Patent Publication Number: US-11383405-B2

Title: Methods for producing ceramic molded body and ceramic structure

Description:
FIELD OF THE INVENTION 
     The present invention relates to methods for producing a ceramic molded body and a ceramic structure. 
     BACKGROUND OF THE INVENTION 
     Ceramic structures are used for various applications. For example, a honeycomb-shaped ceramic structure having partition walls that define a plurality of cells each extending from a first end face to a second end face is widely used for catalyst supports, and for various filters such as diesel particulate filters (DPFs), and gasoline particulate filters (GPFs). 
     The ceramic structure is produced by extruding a ceramic molding material (kneading material) containing ceramic raw materials to obtain a ceramic molded body, and then cutting the ceramic molded body to have a predetermined length, drying and firing it. As used herein, the ceramic molded body refer to a material in a state before firing, and the ceramic structure refers to a material in a state after firing. 
     In recent years, there is a requirement for improvement of dimensional accuracy of the ceramic structure from the viewpoint of increasing productivity of the ceramic structure or the like. To address the requirement, a method for improving the dimensional accuracy of the ceramic molded body before firing has been proposed. For example, Patent Literature 1 proposes a method of controlling extrusion molding process parameters (e.g., an extrusion pressure) by acquiring a shape signal of an outer peripheral surface of a ceramic molded body obtained by extrusion molding, and comparing in real-time the shape signal with a reference shape signal. Further, Patent Literature 2 proposes a method of measuring a dimension of a ceramic molded body that has been extruded and dried, and adjusting an amount of liquid to be added to a kneading material based on the measurement result. 
     CITATION LIST 
     Patent Literatures 
     [Patent Literature 1] Japanese Patent Application Publication No. 2017-536549 A 
     [Patent Literature 2] Japanese Patent No. 6436928 B 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method for producing a ceramic molded body, the method comprising: 
     a molding step of subjecting a ceramic molding material to extrusion molding using an extrusion molding machine equipped with a temperature control portion to provide a ceramic molded body; 
     a cutting step of cutting the ceramic molded body to have a predetermined length; and 
     a dimension measuring step of measure a dimension of the cut ceramic molded body; 
     wherein a relationship between a temperature of the temperature control portion and the dimension of the cut ceramic molded body is previously obtained, and based on the relationship, an appropriate temperature of the temperature control portion is calculated from the dimension of the ceramic molded body measured in the dimension measuring step, and the temperature control portion is controlled to the appropriate temperature in the molding step. 
     Also, the present invention relates to a method for producing a ceramic structure, the method comprising: 
     a drying step of drying the ceramic molded body obtained by the method for producing the ceramic molded body; and 
     a firing step of firing the dried ceramic molded body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view illustrating a schematic structure of an extrusion molding machine suitable for use in a method for producing a ceramic molded body according to an embodiment of the present invention; 
         FIG. 2  is a front view of a temperature control drum as viewed from a drum side; 
         FIG. 3  is a graph showing an illustrative relationship between a temperature of a temperature control portion and a dimension of a ceramic molded body; and 
         FIG. 4  is a graph showing a change in the dimension of the ceramic molded body produced based on the relationship between the temperature of the temperature control portion and the dimension of the ceramic molded body, as a function of time. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     When the ceramic molded body obtained by extrusion molding is cut to have a predetermined length, the shape may be deformed due to the stress accumulated during extrusion molding. Therefore, in the method disclosed in Patent Literature 1 which controls the extrusion process parameters by measuring the shape immediately after extrusion molding, the deformation caused by cutting cannot be considered, and the dimensional accuracy of the ceramic molded body cannot be stably improved. 
     Further, since the drying of the ceramic molded body requires a long period of time, the method disclosed in Patent Literature 2 which measures the shape of the ceramic molded body after drying and adjusts the amount of the liquid to be added to the kneading material requires a long period of time until the adjustment is reflected. Therefore, in some cases, the produced ceramic molded bodies may be wasted until the adjustment is reflected. 
     The present invention has been made to solve the above problems. An object of the present invention is to provide a method for producing a ceramic molded body, which can quickly and stably improve dimensional accuracy of the ceramic molded body. 
