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New Spiral Mandrel Dies1
Peter Fischer, Johannes Wortberg
1 Extended version of a paper presented at the SKZ conference on "Innovations in Extrusion" on 13-14.05.98 in Würzburg
2
Although spiral mandrel dies have been state-of-the-art in
extrusion technology for 25 years, new die concepts are
more topical than ever before. Different melt distribution
systems are combined with spiral mandrels in a cylindrical,
conical or radial configuration. In the case of coextrusion dies, in
particular, this gives rise to good thickness uniformity with a
broader range of processing parameters (raw materials,
throughput, temperatures), short residence times (material or
colour changes), low pressure losses, good thermal control, a
long service life, low space requirements, easy assembly,
dismantling and cleaning and, not least, favourable production
possibilities. Sample designs from sheathing, pipe and tube
(co)extrusion demonstrate the process, production and
operational engineering advantages of innovative spiral
mandrel dies.
1. Optimization criteria
Outstanding results have been achieved with spiral mandrel
dies for the extrusion of rotationally symmetrical
products (blown film, tubes, pipes, hollow articles) for many
years, in the form of a uniform volume-flow and product wall
thickness distribution with no weld lines to impair quality
[1,4,6,9,10,11,12]. Alongside these key aspects, present-day
designs are also required to fulfil
additional criteria, particularly in the case of coextrusion:
Rheological criteria
• low pressure loss and a
short residence time (flow channel cross-sections and
lengths)
• compliance with minimum
and maximum wall shear stresses (risk of deposits
and melt fracture) for the specified range of
processing settings
• avoidance of interfacial
instability in coextrusion, particularly with extreme
layer thickness ratios (individual layers account
for only 1% of the overall layer thickness at times)
• avoidance of non-uniform elastic deformation (and
orientation)
• avoidance of local stagnant
zones and dead spots (material or colour
changes).
Thermodynamic criteria
• attention to dissipation and
heat exchange for the raw materials in question
(heating, insulation) including for the individual
layers in coextrusion dies, where appropriate
• attention to thermal symmetry in the die and
melt flow.
Operational engineering criteria
• rigidity and freedom from warpage
• simple assembly, dismantling and cleaning
(with no melt, or only a small quantity of melt
sticking to the flow channel surfaces)
• tight parting planes
• easy centering
• shortest possible material and colour change times
(for the system in its entirety, including the
extruder, melt filter, adapter and die), and hence a
reduction in the number of rejects.
3
2. Dimensioning
When dimensioning spiral mandrel dies, it is necessary to
consider the relevant areas of the flow channel between the
extruder barrel or screen-change adapter and the annular
slot at the end of the die. The following areas of the flow
channel need to be dimensioned in respect of their
cross-sections and lengths:
1. adapter to the die mount
2. melt flow branches inside the die
3. melt pre-distribution systems (e.g. spider legs, branched
holes, coat-hangers)
4. spiral mandrel melt
distributor
5. restricted flow zones or
relaxation zones
6. the point where the individual
layers are brought together in coextrusion
7. joint flow channel from the point where the melt meets
up to where it reaches the die orifice in coextrusion
8. die gap geometry in the diameter-dependent die unit
(gap width, parallel gap length).
2.1 Simulation
Computed simulations can
sensibly be used to achieve the correct dimensioning of the flow
channels for specific
applications. This involves
calculating the flow processes in circular and annular channels
for single-layer flows, using uni-dimensional formulations with
the assumption of isothermal conditions.
Two-dimensional models (networks) for isothermal flows
have now proved successful for the computation of spiral
mandrel geometries. whether
these be cylindrical, conical or
radial in design (Fig. 1) [10]. Although computation models
for three-dimensional, non-isothermal flow are available
(FEM, BEM), it has so far proved difficult to interpret the additional
information obtained (spatial velocity and temperature
profiles) for purposes of improving on the flow channel
design. Despite this, valuable
Fig. 1: Network model for the computation of a spiral mandrel manifold
Fig. 2: Pressure loss optimization at the die gap
4
qualitative assistance is
obtained for optimizing geometric details, such as feed
throat geometries, deflections and stream divisions.
Apart from optimization of the spiral mandrel geometry, special
importance has to be attached to the establishment and
favourable configuration of the pressure loss. The procedure to
be followed for optimizing a die orifice is shown in Fig. 2 in
qualitative terms.
