Abstract:
The invention relates to a honeycomb, particularly a catalytic converter substrate, with a honeycomb structure comprising a large number of ducts running in the longitudinal direction of the honeycomb, through which a fluid can flow, where the honeycomb displays structured foils arranged one above the other that are arranged to form plane or curved foil layers, and at least one stiffening element, introduced into the honeycomb structure, that extends essentially parallel to the foils, at least in parts. In order to create a honeycomb that displays sufficient stability with high resistance to thermal shocks, that permits the most favourable possible design of the flow ducts in terms of flow and that is simple and inexpensive to manufacture, it is proposed that stiffening elements be introduced into the honeycomb, the dimensions of which transverse to their longitudinal direction are small compared to the dimensions of the honeycomb structure in this direction.

Description:
FIELD OF THE INVENTION 
     The invention relates to a honeycomb, particularly a catalytic converter substrate, pursuant to the generic part of Claim  1  and a process for its manufacture. 
     BACKGROUND OF THE INVENTION 
     A honeycomb of this type is known from DE 27 33 640, which consists of an alternating arrangement of corrugated and plane foil layers. The flow ducts, which display a sinusoidal or triangular cross-section, are, however, unfavourable in terms of the catalytic function, as the gussets of the ducts are virtually ineffective, particularly in the case of laminar flow. In addition, the properties of the honeycomb under fluctuating temperature conditions are unfavourable because the large-area soldering of the plane and corrugated foils results in a very stiff honeycomb structure. As a result, however, local and temporal temperature fluctuations cannot be adequately balanced out, as a result of which the geometry of the flow ducts is subject to irreversible changes and cracks can occur in the cell walls, this being intensified by vibratory stresses on the honeycomb. The service life of honeycombs of this kind is thus in need of improvement. 
     In order to increase the stability of the honeycomb, it is moreover known from EP 0 245 738 that rigid bearing walls extending into the honeycomb are provided. The manufacture of honeycombs of this kind is, however, relatively complex, as the foil layers have to be cut through for this purpose. In addition, the fastening of the thin foils to the comparatively thick, rigid bearing walls presents a problem. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to create a honeycomb that displays sufficient stability with high resistance to thermal shocks, that permits the most favourable possible design of the flow ducts in terms of flow and that is simple and inexpensive to manufacture. 
     According to the invention, this object is solved by a honeycomb with the features of Claim 1. Due to the fact that the dimensions of the stiffening elements, which run essentially parallel to the foil layers, transverse to their longitudinal direction, is small compared to the dimensions of the honeycomb structure in this direction, mass transport within the flow ducts, and thus also the effective reaction cross-section of the ducts, is virtually not reduced. Unfavourable cross-section geometries resulting from the formation of gussets thus virtually do not occur at all, or only in areas of small volume. In addition, owing to their orientation parallel to the foil layers, the stiffening elements can easily be incorporated into the honeycomb structure during its manufacture. A certain degree of stiffening is already achieved by the minimum distance between adjacent foil layers being limited in the case of structured foils, meaning that foil structures, for example, are supported by the stiffening elements. In particular, the stiffening elements can prevent elongation of the honeycomb structure in a direction perpendicular to the flow ducts or foil profiles, which would lead to the undesirable formation of spaces between foil layers of different elongation and thus to unfavourable vibratory stresses in the honeycomb structure. Moreover, as a result of the stiffening elements introduced, the honeycomb according to the invention can be made up of foils with virtually any desired structure or orientation, as it is no longer necessary to fasten the foil layers to each other. 
     The stiffened areas of the honeycomb structure can have punctiform or locally isolated dimensions or, in the case of stiffening elements of corresponding length, they can form stiffening zones. In all cases, the stiffening elements according to the invention locally fix the foils relative to each other more strongly, thus producing larger areas of the honeycomb structure displaying high flexibility. 
     In order to achieve sufficient stabilisation of the honeycomb structure, it suffices in itself for the length of the stiffening elements, regardless of the direction in which they extend, to be equal to or greater than the transverse dimension of a duct in one direction, e.g. height or width, and for them to bridge a flow duct, for example, i.e. to act on opposite walls of a duct or the housing. Also, the stiffening elements can extend only over several duct diameters transverse to the longitudinal direction of the duct, e.g. 5 to 10 duct diameters, or over the entire width of the honeycomb. In the case of non-isometric or non-isogonal ducts, the stiffening elements can also extend over only part of the duct cross-section amounting to a multiple of the duct dimension in the cross-sectional direction of smaller size, e.g. twice this dimension or more. 
     Advantageously, the dimension of the stiffening elements transverse to their longitudinal dimension is small compared to the dimensions of the flow ducts in this direction, e.g. in the range of {fraction (1/10)} to {fraction (1/50)} of the dimension of the flow ducts in this direction or less, without being limited to these values. The transverse dimension of the stiffening elements can be {fraction (1/100)} to  {fraction (1/1000)} or less of the duct length, for example, if these run transverse or at an angle to the ducts. Accordingly, when using the same material, the width of the stiffening elements can be just  0.5 to 10 times, preferably 1 to 5 times, the thickness of the foils making up the honeycomb structure, without being limited to these values. 
     If, for example, a honeycomb is available which has a flow duct length of 100 mm and a flow duct diameter of 1 mm, strip-like stiffening elements with a width of several millimeters and/or stiffening wires with a diameter of several hundredths to several tenths of a millimeter arranged transverse to the flow ducts can be provided. If the stiffening elements are arranged in the longitudinal direction of the ducts, their width can be in the range of 0.01 to 0.5 mm, preferably 0.003 to 0.2 mm. It goes without saying that, given corresponding honeycombs with larger duct diameters, which can easily also be in the region of approx. 1 cm or more for corresponding applications, the stiffening elements can display correspondingly larger diameters or widths. 
     It is also possible for several stiffening elements to be assigned to one foil or one pair of foils or several adjacent foils. 
     The stiffening elements preferably extend over the entire honeycomb structure in their longitudinal direction. 
     The stiffening elements are advantageously designed to be elastically deformable under operating conditions, perpendicular to their longitudinal direction, particularly in the direction of the flow ducts. 
     The stiffening elements can run between adjacent foil layers, although they can also pass through profiled foils or be woven into plane foils, and/or connect adjacent foils to each other. 
