METHOD OF PRODUCING A DIE FOR EXTRUDING A CERAMIC BATCH INTO A HONEYCOMB STRUCTURAL BODY AND A DIE PRODUCED BY SUCH METHOD

Abstract: An extrusion die having lattice-like slits (2′) of very small width (i′) can be easily obtained by plating a metal film (4) on the surface of slits (2) in the matrix (1) of the die. A honeycomb structural body produced by the use of the extrusion die has a very high cell density and a very large surface area, and is suitable as a catalyst carrier for the purification of exhaust gas of automobile.

Description: TITLE: METHOD OF PRODUCING A DIE FOR EXTRUDING A CERAMIC BATCH INTO A HONEYCOMB STRUCTURAL BODY AND A DIE PRODUCED BY SUCH METHOD Background of the Invention The present invention relates to a method of producing die for extruding a ceramic batch into a honeycomb structural body. There has been known a ceramic honeycomb structural body having a large number of parallel cells, which are confined by a relatively thin lattice-like partition wall and extend therethrough in its axial direction, and having an open frontal area of about 60-90 h, preferably 65-85%. Such kind of honeycomb structural body has been used for the catalyst carrier for the purpose of purifying harmful CO, HC and NO from the exhaust gas by passing the exhaust gas through the cells so as to react it with a catalyst supported on the surface of the partition wall. The honeycomb structural body is manufactured by a method, wherein a ceramic batch is extruded through the extrusion die into a ceramic honeycomb shaped body, and the shaped body is dried and fired, as described in U.S. Patent Specification No. 3,824,196. When the above described ceramic honeycomb structural body having a catalyst supported on the partition wall of cell is used for purifying harmful gases in an exhaust gas, the exhaust gas is passed through the honeycomb structural body to react the exhaust gas with the catalyst. Therefore, the reaction area must be large and the pressure loss of the exhaust gas must be low. In order to increase the reaction area of a honeycomb structural body in a limited volume of a purifying apparatus for exhaust gas, the thickness of the partition wall is preferred to be as small as possible. If the dimension between partition walls are decreased without changing the thickness of partition wall in order to increase the cell density, a large reaction area can be obtained, but the pressure loss of the exhaust gas becomes high, and the use of such honeycomb structural body is disadvantageous. A honeycomb structural body having a thin partition wall can be obtained by the use of an extrusion die having a small slit width. However, the slit of such extrusion die has hitherto been produced by the abrasion machining with the aid of a grinder, by the milling machining and by the electric discharge machining subjected to steel. Therefore, the limit of the slit width, which is able to be formed by the above described machining with respect to slits having a given depth, is as large as about 0.30 mm. The production of slits having a width smaller than 0.3 mm is very difficult due to the production accuracy of grinder, milling blade and discharge electrode and due to the machining accuracy when the slits are formed by the use of these tools. Further, Japanese Examined Patent Publication No. 20,435/76 discloses that a honeycomb structural body having a round shape at the crossing corner of partition walls lets the exhaust gas flow smoothly and improves the reaction efficiency of catalyst. This honeycomb structural body is produced by the use of an extrusion die having a round shape at the crossing corner of lattice-like slits, which round shape is obtained by rounding off the crossing corner of the lattice-like slits by a cutting machining or an electric discharge machining. However, in this method, even the formation of slits having a width of as large as about 0.3 mm is difficult, and when it is intended to form slits having a width smaller than 0.3 mm, it is extremely difficult to insert into the slits a cutting tool for rounding off the crossing corner of lattice-like slits of the die and to manufacture accurately a discharge electrode having a small round portion. As the result, in the above described method, it is difficult to form partition walls having a thickness of smaller than 0.3 mm between adjacent cells and further to round off the crossing corner of the partition walls. Therefore, it is impossible to obtain a large catalytic reaction area and to let the exhaust gas flow smoothly. For example, when the honeycomb structural body produced by the above