Patent Publication Number: US-2015086670-A1

Title: Die for forming honeycomb structure and manufacturing method therefor

Description:
TECHNICAL FIELD 
     The present invention relates to a die for forming a honeycomb structure and a method of manufacturing the same, and more particularly, to a die for forming a honeycomb structure and a method of manufacturing the same, capable of suppressing a distortion in the honeycomb structure during manufacturing of the honeycomb structure. 
     BACKGROUND ART 
     Heretofore, a die for forming a honeycomb structure has been used as an extrusion die for manufacturing a ceramic honeycomb structure by extruding a ceramic raw material. In the die for forming a honeycomb structure, for example, a second plate-shaped portion having a plurality of back holes opened on both sides and a first plate-shaped portion having slits communicating with the back holes formed in the second plate-shaped portion are stacked. In such a die for forming a honeycomb structure, for example, a first plate-shaped member and a second plate-shaped member having back holes are bonded through a hot pressing process, and slits communicating with the back holes are formed in the first plate-shaped member (for example, refer to Patent Document 1). 
     Meanwhile, in the aforementioned die for forming a honeycomb structure, a width of the slit is much narrower than a diameter of the back hole. For this reason, as a ceramic raw material is introduced into the back holes, a pressure inside the back holes increases, so that a stress may be concentrated on the slits. Therefore, the slits may be easily worn or deformed disadvantageously. 
     For such problems, a die for forming a honeycomb structure, capable of suppressing wear or deformation of the slit has been discussed (for example, refer to Patent Document 2). 
     The die discussed in Patent document 2 is a die having a forming portion having a forming trench (slit) formed on a front side and a rectangular cross-sectional aperture formed on a back side and a die base portion having a through-hole portion. The forming portion of the die is formed of wear-resistant alloy. 
     CITATION LIST 
     Patent Documents 
     
         
         Patent Document 1: JP-A-2006-51682 
         Patent Document 2: JP-B-6-22806 
       
    
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     In the aforementioned die discussed in Patent document 2, when a ceramic raw material is introduced from the through-hole portion (back hole), the ceramic raw material is introduced into the slit through an aperture (cavity) formed of an wear-resistant material. For this reason, it is possible to solve a problem of wear or deformation. In particular, if the die base portion of the die having a through-hole portion is formed of stainless steel, and the forming portion having an aperture and a slit is formed of cemented carbide, it is possible to suppress wear or deformation. 
     Meanwhile, in the die discussed in Patent document 2, a diameter of the circular cross-sectional shape of the through-hole portion is longer than one side of the rectangular cross-sectional shape of the aperture and is shorter than a diagonal line of the rectangular cross-sectional shape of the aperture. In addition, the center of the aperture and the center of the through-hole portion are nearly concentric. When the aperture has a rectangular (cross-sectional) shape, the through-hole portion has a circular (cross-sectional) shape, and the shapes satisfy the aforementioned condition, it can be said that the aperture and the through-hole portion are arranged in nearly the same position with nearly the same size. Supposing that the aperture and the through-hole portion are arranged in nearly the same position with nearly the same size in this manner, the aperture and the through-hole portion may be deviated from each other when a forming portion (material of the forming portion) and a die base portion (material of the die base portion) are bonded to manufacture the die. Here, the “deviated state” refers to a state that the center positions do not match, and the outer circumference (outer edge) of the open end of the aperture and the outer circumference (outer edge) of the open end of the through-hole portion intersect with each other. In addition, the deviated state also includes a state that the open end of the aperture and the open end of the through-hole portion are not overlapped at all. If the honeycomb structure is formed using the die in which the open end of the aperture and the open end of the through-hole portion are deviated from each other, new problem that a distortion may be generated in the honeycomb structure, formability may be degraded and the like, occurs. This is because the open end of the aperture and the open end of the through-hole portion are deviated from each other, so that a ceramic raw material is not sufficiently introduced into the slit, or a pressure is not uniformly applied to a ceramic raw material when it is extruded from each slit. In addition, even when the open end of the aperture and the open end of the through-hole portion are slightly deviated, formability is influenced, so that the same problem occurs. 
     Therefore, it is necessary to suppress a distortion in the honeycomb structure and improve formability when the honeycomb structure is formed. 
     The present invention has been made in view of the aforementioned problems. In the die for forming a honeycomb structure according to the present invention, the diameter of the cavity is different from the diameter of the back hole. In addition, according to the present invention, the open end of the cavity on the first bonded surface is arranged inside the open end of the back hole on the second bonded surface, or the open end of the back hole on the second bonded surface is arranged inside the open end of the cavity on the first bonded surface. For this reason, according to the present invention, it is possible to suppress a distortion in the honeycomb structure when a honeycomb structure is formed, and provide a die for forming a honeycomb structure capable of improving formability. 
     Means for Solving the Problem 
     According to the present invention, there are provided a die for forming a honeycomb structure and a method of manufacturing the same as follows. 
     [1] According to an aspect of the present invention, there is provided a die for forming a honeycomb structure including: a second plate-shaped portion having a second bonded surface, where a back hole for introducing a forming raw material is formed; and a first plate-shaped portion that has a first bonded surface and is formed of tungsten carbide based cemented carbide, where a slit communicating with the back hole to form a forming raw material is formed, and a cavity communicating with the back hole and the slit is formed in the first bonded surface side, wherein the second plate-shaped portion is formed of a material containing at least one selected from a group consisting of iron, steel, aluminum alloy, copper alloy, titanium alloy, and nickel alloy, the first plate-shaped portion is arranged on the second plate-shaped portion such that the first bonded surface comes into contact with the second bonded surface, an open end of the cavity on the first bonded surface has a diameter different from that of an open end of the back hole on the second bonded surface, and the open end of the cavity on the first bonded surface is arranged inside the open end of the back hole on the second bonded surface, or the open end of the back hole on the second bonded surface is arranged inside the open end of the cavity of the first bonded surface. 
     [2] In the die for forming a honeycomb structure described in [1], the diameter of the open end of the cavity on the first bonded surface may be larger than the diameter of the open end of the back hole on the second bonded surface, and the diameter of the open end of the cavity on the first bonded surface may be 1.01 to 1.50 times of the diameter of the open end of the back hole on the second bonded surface. 
     [3] In the die for forming a honeycomb structure described in [1], the diameter of the open end of the back hole on the second bonded surface may be larger than the diameter of the open end of the cavity on the first bonded surface, and the diameter of the open end of the back hole on the second bonded surface may be 1.01 to 1.50 times of the diameter of the open end of the cavity on the first bonded surface. 
     [4] In the die for forming a honeycomb structure described in any one of [1] to [3], the cavity may have a depth of 0.1 to 90 mm. 
     [5] In the die for forming a honeycomb structure described in any one of [1] to [4], a bottom portion as a head edge of the cavity may have a flat shape, a flat shape having straightly chamfered corners, or an outwardly convex curved shape on a cross section perpendicular to a surface of the first plate-shaped portion. 
     [6] The die for forming a honeycomb structure described in any one of [1] to [5] may further include a buffer portion which is a space that is formed along an end of the slit in the first bonded surface side of the first plate-shaped portion, communicates with the slit, and has a width larger than that of the slit. 
