Particle 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... 

Slip casting of metal powders closely follows ceramic slip casting techniques (see Ceramics). SHp, which is a viscous Hquid containing finely divided metal particles in a stable suspension, is poured into a plaster-of-Paris mold of the shape desired. As the Hquid is absorbed by the mold, the metal particles are carried to the wall and deposited there. This occurs equally in all directions and equally for metal particles of all sizes which gives a uniformly thick layer of powder deposited at the mold wall. 

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... 

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... 

Controlling the stability of suspended particles, and the filtration characteristics of cast suspensions, are very important in the ceramics industry. Here the casting suspensions may comprise clays, silica, oxides, carbides or nitrides [357,793], In slip casting, a suspension is pored into a porous mould and allowed to stand. Water dif-... 

In ceramics, a variation of slip-casting in which a stabilized colloidal dispersion of particles is poured into a mold for sintering. 

In the slip-casting process the ceramic powder is suspended in a fluid vehicle, usually water. The suspension, or slip , has a high solids content, typically 50vol.%, and the individual particles are fine, usually less than 10 /mi. Deflocculants, which modify the electrical environment of each particle so that they repel one another, are added to the slip. The fineness of the powder and consequent high surface area ensure that electrostatic forces dominate gravity forces so that settling does not occur. When exceptionally heavy powder particles... 

Magnetic anisotropy. In conjunction with slip casting, magnetic alignment of anisotropic particles may be used to produce polycrystalline materials with a preferred orientation, or texture. 

Figure 7.24 Slip-casting process (a) a mold, which contains a cavity of the desired shape, is prepared (b) the slip, a slurry or suspension of particles, is poured into the mold (c) liquid is absorbed by the mold, compacting the slip s particles on the mold walls via capillary forces (d) when a cast layer of desired thickness has been built up, the remaining slip is poured out of the mold (e) after drying, the slip-cast part is removed from the mold, which is then dried for reuse.

The slip casting process can be considered a type of filtration process, whereby the liquid filtrate is removed from the slip, leaving behind the filtrant, or ceramic particles, as the cast body. D Arcy s law (D Arcy, 1856), which was developed to quantify the flux of a filtrate (water) through a porous bed (sand), may be used to analyze the slip casting process ... 

Figure 7.25 Alignment of anisometric particles in the slip-cast layer is possible due to the capillary forces acting normal to the mold wall. Particles will tend to follow the mold contours.

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