METHOD OF PRODUCING MOLD FOR SLIP CASTING

Abstract: A mold forming method in which a pattern (l) is made of an organic material such as foamed polystyrene and at least a portion of the outer or inner surface of the mold pattern is covered by a thin coating (3), and filling the surroundings of the pattern (l) with a mold material (6). The pattern (l) is then dissolved by a solvent (7), leaving the coating (3) unsolved. The coating (3) is finally removed by a solvent contained in a slip (9) which is poured into the cavity (8) provided with the coating (3).

Description: The present invention relates to a method of producing a slip casting mold which is used in fabricating formed articles by casting from a slip which contains, for example, ceramics powders, metal powders and carbon powders. More particularly, the present invention is concerned with a method of producing a slip casting mold which is used in molding of articles which necessitate cores and mold parts having such complicated configurations that they cannot be extracted due to inverse tapers. In casting articles having complicated outer and inner configurations from a mold, it has been a common measure to form the mold by assembling a multiplicity of main mold parts and cores. This method, however, requires a number of steps for the preparation of many mold parts and cores. In addition, the quality of the products tends to be impaired due to the presence of burrs. A method for overcoming these problems are disclosed in the specification of British Patent No. 1482436. According to this method, mold parts having complicated configurations are formed from an organic material, while other mold parts of comparatively simple configurations are formed from gypsum. As a slip is poured into a resultant mold, the gypsum absorbing the water contained in the slip to solidify the slip. Thereafter, the organic material is dissolved by a solvent, whereby a wet shaped body generally referred to as a "green body" is obtained. This method, however, suffers from the following disadvantages. (i) Dissolved residue of organic material inevitably remains locally on the surface of the green body. The more complicated the article configuration, the more difficult the removal of the dissolved residue. The dissolved residue remaining on the product surface due to incomplete removal or due to penetration into the green body seriously deteriorates the quality of the product. (ii) An impractically long period of time is required for solidification of green body, because only a limited portion of the mold can absorb water. The specification of United States Patent No. 2830343 discloses a method in which a pattern made of foamed polystyrene is embedded in molding sand. As a molten metal is poured into the mold as it coexists with the pattern, the pattern is made to disappear by the heat of the molten metal, so that the space which has been occupied by the pattern is replaced by the molten metal. This method is generally called as a full-mold casting method. Considering that the removal of the pattern and the substitution by the molten metal have to be conducted simultaneously, it would be very difficult to apply the full-mold casting method to slip casting. Thus, the method disclosed in the specification of British Patent No. 14824365 is disadvantageous in that the dissolved residue of organic material remains on the surface of the green body. As explained before, the more complicated the article configuration, the more difficult the removal of the residue. Any residue remaining on the surface of the green body due to incomplete removal or any fraction of the residue penetrated into the green body impairs the quality of the product. Furthermore, a high degree of skill and experience, as well as a number of steps, is required for the assembly of the mold from many mold parts and cores. On the other hand, the application of the full-mold method disclosed in the specification of United States Patent No. 2830343 to the slip casting is considered to be difficult in view of its feature that the removal of the pattern and the substitution by the molten metal have to be conducted simultaneously. DE-A-3 530 910 describes a process for preparing a mold for casting a slip and a casting mold for casting a metal, particularly for casting articles having a complicated shape of the appearance and/or a cavity of a complicated shape, by forming a pattern from an organic material and removing said pattern by dissolving it with a solvent. The pattern is provided with a coating film made from a material insoluble in said solvent, embedded in a molding material and dissolved with the solvent, whereupon the coating film and the residue of dissolution of the pattern is removed from the mold. The pattern is dissolved either after the hardening of the molding material or before the hardening of the molding material, in case said molding material contains a water-soluble binder and is hardened by heating the green mold. In the latter case the residue of dissolution of the pattern that has been also hardened during the solidification of the molding material is softened with the solvent before removal from the mold together with the coating film. Usual steps for obtaining