A capillary rheometer is an apparatus designed to measure shear viscosity and other rheological (= flow) properties. Capillary rheometers for plastics are piston-die systems designed to measure viscosity of polymer melts as a function of temperature and rate of deformation. They are capable of testing basic polymers, compounds, various composites with small reinforcing particles or fibers, feedstock for metal injection molding and similar materials.
The basic principle is that a thermoplastic sample (originally in the shape of granules, powder or flakes) is made fluid by heating and forced to flow out of a cylinder through a capillary die. The measured quantity is normally the generated pressure under steady state conditions. A flow curve is the typical output, obtained by interpolation of several experimental data. Viscosity is represented as the Greek letter “eta” (η) and expressed (in SI units) in pascal seconds (Pa·s) or newton seconds per square meter (N·s/m2).
Capillary rheometers ensure testing conditions which are truly representative of processing conditions, especially for high-pressure and high-speed techniques like injection molding, therefore they are key for process optimization.
Other rheological properties that can be measured or estimated from capillary rheometers data include extensional viscosity, extrudate swell, thermal stability, wall slip. Ancillary measurements can be carried out for thermal conductivity, density dependence on pressure and temperature (pvT), melt strength.
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