Abstract:
A method for applying powder to a printed sheet includes ejecting air flows from a nozzle bar. The air flows are set in such a way that the air flows which are ejected per meter length of the nozzle bar produce a resultant force acting on the printed sheet of from 0.5 newtons to 16 newtons. A printing press for carrying out the method is also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority, under 35 U.S.C. §119, of German Patent Application DE 10 2006 014 252.7, filed Mar. 28, 2006; the prior application is herewith incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a method for applying powder to a printed sheet, in which air flows are ejected from a nozzle bar. The invention also relates to a printing press for carrying out the method. 
     Printed sheets are powdered in deliveries of printing presses, in order to prevent ink transfer from one sheet to the other in a delivery stack. A defined powder amount has to be applied per sheet in order to ensure that this aim is achieved. For that purpose, powdering apparatuses are used which eject the powder in a powder air flow from a nozzle bar. Since not all of the ejected powder amount adheres to the sheet, operation has to take place with a defined powder excess. However, that powder excess should be kept as small as possible, because it leads to contamination of the delivery. 
     It could be assumed that more effective powdering could be achieved and therefore the powder loss could be reduced, by setting the powder air flow to be more powerful. 
     However, it is apparent from German Patent DE 197 51 383 B4, corresponding to U.S. Pat. No. 6,413,580, that excessively powerful setting of the powder air flow has a disadvantageous effect on the sheet run. It is specified in that prior art that it is disadvantageous to load the sheet with a relatively large “air impulse flow” which lies in a region of 0.04 newtons. 
     Furthermore, German Published, Non-Prosecuted Patent Application DE 2004 053 099 A1 describes a printing press having a delivery which has gripper bars for fixedly holding the respective sheet at its front edge and rear edge at the same time. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a method for applying powder to a printed sheet and a printing press for carrying out the method, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type and with which more effective powdering is ensured. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a method for applying powder to a printed sheet. The method comprises ejecting air flows from a nozzle bar. The ejection of the air flows per meter length of the nozzle bar is set to produce a resultant force acting on the printed sheet of from 0.5 newtons to 16 newtons. The airflows can be powder air flows which contain the powder or supporting air flows which envelop the powder air flows at least partially. 
     The magnitude of the resultant force which lies in the range of from 0.5 newtons to 16 newtons is therefore at least more than ten times the magnitude which is specified in German Patent DE 197 51 383 B4, corresponding to U.S. Pat. No. 6,413,580. 
     In accordance with another feature of the invention, the resultant force lies in a range of from 3.5 newtons to 10 newtons, and preferably in a range of from 4.0 newtons to 5.0 newtons. In the last-mentioned development, the magnitude of the resultant force is therefore at least 100 times the magnitude which is specified in German Patent DE 197 51 383 B4, corresponding to U.S. Pat. No. 6,413,580. 
     In accordance with a further feature of the invention, the printed sheet is transported past the nozzle bar through the use of a transport device, and a rear edge of the printed sheet is fixed in the process through the use of the transport device. In this context, the transport device is understood to be a moving transport device which is therefore different than an immovable sheet guiding device. The rear edge has a substantially constant vertical spacing relative to the nozzle bar, as a result of the rear edge being fixed. The transport device can be a conveyor belt, on which the sheet rests, including its rear edge. Instead of the conveyor belt, a plurality of conveying belts which run in parallel can also be used. The rear edge can be fixed on the transport device only under the action of the air flows, which presses the rear edge against the transport device. In this case, the rear edge is supported by the transport device on the sheet side which faces away from the air flows. However, there can also be provision for the rear edge to be attracted by suction by the transport device, in order to fix the rear edge. For example, the above-mentioned conveyor belt can be a suction belt. 
     In accordance with an added feature of the invention, the transport device includes a front gripper bar and a rear gripper bar, a front edge of the printed sheet is held fixedly through the use of the front gripper bar, and the rear edge of the printed sheet is held fixedly at the same time through the use of the rear gripper bar. Tests have shown that a printed sheet which is clamped at both ends in the gripper bars in this way has a sufficiently stable sheet run which permits loading of the sheet with extraordinarily powerful air flows. Surprisingly, the pressure of the air flows can even be increased to such an extent that the resultant force of the air flows which acts on the sheet achieves the magnitude which was specified in the previous text. It has been proven that fluttering movements of the rear edge of the sheet, which were otherwise caused by the extraordinarily powerful air flows, are suppressed reliably by the rear gripper bar. The rear gripper bar is capable of applying sufficiently high clamping forces, by way of which the sheet is held reliably and the risk of the rear edge of the sheet being ripped out of the rear gripper bar, which is conceivable due to the high resultant force of the air flows, can be precluded. An absolutely stable, undisrupted sheet run is therefore ensured. 