     In their longitudinal direction, the stiffening elements are advantageously connected to the foil layers and/or the housing in a manner capable of absorbing tensile forces, e.g. by means of suitable jointing techniques, such as welded connections, positive, frictional and/or material connections. However, connection of the stiffening elements to the foil layers, in particular, can also be achieved by coating with a ceramic material required to produce a catalytic coating. 
     In order to achieve frictional connection of the stiffening elements to the foil layers, the stiffening elements can be woven into the foil layers, particularly connecting two adjacent foil layers in the process, or be clamped in corresponding folds in the foils. Areas of the foils can be notched out to this end, or the stiffening elements can be inserted into the folds of connecting webs located at the face ends of the foils. Correspondingly, the structured areas, such as the foil corrugations, can also be provided with notched tabs or projections running in the longitudinal direction of the ducts, these being arranged one behind the other, possibly at an offset height, and forming a lead-through for wires or the like running parallel to the flow ducts. 
     However, an increase in the dimensional stability of the honeycomb is already achieved if the stiffening elements loosely support the foils or are loosely passed through one or more foils, e.g. by providing an appropriate profile. 
     Particularly if they are located at the level of a foil layer or between the foil layers, the stiffening elements can also be connected to each other by way of additional stiffening or connecting struts, which can run essentially parallel to the foil layers and/or perpendicular to them. In this way, extended systems of stiffening elements can be constructed, which can extend in two or three dimensions over relatively large areas of the honeycomb or the entire honeycomb. Correspondingly, in order to stiffen the honeycomb structure, expanded-metal layers or wire mesh can also be inserted between the foil layers, these particularly being inserted into indentations of foil layer profiles and possibly secured there in a manner preventing movement. 
     Advantageously, the stiffening elements are connected to the foil layers under axial pretension. This makes it possible not only to increase the stiffness of the honeycomb, but also to calibrate the geometry of the flow ducts or the dimensions of the honeycomb. In this context, the stiffening elements can be connected both to the housing of the honeycomb and to existing partition walls, these being designed as rigid bearing walls or as elastically deformable partition walls composed, for example, of fold areas of the foil layers. The fold areas can be of U, V, W or Z-shaped design, without limitation, in which context individual or several legs of the fold are joined together in order to construct the wall. The folded design of the partition walls means that they are flexible and, at the same time, that they expand under compression, this resulting in good temperature shock resistance. 
     If the stiffening elements running through the honeycomb structure are pretensioned, the pretensioned area of the corresponding foils can be grouped in sections. This makes it possible, for example, to provide block-type areas of high pretension and thus high stiffness within the honeycomb structure that are separated by areas of low pretension and thus increased deformability. 
     This kind of design with pretensioned areas within the honeycomb structure can be produced by the fastening elements on the foil layers for fastening the stiffening elements only being provided in some areas. Thus, for example, the connecting webs of foil strips folded in zigzag fashion can be removed in some areas of the lateral edge zones of the honeycomb, thus providing a zone of increased extensibility adjacent to the housing and producing a honeycomb with particularly favourable mechanical properties. 
     The cross-sectional geometry of the flow ducts can be adjusted by pretensioning stiffening elements fastened to the foil layers. 
     According to another advantageous configuration, the stiffening elements can be formed from partial sections of the foil layers. 
     This is particularly the case if the honeycomb is formed by a foil strip folded in zigzag fashion, where the individual foil layers are connected to each other by web-like connections in the area of the folds. In this context, the connecting webs in the area of the folds can be produced by way of punched tabs, where the fold line of the adjacent folded sections of the foil strip runs through the punched tab. The punched tab can be designed in such a way that a web running along the fold line remains, meaning that the wall areas of a flow duct that are opposite each other along the fold line can be connected to each other. In order to permit corrugation of the foil strip, the web running through the cross-section of a flow duct can be shortened by bending or folding appropriately in its longitudinal direction. 
     In addition, or as an alternative, to the configurations described, it is also possible to provide stiffening elements which are designed as inserts that can be inserted into the face ends of the ducts. The inserts, the outside contours of which can be adapted to the cross-sectional geometry of the ducts, prevent adjacent foil layers from sliding into each other, without substantially affecting the flow cross-section of the ducts. The inserts can be designed in such a way that they display areas that protrude from the face ends of the honeycomb when inserted and act as flow-guiding devices. These areas, which can be integrally moulded, can enable lateral inflow into the inlet area of the honeycomb and/or be arranged at an angle to the longitudinal direction of the honeycomb. 
     The inserts can be designed as separate components and advantageously extend over the width, possibly also over the height, of several ducts, or over the entire width and/or height of the honeycomb structure. Stiffened areas of the honeycomb structure can alternate with areas of increased extensibility in this way. By varying the arrangement of inserts at both face ends of the honeycomb, it is possible, for example, to obtain twistable honeycombs, which may be advantageous for certain fields of application. If the inserts extend over several ducts, they can be arranged both parallel and perpendicular or at an angle to the foil layers. 
     The inserts can also be integrally moulded to the foil layers and produced, for example, by appropriate folding of foil sections. The inserts can be designed to suit the requirements by shaping the foil ends or by punching. 
     Particularly if the stiffening elements are designed as inserts, the flow cross-sections can easily be varied over the length of the flow ducts. For instance, the inserts can be profiled in such a way that the flow ducts have a smaller diameter in the turbulent inlet area of the ducts than in the duct areas with laminar flow inside the honeycomb. In this context, the inlet area is advantageously divided into a large number of flow ducts, so that the total of the flow cross-sections of the ducts in the inlet area is roughly equal to the flow cross-section of the duct in the middle area of the honeycomb. 
     As an alternative, or in addition, to the configurations described above, the stiffening elements can also be designed as webs running along the flow ducts. In this context, the webs display a width which is substantially smaller than that of any profiles provided for producing the honeycomb structure, e.g. one-quarter or one-eighth of the same, or less. In the case of webs consisting of two side walls, both fold legs can, in particular, contact each other, advantageously over virtually the entire height, or only be such a distance apart from each other that the respective coating compound used does not penetrate the space between the legs. 
     The webs can extend over the entire height of the ducts or, advantageously, only over part of the same, so that gas exchange between the constituent ducts is possible. The webs can also display notched tabs, by means of which adjacent foil layers are supported or which serve to increase the catalytically active surface area. The notched tabs or the webs themselves can be used to fasten or support further stiffening elements, such as wires running transverse to them. The webs can, in particular, be designed as fold webs of the foil layers, there being beaded areas at the ends or in the middle area of the fold webs for additional stabilisation, these counteracting any spreading of the fold webs. Wires or the like can additionally be inserted in the fold webs. Fold webs may, for instance, protrude from the foil layers in a direction inclined or perpendicular to the major plane of the foil layers or at least substantially in parallel to the foil layers, which might be achieved for instance by folding the webs in a lateral direction generating double or multiple folded foil layer sections heving for instance 3 to 10 or even more folds. 
