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Screw design and optimization of grooved-barrel extruders
Page 1
Demands for greater output from
modern extrusion equipment is placing
higher requirements on the machines
plasticating and homogenizing capacity.
Such requirements can only be fulfilled
by modifying existing screw designs or
developing new ones.
BASF’s years of
experience with extruder design, combi-
ned with its use of the latest computer
simulation software, is helping customers
get more from their machines.
Technical Information for Experts 03/97e
Screw design
and optimization of
grooved-barrel
extruders
BASF Plastics
Design
Processing
Testing
Materials
Information
System
------------------------------------------------------------------------
Page 2
Grooved-barrel extruders are high-out-
put devices. They are able to provide
high output because the grooves in the
hopper end of the extruder barrel aid
the forced conveyance of material
through the feed zone.
While the design of grooved-barrel
extruders and their associated screws
has become very sophisticated, deve-
lopment work continues in order to
meet the demand for even higher per-
formance. Notable recent develop-
ments are decompression and barrier-
flight screws.
The main demands on the plastification
unit are:
rouble-free feeding, conveying and
compression of the polymer resin
a wide processing window, giving
the machine the ability to process
both virgin and regrind
high output while ensuring good
melt homogeneity (both physical
and thermal)
optimal melt temperature
gentle polymer processing
pulsation-free melt flow
energy-efficient operation
low wear on the screw and barrel
Optimizing the extruder
As many of these requirements conflict,
the problem is how to improve one
without adversely affecting others.
A typical example would be how to
increase the extruder throughput while
improving the quality of the melt and
lowering its temperature. Such
demands often call for priorities to be
set or compromises to be made.
There are several ways of tackling the
optimization problem:
by trial and error
through the use of proven designs
(model laws)
by using computer simulation to
model the extrusion process
In the solution of practical engineering
problems, it is necessary to use a com-
bination of the three.
Because the computer model is based
on a number of assumptions and sim-
plifications, the results obtained are
approximations; they nevertheless tend
in the right direction. The information
gained represents a competitive advan-
tage as it can save a great deal of time
and money. For instance, the effect of
varying the extruder screw’s pitch or
channel depth can be established very
quickly. To do this by trial and error or
by relying purely on experience would
nowadays be unjustifiably costly, parti-
cularly for large screws.
Several important variables used in the
design and optimization of the extruder
screw will be described in the follo-
wing.
Extruder throughput and pressure
profile
From a purely economic viewpoint, the
most important variable is extruder
throughput, which we obviously seek
to maximize. To be able to do this for a
given size of screw, we have to increa-
se the screw’s plasticating capacity –
either by modifying the existing design
or replacing it with a completely new
one.
An additional aim is to reduce the pres-
sure maximum at the end of the extru-
der’s grooved feed zone in order to
reduce mechanical wear (high pressure
increases wear due to friction between
the material and the extruder, as well
as between the flight lands and the wall
of the barrel).
This implies that we have to get away
from the conventional wisdom that the
grooved feed zone must cause as high
a pressure build-up as possible in
order to overcome the resistance of the
die and to compensate for pressure
losses due to the screw. Under such a
scheme, a typical pressure for a hard,
high-melting-point material would be in
the order of 2000 bar. Today we know
that high pressure is not the deciding
factor for optimum extruder operation it
was thought to be.
When it comes to dimensioning the
extruder, it is important to consider the
feed zone (most of which includes the
grooved bushing) in relation to the pla-
sticating capacity of downstream zones
and not in isolation. Only by doing so is
it possible to influence the melting cha-
racteristics and the melt pressure profi-
le along the whole length of the barrel.
Feststoff
Schmelze
Barriereschnecke
2-gängige Schnecke
(Gangsteigungswechsel)
1-gängige Schnecke
(konstante Gangsteigung)
Figure 1:
Types of extruder
screw
------------------------------------------------------------------------
Page 3
A screw redesigned along these lines
produces a much flatter pressure profile.
Figure 2 compares the pressure profiles
of a conventional constant-pitch screw
with a modern screw; the latter’s pitch
and channel depth change at the end
of its feed section. The resultant pres-
sure reduction is clear.
There are a number of factors that
affect the way material is conveyed
through the extruder – the material’s
particle size (whether the material is in
the form of pellet, powder or granules);
it’s particle size distribution (whether
virgin and/or regrind); the shape and
depth of the grooves in the feed zone;
and the depth of channel in the screw’s
feed section. All these must be consi-
dered if the throughput of the extruder
is to be computed with any accuracy.
