FOLDED HONEYCOMB CARDBOARD AND CORE MATERIAL FOR STRUCTURAL APPLICATIONS

Abstract

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

Full text

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FOLDED HONEYCOMB CARDBOARD AND CORE MATERIAL
FOR STRUCTURAL APPLICATIONS
Jochen Pflug*, Ignaas Verpoest*, Dirk Vandepitte
+
Today, most honeycomb cores are produced in a batch wise production
process by cutting from a block. A new continuous production concept for
low cost honeycomb core materials from a single corrugated cardboard
sheet has been developed and patented at the K.U.Leuven. For the
production of this, so called folded honeycomb cardboard core material, the
efficient machinery from the packaging industry is used to a maximum
extent. The low production costs will open new markets for honeycomb
materials for example in the automotive industry. The new core material
and its continuous production processes are presented in this paper. The flat
wise compression properties as well as some potential applications are
discussed. Furthermore, two techniques to measure the local flat wise
compression stiffness variations are presented.
INTRODUCTION
Honeycombs are used in aerospace industries since many decades as the preferred
core material for buckling and bending sensitive sandwich panels and structures. These
aerospace honeycombs are most often hexagonal or over-expanded and are usually
produced for aluminium sheets or phenol resin coated aramid paper by the expansion
process.
In the past decades the interest of other large industries in sandwich core materials
with good specific mechanical properties has increased continuously. Thus, today more
than half of the honeycomb core materials are used in other application areas, e.g. for
panels in trains, trucks and ships, for cladding of buildings and in sporting goods.
Honeycombs produced from low cost paper (Kraftpaper or recycled paper) can be
inexpensive enough to be used as crash elements in automotive interior and packaging
applications.
Low cost honeycomb production today
The main reason for the high costs of traditional expanded honeycomb cores is the
batch 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: Jochen.Pflug@mtm.kuleuven.ac.be
+
K.U.Leuven, Department Mechanical Engineering, Division PMA

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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 many
sheets 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 cost
paper honeycombs are usually above 10 mm, because the expansion process step in
conventional honeycomb production gets more difficult at lower cell sizes.
A second production process for conventional honeycombs is the corrugation
process. This process is not often used and more expensive due to the handling
operations required for the production of the block and the more difficult cutting off
from the expanded block. However, if inexpensive corrugated cardboard sheets are
used, 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 reduce
the production costs of honeycomb cores, produced from paper as well as from
thermoplastic materials.
honeycomb
paper roll stacked sheets unexpanded block slice
corrugated cardboard sheets
corrugated
cardboard
block
honeycomb

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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 honeycomb
materials are used to replace foam products used as inner packaging for a better crash
absorption 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 used
for interior components. Often combinations from different plastics (Polyurethane
(PU) foam and Polypropylene/glass fiber veils) are used, but to enable the
recyclability 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 fibers
for the automotive interior parts.
Figure 4 shows a sandwich panel used for automotive interior with a cardboard
honeycomb core and glass fiber or natural fiber mat skins with a PU matrix which
foams slightly into the core (Paul and Klusmeier (3)). This type of material is used
for sun roof panels, hard tops, rear parcel shelves, spare wheel covers and luggage
floor assemblies. Today core materials for these applications are produced by the
manual process via a block of stacked corrugated cardboard sheets, shown in figure
2.
Figure 4 Sandwich material with paper honeycomb core for automotive
interior applications
paper
honeycomb
top layer
metal insert
bottom layer
decorative
layer
crushed core

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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 formed
attachment surfaces. The recyclability and the weight plays an increasing role.
Honeycomb sandwich construction can offer the solution to several of these
demands.
CORRUGATED CARDBOARD PACKAGING MATERIALS
The most inexpensive sandwich material, the corrugated cardboard, is an efficient
packaging material, widely used for transportation, storage and protection of goods.
The combination of low cardboard paper raw material costs and low corrugated
sandwich production costs has led to a long and ongoing success story. However, the
corrugated cardboard is not an optimum sandwich material.
The edgewise (in-plane) compression resistance (measured by the so called edge
crush test, ECT) and the bending properties are determining the performance of a
packaging box. In figure 5 the structure and the principal directions of corrugated
cardboard as well as edgewise (ECT) and flat wise (FCT) loading directions are
shown. The important ECT value is dependent on the sandwich thickness, the core
structure and the paper properties. Due to the paper production process the skins (the
so called liners) have better properties (2.5 time higher stiffness (Baum et al. (4)) and
1.7 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 not
provide
the
optimum support for
the liners. Especially under edgewise compression loads in MD the buckling of the
liners between the corrugation tops (dimpling failure) occurs at low stress levels
(usually below 20 % of the liner compression strength). Therefore, the direction of the
main load application has to be perpendicular to the fiber orientation in the liners. The
likely liner dimpling affects furthermore the surface quality and the important
printability of the board.
ECT-loading
direction
liner
flute
liner
FCT-loading
direction

