Patent Publication Number: US-2018043595-A1

Title: Honeycomb structure forming die

Description:
“The present application is an application based on JP-2016-156972 filed on Aug. 9, 2016 and JP-2016-220868 filed on Nov. 11, 2016 with Japan Patent Office, the entire contents of which are incorporated herein by reference.” 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a honeycomb structure forming die, and more particularly, it relates to a honeycomb structure forming die which is capable of forming, in high quality, a honeycomb formed body having a central portion different from a circumferential portion in cell structure. 
     Description of the Related Art 
     Heretofore, a honeycomb structure onto which a catalyst is loaded has been used for a purifying treatment of toxic substances such as HC, CO and NO x  included in an exhaust gas emitted from an engine of a car or the like. Further in the honeycomb structure, plugging portions are applied to open ends of cells defined by porous partition walls, whereby the honeycomb structure is also used as an exhaust gas purifying filter. 
     Honeycomb structures are pillar-shaped structures each including partition walls defining a plurality of cells which become through channels for an exhaust gas. Such a honeycomb structure has a cell structure in which a plurality of cells are regularly arranged at predetermined cycles in a plane perpendicular to an extending direction of the cells. Heretofore, there has been one type of cell structure in the above plane of one honeycomb structure, but in recent years, there has been suggested a honeycomb structure having two or more types of cell structures in the above plane for the purpose of improvement of an exhaust gas purifying efficiency, or the like. For example, there has been suggested a honeycomb structure having a central portion different from a circumferential portion in cell density or cell shape in a plane perpendicular to a cell extending direction, whereby the honeycomb structure has two types of cell structures in the above plane. 
     This honeycomb structure is manufactured by forming a kneaded material including a ceramic forming raw material with an extruding die to prepare a honeycomb formed body and then drying and firing the prepared honeycomb formed body. For example, the die is prepared as a honeycomb structure forming die by forming, in a die substrate made of a metal, back holes into which the kneaded material is introduced and slits which communicate with the back holes (e.g., see Patent Documents 1 to 4). Hereinafter, the honeycomb structure forming die will be referred to simply as “a forming die” or “a die” sometimes. 
     [Patent Document 1] JP-A-2015-096310 
     [Patent Document 2] JP-A-2013-132879 
     [Patent Document 3] JP-A-2013-132881 
     [Patent Document 4] JP-A-H04-332604 
     SUMMARY OF THE INVENTION 
     For example, a die described in Patent Document 1 includes a first mold positioned on an upstream side in an extruding direction of a raw material and a second mold positioned on a downstream side. The above first mold has convex regions projecting from a periphery on the downstream side in the extruding direction, and the above second mold has through-holes which fit with the convex regions. In the die described in Patent Document 1, the convex regions of the first mold are inserted into the through-holes of the second mold to combine the first mold and the second mold into one die. 
     However, the die described in Patent Document 1 has the problem that there are restrictions on design when the die is used in forming a honeycomb formed body to prepare a honeycomb structure having two types of cell structures. In other words, usually in the die to form the honeycomb formed body, “latticed slits” corresponding to a cell structure of the honeycomb formed body are formed on the side of a kneaded material discharging surface of a die substrate. Further in such a die, back holes are formed as raw material supply holes which communicate with intersection points of the latticed slits. In the die described in Patent Document 1, in circumstances where the first mold is different from the second mold in shape of the respective slits, it is difficult to completely match all back holes (i.e., first raw material supply holes) in the first mold with all back holes (i.e., second raw material supply holes) in the second mold. When all the back holes do not match one another and when the first mold and the second mold are combined into one piece, movement of the kneaded material in the die is obstructed and it becomes difficult to perform uniform extrusion. Consequently, in the die described in Patent Document 1, it is necessary to select the shapes of the respective slits formed in the first mold and the second mold so that the above-mentioned movement of the kneaded material is not obstructed, and a degree of freedom in design is very low. 
     Each of dies described in Patent Documents 2 and 3 is not a die to form a honeycomb formed body having two types of cell structures, but is a die for the purpose of improvement of cell quality only in an outermost circumferential portion. Furthermore, the dies described in Patent Documents 2 and 3 have the problem that the dies are not compatible with formation of various honeycomb formed bodies in which shapes of two types of cell structures and their formation ranges vary over a broad range. For example, a honeycomb structure having two types of cell structures occasionally has a boundary wall disposed to surround the cell structure of a central portion in a boundary between the cell structure of the central portion and the cell structure of a circumferential portion. During extrusion, the boundary wall requires a large amount of kneaded material of a forming raw material as compared with partition walls constituting the cell structures of the central portion and the circumferential portion. When the honeycomb formed body having such a boundary wall as described above is formed with one of the dies described in Patent Documents 2 and 3, the requirement exceeds supply of the kneaded material to form the boundary wall, and formation defects might be caused in the boundary wall and its vicinity. Furthermore, each of the dies described in Patent Documents 2 and 3 has been developed for the purpose of the improvement of the cell quality only in the outermost circumferential portion, and hence there is a problem in strength of a shell region. When the problem is to be solved only by simply enlarging a forming range of the shell region, there is the fear of the problem that the shell region is deformed. 
     A die described in Patent Document 4 has a structure in which adjacent die bodies are tightened with a wedge or engaged with each other by utilizing a pressing pressure, and hence there is the problem that pressure resisting properties to a kneaded material are low and that the die is easy to break. The die described in Patent Document 4 also has the problem that positional shifts of back holes are likely to be induced and that formation defects are likely to be generated. 
     On the other hand, the honeycomb formed body having two types of cell structures occasionally has a boundary wall defining the two types of cell structures in a boundary between a central portion of the honeycomb formed body and a circumferential portion thereof. An amount of a kneaded material to be consumed for the boundary wall during extrusion is different from that of a kneaded material to be consumed for partition walls to be formed around the boundary wall. Consequently, the honeycomb formed body having the two types of cell structures has the problem that formation defects are likely to be generated especially around the boundary wall. For example, when a type or the like of kneaded material of the forming raw material is changed in performing the extrusion, a fluidity of the kneaded material varies. Therefore, the die to extrude the honeycomb formed body having the boundary wall has very poor versatility. 
     The present invention has been developed in view of the above-mentioned problems, and an object thereof is to provide a honeycomb structure forming die which is capable of forming, in high quality, a honeycomb formed body having a central portion different from a circumferential portion in cell structure. 
     According to the present invention, there is provided a honeycomb structure forming die as follows. 
     [1] A honeycomb structure forming die including: 
     a first die which is disposed on an upstream side in an extruding direction of a kneaded material of a forming raw material and in which a central region on the side of a kneaded material discharging surface has a convex region projecting toward a downstream side in the extruding direction; and 
     a ring-shaped second die which is disposed on the downstream side of the first die and which possesses a shape complementary to the convex region, 
     wherein in the central region of the first die, first kneaded material introducing holes and latticed first slits communicating with the first kneaded material introducing holes are formed, 
     in a circumferential region surrounding the central region of the first die, the first kneaded material introducing holes are formed to pass through the circumferential region of the first die, 
