Iccs5 Torhex Paper

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S A N D W I C H C O N S T R U C T I O N 5

EMAS 1999 1

FOLDED HONEYCOMB CARDBOARD AND CORE MATERIALFOR STRUCTURAL APPLICATIONS

Jochen Pflug*, Ignaas Verpoest*, Dirk Vandepitte+

Today, most honeycomb cores are produced in a batch wise productionprocess by cutting from a block. A new continuous production concept forlow cost honeycomb core materials from a single corrugated cardboardsheet has been developed and patented at the K.U.Leuven. For theproduction of this, so called folded honeycomb cardboard core material, theefficient machinery from the packaging industry is used to a maximumextent. The low production costs will open new markets for honeycombmaterials for example in the automotive industry. The new core materialand its continuous production processes are presented in this paper. The flatwise compression properties as well as some potential applications arediscussed. Furthermore, two techniques to measure the local flat wisecompression stiffness variations are presented.

INTRODUCTION

Honeycombs are used in aerospace industries since many decades as the preferredcore material for buckling and bending sensitive sandwich panels and structures. Theseaerospace honeycombs are most often hexagonal or over-expanded and are usually

produced for aluminium sheets or phenol resin coated aramid paper by the expansionprocess.

In the past decades the interest of other large industries in sandwich core materialswith good specific mechanical properties has increased continuously. Thus, today morethan half of the honeycomb core materials are used in other application areas, e.g. forpanels in trains, trucks and ships, for cladding of buildings and in sporting goods.Honeycombs produced from low cost paper (Kraftpaper or recycled paper) can beinexpensive enough to be used as crash elements in automotive interior and packagingapplications.

Low cost honeycomb production today

The main reason for the high costs of traditional expanded honeycomb cores is thebatch like production process. Honeycomb core production today is labor intensive,discontinuous and not in-line. Most honeycomb cores are adhesive bonded expanded

* K.U.Leuven, Department Metallurgy and Materials Engineering, De Croylaan 2, B-3001 Leuven,Belgium, e-mail: [email protected]+

K.U.Leuven, Department Mechanical Engineering, Division PMA


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EMAS 1999 2

cores (Bitzer (1)). Low cost paper honeycombs are produced with the same process,shown in figure 1. First, glue lines are printed on flat sheets. Second, a stack of manysheets is made and the glue is cured. In the third phase, slices are cut from these blocks.Finally, the sheets are pulled apart, thus expanding into a hexagonal honeycomb core.The residual stresses in paper honeycombs have to be relaxed after expansion by heat.

Figure 1 Expansion production process of conventional paper honeycombs

For low cost applications the degree of automation has exceeded the level reached in

aerospace honeycomb production. However, cell size and core height of these low costpaper honeycombs are usually above 10 mm, because the expansion process step inconventional honeycomb production gets more difficult at lower cell sizes.

A second production process for conventional honeycombs is the corrugationprocess. This process is not often used and more expensive due to the handlingoperations required for the production of the block and the more difficult cutting offfrom the expanded block. However, if inexpensive corrugated cardboard sheets areused, a low cost honeycomb core can be produced with the process shown in figure 2.

Figure 2 Manual production of corrugated cardboard honeycomb cores

The increasing demand for low cost sandwich core materials and their advantageous

mechanical properties have stimulated research activities at the K.U.Leuven to reducethe production costs of honeycomb cores, produced from paper as well as fromthermoplastic materials.

honeycomb

paper roll stacked sheets unexpanded block slice

corrugated cardboard sheets corrugated

cardboardblock

honeycomb


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EMAS 1999 3

Low cost honeycomb applications today

Many expanded paper honeycombs are used in door filling and furniture applications.In packaging applications these recyclable and bio-degradable paper honeycombmaterials are used to replace foam products used as inner packaging for a better crashabsorption behavior. Figure 3 shows paper honeycombs in packaging applications.

