Patent Publication Number: US-2016229083-A1

Title: Method of producing die for extrusion molding and method of producing honeycomb structured body

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional application of application Ser. No. 14/499,504, filed Sep. 29, 2014, which is a continuation application of International Application No. PCT/JP2012/058363, filed Mar. 29, 2012. The contents of these applications are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of producing a die for extrusion molding and a method of producing a honeycomb structured body. 
     2. Discussion of the Background 
     Exhaust gases discharged from internal combustion engines of vehicles (e.g., buses, trucks) and construction machines contain particulate matter, such as soot. Particulate matter has been a problem as it is harmful to the environment and human body. Thus, various particulate filters which include honeycomb structured bodies formed of porous ceramic are proposed. Those filters purify exhaust gases by capturing particulate matter in exhaust gases. 
     For achieving excellent heat resistance and strength, such a honeycomb structured body includes a plurality of prismatic honeycomb fired bodies which are combined with one another with adhesive layers provided therebetween. The honeycomb fired bodies are produced by subjecting a mixture containing ceramic materials (e.g., silicon carbide) to, for example, extrusion molding, degreasing, firing, or other treatments. 
     Generally, in the production of honeycomb structured bodies, a molding raw material is extrusion molded through a die for extrusion molding to produce a honeycomb molded body including a large number of cells which are separated by cell walls and are arranged in parallel with one another in a longitudinal direction. 
     One example of known dies for extrusion molding for producing honeycomb molded bodies is a die that includes raw material supply sections for supplying a molding raw material, and slit grooves for forming the molding raw material into a honeycomb molded body. The slit grooves communicate with the raw material supply sections and are arranged in a lattice pattern. 
     Regarding a method for machining a material of a die into a die having the aforementioned shape, machining with a cutting tool, such as a drill, is widely performed (JP H05-131425 A), for example. In the case where the material of a die consists of a hard ingredient, such as a superhard alloy, or a die to be produced has a complex shape, the machining is performed by electrical discharge machining. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, in a method of producing a die for extrusion molding, a treated surface is provided on an inner wall surface of a second through hole. The die includes a raw material supply section having a first through hole that extends from a first face toward a second face opposite to the first face. The die includes a molding section having the second through hole that extends from the second face toward the first face so as to communicate with the first through hole. In providing the treated surface, a material for the die is machined to form the material into a die shape and to provide a machining-affected layer on the inner wall surface of the second through hole in the molding section, an oxidized layer is provided by heating the machining-affected layer to oxidize the machining-affected layer so as to convert the machining-affected layer into the oxidized layer, and the oxidized layer is removed. 
     According to another aspect of the present invention, in a method of producing a honeycomb structured body, a die for extrusion molding is provided. The die includes a raw material supply section having a first through hole that extends from a first face toward a second face opposite to the first face. The die includes a molding section having a second through hole that extends from the second face toward the first face so as to communicate with the first through hole. In providing the die, a material for the die is machined to form the material into a die shape and to provide a machining-affected layer on an inner wall surface of the second through hole in the molding section, an oxidized layer is provided by heating the machining-affected layer to oxidize the machining-affected layer so as to convert the machining-affected layer into the oxidized layer, and the oxidized layer is removed to provide a treated surface on the inner wall surface of the second through hole. A molding raw material is extrusion molded through the die for extrusion molding to produce at least one honeycomb molded body that includes cells. The cells are separated by cell walls and arranged substantially in parallel with one another in a longitudinal direction of the at least one honeycomb molded body. The at least one honeycomb molded body is fired to produce at least one honeycomb fired body. A ceramic block of the at least one honeycomb fired body is produced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings. 
         FIG. 1A  is a cross-sectional view schematically illustrating one example of a die for extrusion molding according to the first embodiment of the present invention.  FIG. 1B  is a partially enlarged view of the die for extrusion molding shown in  FIG. 1A . 
         FIG. 2  is an SEM image of a cross-section of a surface of a slit groove after the machining step in the method of producing a die for extrusion molding according to the first embodiment of the present invention. 
         FIG. 3  is an enlarged front view of the die for extrusion molding shown in  FIGS. 1A and 1B . 
         FIGS. 4A to 4C  are each a cross-sectional view schematically showing a method of producing a die for extrusion molding according to the first embodiment of the present invention. 
         FIG. 5  is a perspective view schematically illustrating one example of a honeycomb molded body that is extrusion molded through the die for extrusion molding according to the first embodiment of the present invention. 
         FIG. 6A  is a perspective view schematically illustrating one example of a honeycomb fired body to be produced using the die for extrusion molding according to the present embodiment.  FIG. 6B  is an A-A line cross-sectional view of the honeycomb fired body shown in  FIG. 6A . 
         FIG. 7  is a perspective view schematically illustrating one example of a honeycomb structured body that is produced using the die for extrusion molding according to the present embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The embodiments will now be described with reference to the accompanying drawings, wherein like reference numerals designate corresponding or identical elements throughout the various drawings. 
     A die for extrusion molding according to a first aspect of the embodiments of the present invention is devised to achieve the above goal. The die includes: a first face; a second face formed opposite the first face; a raw material supply section with a first through hole that extends from the first face toward the second face; and a molding section with a second through hole that extends from the second face toward the first face so as to communicate with the first through hole, wherein the die includes a treated surface on the inner wall surface of the second through hole, the treated surface being obtainable through the steps of: machining a material of the die to form the material into a die having a predetermined shape and to form a machining-affected layer on the inner wall surface of the second through hole in the molding section; forming an oxidized layer by heating the machining-affected layer to oxidize the layer so as to convert the machining-affected layer into an oxidized layer; and removing the oxidized layer. 
     The treated surface provided through the steps of forming an oxidized layer and removing the oxidized layer is less fragile. 
     The treated surface is less likely to be worn away even after repeated extrusion molding of molding raw materials. Thus, the life of the die can be increased. 
     An increase in the life of the die means preventing abrasion of the inner wall surface of the second through hole during use of the die to avoid an increase in the thickness of the cell walls of extrusion molded honeycomb molded bodies, and also means preventing uneven abrasion of the inner wall surface of the second through hole during use of the die to avoid uneven thickness of the cell walls of extrusion molded honeycomb molded bodies. 
