Abstract:
A cylinder for conveying printed sheets along a UV or electron beam drier has inert-gas openings to which an inert gas is supplied at least temporarily during operation. A method for conveying printed sheets 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 2013 005 913.5, filed Apr. 5, 2013; 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 cylinder for conveying printed sheets along a UV or electron beam drier and a method for conveying printed sheets. 
     In printing processes, inks or varnishes are applied to printing substrates. Those inks or varnishes are cured by UV radiation or by electron beams. 
     German Utility Model DE 29 819 345 U1 discloses an inertization configuration wherein covers that are adjustable relative to the cylinder are provided. In addition, a cover device is provided that has protrusions on the front face relative to the cylinder. 
     German Utility Model DE 29 707 190 U1 describes an inertization configuration wherein sealing bars are adjustable relative to the cylinder. The cylinder has a cylinder gap with a circumferential-side cover, and the inertization space is subdivided by walls. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a cylinder for transporting printed sheets along a UV or electron beam drier and a method for conveying printed sheets, which overcome the hereinafore-mentioned disadvantages of the heretofore-known cylinders and methods of this general type. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a cylinder for conveying printed sheets along a UV or electron beam drier, comprising inert-gas openings to which an inert gas is applied at least temporarily during operation. 
     With the objects of the invention in view, there is also provided a method for conveying printed sheets, which comprises drying the printed sheets in a drying operation using a UV or electron beam drier, conveying the printed sheets over a cylinder along the UV or electron beam drier, and discharging inert gas at least temporarily during the drying operation through inert-gas openings formed in the cylinder. 
     The cylinder of the invention allows the content of residual oxygen in the inertization space of the drier to be minimized, substantially preventing it from affecting the curing of the ink or varnish. 
     In accordance with another feature of the invention, the inert-gas openings are circumferential grooves or rows of holes extending in the circumferential direction of the cylinder. 
     In accordance with a further feature of the invention, the cylinder includes suction openings. 
     In accordance with an added feature of the invention, the suction openings are circumferential grooves or rows of nozzles/holes extending in the circumferential direction of the cylinder. 
     In accordance with an additional feature of the invention, a switching device is provided for switching a subset of the suction openings from being supplied with suction air to being supplied with an inert gas, the inert-gas openings being formed by suction openings that have been switched to inert-gas supply. 
     In accordance with yet another feature of the invention, the inert-gas openings form a row of inert-gas nozzles parallel to the cylinder axis. 
     In accordance with yet a further feature of the invention, the inert-gas nozzles are disposed in a cylinder gap of the cylinder that extends in parallel with the axis of the cylinder or in a circumferential cover of the cylinder gap. 
     In accordance with yet an added feature of the invention, the inert-gas nozzles are located between sheet grippers of the cylinder. 
     In accordance with yet an additional feature of the invention, a control device for supplying an inert gas to the inert-gas openings in accordance with a sheet-conveying cycle is provided. 
     In accordance with a concomitant feature of the invention, the control device is a valve device and the control device includes a cycle valve and a selection valve. 
     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 cylinder for conveying printed sheets along a UV or electron beam drier and a method for conveying printed sheets, 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 SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a diagrammatic, cross-sectional view of a sheet-conveying cylinder with an associated drier; 
         FIG. 2  is an enlarged, fragmentary, perspective view of the cylinder; 
         FIG. 3  is a perspective view of the cylinder with a sheet of printing material held thereon; and 
         FIG. 4  is a block diagram of a control device for controlling an application of a vacuum and of an inert gas to the cylinder. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the figures of the drawings in detail and first, particularly, to  FIG. 1  thereof, there is seen a section of a printing press  1  for lithographic offset printing on printed sheets or signatures  11  (see  FIG. 3 ). The section includes a cylinder  2  for conveying the printed sheets  11  and a drier  4  for drying the printed sheets  11  that are being conveyed on the cylinder  2 . The drier  4  is a UV drier or an electron beam drier. The printing press  1  applies an ink or a layer of varnish to the printed sheet  11 . The ink or varnish is subsequently cured in the drier  4  by UV or electron beams. 
     The drier  4  is formed of UV radiation or electron beam emitters, feed devices for supplying an inert gas to a space between the drier  4  and the cylinder  2  and seals for preventing the inert gas from leaking from the space and for preventing oxygen from infiltrating the space from the environment. The emitters are disposed between the feed devices as viewed in the conveying direction of the sheet  11 , and the feed devices are disposed between the seals. For reasons of simplicity, emitters, feed devices, and seals are not shown in the drawing. The supplied inert gas is nitrogen. 
     The cylinder  2  has a cylinder body with a cylinder gap  3  in which sheet grippers  7  (see  FIG. 2 ) for holding the printed sheets  11  are located. The cylinder gap  3  is largely covered by a circumferential cover  5 . 
     As shown in  FIG. 2 , the sheet grippers  7  are disposed in a row and protrude from the cylinder gap  3  through holes in the circumferential cover  5 . Inert-gas nozzles  8  alternate between the sheet grippers  7  in the row. In addition to the circumferential cover  5 , a lateral cover  6  is provided for the cylinder gap  3  on each of its two ends. 
