14-18 pe111673 pe5 25.04.2014 10:57 uhr seite 15 · blown film extrusion blown film extrusion and...

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LENNART EDERLEH

TILL BERGMANN

INGO PUTSCH

Strong pressure from competition inthe packaging sector continues topush demands on the manufacturers

of raw materials and machines to devel-op more sustainable packaging solutions.

In the flexible packaging sector, pouchesare showing the strongest growth. Theiradvantages of reduced packaging weighttogether with the resulting lower produc-tion and logistic costs are behind its ex-pansion into new applications [1].

Water-Cooled Blown Film Extrusion

Blown film extrusion and cast film extru-sion are currently the conventional

processes for manufacturing films forflexible packaging solutions. Whereasclassic blown film extrusion with plasticsmelt by convective cooling exhibitsschows relatively slow cooling rates of 20to 80 K/m, cast film extrusion can achieveapprox. ten times higher values thanks tocontact cooling on the chill roll. Howev-er, with water-cooled blown film extru-sion, even higher cooling rates can beachieved on the order to 1,000 to3,000 K/m [2]. That means that films

© Carl Hanser Verlag, Munich Kunststoffe international 5/2014

New Packaging SolutionsThanks to Rapid CoolingExtrusion. Water-cooling can achieve very high cooling rates for blown film extru-

sion. The result is soft, elastic films with near-amorphous molecular structures.

The resulting low tendency to curl is a clear advantage when asymmetric multi-

layer film structures are processed further.

Translated from Kunststoffe 5/2014, pp. 26–31Article as PDF-File at www.kunststoffe-international.com; Document Number: PE111673

Thanks to water-cooling, films can beproduced with espe-cially good opticaland mechanicalproperties (figures:

Windmöller & Hölscher)

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manufactured by this technology exhibita range of properties that makes them es-pecially interesting for flexible packaging.Due to such rapid cooling, crystallizationprocesses can be inhibited, thereby en-abling films to be produced with near-amorphous molecular structure whilelending very good surface gloss and hightransparency to such films. At the sametime, the amorphous structure of thefilms makes them very soft and puncture

resistant. Moreover, they can be processedvery well by thermoforming.

In Aquarex brand, water-cooled blownfilm extrusion from Windmöller &Hölscher KG of Lengerich, Germany, thefilm tube is extruded vertically downward(Fig. 1). As in conventional blown filmlines, different materials are plasticized inseveral extruders and combined into amulti-layer annular gap flow. The melttube exiting the die is drawn off down-

ward and simultaneously blown via in-ternal pressure. A controlled water cali-brator then directs water onto the filmbubble so that the molten plastic cools offrapidly. The film tube is then laid flat andpulled off and either subjected to ther-mofixing or transferred to a single or dualwinder. All the time, the proven compo-nents of Windmöller & Hölscher’s entireconventional modular blown-film lineVarex II, e.g., extruders, dies, automationand winders, are in operation. Thus, filmscomposed of as many as 11 layers can beproduced with up to 1,600 mm workingwidth and ranging in thickness from 30to 300 µm. This innovative cooling con-cept achieves high production outputs inconjunction with low overall height ofthe entire line.

Infusion Bags

A typical application for water-cooledblown film are pouches to fill medicinalinfusion solutions with, often referred toas infusion bags (Fig. 2). This type of pack-aging fulfills very high optical and me-chanical requirements. The preferred rawmaterials used here are modified PP typesmanufactured especially for medical ap-plications. Since the films come in con-tact with the infusion solution, rigid re-quirements have to be met in terms ofmaterials asepsis and ambient hygiene inthe production facilities, so that no harm-ful particles reach the film. By usingcleaned air in the film bubble, a hermet-ically sealed space can be relatively sim-

Kunststoffe international 5/2014 www.kunststoffe-international.com

Fig. 1. In water-cooled blown film extru-sion, the film tube is extruded downward

Fig. 2. The strong demands placed on infusion bags can be met when the films are manufactured bywater-cooled blown film extrusion

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ply created, whose cleanliness can be en-sured and that later constitutes the insideof the infusion bag. During flattening inthe pull-off, air is completely removedfrom between the films to prevent soil-ing. Rapid cooling and the resulting tex-ture make the film soft, similar to a softPVC film, even though a PP is involved.The combination of good film weldabil-ity and high puncture resistance is also astrong motive behind this application,since infusion bags have to be tight andrepeatedly survive a 1 m fall without leak-ing. The film must then be soft enough toensure pleasant haptics and very goodemptying. All the requirements men-tioned can be fulfilled using water-cooledblown film extrusion to manufacture thefilms.