     In a die for extrusion molding according to a second aspect of the embodiments of the present invention, the raw material supply section further has a first opening formed at the first face and a second opening formed at a part where the second through hole communicates with the first through hole, and the width of the raw material supply section decreases from the first opening toward the second opening. 
     If the width of the raw material supply section decreases from the first opening toward the second opening, a molding raw material readily flows from the raw material supply section to the molding section. Thus, the molding raw material is prevented from clogging in the die for extrusion molding. 
     In a die for extrusion molding according to a third aspect of the embodiments of the present invention, the molding section includes slit grooves that communicate with a plurality of the second through holes, the slit grooves connecting to one another to form a lattice pattern. 
     If the molding section includes slit grooves that communicate with a plurality of the second through holes, the slit grooves connecting to one another to form a lattice pattern, extrusion molding of a molding raw material through the die for extrusion molding enables a honeycomb molded body having a large number of cells that are separated by cell walls and are arranged in parallel with one another in a longitudinal direction. 
     A die for extrusion molding according to a fourth aspect of the embodiments of the present invention includes a treated surface on the inner wall surface of the first through hole, the treated surface being obtainable through the steps of: machining a material of the die to form the material into a die having a predetermined shape and to form a machining-affected layer on the inner wall surface of the first through hole in the raw material supply section; forming an oxidized layer by heating the machining-affected layer to oxidize the layer so as to convert the machining-affected layer into an oxidized layer; and removing the oxidized layer. 
     If the inner wall surface of the first through hole is provided with the treated surface through the above steps, the inner wall surface of the first through hole is less fragile. This prevents abrasion of the inner wall surface of the first through hole even after repeated extrusion molding of molding raw materials. Thus, the first through hole can maintain the shape that allows a molding raw material to readily flow from the raw material supply section to the molding section. As a result, the molding raw material is prevented from clogging in the die for extrusion molding even after repeated extrusion molding of molding raw materials. 
     In a die for extrusion molding according to a fifth aspect of the embodiments of the present invention, the treated surface has a surface roughness (Ra) of 0.1 to 5.0 μm. 
     If the treated surface has a surface roughness (Ra) of 0.1 to 5.0 μm, abrasion due to contacts between molding raw materials and the treated surface is further suppressed even after repeated extrusion molding of molding raw materials. Thus, the treated surface is much less likely to be worn away. As a result, the life of the die can be further increased. 
     If the treated surface has a surface roughness (Ra) of more than 5.0 μm, abrasion occurs due to contacts between the molding raw materials and the treated surface during repeated extrusion molding of molding raw materials. Thus, the treated surface may be further worn away. 
     In a die for extrusion molding according to a sixth aspect of the embodiments of the present invention, the treated surface has a hardness of 300 to 1500 Hv. 
     A treated surface having a hardness of 300 to 1500 Hv is more likely to prevent abrasion of the surface of the die even after repeated extrusion molding of molding raw materials. Thus, the life of the die can be further increased. 
     If the treated surface has a hardness of less than 300 Hv, the surface of the die may be easily worn away during repeated extrusion molding of molding raw materials. 
     If the treated surface has a hardness of more than 1500 Hv, it may be difficult to produce a die for extrusion molding that includes a treated surface having the hardness. 
     A die for extrusion molding according to a seventh aspect of the embodiments of the present invention is made of a superhard alloy that includes a sintered mixture of tungsten carbide and cobalt. 
     If the die is made of a superhard alloy that includes a sintered mixture of tungsten carbide and cobalt, the surface of the die has higher hardness. This prevents abrasion of the surface of the die even after repeated extrusion molding of molding raw materials. Thus, the life of the die can be further increased. 
     In a die for extrusion molding according to an eighth aspect of the embodiments of the present invention, the step of removing the oxidized layer is performed by flow polishing. 
     The step of removing the oxidized layer by flow polishing enables a treated surface where the oxidized layer is sufficiently removed. Such a treated surface is less fragile than the machining-affected layer, and is much less likely to be worn away even after repeated extrusion molding of molding raw materials. Thus, the life of the die can be further increased. 
     In a die for extrusion molding according to a ninth aspect of the embodiments of the present invention, a molding raw material to be introduced to the raw material supply section includes silicon carbide. 
     Even if the molding raw material includes silicon carbide, which is very hard, the treated surface is less likely to be worn away. Thus, the life of the die can be increased. 
     In a method of producing a die for extrusion molding according to a tenth aspect of the embodiments of the present invention, the die includes: a first face; a second face formed opposite the first face; a raw material supply section with a first through hole that extends from the first face toward the second face; and a molding section with a second through hole that extends from the second face toward the first face so as to communicate with the first through hole, the method including forming a treated surface on the inner wall surface of the second through hole through the steps of: machining a material of the die to form the material into a die having a predetermined shape and to form a machining-affected layer on the inner wall surface of the second through hole in the molding section; forming an oxidized layer by heating the machining-affected layer to oxidize the layer so as to convert the machining-affected layer into an oxidized layer, and removing the oxidized layer. 
     In the step of forming an oxidized layer, the machining-affected layer is heated so that the machining-affected layer is gradually oxidized from the surface to the inside. As a result, an oxidized layer with excellent peelability is formed. Thus, in the step of removing the oxidized layer, the oxidized layer can be readily removed to obtain a treated surface. 
     In a method of producing a die for extrusion molding according to an eleventh aspect of the embodiments of the present invention, the machining step is performed by die-sinking electrical discharge machining. 
     If the machining step is performed by die-sinking electrical discharge machining, the material of the die, even if it is a hard material, can be favorably machined into a predetermined shape. Moreover, if the machining step is performed by die-sinking electrical discharge machining, the material can be favorably machined into the shape of the die to be produced even if the shape is complex. 
     In a method of producing a die for extrusion molding according to a twelfth aspect of the embodiments of the present invention, in the step of forming an oxidized layer, the machining-affected layer is heated to a temperature of 500 to 1000° C. in an oxygen atmosphere. 
     Heating the machining-affected layer to a temperature of 500 to 1000° C. in an oxygen atmosphere can sufficiently oxidize the machining-affected layer. 
     If the machining-affected layer is heated to a temperature of less than 500° C., the machining-affected layer may not be sufficiently oxidized. 
