There are four major steps in manufacturing most ceramic articles, including forming, drying, firing, and finishing. The present invention is applicable to the drying step, which often represents a significant and sometimes heretofore insurmountable problem in ceramic manufacturing.
For example, gelcasting is a useful method for forming complex articles in ceramic and/or metal powders. Drying (removing water or other solvent from) green (freshly cast, wet) gelcast articles is an essential step in the gelcasting manufacturing process.
The present invention is also applicable to other powder forming methods, such as solvent-based injection molding and extrusion. The present invention is not limited to drying green ceramic articles, but is also suitable for drying various other types of particulate articles, as set forth hereinbelow.
Conventional drying methods generally comprise exposing a formed article to air or a selected gas or gas mixture having particular conditions of temperature and humidity that will cause solvent to evaporate from the gelled article and be carried away by the atmosphere surrounding the article.
Some types of articles are difficult to dry properly via conventional drying methods because of solvent gradients therein, resulting in significantly differential drying of various regions thereof which induces structural stresses. For example:
1. Thick articles, especially those having at least a one inch minimum cross section, often crack during drying because the interior dries much more slowly than the surface. PA1 2. Thin plates often curl or warp during drying because one side dries faster than the other. PA1 3. Articles having occluded regions (for example, cones and other closed ended containers, and articles having serpentine internal passageways) are subject to stresses and malformation (warping, cracking, etc.) due to the occluded regions drying much more slowly than the open regions. PA1 4. Articles with sharp edge sections tend to dry and crack in the region of such edges before the bulk of the material can be properly dried. PA1 5. Articles having complex shapes, generally which include a plurality of thick regions, thin regions, sharp edges, and/or occluded regions, etc. PA1 1. U.S. Pat. No. 4,894,194 issued on Jan. 16, 1990 to M. A. Janney entitled "Method for Molding Ceramic Powders." PA1 2. U.S. Pat. No. 5,028,362 issued on Jul. 2, 1991 to M. A. Janney and O. O. Omatete entitled "Method for Molding Ceramic Powders Using a Water-Based Gelcasting." PA1 3. U.S. Pat. No. 5,145,908 issued on Sep. 8, 1992 to M. A. Janney and O. O. Omatete entitled "Method for Molding Ceramic Powders Using a Water-Based Gelcasting Process." PA1 4. A. C. Young, O. O. Omatete, M. A. Janney, and P. A. Menchhofer, "Gelcasting of Alumina," J. Am. Ceram. Soc., 74 3! 612-18 (1991). PA1 5. O. O. Omatete, M. A. Janney, and R. A. Strehlow, "Gelcasting-A New Ceramic Forming Process," Am. Ceram. Soc. Bull., 70 10! 1641-49 (1991). PA1 6. O. O. Omatete, T. N. Tiegs, and A. C. Young, "Gelcast Reaction-Bonded Silicon Nitride Composites," Ceram. Ens. Sci. Proc., 12 7-8! 1257-64 (1991). PA1 7. O. O. Omatete, A. Bleier, C. G. Westmoreland, and A. C. Young, "Gelcast Zirconia-Alumina Composites," Ceram. Eng. Sci. Proc., 12 9-10! 2084-94 (1991). PA1 8. Fanelli, A. J., et al. in "New aqueous injection molding process for ceramic powders", J. Am. Ceram. Soc., 72, 1833-43 (1989) PA1 9. Rivers, R. D., Method of injection molding powder metal parts, U.S. Pat. No. 4,113,480 issued Sep. 12, 1978. PA1 10. Mark A. Janney, "Extrusion and Injection Molding," Chapter 7 in Ceramic Processing, Terpstra, Pels, and DeVries, eds., Elsevier, Amsterdam, 1995. PA1 1. M. A. Janney and C. A. Walls, "Gelcasting compositions having improved drying characteristics and machinability," pending U.S. patent application Ser. No. 08/931,809, filed Sep. 16, 1997. PA1 2. Claudia A. H. Walls, Mark A, Janney, and Glen H. Kirby, "Gelcasting compositions and methods," U.S. patent application Ser. No. 08/931,809, filed Sep. 16, 1997 now abandoned.
Such drying problems create defects (e.g. cracking, warpage, other malformations) in the dried articles, reducing the value of the articles, and often rendering the article useless.
Further background information can be found in the following patents and publications:
The following are incorporated herein by reference: