The Historical Development of the Counter-rotating Twin

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PVC Processing. Counter-rotating, closely intermeshing twin-screw extruders are
primarily used nowadays for the production of pipes, profiles and sheet of unplasticised PVC. This article traces the evolution of this machine technology from its
origins down to the present.
The Historical Development
of the Counter-rotating
Twin-screw Extruder
Following the founding of the Anger
company, the brothers Anton and Wilhelm Anger developed a process in the
early 1950s for extruding powder into a
finished pipe in a single working step
(Fig. 1) [1]. Almost simultaneously, they
also succeeded in solving the problem of
how to join pipes by means of plastic. In
1954, Anger developed a Mapre extruder. Just one year later, the first self-developed twin-screw extruders were built.
These were 3-section screws (without
vent section) and had a length of 12D.
Fig. 1. Technicians during early extrusion trials (Anger, 1953) [1]
HANS-PETER SCHNEIDER
n pipe and profile extrusion, conical
twin-screw extruders are primarily
used in the lower to medium performance range as principal and coextruders. Parallel twin-screw extruders are
mostly used in the medium-to-high-performance area. For pelletizing, where the
maximum output performance is
achieved, parallel extruders are used almost exclusively.
I
Translated from Kunststoffe 5/2005, pp. 44–50
Kunststoffe plast europe 5/2005
Historical Development
PVCu pipes were first laid in 1935 as
pressure pipes for public water supplies.
Pipes of PVCu suffer from neither corrosion nor incrustation. It was the search
for efficient ways of producing these
PVCu pipes for water supplies that
sparked the development of counter-rotating twin-screw extruders. The family
tree of extruder manufacturers, as it
were, has three main branches: Anger
(Mapre), Kestermann and Schloemann
(Pasquetti).
Fig. 2. Twin-screw extruder BT 150/11,5D
(Schloemann, 1964) [2]
After a test period lasting several
years, combined with extensive theoretical
studies and practical trials along with a
whole range of pipe-laying operations,
Anton and Wilhelm Anger, together with
stake holders, founded the company
Kunststoffwerk Gebrüder Anger GmbH &
Co. Its headquarters were in Munich and
the production shop was in Bogen on the
Danube, in the buildings of a former
leather factory. The company’s first extruders were launched in 1959.
In 1960, the Schloemann company
took over the Pasquetti company and al-
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2
so began building twin-screw extruders.
Models BT 50/12D, BT 80/12D and BT
100/12D were followed in 1964 by the BT
150/11.5D, which had an output rate for
pipe of 200 kg/h. It had a 70 kW electric
motor and a heating capacity of 48 kW
(Fig. 2).
Weber produced its first twin-screw
extruder DS 60 in 1961. This had a processing section of 12D but no venting. It
did not launch longer machines with
venting, such as the DS 60–17D, DS
85–16D and DS 120–17D, until the period from 1964 to 1968.
In 1962, the Rheinstahl Group acquired a majority holding in Kunststoffwerk Gebrüder Anger GmbH & Co. By
that time, the plant had 30 pipe-production lines and was one of Europe’s
leading manufacturers. In 1964, Anton
Anger eventually founded the company
AGM (Anton Anger Allgemeine Maschinenbau GmbH), headquartered in
Linz/Austria. His brother Wilhelm’s
company, located in Vienna, called itself
APM (Anger Plastic-Verarbeitungsmaschinen GmbH & Co).
While AGM built conical and parallel
twin-screw extruders, APM concentrated
on the production of one-stage and twostage parallel extruders (Fig. 3).
From 1964 on, Kestermann also started offering single-stage twin-screw extruders without venting (K2-80, K2-100
and K2-120) and two-stage designs with
venting (K2-80/86V, K2-100/107V and
K2-120/130V). Later, the single-stage machines were also fitted out with venting.
In 1968, Reifenhäuser built an extruder with a diameter of 125 mm and a processing unit of 16D (without venting)
(Fig. 4). Reifenhäuser subsequently acquired the twin-screws (Bitruder) of
Schloemann in 1972.
