TorHex paper for 5th Symposium Kassel

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5 Global Wood and Natural Fibre Composites Symposium

April 27 - 28, 2004 in Kassel/Germany

Continuously Produced Paper Honeycomb Sandwich Panels for Furniture Applications J. Pflug 1), B. Vangrimde 1), I. Verpoest 1), D. Vandepitte 2) M. Britzke 3), A. Wagenführ 3) 1) Katholieke Universiteit Leuven, Department MTM 2) Katholieke Universiteit Leuven, Division PMA 3) Technisch Universität Dresden, Institute of Wood and Paper Technologies

Abstract Today mechanical requirements and weight targets demand a lightweight sandwich design in many application areas. The potential of honeycomb sandwich construction in furniture applications is, like in many other application areas mainly determined by the production cost of cores and panels. In the last decade the traditional honeycomb production processes for low cost paper honeycomb cores have been optimised towards concepts with a fully automated continuous in-line sandwich panel production. Sandwich selection charts, a graphical presentation of the effects of sandwich constructions on weight and cost are shown for paper honeycomb sandwich panels. This allows a transparent selection and comparison of sandwich materials for furniture applications. It can be shown that a paper honeycomb sandwich panel can offer cost savings in comparison to uniform chipboard panels. 1. Introduction Optimal structural performance may require the use of material combinations, because a composition can be more efficient if each component is optimised for a certain function. Sandwich construction combines the in-plane properties of a skin material with the out-of-plane properties of a core material. For the bending properties of a panel the stiffness and strength of a material can be used much more efficiently if the stresses act at large distances from the neutral axis. A high moment of inertia is a substantial advantage for bending loaded and buckling sensitive (e.g. in-plane compression loaded) panels. With a uniform material a thickness increase leads to an increase of both weight and material cost of a panel. Sandwich constructions use the fact that the core of a bending loaded uniform (monolithic) panel does not carry much in-plane stresses and does not represent the visible surface of the panel. The core can thus be made from a different, more lightweight and/or less expensive material. However sufficient mechanical properties of the sandwich core material are necessary to prevent relative displacements of the skins with respect to each other in out-of-plane and in-plane directions. Honeycomb core materials can offer the required out-of-plane shear and out-of-plane compression properties at an extremely low density. They are used in aerospace as the preferred core material for sandwich parts and structures since many decades [1, 2]. The demands for low cost and high production capacities in packaging, automotive and furniture industries require an automated and continuous sandwich core and panel production process. Corrugated cardboard, well known for its low cost packaging applications, is for example produced by a very fast and continuous in-line process. In contrast to corrugated core types, honeycomb core types have vertical cell walls which provide a rigid bi-directional support for the skins, leading to much better mechanical properties. However, the high production cost of honeycomb cores and panels often prevented their use in low cost sandwich constructions. During the last two decades the efforts to reduce the production cost of honeycomb sandwich panels have been increased by industry and research institutions. Since the sandwich concept finds applications in many industries, different terminology can be found. The skin material is e.g. also

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called facing, face or liner. Figure 1 clarifies some terms and shows the different production steps from raw material to sandwich constructions. Furthermore the differences between the traditional value chain for aerospace applications and a value chain for low cost applications is shown. upper skin

sandwich constructions

t

bonding / lamination

core production

raw materials (for core and skins)

core

assembling forming sandwich sandwich part structure (assembly of (panel with

h bonding layers

lower skin

sandwich constituents (core, skin and bonding layer)

Core material producer

In-line core preparation

sandwich material (with dimensions this is referred to as a sandwich panel)

edge closures, inserts and/or curvatures)

Sandwich panel producer

Part producer

Sandwich material producer

In-line post processing

sandwich parts)