     Another object of the present invention is to provide a method for producing a ceramic structure, which can stably improve dimensional accuracy of the ceramic structure. 
     As a result of intensive studies to solve the above problems, the present inventors have found that there is a correlation between a dimension of a ceramic molded body cut in a cutting step and a temperature of a temperature control portion in a molding process, and based on this finding, the dimension of the ceramic molded body cut in the cutting step is measured, and the temperature of the temperature control portion in the molding step is controlled based on the measured dimension, whereby the dimensional accuracy of the ceramic molded body can be quickly and stably improved, and they have completed the present invention. 
     According to the present invention, it is possible to provide a method for producing a ceramic molded body, which can quickly and stably improve dimensional accuracy of the ceramic molded body. 
     Also, according to the present invention, it is possible to provide a method for producing a ceramic structure, which can stably improve dimensional accuracy of the ceramic structure. 
     Hereinafter, embodiments of the present invention will be specifically described. It is to understand that the present invention is not limited to the following embodiments, and those which appropriately added changes, improvements and the like to the following embodiments based on knowledge of a person skilled in the art without departing from the spirit of the present invention fall within the scope of the present invention. 
     (1) Method for Producing Ceramic Molded Body 
     A method for producing a ceramic molded body according to an embodiment of the present invention includes: a molding step of subjecting a ceramic molding material to extrusion molding using an extrusion molding machine equipped with a temperature control portion to provide a ceramic molded body; a cutting step of cutting the ceramic molded body to have a predetermined length; and a dimension measuring step of measure a dimension of the cut ceramic molded body. 
     (Molding Step) 
     The molding step is to extrude a ceramic molding material using an extrusion molding machine equipped with a temperature control portion to obtain a ceramic molded body. 
     The extrusion molding machine is not particularly limited as long as it has a temperature control portion, and a machine known in the art can be used. Here,  FIG. 1  shows a schematic view illustrating a schematic structure of a typical extrusion molding machine. 
     As shown in  FIG. 1 , the extrusion molding machine  1  includes; an extruding portion  10 ; a molding portion  20  connected to the extruding portion  10 ; and a rectifier  30  arranged between the extruding portion  10  and the molding portion  20 . The extruding portion  10  has a screw  11  and a barrel  12  capable of housing the screw  11 . Further, the molding portion  20  includes one end and other end, and the one end has a die  21 , and the other end is connected to an extruding port  13  of the extruding portion  10 , and the upstream side of the die  21  is provided with a screen (filtering net)  23  and a temperature control portion  24 . 
     The extruding portion  10  is not particularly limited as long as it has the screw  11  and the barrel  12  capable of housing the screw  11 . Any member known in the art can be used. 
     The screw  11  preferably has a screw shaft  14  and a blade portion  15  that is spirally formed along the screw shaft  14 . 
     Further, the screw  11  is preferably a biaxial screw that rotates in the same direction, from the viewpoint of kneading properties of the ceramic molding material, and more preferably a meshing type biaxial screw. In this case, a pair of screws  11  are arranged in parallel inside the barrel  12 . 
     A base portion of the screw  11  is connected to a drive device  16 . The drive device  16  includes a motor and a gearbox (not shown), and controls a rotation speed so as to obtain a predetermined extrusion pressure and rotates the screw  11 . 
     Provided on an upstream side of the extruding portion  10  is a material introducing portion  17  for feeding a ceramic raw material mixture into the extruding portion  10 . The ceramic raw material mixture fed from the material introducing portion  17  is kneaded by the screw  11  to form a ceramic molding material, which is fed to the molding portion  20 . 
     The molding portion  20  includes a drum  22  having a space therein. One end of the molding portion  20  has a die  21 , and the other end is connected to the extruding portion  13  of the extruding portion  10 . 
     A shape of the drum  22  is not particularly limited, and a part of the dram  22  may have a decreased diameter portion or an increased diameter portion. For example, as shown in  FIG. 1 , the drum  22  has the decreased diameter portion on the extruding port  13  side. The drum  22  having such a structure may be composed of one member or may be composed of a plurality of members. When the drum  22  is composed of a plurality of members, the drum  22  can be obtained by combining an increased diameter drum and a straight drum. 
