CASTING MOLD FOR SLIP CASTING

Abstract: A mould for slip casting consists of a composite of boron nitride with other ceramic materials (such as silicon nitride, alumina, silicon carbide, zirconium oxide etc.), which is porous and has a high water-absorption capacity. Pref. the composite contains 10-50% wt. boron nitride, has a mean pore size 0.1-2um and pore ratio 10-40%. Figure shows the pore ratio for boron nitride/silicon nitride composites against boron nitride content (% wt.).

Description: Field of Technology The present invention relates to a mold for slip casting having fine pores and exhibiting good water-absorptivity, long durability, high strength and excellent mold-releasability. Background Technology Slip casting is a kind of molding techniques of powders applied, in particular, to the field of ceramics and employed from old in potteries for sanitation fixtures, chinas and porcelains and the like. In recent years, this technique is applied also to the field of fine ceramics using alumina (Al₂O₃), silicon nitride (Si₃N₄) and the like and is expected to be a molding technique of parts for industrial use. Gypsum molds known from the oldest time are conventional as a mold for slip casting. The reason therefor include excellent water-absorptivity, mold-releasability and dimensional accuracy as well as the shapeability of large and complicated forms at low costs. On the other side, however, gypsum molds are poor in the water resistance, compressive strength and abrasion resistance and have a defect that the Ca²⁺ ions in the gypsum are dissolved out to enter the ceramic body. In addition, gypsum molds have a problem that the porosity and pore diameter distribution differ from mold to mold to cause variation in the water absorptivity so that definite molding conditions cannot be undertaken. When the mold is used in molding under pressurization in order to increase the productivity, furthermore, the strength is insufficient and a defect is caused that a large number of the molds must be used in parallel. Standing on such a background, a variety of molds of other materials are under development with an object to overcome the defective points of gypsum molds. Major materials include resins, composite materials of a resin and a ceramic, composite materials of a metal and a ceramic, metals and the like. (Yoichi Motoki, "Manufacturing Process of Ceramics", published October 10, 1978 by Gihodo Publishing; official publication of Japanese Patent Kokai 60-166405; and official publication of Japanese Patent Kokai 49-53216). These molds, however, have following defective points. (1) The pore diameter is larger than 3 µm so that clogging takes place on the mold when fine particles are subjected to slip casting. (2) The mold-releasability between the mold and the ceramic body is poor so that compulsory mold-takeoff must be undertaken by passing a gas and other means sometimes to cause damage on the molded body. When polyvalent cations highly ion-exchangeable, such as Ca²⁺ ions, are incorporated into the surface of the mold in order to improve the mold-releasability, they enter the ceramic body. (3) Fabrication of the mold is not an easy matter and the working cost is high to impart full accuracy to the surface of the product or to finish a product of a complicate configuration having an extremely thin-walled portion or an extremely narrow portion with a good dimensional accuracy. In WO-A-8801258 composite refractory materials are disclosed which exhibit good thermal shock resistance, reasonable high mechanical strength and good resistance to corrosion. In particular, said materials comprise a matrix refractory material within which are dispersed particles of a ceramic material which comprises from 1.0% to 40% of the composite material. The composite refractory materials have a porosity which does not exceed 12%. Said composite refractory materials comprising silicon carbide, as well as boron nitride, are useful for the manufacture of kiln furniture. It is the object of the present invention to provide an improved mold for slip casting. Said object is achieved by the subject matter of the single patent claim. The inventive mold for slip casting has the following benificial properties: (1) the mold has fine pore diameters which are free from clogging even in the use of an ultra-fine powder for fine ceramics; (2) the mold is of long durability, has excellent water resistance and has compressive strength without causing denaturation or wearing by abrasion of the surface of the mold even after many times of use; (3) the mold is of high strength