Assembly plant

End user

Figure 1: From raw materials to sandwich constructions Low cost applications demand the combination of efficient sandwich core materials (like honeycomb) from a low cost raw material (e.g. paper) and efficient production processes. 2. Paper honeycomb production processes Most aerospace honeycomb cores are adhesive bonded expanded cores produced in a batch process [2]. Paper honeycombs as well as honeycombs from aluminium foils can be produced by this traditional expansion process. For low cost applications, the degree of automation was increased beyond the level reached in aerospace honeycomb production. Figure 3 shows schematically the different process steps in the production of expanded honeycombs made out of low cost recycled papers (Testliner). At first, adhesive lines are printed on the paper, which may come from one or several paper rolls. Secondly, a stack of several sheets is made and bonded together. Those sheets can be cut to strips prior or after stacking to a slice. In the third step many slices are stacked and bonded together. This results in the unexpanded endless paper honeycomb core. Finally, the sheets are pulled apart expanding the stack into a hexagonal honeycomb core. The residual stresses in paper honeycombs have to be relaxed after expansion by a controlled application of heat. Paper roll

Stacked sheets

Slice

Unexpanded endless core

Expansion process

Expanded honeycomb

Figure 3: Paper honeycomb production by expansion Expanded low cost paper honeycombs are used for door filling and for inner packaging protection elements since decades. Only recently a massive penetration into furniture applications can be

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observed. In the rapidly growing Asian furniture industry a majority of all furniture panels are already paper honeycomb core panels [3, 4]. However, cell size and core height of these paper honeycombs are usually above 10 mm, because the cutting and bonding production steps are more time consuming at lower cell sizes. Corrugated block

Single wall corrugated sheets

Stacked sheets

Cutting from the block

Corrugated honeycomb

Figure 4: Paper honeycomb production from corrugated cardboard A second traditional process for honeycombs is the corrugation process. For aluminium honeycombs this type of process is not so often used since it is more expensive, due to the required handling operations (stacking and bonding of trapezoidal corrugated sheets) and the more difficult cutting from the large block [2]. However, if inexpensive corrugated cardboard sheets are used, low cost honeycomb cores can be produced by this type of process. Figure 4 shows the process with single wall corrugated cardboard. The adhesive to bond the sheets can be applied on the corrugation tops or on the flat liner. The liner is required to prevent deformations of the corrugated layer and to allow a stacking to a block without need for an exact positioning of the sheets. With standard corrugated cardboard sheets, a small cell size of 5 mm can be realized, leading to a larger density compared to expanded honeycomb cores. However, a small cell size is important for the surface quality and the out-of-plane compression properties. A large cell size, especially in combination with thin skins, can result in a print through of the honeycomb pattern. Paper honeycombs for automotive applications are commonly produced by this process via a block of stacked corrugated cardboard sheets. Those sandwich panels with glass fiber or natural fiber reinforced skins are for example used for sun roof panels and spare wheel covers [5]. The low amount of raw material required for the production of honeycomb cores can result in a very cost efficient sandwich material if a suitable raw material and an efficient production method are combined. An overview on the recent advances in production processes which may enable a widespread introduction of honeycomb sandwich constructions into non-aerospace markets is presented in [6]. The packaging industry has very high demands on the cost effectiveness of their production processes. The efficient continuous in-line production technology of the corrugated cardboard packaging industry, shown in figure 5, results in very low cost.

Corrugated cardboard

Figure 5: Continuous in-line production of corrugated cardboard The increasing demand for low cost core materials is the driving force of several research activities at the K.U.Leuven. The goal of this research is a further reduction of production costs of honeycomb cores produced from paper as well as from thermoplastic films. In the framework of the EUREKA

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research projects ThermHex and TorHex, cost efficient honeycomb materials and their continuous production processes have been developed. For the production of those patented, “folded“ honeycombs, production technology and processes of the packaging industry are used to enable honeycomb production by successive in-line operations [7]. The TorHex paper honeycomb process uses the corrugated cardboard production technology to a maximum extent. The principal production concept is shown in figure 6. After the production of a single flute corrugated cardboard, the TorHex honeycomb process requires only a lengthwise slitting step and a folding/turning step [8]. Paper honeycomb cores can hereby be produced at low cost from sheets or from an endless (e.g. fan-folded) web of corrugated cardboard.