     [7] According to another aspect of the present invention, there is provided a method of manufacturing a die for forming a honeycomb structure, including: forming a plurality of back holes in a second plate-shaped member formed of a material containing at least one selected from a group consisting of iron, steel, aluminum alloy, copper alloy, titanium alloy, and nickel alloy; forming a plurality of cavities that have a diameter different from that of the back holes and communicate with the back holes when the second plate-shaped member is bonded to a first bonded surface which is one surface of the first plate-shaped member formed of tungsten carbide based cemented carbide; stacking the first plate-shaped member and the second plate-shaped member and bonding the first plate-shaped member and the second plate-shaped member while a second bonded surface which is one surface of the second plate-shaped member having the back holes faces the first bonded surface of the first plate-shaped member having the cavities; and forming slits communicating with the cavities from a surface side of the first plate-shaped member to manufacture a die for forming a honeycomb structure. 
     Effect of the Invention 
     The die for forming a honeycomb structure according to the present invention includes a second plate-shaped portion and a first plate-shaped portion. The second plate-shaped portion is formed of a material containing at least one selected from a group consisting of iron, steel, aluminum alloy, copper alloy, titanium alloy, and nickel alloy. In addition, the second plate-shaped portion has a second bonded surface where back holes for introducing a formation raw material are formed. The first plate-shaped portion is formed of tungsten carbide based cemented carbide. Furthermore, the first plate-shaped portion has a first bonded surface and slits for forming a forming raw material by communicating with the back holes, and cavities communicating with the back holes and the slits are formed in the first bonded surface side. In addition, the first plate-shaped portion is arranged on the second plate-shaped portion such that the first bonded surface comes into contact with the second bonded surface. Furthermore, the open end of the cavity on the first bonded surface has a diameter different from that of the open end of the back hole on the second bonded surface. Moreover, the open end of the cavity on the first bonded surface is arranged inside the open end of the back hole on the second bonded surface, or the open end of the back hole on the second bonded surface is arranged inside the open end of the cavity on the first bonded surface. Since the die for forming a honeycomb structure according to the present invention has the aforementioned relationship between the back hole and the cavity in this manner, it is possible to suppress a distortion of the formed honeycomb structure when the honeycomb structure is formed, and improve formability. 
     A method of manufacturing a die for forming a honeycomb structure according to the present invention includes following steps. Specifically, a plurality of back holes are formed in a second plate-shaped member formed of a material containing at least one selected from a group consisting of iron, steel, aluminum alloy, copper alloy, titanium alloy, and nickel alloy. In addition, a plurality of cavities that have a diameter different from that of the back holes and communicate with the back holes when the second plate-shaped member is bonded to a first bonded surface which is one surface of the first plate-shaped member formed of tungsten carbide based cemented carbide. In addition, the first plate-shaped member and the second plate-shaped member are stacked and bonded to each other while a second bonded surface which is one surface of the second plate-shaped member having the back holes faces the first bonded surface of the first plate-shaped member having the cavities. Furthermore, slits communicating with the cavities from a surface side of the first plate-shaped member are formed to manufacture a die for forming a honeycomb structure. In this manner, in the method of manufacturing the die for forming a honeycomb structure according to the present invention, the cavity formed in the first plate-shaped member has a diameter different from that of the back hole formed in the second plate-shaped portion. For this reason, it is possible to obtain a die for forming a honeycomb structure having excellent formability. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view schematically showing a die for forming a honeycomb structure according to an embodiment of the present invention as seen from a first plate-shaped portion side where slits are formed; 
         FIG. 2  is a perspective view schematically showing a die for forming a honeycomb structure according to an embodiment of the present invention as seen from a second plate-shaped portion where back holes are formed; 
         FIG. 3  is an enlarged plan view partially showing a surface of the first plate-shaped portion side of the die for forming a honeycomb structure of  FIG. 1 ; 
         FIG. 4A  is a schematic diagram showing a cross section taken along a line A-A′ of the die for forming a honeycomb structure of  FIG. 3 ; 
         FIG. 4B  is a schematic diagram showing a cross section in parallel with a thickness direction of a die for forming a honeycomb structure according to another embodiment of the present invention in a state that open end of the back hole on a second bonded surface are arranged in the inside of the open end of the cavity on a first bonded surface; 
         FIG. 5  is a cross-sectional view partially showing a cross section perpendicular to the slit in a die for forming a honeycomb structure according to further another embodiment of the present invention; 
         FIG. 6  is a cross-sectional view partially showing a cross section perpendicular to the slit in a die for forming a honeycomb structure according to still another embodiment of the present invention; 
         FIG. 7  is a cross-sectional view partially illustrating a cross section perpendicular to the slit in a die for forming a honeycomb structure according to still another embodiment of the present invention; 
         FIG. 8  is a cross-sectional view partially showing a cross section perpendicular to a surface of the first plate-shaped portion in a die for forming a honeycomb structure according to still another embodiment of the present invention; 
         FIG. 9  is a cross-sectional view partially showing a cross section perpendicular to a surface of the first plate-shaped portion in a die for forming a honeycomb structure according to still another embodiment of the present invention; 
         FIG. 10  is a cross-sectional view partially showing a cross section perpendicular to the slit in a die for forming a honeycomb structure according to still another embodiment of the present invention; 
         FIG. 11  is a cross-sectional view partially showing a cross section perpendicular to the slit in a die for forming a honeycomb structure according to still another embodiment of the present invention; 
         FIG. 12  is a cross-sectional view partially showing a cross section perpendicular to the slit in a die for forming a honeycomb structure according to still another embodiment of the present invention; and 
         FIG. 13  is a cross-sectional view partially showing a cross section perpendicular to the slit in a die for forming a honeycomb structure according to still another embodiment of the present invention. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. It would be appreciated by those skilled in the art that the invention is not limited to the following embodiments, and they may be appropriately modified or changed without departing from the spirit and scope of the invention. 
     (1) Die for Forming Honeycomb Structure: 
     A die for forming a honeycomb structure according to an embodiment of the present invention will be described.  FIG. 1  is a perspective view schematically showing a die for forming a honeycomb structure according to an embodiment of the present invention as seen from a first plate-shaped portion where slits are formed.  FIG. 2  is a perspective view schematically showing a die for forming a honeycomb structure according to an embodiment of the present invention as seen from a second plate-shaped portion side where back holes are formed.  FIG. 3  is an enlarged plan view partially showing a surface of the first plate-shaped portion side of the die for forming a honeycomb structure of  FIG. 1 .  FIG. 4A  is a schematic diagram showing a cross section taken along a line A-A′ in the die for forming a honeycomb structure of  FIG. 3 . The die for forming a honeycomb structure of  FIG. 4A  has a state that an open end of a cavity on a first bonded surface is arranged inside an open end of a back hole on a second bonded surface. 
