As the water content is reduced, increased pressure is required on the paste to produce a similar flow Thus, the shaping processes can be divided according to the consistency of the mixture: (1) slip casting, in which the mixture is a slurry(2) plastic-forming methods that shape the clay in a plastic condition(3) semi-dry pressing, in which the clay is moist but possesses low plasticity and(4) dry pressing where the clay is basically dry, containing less than 5% water. Dry clay has no plasticity. The four categories are represented in the chart of Figure 17.4, which compares the categories with the condition of the clay used as starting material. Each category includes several different shaping processes.
Slip Casting Slip casting is used in powder metallurgy, but its application in ceramics shaping is much more common. In slip casting, a suspension of ceramic powders in water, called a slip, is poured into a porous plaster of paris (Casor2HO) mold so that water from the mix is gradually absorbed into the plaster to form a firm layer of clay at the mold surface. The composition of the slip is typically 25% to 40% water, the remainder being clay often mixed with other ingredients. It must be sufficiently fluid to flow into the crevices of the mold cavity, yet lower water content is desirable for faster production rates. Slip casting has two principal variations drain casting and solid casting. In drain casting, which is the traditional process, the mold is inverted to drain excess slip after the semi-solid layer has been formed, thus leaving a hollow part in the mold, the mold is then opened and the part removed. The sequence, which is very similar to slush casting of metals, is illustrated in Figure 17.5. It is used to make tea pots.