     in the ring-shaped second die, there are formed second kneaded material introducing holes into which the kneaded material discharged from the first kneaded material introducing holes formed in the circumferential region of the first die is introduced, and latticed second slits communicating with the second kneaded material introducing holes, and 
     opening positions of the first kneaded material introducing holes of the circumferential region of the first die do not match opening positions of the second kneaded material introducing holes of the second die in at least a part of the honeycomb structure forming die, 
     the honeycomb structure forming die having a clearance region to extrude the kneaded material into a ring shape, between an outer peripheral surface of the convex region of the first die and an inner peripheral surface of the ring-shaped second die, 
     the honeycomb structure forming die further including a space forming member between the first die and the second die, 
     wherein the space forming member is disposed in a circumferential region of the second die on an outer side of a region where the second kneaded material introducing holes are formed, and 
     between the surface of the circumferential region of the first die on the downstream side and the surface of the second die on the upstream side, a space is formed in which movement of the kneaded material is performed between the first kneaded material introducing hole and the second kneaded material introducing hole. 
     [2] The honeycomb structure forming die according to the above [1], wherein a shape of the first slits is different from a shape of the second slits. 
     [3] The honeycomb structure forming die according to the above [1] or [2], wherein a thickness of the space forming member is from 0.03 to 5.00 mm. 
     [4] The honeycomb structure forming die according to any one of the above [1] to [3], which includes two or more space forming members, wherein the space forming member interposed between the first die and the second die is replaced, whereby a distance of the space in the extruding direction is changeable. 
     [5] The honeycomb structure forming die according to any one of the above [1] to [4], including two or more types of second dies which are different in shape of the second slits, wherein the second dies are replaceable. 
     [6] The honeycomb structure forming die according to any one of the above [1] to [5], wherein a ratio of an area of the central region of the first die to an area of an end face of a honeycomb formed body to be extruded is from 30 to 70%. 
     [7] The honeycomb structure forming die according to any one of the above [1] to [6], wherein slits surrounding one cell among the first slits and slits surrounding one cell among the second slits extend in mutually intersecting directions. 
     [8] The honeycomb structure forming die according to any one of the above [1] to [7], wherein an arranging direction of a cell structure of the honeycomb formed body which is to be extruded through the first slits and an arranging direction of a cell structure of the honeycomb formed body which is to be extruded through the second slits extend in mutually intersecting directions. 
     [9] The honeycomb structure forming die according to any one of the above [1] to [8], wherein in the first die, the central region is the same as the circumferential region in an opening diameter of the first kneaded material introducing hole and an interval between the first kneaded material introducing holes. 
     A honeycomb structure forming die of the present invention includes a first die in which a central region on the side of a kneaded material discharging surface has a convex region projecting toward a downstream side in an extruding direction of a kneaded material, and a ring-shaped second die which possesses a shape complementary to the convex region of the first die. The honeycomb structure forming die of the present invention further includes “a space forming member” to form a space in which movement of the kneaded material is performed between the first kneaded material introducing hole and the second kneaded material introducing hole, between the surface of a circumferential region of the first die on the downstream side and the surface of the second die on an upstream side. 
     The honeycomb structure forming die of the present invention is capable of forming, in high quality, a honeycomb formed body having a central portion different from a circumferential portion in cell structure. In other words, even when positions of the first kneaded material introducing holes of the first die do not match positions of the second kneaded material introducing holes of the second die, a flow rate distribution of the kneaded material to be introduced into the second kneaded material introducing holes can be uniformized by the space formed by the space forming member. Therefore, the honeycomb structure forming die of the present invention is capable of uniformizing an amount of the kneaded material to be discharged from second slits of the second die, and forming the honeycomb formed body in high quality. 
     Furthermore, in the honeycomb structure forming die of the present invention, the space forming member is a member separated from the first die and the second die, and hence a size of the space formed by the space forming member is easily changeable. In other words, when a thickness of the space forming member is changed, a distance of the space in the extruding direction is adjustable. Furthermore, by the adjustment of the distance of the space in the extruding direction, it is possible to adjust an amount of the kneaded material to be introduced into a clearance region to extrude a boundary wall. For example, when a type of kneaded material is changed or when the second die is replaced with another second die having a different second slit shape, it is possible to appropriately adjust the amount of the kneaded material to be introduced into the above-mentioned clearance region, and it is possible to form the honeycomb formed body into the high quality. The honeycomb structure forming die of the present invention is remarkably excellent in versatility. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view schematically showing a kneaded material discharging surface side of one embodiment of a honeycomb structure forming die of the present invention; 
         FIG. 2  is a plan view of a kneaded material introducing surface side of the honeycomb structure forming die shown in  FIG. 1 ; 
         FIG. 3  is a plan view of a kneaded material discharging surface side of a first die constituting the honeycomb structure forming die shown in  FIG. 1 ; 
         FIG. 4  is a plan view of a kneaded material discharging surface side of a second die constituting the honeycomb structure forming die shown in  FIG. 1 ; 
         FIG. 5  is a plan view of a kneaded material introducing surface side of the second die constituting the honeycomb structure forming die shown in  FIG. 1 ; 
         FIG. 6  is a plan view of a space forming member constituting the honeycomb structure forming die shown in  FIG. 1 ; 
         FIG. 7  is a cross-sectional view schematically showing a cross section taken along the A-A′ line of the honeycomb structure forming die shown in  FIG. 1 ; 
         FIG. 8  is an enlarged cross-sectional view of an enlarged part of  FIG. 7 ; 
         FIG. 9  is an enlarged cross-sectional view showing a state where the space forming member is replaced in the honeycomb structure forming die shown in  FIG. 8 ; 
         FIG. 10  is a perspective view schematically showing one example of a honeycomb structure prepared by the honeycomb structure forming die of the present invention; 
         FIG. 11  is a plan view schematically showing an inflow end face of the honeycomb structure shown in  FIG. 10 ; 
         FIG. 12  is a cross-sectional view schematically showing a cross section taken along the B-B′ line of  FIG. 11 ; and 
         FIG. 13  is a plan view schematically showing a kneaded material discharging surface side of another embodiment of the honeycomb structure forming die of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention will specifically be described with reference to the drawings. It should be understood that the present invention is not restricted to the following embodiments and that design changes, improvements and the like are suitably addable on the basis of ordinary knowledge of a person skilled in the art without departing from the gist of the present invention. 
     (1) Honeycomb Structure Forming Die: 
