Influencing Factors and Parameters in the Extrusion Process

MASCHINEN UND ANLAGEN
MACHINERY AND EQUIPMENT
Influencing factors rubber extrusion pin type extruder extruder performance material outlet temperature extrudate quality
The performance of an extruder aggregate as a result of the maximum
permitted screw speed is limited by the
maximum permitted material outlet
temperature or by the required level of
homogeneity in the extrudate. As a
basic result of fundamental trials both
the material outlet temperature as well
as the level of reached homogeneity is
corresponding to the duration time for
the compound to stay inside the extruder and the effective shearing rate.
This is mostly being determined by the
back pressure depending on the profile
size and the compound properties and
by the screw design or special installed
shearing elements like pins. Further
important influencing factors, which
may influence the performance of up to
30 % or even more, are the production
conditions such as TCU-settings for
screw, barrels and head, and the pin
configuration.
Einflussfaktoren und Parameter
auf den Extrusionsprozess
Einflussgrößen Kautschukextrusion Stiftextruder Ausstoßleistung Materialaustrittstemperatur Extrudatqualität
Die Leistungsfähigkeit eines Extruders und damit die zulässige
Schneckendrehzahl wird begrenzt
durch die maximal zulässige Materialaustrittstemperatur oder durch die
geforderte Produktqualität, bestimmt durch den erreichten Homogenisierungsgrad im Extrudat. Erkenntnisse aus Grundlagenuntersuchungen hierzu zeigen, das sowohl
die Materialaustrittstemperatur als
auch der erreichte Homogenisierungsgrad von der Verweildauer der
Kautschukmischung im Extruder und
vom Grad ihrer Scherung abhängt.
Diese wiederum werden bestimmt
von dem von der Profilgröße und den
Mischungseigenschaften abhängigen
Gegendruck, der Schneckengeometrie und der Effektivität eventuell
vorhandener Scherelemente wie z. B.
Stiften. Weitere wichtige Einflussgrößen, die die Leistungsfähigkeit bis
zu 30 % oder mehr beeinträchtigen
können, sind außerdem die Betriebsbedingungen wie Temperierung von Schnecke, Zylindern und
Kopf und die Wahl Bestiftung.
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Influencing Factors and
Parameters in the Extrusion
Process
Even if extruder size and configuration as
well as the screw design are important factors for the performance and the product
quality of an extrusion line, there are other
influencing factors such as compound properties, profile size (back pressure), screw
speed, TCU-settings and pin configuration, which mainly influence the actual production and which have to be taken into
consideration for optimisation of the extrusion process individually.
Therefore modern extrusion lines are supplied with computer based memories in
which the optimised production data are
stored as recipes for different profiles
and compounds and can individually be recalled for the corresponding application.
These production data have of course at
first to be optimised for each single profile
due to the actual requirements as productivity, material temperature and appearance according to the performance triangle
of the extrusion shown in Fig. 1.
The extruder output is the main determining factor for the productivity. It is first
of all determined by the extruder size,
the screw speed and the back pressure,
i.e. the profile size.
But on the other hand the output is also
limited either by the permitted material
outlet temperature or by the required quality of the extrudate, i.e. the surface appearance or product homogeneity.
The target of the optimising process is to
find the right combination of the parameters for max. output at lowest material outlet temperature and sufficient appearance.
to reduce the shearing rate and the material outlet temperature on the expense of
output.
Of course the existing diagram may have
to be modified according to other influencing factors such as compound type, consistence and properties, profile size and
others.
Extruder performance diagram
ML(1þ4) 100 8C
Fig. 2 shows the performance of ThyssenKrupp pin type extruders, i.e. the output
[kg/h] of different extruder sizes depending on the screw speed [rpm]. Parameters
are the additionally included estimated
material outlet temperature graphs.
According to the graphs it is shown, that
higher screw speed does not only increase
the output, but also the material outlet
temperature is increased because of additional shearing of the compound.
Exceeding the permitted temperature will
then require reduction of the screw speed
Compounds with high Mooney-viscosity
normally cause high torque because of
high flow resistance inside both the flow
channels of the screw and the extrusion
Influence of back pressure
The actual back pressure in the extrusion
head determines the actual operational
point of the screw, i.e. the actual output
of the entire extruder aggregate. It is depending on the profile size, the design
of the flow channels, the actual viscosity
of the compound (depending on the actual material temperature) and the flow conditions (temperature of the extrusion
head).
Small profiles, long flow channels, high
compound viscosity and low extrusion
head temperatures cause high back pressure and low output as well as high material temperature.
The influence of back pressure on output
and material outlet temperature is shown
in Fig. 3, in which the different back pressure levels are realized by using different
profile sizes:
Influence of the compound
The influence of the compound properties
on material temperature and output is
mainly characterized by the visco-elastic
behaviour of the compound, expressed
by the Mooney-viscosity
L. Köster, Hamburg-Harburg
Corresponding author:
Dr.-Ing. Lothar Köster
ThyssenKrupp Elastomertechnik GmbH
Seevestr. 1
21079 Hamburg-Harburg
Tel.: 0 40/7 71 79-0
KGK Kautschuk Gummi Kunststoffe 58. Jahrgang, Nr. 7-8/2005
Fig. 1. Performance
triangle of extrusion
the material inside the extruder and to reduce the number of shearing elements like
pins, if low material temperature is wanted.