The simulation of multilayer
flows in coextrusion dies is aimed, inter alia, at avoiding
interfacial instability. Only simple basic rules can be derived for
the dimensioning of a specific flow channel, however. Hence, it
can be recommended that the gap widths be adjusted to the
specific layer distribution in the product before and after the
point where a number of different layers are combined
[10]. The frequently-asked question about the bandwidth of
the melt throughput or volumetric flow rate in
coextrusion dies cannot, however, be answered on a
generalized basis. This will depend, among other things, on
the elastic properties of the melt, and taking these into
consideration in flow simulations involves a high outlay,
particularly where complex shear/elongational flows prevail.
Restrictions due to the rheological (in)compatibility of
certain raw materials, on the one hand, and pressure loss and
residence time criteria, on the
other hand, mean it is not possible to predict the limits of
the processing parameter range sufficiently accurately in
theoretical terms. Experimental results are generally only
available for reference products, and hence it is necessary to be
cautious and not always accept the confirmation given with
regard to the feasibility of certain new, multilayered products (e.g.
with throughput ranges of 1:10, or greater).
2.2 Different materials
A spiral mandrel die can and,
indeed, must be optimized for a number of different materials in
most cases, making allowance for their respective processing
temperatures and melt throughputs. Where a conflict
prevails between pressure loss and residence time, the flow
channels must be dimensioned for the melt with the highest
viscosity, specifying a maximum tolerable pressure loss in each
section of the flow. Residence time criteria may be more
important than local pressure losses in areas containing
critical deflections or flow branches, and also for the spiral
geometry, while low pressure losses may take precedence in
simple, circular flow channels (drilled holes), with a still
acceptable residence time.
Shear rates should not be set at
below 5 s-1 or above 50 s-1 for design purposes, so as to
ensure that there is still sufficient wall shear stress to permit an
acceptable material or colour
change. In a few cases, such as with cable and conductor-
sheathing dies, or with low-viscosity melts, shear rates may
be as high as 1000 s-1.
2.3 Geometric degrees of
freedom
In some cases, the simulation
models can provide useful assistance with the specification
of geometric degrees of freedom, such as the type and
extent of melt pre-distribution, the number of turns in the spiral,
spiral overlap (design height), the shape of the spirals and the
spiral inlet and outlet. In all cases, however, it will still be
necessary to make allowance for production engineering
aspects and the cost, such as when determining the number of
spirals and the shape of the spiral cross-section. Computer
simulation has doubtless made less of a contribution to the wide
variety of different design concepts that are available
today than has the creativity of engineers.
A number of design solutions for spiral mandrel dies with different
pre-distribution systems and spiral mandrel configurations for
coextrusion dies are presented and discussed in what follows.
5
3. Melt pre-distributors
For a long time, the only approach adopted was to equip
spiral mandrels with pre-distributor systems comprising
branched drilled holes or circular channels so that each
spiral was fed from a hole of its own. Systems have also
become established today where pre-distribution is
performed in an annular gap. Contrary to the previously-used
smear devices placed behind the spider-leg mandrel [1], the
spiral mandrel manifold is retained as the key element for
definitive melt distribution here.
In other words, almost all the
melt flowing in from the annular
gap on the pre-distributor system is pressed into the spiral
inlets. Following this, the redistribution that results from
the overlap of the axial and spiral flows leads to sickle-
shaped partial flows distributed over the circumference and,
finally, to a uniform melt distribution at the end of the
spiral mandrel manifold (Fig. 3) [14]. The designs that have
proved successful in practice extend from the conventional 1/1
solution (hole/spiral) through to a form of pre-distribution
involving a slot die reproduced on a cylinder, as it were, with a
downstream spiral mandrel manifold (Fig. 4).
Fig. 3: Operating principle of a spiral mandrel manifold
Pre-distributor + Spiral mandrel
n feed holes + n spirals
(„conventional design“)
m feed holes with + n spirals coat - hangers
Important: more than 90% melt from pre-distributor into spirals
Fig. 4: Allocation of pre-distributor and spiral mandrel manifold
6
3.1 Conventional solution
Figure 5 shows a conventional solution by way of example, with
radially oblique pre-branching channels running directly into
the spiral inlets (n = number of holes or circular channels). By
depicting the configuration laid out flat, it is clear that the
hatched areas cannot be made round on a lathe but have to be
machined with a milling cutter. Otherwise, dead spots between
the spiral inlets would impair the functionality of the spiral
mandrel.
