Method for subjecting powder-molded material to isostatic pressing

Abstract: The present invention provides a method for subjecting a powder molded material (2) to isostatic pressing by covering the powder molded material with a water-non-permeable rubber film (24), wherein the surface area of the rubber film is larger than the surface area of the powder molded material. In the method, the rubber film (24) can well follow a complicated shape (e.g. shape having notch(es), throughhole(s), dent(s) and bend(s), or cylindrical shape) of the powder molded material; the damage of the rubber film during isostatic pressing occurs hardly; and the removal of the rubber film after isostatic pressing is easy.

Description: Background of the Invention (1) Field of the Invention The present invention relates to a method of preparing an isostatically pressed body of powder material, using a rubber sheat package. (2) Prior Art In powder molding which comprises molding a ceramic powder, a metal powder or the like into a desired shape, various methods are in use depending upon the desired shape and application purpose of the molded material to be obtained. The methods include, for example, pressing, injection molding, extrusion molding and casting. The powder-molded material obtained by one of these methods is subjected to isostatic pressing after covering the powder-molded material with a water-non-permeable film, in order to improve the density and remove the large defects. The isostatic pressing is also called cold isostatic pressing (CIP) or isotropic pressing, wherein the whole surface of a molded material is pressurized uniformly in a pressure medium. Water is generally used as the pressure medium. In subjecting, to isostatic pressing, a powder-molded material having a complicated shape, i.e. a powder molded material having notch(es), throughhole(s), dent(s), bent(s) and/or the like or having a cylindrical shape, a latex rubber has hitherto been used to cover the surface of the powder-molded material. That is, a latex rubber has been coated on the powder-molded material, or the powder-molded material has been dipped in a latex rubber solution and dried. The step for covering the surface of the powder-molded material with a latex rubber is repeated a plurality of times, as shown in Fig. 8. In the above covering of the powder-molded material surface with a rubber latex, there often happened the breakage of closed holes present in the vicinity of the powder-molded material surface, by isostatic pressing, the resultant formation of open holes (defects) on the surface, the breakage of the latex rubber by the defects, the consequent penetration of a pressure medium into the powder-molded material, and the formation of a lower density portion in the powder-molded material. Also, since the latex rubber sags easily at the edge(s) of the powder-molded material, there often happened no sufficient coverage with latex rubber, the resultant penetration of said pressure medium into the powder-molded material, and the formation of a lower density portion in the powder-molded material. Further, in the step of removing the rubber after the isostatic pressing of the latex rubber-covered powder-molded material, it has been necessary to remove the rubber physically (manually) and then dry up the moisture. Alternatively, the rubber can be removed by firing in a heat treatment but, in this case, a firing furnace has been necessary. EP-A-452936 describes the isostatic pressing of a pre-molded ceramic powder body, using as a protective skin a bag made of rubber foil, which is welded or vulcanized to close it. From a drawing, it may be inferred that the bag has a larger surface area than the molded body. Summary of the Invention In view of the above-mentioned problems of the prior art, the present inventors used a bag-shaped rubber film to cover a powder-molded material and found that the bag-shaped rubber film can well follow powder-molded materials of complicated shape, i.e. powder-molded materials having notch(es), throughhole(s), hollow cylinder(s), ring(s), and/or recess(es) having at least one re-entrant angle of 270°. According to the present invention, there is provided a method for preparing an isostatically pressed body, as set out in claim 1. The invention is especially advantageous where the molded material has a complicated shape, e.g. its surface has at least one concave region and/or at least one recessed region. In the present method, the inside of the rubber film is preferably made vacuum. Further, the powder-molded material is preferably made substantially of a ceramic or a metal. In the present invention, there can be used, as the rubber film, a commercial ice bag, a commercial semi-translucent rubber tube, etc. as they are. The ice bag refers to a bag used for cooling the affected part of a patient by placing ice chips or water therein. Suitably the rubber film has elasticity, i.e. it has a predetermined size when unstressed, as distinct from a plastically deformable latex. In the present invention, the