and is suitable not only for molding under normal pressure but also for molding under pressurization; futhermore, (4) the inventive mold has good mold releasability and exhibits little wettability with ceramic bodies. Brief Description of the Drawing Figure 1 is a graph showing the dependency of the three-point bending strength of Si₃N₄-BN type composite sintered bodies at room temperature on the content of BN; Figure 2 is a graph showing the dependency of the Shore hardness of the above mentioned sintered bodies on the content of BN; Figure 3 is a graph showing the dependency of the porosity of the above mentioned sintered bodies on the content of BN; Figures 4(a) and (b) are each a graph showing the pore diameter distribution of the Si₃N₄-BN type composite sintered bodies with a BN content of 20 and 50% by weight, respectively; and Figure 5 is a graph showing the relationship between the pressure and the velocity of wall deposition in slip casting when the mold according to the present invention is used. The present inventors firstly conducted studies of the material of the mold. The guide line is to select the material from ceramics in view of the abrasion resistance and compressive strength. BN has come to their attention as the base in respect of the little wettability with the ceramic body and good workability. A problem in BN, however, is that BN is hardly sinterable to be sintered usually by the HP method and the abrasion resistance is low. The present inventors accordingly have come to an idea to combine BN with other ceramic constituents capable of being sintered densely by heating under normal pressure or under pressurized atmosphere and having excellent abrasion resistance and compressive strength. The intention therein consists in the synergistic effect given below. (1) BN inhibits sintering of other ceramics and pores are formed thereby in the sintered article. (2) The abrasion resistance is supplemented while the advantages of BN are utilized. Incidentally, the above mentioned pores must be open pores making communication between the inner and outer surfaces of the mold in order to ensure sufficient water absorptivity of the mold and the mold must have air permeability. In the following, description is given with reference to the experiments of the present invention by using BN and Si₃N₄ as the other ceramic. The Si₃N₄ used as the starting material was a powder having an average particle diameter of 0.8 µm and the proportion of the α-phase of 93% and the BN used as the starting material was a powder having an average particle diameter of 0.2 µm and a purity of 97%. These two kinds of powders (including a sintering aid of Si₃N₄) were mixed together in a ball mill using ethanol and, after drying, shaped by slip casting into a test piece having a length of 50 mm, width of 50 mm and thickness of 10 mm. This shaped body was sintered in N₂ under 9 atm. at 800 °C. The weight proportion of Si₃N₄ and BN was varied as Si₃N₄/BN = 100/0, 90/10, 80/20, 70/30, 60/40, 50/50 and 40/60. These sintered bodies were evaluated for various properties. Figure 1, Figure 2 and Figure 3 each show the dependency of the three-point bending strength (σ b3 ), Shore hardness and porosity, respectively, on the content of BN. Figures 4 show the pore diameter distribution in the sintered bodies of 20% by weight BN content in (a) and of 50% by weight BN content in (b). From the results of Figure 1 to Figures 4, increase in the content of BN results in an increase of the porosity though with a decrease in the strength and hardness as a mold. It is also understood that the pore diameter is independent on the content of BN with an average pore diameter of about 0.2 µm in a very sharp distribution zone. It is presumable that BN inhibits sintering of Si₃N₄ and the degree of inhibition is increased with an increase in the content of BN. The compressive strength is also a sufficiently high strength as compared with the strength of gypsum molds of 10 to 50 kg/cm² so that molding can be performed not only under normal pressure but also under pressurization. Furthermore, the pore diameter is very fine and the distribution thereof is sharp so that the mold can be estimated to have sufficient and uniform water absorptivity. It is presumable that the pore diameter can be so small because the starting materials are fine particles. It is expected from the above described test piece experiments that, by using a mold of the present invention, no clogging takes place even in slip casting by using fine particles with good mold-releasability and molding can be performed under