JPH0524876B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical field] Belongs to the field of ceramic green manufacturing technology. More specifically, it belongs to the field of uniform drying technology for ceramics that can obtain greens with desired shrinkage characteristics. Note that the meanings of technical terms used in this invention are as follows. Clay: Clay containing water before being molded. Hydrated green: A ceramic molded product containing water before drying. Green: A ceramic molded product that does not contain water after drying. Fired product: The final product obtained by firing green. Shrinkage...represents shrinkage during firing unless otherwise specified. [Background Art] Ceramic products have come to be used in a very wide range of applications, and accordingly, demands for improved performance in various properties have become increasingly severe. Particularly in the case of ceramics, when fired after being molded into the desired shape, approximately
Causes dimensional shrinkage of 10-20%. Since this dimensional shrinkage largely depends on the green adjustment method, including molding, various ideas have been incorporated into the adjustment method. For example, in the case of a hybrid IC substrate,
Because ICs, etc. are assembled on the board on an automated line, the dimensional accuracy is within ± of the desired dimensions.
A very high level of 0.25% or less is required. The reason why the dimensions become unstable and the variation becomes large is due to the large variation in green density. Immediately after molding, the water content is high, so the drying process is used to bring it to an almost bone-dry state, but the dehydration method at that time creates density and density within the molded product, so the shrinkage rate may vary or, in extreme cases, Cracks may occur. Therefore, uniform drying is an essential condition for obtaining ceramic products with high dimensional accuracy. Conventionally, far infrared rays and hot air drying have been used to dry hydrated greens, and microwave drying has also been used in some cases. In the first two cases, heat from a far-infrared heater or hot air is transferred to the ceramic surface, and the interior is heated by the heat transfer and drying progresses. Therefore, the drying efficiency is poor, and since the vicinity of the surface is heated first, the drying shrinkage of the organic binder on the surface causes the surface to become resistant to the movement of water, and the drying rate is rather reduced. Further, microwave drying does not have the above-mentioned problems because it heats the ceramic from within, but it does have the disadvantage that it heats up rapidly, resulting in an uneven green. Therefore, there was a need to provide a uniform drying method to obtain greens with predictable firing shrinkage characteristics. OBJECTS OF THE INVENTION It is an object of the present invention to provide a drying method for obtaining uniformly dried greens with predictable firing shrinkage characteristics. DISCLOSURE OF THE INVENTION The method of the present invention is applied to systems using water-based binders. As an example, consider a system using methyl cellulose (hereinafter referred to as MC) as a binder. Similar ideas can be applied to systems using other water-based binders. Appropriate amount for ceramics
In the drying process of the hydrated green obtained by molding the mixed and kneaded clay into a desired shape with the addition of MC and, if necessary, a plasticizer, shrinkage occurs due to movement of water (dehydration). Looking at this process in more detail, we can see that during dehydration, ceramic powder is attracted by capillary suction force and coagulates.
Shrinkage occurs due to condensation due to heating and dehydration. The latter is greatly influenced by the heating method, and in some cases, shrinkage may stop with water remaining in the MC molecules, causing non-uniform firing shrinkage. This is closely related to the mechanism of thermal gelation of MC. That is, the gelation mechanism of MC is that highly methoxy-substituted portions of MC are hydrated, become crosslinking points, and form a network structure (gel). Its gelling temperature is 60
The temperature is approximately 90℃, but if it is rapidly heated to a temperature higher than this, water will be trapped inside the network structure.
Does not dehydrate easily. In this case, the green apparent density becomes small. Therefore, it can be seen that the way heat is applied greatly affects the shrinkage characteristics during firing. Next, we will discuss the characteristics of common drying methods. (1) Far-infrared drying method, hot air drying method The hydrous green surface of the ceramic is heated by a far-infrared heater or hot air, and the interior is heated by heat transfer. Since this heating is conductive heating, it is less efficient than microwave drying, which will be described later. Furthermore, since the surface is heated first, the aforementioned gelation of MC proceeds on the surface first, and the surface shrinks before the inside is sufficiently dried. This creates resistance and prevents dehydration, further reducing efficiency. Although there are differences depending on the binder, this tendency is remarkable for acrylic binders. In addition, since the green is stabilized during the continuous drying process, even if the surface temperature is measured, there is a temperature difference between it and the inside, making it difficult to measure accurately and the response speed is slow. (2) Microwave drying Ceramics generally have a dielectric loss coefficient (ε _r・
Since water is large and has a small tan δ), microwave drying is efficient for drying hydrated greens. Moreover, since the material is heated from within, the material has a uniform temperature, and the surface does not shrink first and become a resistance to dehydration. Also, because of its fast response, it is possible to monitor the surface temperature during the continuous drying process and control the microwave output to keep the surface temperature constant. On the other hand, since the material is heated rapidly, it tends to generate hydrous gel as described above, which tends to cause uneven greening. The present invention provides a method for uniformly drying greens, which compensates for the drawbacks of both methods and allows predicting the shrinkage rate during firing. A commonly used method is to sufficiently knead a mixture of ceramic powder, water, a water-based binder, and optionally a plasticizer using a kneader or the like, and mold the mixture into a desired shape using a molding machine. For simplicity, use ceramic powder with alumina as a water-based binder.
We will describe the case in which MC and glycerin are used as plasticizers, are formed into a sheet using an extruder, and are continuously dried and rolled up. Similar ideas can be applied to other molding methods and systems using water-based binders or plasticizers. The relationship between moisture content and drying shrinkage rate of this system is shown in FIG. According to this graph, it can be seen that the drying shrinkage rate changes greatly with respect to the change in the moisture content at high moisture contents (10% or more). When microwaves are introduced to dry this area, hydrogels tend to form due to the rapid shrinkage rate. In addition, interaction due to friction with the belt for conveying the hydrated greens is also large, causing uneven drying. To solve this problem, a preheating oven is installed in front of the microwave dryer. The purpose of this preheating furnace is to preheat and pre-dry the hydrated green before passing it through the microwave dryer, and a hot air circulation type preheating furnace is preferable as the preheating furnace. This is because when heating with a far-infrared heater, the sheet surface is heated by thermal radiation, so there is a stronger tendency for the surface to contract and create resistance, whereas with hot air circulation, this tendency is less, and the surface The efficiency is high because the water evaporated from the water is carried away by the hot air. However, even with this method, the above-mentioned problem of increased surface resistance cannot be avoided, so drying should be limited to an appropriate moisture content. The possible moisture content can be determined by preliminary measurement of the relationship between drying rate and moisture content. That relationship is the second
As shown in the figure, it is possible to dry to about 8 to 10% with the hot air circulation method, and it is possible to dry by preheating in a preheating oven to about 8 to 10%.
Dry and then continuously transfer to a microwave dryer. Note that the degree of the preliminary drying described above differs depending on the drying method used. The hydrated green is preheated when it enters the microwave oven, its moisture content has been reduced to a level that does not cause rapid drying shrinkage, and it has almost no surface resistance, so it can be dried very efficiently. Figure 3 shows a graph plotting the surface temperature of ceramics and the linear shrinkage rate during firing with microwave (2450MHz) power as the horizontal axis. The surface temperature was monitored using an infrared thermometer (manufactured by Minolta Corporation, IR-0510) through CaF ₂ glass through a monitoring window provided on the ceiling near the outlet of the microwave oven. This figure shows that unless the microwave output is high, the surface temperature increases almost linearly, but the linear shrinkage rate remains almost constant up to 2kw.
It can be seen that when the power exceeds 2kw, there is a sudden change. The surface temperature at 2kw is approximately 60℃, which is the gelation point of MC. Therefore, if you select a microwave power that will keep the temperature below 60°C and dry at a constant output, you can obtain a green with a nearly stable linear shrinkage rate. The temperature setting here can be changed depending on the gelling temperature of the binder used. Circulating hot air in the microwave oven will increase the drying ability, so keeping the hot air temperature constant will produce more stable greens more effectively. The dimensions and variations thereof after firing of the greens prepared in this manner are very stable, and no cracks or the like occur. On the other hand, as mentioned above, if drying is completed with the film still on the surface, the density of the green will be low and non-uniform, resulting in large variations and cracks are likely to occur. Such a uniform drying method is more effective for thick items and large volume items. This is because microwaves penetrate into the ceramic molded product and heat and dry it from the inside. As mentioned above, it is understood that it is necessary to stabilize the material temperature in order to stabilize the shrinkage rate. In the above method, it can be stabilized by applying a constant microwave power that brings the material temperature below 60°C, which is the gelling point of MC, but for products that require even higher dimensional accuracy, it is necessary to It is necessary to control it so that it remains constant. To achieve this, we controlled the microwave power to always maintain a constant temperature while monitoring the material temperature using the aforementioned infrared thermometer. The responsiveness of controlling power by sensing temperature is very good, and the temperature fluctuation was within ±1°C, compared to ±4°C with constant output, which is a very good result. Ta. Dimensional accuracy after firing,
This has the effect of making the variation more stable and producing greens that are less prone to cracks. Example Alumina powder (manufactured by Showa Denko K.K., AL-45H)
Part by weight, talc (manufactured by Matsumura Sangyo Co., Ltd., High Filler)
4 parts by weight of HF5000PJ), MC (manufactured by Shin-Etsu Chemical Co., Ltd.),
A clay was prepared by blending 5 parts by weight of SH-6000), 15 parts by weight of water, and 2 parts by weight of glycerin. After mixing and kneading the above clay, it was formed into a sheet (2 mm thick) using an extruder. The obtained sheet was dried in the drying manner shown in Table 1. As shown in Table 1, it can be seen that the method of the present invention is excellent in drying uniformity.