     As shown in  FIGS. 1 to 4A , a die for forming a honeycomb structure  1  according to this embodiment includes a second plate-shaped portion  3  and a first plate-shaped portion  7  formed of tungsten carbide based cemented carbide. The second plate-shaped portion  3  is formed of a material containing at least one selected from a group consisting of iron, steel, aluminum alloy, copper alloy, titanium alloy, and nickel alloy. The second plate-shaped portion  3  has a second bonded surface  6  where a back hole  5  for introducing a forming raw material is formed. The first plate-shaped portion  7  has a first bonded surface  10  where a slit  9  communicating with the back hole  5  for forming the forming raw material is formed, and a cavity  11  communicating with the back hole  5  and the slit  9  is formed in the first bonded surface  10  side. The slit  9  directly communicates with the cavity  11 . That is, it can be said that the slit  9  communicates with the back hole  5  through the cavity  11 . The first plate-shaped portion  7  is arranged in the second plate-shaped portion  3  such that the first bonded surface  10  comes into contact with the second bonded surface  6 . A diameter of the open end of the cavity on the first bonded surface is different from a diameter of the open end of the back hole on the second bonded surface. In addition, the diameter of the open end of the cavity on the first bonded surface is smaller than the diameter of the open end of the back hole on the second bonded surface. Furthermore, an open end  11   a  of the cavity  11  on the first bonded surface  10  is arranged inside an open end  5   a  of the back hole  5  on the second bonded surface  6 . By configuring the die for forming a honeycomb structure  1  ( 1 A) in this manner, it is possible to improve formability of the formed honeycomb structure. The phrase “the open end  11   a  of the cavity  11  is arranged inside the open end  5   a  of the back hole  5 ” means that the back hole  5  having a large opening diameter and the cavity  11  having a small opening diameter communicate with each other, and an outer circumference (outer edge) of the open end of the back hole  5  and an outer circumference (outer edge) of the open end of the cavity  11  do not intersect with each other. It is noted that a state that the outer circumference (outer edge) of the open end of the cavity  11  is inscribed with the outer circumference (outer edge) of the open end of the back hole  5  is included in the meaning of the phrase “the open end  11   a  of the cavity  11  is arranged inside the open end  5   a  of the back hole  5 .” 
     A thickness of the die for forming a honeycomb structure according to this embodiment is preferably set to 5 to 100 mm, but not particularly limited thereto. If the thickness of the die is thinner than 5 mm, the die may be broken during the formation. If the thickness of the die is thicker than 100 mm, a pressure loss may become significant during the formation of the honeycomb structure, so that it may be difficult to perform formation. 
     (1-1) Second Plate-Shaped Portion 
     The second plate-shaped portion  3  is formed of a material containing at least one selected from a group consisting of iron, steel, aluminum alloy, copper alloy, titanium alloy, and nickel alloy. The steel is a material containing at least one selected from a group consisting of stainless steel, die steel, and high-speed steel. Out of such materials, steel is preferably selected as a material of the second plate-shaped portion  3 . More preferably, stainless steel is selected. It is noted that the “material containing at least one selected from a group consisting of iron, steel, aluminum alloy, copper alloy, titanium alloy, and nickel alloy” may be referred to herein as “free cutting material.” The “free cutting material” is a material easily grindable compared to tungsten carbide based cemented carbide. The second plate-shaped portion  3  is less influenced by wear, compared to the first plate-shaped portion  7 , because no slit is formed. Since the second plate-shaped portion  3  is formed of free cutting material, it has excellent workability, compared to tungsten carbide based cemented carbide. In addition, since a cost of the free cutting material is lower than that of tungsten carbide based cemented carbide, it is possible to lower the manufacturing cost. 
     As the aforementioned “stainless steel” as a kind of the material of the second plate-shaped portion  3 , stainless steel well known in the art may be used. For example, the stainless steel may include SUS304, SUS303, and the like. 
     The second plate-shaped portion  3  may have a desired size depending on a utilization purpose without any particular limitation. It is noted that, if the second plate-shaped portion  3  has a disk shape, a diameter of the disk (diameters of one side and the other side) is preferably set to 30 to 500 mm. 
     A thickness of the second plate-shaped portion  3  may be determined appropriately depending on a utilization purpose considering a shape of the slit or a shape of the back hole without any particular limitation. 
     (Back Hole) 
     The back holes  5  for introducing a forming raw material are formed in the second plate-shaped portion  3 . The “back hole  5 ” is a through hole for introducing a forming raw material (the hole opened to both sides of the second plate-shaped portion  3 ). When a honeycomb structure is formed using the die for forming a honeycomb structure  1 , the forming raw material of the honeycomb structure is introduced from the back holes  5 . 
     The shape of the back hole  5  is not particularly limited as long as it can guide the introduced forming raw material into the cavity  11  and the slit  9 . For example, a shape of the back hole in the “cross section perpendicular to a direction where the forming raw material flows (the thickness direction of the second plate-shaped portion)” of the back hole is preferably circular. In addition, a diameter of the open end of the back hole  5  is preferably set to 0.5 to 5.0 mm, and more preferably, 0.8 to 3.0 mm. Such the back hole  5  may be formed, for example, using various machining methods such as electrochemical machining (ECM), electrical discharge machining (EDM), laser machining, and mechanical machining such as drilling. Out of these methods, the electrochemical machining (ECM) is preferably employed because the back holes  5  can be formed efficiently and accurately. A space of the back hole preferably has a cylindrical shape. In this case, a diameter (diameter of the back hole) on the “cross section perpendicular to a direction where the forming raw material flows (thickness direction of the second plate-shaped portion)” of the back hole becomes constant. In this case, the diameter of the back hole is equal to the “diameter of the open end of the back hole on the second bonded surface.” Furthermore, the number of the back holes may be appropriately determined depending on a shape and the like of the honeycomb structure to be manufactured without any particular limitation. 
     (Second Bonded Surface) 
     The second plate-shaped portion  3  has the second bonded surface  6 . As shown in  FIG. 4A , the second bonded surface  6  is a surface of the second plate-shaped portion  3  bonded to (coming into contact with) the first plate-shaped portion  7 . 
     (1-2) First Plate-Shaped Portion 
     The first plate-shaped portion  7  is a plate-shaped member formed of tungsten carbide based cemented carbide. A width of the slit  9  is much narrower than the diameter of the back hole  5 . For this reason, when extrusion is performed for the forming raw material, a pressure inside the back hole  5  increases, so that a stress is concentrated on the slit  9 , or a defect such as wear or deformation may easily occur. Therefore, the first plate-shaped portion  7  is formed of tungsten carbide based cemented carbide having a high wear resistance. 
     The “tungsten carbide based cemented carbide (cemented carbide)” refers to alloy obtained by sintering tungsten carbide and a binder. The binder is metal containing at least one selected from a group consisting of cobalt (Co), iron (Fe), nickel (Ni), titanium (Ti), and chromium (Cr). The tungsten carbide based cemented carbide has an especially excellent wear resistance or mechanical strength. 
     The first plate-shaped portion  7  may have a desired size depending on a utilization purpose without any particular limitation. However, when the first plate-shaped portion  7  has a disk shape, a diameter of the disk is preferably set to 30 to 500 mm. When the first plate-shaped portion  7  and the second plate-shaped portion  3  have a disk shape, a diameter of the first plate-shaped portion  7  is preferably set to 90 to 100% of the diameter of the second plate-shaped member  3 . 
     It is noted that a thickness of the first plate-shaped portion  7  is preferably set to 0.6 to 95 mm, more preferably 0.6 to 30 mm, and particularly preferably 1.0 to 20 mm. In addition, the thickness of the first plate-shaped portion  7  is preferably set to 0.05 to 5 times of the thickness of the second plate-shaped portion  3 . 