Continuous in-line process

Corrugated cardboard (endless or as sheets)

Length wise slitting Turning of the cardboard strips

Folded honeycomb (continuously produced corrugated honeycomb)

Figure 6: TorHex paper honeycomb process The low cost value chain presented in figure 1 requires an in-line production of honeycomb core and sandwich panel. Handling and transport of the honeycomb core must be avoided. The expansion process has hereby the advantage that the transport and storage of the endless unexpanded core requires much less volume compared to corrugated honeycombs. The continuously produced folded honeycombs from corrugated cardboard on the other hand allow to use locally available corrugated cardboard and to produce paper honeycombs in-line with the sandwich panel production. In both cases the final honeycomb production step is performed at the sandwich material producer. With single and double flute corrugated cardboards of different thickness a panel height from 4 mm to about 12 mm can be produced by the TorHex process in a cost efficient way. For furniture applications which require larger heights the use of continuously expanded paper honeycombs or corrugated honeycombs produced from a block should be considered. The small cell size of corrugated paper honeycombs is especially in combination with thinner skins favourable. The optimal skin thickness for a maximal weight and cost advantage is mainly determined by the sandwich panel height. 3. Performance versus cost In engineering structural efficiency is defined as the capability of a structure to carry loads at a minimum weight. If loading and support conditions are not changed (i.e. the structural loading coefficient is constant) the structural efficiency is only dependent on the material efficiency. The material efficiency is usually defined as a mechanical performance per weight. For buckling and 1/3 bending stiffness of panels the material efficiency per weight is E /ρ and for tension and compression stiffness of panels it is E/ρ (with E the elastic modulus and ρ the material density). Materials selection charts (Ashby diagrams) [9] can be used to compare how efficient different materials fulfil a certain 1/3 structural function. For sandwich constructions the material efficiency in bending (MEw = EH /ρH) is especially important because the bending stiffness is the main advantage of the sandwich concept. For structural design, performance is primarily determined by the structural (mechanical) performance. Nevertheless, the non-structural performance (i.e. physical-chemical properties like e.g. thermal properties, surface quality, sound absorption, wear or chemical resistance) can be important issues for structural parts as well.

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Since a maximal economical advantage is often targeted the optimisation of the performance per weight should be part of a performance per cost optimisation. A general procedure for multi-objective optimisation based on value functions has been presented by Ashby [10]. This general concept allows to include the value of every requirement into the selection and optimisation process. Besides the material cost, other cost factors (production cost, operating cost and ecological cost) need to be added, as shown in equation (1). performance cost

≈

structural performance

(1)

material cost + production cost + operating cost + ecological cost affected by weight

Material selection and production process selection determines a large part of the cost of sandwich constructions. Thus the selection of the production process needs to be included into the engineering selection process in the state of screening and comparing different design and material options [9]. Maximization of performance per cost for structural parts requires to find the best compromise between (structural) performance and material cost, production cost and weight. Figure 2 shows how the different options for cost reductions can enable an exploitation of the economical advantage of lightweight constructions. design space

production cost savings

raw material cost savings selection of a more cost efficient raw material C

interact

selection of a more cost efficient production process C /h ap

wH

structural efficiency cost savings interact

less material due to a more lightweight design EH1/3/ρH

interact

effects affect

operating and ecological cost savings less energy consumption due to a lower weight

affect

cwv ρH

Figure 2: Options for cost reduction due to material/process selection and lightweight design The major cost reduction options in the design space interact and contradict often (figure 2). Materials with lower cost often have lower mechanical properties and more lightweight designs usually lead to more complex structures which often require more expensive production processes. This results in complex interactions during the optimisation of a design towards a maximal efficiency per cost. The operating cost (e.g. energy consumption in transport applications) and the ecological cost (e.g. cost of recycling) are both affected by the weight and of increasing importance since many years. To include those aspects into the comparison of materials, the benefit of weight savings has to be determined. This value of weight saving (cwv) may be established by a detailed life cycle analysis or may just be assessed by asking customers how much they would pay for a weight reduction. For furniture applications a weight saving will often not be valued (cwv = 0). However, a weight saving may lead to cost saving just by the lower amount of raw material used. The material efficiency per cost for buckling and bending (MEc) is given by equation (2), which includes the material cost per weight Cw, the value of weight saving and the panel production cost per square meter Cap.

