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Mixing is an essential function in an Extruder. The Goal of the mixing process in an extruder is to increase the uniformity of the composition.
Mixing or the lack of it results from the work done in the extruder that causes ingredients (usually immiscible) to experience forces of shear,
elongation or compression, bending, erosion and impact. Shear and extensional flow are the two common types of flow in a co-rotating twin-
screw extruder. Extensional flow can occur as a result of building up of pressure or during its release.
Kneading
Mixing is complete only if 'wetting' is achieved and the term “kneading” refers to the action in the extruder that causes wetting. Achieving a
chemical union (wetting) of two or more components is an important objective of the compounding process. Complete wetting or 100% wet
state can be defined as the state of a “foreign” or an immiscible particle which is completely surrounded by the fluid molecules (melt) that have
penetrated / bonded to it due to the forces of attraction. Wetting can be “Natural” but facilitated by the use of wetting agents like wax or it can be
“Forced” by the use of a coupling agent since wetting depends upon the magnitude of affinity that exists between the particle and the melt. In
the SEM image of glass fiber in Polypropylene (PP) showing poor wetting,
(Fig. A)
it is clear that the individual fiber are completely surrounded by
PP. However, this did not result in any kind of chemical union. However, the pictures showing good wetting, the glass fibers are not only
surrounded but also coupled with PP. A coupling agent is necessary since the glass fiber does not have natural affinity for PP.
Zone # 4 The MIXING ZONE
(Fig. A)
SEM Images of Poor Wetting (Left) and Good Wetting (Right)
(Source: Sascha Englich, Master's Thesis U. Chemnitz,
Research carried out at STEER Engineering Bangalore)
A kneading action that comprises of folding, pressing, and
stretching as in the case of 'dough kneading by hand' can be
performed in a co-rotating twin-screw extruder. However,
due to limitations in understanding the nature of work
carried out in an extruder, kneading action as a result of
shearing action is the predominant way to create wetting.
This type of action results in wasteful energy input without
useful mixing. Use of Fractional Elements (US Patent
6783270) in the form of kneading blocks can result in higher
wetting action due to inherent design advantages such as
uniformity of shear, increased elongational mixing ability
without sacrificing the cleaning action. If kneading blocks
have to be avoided, continuous mixing elements with
Erdmenger profile or Sakagami profile (SMAP elements) are
highly effective in creating the right circumstances for
wetting. The difference between the design that employs
kneading blocks and continuous mixing elements is in the
dispersive nature of the two types. Dispersion and
Distribution are two important terms that are constantly
encountered while discussing Mixing. Mixing is a broad
term that comprises of both chemical as well as physical
action. In terms of physical action, mixing can be broadly
classified as “Dispersive” or “Distributive”.
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