     If the machining-affected layer is heated to a temperature of more than 1000° C., the quality of the material of the die beneath the machining-affected layer may be changed. Thus, the die may fail to have intrinsic physical properties. 
     In a method of producing a die for extrusion molding according to a thirteenth aspect of the embodiments of the present invention, in the step of forming an oxidized layer, the machining-affected layer is heated to a predetermined temperature of 500 to 1000° C. in a nitrogen atmosphere; and the heated machining-affected layer is left stand at the predetermined temperature in an oxygen atmosphere for 5 to 180 minutes. 
     Dies for extrusion molding used to produce honeycomb molded bodies need to have high dimensional accuracy. 
     In the step of forming an oxidized layer, the oxidation includes introduction of oxygen after the heating and then maintenance of a given temperature. Thus, the oxygen concentration can be controlled during the oxidation. As a result, an oxidized layer with a uniform thickness can be formed, which in turn can produce a die with a high dimensional accuracy. 
     In a method of producing a die for extrusion molding according to a fourteenth aspect of the embodiments of the present invention, the step of removing the oxidized layer is performed by flow polishing. 
     If the step of removing the oxidized layer is performed by flow polishing, the oxidized layer can be readily removed. 
     A method of producing a honeycomb structured body according to a fifteenth aspect of the embodiments of the present invention includes the steps of: extrusion molding a molding raw material through a die for extrusion molding to produce at least one honeycomb molded body that includes a large number of cells, the cells being separated by cell walls and arranged in parallel with one another in a longitudinal direction; firing the at least one honeycomb molded body to produce at least one honeycomb fired body; and producing a ceramic block of the at least one honeycomb fired body, the die including: a first face; a second face formed opposite the first face; a raw material supply section with a first through hole that extends from the first face toward the second face; and a molding section with a second through hole that extends from the second face toward the first face so as to communicate with the first through hole, wherein the die includes a treated surface on the inner wall surface of the second through hole, the treated surface being obtainable through the steps of: machining a material of the die to form the material into a die having a predetermined shape and to form a machining-affected layer on the inner wall surface of the second through hole in the molding section; forming an oxidized layer by heating the machining-affected layer to oxidize the layer so as to convert the machining-affected layer into an oxidized layer; and removing the oxidized layer. 
     In the method of producing a honeycomb structured body according to the fifteenth aspect of the embodiments of the present invention, an increase or variation in the thickness of the cell walls of the honeycomb molded body can be prevented in the step to produce at least one honeycomb molded body. Thus, a honeycomb structured body can be favorably produced. 
     Embodiments of the present invention will be specifically described below. However, the present invention is not limited to those embodiments, and may be appropriately changed to an extent not changing the gist of the present invention. 
     First Embodiment 
     The following describes the die for extrusion molding according to the first embodiment of the present invention, and the method of producing a die for extrusion molding and the method of producing a honeycomb structured body according to the first embodiment, which is one embodiment of the present invention, with reference to drawings. 
     First, a die for extrusion molding according to the present embodiment will be described. 
     The die for extrusion molding of the present embodiment includes: a first face; a second face formed opposite the first face; a raw material supply section with a first through hole that extends from the first face toward the second face; and a molding section with a second through hole that extends from the second face toward the first face so as to communicate with the first through hole, wherein the die includes a treated surface on the inner wall surface of the second through hole, the treated surface being obtainable through the steps of: machining a material of the die to form the material into a die having a predetermined shape and to form a machining-affected layer on the inner wall surface of the second through hole in the molding section; forming an oxidized layer by heating the machining-affected layer to oxidize the layer so as to convert the machining-affected layer into an oxidized layer; and removing the oxidized layer. 
       FIG. 1A  is a cross-sectional view schematically illustrating one example of a die for extrusion molding according to the first embodiment of the present invention.  FIG. 1B  is a partially enlarged view of the die for extrusion molding shown in  FIG. 1A . 
       FIG. 1A  and  FIG. 1B  are cross-sectional views of a die for extrusion molding illustrated in a direction parallel to the direction of extruding a molding raw material. The arrow “a” in  FIG. 1A  and  FIG. 1B  indicates the direction of extruding a molding raw material. 
     As shown in  FIG. 1A  and  FIG. 1B , a die for extrusion molding  100  includes a first face  10   a ; a second face  10   b  formed opposite the first face  10   a ; a raw material supply section  11  with a first through hole  111  that extends from the first face  10   a  toward the second face  10   b ; and a molding section  12  with a second through hole  121  that extends from the second face  10   b  toward the first face  10   a  so as to communicate with the first through hole  111 . The molding section  12  includes slit grooves that communicate with a plurality of the second through holes  121 , the slit grooves connecting to one another to form a lattice pattern. 
     The raw material supply section  11  is formed to supply a molding raw material. The molding section  12  is formed to form a molding raw material passed through the raw material supply section  11  into the shape of a honeycomb molded body. 
     An outer frame  20  for immobilizing the die for extrusion molding  100  may be provided as needed. 
     In the following description, the inner wall surfaces of the second through holes  121  in the molding section  12  correspond to the surfaces of the slit grooves  12 . 
     A treated surface  13  is formed on the surfaces of the slit grooves  12 . 
     The treated surface  13  formed on the surfaces of the slit grooves  12  is an essential element of the die for extrusion molding according to the present embodiment. In addition to the essential element, the treated surface  13  may also be formed on the inner wall surface of the first through hole  111 , the first face  10   a , the second face  10   b , or other parts in the raw material supply section  11 . 
     The die for extrusion molding  100  is preferably made of a superhard alloy that includes a sintered mixture of tungsten carbide and cobalt, a superhard alloy that includes a sintered mixture of tungsten carbide, cobalt, and a trace amount of other particles (for example, TiC, TiN), tool steel, stainless steel, an aluminum alloy, or the like, and more preferably a superhard alloy that includes a sintered mixture of tungsten carbide and cobalt. 
     A superhard alloy that includes a sintered mixture of tungsten carbide and cobalt usually has a hardness of 1000 to 1500 Hv. 
       FIG. 2  is an SEM image of a cross-section of a surface of a slit groove after the machining step in the method of producing a die for extrusion molding according to the first embodiment of the present invention. 
     The die for extrusion molding shown in  FIG. 2  is made of a superhard alloy that includes a sintered mixture of tungsten carbide and cobalt. The slit groove is formed by electrical discharge machining. 
     As shown in  FIG. 2 , after the machining step, a machining-affected layer  26  is formed on the surfaces of the slit grooves  12 . The machining-affected layer  26  is a layer of melted and sintered tungsten carbide and cobalt, which are the materials of the die. The layer has defects, such as cracks and fine pores, caused by the thermal energy during the electrical discharge machining. 