3.2 Coat-hanger
pre-distribution system
One way of feeding an annular
gap at the end of the pre-distribution system is by using a
coat-hanger pre-distributor (frequently used in slot dies and
sometimes on deflector torpedo heads). The variants that have
already been constructed start with just one coat-hanger on the
circumference of the pre-distribution system, which is fed
in axially or radially from a drilled hole or a channel, and go right
through to combined pre-branching systems
incorporating a number of coat-hanger distributors around the
circumference. Decoupling the feed channel and the spirals
means that the number of spirals can be determined in
isolation and can vary from n to a multiple of n, depending on
the task on hand and the design constraints.
On the spiral mandrel in Fig. 6,
the melt is fed laterally into the coat-hanger channel "wound
around" the cylindrical mandrel. The coat-hanger channel then
transfers the melt to the spiral mandrel proper via an annular
gap. A basic die is obtained with just two components (in this
case having a spiral mandrel diameter of ≤100 mm) which
offers all the advantages of the spiral mandrel principle by
comparison with mandrel supports, circular channel and
heart-shaped manifolds.
Fig. 5: Spiral mandrel distributor with central feed and
drilled pre-branching n holes ! n spirals
Fig. 6: Side-fed spiral mandrel
manifold with coat- hanger pre-distribution
1 drilled hole/coat- hanger ! 6 spirals
7
Figures 7, 8 and 9 show different
designs of spiral mandrel with multiple pre-distribution systems
in the form of coat-hangers.
Decoupling the number of
spirals from the pre-distribution system permits a more
favourable rheological and technological design, with an
increased number of turns, particularly in coextrusion dies
where the number of individual pre-branching holes is limited by
design constraints. As a rule, it is best to have a larger number
of spirals with smaller cross-sections. This is more important
for high-molecular raw materials, some of which display a high
visco-elasticity (e.g. PE-HMW-
HD), than for low-viscosity products (e.g. PA).
4. Concentric spiral mandrel dies and stack
dies (pancake dies)
Coextrusion dies with non-
concentric spiral mandrels have been available on the market
since the start of the Nineties, intended particularly for blown
film dies. Their distinguishing design characteristics are the
radial or conical arrangement of the spiral mandrels, with an
upstream pre-branching system, and a melt feed from the side
[2,5,7,15,16]. The relatively flat
design means that coextrusion
dies can be made up by stacking individual discs
(pancakes) on top of each other. The fact that these pancakes are
interchangeable and can be added to (such as for the
extrusion of three to five, or seven layers) means that these
dies offer the user the greatest possible number of
combinations.
While most of the familiar stack
die concepts incorporate a radial arrangement of spiral
mandrel manifolds, side-fed conical designs with pre-
branching potentially constitute the better alternative, since
these permit greater spiral overlap with smaller outside
diameters and lower heights for the individual modules. A
corresponding concept has recently been introduced as the
"Multicone" modular film blowing head [15].
The advantages of these designs have to be set against a
number of drawbacks, however. Table 1 marks an attempt at a
comparative appraisal. What are listed here are fundamental
arguments; the more appropriate concept for an
individual case (application, product, (range of) dimensions)
can only be established through practical experience. Where
alternative concepts may be available, a best-possible
benefits/risk estimate should be conducted on the basis of the
arguments listed before a decision is made [2,5]. Fig. 7: Spiral mandrel with pre-branching (1/2 – 2/4) and coat-hanger
pre-distribution (4/n)
8
Fig. 8: Spiral mandrel from a 6-layer head
Fig. 9: Spiral mandrel with drilled pre-branching holes, coat-hanger pre-distribution and a large number of spirals
9
Table 1: Concentric spiral mandrel dies versus stack dies (pancake dies)
Concentric Spiral Manifold Stack Die (Pancake Die)
Pressure loss Flow resistances identical subsequent to merging. Separate
optimum channel design for individual layers.
Additional flow resistance after each merging point. Unfavourable long flow
paths in large dies.
Residence time, material/colour
changes
Can be determined individually for all the separate layers up to
the point where they merge.
Can no longer be determined individually after each merging point. Unfavourable in
big dies on account of long flow paths.