     One embodiment of a honeycomb structure forming die of the present invention will be described. Here,  FIG. 1  is a plan view schematically showing a kneaded material discharging surface side of the one embodiment of the honeycomb structure forming die of the present invention.  FIG. 2  is a plan view of a kneaded material introducing surface side of the honeycomb structure forming die shown in  FIG. 1 .  FIG. 3  is a plan view of a kneaded material discharging surface side of a first die constituting the honeycomb structure forming die shown in  FIG. 1 .  FIG. 4  is a plan view of a kneaded material discharging surface side of a second die constituting the honeycomb structure forming die shown in  FIG. 1 .  FIG. 5  is a plan view of a kneaded material introducing surface side of the second die constituting the honeycomb structure forming die shown in  FIG. 1 .  FIG. 6  is a plan view of a space forming member constituting the honeycomb structure forming die shown in  FIG. 1 .  FIG. 7  is a cross-sectional view schematically showing a cross section taken along the A-A′ line of the honeycomb structure forming die shown in  FIG. 1 .  FIG. 8  is an enlarged cross-sectional view of an enlarged part of  FIG. 7 . It is to be noted that in  FIG. 3 ,  FIG. 5  and  FIG. 6 , surfaces of the first die, the second die and the space forming member are shown with hatchings, respectively. 
     As shown in  FIG. 1  to  FIG. 8 , a honeycomb structure forming die  100  of the present embodiment includes a first die  10 , a second die  20 , and a space forming member  30 . The first die  10  is disposed on an upstream side in an extruding direction X of a kneaded material of a forming raw material, and a central region  15  on the side of a kneaded material discharging surface  18  has a convex region  16  projecting toward a downstream side in the extruding direction. The second die  20  is a ring-shaped die which is disposed on the downstream side of the first die  10  and which possesses a shape complementary to the convex region  16  of the first die  10 . The space forming member  30  is interposed between the first die  10  and the second die  20 . The space forming member  30  functions as a spacer to form a space  31  between the surface (a downstream surface  14 ) of a circumferential region  17  of the first die  10  on the downstream side and the surface (an upstream surface  24 ) of the second die  20  on an upstream side. Hereinafter, the honeycomb structure forming die  100  of the present embodiment will be referred to simply as “the die  100 ” sometimes. The extruding direction X of the kneaded material is an extruding direction in performing extrusion by use of the die  100  of the present embodiment, and the direction extends from a kneaded material introducing surface  19  toward the kneaded material discharging surface  18 . 
     According to the die  100  of the present embodiment, in the central region  15  of the first die  10 , first kneaded material introducing holes  12  and latticed first slits  11  communicating with the first kneaded material introducing holes  12  are formed. The first kneaded material introducing holes  12  are formed coaxially with intersection points of the latticed first slits  11  in the extruding direction X. In other words, the first kneaded material introducing holes  12  communicate with the intersection points of the latticed first slits  11 . In the circumferential region  17  surrounding the central region  15  of the first die  10 , the first kneaded material introducing holes  12  are formed to pass through the circumferential region  17  of the first die  10 . 
     In the ring-shaped second die  20 , there are formed second kneaded material introducing holes  22  into which the kneaded material discharged from the first kneaded material introducing holes  12  formed in the circumferential region  17  of the first die  10  is introduced, and latticed second slits  21  communicating with the second kneaded material introducing holes  22 . The second kneaded material introducing holes  22  are formed coaxially with intersection points of the latticed second slits  21  in the extruding direction X. In other words, the second kneaded material introducing holes  22  communicate with the intersection points of the latticed second slits  21 . The die  100  of the present embodiment is constituted so that opening positions of the first kneaded material introducing holes  12  of the circumferential region  17  of the first die  10  do not match opening positions of the second kneaded material introducing holes  22  of the second die  20  in at least a part of the die. 
     In the die  100  of the present embodiment, the first die  10  and the second die  20  are combined to sandwich the space forming member  30  therebetween. The space forming member  30  is disposed in a circumferential region of the second die  20  on an outer side of a region where the second kneaded material introducing holes  22  are formed. Hereinafter, an end face of the circumferential region  17  of the first die  10  on the downstream side in the extruding direction X will be referred to as “the downstream surface  14  in the circumferential region  17  of the first die  10 ” and an end face of the ring-shaped second die  20  on the upstream side in the extruding direction X will be referred to as “the upstream surface  24  of the second die  20 ” sometimes. Furthermore, when the upstream side is simply mentioned, the upstream side in the extruding direction X is meant, and when the downstream side is simply mentioned, the downstream side in the extruding direction X is meant. 
     The die  100  of the present embodiment has the space  31  in which the movement of the kneaded material is performed between the first kneaded material introducing hole  12  and the second kneaded material introducing hole  22 , between the downstream surface  14  in the circumferential region  17  of the first die  10  and the upstream surface  24  of the second die  20 . In  FIG. 6  to  FIG. 8 , reference numeral  33  indicates a substrate  33  constituting the space forming member  30 . 
     The die  100  of the present embodiment has a clearance region  35  to extrude the kneaded material into a ring shape, between an outer peripheral surface of the convex region  16  of the first die  10  and an inner peripheral surface of the ring-shaped second die  20 . In other words, in the die  100  of the present embodiment, a cavity region  26  in a central region  25  of the second die  20  is formed to be slightly larger than a peripheral edge of the convex region  16  of the first die  10 . According to this constitution, when the first die is combined with the second die to insert the convex region  16  of the first die  10  into the cavity region  26  of the ring-shaped second die  20 , the ring-shaped clearance region  35  is formed between the convex region  16  of the first die  10  and the second die  20 . The ring-shaped clearance region  35  functions as the clearance region  35  to form a boundary wall of a honeycomb formed body. In the present invention, “the ring-shaped second die  20  possessing a shape complementary to the convex region  16  of the first die  10 ” means a ring-shaped die having the cavity region  26  which is slightly larger than the convex region  16  of the first die  10 . 
     In the die  100  of the present embodiment, an end face of the first die  10  on the upstream side in the extruding direction X is the kneaded material introducing surface  19  of the whole die  100 . Therefore, during extrusion, the kneaded material of the forming raw material is initially introduced into the first kneaded material introducing holes  12  opened in the kneaded material introducing surface  19  of the first die  10 . The kneaded material introduced into the first kneaded material introducing holes  12  of the central region  15  of the first die  10  moves to the latticed first slits  11  communicating with the first kneaded material introducing holes  12 , and is discharged as a formed body corresponding to a shape of the first slits  11  from the kneaded material discharging surface  18  of the first die. On the other hand, the kneaded material introduced into the first kneaded material introducing holes  12  of the circumferential region  17  of the first die  10  is discharged from a downstream surface  14  side of the first die  10 , and introduced into the second kneaded material introducing holes  22  of the second die  20  through the space  31 . In this case, even when positions of the first kneaded material introducing holes  12  of the first die  10  do not match those of the second kneaded material introducing holes  22  of the second die  20 , a flow rate distribution of the kneaded material can be uniformized by the space  31  formed between the first die  10  and the second die  20 . The kneaded material introduced into the second kneaded material introducing holes  22  of the second die  20  moves to the latticed second slits  21  communicating with the second kneaded material introducing holes  22 , and is discharged as a formed body corresponding to a shape of the second slits  21  from a kneaded material discharging surface  28  of the second die. Therefore, the die  100  of the present embodiment is capable of uniformizing an amount of the kneaded material to be discharged from the second slits  21  of the second die  20  and forming the honeycomb formed body in high quality. Furthermore, the space  31  also communicates with the clearance region  35  to extrude the kneaded material into the ring shape, and hence the kneaded material introduced into the first kneaded material introducing holes  12  of the circumferential region  17  of the first die  10  is also introduced into the clearance region  35  through the space  31 . Therefore, a flow rate distribution of the kneaded material to be introduced into the clearance region  35  can be uniformized, and it is also possible to especially effectively inhibit generation of formation defects around the boundary wall of the honeycomb formed body to be extruded. Consequently, the die  100  of the present embodiment is capable of forming, in high quality, the honeycomb formed body having a central portion different from a circumferential portion in cell structure and having the boundary wall in a boundary between the central portion and the circumferential portion. 