In the year 1969, Cincinnati started
selling its conical extruders CM 45
(Fig. 7), CM 55, CM 65 and CM 80. The
program at that time was rounded out
with a two-stage parallel machine (A4/
125/125).
Apart from the companies just mentioned, further producers of counter-rotating twin-screw extruders in the 1960s
were Bausano (Italy), Bandera (Italy),
Mapré (Luxembourg) and Leistritz (Germany).
Rheinstahl eventually acquired Kestermann’s activities in 1970. In so doing, it
acquired the machine design in which the
exchangeable breaker plate was secured
against rotation in the barrel (Fig. 5).
1971 was the birth year of KraussMaffei Extrusionstechnik. Initially, it was
headquartered in Munich. The chief engineers came from AGM and APM. Apart
from a conical KMD50K, it offered two
parallel extruders, namely the KMD9020D (Fig. 6) and the KMD120-20D. In
1974, Krauss-Maffei Austria was founded and engineering and marketing were
switched to Asten near Linz. Production
of the extruders continued in Munich.
The headquarters of Krauss-Maffei Extrusionstechnik were switched back to
Munich in 1979, where they have remained ever since.
Thyssen and Rheinstahl merged in
1972. Three years later, Thyssen Plastik
Maschinen (TPM) was founded. This
company evolved out of the former division of Thyssen Rheinstahl, whose activities were mainly based on Kestermann’s
extrusion technology. After a very short
development period, the new twin-screw
extruder series was presented in 1976.
These were exclusively parallel models
Fig. 3. Two-stage twin-screw extruder A4/80/84
(80 and 84 mm, 11D and 8D), (model APM) [2]
Fig. 4. Twin-screw extruder 125 -16D (Reifenhäuser, 1968) [2]
Fig. 5. Twin-screw extruder (Kestermann, 1970)
[4]
Fig. 6. Twin-screw extruder KMD 90-20D
(Krauss-Maffei, 1973) [6]
Fig. 7. Twin-screw extruder CM 45 (Cincinnati,
1976) [3]
Fig. 8. Twin-screw extruder TPM2- 90-22V (TPM,
1980) [7]
Fig. 9. Conical and parallel twin screws
© Carl Hanser Verlag, München
Kunststoffe plast europe 5/2005
SPECIAL ■
Fig. 11. Conical twin-screw systems
with screw diameters of 50, 60, 85, 107,
130 and 160 mm. The processing units
varied with the application, and were
either 16D or 22D. The TPM 90 (Fig. 8)
was added to the range in 1979. The new
Service and Development Centre in Dornach near Munich opened in 1979, only
to shut again in early 1980. The company then moved to Bad Oeynhausen/Germany. This coincided with the birth of
Battenfeld Extrusionstechnik.
Development of
Conical Extruders
Two designs have prevailed for counterrotating, closely intermeshing twin-screw
extruders: the parallel and conical types.
In the parallel variant, the external screw
diameter and the axial distance remain
constant over the entire screw length,
whereas, in the conical variant, it varies
continuously (Fig. 9).
Parallel extruders have been made industrially since the 1950s. In the formative years, the mechanical reliability of the
parallel twin-screw extruders did not always come up to scratch. The main problems occurred in the region of the driven
shafts. On account of the narrow axial distances, it was not possible, with the bearing technology available then, to accommodate long-term radial and axial forces
by means of appropriate dimensioning
techniques. It was not until the end of the
1960s that bearings became available
which enabled parallel extruders to offer
adequate operational reliability [8].
The problem of safely accommodating
the radial and axial forces was solved with
Kunststoffe plast europe 5/2005
The inventor of the Krauss-Maffei
the development of conical twin-screw
extruders. In these machines, the screw patent co-founded Maplan in 1977 and
external diameter and the axial distance circumvented his own invention. Maplan
increase steadily from the screw to the filed a new patent [11] in 1978 in which
transmission. This offers design advan- the overlapping ratio of the double contages for shaping the distributor drive: ical screws, relative to the Krauss-Maffei
First of all, the two bevel wheels of the dis- variant, was altered slightly and not kept
tributor drive, viewed from the screw tip, constant.
can move back so far that their average
Cincinnati had also been producing
diameter assumes an adequate size for the double conical screws since 1978, which
permanent design. The distributor drive were termed super-conical from 1985 on.
consists of very few parts, which leads to In both screw systems, the degree of overcost advantages during manufacture lapping lies between 18 % and 20 % (as
(Fig. 10). Furthermore, this solution of- in patent claim 2 of the Krauss-Maffei
fers good scope for designing the recep- patent), but the overlapping ratio varies
along the length. As a result, patent claim
tacle for the axial bearings [9].
