JP2001507075A - Method for producing sintered carbide alloy by powder injection molding - Google Patents

Abstract

  (57) [Summary] The present invention provides a method of grinding a powder mixture comprising a powder that forms a hard component and a binder phase, drying the ground powder mixture, and mixing the powder mixture with an organic binder, a wax and a surfactant. Into a feedstock, the feedstock being formed into an object of the desired shape in conventional plastic molding equipment, the binder phase being removed by one or more injection molding techniques, including removing the binder from the object and sintering. And a method for producing a sintered body including a hard component. If the surfactant has already been introduced during the attrition operation, the level of porosity of the object is considerably reduced.

Description

The present invention relates to a method for producing a cemented carb alloy by powder injection molding. Sintered carbide alloys are generally manufactured by powder metallurgy. This powder metallurgy process involves wet milling a powder mixture containing a hard component and a powder that forms a binder phase in an alcohol-water solution, drying the milled mixture by spray drying to obtain a good flow. A powder consisting of agglomerates having a diameter of about 0.1 mm, comprising compressing the dried powder into a body of a desired shape, and finally sintering. An alternative method of making a cemented carbide part is by injection molding. Injection molding is a common manufacturing method in the plastics industry for producing "final shape" or "nearly final shape" parts. A paste consisting of or containing a thermoplastic or thermoset polymer is heated to a suitable temperature and shaped through a nozzle into a shaped body having the desired geometry. When used in powder metallurgy, injection molding is commonly referred to as powder injection molding (PIM). Since PIM is a more expensive part manufacturing method than conventional powder metallurgy technology tool pressing, it is preferable to apply it to continuously manufacture a small number or a large number of parts having a complicated shape. The four main processing steps in powder injection molding are the following I-IV. I. The desired metal or ceramic powder is intimately mixed with an organic binder, with or without a combination of waxes and surfactants, for example, polyolefins such as polyethylene, polypropylene, copolymers, acetates, or polyacetals of these with acrylates. The resulting mixture is commonly called a feed. Surfactants are used as mixing aids. It facilitates the adhesion of the binder polymer to the metal or ceramic powder and allows to increase the amount of powder that can be incorporated into the polymer matrix. In addition, surfactants function as dispersants. That is, it is used as a measure to obtain a uniform powder dispersion throughout the final formulation of the mixture of the polymer component and the powder. In order to obtain the desired thermoplastic properties, the amount of binder must be 55 to 25% by volume and the amount of surfactant corresponds to at least one monolayer of surfactant on the metal / ceramic powder. Should. The mixing step is very important as it forms the basis for other processing steps. The binder must have suitable properties during molding and burn out easily before sintering without leaving any unwanted carbon or other undesired residues. As a result of the mixing, a formulation is obtained. The mixing should take place in the twin screw extruder at a temperature sufficiently higher than the melting section of the polymer components to ensure complete homogenization. II. The molding of the part into the desired shape is carried out with a conventional plastic injection molding machine. Depending on the polymer components used in the polymer matrix, the feed is heated to about 100-240 ° C. and placed into the cavity of the desired shape. After cooling, the molded part is extruded out of the cavity. III. Remove the binder from the molded part. This operation must be performed in such a way as to avoid cracks in the part. Removal of the binder can be accomplished in a number of ways. Generally, the binder is removed by heating or by extraction in a suitable solvent or by a combination of both. IV. Sintering is performed in essentially the same manner as for tool pressed parts. When trying to make a defect-free product, a thorough mixing process is fundamental. When mixing metal or ceramic powders with a binder, three different approaches I-III below are commonly used. I. Simultaneous mixing of all binder components In this case, in a Brabender or Haake type batch mixing apparatus, the binder components are mixed simultaneously with the powder in a molten state. Alternatively, the components are mixed simultaneously in an extruder. II. Dry Premixing of Powder and Surfactant In this case, the surfactant is dry blended with the powder in a batch or continuous mixing apparatus before mixing in a molten state. III. Dry premixing of powder and surfactant, and some binder and surfactant. In this case, the same or another surfactant is mixed with some or the rest of the binder component before dry blending the surfactant with the powder and mixing in the melt as in I and II. I do. As mentioned above, the cemented carbide alloy powder is ground. Milling may be necessary to obtain a uniform distribution of the binder phase in the milled mixture. The grinding operation is performed by a grinding machine of various sizes using a ground body of the sintered carbide alloy. The grinding time is several hours to several days. The attrition operation results in a slurry, which is then spray dried. Spray drying results in a powder consisting essentially of spherical aggregates of a size of about 0.1 mm. When the powder is mixed simultaneously with all the binder components as in I, there is a risk that the adsorption of the surfactant to the powder components and the adsorption of the surfactant to all the various binder components compete with each other. is there. This may result in poor wetting action at the most desired location, i.e. the surface of the powder. As a result of this poor wetting action, the miscibility of the polar metal powder with the non-polar binder is significantly reduced. In the case of II and III this is improved, but rather long mixing times are required to ensure that the surfactant disperses well and adheres equally to the powder. However, a disadvantage of mixing fine-grained powders in this way is that the agglomerates remain intact. As a result, the binder adheres to the powder agglomerates and thereby contributes to keeping the agglomerates together. Even if the agglomerates decompose into particles during the mixing step, homogenization is a challenge due to insufficient wetting of the powder by the