Abstract:
A guiding element for a printing unit is provided to facilitate use of the printing unit in an imprinter function. In one operating situation, a strip is printed as it passes through a printing gap of the printing unit. In another operating situation, the strip is guided through the printing gap by the guiding element in a non-contact manner. The guiding element includes, on the outer surface, a plurality of openings adapted for the discharge of a pressurized fluid. These openings are micro-openings of a diameter less than 500 μm.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This U.S. application is the U.S. national phase, under 35 USC 371, of PCT/DE2003/003473, filed Oct. 20, 2003; published as WO 2004/037537 A2 on May 6, 2004 and claiming priority to DE 102 48 820.7, filed Oct. 19, 2002; to DE 103 07 089.3, filed Feb. 19, 2003; to DE 103 22 651.6, filed May 20, 2003 and to DE 103 31 469.5, filed Jul. 11, 2003, the disclosures of which are expressly incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention is directed to printing units with guide elements. The printing unit is adapted for imprinter functions. In one situation, a web is printed in a printing gap. In another situation, the web is conducted, without printing, through the gap. 
     BACKGROUND OF THE INVENTION 
     A printing unit with two web guide elements, which two web guide elements are arranged respectively in an inlet and in an outlet area of a printing unit in such a way that, with the printing location disengaged, a web can be conducted through the printing location without touching it, is known from DE 93 11 113 U1. The two web guide elements are embodied as rollers, which are rotatably seated in lateral walls of the printing unit. 
     A turning bar is disclosed, in one preferred embodiment, in U.S. Pat. No. 3,744,693. A tube wall element made of a porous material which is permeable to air forms a closed pressure chamber in conjunction with a base body. The porous segment constitutes a wall of the chamber and is embodied to be load-bearing over the width of the latter, without a load-bearing support. In a second example, a segment with through-bores is utilized instead of the porous segment. 
     U.S. Pat. No. 5,423,468 shows a guide element which has an inner body with bores and an outer body of a porous material which is permeable to air. The bores in the inner body are only provided in the expected area of a loop of material which will pass around the guide element. 
     EP 0 705 785 A2 is concerned with the transport and deflection of web-shaped material, for example in the form of film material. In one embodiment, compressed air flows through the pores of a porous wall with mean pore diameters of 7 to 10 μm, and in another embodiment air flows through a wall having micro-bores with openings of 350 μm. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is directed to producing printing units with guide elements for a flying printing former change. 
     In accordance with the present invention, this object is attained by the provision of a guide element of a printing unit, which printing unit is usable in an imprinter function. In one operational situation, a web is imprinted in a printing gap. In a second operational situation, the web is conducted through the gap without contact by a guide element. The guide element includes a micro-porous air permeable material through which air can pass. The openings may have a diameter of less than 500 μm. 
     The advantages to be gained by the use of the present invention consist, in particular, in that a dependably and accurately operating web guide element of a printing unit is provided. By the provision of an air cushion which is formed by the micro-openings, a high degree of homogeneity is accomplished over the length of the air cushion, simultaneously with small losses. In contrast to prior rollers, no inertia must be overcome, in particular in the course of changing speeds. 
     By the provision of air outlet openings, with diameters in the millimeter range, forces can be applied point-by-point to the material, with an impulse of a jet, by the use of which, the material can be kept away from the respective component, or can be placed against another component. By the distribution of micro-openings in the guide element, with a high hole density and with a broad support, as a mailer of priority, the effect of a formed air cushion is applied. The cross-section of bores used in prior devices were, for example, in the range of between 1 and 3 mm. The cross section of the micro-openings, in accordance with the present invention, is smaller by at least the power of ten. Substantially different effects arise from this difference in size. For example, the distance between the surface of the guide element with the openings and the web can be reduced, and because of this, flow losses, which occur outside of the effective areas of the web, can be clearly reduced. 