     A shape of the die  21  is not particularly limited, and it can be appropriately set according to the shape of the ceramic molded body to be produced. For example, when producing a ceramic molded body having a honeycomb shape, the die  21  having slits corresponding to the thickness of the partition walls in the form of honeycomb is used. 
     The screen  23  is provided in the drum  22  (molding portion  20 ), and is formed of a material in the form of mesh. The screen  23  can remove coarse particles or other impurities mixed in the ceramic molding material to stabilize the ceramic molding material fed to the die  21 . 
     The temperature control portion  24  is arranged between the screen  23  and the die  21 . 
     The temperature control portion  24  is not particularly limited as long as it can control the temperature of the ceramic molding material. Any temperature controller known in the art can be used. Among others, it is preferable to use a temperature control drum through which a fluid can flow, as the temperature control portion  24 . Since the temperature control drum can control the temperature by adjusting the temperature of the fluid, it is possible to reduce the consumption of electricity as compared with a case where a heating means such as a heating element is used. For example, the ceramic molding material can be easily and efficiently heated by circulating hot water having a temperature controlled by using a boiler or the like through the temperature control drum. 
     Here,  FIG. 2  shows a front view of the temperature control drum as viewed from the drum  22  side. As shown in  FIG. 2 , the temperature control drum  25  has a fluid feed port  26  and a fluid discharge port  27 , and forms a fluid flow path in the circumferential direction. Although not shown, the feed port  26  and the discharge port  27  are connected to a fluid feed device via a tube or the like. By circulating the fluid while controlling the temperature of the fluid by this feed device, the temperature can be easily controlled. 
     The temperature of the temperature control portion  24  is determined based on results of the dimension measured in the dimension measuring step. More particularly, a relationship between the temperature of the temperature control portion  24  and the dimension of the ceramic molded body cut to have a predetermined length (which may, hereinafter, be abbreviated as a “cut ceramic molded body”) is previously obtained. Based on the relationship, an appropriate temperature of the temperature control portion  24  is calculated from the dimension of the ceramic molded body measured in the dimensional measuring step as described later, and the temperature control portion  24  is controlled to the appropriate temperature. 
     The relationship between the temperature of the temperature control portion  24  and the dimension of the cut ceramic molded body can be obtained based on the past data accumulated by producing the ceramic molded bodies. Further, the relationship can be optimized in real-time by reflecting the data obtained by continuously carrying out the method for producing the ceramic molded body according to the embodiment of the present invention. 
     The relationship between the temperature of the temperature control portion  24  and the dimension of the cut ceramic molded body may differ depending on conditions such as a material and size of the ceramic molded body and a type of the extrusion molding machine  1 . Therefore, it is preferable that the relationship is determined by rendering each of these conditions identical. 
     The dimension of the cut ceramic molded body used to obtain the relationship is not particularly limited. A diameter of a cut surface of the cut ceramic molded body (for example, a radius or diameter when the ceramic molded body has a circular pillar shape) is preferably used, and a value (ΔR) obtained by subtracting a previously defined reference value of a diameter of a cut surface of a reference ceramic molded body from the measured value of the measured diameter of the cut surface of the cut ceramic molded body is more preferably used. The use of these dimensions can allow the correlation of the relationship to be easily obtained. 
     In addition, as used herein, the “reference ceramic molded body” refers to a ceramic molded body having an ideal (target) dimension. 
     Here,  FIG. 3  shows an example of the relationship between the temperature of the temperature control portion  24  and the dimension of the cut ceramic molded body. 
