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I have some conceptual doubts about shear viscosity and would like some help if possible. In general, viscosity depends both on temperature and on shear forces (more strongly on shear forces).

"Shear viscosity" is the viscosity that a molten plastic assumes when the molten fluid is subjected to shear forces that put it in motion, make the fluid layers slide one over the other. This happens when the molten plastic is pushed through a channel/pipe or mixed it. For most fluids, the shear viscosity decreases with shear (thinning).

That said, a capillary rheometer is the device used to measure shear viscosity versus shear strain rate (1/s). This device pushes the molten plastic through a small orifice once a certain pressure is applied at the top. Based on the molten plastic output flow rate, the shear viscosity is calculated. Inside a cylindrical pipe the molten plastic assumes a parabolic profile (more or less). This velocity profile represent the velocity gradient. The v gradient is related to the shear rate of deformation of the plastic. The smaller v gradient the less the fluid layers interact with each other. By knowing the shear strain rate of deformation and the shear forces we can determine the viscosity (slope of the graph).

How does the capillary rheometer calculate the shear stresses? Indirectly, by knowing the pressure applied at the top of the molten plastic?

How does the output flow rate from the rheometer orifice relate to the v gradient and to the shear strain rate of deformation? The larger the output flow rate the larger the strain rate?

thanks!

fog37

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# Shear viscosity and Capillary Rheometer

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