     In the present description, “the cell structure” means a structure of a honeycomb structure which is directed in accordance with a partition wall thickness, a cell density, and a cell shape. Furthermore, “the shape of the slits” means the shape of the slits which is directed in accordance with a width, a depth and a length of each slit formed in the die and a mutually connecting configuration of the slits. 
     Furthermore, although not shown in the drawings, in the die of the present embodiment, a plurality of types of second dies which are different in shape of the second slits may separately be prepared, and the second die is replaceable for use in accordance with the cell structure of the honeycomb formed body to be formed. The second kneaded material introducing holes of the second die are formed coaxially with the intersection points of the latticed second slits in the extruding direction, and hence the respective opening positions of the second kneaded material introducing holes in the upstream surface of the second die vary in accordance with the shape of the second slits of the second die. The die of the present embodiment has the space between the first die and the second die, and hence even when the second die having a different shape of the second slits is used, uniform extrusion can always be achieved in the central region and circumferential region without obstructing the movement of the kneaded material in the die. 
     Further in the die  100  of the present embodiment, the space forming member  30  is a member separated from the first die  10  and the second die  20 , and hence a size of the space  31  formed by the space forming member  30  is easily changeable. In other words, when a thickness of the space forming member  30  in the extruding direction X is changed, a distance of the space  31  in the extruding direction X is adjustable. Then, the distance of the space  31  in the extruding direction X is adjusted, whereby an amount of the kneaded material to be introduced into the clearance region  35  is adjustable. For example, a distance of a space  31 A in the extruding direction X is adjustable by using a space forming member  30 A shown in  FIG. 9 . The space forming member  30 A shown in  FIG. 9  is constituted similarly to the space forming member  30  shown in  FIG. 8  except that a thickness of the space forming member in the extruding direction X varies. Here,  FIG. 9  is an enlarged cross-sectional view showing a state where the space forming member is replaced in the honeycomb structure forming die shown in  FIG. 8 . In the die  100  shown in  FIG. 9 , constituent elements constituted similarly to the die  100  shown in  FIG. 8  are denoted with the same reference numerals as in  FIG. 8 , and description is omitted sometimes. In  FIG. 9 , reference numeral  33 A indicates a substrate  33 A constituting the space forming member  30 A. 
     For example, when a type of kneaded material of the forming raw material is changed during use of the die  100  shown in  FIG. 8 , fluidity of the kneaded material varies, and the flow rate distribution of the kneaded material to be introduced into the clearance region  35  might vary. Consequently, when the type of kneaded material is changed, balance in consumption of kneaded material between the second slits  21  and the clearance region  35  might be lost. Furthermore, also when the second die  20  is replaced with another second die (not shown) having a different shape of second slits, the amount of the kneaded material to be discharged from the second slits  21  of the second die  20  varies, and the balance in consumption of the kneaded material between the second slits  21  and the clearance region  35  might be lost. When the balance in consumption of the kneaded material between the second slits  21  and the clearance region  35  is lost, the thickness of the space forming member  30  is adjusted, and the amount of the kneaded material to be introduced into the clearance region  35  is adjusted, whereby it is possible to effectively inhibit the generation of the formation defects around the boundary wall of the honeycomb formed body. In the die  100  of the present embodiment, also when the change of the type of kneaded material, the replacement of the second die and the like are performed, the distance of the space  31  in the extruding direction X is adjusted, whereby the flow rate distribution of the kneaded material in the die  100  is suitably adjustable. Therefore, the die  100  of the present embodiment produces not only the effect that the second die is replaceable but also the especially remarkable effect that the die is remarkably excellent in versatility. 
     Here, the honeycomb structure prepared by the honeycomb structure forming die of the present embodiment is described.  FIG. 10  is a perspective view schematically showing one example of the honeycomb structure prepared by the honeycomb structure forming die of the present invention.  FIG. 11  is a plan view schematically showing an inflow end face of the honeycomb structure shown in  FIG. 10 .  FIG. 12  is a cross-sectional view schematically showing a cross section taken along the B-B′ line of  FIG. 11 . 
     A honeycomb structure  200  shown in  FIG. 10  to  FIG. 12  includes a pillar-shaped honeycomb structure body  204  having porous partition walls  201  and a circumferential wall  203  disposed to surround a circumference of the partition walls  201 . The partition walls  201  of the honeycomb structure body  204  define a plurality of cells  202  which extend from an inflow end face  211  to an outflow end face  212  and become through channels for fluid. Furthermore, the honeycomb structure body  204  has a central cell structure  215 , a circumferential cell structure  216 , and a boundary wall  208  disposed in a boundary portion between the circumferential cell structure  216  and the central cell structure  215 . In the honeycomb structure body  204 , the central cell structure  215  and the circumferential cell structure  216  are different cell structures. 
     Here, the central cell structure  215  is a cell structure constituted by a plurality of cells  202   a  formed in a central portion of the honeycomb structure body  204  in a plane of the honeycomb structure body  204  which is perpendicular to an extending direction of the cells  202 . The circumferential cell structure  216  is a cell structure constituted by a plurality of cells  202   b  formed closer to a circumference of the honeycomb structure body  204  than to the central portion of the honeycomb structure body in the above plane. 
     “The cell structure” is a structure formed by a set of repeating units, and one repeating unit corresponds to one cell  202  defined by the partition walls  201  or a combination of a plurality of cells  202  in the plane perpendicular to the extending direction of the cells  202 . For example, when the cells having the same shape are regularly arranged in the above plane, a region where the cells having the same shape are present becomes one cell structure. Furthermore, when a combination of a plurality of cells which are different in cell shape constitutes one repeating unit, a region where the repeating unit is present constitutes one cell structure. 
     When it is described that two cell structures are “different cell structures”, it is meant that in case of comparison of the two cell structures, the structures are different in one of a partition wall thickness, a cell density and a cell shape. Here, when “the structures are different in partition wall thickness”, it is meant that in case of the comparison of the partition wall thicknesses of the two cell structures, there is a difference of 25 μm or more. Furthermore, when “the structures are different in cell density”, it is meant that in case of the comparison of the cell densities of the two cell structures, there is a difference of 7 cells/cm 2  or more. 
     The die of the present embodiment is suitably usable in formation of a honeycomb formed body to manufacture the honeycomb structure  200  shown in  FIG. 10  to  FIG. 12 . Hereinafter, a more suitable configuration of the die of the present embodiment will be described. 
