Feed Screws, Designs

Single-Screw Extruders and Barrier Screws

Single-Screw Extruders and Barrier Screws
1
Peter Fischer, Johannes Wortberg
1
Extended version of a paper presented at the VDI conference on "The Single Screw Extruder – Basics and
System Optimization", published by VDI-Verlag Düsseldorf, 1997 „Kunststofftechnik“
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Page 2
2
The
development
of
single-screw plastificating
extruders
In
the
USA,
extruder
development was - and still is -
largely
characterized
by
machines with smooth barrels.
Further development has tended
to concentrate more on the
screws than anything else, with
so-called 'barrier screws' –
screws in which the solid
material is kept separate from
the melt in the melting section –
at the center of attention.
Although the first barrier screw
was actually invented in Europe
in 1959 by Maillefer, most of the
further development work and
the practical application of this
principle took place in the USA.
The first USA patent was not
applied for until 1961 by Geyer
from Uniroyal [1].
Even
today,
smooth-bore
extruders with barrier screws are
superior to grooved barrel
extruders for many applications,
provided the conveying stability
is adequate. This applies in
particular to applications in
which fluctuating proportions of
recycled or regrind material
have a disruptive influence on
the
normal
conveying
characteristics of the solid
material.
In
such
cases,
extrusion is likely to be more
stable with a smooth-bore
extruder.
In Europe, the development of
extruders with heat-separated
grooved bushes in the feed
section began at the end of the
fifties and beginning of the
sixties. Grooves in the barrels to
increase barrel friction and
assist conveying of the solid
material had been tried out long
before
then.
They
were,
however, not enough to process
the newer high-molecular weight
HDPEs in powder and grit form.
This
specifically
European
phenomenon
on
the
raw
materials side has come from
the systematic analysis and
development of the grooved
bush principle.
Extruders with grooved bushes
were initially operated with the
conventionally flighted three-
section screws commonly used
in Europe. To have better
control of the melt temperatures,
vented
screws
were
later
developed, and, to improve the
melt
homogeneity,
were
subsequently equipped with
shearing/mixing sections [2].
One
problem
nevertheless
remained: very high pressures
at the end of the feed section
and, as a result, considerable
wear and tear on the screw and
barrel.
Fig. 1: Development of extruder screws in the USA and Europe
a =
3-zone compression screw
b =
Uniroyal screw
c =
Maillefer screw
d =
compression screw with
UCC(Maddock) ‘mixer’
e =
compression screw with
pin mixer
f =
barrier screw
g =
barrier screw with UCC
(Maddock) ‘mixer’
h =
5-zone decompression screw
with shearing/mixing device
i =
compression screw
with pin mixer
k =
no-compression screw with/
shearing/mixing device
l =
barrier screw with
shearing/mixing device
m =
high-output barrier screw
with shearing/mixing section
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Page 3
3
The development of extruder
technology is basically reflected
in the development of the
screws . For thirty years,
development teams went their
different ways in the USA and
Europe until, at the beginning of
the nineties – also due to
increasing globalization – the
directions
of
development
began
to
converge again.
Combining the grooved bush
principle with barrier screws is
the logical step to optimize
extrusion technology [3].
Screw
designs
and
selection criteria
As already mentioned, the
choice of a suitable extrusion
system
(conventional
or
grooved
bush
concept)
depends on the particular
application. After all, the
design
of
the
screw
determines the quantitative
and the qualitative properties
of the extrudate. In practice,
different screw lengths have
become
established
for
different applications.
For
applications in extrusion blow
molding,
for
example,
relatively short extruders (L:D
= 20:25) are used, whereas
in other applications, such as
film and pipe extrusion,
extruders with longer screws
(L:D >= 30) are generally
employed. As a result, the
way the total screw length is
divided up into the "feed and
compression" and "melting
and homogenizing" sections
can vary considerably.