     In contrast to prior components with openings, or with bores, having opening cross sections in the millimeter range and a hole distance of several millimeters, a substantially more homogeneous surface is provided with the formation of micro-openings on the surface. Here, micro-openings are understood to mean openings in the surface of the component which have a diameter of smaller than or equal to 500 μm, preferably smaller than or equal to 300 μm, and, in particular, smaller than or equal to 150 μm. A “hole density” of the surface provided with micro-openings is at least one micro-opening per 5 mm 2 , which is the equivalent of a density of 0.2hole/mm 2 , and advantageously at least one micro-opening per 3.6 mm 2  which results in a density of 0.28 hole/mm 2 . 
     Because of the embodiment of the openings of the guide element as micro-openings, the air cushion is made more uniform. The flow volume exiting per surface unit is reduced in such a way that a flow loss can be acceptably small also in the areas of the guide element around which the web is not looped. 
     The micro-openings can be advantageously provided as open pores at the surface of a porous, and in particular, at the surface of a micro-porous, air-permeable material, or as openings of penetrating bores of small diameter, which extend through the wall of a supply chamber toward the exterior of the guide element. In another embodiment of the present invention, the micro-bores are configured as openings of penetrating micro-bores. 
     In order to achieve a uniform distribution of air exiting from the surface of the guide element, in the case of employing micro-porous material, and without requiring, at the same time, large layer thicknesses of the material with high flow resistance, it is useful for the guide element to have a rigid air-permeable support, to which support the micro-porous material has been applied as a layer. Such a support can be charged with compressed air, which flows out of the support through the micro-porous layer and, in this way, forms an air cushion on the surface of the component. 
     On the other hand, the support can be porous and can have a better air permeability than the micro-porous material. It can also be formed of a flat material or of a formed material, which encloses a hollow space and which is provided with air outlet openings. Combinations of these alternatives can also be considered. 
     To achieve a uniform air distribution, it is moreover desirable that the thickness of the layer corresponds to at least a distance between adjoining openings. 
     In the case of using micro-bores, an embodiment is advantageous, wherein the side of the guide element which faces the web and which has the micro-openings is embodied as an insert or as several inserts in a support. In a further development of the present invention, the insert can be releasably or, if desired, can be exchangeably connected with the support. In this way, cleaning and/or an exchange of inserts with different micro-perforations, for adaptation to different materials and web widths, is possible. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the present invention are represented in the drawings and will be described in greater detail in what follows. 
       Shown are in: 
         FIG. 1 , a schematic side elevation representation of several printing groups through which a web travels, in 
         FIG. 2 , a cross-sectional view of a first embodiment of a guide element in accordance with the present invention, in 
         FIG. 3 , a cross-sectional view of a second embodiment of a guide element, in 
         FIG. 4 , a perspective view of a third embodiment of a guide element, in 
         FIG. 5 , a cross-sectional view of a fourth embodiment of a guide element, in 
         FIG. 6 , a cross-sectional view of a fifth embodiment of a guide element, in 
         FIG. 7 , a perspective view of a sixth embodiment of a guide element, in 
         FIG. 8 , a cross-sectional view of a seventh embodiment of a guide element, and in 
         FIG. 9 , an end view of an eighth embodiment of a guide element in accordance with the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A schematic, side elevation view of three printing units  05 , for example of three printing groups  05  for sheet work, and in particular of three offset printing groups  05  for sheet work, through which a web  02 , such as, for example, a web  02  of material  02 , or a web  02  of imprinted material, runs sequentially, is shown in  FIG. 1 . These printing groups  05  of a printing press can also be constituted in different ways, for example as three-cylinder offset printing groups  05 , as a direct or flexographic printing group, as a printing group for letterpress or rotogravure printing, or as individual printing units  05  that are different from each other. For example, at least one of the printing groups  05 , which is configured for sheet work, has a guide element  01 , and in particular has a web guide element  01 , at least in an outlet area of its printing gap  10 , for use in changing the direction of travel of a freshly imprinted, not yet dry web  02  at the outlet of the printing group  05 . The web guide element  01  can be used, for example, for conducting the web  02  to the printing gap  10  of the next following printing group  05  in the correct orientation. In  FIG. 1 , the individual printing units  05  are each shown with a web guide element  01  at both an inlet and an outlet area. 
