Drying carriers for honeycomb structures

Abstract: A drying carrier adapted for carrying honeycomb-structures (3) includes a plurality of perforated plates (10) spaced to each other in the longitudinal direction of the drying carrier (1) and made of a material e.g. metal, having a conductivity higher than that of the rest of the drying carrier. Each of said perforated plates (10) is provided with a convex portion (11) having an upper contacting surface of the honeycomb structure (3) to be carried. The surface area (A) of the upper contacting surface of the convex portion is smaller than the bottom surface area of the honeycomb-structure.

Description: This invention relates to a drying apparatus comprising the features as indicated in the preamble of claim 1 and to a method of drying honeycomb-structure articles. The drying carriers used in such a drying apparatus may be used when honeycomb structures are dried through a drying apparatus as shown in Fig. 1 which illustrates a dielectric and ventilation type drying apparatus including a dielectric drying zone 101 and a ventilation drying zone 102. In drying the honeycomb structure, the honeycomb structures 3 are carried on a carrier 1 in such a manner that the open bottom surface of the honeycomb structure 3 contacts the upper surface of the carrier 1. The honeycomb structure 3 is a ceramic green structure which is extrusion molded from a batch of a ceramic material through an extrusion die and typically has a number of parallel through holes separated by partitions having a uniform thickness. The honeycomb structures 3 carried on the carrier 1 are continuously moved through the dielectric drying zone 101 and the ventilation drying zone 102 by means of a dielectric drying conveyor 111 and a ventilation drying conveyor 112 and dried as they move. The dielectric drying zone 101 includes upper and lower electrodes 113 which are arranged parallel to the open end surfaces of the honeycomb structure, and hot air ventilating ducts 114 for blowing hot air into the drying zone to prevent steam generated during drying from condensing on the surfaces of the electrodes 113 or the drying zone 101. The honeycomb structures 3 are then transferred to the ventilation drying zone 102 by means of a ventilation drying conveyor 112 in order to completely dry the honeycomb structures after dielectric drying. In the ventilation drying zone 102, hot air of 80-150°C is blown at a speed 0.3-2.0 m/sec into the ventilation drying zone by means of a hot air ventilating duct 115 to dry the honeycomb structure 3 by ventilating. Thus, the honeycomb structures 3 are subjected to a sequence of dielectric drying and ventilation drying through the dielectric and ventilation drying apparatus by using the carrier. In Fig. 1, a steam discharge duct is denoted by a numeral 116. A dielectric drying apparatus is described in EP-A-0273707. The bottom of a green honeycomb structure is placed on a metallic planar perforated plate and an electrode is placed at the top of the honeycomb. A dielectric drying current is then passed between the bottom perforated plate and the top electrode to dry the honeycomb. This process improves drying efficiency. Another dielectric drying apparatus is described in our US-A-4439929, corresponding to JP-A-57/139278. The two-part form of claim 1 is based on this. Ceramic honeycomb green bodies are placed on discrete, planar perforated metallic plates larger than the area of the bodies. The plates are separated by low-conductivity regions formed by a board supporting the metallic plates. This arrangement improves the drying efficiency since more electric lines of force pass through the metallic plate than through the areas not occupied by the honeycomb. See Figs. 2 and 3: The drying carrier has a hole 2 larger than the bottom of the honeycomb structure 3 to be carried, and the perforated plate 4 made of a material having a conductivity higher than that of the carrier 1 is securely fitted in the hole 2. When the honeycomb structures are dried using such a drying carrier, particularly in dielectric drying, the occurrence of cracks in the honeycomb structure is prevented in the subsequent sintering step. Thus, that type of drying carrier is superior to the conventional drying carrier so that the quality and productivity of various honeycomb structures such as cordierite, mullite, silicon nitride and the like are improved. However, the perforated plate 4 of the drying carrier has an upper contacting surface area larger than that of the bottom surface of the honeycomb structure to be dried, and a flat shape. Therefore, when the honeycomb structure shrinks during drying the shrinkage of the bottom portion of the honeycomb structure is hindered owing to frictional resistance between the surfaces of the bottom of the honeycomb structure and the perforated plate. As a result the honeycomb structure is deformed, reducing dimensional accuracy so that yield is low when high dimensional accuracy is required. Furthermore the friction resistance tends to generate a shrinkage strain in the bottom surface of the honeycomb structure, causing cracks in the bottom surface. Thus, the