Distribution,
dependence on operating point
Number of overlaps almost
unlimited. Broad range of operating points.
Number of overlaps limited. Narrow
range of operating points.
Melt merging Can be performed sequentially or at a single point (depending on
layer structure).
Can only be performed sequentially. Unfavourable with some structures (raw
materials).
Layer/flow
stability
Can be optimized through the
merging configuration.
All multilayer flows (2 to n layers) must be
stable.
Variability of layer
structure
Can only be varied to a very
limited degree (such as by an adapter).
Can be varied by changing discs with
height adjustment of the extruder/adapter. Gap widths and
merging only adjustable to a limited extent.
Temperature control of
individual layers
Not generally possible: can only be influenced via joint die
temperature and melt temperature control upon exit
from the extruder.
Individual temperature control for the individual discs (if necessary, by heat
exchange between the discs on the basis of metal contact).
Number of
components
Relatively few individual parts
and screws; few sealing surfaces.
A large number of separate parts and
screws in some cases; a large number of sealing surfaces.
Suitability for internal ducts and
internal cooling
Depends on feed and pre-branching; good with side feed.
Fundamentally suitable by virtue of the side feed.
10
The extent to which the
advantages of variability in the stack die concept outweigh its
potential drawbacks through the limited adaptability of the melt
merging process and the limited influence that can be exerted on
potential interfacial instabilities, will depend to a large extent on
the application in question. In the case of products with
smaller dimensions, the advantages will potentially be
greater than for larger die diameters. The stack die
principle can offer advantages in extrusion coating or cable
sheathing on account of its side feed (inherent to the concept)
and the relatively low height which will also permit the
system to be retrofitted to
existing dies, between the basic
die and the die orifice. It can be assumed that both design
principles will find their specific applications and that neither
concept will come to dominate. The spiral mandrel die concept,
which offers a convincing performance on many points,
has, after all, still not displaced alternative manifold and die
concepts in all applications, including spider-type dies for
PVC [13].
5. Die units for variable
product dimensions
Where identical or uniform
products are to be manufactured in high volumes,
use is frequently made of special-purpose dies. This holds
particularly true for a large
number of die designs from the USA, on account of the specific
conditions prevailing on the market there. Here in Europe,
the situation is different: what is required is flexibility, including
with regard to the product variants that can be produced
with a single basic system. A typical example are pipe dies,
which are required to cover the largest possible range of pipe
dimensions (diameters and wall thicknesses).
Die units are connected up to the basic spiral mandrel dies
(either directly or via interchangeable intermediate
components) with flow channels that shape the melt to the outlet
cross-section. When designing these areas of the flow channel,
allowance must be made for different criteria as a function of
the product and the material, as has already been mentioned in
Section 2.
Working on from the basic
diameter at the end of the spiral mandrel manifold or, in the case
of extrusion dies, the basic diameter at the point where the
individual layers merge, it is possible to conduct the annular
channel either towards the inside, or straight on or towards
the outside. In pipe extrusion, the channel will generally be
reduced to a smaller diameter while, on blown film dies, it will
generally increase in size towards the die orifice. At all
events, care should be taken to ensure that the flow accelerates
steadily.Fig. 10: Die units for colour strip marking on a sheathing die
11
Figure 10 shows the range of
die units for a spiral mandrel die used to sheathe steel pipes with
PE. The space allowed in the basic die for the biggest pipe to
pass through means that all the die orifices are smaller than the
diameter of the spiral mandrel manifold. On this example,
provision has also been made for a unit to apply colour stripes
to the sheathing.
Figure 11 shows the different-
diameter die units for a pipe die.
6. New requirements
placed on extrusion dies
The complexity of new
products, which in many cases is beyond the limits of "standard"
dies on account of the greater range of raw materials used and
the need for multilayer extrudates, means that suitably-
aligned extrusion technology,
and particularly, die technology,
is now more important than ever before. Apart from the geometric
degrees of freedom, surfacing and coating techniques are
gaining increasing importance. In addition to the proven
galvanic coating processes (chrome plating and nickel
plating) for the customary steels used in die making (e.g. C45 or
C60), innovative coatings offering improved friction and
adhesive properties could also be attractive for extrusion die
technology too in the future. Initial experience gained with
coatings of "Diamond-Like Carbon" (DLC) on die
components is highly promising [8]. Of particular interest here is
the influence on the amount of deposits in the flow channel and
at the die orifice. Since PVD or CVD coating processes may
place additional demands on
the base material and the part
geometry, this provides a new challenge to die designers.