     A second printing group  05 , following the first printing group  05 , also has a web guide element  01  in both the inlet and the outlet area of its printing gap  10 . This allows the second printing group  05  to be able to conduct a previously imprinted web  02  through its printing gap  10  in a contactless manner while the printing location of this second printing group  05  is disengaged. This second printing group  05  can thus be operated as an imprinting-type printing group  05  or as a printing group  05  for accomplishing a flying printing former change, alternatingly with another such printing group  05 . In one operational situation, the web  02  is imprinted by one of the printing groups  05 , while passing, without contact, through the second of these printing groups  05 . In another operational situation, this sequence is reversed. The two web guide elements  01  may be spatially arranged, for example, in such a way that the web  02  extends substantially perpendicularly with respect to a connecting plane of the two cylinders constituting the printing location. During imprinting operations, one of at least two printing units  05  of the printing press shown in  FIG. 1  is in contact with, and imprints the web  02 , while the other printing unit  05  is disengaged from web  02  and the web  02  runs through this other printing unit  05  without contact. The printing press preferably has five printing units  05 . In one mode of operation of the printing press, one of the five printing units is passed through by the web  02  without contact, while the web  02  is imprinted by the remaining four printing units  05  in four colors, for example on both sides. In a, second operational situation, the printing unit  05 , which previously had been passed through, without web contact, is placed into operation in the printing process, while one of the four printing units  05  which had previously been printing the web  02  is now passed by web  02  without contact. At least the two printing units  05 , alternatingly through which passage of the web  02 , without contact, is to occur, have guide elements  01 , as will be described in detail below, in each of the inlet and outlet areas of the respective printing gap  10  of each such printing unit  05 . 
     At least one of the two web guide elements  01  of the printing group  05  configured for alternating printing and specifically at least the web guide element  01  which is arranged in the outlet area of the printing gap  10  of at least one printing unit  05  are or is embodied as a contactless operating web guide element  01 , and in particular, as a rod  01 , around which air flows, in a manner as will be described in what follows, and as may be seen in  FIG. 2 . 
     The surface of the guide element  01  has openings  03 , in the form of, for example, micro-openings  03 , through which a fluid, such as a gas or a mixture, and in particular, air, which is under higher pressure than the surroundings, flows from an inside located hollow space  04 , for example a chamber  04 , in particular a pressure chamber  04 , during operation of the guide element  01 . An appropriate feed line for delivering compressed air into the hollow space  04  is not represented in the drawings. 
     The guide element  01  has the micro-openings  03  at least on the side of its surface cooperating with the web  02 , or on the side of its surface facing the web  02 . Guide element  01  can also have the micro-openings  03  on other sides, not facing the web  02 . Alternatively, it can be made completely of a material which has the micro-openings  03  at least on its longitudinal section which works together with the web  02 . 
     This simplest embodiment, without a preferred direction for the arrangement of the micro-openings  03 , becomes possible because of the provision of the openings  03  as micro-openings  03 . Because of this structure, a thinner, but more homogeneous air cushion is produced. At the same time, a required, or a resulting volume flow, and with that also a flow loss over the “open” side, is considerably reduced. In contrast to openings with a large cross section, the high resistance to fluid flow of the micro-openings  03  has a result that the “non-coverage” of an area of openings  03  does not lead to a sort of short-circuit flow through those non-covered openings. The partial resistance falling off via the openings  03  is given a greater weight in the total resistance. 
     In a first preferred embodiment of several structures of guide elements  01 , as seen in  FIGS. 2 to 6 , the micro-openings  03  are embodied as open pores on the surface of a porous, and in particular, on the surface of a micro-porous, air-permeable material  06 , such as, for example, an open-pored sinter material  06 , and, in particular, a sinter metal. The pores of the air-permeable porous material  06  have a mean diameter or a mean size of less than 150 μm, for example a size of 5 to 60 μm, and in particular a size of 10 to 30 μm. The material is provided with an irregular amorphous structure. 