     The dimension of the cut ceramic molded body used to obtain the relationship as shown in  FIG. 3  employed the value OR obtained by subtracting the previously defined reference value of the radius of the cut surface of the reference ceramic molded body from the measured value of the measured radius of the cut surface of the cut ceramic molded body. Used as the cut ceramic molded body was a circular pillar shaped honeycomb ceramic molded body produced under the same conditions except for the temperature of the temperature control portion  24 . The cut ceramic molded body was produced as follows: 
     Using a cordierite-forming raw material containing alumina, kaolin and talc mixed together as a ceramic raw material, the cordierite-forming raw material was mixed with a binder including an organic binder, a water-absorbent resin as a pore former, and water (42% by mass) as a dispersion medium to form a ceramic raw material mixture, which was fed to the material introducing portion  17  of the extrusion molding machine  1  as shown in  FIG. 1 . The ceramic raw material mixture introduced from the material introducing portion  17  was kneaded in the extruding portion  10  to form the ceramic molding material, and the ceramic molding material was extruded from the die  21  of the molding portion  20  to obtain a ceramic molded body. The obtained ceramic molded body was cut to have a predetermined length using a wire rod hung between a pair of bobbins to obtain a cut ceramic molded body. The cut ceramic molded body has a honeycomb structure including partition walls that define a plurality of cells each extending from a first end face to a second end face, and has a quadrangular cell shape (a cell shape in a cross section orthogonal to a cell extending direction). Further, the ceramic molded body has a water content of 20%. 
     For the cut ceramic molded body obtained above, a radius of an upper half of the cut surface was measured while maintaining the axial direction of the cut ceramic molded body in the horizontal direction, using a method employing an end face inspection machine as described later. The value (ΔR) was calculated by subtracting the previously defined reference value of the radius of the cut surface of the reference ceramic molded body from the measured value of the radius of the cut surface of the cut ceramic molded body thus measured. A plurality of cut ceramic molded bodies were produced by changing the temperatures of the temperature control portion  24 , and the relationship between the temperatures of the temperature control portion  24  and the dimensions (ΔR) of the cut ceramic molded bodies was obtained. A graph showing the relationship is shown in  FIG. 3 . 
     The value of ΔR has a relationship that changes as shown in  FIG. 3  depending on the temperatures of the temperature control portion  24 . Therefore, an appropriate temperature of the temperature control portion  24  may be calculated from the dimension (ΔR) of the cut ceramic molded body measured in the dimension measuring step as described later based on that relationship, and the temperature of the temperature control portion  24  may be controlled. For example, when the temperature of the temperature control portion  24  is set to 25° C. to produce the ceramic molded body, the temperature of the temperature control portion  24  may be controlled to 30° C. when it is desired to decrease the dimension (ΔR) of the cut ceramic molded body measured in the dimensional measuring step described later by 0.1 mm.  FIG. 4  shows a graph illustrating a change (referred to as “after cutting”) in the dimension (ΔR) of the cut ceramic molded body produced by thus controlling the temperature of the temperature control portion  24 , as a function of time. In addition,  FIG. 4  also shows, as a reference, a change (referred to as “before cutting”) in the dimension (ΔR) of the cut ceramic molded body as a function of time, which was produced by determine the same relationship as that described above for the ceramic molded body before cutting and controlling the temperature of the temperature control portion  24 . 
     As shown in  FIG. 4 , when the appropriate temperature of the temperature control portion  24  was controlled based on the relationship using the dimension (ΔR) of the ceramic molded body after cutting (the cut ceramic molded body), the dimension of the cut ceramic molded body could be stably decreased as compared with a case where the appropriate temperature of the temperature control portion  24  was controlled based on the relationship using the dimension (ΔR) of the ceramic molded body before cutting. Therefore, the dimensional accuracy of the cut ceramic molded body can be quickly and stably improved by previously determining the relationship between the temperature of the temperature control portion  24  and the dimension of the cut ceramic molded body, calculating the appropriate temperature of the temperature control portion  24  based on the dimension of the cut ceramic molded body measured in the dimension measuring step, and controlling the temperature of the temperature control portion  24  in the molding step. 
     Optionally, an outer circumference of the drum  22  (molding portion  20 ) may preferably be covered with a heat insulating sheet (not shown). Such a configuration can allow the temperature inside the drum  22  to be maintained at a predetermined temperature, so that the effect of improving the dimensional accuracy of the ceramic molded body can be enhanced. 
     The molding step can carry out using the extrusion molding machine  1  having the structure as described above. In the molding step, a ceramic raw material mixture is fed from the material introducing portion  17  to the interior of the barrel  12 . The ceramic raw material mixture is kneaded while being subjected to shearing force by the rotation of the screw  11  to form a ceramic molding material, which is conveyed to the extruding port  13  side at the tip of the barrel  12 . The ceramic molding material extruded from the extruding port  13  of the barrel  12  passes through the through holes of the rectifier  30 , and passes through the screen  23  to be fed to the die  21 . The ceramic molding material is extruded through the die  21  into a desired shape. 
