net24at four points circumferentially separate by 90°, to measure the distance between the lower surface6aof the sealing film6attached to the lower end surface of the ceramic honeycomb structure10and the upper surface of the metal net24.

(2) Introduction of Plugging Material Slurry

A predetermined volume of the plugging material slurry40supplied to the plugging material slurry reservoir20through the inlet22by a pump (not shown) is introduced into the predetermined cells13aof the ceramic honeycomb structure10through the metal net24[FIG. 8(d)], forming plugs14aof a desired length in the lower end portions15aof the predetermined cells13a.

After the plugging material slurry40is introduced into the predetermined cells13aof the ceramic honeycomb structure10, the air is removed from the elastic member33to release close contact with the ceramic honeycomb structure10[FIG. 8(e)]. Though the rotation of the ceramic honeycomb structure10may start immediately thereafter, it starts preferably after a predetermined time lapse. This lapse time is preferably 1 second or more, and it is preferably 30 seconds or less for production efficiency. This lapse time is more preferably 2-10 seconds, most preferably 3-5 seconds.

(3) Rotation of Ceramic Honeycomb Structure

Immediately or a predetermined period of time after close contact with the ceramic honeycomb structure10is released, the ceramic honeycomb structure10is freed from gripping and rotated [FIG. 8(e)]. By the rotation of ceramic honeycomb structure10, the plugging material slurry layer40abetween the lower end surface16aof the ceramic honeycomb structure10and the metal net24is fluidized, and thus easily separated from the plugs14aformed in the cells of the ceramic honeycomb structure10and the plugging material slurry40under the metal net24.

A rotation angle before the start of lifting is preferably 30° or more, more preferably 60° or more. Because the ceramic honeycomb structure10needs to be rotated only by an angle enough to fluidize the plugging material slurry layer40abetween the lower end surface16aof the ceramic honeycomb structure10and the metal net24, the rotation angle may be 180° or less. The rotation speed is preferably 10-50°/second, more preferably 15-40°/second. When the rotation angle is less than 30° or the rotation speed is less than 10°/second, the plugging material slurry layer40ais not sufficiently fluidized. More than 50°/second of the rotation speed rather deteriorates the separation of the plugging material slurry layer40a.

(4) Lifting of Ceramic Honeycomb Structure

After rotation is started, the ceramic honeycomb structure10provided with the plugs14ais rotationally lifted [FIG. 8(f)]. A lifting speed of the ceramic honeycomb structure10is preferably 20-80 mm/second. When the lifting speed is less than 20 mm/second, the separation effect of the plugging material slurry disappears. On the other hand, when the lifting speed is as high as more than 80 mm/second, the plugs may fall from some cells of the ceramic honeycomb structure10, resulting in large unevenness in length of the plugs in the cells not only in one ceramic honeycomb structure10but also among individual ceramic honeycomb structures10. The lifting speed of the ceramic honeycomb structure10is more preferably 30-70 mm/second.

The rotation conditions may be the same or different between the rotation step and the lifting step. In the latter case, the rotation direction is preferably reversed. The number of reversing the rotation direction is not limited to one, but may be two or more. With the rotation direction reversed, the plugging material slurry layer40ais more easily separated from the plugs14in the cells13and the plugging material slurry40under the metal net24.

The rotation of the ceramic honeycomb structure10need not be performed until the lifting step is finished, but may be terminated in the course of the lifting step as long as the plugs14in the cells become separable from the plugging material slurry layer40aon the metal net24. However, to certainly accomplish the separation, the rotation is preferably performed until the lifting step is finished.

After the plugs14aare formed in one-side end portions15a, the ceramic honeycomb structure10is turned upside down [FIG. 8(g)], and then the steps shown inFIGS. 8(a)-8(f)are repeated. The ceramic honeycomb structure10provided with the plugs14a,14bin both end portions is taken out of the holding member30, and then a new ceramic honeycomb structure10is set in the holding member30. By repeating the steps shown inFIGS. 8(a)-8(g)in this way, plugs can be continuously formed in the ceramic honeycomb structures10with good efficiency.