     (Slit) 
     In the first plate-shaped portion  7 , slits  9  for forming a forming raw material are formed to communicate with the cavities  11 . The slit is a crevice (notch) formed in the first plate-shaped portion  7 . The forming raw material introduced from the back hole  5  enters the slit  9  inside the die for forming a honeycomb structure and is then extruded from the open end of the slit  9 , so that a honeycomb-shaped formed body is formed. 
     An area where the slit is formed on the first plate-shaped portion  7  or a formation pattern of the slit may be appropriately determined depending on a utilization purpose without any particular limitation. For example, the formation pattern of the slit may have a polygonal shape such as triangle, rectangle, pentagon, hexagon, and octagon or a circular shape, or may be a combinational pattern having a plurality of shapes including a polygonal shape and a circular shape on a cross section perpendicular to a direction where the forming raw material is extruded (flows). For example, in the die for forming a honeycomb structure  1  shown in  FIGS. 1 to 3 , a formation pattern of the slit  9  has a rectangular shape on the cross section perpendicular to a direction where the forming raw material is extruded. 
     A width of the slit may be appropriately determined depending on a shape of the honeycomb structure to be formed. For example, when a die for forming a honeycomb structure for extruding a ceramic honeycomb structure for a general exhaust gas filter or a catalyst carrier is manufactured, the slit preferably has a width of 0.05 to 1 mm, and more preferably 0.06 to 0.5 mm. A depth of the slit may be set so that it can communicate with the cavity. The slit preferably has, for example, a depth of 0.5 to 10 mm. 
     (First Bonded Surface) 
     The first plate-shaped portion  7  has the first bonded surface  10 . As shown in  FIG. 4A , the first bonded surface  10  is a surface of the first plate-shaped portion  7  bonded to (coming into contact with) the second plate-shaped portion  3 . 
     (Cavity) 
     The “cavity  11 ” is formed in the first plate-shaped portion  7 . The “cavity  11 ” is formed to communicate with the back hole  5  formed in the second plate-shaped portion  3  and the slit  9  formed in the first plate-shaped portion  7 . In addition, the “cavity  11 ” is a hollow formed in the “first bonded surface  10 ” of the first plate-shaped portion  7 . That is, the cavity  11  is formed to face a surface (surface of the first plate-shaped portion  7 ) opposite to the first bonded surface  10  from the first bonded surface  10  side. It is noted that the “cavity  11 ” does not reach a surface of the first plate-shaped portion  7  as shown in  FIG. 4A . That is, although the “cavity  11 ” communicates with the slit, the cavity  11  itself does not serve as a through hole. Therefore, the “cavity  11 ” has a bottom portion  11   b  which is a leading edge (bottom portion) of the hole inside of the first plate-shaped portion  7 . Since the “cavity  11 ” is formed in this manner, the forming raw material introduced from the back hole  5  formed in the second plate-shaped portion  3  passes through the “cavity  11 ” and enters the slit  9 . In addition, the forming raw material is extruded from an open end  9   a  of the slit to form a honeycomb formed body (honeycomb structure). 
     The cavity  11  preferably has a depth h of 0.1 to 90 mm (refer to  FIG. 4A ), and more preferably 0.5 to 20 mm. In this manner, by setting the depth h of the cavity  11  to this range, it is possible to improve formability. If the depth h of the cavity is smaller than 0.1 mm, a strength of the first plate-shaped member may be degraded when the forming raw material is extruded. If the depth h of the cavity is larger than 90 mm, it is difficult to form a cavity by machining the first plate-shaped member when the die for forming a honeycomb structure according to this embodiment is manufactured. Here, the “depth h of the cavity  11 ” refers to a distance from the first bonded surface of the first plate-shaped portion to the bottom portion  11   b  of the cavity as shown in  FIG. 4A . 
     A diameter of the open end of the cavity  11  is preferably set to 0.5 to 5.0 mm, and more preferably 0.8 to 3.0 mm. Such a cavity  11  may be formed, for example, using various machining methods such as electrochemical machining (ECM), electrical discharge machining (EDM), laser machining, and mechanical machining such as drilling. As a method of forming the cavity  11 , the electrochemical machining (ECM) is preferably employed out of these methods because the cavity  11  can be formed efficiently and accurately. A space of the cavity is preferably cylindrical. In this case, a diameter (diameter of the cavity) on the “cross section perpendicular to a direction where the forming raw material flows (thickness direction of the first plate-shaped portion)” in the cavity becomes constant. In addition, in this case, the diameter of the cavity  11  becomes equal to the “diameter of the open end of the cavity on the first bonded surface.” Furthermore, the number of the cavities  11  is preferably equal to the number of the back holes. 
     (1-3) Relationship Between Open End of Cavity and Open End of Back Hole 
     As shown in  FIG. 4A , in the die  1  ( 1 A) for forming a honeycomb structure  1  ( 1 A) according to this embodiment, a diameter d 1  of the open end  11   a  of the cavity on the first bonded surface  10  is different from a diameter D 1  of the open end  5   a  of the back hole on the second bonded surface  6 . In addition, as shown in  FIG. 4A , in the die for forming a honeycomb structure according to this embodiment, the diameter d 1  of the open end of the cavity  11  on the first bonded surface is formed to be smaller than the diameter D 1  of the open end of the back hole  5  on the second bonded surface. In addition, the open end  11   a  of the cavity  11  of the first bonded surface  10  is arranged inside the open end  5   a  of the back hole  5  on the second bonded surface  6 . 
     The “open end  11   a  of the cavity on the first bonded surface” is an inlet portion of the hollow (cavity) opened on the first bonded surface  10 . It is noted that the diameter d 1  of the open end  11   a  of the cavity is preferably equal to the diameter of the bottom portion  11   b  of the cavity. Similarly, the “open end  5   a  of the back hole on the second bonded surface” is an outlet portion (outlet portion of the forming raw material) opened on the second bonded surface  6  in the second bonded surface  6  side. A ceramic raw material is supplied to the cavity as soon as it passes through the outlet portion. 
     In the die for forming a honeycomb structure according to this embodiment, the open end  11   a  of the cavity on the first bonded surface  10  is arranged inside the open end  5   a  of the back hole on the second bonded surface  6 . When the open end  11   a  of the cavity is arranged inside the open end  5   a  of the back hole in this manner, it is possible to prevent hindrance of “a stable (smooth) flow of the ceramic raw material from the back hole  5  to the slit  9 .” That is, a deviation of the flow of the forming raw material inside the cavity  11  is suppressed, and the forming raw material is introduced into the slit with the same pressure. As a result, it is possible to prevent a distortion of the honeycomb structure when the honeycomb structure is formed. That is, it is possible to obtain a die for forming a honeycomb structure capable of forming a honeycomb structure with excellent formability. 
     In the die for forming a honeycomb structure according to this embodiment, the “diameter D 1  of the open end  5   a  of the back hole on the second bonded surface  6 ” is preferably set to 1.01 to 1.50 times of the “diameter d 1  of the open end  11   a  of the cavity on the first bonded surface  10 .” As a result, it is possible to improve formability when a honeycomb structured formed body (honeycomb structure) is formed. If the ratio of the diameter is smaller than 1.01, the open end of the cavity and the open end of the back hole may be deviated from each other when the first plate-shaped portion and the second plate-shaped portion are bonded to each other during the manufacturing of the die for forming a honeycomb structure. If the open end of the cavity and the open end of the back hole are deviated from each other, the honeycomb structure obtained in forming a honeycomb structure may be easily deformed. If the ratio of the diameter is larger than 1.50 times, it may be difficult to form a desired formed structure by coupling the back holes to each other. The open end of the cavity and the open end of the back hole preferably have the same shape and different sizes although they are not particularly limited. As a result, it is possible to provide a more uniform flow of the forming raw material. In addition, the open end of the cavity and the open end of the back hole preferably have a circular shape. If they have a circular shape, it is possible to provide a more uniform flow of the forming raw material. 
