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It offers an opportunity to work on the material by the application of forces that shear or
smear the material, elongate, re-orient, compress or fold the material while retaining the
control over the time the material is subjected to this rigor. There exists certain limits for
shear rates or strain with respect to one or the other type of stress field, but such limits are in
the neighborhood of the desired values. The overwhelming influence of shear is a decisive
factor. Some applications benefit from high shear rates while in others, it is detrimental.
This work focuses on the beneficial effects of shear particularly in mixing and melting.
Effects of the other forces such as extensional, specific aspects of reactive extrusion and
efforts to reduce shear influence are not being dealt with here.
Fundamental Concepts inMixing:
Separation of bundles, size reduction of agglomerated particles, uniform distribution
(establishing homogeneity) and wetting or impregnation of immiscible constituents in the
matrix are the most common terms used in describing mixing requirements. Unfortunately,
the usage is severely corrupted. For example, the term dispersion is used for all aspects of
mixing including wetting. Essentially, requirement of size reduction or multi axial separation
of agglomerated particles is referred to as dispersion or dispersive mixing. Separation of
bundles without attrition of fibers is referred to as distributive mixing. The last two aspects
namely establishing homogeneity and wetting is normally referred to as kneading. The
essential aspect of kneading is to completely surround every individual immiscible
particulate matter with the wetting medium in a manner that ensures the composition of the
smallest required volume in the material is the same as the entire volume of the material. In
every section, the material appears the same to the naked eye as well as microscopic
examination. Wetting results in establishing a weak bond such as Vanderwaal’s bonds or
polar bonds that may need to be strengthened by stronger bonds generally called as
coupling. Michler (1998) provides evidence of such coupling seen in Figure 1. Inadequate
or incomplete size reduction and re-agglomeration are common problems experienced in
dispersive mixing. Attrition or loss of fiber length and lack of wetting is the usual problem in
distributive mixing.
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Figure 1: SEM Pictures of Glass Fiber Wetting & Coupling
THE EXTRUDER TIMES / ISSUE 08 / JANUARY-MARCH 2009
E N H A N C I N G
Capabilities
in a Twin-Screw Extruder
MELTING
AND
MIXING