a considerably high pressure to enhance the mass-productivity. It is possible thereby to use the same for products under high requirements for qualities such as, for example, automobile parts. Usable other ceramic constituents to be combined with BN include, in addition to Si₃N₄, those materials having excellent anti-abrasion characteristics such as, for example, SiC, Sialon, ZrO₂, Al₂O₃, AlN, mullite and the like. The effect of composite forming of each of them with BN was similar to Si₃N₄. It is the same when two kinds or more of the materials, such as Al₂O₃-ZrO₂, are combined according to the object. Furthermore, the pore diameter and porosity can be freely controlled by means of the particle size and contents of the starting materials so that the particle size and contents of the materials for molding should be adjusted. The content of BN should be freely selected depending on the material to be molded since an increase in BN decreases the compressive strength and hardness though with improvements in the mold-releasability and workability. The present inventors accordingly prepared two kinds of slips (A) and (B) as follows to examine the performance of the mold according to the present invention. (A) a slip for fine ceramic having a viscosity of about 500 cps obtained by mixing 100 parts by weight of alumina of 0.8 µm (Al160SG-4, manufactured by Showa Denko) as the starting material and 25 parts by weight of water followed by adjustment of the viscosity using a polycarboxylic acid as a deflocculating agent. (B) a slip for pottery having a viscosity of about 1000 cps obtained by mixing 60 parts by weight of a pottery material composed of 50% by weight of pottery stone, 25% by weight of clay and 25% by weight of feldspar, having-a particle size constitution of 50% under 10 µm, as the starting material and 40 parts by weight of water followed by controlling of the viscosity with water glass as a deflocculating agent. These slips were each molded under normal pressure (P = 0 kg/cm²) or under pressurization (P = 10 or 20 kg/cm²) by using an Si₃N₄-BN mold sintered in the same manner as described above (a bottomed cylindrical form having an inner diameter of 40 mm, depth of 100 mm and wall thickness of 40 mm) to evaluate the moldability such as the velocity of wall deposition and the like. Various results of the test are shown in Table 1. The abrasive wearing of the mold is shown by the abraded amount after 50 times of testing. And, similar tests were undertaken of the mold of the present invention in which the weight proportion of Si₃N₄/BN was 70/30 and the average pore diameter was varied by the changes in the particle diameter of the BN. The results are shown in Table 2. It was understood from these results that a content of BN should be selected so as to give a remarkable improvement in the mass-productivity by pressurization and to give the highest mold-releasability and abrasion resistance. Further, the mass-productivity can be improved by supplying the slip under pressurization. The mass-productivity can be improved by increasing the content of BN as a consequence of the increase in the porosity. Incidentally, a difficulty is caused in mold-releasing when the weight proportion of BN is smaller than 10% while the abrasion resistance would be poor when it exceeds 50%. Further, the molding velocity would be too low when the average particle diameter is smaller than 0.1 µm and, when it is larger than 2 µm, there appears a tendency that clogging takes place. Furthermore, the molding velocity would be too low when the porosity is smaller than 10% and, when it is larger than 40%, the strength of the mold would be insufficient. It is known that the velocity of wall deposition of a slip in a mold is given by the relationship of the following equation assuming no changes of the slip in the lapse of time: L²/t = k. where L is the thickness of the deposited wall (mm); t is the time (minutes); and k is a constant. k in the above equation is called the coefficient of walldeposition velocity. According to the studies of the inventors, it has been found that the value of this k is approximately proportional to the pressurizing force p on the slurry when a mold of Si₃N₄-SN is used. Figure 5 is a graph showing this fact. Figure 5 illustrates an example in which an Al₂O₃ slurry was subjected to slip casting by using a mold of Si₃N₄-BN type (Si₃N₄:BN = 70:30 and particle diameter of BN 0.5 µm). The velocity of wall deposition is grealy increased by increasing the pressurizing force on the slurry. Consequently, the efficiency of molding of the products