Though the holding, rotation and lifting of the ceramic honeycomb structures10may be conducted by different handling devices60, they can be conducted by only one robot hand for simplification of the apparatus and easy change of settings.

(5) Principle of Separation of Plugging Material Slurry Layer

The plugging material slurry40introduced into the cells turns to plugs14with its water absorbed by the ceramic honeycomb structure10. However, the just-formed plugs14are still at least partially fluid and connected to the plugging material slurry layer40aon the metal net24. When the ceramic honeycomb structure10is lifted in this state, the plugs14in the cells are pulled back by the plugging material slurry layer40a, resulting in not only length unevenness but also dropping off from part of the cells.

As a result of intensive research with the above knowledge, it may be presumed that the effect of the present invention is obtained by the following principle. As shown inFIG. 9, when the ceramic honeycomb structure10placed above the metal net24by a slight distance D is rotated, the plugging material slurry layer40abetween the lower surface6eof the sealing film attached to the lower end surface16aof the ceramic honeycomb structure10and the upper surface and the metal net24is sufficiently fluidized due to a strong shearing force, making the plugging material slurry layer40aeasily separable. In addition, because the ceramic honeycomb structure10is rotated after the sealing is released, air easily enters the fluidized plugging material slurry layer40afrom the surroundings. Though not necessarily clear, this appears to be because the bonding of ceramic particles in the fluidized plugging material slurry layer40ais weak due to so-called “thixotropy,” thereby permitting air to easily enter.

When the ceramic honeycomb structure10is rotationally lifted in such a state, the fluidized plugging material slurry layer40astarts to be separated, and then air enters in separated portions to form partial air layers. With such air layers formed in the plugging material slurry layer40agradually expanding, adjacent air layers are connected to secure the separation of the entire plugging material slurry layer40a.

Even when the plugging material slurry layer40ais sufficiently fluidized, air entering from the surroundings is necessary for the separation of the plugging material slurry layer40a. When the ceramic honeycomb structure10is lifted without air entering from the surroundings, the plugging material slurry layer40ais decompressed, thereby sucking the plugs14in the cells13, resulting in length unevenness and dropping of the plugs14. Thus, the important feature of the present invention is that the plugging material slurry layer40ais fluidized by the rotation of the ceramic honeycomb structure10, while permitting air to enter from the surroundings. Accordingly, the ceramic honeycomb structure10is rotated after the sealing is released, and lifted after the rotation starts.

Because the metal net used as the porous plate24provides the plugging material slurry layer40awith an uneven outer peripheral surface, air easily enters from the surroundings. Therefore, the plugging material slurry layer40ais rapidly separated, securely preventing the plugs14in the cells from being pulled back.

(6) Drying and Sintering

The plugs14a,14bformed in the end portions15a,15bof the predetermined cells13a,13bare dried and sintered. The drying and sintering conditions per se may be known. For example, the drying may be conducted using hot air at 80-150° C., microwaves, high-frequency waves, etc. Among them, high frequency waves are preferable because they can heat and dry only the plugs. The end surfaces16a,16bprovided with the plugs14a,14bmay be preliminarily dried on an electric hot plate before the above drying. The dried plugs may or may not be sintered, but the sintering, if conducted, is preferably at the sintering temperature of the ceramic material powder constituting the plugging material slurry (for example, 1400° C., when the ceramic material powder is made of a cordierite-producing material) for 5 hours. A furnace used for sintering may be batch-type or continuous. As a continuous furnace, a roller hearth kiln is preferable.

The present invention will be explained in more detail by Examples below without intention of restriction.