     In the case of structured foil layers where partial areas of one and the same foil layer are in punctiform or linear contact, the honeycomb can be stabilised by establishing punctiform or linear connections between contacting foil areas in the areas of contact. This has an influence on the longitudinal expansion characteristics of a single foil layer, where the connections can be at a distance from the top side of the structured foil layer, forming expansion legs. The foil layer as a whole thus acts as a stiffening element capable of absorbing tensile and/or compressive forces, which is produced by joining individual preformed sections of the foil layers and extends over individual or several flow ducts or results in layer doubling. Correspondingly, individual sections of the foil layers, e.g. in web form, can be notched out and joined to each other or with the foil layers in a manner capable of absorbing tensile forces in order to produce stiffening elements. 
     The joints within an individual foil layer can be produced by any desired jointing techniques, e.g. by spot welding, and, in particular, positive connections can be produced by means of punched and folded foil areas, the face ends of which engage an adjacent flow duct or apertures correspondingly provided for this purpose, or which are non-positively connected to the wall of a flow duct. 
     In addition to the stiffening elements inserted according to the invention, it is also possible to provide stiffening elements of a wide variety of configurations which extend in a perpendicular direction relative to the foil layers and connect two or more foil layers. Perpendicular direction is generally intended to mean a direction possessing a perpendicular direction component and including an angled course, e.g. at an angle of 45° relative to the foil layers. 
     The perpendicular stiffening elements can be designed as rigid bearing walls, although they are preferably of elastically deformable design, where one-dimensional stiffening elements can be provided, in the form of wires, strips, interconnected foil folds or the like, or two-dimensional elements in the form of deformable outer or partition walls, which particularly consist of folded sections of the foil layers. The stiffening elements running parallel to the foil layers can be fastened to the stiffening elements running vertical to the foil layers in a manner capable of absorbing tensile forces, or they can be loosely passed through or by these. 
     According to another advantageous configuration, the stiffening elements are located upstream, or in the inlet area, of the flow ducts, i.e. in the area of turbulent flow. The stiffening elements, which can particularly extend transverse to the flow ducts, thus form additional catalytically active surfaces at the same time. In addition, or as an alternative, projecting areas with catalytically active surfaces can also be located in the catalytically particularly effective inlet area by other measures. In particular, the stiffening elements in the inlet area, which can also be designed as strips or wires, can have a larger diameter than in the area of laminar flow. The inlet area reinforced with stiffening elements can also display foil layer sections with free ends which permit lateral inflow of a fluid on one or more sides. 
     If the stiffening elements are located upstream of the face ends of the flow ducts, it has proven advantageous for the distance between the outer edge of the stiffening elements facing away from the flow ducts and the face ends of the flow ducts to be in the range of 0.1 to 3 times the diameter of the ducts. If the foils are connected to each other by fold webs lying on fold lines located upstream of the face ends of the ducts, the same applies to the distance between the fold lines and the face edge of the inlet or outlet apertures of the ducts. This applies regardless of whether or not stiffening elements are located in the fold areas. 
     In order to improve the flow conditions, particularly in the event of angular inflow into the ducts, the duct ends can be of scoop-like design. It is also possible to provide window-like foil folds at the duct ends in order to enlarge the inlet areas with turbulent flow. 
     It goes without saying that the honeycomb according to the invention can be constructed not only from a profiled foil strip laid in zigzag fashion, but also from individual profiled foils, between which non-profiled foils may possibly also be arranged. In particular, individual foil layers can also be arranged one above the other in such a way that the flow ducts are produced by the profiles of opposite foil layers. The stiffening elements can also be provided within the honeycomb at a distance from the face ends of the same. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An example of the invention is described below and explained on the basis of the figures, wherein: 
     FIG. 1 is a schematic representation of a cross-section perpendicular to the longitudinal axis of a honeycomb according to the present invention. 
     FIG. 2 a  is a cut apart perspective view showing a cross-section of a honeycomb made of trapezoidally structured thin foil layers. 
     FIG. 2 b  is a similar cut-away perspective view showing a cross-section of a honeycomb made of single-corrugated or sinusoidally structured thin foil layers. 
     FIG. 3 is a similar cut-away perspective view showing a section of a honeycomb formed of a foil strip laid in zigzag fashion with corrugated foil layers. 
     FIG. 4 is a partial perspective view showing a section of the FIG. 3 honeycomb structure in only partially folded condition. 
     FIG. 5 shows a section of an originally smooth, thin foil strip fro manufacturing a honeycomb structure according to the present invention. 
     FIG. 6 is a vertical longitudinal section through a honeycomb according to the present invention. 
     FIG. 7 a  is a perspective view showing a foil layer having rectangular punched holes for manufacturing a honeycomb according to the present invention. 
     FIG. 7 b  is a perspective view of a foil layer according to FIG. 7 a  being folded along the fold lines. 
     FIG. 8 a  is a top view on a foil layer for manufacturing a further embodiment of a honeycomb according to the present invention. 
     FIG. 8 b  is an enlarged perspective view of the foil layer according to FIG. 8 a.    
     FIG. 8 c  is a perspective view of the part of the foil layer according to FIG. 8 b.    
     FIG. 8 d  is a perspective view of a part of the foil layer according to FIG. 8 a , being folded. 
     FIG. 8 e  is a perspective view of a zig-zag folded foil layer, according to FIG. 8 a.    
     FIG. 8 f  is an enlarged view of a part of FIG. 8 e.    
     FIG. 8 g  is an enlarged view of a part of the honeycomb structure according to FIG. 8 e  having notched tabs. 
     FIGS. 9 and 9 a  illustrate another version wherein stabilization of the honeycomb structure is provided with wires. 
     FIG. 10 is a schematic illustration of how the cross-section of the flow ducts can be calibrated or altered using pre-tensioned wires. 
     FIG. 11 shows profiled foil layers and wires clamped between webs. 
     FIG. 12 a  is a diagrammatic, cross-sectional view of a honeycomb according to the invention showing two different arrangements of foil layers. 
     FIGS. 12 b  and  12   c  are enlarged views of the two different foil layer arrangements according to FIG. 12 a , wherein the arrows designate the middle plane of the honeycomb according to the figures. 
