Influence of solids loading on aqueous injection moulding paste

Model aqueous injection moulding (AIM) pastes were investigated by capillary rheology at five levels of solids loading. The critical volume fraction of solids (phi(m)) was measured by water absorption and estimated by linear and non-linear (where appropriate) extrapolation of the rheological parameters, reciprocal apparent viscosity and reciprocal bulk yield str ess. Three models which consider the influence of volume fraction of solids on relative viscosity were considered: the Chong, German, and Krieger-Dougherty equations. An iterative method for estimation of phi(m), dependent on the model used, was also developed for comparison. The German model using the iterative method for estimating phi(m) gave the best prediction of overall rheological behaviour. The Krieger-Dougherty method using the curve fitted reciprocal bulk shear stress to find phi(m) also gave low errors and correctly predicted a relative viscosity of unity for the pure binder, suggesting an improved prediction over a wider range of volume fraction of solids than the German equation. The power law exponent n was shown to increase with increasing volume fraction of solids. This anomalous behaviour is explained in terms of a change in the flow mechanism fi om shear pow to slip flow through a transitionary region.