Form-Fill-Seal Packaging

Infusion bags represent a special shape ofpackaging produced by the Form-Fill-Seal process (FFS). In addition to medicaltechnology, these products are also usedin industry and food packaging sector(Fig. 3). Stand-up pouches, bags with a sidegusset and simple pillow pouches are inwidespread use. Depending on the designof the filling equipment, a distinction ismade between horizontal and vertical sys-tems. The film can be fed either as a tube

or as flat rollstock that is welded to a tubeat first. The actual bags then take form viatransverse welds. Besides mechanical sta-bility and the film’s optical properties,sealability and strength in the vicinity ofthe seal seams are of primary importance.

Since the described pouch version issuitable in principle for all flowable andfree-flowing packaged goods, it finds wideuse in the food sector. For example, inAsian countries, milk is packaged almostexclusively in pillow pouches. In the Mid-dle East, the main packaged goods are ed-ible oils, ghee (clarified butter), rice, nuts

and dried fruit. Where perishable foodstuffs are involved, or longer shelf life isdemanded, multi-layer barrier films areused. They are characterized by specialfunctional layers that reduce permeabil-ity to water and/or moisture, gases andaromas. Typically speaking, these filmshave a symmetric structure with 5, 7, 9,or 11 layers, whereby the functional lay-ers of PA and EVOH are arranged sym-metrically in the middle of the structure(Fig. 4). Since these materials are incom-patible with PE, bonding layers have to beincluded in order to ensure sufficient in-ter-layer adhesion. Each of the outer PElayers has to exhibit good printability orhigh surface gloss. On the other hand,good sealability must be ensured in orderto produce pillow packaging.

Asymmetric Multi-Layer Films

Wherever symmetric structures are in-volved, sealing presents a challenge. Onthe one hand, a processing point has tobe found at which a stabile seal seamforms. On the other hand, the side of thefilm in contact with the sealing jaw mustnot get too hot; otherwise it sticks to the

sealing jaw. Impurities can then arise, andthe filling equipment has to be shut downin order for the impurities to be wipedaway, or to replace the sealing jaws. In or-der to avoid sticking, the film surface incontact with the sealing jaw can beequipped with a thermally stable materi-al, e.g., PA. This can be achieved by lam-inating a second film, but every addition-al processing step admittedly generatesadditional costs. As an alternative, anasymmetric film structure can also bemanufactured by co-extrusion in whichthe thermally more stable material is in-

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Fig. 3. The vertical Form-Fill-Sealprocess is illustrated here by thisheavy duty sack production

Fig. 4. Typicalbarrier films withsymmetric andasymmetricstructure. Asym-metric films withPA in the borderlayer acceleratesealing at thebottling plant

7-layer 9-layerSymmetric SymmetricAsymmetric Asymmetric

PAHV

PE

PE

EVOHPA

PA

HV

HV

y y y yPH

P

P

EP

P

H

H

© Kunststoffe

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cluded in a border layer of a multi-lay-ered film (Fig. 4). This approach eliminatesthe laminating process, thereby reducingthe overall cost of manufacturing the film.In both cases, higher temperatures can re-duce sealing time in the FFS equipmentand with it the cycle time.

The marked difference in the meltingtemperatures of PA and PE certainly hasits disadvantages when processing is doneby conventional blown film extrusion(Fig. 5). In a 3-layer structure from PA andPE where the PA layer is on the inside ofthe tube, the PA very quickly devolves toits solid state after exiting the die open-ing. It solidifies at the frost line of the PA,while the PE is still present in its moltenstate and thus can compensate for volumeshrinkage in the PA by elastic deforma-tion. In the area of the PE frost line, thePA has already solidified and can nolonger compensate for PE shrinkage byelastic deformation. The film has a strongtendency to roll up. This effect is termedcurling. Such curling is disadvantageousfor further processing in the FFS process,since it complicates film handling in theFFS machine, thereby slowing down theprocess. Similar effects can be observedto a lesser degree when the melt is cooledon one side on the chill roll in a cast filmline. In the case of water-cooled blownfilm extrusion, the curling phenomenonalmost never appears. Thanks to inten-sive and rapid melt cooling by contactwith cold water, the frost lines of PA andPE lie to a certain degree on the same lev-el, so that no curling occurs (Fig. 6). Con-sequently, the restrictions to further pro-cessing disappear.