     FIGS. 13 a  and  13   b  are schematic views of the face side (FIG. 13 a ) and lateral side (FIG. 13 b ) of a further embodiment of foil layers for manufacturing a honeycomb of the invention. 
     FIGS. 14 a  and  14   b  are a partial face views of a further embodiment of the honeycomb of the invention (FIG. 14 a ) and a side view of foil layers in a partially assembled state for manufacturing a honeycomb of the invention. 
     FIG. 14 c  is a side view of a partially folded foil layer for manufacturing a honeycomb as shown in FIG. 14 b.    
     FIG. 15 is an elevational view, partly broken away of a configuration with a separate insert. 
     FIGS. 16 and 17 are perspective views of other configurations. 
     FIG. 18 a  is a view on a part of a foil layer for manufacturing a honeycomb of the invention. 
     FIG. 18 b  is a perspective view of a part of a foil layer of FIG. 18 a , being folded. 
     FIG. 18 c  is an enlarged view of the foil layer according to FIG. 18 b  in a partially folded state. 
     FIG. 19 a  is a schematic cross-sectional view of a further embodiment of a honeycomb of the present invention. 
     FIG. 19 b  is a perspective view of a part of the honeycomb according the FIG. 19 a  and FIG. 19 c  is an enlarged perspective view of a part of FIG. 19 b.    
     FIG. 19 d  is an enlarged view of a part of FIG. 19 a.    
     FIG. 20 a  is a top view on a foil layer for manufacturing a further embodiment of a honeycomb of the present invention. 
     FIG. 20 b  is a perspective view of a zig-zag-folded foil layer according to FIG. 20 a.    
     FIG. 20 c  is a perspective view of a modified foil layer of FIG. 20 b  having additional folded webs and additional stiffening elements. 
     FIG. 20 d  is a schematic face end view of the structure of the folded foil layer according to FIG. 20 b.    
     FIG. 20 e  is a top view of a modified form of a foil layer according to FIG. 20 a.    
     FIG. 21 a  is a schematic perspective view of a further embodiment of a honeycomb according to the present invention. 
     FIG. 21 b  is an enlarged face end view of the honeycomb structure according to FIG. 21 a.    
     FIG. 22 is a perspective view of another arrangement. 
     FIG. 23 a  id a cross-sectional side view of a further embodiment of a honeycomb being arranged in a housing. 
     FIG. 23 b  is an enlarged view of a part of FIG. 23 a.    
     FIG. 24 a  is a perspective view showing a foil layer for manufacturing a further embodiment of a honeycomb according to the present invention. 
     FIG. 24 b  is a perspective view of a foil layer according to FIG. 24 a , being folded. 
     FIG. 25 a  is a perspective view of a foil layer for manufacturing a further embodiment of a honeycomb according to the present invention. 
     FIG. 25 b  is a perspective view of the foil layer according to FIG. 25 a , being folded. 
     FIG. 25 c  is a side view of a zig-zag-folded foil layer according to FIG. 25 a.    
     FIG. 26 a  is a schematic face view of a part of a honeycomb according to the present invention. 
     FIG. 26 b  is a perspective view of a foil layer for manufacturing a honeycomb according to FIG. 26 a  in a partially unfolded state and having modified ends of the crests and depressions of the corrugations. 
     FIG. 26 c  is a schematic side view of a part of a honeycomb built by a foil layer according to FIG. 26 b  in a partially unfolded state. 
     FIG. 27 a  is a perspective view of a foil layer for manufacturing a honeycomb according to the preset invention with additional stiffening elements. 
     FIG. 27 b  is an enlarged view of a part of FIG. 27 a.    
     FIG. 27 c  is a side view of a honeycomb being manufactured by staking foil layers according to FIG. 27 a.    
     FIG. 28 a  is a schematic side view of a further embodiment of a honeycomb according to the present invention. 
     FIGS. 28 b ,  28   c  and  28   d  are perspective enlarged views of parts of the honeycomb of FIG. 28 a  shown with stiffening elements (FIGS. 28 b  and  28   c ) or without stiffening elements (FIG. 28 d ). 
     FIG. 29 a  is a schematic cross-sectional view of a further embodiment of a honeycomb according to the invention being arranged in a housing. 
     FIG. 29 b  is an enlarged view of a part of the honeycomb according to FIG. 29 a.    
     FIG. 29 c  is a schematic side view of the zigzag folded foil layers of the honeycomb of FIG. 29 a  in a partially unfolded state. 
     FIG. 30 shows stiffening elements in the design of multiple folded single layered foils. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows, in a schematic representation of a cross-section perpendicular to the longitudinal axis of the honeycomb, a cuboid honeycomb with a honeycomb structure  11  consisting of a single constituent honeycomb in a housing  10 . The honeycomb structure consists of a thin foil strip with single-corrugated, structured foil sections  13  which are stacked one above the other in plane fashion. At the two longitudinal sides of the constituent honeycomb, free ends  14  of sections  13  are, by bending through an angle of approx. 90 degrees, built up into outer wall areas  22 , ends  16  of which are firmly connected to housing  10  by means of beads  17 . 
     Wires  30  are inserted between every second foil layer in order to stabilise the honeycomb structure, their ends  30   a  being firmly integrated in the two lateral wall areas of the constituent honeycomb. Moreover, wires  33 , which run diagonally, are woven into the honeycomb structure in order to stabilise the honeycomb structure. 
     FIG. 2 a  shows a cross-section of a section of a honeycomb made of trapezoidally structured thin foil layers  13   a, b, c  stacked one above the other in plane fashion, the fold ends of which are connected to each other by means of webs  34 . Opposite foil layers form hexagonal flow ducts. The honeycomb structure is stabilised by wires running parallel to foil layers  13   a, b, c . Wire  30 , located in the fold line, is clamped in fold webs  34 , while wire  31 , part of the way up the foil layer, is loosely guided through the foil through holes  35   a  and wire  32 , at the top edge of foil layer a, is woven into the foil with the aid of holes  35   b , with a difference in height at each corrugation, and fixed to it by a frictional connection. The individual wires  30 ,  31 ,  32  can be provided in the honeycomb structure either alternatively or simultaneously, and wires  31 ,  32  can also run at an angle to the corrugations. 
     FIG. 2 b  shows a corresponding section of a honeycomb structure made of single-corrugated or sinusoidally structured thin foil layers stacked one above the other in plane fashion, where stabilisation is accomplished by wires  30 ,  31 ,  32 , as in FIG. 2 a.    