can be greatly enhanced. When the strength of the mold is taken into consideration, incidentally, it is a fair assumption that the process can be run even under a still higher pressure of, for example, 500 kg/cm² or higher. Conventional gypsum molds are destroyed when the pressurizing force of the slurry exceeds 10 kg/cm² so that a pressurizing force of high pressure cannot be applied. As compared therewith, accordingly, high-pressure, high-efficiency operation can be performed by the slip casting method using the mold of the present invention. Example 1. In the following, a description is given with reference to the examples of the present invention. As the mold according to the present invention, molds were prepared in such compositions of SiC-BN, ZrO₂-BN, Al₂O₃-BN, mullite-BN, Sialon-BN and Al₂0₃-ZrO₂-BN (weight proportion of Al₂O₃/ZrO₂ = 80/20) that the content of BN in each was 30% in a weight proportion. As the comparative examples, gypsum molds and epoxy resin molds were prepared. The configuration of the molds was the same as described above and the various properties are shown in Table 3. Slip casting of the products was performed by using the above described Al₂O₃ slip under normal pressure and under pressurization of 10 or 20 kg/cm². The test was repeated 50 times and the results are shown in Table 4. As is clear from the results in Table 4, the molds of the present invention were satisfactory in all of the items. In contrast thereto, clogging rapidly takes place in the epoxy resin molds and the mass-productivity of the gypsum molds is low with the molding pressure limited to 10 kg/cm² along with rapid abrasive wearing. Example 2. In the following a description is given of further examples of the present invention. Conditions of the slip casting are as follows. Mold of example: Material: Si₃N₄-BN based mold (Si₃N₄:BN=70:30) Dimensions: 20 mm diameter x 100 mm length Mold of comparative example: Material: 2 kinds including gypsum and epoxy resin Dimensions: the same as in the example Molding materials: (1) Base material of pottery (pottery stone 50% by weight; clay : 25% by weight; feldspar 25% by weight) (2) Al₂O₃ (average particle diameter 0.8 µm) (3) Si₃N₄ (average particle diameter 0.8 µm) Molding pressure: 10, 50, 100 and 200 kg/cm² Firing conditions of molded body: Potteries: 1300 °C, in air A1₂O₃: 1600 °C, in air Si₃N₄: 1800 °C, 9 kg/cm², in N₂ Table 5 to Table 7 show the mold-releasability, surface condition and bulk density of the molded bodies. The bulk density and three-point bending strength were measured of the sintered bodies obtained by firing under the respective conditions necessary to effect sintering and the moldability in slip casting was evaluated for the respective molds. The results of the evaluation are shown also in Table 5 to Table 7. As is clear from the results in Table 5 to Table 7, molded bodies having good surface condition without defects could be obtained in a high efficiency by conducting molding of slip casting using a mold of an Si₃N₄-BN type sintered body of the present invention. And, the results of the bulk density of the molded body and the bulk density and bending strength of the sintered body were at least equivalent to the products by a gypsum mold and epoxy resin mold. Industrial Applicability According to the present invention, an excellent mold for slip casting can be obtained by using a mold constituted from boron nitride and other ceramic consituents. Further, molding of ceramic products and the like can be performed with good mold-releasability, without clogging and with high productivity by conducting slip casting using this mold.

Claims: A mold for slip casting characterized by being constituted from boron nitride and one or more other ceramic constituents, selected from silicon nitride, silicon carbide, sialon, zirconium oxide, aluminum oxide, aluminum nitride, and mullite, and having air permeability, in which the weight proportion of boron nitride relative to the total weight of boron nitride and said at least one further ceramic constituent is 10 to 50%, average pore diameter is 0.1 to 2µm and the porosity is 10 to 40%.

Inventors: FUNAHASHI TOSHIHIKO RESEARCH L, ISOMURA KEIICHIRO RESEARCH LAB, OGASAWARA KAZUKI TOKYO MAIN OF, UCHIMURA RYOJI RESEARCH LABORA, FUNAHASHI, TOSHIHIKO RESEARCH LABORATORIES OF, ISOMURA, KEIICHIRO RESEARCH LABORATORIES OF, OGASAWARA, KAZUKI TOKYO MAIN OFFICE OF KAWASAKI, UCHIMURA, RYOJI RESEARCH LABORATORIES OF
Applicants: KAWASAKI STEEL CO, KAWASAKI STEEL CORPORATION
Application year: 1989
Main classification: B28B1
IPC codes: C04B38, B28B1