binder. It is an object of the present invention to provide a method for promoting the dispersion of surfactants and ensuring that the surfactants adhere to particles and relatively small aggregates, not only to relatively large aggregates. It is. It has now surprisingly been found that the addition of surfactant during the grinding process of the cemented carbide alloy powder results in a sintered structure with improved properties. Compared to the prior art, the degree of porosity of the parts manufactured according to the invention is considerably reduced. According to the invention, a powder making the hard component (<0.5 μm to 10 μm) and a powder making the binder phase (3 to 20% by weight of the hard component) are used for conventional tool pressing. Grind with surfactant instead of lubricant added to the powder. In this case, the grinding liquid is assumed to be capable of dissolving or at least partially dissolving the surfactant. The amount of surfactant should be sufficient to cover the surface of all of the powder, and preferably the excess is no more than can be dissolved or miscible with the rest of the binder component Should. The surfactant is a single fatty acid, such as hexadecanoic acid, tetradecanoic acid, 9,10-octadecanoic acid, 9,12-octadienoic acid, or 9,12,5-acid mixed with the powder in ethanol, acetone and benzene. Octadecatrienoic acid may be used. Further, the surfactant may be certain organometallic compounds, Zn-stearate, or alcohols corresponding to fatty acids, such as 1-hexadecanol. It may be an amine, for example octadecylamine. All these surfactants can be ground in ethanol. As surfactants, Zn salts of high molecular weight unsaturated fatty acids having a melting point of 75-95 ° C., which can be milled with benzene, ethanol, xylol, or most of the high molecular weight unsaturated salts having a melting point of 97-105 ° C. Zn salts of saturated fatty acids can also be used. Preferably, the surfactant is a Zn salt of a high molecular weight, most unsaturated fatty acid or hexadecanol. After a sufficient amount of attrition, the resulting slurry is dried, preferably by spray drying. The dried powder is then mixed with the remainder of the binder at a temperature well above the melting points of these components. The composition of the binder component is a wax mixed with a polyolefin, that is, any one of EVA (ethene vinyl acetate), EBA (ethene butyl acrylate), EAA (ethene acrylic acid), PE (polyethylene), PP (polypropylene) or these May be paraffin wax or microcrystalline wax mixed with the combination of, or simply mixed with wax. A preferred binder is paraffin wax mixed with PP. After mixing, molding into parts having the desired shape is carried out in a conventional plastic injection molding machine. Preferably, the binder is removed from the molded part by extraction with a bath containing para-mentha-1,8-diene or methyl-ethyl-ketone and 2-propanol and subsequent drying by vacuum or heating. Finally, sintering is performed in essentially the same way as for tool pressed parts. The reason for the observed improvement is probably due to the fact that a homogeneous coating of the powder particles with the surfactant was obtained. Wet mixing of the powder with the surfactant while grinding the powder allows the surfactant to coat the powder particles, and this operation can break down most agglomerates. This allows the surfactant to coat each particle, rather than each aggregate. When the powder coated in the twin screw extruder is mixed with the polymer component in a later step, the risk of including powder agglomerates is minimized. Although the invention has been described with reference to sintered carbide alloy powder, it is clear that the invention can be applied to titanium-based carbonitride powders, often referred to as cermets. Example 1 (invention) 30 kg of WC powder with an average particle size of 1.3 to 2.9 μm and 3 kg of Co powder are mixed with 0.5 kg of cetyl alcohol and mixed in a 90:10 alcohol-water solution with 30 kg. Wet milled over time. The resulting slurry was spray dried to a powder. The spray dried powder was mixed with PP (polypropylene) and paraffin wax and pelletized in an extruder. The pellets were fed to a conventional extruder and formed into tangential inserts at a cylinder temperature of 125-165 ° C. The molded parts were freed of binder by first extraction in methyl-ethyl-ketone and 1-propanol and then by heating to a temperature of 400 ° C. with flowing H ₂ gas at atmospheric pressure. After the binder removal step, the part was sintered in a standard manner. Sintered parts, the porosity level in IS O4505 criteria were found to be present 1-2 per macropores 1c m ² is A00 + B02 + C00. Example 2 (invention) The procedure of Example 1 was repeated, except that 0.7 kg of stearic acid was ground together with the powder in an ethanol solution and then spray dried and mixed as in Example 1. The sintered parts were found to have a porosity level of A00 + B00 + C00 and 1-2 macropores / cm ² . Example 3 (invention) 24 kg of WC powder and 2 kg of Co powder with an average particle size of 3-4 μm are mixed with 0.17 kg of Zn salt of a high molecular weight unsaturated fatty acid, and then mixed with a 90:10 alcohol-water solution. For 22 hours. The resulting slurry was spray dried to a powder. The spray dried powder was mixed with PP (polypropylene) and paraffin wax and pelletized in an extruder. The pellets were fed to a conventional injection molding machine and molded into Q-cut inserts at a cylinder temperature of 150-170 ° C. The molded parts were freed of binder by first extracting in para-mentor-1,8-diene and then drying at 50 ° C. under vacuum. After the binder removal step, the part was sintered in a standard manner. The sintered part was found to have a porosity level of A00 + B00 + C00 based on IS O4505 and no macropores. Example 4 (Prior Art) Example 1 was repeated except that cetyl alcohol was added with the powder during the mixing step. The molded part was sintered together with the part obtained in Example 1. The sintered parts were found to have a porosity level of A00 + B02 + C00 with 8-10 macropores / cm ² .

Claims (1)

[Claims]   1. The powders that make up the hard constituents and the binder phase are mixed with organic compounds, waxes and surfactants. Mixed with a binder containing a surfactant to form a feed material, and the feed material is mixed with a normal plastic. Molding into an object of a desired shape with a back molding apparatus, removing the binder from the object, And one or more hard phases in the binder phase by injection molding techniques including sintering. A method for making a sintered body including constituents, wherein the powder forms a hard constituent and a binder phase. Wet milling with surfactant, drying and adding the rest of the binder A method characterized by the following.