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In Figure 6 some corrugated cardboard core structures (flute types) and their geometric
parameters are shown.
Flute type Flute height
t
F
[mm]
Wave length λ
[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 raw
material and the production
costs
of those double flute structures are higher.
To improve the printability and the cardboard properties, a trend towards flute types
with a smaller wave length such as the E-Flute can be observed in the packaging
market. The EB-Flute has become an alternative to the C-Flute. An EE-flute targets the
B-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 and
continuous production, to be competitive to the corrugated cardboard. Furthermore,
better mechanical properties to allow weight and raw material cost savings are
essential for packaging applications as well as for structural applications.
FOLDED HONEYCOMB CARDBOARD
The folded honeycomb material concept has been developed and patented by the
K.U.Leuven. Both, the material concept and its innovative production from a single
continuous sheet by successive in-line operations have been investigated. In the
recently concluded feasibility phase of the EUREKA research project to develop
folded honeycomb cores for packaging and structural applications, the technical
feasibility of two core versions (TorHex and ThermHex) has been proven.
EB-Flute
C-Flute
BC-Flute
t
λλ
t
M
t
C
t
L
t
L
BCE-Flute
EE-Flute
ECE-Flute
t
F
= t
C
+ t
M

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The TorHex core is a folded honeycomb that allows for an exceptionally cost efficient
production, since the production process uses the know how and the components of the
corrugated cardboard production line to a maximum extend. The inner core structure is
a honeycomb with sinusoidal corrugated cell walls and a folded reinforcing mid layer.
To produce a TorHex honeycomb cardboard two liners are laminated onto the
lightweight TorHex core.
Figure 7 shows the two principle directions (CD and MD) of the TorHex material
as 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 the
cell size of the honeycomb. A cell size of about 4.5
mm (A-Flute) is sufficient to
support 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 main
geometric 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-loading
direction
liner
TorHex core
liner
FCT-loading
direction

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The reinforcing mid layers reduce the cell size and improve the material properties in
the machine direction further. They can carry tension forces in the production direction
and enable a fast transport of the material. All material of the corrugated cardboard is
used very efficiently in the TorHex honeycomb. Table 1 shows the weight of two
TorHex types.
TABLE 1 - TorHex types
TorHex type
Core weight
[g/m
2
]
Height
[mm]
Board weight
[g/m
2
]
TorHex I 335 5 625
TorHex II 374 5 664
The ECT-values, the bending properties and the FCT-values of TorHex honeycomb
cardboard have been measured. These tests have verified the excellent properties of
the new material and proved the large potential of this technology.
Compared to a corrugated core, a honeycomb provides an optimal isotropic support
for the skins. This allows for substantial weight and raw material savings and results in
an improved surface quality and in a better printability.
This continuously produced low cost cardboard honeycomb core fits to the demands
of the automotive industry for lightweight cores for interior parts. The complete
automation 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 automotive
applications 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 of
different production concepts. Detailed concept studies and tests with lab scale
machine designs as well as process simulations have enabled an optimization of the
material and the process. In the two year feasibility phase the originally proposed
cross-wise production principle of folded honeycombs (earlier published by the
authors (5)), has been further developed resulting in a new concept for cardboard
honeycomb production. The key advantage of this new concept is the possibility for a
high speed and low cost automated continuous production.
The principle production concept is shown in Figure 9. Compared to the single flute
corrugated cardboard production process, the TorHex honeycomb production requires
additionally a lengthwise slitting step and a folding/turning process.