frictional resistance adversely affects the quality of the products. The problem addressed herein is to provide new drying apparatus and drying methods for honeycomb structures, reducing the previous tendency to dimensional deformation and cracking caused by frictional resistance at the bottom surface of the honeycomb structure. In one aspect we provide drying apparatus including a dielectric drying zone and a drying carrier for conveying ceramic honeycomb-structure articles having generally flat bottom surfaces through the dielectric drying zone in a longitudinal direction, for drying therein, the drying carrier comprising a longitudinally-extending substrate of low electrical conductivity and plural perforated plates of relatively high electrical conductivity distributed in the longitudinal direction along the substrate and supported thereby, the perforated plates providing a longitudinal series of foraminous support portions having generally flat contact areas for supporting contact against the bottom surfaces of the honeycomb-structure articles, characterised in that the portions of the perforated plates providing said generally flat contact areas are raised relative to surrounding surfaces thereof so that the bottom surfaces of honeycomb-structure articles supported thereon can project beyond the raised portions to overhang the surrounding surfaces. In another aspect we provide a method of drying honeycomb-structure articles having generally flat bottom surfaces, comprising supporting the honeycomb-structure articles on foraminous support portions of a drying carrier as defined above the raised contact areas of the foraminous support portions of the drying carrier having an area smaller than, but not less than 60% of, the respective bottom surface areas of the honeycomb-structure articles whereby part of the bottom surface of each article overhangs a surrounding surface of the drying carrier; conveying the honeycomb-structure articles on the drying carrier, and drying the conveyed honeycomb-structure articles by dielectric drying. Preferred features are now mentioned, and specific embodiments described, with reference to the drawings in which: Fig. 1 is a diagrammatic view of a dielectric and ventilation type drying apparatus; Fig. 2 is a perspective view illustrating an example of a drying carrier proposed by the applicant prior to the present invention; Fig. 3 is an enlarged sectional view of a product carried on the drying carrier Shown in Fig. 2; Fig. 4 is a sectional view illustrating an embodiment of a drying carrier embodying the invention; Fig. 5 is a diagrammatic view showing a relationship between a perforated plate and a product; Fig. 6 is a diagrammatic view illustrating a sample used in a test; Fig. 7 is a diagrammatic view of a product; Figs. 8a and 8b are diagrammatic views illustrating an embodiment of a perforated plate adapted for preventing electric discharge; Figs. 9a and 9b are diagrammatic views illustrating a perforated plate having an aperture formed by punching and a product set on the perforated plate; Figs. 10a and 10b are diagrammatic views illustrating another embodiment of the perforated plate according to the invention; and Fig. 11 is a diagrammatic view illustrating a further embodiment of the perforated plate. Referring to Fig. 4 illustrating an embodiment of the present invention, a perforated plate 10 is made of a material electrical conductivity higher than carrier substrate 1 and is fitted on a shoulder 5 of a hole 2 formed in the carrier 1. The hole 2 in the carrier has a sectional area larger than that of the bottom of the honeycomb 3 to be carried. Therefore, the perforated plate 10 to be fitted in the hole 2 has a general area dimension larger than the dimension of the bottom surface of the honeycomb structure 3, but the perforated plate 10 has a convex shape. In the embodiment, the perforated plate 10 has a convex portion 11 formed at the central portion of the perforated plate so that the honeycomb structure is set on the upper contacting surface 6 of the convex portion. The area (A) of the upper contacting surface 6 of the perforated plate 10 is smaller than the bottom surface area (B) of the honeycomb structure and is not less than 0.6 B. The drying carrier is preferably made of plastic or other synthetic resins having a low conductivity, plasterboard or wood. The perforated plate is made of a material having a conductivity higher than that of the material of the substrate, for example aluminum, copper, aluminum alloy or some combination thereof. The perforated plate 10 has a number of apertures 12 as shown in Fig. 1 with an open percentage of 20∼90%, preferably 40∼80%. The shape of the apertures 12 in the perforated plate 10 may be a circle, rectangular, slit or other. If the open percentage of the perforated plate 10 is lower than 20%, steam generated from the bottom surface portion of the honeycomb structure 3 during the step of drying tends to be insufficiently diffused and condenses between the perforated plate and the bottom surface of the honeycomb structure so that the bottom portion of the honeycomb structure is not completely