     The ceramic honeycomb structure extruded using the die for forming a honeycomb structure  1  ( 1 A) according to this embodiment is a ceramic honeycomb structure having a porous partition wall defining and forming a plurality of cells extending in a fluid flow direction. The ceramic raw material used in the manufacturing of the ceramic honeycomb structure using the die for forming a honeycomb structure  1  ( 1 A) according to this embodiment is a raw material obtained by mixing and kneading water, a binder, a pore former, and the like with ceramic powder. 
     (1-4) Another Embodiment of Die for Forming Honeycomb Structure 
     Next, a description will be made for a die for forming a honeycomb structure according to another embodiment of the present invention. The die for forming a honeycomb structure according to this embodiment is configured as described below. Specifically, as shown in  FIG. 4B , in the die for forming a honeycomb structure according to an embodiment of the present invention (refer to  FIG. 4A ), the open end  5   a  of the back hole  5  on the second bonded surface  6  is arranged inside the open end  11   a  of the cavity  11  on the first bonded surface  10 . In the die for forming a honeycomb structure according to this embodiment, the open end  5   a  of the back hole  5  on the second bonded surface  6  is smaller than the open end  11   a  of the cavity  11  on the first bonded surface  10 . The phrase “the open end  5   a  of the back hole  5  is arranged inside the open end  11   a  of the cavity  11 ” means that the back hole  5  having a small opening diameter and the cavity  11  having a large opening diameter communicate with each other, and the outer circumference (outer edge) of the open end of the back hole  5  and the outer circumference (outer edge) of the open end of the cavity  11  do not intersect with each other. It is noted that a state that the outer circumference (outer edge) of the open end of the back hole  5  is inscribed with the outer circumference (outer edge) of the open end of the cavity  11  is included in the meaning of the phrase “the open end  5   a  of the back hole  5  is arranged inside the open end  11   a  of the cavity  11 .”  FIG. 4B  is a schematic diagram showing a “cross section in parallel with the thickness direction” of the die for forming a honeycomb structure according to another embodiment of the present invention, in which the open end of the back hole on the second bonded surface is arranged inside the open end of the cavity on the first bonded surface. 
     In a die  1  for forming a honeycomb structure  1  ( 1 B) according to this embodiment, since “the open end  5   a  of the back hole  5  is arranged inside the open end  11   a  of the cavity  11 ” in this manner, the forming raw material inside the cavity  11  forms a uniform flow and is introduced into the slit with a uniform pressure. As a result, it is possible to prevent a deformation of the shape of the honeycomb-shaped formed body to be formed. 
     In the die for forming a honeycomb structure according to this embodiment, the “diameter d 1  of the open end  11   a  of the cavity on the first bonded surface  10 ” is preferably 1.01 to 1.50 times of the “diameter D 1  of the open end  5   a  of the back hole on the second bonded surface  6 .” As a result, it is possible to improve formability when the honeycomb structured formed body is formed. If the ratio of the diameter is smaller than 1.01, the open end of the cavity and the open end of the back hole may be deviated from each other when the first plate-shaped portion and second plate-shaped portion are bonded to each other during the manufacturing of the die for forming a honeycomb structure. If the ratio of the diameter is larger than 1.50, it may be difficult to form a desired formed structure by coupling the cavities to each other. 
     In die for forming a honeycomb structure according to the present invention, as shown in  FIG. 5 , “a bottom portion  9   b  of the slit preferably has an outwardly convex curved shape on the cross section perpendicular to the slit  9 .” Herein, the “cross section perpendicular to the slit  9 ” means a cross section perpendicular to the slit  9  in parallel with the depth direction of the slit  9  (directed from the surface of the first plate-shaped portion to the inside). The bottom portion  9   b  of the slit is an end of the slit  9  in the “first bonded surface  10  side of the first plate-shaped portion  7 ” on the cross section perpendicular to the slit  9 . It is noted that an end of the slit  9  in a “surface  7   a  side of the first plate-shaped portion  7 ” corresponds to the open end  9   a  of the slit  9 . In addition, in the die for forming a honeycomb structure according to the present invention, it is preferable that the “bottom portion  9   b  of the slit preferably have an outwardly convex V-shape on the cross section perpendicular to the slit  9 ” as shown in  FIG. 6 . In addition, in the die for forming a honeycomb structure according to the present invention, it is preferable that “the bottom portion of the slit have a “flat shape (straight shape) of which corners are straightly chamfered (C-chamfered shape)” on the cross section perpendicular to the slit.” Furthermore, in the die for forming a honeycomb structure according to the present invention, it is preferable that “the slit  9  have a tapered shape narrowed from the open end  9   a  to the bottom portion  9   b  of the slit on the cross section perpendicular to the slit  9 ” as shown in  FIG. 7 . The slit may have various shapes as described above.  FIG. 5  is a cross-sectional view partially showing the cross section perpendicular to the slit  9  in the die for forming a honeycomb structure according to further another embodiment of the present invention (die for forming a honeycomb structure  1 C).  FIG. 6  is a cross-sectional view partially showing the cross section perpendicular to the slit  9  in the die for forming a honeycomb structure according to still another embodiment of the present invention (die for forming a honeycomb structure  1 D).  FIG. 7  is a cross-sectional view partially showing the cross section perpendicular to the slit  9  in the die for forming a honeycomb structure according to still another embodiment of the present invention (die for forming a honeycomb structure  1 E). 
     In the die for forming a honeycomb structure according to the present invention, it is preferable that the bottom portion  11   b  of the cavity  11  have an outwardly convex curved shape on the cross section perpendicular to the surface  7   a  of the first plate-shaped portion  7  as shown in  FIG. 8 . In addition, in the die for forming a honeycomb structure according to the present invention, it is preferable that “the bottom portion of the cavity have a ‘ flat shape (straight shape) of which corners are formed in an outwardly convex curved shape’ on the cross section perpendicular to the surface of the first plate-shaped portion.” Furthermore, in the die for forming a honeycomb structure according to the present invention, the bottom portion  11   b  of the cavity  11  preferably has one of the shapes described below as shown in  FIG. 9 . Specifically, it is preferable that the bottom portion  11   b  of the cavity  11  have a “‘flat shape (straight shape) of which corners are straightly chamfered (C-chamfered)’ on the cross section perpendicular to the surface  7   a  of the first plate-shaped portion  7 .” The bottom portion  11   b  of the cavity  11  may have various shapes as described above. It is noted that, in the die for forming a honeycomb structures  1 F and  1 G shown in  FIGS. 8 and 9 , the bottom portion  9   b  of the slit  9  has a flat shape (straight shape) on the cross section perpendicular to the slit  9 . This may also be referred to as the bottom portion  9   b  of the slit  9  has a shape of which corners have right angel on the cross section perpendicular to the slit  9 .  FIG. 8  is a cross-sectional view partially showing a cross section perpendicular to the surface  7   a  of the first plate-shaped portion  7  in the die for forming a honeycomb structure according to still another embodiment of the present invention (die for forming a honeycomb structure  1 F).  FIG. 9  is a cross-sectional view partially showing a cross section perpendicular to the surface  7   a  of the first plate-shaped portion  7  in the die for forming a honeycomb structure according to still another embodiment of the present invention (the die for forming a honeycomb structure  1 G). 
