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The kind of work that can be carried out
inside the extrusion apparatus involves
application of forces that causes
shearing or smearing, elongation,
bending, torsion and compression, a
cocktail of forces that have a general
term called kneading applied to it.
Origin of Co-rotating Twin-Screw Extrusion:
By the middle of the 20th century, extrusion equipment based on a single screw was well
established in rubber applications and in the process industries such as soap and brick
making. Certain specialized devices such as Banbury mixers were also available for
specialized mixing of viscous materials. However, the need for a device that can
continuously process a chemical reaction that results in increase of molecular weight or
viscosity without gelling spurred the development of newer extrusion technology that would
later change plastics compounding forever.
The early co-rotating twin-screw extruder was conceived as a reactor vessel where control
was needed in the positive movement of the molecules with the possibility of mixing or
stirring action. Roberto Colombo (1938) of LMP, Turing developed the first commercial co-
rotating twin-screw and sold it to I.G. Farben. This design was licensed to manufacturers in
France (now Clextral), England (R. H. Windsor) and Japan (Ikegai). Bayer-werk of I.G.
Farben formed the High Viscosity group to develop further the extrusion technology for
reactive extrusion. Booy (1978) provides the details of the work by Erdmenger (1954) who
was assigned the task of engineering the corotating extrusion technology which was one of
the three extrusion concepts that the high viscosity group led by Meskat pursued. Counter-
rotating twin-screw extrusion developed by Kiesskalt and reciprocating single screw devices
called Co-kneaders developed by List were the other two competing technologies.
Erdmenger (1954) conceived the exact geometry that leads to perfect wiping of the screw
profile in a co-rotating twin-screw extruder. During the last few decades, this technology has
revolutionized plastics processing in the same way a personal computer transformed
information processing. Many manufacturers making devices with competing technologies
have changed their products to co-rotating technology. While a counter-rotating extruder
becomes a positive displacement “plug” flow device, a co-rotating extruder works similar to
a single screw with some unusual features.
The kind of work that can be carried out inside the
extrusion apparatus involves application of forces that
causes shearing or smearing, elongation, bending, torsion
and compression, a cocktail of forces that have a general
term called kneading applied to it.
In a co-rotating twin-screw extrusion system that is setup
to deliver the right kind and amount of work (as a result of
application of the right forces) at the right place (in the
various extruder processing zones) for the right amount of
time (resulting in a narrow residence time distribution
considering all the molecules in a given volume), the
progress of material through the extruder is generally
highly controlled and such a system can be used to
conduct sophisticated chemical reactions apart from the
afore mentioned physical process to prepare materials
with unique properties.
An extruder is a vessel for continuous processing of
polymer and other materials where fusion of all ingredients
can occur that may be accompanied by a chemical reaction
or volatile extraction. The various ingredients in the
material are forced to interact closely while undergoing
size reduction (dispersion) to create a homogeneous mass
as a result of weak adhesive bonds.
The interaction of the materials in different states needs
the presence of at least one polymer material that becomes
the wetting medium or the matrix. Generally, the wetting
medium is not present in a viscous state at the time of
commencement of the process and therefore primary
fusion of the matrix has to occur preceding the mixing or
homogenization process.
E N H A N C I N G
Capabilities
in a Twin-Screw Extruder
MELTING
AND
MIXING
Melting and Mixing Capability Enhancement: Technology Revealed
Babu Padmanabhan
Keywords
: Continuous Processing; Co-
rotating twin-screw extrusion; Kneading;
Narrow Residence Time Distribution;
Material Preparation.