Abstract:
A filter includes a filter element ( 1 ) having a filter medium ( 106 ) pleated in a zigzag manner, with a raw side ( 105 ) and a clean side. On the raw side ( 105 ) and/or on the clean side of the filter medium ( 14; 106 ), a plurality of oblong adhesive sections ( 122 ) are disposed along at least two adhesive tracks ( 101 ) on the filter medium ( 106 ), which run obliquely or perpendicularly to the pleat edges (F) at least in some sections. Each adhesive track ( 101 ) having at least one adhesive section ( 122 ) and at least one adhesive interruption ( 107 ). The adhesive interruptions ( 107 ) are offset from each other such that in a medium section ( 120 ) the adhesive interruptions ( 107 ) do not form a passage ( 126 ) that runs continuously parallel to the pleat edges (F).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is US National Stage Entry of international patent application no. PCT/EP2010/063131, filed Sep. 7, 2010 designating the United States of America, the entire disclosure of which is incorporated herein by reference. Priority is claimed based on Federal Republic of Germany patent application no. 10 2009 040 202.0, filed Sep. 7, 2009, the entire disclosure of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The invention concerns a filter for filtering fluids, in particular gases, in particular intake air, fuel or motor oil, in particular of an internal combustion engine, in particular of a motor vehicle, or ambient air for introduction into venting systems of buildings or vehicles, comprising a filter element with a zigzag-folded filter medium having a raw side and a clean side, wherein at the raw side and/or at the clean side of the filter medium a plurality of elongate adhesive sections along at least two lines of adhesive are arranged that extend, at least sectionwise, at a slant or perpendicular to fold edges, wherein on each line of adhesive at least one adhesive section and at least one interruption of the adhesive are arranged. The lines of adhesive serve in this connection in particular for stabilizing the filter element and/or for stabilizing the folds. 
     BACKGROUND OF THE INVENTION 
     The prior art discloses different filters with folded filter media. In the known flat non-cylindrical folded filter elements, the achievable fold heights are limited, for example, for stability reasons. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a stable filter element with fold heights as large as possible and a filter efficiency as high as possible. 
     This object is solved according to the invention in that the interruptions of the adhesive of the different, in particular adjacently positioned, lines of adhesive, viewed in the direction of the fold edges, are displaced relative to each other such that by means of the interruptions of the adhesive in a medium section that extends between two neighboring fold edges no passage is realized that extends continuously parallel to the fold edges across two or more adjacently positioned lines of adhesive. 
     According to the invention, elongate adhesive sections are thus provided with which the folds and thus the filter element are stabilized. In this way, filter elements that are tall relative to the fold distance, i.e., in particular filter elements with tall folds, of a stable configuration can be built. By means of the adhesive sections, the flow course of the fluid to be filtered is moreover optimized with respect to the filtration. Between neighboring lines of adhesive, by means of the adhesive sections flow areas are realized in which the fluid to be filtered is guided substantially perpendicularly to the fold edges. In order to load the flow areas uniformly with fluid, interruptions of the adhesive are provided that represent fluid connections between the neighboring flow areas and enable compensation flows between the neighboring flow areas. The fluid connections ensure a pressure distribution as uniform as possible between the flow areas. 
     In one embodiment, for a majority of the interruptions of the adhesive, in particular for all interruptions of the adhesive, the length of the interruptions of the adhesive is less than the length of the adhesive sections delimiting the respective interruptions of the adhesive. 
     An underpressure that is present at the filter element, in particular underpressure generated upon operation of the internal combustion engine in the intake manifold, can cause in the flow areas a collapse of the intermediate fold spaces locally or even across the entire width of the filter medium, i.e., oppositely positioned filter media sections at the clean side will come into direct contact. Because of material irregularities, in particular with regard to non-uniform impregnation, also stress or deformations may occur that lead to collapse at the raw side. Such collapsed areas are no longer flowed through so that the filtration performance of the filter element is restricted. With the displaced interruptions of the adhesive, firstly a passage is realized that extends at a slant or at least does not extend continuously parallel to the fold edges and, by means of the passage, compensation flows between neighboring flow areas that are separated by the lines of adhesive are enabled. As a passage in the meaning of the invention a channel-type area is to be understood that is delimited by two virtual curves that each extend through the corner points of the adhesive sections of neighboring lines of adhesive. Because of the displaced interruptions of the adhesive, it is secondly achieved that an area that is weakened by an interruption of the lines of adhesive is supported by neighboring lines of adhesive so that the risk of collapse is reduced. In this way, the risk of collapse at the clean side as well as at the raw side is avoided. 
     In an advantageous embodiment, the passage can extend in a zigzag shape, in particular in a V-shape or W-shape. Because of the change in direction, within the passage a symmetrical structure in particular with regard to mechanical stability is enabled. 
     In a further advantageous embodiment, the lines of adhesive can be straight lines of adhesive that extend parallel to each other and perpendicular to the fold edges. Along the straight lines of adhesive, the adhesive sections can be realized in a simple way. Preferably, the adhesive sections can be applied with applicator nozzles onto the filter medium while the filter medium is moved in a transport direction past the applicator nozzle. The applicator nozzle can be arranged stationarily above a transport belt for the filter medium. 
     Advantageously, the lines of adhesive can be arranged equidistantly. In this way, between neighboring adhesive sections flow areas are realized that are of the same width. Moreover, the folds are supported uniformly across the entire width of the filter element so that the risk of a collapse of the folds is reduced. 
     Alternatively, the spacings of neighboring lines of adhesive, viewed in the direction of the fold edges, can advantageously increase or decrease from one end face of the filter element to the other end face. In particular in case of an off-center inflow, this has a positive effect on pressure losses and dust capacity in the flow areas and on the required adhesive quantity. 
     In a further alternative that is in particular advantageous in case of inflow at the middle of or centrally to the filter element, the spacings of adjacent lines of adhesive can increase in particular in outward direction relative to a mirror plane that extends perpendicularly to the fold edges, preferably is centrally arranged relative to the extension of the filter medium in the direction of the fold edges. In this way, toward the end faces adhesive sections, and thus adhesive, are saved and the effective filter surface area is enlarged. Moreover, the flow areas relative to the edges are wider so that locally the pressure loss is lowered and the dust capacity is increased. Moreover, the stability of the filter element is adjusted to the locally acting mechanical load. 
     In a further advantageous embodiment, at the raw side and at the clean side adhesive sections can be arranged such that at least one of the adhesive sections at the raw side projects on both sides past at least one of the interruptions of the adhesive at the clean side and overlaps with its ends the adhesive sections at the clean side that adjoin the interruptions of the adhesive, and/or at least one of the adhesive sections at the clean side projects on both sides at least one of the interruptions of the adhesive at the raw side and over laps with its ends the adhesive sections at the raw side that adjoin the interruptions of the adhesive. 
     As a result of the mutually overlapping adhesive sections at the clean side as well as at the raw side, the stability of the folds as a whole is increased. In this way, gaps in the support action of the filter medium are avoided. Overlap of the adhesive sections enables force transmission of the adhesive sections on the clean side onto the corresponding adhesive sections at the raw side. 