     In the die for forming a honeycomb structure according to the present invention, it is preferable that a buffer portion be provided as “a space that is formed along the “end of the slit in the first bonded surface side of the first plate-shaped portion,” communicates with the slit, and has a width wider than that of the slit”. As shown in  FIG. 10 , in the die for forming a honeycomb structure according to still another embodiment of the present invention, a buffer portion  21  which is a space communicating with the end of the slit  9  “in the first bonded surface  10  side of the first plate-shaped portion  7 ” is provided on the cross section perpendicular to the slit  9 . A width A of the buffer portion  21  is larger than that of the slit  9 . The buffer portion  21  is formed along the end of the entire slit  9  “in the first bonded surface  10  side of the first plate-shaped portion  7 .” Since a die for forming a honeycomb structure  1 H has the buffer portion  21 , the forming raw material flowing from the back hole  5  can be easily widened inside the buffer portion  21  having a wide width before it flows into the slit  9  having a narrow width. In addition, it is possible to flow the forming raw material from the buffer portion  21  into the slit  9 . As a result, it is possible to easily and uniformly flow the forming raw material to the entire slit  9 . Furthermore, since the die for forming a honeycomb structure  1 H has the buffer portion  21 , it is possible to reduce a pressure loss when the forming raw material is extruded. The width A of the buffer portion  21  is preferably set to 0.1 to 4.0 mm, more preferably 0.2 to 3.0 mm, and particularly preferably 0.5 to 2.0 mm. If the width A of the buffer portion  21  is narrower than 0.1 mm, an effect of easily and uniformly flowing the forming raw material to the entire slit  9  may be degraded. If the width A of the buffer portion  21  is wider than 4.0 mm, the die may be easily broken during the extrusion. In addition, a depth B of the buffer portion  21  is preferably set to 0.05 to 5.0 mm, more preferably 0.1 to 4.0 mm, and particularly preferably 0.5 to 2.0 mm. If the depth B of the buffer portion  21  is shallower than 0.05 mm, an effect of easily and uniformly flowing the forming raw material to the entire slit  9  may be degraded. If the depth B of the buffer portion  21  is deeper than 5.0 mm, fabrication may become difficult. The depth of the buffer portion  21  is a length of the buffer portion  21  in the “thickness direction of the die for forming a honeycomb structure  1 H.” It is noted that the buffer portion  21  of the die for forming a honeycomb structure  1 H has a rectangular shape on the cross section perpendicular to the slit  9 . That is, a bottom portion  21   a  of the buffer portion  21  has a “flat shape (straight shape)” on the cross section perpendicular to the slit  9 . In addition, a position of the buffer portion  21  “in an end  21   b  in the side opposite to the bottom portion  21   a ” in the thickness direction of the die for forming a honeycomb structure  1 H is preferably set to the same position as the bottom portion  11   b  of the cavity  11  on the cross section perpendicular to the slit  9 . “The end  21   b  in the side opposite to the bottom portion  21   a ” of the buffer portion  21  is the end of the buffer portion  21  “coupled to the slit  9 .”  FIG. 10  is a cross-sectional view partially showing the cross section perpendicular to the slit  9  in the die for forming a honeycomb structure according to still another embodiment of the present invention (die for forming a honeycomb structure  1 H). 
     In the die for forming a honeycomb structure according to the present invention, it is preferable that, while the buffer portion  21  is formed, the bottom portion  21   a  of the buffer portion  21  have an outwardly convex curved shape on the cross section perpendicular to the slit  9  as shown in  FIG. 11 .  FIG. 11  is a cross-sectional view partially showing the cross section perpendicular to the slit  9  in the die for forming a honeycomb structure according to still another embodiment of the present invention (die for forming a honeycomb structure  1 I). 
     In the die for forming a honeycomb structure according to the present invention, it is preferable that, while the buffer portion  21  is formed, the buffer portion  21  have the following shape as shown in  FIG. 12 . Specifically, it is preferable that the buffer portion  21  be shaped such that “the bottom portion  21   a  has a ‘flat shape (straight shape) of which corners are straightly chamfered (C-chamfered shape)’” on the cross section perpendicular to the slit  9 . In addition, it is preferable that the bottom portion of the die for forming a honeycomb structure have a “V-shaped” buffer portion on the cross section perpendicular to the slit.  FIG. 12  is a cross-sectional view partially showing a cross section perpendicular to the slit  9  in the die for forming a honeycomb structure according to still another embodiment of the present invention (die for forming a honeycomb structure  1 J). 
     In the die for forming a honeycomb structure according to the present invention, while the buffer portion  21  is formed, the buffer portion  21  is preferably arranged as described below as shown in  FIG. 13 . Specifically, it is one of a preferable aspect that the bottom portion  21   a  of the buffer portion be formed (arranged) to overlap with the bottom portion  11   b  of the cavity  11  in the thickness direction of a die  1 K for forming a honeycomb structure.  FIG. 13  is a cross-sectional view partially illustrating a cross section perpendicular to the slit  9  in the die for forming a honeycomb structure according to still another embodiment of the present invention (die for forming a honeycomb structure  1 K). The buffer portion  21  may, for example, have various shapes as described above for the buffer portions of the dies  1 H to  1 K for forming a honeycomb structure shown in  FIGS. 10 to 13 . 
     It is noted that, in the dies for forming a honeycomb structure  1 C to  1 K shown in  FIGS. 5 to 13 , the diameter of the open end of the cavity is smaller than the diameter of the open end of the back hole, and the open end of the cavity is arranged inside the open end of the back hole. In addition, in the dies for forming a honeycomb structure  1 C to  1 K, it is preferable that the diameter of the open end of the cavity be larger than the diameter of the open end of the back hole, and the open end of the back hole be arranged inside the open end of the cavity. In addition, the slit  9  formed in the die for forming a honeycomb structure according to the present invention may have a shape obtained by combining two or more shapes of the same shapes the slits  9  of the dies for forming a honeycomb structure  1 C to  1 F shown in  FIGS. 5 to 8 . Furthermore, the cavity formed in the die for forming a honeycomb structure according to the present invention may have a shape obtained by combining two or more shapes of the same shapes of the cavities  11  of the dies  1 F to  1 H for forming a honeycomb structure shown in  FIGS. 8 to 10 . Moreover, the buffer portion formed in the die for forming a honeycomb structure according to the present invention may have a shape obtained by combining two or more shapes of the same shapes of the buffer portions  21  of the dies for forming a honeycomb structure  1 H to  1 K shown in  FIGS. 10 to 13 . 