described method is used as a catalyst carrier in the purification of the exhaust gas of an automobile, the honeycomb structural body is large in weight, and further the catalyst requires a long time for the temperature raising at the starting of engine, and the exhaust gas can not be sufficiently purified during the starting of the engine. While, even in the precision machining, the dimensional accuracy of slit width has always been about 10% of working error. For example, when it is intended to produce slits having a width of 0.3 mm, slits having a width of about 0.27-0.33 mm are obtained. When this tolerance is not accepted in practice, the extrusion die must be discarded. Further, when a slit having a width of 0.33 mm is obtained, and the upper limit of the slit width is 0.35 mm, only a very small amount of honeycomb structural body can be extruded until the width 0.33 mm of the slit becomes 0.35 mm due to the abrasion of the slit. Further, it has been described in U.S. Patent No. 3,930,522, column 6, lines 56-58 and U.S. Patent No. 4,021,134, column 5, lines 46-50 that a die for extruding a honeycomb structural body is subjected to the electroless nickel plating. However, these U.S. Patents only aim to prevent the rusting of the die and to decrease the friction coefficient between the die and ceramic batch, and disclose neither the method of the present invention, wherein the slit width is controlled by the thickness of plating metal film so as to form slits having a width smaller than the width of slits formed by any conventional methods in a very simple process, nor the effect attained in the present invention. Further, even if a slit having a- necessary width has been obtained by the electroless nickel plating, the plated nickel film is very poor in the abrasion resistance against ceramic batch. The inventors have found that, even when slit having nickel film plated thereon is heat treated at a temperature of 600-7000C, which heat treatment temperature has hitherto been believed to be effective for improving the abrasion resistance of plated nickel film, the temperature range of the heat treatment is not effective at all for the improvement of abrasion resistance of the nickel film against ceramic batch. That is, the slit is worn by a small amount of extrusion of ceramic batch, and as the result the slit width becomes larger than the defined width, and the die can not at all be used for the production of honeycomb structural body. Of course, nickel film having a thickness of about 0.01-0.05 mm, which is commonly used for the purpose of the prevention of rusting and the lowering of friction coefficient, is worn and lost rapidly due to the-abrasion by ceramic batch, and can not attain the above described objects. Summary of the Invention The object of the present invention is to solve the above described drawbacks of the conventional die for extruding a honeycomb structural body. That is, the feature of the present invention is the provision of a method of producing a die for extruding a honeycomb structural body, wherein forming a lattice of slits having a given depth in the front surface of the matrix of the die in a shape substantially corresponding to the geometrical shape of a honeycomb structural body to be extruded, and forming a plurality of independent inlets for feeding a ceramic batch and communicated to the slits in the rear surface of the matrix, an improvement comprising forming a metal film on the surface of at least the slits by plating, and adjusting the thickness of the metal film so that the slits have a given width. Brief Description of the Drawings Figure 1 is a vertical sectional view of the matrix of an extrusion die; Figure 2 is a plan view of the matrix of the die shown in Figure 1, Figure 3 is an enlarged vertical sectional view of a part of an extrusion die according to the present invention, and Figure 4 is an enlarged horizontal sectional view of the die shown in Figure 3 taken along a line IV-IV in Figure 3. Detailed Description of the Preferred Embodiments The present invention will be explained in more detail with reference to the accompanying drawings. In Figures 1-4, the numeral 1 represents a matrix of anectrusion die manufactured by a conventional mechanical machining or electric discharge machining, which is provided at its front surface with slits 2 crossed at a right angle with each other in the form of lines of checkerboard and bored up to a given depth in the matrix towards its rear surface, and at its rear surface with a large number of feed inlets 3 for a ceramic batch, which has a circular cross-sectional