These cores are produced by the process shown in figure 1.

Figure 3 Inner packaging with paper honeycomb corner and edge elements

In the automotive industry more and more light weight sandwich materials are usedfor interior components. Often combinations from different plastics (Polyurethane(PU) foam and Polypropylene/glass fiber veils) are used, but to enable therecyclability of these parts, the trend goes towards monomaterial sandwiches (Eller(2)). Another trend is to use natural raw materials such as flax, hemp or sisal fibersfor the automotive interior parts.

Figure 4 shows a sandwich panel used for automotive interior with a cardboardhoneycomb core and glass fiber or natural fiber mat skins with a PU matrix whichfoams slightly into the core (Paul and Klusmeier (3)). This type of material is usedfor sunroofpanels,hard tops,rear parcel shelves,spare wheel covers and luggage

floor assemblies. Today core materials for these applications are produced by themanual process via a block of stacked corrugated cardboard sheets, shown in figure2.

Figure 4 Sandwich material with paper honeycomb core for automotive

interior applications

paperhoneycomb

top layermetal insert

bottom layer

decorativelayer

crushed core


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EMAS 1999 4

In modern automobile design, integrated interior modules such as door trim panels,headliners, package trays have multiple functions such as structural support,acoustic damping, water/dust barrier and they are three dimensionally formedattachment surfaces. The recyclability and the weight plays an increasing role.Honeycomb sandwich construction can offer the solution to several of thesedemands.

CORRUGATED CARDBOARD PACKAGING MATERIALS

The most inexpensive sandwich material, the corrugated cardboard, is an efficientpackaging material, widely used for transportation, storage and protection of goods.The combination of low cardboard paper raw material costs and low corrugatedsandwich production costs has led to a long and ongoing success story. However, thecorrugated cardboard is not an optimum sandwich material.

The edgewise (in-plane) compression resistance (measured by the so called edgecrush test, ECT) and the bending properties are determining the performance of a

packaging box. In figure 5 the structure and the principal directions of corrugatedcardboard as well as edgewise (ECT) and flat wise (FCT) loading directions areshown. The important ECT value is dependent on the sandwich thickness, the corestructure and the paper properties. Due to the paper production process the skins (theso called liners) have better properties (2.5 time higher stiffness (Baum et al. (4)) and1.7 times higher strength) in the machine direction (MD) than in the cross direction(CD).

Figure 5 Structure and principal directions of corrugated cardboard

The corrugated core (the so called flute)does notprovidetheoptimum support forthe liners. Especially under edgewise compression loads in MD the buckling of theliners between the corrugation tops (dimpling failure) occurs at low stress levels(usually below 20 % of the liner compression strength). Therefore, the direction of themain load application has to be perpendicular to the fiber orientation in the liners. Thelikely liner dimpling affects furthermore the surface quality and the importantprintability of the board.

ECT-loadingdirection

liner

flute

liner

FCT-loadingdirection


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EMAS 1999 5

In Figure 6 some corrugated cardboard core structures (flute types) and their geometricparameters are shown.

Flute type Flute heighttF [mm]

Wave length

[mm]

A-Flute 4.7 8.6

C-Flute 3.6 7.2

B-Flute 2.5 6.1

E-Flute 1.1 3.4

Figure 6 Geometric dimensions and corrugated cardboard flute types

For a larger sandwich thickness a second corrugated core and a mid-layer are required,reducing the moment of inertia per weight drastically. Thus the amount of used rawmaterial and the productioncostsof those double flute structures are higher.

To improve the printability and the cardboard properties, a trend towards flute typeswith a smaller wave length such as the E-Flute can be observed in the packagingmarket. The EB-Flute has become an alternative to the C-Flute. An EE-flute targets theB-Flute market and triple flute boards with the E-flute such as BCE- and ECE-Flute

have been developed to compete with the BC-Flute.