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It is expected that this process will be not much more expensive than the additional
corrugation 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 at
the K.U.Leuven.
FLAT WISE COMPRESSION PROPERTIES OF CORRUGATED AND
HONEYCOMB CARDBOARD
Due to the vertical cell walls, the flat wise compression resistance of the TorHex core
are expected to be higher compared to the values of corrugated cardboard. Table 3
shows the total weights and the FCT properties of some corrugated cardboard types
and results from flat wise compression tests with the TorHex material.
TABLE 2 - Flat wise compression test results
Cardboard types
Height
[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 at
constant production width
b
Corrugated
= b
Honeycomb
corrugated
cardboard
length wise
slitting
turning of the
cardboard strips
b
Corrugated
b
Honeycomb
TorHex
core

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The FCT properties of double flute boards are usually not measured since the
corrugated core tends to collapse asymmetrical. However, the values are close to the
FCT properties of a single flute board with the larger flute.
For comparison the complete force displacement curves during the compression
tests 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 to
the FCT-value of the core. The energy absorbed during the flat wise compression
failure 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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LOCAL FLAT WISE COMPRESSION PROPERTIES OF CORRUGATED AND
HONEYCOMB CARDBOARD
For the edgewise compression resistance (ECT) the local compression stiffness of the
core to prevent out of plane deformation of the skins is of key importance. The local
compression 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 large
wave length of the corrugated medium. To reduce the surface unevenness due to the
local buckling between the flute tops (liner dimpling) double corrugated core
constructions are used. Usually an inner layer between the corrugated layers is used to
be able to use a smaller wave length at the printed outer side.
The local compression stiffness of double flute boards varies depending on the
position of the two flutes towards each other. The maximum compression stiffness is
reached at the position where the two flute tops meet. In some recently developed
board 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 improved
specific 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 not
much improved. The TorHex core structure is in principle independent from the core
height 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 to
investigate the local compression resistance. A constant point pressure is applied by a
probe (2
mm in diameter). The z-coordinates measured with a very low contact load of
F = 0.18 N (probe under 45
o
) approximately represent the surface unevenness of the
sample. The differences to a measurement with a contact load of F = 1.09 N (probe
under 7.5
o
) 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 an
angle of 45
o
cardboard
sample

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The flat wise compression deformations at 1875 points of a surface area of 7.5 mm x
11 mm are shown. The spacing of the measurement grid was 0.2 mm. The TorHex II
sample in figure 12 exhibits a very equal cell wall stiffness.
Furthermore, the variations of the local deformations under a uniform compression
load 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. The
measurement of z-displacement was performed by the optical strain mapping system of
the company GOM, Braunschweig, Germany. Deflections were measured with 0.8 bar
pressure onto the samples. The tested corrugated cardboard samples and the TorHex
honeycomb cardboard had all the same liner weight per unit area. The out of plane
displacements 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. While
the BC-Flute shows larger but smoother deformations due to the double flute
construction. 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
0.60
0.03
cardboard
sample in a
vacuum bag
two digital
cameras

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TorHex cardboard shows only very small deformation differences of 0.05 mm, with
deformations 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 material
savings compared to corrugated cardboard. The FCT values and the complete flat wise
compression resistance of the TorHex material are much better than the properties of
any available corrugated cardboard of comparable thickness. The small local stiffness
variations indicate that the printability of TorHex cardboard as well as the support of
the liners during edgewise loading will be very good.
The TorHex core is an environmental friendly honeycomb. Because of the good
mechanical properties, the low paper material costs and the extremely low production
costs, it can be expected that the TorHex material will find many structural applications
in panels for cars, floors and furniture.
ACKNOWLEDGEMENTS
The authors acknowledge the support of the Flemish Institute for the Promotion of
Scientific and Technological Research in Industry (IWT) and the Belgian program on
Interuniversity Poles of Attraction initiated by the Belgian State, Prime Minister's
Office, Science Policy Programming. The authors gratefully acknowledge furthermore
the contribution and financial support provided by AssiDomän Packaging.
REFERENCE
(1) Bitzer, T.N., Recent Honeycomb Core Developments, Sandwich Construction
Conference 3, Edited by H.G. Allen, Southampton, 1995, pp. 555–562
(2) Eller, R., End of Life Vehicle (ELV) concerns impact interior material
substitution, Automotive & Transportation Interiors, Vol. 9, 1999, pp. 218–232.
(3) Paul, R. and Klusmeier, W., Structhan
®
– A Composite with a Future, Status
Report, Bayer AG, Leverkusen, 1997.
(4) Baum, G.A., Brennan, D.C and Habeger, C.C., Orthotropic elsatic constants of
paper, Tappi Journal, Vol. 64, No.2, 1981, pp.97–101.
(5) Pflug, J., Verpoest, I. and Vandepitte, D., Folded Honeycombs – Fast and
continuous 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.27–34.