Rheology and fiber orientation during injection molding of ceramic suspensions

The property performance of whisker reinforced ceramic composites is controlled by whisker location and orientation. This research was focused on the rheology of ceramic pastes and its use in predicting and controlling the whisker orientation during injection molding. In particular, suspensions of silicon particles and silicon carbide whiskers in polyethylene were studied. Both capillary and cone and plate rheometers were used to characterize the suspensions.; Concentrated suspensions exhibit a yield stress which represents initial resistance to deformation. A rheological constitutive equation has been proposed to explain the steady flow behavior of materials with yield values and a correlation was developed to relate the steady and oscillatory shear properties. This correlation allows for estimation of the model parameters from the dynamic measurements alone. The model has been verified with experimental data on a variety of ceramic suspensions and the model parameters were obtained from the dynamic measurements. The steady state model was later extended to explain the transient behavior in thixotropic fluids such as stress overshoot during the startup of a steady shear flow and stress relaxation after the cessation of flow. The modification was based on kinetics of the processes which are responsible for these structural changes, i.e., the breakdown and buildup under the action of external shear. The model can now explain the rheological behavior of a wide spectrum of thixotropic fluids and the predictions for oscillatory shear flow were found to be in good agreement with the experimental data.; This rheological information was used to predict the velocity profiles and whisker orientation during mold filling. The finite element program POLYFLOW was used to obtain the velocity profiles and orientation predictions were obtained using the Folgar-Tucker approach for fiber rotation in concentrated suspensions. The numerical algorithm was used to demonstrate the strong influence of gate location on the velocity profiles and hence on the fiber orientation. Orientation predictions from the numerical simulations were found to be in excellent agreement with the experimental data provided by GTE Laboratories for both single-gated and double-gated mold cavities.