In the blow moulding process, a high
percentage of flash is reground and
returned to the extruder. The shape of
regrind is very different from that of vir-
gin material. The extruder’s material-
conveying characteristics are affected
profoundly by the percentage and
shape of the regrind added.
Figure 3 shows the different ways
material can be conveyed in the extru-
der’s grooved feed zone. A good com-
puter model will take these situations
into account.
The way the particles of material are
conveyed depends on their size relative
to the depth of the groove and screw
channel. If the particles are larger than
the groove they are driven along it by
the screw flight (cases 1a & 1b). If the
depth of the screw’s flight is shallower
than twice the particle diameter
(case1a), the particles interlock and
maximum material throughput is
achieved. If the screw flights are dee-
per (case 1b), as is the case for larger
screw diameters, particle interlocking
only occurs in the upper part of the
screw channel- this explains why the
throughput of the grooved bushing
approaches that of a smooth barrelled
one at larger screw diameters.
In cases 2a and 2b the particles are
smaller than the grooves. Here, the
amount of material that moves along
the groove largely depends on friction
between individual particles rather than
the driving force of the screw flight.
Taking the different conveying mecha-
nisms into account enables the throug-
hput of the extruder to be computed to
within ±10% of the actual value. Figure
4 compares computed and measured
throughputs for different screw sizes
and shows just how accurate the com-
putation can be.
1600
1200
800
400
0
Druck [bar]
klassisches Konzept
neues Konzept
Figure 2:
Pressure profile
along the extru-
der barrel produ-
ced by different
screw designs
d
Fall 1a: h<2d, t
Fall 2a: h,t >d
Gleitebene
Grieß
Gangtiefe h
Nuttiefe t
Figure 3:
Idealized convey-
ing mechanisms
140
120
100
80
60
40
20
40
40
40
60
60
70
80
90 100 120 120 150
0
Neugut
Mischung
Lupolen
®
4261 A und 5261 Z
gemessener Durchsatz/berechneter Durchsatz [%]
Schneckendurchmesser [mm]
Fig. 4 :
Relation of mea-
sured to compu-
ted extruder
throughputs
------------------------------------------------------------------------
Page 4
Critical screw speed
As the extruder’s screw speed increa-
ses from zero, the throughput of the
extruder rises uniformly until a critical
speed is reached. Up to this point, the
throughput of material is unaffected by
back pressure because the material in
the feed zone is largely solid. Around
the critical screw speed the material in
the feed zone starts to melt faster
because of the excessive frictional hea-
ting caused by the high pressure build-
up; conveying performance starts to be
lost as a result (see figure 5). A further
increase in screw speed beyond the cri-
tical value causes further melting and a
drop in pressure; throughput continues
to increase, but at a lower rate. This
behaviour is typical of extruders with
large-diameter screws.
With a conventional screw design it is
possible to increase the throughput of
the extruder by employing a screw with
deeper flights, but this is likely to lead
to a drastic drop in plasticating perfor-
mance and poor melt quality. To com-
pensate, the screw must also be made
longer to give the material more time to
homogenize. This thinking is reflected
in new blow-moulding extruders in
which screw lengths have increased
from 20 D to 24 D, or even to 30 D in
some cases.
Bigger screws may well increase the
extruder’s specific throughput – the
amount of extrudate per screw revolu-
tion – but the pressure at the end of
the grooved bushing and the critical
screw speed remain the same. Other
things have to be done in order to
reduce the pressure. For example, the
screw’s channel depth and/or pitch can
be increased just after the end of the
grooved bushing. Apart from reducing
the pressure drop along the screw’s
homogenizing section, the bushing
end pressure is also minimized (see
figure 2).
Figure 6 compares the throughput cha-
racteristics of two screws each having
the same overall dimensions. One is a
conventional screw (“old” screw) with
constant pitch, the other a modern
screw (“new” screw) with a pitch chan-
ge. As expected, a bend occurs in the
line at the conventional screw’s critical
screw speed. At the same speed, no
such bend is present in the modern
screw’s characteristic. This indicates
that the modern screw has a higher
critical screw speed and therefore
capable of higher throughput.
Melting and homogenization
There is little point in trying to increase
the throughput of the extruder if good
melt homogeneity is lost in doing so.
The course of melting can be followed
by finding how the proportion of solid
material in the screw channel changes.
This proportion can be described as
the solid-bed ratio
Y = x/b,
where x is the width of the bed of solid
material and b the width of the screw
channel (or solids channel in the case
of barrier screws).