     FIG. 3 shows a section of a honeycomb consisting of foil strip  12   a , laid in zigzag fashion, with corrugated foil layers  13   a ,  13   b  and  13   c . The stiffening element is designed as a fold web  37 , integrally moulded on the foil layers, which projects outwards from the face ends of flow ducts  38  and runs parallel to the centre plane of flow ducts  38 . Fold web  37 , which is separated from corrugated wall  39  of the flow duct by cutting or punching, displays a fold  36  at the level of each flow duct  38  such that the fold web is shortened to the diameter of flow duct  38 . Fold  36  can be formed together with the corrugation of foil layers  13   a ,  13   b  and  13   c , or starting from a curved fold web. Additional stabilising wires  30  are clamped in fold web  37 , which extends over the entire honeycomb or part thereof. 
     FIG. 4 shows a section of the honeycomb structure according to FIG. 3 in only partially folded condition, where the foil tuck for forming fold  35  can be seen. 
     FIG. 5 shows a section of an originally smooth, thin foil strip for manufacturing the honeycomb according to the invention. In order to pre-shape thin foil strip  12   a , grooves  41  are stamped into the strip in its longitudinal direction, either hot using stamping rolls, or by means of laser beams. Fold lines  40  run perpendicular to these in order to permit folding of the foil strip in zigzag fashion in order to produce a stack. At the level of fold lines  40 , narrow webs  42  of width b are formed by punched holes  43  of dimension a in the longitudinal direction of the strip. Dimension a determines the inlet apertures of the individual ducts of the finished honeycomb structure as regards their height or the maximum width of stiffening elements inserted between the webs. Adjacent to foil sections  12   a , areas can also be left in the punched holes, by means of which stiffening elements running perpendicular or parallel to the foils can be fastened in positive fashion. 
     FIG. 6 shows a vertical longitudinal section through a honeycomb according to the invention. When placed one on top of the other, pairs of channels in foil strip  12   a  form flow ducts, through which an exhaust gas can flow in flow direction S. Narrow, thin strips  34   a , which are held by webs  42 , are inserted in fold lines  40  at the inlet side E and the outlet side A of the honeycomb structure. Webs  42  are located upstream of inlet openings E and their cross-section is widened by the inserted strips, this resulting in a catalytically more effective arrangement. 
     As illustrated on the left in FIG. 7, fold web  42 , which connects foil layers  13   a ,  13   b , can have a certain height h, so that foil layers  13   a ,  13   b  are some distance apart. In order to prevent buckling of web  42 , an appropriately dimensioned strip or profiled section can be inserted between foil layers  13   a ,  13   b , and it is also possible to provide notched tabs on the foil layers that rest on the opposite foil layer. 
     According to the right-hand side of FIG. 7, the structure according to the left-hand side of FIG. 7 can be produced by rectangular punched holes  43   b  and the formation of fold lines or deformation zones, followed by folding and compression of the foil layers. 
     According to FIG. 8, a honeycomb with essentially rectangular flow ducts  38  can be constructed by appropriately profiling a foil strip  12   a . In order to produce walls  45  in the form of double folds, which form the lateral borders of flow ducts  38 , fold lines  41  are made in the foil strip. Webs  42 , which connect foil layers  13   a ,  13   b  and  13   c  and are produced by rectangular lead-throughs  43  in foil strip  12   a , are cut on both sides of the fold lines, resulting in foil sections  44 , which are connected to webs  42  on one side and are folded into the flow duct in order to stabilise the inlet area. At the same time, foil sections  44  can serve to fasten wires guided parallel or perpendicular to foil layers  13   a ,  13   b  and  13   c . Fold lines  46   a ,  46   b ,  46   c ,  46   d  and cuts  47  are made in the foil layers parallel to lead-throughs  43  and a distance from them. Foil sections  48   a  are folded over in order to shorten fold webs  45  and produce folds  48   b , with the result that foil sections  49  are angled and cuts  47  are enlarged to form triangular gaps (see middle of FIG.  8 ). 
     As shown at the top of FIG. 8, foil layers  13   a ,  13   b ,  13   c  are positioned on top of each other with webs  45  arranged one above the other in parallel alignment. Angled foil sections  49  then reach around web  45  of the foil layer below, so that adjacent foil layers are connected to each other in positive fashion by stiffening wires  31 , in which context wires  31  are located above foil sections  49  and guided through cuts  47  and the upper area of adjacent web  49 . 
     In addition, cuts are made on the top side of webs  45 , forming tabs  45   b  which are inserted into the fold of web  45  above and fixed in it in non-positive fashion. As a result, partition walls  45   a  are created within the honeycomb structure, these acting as stiffening zones and additionally stabilising the honeycomb structure. 
     Furthermore, the base of flow ducts  38  is provided with notched tabs  45   c , which serve the exchange of gas between adjacent flow ducts and/or to provide support at the base of the flow duct below. 
     FIG. 9 illustrates another version of the stabilisation of the honeycomb structure with wires  50 , which simultaneously connect adjacent foil layers  51   a ,  51   b  to each other. For this purpose, cuts are made at intervals in some of the corrugations  52  of the foil layers, this resulting in alternating upward-pointing areas  55  and downward-pointing areas  56  following compression of these corrugations. After folding the foil strip, areas  55  of adjacent foil layers, which engage areas  56 , are arranged in line with each other, as shown in FIG. 9 a , thus producing a duct  57 , through which wire  50  can be inserted in a manner connecting adjacent foil layers to each other in positive fashion. Like the foil strip, wire  50  is folded in zigzag fashion, although it can also extend over only one foil layer, if appropriate. 
     In addition to wires  50  and running perpendicular to them, wires  59  are provided, which are located at the level of the cuts bordering areas  55 ,  56  and laterally bordered by these. Moreover, wires  59  are connected to fold legs  53 , which laterally border foil layers  51   a ,  51   b  and are connected to each other by jointing, thus resulting in a closed side wall. In addition, wires  59  are pretensioned in order to increase the stiffness of the honeycomb, as a result of which calibration of the honeycomb can be achieved at the same time by setting an appropriate pretension of wires  59  by way of localised electric heating of the honeycomb via electrodes  60 . Moreover, wires  54  are also provided, which run perpendicular to the foil layers, penetrate the compressed corrugations and are connected to wires  59 . 
