JPS6246969A - Method of dewaxing ceramic formed body - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for degreasing a ceramic molded body containing an organic binder. The degreasing method of the present invention can be used in the automobile industry, etc., which requires large quantities of ceramic sintered bodies with relatively complex shapes.

[Background Art] In recent years, the use of ceramic materials in automotive parts, heat-resistant materials, electronic materials, etc. is rapidly expanding, and the shapes of the products are also becoming more complex. Therefore, injection molding or extrusion molding methods, which mold a mixture of an appropriate organic substance and ceramic material, have been adopted as methods for accurately and efficiently molding ceramic materials into molded products with complex shapes. There is. However, the molded article molded according to the method described above must be removed after molding, that is, degreased.

Conventionally, degreasing of ceramic molded bodies has been carried out by heating the molded bodies in air, in an inert gas atmosphere, or in vacuum.

[Problems to be Solved by the Invention] In the conventional degreasing method described in -F, the viscosity of the organic binder first decreases due to heating. When the temperature of the molded body exceeds a certain value, the organic binder starts decomposing and decomposition gas is generated. Therefore, the volume of the molded body expands due to the pressure of the gas, which, combined with a decrease in shape retention due to the decrease in the viscosity of the binder, causes problems such as cracking, swelling, and swelling of the ceramic molded body. Ta.

The above problems can be solved by lowering the pressure of the cracked gas. For example, this can be solved by raising the temperature of the molded body extremely slowly at 1 to 2"C/Hr. However, this method has the disadvantage that degreasing takes a very long time and is not efficient. In order to degrease a cylindrical ceramic molded body with a height of 30 mm without generating defects such as cracks and swelling, it takes more than 150 hours, depending on the type of ceramic powder, particle size, and type of binder. Sometimes it takes a long time of 300 to 400 hours.

A method of heating the molded body in a pressurized atmosphere is also used. The relationship between the pressure and volume of cracked gas is approximated by the gas equation of state (PV-n RT). For example, when the pressure increases from 1 atm to 5 atm, the volume of the gas becomes 115 if the temperature is the same. . Therefore, by heating in a pressurized atmosphere, the volume of the decomposed gas can be reduced, and the defects described above can be prevented. However, with this method, due to the phenomenon that the decomposition temperature of the binder increases as the pressure increases, as shown in Figure 1, if pressure is continued until degreasing is completed, a considerably high temperature is required, and the equipment However, there are problems in that it is expensive and takes a long time to cool down. Furthermore, the binder may be in a carbonized state, remaining in the form of carbon inside the molded body after degreasing, causing it to turn brown during firing or reacting with the molding material to produce unnecessary compounds. For this reason, there were problems in that predetermined electrical and magnetic properties could not be obtained, and that the strength and durability of 3i 3N4 at high temperatures in particular deteriorated.

The present invention has been made in view of the above-mentioned problems, and it is an object of the present invention to provide a degreasing method that prevents the occurrence of defects in molded articles as described above and requires a relatively short degreasing time.

[Means for Solving the Problems] The degreasing method of the present invention involves heating a ceramic molded body obtained by molding a mixture of a molding material made of ceramic powder and a binder made of an organic substance, and degreasing the molded body from the molded body. In the method for degreasing a ceramic molded body in which the binder is removed, from the start of substantial decomposition of the binder until the degreasing M reaches a critical volume % in the range of at least 20 to 30 volume % of the binder. In the first region, the molded body is heated under a pressure atmosphere of ) Jl, and in the second region where the amount of degreasing exceeds the critical volume %, the molded body is heated under a pressure atmosphere lower than the pressure in the first region. It is characterized by “doing.”

The ceramic molded body used in the degreasing method of the present invention is manufactured by injection molding a mixed raw material containing ceramic powder and a Ti binder mainly composed of resin. To manufacture this ceramic molded body, the same injection molding method as the method for producing conventional ceramic molded bodies can be used as is.

The ceramic powder is, for example, silicon nitride (S+3N4),
Nitrides such as aluminum nitride (AIN>, boron nitride (BN>), silicon carbide (S i C>, titanium carbide (T
+C) carbides, alumina (A, 2Zo3>, zirconia (Zr02), :]-gelite, barium titanate (BaTi03), barium oxide (Bad)
, titanium tungsten (TiOz), or silicates, titanium boride (TiBz), zirconium boride (Zr
The same materials as conventional ones, such as borides such as Bz) or oxynitrides such as Sialon, can be used.