shape and are uniformly bored in the matrix towards its front surface and are communicated to the slits 2 so as to supply the ceramic batch into the slit 2. The width of the slit 2 formed in the matrix is measured by means of a measuring instrument, such as a projector. A metal film 4 having a thickness of (i-i')/2, wherein i represents the measured width of the above formed slit 2 and i' represents the desired width of slit to be formed, is formed by a plating on the surface of the slit 2 only or on the area ranging from the surface of the slit 2 to the surface of the feed inlet 3 for ceramic batch. When a curve showing a relation between the plating time and the thickness of plated metal film is previously determined, a metal film 4 having a given thickness dan be easily obtained by a plating in a highly accurately adjusted plating bath for the time calculated from the curve. Alternatively, the metal film 4 having a given thickness can be formed in the following manner. The matrix 1 and a test piece made of the same steel as the matrix 1 are immersed in a plating bath, the test piece is taken out from the bath at intervals with the lapse of time, and the thickness of the metal film plated on the test piece is measured. When the thickness of the metal film has reached the given thickness, the plating of metal on the matrix 1 is stopped. The metal film 4 formed on the surface of at least the slit 2 of the matrix 1 may be heat treated in a conventional process in order to increase the abrasion resistance. Particularly, when nickel layer is formed as a metal film, the nickel film is very poor in the abrasion resistance against ceramic batch. Therefore, it is necessary that the amount of hypophosphorous acid ions and nickel ions in the plating bath are properly adjusted so as to form a nickel-phosphorus alloy film (Ni: 91-92%, P: 8-9%), which composition has a high abrasion resistance against ceramic batch, and further it is necessary that the resulting nickel-phosphorus alloy film is heat treated at 400-500 C. The inventors have found that the above obtained nickel-phosphorus alloy film has an abrasion resistance against ceramic batch as high as about 5 times of the abrasion resistance of nickel film having a conventional composition without heat treatment or with a heat treatment at 600-700 C, which is considered to be most effective for improving the abrasion resistance of nickel film. That is, in this method, the durability of the plated extrusion die is fairly improved, and the plated die can be used practically for the production of honeycomb structural bodies. Moreover, the slit 2 of the above described matrix 1, which has been manufactured by a conventional method, is not round at the crossing corner 5 of the slits similarly to a conventional matrix as shown in Figure 4, and when a metal film 4 is plated on the slit 2, the surface of the metal film 4 located at the crossing corner 5 of the slits can be easily made into a round shape having a radius of curvature, the value of which is equal to the thickness of the metal film 4. Of course, in a matrix 1 having round portions at the crossing corner of the slits 2, the round portions can be made into a larger round shape by the formation of a metal film 4 on the round portions. In the extrusion die produced in the above described method, the thickness of the metal film 4 formed on the surface of the slit 2 arranged in the front surface of the matrix 1 is controlled very accurately to a uniform thickness of (i-i')/2, wherein i represents the width of the slit 2 and i' represents the desired width of slit 2' to be formed, by the above described method. Therefore, a ceramic batch supplied into the slits 2' through the feed inlets 3, which are arranged in the rear surface side of the die and communicated to the slits 2', can be extruded through the slits 2' into a proper honeycomb structural'body, whose partition walls between adjacent cells having a uniform thickness. Moreover, the above described slit 2' of this die is not directly formed by a mechanical machining or an electric discharge machining, but is formed so as to have a given width by plating a necessary thickness of a metal film 4 on the slit 2 of a matrix 1 manufactured by a conventional method. Therefore, any slit width less than 0.3 mm can be easily obtained in a high accuracy. Accordingly, honeycomb structural bodies, whose partition walls between adjacent cells have any desired thickness of less than 0.3 mm, can be produced, and a catalyst carrier, which has a very thin partition wall having a desired thickness between adjacent cells, can