Presentation of melt flow data

Melt rheometers either impose a fixed flow rate and measure the pressure drop across a die, or, as in the melt flow indexer, impose a fixed pressure and measure the flow rate. Equation (B.5) gives the shear stress, but Eq. (B.IO) requires knowledge of n to calculate the shear strain rate. It is conventional to plot shear stress data against the apparent shear rate y, calculated using n = 1 (assuming Newtonian behaviour). If the data is used subsequently to compute the pressure drop in a cylindrical die, there will be no error. However, if a flow curve determined with a cylindrical die is used to predict 

The apparent shear viscosity decreases as the shear strain rate increases. 

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