As the organic binder, thermoplastic resins such as polystyrene, atactic polypropylene, ethylene-vinyl acetate copolymer, polyethylene, polyacetal, and acrylic can be used as in the conventional case. In addition to the above resin, mineral oil, aliphatic compounds such as paraffin, aromatic compounds such as naphthalene, surfactants such as stearic acid, and other additives may also be used.

As a result of intensive research, the present inventors have discovered the relationship between the rate of temperature rise under various pressure atmospheres, the decomposition of the organic binder, and the failure of the molded article, and have completed the present invention.

That is, the greatest feature of the present invention is that the degreasing amount is at least 20 to 3% of the total amount of the organic binder from the start of substantial decomposition of the organic binder.
The temperature in the first region is increased in a pressurized atmosphere until it reaches a critical volume % in the range of 0 volume %, and the temperature in the second (rI region) exceeding the critical volume % is increased from the pressure in the first region. The reason is that it is carried out under a low pressure atmosphere.

Here, the critical volume % means that the pores created by the decomposition of the organic binder in the molded object communicate with each other and allow the decomposed gas inside the molded object to escape to the outside without causing abnormalities in the molded object. It means the minimum amount of fat removed that is possible. This critical volume % varies depending on the type of organic binder, but is generally in the range of 20 to 30 volume % of the total amount of organic binder. For example, the inventors attached plate-shaped injection molded bodies with various degreasing rates to one end of an air flow path and measured the time it took for a certain amount of air to pass through each molded body, as shown in Figure 2. Furthermore, it was found that when the degreasing rate exceeds about 25%, the air passage time becomes rapidly shortened.

This indicates that the cavities formed by the removal of the binder do not communicate until the degreasing rate is about 25%, but when the degreasing rate exceeds about 25%, communicating pores are formed. There is.

Further, when the organic binder contains a low molecular weight substance, the weight may decrease due to evaporation of the low molecular weight substance even at a low temperature, such as from room temperature to 60 to 120° C., before the start of thermal decomposition. The term "substantial decomposition" as used in the present invention includes such transpiration and the like.

The features are explained below.

In the region from room temperature to the actual decomposition temperature of the organic binder, the viscosity of the organic binder decreases as the temperature rises, but the vapor pressure of the organic binder is low and it does not substantially gasify. . For this reason, even if the heating rate is increased within this range, there will be little trouble due to decomposition of the organic binder.

Also, there is no influence of atmospheric pressure. Therefore, the temperature distribution of the molded body becomes non-uniform, and the temperature can be increased to an extent that does not cause deformation or cracking due to thermal strain, for example, 5 to b. Further, the pressure of the atmosphere may be reduced pressure, increased pressure, or atmospheric pressure.

In the first region where the temperature of the molded body is from when the organic binder starts to substantially decompose until the amount of degreasing reaches at least a critical volume %, the viscosity of the organic binder further decreases, and part of the organic binder or All of them begin to thermally decompose and gasify on the surface of the compact. However, in this first region, the strength of the compact is weak because the viscosity of the organic binder is reduced, and the pore passages through which the decomposition gas generated inside the compact escapes to the outside are hardly connected. Therefore, in the past, if the temperature rose a little too quickly, it would deform, and cracks and bulges would occur due to the high internal gas pressure. Therefore, in the present invention, the temperature of this first region is increased under a pressurized atmosphere.

This pressurization can be applied at a pressure of 2 to 100 atmospheres, preferably 4 to 100 atmospheres. If it is lower than 2 atm, the effect of pressurization will not be fully utilized, and if it is higher than 100 atm, the equipment for pressurization will be large-scale from a safety standpoint, which will cause problems in terms of cost and other issues. Note that the pressure in this first region may be constant, or this region may be further divided into smaller parts, and the pressure from the actual start of decomposition until the amount of degreasing reaches a predetermined volume % can be adjusted from the predetermined volume % to the critical volume %. The pressure can be divided into multiple stages, such as increasing the pressure to a level greater than .

By increasing the pressure in this first region, the temperature increase rate in this region can be increased, for example, 2 to 20 times the conventional rate, and the degreasing speed is significantly improved. The rate of temperature increase must be determined by taking into account the shape, size, thickness, etc. of the molded body, the type of molding material, the type of organic binder, etc.

