DIELECTRIC DRYING PROCESS FOR HONEYCOMB STRUCTURES

Abstract: A dielectric-drying process of a honeycomb structure is carried out by placing the honeycomb structure (1) on a drying support board (2) provided with a perforated plate (3), and passing an electric current between an electrode arranged above the upper opening end face of the honeycomb structure and an electrode arranged beneath the lower opening end face thereof to conduct the drying. In this process in order to improve the uniformity of drying, an upper plate (4) having a conductivity higher than that of the honeycomb structure (1) is placed on the upper opening end face of the honeycomb structure (1).

Description: This invention relates to a dielectric drying process for honeycomb structures. A dielectric drying process has been carried out in order to dry a honeycomb structure of a ceramic green structural body obtained by extruding a ceramic material through a die and having many parallel through-holes isolated from each other by partition members each having an approximately uniform wall thickness. That is, the honeycomb structure was set between opposed electrodes and then an electric current was applied across the electrodes to cause molecular motion of dipole of water in the inside of the honeycomb structure through the generated high frequency energy, during which the honeycomb structure was dried by the resulting friction heat. However, when the honeycomb structure is dried by the above dielectric drying process, there arises a drawback that the density of electric force line passing through the honeycomb structure becomes ununiform. In order to solve this drawback, the inventor has previously proposed a drying support board composed of a perforated plate, a predetermined region of which inclusive of a portion contacting with a lower opening end face of the honeycomb structure has a conductivity higher than that of the other remaining peripheral portion thereof (see US Patent No. 4,439,929). When the honeycomb structure is subjected to a dielectric drying by using the above drying support board, the density distribution of the electric force line becomes more uniform, but the density in the upper portion of the honeycomb structure is still non-uniform, and consequently the drying of the upper portion in the honeycomb structure becomes slow as compared with the other remaining portion. That is, the drying shrinkage in the slow-drying portion is small as compared with that of the other portion, so that the dimensional scattering is caused between the upper portion and the lower portion in the honeycomb structure after the dielectric drying and hence the dimensional accuracy is reduced. As a result, the size of the upper portion becomes undesirably larger than that of the lower portion. Furthermore, when the drying of the upper portion in the honeycomb structure is delayed to form a high-moisture region in this upper portion, if draft drying or firing is carried out after the dielectric drying, shrinkage is large only in the high-moisture region and cracks are apt to be caused. Therefore, there is a demand for development of a technique capable of uniformly subjecting the honeycomb structure to a dielectric drying without delaying the drying of the honeycomb structure as a whole. It is, therefore, an object of the invention to solve the aforementioned drawbacks of the conventional technique and provide a dielectric drying process for the production of honeycomb structures having an improved dimensional accuracy. According to the invention, there is provided a process for dielectric-drying a honeycomb structure by placing the honeycomb structure on a drying support board composed of a perforated plate, a region of which inclusive of a portion contacting a lower opening end face of the honeycomb structure has a conductivity higher than that of at least one other portion, and passing an electric current between an electrode arranged above the upper opening end face of the honeycomb structure and an electrode arranged beneath the lower opening end face thereof to perform the drying, characterized in that an upper plate having a conductivity higher than that of the honeycomb structure is placed on the upper opening end face of the honeycomb structure. Embodiments of the invention will be described with reference to the accompanying drawings, wherein: Fig. 1 is a perspective view of an embodiment of the dielectric drying process of the honeycomb structure according to the invention; Fig. 2 is a diagrammatical view of one drying apparatus for practising the dielectric drying process according to the invention; and Fig. 3 is a graph showing a change of moisture content. According to the invention, not only the density of electric force line at the lower portion of the honeycomb structure is made more uniform by the conventional support board, but also the density of electric force line at the upper portion of the honeycomb structure can be made more uniform by the upper plate arranged on the upper opening end face of the honeycomb structure, so that the drying of the honeycomb structure is uniformly performed as a whole and consequently the dimensional accuracy of the honeycomb structure as a whole is improved and highly uniform moisture distribution can be achieved and no crack occurs. Further, the density of electric force line can optionally be changed by varying the surface area of the upper plate, so that the moisture distribution in the honeycomb structure after the drying can optionally be controlled and consequently the shape thereof can be well controlled. That is, the ceramic honeycomb structure can be dried with a high dimensional accuracy. In Fig. 1 is shown a perspective view illustrating the dielectric drying process of the honeycomb structure according to the invention, wherein plural honeycomb structures 1 are placed on a perforated plate 3 arranged in a support board 2 and also a perforated plate 4 as an upper plate is placed on the upper opening end face of each of the honeycomb structures 1. The perforated plate 4 has a conductivity higher than that of the honeycomb structure 1 and is