Rheology of a thermoplastic paste through the mushy state transition

The rheology of a highly filled thermoplastic paste employed in a novel continuous casting process was investigated across the melt-mushy zone temperature transition. The paste comprises of an ultrafine (or nano-particulate) zinc oxide powder (d1,0=85nm, 50 vol%) and a continuous phase consisting of a blend of organic waxes with an onset of solidification around 54∘C. Data were collected over nine decades of shear rate, using controlled strain and controlled stress devices, steady shear and oscillatory modes, rotational rheometry, and capillary rheometry using the multi-pass technique. The melt rheology could be described using a Carreau-type constitutive model, with strong evidence of a low shear rate viscosity plateau and a possible high shear rate viscosity plateau, with a transition between the two described by a highly shear-thinning power-law region. Nearer the solidification temperature the material exhibited strong pseudo-plastic behaviour, and capillary flow appears to be determined by pressure-dependent slip behaviour.