The first conical extruders were built 1 of the Krauss-Maffei patent was not inby Anger (AGM) (from 1964 on), fringed.
Cincinnati never registered a patent
Cincinnati (since 1969), Weber (since
1981) and Krauss-Maffei (since 1973). for the double conical or super-conical
These are so-called single conical screws. screws. However, in 1983, Cincinnati
In this type of screw, the flight depth re- registered a working model [12] in
mains constant along the length of the which the barrel features at least two axial sections of different conical angles.
screw.
In 1974, Krauss-Maffei registered its The use of different conical angles in the
patent for a socalled
double
conical screw [10].
In double conical
screws, the flight
depth decreases
continuously from
the feed section
to the metering
section (Fig. 11).
Without
any
change to the axial angle of the Fig. 10. Conical distributor drive
transmission for
the length of the barrel unit, the external screws and thus in the barrel was never
diameter at the start of the feed section implemented, but would nonetheless
was enlarged under the terms of patent have been possible since the conical
claim No 1 such that the “ratio of over- Cincinnati cylinders consist of several
lapping with the local external screw di- segments.
The conical screws designed by Weber
ameter along the full length is approximately constant”and, according to patent have been of the single conical type since
claim No 2 “amounts to between 18 % 1981. In 2000, Battenfeld, which until
and 20% of the local screw diameter” (by then had only produced parallel extrud“overlapping” is meant the flight depth ers, presented its so-called negative conical screw design [13], which today is
minus the flight clearance).
This change to the diameter ratios pro- called the active conical design. In this deduced constant overlapping at every point sign, the flight depth increases continualong the screw and, to be sure, in the ously from the feed section to the end of
same ratio as in the parallel Krauss-Maf- the metering section.
fei screws. The main reason for developing a double conical screw was to increase Development of
the output rate while retaining the barrel Parallel Extruders
length and the axial angle, and thus to use
the same transmission.
The delivery rate of an extruder at a givNaturally, further performance en- en screw diameter depends on three key
hancements in the pipe and profile area factors, namely the installed screw torque,
necessitated higher screw torques for the the maximum screw speed and the spedouble conical extruders.
cific drive energy.
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Extending the Processing Unit
L/D ratios
© Kunststoffe
Fig. 12. Evolution of L/D ratios in twin-screw extruders of different manufacturers
The introduction of venting and screw
temperature control enabled substantial
increases to be made in screw operational
speeds from the mid-1960s onwards,
but, to avoid greater wear, not to the same
extent as in operating torque [14].
Practical experience over recent
decades has shown that certain screw
and peripheral speeds should not be exceeded in the screw designs used so far
as, otherwise, this may lead to partial
over-shearing of material and thus to
temperature inhomogeneities. Further
limiting factors on the screw speed are
the recipe and, especially, wear on the
processing unit.
The delivery rate of a twin-screw extruder of given screw diameter can therefore not be increased by means of higher
screw speeds.
Instead, the extra performance must
come from an increase in specific
throughput. The specific output is the
ratio of the output rate to the screw speed.
For a given screw diameter, the specific
output rate can therefore only be increased by increasing the screw torque or
reducing the specific drive energy.
construction and a boom in the plastic
windows sector increased demand for
high-performance extruders which could
be used non-stop for the main window
profiles.
Overlapping
Torque Increase
The first twin-screw extruders still featured comparatively low screw torques.