     FIG. 10 illustrates how the cross-section of the flow ducts can be calibrated or altered by means of pretensioned wires  61 , which are drawn through a profiled foil layer  62 . In the practical example shown, wire  61  is drawn through corrugated foil layer  62  half way up and fixed to the end of the foil layer. By exerting a tensile force on wire  61  or by compressing foil layer  62  in the longitudinal direction of the wire, the corrugated shape of the foil layer represented by the solid line can be transformed into shape  63 , represented by the broken line and illustrated alongside, wire  61  being pretensioned in this context. Correspondingly, stiffening elements can also be provided that pass through the foil layers transverse to their corrugation at several different levels, e.g. at ¼, ½ and ¾ of the height of the same, as a result of which different duct cross-sections can be produced. 
     According to FIG. 11, given foil layers profiled according to FIGS. 2 a ,  2   b ,  10  and  11 , wires  65  can be clamped between webs  64 , where the ends of wires  65  are integrated in the side walls made up of downward folds in end areas  66  of the foil layers in a manner capable of absorbing tensile forces. The end areas of foil layers  66  are provided with notched tabs  68 , which serve to fasten the honeycomb to housing  67 . Instead of wires  65 , it is also possible to clamp strips in fold webs  64 , which can be fixed to housing  67 . 
     According to FIG. 12 a , a middle partial area of foil layers  70  can have a vertical offset, where the offset lines  71  of adjacent foil layers can be located in a plane  72  that preferably runs parallel to the direction of flow. If, as illustrated in the figure, the vertical offset amounts to the height a of a foil layer, wires  73  run at the level of webs  74 , in which they are clamped in a manner capable of absorbing tensile forces, over half the width of a foil layer and loosely between adjacent foil layers  70   a ,  70   b  over the other half of the foil layer. In this context, the ends of wires  73  are integrated in the side walls made up of the foil ends. The width of the honeycomb can be calibrated or altered by applying a tensile force to wires  73 . In this context, either only the right-hand half or the left-hand half of the foil layer can be specifically calibrated or deformed by applying tensile forces only to every other wire  73 . 
     As illustrated in FIG. 12 b , the vertical offset of the foil layers can also be twice the foil height b, this increasing the length of expansion legs  76 . In this way, the areas of the honeycomb separated by the expansion legs can be isolated from each other as regards forces. 
     FIG. 13 (left) shows stiffening elements in the form of inserts  80 , which are integrally moulded on the two ends of foil layers  81   a  and can be inserted into the flow duct formed by foil layers  81   b ,  81   c  and fixed there by means of a frictional connection. FIG. 13 (right) shows a front view of a foil layer of this type. The lateral end areas of foil layer  81   a  are provided with downward folds  82 , which can be connected to corresponding downward folds of foil layers located above or below them and fixed in beads of the corresponding housing. 
     FIG. 14 shows a configuration in which the honeycomb is constructed of identical foil sections  90   a,b,c , each of which displays two corrugated sections  92 ,  93 , which are connected to each other via a bending area  91  and on the free ends of which inserts  95  are integrally moulded via connecting webs  94 . In folded condition, the two inserts  95  are located on the same side of foil section  90 , meaning that, when foil sections  90  are arranged so as to be rotated through 180° relative to each other, the inserts of the adjacent foil layers can engage the face end of flow duct  96  formed by foil section  90 . 
     FIG. 15 shows a configuration with a separate insert  100 , the height of which extends over several flow ducts of foil strip  101 , which is laid in zigzag fashion, and can engage the face ends of these. A corresponding insert can also be inserted at the opposite face end of the laid foil strip. In the practical examples illustrated, the contour of the inserts corresponds to that of the flow ducts, although this is not necessary. 
     FIG. 16 shows a configuration in which, in addition to stiffening wires  111  located between the individual foil layers  110 , additional stiffening webs  112  are provided in the form of doubled foil areas, which simultaneously act as flow ribs, where the height of the webs is roughly half the height of flow paths  113 , indicated by broken lines. The foil layers forming rectangular flow ducts are arranged in congruent fashion relative to each other, where wires  111  prevent slipping of the troughs of one foil layer into the depressions in the foil layer below. The foil layers can also be arranged in the opposite direction relative to each other, meaning that the stiffening webs of a first foil layer are opposite to those of the second foil layer and flow ducts of twice the height are formed. 
     FIG. 17 shows a foil strip  115 , laid in zigzag fashion, with heart-shaped flow ducts  116 , which are divided by stiffening webs  117 , formed by doubling the foil layer. As indicated by the broken circles, this combines groups of three flow paths  114  into a single, larger flow path, enabling gas exchange in the process. In this context, adjacent foil layers  118 ,  119  are connected to each other by connecting webs  120 . For additional stiffening, corner areas  121  of stiffening webs  117  are beaded, and beaded areas  122  are provided in the area of the doubling of the contacting side walls of adjacent flow ducts by making cuts. In addition, stiffening webs  117  display notched tabs  123 , which reach into the flow duct and increase the catalytically active surface in the flow ducts. The beading of stiffening webs  117  and of areas  122  produces isolated stiffening elements which prevent elongation of the honeycomb structure in a direction transverse to the flow ducts and parallel to the foil layers. In this context, the beads can be provided both at the face ends of the flow ducts and in their interior. 
     FIG. 18 shows a foil strip  130 , laid in zigzag fashion, where the height h of the virtually rectangular corrugations  131  is a multiple of the width b of the same (approx. 4:1). The corrugations are laterally offset by a fraction of the width of the same relative to each other and open in the direction of the opposite foil layer. The individual corrugations, which are inclined relative to the plane of the foil layer, are connected to each other by connecting webs  132  (see enlarged section on the right in FIG.  18 ). FIG. 18 (bottom) shows the foil strip before being folded. The asymmetrical shape of punched holes  133  relative to the longitudinal direction of foil strip  131  leads to an offset x of the corners of the parallelogram and brings about an offset of the upper or lower vertex  134 ,  135  of the foil corrugations, meaning that the lower vertex of a foil corrugation is located above the open side of the flow duct of the foil layer below. Offset y defines the face-end inclination or the axial offset of the upper vertex relative to the lower face edge of the foil corrugations. In all, this thus results in a configuration where gaps extending over the entire width of the honeycomb are formed between opposite corrugations, meaning that an exchange of medium also takes place over the entire width of the honeycomb through the adjacent gaps. As the punched holes along a fold line can generally also be of different design, the width of the honeycomb over which an exchange of medium is possible in the transverse direction can be adjusted. Stiffening wires not shown in the illustration are inserted between the foil layers. 