     The machining-affected layer  26  preferably has a thickness (the length indicated by the double-headed arrow “e” in  FIGS. 2 ) of 0.1 to 20 μm. 
     The thickness of the machining-affected layer can be measured based on an SEM image. The minimum value of the above range is an average of the thickness of any 10 sites where the thickness of the machining-affected layer  26  is apparently small. The maximum value of the above range is an average of the thickness of any 10 sites where the thickness of the machining-affected layer  26  is apparently large. 
     A normal layer  27  including tungsten carbide particles  201  and cobalt  202  as a binder exists beneath the machining-affected layer  26 . 
     The tungsten carbide particles  201  preferably have an average particle diameter of 0.1 to 10 μm. The cobalt  202  content is preferably 3 to 20%. 
     As shown in  FIG. 1A  and  FIG. 1B , the treated surface  13  is formed on the surfaces of the slit grooves  12  of the die for extrusion molding according to the first embodiment of the present invention. That is, the surfaces of the slit grooves  12  provided with the machining-affected layer  26  (shown in  FIG. 2 ) thereon undergo the step of forming an oxidized layer and the step of removing the oxidized layer so that the machining-affected layer  26  is removed, and thereby the treated surface  13  is obtained. Accordingly, the treated surface  13  is a surface obtained by removing the machining-affected layer  26 . 
     The normal layer  27  including the tungsten carbide particles  201  and the cobalt  202  as a binder exists beneath the treated surface  13 . 
     The production methods will be described in detail in the method of producing a die for extrusion molding according to the present embodiment below. 
     The treated surface  13  preferably has a surface roughness (Ra) of 0.1 to 5.0 μm. 
     The surface roughness (Ra) refers to a center-line average roughness in accordance with JIS standard (Standard No.: JIS B 0601), and can be measured with, for example, a stylus type surface roughness tester. 
     The treated surface  13  has a hardness of preferably 300 to 1500 Hv. The hardness refers to a Vickers hardness measured in accordance with JIS standard (Standard No. JIS Z 2244). 
     A Vickers hardness test is performed as follows. A needle-shaped object having a diamond-shaped tip (angle between faces:136°), called a diamond indenter, is pressed at a test force F (kgf) to the surface of a substrate to be measured for the hardness. The surface area S (mm 2 ) of a resulting impression is calculated from the length d (average of the two-direction diagonal lines) of the diagonal lines. The hardness can be calculated from the length d, the surface area S, and the test force F (kgf) based on the following formula. 
       Hardness ( Hv )= F  (kgf)/ S  (mm 2 )=0.1892 F  (kgf)/ d   2  (mm 2 ) 
     The length of the raw material supply section  11  in a direction parallel to the direction of extruding a molding raw material is preferably, but not limited to, 3 to 20 mm. 
     If the length of the raw material supply section  11  in a direction parallel to the direction of extruding a molding raw material is within the above range, a molding raw material can be readily extrusion molded. 
     The width (the length indicated by the double-headed arrow “b” in  FIG. 1B ) of the raw material supply section  11  is preferably, but not limited to, 1.0 to 1.5 mm. 
     If the width of the raw material supply section  11  is within the above range, a molding raw material can be readily extrusion molded. 
     In the case of the raw material supply section  11  having a round cross section, the width of the raw material supply section  11  refers to the diameter of the round. In the case of the raw material supply section  11  having a polygonal cross section, the width refers to the diameter of a hypothetical circumscribed round touching the vertices of the polygon. 
     As shown in  FIG. 1B , the raw material supply section  11  further includes a first opening  112  formed at the first face  10   a , and a second opening  113  formed at a part where the second through hole  121  communicates with the first through hole  111 . The width (the length indicated by the double-headed arrow “b” in  FIG. 1B ) of the raw material supply section  11  decreases from the first opening  112  toward the second opening  113 . 
     The slit grooves  12  each has a slit width (the length indicated by the double-headed arrow “c” in  FIG. 1 ) that corresponds to the thickness of each cell wall or the thickness of the outer peripheral wall of the honeycomb molded body. The slit width is preferably 30 to 1000 μm, and more preferably 60 to 500 μm. 
     The length of the slit grooves  12  in a direction parallel to the direction of extruding a molding raw material is preferably, but not limited to, 1 to 4 mm. 
     If the length of the slit grooves  12  in a direction parallel to the direction of extruding a molding raw material is within the above range, a molding raw material can be readily extrusion molded. 
       FIG. 3  is an enlarged front view of the die for extrusion molding shown in  FIGS. 1A and 1B . 
     As shown in  FIG. 3 , the slit grooves  12  communicate with the raw material supply sections  11  and form a lattice pattern. 
     Supposing that points at which the slit grooves  12  intersect are intersections  14 , the number of the intersections  14  is preferably 100 to 500 per square inch, and more preferably 200 to 400 per square inch. 
     Each of the raw material supply sections  11  is usually disposed at an intersection of the slit grooves  12 . 
     Specifically, as shown in  FIG. 3 , supposing that adjacent ones among the intersections of the slit grooves  12  are intersections  14   a  and  14   b , one of the raw material supply sections  11  is disposed on the intersection  14   a.    
     The molding raw material may be selected depending on the materials of a honeycomb molded body (honeycomb structured body) to be produced. 
     Examples of the molding raw material include: nitride ceramics, such as aluminum nitride, silicon nitride, boron nitride, or titanium nitride; carbide ceramics, such as silicon carbide, zirconium carbide, titanium carbide, tantalum carbide, or tungsten carbide; and oxide ceramics, such as alumina, zirconia, cordierite, mullite, silica, or aluminum titanate. Silicon carbide is especially preferable. 
     The following describes a method of producing a die for extrusion molding according to the present embodiment. 
     The method of producing a die for extrusion molding according to the present embodiment is a method of producing a die for extrusion molding, the die including: a first face; a second face formed opposite the first face; a raw material supply section with a first through hole that extends from the first face toward the second face; and a molding section with a second through hole that extends from the second face toward the first face so as to communicate with the first through hole, the method including forming a treated surface on the inner wall surface of the second through hole through the steps of: machining a material of the die to form the material into a die having a predetermined shape and to form a machining-affected layer on the inner wall surface of the second through hole in the molding section; forming an oxidized layer by heating the machining-affected layer to oxidize the layer so as to convert the machining-affected layer into an oxidized layer, and removing the oxidized layer. 
       FIGS. 4A to 4C  are each a cross-sectional view schematically showing a method of producing a die for extrusion molding according to the first embodiment of the present invention.  FIG. 4A  to  FIG. 4C  are cross-sectional views of a die for extrusion molding illustrated in a direction parallel to the direction of extruding a molding raw material. The arrow “a” in  FIG. 4A  to  FIG. 4C  indicates the direction of extruding a molding raw material. 
     First, a machining step is performed for machining a material of the die to form the material into a die having a predetermined shape. 