     Advantageously, the extension of the adhesive sections perpendicularly to the filter medium can vary along a line of adhesive so that in the folded filter medium the adhesive sections each can rest flat with their free side that is facing away from the filter medium on a corresponding free side of an adhesive section positioned oppositely in the intermediate fold space or on the surface of the medium section that is positioned oppositely in the intermediate fold space. In this way, the two medium sections that delimit an intermediate fold space are supported relative to each other by means of the adhesive sections. In this way, it is prevented that the medium sections are compressed or the folds can collapse there. The stability of the entire filter element is increased in this way. Moreover, by means of the adhesive sections, support and flow guiding walls are realized that extend transversely to the fold edges in the intermediate fold space, wherein by means of the interruptions of the adhesive fluid connections between the neighboring flow areas that are delimited by the support and flow guiding walls are realized. 
     In a further advantageous embodiment, filter medium sections which extend on both sides of raw-side fold tips to the neighboring raw-side fold bases, viewed from the raw-side fold tips, can have a first bend toward the raw side and therebehind a second bend toward the clean side. In this way, the fold tips at the inflow side for the fluid to be filtered are fluidically optimally shaped so that inflow of the fluid to be purified at the raw side of the filter medium is optimal. In this way, pressure losses are reduced and the load capacity of the filter element for foreign material, in particular dust, is increased. The special bends in the area of raw-side fold tips are produced advantageously in that the fold tips in a narrow area after folding of the filter medium are tightly crimped together so that the areas behind the second bends are spread apart and the filter medium sections extend relatively steeply to the raw-side filter bases. A steep course of the filter medium sections has a positive effect in relation to pressure losses and the load capacity. Moreover, the bends effect a stabilization of the fold walls. Accordingly, filter elements with a large fold height relative to the fold spacing can be built with a stable configuration. 
     In a further advantageous embodiment, at the raw side a plurality of elongate depressions can be formed in the filter medium that extend between the raw-side fold tips and the raw-side fold bases approximately perpendicularly to the fold edges of the filter medium and realize at the clean side corresponding projections in such a way that in a raw-side intermediate fold space two depressions, respectively, are positioned directly opposite each other on the two medium sections delimiting the intermediate fold space and each form partially a flow passage. The raw-side intermediate fold spaces can thus be widened like a channel by means of the depressions at several locations. Because of the flow passage the course of the flow of the supplied fluid is optimized for improving the filtration efficiency, reducing pressure losses at the filter element, and increasing the load or dust capacity. The fluid to be filtered can therefore be guided uniformly to the base of the folds so that as much as possible the entire surface area of the filter medium is flowed through. This leads to uniform loading of the raw side with the separated foreign materials and an increase of the load capacity of the filter element. The service life is thus extended. Furthermore, the areas of the filter element that are formed as depressions and projections stabilize the folded filter medium. 
     In one embodiment of the invention, the filter element is formed of a filter medium that is comprised of cellulose, meltblown fibers, microfibers or nanofibers, woven or knitted fibers, of a nonwoven or a combination of these materials. 
     In one embodiment, the filter medium comprises a layer of a support and a filter layer at the inflow side wherein the filter layer at the inflow side has a filter layer of fine fibers. 
     This arrangement has the advantage that at the inflow side the deposition of particles occurs surface-near within the fine fiber layer or in case of appropriately thin fibers, generally nanofibers in this context, completely at the surface of the fine fiber layer. In this way, it is prevented that small particles, in particular soot particles, can penetrate into the filter medium in the area of high flow velocities and clog its interior. 
     This is in particular advantageous for large fold heights because here the different sections of the filter medium, depending on the position in the filter element, are subjected to loads of different strength. As a result of the inhomogeneous velocity distribution, local blockage of the filter surface area may result therefore in elements with tall folds. Since, as a result of the higher pressure loss, the blocked areas over time are flowed through less, the excess flow velocity area can migrate so that further areas begin to become blocked so that the overall operating life (service life) of the element would be reduced. 
     In one embodiment, a volumetric ultrafine fiber layer is used and with it a depth filter layer of ultrafine fibers is formed that stores the separated particles distributed across the entire thickness of the ultrafine fiber layer and thus prevents buildup of a dense filter cake. For this purpose, an ultrafine fiber layer of meltblown fibers is used preferably. 
     In one embodiment, the filter comprises a filter medium in which in the flow-through direction at least one layer of a filter medium is arranged on a support wherein the inflow filter layer comprises a filter layer of fine fibers wherein the fine fibers are arranged in the interior of the filter medium. 
     In one embodiment, the fine fibers are microfibers or nanofibers. 
     In one embodiment, at least one layer of a filter medium is arranged on a support wherein the inflow filter layer has a filter layer of fine fibers and the support is a grid of polyamide 6.6. 
     In one embodiment, the mesh width of the grid is between 100 and 150 μm. 
     In one embodiment, the filter layer has fibers with an average fiber diameter of ˜2 μm. 
     In one embodiment, in the flow-through direction at least one layer of a filter medium is arranged on a support, in particular a cellulose support or a cellulose filter medium, wherein the inflow filter layer has a filter layer of fine fibers. 
     In one embodiment, the inflow filter layer is a voluminous ultrafine filter layer of meltblown fibers with a thickness in the range of 0.01-0.3 mm wherein the average fiber diameter of the meltblown fibers is in particular approximately 2 μm. In this connection, in particular the lower fiber diameter d50-2σ is approximately 700 nm. In an advantageous further embodiment, the weight of the ultrafine fiber layer is in the range of 5-20 g/m 2 . 
     In one embodiment, the ultrafine filter layer comprises nanofibers with diameters of 0.01 to 0.5 micrometers. The ultrafine fiber layer of nanofibers has in particular a thickness of less than 1 μm. In this way, the separated particles are deposited on the surface and form a filter cake. The fibers can be produced, for example, by means of electrospray. 
     In one embodiment, the thickness of the ultrafine fiber layer is 0.08-0.13 mm. 
     In one embodiment, the weight of the ultrafine fiber layer is 10 g/m 2 . 
     In one embodiment, the air permeability of the ultrafine fiber layer is in the range of 500-5,000 l/m 2 s. 
     In one embodiment, the air permeability of the ultrafine fiber layer is in the range of 1,000-1,500 l/m 2 s. 
     In one embodiment, the material of the ultrafine fiber layer is selected from the group consisting of polybutylene terephthalate, polycarbonate, polypropylene, polyamide, polyethylene terephthalate, polyvinyl alcohol, polyvinyl nitrate, polyvinyl acetate, polyvinyl halide, polyester, polyalkylene terephthalate, polyalkylene naphthalate or polyurethane. 
     In one embodiment, the support is comprised of a cellulose-based filter medium. 
     In one embodiment, the weight per surface area of the support is between 50 and 200 g/m 2 . 
     In one embodiment, the support has an air permeability between 50 and 100 l/m 2 s. 
     In one embodiment, the thickness of the support material is in the range of 0.2-0.5 mm. 
     In one embodiment, the support is impregnated to be flame resistant. 
     In one embodiment, the connection between support and ultrafine fiber layer is produced by calendering. 
     In one embodiment, the ultrafine fiber layer is glued onto the support. 