surface area of the rubber film is larger than the surface area of the powder-molded material. This requirement cannot be met in the case of covering the powder-molded material with a latex, because the surface area of the resulting rubber film is equal to the surface area of the powder-molded material. Herein, the surface area of the rubber film is measured when the rubber film is tightly adhered to the powder-molded material. In view of, for example, the necessity of sealing the rubber film to make the film inside vacuum, the surface area of the rubber film must be larger than the surface area of the powder-molded material. Brief Description of the Drawings Fig. 1A, Fig. 1B and Fig. 1C are drawings showing powder-molded materials to which the present invention is applicable. Fig. 2 is a drawing showing an example of the application of the present invention to a powder-molded material. Fig. 3 is a drawing showing the notches of a powder-molded material to which the present invention is applicable. Fig. 4A and Fig.4B are drawings showing the throughholes of a powder-molded material to which the present invention is applicable. Fig. 5 is a drawing showing a dynamic blade of gas turbine to which the present invention is applicable. Fig. 6 is a drawing showing a static blade of gas turbine to which the present invention is applicable. Fig. 7A and Fig. 7B are drawings showing a rotor of turbine to which the present invention is applicable. Fig. 7B is a drawing when the rotor is viewed from below, and Fig. 7A is a sectional view of the rotor taken at the P-P' line of Fig. 7B. Fig. 8 is a conventional procedure used for covering a powder-molded material with a rubber. Detailed Description of the Prefered Embodiments The present invention is hereinafter described in more detail by way of Examples. However, the present invention is not restricted by these Examples. Examples 1-3 and Comparative Examples 1-3 Fig. 1 shows examples of parts to which the present invention is applicable. Each of Fig. 1A, Fig. 1B and Fig. 1C shows a powder-molded material having a cylindrical shape. In the powder-molded material of Fig. 1A, the side wall 10a surrounds a space extending from the end 12a to the end 14a in the axial direction, at the circumference of the space. Both the end 12a and the 14a are open. In the powder-molded material of Fig. 1B, the side wall 10b surrounds a space extending from the end 12b to the end 14b in the axial direction, at the circumference of the space. A notch 16b is formed at the end 12b, and a throughhole 17b extending in the diameter direction of the powder-molded material is formed in the side wall 10b. Both the end 12b and the end 14b are open. Needless to say, two or more notches or two or more throughholes may be formed. In the powder-molded material of Fig. 1C, the side wall 10c surrounds a space extending from the end 12c to the end 14c in the axial direction, at the circumference of the space. At the end 12c and the end 14c are provided a flange llc and a flange 13c each of ring shape, respectively. Both the end 12c and the end 14c are open. The flange 11c and the side wall 10c are in contact at the bent 18c, and the flange 13c and the side wall 10c are in contact at the bent 19c. There were mixed 100 parts by weight of a silicon nitride powder, 1.5 parts by weight of a SrCO 3 powder, 4.4 parts by weight of a MgO powder and 5.5 parts by weight of a CeO 2 powder all having an average particle diameter of 0.6 µm. 100 parts by weight of the resulting compound was mixed with 2 parts by weight of a polyvinyl alcohol (a binder). The resulting mixture was spray-dried to obtain a powder having an average particle diameter of 50 µm. The powder was molded, by the use of a press and a mandrel, to obtain a cylindrical molded material having a shape of Fig. 1A. Then, in order to remove the polyvinyl alcohol from the molded material, the molded material was subjected to degreasing at 500°C for 10 hours in an atmosphere consisting of 50% by weight of nitrogen and 50% by weight of air. The powder-molded material after degreasing was cut to form a notch 16b and a throughhole 17b to obtain a molded material having a shape of Fig. 1B. Incidentally, a dent may be formed in place of the throughhole, by cutting the side wall to an extent that no throughhole is formed. Separately, the portion of the side wall excluding the two ends was cut at the outside to form a flange 11c and a flange 13c to obtain a powder-molded material having a shape of Fig. 1C. These flanges may be formed by bonding. Next, each of the three molded materials was covered with a semi-translucent rubber tube. The tube had a thickness of about 1 mm. Fig. 2 is a drawing showing a state in which a powder-molded material 20 having a cylindrical shape is covered with a rubber tube 24. As the rubber tube 24, there was used, for example, a commercial semi-translucent rubber tube having high stretchability and elasticity. The powder-molded material 20 was inserted into the rubber tube 24; the rubber tube 24 was folded