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7/31/2019 98 en Single Screw Extruders
1/17
Sing le-Screw Ext ruders and Bar r ie r Screw s1
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 Dsseldorf, 1997 Kunststofftechnik
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2
The development of
s ing le -sc rew p las t i f i ca t ing
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 comp ression screwb = Uniroyal screwc = Maillefer screwd = compression screw with
UCC(Maddock) mixere = compression screw with
pin mixerf = barrier screwg = barrier screw with UCC
(Maddock) m ixerh = 5-zone decompression screw
with shearing/mixing devicei = compression screw
with pin mixerk = no-compression screw with/
shearing/mixing devicel = barrier screw with
shearing/mixing device
m = high-output barrier screwwith shearing/mixing section
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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 des igns and
se lec t ion c r i te r ia
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 screwdetermines 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 generallycarried 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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4
screw developments have
moved in the direction of
designs which conform, to a
very large extent, to the princip le
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 b lowing 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 doubleflighted/paired screw of 150 mm diameter
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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 / h igh-
ou tpu t sc rews
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-calledgrooved barrel extruders with
barrier screws have proved to
be the most suitable systems
among sing le-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 meltingsection. 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
350400
450
500
0 50 100 150 200 250
screw speed [rpm]
pressure[bar]
meltpressure in front of the screw tip
meltpressure after grooved feed bush
PE - HD Host alen 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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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 g rooved 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
Homogen izing e lements Bar r ie r p las t i f i ca t ingsec t ion
Feed sect ion
Good d ispersive/distributive mixing effect
Heat transfer to the barrel
Low pressure loss
Effective separation of solidsfrom melt
High homogenizing effect
Good control of melttemperature
Clear pressure build-up
Melt throughput gearedto homogenizing capacity
Low pressure level
Low torque
Reduced wear and tear
Fig. 6: Barrier screw concept with homogenizing elements
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Fig. 6 summarizes the three
sections and shows the special
characteristics of a barrier screw
with homog enizing 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 %.
Opera t ion w i th bar r ie r
sc rews
With barrier screws, designed
according to the principle ofDray 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 settingsare 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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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.
Prac t ica l exper ience
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 50mm/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 upto 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 b arrier screws
Fig. 9: Specific throughput vs. Screw speed (Quelle: KKM)
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Fig. 10 gives some examples of
results obtained with and
without a gear pump. Using this
concep t, the targets were readily
achieved, which meant that the
installed d rive 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 ismaterial 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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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, thesecond 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 canbe 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 [r pm]
Throughput[kg/h]
190
195
200
205
210
215
Melttemperature[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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Extruder concepts for
d i f ferent p last ics / new
high-per formance
mate r ia l s
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 otherwords 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
come on to the market, the
question continually arises
about the most suitable extruder
and whether they can be
processed on existing
machines. This was the case
with LLDPE, and it is now
happening with the metallocene
polyolefins, for example
mLLDPE. The goal of the
chemical industry is where this
has not happened already to
make it possible to run mPE on
existing extruders by modifying
the molecular structure.
One aspect not being examined
here, but nevertheless of
considerable importance, is the
question of how the material
behaves in the dies and after
emerging from the die. With
blown film extrusion, for
example, this would concern the
melt elasticity and the bubble
stability.
D
N
DT
L
A
A
BH
H = 3 - 3,5mm
= 7 - 8L = 3 - 3,5 D
n = D(mm)/5D - D = ca.4mmB = 7 - 8mm
NT
Fig. 13 : Lay-out data for grooved feed sections
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Basically, it can be said that
mPE can also be run on
extruders used for processing
LLDPE. This applies both to
grooved barrel machines andsmooth-bore extruders [6].
However, because of the
specific material properties,
there is a difference in the
throughput rates (Fig. 14) which,
in turn, leads to differences in
melt temperature (Fig. 15) and
outputs. On the other hand, this
phenomenon is not specific to
metallocene.
One of the most important
factors concerning the
conveying properties in the feed
section is evidently not the free
flowing characteristics or the low
degree of hardness of the
granules. In fact, the large
influence of lubricants as can
be seen in Fig. 16 indicatesthat friction on the surface of the
screw plays a major role,
something which has also been
encountered with "normal"
polyolefins (cf. Fig. 9). One way
of countering the poor
conveying characteristics of thesolid material is to add a
lubricant or material containing a
lubricant. Another possibility,
this time on the machine side, is
to reduce the coefficient of
friction by cooling the screw,
coating the surface etc.