     The selection of the material, its dimensioning and its charging with fluid under pressure have been made in such a way that 1 to 20 standard cubic meters of fluid per m 2  of surface, and, in particular, 2 to 15 standard cubic meters of fluid per m 2  of surface, exit from the air outlet surface of the sinter material. An air escape of 3 to 7 standard cubic meters per m 2  of surface is particularly advantageous. 
     In an advantageous manner, the sinter surface of the guide element  01  is charged with an excess pressure of at least 1 bar, and in particular of more than 4 bar, out of the hollow chamber  04 . Charging the sinter surface of the guide element  01  with an excess pressure of 5 to 7 bar is particularly advantageous. 
     If the hollow space  04  of the guide element  01  is essentially defined only by a body of porous material  06  enclosing the hollow space  04 , i.e. without any further load-bearing layers, at least at its longitudinal section, which is acting together with the web  02 , this body may be, for example, embodied in the form of a tube, and is embodied to be substantially self-supporting with a wall thickness of more than or at least equal to 2 mm, and in particular with a wall thickness of more than or at least equal to 3 mm, as seen in  FIG. 2 . If necessary, a support can extend through the hollow space  04 , on which support the porous material body can be supported at points, or in certain areas, but which support is not in active contact with the body. Such a body of porous material  06  can also be embodied in the form of a half shell, as represented in  FIG. 3 . 
     To achieve a uniform distribution of the air exiting at the surface of the micro-porous material  06 , without at the same requiring large layer thicknesses of the material  06 , with a resultant correspondingly high flow resistance, it is useful, in an advantageous embodiment of the present invention, that the guide elements  01  have a solid support  07 , which is air-permeable at least in part and on which solid support  07  the micro-porous material  06  has been applied as an outer layer  06 , as shown in  FIGS. 4 ,  5  and  6 . Such a support  07  can be charged with compressed air, which compressed air flows out of the support  07  through the micro-porous layer  06  and in this way forms an air cushion at the surface of the guide element  01 . In a particularly advantageous embodiment of the present invention, the porous material  06  is therefore not embodied as a supporting solid body, either with or without a frame structure, but instead is provided as a layer  06  on a support material  07 , which support material  07  has passages  08  or through-openings  08  and which is, in particular, made of metal. A structure is understood to be the “non-supporting” layer  06  together with the support  07 , in contrast to, for example, the above mentioned “self-supporting” layers, wherein the micro-porous layer  06  is supported, over its entire layer length and its entire layer width, on a multitude of support points of the support  07 . For example, the support  07  has, over its width and length which is active together with the micro-porous layer  06 , a plurality of non-connected passages  08 . This embodiment is clearly different from the embodiment in which a porous material  06 , which is extending over the entire width and which is active together with the web  02 , is configured to be self-supporting over this distance, and is only supported in the end area on a frame or a support, and therefore must have an appropriate thickness. 
     In the preferred embodiment represented in  FIGS. 4 ,  5  and  6 , the underlying support material absorbs substantially all of the weight, torsion, bending and/or shearing forces of the component, for which reason an appropriate wall thickness, for example greater than 3 mm, and in particular greater than 5 mm of the support  07  and/or an appropriately reinforced construction has been selected. The support  07  which, for example, defines the hollow space  04  facing toward the micro-porous layer  06 , or which constitutes the hollow space  04  by an appropriate shaping, for example by being tube-shaped, as seen in  FIG. 4 , has, on the side of support  07  that is coated with the micro-porous material  06 , a plurality of openings  09  for the supply of compressed air to the micro-porous material  06 . Micro-porous material  06  can also be partially located in the openings  09  of the support  07  in the area of the walls. 
     As represented in  FIGS. 4 ,  5  and  6 , the guide element  01  has the support  07 , which is also called the base body  07 , with the hollow or inner space  04 , and which may be, for example, a tube-shaped support  07 , as seen in  FIG. 4 , which support  07  has a plurality of the penetrating openings  09  in its wall and extending radially as far as the surface. In principle, the support  07  can be configured with any arbitrary hollow profile, but advantageously it is configured with a ring-shaped profile. During the operation, a fluid, for example gas, which is at a pressure P that is greater than the ambient pressure, is blown through the hollow space  04  and out through the openings  09 , for example by the use of a compressor, which is not specifically represented. At least in the section provided with the openings  09 , the surface of the support  07  has the layer  06  of a micro-porous material, which layer  06  also covers the openings  09  and extends continuously over the area of the guide element  01  which is working together with the web  02 , i.e. a continuous surface at least in the area of the guide element  01  which is provided for looping the web  02 . 
