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Single Screw Extruders and Barrier Screws

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					Single-Screw Extruders and Barrier Screws1
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“
                                     provided the conveying stability          development of the grooved
The     development      of          is adequate. This applies in              bush principle.
single-screw plastificating          particular to applications in             Extruders with grooved bushes
extruders                            which fluctuating proportions of          were initially operated with the
In    the   USA,     extruder        recycled or regrind material              conventionally flighted three-
development was - and still is -     have a disruptive influence on            section screws commonly used
largely     characterized    by      the      normal        conveying          in Europe. To have better
machines with smooth barrels.        characteristics of    the solid           control of the melt temperatures,
Further development has tended       material.    In     such    cases,        vented screws were later
to concentrate more on the           extrusion is likely to be more            developed, and, to improve the
screws than anything else, with      stable with a smooth-bore                 melt      homogeneity,       were
so-called 'barrier screws' –         extruder.                                 subsequently equipped with
screws in which the solid            In Europe, the development of             shearing/mixing sections [2].
material is kept separate from       extruders with heat-separated             One     problem       nevertheless
the melt in the melting section –    grooved bushes in the feed                remained: very high pressures
at the center of attention.          section began at the end of the           at the end of the feed section
Although the first barrier screw     fifties and beginning of the              and, as a result, considerable
was actually invented in Europe      sixties. Grooves in the barrels to        wear and tear on the screw and
in 1959 by Maillefer, most of the    increase barrel friction and              barrel.
further development work and         assist conveying of the solid
the practical application of this    material had been tried out long
principle took place in the USA.     before     then.     They    were,
The first USA patent was not         however, not enough to process
applied for until 1961 by Geyer      the newer high-molecular weight
from Uniroyal [1].                   HDPEs in powder and grit form.
Even      today,     smooth-bore     This     specifically    European
extruders with barrier screws are    phenomenon        on the raw
superior to grooved barrel           materials side has come from
extruders for many applications,     the systematic analysis and

                                                                          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
Fig. 1: Development of extruder screws in the USA and Europe


2
 The development of extruder             applications in extrusion blow      screw       (dynamic       mixing
 technology is basically reflected       molding,      for     example,      sections), but also static mixing
 in the development of the               relatively short extruders (L:D     elements.
 screws . For thirty years,              = 20:25) are used, whereas
 development teams went their            in other applications, such as      The various constructions of
 different ways in the USA and           film and pipe extrusion,            homogenizing elements will be
 Europe until, at the beginning of       extruders with longer screws        dealt with in more detail later.
 the nineties – also due to              (L:D ≥ 30) are generally            While a wide variety of screw
 increasing globalization – the          employed. As a result, the          concepts are still in use, current
 directions   of    development          way the total screw length is       developments are concentrating
 began to converge again.                divided up into the "feed and       very much on barrier screws.
 Combining the grooved bush              compression" and "melting           For this reason, this report will
 principle with barrier screws is        and homogenizing" sections          concentrate on such models
 the logical step to optimize            can vary considerably.              while taking a wider look at the
 extrusion technology [3].                                                   topic of single-screw extrusion.
                                         First of all, for a specific
 Screw      designs          and         application, a decision has to be   Fig. 2 shows schematically the
 selection criteria                      taken as to what proportion of      basic concept of barrier screws
                                         the total screw length should be    for different lengths of extruders,
 As already mentioned, the               reserved for homogenizing the       with and without barrel venting.
 choice of a suitable extrusion          plastificated melt. This question   The concept is the same for new
 system     (conventional     or         can      nowadays      only    be   extruders as it is for the
 grooved      bush    concept)           answered on the basis of            retrofitting of existing machines.
 depends on the particular               experience or following an
 application. After all, the                                                 The evaluation of a barrier
                                         appraisal of the demands made
 design     of    the    screw                                               plastificating section is generally
                                         on the melt quality. Even
 determines the quantitative                                                 carried out by looking at the
                                         specifying the necessary melt
 and the qualitative properties                                              differences in the pitch and flight
                                         quality can sometimes cause
 of the extrudate. In practice,                                              depths and the design of the
                                         problems. Complying with an
 different screw lengths have                                                feed section and outlet area of
                                         imprecisely defined melt quality
 become      established     for                                             the barrier flights. Both North
                                         can    necessitate   not   only
 different applications. For                                                 American and European barrier
                                         homogenizing elements on the




