JPH0420361B2 - - Google Patents

Description

Detailed Description of the Invention [Field of Industrial Application] This invention involves dispersing metal powder or ceramic powder in a liquid dispersion medium to form a slurry, casting the slurry, and then converting the remaining dispersion medium into a supercritical state. This relates to a method of extracting and removing with monochlorotrifluoromethane.

[Conventional technology]

For example, 2% Ni-98% Fe powder, SUS316 powder, metal powder such as stellite powder, ceramic powder such as alumina, silicon carbide, silicon nitride, zirconia, molybdenum carbide, titanium carbide, etc., tungsten carbide-cobalt carbide, carbide, etc. Titanium
There is a casting method as a method for obtaining a molded body from a mixed powder of ceramic such as nickel and metal.

In this method, raw material powder is dispersed in a liquid dispersion medium to form a slurry, and this slurry is molded into a liquid-absorbing mold that is formed by molding a plaster mold that has been treated with a mold release agent or other mold treatment. A part of the dispersion medium is absorbed into the mold to form an adhesive layer along the mold surface, which grows over time to ensure molding strength, and after demolding, the dispersion medium is removed. A molded body is obtained by heating and removing.

Since the above-mentioned water absorption phenomenon of the mold occurs continuously, the water content in the ink layer gradually decreases, and as a result, the cast body increases in hardness, contracts, and separates from the mold.

Once these steps are reached, the cast body is removed from the mold and, if necessary, green processed and dried. At the same time, the plaster that has absorbed water is also dried so that it can be used repeatedly.

The raw material powder used here is made into a slurry by pulverizing and mixing in a ball mill etc. with a dispersion medium consisting of water and a small amount of dispersant, etc., but in order to make a stable slurry, this mixing is necessary. It is obtained by stirring for several days, adjusting the moisture content and viscosity, and then defoaming in vacuum.

In such a manufacturing method, it is known that various alcohols can be used as the dispersion medium.

In addition, instead of plaster, plastics and metals, which have excellent mechanical strength and wear resistance, can be used as mold materials.
It is also known to use absorbent molds made of ceramics, fiber composite materials, ceramics such as silicon carbide, and the like.

However, the conventional casting method using metal powder or ceramic powder has the following problems.

(1) The process from casting to demolding takes a long time. The time required varies depending on the shape, dimensions, etc. of the desired molded object, but cannot be generalized, but for example, if a cylindrical object of 12 mmφ x 20 mm is obtained by casting a water-based slurry of alumina, it will take approximately 8 hours. It takes.

(2) It takes a long time to dry the molded body.

For example, if the above-mentioned molded product is molded and then subjected to air drying, it will take approximately 24 hours.

(3) A mold drying process is required.

Since this process requires a long time, it reduces productivity, and if possible, it is desirable to use a type that can be omitted.

(4) In the drying process, distortions and cracks are likely to occur in the molded product.

This is because when the dispersion medium evaporates from the surface of the molded object, pores are formed inside the molded object, and at the gas-liquid surface, gas-solid-liquid contact points, etc., forces due to capillarity act, causing adjacent particles to It acts to attract comrades together, and due to this phenomenon, shrinkage progresses from the surface layer, causing or easily causing distortion and cracks in the molded body.

In order to solve these problems, the present applicant has developed a method of converting metal powder or ceramic powder into a material whose main component is a substance that can be extracted with liquid or supercritical carbon dioxide and has a melting point of 0 to 100°C. The slurry is dispersed in a dispersion medium to form a slurry, and this slurry is cast into a non-liquid-absorbing mold, cooled and solidified to form a molded body, which is then demolded, and then molded using liquid or supercritical carbon dioxide. We applied for a method for extracting and removing the main part of the dispersion medium in the body (Japanese Patent Application No. 23863/1982).

According to this method, the molding time can be shortened from the conventional 8 hours to 1 minute, the mold drying process that takes 24 hours can be omitted, and it is also possible to prevent the occurrence of distortions, cracks, etc. in the molded product. It was an extremely excellent method.

[Problems to be solved by the invention] However, although the molding method has been improved in this way, there is still room for improvement regarding the following problems. .

In other words, when non-oxide ceramic powder such as nitride, carbide, or boride, or metal powder is used, oxidation of the surface may proceed due to contact with high-pressure carbon dioxide. When an oxide film is formed, in the case of non-oxide ceramic powder, the mechanical strength of the sintered body at high temperatures decreases, and in the case of metal powder, it impedes sinterability. was recognized.

Therefore, it has been desired to select a substance as an extractant that is not likely to oxidize the surface of the powder by converting into carbon dioxide.

[Means for solving problems]

The present invention was arrived at as a result of various studies to further improve the conventional slurry casting technology as described above. This invention relates to a method for forming metal powder or ceramic powder, which is characterized by extraction and removal using critical monochlorotrifluoromethane.

[Effect]

In the process of the invention, a substance is used as a dispersant which is extractable with monochlorotrifluoromethane at a critical pressure of 39 atmospheres or higher and has a melting point of 0°C to 100°C.

Examples of such substances include methyl carbonate (melting point: 0.5°C), t-butyl alcohol (melting point: 25.6°C),
°C), tetrachlorodifluoroethane (same temperature, 26
There are many substances such as stearyl alcohol (59℃), stearic acid (70.1℃).

In the method of the present invention, if a substance with a melting point that is too low is used, the production equipment for the low-temperature refrigerant for freezing and solidifying the slurry will be large-scale, which is undesirable.On the other hand, if the melting point is too high, the slurry will stick when the equipment is stopped. This is undesirable because it starts to show inconvenient phenomena such as difficulty in cleaning, and it is not desirable to reduce the melting point range to 0.
It is preferable to set it as ℃~100℃.