     In another embodiment of the present invention, as seen in  FIGS. 5 and 6 , the hollow space  04  is not constituted by a tube with a support  07  configured in a ring shape, but which instead is structured with a different geometry. Advantageously, the support  07  has a wall  15  in the shape of a segment of a circle, or a wall  15 , in particular with a fixed radius, or with a radius of curvature R 07  or R 15  in relation to a fixed center M 07 , which is closed on its open side, for example by a cover  20 . This wall  15 , in the shape of a segment of a circle with the cover  20 , can be embodied as one piece or as several pieces, which are however connected with each other. In  FIG. 5  the angle γ of the partial circle of the wall  15  having the openings  09  has been selected to be approximately 180°. With a defined width b 01  of the guide element  01 , as seen in  FIG. 6  and with this defined width being limited, for example because of a maximum width which is predetermined for reasons of structural space, the largest possible area of the guide element  01  can be achieved with this step. With a desired or with a predetermined width b 01 , the radius R 15  of the partial circle, or of the tube used as the raw material is selected on the basis of the desired deflection, deflection angle α of the change of direction of the web  02 ; as seen in  FIG. 1 , and an appropriate partial circle is used. In this way, a change of direction takes place as “softly” as possible and is supported by the air cushion over the largest possible area in the available structural space. 
     In the representation of  FIG. 6 , the angle γ of the partial circle is less than 180°, and, for example, is between 10° and 150°, and in particular is approximately 90° here. In a preferred embodiment, for use in the area of the printing gap, either upstream and/or downstream of the printing unit  05 , the angle γ of the partial circle has been selected to be 10° to 45°, and in particular, between 15° and 35°. The width b 01  has been selected to be, for example, between 30 to 150 mm, and in particular to be between 50 to 110 mm. The radius of curvature R 15  of the wall  15  of the support  07  is, for example, between 120 and 150 mm, and in particular, is between 140 and 200 mm. As was the case in  FIG. 5 , the micro-porous layer  06  can be extended as far as the front cover  20 , or it can only cover the curved wall  15  of support  07  containing the openings  09 . In its end areas, the micro-porous layer  06  can also be flattened to form a soft transition. 
     By the above-mentioned steps, a surface of an air cushion, which is as large as possible and which acts as a support, can be achieved at a width b01 of the guide element  01  or at a width b 07  of the support  07 , such as for example, a maximum width that may be preset for reasons of structural spacing. At a desired or at a predetermined width b 01 , the radius R 07  of the partial circle, or of the tube used as the raw material is selected on the basis of the required web directional change, represented by way of example as the deflection α of the change of direction of the web  02  in  FIG. 1  in the first printing unit  05 , and an appropriate partial circle is used. By this selection, a change of direction takes place as “softly” as possible and is aided by the air cushion over the largest possible area in the structural space available. 
     In an advantageous embodiment of the present invention the configuration of the guide element  01  is such that the partial circle angle γ of the wall  15  is formed from the deflection angle α desired for the course of the web  02 , wherein γ=α+Δ, and wherein Δ is an addition for an assumed run-up and run-off of the web  02  and is selected to lie between 0° and 50°, and in particular is selected to lie between 10° and 30°. The radius of curvature R 07  of the support  07  is then selected to be such that, taking the addition A into consideration, the desired width b 01  or b 07  is maintained. The radius of curvature R 15 , or R 07  is then selected to be R 15  or R 07 =b 01 /(a*sin(y/2)). An excess projection possibly created by the layer thickness is negligible because of the slight thickness. Thus, while taking dependability into consideration, a large active surface is formed, together with an optimal use of the space. 
     With needed deflection angles α starting at, for example, 120°, a semi-circular profile or even a full circle profile can be of advantage for the guide element  01 , for reasons of simplification. In this case, the opening  09  and/or the micro-porous layer  06  can include the full 360° angle, or only a partial circle. 