First of all, for a specific
application, a decision has to be
taken as to what proportion of
the total screw length should be
reserved for homogenizing the
plastificated melt. This question
can
nowadays
only
be
answered on the basis of
experience or following an
appraisal of the demands made
on the melt quality. Even
specifying the necessary melt
quality can sometimes cause
problems. Complying with an
imprecisely defined melt quality
can necessitate not only
homogenizing elements on the
screw
(dynamic
mixing
sections), but also static mixing
elements.
The various constructions of
homogenizing elements will be
dealt with in more detail later.
While a wide variety of screw
concepts are still in use, current
developments are concentrating
very much on barrier screws.
For this reason, this report will
concentrate on such models
while taking a wider look at the
topic of single-screw extrusion.
Fig. 2 shows schematically the
basic concept of barrier screws
for different lengths of extruders,
with and without barrel venting.
The concept is the same for new
extruders as it is for the
retrofitting of existing machines.
The evaluation of a barrier
plastificating section is generally
carried out by looking at the
differences in the pitch and flight
depths and the design of the
feed section and outlet area of
the barrier flights. Both North
American and European barrier
Fig. 2 Basic concept of barrier screws
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Page 4
4
screw
developments
have
moved in the direction of
designs which conform, to a
very large extent, to the principle
of the Dray and Lawrence screw.
The characteristic features of
these screws are that, through
elevations in the respective
pitches of the main flight of the
screw and the barrier flight, a
sufficiently wide channel is
created in the solids channel –
this encourages plastificating –
and that, through a variable
adjustment of the flight depth
profiles, the melt temperature
curve can also be adjusted, with
the aim being to keep the melt
temperatures
as
low
as
possible. Although barrier screw
designs still exist today with a
solids channel that is not sealed
off, the only way of ensuring
complete melting in the barrier
plastificating section is to use
solids channels with a 'dead-
end' groove (Fig. 3).
The front of the barrier flight at
the beginning of the barrier
plastificating section can be
designed with the melt channel
closed at the rear end or with an
open melt channel. In this case,
even if
we
assume
that
unmelted material enters the
melt
channel,
complete
plastification
is
nevertheless
ensured by the end of the
barrier section because of the
long residence time in the melt
channel. A detailed description
of
different
barrier screw
concepts,
including
their
characteristic features, is given
in [4].
For extrusion applications in
which relatively high extrusion
temperatures are required (e.g.
paper
coating),
the screw
geometries must be modified by
making the flight depths in the
melt-filled sections smaller so
that,
due to
the higher
dissipation energy, the target
melt temperatures are reached.
This could possibly also be
done by adapting the feed
sections to reduce the specific
melt throughputs. Last but not
least, the shearing sections
used for such applications can -
and must - be dimensioned in
such a way that the necessary
temperature
increase
is
reached.
For other applications,
for
example foam extrusion, exactly
the opposite course must be
taken
to
keep the
melt
temperatures as low as possible
after injecting the blowing agent.
Here, the best solution is to
regard the extrusion system as a
highly effective heat exchanger,
and to enhance its effectiveness
through reduced dissipation in
large-dimensioned
screw
channels and through frequent
interface renewal by the screw
flights on the inner wall of the
barrel.
Fig. 3: Transition between feed section and barrier section on a double
flighted/paired screw of 150 mm diameter
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Page 5
5
We will now deal with the
influences of the raw material
and the extruder feed section
geometry, which determine the
conveying properties of an
extruder.
Universal screws / high-
output screws
For the user, the ideal situation
would be to have a screw on
which as many different plastics
as possible could be processed
at high throughput speeds and
with good melt homogeneity.
Some of the most important
requirements are:
• Processability of mixtures
with different sized and
different shaped granules
• High
plastificating
performance
• Gentle
but
complete
plastification
• Good melt homogeneity
• Controlled melt temperatures
• Minimal
change
in
the
material through degradation
or crosslinking
• High level of versatility: ability
to process a broad selection
of raw materials with a wide
range of throughput rates
• Low
performance-related
investment and operating
costs
In recent
years,
so-called
grooved barrel extruders with
barrier screws have proved to
be the most suitable systems
among single-screw machines.
