Viscosity is one method used by foundries to control their slurries and, ultimately, the characteristics of their shells. In this paper, the relationship between slurry viscosity and shell properties such as strength, thickness and permeability is explored. A series of experiments was undertaken to establish the effect of backup slurry viscosity on these key shell characteristics. Test shells were made using a common primary coat and four backup slurries of varying viscosity. Viscosities were measured and controlled using the Zahn flow cups commonly used in the investment casting industry. In addition, the effect of stucco particle size on shell thickness was investigated by using three different stucco grades to build the test specimens. Each set of shells were tested using industry standard methods to evaluate the effect of backup slurry viscosity and stucco size on green, hot and as-fired properties. Numeric and graphical results from the experiments are presented. The conclusions drawn will help the investment caster understand the complex interactions involved during shell build as well as indicating how systems can be modified to optimize shell properties.
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