     In one embodiment of the invention, the filter element comprises a filter medium that is folded multiple times alternatingly in an accordion-like way. In this connection, by means of the filter medium the raw side where the filter element is loaded with the fluid to be purified, is separated from the clean side. Along the fold lines at the filter medium alternatingly raw-side and clean-side fold edges are formed that form fold tips that alternatingly are oriented in the direction of the clean side and the raw side. The spacing between the planes that are formed by the clean-side and raw-side fold tips is defined as fold height. The spacing between two neighboring fold tips is defined as fold distance. The fold base is positioned opposite the fold tips on the other side of the filter medium, i.e., the fold base is the space that is enclosed by the respective fold tip. Accordingly, the clean-side fold tip and the raw-side fold base are positioned at the same fold edge on the opposite sides of the filter medium and vice versa. The two edges of the filter material that extend perpendicularly to the fold edges and alternatingly between raw-side and clean-side fold edges are referred to as end face edges. In the folded state, the end face edges form two opposite end faces. The sides on which the ends of the filter medium are located and that extend in particular parallel to the fold edges are referred to as terminal sides. 
     In an advantageous embodiment, the fold height is at least 50 mm, advantageously 100 mm or 150 mm and particularly advantageously 200 mm. In a further advantageous embodiment, the filter element has fold heights of at least 300 mm. In this way, the filter element, in particular in case of a small inflow surface area, can be provided with a particularly large filter surface area. 
     In one embodiment, the ratio of fold height to fold distance is at least between 50:1 and 180:1, preferably between 100:1 and 160:1, in particular preferred between 120:1 and 140:1. 
     In one embodiment, the fold distance is between 7.5 mm and 1.8 mm, advantageously between 2.5 mm and 3 mm. 
     In one embodiment, the filter medium is grooved or provided with knobs wherein the knobs are introduced into both sides of the filter medium and, in the folded state, are in contact with the respective oppositely positioned filter medium section. This achieves a stabilization of the filter medium. 
     Alternatively or in addition, embossments can be introduced into the medium which have a large length/width ratio and extend between fold tips and fold base. The embossments are advantageously designed such that in the intermediate fold space two or several embossments are positioned directly opposite each other on opposite medium sections and thus an embossment is supported on an oppositely positioned embossment. It is particularly advantageous to embody the embossments in a wedge shape wherein the wedge, depending on the shape of the intermediate fold space decreases or increases in the direction of the fold base. Furthermore, the embossments can be advantageously glued to the oppositely positioned embossment, for example, by means of a hotmelt. 
     In one embodiment, lines of adhesive, for example, of hotmelt adhesive, are introduced in the folds and extend between fold tips and fold base. In this connection, on the clean side as well as on the raw side lines of adhesive are provided. In this connection, before erecting the individual folds, at least two lines of adhesive are applied parallel to each other and perpendicularly to the direction of the fold edges onto the filter medium. 
     In one embodiment, the lines of adhesive are not continuous but interrupted at regular spacings. 
     In one embodiment, the lines of adhesive extend between fold tips and fold base but on the clean side and/or the raw side are interrupted regularly. 
     In an advantageous embodiment, the line of adhesive is interrupted at the raw side at least once between fold tip and fold base, respectively. The interruption is provided in this connection, for example, centrally between fold tip and fold base and has a length of between 10 mm and 80 mm, preferably between 30 mm and 60 mm. 
     In an advantageous embodiment, at the clean side an interruption of the line of adhesive is provided that surrounds the clean-side fold tip. In this way, the folds at the clean side are not glued together in the area of the fold tips. 
     Advantageously, at least one, preferably all, lines of adhesive at the raw side can be directly opposite one/the line(s) of adhesive at the clean side. 
     In an advantageous embodiment, the lines of adhesive that have interruptions are designed such that the interruption of the clean-side line of adhesive and of the raw-side line of adhesive do not overlap. In this way, it is ensured that in the area of the clean side fold tips as well as in the vicinity of the clean-side fold base an overlap of the clean-side line of adhesive and raw-side line of adhesive is formed. 
     In one embodiment, the at least two lines of adhesive have interruptions that are arranged at the same spacing relative the fold tips and relative to the fold base. The straight lines that are formed by the starting and end points of the individual sections of the lines of adhesive extend in this way parallel to the fold edges. 
     In an alternative embodiment, the starting and end points of the interruptions of the adhesive are oriented along a plurality of straight lines that extend parallel to each other and are positioned relative to the fold edges at an angle of 10-80°, preferably 45+/−15°. 
     In a preferred embodiment, the starting and end points of the interruptions of the lines of adhesive extend on at least two sets of straight lines wherein each set of straight lines comprises straight lines parallel to each other wherein the corresponding straight lines on the filter medium cross each other so that an arrow-shaped or zigzag-shaped extension of the interruptions of the line of adhesive on the filter medium is formed. 
     Alternatively, the starting and end points of the interruptions of the lines of adhesive can be defined by a set of curves wherein the curves in particular have the same shape but in a direction perpendicular (+/−30°) to the fold edges are displaced relative to each other. In this connection, for example, circle or ellipsis sections, sine shapes or other regular oscillation shapes can be used. 
     In one embodiment, the filter has either at the clean side and/or at the raw side a surface formed by the fold edges that, at least in a partial area, is not positioned parallel to the plane that is formed by the oppositely positioned fold tips. In this way, the filter element can be adapted in a simple way by better space utilization in the area of this side of the filter insert to complex structures in the intake manifold of an internal combustion engine for optimizing the filtering action. By means of good contact of the filter element at the geometry in the intake path, the invention serves for improving the flow behavior at this air inlet side of the filter element. 
     In an advantageous embodiment at least one part of the surface that is formed by the fold tips at the raw side is positioned at an angle of in particular 0-80°, preferably 10°-45°, especially preferred 10°-25°, to the surface that is formed by the fold tips at the clean side. 
     In a particularly advantageous embodiment, the surface that is formed by the raw-side fold tips extends in an area of the filter element parallel to a surface that is formed by the clean-side fold tips and in at least a second edge area of the filter element at an angle of 0-80°, preferably 10°-45°, especially preferred 10°-25°, to the surface that is formed by the clean-side fold tips, wherein relative to the edge of the filter element the fold height decreases continuously. In this way, advantageously the inflow conditions in the air filter housing can be adapted to the mounting space conditions. For example, in areas that are loaded less by the flow, in which the particle load of the filter element is reduced, a reduced fold height can be provided. Moreover, in this way the pressure loss at the filter element can be lowered. 
     In one embodiment, the filter element has at least two sections in which the surfaces that are formed by the clean-side or raw-side fold tips extend parallel but a different spacing to the respective oppositely positioned surface. The difference in fold height of the at least two sections results in a stepped element which enables better space utilization in mounting spaces of complex shape. 
     In a further embodiment, also slants, angled or curved contours can be provided at the appropriate side of the filter insert, wherein by means of the continuous zigzag folding a substantially uniform filtering action across the entire filter surface area is ensured. On the other side of the filter element, a uniform planar surface without connecting webs is provided here because the areas of different fold height pass continuously into each other. 
