A3
Version: 2
Issued: 15. February 2016.
BUDAPEST UNIVERSITY OF TECHNOLOGY AND ECONOMICS
FACULTY OF MECHANICAL ENGINEERING
DEPARTMENT OF POLYMER ENGINEERING
Extrusion
THE VALIDITY OF THIS PAPER HAS TO BE VERIFIED ON THE DEPARTMENT WEBSITE!
WWW.PT.BME.HU
A3 – EXTRUSION
Version: 2
15. February 2016.
THE LOCATION OF THE TRAINING
TABLE OF CONTENTS
1.
THE AIM OF THE TRAINING.............................................................................................................. 3
2.
THEORETICAL BACKGROUND......................................................................................................... 3
2.1.
INTRODUCTION .................................................................................................................................. 3
2.2.
THE SECTIONS OF AN EXTRUDER SCREW ............................................................................................ 5
2.3.
THE MELT-CONVEYING CAPACITY OF THE EXTRUDER ........................................................................ 7
2.4.
EXTRUDER TOOLS ............................................................................................................................ 10
3.
EXPERIMENT ....................................................................................................................................... 16
4.
THE MACHINES, EQUIPMENT, TOOLS USED FOR THE EXPERIMENT ............................... 16
TEST REPORT ................................................................................................................................................ 17
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1. The aim of the training
Becoming acquainted with the extrusion technology of thermoplastic polymers. During the
training a rod shaped pre-product of one material will be produced, then chopped (pelletized).
During the production the influence of the screw rotation speed on the material output of the
extruder will be examined.
2. Theoretical background
2.1. Introduction
Extrusion is one of the most efficient, most significant technologies of polymer processing,
during which the (mostly thermoplastic) polymer will be

taken into a plastic (melt) state by the extruder, then the viscous melt is

placed under pressure (compressed),

homogenized,

pressed through an open die with a given, constant cross-section, and after that,

