Rheology testing is measuring the deformation of matter under the influence of imposed stress,
by analysing the internal response of materials to forces. Polymer flow characteristics are critical
in all the conversion and production processes: the material is forced to flow and the rheological
characteristics determine the processability.
Why is Rheology Testing Important?
Firstly, flow is involved in the processing and fabrication of such materials in order to make
useful objects. Thus, fluid rheology is relevant to polymer processing and determines stress levels
in operations such as extrusion, calendaring, fibre spinning, and film blowing. Similarly, rheology
influences residual stresses, cycle times, and void content in composite processing operations
such as blow moulding, compression moulding, and injection moulding.
Secondly, in the use of
polymers the mechanical properties are very important, and the mechanical behaviour is also influenced
by rheology. Then, rheological measurements are also employed for product characterisation
and quality control purposes. Such measurements are often used for examining and understanding the
interaction of the different constituents of a multi-component or multiphase mixture and their influence
on the flow and other properties of such materials.
Different Types of Rheological Behaviour
All the rheological properties of complex fluids, like polymer melts, depend upon the rate of deformation
(shear rate), the molecular weight and the structure, the concentration of various additives and fillers,
as well as the temperature. In addition, even at a constant rate of deformation, stresses are found to
depend on time.
Different viscosity (or Melt Flow Index) =
Different behaviour during flow
Viscosity = Resistance to the flow =
Opposite of fluidity = Inversely proportional
to Melt Flow Index (MFI, MFR)