back, at one end 21 of the powder-molded material 20, into the inside (hollow portion) of the powder-molded material 20; and one end 26 of the rubber tube 24 was taken out from the other end 22 of the powder-molded material 20. Then, the one end 26 and an end 27 corresponding thereto were adhered to each other with a rubber paste. Degassing was conducted from other end 26 of the rubber film 24 by the use of a vacuum pump or the like to make vacuum the inside of the rubber film 24. Thereafter, the other end 26 and an end 27 corresponding thereto were adhered to each other with a rubber paste. "Vacuum" is necessary to allow the rubber tube 24 to tightly adhere to the surface of the powder-molded material 20. The adhesion between the end 26 and the end 27 is necessary to avoid the leakage of a pressure medium into the rubber tube 24 during isostatic pressing. The rubber tube 24 and the powder-molded material 20 were allowed to stand for about 5 minutes to confirm that there was no leakage from the rubber tube 24. Then, isostatic pressing was conducted at a pressure of 7,000 kgf/cm 2 . As the pressure medium, there was used a 50:50 (by weight) mixed solution consisting of water and ethylene glycol. Thereafter, sintering was conducted at 1,700°C for 3 hours in a nitrogen atmosphere of 9.5 atom. Meanwhile, in Comparative Examples 1-3, the production of each powder-molded material, the isostatic pressing step and the sintering step were the same as in Examples 1-3 but the method of rubber film formation was different. In Comparative Examples 1-3, a latex rubber was brush-coated in a thickness of about 0.2 mm according to the procedure shown in Fig. 8. In Comparative Examples 1-3, each molded material after isostatic pressing had several discolored areas each of 2-30 mm in diameter. At these areas, the latex rubber was broken and, as a result, the pressure medium penetrated into the molded material. The number of these areas was larger at the ends, notch, throughhole and bents than at the side wall. Fig. 3 shows notches 32 formed in a powder-molded material 30. Description is made on the.conditions under which the present invention is favorably applicable to such a powder-molded material 30. The height H of each notch 32 is preferably two times or less the width W of the notch, more preferably 1.6 times or less the width W because such a height enables easy adhesion of a rubber film to the bottom 34 of the notch 32. For the same reason, the distance I between two adjacent notches 32 is preferably at least the same as the width W of the notch 32, more preferably at least 1.5 times the width W, most preferably at least 2 times the width W. Fig. 4A and Fig. 4B show throughholes 42 formed in the side wall 40 of a powder-molded material. Description is made on the conditions under which the present invention is favorably applicable to such a powder-molded material. The thickness T of the side wall 40 is preferably 2 times or less the diameter D of each throughhole 42, preferably 1.6 times or less the diameter D, because such a thickness enables easy adhesion of a rubber film to the surface 42S of the throughhole 42. For the same reason, the distance I between two adjacent throughholes 42 is preferably at least the same as the diameter D of the throughhole 42, more preferably at least 1.5 times the diameter D, most preferably at least 2 times the diameter D. Examples 4-5 and Comparative Examples 4-5 As examples of small-sized powder-molded materials having a complicated shape to which the present invention is applicable, a dynamic blade 50 and a static blade 60 both of gas turbine are shown in Fig. 5. In Fig. 5, the dynamic blade 50 comprises a blade 52, a shroud 54 bonded to the blade 52, and a root 56 bonded to the shroud 54. The shroud 54 has a plate shape; on one side of the plate is provided the blade 52; and on other side of the plate is provided the root 56. The horizontal section of the blade 52 is streamlined and tapered along the flow of a fluid. In Fig. 6, the static blade 60 comprises a blade 62 and a pair of shrouds 64. The horizontal section of the blade 62 is streamlined and tapered along the flow of a fluid. There were mixed 100 parts by weight of a silicon nitride powder, 2 parts by weight of a SrCO 3 powder, 3 parts by weight of a MgO powder and 3 parts by weight of a CeO 2 powder all having an average particle diameter of 1.0 µm. To 100 parts by weight of the resulting compound was added 20 parts by weight of an organic binder. The organic binder had a melting point of 62°C and was composed mainly of a paraffin wax. The resulting mixture was subjected to injection molding to obtain a powder-molded material 50. The powder-molded material was completely buried in an inorganic powder placed in a stainless steel vat. The inorganic powder was composed mainly of cordierite, and was dried beforehand and had a water content of not higher than 1%. Then, the buried powder-molded material was subjected to degreasing. The degreasing step consisted of the following operation. Firstly, the