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Table 2. Densities and Porosities of Industrial Shells Used in This Study Bulk Density, g/cm3 1.64


Shell #1 Shell #2 Shell #3 Shell #4 Shell #5 Shell #6 Shell #7


Theoretical density, g/cm3 2.41


1.53 1.63 1.93 2.24 1.98 1.96


ity and thermal conductivity of the shell. For higher specific heat capacity, more energy is needed to heat up the shell to a certain temperature. Tus, the solidification time will be shorter. Higher thermal conductivity will allow the heat of the liquid metal to flow through the shell more quickly, which also shortens the solidification time and increases the maximum tempera- ture of the shell. Sensitivity testing by modeling also showed the exter- nal heat transfer coefficient mainly effected the shell and casting cooling rates after solidification. To evaluate the shell density and


porosity, whole pieces of the shell containing all layers were examined and the overall bulk density and open porosity accessible for water were calculated. Seven industrial ceramic shells


(Tables 1 & 2) were evaluated in the study and a thermal property data- base was developed. According to the results of the tests, temperature- dependent specific heat capacities in all shells had a similar trend, but the average and maximum values mainly depended on the phase of the start- ing materials and the reactions and transformations during the thermal processing, which were not readily predictable (Fig. 2). Generally, at above room tem-


perature, the thermal conductivity of the most dense ceramics decreased with increasing temperature because phonon scattering is more intense from the vibrating lattice at a higher temperature. However, the investment casting shells, where the colloidal silica is used as a binder in most cases and a significant amount of fused silica is utilized as flour and stucco, more often showed an increasing thermal conductivity at higher temperatures due to the photon radiation becoming dominant at higher temperature in semi-transparent silica.


-


2.42 2.90 3.30 3.18 3.26


Open porosity, % 21.7


25.7 23.0 23.8 21.0 26.1 26.7


Closed porosity, % 10.0


-


9.9 9.7


11.1 11.6 13.1


Total porosity, % 31.7


-


32.9 33.5 32.1 37.7 39.8


Fig. 3. Graphs show (a) specific heat capacity and (b) thermal conductivity values of the shells studied and determined by the improved laser flash method.


January 2016 MODERN CASTING | 45


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