     Specifically, as shown in  FIGS. 1A and 1B , a material of the die is machined to form the first through hole  111  that extends from the first face  10   a  toward the second face  10   b  and then form the second through hole  121  that extends from the second face  10   b  toward the first face  10   a  so as to communicate with the first through hole  111 . The molding section  12  includes slit grooves that communicate with a plurality of the second through holes  121 , the slit grooves connecting to one another to form a lattice pattern. 
     In the following description, the inner wall surfaces of the second through holes  12  in the molding section  12  correspond to the surfaces of the slit grooves  12 . 
     The shapes of the raw material supply sections  11  and the slit grooves  12  are described above, and thus the specific description thereof is omitted. 
     Examples of the methods for forming the raw material supply sections and the slit grooves include, but not particularly limited to, machining with a cutting tool, such as a drill. 
     If the die is made of a hard material, such as a superhard alloy, or a die to be produced has a complex shape, examples of the methods include electrical discharge machining. Electrical discharge machining is a method of applying voltages between a workpiece and the tool electrode to generate electric discharges, and gradually removing the workpiece by the spark energy of the electric discharge. The following three types of electrical discharge machining are known: die-sinking electrical discharge machining that includes transfer machining of a workpiece by use of a shaped electrode; wire electrical discharge machining that includes cutting a workpiece into a desired shape by use of a thin wire electrode; and small hole electrical discharge machining which enables formation of holes having a very small diameter by use of a rod electrode. Die-sinking electrical discharge machining is especially preferred. 
     The machining-affected layer  46  is formed on the surfaces of the slit grooves  12  through the machining step as shown in FIG.  4 A. The machining-affected layer  46  has defects  48 , such as cracks and fine pores. The machining-affected layer  46  shown in  FIG. 4A  corresponds to the machining-affected layer  26  shown in  FIG. 2 . The machining-affected layer is already described, and thus the detailed description thereof is omitted. 
     A normal layer  47  having no defects, such as cracks and fine pores, exists beneath the machining-affected layer  46 . 
     Next, the step of forming an oxidized layer is performed by heating the machining-affected layer to oxidize the layer so as to convert the machining-affected layer into an oxidized layer. 
     Specifically, the step is preferably performed by heating the machining-affected layer  46  to a temperature of 500 to 1000° C. in an oxygen atmosphere. The step is more preferably performed by heating the machining-affected layer  46  to a predetermined temperature of 500 to 1000° C. in a nitrogen atmosphere; and then allowing the heated machining-affected layer to stand at the predetermined temperature in an oxygen atmosphere for 5 to 180 minutes. 
     Preferably, an electric furnace is used for the heating. 
     The machining-affected layer  46  is converted to an oxidized layer  49  through the step of forming an oxidized layer as shown in  FIG. 4B . 
     In the case, for example, where the machining-affected layer  46  is a layer including tungsten carbide (WC) and cobalt (Co), the oxidation converts the machining-affected layer  46  into the oxidized layer  49  including tungsten oxide (WO 3 ), an oxide (CoWO 4 ) of mixture of tungsten and cobalt, and cobalt oxide (CoO). The WC or Co increases its volume when it is oxidized to WO 3 , CoWO 4 , or CoO, which increases the defects  48 , such as cracks and small pores. As a result, an oxidized layer with excellent peelability is formed. 
     Lastly, the step of removing the oxidized layer is performed. 
     The removal of the oxidized layer is preferably performed by flow polishing. Specifically, the flow polishing is performed by repeating a series of introducing an abrasive uniformly into the raw material supply sections and extruding the abrasive from the slit grooves. 
     The abrasive is preferably silicon carbide having a grain size from #100 to #1000, and especially preferably silicon carbide having a grain size #600 (average grain size: 25.8 μm). 
     The polishing is preferably performed under the condition of a polishing pressure of 1 to 10 MPa, a polishing temperature of 10 to 50° C., and a polishing time period of 5 to 48 hours. 
     If the polishing pressure, the polishing temperature and the polishing time period are each within the above range, the oxidized layer is sufficiently removed so that a treated surface with a flat surface can be obtained. 
     The oxidized layer  49  is removed through the step of removing the oxidized layer as shown in  FIG. 4C  so that the treated surface  43  is obtained. The normal layer  47  having no defects, such as cracks and fine pores, exists beneath the treated surface  43 . 
     The treated surface is already described above, and thus the detailed description is omitted 
     A die for extrusion molding according to the present embodiment can be produced through the above steps. 
     Lastly, the following describes one example of a method of producing a honeycomb structured body using the die for extrusion molding according to the present embodiment. 
     The method of producing a honeycomb structured body according to the present embodiment includes the steps of: extrusion molding a molding raw material through a die for extrusion molding to produce at least one honeycomb molded body that includes a large number of cells, the cells being separated by cell walls and arranged in parallel with one another in a longitudinal direction; firing the at least one honeycomb molded body to produce at least one honeycomb fired body; and producing a ceramic block of the at least one honeycomb fired body, the die including: a first face; a second face formed opposite the first face; a raw material supply section with a first through hole that extends from the first face toward the second face; and a molding section with a second through hole that extends from the second face toward the first face so as to communicate with the first through hole, wherein the die includes a treated surface on the inner wall surface of the second through hole, the treated surface being obtainable through the steps of: machining a material of the die to form the material into a die having a predetermined shape and to form a machining-affected layer on the inner wall surface of the second through hole in the molding section; forming an oxidized layer by heating the machining-affected layer to oxidize the layer so as to convert the machining-affected layer into an oxidized layer; and removing the oxidized layer. 
     (1) First, a wet mixture (molding raw material) of ceramic powders and a binder is prepared. 
     Specifically, ceramic powders, an organic binder, a liquid plasticizer, a lubricant, and water are mixed to prepare a wet mixture for producing a honeycomb molded body. 
     The ceramic powders may be selected depending on the materials of a honeycomb molded body (honeycomb structured body) to be produced. 
     Examples of the main component of the materials of the honeycomb molded body include nitride ceramics, such as aluminum nitride, silicon nitride, boron nitride, or titanium nitride; carbide ceramics, such as silicon carbide, zirconium carbide, titanium carbide, tantalum carbide, or tungsten carbide; and oxide ceramics, such as alumina, zirconia, cordierite, mullite, silica, or aluminum titanate. 
     The main component of the materials of the honeycomb molded body is preferably a non-oxide ceramic, and particularly preferably silicon carbide because such materials have excellent heat resistance, mechanical strength, thermal conductivity, or the like. 
     Herein, the expression “the main component is silicon carbide” means that the silicon carbide content in the ceramic powders is not less than 60% by weight. In the case where the main component is silicon carbide, the main component may also include silicon-bonded silicon carbide. The same is true to the case where the main component is a component of the materials other than silicon carbide. 
