METAL BINDER AND MOLDING COMPOSITION

Abstract: A metal binder comprising a base binder agent and an auxiliary binder agent which comprises at least one member selected from a group consisting of adamantane, trimethylene norbornane and cyclododecane and a molding composition comprising a metal powder blended with the base binder and the auxiliary binder agent which comprises at least one member selected from the group consisting of adamantane, trimethylene norbornane and cyclododecane.

Description: a. Field of the Invention This invention relates to a metal binder suitably usable in the injection molding for forming shaped articles from metal powder and a molding composition in which the metal binder is blended. In conventional powder metallurgy wherein powders of various metals (the word "metal" used here also includes an alloy and a sintered hard alloy) are molded and then sintered to produce sintered metal articles, the molding is generally carried out by press molding. However, the press molding is not satisfactory to form complicatedly-shaped objects. Besides, the press molding can not assure high sintering density and sufficient strength in the succeeding sintering step. To solve these problems, there has been proposed an injection molding in the field of metal powder molding and some proposals have actually been put into practice. The injection molding techniques for metal powders are disclosed, for example, in US-A-4,305,756, US-A-4,404,166, US-A-4,415,528, US-A-4,445,936, US-A-4,602,953, US-A-4,661,315. This injection molding technique can form complicated shapes and can be utilized for the molding of various objects. In addition, this technique has such advantages that the kneading, feeding and molding of the metal powders and a binder or binders are carried out in one process, a high molding accuracy can be attained and forming step can be omitted or simplified. In this connection, it is to be noted that binders used in the conventional powder metallurgy arts include polymeric materials such as ethylene-vinyl acetate copolymer, poly(meta)acrylate, polypropylene, plasticizers such as dibutyl phthalate, and waxes such as paraffin wax. Such binders are used also in the injection molding of the metal powders. These binders heretofore used, however, have such disadvantages that they are rather difficult to be debindered and it takes considerable time for them to be debindered, cracking or swelling are liable to occur when debindered, and high sintering density and high dimensional accuracy are not assured. Shaped articles formed by injection molding using a conventional binder have a further disadvantage that they can not retain their shapes by fluidity caused by softening of the articles unless debindering and sintering are carried out with the ariticles placed in powders. By these reasons, it is difficult to practically use the injection molding in the field of powder metallurgy, though the injection molding is known to be desirable and it is especially difficult, almost impossible, to utilize the injection molding for the production of precision sintered articles. On the other hand, ceramic binder using adamantane and/or trimethylene norbornane as an auxiliary agent is know in the field of the preparation of ceramic products. Thus, EP-A-206342 discloses a binder for ceramics, which contains an organic binder agent, and, as auxiliary binder agent, adamantane and/or trimethylene norbornane. In the known ceramic binder a preferred proportion of the organic binder agent is 0.5 to 50 parts by weight while a preferred proportion of the auxiliary binder agent is 0.1 to 15 parts by weight and 2 to 8 parts by weight. It is therefore an object of the present invention to provide a metal binder which is capable of effecting removal of the binder rapidly to reduce residue in the molded article and preventing possible occurrence of cracking and swelling in the step of debindering, assuring the quality of the resulting sintered articles and thereby enabling the injection molding to be practical in the field of powder metallurgy, and a molding composition in which the metal binder is blended with material metal powders. A metal binder of the present invention comprises a metal binder comprising a base binder agent and an auxiliary binder agent, said base binder agent comprising 1.2 to 6.1 parts by weight of a polymer, 1.8 to 4.7 parts by weight of a wax and 0.8 to 1.3 parts by weight of a plasticizer and said auxiliary binder agent comprising at least one member selected from a group consisting of adamantane, trimethylene norbornane and cyclododecane, and being blended in an amount of 0.5 to 5.0 parts by weight with said base binder agent. Further, a molding composition of the present invention comprises a metal binder as claimed in claim 1, in which said base binder agent comprises 2.0 to 3.4 parts by weight of ethylene-vinyl acetate copolymer, 1.2 to 2.7 parts by weight of an acrylic resin, 1.8 to 4.7 parts by weight of a wax and 0.8 to 1.3 parts by weight of dibutyl phthalate. As examples of metal powders preferably employable in the molding composition, there may be mentioned powders of metals belonging to group VIII of the Periodic table of the elements, for example, iron, nickel, cobalt, etc. or alloys containing these metals, for example, Cr-Ni-C or Fe-Ni. With the composition as described above, the debindering time in the production of the metal shaped article can be curtailed and cracking and/or swelling which would otherwise be caused in the debindering can be prevented. In addition, high sintering density and high dimensional accuracy are assured to improve the quality of the products, realizing provision of precision sintering parts in the field of powder metallurgy. Details of the invention will become apparent from the following description in connection with the enclosed drawing. In the drawing the only Figure is a block diagram showing a process for preparing sintered articles by using the metal binder of the present