     FIG. 19 shows a foil strip  140 , laid in meandering fashion, with virtually rectangular corrugations, where lower vertices  141  of a foil layer are located above the open sides  142  of the foil layer below. At certain intervals, the foil layers display a vertical offset  143 , forming legs, which extends over an integral multiple of the height h of the corrugations and where the legs can be connected to form partition walls. Stiffening wires  144 , arranged perpendicular to the corrugations, lie on flattened areas  145 , which are produced by slits cut or grooves impressed into the foil layers, so that the lower vertex  141  of a foil layer is located below the upper vertex  146  of the foil layer below. Connecting wires are passed through doubled foil areas  147 , which are produced by notched foil sections on the face ends or in the interior of the flow ducts. Furthermore, web-like notched tabs  148 , running in the longitudinal direction of the flow ducts, are provided, which rest on the opposite side wall of the flow duct and simultaneously permit the exchange of gas through lead-throughs  149 . 
     FIG. 20 shows another configuration of a honeycomb in a section displaying an asymmetrical cross-section of flow ducts  151  in relation to the plane of foil layers  150 . To this end, foil strip  152  is provided with rectangular punched holes  155  in the area of fold lines  154 , the fold line running along their diagonal. For additional stiffening, fold webs  156  are provided, which run in the longitudinal direction of the flow ducts and whose notched tabs  157  rest on stiffening wires  158 , in place of which layers of expanded metal or wire mesh can also be provided. Moreover, stiffening wires  159  are clamped in the connecting webs of the foil layers. Given an appropriate height of the fold webs, these can also rest directly on the wires. 
     FIG. 21 shows a section of a honeycomb with a foil strip  251 , laid in meandering fashion, and expanded-metal layers  258  extending transverse to it, the ends of which that project beyond the individual foil layers  257  are folded around the respective foil layer ends and engage the adjacent flow duct. End areas  262  of expanded-metal layers  258  then stand perpendicular to the respectively opposite foil layers  257  and either rest on them or support them. For purposes of stabilisation, each of the foil layers of the foil strip displays two stiffening ribs  253 ,  254  running transverse to the direction of flow indicated by the arrows, the length of fold webs  255  being large compared to the distance between foil layers. In order to form the upward and downward-pointing ribs  253 ,  254 , foil sections  256 , located in the area of the fold, are laterally folded outwards, simultaneously serving to fasten the honeycomb to a housing (not shown). 
     Expanded-metal layers  258  display sections  259 , extending in the longitudinal direction of the flow ducts and resting on the foil layers at the face end, which stand vertically on the foil layers and support the foil layer above. Sections  259  are provided with lateral bulges  260  to increase the stiffness in the event of compressive forces acting vertically on the foil layers, and are guided through ribs  253  without play in vertical gaps. Foil sections  259  can also display areas of lower height, which can be arranged between bulges  260  and enable an exchange of fluid. Foil sections  259  are connected to each other by intersecting connecting webs  261 , which are integrally moulded together at the points of intersection, can be produced by making cuts in the foil layers and also permit an exchange of fluid in the transverse direction. Instead of using individual expanded-metal layers, these can, like foil layers  257 , be connected to each other to form an endless expanded-metal strip that is laid in meandering fashion. The expanded-metal layers simultaneously serve to increase the active catalyst surface. 
     According to FIG. 22, expanded-metal layers  287  can be inserted between the individual foil layers  277  with corrugated profile, this simultaneously allowing the distance between foil layers to be adjusted as required. In this context, the one-piece expanded-metal layers display elongated sections  289  in the form of narrow strips arranged perpendicular to the principal planes of the foil layers, as well as connecting webs  291 , which are connected to each other via points of intersection. The points of intersection are inserted in recesses  290  in the foil layers in a manner preventing movement and can be additionally fastened here, e.g. by soldered connections or by stiffening wires running vertical to the foil layers. The expanded-metal layers can be fixed on lateral fold webs of the foil layers. Corresponding to the foil strip, the expanded-metal layers can also be designed as a strip laid in meandering fashion. Profiles  289 , around which flow is possible, simultaneously improve pollutant conversion in the ducts. 
     FIG. 23 shows a honeycomb  160 , which is provided with a gas inlet  162  and a gas outlet  163  in a housing  161 . In this context, inflow into flow ducts  164  of the honeycomb is at an angle. In the inlet and outlet area, the individual foil layers of the honeycomb are (see detail view) connected to each other by separate inserts  165 , the free ends of which are at the same time shaped by the provision of bevelled areas  166  in such a way that the inlet aperture into the flow ducts in the direction of flow is greater than the cross-section of the flow ducts in a plane perpendicular to their longitudinal direction. This makes it possible to increase the catalytic efficiency of the honeycomb in the area where the gas to be purified flows into the flow ducts and to reduce pressure losses caused by turbulence in the outlet area. At the same time, the inserts stabilise the inlet and outlet areas of the honeycomb, e.g. by increasing the wall thickness, whereas the ducts retain their larger cross-section unchanged in the middle area of the honeycomb, e.g. with gaps extending over the entire width which, although they are less stable, permit an unobstructed exchange of fluid transverse to the honeycomb. 
     FIGS. 24 and 25 illustrate that, given an appropriate shape of punched holes  170  and cuts  171 , which are of rhombic and V shape in the practical example shown, end areas  169  of the individual flow ducts can be structured in such a way that they display a different cross-section or contour at the face end than the areas of the flow ducts a distance away from the ends of the honeycomb or the envelope of the face area of the entire honeycomb, this being the case if, for example, the face ends of the flow ducts do not lie perpendicular to the longitudinal direction of the flow ducts. The flow conditions upstream of the inlet areas into the flow ducts, which are defined by the projecting foil sections  172 ,  173 , and also their position, can be defined in this way, meaning that the inlet areas of the honeycomb can be adapted to suit the respective requirements. 