Fig. 2 Basic concept of barrier screws




                                                                                                                3
screw     developments       have      plastificating section can be        melt throughputs. Last but not
moved in the direction of              designed with the melt channel       least, the shearing sections
designs which conform, to a            closed at the rear end or with an    used for such applications can -
very large extent, to the principle    open melt channel. In this case,     and must - be dimensioned in
of the Dray and Lawrence screw.        even if we assume that               such a way that the necessary
The characteristic features of         unmelted material enters the         temperature    increase    is
these screws are that, through         melt    channel,    complete         reached.
elevations in the respective           plastification is nevertheless
pitches of the main flight of the      ensured by the end of the            For other applications, for
screw and the barrier flight, a        barrier section because of the       example foam extrusion, exactly
sufficiently wide channel is           long residence time in the melt      the opposite course must be
created in the solids channel –        channel. A detailed description      taken to keep the melt
this encourages plastificating –       of  different  barrier   screw       temperatures as low as possible
and that, through a variable           concepts,      including     their   after injecting the blowing agent.
adjustment of the flight depth         characteristic features, is given    Here, the best solution is to
profiles, the melt temperature         in [4].                              regard the extrusion system as a
curve can also be adjusted, with                                            highly effective heat exchanger,
the aim being to keep the melt         For extrusion applications in        and to enhance its effectiveness
temperatures      as   low    as       which relatively high extrusion      through reduced dissipation in
possible. Although barrier screw       temperatures are required (e.g.      large-dimensioned          screw
designs still exist today with a       paper coating), the screw            channels and through frequent
solids channel that is not sealed      geometries must be modified by       interface renewal by the screw
off, the only way of ensuring          making the flight depths in the      flights on the inner wall of the
complete melting in the barrier        melt-filled sections smaller so      barrel.
plastificating section is to use       that, due to the higher
solids channels with a 'dead-          dissipation energy, the target
end' groove (Fig. 3).                  melt temperatures are reached.
                                       This could possibly also be
The front of the barrier flight at     done by adapting the feed
the beginning of the barrier           sections to reduce the specific




Fig. 3: Transition between feed section and barrier section on a double
flighted/paired screw of 150 mm diameter


4
We will now deal with the                                               • Controlled melt temperatures          homogeneity problems.
influences of the raw material
and the extruder feed section                                           • Minimal change in the                 A better solution than a screw
geometry, which determine the                                             material through degradation          with a stepped pitch or channel
conveying properties of an                                                    or crosslinking                   depth is a barrier screw. At the
extruder.                                                                                                       beginning    of    the    barrier
                                                                        • High level of versatility: ability    plastificating  section,   the
                                                                              to process a broad selection      conveying flight changes to a
Universal screws / high-                                                      of raw materials with a wide      greater pitch; the beginning of
output screws                                                                 range of throughput rates         the barrier flight also has a
For the user, the ideal situation                                                                               higher pitch. The depth of the
                                                                        • Low     performance-related
would be to have a screw on                                                                                     channels is adjusted to the
                                                                          investment and operating
which as many different plastics                                                                                desired conveying and melting
                                                                          costs
as possible could be processed                                                                                  characteristics. These   two
at high throughput speeds and                                                                                   measures result in a fairly
                                                                        In recent years, so-called
with good melt homogeneity.                                                                                     balanced, low pressure profile,
                                                                        grooved barrel extruders with
Some of the most important                                                                                      or even in a pressure build-up
                                                                        barrier screws have proved to
requirements are:                                                                                               towards the end of the screw
                                                                        be the most suitable systems
                                                                                                                (Fig. 4).
                                                                        among single-screw machines.
• Processability of mixtures
  with different sized and                                                                                      When          assessing        the
                                                                        With many grooved barrel
  different shaped granules                                                                                     "universality" of a screw, the
                                                                        extruders, the pressure build-up
                                                                                                                homogeneity of the melt plays a
                                                                        at the end of the feed section is
• High                                      plastificating                                                      dominant       role.    This     is
                                                                        too high, encouraging wear and
  performance                                                                                                   particularly       true      when
                                                                        tear and impairing the stability of
                                                                                                                processing mixtures of different
• Gentle        but                              complete               the process. This can be
                                                                                                                materials, and also with regrind
  plastification                                                        countered by enlarging the pitch
                                                                                                                material,     with       color
                                                                        or making the screw channel
                                                                                                                masterbatches and with the so-
• Good melt homogeneity                                                 deeper, although this involves
                                                                                                                called 'direct extrusion' process
                                                                        the risk of plastification and
                                                                                                                (combining compounding and
                                                                                                                extrusion in one step, "in-line").