cooled down by the follow-up equipments to ensure dimensional consistency, so

a polymer product with a constant cross-section and an optional length will be produced in
continuous operation.
Almost 40% of polymer products are produced by extrusion. One of the essential properties
of the technology is, that one dimension of the 3D structure of the product can be infinite, it can be
shaped like a tube, a flat sheet, a prismatic rod, film, etc. The last phase of the process is winding or
cutting. The feedstock can be used as powder or in pelletized form, additives can be mixed to it.
Such additives can be thermal stabilizers, which can decrease the tendency of degradation, UV
stabilizers, which can provide protection against natural light, plasticizers, which can ease
processing, flame retardants, lubricants, which decrease screw friction. The additives can be
introduced in (cold, hot) powder mixers, the pellets can be produced using so-called pelletizerextruders. The first extruders were used in the food industry by bakers for noodle production. The
first light industrial application was by H. Bewley and R. A. Brooman in 1845, but the first extruder
has been patented by M. Grey. These extruders were mainly used in the rubber industry. The
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nowadays used modern extruders (Figure 1.) do not differ from the old ones significantly in
buildup, but the geometry of the screw used for thermoplastic polymers went through a lot of
changes.
Figure 1. The buildup of an extruder
The size of the screw was affected mainly by these changes. Thermoplastic polymers need a
certain dwell time, to reach total melt state. The optimal screw size has been found at around a
length/diameter, l/d ratio of 20. The needed amount of heat is provided by built in heating elements
around the barrel wall. The optimal processing temperature is set and maintained by both heating
and cooling of the barrel. The processing temperature used for extrusion is lower than the one for
injection moulding because of the longer dwell time and higher melt viscosity. The melt pressure is
usually also one order of magnitude lower than the one for injection moulding, and there is a
characteristic pressure-distribution curve for each screw build (Figure 2.).
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Figure 2. The change of pressure in a one-screw extruder, sections of the extruder screw
2.2. The sections of an extruder screw
The extruder screw can be mainly divided into three sections: feed, compression (transition),
and homogenizing or metering section. For special needs further sections are imaginable (for
example: degassing (venting) section).
The function of the feed section is to convey the solid material towards the compression
section. To achieve this in one-screw extruders the friction between the polymer and the screw
should be less than between the barrel and the screw. In twin-screw extruders, which have been
developed for achieving higher homogeneity, - the screw threads are overlapping (intermeshing),
the screws can rotate in the same or opposite direction (co – or counter rotating, respectively).
Besides the drag induced flow, which is the sole mode of polymer transport in single screw
extruders, in twin screw extruders there is a positive displacement characteristics in the
intermeshing region, similar to gear pumps.
The compression section bears two functions. One of them is to completely melt the
material (Figure 3.), the other is to provide sufficient pressure for the forcing-through of the
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material through the extruder-die. The melting process starts at the material particles in contact with
the warmer cylinder wall, then a circulating flow emerges in the thread-groove, which fastens the
melting of the polymer.
Figure 3. The melting process
The other function, the increasing of the pressure can be achieved mainly by two methods.
The first is through increasing screw core diameter (core-progressive), the other is through
decreasing the screw pitch (pitch-degressive). A third, not too widespread method is the increase of
the thickness of the thread-flights. The so generated pressure helps not only the pushing through of
the melt through the die, but also in most cases the elimination of air trapped between the particles.
If this degassing is insufficient for the process technology, a so-called degassing extruder, specially
developed for such cases, is needed.
The third section is the so-called homogenizing, metering section. Homogenization is not
only needed when mixing additives, or reinforcing materials to the polymer material, but also when
extruding “pure” polymer. The reason is, that after melting the high temperature of the material
contacting the barrel (where the heat transfer occurs) decreases with the distance. Not overcoming
this problem can worsen the quality of the product. Mixing elements can be used not only at the end
of the screw, but also in its other sections (Figure 4.).
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Figure 4. Mixing and shear elements of extruder screws
2.3. The melt-conveying capacity of the extruder
The main dimensions and the emerging flow components can be seen in Figure 5.
Figure 5. The main dimensions of the extruder screw
The melt-conveying capacity of an extruder in terms of volume flow rate is:
.
.
.
.
V e  V s V t V r
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.
Where V e is the total volume rate (m3/s), V s is the drag volume rate, which is responsible
.
for material transport, and is caused by the rotation of the screw, V t is the back pressure volume
.
rate, which is opposite to the drag volume rate and is caused by the resistance of the die. V r is the
gap volume rate, which is caused by the fitting gap (clearance) and reduces melt-conveying
capacity.
The gap between the screw and the barrel, the clearance is small, typically between 0,002 D
and 0,005 D, it is sealed and lubricated by the viscous polymer melt during the rotation. This is why
it can be neglected at the first approximation. The other two components will be examined through
the simplest one-way flow at presumed isothermal circumstances and Newtonian medium.
The model of the drag flow (Figure 6.) is as follows: the material flows between two flat
plates (the upper plate corresponds to the inner surface of the barrel, the lower plate corresponds to
the screw core). The upper plate moves by a constant v0 velocity. There is no resistance and
pressure drop in the flow direction (open channel).
Figure 6. The velocity distribution of the drag flow
The drag flow emerges between the screw core and the cylinder wall, induced by the v0
velocity arising from the rotation of the screw. The vs velocity distribution is:
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vs ( y )  v0
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y
h
(6.2)
.
So the V s volume rate can be calculated easily:
.
bv
Vs  0
h
.
d V s  vs ( y)dA
(6.3)
dA  bdy
(6.4)
h
bv0  y 2 
v0
ydy

   bh
0
h  2 0
2
h
(6.5)
.
v0 is proportional to the screw speed, n: V s ≈ b  h  n ~ h  n . So the drag flow depends firstly
on the dimensions of the screw-groove and the rotational speed of the extruder screw, so the yield
increases for larger groove and higher rpm.
The back pressure flow is induced mainly by die resistance (pressure) (Figure 7.) and other
resistances in the way of the flow.
Figure 7. The velocity distribution caused by the back pressure flow
Its calculation can be derived from the Hagen-Poiseuille equation for the flow of polymer
melt in a rectangular duct.
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.
Vt 
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1 bh 3 p