invention. The metal binder of the present invention comprises a polymer, a plasticizer and a wax as a base binder agent and at least one member selected from a group consisting of adamantane, trimethylene norbornane and cyclododecane as an auxiliary binder agent comprising a sublimable material. As examples of the polymer usable as the base binder agent, there may be mentioned polymers which can retain strength when molded and which can be easily decomposable and can be readily reduced to lower molecular-weight materials and removed easily in the debindering, such as ethylene-vinyl acetate copolymer, acrylic resin, polybutyl methacrylate, polyethylene, atactic polypropylene, etc. As examples of plasticizer, there can be mentioned phthalate plasticizers, for example, DBP (dibutyl phthalate) for plasticizing a polymeric component of the binder. As examples of wax, there can be mentioned paraffin wax, polyethylene wax, liquid paraffin, etc. Loadings of the polymer, plasticizer and wax in the base binder agent depend upon the material metal Preferably, 2.0 to 3.4 parts by weight of ethylene-vinyl acetate copolymer, 1.2 to 2.7 parts by weight of acrylic resin, 1.8 to 4.7 parts by weight of wax and 0.8 to 1.3 parts by weight of DBP are blended. Additives other than the plasticiser or wax, for example, a lubricant, may be added to the base binder agent. The metal binder of the present invention comprises the base binder agent as specified above and at least one member selected from a group consisting of adamantane, trimethylene norbornane and cyclododecane which is blended as an auxiliary binder agent. Components of the auxiliary binder agent, especially, adamantane are less toxic as compared with the conventional auxiliary binder agent such as naphthalene, camphor, etc. They have another advantage that they rarely produce carbonaceous products through reaction with other materials or self-decomposition. Therefore, contents of carbonaceous impurities in the final products can be reduced. In addition, tinting or coloring and sintering properties can be improved. When adamantane and trimethylene norbornane are both used, they are added in a ratio of 1 : 9 or more, preferably 1 : 9 to 4 : 1. The loading of the base binder agent and the auxiliary binder agent depends upon the material metals, but in general, the auxiliary binder agent is added in a preferred amount of 1.0 to 2.0 parts by weight based on 100 parts by weight of material metals. The so prepared metal binder is suitably used with powders of metals such as iron, nickel, copper, stainless steel, etc or alloys such as ferrite or sintered hard alloys such as WC, TiC, TaC/Co, Ni for preparing sintered articles. The metal powders as recited above include powders of alloys or sintered hard alloys partially mixed with ceramics. A process for preparing sintered article from metal powders using a metal binder of the present invention will now be described. A base binder agent comprising metal powders pulverized mechanically or by plasma and a plasticizer and/or a wax added if desired, and an auxiliary binder agent comprising adamantane composition are kneaded by a mixer (step 101). The kneading is carried out, for example, at a temperature of 80 to 150°C, preferably 100 to 120°C for 0.5 to 3 hours, preferably 0.5 to 1 hour. The kneading is effected by using a kneader which provides a shearing force, while applying heat, such as a Henschel mixer, a muller mixer, a blast mill, a hot kneader, a Ko-kneader, or the like. The blending procedures may be such that the primary binder agent is mixed well with the auxiliary binder agent to prepare the metal binder and the obtained metal binder is then blended with the metal powder or metal powders, or the metal powder or powders, the base binder agent and the auxiliary binder agent are blended all at once or sequentially. The so kneaded materials are then formed into granules by rolls or formed into pellets by a pelletizer to prepare a molding material (step 102). The resulting molding material is shaped into a desired shape by slip casting, pressure casting, press molding, jiggering, extrusion molding, rubber press (CIP), rumming, high-temperature press molding, injection molding, doctor blade (sheet forming), roller machine or the like. The molding material of the present invention is most suitably shaped by the injection molding. The molding material in which the metal binder of the present invention is blended can be molded well by injection under the conditions of low injection pressure and low injection temperature. Besides, this molding material can curtail the time required for removing the binder after injection molding. When the injection molding is employed, the molding material is supplied to a plunger type, preplasticizer type, or screw-in-line type injection molding machine to obtain a shaped objects by the injection molding (step 103). The injection molding is carried out, for example, at a temperature of 120 to 200°C under a pressure of 300 to 1500 kg/cm². The kneaded material of metal powders, base binder agent and auxiliary binder agent may be supplied to the injection molding machine, as it is, without being pelletized. Thereafter, the shaped objects are subjected to debinderizing to remove the metal binders (step 104). At this time, it suffices for the shaped objects to be only placed on a stand without being buried in powders. The debinderizing treatment is carried out at a temperature of 20 to 600°C for 20 to 120 hours, preferably 50 to 100 hours. At an early stage of the debinderizing step, sublimable materials of the auxiliary binder agent are removed, which makes removal of the remaining binder components easier, more uniformly and more rapidly. The metal binder of the present invention can be removed more rapidly and more completely as compared with the conventional metal binder to reduce the residue of the binder very much. The shaped objects are sintered after the debinderizing treatment to obtain sintered articles (step 