a product by slip casting are as follows: (1) preparing a pattern; (2) providing a mold (forming a cavity); (3) casting a slip; (4) solidifying the slip (a liquid medium contained in the slip is absorbed in mold walls so that a green body is obtained); (5) removing the mold (the green body is obtained); and (6) sintering the green body. With the exception of a case of a product having such simple shape as plate-like shape, straight shape and etc., there have existed problems in the steps 2 (the providing of the mold), 4 (the solidification of the slip) and 5 (the removing of the mold) when obtaining a product having a reversed taper or having many three-dimensional, convex and concave portions (described hereinafter as "complicated shape product"), and it has been desired to find out measures for improving the problems. That is, when molding a complicated shape product, it is first necessary to provide a mold cavity of a complicated shape. For preparing the mold cavity of a complicated shape, it has been necessary to use many main molds and cores because of the following reasons: (a) Unless assembling many mold portions (main molds and cores), it is impossible to provide a mold cavity of a complicated shape (regarding the above-described mold-forming step 2); (b) A green body is apt to suffer stress from both the rigid cores and main molds with the result that cracks are apt to occur in the green body, during the solidification of the slip and during the shrinkage of the green body caused due to the removing of water and due to drying of the green body (the step 4 of solidifying the slip); and (c) In order to remove a mold without causing flaws on the green body after the green body is prepared by the solidifying of the slip, the mold must be divided into many mold portions because the strength of the green body is very low in comparison with the mold, that is, the strength of the green body is not more than one fifth that of the mold (the step 5 of removing the mold). Accordingly, an object of the present invention is to readily provide an integrated mold suitable for use in obtaining an article with a complicated configuration by slip-casting. Another object of the present invention is to provide a method of producing an integrated mold which is suitable for use in slip casting for the production of an article having both a complicated outer configuration and a complicated shape of internal cavity, without any dissolved residue of pattern material on the surface of the mold cavity. Still another object of the present invention is to provide a method of producing a mold which can completely prevent the crack of a green body from occurring due to core constraint attributable to drying shrinkage of the green body. It is, therefore, an object of the invention to provide a method of producing a mold for slip casting comprising the steps of preparing a pattern made of an organic material soluble in a predetermined solvent A, providing a coating on at least a portion of the surface of said pattern which coating is made of a material soluble in another predetermined solvent B but insoluble in said solvent A, filling surroundings of the pattern with a molding material, contacting said pattern with said solvent A so as to contract or dissolve said pattern, and leaving the coating in place and introducing a slip containing solvent B in the cavity left by the removed organic material and removing the mold. Furthermore, it is an object of the invention to provide a method of producing a mold for slip casting comprising the steps of preparing a pattern made of an organic material soluble in a predetermined solvent, providing a coating on at least a portion of the surface of the pattern which coating is made from a material insoluble in said solvent, leaving the coating in place, heating the mold, removing the coating by burning, introducing the slip in the cavity left by the removed organic material and removing the mold. The invention simultaneously improves the problems (a), (b) and (c) as shown above which have been caused in conventional slip casting method of producing a product of a complicated shape. That is, in the invention it is possible to use a single integrated mold even if the mold has a cavity complicated in shape. Since the mold has already lost strength for integration after the forming of the mold and before the casting of the slip or since the mold loses the strength for integration as the mold absorbs a solvent contained in the slip, a resultant green body receives no stress during the drying and shrinking thereof, with the result that no crack occurs. Since, in any case, the strength for integrating the mold is lost after the green body is formed, it is very easy to remove the mold. Thus, the green body is separated from the mold while having a good state without flaws. Next, the invention is exemplified by showing specific steps. First, there is prepared a pattern of foamed polystyrene having the same shape as that of a product to be produced (the dimensions of the pattern is made to be somewhat larger to compensate shrinkage caused during the solidification of the slip), onto the surface of which pattern is formed a coating of polyvinyl alcohol. Then, the