     In one embodiment, the fold tips have a shape that corresponds to a wedge or alternatively a truncated wedge. This means that the fold tip in the case of a truncated wedge shape at the center along the fold edge has a plateau which is substantially parallel to the plane that is formed by the fold tips. On either side of the plateau, narrow transition areas of the fold tips adjoin that, relative to the plateau, are positioned at an angle of 45 (+35/6−30°), preferably 45°-80°. The width of the plateau area is at most one fourth, preferably at most one fifth, and particularly preferred at most one sixth of the fold distance. In the case of the wedge shape, the transition areas are positioned at an angle of 90+/−30°, preferably 60°-90°, and pass with a pointed fold edge directly into each other. The width of the transition areas in the case of the wedge shape is at most 35%, preferably at most 25%, of the fold distance, in case of the truncate wedge at most one fourth, preferably at most one fifth, and especially preferred one sixth, of the fold distance. 
     In one embodiment, the fold tips have a multi-step shape wherein in a first area, beginning at the fold edge, the two filter medium sections are substantially resting directly on each other or are positioned at an angle of less than 10°, preferably less than 5°. The first area extends in this connection across a length of less than 10 mm, preferably 5 mm+/−0.2 mm. In an adjoining transition area with a length of less than 10 mm, preferably 5 mm+/−0.2 mm, the oppositely positioned filter medium sections are positioned relative to each other at an angle of 10°-40°, preferably of 18°-30°, particularly preferred 20°-25°. Across the remaining extension of the fold, the oppositely positioned filter medium sections extend substantially parallel and preferably are positioned relative to each other at an angle of 0° to 0.2° or alternatively 0° to −0.2°. 
     In one embodiment, the open cross-sectional surface between two folds formed at the raw side and/or clean side where the medium to be purified flows in or where the purified medium flows out is smaller than the base surface of the correlated fold base. This means that the distance, which two filter medium sections defining a fold of the filter bellows have relative to each other, is smaller in an end area at the open side of this fold than in the area in which the filter medium sections converge and are connected. 
     In a preferred embodiment, the ratio of the raw-side open cross-section between two fold tips at the raw side relative to the base surface positioned within the same intermediate space at the bottom and formed by the fold base is smaller than 1, preferably smaller than 0.85, further preferred smaller than 0.7 and particularly preferred smaller than 0.4. 
     In one embodiment, the end faces are sealed by means of continuous lines of adhesive. In this connection, at least on the clean side each fold is closed off by hotmelt that, in the unfolded state is applied in a continuous line at the edge of the filter medium and upon erection of the folds closes off the latter relative to the end face. In a further embodiment, also in the edge area also a continuous or interrupted line of adhesive is applied onto the raw side of the filter element for stabilization of the filter element. 
     In one embodiment, the sealing action at the end faces is realized by a textile material or fabric, for example, nonwoven, in particular polyester nonwoven, that is provided with a coating of hotmelt. For sealing the end faces, the coated side of the fabric is contacted with the filter bellows. In this connection, the coating of hotmelt can be heated by means of infrared radiation before application to the end face and subsequently, in the plastic state, is attached to the filter bellows. The hotmelt penetrates into the folds of the filter medium and solidifies upon cooling. Alternatively or additionally, the coating can be heated from the exterior by infrared radiation or by contact with a hot counter member when the coating of hotmelt is in contact with the end face of the filter bellows. 
     In a further embodiment, the filter element has a flat seal at the end faces with plastic material. 
     In one embodiment, the flat seal is formed by a substantially plate-shaped plastic part that is adhesively attached flat to the end face of the filter bellows by means of a hotmelt. 
     In one embodiment, the flat seal is formed by a cast polyamide, for example, polyamide 6 with starting material c-caprolactame. It is provided in a casting mold in liquid state, subsequently the end face of the filter element is immersed into the still liquid plastic material. The plastic material hardens in the shape of the casting mold at the end face of the filter element and thus closes off the end face. Alternatively, into the casting mold a polyurethane can be introduced which upon curing will foam and penetrate between the folds at the end face. In this way, a lightweight and flexible seal can be achieved. 
     In another embodiment, the flat seal at the end faces is realized by means of a thermally softenable plastic material, for example, polypropylene, polyamide, poly-oxymethylene. In this connection, a substantially plate-shaped plastic part is heated at the side that is to be attached to the end face by infrared radiation up to the range of the melting point and subsequently is pressed against the end face of the filter bellows. In this way, the filter medium penetrates with its end face edges into the softened plastic material and is joined thereto upon solidification of the plastic material. 
     In another embodiment, the flat seal at the end faces is realized by a thermally softenable material that swells when exposed to heat, in particular nitrile rubber. In this connection, a film of this material is pressed against the end faces of the filter bellows and heated at the same time. This causes the material to soften, swell, enclose the end face edges of the filter medium. Subsequently, the material hardens so that a fast and positive-locking connection of the material with the filter medium is produced. In an advantageous further embodiment, during the connecting process a nonwoven layer is introduced into the outwardly positioned surface of the thermally softenable material that is facing away from the filter medium. It is connected with the thermally softenable material in the same way as the filter medium. This has the advantage that a uniform surface and, moreover, an additional stabilization of the flat seal are achieved. 
     In one embodiment, the filter element has a circumferentially extending frame made of plastic material that covers at least partially the surfaces of the filter element that are not flowed through. In this connection, the frame parts at the end faces can be formed by the flat seal of plastic material or additional frame parts can be provided that surround the end faces. 
     In an advantageous embodiment, the frame has a circumferentially extending seal which serves for separating the raw side and the clean side. It can act axially and can be attached to the frame on the raw side of the filter element. In this connection, the seal can be oriented in axial direction perpendicularly to the raw-side inflow surface that is formed by the raw-side fold tips and can be contacted with a seal surface of the housing above the inflow surface. Alternatively, the seal can be provided below the inflow surface so as to extend circumferentially about the filter element wherein the sealing action is realized circumferentially about the filter element in a plane between inflow side and outflow side. 