                  500                                                                                           Barrier screws, too, must be
                            PE - HD Hostalen GM 7746
                  450
                            Extruder 50 mm, 28:1 L/D                                                            provided with elements for
                  400
                                                                                                                homogenizing after the melting
                  350
                                                                                                                section. Depending on the
 pressure [bar]




                  300

                  250                                                                                           requirements     of    the    raw
                  200
                                                                                                                materials and the demands
                  150

                  100
                                                                                                                made on the product, shearing
                                                                 meltpressure in front of the screw tip
                  50
                                                                 meltpressure after grooved feed bush
                                                                                                                sections must be used for
                   0
                                                                                                                dispersion (for example for color
                        0              50              100              150                 200           250
                                                        screw speed [rpm]                                       pigments), and/or distributing
                                                                                                                mixing elements must be
Fig. 4: Meltpressure in front of the screw tip and after grooved feed bush                                      provided for axial and transverse
        (source: KKM)                                                                                           mixing.




                                                                                                                                                 5
Fig. 5: Spiral shearing section and faceted mixing section after a barrier section

In practice, barrier screws with        renewal of the surfaces and/or           excessively large variations in
neutral-pressure,    (multiple-         interfaces between the moving            the raw material properties –
)spiral shearing elements and           screw elements and the fixed             especially the bulk density, flow
with faceted mixing sections            inner surface of the barrel. A           properties       and       friction
designed to give good flow              further influence can be exerted         coefficients – are to be
properties       have    proved         on the temperature of the melt           expected, it is probable that
successful,     also for direct         by taking additional measures,           using the grooved bush concept
coloring with a color masterbatch       for example, by fitting a                will     lead    to     excessive
(Fig. 5).                               temperature control system on            fluctuations in melt throughput
                                        the inside of the screw (e.g. as a       due to the fact that the output
With homogenizing elements of           closed cooling system),                  characteristics are governed by
this kind, the best way of                                                       the solids conveying in the feed
maintaining full control of the         As    regards    extruders    for        section. In such cases a
general thermal conditions, and         universal applications - in other        smooth-bore extrusion system
thus of keeping the melt                words, extruders capable of              can or must be used. This is
temperature      closely     under      processing a very wide range of          particularly true for processing
control, is to ensure that good         raw materials, including regrind         raw materials with a fairly high
heat transfer by convection to          (fluctuating        proportions,         shredded        content        and
the temperature control system          recycled material etc.) - a              consequently a low bulk density.
of the extruder barrel is possible,     decision has to be taken in each         Another possibility is to pre-
both in the area of the spiral          individual case whether or not to        compact the shredded material
shearing elements and in the            use the grooved barrel extrusion         so that grooved barrel machines
area of the faceted mixing              concept. The decision is not             can function perfectly.
sections,    through      constant      always an easy one to make. If


       Homogenizing elements            Barrier           plastificating Feed section
                                        section


       •    Good dispersive/              • Effective separation of solids • Melt throughput geared
           distributive mixing effect        from melt                       to homogenizing capacity
                                          • High homogenizing effect
       •      Heat transfer to the barrel                                  • Low pressure level
                                          • Good      control    of  melt
       •      Low pressure loss              temperature                   • Low torque
                                          • Clear pressure build-up
                                                                           • Reduced wear and tear