pattern is embedded in a thermally collapsing mold slurry (consisting of α-gypsum of 30 weight parts, SiO₂ fine powder of 70 weight parts, and water of 50 weight parts), and the mold slurry is solidified. After that, trichloroethane is pored through a poring gate to thereby dissolve the pattern, and the dissolved pattern is discharged outside of the mold. Since the coating is insoluble in the trichloroethane, the coating remains on the inner wall of the mold. Then, by heating the mold (at 300°C for about 2 hours), the strength of the mold is decreased very much to have a value less than 1 Kg/cm² due to the characteristics of gypsum, while the coating burns out due to the heating. Then, by pouring in the mold cavity a ceramic slip containing water as a solvent, the water in the slip is absorbed in the mold, the slip being gradually solidified to form a green body. In this case, since the strength of the mold becomes very low, there is caused no crack in the green body and in addition the mold is readily removed, with the result that a good green body can be obtained. Another aspect of the invention relates to a method of producing a mold used for casting slip. In the method, at least a part of the surface of a pattern made of an organic material soluble in a predetermined solvent (A) is covered with a coating soluble in a predetermined solvent (B) and insoluble in the solvent (A) which coating is not permeable to the solvent (A). Then, the surroundings of the pattern is filled with a slurry comprising an inorganic powder as aggregate, gypsum as a binder, and water and preferably with a mold material the strength of which decreases very much when it is heated. Next, the solvent (A) is made to contact with the pattern so that the pattern shrinks and/or is dissolved in the solvent, the pattern being then discharged outside of the mold so as to form a mold cavity. After completing the casting of the slip, the strength of the mold is decreased preferably by heating the mold. By casting a slip containing water or ethyl alcohol or acetone as a solvent into the cavity of the mold obtained in accordance with this aspect of the invention, it is also possible to solidify the slip. In this case, after the slip is solidified to become a green body, the mold is removed. Since the strength of the mold decreases in a great degree, the mold can be readily removed from the green body. The pattern used is preferably formed from a foamed plastic compact which is easily soluble to the solvent A. On the other hand, the pattern used is preferably made of a foamed plastic compact which is easily soluble to organic solvents. More preferably, the material of the pattern is selected from a group consisting of foamed polystyrene, foamed polyethylene and p-dichlorobenzene. Any solvent which is capable of dissolving the pattern may be used as the solvent A. Preferably, the solvent A is one organic solvent selected from a group consisting of trichloroethane, trichloroethylene, tetrachloroethylene, methyl ethyl keton, toluene and ethyl acetate or is a mixture of two or more of these substances. When the mold pattern is made of foamed polyethylene, trichloroethylene or trichloroethane or methyl ethyl ketone is suitably used, whereas, when pdichlorobenzene is used as the pattern material, toluene is used suitably as the solvent. Tetrachloroethylene and ethyl acetate dissolves styrene very well, but the use of ethyl acetate requires much attention because it easily catches fire. The material of the solvent B which is used as a solvent of the slip can conveniently be selected from a group consisting of water, alcohol and acetone. In view of the fact that most of the solvents of the slip is water, most of coating are soluble in water. For example, a "heatcollapsing mold" may be used which is made of a slurry prepared by adding inorganic fine powder such as SiO₂ powder etc. in gypsum and then by adding water of a predetermined amount thereto. The material of coating varies depending on the solvent. In case of a water-soluble coating, the coating material suitably used is an aqueous solution of one or more selected from the group consisting of polyvinyl alcohol (PVA), water-soluble isobutane-maleic anhydride copolymer, polyacrylamide (PAAm), polyethylene oxide (PEO), polyvinyl pyrolidone (PVP), water-soluble vinyl acetate copolymer, acrylic copolymer, polyethylene glycol (PEG), methyl cellulose (MC) carboxy methyl cellulose (CMC), hydroxypropyl cellulose (HPC), water-soluble wax, starch, glue and gum arabic. For example, since polyvinyl alcohol (PVA) is soluble both in water and alcohol (, e.g., ethanol), PVA can be used suitably when the solvent B is water or alcohol. The coating is formed by applying the coating material substantially uniformly on the outer or inner surface of the pattern in a small thickness. The application is preferably conducted by means of brush, spray or a spinner. The thickness of the coating finally ranges between 5 and 100 µm, preferably between 5 and 30 µm. Any thickness variation due to pattern configuration or location of the application is permissible. Fig. 1 is an illustration of a pattern; Fig. 2 is an illustration of the pattern after formation of a coating; Fig. 3 is an