With many grooved barrel
extruders, the pressure build-up
at the end of the feed section is
too high, encouraging wear and
tear and impairing the stability of
the process. This can be
countered by enlarging the pitch
or making the screw channel
deeper, although this involves
the risk of plastification and
homogeneity problems.
A better solution than a screw
with a stepped pitch or channel
depth is a barrier screw. At the
beginning
of
the
barrier
plastificating
section,
the
conveying flight changes to a
greater pitch; the beginning of
the barrier flight also has a
higher pitch. The depth of the
channels is adjusted to the
desired conveying and melting
characteristics.
These
two
measures result in a fairly
balanced, low pressure profile,
or even in a pressure build-up
towards the end of the screw
(Fig. 4).
When
assessing
the
"universality" of a screw, the
homogeneity of the melt plays a
dominant
role.
This
is
particularly
true
when
processing mixtures of different
materials, and also with regrind
material,
with
color
masterbatches and with the so-
called 'direct extrusion' process
(combining compounding and
extrusion in one step, "in-line").
Barrier screws, too, must be
provided with elements for
homogenizing after the melting
section. Depending on the
requirements
of
the
raw
materials and the demands
made on the product, shearing
sections must be used for
dispersion (for example for color
pigments), and/or distributing
mixing elements must be
provided for axial and transverse
mixing.
0
50
100
150
200
250
300
350
400
450
500
0
50
100
150
200
250
screw speed [rpm]
p
r
e
ssu
r
e
[
b
a
r
]
meltpressure in front of the screw tip
meltpressure after grooved feed bush
PE - HD Hostalen GM 7746
Extruder 50 mm, 28:1 L/D
Fig. 4: Meltpressure in front of the screw tip and after grooved feed bush
(source: KKM)
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Page 6
6
In practice, barrier screws with
neutral-pressure,
(multiple-
)spiral shearing elements and
with faceted mixing sections
designed to give good flow
properties
have
proved
successful,
also
for
direct
coloring with a color masterbatch
(Fig. 5).
With homogenizing elements of
this kind, the best way of
maintaining full control of the
general thermal conditions, and
thus of keeping the melt
temperature
closely
under
control, is to ensure that good
heat transfer by convection to
the temperature control system
of the extruder barrel is possible,
both in the area of the spiral
shearing elements and in the
area of the faceted mixing
sections,
through
constant
renewal of the surfaces and/or
interfaces between the moving
screw elements and the fixed
inner surface of the barrel. A
further influence can be exerted
on the temperature of the melt
by taking additional measures,
for example,
by
fitting a
temperature control system on
the inside of the screw (e.g. as a
closed cooling system),
As
regards
extruders
for
universal applications - in other
words, extruders capable of
processing a very wide range of
raw materials, including regrind
(fluctuating
proportions,
recycled material etc.) - a
decision has to be taken in each
individual case whether or not to
use the grooved barrel extrusion
concept. The decision is not
always an easy one to make. If
excessively large variations in
the raw material properties –
especially the bulk density, flow
properties
and
friction
coefficients – are to
be
expected, it is probable that
using the grooved bush concept
will
lead
to
excessive
fluctuations in melt throughput
due to the fact that the output
characteristics are governed by
the solids conveying in the feed
section. In such cases a
smooth-bore extrusion system
can or must be used. This is
particularly true for processing
raw materials with a fairly high
shredded
content
and
consequently a low bulk density.
Another possibility is to pre-
compact the shredded material
so that grooved barrel machines
can function perfectly.
Fig. 5: Spiral shearing section and faceted mixing section after a barrier section
Homogenizing elements Barrier
plastificating
section
Feed section
• Good dispersive/
distributive mixing effect
• Heat transfer to the barrel
• Low pressure loss
• Effective separation of solids
from melt
• High homogenizing effect
• Good
control
of
melt
temperature
• Clear pressure build-up
• Melt throughput geared
to homogenizing capacity
• Low pressure level
• Low torque
• Reduced wear and tear
Fig. 6: Barrier screw concept with homogenizing elements
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Page 7
7
Fig. 6 summarizes the three
sections and shows the special
characteristics of a barrier screw
with homogenizing elements.