MEc = EH1/3/Cv = EH1/3 / (CwH ρH + cwv ρH + Cap/h)

(2)

The sandwich bending modulus EH, the sandwich material density ρH as well as the material cost per

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volume Cv = CwH ρH can be expressed dimensionless with the ratio between the skin thickness t, the sandwich height h (see figure 1) and the properties of the core (index c) and skin (index s) materials.

 t t2 t3  EH = Ec + ( Es − Ec )  6 − 12 2 + 8 3   h h h  t ρH = ρc + ( ρs − ρc ) 2 h t  CwH = Cwc ρc + ( Cws ρs − Cwc ρc ) 2  ρH h 

(3) (4) (5)

For low cost applications it is essential that the knowledge how to design and to optimise sandwich panels is implemented by the panel producer for a limited variety of standard panels. Those standard panels have to be optimised independently from the later loading condition. Nevertheless, engineers need to be aware of the different behaviour and the different failure modes of sandwich materials. 4. Sandwich selection charts For sandwich material selection the performance per cost of different sandwich panels needs to be compared to each other and to uniform panels. The material efficiency can be presented graphically as a function of the thickness ratio t/h with the help of the equations (3),(4) and (5). It has been proposed earlier to display the properties of sandwich material combinations as a function of the thickness ratio t/h in materials selection charts to facilitate the selection of sandwich materials [11]. This graphical presentation enables to directly assess weight and cost savings in function of the thickness ratio.

t/h = 0.1 t/h opt weight

Panel example : h = 10 mm, t = 1 mm

Figure 7: Sandwich material selection chart for optimal weight Figure 7 shows the curve for a sandwich material combination (chipboard skins on a paper honeycomb core) in a modulus versus density materials selection chart. The properties of all possible

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sandwich panels with this material combination (thickness ratios from 0 to 0.5) can be found on this curve. The density on the x-axis is recalculated to the weight of a 10 mm thick panel. The maximum 1/3 material efficiency per weight in bending is at high E /ρ values in the upper left corner of the diagram in figure 7. The properties of a panel with a thickness ratio of t/h = 0.1 are shown in the diagram. The maximal weight saving requires a very low thickness ratio t/hopt weight. 1/3 The maximum material efficiency per cost in bending E /Cv of the same material combination is shown in figure 8. The optimal thickness ratio t/hopt cost is reached at a thickness ratio close to t/h = 0.1.

t/h = 0.1

t/h opt cost

Panel example : h = 10 mm, t = 1 mm

Figure 8: Sandwich material selection chart for optimal cost Often a larger skin thickness needs to be selected because of strength requirements, dimpling or wrinkling failure modes, surface quality requirements or production constraints. In most cases a rather slow decrease of the material efficiency close to the optimum allows to increase the thickness ratio t/h slightly without a large penalty. The proposed sandwich selection diagrams allow to assess the effects of non-optimal thickness ratios. Shear deformations of the core can be included into the optimisation of sandwich constructions and a similar optimisation towards a maximum bending strength, involving different failure modes, can be performed. Those optimisations have been discussed in detail earlier [12, 13]. To find an optimum compromise between the shear deflection and bending deflection it is possible to calculate a shear reduced flexural modulus. The inclusion of shear deformation and strength leads to a dependency on the span length as well as on the loading and support conditions. However the graphical presentation in function of the thickness ratio in sandwich selection charts has the potential to facilitate the optimal selection and comparison of sandwich materials because those diagrams can make the complex effects of sandwich materials selection and design somewhat more transparent. For industry the material efficiency is especially useful because the ratios between the material efficiency of a material option and the efficiency of a reference material are saving factors. Those saving factors enable to determine directly the potential weight savings and cost savings of different