     Moreover, a radially outwardly acting seal can also be provided which is connected to a circumferentially extending frame part which projects past the raw-side inflow surface that is formed by the raw-side folds tips. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages, features, and details of the invention result from the following description in which embodiments of the invention will be explained in more detailed with the aid of the Figures. A person of skill in the art will expediently consider the features disclosed in combination in the drawing, the description and the claims also individually and combine them to other meaningful combinations. It is shown in: 
         FIG. 1 , a view of an embodiment of a filter in accordance with the invention; 
         FIG. 2 , a view of a further embodiment of a filter according to the invention; 
         FIG. 3 , a detail view of an arrangement of the lines of adhesive on a filter according to the invention; 
         FIG. 4 , a detail view of an alternative arrangement of the lines of adhesive on a filter according to the invention; 
         FIG. 5 , an embodiment of the outer shape of a filter according to the invention; 
         FIG. 6 , a further embodiment of the outer shape of a filter according to the invention; 
         FIG. 7 , an embodiment of a fold tip of a filter according to the invention; 
         FIGS. 8 to 12 , schematic detail views of different arrangements of lines of adhesive on a filter medium similar to the arrangement of  FIG. 4 ; 
         FIGS. 13 to 15 , schematic isometric illustrations of a fold of the filter element of the filter similar to the filter of  FIGS. 1 to 7 , wherein here the lines of adhesive are arranged at different spacings relative to each other; 
         FIG. 16 , schematically in section a filter medium, not folded, of a filter similar to the filter of  FIGS. 1 to 15 , in which at the raw side and at the clean side lines of adhesive are arranged, wherein the height of the lines of adhesive perpendicular to the filter medium alternatingly increases and decreases with their spacing relative to the neighboring fold edges; 
         FIG. 17 , schematically in section a detail of a filter element with a folded filter medium of a filter similar to the filter of  FIGS. 1 to 16 , in which the free sides of the adhesive sections of a filter medium section are each resting flat on a corresponding free side of the adhesive sections of the filter medium section that is oppositely positioned in the intermediate fold space; 
         FIG. 18 , schematically in section an alternative filter element of a filter similar to the filters of  FIGS. 1 through 17  in which the raw-side fold tips are crimped and the raw-side fold bases are widened; 
         FIG. 19 , an isometric detail view of a further embodiment of a filter element in which at the raw side depressions that extend perpendicularly from the fold base to the fold tips are formed in the filter medium; 
         FIG. 20 , an isometric detail view of a further embodiment of a filter element similar to the filter element of  FIG. 19  in which the raw-side fold bases in comparison to the raw-side fold tips are widened; 
         FIG. 21 , a section of a further embodiment of a filter element similar to the filter elements of  FIGS. 19 and 20  in which the depths of the depressions across the fold height is approximately constant; 
         FIG. 22 , a section of a filter element similar to the filter element of  FIG. 21  in which the depth of the depressions from the raw-side fold tips to the fold bases at the raw side decreases in a wedge shape; 
         FIG. 23 , a section of a filter element similar to that of the filter elements of  FIGS. 19 to 22 , along a section plane parallel to a plane through the raw-side fold tips in which, between neighboring depressions, a line of adhesive is arranged, respectively; 
         FIG. 24 , a section of a filter element which is similar to the filter elements of  FIGS. 19 to 22 , along a section plane parallel to a plane through the raw-side fold tips in which oppositely positioned supports that are formed on the medium sections that are positioned opposite each other in the clean-side intermediate fold spaces support each other; 
         FIG. 25 , an isometric illustration of a filter medium, not folded, for use in a filter element similar to the filter elements of  FIGS. 19 to 24  in which the depressions are realized by means of grooves that extend perpendicularly to the fold edges; 
         FIG. 26 , an uncoated PA 6.6 grid with a mesh width of 100±10 μm; 
         FIG. 27 , the grid of  FIG. 26  with a coating of nanofibers; 
         FIG. 28 , the coated grid of  FIG. 27  in cross-section; 
         FIG. 29 , an uncoated PA 6.6 grid with a mesh width of 102±6 μm; 
         FIG. 30 , the grid of  FIG. 29  with a coating of nanofibers; 
         FIG. 31 , the coated grid of  FIG. 30  in cross-section; 
         FIG. 32 , an uncoated PA 6.6 grid with a mesh width of 150±9 μm; 
         FIG. 33 , the grid of  FIG. 32  with a coating of nanofibers; and 
         FIG. 34 , the coated grid of  FIG. 33  in cross-section. 
     
    
    
     Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention. 
     DETAILED DESCRIPTION 
       FIG. 1  shows a filter element  1  with a raw-side inflow side  2  and a clean-side outflow side  3 . The filter element is formed by a filter medium  14  that is folded multiple times wherein the folds extend between the inflow side and the outflow sides, i.e., fold tips are positioned at the inflow side and at the outflow side, respectively. The lateral surfaces of the filter element  4  that are not flowed through are enclosed by a polyester nonwoven that is provided on the side that is facing the filter element with a hotmelt layer. This hotmelt layer provides a flat adhesive connection of the polyester nonwoven with the filter element whereby also the end face  5  of the filter bellows is sealed. The filter element  1  comprises a main frame  6  and an auxiliary frame  7  wherein the main frame supports an axial seal  8  that seals in the direction of the outflow side  3  and is introduced into a groove of the main frame or into a groove between main frame and lateral surfaces  4 . The auxiliary frame is connected to the lateral surfaces  4  by an adhesive connection and has radial surfaces  9  and axial surfaces  10  for supporting the filter element in a housing, not illustrated. 
       FIG. 2  shows an embodiment of a filter element  1  with a raw-side inflow side  2  and a clean-side outflow side  3 . At the lateral surfaces  4  of the filter element  14  that are not flowed through a plastic frame  16  is attached by means of a hotmelt connection wherein on the end faces  5  by means of the hotmelt also the sealing action of the end face is realized. On the end faces  15  openings  11  are introduced into the plastic frame  16 . The plastic frame  16  supports on the inflow side  2  an axial seal  12  which can be engaged by a sealing surface of a housing, not shown. Moreover, a grip  13  is provided that is connected with the plastic frame  16  and serves for improved handling of the filter element  1 . 
       FIG. 3  shows a detail view of an arrangement of the lines of adhesive on a filter element according to the invention. In this embodiment, lines of adhesive  101  of hotmelt are introduced into the folds and extend between the fold tips  102  and the fold base  103 . In this connection, on the clean side  104  as well as on the raw side  105  lines of adhesive  101  are applied. Before erecting the individual folds, at least two lines of adhesive  101  are applied parallel to each other and perpendicular to the direction of the fold edges  102 ,  103  onto the filter medium  106 . The lines of adhesive  101  are not continuous but are interrupted at regular spacings by an interruption  107 . At the raw side  105  the line of adhesive is interrupted once between fold tip and fold base. The interruption  107  is located centrally between fold tip and fold base and corresponds in its length to one third up to one half of the fold height. The interruption  107  begins at a spacing b from the raw-side fold base  103 . Accordingly, the folds at the raw side are glued only in the area of the fold base and fold tips. At the clean side, an interruption of the line of adhesive across a length a is provided which surrounds the raw-side fold tip. Accordingly, the folds at the raw side are not glued in the area of the fold tips. 
     The spacings a and b of the raw-side fold base and clean-side fold tip are configured such that the interruption  107  of the clean-side and the raw-side lines of adhesive do not overlap. In this way, it is ensured that in the area of the clean-side fold tips as well as in the vicinity of the clean-side fold base an overlap of the line of adhesive  101  of the clean side  104  and raw side  105  is formed. 
     The straight lines x and y formed by the starting and end points of the individual sections of the lines of adhesive extend parallel to the fold edges. 
       FIG. 4  shows an alternative arrangement of the lines of adhesive on a filter element according to the invention. In this embodiment, the start and end points of interruptions of the lines of adhesive  101  are aligned relative to a plurality of straight lines z that extend parallel to each other and form with the fold edges F an angle α of 10-80°, preferably 45+/−15°. In this connection, the starting and end points of the interruptions of the lines of adhesive extend along at least two sets of straight lines z′ and z″, wherein each set of straight lines comprises straight lines parallel to each other, wherein the corresponding straight lines on the filter medium cross each other so that an arrow-shaped or zigzag-shaped course of the interruptions  107  of the lines of adhesive  101  is formed on the filter medium. 
