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Modern high performance extruders are distinguished by their
specialized features. Although, performance measurements are
common to the industry, new developments have resulted in a large
pool of useful information that needs to be disseminated. New
material grades with specific process requirements are being
developed and extruder manufacturers need to accept fresh
Within the span of several decades, necessary improvements were
challenges to provide solutions that give processors improved
made to the twin-screw extruder gearbox and thrust bearing design
process capability to achieve the desired quality at economically
which resulted in increased throughput and greatly extended the
viable throughput rate. Yet, most twin-screw extruder suppliers rely
service life. The twin-screw extruder design evolved to include a
on traditional design that can limit the machine’s range of
modular barrel and segmented screw which enabled a high level of
performance.
flexibility for quickly adapting to various processing requirements.
Several notable companies (Werner Pfleiderer, Leistritz, Berstorff,
What is a TSE?
JSW, Toshiba and others) have made significant contributions to the
The basis for a twin-screw extruder (TSE) is in essence two
advancement of the co-rotating twin-screw extruder design. In more
Archimedean screws positioned in a parallel configuration. The use
recent years, STEER has introduced innovative technologies which
of dual screws as opposed to a single screw enables a degree of
have further advanced twin-screw extruder performance. STEER has
positive conveying as a means for material transport in contrast to a
created a paradigm shift that has “Game Changing” consequences to
single screw’s drag flow mode of conveyance. Twin-screw extruders
the customer by giving the ability to treat the extruder as a mixing
were first applied to plastics processing in the first half of the 20th
vessel. STEER’s “
Generation Next
” slogan embodies its corporate
century. Two distinct design approaches were successfully
philosophy to advance twin-screw extruder performance to the
implemented and characterized by the direction of rotation of the
highest degree.
screws in relation to one another, namely counter-rotating and co-
rotating screws. Another aspect in twin-screw design is the degree
Why a TSE and how TSEs over the years have become a
of intermesh between the two screws which ranges from zero or non-
necessity?
intermeshing to fully intermeshing. The focus of this article will be on
the closely intermeshing, co-rotating twin-screw extruder.
The invention of the intermeshing, co-rotating, twin-screw extruder
is attributed to Roberto Colombo of Italy with early application in the
processing of cellulose acetate and PVC.
A major adaptation of the co-rotating twin-screw extruder came by
the way of Rudolf Erdmenger, a German engineer employed by Bayer
AG in Germany. Erdmenger’s keen insight into the science of
kinematics introduced a “self-wiping”, closely intermeshing profile to
the design of extruder screws. This resulted in self-cleaning of the
screws as the flight of one screw wiped the root of the adjacent
screw. This invention also provided for a degree of positive
conveying of the material which reduced average residence time of
the material in the extruder.
Twin-screw extruders have become the preferred choice for
compounding and continuous reactive and devolatilization processes.
In fact, most plastic in use today has undergone a compounding step
prior to becoming an end-use product. While single screw extruders
still have a major stake in extrusion, twin-screw extruders offer a
number of advantages over single screw extruders:
1. Stable intake of a wide range of materials due
to higher conveying efficiency
2. Ability to perform multiple process operations
in one machine
3. Flexibility to change formulations
4. Capable of feeding multiple components
5. Adjustment of mixing intensity or residence
time by screw rpm
6. Self cleaning aspect of the screws
7. Shorter length required to melt
8. Flow patterns in the twin-screw extruder offer
much improved mixing over the single screw
extruder
9. Shorter residence time
10. Rate independent of discharge pressure