2
Melt rheology and rheometry
Two main groups of material property data are needed to analyse and simulate flow in
the injection moulding process:
1
Properties of resistance to flow under stress (
rheology
):
expressed by shear stress
vs
. shear strain rate data;
2
Properties of static compressibility and thermal expansion:
expressed using PVT
data.
These notes deal with the first group:
rheological properties and their measurement
(
rheometry
).
Rheometric methods have to be simple and analysable, so they provide a clear
introduction to the analysis of melt flow in injection moulding and a warning about
possible problems.
Learning outcomes
After completing this section you should be able to…
1
Demonstrate an understanding of the theoretical basis and practical application
of a Capillary Rheometer and a Cone and Plate rheometer.
2
Manipulate power-law, Cross, Cross-Arrhenius
and
Cross-WLF
constitutive
equations for polymer melts, and use them to represent data.
3
Assess whether Bagley or Rabinowitsch corrections are needed in calculations for
polymer melt flow in channels (e.g. rheometer capillaries, mould runners and
gates), and apply them if necessary.
4
Demonstrate understanding and make appropriate use of the following
terminology:
apparent viscosity, Bagley correction, consistency, Cross model,
degradation, die, extensional flow, extensional viscosity, flow curve, melt, melt
slip, MFI, newtonian viscosity, power-law flow, pseudoplasticity, Rabinowitsch
correction, reference viscosity, reference shear rate, jetting, rheometer (capillary,
cone-plate), shear rate (= shear strain rate), temperature coefficient of viscosity,
viscometer = rheometer, zero-shear viscosity.
2.1
Revision
SPAP introduced:
1
The
MFI test
:
a widely-used industrial test which provides a single-point ‘index’ of
melt viscosity.
ME4
Polymer Processing Technology
PSL
19 October 2009