Fig. 6: Barrier screw concept with homogenizing elements

6
Fig. 6 summarizes the three          They are marketed under       such     so that it actually drops from the
sections and shows the special       names     as    CTM,          TMR,     feed section to the end of the
characteristics of a barrier screw   STAROMIX and 3-DD               [5].   barrel.
with homogenizing elements.          Although they have a          good
                                     mixing action, some of        them     The temperature at the end of
For universal application, use       have    an  inadequate self-           the barrel is the same as in a
can generally be made of             cleaning system and others             conventional screw, in other
screws designed according to         have problems with wear and            words it is geared to the melt
the above concept in lengths of      tear. Apart from this, it is           temperature. In the first heating
between 20 and 30 x D. The           essential that the melt is 100 %.      zone after the grooved bush, it is
feed section consists either of a                                           perfectly in order to work at a
shallow-flighted feed section        Operation       with      barrier      temperature which is about 20 –
with subsequent decompression        screws                                 30 °C higher. In the lower to
(grooved bush concept) or a                                                 medium speed range, the
constantly deep-flighted feed        With barrier screws, designed          temperature in the final barrel
section (smooth-bore extruder),      according to the principle of          section is set at the same level
followed      by    the   barrier    Dray and Lawrence screws, the          as the melt temperature.
plastificating section and the       solids channel has been made
homogenizing sections.The best       wider. This provides a larger          The temperature at the end of
solution is first to have            contact surface area for the           the barrel is the same as in a
dispersively     acting   mixing     material being melted so as to         conventional screw, in other
elements, and then distributively    introduce energy via the barrel        words it is geared to the melt
acting elements. Fig. 7 shows        heating. This means that, with         temperature. In the first heating
possible and commonly used           barrier screws,     the   heating      zone after the grooved bush, it is
constructions.                       process   must       be   started      perfectly in order to work at a
                                     immediately after the material is      temperature which is about 20 –
In recent years, "static-dynamic"    fed in. Either a constant              30 °C higher. In the lower to
mixer systems with spherical         temperature program must be            medium speed range, the
indentations in a rotor and stator   set over the length of the barrel,     temperature in the final barrel
have become quite popular.           or the temperature must be set         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
Fig. 7: Executions of shearing and mixing elements (source: KTP)            process stability.

                                                                                                            7
                                                                           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
Fig. 8: Temperature program for barrier screws                             temperatures    as    low   as
In practice, it is not difficult to   maximum melt throughputs in          possible. In addition, the system
establish       the     optimum       the specified melt quality. For      had to have outstanding self-
temperature settings.                 this purpose, the combination of     cleaning       and        material
                                      a grooved barrel extruder and a      changeover characteristics.
Because       of    the    special    barrier      screw    with      a
characteristics of barrier screws,    homogenizing       section     is
it   has    often     proved an       particularly recommended. With
advantage to turn up the heating      larger screw diameters, a
output in the first and second        double-flighted    or   twin-pair
barrel sections, and to increase      screw system can be used to
the fan or cooling capacity in the    improve     the   conveying
two sections at the end.              properties in the feed section

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
                                      Fig. 9: Specific throughput vs. Screw speed (Quelle: KKM)
materials, but so as to achieve


8
Fig. 10 gives some examples of        of the peripheral speeds of the       into the machine.
results obtained with and             screw.
without a gear pump. Using this
concept, the targets were readily     The production of fuel tanks is
achieved, which meant that the        an impressive example of the
installed drive power was almost      direct recycling of production
completely utilized for the given     scrap. A problem here is that,
range of raw materials. Further       with      blow       molding,
increases in throughput are           comparatively short extruders
conceivable over and above            are used.
these figures. However, this
                                      Depending on the shape of the
would make it necessary to
                                      tank and on other boundary
adapt the drive unit by raising
                                      conditions, between 40 and 60
the     motor    power     and
                                      % flash occurs as scrap at the
proportionally increasing the
                                      production machine. This is
screw speed. It also becomes
                                      material which was cut off at the
increasingly important to take
                                      top and bottom of the parison
special measures to prevent
                                      and from the pinch-off edges of
excessive    wear   and      tear
                                      the blow mould. This material is
because of the greater influence
                                      directly ground and fed back