     The ceramic molding material can be obtained by kneading the ceramic raw material mixture. 
     The ceramic raw material mixture may contain ceramic raw materials and water, although not particularly limited thereto. 
     Examples of the ceramic raw materials that can be used include, but not particularly limited to, cordierite-forming raw materials, cordierite, silicon carbide, silicon-silicon carbide composite materials, mullite, aluminum titanate, and the like. These can be used alone or in combination of two or more. As used herein, the cordierite-forming raw material refers to a ceramic raw material formulated so as to have a chemical composition in which silica is in a range of from 42 to 56% by mass, alumina is in a range of from 30 to 45% by mass, and magnesia is in a range of from 12 to 16% by mass. The cordierite-forming raw material is calcined to form cordierite. 
     In addition to the ceramic raw materials and water, the ceramic raw material mixture may also contain a dispersion medium other than water, a binder (for example, an organic binder, an inorganic binder, or the like), a pore former, a surfactant, and the like. Each raw material has any composition ratio, and preferably has a composition ratio according to the structure, material, and the like of the ceramic molded body to be produced. 
     The ceramic molded body obtained by extrusion molding preferably has a water content of from 10 to 50%. The ceramic molded body having the water content in such a range can allow the dimensional accuracy of the ceramic molded body to be stably improved by the method for producing the ceramic molded body according to the embodiment of the present invention. 
     As used herein, the water content of the ceramic molded body refers to a water content measured by an infrared heating type moisture meter. 
     (Cutting Step) 
     The cutting step is to cut the ceramic molded body obtained by extrusion molding to have a predetermined length. 
     The cutting method is not particularly limited, and a method known in the art can be used. For example, the ceramic molded body can be cut by using a wire rod hung between a pair of bobbins. 
     The length of the ceramic molded body to be cut is not particularly limited, and the ceramic molded body may be cut to have an appropriate length depending on applications. 
     The structure of the cut ceramic molded body preferably has a honeycomb structure including partition walls that define a plurality of cells each extending from a first end face to a second end face, although not particularly limited thereto. 
     The ceramic molded body having a honeycomb structure (hereinafter referred to as a “honeycomb molded body”) may have a non-limiting shape, including a circular pillar shape, an elliptical pillar shape, and a polygonal pillar shape having end faces in the form of square, rectangular, triangular, pentagonal, hexagonal, octagonal, and the like. 
     Further, the honeycomb molded body may have a non-limiting cell shape (a cell shape in the cross section orthogonal to the cell extending direction), including a triangle, a quadrangle, a hexagon, an octagon, a circle, or a combination thereof. 
     (Dimension Measuring Step) 
     The dimensional measuring step is to measure the dimension of the cut ceramic molded body. 
     The method for measuring the dimension of the cut ceramic molded body is not particularly limited, and a method known in the art can be used. For example, the dimension of the cut ceramic molded body can be measured using an end face inspection machine, a laser type outer diameter dimension measuring device, or the like. 
     However, it is preferable that the measuring method used in the dimension measuring step is the same as the measuring method for the dimension of the ceramic molded body used to obtain the relationship between the temperature of the temperature control portion  24  and the dimension of the cut ceramic molded body. Further, the dimension of the cut ceramic molded body measured in the dimension measuring step is preferably the same as the dimension of the ceramic molded body used to obtain the relationship between the temperature of the temperature control portion  24  and the dimension of the cut ceramic molded body. 
     The dimension of the cut ceramic molded body measured in the dimension measuring step is not particularly limited. A diameter of a cut surface of the cut ceramic molded body (for example, a radius or diameter of the cut surface when the ceramic molded body has a circular pillar shape) is preferably used, and a value (ΔR) obtained by subtracting a previously defined reference value of a diameter of a cut surface of a reference ceramic molded body from the measured value of the measured diameter of the cut surface of the cut ceramic molded body is more preferably used. The diameter of the cut surface of the cut ceramic molded body can be calculated by measuring the radii at a plurality of positions after a correction process of a center position as described later and averaging the measured values. 