[Table] [Effects of the Invention] This invention provides ceramic clay prepared by blending water and an aqueous binder with ceramic powder, and further adding a plasticizer as necessary.
When drying after forming into a desired shape, the method is characterized by pre-drying and then continuous drying using a microwave dryer, which has the effect of providing uniformly dried greens. There is.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between moisture content and drying shrinkage rate of hydrated green, Figure 2 is a graph showing the relationship between moisture content and drying rate of hydrated green, and Figure 3 is a graph showing the relationship between the moisture content and drying rate of hydrated green. 2 is a graph showing the relationship between the surface temperature of ceramics and the linear shrinkage rate during sintering of ceramics.

Claims (1)

[Scope of Claims] 1. Water-containing green obtained by molding ceramic clay prepared by blending water and a water binder with ceramic powder and adding a plasticizer as needed into a desired shape. A method of drying ceramics in which a pre-drying means is applied when drying the ceramics, and then a microwave dryer is used to continuously dry the ceramics, the pre-drying means using a hot air dryer, and The ceramics are pre-dried to a water content of drying method. 2. As a means of controlling the temperature inside the oven of the microwave dryer, the temperature of the ceramic hydrated green is monitored using an infrared thermometer, and the microwave output is adjusted so that the surface temperature of the molded product is constant. A method for drying ceramics according to claim 1, characterized in that a method for drying ceramics is used.