Method of removing binder from powder moldings

Abstract: A method of mixing a powder material and a binder in a predetermined ratio and kneading the mixture so that substantially whole surface of the powder particles are covered by the binder, molding the kneaded mixture into the shapes of desired products, then removing the binder from the moldings without causing expansion or cracks in the moldings before sintering. The binder comprises one or more organic binding substances soluble to an alcohol or to an mixed solvent of an alcohol and benzene or one or more ketones and one or more organic binding substances insoluble to the alcohol or to the mixed solvent. The moldings are put in contact with the alcohol or the mixed solvent to extract the organic binding substances soluble to the alcohol or to the mixed solvent from the moldings. The organic binding substances insoluble to the alcohol or to the mixed solvent are then removed by heat-decomposition. The dissolution of the organic binding substances soluble to the alcohol or to the mixed solvent can be controlled to an appropriate rate by the constituents of the mixed solvent and their ratio of mixture. By this method, it is made possible to remove the binder from the moldings made of powders with small particle sizes and large specific surface areas, which are difficult to remove the binder from without causing defects by the conventional methods, in short time without causing expansion or cracks in them.

Description: BACKGROUND OF THE INVENTION a) Field of the Invention The present invention relates to improved methods of kneading a powder of metal, ceramic, cerment, or other materials with a binder, molding the kneaded powder, and removing the binder from the moldings before sintering them to obtain products. b) Description of the Prior Art Injection molding is widely used as a method of forming complicate-shaped metal and ceramic products out of powders. In the injection molding, various organic compounds or thermoplastic resins are added to a powder to fluidize; the powder with a binder added to is hot-kneaded, and injection-molded; the binder is removed from the moldings obtained before sintering; and the moldings are sintered into products. In this process, removal of the binders is most important to obtain products without defects. There are two methods conventionally used for removing the binder from powder moldings: the heat decomposition method in which an organic binder is decomposed and gasified by heating, and the dissolution method in which an organic binder is extracted from moldings with an organic solvent. In the thermal decomposition method, rapid decomposition and gasification of an organic binder raise the internal pressure of moldings and thereby cause cracks or expansion in them. The decomposition and gasification of a binder therefore must be carried out gradually, and take a long time. Especially for a powder with a small particle size and a large specific surface area to which a larger amount of binder must be added to fluidize for injection molding, complicate techniques such as heating moldings step by step to higher temperatures are necessary for the removal of the binder, and the removal of the binder takes a longer time. Though addition of volatile substances to binders is thought of, those substances can evaporate during kneading and molding, and make it difficult to reuse the sprue and runner part of moldings. In the dissolution method, an organic binder is extracted from moldings with an organic solvent, and the pores left by the binder dissolved out serve as passages of decomposed and gasified binder. Therefore, remaining binder is easily removed by heat-decomposition without causing defects in the moldings. In the dissolution method, however, liquid materials such as mineral oils, fatty acid-derived oils, or natural oils are used in large quantities for a binder. But these oils can separate from powder during kneading and ooze out of moldings, so as to make it difficult to store the kneaded powder and moldings stable for a long time. Moreover, oils move to the surface of moldings during a storage and can cause cracks or expansion in the moldings during extraction. A method of dissolving out a plasticizer and a lubricant in an organic binder using hot water is also thought of. In this method, however, the stability of the kneaded powder containing them and the strength of moldings decrease as the proportion of the plasticizer and the lubricant added increase, whereas cracks and expansion can be caused by heat decomposition and gasification of the binder if their proportion is low. Disclosed in Japanese Patent Provisional Publication gazette No. 101101/1990 is another dissolution method in which water-soluble resins are used for a binder and they are dissolved out with water. This method has an advantage of using water lower in cost and safer to handle than an organic binder. Water, though, is inferior to an organic solvent in the permeating ability into moldings and evaporativity. Therefore, extraction of the binder and drying of moldings after the extraction take a longer time, especially for ceramic or metal powders with 10 m 2 /g or greater specific surface area to which a large quantity of an organic binder must be added. Moreover, when heating moldings to remove the water permeated into moldings, the problems of cracks and expansion can arise as a higher temperature is used. In addition, powders of materials which can rust in contact with water, such