108 © Carl Hanser Verlag, München Kunststoffe 9/2010

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GEORG BURMANN

PETER FISCHER

JOHANNES WORTBERG

Aspiral mandrel or a flat spiral die?”This was the question that had tobe asked in 2006 when considering

which type of extrusion die is the best touse for the manufacturing of small tubes,hoses, blown films and blow molded ar-ticles [1]. Today, it is clear that the systemsthat have become established are those inwhich the distribution of the melt (or thedifferent melts in the case of multi-layerdies) takes place on a single plane – andnot just for special applications. Circulardistributors, which are also known as flatspiral dies or pancake dies, are state of the

art. This applies particularly to the pro-duction of media-conveying lines for carsand other vehicles, for medical technolo-gy, for hydraulic and pneumatic systemsand also for underfloor heating pipesbased on PE-HD and PP, as well as for theproduction of barrier-layer films, med-ical films and multilayer blow molded ar-ticles.

Design Characteristics

In the same way as the “classic” axial spi-ral mandrel distributors of a cylindricalor conical design, the flat radial distribu-tors are characterized by a uniform vol-ume flow and hence an excellent (layer)thickness distribution, as well as the ab-sence of weld lines and other weak points.The circular distributor technique pro-vides ideal conditions for a modular builtblock system. The melts are fed in, pre-

distributed and then circumferentiallydistributed in a block (a module), whichis made up of just a few round discs. Toproduce multi-layer coextruded prod-ucts, it is possible to stack several mod-ules (i. e. as many modules as are re-quired) on top of each other to give a stackdie. The components of the individualmodules are either completely identical,or more or less identical. A mandrel isarranged in the center for the formationof the annular gap space for the melt. Thiscan be made with a hole in it for the pas-

Melt Flow in aCircular Distributor

Extrusion Dies. Circular distributor technology makes it possible to achieve a

modular building-block system for coextruded pipes and hoses. Rapid material

color change and the possibility to fulfill product specific requirements open up

a broad range of applications.

Special die with an inversecircular distributor (CVI)

ETA Kunststofftechnologie GmbHD-53842 Troisdorf / GermanyTEL +49 2241 949707> www.eta-gmbh.de

Contacti

Translated from Kunststoffe 9/2010, pp. 150–154Article as PDF-File at www.kunststoffe-international.com; Document Number: PE110505

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sage of air or another(cooling) medium(Fig. 1). By way of an op-tion, this mandrel canbe equipped with in-ternal heating to in-crease the variabilityof layer structures andmaterials that can beprocessed.

Apart from these ben-efits – good circumferen-tial distribution, no weldlines,modularity, small dimen-sions, low-cost manufacture – thesystem also offers a series of process-en-gineering and operational-engineeringadvantages:� Short flow paths and a low melt vol-

ume, leading to short residence timesand rapid material and color changes,low wall-shear velocity, and hence low

� dissipation (temperature increase),� low pressure losses, and hence a high

throughput potential,� high flexibility in terms of layer struc-

ture (thick layers/thin layers, materi-als/layer structures) and number oflayers,

� thermal separation and separate tem-perature control of the individualmodules is possible,

� special versions of individual modules,e.g. for corrosion protection,

� simple and rapid dismantling, clean-ing and re-assembly,

� feasibility of upgrading.It is, however, also important to point outthe drawbacks:� Limits on high melt throughputs� melts can only be merged sequentially

(one after the other), which is not sogood for certain structures (i.e. poly-mers with great differences in viscosity),

� pressurized areas, which place strin-gent requirements on the manufactur-

ing process and requires careful brac-ing of the individual parts.

For extruded products with small dimen-sions, however, the advantages clearlyoutweigh the drawbacks.

Layout and Design

Flow analyses (simulations) are conduct-ed with analytical or numerical models inorder to support the process-engineeringlayout and design [2, 3, 4]. The networktheory developed for two-dimensional,isothermal flows is frequently used for thepreliminary layout. The network modeland the way in which the circular distrib-utor works is set out in Figure 2.

It is clear from this schematic diagramalready that the design with oval channelsin the two neighboring circular discs isadvantageous. The distribution channelsdo not have any edges or dead zones atwhich the flow could stagnate. By super-imposing the part-flows from the twochannel halves as they flow into the jointgap,a finer structure is obtained than witha single-sided channel, which means thatthe channel length can be reduced, pro-ducing a favorable influence on the pres-

sure consumption and the residence time.Observations conducted in practice

confirm this advantage in the caseof product changes. The purg-

ing times are considerablyshorter compared with diesthat have axial spiral mandreldistributors.