     Generally speaking, means  175  are, as also illustrated in FIG. 25 (bottom), thus provided upstream or downstream of the inlet and/or outlet area of a structural body (cf. also FIGS. 3,  26 ), which project axially from face ends  176  of the honeycomb or structural body (as shown at the bottom of FIG. 25) and bring about deflection of the flow of a medium in relation to the principal direction of flow within the structural body or its longitudinal direction. Flow deflection can, for example, take place in the manner of a macroscopic change in the direction of flow (see FIGS. 22,  26 , for example) or also, for example, in the manner of swirling, as at the front or deflecting edges  175  of the cuts in FIG. 25 (bottom). The means can be separately assigned to each flow duct or flow path. The dimensions of the means in the direction of flow can be small compared to the length of the structural body, e.g. in the region of several times (e.g. 10), or less than, the diameter of the duct or flow path. There is thus a continuous or stepwise change in the flow conditions by structural elements in the inlet area of the structural body, starting from the face-end envelop  177  of the projecting flow deflectors and extending over a certain depth which can, for example, correspond to 0.5 to 10 times (without limitation) the width of the flow ducts or the distance between the core flows, i.e. the flow paths with the highest flow velocities. The means can be integrally moulded on the foil layers, e.g. by the cuts described above, or produced by the axial elongation of structured or plane foil layers, or designed as separate components, e.g. in the form of axially extending wires. The means can be arranged concentrically in relation to the flow ducts or flow paths with the highest flow velocity, or also between these. The axial projection of the flow deflectors refers to the face end or the face-end envelope  178  of the honeycomb, which defines the start of the individual flow ducts or flow paths, which arise from the splitting of the overall flow hitting the structural body into component flows, in the inlet area of the structural body. The projecting means can thus correspondingly also be provided on honeycombs having a conically shaped face end, which can be produced, for example, by telescoping a coiled strip. In particular, the means can, if appropriate, also be realised on structural bodies without stiffening elements according to the invention, and also, if appropriate, on structural bodies with partially or completely unobstructed fluid exchange in one or two transverse directions. The free ends of the projecting areas can enclose and angle of between 150° and 20°, preferably 90° and 30°, with the face-end area lying centrally between them. 
     According to FIG. 26, inlet areas  180  into the honeycomb, in which turbulent gas flow prevails, can be designed in scoop-like form, enlarging the inner deflection radius of the flow paths, in order to enlarge the inlet aperture of the flow ducts, align it approximately perpendicular to the direction of flow indicated by arrows  181  and provide an axial offset. To this end, flattened areas  183 , which enlarge the deflection radius, are provided on the end of the flow duct ends facing away from the direction of flow, where slits  184  are made in the opposite areas and free ends  185 ,  186  are bent outwards until they almost come into contact with the flattened areas of the opposite flow duct. Moreover, fold webs  187 , which are reinforced and thickened by inserted strips, are angled in the direction of the direction of inflow and thus act as upstream guide vanes. A correspondingly opposite shape can be provided in the outlet area of the honeycomb. 
     FIG. 27 shows an arrangement of foil layers  263  with fold webs  267 , running along the direction of flow, for stiffening the honeycomb structure and increasing the degree of conversion. Foil layer sections  264   a ,  265   a  of end areas  264 ,  265  of the honeycomb are angled in relation to middle area  266 , cuts being made in fold webs  267  for this purpose. This reduces pressure losses in the inlet area of the honeycomb in the event of inflow at an angle relative to foil layers  263 . At the level of the cuts and in the inlet areas, the foil layers are supported by stiffening wires  269 , which are inserted in webs  267  and extend transverse to the direction of flow. Moreover, stiffening wires  268  are provided, which run vertical to foil layers  263  and are partly connected to wires  269 . 
     FIG. 28 shows a cuboid honeycomb comprising individual foils  277  with triangular channels  279  and flow ducts, which can also be of isometric design, extending over the entire width of the foil layers. To achieve low-turbulence inflow into the honeycomb at an angle to the principal plane of the foil layers, the ends of each of channels  279  are provided with a bevel  280 , which points towards the free end of the honeycomb and is angled towards the direction of inflow. Face-side end areas  281  of foil layers  277  are provided with stiffening beads  282 , into which additional stiffening wires  283  are clamped. Moreover, strips  284  are provided, which run transverse to foil layers  277 , rest on the top edges of channels  279  and support the foil layer above. 
     In the middle area of the honeycomb, the channel-shaped profile of foil layers  277  is interrupted by flattened area  286 , which extends over the entire width of the foil layer, runs at the level of the top edge of channel  279  and into which stiffening wires  287  are woven. As a result of this folding, flow channels  279  are continued with a lateral and a vertical offset (see arrow  288 ), with the result that the fluid passing through one duct area is automatically mixed with fluid passing through adjacent duct areas. 
     FIG. 29 shows a honeycomb which consists of three component honeycombs  190   a,b,c  and is fastened in housing  192  via beads  191 . The component honeycombs are each produced from a foil strip laid in zigzag fashion, where stiffening wires  193  or foils are inserted between the individual foil layers. FIG. 29 (middle) shows an enlarged face-end view of the honeycomb structure. According to this, stiffening wires are clamped in webs  194 , which connect the individual foil layers, in the hatched area in FIG. 29 (top), thus producing a structure with comparatively great flexural resistance. The connecting webs are removed in areas  195 , adjacent to the housing, so that the foil layers can be designed in arched form over the stiffening wires or films set back from the plane of the face end in this area, in that the vertices of the corrugations can shift relative to each other layer by layer, without slipping into each other. This results in expansion zones with a comparatively flexible honeycomb structure and the honeycombs can easily be adapted to non-rectangular housing shapes. Areas of arched shape or that can bend by way of relative shifting of the layers can also be provided within a component honeycomb by removing webs  194  at the face ends of these areas. 
     FIGS. 30 a,b,c  shows stiffening elements  25 ,  26 ,  27  and  28  in the design of multiple folded single layered foils  21   a ,  21   b  having a smooth or profiled, for instance waved design. The folded sections seperated by folding lines preferably are compressed ao that adjacent folding sections are engaging each other by an engaging area. Additional stiffening elements being designed as wires  29   a  or spirals  29   b  are provided extending perpendicular to the foil layers and intersecting the mutiple folded layer sections. 
     It should be noted that the honeycombs provided with stiffening elements according to the invention can also display stacks of greatly curved foil layers, these being obtained, for example, if entire stacks of foil layers are folded or wrapped around a central fold line. In this context, the stiffening elements can follow the bending of the foil layers and be connected to them, possibly in a manner capable of absorbing tensile forces. 
     Regardless of the design of the honeycomb, the stiffening elements according to the invention can be of essentially rigid, or particularly also elastic design, where the elasticity can be less or, given an appropriate arrangement, also greater than that of the foil layers. The elastic properties can refer to the honeycomb under operating conditions, while they advantageously exist over the entire range between operating and room temperature.