     In the die of the present embodiment, it is preferable that the shape of the first slits  11  is different from the shape of the second slits  21  as shown in  FIG. 1  to  FIG. 8 . There are not any special restrictions on the shape of the first slits  11  and the shape of the second slits  21 , and the shapes are suitably selectable in accordance with the cell structure of the honeycomb formed body to be formed. Furthermore, the die may include two or more types of second dies  20  which are different in shape of the second slits  21 , and may be constituted so that the second die  20  is replaceable. Furthermore, the die may include two or more types of first dies  10  which are different in shape of the first slits  11 , and may be constituted so that the first die  10  is replaceable. 
     The space forming member  30  is constituted of the plate-shaped substrate  33  having the space  31  in its region corresponding to the central region  15  and the circumferential region  17  of the first die  10 . 
     A thickness of the space forming member  30  in the extruding direction X is preferably from 0.03 to 5.00 mm and especially preferably from 0.05 to 1.00 mm. When the thickness of the space forming member  30  is smaller than 0.03 mm, a volume in the space  31  is excessively small, and it might be difficult to uniformize the flow rate distribution of the kneaded material. When the thickness of the space forming member  30  is in excess of 5.00 mm, a through channel forming the clearance region  35  is excessively short, and hence the boundary wall might not sufficiently be formed. 
     The die  100  of the present embodiment may include two or more space forming members  30 . For example, as shown in  FIG. 8  and  FIG. 9 , two or more space forming members  30  and  30 A which are different in thickness may beforehand be prepared. Furthermore, the space forming member in which the distance of the space  31 , i.e., the volume in the space  31  is optimum may suitably be selected for use from the two or more space forming members  30  and  30 A in accordance with forming conditions during the extrusion. 
     A ratio of an area of the central region  15  of the first die  10  to an area of an end face of the honeycomb formed body to be extruded can suitably be determined in accordance with the cell structures of the central portion and circumferential portion of the honeycomb formed body to be formed (e.g., see  FIG. 10  to  FIG. 12 ). It is to be noted that in the die of the present embodiment, the above ratio is preferably from 30 to 70% and especially preferably from 40 to 60%. 
     As described above, in the die  100 , the cavity region  26  in the central region  25  of the second die  20  is fainted to be slightly larger than the peripheral edge of the convex region  16  of the first die  10 , and the ring-shaped clearance region  35  is formed between the convex region  16  of the first die  10  and the second die  20 . There is not any special restriction on an interval between the ring-shaped clearance regions  35  mentioned above, and the interval is suitably selectable in accordance with a thickness of the boundary wall of the honeycomb formed body to be formed. For example, it is preferable that the interval between the ring-shaped clearance regions  35  is from 0.04 to 0.50 mm. 
     In the first die  10 , it is preferable that the central region  15  is the same as the circumferential region  17  in an opening diameter of the first kneaded material introducing hole  12  and an interval between the first kneaded material introducing holes  12 . According to such a constitution, for example, it is possible to easily manufacture the first die  10  at low cost. 
     Furthermore, in the die of the present embodiment, slits surrounding one cell among the first slits and slits surrounding one cell among the second slits may extend in mutually intersecting directions. Here, “the cell” means a space defined by partition walls in the honeycomb formed body to be formed. For example, a die  300  shown in  FIG. 13  is prepared in a state where the first slits  11  of the first die  10  are rotated clockwise as much as 45° from the die  100  shown in  FIG. 1 . Consequently, in the die  300 , slits surrounding one cell among the first slits  11  and slits surrounding one cell among the second slits  21  do not have a parallel positional relation. In the die of the present embodiment, even when the slits surrounding the respective cells intersect with one another in the first slits and the second slits as described above, it is possible to uniformize the flow rate distribution of the kneaded material. Therefore, it is possible to uniformize the amount of the kneaded material to be discharged from the second slits of the second die and it is possible to form the honeycomb formed body in high quality. 
     In the die  100  of the present embodiment, as shown in  FIG. 1 , an arranging direction of the cell structure of the honeycomb formed body to be extruded through the first slits  11  is parallel to an arranging direction of the cell structure of the honeycomb formed body to be extruded through the second slits  21 . In other words, an extending direction of the first slits  11  of the first die  10  is parallel to an extending direction of the second slits  21  of the second die  20 . However, as in the die  300  shown in  FIG. 13 , the arranging direction of the cell structure of the honeycomb formed body to be extruded through the first slits  11  and the arranging direction of the cell structure of the honeycomb formed body to be extruded through the second slits  21  may extend in mutually intersecting directions.  FIG. 13  is a plan view schematically showing a kneaded material discharging surface side of another embodiment of the honeycomb structure forming die of the present invention. In the die  300  shown in  FIG. 13 , constituent elements similar to those of the die  100  shown in  FIG. 1  are denoted with the same reference numerals and description thereof is omitted sometimes. 
     The die  300  shown in  FIG. 13  also includes a space forming member  30  (see  FIG. 7 ) between a first die  10  and a second die  20 . Consequently, also when an extending direction of first slits  11  of the first die  10  and an extending direction of second slits  21  of the second die  20  intersect with each other as in the die  300  shown in  FIG. 13 , it is possible to uniformize the flow rate distribution of the kneaded material. Therefore, it is possible to uniformize an amount of the kneaded material to be discharged from the second slits  21  of the second die  20  and it is possible to form the honeycomb formed body in high quality. 
     The die  300  shown in  FIG. 13  is beforehand prepared so that the first slits  11  of the first die  10  are rotated clockwise as much as 45°. However, for example, the die  100  shown in  FIG. 1  is usable by rotating the first die  10  clockwise as much as 45°. Also when the first die  10  of the die  100  shown in  FIG. 1  is rotated clockwise as much as 45°, the movement of the kneaded material in the die  100  is not obstructed, because the space forming member  30  is present between the first die  10  and the second die  20  as shown in  FIG. 7  and  FIG. 8 . A conventional die which does not include the space forming member  30  is designed so that, for example, the first kneaded material introducing holes of the first die match the second kneaded material introducing holes of the second die in the extruding direction of the kneaded material. Consequently, when the first die is only rotated for use, the movement of the kneaded material in the die might be obstructed. 
     In the die of the present embodiment shown in  FIG. 1  to  FIG. 8 , there are not any special restrictions on a thickness of the first die  10 , a projecting height of the convex region  16  of the first die  10  and thickness of the second die  20 . It is preferable that the thickness of the first die  10  is from 10 to 50 mm. It is preferable that the projecting height of the convex region  16  of the first die  10  is from 5 to 30 mm. It is preferable that the thickness of the second die  20  is from 5 to 30 mm. It is to be noted that the projecting height of the convex region  16  of the first die  10  may be the same as or different from the thickness of the second die  20 . For example, when the convex region  16  of the first die  10  is inserted into the cavity region  26  of the ring-shaped second die  20 , a position of the kneaded material discharging surface  18  of the first die  10  may match or does not have to match a position of the kneaded material discharging surface  28  of the second die  20 . 
     An example of a material of a first die substrate  13  constituting the first die  10  and a second die substrate  23  constituting the second die  20  is a metal or an alloy which is usually used as a material of the honeycomb structure forming die. Hereinafter, the first die substrate and the second die substrate will generically be referred to simply as “the die substrate” sometimes. An example of the material of the die substrate is a metal or an alloy including at least one metal selected from the group consisting of iron (Fe), titanium (Ti), nickel (Ni), copper (Cu) and aluminum (Al). 
     An example of the alloy for use as the material of the die substrate is stainless alloy, more specifically SUS630. This stainless alloy is an inexpensive material which is comparatively easy to be processed. Another example of the alloy constituting the die substrate is tungsten carbide based cemented carbide which is excellent in wear resistance. By use of the die substrate made of tungsten carbide based cemented carbide or the like, it is possible to manufacture the honeycomb structure forming die in which the slits wear away less. 