Continuous improvements to radial and
axial bearings made it possible to more or
less treble screw torques between 1960
and 1977 [15]. While the rise in screw
torque between 1960 and 1990 was virtually linear, it increased dramatically
thereafter into the mid-1990s. This was
triggered primarily by an enormous performance increase in the area of window
profiles. Innovative developments in die
4
Not only is shear energy incorporated
into the dry blend, but also heat energy
via the barrel heaters. A certain amount
of energy is required to plasticate 1 kg of
PVC dry blend. The specific total energy,
i.e. the sum of specific heat and shear
energy, is therefore almost constant.
Heat energy and shear energy are accordingly closely related to each other.
In practice, it is found that the shear energy fraction increases with increase in
screw speed.
On the other hand, more shear energy
has to be incorporated into the material
via the screws in the case of a short processing unit relative to a long processing
unit.
Given the same specific output and the
same screw speed, the dwell time of material in the extruder with the long processing unit is greater than in the short
extruder of the same screw diameter and
axial distance. In other words, a longer
processing unit can be used to reduce the
specific drive energy, provided that all
other construction factors are kept constant. In practice, this is accomplished by
using screw geometries of lower compression.
The processing units of twin-screw
extruders have accordingly become
longer as time has passed. The first twinscrew extruders had a processing length
of 8D. Some 35 years ago, processing
lengths were still 12–18D, whereas nowadays they are between 22 and 36D. The
L/D ratio has risen more or less linearly
from 1955 to 2001. Outstanding extensions to the processing units are the extension of the Krauss-Maffei pipe extruders to 36D in 2001 and the extension
of the processing units of the KraussMaffei profile extruder to 32D in the year
2003 (Fig. 12).
Fig. 13. Preheating device with vanes
(Krauss-Maffei model)
Extending the processing unit is not the
only way to increase the dwell time of the
material in the extruder. It can also be
done by increasing the level of overlapping of the screws (D/a). While the early twin screws still had relatively short
overlapping ratios of 1.15 to 1.20, these
now lie between 1.19 and 1.23. This has
proven to be the optimum value in practice as regards shear rate of external diameter and root diameter, maximum
possible root drilling for internal temperature control of the screws, and screw
strength.
© Carl Hanser Verlag, München
Kunststoffe plast europe 5/2005
SPECIAL ■
Fig. 14. Multi-screw
extruder (KraussMaffei model)
Increase in Throughput
The main engine for driving the development of twin-screw extruders was the
efficient production of semi-finished
plastic goods of PVC, initially pipes, but
later profiles and sheet. Recent decades
have seen steady increases in output rates,
especially of the parallel twin-screw extruders.
Material Preheating
In the early 1970s, Krauss-Maffei looked
for other ways of increasing the performance of twin-screw extruders. Increasing
the output by extending the processing
unit, coupled with a torque increase,
seemed to be feasible only with a huge effort. More powerful transmissions were
not available. Reasons of quality and wear
protection prevented any consideration
from being accorded to increasing performance by increasing the screw speed.
Accordingly, a preheating device was
developed, and presented to the public for
the first time in 1975. This preheating device was mounted on top of the feed
opening of the extruder barrel and its
purpose was to incorporate into the material some of the energy required for
plastication prior to the actual extrusion
process.
It features a motor that employs a
speed reduction gear to drive a shaft on
which vanes are mounted (Fig. 13). The
vanes slide on heated, circular plates that
are arranged in several levels. The powder passes from the hopper into the upper level, is transported further by the
vanes until after 7/8ths of a revolution,
it falls through an opening into the next
level. This process repeats itself in subsequent levels until finally the heated powder is transferred to the extruder screws
[16]. The preheating device also functions
at the same time as a feed unit.
The use of this type of preheating in
combination with appropriate downKunststoffe plast europe 5/2005
stream processing unit can increase the
output rates of the various extruder types
by 20–35 %. The greater output is
achieved at the same screw speed and motor load.
Maplan later launched preheating devices that initially consisted of two individually driven screws which conveyed
the material through a heated housing.
Additional heating of the material was effected by means of the oil-heating system
for the screws. Nowadays, Maplan offers
preheating devices that consist of a heated barrel and co-rotating, oil-heated conveying screws. Apart from material preheating, these devices have a metering
function.