dried. If the open percentage of the perforated plate exceeds 90%, the strength of the perforated plate becomes low and it may tend to become deformed, e.g. by the weight of honeycomb structures. The apertures 12 may be formed in the perforated plate 10 by punching e.g. as mentioned later. The convex portion 11 may be formed by pressing e.g. in the punching step. The convex portion 11 has an upper contacting surface 6 having a surface area A smaller than the bottom surface area B of the honeycomb structure as mentioned above in a relationship of 0.6B≦A<B, preferably 0.7B≦A<0.9B. An angle ϑ is preferably in a range of 30°∼60°. Furthermore, the height of the convex portion 11, that ii the distance to the upper contacting surface 6 from the flat surface at the peripheral portion of the perforated plate 10 is preferably in a range of 1 mm∼5 mm, preferably 1.5 mm∼2.5 mm. If the upper contacting surface area (A) of the convex portion 11 is smaller than 0.6B, the honeycomb structure tends to be deformed by own weight so that when the structures are moved by a conveyor as shown in Fig. 1, the structures become unstable and then contact with each other to thereby be broken owing to a collision. If the upper contacting surface area A is larger than the bottom surface area B, all the bottom surface of the honeycomb structure contacts the upper contacting surface of the convex portion and as a result dimensional deformation or cracking tends to occur in the bottom portion of the honeycomb structure owing to the friction resistance between the contacting surfaces. Furthermore, if the height of the convex portion 11 is less thin 1 mm, the bottom peripheral portion of the honeycomb structure 3 may be easily contacted with the flat peripheral portion of the perforated plate 10 as shown in Fig. 5. When the bottom peripheral portion of the honeycomb structure 3 is contacted with the flat peripheral portion of the perforated plate 10, the effect of reducing the friction resistance intended by providing the convex shape is nullified so that the occurring of the dimensional deformation and the crack during the step of drying is not prevented. However, if the height of the convex portion 11 is more than 5 mm, the honeycomb structure set on the convex portion may be unstable so that the honeycomb structures may be contacted with each other to thereby be broken by a collision as mentioned above. In particular, when the convex portion has the upper contacting surface area A smaller than 0.6B and the height higher than 5 mm, the honeycomb structure can be greatly deformed by own weight and resulted in occurring cracks. The perforated plates 10 having the convex portion 11 formed according to the aforementioned conditions are fitted in holes formed in the carrier 1, respectively, and fixed with an adhesive having a low conductivity and a heat resistance, for example, for 150°C corresponding to a predetermined ventilation temperature condition in the drying apparatus. The drying carrier 1 provided with perforated plates 10 as shown in Fig. 4 can be used in a drying apparatus as shown in Fig. 1. An example of the material and dimensions of the carrier and perforated plates will be described by referring to Fig. 4. The material of the carrier is wood, the material of the convex shaped perforated plate is aluminum, diameter D₁ is 146 and D₂ is 52 mm, the open percentage of the perforated plate is determined by the diameter (d) of the aperture 12 and a pitch (p) between apertures 12, for example, in the embodiment illustrated in Fig. 4, (d) is 5.5 mm, (p) is 7.0 mm and the open percentage is 48.5%. The honeycomb structure 3 set on the upper contacting surface 16 of the convex portion 11 of the perforated plate 10 has a diameter (D) of 118 mm so that the contacting surface area (A) of the perforated plate 10 is 0,8 (B), B is the bottom surface area of the honeycomb structure to be dried, and the height (h) of the convex portion 11 is 2 mm. In order to confirm the effect of the construction, a test was performed by using convex shaped perforated plates 10 as in Fig. 6 having various contacting surface areas (A), angles ϑ, and heights (h) of the convex portions. The results of the test are shown in table 1 together with a comparative prior art. In the table 1, the dimension difference is a difference (f) between diameters of upper and lower portions of each honeycomb structure (products) as shown in Fig. 7. Contacting area A Angle ϑ° Height h mm Dimension difference mm Bottom crack occurring % Deformation owing to own weight Example 1 0.8 B 45 2 0.5 0 O ○ Example 2 0.5 B 45 2 0.5 0 X Example 3 0.9 B 45 2 0.5 0 O ○ Example 4 0.8 B 45 1 0.9 20 O ○ Example 5 0.5 B 40 5 0.7 15 X Example 6 0.6 B 45 2 0.5 0 ○ Example 7 0.9 B 60 2 0.5 0 O ○ Prior art 1.0 B 0 - 1.2 40 O ○ It is seen from the results shown in the table 1 that, in the embodiments, the dimensional deformation and cracks caused by the friction resistance during the step of drying are greatly reduced or prevented in comparing with the prior art wherein all the bottom surface of the honeycomb structure contacts the surface of the perforated plate without the convex portion. In