Curing kinetics of ceramic slurries used in investment casting with ice patterns

Abstract

Ice patterns can be used to make ceramic investment moulds for metal castings. Using ice means that the ceramic slurries must be poured around the pattern and cured at sub freezing temperatures. Success of this process depends significantly on the curing kinetics of the slurries. This paper describes the experimental results of an investigation into the curing kinetics of slurries with differing compositions. The variables considered include volume percent solids, temperature and the volume of the catalyst. Spindle speed was also included as an experimental variable to see if the viscosity exhibited shear rate dependency. The Taguchi method of experimental design was used to reduce the number of trial runs. The curing kinetics of the slurries was studied by measuring the viscosity variation of the slurries with time. The data suggests that there is an initiation and propagation stage and a reactant depletion stage during the slurry gelation process. Three models are evaluated for fitting the time dependence of the viscosity data: a simple exponential in time, a Dougerty–Krieger exponential where there is a defined end point time, and a two stage model developed by superposing a simple exponential for the second stage with an Avrami kinetics model for the first stage. The results indicate that the two stage model can best describe the curing. Significance and effects of the process variables are discussed. For stage one, the characteristic time is strongly dependent upon the amount of catalyst, the spindle speed and the solids loading. For stage two, the characteristic time is strongly dependent upon the amount of catalyst, as well as temperature. A cylindrical ice part is used as an example to investigate the dimensional accuracy. The measured outer diameters of the castings are very close to the nominal outer diameter.