     (2) Method of Manufacturing Die for Forming Honeycomb Structure: 
     Next, a description will be made for a method of manufacturing a die for forming a honeycomb structure according to an embodiment of the present invention. A method of manufacturing a die for forming a honeycomb structure according to this embodiment is a method of manufacturing a die for forming a honeycomb structure according to an embodiment of the present invention shown in  FIGS. 1 to 4A  (die for forming a honeycomb structure  1 ). It is noted that the die for forming a honeycomb structure according to another embodiment of the present invention shown in  FIG. 4B  may be manufactured similarly except that the size of the open end is different between the back hole and the cavity. 
     First, a plurality of back holes  5  are formed in the second plate-shaped member  3  having a disk shape formed of free cutting material (process (1)). Each condition such as the “diameter of the open end” of the back hole  5  is preferably set to the same condition as the preferable condition of the aforementioned die for forming a honeycomb structure according to an embodiment of the present invention. 
     Such back holes  5  may be formed, for example, preferably using various mechanical machining methods such as electrochemical machining (ECM), electrical discharge machining (EDM), laser machining, and mechanical machining such as drilling without any particular limitation. Out of these methods, the electrochemical machining (ECM) is preferably employed. Using the electrochemical machining (ECM), it is possible to efficiently form the back holes with high dimensional precision. 
     Then, the cavities  11  are formed on one surface of the first plate-shaped member  7  (first bonded surface  10 ) formed of tungsten carbide based cemented carbide (cemented carbide) (process (2)). One surface of the first plate-shaped member  7  is a surface of the side bonded to the second plate-shaped member  3  (first bonded surface  10 ). The cavity  11  is formed such that the diameter d 1  of the cavity  11  is smaller than the diameter D 1  of the back hole  5  (refer to  FIG. 4A ). The number of cavities  11  is equal to the number of back holes  5 , and the cavities  11  are arranged in the same positions as those of the back holes  5  when the first plate-shaped member  7  and the second plate-shaped member  3  are bonded to each other. The arrangement of the back holes  5  in the second plate-shaped member  3  and the arrangement of the cavities  11  in the first plate-shaped member  7  will be described below in detail. Specifically, the cavities  11  are arranged such that the open end  11   a  of the cavity is arranged inside the open end of the back hole when the first plate-shaped member  7  and the second plate-shaped member  3  are bonded to each other. 
     The cavity  11  may be formed, for example, preferably using various machining methods such as electrochemical machining (ECM), electrical discharge machining (EDM), laser machining, and mechanical machining such as drilling without any particular limitation. Out of these methods, the electrochemical machining (ECM) is preferably employed. Using the electrochemical machining (ECM), it is possible to efficiently form the back holes with high dimensional precision. 
     Then, while the second bonded surface  6  which is one surface of the second plate-shaped member  3  faces the first bonded surface  10  of the first plate-shaped member  7 , the first plate-shaped member  7  and the second plate-shaped member  3  are stacked and the first plate-shaped member  7  and the second plate-shaped member  3  are bonded to each other (process (3)). As a result, the second bonded surface  6  of the second plate-shaped member  3  and the first bonded surface  10  of the first plate-shaped member  7  are bonded to each other. 
     It is preferable that a bonding material be interposed between the first plate-shaped member  7  and the second plate-shaped member  3  when the first plate-shaped member  7  and the second plate-shaped member  3  are stacked. In addition, it is preferable that the first plate-shaped member  7  and the second plate-shaped member  3  be bonded to each other while the bonding material is interposed between the first plate-shaped member  7  and the second plate-shaped member  3 . The bonding material preferably has a film shape, a sheet shape, a plate shape, and the like. 
     The bonding material is preferably formed of, for example, metal or alloy containing at least one selected from a group consisting of copper (Cu), silver (Ag), gold (Au), nickel (Ni), and aluminum (Al). In addition, it is preferable that the bonding material penetrate into the inside of at least one of the first plate-shaped portion  7  and the second plate-shaped portion  3  when pressing (hot pressing) while heating are performed while the bonding material is interposed between the first plate-shaped member  7  and the second plate-shaped member  3 . If the bonding material is configured in this manner, it is possible to improve bonding between the first plate-shaped portion  7  and the second plate-shaped portion  3 . 
     The bonding material may further contain an additive such as palladium (Pd), silicon (Si), tin (Sn), cobalt (Co), phosphorus (P), manganese (Mn), zinc (Zn), and boron (B). If such an additive is further contained, it is possible to lower a bonding temperature and improve reliability. 
     In order to stack and bond the first plate-shaped member  7  and the second plate-shaped member  3 , a stack of the first plate-shaped member  7  and the second plate-shaped member  3  is preferably bonded through hot pressing. A temperature of the hot pressing is preferably set to 900 to 1200° C., and more preferably 1000 to 1150° C. Heating at such a temperature enables excellent bonding between the first plate-shaped member  7  and the second plate-shaped member  3  and prevents degradation of the strength of the second plate-shaped member  3 . In addition, a hot pressing time is preferably set to 1 minute to 1 hour, and more preferably 10 to 45 minutes. If the hot pressing time is shorter than 1 minute, it may be difficult to bond the first plate-shaped member  7  and the second plate-shaped member  3  with a strong bonding strength. If the hot pressing time is longer than 1 hour, base materials of the first and second plate-shaped members may be easily deteriorated. A pressure of the hot pressing may be appropriately determined depending on shapes, sizes, and the like of the first plate-shaped member  7  and second plate-shaped member  3 . For example, the pressure is preferably set to 0.01 to 100 MPa, and more preferably 0.1 to 10 MPa. As a hot pressing machine, for example, a FVHP-R manufactured by “Fujidempa Kogyo Co., Ltd.” may be employed. 
     Then, slits  9  communicating with the cavities  11  are formed from “the surface opposite to the first bonded surface” (surface side) of the first plate-shaped portion  7  so that the die for forming a honeycomb structure  1  is obtained (refer to  FIGS. 1 to 4A ) (process (4)). It can be said that the slit  9  communicates with the back hole  5  through the cavity  11  since it also communicates with the “cavity  11  communicating with the back hole  5 .” As a method of forming slits in the first plate-shaped member, any one of methods known in the art such as grinding using a diamond grinding wheel may be appropriately employed without a particular limitation. In the die for forming a honeycomb structure  1  of  FIGS. 1 to 4A , a planar shape (slit formation pattern) of a cell block  13  formed by the slits  9  is rectangular. Each condition of the slit such as the slit formation pattern is preferably set to the same condition as that described in relation to the aforementioned die for forming a honeycomb structure according to an embodiment of the present invention. 
     In order to form the slit  9  in the die for forming a honeycomb structure  1 C of  FIG. 5 , for example, a diamond grinding wheel having a head (outer circumference) having an “outwardly convex curved shape” is preferably employed. In addition, in order to form the slit  9  in the die for forming a honeycomb structure  1 D of  FIG. 6 , for example, a diamond grinding wheel having a “V-shaped” head (outer circumference) is preferably employed. Furthermore, in order to form the slit  9  in the die for forming a honeycomb structure  1 E of  FIG. 7 , for example, a diamond grinding wheel having a generally V-shaped cross section is preferably employed. 