Figure 7 shows good and bad melt
profiles. Such plots provide information
about the screw’s combined conveying
and plasticating characteristics, and
enable something to be said about the
homogeneity of the melt in front of the
screw. The bad melt profile in figure 7
shows two unfavourable situations: an
increase in the proportion of solid
material in the screw channel (in this
case in the screw’s compression sec-
tion) and solid material still in the screw
channel at the start of the screw’s
shear section.
An increase in the solid-bed ratio
should be avoided to prevent the com-
pacted slug of polymer particles brea-
king up due to high deformation stres-
ses. Such an occurrence would produ-
ce islands of solid material surrounded
by melt. These can no longer be mel-
ted effectively by the action of shear
forces, and so lead to inhomogeneities
in the melt. If the solid-bed ratio increa-
ses back up to 1, the screw becomes
choked. This is can happen particularly
when a high-melting-point resin is
being processed with a screw that
compresses the material too much or
Dur
chsatz [kg/h]
450
400
350
300
250
200
150
100
50
0
0
20
40
60
Drehzahl [1/min]
„alte“ Schnecke
„neue“ Schnecke
Lupolen
®
5261 Z
Durchmesser: 120 mm
Länge: 20 D
Figure 6:
Throughput cha-
racteristics;
“new” screw with
pitch change
Drehzahl
Grenz-
drehzahl
fördersteif
Feststoff-
reibung
Misch-
reibung
wandernde
Schmelzfilm-
front
stabile
Verhältnisse
maximaler Nutbuchsenenddruck
Dur
chsatz
Figure 5:
Critical screw
speed
------------------------------------------------------------------------
Page 5
too early. The consequences of this
would be a reduction in extruder through-
put and homogeneity problems.
The plastic should be completely mel-
ted by the time it reaches the screw’s
shear section. Small particles of solid
material would cause an inhomoge-
neous melt, large ones possible partial
blockage and a fluctuation in extruder
throughput. Moreover, clogged screw
clearances would also cause large
pressure losses.
Wall slippage
One problem encountered in compu-
ting polymer heating and melting is
that, above a certain shear stress,
some high-molecular-weight (ie, high
viscosity) resins start to slip on the wall
of the barrel, resulting in a loss of shear
heating. (Earlier computer models
assumed this does not happen.)
The effects of wall slippage can be
estimated by performing the computa-
tion under conditions of wall slippage
and non-slippage. Figure 8 shows the
results for two such cases; all other
variables were taken to be the same
in each instance.
It is obvious that the extruder must
have higher plasticating efficiency to
ensure thorough melting when a wall-
slipping polymer is being processed.
On the other hand, high slippage is
desirable in applications like blow
moulding – here the reduced shear
heating produces lower melt tempera-
ture, which is necessary for adequate
mechanical strength of the extruded
parison. The simulation program is able
to faithfully model the effect of wall slip-
page on melt temperature.
Barrier (flight) screws
The barrier screw was invented at the
end of the 1950s by Maillefer. Since
then the concept has been developed
further. Much of the work has taken
place in the US, mostly on conventional
smooth-barrel extruders. Recent efforts
in Europe have been aimed at combi-
ning the benefits of the barrier screws
with those of the grooved feed zone.
The principle behind all barrier screws
is the same: the main melt channel for-
med by the primary flight is separated
into a solids channel and a melt chan-
nel by a shallower barrier flight. Melt
that forms in the solids channel travels
over the barrier flight into the melt
channel (the same is true of smaller
particles of material, which are subjec-
ted to additional shear heating as they
are transferred to the melt channel).
The barrier flight also helps to homo-
genize the melt.
The upper drawing in figure 9 is of a
Maillefer screw. Note how the width of
the solids channel diminishes while that
of the melt channel increases.
The lower drawing in figure 9 is of a
modern barrier screw. In the barrier
section, the screw features a varying
channel depth and an increased flight
pitch. The solids and melt channels run
in parallel over a large section of the
screw. This and the fact that their com-
bined width is larger than in the Maille-
fer screw means lower pressure requi-
rements from the grooved feed section.
Also, the width of the solids channel is
usually the same as that before the
barrier section; this avoids a sudden
deformation of the solids bed at the
transition to the barrier section. Com-
pared with Maillefer screws, modern
barrier screws offer better plasticating
performance because their wider solids
bed enables the polymer to obtain
more heat energy from the wall of the
barrel. In addition, the larger pitch has
the advantage of producing higher rela-
tive velocities within the melt, thus fur-
ther improving the melting behaviour.