 Fig. 10: Production data with extruder 150mm/33:1 for steel pipe coating




                                                                                                9
                            700                                                                               215

                                       Extruder 150 mm / 20 D
                            600
                                        PE-HD Lupolen 4261A
                                                                                                              210
                                          with 50 % regrind
                            500




                                                                                                                    Melt temperature [°C]
        Throughput [kg/h]




                                                                                                              205
                            400

                            300
                                                                                                              200

                            200                                                               [kg/h]
                                                                                              [°C]            195
                            100

                             0                                                                                190
                                  5   10     15        20        25        30          35       40       45
                                                        screw speed [ rpm]

Fig. 11: Production data with extruder 150mm/20:1 for industrial blow moulding

Fig. 11 gives the key operating                   carried out in the first stage of         polystyrene, polycarbonate and
data for a grooved barrel                         the screw.                                PMMA.
extruder with a diameter of
150 mm/20 D equipped with a                       Fig. 12 shows the concept of a
barrier shearing/mixing section                   vented screw with a barrier
screw for processing high-                        plastificating section in stage 1,
molecular weight HDPE grit                        and a three-zone profile plus
containing regrind material.                      faceted mixing section in stage
                                                  2. Such vented screws with
One notable application for                       barrier plastification are being
'specialty screws' are vented                     successfully used for e.g.
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              Fig. 12: barrier screw configuration for vented extruder 90mm/30:1


10
Extruder      concepts       for       An optimized thermal layout is       mLLDPE.     The   goal     of   the
different plastics / new               also essential for the rest of the   chemical industry is – where this
high-performance                       barrel. Even though the target is    has not happened already – to
materials                              to generate as little excess heat    make it possible to run mPE on
                                       energy as possible via the           existing extruders by modifying
For most applications, grooved         screw, it is not possible with       the molecular structure.
barrel extruders with a heat           high-speed     extrusion    to
separation system between the          dispense with good cooling of        One aspect not being examined
feed section and the subsequent        the barrel, at least not in some     here, but nevertheless of
barrel have become established         sections. Special companies          considerable importance, is the
in Europe. As a rule, the feed         offer             heating/cooling    question of how the material
bushes are axially grooved and         combinations for this purpose,       behaves in the dies and after
correspond to the construction         in which a great deal of heat can    emerging from the die. With
concept shown in Fig. 13.              be dissipated via aluminum or        blown      film  extrusion,  for
                                       copper      elements.      Some      example, this would concern the
A good thermal layout - in other                                            melt elasticity and the bubble
                                       machine manufacturers also
words sufficient and uniform                                                stability.
                                       supply customized systems of
heat dissipation in the area of
                                       this kind. When new materials
the grooved bush - is of
                                       come on to the market, the
particular importance. For this
                                       question    continually   arises
purpose, the cooling channels
                                       about the most suitable extruder
and heat transfer resistances
                                       and whether they        can be
must be optimized. In many
                                       processed   on           existing
cases, temperature control units
                                       machines. This was the case
or installations with bypass
                                       with LLDPE, and it is now
control etc. are fitted to create
                                       happening with the metallocene
constant thermal conditions.
                                       polyolefins,   for    example


                                  A
                                               α
DN




                            DT




                                                                                  B
                                                                                               H




                                  A
                              L
        n = D(mm)/5                      H = 3 - 3,5mm
        DT - DN= ca.4mm                  α = 7 - 8°
        B = 7 - 8mm                      L = 3 - 3,5 D
Fig. 13 : Lay-out data for grooved feed sections




                                                                                                            11
 Fig. 14: Specific throughput vs. Screw speed for grooved feed extruder Ø80 mm/30:1
          (source: Reifenhäuser [6])

Basically, it can be said that        something which has also been         friction by cooling the screw,
mPE can also be run on                encountered       with     "normal"   coating the surface etc.
extruders used for processing         polyolefins (cf. Fig. 9). One way
LLDPE. This applies both to           of    countering       the    poor
grooved barrel machines and           conveying characteristics of the
smooth-bore     extruders    [6].     solid material is to add a
However, because of the               lubricant or material containing a
specific material properties,         lubricant. Another possibility,
there is a difference in the          this time on the machine side, is
throughput rates (Fig. 14) which,     to reduce the coefficient of
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 – indicates
that friction on the surface of the
screw plays a major role,             Fig. 15: Melt temperature vs. Screw speed (source: KKM)


