Control of yield stress in low-pressure ceramic injection moldings

Wax-based zirconia mixtures with solid content ranging from 55 to 60 vol% were prepared for low-pressure injection molding (LPIM) application. In LPIM, an appropriate yield stress of the suspension may be critical for two specific processes, namely, molding and thermolytic debinding. The former requires a low viscosity and yield stress which facilitates conveying and molding of the suspension at elevated temperatures. However, the latter requires the molded artifacts to have sufficient yield stress preventing deformation on thermolytic debinding. Therefore, to properly manipulate the suspension property, a better understanding and control of the yield stress of the suspensions becomes critical for optimization between the two processing stages in the LPIM process. In this investigation, the yield value of the suspensions together with the corresponding value for a successful LPIM application is described with an additional help by visual examination of the molded specimens during thermal debinding. The yield stress is correlated linearly with a dimensionless parameter, φ/A - φ defined as flow resistance parameter, where A represents the maximum particle packing density and is shown to be related dynamically to the shear force. A further analysis also explain the yield stress as a result of the van der Waals attraction in the suspensions. A model is proposed for the determination of interparticle distance (λ) which produces the λ consistent in magnitude with those reported in literature.