Figures 12 and 13 show the results of laboratory tests
conducted with a blown film die whose mandrel received
different coatings. The extent of the deposits was taken as a
reference to assess the surface properties.
Fig. 11: RW-1-25/125 pipe die with dies for pipes – Ø 32 and 110 mm
12
0,509
0,582
0,421 0,4310,45
0,095
0
0,05
0,1
0,15
0,2
0,25
0,3
0,35
0,4
0,45
0,5
0,55
0,6D
epos
it af
ter 4
hou
rs [g
]
Reference NiP+PTFE 4S-T CrN (Mplas) CrN (MVT) DLC+Si
Type of coating
Weight
Fig. 12: Property comparison for different coatings (Source: 'KKM')
0,043
0,026 0,028
0,010
0,01
0,02
0,03
0,04
0,05
0,06
0,07
0,08
0,09
0,1
Dep
osit
afte
r 4 h
ours
[g]
NiP Cr NiP blasted Cr blasted
Type of coating
Weight
Fig. 13: Property comparison for chromium and nickel coatings (Source: 'KKM')
13
7. Examples of dies [17]
The dies shown in Figs. 14 to 23 have all proved successful in
practical use and demonstrate the potential range of different
designs for the spiral mandrel principle. An optimum problem
solution can be obtained by establishing a concept that is
suitably tailored to the task and the requirements, and
employing a customized rheological, mechanical and
thermal layout.
Figure 14 shows the cross-
section and a laid-flat view of the pre-distribution system with a
spiral mandrel for an extrusion die that is used to sheathe steel
pipes. The melt is fed in from the side via a branching system
incorporated in the circumference, before being
uniformly pre-distributed over four coat-hangers.
Following this, the final
distribution of the melt takes place in a spiral mandrel
manifold with 16 turns (for this particular size). The basic die is
made up of only two parts and is therefore easy to assemble and
dismantle.
Figure 15 shows the pipe
sheathing die, which also marks the pipe with colour stripes.
Fig. 14: Spiral mandrel die for
sheathing, with a side feed, quadruple pre-branching and coat-hanger pre-distributors
Fig. 15: Sheathing die for steel pipes
14
Figure 16 shows a concept for a
three-layer pipe extrusion die that has been designed for a
layer structure with a relatively thick inner layer and thin central
and outside layers. The pre-distribution system for the inner
layer is a "conventional" system with one feed hole per spiral
turn and an axial central feed. The central and outside layers,
by contrast, have an axially eccentric feed via a single hole
in each case. For each of the layers, a coat-hanger pre-
distribution system performs the pre-distribution of the melt in an
annular gap. Following this, the distribution proper is conducted
in spiral mandrel manifolds, with six spiral turns in this case. In
this "coating task" with a thin
central and outside layer, the
thin layers are first brought together and then the inner layer
added on.
At this point, it should be stated
that the way in which the melts are brought together in
coextrusion - either sequentially, at a single point, or through a
combination of both - is of major importance and can determine
the overall serviceability of the system.
The three-layer pipe extrusion die for small corrugated pipes
that is shown in Fig. 17 has been designed for one third of
the melt to pass via each of the spiral mandrel manifolds. This
example is designed to show
that spiral mandrel technology can be used for smaller pipe
dimensions as well. This also holds true for smaller pipes
based on polyamide which are used as single-layer or
multilayer pipes in the automotive sector, such as for
fuel lines.
Figure 18 shows this die with a
protective cover.
Fig. 16: Three-layer spiral mandrel
die for coating the exterior of pipes
Inside:n holes ! n spirals Centre/outside: 1 hole !
1 coat-hanger ! n spirals
Fig. 17: Three-layer spiral mandrel die for corrugated pipes; 1 drilled hole ! m drilled holes / coat-hangers ! n spirals
15
Apart from fixed covers (see
also Fig. 19) contact protection can also be provided through
insulation (Fig. 20).