     An example of a material of the substrate  33  constituting the space forming member  30  is the metal or the alloy which is the example of the material of the die substrate. 
     There is not any special restriction on a method of manufacturing the die of the present embodiment. For example, the die of the present embodiment can be manufactured in conformity with a heretofore known die manufacturing method. 
     The first kneaded material introducing holes and the second kneaded material introducing holes can be formed by using known machine processing such as drill processing, discharge processing, electrolytic processing or laser processing to the first die substrate and the second die substrate. 
     The first slits and the second slits can be formed by using known machine processing such as grind processing, discharge processing, electrolytic processing or laser processing to the first die substrate and the second die substrate. 
     The convex region constituting the central region of the first die can be formed by grind processing, discharge processing, or bonding of two members. 
     EXAMPLES 
     Hereinafter, the present invention will further specifically be described with reference to examples, but the present invention is not restricted by these examples. 
     Example 1 
     In Example 1, a die was manufactured to prepare a honeycomb structure  200  of a cell structure in which a central cell structure  215  of a honeycomb structure body  204  was different from a circumferential cell structure  216  as shown in  FIG. 10  to  FIG. 12 . More specifically, in Example 1, the die was prepared so that the honeycomb structure of a final product was constituted as follows. The honeycomb structure of the final product had a round pillar shape in which a diameter of an end face was 100 mm, and a diameter of the central cell structure in the end face was 70 mm. The honeycomb structure had a boundary wall having a thickness of 0.1 mm in a boundary between the central cell structure and the circumferential cell structure. In the central cell structure, a cell shape was quadrangular, a partition wall thickness was 0.09 mm, and a cell density was 93 cells/cm 2 . In the circumferential cell structure, a cell shape was quadrangular, a partition wall thickness was 0.11 mm, and a cell density was 62 cells/cm 2 . It is to be noted that each dimension of the above honeycomb structure does not include any manufacturing tolerances. 
     Initially in Example 1, there was prepared a plate-shaped first die substrate in which a vertical dimension was 200 mm, a lateral dimension was 200 mm and a thickness was 20 mm. One surface of the prepared first die substrate was defined as a kneaded material discharging surface, and in a central region on the side of the kneaded material discharging surface, a convex region was formed by discharge processing so that a projecting length was 10 mm. 
     Next, in the kneaded material discharging surface of the convex region of the first die substrate, latticed first slits were formed. The latticed first slits were formed into a slit shape to extrude partition walls constituting the central cell structure of the above-mentioned honeycomb structure of the final product. The first slits were formed by grind processing. 
     Next, in a kneaded material introducing surface of the first die substrate, first kneaded material introducing holes having an opening diameter of 1.2 mm were formed to communicate with intersection points of the first slits. Also in a circumferential region of the first die substrate which did not have a convex region, the first kneaded material introducing holes were formed at the same pitch as in a central region having the convex region. The first kneaded material introducing holes of the circumferential region of the first die substrate were through-holes extending from a kneaded material introducing surface of the first die substrate to a downstream surface of the circumferential region. As described above, a first die in the die of Example 1 was prepared. 
     Next, there was prepared a plate-shaped second die substrate in which a vertical dimension was 200 mm, a lateral dimension was 200 mm and a thickness was 10 mm. The die substrate was made of stainless steel. A central region of the prepared second die substrate was hollowed out into a round shape to form the second die substrate into a ring shape. 
     Next, in a kneaded material discharging surface of a convex region of the second die substrate, latticed second slits were formed. The latticed second slits were formed into a slit shape to extrude partition walls constituting the circumferential cell structure of the above-mentioned honeycomb structure of the final product. The second slits were formed by grind processing. 
     Next, in the surface of the second die substrate opposite to the kneaded material discharging surface, second kneaded material introducing holes having an opening diameter of 1.2 mm were formed to communicate with intersection points of the second slits. 
     Next, there was prepared a plate-shaped substrate for a space forming member in which a vertical dimension was 200 mm, a lateral dimension was 200 mm and a thickness was 0.5 mm. The substrate was made of stainless steel. A central region of the prepared substrate was hollowed out into a round shape having a diameter of 120 mm to prepare the ring-shaped space forming member having a space in a region corresponding to the central region and circumferential region of the first die. 
     Next, in a state where the space forming member was disposed on the side of the kneaded material discharging surface on an outer side of the circumferential region of the first die, the convex region of the central region of the first die was inserted into a cavity region of the central region of the second die to sandwich the space forming member between the first die and the second die, and thus, the die of Example 1 was manufactured. 
     Table 1 shows “a die structure”, “a pitch (mm) of the second slits”, “an interval (mm) between boundary portion forming clearance regions”, “presence” of the space forming member, and “a thickness (mm)”. It is to be noted that the table shows a die as “two-body assembly” in a column of “the die structure” when the die is manufactured by combining the first die and the second die into one die to sandwich the space forming member therebetween as in the die of Example 1. On the other hand, the table shows a die as “a monolithic structure” in the column of “the die structure” when the die is manufactured by preparing one die substrate so that a central region is different from a circumferential region in shape of slits. 
     A total operation time required in manufacturing the die of Example 1 was 70 hours. “A formed body quality” was evaluated by using the die of Example 1 by such a method as described below. Table 1 shows the results. 
     (Formed Body Quality) 
     A honeycomb structure constituted of a cordierite composition was extruded by using the prepared die. The extruded honeycomb formed body was visually confirmed, and the quality of the honeycomb formed body was evaluated in accordance with the following evaluation standards. When all partition walls and a boundary wall are formed in an extruded honeycomb formed body, the formed body is evaluated as “excellent”. When the number of non-formed portions per 25.4 mm 2  (inch) is one or more and smaller than three in the partition walls and boundary wall of the extruded honeycomb formed body, the formed body is evaluated as “good”. When the number of the non-formed portions per 25.4 mm 2  (inch) is three or more and smaller than 10 in the partition walls and boundary wall of the extruded honeycomb formed body, the formed body is evaluated as “passable”. When the number of the non-formed portions per 25.4 mm 2  is 10 or more in the partition walls and boundary wall of the extruded honeycomb formed body or when the walls are not formable, the formed body is evaluated as “failure”. It is to be noted that Table 1 shows a type of kneaded material used in the evaluation of the formed body quality in a column of “type of kneaded material”. As shown in Table 1, there were used two types of kneaded materials, i.e., “material A” and “material B” which was different from the material A. The material A and the material B were both kneaded cordierite forming raw materials, and were different in type of additive and water content. Differences in characteristics between the material A and the material B have influences on shape retaining properties and partition wall quality of the honeycomb structure to be extruded. The material A is a composition which is poor in shape retaining properties and excellent in partition wall quality as compared with the material B. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Interval between 
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                 boundary wall 
                 Space forming 
                   