Kaolin powder, talc powder, silica powder and alumina powder were mixed to prepare cordierite-producing material powder comprising, by mass, 48-52% of SiO2, 33-37% of Al2O3, and 12-15% of MgO, which was then fully mixed with methylcellulose (binder), a lubricant, and foamed resin balloons (pore-forming material) in a dry state. With a predetermined amount of water added, they were sufficiently blended to prepare a plasticized moldable ceramic material. The moldable ceramic material was molded by an extrusion-molding die, and cut to a honeycomb green body of 270 mm in diameter and 300 mm in length. The honeycomb green body was dried and sintered to obtain a cordierite-type ceramic honeycomb structure having a cell wall thickness of 0.3 mm, a cell pitch of 1.5 mm, porosity of 62%, and an average pore size of 21 μm. The outer periphery of the ceramic honeycomb structure was cut to an outer diameter of 265 mm, and end portions15a,15bwere then ground to obtain the ceramic honeycomb structure10of final shape [FIG. 1(a)].

A sealing resin film of 0.09 mm in thickness was attached to each of both ends of the ceramic honeycomb structure10[FIG. 1(b)], and each sealing resin film was provided with penetrating pores at positions corresponding to cells to be plugged in a checkerboard pattern by laser beams [FIG. 1(c)].

100 parts by mass of ceramic material powder (cordierite-producing material powder, average particle size: 53 μm) was mixed with 1 part by mass of methylcellulose (binder), and then blended with 3 parts by mass of a dispersant and 57 parts by mass of ion-exchanged water, to prepare a plugging material slurry. The cordierite-producing material obtained by mixing, by mass, 6.3% of Kaolin, 41.1% of talc, 18.2% of silica, 23.3% of alumina, and 11.1% of aluminum hydroxide, was used. The plugging material slurry had viscosity of 15.0 Pa·s.

In the apparatus shown inFIGS. 2-4 and 8, the plugging material slurry40was supplied to the reservoir20through the inlet22by a Mohno pump (not shown). The metal net24[mesh size: 0.83 mm (55% of the cell pitch), wire diameter: 0.25 mm, opening ratio: 59%] was disposed on the upper opening23of the reservoir20, and then the holding member30was fixed thereon. The inner diameter of the holding member30was 270 mm, which was the same as the size of the upper opening23of the reservoir20, and larger than the 266-mm outer diameter of the ceramic honeycomb structure.

The holding member30had the fixing member34made of stainless steel, and the elastic member33made of silicone rubber was disposed on its inner periphery. With both end portions33b,33binserted into the grooves34a,34aof the fixing member34, the elastic member33was fixed to the fixing member34by the annular pressing plates36a,36bmade of stainless steel and screws37.

The plugging material slurry40was charged into the plugging material slurry reservoir20by a Mohno pump (not shown) so that it covered the upper surface of the metal net24. A gap between a surface of the plugging material slurry on the metal net24and the lower end surface of the ceramic honeycomb structure10had a distance D. With the outer peripheral surface110of the ceramic honeycomb structure10gripped by the handling device60, the lower surface6eof the sealing film6attached to the lower end surface of the ceramic honeycomb structure10was apart from the upper surface of the metal net24by the distance D of 0.1 mm.

The distance D was measured by placing the ceramic honeycomb structure gripped by the handling device60above the metal net without the holding member30, and inserting a feeler gauge into a gap between the surface of the sealing film attached to the lower end surface of the honeycomb structure and the metal net at four points circumferentially separate by 90°.

With air injected into a closed space35between the elastic member33and the inner periphery of the fixing member34through injection holes38, the elastic member33was inflated to come into close contact with the outer peripheral surface of the ceramic honeycomb structure10. In this state, a predetermined volume (for example, 200 cm3) of a plugging material slurry40was supplied to the reservoir20by a Mohno pump (not shown), to introduce 200 cm3of the plugging material slurry40into the predetermined cells13aof the ceramic honeycomb structure10through the penetrating pores6aof the sealing film6.