Massive application of honeycombs in packaging would require a cost efficient andcontinuous production, to be competitive to the corrugated cardboard. Furthermore,better mechanical properties to allow weight and raw material cost savings areessential for packaging applications as well as for structural applications.

FOLDED HONEYCOMB CARDBOARD

The folded honeycomb material concept has been developed and patented by theK.U.Leuven. Both, the material concept and its innovative production from a singlecontinuous sheet by successive in-line operations have been investigated. In the

recently concluded feasibility phase of the EUREKA research project to developfolded honeycomb cores for packaging and structural applications, the technicalfeasibility of two core versions (TorHex and ThermHex) has been proven.

EB-Flute

C-Flute

BC-Flute

t

tMtC

tL

tL

BCE-Flute

EE-Flute

ECE-Flute

tF = tC + tM


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EMAS 1999 6

The TorHex core is a folded honeycomb that allows for an exceptionally cost efficientproduction, since the production process uses the know how and the components of thecorrugated cardboard production line to a maximum extend. The inner core structure isa honeycomb with sinusoidal corrugated cell walls and a folded reinforcing mid layer.To produce a TorHex honeycomb cardboard two liners are laminated onto thelightweight TorHex core.

Figure 7 shows the two principle directions (CD and MD) of the TorHex materialas well as the ECT and the FCT loading directions.

Figure 7 Structure and principal directions of the honeycomb cardboard

The thickness of the corrugated cardboard (the height of the corrugations) defines thecell size of the honeycomb. A cell size of about 4.5 mm (A-Flute) is sufficient tosupport the skins, to prevent the buckling of the skins into the cells (dimpling failure)due to compression loads in both directions (CD and MD). Figure 8 shows the maingeometric parameters as well as a TorHex sample.

Figure 8 Main geometric parameters and view onto a TorHex sample

TorHex cardboard liner (skin)

folded core liner flute (corrugated medium)

CD MD

height hc

cell size c

CD

CD

CD

CD

MD

MD

MD

MD

ECT-loadingdirection

liner

TorHex core

liner

FCT-loadingdirection


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EMAS 1999 7

The reinforcing mid layers reduce the cell size and improve the material properties inthe machine direction further. They can carry tension forces in the production directionand enable a fast transport of the material. All material of the corrugated cardboard isused very efficiently in the TorHex honeycomb. Table 1 shows the weight of twoTorHex types.

TABLE 1 - TorHex types

TorHex typeCore weight

[g/m2]Height[mm]

Board weight[g/m2]

TorHex I 335 5 625

TorHex II 374 5 664

The ECT-values, the bending properties and the FCT-values of TorHex honeycombcardboard have been measured. These tests have verified the excellent properties ofthe new material and proved the large potential of this technology.

Compared to a corrugated core, a honeycomb provides an optimal isotropic supportfor the skins. This allows for substantial weight and raw material savings and results inan improved surface quality and in a better printability.

This continuously produced low cost cardboard honeycomb core fits to the demandsof the automotive industry for lightweight cores for interior parts. The completeautomation of the production process for materials and parts is a general demand for

process used by automotive suppliers. Already today double flute corrugated

cardboard panels with just a Polyurethane impregnation are frequently used to

stiffen the thin steel roof of cars. Apart from packaging and potential automotiveapplications especially the furniture industry will provide applications.

FOLDED HONEYCOMB CARDBOARD PRODUCTION PROCESS

The two year feasibility phase of the project has allowed a detailed investigation ofdifferent production concepts. Detailed concept studies and tests with lab scalemachine designs as well as process simulations have enabled an optimization of thematerial and the process. In the two year feasibility phase the originally proposedcross-wise production principle of folded honeycombs (earlier published by theauthors (5)), has been further developed resulting in a new concept for cardboardhoneycomb production. The key advantage of this new concept is the possibility for ahigh speed and low cost automated continuous production.

The principle production concept is shown in Figure 9. Compared to the single flutecorrugated cardboard production process, the TorHex honeycomb production requiresadditionally a lengthwise slitting step and a folding/turning process.