                 Total operation 
                   
               
               
                   
                 Pitch of 
                 forming clearance 
                 member 
                 Type of 
                 time required in 
                 Formed 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 second slits 
                 regions 
                   
                 Thickness 
                 kneaded 
                 manufacturing 
                 body 
               
               
                   
                 Die structure 
                 (mm) 
                 (mm) 
                 Presence 
                 (mm) 
                 material 
                 (hours) 
                 quality 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Example 1 
                 Two-body 
                 1.4 
                 0.1 
                 Present 
                 0.1 
                 Material A 
                 70 
                 Good 
               
               
                   
                 assembly 
               
               
                 Example 2 
                 Two-body 
                 1.4 
                 0.1 
                 Present 
                 0.3 
                 Material A 
                 70 
                 Passable 
               
               
                   
                 assembly 
               
               
                 Example 3 
                 Two-body 
                 1.4 
                 0.1 
                 Present 
                 0.5 
                 Material A 
                 70 
                 Excellent 
               
               
                   
                 assembly 
               
               
                 Example 4 
                 Two-body 
                 1.4 
                 0.1 
                 Present 
                 0.5 
                 Material B 
                 70 
                 Good 
               
               
                   
                 assembly 
               
               
                 Example 5 
                 Two-body 
                 1.4 
                 0.1 
                 Present 
                 0.7 
                 Material B 
                 70 
                 Excellent 
               
               
                   
                 assembly 
               
               
                 Example 6 
                 Two-body 
                 1.4 
                 0.2 
                 Present 
                 0.3 
                 Material B 
                 70 
                 Excellent 
               
               
                   
                 assembly 
               
               
                 Comparative 
                 Monolithic 
                 1.4 
                 — 
                 — 
                 — 
                 Material A 
                 100 
                 Failure 
               
