Casting slip

Other materials are used for the production of casting molds with high mechanical strength and high hardness, for instance, epoxy resins filled or non-filled with ceramic powders. 

As the casting process is an operation of filtration with formation of deposit, its kinetics is generally evaluated nsing Darcy s law [TIL 86]. By considering that rheological properties of the suspension remain constant during filtration, that there is no sedimentation and that the deposit is incompressible, its thickness e can then be 

The setting rate will be all the higher as the permeabihty of the deposited layer inereases (low Rj). The specifie resistance of the deposit Rj is generally determined by the Kozeny-Carman model [CAR 56], which expresses this resistance according to the porosity of the deposit (porous volume, surface of the pores per unit volume Sy, and tortuosity represented by a parameter T, generally taken as equal to 5)  

The setting time can vary from a few minutes to produce a thin wall with a partially coagulated porcelain suspension, to one hoirr in the case of a perfectly dispersed suspension of submicronic particles. It can take several days for the casting of very thick refractory parts (10 cm). 

Obtaining ceramic parts, with satisfactory properties in a reproducible way by slip casting requires a judicious choice of the grain size and the control of the particle surface chemistry. The rheological behavior and the viscosity of the suspensions in fact depend directly on the grain size of the powders, the inter-particle interactions (state of dispersion) and the particle concentration. 

This is analc us to filtration, as noted by Adcock and McDowall [14]. With slip casting there are two resistances to fiow the cake and the mold [15]. Considering only one porous medium, the fiow rate, Q, 

FIGURE 13.4 Schematic diagram of slip casting with a mold. 

This equation can be used to determine the specific cake resistance from the average particle size of the cake. Using a bundle of tubes to model the porous medium, the sphere diameter. Dp, in the preceding equation is replaced by 

Using the filtration concept with two resistances to flow, Darcy s law becomes 

Substituting this equation for Vq into Darcy s law for the combined resistance of cake and mold (ecpiation 13.5), we have 

A slip is prepared by ballmilling for several hours and removing any entrained air by evacuation. It is helpful to use a deflocculating agent. Sano, with other workers has reported on slip casting [168-172]. 

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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. 

Slip Casting. Slip casting of metal powders into useful articles is an interesting process but has only limited industrial application (30,31). It is sometimes used to produce large, very complicated parts from refractory metals (see Refractories). 

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. 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 process for making hollow articles from latex. A heat-sensitised compounded latex is poured into a hollow non-porous mould which is then rotated about several axes until the latex has gelled on the surface of the mould. See Rotational Moulding. Similar to slip casting of ceramics. Ketones... 

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 2.8. Synthesis route of alumina (a) the colloidal suspension route and (b) the slip-casting process (Larbot et al. 1987, Uhlhorn et al. 1989).

The gel layer thickness increases linearly with the square root of dipping time indicating that indeed a slip-casting process is operative. The rate constant depends on gel structure and pore size of the support. If the modal pore size of the support is increased from 0.12 /im (type 1 support) to 0.34 /xm (type 2 support) the casting rate is decreased in accordance with theory. Typical casting rates for type 1 and type 2 supports are 4.4 /xm/s and 2.8 /xm/s, respectively for HNO3-stabilized sols with a concentration of 1.22 mol boehmite/L. 

The synthesis of 5 lan thick Ti02 Si02 layers on a porous support can be performed using the procedure given below. First a mixed Ti[(OMe)3]4 alkoxide is synthesized by reacting partially hydrolyzed Si(OMe)4 with Ti-isopropoxide. This inorganic polymer is hydrolyzed at pH 11.0 and treated with 2-methyl-2-4-pentanediol and a binder. This solution is then slip-cast onto a porous support, dried and calcined at 700°C. The membrane can be useful in reverse osmosis applications. 

Tiller, F. M. and Chum-Dar Tsai. 1986. Theory of filtration of ceramics I, Slip casting. J. Amer. Ceram. Soc. 69(12) 882-87. 

Figure 7.28 Schematic illustration of (a) drain and (b) solid types of slip-casting processes. Reprinted, by permission, from H. Yanagida, K. Koumoto, and M. Miyayama, The Chemistry of Ceramics, p. 160. Copyright 1996 by John Wiley Sons, Inc.

Table 7.8 Composition of Some Common Slip Casting Slurries...

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.

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