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EMAS 1999 8

It is expected that this process will be not much more expensive than the additionalcorrugation and gluing steps for a double flute corrugated cardboard.

Figure 9 Production principle of TorHex folded honeycomb cardboard core

For production of small TorHex core samples, lab-scale machinery has been built atthe K.U.Leuven.

FLAT WISE COMPRESSION PROPERTIES OF CORRUGATED ANDHONEYCOMB CARDBOARD

Due to the vertical cell walls, the flat wise compression resistance of the TorHex coreare expected to be higher compared to the values of corrugated cardboard. Table 3shows the total weights and the FCT properties of some corrugated cardboard typesand results from flat wise compression tests with the TorHex material.

TABLE 2 - Flat wise compression test results

Cardboard typesHeight[mm]

Total weight[g/m

2]

FCT measured[kPa]

FCT single flute[kPa]

TorHex core I 5 335 388 -

TorHex board I 5.2 625 497 -

TorHex core II 5 374 544 -

TorHex board II 5.2 664 724 -

A-Flute board 5 487 115 100

BC-Flute board 7 725 96 150 (C-Flute)

EB-Flute board 4.6 789 260 250 (B-Flute)

continuous production atconstant production width

b Corrugated = b Honeycomb

corrugatedcardboard

length wiseslitting

turning of thecardboard strips

b Corrugated

b Honeycomb

TorHexcore


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EMAS 1999 9

The FCT properties of double flute boards are usually not measured since thecorrugated core tends to collapse asymmetrical. However, the values are close to theFCT properties of a single flute board with the larger flute.

For comparison the complete force displacement curves during the compressiontests has been measured as well. In Figure 10 the averaged curves from 6 tests on each

cardboard type are shown.

Figure 10 Comparison of the flat wise compression test results

The TorHex cardboard shows higher FCT values than the TorHex core without liners.The higher buckling load of the supported cell walls result in a larger initial peak in the

compression load curve.

Figure 11 TorHex samples after flat wise compression tests

The further compression of the board occurs at a high compression load level, close tothe FCT-value of the core. The energy absorbed during the flat wise compressionfailure is much larger as with a corrugated core, resulting in superior impact behavior.

TorHex core buckling(free cell walls)

TorHex board buckling(liner supported cell walls)


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EMAS 1999 10

LOCAL FLAT WISE COMPRESSION PROPERTIES OF CORRUGATED ANDHONEYCOMB CARDBOARD

For the edgewise compression resistance (ECT) the local compression stiffness of thecore to prevent out of plane deformation of the skins is of key importance. The localcompression stiffness variations are furthermore crucial for the surface quality and the

printability of the board. A large height of the corrugated cardboard results in a largewave length of the corrugated medium. To reduce the surface unevenness due to thelocal buckling between the flute tops (liner dimpling) double corrugated coreconstructions are used. Usually an inner layer between the corrugated layers is used tobe able to use a smaller wave length at the printed outer side.

The local compression stiffness of double flute boards varies depending on theposition of the two flutes towards each other. The maximum compression stiffness isreached at the position where the two flute tops meet. In some recently developedboard types two flutes of the same wave length are used and bonded at the flute tops.This allows to eliminate the middle layer and results consequently in an improvedspecific bending stiffness for an equal board weight (Shaw et al. (6)). However, the

flat wise compression strength, the liner support in MD and the shear properties are notmuch improved. The TorHex core structure is in principle independent from the coreheight and offers perfect liner support in both directions at each board thickness.

The variations of the local deformations under a point load have been measured toinvestigate the local compression resistance. A constant point pressure is applied by aprobe (2mm in diameter). The z-coordinates measured with a very low contact load ofF = 0.18 N (probe under 45o) approximately represent the surface unevenness of thesample. The differences to a measurement with a contact load of F = 1.09 N (probeunder 7.5o) represent the deformations of the sample under a certain local load.