The computer-supportedlayout is aimed at achieving a

low pressure loss for the spec-ified melt throughput, good cir-

cumferential distribution andshort material change-over times. The

production related limits and material-specific limits must be kept in mind forthe layout, i.e. shear rates which ensure arapid material or color change, on the onehand (lower limit) and do not lead tooverheating or inadmissibly high pressurelosses, on the other hand (upper limit).

More detailed observations of the flowprocesses are possible by conducting a nu-merical 3-D-CFD simulation with finite

element models. Apart from the evalua-tion criteria mentioned so far, it is possi-ble to show other physical data, such aspressure, velocity and temperature distri-bution, plus flow lines and distributionspectra. This data can be used to assessthe operating-point dependence of a diefor different throughputs and materials(see Figure 3 by way of example).

A number of scientific papers focus, inparticular, on the problem of die deflec-tion due to the high melt pressure [5].Thecoupled calculation of the flow and thedeflection is of fundamental importancefor modeling and simulation. When itcomes to the practical dimensioning ofthe plates, however, this term can be ig-nored. In multilayer flat die systems, theintermediate modules are only subject toloading from the resultant compressiveforces of the neighboring layers in eachcase. The pressure differences are so low

Fig. 1.Opened block

(module) with circu-lar distribution and inside mandrel

Fig. 2. Operating mode and layer structure for a circular distributor

Fig. 3. CFD flowline diagram for assessing over-laps and operating-point dependence

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that the discs can be manufactured to bethin (favorable from a manufacturingtechnology view). The rear and frontmodules, or connecting plates, must besolid, for design reasons, so that the de-flections occurring there are of the orderof magnitude of the production toler-ances.

Pipe Dies

Whenever the requirements on the meltthroughputs for the thick layers in mul-tilayer composites are not too high, thecircular distributor principle has becomeestablished for dies for small pipes andhoses. This applies particularly for appli-cations in the automotive industry, e.g.for 3 or 5-layer fuel lines based onpolyamide. The modular structure is par-ticularly appreciated, which makes it pos-sible, for example, to expand a three-lay-er system into a five-layer system (Figs. 4and 5).

The special features clear from the lon-gitudinal cross-section in Figure 4 are thepossibility of thermally separating the in-dividual modules through air gaps andthe use of heating/cooling units for im-proved thermal control of the system, i.e.separate, polymer-specific heating at dif-ferent temperature levels.An optional in-ternal heating system that can be fittedcan also bring additional advantages.

The nozzle unit incorporates a two-part mouthpiece. The end section sits ina clamped platen (ring), which remainson the system, in its centered position,when there are changes to the dimen-sions. The new end section is placed, in afunctionally accurate manner, in the cen-tering seat on the clamped centered plate.

This system permits considerably short-er changeover times.

The heating/cooling unit has chains ofribbed ceramic stones with heating con-ductors and tension ropes, which alignthemselves uniformly on the housingwall. The special ceramics with optimizedheat conduction ensures very good oper-ating characteristics for heating and cool-ing as well as good energy efficiency.

The 3-D model in Figure 5 similarlyshows these heating/cooling units and, inaddition, the complete structure with theconnection adapters for the extruder anda mandrel design for die core centeringin the case of corrugated pipe dies. Thesetup time can be considerably shortenedthrough internal die core centering. Thismodel also clearly shows the modularityand, with this, the option of varying thelayer structure in the product by switch-ing modules. In the normal case, the in-ternal mandrel has to be changed in or-der to guarantee perfect flow conditionsat the merger points and in the parallelzones.

In the circular distributor, the melt isnormally conveyed from the outside in-wards. The direction can also be reversed,however, so that the melt runs from theinside outwards (CVI system). Distribu-tors of this type are employed for special,single-layer products. The aim of the CVIsystem is to give the melt the dimensionsof the product over as short a route as pos-sible once it has left the extruder or themelt pump. To do this, the melt is distrib-uted by circular channels in a radial con-figuration pointing towards the outsideonce it has passed through a short pre-

distributor, which is necessary for sup-porting the mandrel and for allowing themedium to pass (Title photo, Fig. 6). This isfollowed by a flow channel, which shouldbe as short as possible, up to the die open-ing. All in all, CVI dies offer the best con-ditions for gentle treatment of the mate-rial.

