a. What is viscoelasticty?
When liquids are subjected to a force, the deformation is permanent. This
property is called viscosity. Viscosity also is an indicator of the resistance
to flow. Liquids are said to be viscous. When solids, example: metals, are
subjected to a certain limited force, the deformation is not permanent. When
the force is released, the solid recovers to its original shape. This property
is called elasticity. The word ‘limited’ is used because excessive force causes
permanent deformation, another topic of discussion. Plastics exhibit both a
viscous component and an elastic component. They are said to be viscoelastic
and the behavior is called visoelasticity. This can be visualized with the
spring and dash pot model.
b. What are Newtonian and Non-Newtonian fluids
Newtonian Fluid - A fluid whose viscosity is not influenced by the shear
rate that is being applied to it. The viscosity remains constant as the shear
changes. Shear rate in injection molding is synonymous to injection speed.
Non-Newtonian Fluid - A fluid whose viscosity changes with the change in the
shear rate that is being applied to it. The viscosity does not remain constant
as the shear changes.
c. How is the melt viscosity in polymers different in
All plastics are non-Newtonian. This means that their viscosity does not remain
constant over a given range of shear rates. In the strict sense, the
rheological behavior of a plastic is a combination of non – Newtonian and
Newtonian behavior. At lower shear rates, the plastic is non-Newtonian but as
the shear rate increases, the plastic tends to exhibit a Newtonian behavior.
This happens because with increasing shear rate, the polymer molecules start to
untangle from each other and start to align themselves in the direction of
flow. Refer to the picture below.