     The dimension of the cut ceramic molded body can be measured at the end face (cut surface) or the side surface of the cut ceramic molded body. 
     Further, the dimension of the cut ceramic molded body may be measured over the entire end face or the entire side surface of the cut ceramic molded body, but it is preferably measured at a part of the end face or the side surface of the cut ceramic molded body. 
     When measuring the dimension of the entire end face or the entire side surface of the cut ceramic molded body, an axial direction (extrusion molding direction) of the cut ceramic molded body should be matched to a vertical direction, so that the cut ceramic molded body has to be rotated by 90° to measure the dimension, which requires a long period of time. Therefore, it is preferable to leave the axial direction (extrusion molding direction) of the cut ceramic molded body in the horizontal direction and measure the dimension a part (for example, an upper half) of the end face or the side surface of the cut ceramic molded body from the upper direction. Such a measuring method can allow a measurement time to be shortened. In this case, it is preferable to perform a correction process of the center position of the measured shape so that an error from a reference shape is minimized. 
     When measuring the dimension of the end face of the cut ceramic molded body, an end face image of the cut ceramic molded body is taken by an imaging camera of the end face inspection machine. The contour of the cut ceramic molded body may be detected from the obtained end face image by means of image analysis, and the dimension (outer diameter, radius) of the cut ceramic molded body may be calculated. 
     When measuring the dimension of the side surface of the cut ceramic molded body, the side surface of the cut ceramic molded body is irradiated with a laser from a laser displacement meter of a laser type outer diameter dimension measuring device. The laser oscillated from the laser displacement meter reaches the side surface of the cut ceramic molded body and is reflected. The reflected laser may be detected by a light receiving element, and the dimension of the cut ceramic molded body may be calculated based on the principle of the triangulation ranging method. 
     In the method for producing the ceramic molded body according to the embodiment of the present invention including the above steps, the temperature of the temperature control portion  24  in the molding step is controlled to the appropriate temperature based on the measurement result of the dimension of the cut ceramic molded body. Therefore, the dimensional accuracy of the ceramic molded body can be quickly and stably improved. 
     (2) Method for Producing Ceramic Structure 
     The method for producing the ceramic structure according to the embodiment of the present invention includes a drying step of drying the ceramic molded body obtained by the method for producing the ceramic molded body as described above, and a firing step of firing the dried ceramic molded body. 
     (Drying Step) 
     The drying step is to dry the ceramic molded body. 
     The method for drying the ceramic molded body is not particularly limited, and a method known in the art can be used. For example, the ceramic molded body may be placed on a drying cradle, conveyed between a pair of electrodes, and subjected to dielectric drying by passing an electric current between the electrodes. The dielectric drying is to subject dipoles of water in the ceramic molding body to molecular motion by a high frequency energy generated by passing the electric current between the pair of electrodes, and dry the ceramic molding body by frictional heat caused by the molecular motion. 
     In addition, as the drying conditions, known conditions can be appropriately selected according to the outer shape and material of the honeycomb structure to be produced. 
     (Firing Step) 
     The firing step is to fire the dried ceramic molded body. 
     The method for firing the ceramic molded body is not particularly limited, and a method known in the art can be used. For example, the ceramic molded body may be fired in a firing furnace. 
     In addition, as the firing conditions, known conditions can be appropriately selected according to the outer shape and material of the honeycomb structure to be produced. Further, organic substances such as the binder may be removed by calcination prior to the firing. 
     Since the method for producing the ceramic structure according to the embodiment of the present invention including the above steps uses the ceramic molded body obtained by the method for producing the ceramic molded body as described above, the dimensional accuracy of the ceramic structure can be stably improved. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           1  extrusion molding machine 
           10  extruding portion 
           11  screw 
           12  barrel 
           13  extruding port 
           14  screw shaft 
           15  blade portion 
           16  drive device 
           17  material introducing portion 
           20  molding portion 
           21  die 
           22  drum 
           23  screen 
           24  temperature control portion 
           25  temperature control drum 
           26  feed port 
           27  discharge port