Fig. 14: Specific throughput vs. Screw speed for grooved feed extruder 80 mm/30:1(source: Reifenhuser [6] )
Fig. 15: Melt temperature vs. Screw speed (source: KKM)
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For engineering plastics (i.e.
polyamides, polyesters,
polyurethanes or thermoplastic
elastomers), use is nowadays
predominantly made of smooth-
bore extruders. This is due not
only to "tradition", but also to the
predominantly low throughputs
involved. Plastics of this kind
can, however, also be
processed without problem on
grooved barrel extruders, as is
shown in [8]. For blow molding
with PA6, a grooved bush/screw
concept similar to the one used
for PE has given good results
[9].
Grooved feed sections are also
being increasingly used in
extruders with a barrel venting
system. The grooves with a
semi-circular, sickle, saw-tooth
or rectangular cross-section
are either cut in the one-part
barrel, or a normal grooved
bush concept is used. This
means that the construction of
vented extruders is currently in a
process of change, as was
already explained with the
barrier vented screws.
Direc t com pound ing in the
ex t ruder
This term is used to explain the
combination of compounding
and extruding in one step. The
process, which aims primarily to
cut down costs, is still in its
infancy, despite all the efforts
being made by machine
manufacturers and plasticsproducers. For processing, use
is made primarily of co-rotating
twin-screw extruders, which offer
far more possibilities in terms of
process technology. They can,
for example, be used for
incorporating fillers and
reinforcing agents, for blending
different polymers or for
simultaneously carrying out
reactions (reactive
treatment/extrusion).
There has, however, been no
lack of attempts to also use
single-screw extruders for
compounding or for the so-
called in-line extrusion. Systems
of this kind are repeatedly
shown and marketed. The
possibilities and limitations are
obvious.
One special kind of in-line
extrusion is the blow molding of
tubular film from mixed film
waste (DSD fraction). After a
temporary phase of euphoria,
normality has, however,
returned. Apart from technical
problems with individual
components of the plant and the
doubts about the product
quality, it has been found that
the cost structure is also
negative over the long term.
EXCEED in BlendsEffect of slip in the LDPE blend component on specific output
on grooved barrelextruders
2
2.2
2.4
2.6
2.8
3
3.2
3.4
80mm
24 L/D
grooved
75mm
25 L/D
grooved
80mm
30 L/D
grooved
Increase: 10 % 15 % 20 %
EXCEED
1MI /0.918D
EXCEED
1MI /0.918D
EXCEED
1MI /0.918D
+ 10 %
LDPE
+ 20 %
LDPE
+ 30 %
LDPE
with slip
Slip additivated LDPE as blend partner gives 10 - 20 % higher specificversus no slip in the LDPE
+ 5 %
LDPE + 5 %
LDPE
withslip
withslip
+ 5 %
LDPE
no slip
kg/rpm/h
Fig 16: Influence of slip agent on specific throughput with mPE (source: EXXON [7])
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Despite all the progress being
made to adapt extruders and
extrusion lines to the
requirements of waste
processing, it must be said that
the production of extruded
quality products using recyclate
is limited. Not so much because
of the machine and processing
technologies, but more because
of the products and the
specified quality. For the time
being, in extrusion, the recycling
of production scrap will continue
to have priority over the
processing of post-consumer
recyclate.
Possib le appl ica t ions and
l im i ta t i ons
At this point, it should be said
that the possible range of
applications for grooved barrel
extruders with barrier screws is
almost "boundless". This is
shown elsewhere.
Retro f i t t ing to op t imize
the sys tem
Whenever funds for capital
expenditures become short, the
purchase of new machines
tends to be put back or
eliminated completely. In such
circumstances, optimizing the
existing system can be a help.
Where there is a need to modify
existing extrusion lines to cope
with a higher output and/orimproved melt quality, a modern
screw concept can be adapted
to the given circumstances.
When making a modification of
this kind, it first has to be borne
in mind that the machine in
question is of a given length
(e.g. frequently between 20 and
25 x D), which can not be
changed, and that it has an
existing drive unit. This
sometimes leads to restrictions
as far as the attainable specific
melt throughput is concerned,
due to bottlenecks with the
torque of the screw drive unit.
The torque results from the
installed motor power and the
installed gear reduction. In some
cases, the gear reduction can
be adapted so that a higher
drive torque is produced on the
screw shank. Since there is a
directly proportional relationship
between the specific output and
the screw drive torque, it is
possible in such cases to
achieve an increase in the
specific melt throughput equal
to the increase in torque.
Fig. 17 gives an example of a
successful retrofit.
Fig. 17: Production data with grooved feed extruder 60mm/24:1 after installing a barrier mixing screw(source: Kuhne)
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Protec t ing the sc rew and
bar re l aga ins t wear and
tea r
One important aspect
concerned with protection
against wear and tear has been
discussed already, namely wear-
reducing screw geometries.
Quite astonishing results can be
obtained by harmonizing the
conveying characteristics and
the pressure build-up, and by
optimizing the melting process
from time to time in conjunction
with a multiple flight design.