For universal application, use
can generally be made of
screws designed according to
the above concept in lengths of
between 20 and 30 x D. The
feed section consists either of a
shallow-flighted feed section
with subsequent decompression
(grooved bush concept) or a
constantly deep-flighted feed
section (smooth-bore extruder),
followed
by
the
barrier
plastificating section and the
homogenizing sections.The best
solution
is
first to
have
dispersively
acting
mixing
elements, and then distributively
acting elements. Fig. 7 shows
possible and commonly used
constructions.
In recent years, "static-dynamic"
mixer systems with spherical
indentations in a rotor and stator
have become quite popular.
They are marketed under such
names
as
CTM,
TMR,
STAROMIX
and 3-DD
[5].
Although they have a good
mixing action, some of them
have
an
inadequate self-
cleaning system and others
have problems with wear and
tear. Apart from this, it is
essential that the melt is 100 %.
Operation
with
barrier
screws
With barrier screws, designed
according to the principle of
Dray and Lawrence screws, the
solids channel has been made
wider. This provides a larger
contact surface area for the
material being melted so as to
introduce energy via the barrel
heating. This means that, with
barrier screws, the heating
process
must
be
started
immediately after the material is
fed in.
Either a constant
temperature program must be
set over the length of the barrel,
or the temperature must be set
so that it actually drops from the
feed section to the end of the
barrel.
The temperature at the end of
the barrel is the same as in a
conventional screw, in other
words it is geared to the melt
temperature. In the first heating
zone after the grooved bush, it is
perfectly in order to work at a
temperature which is about 20 –
30 °C higher. In the lower to
medium speed range,
the
temperature in the final barrel
section is set at the same level
as the melt temperature.
The temperature at the end of
the barrel is the same as in a
conventional screw, in other
words it is geared to the melt
temperature. In the first heating
zone after the grooved bush, it is
perfectly in order to work at a
temperature which is about 20 –
30 °C higher. In the lower to
medium speed range,
the
temperature in the final barrel
section is set at the same level
as the melt temperature.
With low-viscosity melts, or in
cases in which high melt
temperatures
are
required,
correspondingly higher settings
are recommended. It is also
advisable to keep a watch on
the relative periods in which the
heating and cooling units are
switched on (controller output
signals), so as to work in the
medium range of settings (20 ...
80 %). As a rule, this will mean
that the deviations between
target values and actual values
are sufficiently small to ensure
process stability.
Fig. 7: Executions of shearing and mixing elements (source: KTP)
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Page 8
8
In practice, it is not difficult to
establish
the
optimum
temperature settings.
Because
of
the
special
characteristics of barrier screws,
it
has often proved an
advantage to turn up the heating
output in the first and second
barrel sections, and to increase
the fan or cooling capacity in the
two sections at the end.
Practical experience
The broad range of application
of barrier screws for polyolefins
can be seen in Fig. 9. All these
materials were successfully
processed with the same 50
mm/28 D screw with a twin-
spiral shearing section and a
faceted mixing section on a
grooved barrel extruder. Further
details are contained in one of
the later articles.
High-speed extrusion systems
are generally characterized by
the fact that the system is set up
to suit a limited range of raw
materials, but so as to achieve
maximum melt throughputs in
the specified melt quality. For
this purpose, the combination of
a grooved barrel extruder and a
barrier
screw
with
a
homogenizing
section
is
particularly recommended. With
larger screw
diameters,
a
double-flighted
or
twin-pair
screw system can be used to
improve
the
conveying
properties in the feed section
and to
raise the melting
capacity.
One example at the upper end
of the speed scale involves
retrofitting an existing 150 mm
33 D extruder for working with
MDPE and LDPE for the
sheathing of steel pipes. The
objective in this case was quite
clearly to achieve maximum
possible melt output with high
product homogeneity (specified
in reference samples) and, at
the same time, to keep the melt
temperatures
as
low
as
possible. In addition, the system
had to have outstanding self-
cleaning
and
material
changeover characteristics.