be also obtained by firing the honeycomb structural body. That is, when the honeycomb structural body is used as an exhaust gas purifying apparatus, the cell density can be enhanced to the extreme value of honeycomb structure without increasing the pressure loss of the exhausted gas. Therefore, a large reaction area can be obtained in a limited volume of exhaust gas-purifying apparatus. Further, the honeycomb structural body is small in the weight as a carrier for catalyst for purifying exhaust gas of automobile and is not required a long temperature-raising time at the starting of engine. Accordingly, the catalyst supported on the honeycomb structural body can be used without any troubles in the purification of exhaust gas. In the present invention, an extrusion die having slits, whose crossing corners have a round shape formed by plating the metal film 4 on the surface of the crossing corners 5 of the slits 2 of a matrix 1, is used. Therefore, a honeycomb structural catalyst carrier extruded from the die has a round shape at the crossing corner of the partition walls and when the catalyst carrier is used, exhaust gas flows smoothly and the reaction efficiency of the catalyst is high. Further, even if there is some dimensional error in the width of slit 2 formed on the matrix 1 by the mechanical machining or electric discharge machining, slit 2' having a uniform and proper width can be obtained by controlling the thickness of the metal film 4 to be formed on the slit 2. Therefore, even such matrix 1 can be used without discarding. As described above, the present invention has solved the drawbacks of dies produced by the conventional mechanical machining or electrical discharge machining and used for extruding honeycomb structural body, and the extrusion die of the present invention is very valuable for industrial purposes. The following Examples are given for the purpose of illustration of this invention and are not intended as limitations thereof. Example 1 As illustrated in Figures 1-4, feed-inlets 3 for ceramic batch, which has a diameter of 2 mm and a depth of 35 mm, were formed in the rear surface of a steel sheet (Diameter a: 120 mm, diameter b of slit portion: 60 mm, thickness c: 40 mm, thickness d of feed inlet portion: 35 mm and thickness e of slit portion: 5 mm) by a drill at a pitch P of 3 mm, and then the steel sheet was subjected to an electric discharge machining for the purpose of forming slits 2 having a width i of 0.32 mm and a depth of 5 mm in the front surface of the steel sheets at a pitch P of 3 mm, thereby a matrix 1 was obtained. The width of the slits 2 in the matrix 1 was measured by means of a projector and ascertained to be 0.33 mm. When the target slit width is 0.15 mm, the thickness of metal film 4 to be plated is calculated as (0.33 mm-0.15 mm) , 2 = 0.09 mm. Therefore, a nickel-phosphorus alloy film 4 having a thickness of 0.09 mm was formed on the area ranging from the surface of the slits 2 to the surface of the feed inlets 3 for ceramic batch by the electroless nickel plating. From a previously prepared curve showing a relation between the plating time and the thickness of metal film by the use of an electroless plating bath having a nickel concentration of 6.2 g/e and a pH of 4.3 and kept at 880C, it was ascertained that a nickel phosphorus alloy film having a thickness of 0.09 mm can be obtained by carrying out an electroless plating in the bath for 495 minutes. That is, the above described nickel plating of the nickel-phosphorus alloy film 4 was carried out in the above described plating bath kept at 880C for 495 minutes. The resulting nickel-phosphorus alloy film 4 having a thickness t of 0.09 mm was subjected to a heat treatment, wherein the film 4 was heated up to 4500C in 1 hour, kept at 4500C for 3 hours and then cooled in air, and the aimed extrusion die, whose slits 2' had a width i' of 0.15 mm, was produced. When a catalyst carrier produced by the use of the above obtained extrusion die was compared with a catalyst carrier produced by the use of a conventional extrusion die having a slit width of about 0.3 mm and has the same reaction area as that of the former carrier, it was ascertained that the former carrier was about 35% smaller in the weight than the latter carrier and further the former carrier is smaller in size and is lower in heat capacity and in fuel cost than the latter carrier. Example 2 According to Example 1, a matrix 1 having slits 2 of 0.492 mm width i and having no round shape at the crossing corner 5 of the slits 2 was produced, and a nickel-phosphorus alloy film 4 having a thickness