       FIG. 5  shows an embodiment of the outer shape of a filter element  1  according to the invention. In this embodiment, the filter element has at least two sections ( 301 ,  302 ) in which the surfaces  303  and  304  that are formed by the raw-side fold tips extend parallel but at a different spacing H(H′, H″) relative to the respective oppositely positioned surface  305 . The difference in fold height of the at least two sections results in a stepped element that enables a better spatial utilization in mounting spaces of a complex shape. 
       FIG. 6  shows an embodiment of the outer shape of a filter element  1  according to the invention. In this embodiment, the surface area that is formed by the fold tips at the raw side extend in an area  201  of the filter element parallel to the surface that is formed by the clean-side fold tips and in at least one second edge area  202  of the filter element at an angle β of 30° relative to the surface  203  formed by the clean-side fold tips wherein relative to the edge  204  of the filter element  1  the fold height h (h′, h″) decreases continuously. 
       FIG. 7  shows an embodiment of a raw-side fold tip of a filter according to the invention. In this embodiment, the fold tips have a multi-step shape wherein in a first area Z beginning at the fold edge the two filter medium sections have an angle ζ relative to each other that is smaller than 5°. The first area extends in this context across a length Z of approximately 5 mm. In an adjoining transition area Σ with a length Σ of approximately 5 mm the oppositely positioned filter medium sections are positioned an angle σ of approximately 24° relative to each other. Across the remaining course R of the fold, the oppositely positioned filter medium sections extend substantially parallel and, in comparison to the aforementioned angles, are positioned relative to each other at a negative angle of approximately 0.2° so that the clean-side spacing of the two filter medium sections becomes smaller along the further course in the flow-through direction δ. This has the result that the cross-section of the intermediate spaces at the raw side adjoining this fold at both sides to the right and to the left become larger in the direction of the flow-through direction δ. 
     In  FIG. 8 , a section of a filter medium  106  is shown which is similar to the filter media  106  of  FIG. 3  and is used in filter elements for filters similar to the filter elements  1  of  FIGS. 1 ,  2 ,  5  and  6 .  FIG. 8  shows the raw side  105  of a medium section  120  of the filter medium  106  that extends between two fold edges F. The fold edges F are embossed as folding lines in the unfolded filter medium  106 . In the folded filter medium  106 , the fold tips  102  surround the fold edges F. 
     On the filter medium  106 , a plurality of elongate adhesive sections  122  extends along straight lines of adhesive  101 . The lines of adhesive  101  extend equidistantly parallel to each other and perpendicularly to the fold edges F. A spacing  132  between two neighboring lines of adhesive  101  is approximately 25 mm, respectively. 
     Along each of the lines of adhesive  101 , two interruptions  107  of the adhesive of identical length are arranged between the adhesive sections  122 . The width  134  of the interruptions  107  of the adhesive perpendicular to the fold edges F is approximately 15 mm. The interruptions  107  of the adhesive of neighboring lines of adhesive  101 , viewed in the direction of the fold edges F, are displaced relative to each other. 
     The adhesive sections  122  of neighboring lines of adhesive  101  delimit each a passage part  124  of a passage that extends in the raw-side intermediate fold space  148   a  of the folded filter medium  106  between the fold edges F, similar to the embodiment to be explained farther down in connection with  FIG. 19 . The interruptions  107  of the adhesive connect the respective neighboring passage parts  124 . 
     The interruptions  107  of the adhesive of the neighboring lines of adhesive  101  realize two passages  126  that, in the embodiment shown in  FIG. 8 , are extending parallel in a V-shape and slanted relative to the fold edges F, respectively. The passages  126  in  FIG. 8  are defined by two V-shaped curves indicated in dashed lines in the form of imaginary straight lines  128 . The straight lines  128  are similar to the sets of straight lines z′, z″ according to the embodiment of  FIG. 4 . The straight lines  128  extend through the corresponding end points of the adhesive sections  122 . Because of the displaced arrangement of the interruptions  107  of the adhesive it is avoided that the passages  126  extend continuously parallel to the fold edges F. 
     In  FIG. 8 , risk areas  130  of the filter medium  106  which, upon operation of the filter without use of the indicated adhesive sections  122  with interruptions  107  of the adhesive according to the invention, could collapse, for example, as a result of underpressure, are shown as elliptical curves in an exemplary fashion. 
     In  FIG. 9 , a further embodiment of a filter medium  106  is shown which is similar to the embodiment of  FIG. 8 . In contrast to the embodiment of  FIG. 8 , the interruptions  107  of the adhesive are arranged in the embodiment of  FIG. 9  such that the passage  126  shown at the top in  FIG. 9  extends zigzag-shaped wherein the flanks  126   a  extend across three neighboring lines of adhesive  101 , respectively. 
     The passage  126  shown in  FIG. 9  at the bottom extends also in a zigzag-shape. However, in this configuration, the flanks  126   a  extend across two neighboring lines of adhesive  101 , respectively. The use of two differently extending passages  126  improves the stability of the filter medium  106 . 
     A further embodiment of a filter medium  106  according to  FIG. 10  is similar to the filter medium  106  of the embodiments according to  FIGS. 8 and 9 . In this embodiment, in  FIG. 10  at the top a passage  126  in the form of an upside down “V”, similar to the “V” in the embodiment of  FIG. 8 , and at the bottom a passage  126  with zigzag-shaped course, as in the embodiment of  FIG. 9 , are realized. 
     A further embodiment according to  FIG. 11  is similar to the embodiment of  FIG. 10 , wherein here the passage  126  shown at the top in the Figure is oriented as a “V”, as in the embodiment of  FIG. 8 . 
     In a further embodiment of a filter medium  106  according to  FIG. 12 , which is similar to the embodiment of  FIGS. 8 to 11 , an upper passage  126  at the upper end of the  FIG. 12  has the shape of a “W”. A passage  126  shown at the bottom of  FIG. 12  has the shape of an upside-down “W”. 
     The passages  126 , as they are shown in  FIGS. 8 to 12 , can be combined in other ways also. For example, more or fewer than two passages  126  can be provided between two fold edges F. Identical or different lengths  134  of the interruptions  107  of the adhesive can be provided along the lines of adhesive  101  and/or in case of different lines of adhesive  101 . The lengths  134  can also be smaller or greater than 20 mm. Also, the spacings  132  of the lines of adhesive  101  can be different, also smaller or greater than 25 mm. 
     On the clean sides of the medium sections  120 , not shown in  FIGS. 8 to 12 , also adhesive sections and interruptions of the adhesive along the lines of adhesive can be arranged. They can be arranged preferably such that the adhesive sections  122  at the clean side  105  project on both sides past one of the interruptions of the adhesive on the clean side, respectively, and overlap the adhesive sections neighboring the corresponding clean-side interruption of the adhesive at the ends. On the other hand, the adhesive sections on the clean side can likewise project past the interruptions  107  of the adhesive at the raw side  105 . 
     In  FIG. 13 , a detail of a filter element  1  is illustrated which is similar to the filter elements  1  of  FIGS. 1 ,  2 ,  5  and  6 . In  FIG. 13 , two medium sections  120  are shown that extend on either side of one of the fold tips  102   a  at the raw side. 