preferably made from at least one material selected from the group consisting of non-magnetic aluminum, copper, aluminum alloy, copper alloy and graphite. As the perforated plate 4, there are provided several plates having different areas, among which a perforated plate having a desired form is selected. That is, the difference in size between the upper opening end face and the lower opening end face in the honeycomb structure can be controlled to about few milimeters by varying the surface area of the perforated plate 4 as an upper plate though this size difference is dependent upon the size of the honeycomb structure. On the other hand, the support board 2 is comprised by cutting out a portion wider by a given size than the end face of the honeycomb structure from the support board to form a hole 5 and then fitting a perforated plate 3 having a conductivity higher than that of the support board 2 and a surface area larger by a given ratio than the opening end area of the honeycomb structure into the hole 5 formed in the support board. In Fig. 2 is diagrammatically shown the drying apparatus suitable for practising the dielectric drying process according to the invention. In the illustrated drying apparatus, a dielectric drying unit 11 and a draft drying unit 12 for completely drying the honeycomb structure are continuously connected to each other with a conveyor 13 for dielectric drying and a conveyor 14 for draft drying. The dielectric drying unit 11 is constructed with the conveyor 13 for dielectric drying, electrodes 15-1, 15-2, arranged above the upper opening end face and beneath the lower opening end face so as to be parallel with the opening end faces of the honeycomb structure, and hot air ventilating holes 16 for ventilating hot air so as to prevent the dew formation from steam generated in the drying onto the electrodes 15-1, 15-2 and the like. On the other hand, the draft drying unit 12 is provided with a hot air circulating duct 17 for completely drying the honeycomb structure after the dielectric drying so as to enable the cutting with a whetstone or to prevent the occurrence of cracks due to mon-uniform shrinkage in the firing. For instance, hot air heated to a temperature of 80 ∼ 150°C is fed from the hot air circulating duct 17 at a wind speed of 0.3 ∼ 2.0 m/sec into the through-holes of the honeycomb structure. The following example is given in illustration of the invention and is not intended as limitation thereof. Example There were provided ceramic honeycomb structures of 150 mm in height and 120 mm in diameter each made from cordierite, which were subjected to a dielectric drying with the use of upper plates having various shapes, areas and materials as shown in the following Table 1 to obtain samples No. 1 ∼ 7 according to the invention. The term "area" used herein means a ratio relative to a surface area of the opening end face, wherein the area which is the same as the end face area is represented by 100%. On the other hand, samples No. 8 ∼ 9 of Comparative Examples were obtained by the same dielectric drying process as described in US Patent No. 4,439,929 without using an upper plate for the honeycomb structure. The moisture content in the central portion of the resulting sample after the drying was measured at upper, middle and lower positions in the height direction, and the diameters D₁ and D₃ of the opening end faces at the lower and upper ends were measured. The measured results are shown in Table 1. As seen from Table 1, the moisture content at the upper position in the samples No. 1 ∼ 7 according to the invention is clearly lower than that of the samples No. 8 ∼ 9 of Comparative Examples, and also the difference between the diameter D₁ of the lower end and the diameter D₃ of the upper end is very small. Moreover, the change of moisture content at each position in the central portion of the product in the samples No. 1 and 8 is shown in Fig. 3. Furthermore, as seen from the results of samples No. 5 ∼ 7 in Table 1, the difference in diameter between upper opening end and lower opening end is changed by varying the surface area of the upper plate, whereby the shape of the honeycomb structure after the drying can be controlled. As mentioned above, according to the invention, the dielectric drying process is carried out by placing an upper plate on the upper opening end face of the honeycomb structure placed on the support board provided with the given perforated plate, whereby the drying speed at each portion of the honeycomb structure is made more uniform and the honeycomb structure having a more uniform moisture distribution can be obtained and consequently the honeycomb structure having a good dimensional accuracy can be obtained. Furthermore, the moisture distribution can be controlled by varying the surface area of the upper plate, and consequently the shape of the honeycomb structure after the drying can be controlled.

Claims: 1. A process for dielectric-drying a honeycomb structure by placing the honeycomb structure on a drying support board composed of a perforated plate, a region of which inclusive of a portion contacting a lower opening end face of the honeycomb structure has a conductivity higher than that of at least one other portion, and passing an electric current between an electrode arranged above the upper opening end face of the honeycomb structure and an electrode arranged beneath the lower opening end face thereof to perform the drying, characterized in that an upper plate having a conductivity higher than that of the honeycomb structure is placed on the upper opening end face of the honeycomb structure. 2. The process according to claim 1, wherein said upper plate is a perforated plate and is made from at least one material selected from the group consisting of aluminum, copper, aluminum alloy, copper alloy and graphite. 3. The process according to claim 1 or claim 2 wherein an area of said upper plate is varied to control the shape of the honeycomb structure after the drying.

Inventors: MIZUTANI, ISAO
Applicants: NGK INSULATORS, LTD.
Application year: 1987
Main classification: F26B3
IPC codes: C04B33, F26B15, F26B3, F26B23