A special feature is visible on the opendistributor: the variable-length gap areabetween the channels, which constitutes

Fig. 4. Diagram of a circular distributor die for three-layer pipes with thermal separation

Fig. 5. 3-D model of a five-layer die for attachingnozzles for smooth and corrugated pipes

Fig. 6. Diagram of a special die with an inversecircular distributor

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a further degree of freedom for achievingan optimum distribution, alongside thechannel length, the channel depth profileand the gap width. Here too, it is clear thatthe distributor geometry does not take theform of an Archimedean spiral, which iswhy the term “circular distributor” hasbeen selected.

Medical Technology

The extrusion of single-lumen and mul-ti-lumen medical hoses is particularly de-manding – firstly, in respect of the strin-gent specifications that apply when (spe-cial) plastics are used in medicine (toxic-

ity, purity, hygiene, etc.), and, secondly,through the compulsion to miniaturize.

The dies were initially derived from thefamiliar designs for thin cables and linesand designed as so-called cross-head dieswith mandrel distributors and, subse-quently, axial spiral mandrel distributors[6]. Since 2004, extrusion dies for micro-hoses have also been built employing cir-cular distributor technology, for bothmono and co-extrusion [1, 4]. The ad-vantages outlined at the outset are ofoverriding importance here.

Figure 7 shows a distributor disc whereall the channels for the pre-distributionof the melt and for full circumferentialdistribution are in a single plane.“Single-

platen dies”of this type have a par-ticularly small melt volume, whichis one of the main requirements interms of residence time, tempera-ture control, and handling duringassembly and cleaning, etc. In the3-D model, it is possible to see thepre-distribution in a second plane.The individual design will be afunction of the specific circum-stances.

Circular distributor technologyis particularly suitable for small co-extrusion dies for two and three-layer products (Fig. 8). Two-layerdies can be designed in such a waythat, by rotating one platen, it ispossible to switch the allocation ofthe extruders for the inside andoutside layer.

In addition to being used in thesmall-dimension range for catheterapplications and infusion hoses

and the like, circular distrib-utor dies are also in use forbigger hose dimensions –from single-layer plasticized

PVC hoses through to five-layer specialproducts for a large number of applica-tions in medical and pharmaceuticaltechnology.

Barrier Films

Stack dies or pancake dies have been inuse for many years in the production ofhigh-grade packaging films [8] – withseven, nine (Fig. 9) or eleven layers in re-cent times. The circular distributor con-cept is particularly suitable for systemswith small die diameters for the produc-tion of primary bubble on double-bub-ble or triple-bubble systems [7]. The tech-nology has become established preciselyfor this application (Fig. 10).

In the case of blown film dies, the freespace in the center is particularly impor-tant for the passage of heating and cool-ing media. Alongside this, all the advan-tages of circular distributor technologycome into play, right through to the op-tion of retrofitting additional layers.

Prospects

Traditionally, the parison dies for the blowmolding of hollow articles have beenequipped with mandrel distributors.When it comes to continuous coextru-sion, however, heads with axial spiralmandrel distributors have become estab-lished, both for multilayer food packag-ing and for large-scale industrial packag-ing and six-layer fuel tanks with excellentbarrier properties. On account of theanalogy with pipe extrusion, ideas havebeen put forward for transposing circu-lar distributor technology to parison dieheads [9]. The open center of the man-drel offers space for the pull rod of thewall thickness adjustment unit (axialmandrel adjustment, Fig. 11).

In the case of pipe dies, the limits forhigh layer throughputs have been ad-dressed, i.e. limits due to an excessivelyhigh pressure loss, which can lead toproblems with maintaining tightness. Asfar as the pressure loss is concerned, thepredominant share is that of the flow re-sistance in the die unit (die exit). Thethroughput can be increased by the ex-tent to which this resistance can be re-duced. The selective approach to beadopted here is to increase the draw downratio, i.e. to extrude the polymer from abigger die (orifice diameter and gapwidth). In the case of polyamide, high un-der-drawing ratios are standard practice,and with PE-HD and PP, the initial devel-opments are highly promising.