     Basically, other profiles, differing from partial circles, are conceivable for the area of the guide element  01  or of its curved wall  15  interacting with the web  02 , such as, for example, a section of an ellipse, parabola or hyperbola. In this connection, the curved shape of the directional change can be optimized in view of a “soft” directional change. However, the partial circle shape has advantages with respect to standardization, to material use and for simplified manufacture. 
     In contrast with the embodiment of a guide element  01 , wherein the micro-porous material  06  is not underlaid, to a great extent, by a support  07  or by a base body  07  having openings  09 , but instead is only supported, for example, in a bridge-like manner, on a frame-like support in edge areas, the embodiment of the shape of a base body  07  in the shape of a partial circle, an ellipse, a parabola or a hyperbola, directly underneath the micro-porous layer  06 , has great advantages with respect to manufacture, to dimensional stability, to costs and to handling. For example, with this embodiment, at least half of the surface of the micro-porous layer  06 , working together with the web  02 , is underlaid by the support  07 , or by its curved wall  15 , and/or by openings  09  or free cross sections have a diameter or a maximum inside width of 10 mm, and in particular off less than or equal to 5 mm. 
     In connection with the above-mentioned examples embodied with the support  07 , the micro-porous material  06  located outside of the passage  08  has a layer thickness which is less than 1 mm. A layer thickness of this micro-porous material  06 , between 0.05 mm and 0.3 mm is particularly advantageous. A proportion of the open face, in the area of the effective surface of the porous material, here called degree of opening, lies between 3% and 30%, and preferably lies between 10% and 25%. To achieve an even distribution of air it is furthermore desirable for the thickness of the micro-porous layer  06  to correspond at least to the distance between adjoining openings  09  in the support  07 . 
     The wall thickness of the support  07  is, at least in the area with the layer, preferably greater than 3 mm, in particular is greater than 5 mm. 
     The support  07 , provided with a hollow profile, if desired, can itself also be made of a porous material, but with a better air permeability, for example with a greater pore size, than that of the micro-porous material of the layer  06 . In this case, the openings  09  of the support  07  are constituted by open pores in the area of the surface, and the passages  08  are constituted by channels which are incidentally formed in the interior because of the pores. However, the support  07  can also be constituted by any arbitrary flat material enclosing the hollow space  04  and which is provided with passages  08 , or by formed material. Combinations of this alternative can also be considered. 
     In a second preferred embodiment of the present invention, as seen in  FIGS. 7 to 9 , the micro-openings  03  are configured as openings of penetrating bores  11 , and in particular of micro-bores  11 , which extend outward through a wall  12 , for example a chamber wall  12 , which chamber wall is bordering a hollow chamber  04 , for example configured as a pressure chamber  04 . For example, the micro-bores  11  have a diameter, at least in the area of the openings  03 , of less than or equal to 500 μm, advantageously less than or equal to 300 μm, and in particular between 60 and 150 μm. The degree of opening lies between 3% to 25%, and in particular lies between 5% to 15%, for example. The hole density is at least ⅕ mm 2 , and in particular is at least 1/mm 2  up to 4/mm 2 . Therefore, the wall  12  of the web guide element  01  has a micro-perforation, at least in an area located opposite the web  02 . The micro-perforation advantageously extends over the area which works together with the web  02 . However, it can extend as the passages  08  and the micro-porous layer  06  in the first preferred embodiment, over the full circumference of 360° since, as mentioned, the losses are kept within limits. 
     In a second preferred embodiment of the guide element  01  with micro-bores  11 , as seen in  FIG. 8 , the chamber wall  12  has, on the side facing the web  02 , a curved wall  14  or a curved wall section  14 , which is comparable with the curved wall  15  described in connection with  FIGS. 5 and 6 , which has the micro-bores  11 . What has been said in connection with the angles α, γ, Δ and in connection with the width b 01  or b 07 , here b 01  or b 12  and the radius R 15  here R 14  in connection with  FIGS. 5 and 6 , as well as with the way of proceeding and the selection of the radii of curvature, should be applied in the same way to the described example. 