as iron and copper, and those of materials which react with water and generate gases, such as ceramic Si 3 N 4 which generates ammonia gas, cannot be used. SUMMARY OF THE INVENTION The object of the present invention is to provide an improved method of removing a binder from moldings formed of powders of ceramic, metal, cerment or other materials, especially from those formed of powders with small particle sizes and large specific surface areas which are difficult to remove a binder from by the above conventional methods, in a shorter time without causing defects. The method of the present invention comprises mixing a powder material and a binder in a predetermined ratio and kneading the mixture so that substantially the entire surfaces of the powder particles are covered by the binder, molding the kneaded mixture into the shape of a desired product, then removing the binder from the molding without causing expansion or cracks in the molding before sintering, wherein (a) said binder comprises (i) one or more first organic binding substances soluble in an alcohol and in an organic solvent selected from benzene, xylene, toluene, on or more ketones, and mixtures of benzene and one or more ketones, and (ii) one or more second organic binding substances insoluble in said alcohol and in said organic solvent; (b) said molding is contacted with a mixed solvent to extract from said molding said organic binding substances soluble in said mixed solvent; and then (c) said second organic binding substance, insoluble in said mixed solvent, and remaining in said molding, is removed by heat-decomposition, characterised in that the said mixed solvent consists of said alcohol and said organic solvent, the mixture ratio of said mixed solvent being determined so as to control the dissolution of the first organic binding substance to an appropriate rate so as to prevent cracking and collapsing of said molding. The present invention makes it possible to remove binder from moldings formed of powder with small particle sizes and large specific surface areas in a shorter time without causing defects. Further, these methods are applicable to wider materials than the above conventional method which uses water as the extraction medium. DETAILED DESCRIPTION OF THE INVENTION Organic binding substances insoluble in an alcohol and in an organic solvent as defined above include polyethylene, polypropylene, polystyrene, ethylene-vinyl acetate copolymer, nylon, paraffin wax, atactic polypropylene, or polybutyl methacrylate. Organic binding substances soluble in both an alcohol and an organic solvent comprising benzene, one or more ketones, or a mixture of them, include carboxy-vinyl polymer, polyethylene glycol, polypropylene glycol, vinyl acetate resin, polyethylene glycol monostearate, monoglyceride oleate, diglyceride oleate, polyoxyethylene sorbitan monolaurate, or sorbitan monolaurate, for example. Organic binding substances insoluble in the mixed solvent include polyethylene, polypropylene, polystyrene, ethylene-vinyl acetate copolymer, nylon, paraffin wax, atactic polypropylene, or polybutyl methacrylate. A preferred substance for the organic binding substance soluble to alcohol or in the mixed solvent is polyethylene glycol. Taking into consideration the solubility in the alcohol or in the mixed solvent, polyethylene glycol with molecular weight of 200 to 4,000 is desirable. By using polyethylene glycol of different molecular weights in mixture, the strength of moldings can be increased and its dissolution into the alcohol or the mixed solvent can be controlled. Ethylene carbonate, diethylene glycol, glycerol ether, and polyethylene glycol have good compatibility with ethylene-vinyl acetate copolymer, atactic polypropylene, polybutyl methacrylate, and polystyrene used as the thermoplastic resin insoluble in alcohol and in the mixed solvent, serve as a plasticizer and have the effects of lowering the molding temperature. In addition, they quickly dissolve in the mixed solvent at ordinary temperature, and therefore can be easily extracted from moldings with the mixed solvent without causing any problem. Next described in detail is the molding of a powder and removal of the binder from moldings by the method of the present invention. A ceramic or metal powder is mixed with a binder, the mixture is hot-kneaded, the kneaded mixture is formed into pellets, the pellets are used for injection-mold. The binder is then removed from the moldings by the following removal process. First, the organic binding substances are extracted using the mixed solvent. For this extraction process, methods such as putting the moldings in the solvent with continuous stirring, and spraying the solvent continuously over the moldings, are preferable to shorten the time taken for the extraction. After the extraction of the organic binding substances, the moldings are subjected to a debinding process, in which the remaining thermoplastic substances insoluble in the mixed solvent are removed by heating in ordinary pressure, a reduced pressure, or a vacuum in a debinding furnace. In the extraction of the organic binding substances, the dissolution rate can be controlled by the kind of alcohol or ketones used or by the mixture ratio of the alcohol, benzene, and the ketones. The following advantage results from this controllability of the dissolution rate: too rapid dissolution can be prevented by increasing the proportion of benzene or ketones to an alcohol when the powder used has a small average