     At the raw side  105  of the filter medium  106 , similar to the embodiment of  FIGS. 3 ,  4 , and  8  to  12 , a plurality of adhesive sections  122  and interruptions  107  of the adhesive along equidistant lines of adhesive  101  are arranged. The lines of adhesive  101  extend perpendicularly to the fold edges F. 
     In an embodiment illustrated in  FIG. 14  that is similar to the embodiment of  FIG. 13 , the spacings  132   a ,  132   b ,  132   c  between the lines of adhesive  101  increase from an edge  136   a  of the filter element  1  to the right of  FIG. 14  to a left edge  136   b.    
     In a further embodiment of a filter element  1  illustrated in  FIG. 15 , the spacings  132   e  to  132   g  increase mirror-symmetrically relative to a central plane  138  that extends centrally between the edges  136   a  and  136   b  from the interior to the exterior. 
     In  FIG. 16 , a medium section  120  of the filter medium  106 , which is similar to the filter media  14 ;  106  of  FIGS. 1 to 15 , is shown in the unfolded state. The filter medium  106  is moved horizontally in a transport direction  140  between an upper applicator nozzle  142   a  and a lower applicator nozzle  142   b . The raw side  105  of the filter medium is at the top in  FIG. 16 , the clean side  104  at the bottom. By means of the applicator nozzles  142   a  and  142   b , the adhesive sections  122   a  are applied to the raw side  105  and the adhesive sections  122   b  to the clean side  104  of the filter medium  106 . On the raw side  105 , the height  144  of the adhesive sections  122  perpendicularly to the filter medium  106  increases continuously from the leading fold edge F in the transport direction  140 , in  FIG. 16  to the right, toward the trailing fold edge F, to the left, within an adhesive section  122   a  as well as between the adhesive sections  122   a  that are separated from each other by interruptions  107  of the adhesive. At the raw side  107  the height  144  of the adhesive sections  122   b  provided thereat decreases correspondingly continuously from the leading fold edge F to the rearward fold edge F. 
     The raw-side adhesive sections  122   a  overlap the clean-side adhesive sections  122   b  so that the raw-side adhesive sections  122   a  project past the clean-side interruptions  107  of the adhesive and, vice versa, the clean-side adhesive sections  122   b  project past the raw-side interruptions  107  of the adhesive. 
     In a medium section not shown in  FIG. 16  that follows the medium section  120  in the transport direction  140 , the heights  144  of the raw-side adhesive sections  122   a  decrease from the rearward fold edge F in the transport direction  140  to the next fold edge. Accordingly, on the clean side  104 , in the next medium section, the height  144  of the adhesive section  122   b  increases from the rearward fold edge F to the next fold edge. In analogy, a medium section, that is also not shown in  FIG. 16  and that precedes the illustrated medium section  120  in transport direction  140 , is constructed with respect to the leading fold edge F mirror-symmetrical to the illustrated medium section  120 . 
     In  FIG. 17 , a filter element  1  is illustrated in longitudinal section; it is folded of a filter medium  106  that is similar to the filter element  106  in  FIG. 16 . In the raw-side intermediate fold spaces  148   a , the free sides of the raw-side adhesive sections  122   a  of a medium section  120  that are facing away from the surface of the filter element  106  are resting on the corresponding free sides of the adhesive sections  122   a  of the respective oppositely positioned medium section  120 . 
     By means of the adhesive sections  122   a  resting on each other, neighboring medium sections  120  of an intermediate fold space  148  are supported relative to each other and their shape is maintained. Also, the adhesive sections  122  form boundaries for the passage sections  124  that are shown in  FIGS. 3 ,  4  and  8  to  15 . 
     As on the raw side  105 , on the clean side the free sides of the adhesive sections  122   b  of a clean-side intermediate fold space  144   b  are resting flat against each other and prevent in this way a collapse of the clean-side intermediate fold spaces  148   b.    
     In  FIG. 18 , a section of a further embodiment of a filter element  1  is shown in which the raw-side fold tips  102   a , similar to the embodiment of  FIG. 7 , have a multi-step shape. The total fold height  550  is approximately 5 cm to 40 cm, in particular 20 cm to 40 cm. The thickness  151  of the filter medium  106  is approximately 0.5 mm. 
     The filter medium sections  120  that extend on both sides of the raw-side fold tips  102   a  toward the neighboring raw-side fold bases  103   a , each have at the end of a first section  120   a  a first bend  552  toward the raw side  105  at a spacing Z from the fold edge F. The length Z of the first area  120   a  is approximately 5 mm. The first areas  120   a  are positioned at an angle ζ of approximately 5° relative to each other. Instead, they can also be positioned relative to each other at an angle ζ of less than 5° or between 5° and approximately 10°. 
     At a spacing Σ from the first bend  552  the filter medium sections  120  have a second bend  554  toward the clean side  104 . The length Σ of a second area  120   b  between the first bend  552  and the second bend  554  is approximately 5 mm. The second areas  120   b  are positioned at an angle δ of approximately 24° relative to each other. They can instead also be positioned at an angle between 10° and 24° or between 24° and 40° relative to each other. 
     The lengths Σ and/or Z can also be less than 5 mm or up to approximately 10 mm. 
     Adjoining the second area  120   b , third areas  120   c  extend up to the raw-side fold bases  103   a , respectively. The third areas  120   c  are positioned at an angle of approximately 0.2° relative to each other. Instead, they can also be positioned at a different angle, preferably between approximately −5° and approximately +5° relative to each other. 
     The raw-side fold bases  103   a  have an approximately V-shaped profile that is tapering in a pointed shape to the raw-side fold edge  102   a . In contrast thereto, the clean-side fold bases  103   b  have an approximately U-shaped profile. Clean-side filter base widths  556  of the clean-side filter bases  103   b , at the level of the two second bends  554  at a distance  560  from the appropriate raw-side fold edge  102   a , are smaller than raw side filter base widths  558  of the raw side filter bases  103   a  at the corresponding distance  560  from the clean-side fold edges  102   b.    
     The shape of the raw-side fold tips  102   a  is produced during manufacture in that the filter medium  106  is first folded along the fold edges F and, subsequently, crimped in the first areas  120   a . In doing so, the material of the filter medium  106  is compressed in the first area  120   a  and, at the same time, the first bends  552  and the second bends  554  are created. 
     In addition, as illustrated in an exemplary fashion in  FIG. 18  on the medium section  120  shown to the far right, similar to the embodiments of  FIGS. 1 to 3 ,  4 , and  8  to  17 , adhesive sections  122  and interruptions  107  of the adhesive along the corresponding lines of adhesive  101  may be arranged optionally on the medium sections  120  at the raw side  105  and/or the clean side  104 . 
     Moreover, similar to the embodiments described farther down in connection with  FIGS. 19 to 25  and as shown in an exemplary fashion in  FIG. 18  on the medium section  120  all the way to the left, the medium sections  120  may be provided optionally on the clean side  105  with depressions  656  that, on the raw sides  104 , produce correspondingly shaped projections  658 . 