105). The debinderizing step (104) and the burning step (105) may be carried out continuously. The molding composition of the present invention will now be described. The molding composition of the present invention comprises metal powders which are blended with a polymer, a plasticizer and a wax as a base binder agent and at least one member selected from a group consisting of adamantane, trimethylene norbornane and cyclododecane as an auxiliary binder agent comprising a sublimable material. The metal powders usable in the molding composition may be powders of group VIII metals such as iron, nickel, cobalt, etc. or powders of alloys containing such metals, for example, Cr-Ni-C, Fe-Ni, etc. The base binder agent and the auxiliary binder agent usable in the molding composition may be the same as those used in the metal binder of the present invention as described above. More particularly, the base binder agent usable in the present molding composition may comprise a polymer, a plasticizer, a wax, etc. and the auxiliary binder agent of sublimable materials which is usable in the composition may comprise at least one member selected from a group consisting of adamantane, trimethylene norbornane and cyclododecane. As examples of the polymer usable as the base binder agent, there may be mentioned ethylene-vinyl acetate copolymer, acrylic resin, polybutyl methacrylate, polyethylene, atactic polypropylene, etc. As examples of the plasticizer, there can be mentioned phthalate plasticizers, for example, DBP (dibutyl phthalate) for plasticizing a polymeric component of the binder. As examples of the wax, there can be mentioned paraffin wax, polyethylene wax, liquid paraffin, etc. EXAMPLES The invention will now be more particularly described, referring to examples in comparison with comparative examples, In the examples of the present invention, metal powders blended with the metal binders of the present invention were used as materials for forming shaped articles by injection molding. Table 1 shows formulation of metal powders, base binder agent and auxiliary binder agent, Table 2 shows conditions of molding, debindering and sintering, Table 3 shows evaluation of obtained sintered bodies. The base binder agents and the auxiliary binder agents as listed in Table 1 were charged in amounts as specified in the same table and they were kneaded by laboratory blast mill at a temperature of 100 + 10°C for 30 minutes. The torque was 150 kg cm. Then, the kneaded materials were broken by a manual press and further subjected to crushing to be formed into bulk materials having a particle size of 5 to 7mm. The obtained bulk materials were shaped by an injection molding machine (vertical, plunger type injection molding machine manufactured and sold by Yamashiro Seiki Kabushiki Kaisha) under the conditions as specified in Table 2. The resulting shaped bodies were subjected to debindering treatment under the conditions as specified in Table 2 and then sintered under the conditions as specified in the same table to obtain sintered metal products. The evaluation of the obtained sintered metal products (sintered bodies) were made in terms of appearance, density and residual carbon amount. Besides the results as summarized in the table, there were found the following: (1) When the inside of the obtained sintered metal products were inspected by X-ray photograph, no cracks were found in the products obtained according to the examples of the present invention, while some cracks were found in the products of the comparative examples. (2) Adamantane, trimethylene norbornane and cyclododecane, at least one of which were used as the auxiliary binder agent, only showed very low toxicity as a sublimable material. With the molding composition of the present invention in which the metal binder is blended in the metal powders, similar results were obtained.

Claims: A metal binder comprising a base binder agent and an auxiliary binder agent, said base binder agent comprising 1.2 to 6.1 parts by weight of a polymer, 1.8 to 4.7 parts by weight of a wax and 0.8 to 1.3 parts by weight of a plasticizer and said auxiliary binder agent comprising at least one member selected from a group consisting of adamantane, trimethylene norbornane and cyclododecane and being blended in an amount of 0.5 to 5.0 parts by weight with said base binder agent. The metal binder as claimed in claim 1, in which said base binder agent comprises 2.0 to 3.4 parts by weight of ethylene-vinyl acetate copolymer, 1.2 to 2.7 parts by weight of an acrylic resin, 1.8 to 4.7 parts by weight of a wax and 0.8 to 1.3 parts by weight of dibutyl phthalate. A molding composition comprising 100 parts by weight of a metal powder, a base binder agent and an auxiliary binder agent, said base binder agent comprising 1.2 to 6.1 parts by weight of a polymer, 1.8 to 4.7 parts by weight of a wax and 0.8 to 1.3 parts by weight of a plasticizer and said auxiliary binder agent comprising at least one member selected from a group consisting of adamantane, trimethylene norbornane and cyclododecane, and being blended in an amount of 0.5 to 5.0 parts by weight with said other components. The molding composition as claimed in claim 3, in which said base binder agent comprises 2.0 to 3.4 parts by weight of ethylene-vinyl acetate copolymer, 1.2 to 2.7 parts by weight of an acrylic resin, 1.8 to 4.7 parts by weight of a wax and 0.8 to 1.3 parts by weight of dibutyl phthalate. The molding composition as claimed in claim 3, in which said metal powder is made of a metal of group VIII or an alloy containing metal of group VIII of the Periodic table of the elements. Using a binder comprising a base binder agent and an auxiliary binder agent which comprises at least one member selected from a group consisting of adamantane, trimethylene norbornane and cyclododecane as a metal binder.

Inventors: HANAMURA, TOMOAKI, SAITOH, KATUYOSHI
Applicants: IDEMITSU PETROCHEMICAL CO. LTD.
Application year: 1988
Main classification: B22F3
IPC codes: B22F1, B22F3