On the other side especially homogeneitycritical compounds like natural rubber (NR)
may require a high grade of shearing for
good plastification and homogenizing to
get good appearance.
This will either need special shearing elements like e.g. a high number of pins or
special screw design, or it can be influenced by selecting a TCU-setting, which reduces the output for a longer time of duration of the material inside the extruder.
High output means low duration time for
the material inside the extruder, i.e. low
shearing rate, low level of uniformity
and low material outlet temperature.
Low output means longer duration time
for the material inside the extruder, i.e.
high shearing rate, high level of uniformity
and homogeneity, but also higher material
outlet temperature.
The influence of different TCU-settings
due to their effect on output and material
outlet temperature is shown in Fig. 5.
A guideline for correct TCU-settings according to required output and material outlet
temperature or product quality is shown in
following list:
Influence of pin configuration
Fig. 2. Extruder performance diagram
head. Therefore a high level of shearing is
needed. This causes high material temperatures and, corresponding hereto, a reduced output according to the graph in
Fig. 4.
Low material temperature always means,
that the compound needs to be sheared
in a low range only. Corresponding hereto
it is necessary to reduce the duration for
According to a.m. statements the number
of pins in a pin type extruder is of important influence as well for output, i.e. productivity, as also for shearing and plasticizing of the compound, i.e. product quality.
A great number of pins will always increase
the shearing rate and by that give good
homogeneity and appearance. But it will
also increase the material temperature.
Influence of TCU-settings
The simplest and therefore most important
possibility for the operator to influence as
well the product quality as also the output is
given by changing the TCU-settings according to the requirements of the compound.
But since rubber is a bad heat conductor it
is a mistake to believe, that the material
temperature can be influenced by cooling
the extruder or the extrusion head to draw
out the internal heat of the rubber.
But instead of it is possible to influence the
adhesion between material and barrel or
flow channel walls by selecting the correct
temperature.
Fig. 3. Influence of back pressure
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363
On the other hand a low number of pins
will reduce the shearing rate and by that
reduce the material temperature. But on
the other hand the result may be an insufficient homogeneity and insufficient appearance, especially for homogeneity-critical compounds like natural rubber.
The target of starting up an extrusion line
therefore is to find the lowest necessary
number of pins to achieve as low a material
outlet temperature as possible and at the
same time to achieve an acceptable surface appearance.
Fig. 6 shows the effect of different pin configurations on a KGS 150/16 pin type extruder on output and material outlet temperature:
Fig. 4. Influence of different Mooney-viscosity
Influence of screw design
Fig. 5. Influence of
different TCU-settings on output and
material temperature
Since shearing rate and duration time inside the extruder on one hand increase the
product quality because of increased homogeneity and uniformity, it on the other
hand also will increase the material outlet
temperature and so limit the productivity.
To reduce the shearing rate but still to have
the effect of good mixing and uniformity
ThyssenKrupp Elastomertechnik developed a new series of high performance
screws with improved shearing and mixing
zones by flight separation in the pin section of the screw as shown in Fig. 7.
The separation of the flights and the flow
channels in the pin section of the screw
continuously distributes and collects the
flow of the compound and mix the different flows for good uniformity without additional shearing.
By that for the same or even better level of
uniformity and product quality the number
of pins may be reduced, so that either the
material outlet temperature will be decreased or the screw speed might be increased
for more output at same temperature.
A comparison of the output / temperature
relationship between 2 similar pin type
screws KGS 150/16 with/without flight separation is shown in Fig. 8.
Summary
According to a.m. comments regarding
the different influence factors and operational conditions and there influence on
productivity and product quality of an extrusion process, it can be stated, that each
application and each product normally require its own optimisation due to the compound and production requirements.
Fig. 6. Influence of
pin configuration
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KGK Kautschuk Gummi Kunststoffe 58. Jahrgang, Nr. 7-8/2005
List 1. Guideline TCU-Settings for TKEL-Pin Type Extruders
Screw
Inlet Barrel
Pin Barrel 1
Pin 2 þ Ext.
Head
High Output
Low Temperature
90 – 95 8C
20 – 30 8C
40 8C
60 8C
90 – 100 8C
Low Output
Good Homogeneity
70 – 80 8C
40 – 50 8C
60 – 70 8C
80 – 90 8C
90 – 100 8C
Fig. 7. High output
pin type screw KGS
150/16 with flight
separation
The qualified operator has a number of different possibilities to his disposal to optimise the process and to get best results.
On the other hand, unqualified operation
may lead to losses or scrap production,
which normally are not in the responsibility
of the machine.
But anyway, no extruder or extrusion line,
even when it is operated by the most qualified operator, can compensate problems
in the process, which are caused by insufficient compound.
The result of any extrusion process is a
combination of all the influencing factors,
starting with the quality of the ingredients
and the consistence and properties of the
compound, continuing with the qualification of the operators and the operational
conditions and ending with the performance of the extruders and the entire extrusion line.
Only when all these parameters are taken
into consideration and respected, the result can be a good tire.
The author
Dr.-Ing. Lothar Köster ist Leiter der Verfahrenstechnik / Extrusion bei der Firma ThyssenKrupp Elastomertechnik GmbH, Hamburg-Harburg.
Fig. 8. Output / temperature relationship
with/without flight
separation
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