In the second region where the amount of degreasing exceeds at least the critical volume %, most of the cavities formed after the organic binder is vaporized communicate with each other to form communicating pores.

Furthermore, since the ceramic powder occupies a large proportion of the organic binder in the entire molded body, the molded body itself becomes difficult to deform even if the viscosity of the bound binder is low. Therefore, the thermally decomposed gas easily diffuses from the surface of the compact through the communication holes, and high gas pressure is not generated inside the compact. Therefore, even if the atmospheric pressure is lowered, cracks and swelling will not occur.

It should be noted that the higher the pressure, the faster the temperature increase rate can be, and a larger amount can be degreased.

However, as shown in FIG. 1, as the degreasing rate increases, high temperatures are required, causing problems such as carbonization of the binder, heat resistance of the equipment, and energy issues. Therefore, in the present invention, degreasing is performed at least in the second region exceeding the critical volume % at a lower pressure than in the first region.

The pressure in the second region may be a constant value lower than the pressure in the first region, or the pressure may be gradually reduced. It is also possible to reduce the pressure. At the same time, it is desirable to take measures such as back-heating termination or cooling to cope with the decrease in decomposition temperature caused by the decrease in pressure. This is especially necessary when degreasing is carried out in the atmosphere. This prevents heat generation due to rapid decomposition of the binder and prevents carbonization of the binder.

In addition, when the molded body is a complex structure having a thin wall part and a thick wall part, it is possible to divide the first region and the second region at the time when the amount of degreasing of the thick wall part reaches a critical volume %. is necessary. Degreasing progresses faster in thin-walled areas than in thick-walled areas, and when the amount of degreased in thin-walled areas reaches the critical volume %, the amount of degreased in thick-walled areas may not reach the critical volume %, and the molded product Problems such as swelling and clutter begin to occur. Therefore, when there is a considerable difference in the thickness of the molded bodies, it is also preferable to shift from the first region to the second region when the amount of degreasing reaches 30 to 50% by volume on average.

The atmosphere during degreasing may be air or a non-oxidizing atmosphere, and can be selected in consideration of the type of organic binder used and the like. Depending on its shape, the molded body may be degreased by being stored in a container with a support or the like provided, or it may be degreased by being embedded in a degreasing material such as ceramic powder.

The molded body obtained by the degreasing method of the present invention and from which the organic binder has been removed is eroded and fired to form a fired body. This firing is carried out by heating to the firing temperature of the ceramic powders constituting the molded body, and sintering the ceramic powders together. Note that ceramics such as nitrides and carbides that undergo oxidative decomposition when heated in an oxidizing atmosphere need to be fired in a non-oxidizing atmosphere such as nitrogen gas, sometimes in a vacuum.

[Operations and Effects of the Invention] The present invention provides that when the organic binder is decomposed and removed by heating from a molded body consisting of an organic binder and ceramic powder, cavities are formed when a critical volume % of the organic binder is degreased. It is based on the clarification of the facts that are connected.

In the first region where defects are likely to occur in the molded body, degreasing is performed in a pressurized atmosphere, thereby making it possible to shorten the degreasing time. Further, in the second region where defects are less likely to occur in the compact, degreasing is performed under a pressure atmosphere lower than the 19th rI region to prevent carbonization of the binder and prevent defects in the compact after firing.

Ru. Furthermore, since the decomposition temperature is lower in the second region, the Sonetsuzan can be kept low, resulting in energy savings.

[Example] ' (First example) 84 overlapping parts of silicon nitride powder with a particle size of 0.6μ, 6 parts by weight of polyethylene wax, and 6 parts by weight of attack polypropylene
Parts by weight of paraffin wax, 2 parts by weight of paraffin wax, and Diocchisiftale-82 heavily affected parts were heated and kneaded at 160°C, and molded into a bellet for injection molding, and a diesel engine pre-chamber body in the shape of a cylinder with a bottom was molded by injection molding. . This molded body is 2
As shown in Table 1, in a nitrogen gas atmosphere,
Under the conditions of 20"C/Hr, the pressure of the atmosphere in the first region of room temperature 20°C to 4'OO°C was 7 atm, and the temperature was 400°C to 500°C.
Degreasing was performed by lowering the pressure of the atmosphere in the second region at a temperature of 7 atm to atmospheric pressure at a constant rate. The total degreasing time per molded body was 24:1. Here, 2
The first head area from 0℃ to 400℃ is heated to 20℃ under a pressure of 7 atmospheres.
When the temperature is increased at a rate of 1r/t, the organic binder
Preliminary experiments conducted in advance have shown that % by volume is degreased.