illustration of a step after filling with a mold material; Fig. 4 is an illustration of a step in which the pattern is being dissolved; Fig. 5 is an illustration of a mold after removal of the pattern; Fig. 6 is an illustration of a step of casting a slip; and Fig. 7 is a sectional view of a green body obtained. Fig. 8 is an illustration of a pattern after formation of a coating thereon; Figs. 9 and 10 are illustrations of a step after filling with a mold material; Fig. 11 is an illustration of a step of dissolving the pattern; and Fig. 12 is an illustration of a step after dissolving the pattern. Fig. 13 is an illustration of the state after filling with a mold material; Figs. 14 to 18 are illustrations of steps of dissolving a pattern, contraction of the pattern, and removal of dissolved residue; and Fig. 19 is a sectional view of a resultant mold. Fig. 20 is an illustration of a pattern of female type; Fig. 21 is an illustration of the pattern provided with coating; Fig. 22 is an illustration of a mold material filled in the pattern; and Fig. 23 is an illustration of a step of dissolving the pattern. Fig. 24 is a sectional view of a pattern made of foamed styrene and having a configuration corresponding to that of a turbo-charger casing; and Fig. 25 is an illustration of the appearance of a sintered product of a turbo-charger casing made of silicon nitride. Fig. 26 is a sectional view of a pattern made of foamed styrene and having a configuration corresponding to that of a screw rotor; Fig. 27 is a sectional view of the pattern embedded in a mold material; Fig. 28 is a sectional view of the mold after removal of the foamed styrene; and Fig. 29 is a sectional view of the mold illustrating the pouring of a slip into the mold cavity. Fig. 30 shows the appearance of a pattern made of foamed styrene and having a configuration corresponding to that of a casing; and Fig. 31 is an illustration of the pattern embedded in a mold material. Fig. 32 is an illustration of a pattern having a three-dimensional configuration with many convex and concave portions. The method of producing a slip casting mold has the steps of: preparing a pattern made of an organic material soluble to a solvent, e.g., a foamed styrene; forming on the surface of the pattern a coating soluble in water but not soluble to the solvent; filling the surroundings of the pattern with a mold material collapsing by water; hardening the mold material; dissolving the pattern by the solvent capable of completely dissolving the pattern; and removing the pattern to form a mold cavity. That is, the method of this embodiment is carried out in accordance with the following procedure: (1) A pattern 1 (see Fig. 1) was formed of a resin (, for example, foamed styrene) soluble to a solvent. (2) A cylindrical sprue 2 (see Fig. 2) made of a material which does not absorb water, e.g., vinyl chloride, was bonded to the mold pattern 1, and a material soluble in water but not soluble in the solvent was coated onto the surface of the mold pattern to a thickness of 5 to 100 µm, preferably 5 to 30 µm, thus forming a coating 3. The coating was then dried. (3) A flask 5 was placed on a molding board 4 and the pattern was placed in the flask. Then, a water-collapsible sand 6 containing a powder which is hardly soluble in water such as a refractory powder with a water-soluble binder dispersed therein was filled around the pattern 1 as shown in Fig. 3. (4) A solvent 7 such as trichloroethane capable of dissolving the pattern material was poured through the sprue 2 as shown in Fig. 4, thereby dissolving the pattern 1. After the pattern was dissolved apparently completely, the solution of the solvent 7 and the pattern material dissolved therein was removed to the outside of the mold. If there is any residue of the pattern material on a portion of the wall surface of the mold, the solvent 7 be charged again to completely remove the residue, as shown in Fig. 4. It is to be noted that the material of the coating 3 is not at all soluble in the solvent which is used for dissolving the pattern material, so that the solvent does not penetrate into the mold. Therefore, the removal of the pattern is possible merely by using only a small amount of solvent corresponding to the volume of the pattern. The solution of both the solvent 7 and the material of the pattern 1 dissolved therein cannot penetrate either into the mold, so that the water-absorbability of the mold sand is not impaired. (5) A mold cavity 8 having the same configuration as the pattern 1 was formed by these steps, as shown in Fig. 5. (6) A ceramic slip 9 was prepared by mixing a ceramic powder with water, and the thus prepared slip 9 was poured into the mold cavity 8 as shown in Fig. 6. Since the coating 3 is soluble in water and is permeable to water, the water in the slip 9 was progressively absorbed into the water-collapsible mold 6. In consequence the slip 9 was solidified to form a green body. In addition, the adhesion between particles of the mold was then decreased so that the strength of the mold was reduced to a level which is 1/10 to 1/20 of the initial strength. This tendency was greater in the region closer to the green body. (7) After the solidification of the slip, the green body 