     (2) Next, the wet mixture (molding raw material) is extrusion molded into a honeycomb molded body having a predetermined shape. 
     The die for extrusion molding according to the present embodiment is used for the extrusion molding. 
       FIG. 5  is a perspective view schematically illustrating one example of a honeycomb molded body that is extrusion molded through the die for extrusion molding according to the first embodiment of the present invention. A honeycomb molded body  500  shown in  FIG. 5  includes a large number of cells  501  that are separated by cell walls  502  and arranged in parallel with one another in a longitudinal direction (the direction of the double-headed arrow “f” in  FIG. 5 ). An outer peripheral wall  503  is formed on the circumference of the cells  501  and the cell walls  502 . 
     (3) Thereafter, the honeycomb molded body is dried using a drier, such as a microwave drier, a hot air drier, a dielectric drier, a reduced pressure drier, a vacuum drier, or a freeze drier, to thereby produce a dried honeycomb dried body. 
     The dried honeycomb molded body is degreased (for example, at 200 to 500° C.) and fired (for example, at 1400 to 2300° C.) under predetermined conditions. 
     Through the above steps, a honeycomb fired body including: a large number of cells that are separated by cell walls and arranged in parallel with one another in a longitudinal direction; and an outer peripheral wall provided on the circumference thereof can be produced. 
     The degreasing and firing of the dried honeycomb molded body may be performed under conventional conditions for producing honeycomb fired bodies. 
     The method of producing a honeycomb structured body using the die for extrusion molding according to the present embodiment enables production of a honeycomb fired body in which one of end portions of each cell is plugged. In this case, after the drying in the step (3), predetermined end portions of the cells of the dried honeycomb molded body are filled with a predetermined amount of a plug material paste, which becomes a plug, to plug the cells. Then, the honeycomb molded body is degreased and fired as described above so that a honeycomb fired body in which one of end portions of each cell is plugged can be produced. 
     The wet mixture may be used as the plug material paste. 
       FIG. 6A  is a perspective view schematically illustrating one example of a honeycomb fired body that is produced using the die for extrusion molding according to the present embodiment.  FIG. 6B  is an A-A line cross-sectional view of the honeycomb fired body shown in  FIG. 6A . 
     A honeycomb fired body  600  shown in  FIG. 6A  and  FIG. 6B  includes: a large number of cells  601  that are separated by cell walls  602  and arranged in parallel with one another in a longitudinal direction (the direction of an arrow “g” in  FIG. 6A ); and an outer peripheral wall  603  provided on the circumference of the cells  601  and the cell walls  602 . One of the end portions of each cell  601  is plugged with a plug  604 . 
     Thus, exhaust gas G (exhaust gas is indicated by G, and the flow of the exhaust gas is indicated by an arrow in  FIG. 6B ) which enters one of the cells  601  with one end opened will always pass through the cell wall  602  separating the cells  601  to flow out from another one of the cells  601  with an another end opened. PMs or the like in exhaust gas are captured when the exhaust gas G passes through the cell walls  602 . The cell walls  602  thus function as a filter. 
     As mentioned earlier, a honeycomb structured body including the honeycomb fired body in which one of the end portions of each cell is plugged can be favorably used as a ceramic filter. Furthermore, a honeycomb structured body including a honeycomb fired body in which none of the end portions of the cells is plugged can be favorably used as a catalyst carrier. 
     (4) Next, a ceramic block of at least one honeycomb fired body is produced. 
     The following describes one example of a method of producing a ceramic block of a plurality of honeycomb fired bodies which are combined with adhesive layers. 
     First, an adhesive paste, which becomes an adhesive layer, is applied to a predetermined side face of one of the honeycomb fired bodies to form an adhesive paste layer. Another honeycomb fired body is then stacked on the adhesive paste layer. Through repetition of this process, an aggregate of the honeycomb fired bodies is produced. 
     Next, the aggregate of the honeycomb fired bodies is heated to dry and solidify the adhesive paste layers so that a ceramic block is produced. 
     The adhesive paste is, for example, one including an inorganic binder, an organic binder, and inorganic particles. The adhesive paste may also include inorganic fibers and/or a whisker. 
     (5) Thereafter, a ceramic block is machined. 
     Specifically, the outer periphery of the ceramic block is machined with a cutter, such as a diamond cutter, to produce a ceramic block with a round pillar-shaped outer periphery. 
     (6) Next, an outer periphery coating material paste is applied to an outer peripheral surface of the round pillar-shaped ceramic block, and is then solidified by drying to form an outer periphery coat layer. 
     The adhesive paste may be used as the outer periphery coating material paste. The outer periphery coating material paste may be a paste having different composition from that of the adhesive paste. 
     The outer periphery coat layer is not essential, but may be provided as needed. 
     Through the above steps, a honeycomb structured body can be produced. 
       FIG. 7  is a perspective view schematically illustrating one example of a honeycomb structured body that is produced using the die for extrusion molding according to the present embodiment. 
     A honeycomb structured body  700  shown in  FIG. 7  includes a ceramic block  703  and an outer periphery coat layer  702  formed on an outer periphery of the ceramic block  703 . The ceramic block includes a plurality of honeycomb fired bodies  600  which are combined with one another with adhesive layers  701  provided therebetween. The outer periphery coat layer may be formed as needed. 
     The honeycomb structured body including a plurality of honeycomb fired bodies combined is also referred to as an aggregated honeycomb structured body. 
     The effects of the die for extrusion molding and the method of producing a die for extrusion molding according to the present embodiment are listed below. 
     (1) In the die for extrusion molding according to the present embodiment, the treated surface is obtained through the steps of forming an oxidized layer by heating the machining-affected layer to oxidize the layer so as to convert the machining-affected layer into an oxidized layer; and removing the oxidized layer. The treated surface obtained through the steps of forming an oxidized layer and removing the oxidized layer is less fragile. Thus, the treated surface is less likely to be worn away even after repeated extrusion molding of molding rawmaterials. Thus, the life of the die can be increased. 
     (2) In the die for extrusion molding according to the present embodiment, the treated surface has a surface roughness (Ra) of 0.1 to 5.0 μm. If the treated surface has a surface roughness (Ra) within the above range, abrasion due to contacts between molding raw materials and the treated surface is further suppressed even after repeated extrusion molding of molding raw materials. Thus, the treated surface is much less likely to be worn away. As a result, the life of the die can be further increased. 
     (3) In the die for extrusion molding according to the present embodiment, the treated surface has a hardness of 300 to 1500 Hv. A treated surface having a hardness within the above range is more likely to prevent abrasion of the surface of the die even after repeated extrusion molding of molding raw materials. Thus, the life of the die can be further increased. 
     (4) The material of the die for extrusion molding according to the present embodiment includes a superhard alloy that includes a sintered mixture of tungsten carbide and cobalt. If the die is made of the above superhard alloy, the surface of the die has higher hardness. This prevents abrasion of the surface of the die even after repeated extrusion molding of molding raw materials. Thus, the life of the die can be further increased. 
