JPH0710526B2 - How to dry a green compact - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for drying a green molded body, and more specifically, fine powder, water and an organic binder (for example, hydroxypropylmethylcellulose (HPM).
The present invention relates to a method for drying a raw molded body formed by kneading a highly polymerized product of C) and water).

[0002]

2. Description of the Related Art In manufacturing a molybdenum disilicide-based ceramic heating element, a fine ceramic powder having a particle size of less than 1 μm is used as a binder of a high polymer of HPMC, and an appropriate amount of water is added and kneaded to form a shape. A method is adopted in which a kneaded product having a holding property is prepared, and the kneaded product is extruded into a rod-shaped molded product.

When the diameter of the molded product is 5 mm or more, the water distribution in the cross section becomes non-uniform, and generally, the water content in the central portion tends to be larger than that in the surface layer portion. This tendency becomes particularly remarkable when the diameter of the molded product is 10 mm or more, and further, the green density is generally higher in the surface layer portion than in the central portion. In such a green molded body, it is extremely difficult to surely remove the water in the central portion by natural drying. If the removal of water is incomplete, defects such as cracks will occur in the subsequent binder removal step.

[0004] In order to ensure the drying, if a fresh green body immediately after molding is dried with hot air of 80 ° C or more, a uniform drying speed is not obtained between the surface layer portion and the center portion, and further, HPMC of the binder is not used. When it reaches the temperature at which the viscosity changes, a rapid gelation causes a rapid shrinkage, which causes cracking.

As a simple drying method, there are the following methods. The green molded body is covered with a vinyl sheet and left for 2 to 3 days to make the water content in the molded body substantially uniform, and then the vinyl sheet is removed and left to stand in the atmosphere for 2 to 3 days for preliminary drying. Next, hot air drying is performed in a state where the surface layer portion is not completely dried and a moisture escape passage in the central portion is left. However,
According to this method, it takes a long time to dry, and the progress of preliminary drying depends on the season and atmospheric conditions, and it is difficult to stably obtain a good molded product from which water is completely removed.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and is capable of reliably drying a green compact in a short time,
It is an object of the present invention to provide a method for drying a green molded body, which can obtain a sound dry molded body without defects.

[0007]

According to a first aspect of the present invention, when drying a raw molded body formed by a kneaded product obtained by kneading fine powder, water and an organic binder, the raw molded body immediately after molding is subjected to an atmosphere. Maintaining high relative humidity (eg 80 to 90%) and gradually raising the temperature from room temperature (eg raising temperature to 40 to 50 ° C) so that the difference in water content between the surface layer and the center is substantially eliminated. And the relative humidity of the atmosphere is gradually reduced (for example, 50 to 60 ° C) while maintaining the molded body at a constant temperature (for example, 40 to 50 ° C).
%), And in this state, the second step of holding until the moisture in the molded body and the moisture in the atmosphere are substantially equilibrated, and the relative humidity of the atmosphere is further reduced (for example, to about 10%). The temperature of the molded body is lowered and the temperature of the molded body is raised (for example, to 60 to 80 ° C.), and this state is maintained until the moisture in the molded body and the moisture in the atmosphere are substantially equilibrated. And a method for drying a green molded body, the method including:

The second aspect of the present invention is to dry a green compact formed of a kneaded material obtained by kneading fine powder, water and an organic binder, and the humidity of the atmosphere is from 80 to 90% relative humidity to about 10 %. %, And a step of gradually raising the temperature of the green molded body immediately after molding from room temperature to 60 to 80 ° C. in the atmosphere, and relates to a method for drying a green molded body.

[0009]

EXAMPLES Examples of the present invention will be described below. First, an embodiment of the first invention will be described. Mixed powder of molybdenum disilicide (MoSi ₂ ) powder and molybdenum boride (α-MoB) powder (both particle size less than 1 μm) (mixing ratio is 85:15)
1 to 10% by weight (6.5% by weight in this example) as a binder of HPMC water-soluble polymer, and 5 to 15% of water.
% (13% by weight in this example) was added and kneaded. This kneaded product was made into a rod-shaped green (raw) molded body having a diameter of about 15 mm by using an extruder (screw type extruder).

Immediately after the molding, the surface layer of the molded body is covered with the film water, and when the film is exposed to the atmosphere, it starts to dry rapidly and the difference in water content between the surface layer and the inside becomes large, resulting in uneven shrinkage. It happens. Therefore, the green compact having the molding diameter was placed in a drying chamber kept at a relative humidity of 90%, and the temperature inside the drying chamber was raised from room temperature to 40 ° C. over 10 hours. During this period, rapid drying of the surface layer portion of the molded body was suppressed, and the difference in water content between the surface layer portion and the central portion (water content gradient) was made small enough to be ignored. Further, in this step, the internal stress generated by the extrusion molding is relaxed. Hereinafter, this step is referred to as a preliminary drying step.