     In  FIG. 2 , the printed sheet  11  conveyed by the cylinder  2  is not illustrated in order for a row of circumferential grooves  9 ,  10  in the cylinder body to be visible. The circumferential grooves  9 ,  10  are suction grooves and are used to hold the printed sheets  11  by providing a vacuum. Each circumferential groove  9 ,  10  extends in the circumferential direction of the cylinder  2  and the row of circumferential grooves  9 ,  10 , like the row of sheet grippers  7  and the row of inert-gas nozzles  8  integrated therein, extends along the axial direction of the cylinder  2  across the width of the printing format. 
       FIG. 3  shows the cylinder  2  holding a printed sheet  11 . The printed sheet  11  is clamped on its front edge by the sheet grippers  7  and, with the exception of the front edge needed for clamping, is held by suction along its entire sheet length by the circumferential grooves  9  located underneath the printed sheet  11 . 
     It can be seen that not all of the provided circumferential grooves  9 ,  10  are located underneath the printed sheet  11  and covered by the latter. Depending on the respective format of the printed sheets  11 , a larger or smaller number of circumferential grooves  10  is located outside the printed sheet  11  next to its side edges. One half of these non-covered circumferential grooves  10  is close to one cylinder end and the other half is close to the other cylinder end. In the illustrated exemplary embodiment, the printed sheet  11  is located between two non-covered circumferential grooves  10 , one of which is on one cylinder end and the other of which is on the other cylinder end. 
     While suction air or a vacuum is applied to the covered circumferential grooves  9 , an inert gas, which is nitrogen in the illustrated example, is applied to the non-covered circumferential grooves  10 . As is the case with the inert-gas nozzles  8  (se  FIG. 2 ), the inert gas is fed to these non-covered circumferential grooves  10  from the interior of the cylinder  2 . By filling the non-covered circumferential grooves  10  with an inert gas, air or oxygen contained therein is prevented from entering the aforementioned space between the drier  4  and the cylinder  2  due to the rotation of the cylinder  2 . 
     The space forms an inertization chamber in which the protective gas protects the radiation-curable coating on the printed sheet  11  against the effects of oxygen on the curing process while the coating is irradiated and cured. 
     Likewise, the fact that the recesses for the sheet grippers  7  in the circumferential cover  5  are filled with the inert gas prevents ambient air from infiltrating the space through these recesses. 
       FIG. 4  illustrates a supply system applying suction air or a vacuum to the covered circumferential grooves  9  (see  FIG. 3 ) and supplying the inert gas to the inert-gas nozzles  8 . The supply system includes an inert-gas source  12  and a vacuum source  13 . The inert-gas source  12  supplies the inert gas to a pulsed valve  14  that is connected to the inert-gas nozzles  8  on one hand and to a selection valve or switch  15  on the other hand. The circumferential grooves  9 ,  10  are connected to the selection valve  15 . The selection valve  15  can be used to adjust the subset of circumferential grooves  9 ,  10  to which the vacuum from vacuum source  13  is applied—namely to those circumferential grooves  9  that are covered by the printed sheet—and the subset of circumferential grooves  9 ,  10  to which the inert gas is supplied by the pulsed valve  14 , namely to those circumferential grooves  10  that are located outside the respective sheet format. 
     The pulsed valve  14  pulses the supply of inert gas in accordance with the conveying cycle of the printed sheets  11 . In other words, in the case of a cylinder  2  that has two diametrical sheet-gripper systems, there will be two inert-gas pulses per cylinder revolution. The periodical opening and interruption of the inert-gas supply by using the pulsed valve  14  prevents the inert gas from being expelled from the inert-gas nozzles  8  and from the circumferential grooves  10  during that cylinder revolution phase in which the inert-gas nozzles  8  and the circumferential grooves  10  are not located opposite the drier  4  (see  FIG. 1 ). This saves inert gas. Alternatively, the pulsed valve  14  might operate in such a way that the inert gas is supplied to the inert-gas openings, i.e. the non-covered circumferential grooves  10  and the inert-gas nozzles  8  of the cylinder  2 , shortly before they enter the drying chamber or space between the drier  4  and the cylinder  2  in the course of the rotation of the cylinder  2 . 
     The supply device shown in  FIG. 4  may be connected to the cylinder  2  by a rotary valve or a rotary joint. The rotary joint transmits the inert-gas stream from the stationary supply device external to the cylinder to the rotating cylinder  2  and the suction-air stream from the rotating cylinder  2  to the supply device that is external to the cylinder  2  and does not co-rotate with the latter. 
     In accordance with a modified embodiment that is not shown in the drawing, each of the circumferential grooves  9 ,  10  is replaced by a row of nozzles or openings. Like the circumferential grooves  9 ,  10 , these rows of nozzles extend in the circumferential direction of the cylinder  2  and have the same function, i.e. they hold the printed sheet by suction where the nozzles are covered by the printed sheet and expel inert gas where the nozzles are not covered by the printed sheet. The nozzles are accordingly supplied with suction air and inert gas by the supply device shown in  FIG. 4 .