Abstract
AbstractFor Abstract see ChemInform Abstract in Full Text.
Highlights
Introduction to SemiSolid Processing Mascara, honey and certain kinds of paint are all thixotropic
Modelling 6.1 Model of Brown and Co-workers 6.2 Finite Difference Modelling 6.2.1 One Phase Finite Difference Based on the Brown et al Model 6.2.2 One Phase Finite Difference Based on FLOW3D 6.2.3 One Phase Finite Difference Based on MAGMAsoft 6.2.4 One Phase Finite Difference with Adstefan 6.2.5 Two Phase Finite Difference 6.3 Finite Element Modelling 6.3.1 One Phase Finite Element 6.3.2 Two Phase Finite Element 6.3.3 Micro-Modelling 7
Stirring breaks up the dendrites which would normally be present so that the microstructure in the semi-solid state consists of spheroids of solid surrounded by liquid (Fig.1)
Summary
In a Newtonian fluid, the shear stress,τ is proportional to the shear rate, γ , and the constant of proportionality is the viscosity, η. Thixotropic fluids are non-Newtonian i.e. the shear stress is not proportional to the shear rate. If a fluid exhibits a yield stress and gives a linear relationship between shear stress and shear rate, it is termed a Bingham material (Fig.). It is thought that at very high shear rates and at very low shear rates, thixotropic fluids effectively become Newtonian This is expressed in the Cross model [67]: η. The fraction solid decreases with increase in temperature, with a consequent effect on viscosity (see Fig. 14). For a thixotropic material at rest, when a step increase in shear rate is imposed, the shear stress will peak and gradually decrease until it reaches an equilibrium value for the shear rate over time (Fig. 15). The peak viscosity encountered will increase with increasing rest time before it recovers back to the equilibrium viscosity of the shear rate specified
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