12
                                               EXCEED in Blends
                     Effect of slip in the LDPE blend component on specific output
                                        on grooved barrelextruders

           3.4       Slip additivated LDPE as blend partner gives 10 - 20 % higher specific
           3.2                              versus no slip in the LDPE
kg/rpm/h




             3   Increase: 10 %                           15 %                                                   20 %
           2.8

           2.6   EXCEED
                  1MI /             +5%                                              EXCEED
                                                                                               + 10 %    + 20 %     + 30 %
                 0.918D             LDPE        EXCEED    +5%                         1MI /
           2.4                                            LDPE                                  LDPE      LDPE      LDPE
                                                  1MI /                              0.918D
                          +5%                    0.918D
           2.2            LDPE      with                  with                                          with slip
                          no slip   slip                  slip
             2

                            80mm                            75mm                                 80mm
                            24 L/D                          25 L/D                               30 L/D
                           grooved                         grooved                              grooved

Fig 16: Influence of slip agent on specific throughput with mPE (source: EXXON [7])

For engineering plastics (i.e.               vented extruders is currently in a               There has, however, been no
polyamides,          polyesters,             process of change, as was                        lack of attempts to also use
polyurethanes or thermoplastic               already explained with           the             single-screw      extruders    for
elastomers), use is nowadays                 barrier vented screws.                           compounding or for the so-
predominantly made of smooth-                                                                 called in-line extrusion. Systems
bore extruders. This is due not              Direct compounding in the                        of this kind are repeatedly
only to "tradition", but also to the         extruder                                         shown and marketed. The
predominantly low throughputs                                                                 possibilities and limitations are
                                             This term is used to explain the
involved. Plastics of this kind                                                               obvious.
                                             combination of compounding
can,    however,      also   be
                                             and extruding in one step. The                   One special kind of in-line
processed without problem on
                                             process, which aims primarily to                 extrusion is the blow molding of
grooved barrel extruders, as is
                                             cut down costs, is still in its                  tubular film from mixed film
shown in [8]. For blow molding
                                             infancy, despite all the efforts                 waste (DSD fraction). After a
with PA6, a grooved bush/screw
                                             being     made     by       machine              temporary phase of euphoria,
concept similar to the one used
                                             manufacturers      and       plastics            normality      has,      however,
for PE has given good results
                                             producers. For processing, use                   returned. Apart from technical
[9].
                                             is made primarily of co-rotating                 problems       with     individual
Grooved feed sections are also               twin-screw extruders, which offer                components of the plant and the
being increasingly used in                   far more possibilities in terms of               doubts about the product
extruders with a barrel venting              process technology. They can,                    quality, it has been found that
system. The grooves – with a                 for example, be used for                         the cost structure is also
semi-circular, sickle, saw-tooth             incorporating       fillers      and             negative over the long term.
or rectangular cross-section –               reinforcing agents, for blending
are either cut in the one-part               different   polymers        or    for
barrel, or a normal grooved                  simultaneously carrying out
bush concept is used. This                   reactions                   (reactive
means that the construction of               treatment/extrusion).