Fig. 18: Three-layer die for corrugated pipes
Fig. 19: Pipe die with a protective cover
16
Figure 21 shows a five-layer die
for small pipes and hoses. Pre-distribution is conducted in
annular channels here, given the need for a central passage for
coating tasks or for a
temperature-control medium. This is after the melt has been
fed in from the side and deflected in the appropriate
manner. With this type of pre-
branching, 8 or 16 drilled feed holes are required for the spiral
mandrel in question, in line with the 2n power series. The hole
diameters and the flow channel cross-sections in the spiral
mandrel manifolds are aligned to the product specifications (as
explained above) in order to achieve an optimum
compromise between flow resistance and residence time.
Fig. 20: Pipe die with insulation/contact protection
Fig. 21: Five-layer spiral mandrel die with side melt feed and annular channel pre-branching;
1 hole ! 2 channels ! 2n channels ! 2n spirals
17
Figure 22 shows the 5 spiral
mandrels from this die.
The six-layer die shown in Fig.
23 is used for continuous parison production in blow
moulding units for fuel tanks and
filling spouts. Three thin layers (EVOH as a barrier, and two
coupling agents) are embedded in two layers of high-molecular
PE-HD here. The sixth and thickest layer (2nd layer from the
outside) is a coextruded layer with a high percentage of
regrind (parison waste, etc.). One particularly important
feature is the low percentage of the overall composite that is
accounted for by the thin layers. This is only feasible with a
narrow layer thickness distribution [12]. The thick layers
are fed according to the conventional concept (n holes /
n spirals) while the thin layers are fed with the combination of
hole(s) / coat-hangers / n spirals.
Fig. 22: Spiral mandrel manifolds of a five-layer die
Fig. 23: Six-layer spiral mandrel parison die
thick layers: n holes ! n spirals thin layers: 1 hole ! m holes / coat-hanger ! n spirals
18
Figure 24 shows the very good
layer thickness uniformity, including for thin layers that
account for less than 1% of the overall wall thickness in each
case.
The examples given here
demonstrate the flexibility in
design offered by the spiral
mandrel manifold principle in conjunction with different pre-
branching and pre-distribution configurations for specific
applications. Work is now concentrating on the process
and production engineering optimization of die and flow
channel details that are not (yet)
accessible to straightforward mathematical optimization. It
would also seem feasible and worthwhile improving on the
temperature control of extrusion dies.
Fig. 24: Layer thickness distribution in the plastic fuel tank wall
19
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[2] Perdikoulias, J Petric, J.
Developments in annular coextrusion die design. SPE-RETEC ’91: Coextrusion VI, Arlington Heights
[3] Wortberg, J. Rechnerunterstützung bei der Blaskopfauslegung. In: Rechnereinsatz beim Blasformen, VDI-Verlag, Düsseldorf, 1993
[4] Wortberg, J. Rohrwerkzeuge für Polyolefine und technische Kunststoffe. SKZ-Fachtagung: Extrusionswerkzeuge für Profil und Rohr Würzburg, 31.05.1995
[5] Bode, W. W Design considerations for concentric mandrel and stackable coextrusion blown film dies. SPE-ANTEC ´95, Boston, pp. 32 - 39
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[7] N.N. Neuer Fünf-Schichtblaskopf (FBF) mit Spiralverteiler. Firmenschrift Hosokawa Alpine AG, Augsburg, 1995
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[10] Burmann, G. Wortberg, J.
Wendelverteiler für Folien, Rohre und Vorformlinge. Plastics No. One (1995) 11, S. 21 - 29
[11] Schmitz, G. Blasfolienextrusion - Detailverbesserungen bei Anlagen und Automatisierung. Kunststoffe 85 (1995) 12, S. 2118 - 2122
[12] Esser, Franke, M.
Wortberg, J.
Sechs Schichten halten dicht - Coextrusionsblasgeformte Kunststoffkraftstoffbehälter. Kunststoffe 86 (1996) 8,
S. 1120 – 1122
[13] N.N. Rohrköpfe für PVC und ABS. Plastics-Special 7-8 (1996) 12, S. 24-25
[14] Fischer, P. Extruder- und Werkzeugtechnik. Plastics-Special (1996) 12, S. 12 - 14
[15] N.N Multicone - modularer Folienblaskopf. Firmenschrift Windmöller & Hölscher, Lengerich1997
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Blasfolientechnik im Spannungsfeld von Preis und Leistung. Kunststoffe 87 (1997) 11, S. 1584 - 1592
[17] Wortberg, J. Neue Wendelverteilerwerkzeuge. Kunststoffe 88 (1998) 2, S. 175-180
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