After a predetermined period of time (for example, 10 seconds) passed, the air introduced into the closed space35between the elastic member33and the inner periphery of the fixing member34was released. Thereafter, the ceramic honeycomb structure10whose outer peripheral surface was gripped by the handling device60was rotated in one direction by an angle of 90° at a speed of 20°/second (rotation step), and then lifted at a speed of 50 mm/second while rotating in the opposite direction at the same speed (lifting step). The holding member30was then taken out.

The plugs14bwere formed in the other-side end portions15bof the ceramic honeycomb structure10in the same manner. Both plugs14a,14bwere dried and sintered. An outer peripheral wall was formed on a peripheral surface of the ceramic honeycomb structure. In this way, 10 ceramic honeycomb filters were obtained. Each ceramic honeycomb filter had plugs of 10 mm in length.

10 ceramic honeycomb filters were obtained, with the plugs formed in the predetermined cells of a ceramic honeycomb structure in the same manner as in Example 1, except for changing the rotation angle to 60° in the rotation step.

10 ceramic honeycomb filters were obtained, with the plugs formed in the predetermined cells of a ceramic honeycomb structure in the same manner as in Example 1, except for changing the rotation speed to 10°/second in the rotation step.

10 ceramic honeycomb filters were obtained, with the plugs formed in the predetermined cells of the ceramic honeycomb structure in the same manner as in Example 1, except for changing a lifting speed to 20 mm/second in the lifting step.

10 ceramic honeycomb filters were obtained, with the plugs formed in the ceramic honeycomb structure in the same manner as in Example 1, except that flexible, urethane rubber rings of 5 cm in width were attached to lower end portions of the outer peripheral surface of the ceramic honeycomb structure as the second elastic member. With the attached urethane rubber rings, the outer peripheral surface of the ceramic honeycomb structure was brought into closer contact with the holding member, remarkably reducing the amount of a plugging material slurry attached to the outer peripheral surface of the ceramic honeycomb structure and the inner peripheral surface of the holding member.

10 ceramic honeycomb filters were obtained, with the plugs formed in the ceramic honeycomb structure in the same manner as in Example 5, except for using the apparatus shown inFIGS. 5 and 6. The baffle plate25inFIG. 6had pores of 5.0 mm in diameter and an opening ratio of 35.4%.

10 ceramic honeycomb filters were obtained, with the plugs formed in the ceramic honeycomb structure in the same manner as in Example 1, except for using the metal net covering the upper opening of the reservoir20and having a mesh size of 1.7 mm (113% of cell pitch), a wire diameter of 0.25 mm and an opening ratio of 76%.

50% by mass of cordierite-producing material powder having an average particle size of 10 μm and 50% by mass of cordierite powder having an average particle size of 120 μm were mixed, to obtain ceramic material powder (average particle size: 72.5 μm) having a particle size distribution having a first peak at 105 μm and a second peak at 8.5 μm, the frequency of the first peak being higher than that of the second peak. The viscosity of the plugging material slurry contained in this ceramic material powder was 8.5 Pa·s. 10 ceramic honeycomb filters were obtained, with the plugs formed in the ceramic honeycomb structure in the same manner as in Example 1 except for using this plugging material slurry. The particle size distribution of the ceramic material powder was measured by a particle size distribution meter (Microtrack MT3000 available from Nikkiso Co., Ltd.).

COMPARATIVE EXAMPLE 1

10 ceramic honeycomb filters were obtained, with the plugs formed in the predetermined cells of the ceramic honeycomb structure in the same manner as in Example 1, except that the ceramic honeycomb structure was lifted at a speed of 50 mm/second without rotation.

10 ceramic honeycomb filters were obtained, with the plugs formed in the predetermined cells of the ceramic honeycomb structure in the same manner as in Example 1, except for changing the rotation angle to 10° in the rotation step.

10 ceramic honeycomb filters were obtained, with the plugs formed in the predetermined cells of the ceramic honeycomb structure in the same manner as in Example 1, except for changing the rotation speed to 60°/second in the rotation step.