     In a preferred embodiment of the present invention, in accordance with  FIG. 9 , the wall  14  with the micro-bores  11  is embodied as an insert  14  or as several inserts  14  which may be arranged side by side in a support  16 . Each insert  14  can be connected, either fixedly or releasably, or exchangeably in the support  16 . The releasable connection is advantageous in view of possible cleaning or of an exchange of inserts  14  with different micro-perforations for adaptation to different materials, with a different mass and/or surface structure, and web widths. In the variation of this embodiment of the present invention, with inserts  14  and/or with micro-openings substantially arranged over the full circumference, such inserts  14  can, for example, be arranged on a support  16  extending in the hollow space  04 . However, an embodiment of the present invention is also advantageous wherein, as represented in  FIG. 9 , the insert  14  with the openings  09  is only embodied over an angle segment with a curvature, in particular with a curvature that is matched to the path of the web. 
     Again, what was previously said in connection with the angles α, γ, Δ and the width b 01  or b 07 , here b 01  or b 12  and the radius R 15  here R 14  in connection with  FIGS. 5 and 6 , as well as with the way of proceeding and the selection of the radii of curvature, should be applied in the same way to the present example for embodying the curved surface of the insert  14 , or inserts  14 . However, here a projection or a difference between an insert width and a support width must possibly be taken into consideration. The curvature can be forced, for example, by an intentional excess width of the insert  14  with respect to the support  16 , or the fastening arrangement of the latter in the form of a resultant bending. 
     As represented in  FIG. 9 , the releasable connection between the inserts  14  and the support  16  can be realized, for example, by the provision of grooves  17  in the support  16 , which grooves  17  receive the ends of the insert  14 . In addition, or instead, a connection can also be made by screwing or clamping. 
     A wall thickness of the chamber wall  12  or the insert wall  14  or of the insert  14  containing the bores  11  which thickness, inter alia, affects the flow resistance, can be between 0.2 to 0.3 mm, is advantageously between 0.2 to 1.5 mm, and in particular is set at 0.3 to 0.8 mm, for all of the examples concerned. With the smaller ones of the wall thicknesses mentioned in particular, a reinforcing structure, such as, for example, a support extending in the longitudinal direction of the guide element  01 , and in particular a metal support, can be arranged in the interior of the guide element  01 , and in particular can be arranged in the hollow space  04 , on which the chamber wall  12 . the wall  14 , or the insert  14  are supported at least in part or at points. This support can, for example, be provided by ribs which are spaced apart from each other in the axial direction. 
     In connection with the embodiment of the micro-openings  03  in the form of bores  11 , an excess pressure in the chamber  04  of, for example, 0.5 to 2 bar, and in particular of 0.5 to 1.0 bar, is advantageous. 
     The bores  11  can be configured to be cylindrical, funnel-shaped or in another special shape, such as, for example, in the form of a Laval nozzle. 
     The micro-perforation, i.e. the making of the bores  11 , preferably takes place by drilling by the use of accelerated particles, such as, for example, a liquid, such as a water jet, ions or elementary particles, or by the use of electromagnetic radiation of high energy density, for example by light in the form of a laser beam. The making of the micro-perforations by the use of an electron beam is particularly advantageous. 
     The side of the wall  12  or  14  having the bores  11  and facing the web  02 , for example a wall  12  or  14  made of special steel, in a preferred embodiment has a dirt and inkrepelling finish. It has a coating which is not specifically represented of, for example nickel or advantageously chromium which coating does not cover the openings  03  or bores  11 , and which coating has, for example, been additionally treated for example with micro-ribs or is structured in a lotus flower-effect, or which preferably has been polished to a high gloss. 
     While preferred embodiments of a printing unit with guide elements, in accordance with the present invention, have been set forth fully and completely hereinabove, it will be apparent to one of skill in the art that various changes in, for example the structure of the printing units, the source of supply of the fluid under pressure and the like could be made without departing from the spirit and scope of the present invention which is accordingly to be limited only by the appended claims.