Important Stages of the Production Process - Introduction to Polymeric Composites with Rice Hulls - page 88

several new manufacturers of twin-screw compounding extruders have entered the
North American market during the past few years and this fact, combined with the
expanding product lines from well- known companies, has widened the field of choice
enormously.
The two main types of twin-screw extruders for compounding are co-rotating
intermeshing and counter-rotating non-intermeshing extruders. A third, a twin-
screw counter-rotating intermeshing extruder, operates at low screw speeds and is
used primarily in polyvinyl chloride (PVC) processing. The second major group of
extruders considered are twin-rotor continuous mixers, which use 'wing-type' rotors,
similar to those in Banbury batch mixers, rather than screws to accomplish mixing
and do not generate high pressures. These machines are based on the concept of
continuous mixing but, for compounding mixtures for composites with rice hulls, it
is recommended that twin-screw compounding extruders be used.
7.5.1 Basic Characteristics
Twin-screw compounding extruders use varying degrees of distributive and
dispersive mixing to combine two or more materials into a homogenous mass.
Distributive mixing blends the components uniformly in the mixture. In dispersive
mixing, materials undergo changes in physical characteristics such as reduction
in agglomerate size. Both types of mixing require mechanical shear energy to be
transmitted into the polymer matrix, created by the screws rotating against the walls
of the extruder barrel. Where these machines differ the most is in what happens to
the material in the shear field at the apex, where the two screws meet.
Screws of the co-rotating intermeshing types convey materials forward in a 'figure-
eight' pattern. In general, materials alternate between moderate shear against the walls
of the extruder barrel and high shear in the apex region. Shear-inducing kneading
blocks can be added to increase dispersive mixing and specialised screw elements can
be added for distributive mixing. Non-intermeshing screws impart only minimal shear
at the apex. Materials processed with non-intermeshing screws experience moderate
shear against the barrel walls, then relax in the apex region. In these machines,
the rotation of material in the screw channel and interchange of material between
the screws is said to provide constant distributive mixing. Compounder sections can
be added to provide higher shear for dispersive mixing.
Also using the counter-rotating non-intermeshing principle is a number of twin-rotor
continuous mixers. One feature of these machines is that they separate mixing and
pumping functions. Freed of the restrictions of combining pumping and mixing in
the same barrel, these types of machines reportedly have the flexibility to optimise