     With the objects of the invention in view, there is also provided a printing press for carrying out the method according to the invention. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a method for applying powder to a printed sheet and a printing press for carrying out the method, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic, side-elevational view of a printing press having a delivery; 
         FIG. 2  is a top-plan view of the delivery, as seen from a viewing direction II in  FIG. 1 ; and 
         FIG. 3  is a rear-elevational view of a nozzle bar of the printing press, as seen from a viewing direction III in  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the figures of the drawings in detail and first, particularly, to  FIG. 1  thereof, there is seen a printing press  1  having a lithographic printing unit  2  and a delivery  3  for sheets  4  made of printing material. The printing unit  2  includes, for offset printing, a printing form cylinder  5 , a blanket cylinder  6  and an impression cylinder  7 . The delivery  3  has a sheet transport device  77  having a first chain conveyor  8  and a second chain conveyor  9  which transport the sheets  4  to a delivery stack  10  and rotate synchronously with one another in the process. The first chain conveyor  8  includes front gripper bars  11  for holding the sheets  4  at their respective leading sheet end  4 . 1  or edge (front edge), and the second chain conveyor  9  includes rear gripper bars  12  for holding the sheets  4  at their respective trailing sheet end  4 . 2  or edge (rear edge). The gripper bars  11 ,  12  act as holding devices for holding the sheets  4  at both ends. 
     With reference to  FIG. 2 , it is seen that the first chain conveyor  8  includes a pair of endless chains  90  which carry the front gripper bars  11  between them, and the second chain conveyor  9  includes another pair of endless chains  140  which carry the rear gripper bars  12  between them. Each sheet  4  is therefore held fixedly, during its transport which takes place in a running direction  13 , through the use of a front gripper bar  11  and at the same time through the use of a rear gripper bar  12 . 
     A powdering apparatus seen in  FIG. 1  has a nozzle bar  14  which is disposed within circulating paths of the chain conveyors  8 ,  9 . The nozzle bar  14  is disposed at a spacing a relative to the sheet transport path and substantially also with respect to the printed sheet  4  which sags a little between the gripper bars  11 ,  12 . The spacing a is at least 80 mm and at most 300 mm. The spacing preferably lies in a range of from 100 mm to 200 mm. 
     As is shown in  FIG. 2 , the first chain conveyor  8  has one gearwheel or chain sprocket  80  on each of a drive side and an operating side and the respective endless chain  90  which circulates around the latter. The endless chains  90  of the first chain conveyor  8  carry the gripper bars  11  which lead between them in the running direction  13 , in order to hold the leading sheet ends  4 . 1  of the sheets  4 . The second chain conveyor  9  likewise includes a chain sprocket  130  on each of the two machine sides and the respective endless chain  140  which circulates around the latter. The endless chains  140  of the second chain conveyor  9  carry the trailing gripper bars  12  between them, in order to hold the sheet ends  4 . 2  which trail in the running direction  13 . Each one of the leading gripper bars  11 , together with a respective one of the trailing gripper bars  12 , forms a gripper bar pair which holds the respective sheet  4  fixedly at both ends during its transport which takes place toward the delivery stack  10 . As a result of a phase adjustment of one chain conveyor  8  relative to the other chain conveyor  9 , the format of a gripper bar spacing between the rear gripper bar  12  and the front gripper bar  11  of each gripper bar pair can be set as a function of the sheet length of the respective print job. 
     A sheet support  180  which lies toward the drive side and a sheet support  190  which lies toward the operating side, are structurally identical with one another and serve to press the respective sheet  4  against the circumferential surface of the impression cylinder  7 . The sheet supports  180 ,  190  are constituent parts of a delivery drum  430  of the delivery  3 . The delivery drum  430  is configured in a skeleton construction, and can be adjusted along its geometrical rotational axis  200 , which is also the rotational axis of the chain sprockets  80 ,  130 , in an infinitely variable manner. Therefore, the delivery drum  430  can be adjusted from a format setting (shown with a solid line in  FIG. 2 ) for a maximum sheet width of the sheets  4  to a format setting (indicated with a phantom line in  FIG. 2 ) for a minimum sheet width, as well as into intermediate positions which lie between these two extreme positions for medium sheet widths. In each format setting, the drive-side sheet support  180  is aligned with one printfree side edge and the operating-side sheet support  190  is aligned with another printfree side edge, of the respective sheet  4 . The sheet supports  180 ,  190  are mounted in such a way that they can be displaced axially by motor, between those chain sprockets of the chain conveyors  8 ,  9  which are disposed on the drive side and those chain sprockets which are disposed on the operating side. The drive (motor, gear mechanism) which is required for the axial displacement of the sheet supports  180 ,  190  that can take place optionally toward one another or away from one another, is not shown in  FIG. 2  for reasons of improved clarity. 
       FIG. 3  shows the nozzle bar  14  which extends substantially over the entire width of the printed sheet  4 . The nozzle bar  14  includes nozzles  78  which are disposed in a row. In the simplest case, the nozzle bar  14  could be a tube, in the wall of which the nozzles  78  are formed as bores. In the case which is more relevant in practice, the nozzle bar  14  includes a holding crossmember, to which the nozzles  78  are fastened as nozzle heads. In the example which is shown in the drawing, an effective length L of the nozzle bar  14  is precisely 1 meter. Each nozzle  78  ejects an air flow in the form of a powder air flow  79 , which guides the powder in the direction of the printed sheet  4 . The outlet velocity of the air flow from the nozzle  78  is approximately 170 meters per second. The powder air flows  79  exert forces on the printed sheet  4  when they impinge on it. The force which results from these forces is denoted by the designation F and is 4.0 newtons. The force F is a specific force which is related to a length unit (1 meter) of the nozzle bar  14 .