10 was taken out of the mold 6 as shown in Fig. 7. This was conducted very easily because the mold 6 became very low in strength. Finally, the green body 10 was sintered under a predetermined condition so as to form a complete sintered ceramic product. Although the illustrated embodiment employs a pattern of a simple configuration, the advantage of this embodiment is more remarkable when applied to the molding of articles having complicated configurations. Namely, since mechanical removing of the pattern is unnecessary in any case of producing a product having remarkably complicated shape, it is possible to easily form an integrated mold, so that troublesome works required in conventional methods such as preparation and assembly of a number of mold parts and cores are completely unnecessary in the invention. It is to be noted also that, since the mold becomes soft as a result of absorption of water as explained in paragraph No. 6, the green body of ceramics is not constrained by mold part, particularly by a core, when the green body of the ceramics contracts due to the decrease of water contained therein. It is therefore possible to completely prevent the green body from being cracked even when it has a complicated configuration having an internal cavity therein. The described embodiment can be applied not only to the slip casting of ceramics but also to the slip casting of metal or resin powders, and there will be obtained the same advantageous effect as that obtained in the production of ceramics products if water is used as a liquid medium of the slip. An aqueous solution of 15% polyvinyl alcohol was applied onto the whole surface of a foamed polystyrene test piece 11 (foaming magnification of 40 times) shown in Fig. 8 with the exception of the upper end, so as to form a coating 12 of about 50 µm in thickness. The test piece was fixed in the center of a wooden flask 14 placed on a molding board 13, and a slurry 15 of kneaded mixture of 100 weight parts of gypsum and 60 weight parts of water was poured around the test piece 11. Two molds having the foamed styrene test piece 11 embedded therein were thus prepared. The molds were then turned upside down and the boards 13 and the flasks 14 were removed as shown in Fig. 10. Acetone was charged into the pattern test piece 11 in the first mold from the top thereof having no coating of polyvinyl alcohol. In this case, the pattern was dissolved quickly but a large quantity of dissolved residue 17 remained on the surface of the mold, particularly on the bottom. Thus, acetone seems to contract the pattern rather than to dissolve the same. Regarding the second mold, trichloroethane was charged from the top of the pattern test piece. In this case, the pattern was progressively dissolved, so that a mold cavity 18 having no dissolved residue was obtained as shown in Fig. 12 after the removal of the solution composed of trichloroethane and the pattern material. It was thus possible to obtain a water-collapsible mold having a desired mold cavity having no dissolved residue at all by providing a coating of a water-soluble and solvent-insoluble material on the pattern surface and by using trichloroethane as the solvent for dissolving the pattern material. Mold No. Pattern dissolving solvent Adhesion of dissolved residue onto inner wall of mold Evaluation First Mold Acetone Residue found particularly on the bottom of mold cavity good Second Mold Trichloro-ethane No residue (polystyrene completely dissolved in the solvent) Excellent Thus, the use of acetone as the solvent is not preferred because it only causes the pattern to be contracted, necessitating troublesome works for the removal of the dissolved residue or contracted pattern. However, in the invention it is possible to satisfactorily adopt the combination of contraction and dissolution as in the following embodiment. As shown in Figs. 13 to 19, this embodiment makes use of contraction of the pattern by the use of acetone. As the first step, a coating 3 was formed on the pattern 1 of foamed polystyrene in the same manner as that in the second embodiment, and a mold material 6 is filled around the pattern 1 as shown in Fig. 13. Then, acetone was poured to the mold pattern 1 so that the pattern 1 is seemingly contracted as shown in Fig. 14, leaving both acetone and contracted body as residue 34 on the bottom of the cavity formed in the mold, as shown in Fig. 15. Then, trichloroethane 35 was poured as shown in Fig. 16 into the cavity so that a homogeneous solution 36 was formed in the cavity as shown in Fig. 17. Then, the solution was discharged to the outside of the mold as shown in Fig. 1. whereby a mold of this embodiment was completed as shown in Fig. 19. Acetone can dissolve polystyrene but the degree of magnitude of the dissolving is very small as compared with trichloroethane and trichloroethylene, with the result that the polystyrene pattern is merely seemingly contracted into a shape like a rice cake in a case of using acetone. In contrast, trichloroethane or trichloroethylene can completely dissolve the polystyrene pattern to thereby provide a homogeneous solution. It is, therefore, advisable to provide such a homogeneous solution and then to discharge the solution to the