10 ceramic honeycomb filters were obtained, with the plugs formed in the predetermined cells of the ceramic honeycomb structure in the same manner as in Example 1, except for changing a lifting speed to 90 mm/second in the lifting step.

COMPARATIVE EXAMPLE 2

10 ceramic honeycomb filters were obtained, with the plugs formed in the ceramic honeycomb structure in the same manner as in Example 1, except that the rotation step was conducted with the lower end surface of the ceramic honeycomb structure in contact with the upper surface of the metal net.

CONVENTIONAL EXAMPLE 1

In the conventional apparatus shown inFIG. 13, the ceramic honeycomb structure10was pressed downward (shown by the arrow F) to immerse the end portions15ain a plugging material slurry in the vessel90, so that the plugging material slurry was introduced into the predetermined cells to form plugs. Then, the ceramic honeycomb structure was lifted without rotation, and taken out of the vessel90. 10 ceramic honeycomb filters were obtained by this method.

As shown inFIG. 10, the lengths of the plugs (shown by X) were measured at 17 points for each end portion (34 points for both end portions) of each ceramic honeycomb filter. As illustrated inFIG. 11, the measuring method comprised inserting a rod-shaped SUS member R of 0.8 mm in diameter (having a mark Q at a position separate from an end E1by a distance corresponding to the length Lc of the ceramic honeycomb filter) into each cell13until it came into contact with the plug14, and then measuring a length L between a position P where the rod-shaped member R crossed the end surface16of the ceramic honeycomb filter and the mark Q. The measured length L was regarded as the length L of the plugs14.

In each of Examples 1-11, Comparative Examples 1 and 2, and Conventional Example 1, the unevenness in length L of the plugs in one ceramic honeycomb filter was evaluated by calculating standard deviation of the lengths L of 34 plugs, according to the following standards. The unevenness in length L of the plugs among individual ceramic honeycomb filters was evaluated by determining the average length Lav of the plugs in each ceramic honeycomb filter, and calculating standard deviation of the average lengths Lav of 10 ceramic honeycomb filters, according to the following standards.

Excellent: The standard deviation was less than 0.50,

Good: The standard deviation was 0.50 or more and less than 0.60, and

Poor: The standard deviation was 0.60 or more.

The result is shown in Table 1.

As is clear from Table 1, (a) in Examples 1-11, the length unevenness of the plugs was small not only in one ceramic honeycomb structure but also among individual ceramic honeycomb structures, and no dropping of the plugs was observed, but (b) in Comparative Examples 1 and 2, and Conventional Example 1, the length unevenness of the plugs was large both in one ceramic honeycomb structure and among individual ceramic honeycomb structures, and some adjacent plugs dropped in some ceramic honeycomb filters.

DESCRIPTION OF REFERENCE NUMERALS

1: Ceramic honeycomb filter6: Sealing film6a,6b: Penetrating pore of sealing film6e: Lower surface of sealing film10: Ceramic honeycomb structure11: Peripheral wall12: Cell wall13,13a,13b: Cell14a,14b: Plug15a,15b: End portion of ceramic honeycomb structure16,16a,16b: End surface of ceramic honeycomb structure20: Plugging material slurry reservoir21: Reservoir body22: Inlet of reservoir23: Upper opening of reservoir24: Porous plate (metal net)24a: Porous portion of porous plate24b: Ring portion of porous plate25: Baffle plate26: Leg of baffle plate30: Holding member31: Cavity33: Elastic member33a: Inflatable portion of elastic member33b: End portion of elastic member34: Fixing member34a: Groove of fixing member35: Closed space36a,36b: Annular pressing plate37: Screw38: Injection hole40: Plugging material slurry40a: Plugging material slurry layer between a lower surface of a ceramic honeycomb structure and an upper surface of a porous plate50: Pipe52: Pump53: Tank54: BladeD: Distance between a lower surface of a sealing film attached to a lower end surface of a ceramic honeycomb structure and an upper surface of a porous plate