Product Development asIllustrated by Oil Packaging

Since texture alters mechanical propertiesof the film, a conventional blown film for-mula cannot be directly applied to theversion produced with water-cooling. Todemonstrate the potential of this technol-ogy, Windmöller & Hölscher performeda complete product development fromformula to qualification of the finishedpackaging solution. Pillow packaging foredible oil was developed in a joint proj-ect with one of India’s largest producersof such oils (Fig. 7). This edible oil is filledin a bottling plant mainly in clean 3-lay-er PE film by the vertical FFS method.Since edible oil diffuses through PE, thesepackaging bags are soon moistened bythin oil film in a supermarket display. Theuse of an asymmetric composite with PA

on the outside can remedy the situationin two ways. For one thing, the barrier ef-fect of PA against oil seepage is marked-ly higher; for another, sealing at the bot-tling plant could be accelerated by PA’sthermal stability, and cycle time short-ened at the same time. This film is cur-rently being manufactured at consider-able expense by laminating a biaxial ori-ented PA film onto a PE film.

If an asymmetric film were developedwith PA in the border layer for water-cooled blown film extrusion, a packagingsolution could be presented that not on-ly has the above-mentioned positiveproperties for further processing, but ad-vantages from an economic point of view,as well. Moreover, such films havebrighter surface gloss and greater trans-parency than the standard films availableon the market. Thanks to the PA outerlayer, bottling plant cycle times could be

increased from approx. 30 to 40 cycles perminute. Thus corresponds to the me-chanical capacity of the machines. At thesame time, sealing jaw life would increasesignificantly, thereby additionally im-proving the economics of the overallprocess.

Interesting in this connection is thequalification according to the “IndianStandard” of the oil packaging filledthereby. It allows for the packaging of pil-low pouches in normal boxes and truck-ing them more than 2,000 km on Indianhighways. Subsequently, a count is takenof the packages damaged underway. Ac-cording to the oil producer, no damagedpouches could be found following trans-port – something that had never hap-pened in the past. The thus developedpackaging solution eliminates the addi-tional laminating process, thereby im-

Kunststoffe international 5/2014 www.kunststoffe-international.com

Fig. 6. Curling during the manufacture of asymmetric films can be avoided by water-cooled blownfilm extrusion

Water-cooling:PA + PE solidify + shrink

Die orifice:>220 °CEntirely molten

PE PA

PE PA

© Kunststoffe

Fig. 5. With conventional blown film extrusion, curling often occurs on asymmetric structures

PE Frost line:120–80 °CPE solidifies + shrinksPA solid

PA Frost line:180–140 °CPA solidifies + shrinksPE molten + elastic

Die orifice:>220 °CEntirely molten

PE PA

PE PA

PE PA

© Kunststoffe

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proving the economics of bottling andproviding improved protection for thepackaged goods.

Conclusion

The demands placed on today’s packag-ing solutions make it necessary to contin-

ue to develop existing raw materials andproduction methods. Using water-cooledblown film extrusion, films can be manu-factured with properties spectra thatclearly differ from those obtained byblown and cast film extrusion. Rapid meltcooling with water leads to near-amor-phous molecular structure. The result is

a soft, elastic film characterized by verygood weldability. The resulting low ten-dency to curl is a clear advantage whenasymmetric multi-layer composites areprocessed further.�

REFERENCES

1 N.N.: Converted Flexible Packaging - IndustryMarket Research, Market Share, Market Size,Sales, Demand Forecast, Market Leaders, Compa-ny Profiles, Industry Trends. Industry Study 3094,The Freedonia Group Inc., Cleveland, OH, USA,November 2013

2 Leopold, T.: Film Production: System Options forWafer-Thin Protection, Kunststoffe international102 (2012) 7, pp. 45-48

THE AUTHORS

DIPL.-ING. LENNART EDERLEH, born in 1975, isTechnical Sales Director for Windmöller & Hölscher’sblown film lines.

DIPL.-ING. TILL BERGMANN, born in 1981, isprocess specialist for water-cooled blown film extru-sion at Windmöller & Hölscher.

DIPL.-ING. INGO PUTSCH, born in 1967, is Techni-cal Sales Manager at Windmöller & Hölscher and ad-ditionally responsible for Aquarex water-cooled extru-sion lines.


Fig. 7. Pillow pouches for edible oil in an Indian supermarket

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