Next, the relative temperature in the drying chamber kept at 40 ° C. is lowered from 90% to 60% until the moisture of the molded body and the moisture in the atmosphere reach a substantially equilibrium state.
The temperature and humidity were maintained constant for 30 to 40 hours (40 hours in this example). Hereinafter, this step is referred to as a constant rate drying step.

Next, the relative humidity of the atmosphere was lowered from 60% to 10% while gradually raising the temperature in the drying chamber to 80 ° C. It takes 10 to 20 hours (20 in this example).
Time). Hereinafter, this step is called a forced drying step.

The above-mentioned changes in the humidity and humidity of the atmosphere over time are as shown in FIG.

The dried molded body thus obtained has almost no water gradient in the molded body in the preliminary drying step.
Drying proceeds while maintaining this state in the next constant rate drying step, and most of the water is removed in the final forced drying step to complete the drying. The total time required for this drying is 50-70
It doesn't take a long time.

The dried molded body is subjected to binder removal processing and then sintered to form a rod-shaped sintered body. This rod-shaped sintered body had a diameter of about 12 mm, was 20% in size shrinkage due to drying and sintering, and was a good sintered body without defects. In addition, it is desirable that the molded body that has completed the forced drying step be kept in the state of the forced drying step until the next binder removal processing.

Next, an embodiment of the second invention will be described. The atmosphere may be such that the relative humidity is gradually decreased and the humidity is gradually increased throughout the entire process from the start of drying to the end of drying.

The green compact in this example was made under the same conditions as in the previous example. This molded body,
Immediately after charging, the drying chamber with a relative humidity of 90% is gradually reduced to a relative humidity of about 10%, and the atmosphere
The temperature was raised slowly to 60 ° C. from room temperature. Drying starting total duration leading to completion of drying from 50 to 70 hours (the same 70 hours as in the examples in this example).

The relationship between relative humidity and temperature of the elapsed time and the atmosphere is as shown in FIG. In this example, changes in relative humidity and temperature over time are substantially linear.

The state of the molded body after drying was the same as in the above example and was sound.

Although the embodiments of the present invention have been described above, the present invention can be applied even when the shape of the molded body and the molding method are other shapes and molding methods. Further, the powder material and the binder forming the molded body are not limited to the above. That is, the present invention is applicable to the drying of green compacts containing a considerable amount of water.

[0021]

According to the first aspect of the present invention, the first step of gradually raising the temperature of a green compact immediately after molding by keeping the atmosphere at a high relative humidity, and the constant atmosphere of this humidity are used. Since the second step of gradually lowering the relative humidity and the third step of further lowering the atmospheric humidity and heating and holding the molded body, the moisture inside the molded body in the first step is It becomes substantially uniform, the drying progresses in a state where the water content is uniform in the second step, and the drying is completed in the third step.

As described above, in the first step, the moisture inside the molded body becomes substantially uniform in a short time after the start of drying, and the drying proceeds while maintaining this state. The molded body is dried without causing defects, has high reliability, and the drying progresses efficiently, so that the drying does not take a long time.

The second aspect of the present invention is that the relative humidity is 80%.
Since it is gradually decreased from ~ 90% to about 10% and the temperature of the green compact immediately after molding is gradually raised from room temperature to 60-80 ° C in the atmosphere, The moisture inside the body becomes substantially uniform, and the same effects as the effects according to the first aspect of the invention can be obtained.

[Brief description of drawings]

FIG. 1 is a graph showing changes in humidity and temperature of an atmosphere depending on elapsed time in drying a molded body according to an example of the first invention.

FIG. 2 is a graph showing changes in the humidity and temperature of the atmosphere depending on the elapsed time in drying the molded body of the example of the second invention.

Claims (2)

[Claims] 1. When drying a raw molded body formed by a kneaded product obtained by kneading fine powder, water and an organic binder, the raw molded body immediately after molding is kept at a high relative humidity atmosphere. The first step of gradually raising the temperature from room temperature so that the difference in water content between the surface layer portion and the central portion is substantially eliminated, and gradually changing the relative humidity of the atmosphere while keeping the temperature of the molded body constant. The second step of lowering the water content and keeping it in this state until the water content in the molded body and the water content in the atmosphere are approximately equilibrated, and further lowering the relative humidity of the atmosphere and raising the temperature of the molded body. In this state, a third step of holding until the water in the molded body and the water in the atmosphere are substantially equilibrated, the method for drying a green molded body. 2. When drying a green compact formed by kneading a fine powder, water and an organic binder, the humidity of the atmosphere is gradually changed from 80 to 90% relative humidity to about 10% relative humidity. A method for drying a green molded body, which comprises a step of gradually lowering the temperature of the green molded body immediately after molding from room temperature to 60 to 80 ° C. in the atmosphere while the temperature is gradually lowered.