With grooved barrel extruders,
for example, through optimized
harmonization of the system
(adapting the feed section
geometry to the downstream
sections and vice versa),
grooved bushes made of nitride
steel can be used instead of
hard metal or PM-HIP material
(see below), because of the
much lower pressures involved.
On the other hand, new plastics
- in some cases necessitating
higher processing temperatures
- fillers and reinforcing agents or
pigments, higher peripheral
velocities of the screw etc. etc.
are resulting in higher and
higher stresses. They can only
be countered by taking special
measures to increase the
protection against wear and
tear, as is state-of-the-art already
in injection molding [10].
In the USA, so-called bimetal
barrels have been used in
extruders for years and years. In
Europe, too, instead of the
conventional nitride steel
barrels, processors are making
increasing use of barrels that
have been given a centrifuged,
wear-resistant and, if needed,
corrosion-resistant armored
layer. Apart from the fact that
this approx. 1.5 2 mm thick
coat can be adapted to suit the
particular type of stress, it also
offers, with its consistent
properties, a certain "reserve" of
wear and tear, even if the
process engineering parameters
are not quite right.
For small and medium-sized
wear-protected screws (up to
approx. 50 mm), fully hardened
tool steel is used, particularly
cold work steel X 155 CrVmO
12.1 (DIN 1.2379). To achieve a
(limited) corrosion resistance,
frequent use is made of
rustproof, acid-resistant 17 %
chrome steel X 35 CrMo17 (DIN
1.4122). By ionitriding to
increase the surface hardness,
however, this material loses
some of its corrosion resistance.
For very high corrosion
protection, it is preferable to
choose special materials, e.g.
Inconel 625.
With larger screws, it is common
to armor-plate the screw flights,
which are particularly prone to
wear and tear. This involves
using the tungsten inert gas arc
welding or the plasma-powder
application (PPA) welding
method. The most popular
materials for this are nitride
steel, 30 CrMoV9 (DIN 1.8519)
and 14 CrMoV6.9 (DIN 1.7735),
or chrome steel X 35 CrMo17
(DIN 1.4122). Hard alloys such
as Stellite 12, Colmonoy 50,
Colmonoy 56, Colmonoy 83 etc.
are also used for armor-plating.
The screw root surface and
flanks can also be protected by
nitriding, by a hard chrome layer
or by armor-plating.
Hot isostatic pressed materials
(HIP) [10, 11] produced by
powder metallurgy are gaining
increasing importance. Both
"natural hard" and hardenable
alloys are used. The materials
can be produced either as
homogeneous systems or as
composite systems, in the latter
case, either in conjunction with
steel, e.g. as the core with an
external hard shell for screws or
screw elements, or as a
composite of the hard alloy
powder with inserted hard
substances.
The PM-HIP materials have the
advantage that a fine,
homogeneous and pore-free
structure is formed, which is
much preferable to the
conventionally produced
materials. The wear-inhibiting
hard phases (usually carbides)
are also distributed more finely
and evenly in the fine-grain
structure, which means that less
surface area is open to attack in
the matrix. The components can
be equipped specifically to cope
with the expected stresses.
Hardness values of up to 72
HRC can be attained. Fig. 18
shows the overall properties and
behavior of PM-HIP materials.
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With materials examined on a
universal disk tribometer, it was
found that the relative wear
decreases significantly with an
increasing proportion of
vanadium carbide.
If we look at the market as a
whole, solutions in which PM-
HIP materials are used in single-
screw extruders are still the
exception rather than the rule.
The higher the demands made
on the plastics and their
additives, the more popular the
new systems will become, also
for these machines.
0
1
2
3
4
5
6
0 10 20 30 40
Vol.-% VC
Rel.Wear
Rel. Volumetric wear
Material Element [Gew.- %] VC [Vol.- %]C Cr V
X 220 CrVMoW 20 4 2,2 20 4,1 6,9
X 250 CrVMoW 22 6 2,5 21,6 6 10,3
X 260 CrVMo 26 4 2,6 26 4 6,2
X 270 CrVMoW 17 9 2,7 17 9 15,7
X 310 CrVMoW 15 10 3,1 15,2 10,3 18
X 340 VCrWMo 13 13 3,4 12,8 13,3 23,4
X 350 VCrMoW 13 9 3,5 8,5 13 22,8
X 380 VCrWMo 17 13 3,8 12,5 17 29,7
X 410 VCrWMo 17 14 4,1 14 17 29,5
X 450 VCrWMo 18 13 4,5 13 18 31,1
X 500 VCrWMo 20 13 5 13 19,5 33,4
Fig 18: Relative abrasive wear of PM-HIP materialsdependent from VC content (source: Reiloy)
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