Forming slip casting

Common consolidation methods include variations on pressing powders, plastic forming, slip casting and tape casting. The choice of which method to use for a given application is very dependent upon the shape, size and required dimensional tolerances of the final product, available capital outlay and desired productivity. Each of these methods is examined in turn below, followed by sintering, which turns the consolidated powders into a monolithic body with mechanical integrity. 

Slip Casting. SHp casting (38,40—42,45,59—62), the process in which a cast is formed from a slurry using a porous mold, is used to form sinks and other sanitary ware, figurines, refractory cmcibles, porous thermal insulation, fine china, and complex shape stmctural ceramics such as multivane rotors. 

In slip easting a thin slurry, or suspension, of clay in water is poured into a porous mould. Water is absorbed into the mould wall, causing a layer of clay to form and adhere to it. The excess slurry is tipped out of the mould and the slip-cast shell, now dry enough to have strength, is taken out and fired. The process allows intricate shapes (like plates, cups, vases) to be made quickly and accurately. 

As forming processes, slip casting, extrusion, isostatic pressing and electrophoretic forming have been applied. There are some advantages to isostatic pressing ... 

Slip casting is common in the ceramics industry. The material to be cast is milled to a mean particle size of a few microns. A slip is made by mixing the finely divided material with a liquid suspending medium. The slip is then poured into a suitable mold (e.g., of plaster of pans). The liquid in the slip is drawn into the mold by capillary forces and the solids are deposited in a coherent form. For TiBj, ZrBj and CrBj a suspending medium of 5-7 wt% cyclopentadiene in xylene is recommended. A 3 wt% aqueous solution of carboxymethylcellulose is the best dispersing medium... 

Conventional ceramics initially take shape on a potter s wheel or by slip casting and are fired (sintered) in kilns advanced ceramics are formed and sintered in more complex processes such as hot isostatic pressing. 

A common method to slip-cast ceramic membranes is to start with a colloidal suspension or polymeric solution as described in the previous section. This is called a slip . The porous support system is dipped in the slip and the dispersion medium (in most cases water or alcohol-water mixtures) is forced into the pores of the support by a pressure drop (APJ created by capillary action of the microporous support. At the interface the solid particles are retained and concentrated at the entrance of pores to form a gel layer as in the case of sol-gel processes. It is important that formation of the gel layer starts... 

Figure 7.29 Schematic illustration of the formation of a slip-cast layer formed in the extraction of water by capillary action from a mold. From Introduction to Ceramics, by W. D. Kingery, H. K. Bowen and D. R. Uhlmann, Copyright 1976 by John Wiley Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

Transparent vitreous silica cmcibles can be produced by firing slip-cast pieces in a vacuum or a helium atmosphere (48,49), or in helium followed by argon (50). The cmcible is usually supported on a graphite form to minimize distortion. 

Slip-casting has been a major part of traditional ceramics processing technology for at least 250 years. It was introduced for the casting of alumina ware in the early part of the last century and since then has been extensively exploited for forming advanced ceramics. 

Simple shapes are formed by die-pressing, long bodies of uniform section are formed by extrusion, thin plates are formed by band-casting or calendering, and large rings and more intricate shapes are formed by slip-casting. 

Forming from mulations are mixed to a plastic mass using solvent and/or moistening agents, forced into a mold, pushed out and dried. 1. Stable suspension of particles Slip casting automatic tablet molding machines... 

Ceramic Ablators, Several types of subliming or melting ceramic ablators have been used or considered tor use in dielectric applications particulady with quartz or boron nitride [10043-11-5] fiber reinforcements to form a nonconductive char. Fused silica is available in both nonporous (optically transparent) and porous (slip cast) forms. Ford Aerospace manufactures a 3D silica- fiber-reinfo reed composite densified with colloidal silica (37). The material, designated AS-3DX, demonstrates improved mechanical toughness compared to monolithic ceramics. Other dielectric ceramic composites have been used with performance improvements over monolithic ceramics (see Composite materials, ceramic matrix). 

A more advanced view of slip casting with two types of cake, one dense and the other dilute, has been presented [19,20]. But the results are of the same form as the preceding equation with F, the proportionality constant, having a more complicated definition. 

---