outside of the mold. As will be seen from Figs. 20 to 23, this embodiment relates to the formation of a mold core. Thus, the pattern used in the initial step was a female-type pattern as shown in Fig. 20. More specifically, as the first step, the female-type pattern was prepared as shown in Fig. 20 which is made of foamed polystyrene, and a coating was provided on the inner surface of the pattern as shown in Fig. 21. Subsequently, the cavity of the pattern was filled with a mold material shown in Fig. 22, the pattern being then dissolved by a suitable solvent as shown in Fig. 23 after the hardening of the mold, whereby a mold of this embodiment was obtained. A pattern 19 was formed by use of foamed polystyrene into a shape identical to that of an automotive turbo-charger casing, as shown in Fig. 24. The volume of the pattern was 200 cm³, while the foaming magnification of the polystyrene was 20 times. Then, a aqueous solution of 15% polyvinyl alcohol was applied onto the surface of the pattern 19 and then dried so as to form a coating 20 of about 50 µm in thickness. The pattern was placed on a molding board in the same manner as in the first embodiment. A molding sand was prepared by kneading both 100 weight parts of alumina (350 to 325 mesh) and 35 weight parts of aqueous solution of 8% polyvinyl alcohol, and then the surroundings of the polystyrene pattern is filled with the molding sand to embed the pattern. The formed mold was turned upside down after tamping the sand. Then, a predetermined amount of trichloroethane was charged in a sprue portion of the pattern where there is provided no coating, after removal of the wooden flask and the molding board. The pattern 19 was quickly dissolved from the upper side thereof as a result of contact with trichloroethane. Then, the trichloroethane solution with polystyrene dissolved therein was discharged to the outside of the mold by means of a hand pump. The mold thus formed was then placed in an electronic oven so as to be irradiated with microwave for 10 minutes for drying and curing the mold. As a result, a mold was formed to have a required mold cavity with a smooth wall surface having no pattern residue at all and with a high dimensional precision. An Si₃N₄ slip was prepared which comprises Si₃N₄ powder of mean particle size of 0.5 µm as a main constituent thereof, a deflocculation agent, a binder, and distilled water. The slip was poured into the cavity of the mold and the mold was collapsed after elapse of two hours, while confirming the hardening of the green body. In order to promote the collapse of the mold, water was sprayed onto the mold, so that the mold material could be removed very easily, whereby a green body of Si₃N₄ with highly smooth surface with no burr and having a high dimensional precision was obtained. The green body was placed in a nitriding furnace and was gradually heated up from room temperature and maintained at 1850°C for 2 hours followed by slow cooling. As a result, a turbo-charger casing 21 as a complete sintered product of Si₃N₄ having a high density was obtained as shown in Fig. 25. A pattern 22 was formed by use of foamed polystyrene (forming magnitude of 20 times) into a shape identical to that of a male type rotor of a screw compressor, as shown in Fig. 26. Then, an aqueous solution of 15% polyvinyl pyrolidone was applied onto the surface of the pattern 22 except for the top surface thereof so as to form a coating 23 of about 50 µm in thickness. The pattern was placed on a molding board 24 in the same manner as the first embodiment as shown in Fig. 27 and a sprue was bonded by an adhesive to the pattern. The pattern was then surrounded by a wooden flask 25. A molding sand was preparing by kneading 100 weight parts of alumina (250 to 325 mesh) and 40 weight parts of aqueous solution of 1.7% carboxy methyl cellulose (CMC), and the surroundings of the polystyrene pattern is filed with the sand so as to embed the pattern while tamping the sand. The thus formed mold was immediately turned upside down and, after removal of the molding board 24, a predetermined amount of trichloroethylene was poured through the sprue portion of the pattern 22. The pattern 6 was quickly dissolved from the upper side thereof as a result of contact with trichloroethylene. The mold was again turned upside down so that the excess trichloroethylene was removed and was left for 1 hour in the atmosphere whereby the solvent was evaporated and removed. The mold thus formed was irradiated with microwave for 20 minutes for drying and curing purpose in the same manner as that in the second embodiment. As a result, a mold 27 was formed to have a required mold cavity 26 as shown in Fig. 28. A zirconia slip was prepared which comprises 80% zirconia powder as a main constituent, which zirconia contains solid solution of Y₂O₃, a deflocculation agent, a binder, and distilled water. The slip was poured into the cavity 26 of the mold 27 and the mold was collapsed after elapse of 5 hours, while confirming the hardening of the green body. In order to promote the collapse of the mold, water was sprayed in the same manner as in the second embodiment, so that the mold material could be removed very easily, whereby a zirconia green body