particle size and a large specific surface area and therefore the volume of the organic binding substances to be added is large compared to that of the powder, and when the molding is thin; the time required for the dissolution of the organic binding substances and the evaporation of the solvent thereafter can be made shorter by increasing the proportion of benzene or ketones to an alcohol when the powder has a large average particle size and has a small specific surface area and therefore the volume of the organic binding substances to be added is small compared to that of the powder, and when the molding is thick. After the binder is removed, the moldings are sintered into products. Example 13 parts by weight of polyethylene glycol as a thermoplastic resin soluble in an alcohol and an organic solvent, and 5 parts by weight of ethylene-vinyl acetate resin solvent and 3 parts by weight of atactic polypropylene resin as a thermoplastic resin insoluble in alcohol and in an organic solvent were added to 100 parts by weight of alumina ceramic powder (average particle size: 0.6 µm), and the mixture was kneaded at 140 °C for 60 minutes in a kneader. The mixture kneaded was ground and molded into cylindrical rods 6 mm in diameter by a screw-type injection mold. These samples were put in different alcohols or organic solvents or mixtures of them at ordinary temperature (about 25 °C) for two hours, then vacuum-dried for 30 minutes. In this extraction treatment, the dissolution rate in each solvent was measured, and the results were shown in Tables 1 to 8. Mixture ratio (% by vol.) Dissolution (%) Ethanol Benzene 100 0 19.5 80 20 23.5 50 50 Cracked 20 80 Collapsed 0 100 Collapsed Mixture ratio (% by vol.) Dissolution (%) Ethanol Acetone 100 0 19.5 80 20 22.6 50 50 27.7 20 80 31.5 0 100 31.9 Mixture ratio (% by vol.) Dissolution (%) Ethanol Xylene 100 0 19.5 90 10 20.8 80 20 21.3 50 50 Cracked Mixture ratio (% by vol.) Dissolution (%) Ethanol Toluene 100 0 19.5 90 10 22.2 80 20 23.9 50 50 Cracked Mixture ratio (% by vol.) Dissolution (%) Isopropyl alcohol Benzene 100 0 12.8 80 20 20.6 50 50 32.9 20 80 Cracked 0 100 Dissolved Mixture ratio (% by vol.) Dissolution (%) Isopropyl alcohol Acetone 100 0 12.8 80 20 18.6 50 50 25.7 20 80 30.2 0 100 Cracked Mixture ratio (% by vol.) Dissolution (%) Isopropyl alcohol Xylene 100 0 12.8 90 10 15.1 80 20 18.6 50 50 Cracked Mixture ratio (% by vol.) Dissolution (%) Isopropyl alcohol Toluene 100 0 12.8 90 10 14.7 80 20 18.2 50 50 Cracked

Claims: A method of mixing a powder material and a binder in a predetermined ratio and kneading the mixture so that substantially the entire surfaces of the powder particles are covered by the binder, molding the kneaded mixture into the shape of a desired product, then removing the binder from the molding without causing expansion or cracks in the molding before sintering, wherein (a) said binder comprises (i) one or more first organic binding substances soluble in an alcohol and in an organic solvent selected from benzene, xylene, toluene, one of more ketones, and mixtures of benzene and one or more ketones, and (ii) one or more second organic binding substances insoluble in said alcohol and in said organic solvent; (b) said molding is contacted with a mixed solvent to extract from said molding said organic binding substances soluble in said mixed solvent; and then (c) said second organic binding substance, insoluble in said mixed solvent, and remaining in said molding, is removed by heat-decomposition, characterised in that the said mixed solvent consists of said alcohol and said organic solvent, the mixture ratio of said mixed solvent being determined so as to control the dissolution of the first organic binding substance to an appropriate rate so as to prevent cracking and collapsing of said molding. The method of claim 1, in which said first organic binding substance is selected from carboxy-vinyl polymer, polyethylene glycol, polypropylene glycol, vinyl acetate resin, polyethylene glycol monostearate, monoglyceride oleate, diglyceride oleate, polyoxyethylene sorbitan monolaurate, and sorbitan monolaurate. The method of claim 1 or claim 2, in which said second organic binding substance is selected from polyethylene, polypropylene, polystyrene, ethylene-vinyl acetate copolymer, nylon, paraffin wax, atactic polypropylene, and polybutyl methacrylate. The method of any of claims 1 to 3, in which said powder material is a metal, ceramic or cerment. The method of any preceding claim, in which said first organic binding substance is a mixture of polyethylene glycols with molecular weights of 200 to 4000. The method of any preceding claim, in which said alcohol is selected from ethanol and isopropyl alcohol. The method of any preceding claim, in which said powder material has a specific surface area of at least 10 m 2 /g. The method of claim 7, in which said first organic binding substance is polyethylene glycol having a molecular weight of from 200 to 4000, said second organic binding substance is a mixture of atactic polypropylene and ethylene-vinyl acetate copolymer, said alcohol is selected from ethanol and isopropyl alcohol, and said organic solvent is selected from benzene, xylene, toluene and acetone.

Inventors: KANEKO YASUMARI, KANKAWA YOSHIMITSU, KASAHARA NORIO, KANEKO, YASUMARI, KANKAWA, YOSHIMITSU, KASAHARA, NORIO
Applicants: MITSUI MINING & SMELTING CO, MITSUI MINING & SMELTING CO., LTD.
Application year: 1992
Main classification: B22F3
IPC codes: B22F3, C04B35, B22F1