order to prevent undesirable electric discharge during the step of dielectric drying, the perforated plate in the drying carrier may be shaped as shown in Figs. 8 and 9. Referring to Fig. 8a, corners 13 of the perforated plate 10 is preferably curved by a radius of curvature (R) in a range of 5∼15 mm. If the radius of curvature (R) is not more than 5 mm, the corner has an edge so that an electric charge concentrates in the edge during dielectric drying to cause an electric discharge. Furthermore, the perforated plate 10 is preferably provided with a peripheral portion 14 having a predetermined width (w). If the perforated plate 10 has no unperforated edge, as shown in Fig. 8b, edges 10E are formed when the apertures 12 are punched in the plate. Such an edge can be a cause of electric charge concentration. Referring to Fig. 9a, when the apertures 12 are punched in the plate, an edge 10a is formed on the opposite side of the plate to the punching direction. Therefore, the perforated plate 10 must be fixed to the drying carrier 1 in such a manner that the plate side formed with the edges 10a is the under side of the convex portion 11 of the perforated plate 10 on which the honeycomb structure 3 is set as shown in Fig. 9b. Thus, the electric discharge owing to the edges 10a is reduced, also friction resistance is better reduced. Figs. 10 and 11 illustrate other embodiments of the perforated plates 10 of the drying carrier. The perforated plates shown in Figs. 10 and 11 are made of aluminum punched plates. In Fig. 10, the convex portion is formed with a height (h) of 3 mm by punching. The dimensions (mm) denoted in Fig. 7 are as follows: D₁₁ = 142 D₁₂ = 132 D₂₁ = 85 L₁, L₃ = 4.5 L₂ = 133 L₄, L₆ = 4.75 L₅ = 117 t = 2, h = 3, H = 5 P₁, P₂ = 7.5 P₃, P₄, P₅ = 6.5 d = 5.5 R = 10 (R is a radius of curvature of corners) The dimensions (mm) denoted in Fig. 11 are as follows. D₁₃ = 188 D₁₄ = 114 L₇ = 8 L₈ = 12 L₉ = 15 L₁₀ = 5 L₁₁ = 6.5 L₁₂ = 13 d = 5.5 D₂₂ = 122 D₂₃ = 65 R = 10 R₁ = 28 Moreover, a punched portion 11a has an oval shape which is similar to that shown in Fig. 6. In this embodiment, the perforated plate 10 has a similar peripheral portion of a width in a range of 2.2∼7.5 mm, as shown in Fig. 10 in order to prevent the electric discharge caused by edge portions. According to the present teachings, the friction resistance generated during the step of drying can be reduced to help avoid dimensional deformations and cracks, thereby improving quality of the honeycomb structures.

Claims: Drying apparatus including a dielectric drying zone and a drying carrier (1) for conveying ceramic honeycomb-structure articles having generally flat bottom surfaces through the dielectric drying zone in a longitudinal direction, for drying therein, the drying carrier (1) comprising a longitudinally-extending substrate of low electrical conductivity and plural perforated plates (10) of relatively high electrical conductivity distributed in the longitudinal direction along the substrate and supported thereby, the perforated plates (10) providing a longitudinal series of foraminous support portions having generally flat contact areas for supporting contact against the bottom surfaces of the honeycomb-structure articles, characterised in that the portions (11) of the perforated plates (10) providing said generally flat contact areas are raised relative to surrounding surfaces thereof so that the bottom surfaces of honeycomb-structure articles supported thereon can project beyond the raised portions (11) to overhang the surrounding surfaces. Drying apparatus according to claim 1, in which the substrate of low electrical conductivity is of plastics material or a synthetic resin material having a low conductivity, plasterboard or wood. Drying apparatus according to claim 1 or claim 2 in which the perforated plates (10) are made of aluminium, copper, an aluminium alloy or some combination of these. Drying apparatus according to any one of claims 1 to 3 in which the open percentage of the foraminous support portions (10) is 20% to 90%. Drying apparatus according to any one of the preceding claims, in which the raised contact area (11) is raised by 1 to 5mm relative to said surrounding surface. A method of drying honeycomb-structure articles (3) having generally flat bottom surfaces, comprising supporting the honeycomb-structure articles (3) on foraminous support portions (10) of a drying carrier (1) as defined in any one of claims 1 to 5, the raised contact areas (11) of the foraminous support portions (10) of the drying carrier (1) having an area smaller than, but not less than 60% of, the respective bottom surface areas of the honeycomb-structure articles (3) whereby part of the bottom surface of each article (3) overhangs a surrounding surface of the drying carrier (1); conveying the honeycomb-structure articles (3) on the drying carrier (1), and drying the conveyed honeycomb-structure articles (3) by dielectric drying.

Inventors: JIKUMARU SUEHARU, WADA YUKIHISA, JIKUMARU, SUEHARU, WADA, YUKIHISA
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1992
Main classification: F26B15
IPC codes: F26B25, F26B3, F26B15