     The buffer portions  21  formed in the dies for forming a honeycomb structure  1 H to  1 K of  FIGS. 10 to 13  are preferably formed through the method described below. For example, a method of forming the buffer portion  21  from the slit of the first plate-shaped member using a wire electric discharge machine is preferably employed. In addition, the slit may be formed from the surface side of the first plate-shaped member using a wire electric discharge machine. In this case, the buffer portion having a wide hole width is formed in the head of the slit. When the buffer portion is formed, a shape of the buffer portion may be adjusted depending on a wire diameter, a fabricating condition, a wiring locus, and the like. 
     EXAMPLES 
     Hereinafter, examples of the present invention will be described in more detail. Note that such examples are not intended to limit the invention. 
     Example 1 
     First, a back hole (through hole) having a diameter of 2 mm was formed in a plate-shaped member (second plate-shaped member) formed of stainless steel (SUS303) through electrical discharge machining (EDM). As a result, the “diameter of the open end of the back hole on the second bonded surface” became 2 mm. The second plate-shaped member had a disk shape having a diameter of 200 mm. In addition, the second plate-shaped member had a thickness of 50 mm. The area where the back hole is formed (back hole formation area) had a circular shape centered at the center of the first plate-shaped member, and had a diameter of 150 mm. The back hole had a pitch of 5 mm. 
     Then, a cavity (hollow) was formed in the surface (first bonded surface) side of the first plate-shaped member formed of tungsten carbide based cemented carbide through electrochemical machining (ECM). The cavity had a diameter of 1.5 mm and a depth of 5 mm. As a result, the open end of the cavity on the first bonded surface had a diameter of 1.5 mm. In addition, the first plate-shaped member had a disk shape having a diameter of 200 mm. Furthermore, the first plate-shaped member had a thickness of 10 mm. The tungsten carbide based cemented carbide as a material of the first plate-shaped member was obtained by mixing tungsten carbide and cobalt of 2 mass %. 
     Then, the first and second plate-shaped members were stacked by interposing a bonding material therebetween. In order to stack the first plate-shaped member and second plate-shaped member, they were arranged such that the first bonded surface of the first plate-shaped member faces the second bonded surface of the second plate-shaped member. As the bonding material, an film-shaped aluminum (Al) having a thickness of 0.01 mm was employed. 
     Then, the stack of the first plate-shaped member and second plate-shaped member was hot-pressed under a pressure of 0.5 MPa at a temperature of 900° C. for 0.5 hours to bond the first plate-shaped member and second plate-shaped member to each other. In the hot pressing, it is preferable that “the stack of the first plate-shaped member and second plate-shaped member” be interposed and pressed using a “plate-shaped pressing member” having a size larger than those of the first plate-shaped member and second plate-shaped member. As a result, it is possible to uniformly press the stack. It is noted that the “hot pressing” means that “pressing while heating.” 
     The assembly obtained by “bonding the first plate-shaped member and second plate-shaped member” in this manner was cooled to an ambient temperature, and the slits were formed in the first plate-shaped member, so that the die for forming a honeycomb structure having the structure illustrated in  FIGS. 1 to 4A  was obtained. The slits were formed in a lattice shape communicating with the cavities using a diamond grinding wheel. The slit had a width of 0.5 mm and a pitch of 5 mm. The slits were formed such that the cavities are positioned in the intersection between the slits. In the die for forming a honeycomb structure obtained in this manner, the open end of the cavity on the first bonded surface was arranged inside the open end of the back hole on the second bonded surface. A “formation test” was performed for the obtained die for forming a honeycomb structure as described below. A result of the formation test is shown in Table 1. 
     (Formation Test) 
     As a ceramic raw material, a mixture of alumina, talc, and kaolin is used. An organic binder is mixed with this mixture, water is added, and kneading is performed, so that kneaded material (formation raw material) is prepared using a vacuum pugmill. The obtained kneaded material is formed using an extrusion machine installed with the die for forming a honeycomb structure to obtain a cylindrical honeycomb formed body. Then, the obtained honeycomb formed body is dried using a dielectric drier and is then fired at a high temperature using a firing furnace to obtain a honeycomb structure. Through this method, 100 honeycomb structures are prepared. A visual inspection is performed for the obtained honeycomb structures on whether or not there is a “distortion” in the cell. A honeycomb structure having no “distortion” is classified as a quality product, and a honeycomb structure having a “distortion” is classified as a defective product, so that a yield of the quality product is calculated. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Yield (%) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Example 1 
                 99 
               
               
                   
                 Example 2 
                 99 
               
               
                   
                 Comparative Example 1 
                 0 
               
               
                   
                   
               
            
           
         
       
     
     Example 2 
     A die for forming a honeycomb structure was prepared as in Example 1 except that the diameter of the back hole of the second plate-shaped portion is smaller than the diameter of the cavity of the first plate-shaped portion. It is noted that the open end of the back hole on the second bonded surface was arranged inside the open end of the cavity on the first bonded surface. The open end of the cavity on the first bonded surface had a diameter of 2.0 mm. In addition, the cavity had a depth of 5 mm. In addition, the open end of the back hole on the second bonded surface had a diameter of 1.5 mm. The “formation test” was performed for the obtained die for forming a honeycomb structure. A result of the test is shown in Table 1. 
     Comparative Example 1 
     A die for forming a honeycomb structure was prepared as in Example 1 except that the back hole of the second plate-shaped member has the same diameter as that of the cavity of the first plate-shaped member. Both the open end of the cavity on the first bonded surface and the open end of the back hole on the second bonded surface had a diameter of 2.0 mm. In the obtained die for forming a honeycomb structure, plate-shaped member the first and second plate-shaped member were bonded to each other such that the “outer circumference of the open end of the cavity on the first bonded surface” and the “outer circumference of the open end of the back hole of the second bonded surface” intersect with each other. That is, the first plate-shaped member and second plate-shaped member were bonded while the back holes and the cavities are deviated from each other. 
     Referring to Table 1, it is recognized that excellent formability is obtained if a honeycomb structure is formed using the dies for forming a honeycomb structure in Examples 1 and 2. Meanwhile, in the die for forming a honeycomb structure of Comparative Example 1, the first plate-shaped member and second plate-shaped member are bonded to each other while the back hole and the cavity are deviated. Therefore, it was revealed that a distortion is easily generated in the obtained honeycomb-structured formed body, and a yield is degraded in the manufacturing of the honeycomb structure. 
     INDUSTRIAL APPLICABILITY 
     The die for forming a honeycomb structure according to the present invention may be employed in manufacturing of a honeycomb structure used in a catalyst carrier, a filter that catches fine particles in an exhaust gas, and the like. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
         
           
               1 ,  1 A,  1 B,  1 C,  1 D,  1 E,  1 F,  1 G,  1 H,  1 I,  1 J,  1 K: die for forming honeycomb structure,  3 : second plate-shaped portion (second plate-shaped member),  5 : back hole,  5   a : open end of back hole,  6 : second bonded surface,  7 : first plate-shaped portion (first plate-shaped member),  7   a : surface (surface of first plate-shaped portion),  9 : slit,  9   a : open end of slit,  9   b : bottom portion of slit,  10 : first bonded surface,  11 : cavity,  11   a : open end of cavity,  11   b : bottom portion,  13 : cell block,  21 : buffer portion,  21   a : bottom portion of buffer portion,  21   b : end (end of the buffer portion in the side opposite to the bottom portion), d 1 : diameter of open end of cavity, D 1 : diameter of open end of back hole, h: depth of cavity, A: width of buffer portion, B: depth of buffer portion.