                                                                                                                             13
Despite all the progress being         almost   "boundless".        This   is   changed, and that it has an
made to adapt extruders and            shown elsewhere.                         existing drive    unit.    This
extrusion lines    to    the                                                    sometimes leads to restrictions
requirements      of      waste        Retrofitting      to    optimize         as far as the attainable specific
processing, it must be said that       the system                               melt throughput is concerned,
the production of extruded                                                      due to bottlenecks with the
                                       Whenever     funds     for    capital
quality products using recyclate                                                torque of the screw drive unit.
                                       expenditures become short, the
is limited. Not so much because                                                 The torque results from the
                                       purchase of new machines
of the machine and processing                                                   installed motor power and the
                                       tends to be put back or
technologies, but more because                                                  installed gear reduction. In some
                                       eliminated completely. In such
of the products and the                                                         cases, the gear reduction can
                                       circumstances, optimizing the
specified quality. For the time                                                 be adapted so that a higher
                                       existing system can be a help.
being, in extrusion, the recycling                                              drive torque is produced on the
                                       Where there is a need to modify
of production scrap will continue                                               screw shank. Since there is a
                                       existing extrusion lines to cope
to have priority over the                                                       directly proportional relationship
                                       with a higher output and/or
processing    of   post-consumer                                                between the specific output and
                                       improved melt quality, a modern
recyclate.                                                                      the screw drive torque, it is
                                       screw concept can be adapted
                                                                                possible in such cases to
                                       to the given circumstances.
                                                                                achieve an increase in the
Possible applications and
                                                                                specific melt throughput equal
limitations                             When making a modification of
                                                                                to the increase in torque.
                                       this kind, it first has to be borne
At this point, it should be said       in mind that the machine in
                                                                                Fig. 17 gives an example of a
that the possible range of             question is of a given length
                                                                                successful retrofit.
applications for grooved barrel        (e.g. frequently between 20 and
extruders with barrier screws is       25 x D), which can not be




Fig. 17: Production data with grooved feed extruder 60mm/24:1 after installing a barrier mixing screw
         (source: Kuhne)

14
Protecting the screw and               increasing use of barrels that       are also used for armor-plating.
barrel against wear and                have been given a centrifuged,       The screw root surface and
tear                                   wear-resistant and, if needed,       flanks can also be protected by
                                       corrosion-resistant      armored     nitriding, by a hard chrome layer
One      important         aspect      layer. Apart from the fact that      or by armor-plating.
concerned   with        protection     this approx. 1.5 – 2 mm thick
against wear and tear has been         coat can be adapted to suit the      Hot isostatic pressed materials
discussed already, namely wear-        particular type of stress, it also   (HIP) [10, 11] produced by
reducing screw geometries.             offers, with its       consistent    powder metallurgy are gaining
Quite astonishing results can be       properties, a certain "reserve" of   increasing importance. Both
obtained by harmonizing the            wear and tear, even if the           "natural hard" and hardenable
conveying characteristics and          process engineering parameters       alloys are used. The materials
the pressure build-up, and by          are not quite right.                 can be produced either as
optimizing the melting process –                                            homogeneous systems or as
from time to time in conjunction       For small and medium-sized           composite systems, in the latter
with a multiple flight design.         wear-protected screws (up to         case, either in conjunction with
With grooved barrel extruders,         approx. 50 mm), fully hardened       steel, e.g. as the core with an
for example, through optimized         tool steel is used, particularly     external hard shell for screws or
harmonization of the system            cold work steel X 155 CrVmO          screw elements, or as a
(adapting the feed section             12.1 (DIN 1.2379). To achieve a      composite of the hard alloy
geometry to the downstream             (limited) corrosion resistance,      powder with      inserted   hard
sections and vice versa),              frequent use is made of              substances.
grooved bushes made of nitride         rustproof, acid-resistant 17 %
steel can be used instead of           chrome steel X 35 CrMo17 (DIN        The PM-HIP materials have the
hard metal or PM-HIP material          1.4122).   By   ionitriding to       advantage   that    a    fine,
(see below), because of the            increase the surface hardness,       homogeneous and pore-free
much lower pressures involved.         however, this material loses         structure is formed, which is
                                       some of its corrosion resistance.    much     preferable     to   the
On the other hand, new plastics        For    very    high   corrosion      conventionally         produced
- in some cases necessitating          protection, it is preferable to      materials. The wear-inhibiting
higher processing temperatures         choose special materials, e.g.       hard phases (usually carbides)
- fillers and reinforcing agents or    Inconel 625.                         are also distributed more finely
pigments, higher peripheral                                                 and evenly in the fine-grain
velocities of the screw etc. etc.      With larger screws, it is common     structure, which means that less
are resulting in higher and            to armor-plate the screw flights,    surface area is open to attack in
higher stresses. They can only         which are particularly prone to      the matrix. The components can
be countered by taking special         wear and tear. This involves         be equipped specifically to cope
measures to increase the               using the tungsten inert gas arc     with the expected stresses.
protection against wear and            welding or the plasma-powder         Hardness values of up to 72
tear, as is state-of-the-art already   application    (PPA)       welding   HRC can be attained. Fig. 18
in injection molding [10].             method. The most popular             shows the overall properties and
                                       materials for this are nitride       behavior of PM-HIP materials.
In the USA, so-called bimetal          steel, 30 CrMoV9 (DIN 1.8519)
barrels have been used in              and 14 CrMoV6.9 (DIN 1.7735),
extruders for years and years. In      or chrome steel X 35 CrMo17
Europe, too, instead of the            (DIN 1.4122). Hard alloys such
conventional     nitride   steel       as Stellite 12, Colmonoy 50,
barrels, processors are making         Colmonoy 56, Colmonoy 83 etc.