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material options. To quantify the increase of mechanical performance per weight due to structuring, e.g. in an I-beam, a shape factor has been described by Ashby in [14]. This approach defines a ratio between the material efficiency coefficient of the structured material and the material efficiency coefficient of the same unstructured material. For sandwich materials the saving factors have been defined as the ratio of the efficiencies of a sandwich material combination and a uniform panel from the skin material, because the skin material choice is often restricted due to mechanical requirements (e.g. hardness) and/or optical requirements on the outer surface [5]. The resulting weight shape factor (Phi weight) is the factor of weight saved due the use of a sandwich construction with the lightweight core material. The cost saving factor (Phi cost) can include material cost savings, the value of weight saving as well as the production cost for the sandwich panel. The cost comparison in figure 8 confirms that cost advantages compared to uniform chipboard panels are possible. Due to the inclusion of the production cost per square meter the optima shifts to a larger thickness ratio and becomes dependent on the panel height. The production costs for furniture paper honeycomb sandwich panels have been presented in [4]. If the sides of the board are closed by a frame the cost saving factor becomes furthermore dependent on the panel length and width. For thinner sandwich materials the approximately constant production costs per unit area are an increasing percentage of the sandwich panel cost. Therefore, for thin furniture panels the demand for a low cost automated in-line production is even higher. 5. Conclusion The basic reason to use honeycomb sandwich construction is that it provides the highest strength to weight and stiffness to weight ratios. However the furniture industry, like many industries requests a performance per cost advantage additional to a performance per weight benefit. The automated in-line production of paper honeycombs with a continuous in-line panel material production enables to reduce the production costs of sandwich panels. The raw material weight savings result directly in cost savings and can thus lead to the exploitation of the economical advantages of sandwich constructions. For the development of low cost sandwich applications it is essential to enable the designer to gain a better understanding and more confidence in the advantages of sandwich constructions. The presented sandwich selection charts allow to assess the potential weight and cost savings of different sandwich material combinations. References: [1] Zenkert, D., The handbook of sandwich construction, London, UK, EMAS Chameleon Press Ltd., [2] Bitzer, T., Honeycomb Technology - Materials, design, manufacturing, applications and testing, London, Chapman & Hall, 1997 [3] Engelen, G., Die Homag AG geht innovativ das Thema Leichtplattenbau an, Homag press release, 2003 [4] Weber, H.-G., Schatz, J., Honeycomb technology - Homag plant concept for the production of lightweight paper honeycomb panels in through feed, Honeycomb Colloquium 1, Schopfloch, 2004 [5] Paul, R., Klusmeier, W., Structhan® – A Composite with a Future, Status Report, Bayer AG, Leverkusen, 1997 [6] Pflug, J., et al., Honeycomb core materials: New concepts for continuous production, Sampe Journal, 39 (6), p. 22-30, 2003 [7] Pflug, J., Verpoest, I., Folded honeycomb structure consisting of corrugated paperboard and method and machinery for producing the same, K.U.Leuven Research & Development, WO 00/58080 International patent publication, 2000 [8] Pflug, J., Verpoest, I., Vandepitte, D., Folded Honeycomb cardboard and core material for structural applications, Sandwich Construction 5, Zurich, 2000 [9] Ashby, M. F., Materials and process selection in mechanical design, Butterworth Heinemann, Oxford, 1999

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[10] Ashby, M. F., Multi-Objective Optimization in Material Design and Selection, Acta Metall. Mater., vol. 48/1, pp. 359-369, 2000 [11] Pflug, J., Vangrimde, B., Verpoest, I., Material efficiency and cost effectiveness of sandwich materials, Sampe Conference, Longbeach, USA, 2003 [12] Ashby, M.F., Gibson, L. J., Cellular Solids - Structure and properties, Cambridge / UK, Cambridge University Press, 1997 - has been already reference 27 in part 1 [13] Wiedemann, J., Leichtbau Konstruktion, vol. 2, Springer-Verlag, Berlin, 1997 [14] Ashby, M. F., Materials and Shape, Acta Metall. Mater., vol. 39/6, pp. 1025-1039, 1991