with highly smooth surface with no burr and having a high dimensional precision was obtained. The green body was placed in a firing furnace and was heated up gradually. After heating at 1500°C for 3 hours, a homogeneous zirconia sintered product was obtained. A pattern 29 was formed by use of foamed polystyrene into a shape identical to that of a casing, as shown in Fig. 30. The foaming magnification of the polystyrene was 50 times. Then, a water-soluble wax was applied onto the surface of the pattern 29 so as to form a coating 30 of about 70 µm in thickness. The pattern 29 was placed on a molding board 30 in the manner as shown in Fig. 31, and a split-type metal flask 31 was placed around the pattern 29. A mold material was prepared by kneading 100 weight parts of alumina (250 to 325 mesh), 12 weight parts of K₂CO₃ and 13 weight parts of water, and was filled and tamped down around the polystyrene pattern so as to embed the pattern. The thus formed mold 30 was immediately turned upside down and was maintained for 30 minutes in the vapor of trichloroethane after removal of the flask 32 and the molding board 30. The pattern 29 was quickly dissolved and dripped from the upper side thereof as a result of contact with trichloroethane vapor and the drip was discharged to the outside of the mold. Then, the mold was dried and cured in a drying oven at 200°C. An alumina slip was prepared which comprises alumina powder of mean particle size of 2.5 µm as a main constituent thereof, a sintering-assisting agent, deflocculation agent, and distilled water. The slip was poured into the cavity of the mold 33 and the mold 33 was placed in the atmosphere for 3 hours. After confirmation of the curing of the green body, water was sprayed to the water-soluble mold, so that the alumina green body could be removed from the mold very easily. The green body was placed in a gas furnace and was heated up from room temperature and maintained at 1650°C for 3 hours followed by slow cooling. As a result, a casing as a complete sintered product of alumina was obtained. In the embodiments described above, an integrated mold can be obtained without difficulty, so that products having complicated configurations such as a turbo-charger casing can be produced by using only one piece of mold. As explained before, a conventional casting of articles having complicated configurations has usually required more than 20 mold parts including main mold parts and cores, necessitating impractically large number of preparation and assembly steps, as well as high degree of skill and experience. In addition, the quality of the product is impaired due to presence of burrs. The smoothness and the dimensional precision of the conventional product also are inferior to those of the product produced by the mold in accordance with the invention. The advantageous effect brought about in this embodiment is also applied to the cases of all of the prior embodiments of the invention described above. A pattern having a section shown in Fig. 32 encounters a difficulty in providing a coating because of its complicated internal configuration, i.e., due to the presence of many convexities and concavities. In such a case, the pattern was previously divided into sections such as sections A, B and C and coating operation was conducted on each of these sections, and then these sections were bonded into an integral pattern. The thus prepared pattern was used in the production of a mold in accordance with any one of the preceding methods. An aqueous solution of 25% polyvinyl alcohol was applied onto the whole surface of the foamed polystyrene test piece 11 (foaming magnification of 40 times) shown in Fig. 8 with the exception of the upper end to thereby form a coating 12 of about 30 µm in thickness. Then the test piece was fixed in the center of a space defined by a wooden flask 14 placed on a molding board 13 as shown in Fig. 9, and a slurry of kneaded mixture of 20 weight parts of gypsum, 80 weight parts of silica fine powder (having a grain size of not more than about 300 mesh), and 60 weight parts of water was poured around the test piece. After the mold was hardened, the mold was turned upside down, the board and the wooden flasks being removed as shown in Fig. 10. Then, by pouring trichloroethane from the top (having no coating of polyvinyl alcohol) of the test piece 11, the pattern was gradually dissolved. The trichloroethane in which the pattern was dissolved was discharged outside of the mold, whereby there was obtained a mold cavity on the surface of which no residue was observed. The resultant mold was heated at 400°C for 3 hours, so that the adhesion of the gypsum was lost with the result that the strength of the mold became a level not more than about 0.8 Kg/cm² in compression strength. In addition, the coating 23 disappeared by the heating. Then, there was poured into the mold cavity 26 a zirconia slip comprising, as a matrix, zirconia powder of 80% having a grain size of not more than 1 µm in which zirconia a solid-solution of Y₂O₃ is included, a deflocculation agent, a binder and distilled water, all of which were uniformly mixed. After elapsing 5 hours under atmospheric air, the hardening of the green body was confirmed. The mold