Y =
x
b
dimensionslose Feststof
fbettbr
eite
0,9
1,0
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0
25
20
10
15
5
0
OSW
ungünstig
günstig
Schneckenlänge [L/D]
x
b
;;;;;;
;;;;;;
Figure 7:
Principle melt
profiles
0
1
dimensionslose Feststof
fbettbr
eite
Lupolen
®
5261 Z
Durchmesser: 60 mm
Durchsatz: 85 kg/h
wandgleitend
wandhaftend
Figure 8:
Melting profiles
for wall-adhering
and wall-slipping
materials
------------------------------------------------------------------------
Page 6
Figure 10 shows the computed melt
profiles for the above barrier screws for
the same throughput. The Maillefer
screw has a poor melt profile since the
solid-bed ratio begins to climb again at
the start of the barrier section. The rea-
son for this is the continuous decrease
in the width of the solids channel, which
causes a continuous decrease in mel-
ting efficiency. If an attempt is made to
increase the throughput by increasing
the screw speed, the solids channel
may become choked with material.
Extra pressure is then required to force
the solids over the barrier flight. If this
happens, the screw’s conveying capa-
city then becomes limited by the back
pressure. This type of design is therefo-
re only suitable for smaller throughputs.
As with screws in general, the achiev-
able melt throughput also depends on
the amount of energy needed to melt
the polymer and the viscosity of the
molten material. It is here that modern
barrier screws – thanks to their larger-
pitch barrier sections – offer the
advantages of better melting perfor-
mance and „pressure-independent“
conveying characteristics (see flattish
pressure profile in figure 11).
Since the pressure profile is constant
over a large section of the screw, drag
flow becomes the dominant transport
mechanism in the screw channel. Such
a situation minimizes any rheological
differences in the melts of different
polymers.
When designing a screw, attention
must be paid to the polymer’s enthalpy
requirements and melt viscosity. Failure
to do so could lead to blockages in the
screw’s shear section and in barrier
sections that feature closed-off entry
and exit channels. This can happen if
the screw’s melting performance is too
small or if the melt vortex only first
appears in the barrier section.
Because the barrier section is long,
blockages there are likely to be only
localized. This is not the case with
shear sections, which are shorter.
However, even local blockages can
lead to undesirable fluctuations in
throughput and should therefore be
avoided.
Computer simulation is particularly
helpful when designing barrier screws
since barrier screws inherently have
more degrees of freedom, enabling
Schnecke mit Gangbreitenvariation (Maillefer-Prinzip)
Schnecke mit Gangtiefenvariation
Schmelzekanal
Schmelzekanal
Feststoffkanal
Feststoffkanal
Barrieresteg
Barrieresteg
Figure 9:
Different types of
barrier screw
0
1
dimensionslose Feststof
fbettbr
eite
Lupolen
®
2441 D
Durchmesser: 90 mm
Durchsatz: 320 kg/h
Maillefer
Figure 10:
Melt profiles for
modern and
Maillefer barrier
screws
------------------------------------------------------------------------
Page 7
them to better match the requirements
of a particular application. On the other
hand, the screw may respond more
sensitively under certain circumstances;
there are also more opportunities for
making errors.
Even in the age of CNC machining, the
complexity of barrier screws still makes
them costlier to manufacture. The en-
gineer must therefore weigh up whether
the extra cost can be justified by the
advantages they offer.
If not, the easier and cheaper option of
modifying the geometry of a conventio-
nal screw should be considered. One
such option – increasing the screw’s
pitch – has proved to be the most
effective way of reducing the pressure
peak at the end of the grooved bus-
hing and flattening the pressure profile
along the screw. Better melting and
homogenizing performance is also pos-
sible in theory – something which can
be further improved by use of a multi-
ple-flight screws.
The controlled shearing action of the
barrier flight on barrier screws can be
supplemented by fitting a device such
as a Maddock shear section. This is
yet another example which shows the
opportunities for improving the conven-
tional grooved-barrel extruder are far
from exhausted.
600
450
300
150
0
Druck [bar]
Lupolen
®
2441 D
Durchmesser: 90 mm
Durchsatz: 320 kg/h
Figure 11:
Pressure profile
of a modern bar-
rier screw
------------------------------------------------------------------------
Page 8
NOTE
The information contained in this publication is
based on our current knowledge and experience.
Because of the multiplicity of possible effects
during the processing and use of our products,
this information does not free the processor from
carrying out his own tests and experiments. Our
information does not provide legally binding assu-
rances of specific properties or of suitability for a
specific application. The user of our products is
obliged to observe any patent rights and existing
laws and regulations at his own responsibility.
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