Warping and Cracking during Drying

Volume fraction profile during constant rate period (i.e., boundary layer mass or heat transport is the rate determining step)  

FIGURE 14J9 Log-log plot of wei it loss versus time for disc shaped Si powder green body made with different solvents and dried at different conditions (a) solvent-octanol, T = 200°C, Ni velocity 0.1 cm/sec (b) solvent-methanol, T = 80X , JV2 velocity 1.3 cm/ sec (c) solvent-acetcmitrile, T = 2TC N2 velodty 0 cm/sec. Takm from Castro et al. [25]. Reprinted by permission of the American Ceramic Society. 

The following analysis of stresses assumes that the green body is purely elastic. Certainly this is not the complete picture because wet, sticlq powders or gels are not elastic but plastic, showing deformation of the particle network by the frictional movement of particles against each other. We have discussed these mecheinical properties in Sections 

5 and 13.5 where the plastic nature of cohesive powder packings was discussed. The plastic nature of a green body greatly complicates the analysis of stress and for this reason is not presented here. 

Two types of stress are important in drying. The first is the total stress, which corresponds to the force per unit area acting on both the liquid emd the particle network. When the pores are filled with liquid, the stress is spread evenly over the whole green body, because the essentially incompressible liquid distributes the stress evenly in all directions. The second t3q e of stress is the network stress, which is the force per unit area acting only on the particle network. When we consider the warping and cracking of the particle network, the stress on the particle network is important not the total stress. 

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Warping and Cracking during Drying TABLE 14A Evaporative Drying of a Flat Green Body... 

Shrinkage during drying is one of the most important factors for the quality control of molded products. It is not only related to the deformation and the generation of crack, warp. 

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