Figure 12 Test set-up and measured local deformations of TorHex sample II

probe under anangle of 45o

cardboardsample


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EMAS 1999 11

The flat wise compression deformations at 1875 points of a surface area of 7.5 mm x11 mm are shown. The spacing of the measurement grid was 0.2 mm. The TorHex IIsample in figure 12 exhibits a very equal cell wall stiffness.

Furthermore, the variations of the local deformations under a uniform compressionload have been measured by an optical technique as shown in figure 13.

Figure 13 Test set-up and view onto an A-Flute sample during a test

A constant surface pressure was applied with the help of a vacuum bag set-up. Themeasurement of z-displacement was performed by the optical strain mapping system ofthe company GOM, Braunschweig, Germany. Deflections were measured with 0.8 barpressure onto the samples. The tested corrugated cardboard samples and the TorHexhoneycomb cardboard had all the same liner weight per unit area. The out of planedisplacements in mm due to the 80 kPa pressure are shown in figure 14.

Figure 14 Local deformations (in mm) due to uniform compression load

The A-Flute shows at 0.8 bar large 0.4 mm deep dimples with steep transitions. Whilethe BC-Flute shows larger but smoother deformations due to the double fluteconstruction. The measured differences in compression deformation are 0.6 mm. The

A-Flute BC-Flute TorHex I

0.42

-0.06

0.90

0.30

0.06

0.01

0.20

0.600.03

cardboard

sample in avacuum bag

two digitalcameras


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EMAS 1999 12

TorHex cardboard shows only very small deformation differences of 0.05 mm, withdeformations into the cells close to the resolution of the measurement system.

CONCLUSIONS

The TorHex concept allows a very cost efficient production of paper honeycomb cores.The TorHex cardboard offers superior properties as well as weight and raw materialsavings compared to corrugated cardboard. The FCT values and the complete flat wisecompression resistance of the TorHex material are much better than the properties ofany available corrugated cardboard of comparable thickness. The small local stiffnessvariations indicate that the printability of TorHex cardboard as well as the support ofthe liners during edgewise loading will be very good.

The TorHex core is an environmental friendly honeycomb. Because of the goodmechanical properties, the low paper material costs and the extremely low productioncosts, it can be expected that the TorHex material will find many structural applicationsin panels for cars, floors and furniture.

ACKNOWLEDGEMENTS

The authors acknowledge the support of the Flemish Institute for the Promotion ofScientific and Technological Research in Industry (IWT) and the Belgian program onInteruniversity Poles of Attraction initiated by the Belgian State, Prime Minister'sOffice, Science Policy Programming. The authors gratefully acknowledge furthermorethe contribution and financial support provided by AssiDomn Packaging.

REFERENCE

(1) Bitzer, T.N., Recent Honeycomb Core Developments, Sandwich ConstructionConference 3, Edited by H.G. Allen, Southampton, 1995, pp. 555562

(2) Eller, R., End of Life Vehicle (ELV) concerns impact interior material

substitution,Automotive & Transportation Interiors, Vol. 9, 1999, pp. 218232.

(3) Paul, R. and Klusmeier, W., Structhan A Composite with a Future, StatusReport, Bayer AG, Leverkusen, 1997.

(4) Baum, G.A., Brennan, D.C and Habeger, C.C., Orthotropic elsatic constants ofpaper, Tappi Journal, Vol. 64, No.2, 1981, pp.97101.

(5) Pflug, J., Verpoest, I. and Vandepitte, D., Folded Honeycombs Fast andcontinuous production of the core and a reliable core skin bond, International

Conference on Composite Materials, Edited by T. Massard, Paris, 1999.(6) Shaw, N.W., Selway, J.W. and McKinlay, P.R., Revolution in Board Design and

Manufacture, Tappi Journal, Vol. 81, No.10, 1998, pp.2734.

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