     In  FIG. 19  a section of a filter element  1  according to a further embodiment is illustrated. On the raw side  105  the filter medium  106  is embossed with a plurality of elongate depressions  656 . The depressions  656  extend between the raw-side fold tips  102   a  and the raw-side fold bases  103   a  perpendicularly to the fold edges F. The depressions  656  each have a depth  664   a  of approximately 1 mm. A width  666  of the depressions  656  in the direction of the fold edges F is approximately 25 mm to 35 mm. The depth  664  of the depressions  656 , as shown in  FIG. 21  in cross-section, is substantially constant across the height of the folds. Alternatively, as shown in  FIG. 22 , the depth  664   b  of the depressions  656   b  can decrease from the raw-side fold tips  102   a  to the raw-side fold bases  103   a.    
     In the left folds illustrated in  FIGS. 21 and 22 , in an exemplary fashion the raw-side fold tips  102   a , similar to the embodiments of  FIGS. 7 and 18 , are crimped and of a multi-step configuration. 
     The depressions  656  are realized as grooves that form on the clean side  104  corresponding projections  658 . At the raw-side intermediate fold space  148   a , the depressions  656  of the two medium sections  120  that delimit the intermediate fold space  148   a  are positioned directly opposite each other and form each partially a flow passage  660 . The flow passages  660  extend perpendicularly to the fold edges F. 
     In the area of the right depressions  656  in  FIG. 19  a support  662  is formed in an exemplary fashion in the medium sections  120 . The supports  662  project in the direction toward the clean side  104 . 
     In an exemplary fashion, on the clean side  104  a glue bead  622  is arranged, respectively, instead of the supports  262  in the left flow passage  660  in  FIG. 19 . The glue beads  622  serve for stabilization of the flow passages  660 . 
     The supports  662  can also be combined with the glue beads  622 , for example, in such a way that the glue beads  622  are located on the supports  662 . 
     Between the flow passages  660  on the raw side  105  an adhesive section  122  with interruptions  107  of the adhesive is arranged, respectively. These adhesive sections  122  extend similar to the embodiments of  FIGS. 3 ,  4  and  8  to  18  along the lines of adhesive  101 . 
     In  FIG. 20 , a further embodiment of a filter element  1  is illustrated which is similar to the filter element  1  of  FIG. 19 . In contrast to the filter element  1  of  FIG. 19 , the raw-side fold bases  103   a  in the embodiment of  FIG. 20  are widened in comparison to the clean-side fold bases  103   b . As a whole, the intermediate fold spaces  148   a  and  148   b  thus have a similar effect as the intermediate fold spaces  148   a  and  148   b  of the filter element  1  of  FIG. 18 . 
     In  FIG. 23  a section of the filter element  1  of  FIG. 19  parallel to a plane through the raw-side fold tips  102   a  is shown. In the area of one of the raw-side intermediate fold spaces  148   a , the adhesive sections  122  of the oppositely positioned medium sections  120  are resting flat against each other, similar to the embodiment of  FIG. 17 , and delimit in this way the flow passages  660 . 
     In  FIG. 24 , a filter element  1  that is similar to the filter element  1  of  FIG. 20  is shown also in a section parallel to a plane through the raw-side fold tips  102   a . In the clean-side intermediate fold spaces  148   b , the supports  662  of the medium sections  120  are resting against each other and stabilize in this way the folds and the filter element  1 . In contrast to the embodiment of  FIG. 20 , in the embodiment of  FIG. 24 , as in the embodiment of  FIG. 19 , in the raw-side intermediates fold spaces  148   a  on both sides of the flow passages  660  adhesive sections  122  with interruptions  107  of the adhesive are provided. The heights  144  of the adhesive sections  122  perpendicular to the respective raw-side surfaces of the filter medium  106  vary in the embodiments of FIGS.  23  and  24  in analogy to the embodiments of  FIGS. 16 and 17  so that the adhesive sections  122  are resting flat against each other and support each other. 
     In  FIG. 25 , an unfolded filter medium  106  is shown that is provided with grooves for realizing the depressions  656  and the projections  658  in accordance with the embodiments of  FIGS. 19 to 24 . The grooves extend perpendicularly to the fold edges F. The projections  658  and depressions  656  can be realized also by grooves that are interrupted at the fold edges F. 
     In all of the above described embodiments of a filter element  1  and of a method for producing a filter element  1 , the following modifications are possible inter alia. 
     The arrangements of the adhesive sections  122  and of the interruptions  107  of the adhesive along the lines of adhesive  101 , as described in  FIGS. 3 ,  4  and  8  to  16 , the configurations of the raw-side fold tips  102 , as described in  FIGS. 7 and 18 , and the arrangements of depressions  656  and projections  658  on the filter medium  106 , as described in  FIGS. 19 to 25 , can be combined with each other in a meaningful way in a filter element  1 . 
     The straight lines of adhesive  101 , instead of being perpendicular, can also be positioned at a slant, or sectionwise at a slant, relative to the fold edges F. 
     The adhesive sections  122  and the interruptions  107  of the adhesive, instead of extending along the straight lines of adhesive  101  can also extend along lines of adhesive that extend differently, for example, along convoluted or meandering lines of adhesive. 
     Instead of being V-shaped, W-shaped, or zigzag-shaped, the passages  126  can be ex-tending also in a different way, at least sectionwise, non-parallel to the fold edges F. 
     In the embodiments illustrated in  FIGS. 16 and 17 , also every second medium section can be free of adhesive sections. In this case, the height  144  of the adhesive sections  122  on the other medium sections  120  can extend across the entire intermediate fold space. The free sides of the adhesive sections  192  are then resting flat on the respective oppositely positioned surface of the local medium section  120  not provided with adhesive sections. 
     The  FIGS. 26-34  show in exemplary fashion some embodiments of filter media which, in addition to further filter media, can be used for a filter according to the invention. 
       FIG. 26  shows an uncoated PA 6.6 grid with a mesh width of 100±10 μm,  FIG. 27  such a grid with a coating of nanofibers.  FIG. 28  shows the coated grid in cross-section. The air permeability of the uncoated grid is ˜10,000 l/(m 2 s), that of the coated grid ˜800-1,500 l/(m 2 s). The thread count is 69.9 n/cm, the thickness 70 μm and the weight per surface area 24 g/cm 2 . In  FIG. 29 , a PP 6.6 grid is illustrated that has a mesh width of 102±6 μm.  FIG. 30  shows the coated grid and  FIG. 31  a cross-sectional view of the coated grid. The air permeability of the uncoated grid is ˜8,600 l/(m 2 s), that of the coated grid ˜1.500 l/(m 2 s). The thread count is 65 n/cm, the thickness 80 μm, and the weight per surface area is 35 g/cm 2 . A further PA 6.6 grid is illustrated in  FIGS. 32 to 34 . The mesh width here is 150±9 μm. The air permeability of the uncoated grid is ˜11,000 l/(m2s), that of the coated grid ˜650-840 l/(m 2 s). The thread count is 47 n/cm, the thickness is 95 μm and the weight per surface area is 35 g/cm 2 .  FIG. 32  shows the uncoated grid,  FIG. 33  the coated grid, and  FIG. 34  the coated grid in cross-section. 
     In the foregoing specification, specific embodiments of the present invention have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present invention. The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued.