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For fluids where this proportionality does not hold, that is, where they possess nonlinear behavior, they are broadly classified as non-Newtonian. Some well-known categories of non-Newtonian fluid flow are dilatant, viscoelastic, and Bingham.

When discussing the flow characteristics of the feedstock, only viscous, plastic, and elastic effects are considered (Ref 6). In the extrusion process, to facilitate flow of the feedstock in the extruder and die assemblies, it should be viscoelastic, that is, the feedstock should possess an initially high strain rate that decreases over time as elastic stresses are developed. This type of behavior is more commonly referred to as "strain-thinning" behavior, or pseudoplasticity. Paints possess a strain-thinning characteristic; that is, they have a low viscosity at high shear rates (smooth paint flow during brushing) and a high viscosity at low shear rates (particles remain in suspension during storage) (Ref 2). Thus, the characteristic of major import in the extrusion process is the decrease in viscosity of the feedstock with increasing shear strain rate. Consequently, the flow stress (T) for a pseudoplastic material, having a yield stress (T0), is given by:

where 7is the shear rate equal to cffldt, '/is the shear rate factor, or the viscosity, and n is the exponent of shear rate. In the case of pseudoplastic materials, the value of the exponent is less than 1.

During compression, when the feedstock flows through the barrel, shear strain rates are low and high viscosity prevents phase separation and promotes consolidation. By contrast, during shaping, when the feedstock deforms as it flows through the die, shear strain rates are high and a low-viscosity feedstock minimizes the required extrusion pressure. A feedstock possessing pseudoplastic behavior, therefore, possesses both a high viscosity at low shear strain rates and a low viscosity at high shear strain rates, which is why it is the desirable feedstock flow behavior for binder-assisted extrusion.

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