As shown in Table 2, the yield rate of the 20 molded bodies obtained was 95%, and the yield rate of the fired bodies obtained by firing at 1750°C for 2 hours using a conventional method was as follows. 89%
It was favorable.

The yield rate was determined by visual inspection of the surfaces of the molded body (degreased body) and fired body after degreasing and by X1m transmission method. In addition, the non-defective rate of fired bodies was determined based on the degreased body in which no defects were observed.

(Second Example to Sixth Example) Two molded bodies, each of which was the same as that molded in the first example, were
Degreasing was performed under the conditions shown in Table 1 using 0 pieces each. Then, the non-defective product rates of the degreased body and the fired body obtained by firing in the same manner as in the first example were determined in the same manner as in the first example, and the results are shown in Table 2.

(First Comparative Example) Using 20 molded bodies identical to those used in the first example,
As shown in Table 1, the temperature is 7 atm constant and the V temperature is 0℃~
Degreasing was carried out up to 500° C. at a temperature increase rate of 20° C./t]r. Then, the non-defective product rates of the degreased body and the fired body obtained by firing in the same manner as in the first example were determined in the same manner as in the first example, and the results are shown in Table 2.

(Second Comparative Example) Degreasing was carried out in the same manner as in the first comparative example, except that the degreasing conditions were a constant 5 atm and a temperature increase rate of 10° C./Hr. Then, the non-defective product rates of the degreased body and the fired body obtained by firing in the same manner as in the first example were determined in the same manner as in the first example, and the results are shown in Table 2.

(Third Comparative Example) The conditions during degreasing were constant atmospheric pressure, 20°C to 500°C.
Degreasing was carried out in the same manner as in the first comparative example except that the temperature increase rate was 3°C/hr. Then, the non-defective product rates of the degreased body and the fired body obtained by firing in the same manner as in the first example were determined in the same manner as in the first example, and the results are shown in Table 2.

Table 1 (margin below) Table 2 (test examples) Using the fired bodies of the first example, the sixth example, and the second comparative example, each was held at 1000°C for 1 hour, and then at air temperature for 15 minutes. A temperature cycle test was conducted for 1000 cycles in which holding was considered as one cycle, and mechanical strength was determined based on the difference in fracture temperature due to quenching in water. As shown in Table 2, the difference in fracture temperature due to quenching in water was 380°C and 360°C in the first example and the sixth example, while it was 280°C in the second comparative example.

(Evaluation) It is clear from Table 2 that the manufacturing method of the present invention requires only a short degreasing time, and the obtained degreased bodies and fired bodies have an excellent yield rate. In addition, in terms of mechanical strength, no abnormality was observed in the fired product of the example, whereas the mechanical strength of the fired product of the second comparative example was significantly reduced, although the yield rate was excellent. There is. The reason for this is that in the fired body of the comparative example, S+C (silicon carbide) is generated due to residual carbon, and SiC and SI
It is presumed that thermal stress due to the difference in thermal expansion with 3N4 acted during the temperature test cycle, causing the strength deterioration.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between temperature and degreasing rate under various pressure atmospheres. FIG. 2 is a graph showing the relationship between the degreasing rate of a molded article and the time for air to pass through the molded article having each degreasing rate. Patent applicant Toyota Central Research Institute Co., Ltd. Agent
Patent Attorney Hiroshi OkawaFigure 1

Claims (2)

[Claims] (1) A method for degreasing a ceramic molded body, which involves heating a ceramic molded body obtained by molding a mixture of a molding material made of ceramic powder and a binder made of an organic substance, and removing the binder from the molded body, In the first region from the start of substantial decomposition of the binder until the amount of degreasing reaches a critical volume % in the range of at least 20 to 30 volume % of the total amount of the binder, the molded body is heated under a pressurized atmosphere. However, in the second region where the amount of degreasing exceeds the critical volume %,
A method for degreasing a ceramic molded body, comprising heating the molded body under a pressure atmosphere lower than the pressure in the first region. (2) The method for degreasing a ceramic molded body according to claim 1, wherein heating is performed while gradually reducing the pressure in the second region.