               
                 Example 1 
                 structure 
               
               
                 Comparative 
                 Two-body 
                 1.4 
                 — 
                 None 
                 — 
                 Material A 
                 80 
                 Failure 
               
               
                 Example 2 
                 assembly 
               
               
                 Comparative 
                 Two-body 
                 1.4 
                 0.1 
                 None 
                 0.7 
                 Material A 
                 70 
                 Good 
               
               
                 Example 3 
                 assembly 
                   
                   
                 (space is 
                   
                 Material B 
                   
                 Failure 
               
               
                   
                   
                   
                   
                 present) 
               
               
                   
               
            
           
         
       
     
     Examples 2 to 6 
     The procedure of Example 1 was repeated except that “a pitch (mm) of second slits”, “an interval (mm) between boundary portion forming clearance regions” and “a thickness (mm)” of a space forming member” were changed to such values as shown in Table 1, to manufacture dies. The procedure of Example 1 was repeated to perform evaluation of “formed body quality” by use of the dies of Examples 2 to 6. Table 1 shows the results. 
     Comparative Example 1 
     In Comparative Example 1, there was prepared a plate-shaped die substrate in which a vertical dimension was 200 mm, a lateral dimension was 200 mm and a thickness was 20 mm. The die substrate was made of stainless steel. One surface of the prepared die substrate was defined as a kneaded material discharging surface, and in a central region on the side of a kneaded material discharging surface, slits having the same shape as in first slits of a die of Example 1 were formed. Next, in a circumferential region on the side of the kneaded material discharging surface of the die substrate, slits having the same shape as in second slits of the die of Example 1 were formed. Next, at a position corresponding to a clearance region of Example 1, a ring-shaped slit was Ruined to connect an end of each first slit to an end of each second slit. Next, from the side of a kneaded material introducing surface of the die substrate, kneaded material introducing holes having an opening diameter of 1.2 mm were formed to communicate with intersection points of the respective slits. As described above, a die of Comparative Example 1 was manufactured. 
     A total operation time required in manufacturing the die of Comparative Example 1 was 100 hours. The procedure of Example 1 was repeated to perform evaluation of “formed body quality” by use of the die of Comparative Example 1. Table 1 shows the results. 
     Comparative Example 2 
     In Comparative Example 2, there were initially prepared a first die and a second die constituted in the same manner as in the first die and the second die of the die of Example 1. Next, in Comparative Example 2, the second die and the first die were combined to insert a convex region of a central region of the first die into a cavity region of a central region of the prepared second die, and thus, a die of Comparative Example 2 was manufactured. In other words, according to Comparative Example 2, the die was prepared without interposing a space forming member between the first die and the second die. 
     A total operation time required in manufacturing the die of Comparative Example 2 was 80 hours. The procedure of Example 1 was repeated to perform evaluation of “formed body quality” by use of the die of Comparative Example 2. Table 1 shows the results. 
     Comparative Example 3 
     In Comparative Example 3, there was initially prepared a first die constituted in the same manner as in the first die of the die of Example 1. Next, in Comparative Example 3, a second die was prepared in which a region of 0.1 mm from an upstream surface of a second die substrate in an extruding direction was removed by grind processing. The first die and the second die were combined to insert a convex region of a central region of the first die into a cavity region of a central region of the second die, and thus, a die of Comparative Example 3 was manufactured. In the die of Comparative Example 3, a downstream surface of a circumferential region of the first die did not abut on an upstream surface of the second die, and the second die in a cantilever state was combined with the first die. 
     A total operation time required in manufacturing the die of Comparative Example 3 was 70 hours. The procedure of Example 1 was repeated to perform evaluation of “formed body quality” by use of the die of Comparative Example 3. Table 1 shows the results. 
     (Results) 
     In the dies of Examples 1 to 6, it was possible to shorten the manufacturing time and evaluations of the formed body qualities were suitable, as compared with the die of Comparative Example 1. 
     The die of Comparative Example 1 had a result of failure in the evaluation of the formed body quality. The reasons for this result are supposedly that when the central region of the die is different from the circumferential region thereof in shape of slits, a flow rate distribution of the kneaded material in the die easily becomes non-uniform, and an amount of the kneaded material to be discharged from the slits does not become uniform. Especially when extrusion was performed by using the die of Comparative Example 1, requirements exceeded supply of the kneaded material to form the boundary wall, and a lot of formation defects were confirmed in the boundary wall and its vicinity. 
     As to the die of Comparative Example 2, in a region where positions of first kneaded material introducing holes of the first die did not match those of second kneaded material introducing holes of the second die, movement of the kneaded material was obstructed, and a result of failure was obtained in the evaluation of the formed body quality. Furthermore, when extrusion was performed by using the die of Comparative Example 2, requirements exceeded supply of the kneaded material to form the boundary wall, and formation defects were remarkably confirmed in the boundary wall and its vicinity. 
     In the die of Comparative Example 3, when the material A was used as the kneaded material, the evaluation of the formed body quality was good, but when the material B was used as the kneaded material, the evaluation of the formed body quality was failure. 
     A honeycomb structure forming die of the present invention is utilizable in manufacturing a honeycomb formed body having a central portion different from a circumferential portion in cell structure. 
     DESCRIPTION OF REFERENCE NUMERALS 
       10 : first die,  11 : first slit,  12 : first kneaded material introducing hole,  13 : first die substrate,  14 : downstream surface (the downstream surface of a circumferential region of the first die),  15 : central region,  16 : convex region,  17 : circumferential region,  18 : kneaded material discharging surface (the kneaded material discharging surface of the first die),  19 : kneaded material introducing surface,  20 : second die,  21 : second slit,  22 : second kneaded material introducing hole,  23 : second die substrate,  24 : upstream surface (the upstream surface of the second die),  25 : central region,  26 : cavity region,  27 : circumferential region,  28 : kneaded material discharging surface (the kneaded material discharging surface of the second die),  30  and  30 A: space forming member,  31  and  31 A: space,  33  and  33 A: substrate,  35 : clearance region (a boundary wall forming clearance region),  100  and  300 : honeycomb structure forming die (the die),  201 : partition wall,  202 : cell,  202   a : cell (cells of a central cell structure),  202   b : cell (cells of a circumferential cell structure),  203 : circumferential wall,  204 : honeycomb structure body,  208 : boundary wall,  211 : inflow end face,  212 : outflow end face,  215 : central cell structure,  216 : circumferential cell structure,  200 : honeycomb structure, and X: extruding direction.