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85. Cawley, J.D.; Heuer, A.H.; Newman, W.S.; Mathewson, B.B.: Computer-aided manufacturing of laminated engineering materials. Am. Ceram. Soc. Bull. 75, 1996, 75- 86. Argawala, M.K.; Jamalabad; V.R.; Langrana, N.A.; Safari, A.; Whalen, P.J.; Danforth, S.C.: Structural Quality of Parts Processed by Fused Deposition. Rapid Prototyping Journal 2, 1996, 4-19. 87. Heinrich, J.G.; Ries, C.; Görke, R.; Krause, T.: Lasersintern keramischer Werkstoffe. In: J. Heinrich, G. Ziegler, W. Hermel, H. Riedel (Hrsg.): Keramik. Werkstoffwoche 1998, DGM Informationsgesellschaft mbH, Verlag der Deutschen Gesellschaft für Materialkunde e.V., Frankfurt. 180 5. Thermal processes 5.1 Drying After shaping of the ceramic parts, the parts follow to drying. During drying, the particles that have been surrounded by more or less water, depending on the forming techniques get closer and the green body smaller, what is called shrinkage (Fig. 5.1.1). Shrinkage is finished as soon as the powder particles touch each other. The remaining water now has to leave the system through the small pore or channels. In the Bourry diagram (Fig. 5.1.2) the water content is plotted versus the drying time. Water content diminishes at the beginning of the drying process and the part shrinks. After about 72 hours, water can only be found in the pores. Water content further decreases, but shrinkage does not change any more. The dry bending strength increases as the green bodies’ water content decreases (Fig. 5.1.3). As a result the particles can be handled and, for example, be set on conveyor bands mechanically or by hand. Fig. 5.1.1: Illustration of the drying stages in ceramic compounds . Fig. 5.1.2: Bourry drying diagram for clay components. 181 Fig. 5.1.3: Dry bending strenght as a function of the drying temperature . The graphic of the mass loss (Fig. 5.1.4) shows the moisture content as a function of the drying time. At the beginning, the reduction on the moisture content is linear. In this case, the water has direct access to the surface of the tested pieces. When the water is trapped into the pores, the curve deviates from the straight line. The differentiated mass loss graph (Fig. 5.1.4b) shows the area of constant drying rates even clearer. Drying speed decreases with the water’s retraction into the pores. The drying speed in dependence of the moisture content (Fig. 5.1.4c) is also a standard presentation in the literature. The diagram has to be read from right to the left. If the moisture content is high, the drying speed at first is constant, and decreases as the water is trapped into the pores. Mass loss curve differentiated mass loss curve drying curve Fig. 5.1.4: Drying diagram according to Scholz. 182 Drying rate can be increased by raising drying air speed (Fig. 5.1.5) and temperature (Fig. 5.1.6). Furthermore, the drying speed is affected by specimen geometry and density of the components (Fig. 5.1.7). If the total porosity is high (decreasing density), the water can leave the system relatively quickly. At very low moisture degrees we see again a deviation in the curve. This is the point at which only OH groups are found at the surface. Now, if temperature is further increased, drying speed is again reduced and the OH groups leave the surface of the test sample. This is the third drying stage which, however, is not relevant in practice. Fig. 5.1.5: Drying curve for clay minerals, two-stage drying of samples with thickness of 3 cm, in a air humidity of 53.7% and temperature of 25°C according to Kamei. Fig. 5.1.6: Drying velocity of granulates used for floor-ware tile compounds according to Schrader. 183 Fig. 5.1.7: Drying curves of different samples of brick tiles according to Schmidt. Several drying plants will be presented in the following. When the drying process begins the relative moisture is high. As the drying process proceeds and water starts to get trapped into the pores, temperature is increased and humidity reduced (Fig. 5.1.8). These complex correlations are discussed in detail in other lectures and shall not be further explained in this one. 184 Fig. 5.1.8: Variation of the air humidity and temperature in a drying chamber according to Krause. Transitions in a real furnace aggregate are more sliding (Fig. 5.1.9). In a progressive tunnel dryer, the ware is brought in from the right side. Temperature is relatively low at the beginning while humidity is high. In the course through the furnace, temperature increases and humidity decreases. Therefore, an almost constant drying speed can be adjusted for the complete drying process. Fig. 5.1.9: Combined contra and parelell air current in a tunnel dryer according to Krause. 185 Porcelain articles are sometimes dried together with the plaster form used for shaping. Plates, cups or bowls still lying on their plaster forms are, for example, directly blown on and dried at the same time (Fig. 5.1.10). Fig. 5.1.10: Principle of the blowing dryer. Bricks are dried in big chamber dryers by blowing the air directly between the bricks (Fig. 5.1.11). Chamber dryers are discontinuous aggregates. cross-section top view Fig. 5.1.11: Rotomixair (according to Thoma). A roller dryer (Fig. 5.1.12) can perform the drying continuously. In a flat chamber, the temperature across the whole channel can be maintained constant. For example, tiles are transported on the rollers without firing support. This drying process is particularly qualified for mass production. 186 Fig. 5.1.12: Roller dryer (Heimsoth). 5.2 Sintering Drying is followed by a further thermal treatment, considerably below melting point of the ceramics, the so-called sintering. Sintering promotes consolidation combined with further shrinkage. Now the ceramic products get their typical structure and their characteristics. 5.2.1 Sintering of porcelain, silicate and oxide ceramics Porcelain is made from the natural raw materials kaolin, feldspar and quartz (50:25:25 weight-%). Before chemical reactions between these raw materials take place, some physical and chemically bonded water is dehydrated (Fig. 5.2.1). Furthermore, some phase transformation can happen, which is in most of the cases related to volume changes. This may cause stresses during cooling and, if the temperature is inappropriately adjusted, lead to cracks in the components. 187 Fig. 18.104.22.168: DTA analysis of a kaolin . When quartz is used as raw material, at 573°C there is the low to high quartz temperature modification. This is combined with a volume expansion. At 870°C the formation of tridymite which at 1470°C is transformed into cristobalite. Both transformations are combined with volume changes. Tridymite can be transformed during cooling into its low temperature modification, as well as cristobalite (Fig. 22.214.171.124). To avoid stresses or crack formation, threshold times are adjusted in the T-t program during cooling. Fig. 126.96.36.199: Phase transformation diagram in the SiO2-system with the temperature. 188 Crystal transformations and also chemical reactions may occur during the sintering of porcelain. After the water dehydration, kaolin reacts with quartz to build mullite. The mullite content grows as the temperature increases (Fig. 188.8.131.52). Part of the quartz disperses in the melted feldspar, therefore the quartz content of the structure decreases as the temperature is increased (Fig. 184.108.40.206).