                                                                                                         15
With materials examined on a         exception rather than the rule.
universal disk tribometer, it was    The higher the demands made
found that the relative wear         on the plastics and their
decreases significantly with an      additives, the more popular the
increasing    proportion      of     new systems will become, also
vanadium carbide.                    for these machines.

If we look at the market as a
whole, solutions in which PM-
HIP materials are used in single-
screw extruders are still the

                       6

                       5

                       4
           Rel. Wear




                       3
                                             Rel. Volumetric wear
                       2

                       1

                       0
                           0   10       20          30          40
                                     Vol.-% VC

             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 materials
                 dependent from VC content (source: Reiloy)




16
References


[1]      Rauwendaal, C.: Polymer Extrusion. München: Carl Hanser Verlag 1990

[2]      Fischer P.: Stand der Einschneckenextruder in Europa. Kunststoffberater (1984) 4, S. 20-
         23

[3]      Fischer, P., Wortberg, J.: Hochleistungs- und Unversal-Schnecken für die Extrusion.
         Plastics No. One 8/95, S. 21-27

[4]      Wortberg, J.: Schneckenkonzepte für die Hochleistungsextrusion - Barriereschnecken. In
         „Einschneckenextruder          -      Grundlagen      und       Systemoptimierung“.     Reihe
         “Kunststofftechnik“. Düsseldorf: VDI-Verlag 1991, S. 107-139

[5]      Bos, H. L., Meijer H. E. H.: Mischen und Kneten im Einschneckenextruder. In
         „Einschneckenextruder          -      Grundlagen      und       Systemoptimierung“.     Reihe
         “Kunststofftechnik“. Düsseldorf: VDI-Verlag 1991, S. 25-58

[6]      Schröter, B.: Processing of Metallocene PE (mPE) on Blown Film Lines. METALLOCENES
         EUROPE ’97, S. 353-373

[7]      EXCEED POLYETHYLENE, Film Processing Facts. Firmenschrift EXXON CHEMICAL
         EUROPE, Brüssel 1996

[8]      Michels, R., Wortberg, J.: Innovative Entwicklungen bei Einschneckenextrudern. In “Der
         Einschneckenextruder“. Reihe “Kunststofftechnik“. Düsseldorf: VDI-Verlag 1997

[9]      Völkel, M.: Eigenschaften und Anwendungen von verstärkten und                   unverstärkten
         Polyamiden.       In    “Blasformen    ’97   -   Innovationen    und   Perspektiven“.   Reihe
         “Kunststofftechnik“. Düsseldorf: VDI-Verlag 1997, S. 217-233

[10]     Lülsdorf,   P.:    40     Jahre    Verschleißschutz   für   Schnecken     und   Zylinder   in
         Spritzgießmaschinen. Vortrag auf dem KKM / IKM - Fachkolloquium “Spritzgießtechnik“,
         Essen 26.01.96

[11]     Deppe, E.: Verschleißschutzmaßnahmen an Schnecken und Zylindern in Extrudern und
         Spritzgießmaschinen. Vortrag auf dem VDI - Seminar “Technischer Oberflächenschutz“,
         Düsseldorf 19. und 20.06.97




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