Furthermore, in the present invention, monochlorotrifluoromethane is used as a dispersion medium extractant instead of carbon dioxide in Japanese Patent Application No. 61-32863, which is a stable compound consisting of carbon, chlorine, and fluorine. Therefore, there is no risk of oxidizing the metal or ceramic surface.

By using this, the problems of the prior art can be solved.

If the viscosity of the slurry immediately before being cast into a non-absorbing mold is lower than a predetermined value, it will be easy to cast into a mold with a complex shape, but solid-liquid separation will easily occur, and the molded product will not be formed after the dispersion medium is removed. Therefore, it is desirable that the viscosity is 50 centipoise or higher.

On the other hand, if the viscosity is too high, solid-liquid separation becomes difficult to occur, but fluidity decreases, making it difficult to cast into molds with complex shapes. Therefore, it is desirable that the viscosity be 10 ⁴ centipoise or less.

The non-liquid-absorbing mold may be externally atmosphere cooled or may have an internal cooling medium circuit.

Mold materials include tool steel, wood, hard rubber,
Inexpensive materials such as aluminum and stainless steel can be used.

In order to improve the fluidity of the slurry and to give the molded product strength that can withstand handling after the main portion of the dispersion medium has been extracted and removed, binders, plasticizers, etc. Dispersants, wetting agents, lubricants, etc. may be added as appropriate.

As the binder, cellulose acetate butyrate, nitrocellulose, petroleum resin, polyethylene, polyacrylic acid ester, polymethyl methacrylate, polyvinyl alcohol, polyvinyl butyral, vinyl chloride, polymethacrylic acid ester, ethyl cellulose, abietic acid resin, etc. can be used. .

As a plasticizer, butylbenzyl phthalate,
Dibutyl phthalate, butyl phthalate, dimethyl phthalate, methyl agitate, mixtures of phthalate esters, derivatives of polyethylene glycol, tricresol phosphate, etc. can be used.

As a dispersant, glycerin trioleate,
Natural fish oil, synthetic surfactants, benzenesulfonic acid, oleic acid, methyloctadiene, etc. can be used.

Further, as wetting agents, alkylaryl polyether alcohol, ethyl ether of polyethylene glycol, ethyl phenyl glycol, polyoxyethylene ester, monooleate of glycerin, alcohols, etc. can be used.

As the lubricant, stearic acid, stearyl alcohol, etc. can be used.

In addition, the pressure of monochlorotrifluoromethane during the extraction operation must be at least 39 atmospheres or higher, which is a critical pressure in order to give the extractant the ability to dissolve it. Since energy is wasted unnecessarily without increasing the pressure, preferable results are likely to be obtained by setting the upper limit to approximately 500 atm, usually around 300 atm.

It has been shown that pressurizing the slurry and injecting it into the mold without solid-liquid separation is extremely effective for slurries with high viscosity and high powder concentration, and therefore with a low amount of dispersion medium that needs to be removed. However, in the range of 40 to 75 volume % that can achieve imaging, a pressure of 10 Kg/cm ² or less is desirable from the viewpoint of preventing solid-liquid separation.

[Embodiments of the invention]

A slurry of silicon nitride powder (Si ₃ N ₄ ) was prepared according to the following procedure.

First, 92.0 parts by weight of Si ₃ N ₄ and sintering aid
6.0 parts by weight of Y ₂ O ₃ and 2.0 parts by weight of Al ₂ O ₃ were gradually added little by little to 51.1 parts by weight of tetrachlorodifluoroethane containing 0.5 parts by weight of oleic acid as a dispersant, and mixed in a ball mill for 24 hours.

The slurry thus prepared was transferred to a closed stirring tank, the temperature was adjusted to 30°C, and the mixture was stirred and mixed for 2 hours, and then the slurry was degassed by suction from the upper space of the tank using a vacuum pump.

On the other hand, a stainless steel mold with a flat cavity of 70 mm x 100 mm x 10 mm and a built-in cooling water pipe was cooled with cooling water at 5°C, while a pressure of 7 kg/cm ² G was applied to the upper space of the slurry tank. The slurry was injected into the mold through a feed pipe whose temperature was controlled to 30°C by applying nitrogen gas, and while maintaining the pressure at 7 kg/cm ² G for 1 minute, the nitrogen was exhausted and the pressure on the slurry was returned to the atmosphere to form a flat plate-shaped molded body. I took it out.

The plate-shaped molded body thus obtained was held in an extraction tank, and monochlorotrifluoromethane at 35°C was supplied to the extraction tank while adjusting the pressure to 200 atm with a pump.
The tetrachlorodifluoroethane in the molded body was extracted and removed by holding for 2 hours, and then the supply of monochlorotrifluoromethane was stopped and the mixture was heated for 1 hour while exhausting.
Return to atmospheric pressure over time, then use a vacuum pump to
The mixture was evacuated to 10 ^-2 Torr, then nitrogen was supplied to bring the pressure to atmospheric pressure, and the compact was taken out.

There were no cracks, cracks, deformation, or other changes observed in the molded product after the dispersion medium removal operation as described above, and the difference in weight between the molded products before and after the removal of the dispersion medium was due to the amount of tetrachlorodifluoroethane used. It was found that they were almost compatible.

〔Effect of the invention〕

Trifluoromethane used in the present invention has a structure that does not contain oxygen in its molecules and is chemically stable, and there is no possibility of oxidizing the raw material powder, so it completely eliminates the disadvantages caused by oxidation of the raw material powder. It has the effect of being avoidable.

Claims (1)

[Claims] 1. A method for forming metal powder or ceramic powder, which comprises casting a slurry of metal powder or ceramic powder and then extracting and removing the dispersion medium with supercritical monochlorotrifluoromethane.