     (5) In the die for extrusion molding according to the present embodiment, the step of removing the oxidized layer is performed by flow polishing. The flow polishing enables a treated surface where the oxidized layer is sufficiently removed. Such a treated surface is less fragile than the machining-affected layer, and is much less likely to be worn away even after repeated extrusion molding of molding raw materials. Thus, the life of the die can be further increased. 
     (6) In the die for extrusion molding according to the present embodiment, the molding raw material to be supplied to the raw material supply section includes silicon carbide. Even if the molding raw material includes silicon carbide, which is very hard, the surface of the die is less likely to be worn away. Thus, the life of the die can be increased. 
     (7) In the die for extrusion molding according to the present embodiment, the raw material supply section further includes a first opening formed at the first face and a second opening formed at a part where the second through hole communicates with the first through hole. Also, the width of the raw material supply section decreases from the first opening toward the second opening. 
     If the width of the raw material supply section decreases from the first opening toward the second opening, a molding raw material readily flows from the raw material supply section to the molding section. Thus, the molding raw material is prevented from clogging in the die for extrusion molding. 
     (8) The method of producing a die for extrusion molding according to the present embodiment includes the steps of: forming an oxidized layer by heating the machining-affected layer to oxidize the layer so as to convert the machining-affected layer into an oxidized layer; and removing the oxidized layer. In the step of forming an oxidized layer, the machining-affected layer is heated so that the machining-affected layer is gradually oxidized from the surface to the inside. As a result, an oxidized layer with excellent peelability is formed. Thus, in the step of removing the oxidized layer, the oxidized layer can be readily removed to obtain a treated surface. 
     (9) In the method of producing a die for extrusion molding according to the present embodiment, the machining step is performed by die-sinking electrical discharge machining. If the machining step is performed by die-sinking electrical discharge machining, the material of the die, even if it is a hard material, can be favorably machined into a predetermined shape. Moreover, if the machining step is performed by die-sinking electrical discharge machining, the material can be favorably machined into the shape of the die to be produced even if the shape is complex. 
     (10) In the method of producing a die for extrusion molding according to the present embodiment, in the step of forming an oxidized layer, the machining-affected layer is heated to a temperature of 500 to 1000° C. in an oxygen atmosphere. Heating the machining-affected layer to a temperature within the above range in an oxygen atmosphere can sufficiently oxidize the machining-affected layer. 
     (11) In the method of producing a die for extrusion molding according to the present embodiment, in the step of forming an oxidized layer, the machining-affected layer is heated to a predetermined temperature of 500 to 1000° C. in a nitrogen atmosphere; and the heated machining-affected layer is left stand at the predetermined temperature in an oxygen atmosphere for 5 to 180 minutes. Dies for extrusion molding used to produce honeycomb molded bodies need to have high dimensional accuracy. In the step of forming an oxidized layer, the oxidation includes introduction of oxygen after the heating and then maintenance of a given temperature. Thus, the oxygen concentration can be controlled during the oxidation. As a result, an oxidized layer with a uniform thickness can be formed, which in turn can produce a die with a high dimensional accuracy. 
     (12) In the method of producing a die for extrusion molding according to the present embodiment, the step of removing the oxidized layer is performed by flow polishing. The flow polishing enables easy removal of the oxidized layer. 
     EXAMPLES 
     The following examples more specifically describe the present embodiment. The present invention is not limited to the examples. 
     Example 1 more specifically describes the first embodiment of the present invention. 
     Example 1 
     A superhard alloy that includes a sintered mixture of tungsten carbide and cobalt was prepared as a material of a die. The material of a die had a hardness of 1200 Hv. 
     The material of a die was subjected to die-sinking electrical discharge machining so as to form the material into the shape shown in  FIG. 1A . Specifically, the outer periphery was machined to form a protruded second face where a second through hole was to be formed. Then, a first through hole having a round cross section was formed from the first face toward the second face. Next, the second through hole was formed from the second face toward the first face so as to communicate with the first through hole. 
     An electrical discharge machining sinker (EA8PV, produced by Mitsubishi Electric Corporation) was used for the die-sinking electrical discharge machining under a peak current of 5 to 20 A. 
     A machining-affected layer was formed on the surfaces of the slit grooves by the machining. 
     Next, the machining-affected layer was oxidized by heating so as to convert the machining-affected layer into an oxidized layer. 
     The machining-affected layer was oxidized by firstly heating the machining-affected layer from room temperature to 700° C. over one hour in a nitrogen atmosphere, and then keeping the temperature for 105 minutes in an oxygen atmosphere (oxygen concentration: 20.8%). Thereafter, the temperature was decreased to room temperature in a nitrogen atmosphere. An electric furnace was used for the heating. 
     Lastly, the oxidized layer was removed. 
     The oxidized layer was removed by flow polishing. The flow polishing was performed by repeating a series of introducing an abrasive uniformly into the raw material supply sections and extruding the abrasive from the slit grooves. The abrasive used was silicon carbide having a grain size #600 (average grain size: 25.8 μm). The polishing pressure was 6 MPa, the polishing temperature was 30° C., and the polishing time period was 24 hours. 
     The removal of the oxidized layer was checked by observing a cross-section of the surfaces of the slit grooves using a microscope (VHK-100, produced by KEYENCE CORPORATION). 
     A die for extrusion molding was produced through the above steps. 
     The die for extrusion molding produced in Example 1 was measured for the abrasion loss in the slit width as described below. 
     The abrasion loss in the slit width was measured at a part located 30 μm, in the depth direction, from the second face where the second through hole was formed. Moreover, the abrasion loss in the slit width was measured at any 10 sites of the slit grooves which correspond to the cell walls of a honeycomb molded body. Table 1 shows the average of the values measured at the  10  sites. 
     (Measurement of Abrasion Loss in Slit Width) 
     First, flow polishing of the surfaces of the slit grooves was performed. The flow polishing was performed by repeating a series of introducing an abrasive uniformly into the raw material supply sections and extruding the abrasive from the slit grooves. The abrasive used was silicon carbide having a grain size #320 (average grain size: 46.2 μm). The polishing pressure was 6 MPa; the polishing temperature was 30° C.; and the polishing time was 24 hours. 
     Then, the abrasion loss in the slit width was measured using a sizer (UMAP 302, produced by Mitsutoyo Corporation). 
     Comparative Example 1 
     In Comparative Example 1, a die for extrusion molding was produced in the same manner as in Example 1, except that the step of forming an oxidized layer and the step of removing the oxidized layer were not performed. The abrasion loss in the slit width was measured. Table 1 shows the result of the measurement. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Abrasion loss in 
               
