Packing slip casting

Slurry or slip casting provides a relatively inexpensive way to fabricate unifonn-thickness, thin-wall, or large cross section shapes [4o, 44, 45, 46, 42 aiid 48]. For slip casting, a slurry is first poured into a porous mould. Capillary suction then draws the liquid from the slurry to fonn a higher solids content, close-packed, leather-hard cast on the inner surface of the mould. In a fixed time, a given wall thickness is fonned, after which the excess slurry is drained. 

A 10% volume SiC ethanol suspension Dg = 0.3 pm cr = 2) is slip cast with a conical plaster of Paris mold to make a radome for an airplane. The pores in the mold are 50 pm in diameter and the permeability of the mold is negligible compared to that of the powder. Determine the time necessary to slip cast a conical green body with a wall thickness of 1.5 cm. Assume that the ultimate packing density of the SiC powder is 80% theoretical. Data for ethanol density 0.7893 gm/cc viscosity 1.2 cP and yi = 24 dynes/cm. 

Finally, Vroon et al. [82,97] reported the synthesis of continuous porous films of ZSM5 on top of y-alumina supported membranes (pore diameter 4 nm) by slip-casting with a zeolite crystal suspension. The porous zeolite layers (thickness 1-2.5 pm) consist of densely packed zeolite crystals with a diameter of 70-80 nm and with micropores in the zeolite and mesopores (diameter 8-24 nm) between the zeolite particles. This zeolite layer can be used as a support for further processing, e.g., pore filling of the mesopores or deposition of catalysts. First experiments by Vroon et al. to fill the mesopores by in situ crystallisation of MFI in the pores did not result in gas-tight membranes... 

Concerning microfiltration, in which pores of less than 1 pm are needed (typically 0.1-1 pm), suspensions of submicrometer powders must be processed using slip-casting or tape-casting techniques, depending on the support shape (flat or tubular). Smaller pore sizes are required for ultrafiltration. In this case pores result from the packing of colloidal particles, which cannot... 

The colloidal techniques described in Chapter 4 provide considerable benefits for the control of the packing uniformity of the green body. The production of green bodies with uniform microstructure from a fully stabilized colloidal suspension of spherical, fine, monodisperse particles has not been incorporated into industrial applications where mass production is desired and fabrication cost is a serious consideration. Colloidal techniques, however, play an important role in the low-cost forming methods of slip casting and tape casting, as well as in the less commonly used methods of eletrophoretic deposition and gel casting. 

Figure 11.6 Scanning electron micrographs of the top surfaces of slip-cast samples of AI2O3 formed from (a) well-dispersed (pH 4) and (b) flocculated (pH 9) suspensions. Note the difference in packing homogeneity between the samples. (From Ref. 19.)...

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