12 
l
(6.6)
where b and h are known (Figure 5.), η is a material property, l is the length of the examined
segment, p is the pressure drop. The gap flow is usually neglected, and for a not worn screw it can
really be neglected thanks to the good sealing properties of the polymer film. However, in case of
wear it increases proportionately the third power of the gap size δ.
The mean velocity distribution will be the sum of the two components (Figure 8.).
Figure 8. The velocity distribution as the superposition of the drag and back pressure flows
The pressure to drag flow ratio (a) characterizes the relation of the two flow rates. The melt
flow is greatly dependent on the viscosity properties of the raw material.
a
Vt
Vs
(6.7)
2.4. Extruder tools
The rod shaped polymer melt leaving the extruder screw can be theoretically formed to any
regular or irregular form, and after the cooling, processed to a pre-product, tube, sheet, profiled etc.
structural element. But the viscoelastic character of polymers, mostly the unconventional flow
phenomena (for example die swell) hinders the freedom of shaping. The remaining frozen-in
stresses, emerging after the cooling of the extruded product can lead to locally different relaxation
behavior, dimension change, shape distortion, cracks and premature failure. Most of these can be
avoided by a properly designed die. So it is practical to review the most commonly used dies of
polymer extrusion.
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The common feature of an extruder die is to lead the material flow through the sections
below:
–
transition section
–
forming section
–
finishing („ironing”) section
The transition section conducts the circular cross-sectioned viscous material flow. The
forming section forms the desired cross-section. This cross section is to be narrowed in the next
section: the profile will be stabilized: it will be “ironed” by mildly increased pressure.
The polymer flow leaving the tool is not fully solid. The final solidification, where the
needed dimensional accuracy is ensured, takes place in the calibrating unit following the extruder.
The tool for sheet production
In the plastics industry a flat product with a thickness higher than 0,5 mm (most often higher
than 1 mm) is called a sheet, because the thinner product, the film with a thickness of some hundred
microns has fairly different applications. Films are mostly used temporarily (packaging, agricultural
films), and distributed in rolls. Sheets, pre-products for a wide range of applications can be
separated from films also in terms of the tool used for production. The typical tool used for
production, the coathanger sheet die can be seen below (Figure 9.).
Figure 9. Coathanger sheet die
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The main goal is to transform the circular material flow into a sheet having a maximum
width of more than 2000 mm and a maximum thickness exceeding 15 mm. The melt flows first
through a runner system shaped like a coat-hanger, then comes against the choker bar, bearing a key
function in the even distribution of the material flow, apart from its distributive function it also
homogenizes the flow. The evenly distributed material bears the shape of the tail of a dolphin or a
shark (Figure 10.). The material leaves the die trough an adjustable lip, which provides more
uniform product thickness.
Figure 10. Pressure distribution in a coathanger sheet die
The as-produced sheet is then lead through rolls for setting the final sheet thickness and
roughness, and cooling.
The tool of tube-making
The most important application of synthetic polymers for architecture and building
engineering is the production of plastic pipes. A typical die for pipe-production can be seen above
(Figure 11.). The melt flow comes around an axissymmetric core, then leaves the die narrowed to
the final size.
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Figure 11. Pipe die
1, mandrel support; 2, tool housing; 3, mandrel/pin; 4, pipe die; 5, centering bolt; 6, torpedo;
7, heating ribbon; 8, threaded connector to the extruder
The extruder is followed by a calibrator. Calibrators can use vacuum or overpressure. The
vacuum-calibration is performed in a water-cooled calibrating tool (Figure 12.). In the calibrator
small holes or vents lead to the inner surface of the calibrator, the plastic tube, which is thereby
pressed to the inner surface of the tool by the atmospheric pressure.
Figure 12. Calibration of the outer diameter of a pipe in a vacuum-caliber
1, pin; 2, pipe die; 3, cooling water; 4, spillway; 5, vacuum-caliber
Another technology for calibration is pushing the plastic tube to the inner wall of a calibrator
tool by generating overpressure in it (Figure 13.). The compressed air inlet is at the side of the die
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(Figure 11.). The function of the floating plug is to keep the pressure in the tube. When the inner
diameter of the tube needs to be calibrated, the tube is pulled over a longer, cooled plug.
Figure 13. Calibration of the outer diameter of a tube using overpressure inside
a, outlet of the pipe die; b, compressed air inlet; c, cooled calibrating tool; d, floating plug
Profile tools
The extrusion technology makes the production of multiple profiles of complicated shape
and multiple cavities. The good mechanical properties, stiffness of the dimensionally stable, stormproof PVC window profile is also due to its complicated, multiple cavity structure.
The function of the die is likely to the case of a pipe-die: the plastic flow has to be divided to
envelop the cores of the die, then united again to achieve the profile (Figure 14.).
Figure 14. Cross section of a profile die
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In this case the calibration is usually carried out by vacuum, the calibration by inside
pressure cannot be carried out, because multiple, complicated floating plugs would be needed.
Die for film blowing
From the useful bags of packaging technology to the 0.2mm thick and 16m wide agricultural
films the thin PE films are mainly produced by blown film extrusion. The film blowing utilizes the
good extendibility of molten state polymers and the increase in strength caused by it. Film blowing
is specially optimized for LDPE, but can be also applied for the processing of HDPE and LLDPE
too (Figure 15.).
Figure 15. Blown film extrusion
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3. Experiment
The course of the experiment:
1. Starting the extruder
2. Measuring the material flow output at set conditions
3. Evaluation
4. The machines, equipment, tools used for the experiment

Labtech scientific 25-30C single screw extruder

Conveyor

Scale

Granulator
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TEST REPORT
Name:
Grade:
Neptun ID:
Date:
Verified by:
Instructor:
1. Experiment
Measurement of the material flow output at set extruder screw rpm.
2. Basic data, measured and calculated results
Tested material:
Screw rpm
[1/min]
Measured
weight [g]
Test time
[s]
Yield
[g/min]
3. Plot of results
Yield
[g/min]
Screw rpm
[1/min]
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