220 mm diameter) with which, from opposing sides, a small screw engages (of
110 mm diameter) (Fig. 14). The same
peripheral velocity is obtained by halving
the screw speed of the central screw relative to the side screws. The more
favourable ratio of surface area to
throughput obtained with smaller screw
diameters enables a great deal of heat energy to be input from outside. Heat is also introduced via the heated central
screw. The fact that the number of intermeshing zones is twice as high leads to
better compounding of the dry blend.
The material ejected from the chambers
is collected in the mixing head, homogenised and fed to the adapter.
Multi-screw Extruders
Development of
Throttle Designs
A further alternative to conventional
twin-screw extruders was presented by Plastication of PVC dry blend requires
Krauss-Maffei at Europlastique in Paris compression of the material in addition
in 1974, namely a multi-screw extruder to the input of heat energy and shear enfor large pipe production, i.e. for
output rates of between 800 and
1000 kg/h.
The
maximum screw
diameter
that
could be managed
at that time was
130 mm. As yet
unresolved wear
problems made it
risky to build machines of even
greater screw diameter. Accordingly, multi-screw
systems were developed that combine two pairs of
screws in one barrel. One twinscrew assembly is
replaced by the
central screw (of Fig. 15. Comparison of different types of screws
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zones (Fig. 15) and composite vent screws
featuring five zones.
In 1975, TPM acquired Kestermann’s
machinery programme. At K 1976, six
completely new, parallel twin-screw extruders (50–160 mm) were presented.
Instead of the breaker plate, the profile
screws were fitted out with a double
flighted, closely intermeshing throttle
while the pelletizing and pipe screws
featured so-called baffles.
While the pipe screws were a one-part
design, the pelletizing screws came apart
so that the baffles could be exchanged.
The degree of plastication could be adjusted to suit the material via the number and contour of grooves in the baffles.
Nowadays, machine manufacturers
usually use throttle elements of the same
pitch as the other screw zones in their
twin screws. The screws are normally
made in one piece. The throttles vary in
pitch, number of flights, effective length
and the flight and roller gaps (Fig. 18).
Extruder manufacturers now mostly
offer different geometries for different application areas. For any particular application area, such as profile extrusion, the
various geometries often differ only in the
throttle zone. ■
Fig. 16. Two-stage vent screws (Kestermann model, 1968) [17]
ergy. In the multi-part vented screws, the
various manufacturers have incorporated different compression elements or
throttles. The purpose of the throttle is
twofold: First, the material is compressed
and slightly plasticised and, second, the
venting section is hermetically sealed off
from the feed section. Over the course of
time, the machine manufacturers have
devised a range of designs for the throttles.
In the 1960s, Kestermann built socalled two-stage vented screws (Fig. 16).
Each stage has the characteristic features
of the single-flighted feed zone, the thread
transformation zone and the multi-flighted delivery zone. Between the first and
second screw flight, the area in the flightfree screws contains a breaker plate immobilised in the barrel such that it cannot rotate (Fig. 17). In the first screw
flight, the material is drawn in and fed towards the breaker plate. A concomitant
pressure builds up that causes the preheated, partly sheared material to agglomerate. The pressure build-up can be
influenced by exchanging the perforated
breaker plate for another with different
flow-through cross-section.
Back in the 1950s, Anger used a single
flighted zone of relatively small pitch and
6
Fig. 17. Non-rotating breaker disc in screw
barrel (Kestermann model, 1968) [12]
Fig. 18. Double flighted throttle in counterrotating twin screws
REFERENCES
The bibliography can be called are up under
www.kunststoffe.de/A012
narrow flight and roller gaps as throttle
element. The compression could be varied via the height of the pitch.
In the 1960s, APM manufactured fivezone screws with a compression zone, seven-zone screws with two compression
THE AUTHOR
HANS-PETER SCHNEIDER, born in 1955, works for
Krauss-Maffei Kunststofftechnik GmbH, Munich/Germany, where he is project leader for the process-engineering of twin-screw extruders.
© Carl Hanser Verlag, München
Kunststoffe plast europe 5/2005