               
                   
                 slit width 
               
               
                   
                 (μm/h) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Example 1 
                 0.067 
               
               
                   
                 Comparative 
                 0.088 
               
               
                   
                 Example 1 
               
               
                   
                   
               
            
           
         
       
     
     The results in Table 1 indicate that the surfaces of the slit grooves are less fragile and the abrasion loss in the slit width is smaller in the die produced in Example 1 in which a treated surface is formed on the surfaces of the slit grooves than the surfaces of the slit grooves and the abrasion loss in the die produced in Comparative Example 1 in which no treated surface is formed on the surfaces of the slit grooves. Thus, the life of the die produced in Example 1 in which the treated surface is formed on the surfaces of the slit grooves can be increased. 
     Other Embodiments 
     The honeycomb structured body produced using the die for extrusion molding according to the first embodiment of the present invention is an aggregated honeycomb structured body but may be a honeycomb structured body (integrated honeycomb structured body) including a single honeycomb fired body. 
     For producing an integrated honeycomb structured body, a honeycomb molded body is produced in the same manner as in the first embodiment of the present invention, except that a honeycomb molded body to be extrusion molded is larger than and has a different profile from the honeycomb molded body described in the first embodiment of the present invention. 
     In other words, the honeycomb molded body may be produced using a die for extrusion molding that has the same structure as that of the die for extrusion molding according to the first embodiment of the present invention, and has a cross-sectional shape corresponding to the shape of the honeycomb molded body to be obtained. 
     The other steps are the same as those described in the method of producing a honeycomb structured body according to the first embodiment of the present invention. Since the honeycomb structured body includes a single honeycomb fired body, production of an aggregate of the honeycomb fired bodies is not necessary. In the case of producing a round pillar-shaped honeycomb molded body, machining of the outer periphery of the ceramic block is not necessary. 
     In the die for extrusion molding according to the embodiments of the present invention, the raw material supply section in the die may have any shape. Examples of the cross-sectional shape of the raw material supply section parallel to the direction of extruding a molding raw material include a rectangular shape, a tapered shape, and a trapezoidal shape. 
     A tapered cross-sectional shape is especially preferable for easy extrusion of a molding raw material. 
     Similarly, in the die for extrusion molding according to the embodiments of the present invention, the slit groove of the die may have any shape. Examples of the cross-sectional shape of the slit groove parallel to the direction of extruding a molding raw material include a rectangular shape and a tapered shape. 
     A rectangular cross-sectional shape is especially preferable for easy formation of the slit grooves. 
     The essential feature of the die for extrusion molding of the embodiments of the present invention is that the die includes: a first face; a second face formed opposite the first face; a raw material supply section with a first through hole that extends from the first face toward the second face; and a molding section with a second through hole that extends from the second face toward the first face so as to communicate with the first through hole, wherein the die includes a treated surface on the inner wall surface of the second through hole, the treated surface being obtainable through the steps of: machining a material of the die to form the material into a die having a predetermined shape and to form a machining-affected layer on the inner wall surface of the second through hole in the molding section; forming an oxidized layer by heating the machining-affected layer to oxidize the layer so as to convert the machining-affected layer into an oxidized layer; and removing the oxidized layer. 
     The essential feature of the method of producing a die for extrusion molding of the embodiments of the present invention is that the die includes: a first face; a second face formed opposite the first face; a raw material supply section with a first through hole that extends from the first face toward the second face; and a molding section with a second through hole that extends from the second face toward the first face so as to communicate with the first through hole, the method including forming a treated surface on the inner wall surface of the second through hole through the steps of: machining a material of the die to form the material into a die having a predetermined shape and to form a machining-affected layer on the inner wall surface of the second through hole in the molding section; forming an oxidized layer by heating the machining-affected layer to oxidize the layer so as to convert the machining-affected layer into an oxidized layer, and removing the oxidized layer. 
     The essential feature of the method of producing a honeycomb structured body of the embodiments of the present invention is that the method includes the steps of: extrusion molding a molding raw material through a die for extrusion molding to produce at least one honeycomb molded body that includes a large number of cells, the cells being separated by cell walls and arranged in parallel with one another in a longitudinal direction; firing the at least one honeycomb molded body to produce at least one honeycomb fired body; and producing a ceramic block of the at least one honeycomb fired body, the die including: a first face; a second face formed opposite the first face; a raw material supply section with a first through hole that extends from the first face toward the second face; and a molding section with a second through hole that extends from the second face toward the first face so as to communicate with the first through hole, wherein the die includes a treated surface on the inner wall surface of the second through hole, the treated surface being obtainable through the steps of: machining a material of the die to form the material into a die having a predetermined shape and to form a machining-affected layer on the inner wall surface of the second through hole in the molding section; forming an oxidized layer by heating the machining-affected layer to oxidize the layer so as to convert the machining-affected layer into an oxidized layer; and removing the oxidized layer. 
     Desired effects can be obtained by appropriately combining the essential features with the various structures (for example, the shape of the raw material supply sections, the shape of the slit grooves, or the like) mentioned in detail in the above description of the first embodiment of the present invention and other embodiments of the present invention. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.