Fig. 7. Circular distributor plate and single-layerdie for micro-hoses

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Fig. 8. Diagram of a three-layer die for micro-hoses

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In many areas, use is increasingly be-ing made of plastics that place special re-quirements in terms of thermostabilityand temperature control. These includefluoropolymers (PFA, ETFE, FEP, etc.),high-temperature thermoplastics (PAEK,PSU, PPS, PI, etc.) and cross-linking plas-tics (PE-X). The circular distributors withchannels in the two neighboring discs aresuperior to nearly all other die de-signs for these “sensitive”materials.Surface coatings are frequently suf-ficient for corrosion protection. Other-wise, corrosion-resistant materials willbe used.

When multilayer dies are built, it isalso possible to combine dissimilar meltdistribution systems, such as coaxial spi-ral mandrel dies for the main layers andcircular distributors for the thin adhe-sive layers.Customized solutions are re-peatedly seen to be superior to the stan-dard versions.

Progress in the layout and design ofthe spiral mandrel distributor systemscan be expected with optimization al-gorithms for the automatic optimiza-

tion of the distributors.These are current-ly being developed at different places [10,11].�

REFERENCES

1 Fischer, P.: Wendel- oder Spiralverteiler? Extrusion04/2006, pp. 43–45

2 Michaeli, W.; Blömer, P.: Flat spiral dies – rheolog-ical design with network theory, Journal of Poly-mer Engineering 24 (2004) 1–3, pp. 137–153

3 Fritz, H. G.; Cretu, M.: Design Concept forStacked-type Spiral Dies, Kunststoffe internation-al 94 (2004) 6, document number PE102895

4 Burmann, G.: Extrusionswerkzeuge für dieSchlauchextrusion. Handbuch zur 3. DuisburgerExtrusionstagung „Extrudierte Produkte/Verpack-ungen für die Medizin- und Pharmaindustrie“, 8/9March 2006

5 Michaeli, W.; Blömer, P.; Scharf, M.: Rheologicaland Mechanical Design of Pancake Dies, Kunst-stoffe international 97 (2007) 1, pp. 30–33

6 Lechmann, U.: Werkzeugtechnik für dieMikroschlauchextrusion, Handbuch zur SKZ-Fachtagung „Mikroschlauchextrusion für dieMedizintechnik“, Würzburg 10. November 2005

7 Burmann, G.: Werkzeugtechnik für biaxial gereckteSchläuche, Handbuch zur 5. Duisburger Extrusion-

stagung „Technologien und Trends extrudierterFolien – mehrschichtig, orientiert, veredelt“,12/13 March 20088 Wortberg, J.: New Spiral Mandrel Dies, Kun-

ststoffe plast europe 88 (1998) 2, pp. 12–159 Fischer, P.; Michels, R.: Moderne

Schlauchköpfe mit Wendelverteilern. Hand-buch zur SKZ-Fachtagung „Blasformtechnik2006“, Würzburg, 26/27 April 2006

10 Cretu, I. M.: Analyse, Auslegung und Opti-mierung von Wendelverteilersystemen, Dis-sertation, Stuttgart University, 2008

11 Wortberg, J.; Saul, K.; Köhler, P.; Lupa, N.:Automatisierte Optimierung von Strö-mungskanälen zur Verbesserung desSpülverhaltens von Kunststoff-Extrusion-swerkzeugen, WAK Zeitschrift Kunst-stofftechnik / Journal of Plastics Technology5 (2009) 2, pp.130–153

THE AUTHORS

DIPL.-ING. GEORG BURMANN, born in 1961,has been working since 1993 as an engineer inthe development department, and also as a proj-ect manager, at ETA KunststofftechnologieGmbH, Troisdorf, Germany.

DR.-ING. PETER FISCHER, born in 1938, isthe owner of IPF engineering consultants,Königswinter, and co-partner of ETA Kunst-stofftechnologie GmbH.

PROF. DR.-ING. JOHANNES WORTBERG,born in 1951, is a professor at the University ofDuisburg-Essen, Germany, and co-partner of ETAKunststofftechnologie GmbH.

Fig. 10. Seven-layer blown film die for biaxially-stretched films (double/triple bubble)

Fig. 11. Diagram of a multi-layer parison die head with axial and radial wall thickness control (figures: ETA Kunststofftechnologie)

Fig. 9. 3-D model of a nine-layer blown film die (Ø 500 mm)

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