powder-molded material was subjected to temperature elevation from room temperature to 60°C at a rate of 1°C/hr and kept at 60°C for 30 minutes. Then, the resulting material was subjected to temperature elevation from 60°C to 180°C at a rate of 2°C/hr and kept at 180°C for 20 hours. Lastly, the resulting material was subjected to temperature elevation from 180°C to 450°C at a rate of 3°C/hr, kept at 450°C for 10 hours, and cooled from 450°C to room temperature. The material after degreasing was subjected to machining (e.g. shaving off of edges). The resulting three powder-molded materials (three dynamic blades or three static blades) were placed in one ice bag. The ice bag had a thickness of about 0.2-0.3 mm. The inside of the ice bag was made vacuum by a vacuum pump or the like to an extent that each powder-molded material made no movement in the ice bag, after which the mouth of the ice bag was closed and sealed physically. Then, the ice bag was allowed to stand for about 5 minutes to confirm that the ice bag had no leakage. Owing to no leakage, an excellent yield was possible in Examples 4-5. Thereafter, isostatic pressing was conducted in the same manner as in Examples 1-3; the ice bag was removed; and sintering was conducted in the same manner as in Examples 1-3. Meanwhile, in Comparative Examples 4-5, the production of powder-molded material, the isostatic pressing step and the sintering step were conducted in the same manner as in Examples 4-5. However, only the formation of a rubber film was different; that is, a latex rubber was brush-coated according to the procedure shown in Fig. 8. In Comparative Examples 4-5, each powder-molded material after isostatic pressing showed discoloration at several areas (these areas had a diameter of 2-30 mm); at these areas, the latex rubber was broken and a pressure medium penetrated into the powder-molded material. The number of these areas was larger at the edges than at the side wall. Examples 6 and Comparative Example 6 In each of Example 6 and Comparative Example 6, a turbine rotor shown in Fig. 7A and Fig. 7B were produced by injection molding. The production process was the same as in Examples 4-5 and Comparative Examples 4-5 except for the difference in shape of molded material. In Fig. 7A and Fig. 7B, the turbine rotor 70 is provided about a shaft 79 which is a part independent from the turbine rotor 70. The turbine rotor 70 comprises a plurality of blades 72 extending outwardly from the shaft 79 in the shaft diameter direction. Each blade 72 is a curved plate. The results (including the yield after molding) of Examples 1-6 and Comparative Examples 1-6 are shown in Table 1. In the present invention, a powder-molded material having a complicated shape, for example, a powder-molded material having notch(es), throughhole(s), dent(s) or bent(s) or having a cylindrical shape is covered with a rubber film having a surface area larger than the surface area of the powder-molded material and then subjected to isostatic pressing. As a result, the breakage of the rubber film during isostatic pressing has been reduced significantly and the removal of the rubber film after isostatic pressing has become easy.

Claims: A method of preparing an isostatically pressed body, including the steps of: (a) moulding powder material into a shaped body (10a, 10b, 10c, 20,30, 40, 50, 60, 70), said shaped body having a complicated shape having at least one shape feature selected from notch, throughhole, hollow cylinder, ring, and recess having at least one re-entrant angle of 270°, (b) covering the shaped body (10a, 10b, 10c, 20,30, 40, 50, 60, 70) after its moulding with a water-impermeable rubber sheet package (24, 44) having a surface area larger than that of the shaped body, wherein said rubber sheet has a thickness in the range 0.2 to 1 mm, (c) subsequently evacuating the inside of the rubber sheet package covering the shaped body, (d) sealing the evacuated rubber sheet package covering the shaped body, and, (e) isostatically pressing the shaped body in the rubber sheet package. The method according to claim 1, wherein said shaped body has a complicated shape having at least one notch (32), wherein said notch has a height (H) which is no more than twice its width (W), and wherein the distance between said notch and a second notch, if present, is equal to or greater than the width (W) of said notch. The method according to claim 1, wherein said shaped body has a complicated shape having at least one throughhole (42), wherein said throughhole has a thickness (T) which is no more than twice its diameter (D), and wherein the distance between said throughhole and a second throughhole, if present, is equal to or greater than the diameter (D) of said throughhole. A method according to any one of claims 1 to 3, wherein the powder material is a ceramic material or a metal material.

Inventors: TSUTSUMI TAKETOSHI, TSUTSUMI, TAKETOSHI
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1995
Main classification: B22F3
IPC codes: B29C43, B28B21, B30B11, B22F3, B28B3