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The measurements cover shear rates from 1.4 to 1400 sec-1 and illustrate the early portion of the behavior shown in Fig. 1).įigure 2 shows an actual viscosity/shear rate curve for a commercial acetal copolymer with a nominal melt flow rate (MFR) of 9 g/10 min. But logarithmic plots of viscosity versus shear rate tend to level off as the shear rate approaches zero, so we can extrapolate the curve back to the y-axis to arrive at the value for zero-shear viscosity (see Fig. So to measure resistance to flow you have to make the polymer flow, and as soon as you do so the shear rate becomes some non-zero value. In practice you cannot measure viscosity at a shear rate of zero because viscosity is a measurement of resistance to flow. Zero shear rate is a concept only a mathematician could love.