material was very readily removed, and there was obtained a zirconia green body having no burr, having a smooth surface and superior in dimension accuracy. The green body was then placed in a firing furnace, the temperature of the furnace being gradually raised up to 1500°C, at the temperature of 1500°C the green body being maintained for 3 hours, whereby a uniform complete zirconia sintered body was obtained. An aqueous solution of 15% polyvinylpyrolidone of about 50 µm in thickness was applied onto the whole surface of a male-type rotor pattern 22 for a screw compressor, which pattern was made of foamed polystyrene (foaming magnification of 20 times), with the exception of the upper end thereof to thereby form a coating 23 as shown in Fig. 26. The pattern was placed on a molding board 24 in the same manner as the first embodiment as shown in Fig. 27, and a sprue was bonded by an adhesive to the pattern. The pattern was then surrounded by a wooden flask 25. A space defined between the pattern and the flask 25 is filled with a mold slurry prepared by kneading a mixture comprising 70 weight parts of alumina (250 to 325 mesh in grain size), 30 weight parts of gypsum, 8 weight parts of cellulose powder (not more than 300 mesh in grain size), and 75 weight parts of water. After hardening the mold, the mold was immediately turned upside down, the molding board being removed, and trichloroethylene was poured through the sprue portion of the pattern to thereby dissolve the pattern 22. The pattern was rapidly dissolved in the trichloroethylene from the upper side to the bottom thereof as a result of contact with the trichloroethylene. The mold was again turned upside down to discharge the solution outside of the mold. The mold was heated at 400°C for 3 hours, so that the adhesion of the gypsum was lost with the result that the compression strength of the mold became a level not more than 0.8 Kg/cm² while the coating disappeared due to the heating. Thus, it was possible to obtain a desired mold cavity having a smooth surface without any residue of the pattern and having a high dimensional precision. Then, there was prepared a Si₃N₄ slip comprising Si₃N₄ powder of 0.5 µm in mean grain size as a main constituent thereof, deflocculant, and distilled water. The slip was then poured in the mold cavity, a resultant green body being confirmed to be hardened after elapsing two hours therefrom, and thereafter the mold was removed. When effecting the removal of the mold, water was sprayed onto the mold to promote the collapsing of the mold so that the mold material was removed very readily, with the result that there was obtained a green body of Si₃N₄ having a smooth surface without any burr and having good dimensional precision. The green body was placed in a nitrising furnace and was gradually heated to 1850°C, at which temperature it has maintained for 2 hours, and the green body was gradually cooled. Thus, there was obtained a turbo-charger casing 21 of a complete Si₃N₄ sintered product having a high density as shown in Fig. 25. As has been described, according to the invention, it is easily possible to form an integrated mold which is capable of molding an article having a complicated internal and/or external configuration by slip casting, thus making it unnecessary to prepare and to assemble a number of mold parts and cores which are necessary in the conventional processes. The present invention also brings about the following additional advantages: (1) It is possible to obtain a mold having a mold cavity with a cavity wall surface having no dissolved residue at all. (2) It is possible to completely prevent the occurrence of cracking in the green body such as core-constraint cracking due to drying and contraction. These additional advantages (1) and (2) in turn enables articles of complicated shapes to be cast easily without causing any burr.

Claims: A method of producing a mold for slip casting comprising the steps of: preparing a pattern (1) made of an organic material soluble in a predetermined solvent A (7); providing a coating (3) on at least a portion of the surface of said pattern which coating is made of a material soluble in another predetermined solvent B but insoluble in said solvent A; filling surroundings of the pattern with a molding material; contacting said pattern (1) with said solvent A (7) so as to contract or dissolve said pattern (1); and leaving the coating (3) in place and introducing a slip (9) containing solvent B in the cavity (8) left by the removed organic material and removing the mold (6). A method of producing a mold for slip casting comprising the steps of: preparing a pattern (1) made of an organic material soluble in a predetermined solvent; providing a coating (3) on at least a portion of the surface of the pattern which coating is made from a material insoluble in said solvent; leaving the coating (3) in place; heating the mold; removing the coating (3) by burning; introducing the slip in the cavity (8) left by the removed organic material and removing the mold (6).

Inventors: NATORI, TATSUO, SHIMAGUCHI, TAKASHI, YAMADA, TOSHIHIRO, YOKOI, KAZUAKI
Applicants: HITACHI, LTD.
Application year: 1987
Main classification: B28B1
IPC codes: B28B1, B22F3