t of 0.094 mm was formed on the surface of the slits 2 to produce an extrusion die having slits 2' of 0.304 mm width i' and having a round shape, whcih had a radius of 0.094 mm at the crossing corner of the slits 2'. A catalyst carrier produced by the use of the resulting extrusion die had a round shape having a radius of 0.094 mm at the crossing corner of the partition walls, and was higher than the conventional catalyst carrier in the reaction efficiency of catalyst. Example 3 According to Example 1, a matrix 1 having slits 2 of 0.402 mm width 1 and having no round shape at the crossing corner 5 of the slits 2 was produced, and a nickel-phosphorus alloy film 4 having a thickness t of 0.158 mm was formed on the surface of the slits 2 to produce an extrusion die having slits 2' of 0.086 mm width i' and having a round shape, which had a radius of 0.158 mm at the crossing corner of the slits 2'. Therefore, according to the present invention, an extrusion die having a very small slit width and further having a round shape at the crossing corner of such thin slits can be easily obtained. It has hitherto been considered to be impossible to produce slits having the above described small width and to work in the crossing corner of slits having such small width. The honeycomb structural body produced by the use of the above obtained extrusion die had excellent properties described in both Examples 1 and 2. Example 4 According to Example 1, five matrices 1 having slits 2 of 0.3 mm target width i was produced, and nickel-phosphorus film 4 having a variant thickness was formed on the surface of the slits 2 corresponding to the above obtained actual width i of the slit 2 in each matrix 1 so that the resulting slits 2' would have a target width i' of 0.15 mm. The obtained results are shown in the following table. It has been ascertained from the table that the variance of the slit width in the die of the present invention is fairly smaller than the variance of the slit width in the matrix. EMI12.1 <tb> <SEP> Slit <SEP> width <SEP> i <SEP> Thickness <SEP> t <SEP> Slit <SEP> width <SEP> i' <SEP> in <SEP> the <tb> Sample <SEP> die <SEP> of <SEP> the <SEP> present <tb> <SEP> No. <SEP> in <SEP> the <SEP> matrix <SEP> (mm) <SEP> of <SEP> Ni-P <SEP> alloy <SEP> invention <SEP> (mm) <SEP> <tb> <SEP> (target <SEP> width:0.3mm) <SEP> film <SEP> (mm) <SEP> target <SEP> width:0.l5mm) <SEP> <tb> <SEP> 1 <SEP> 0.306 <SEP> 0.0i7 <SEP> 0.152 <tb> <SEP> 2 <SEP> 0.281 <SEP> 0.067 <SEP> 0.147 <tb> <SEP> 3 <SEP> 0.314 <SEP> 0.081 <SEP> 0.152 <tb> <SEP> 4 <SEP> 0.327 <SEP> 0.090 <SEP> 0.147 <tb> <SEP> 5 <SEP> 0.293 <SEP> 0.072 <SEP> 0.149 <tb>

Claims: CLAIMS 1. In a method of producing a die for extruding a honeycomb structural body, wherein forming a lattice of slits having a given depth in the front surface of the matrix of the die in a shape substantially corresponding to the geometrical shape of a honeycomb structural body to be extruded, and forming a plurality of independent inlets for feeding a ceramic batch and communicated to the slits in the rear surface of the matrix, an improvement comprising forming a metal film on the surface of at least the slits by plating, and adjusting the thickness of the metal film so that the slits have a given width. 2. A method according to claim 1, wherein a round shape having a given radius is formed at the crossing corner of the slits by adjusting the thickness of the metal film. 3. A method according to claim 2, wherein the plating is carried out so that the thickness of the metal film formed on the slit is equal to the value of radius of the round shape at the crossing corner of the slits. 4. A method according to claim 1, wherein the plating of the metal film is carried out so that the thickness of the metal film is one-half of the difference obtained by subtracting the target slit width from the slit within the matrix. 5. A method according to claim 1, wherein said plating is an electroless plating. 6. A method according to claim 5, wherein said electroless plating is an electroless nickel plating. 7. A method according to claim 6, wherein said electroless nickel plating is an electroless nickel-phosphorus alloy plating. 8. A method according to claim 7, wherein the electroless nickel-phosphorus alloy film is heat treated. 9. A method according to claim 8, wherein said heat treatment is carried out at a temperature of 400-5000C. 10. A die produced by the method of any one of the preceding claims.

Inventors: IHARA, MASAHIRO
Applicants: NGK INSULATORS, LTD.
Application year: 1979
Main classification: B28B3
IPC codes: B23P15, B28B3, C23C18