Introduction and Motivation

Since homogenous melts are covered in a later account of pressure build-up and power input in the extruder (Chapter 7), this chapter confines itself to the flow behavior of homogenous unfilled polymer melts and on the introduction of the most important rheological parameters such as viscosity, shear thinning, elasticity, and extensional viscosity. The influence of these rheological properties on simple pressure- and drag flows is demonstrated, while the influence of rheological parameters on pressure build-up and power input in the extruder is described in more detail in Chapter 7. 

Even at a phenomenological level, the flow behavior of polymers differs significandy from that of low-viscous liquids such as water. The flow behavior of polymer melts is markedly dependent on the applied shear stress o, the shear rate y, and on time. Elastic effects and normal stresses also occur in practice, and their effects can either help or hinder the processes underway. 

A fundamental rheological value is the viscosity function, i.e., the shear viscosity as a function of the shear rate r] = f(y). The relationship between rj and y is determined by experiments and is described by means of heuristic functions. The viscosity measurement of fluids is shown here by shearing a fluid between two parallel plates (Fig. 3.1). The lower plate is fixed [u(x2 = 0)=0] and the upper plate is drawn across it at a speed of uo [u(x2 = H) = Uo], for which a force F is required. The shear rate is defined as a time derivation of the deformation 6L relative to the height H in a time period dt, or as a quotient  

The viscosity is derived from the shear stress a—F/A and the shear rate y  

At infinitesimally small shear rates, the shear viscosity is described as zero shear viscosity  

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