Fig. 8: Temperature program for barrier screws
Fig. 9: Specific throughput vs. Screw speed (Quelle: KKM)
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Page 9
9
Fig. 10 gives some examples of
results obtained with and
without a gear pump. Using this
concept, the targets were readily
achieved, which meant that the
installed drive power was almost
completely utilized for the given
range of raw materials. Further
increases in throughput are
conceivable over and above
these figures. However, this
would make it necessary to
adapt the drive unit by raising
the
motor
power
and
proportionally increasing the
screw speed. It also becomes
increasingly important to take
special measures to prevent
excessive
wear
and
tear
because of the greater influence
of the peripheral speeds of the
screw.
The production of fuel tanks is
an impressive example of the
direct recycling of production
scrap. A problem here is that,
with
blow
molding,
comparatively short extruders
are used.
Depending on the shape of the
tank and on other boundary
conditions, between 40 and 60
% flash occurs as scrap at the
production machine. This is
material which was cut off at the
top and bottom of the parison
and from the pinch-off edges of
the blow mould. This material is
directly ground and fed back
into the machine.
Fig. 10: Production data with extruder 150mm/33:1 for steel pipe coating
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Page 10
10
Fig. 11 gives the key operating
data for a grooved barrel
extruder with a diameter of
150 mm/20 D equipped with a
barrier shearing/mixing section
screw for processing high-
molecular weight HDPE grit
containing regrind material.
One notable application for
'specialty screws' are vented
extruders, which are used, for
example, in plants producing flat
film and sheets. Because such
machines are being increasingly
combined with melt pumps, the
second stage of the screw only
needs to convey the melt
against the pump pre-pressure
(and possibly
against the
resistance of melt filters), but
does not have to overcome the
resistance of the connection,
possibly a static mixing element,
and the die. Consequently,
much higher throughputs can
be achieved, with plastification
and
homogenization
being
carried out in the first stage of
the screw.
Fig. 12 shows the concept of a
vented screw with a barrier
plastificating section in stage 1,
and a three-zone profile plus
faceted mixing section in stage
2. Such vented screws with
barrier plastification are being
successfully used for e.g.
polystyrene, polycarbonate and
PMMA.
Extruder 150 mm / 20 D
PE-HD Lupolen 4261A
with 50 % regrind
0
100
200
300
400
500
600
700
5
10
15
20
25
30
35
40
45
screw speed [rpm]
T
h
r
oughput
[
k
g/
h]
190
195
200
205
210
215
M
e
lt
t
e
m
per
at
ur
e [
°
C]
[kg/h]
[°C]
Fig. 11: Production data with extruder 150mm/20:1 for industrial blow moulding
Fig. 12:barrier screw configuration for vented extruder 90mm/30:1
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Page 11
11
Extruder
concepts
for
different plastics / new
high-performance
materials
For most applications, grooved
barrel extruders with a heat
separation system between the
feed section and the subsequent
barrel have become established
in Europe. As a rule, the feed
bushes are axially grooved and
correspond to the construction
concept shown in Fig. 13.
A good thermal layout - in other
words sufficient and uniform
heat dissipation in the area of
the grooved bush - is of
particular importance. For this
purpose, the cooling channels
and heat transfer resistances
must be optimized. In many
cases, temperature control units
or installations with bypass
control etc. are fitted to create
constant thermal conditions.
An optimized thermal layout is
also essential for the rest of the
barrel. Even though the target is
to generate as little excess heat
energy as possible via the
screw, it is not possible with
high-speed
extrusion
to
dispense with good cooling of
the barrel, at least not in some
sections. Special companies
offer
heating/cooling
combinations for this purpose,
in which a great deal of heat can
be dissipated via aluminum or
copper
elements.
Some
machine manufacturers also
supply customized systems of
this kind. When new materials

Peter Fischer, Johannes Wortberg