Abstract:
A sheet-fed offset printing press for printing multiple colors on both sides of sheets, preferably on sheets of paper, includes a first row of in-line printing units for printing a first side of the sheet, followed by a reversing device, followed by a second row of in-line printing units for printing the other side of the sheets, and followed by one or more varnishing units that are disposed in such a way, or include varnishing blanket cylinders that are disposed in such a way, that the front sides and the back sides of the passing sheets are varnished. A further reversing device is inserted between two varnishing units. A method of printing multiple colors on both sides of 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 2006 033 104.4, filed Jul. 18, 2006; the prior application is herewith incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a sheet-fed offset printing press for double-sided multicolor printing, preferably on sheets of paper. The printing press includes a first row of printing units disposed directly in line to print on a first side of the sheet, followed by a reversing device, which is in turn followed by a second row of printing units disposed in line to print on the other side of the sheet, and finally by one or more varnishing units. In-line printing presses of that type, which include one or more varnishing units provided at the end of the press for varnishing the sheet side that was printed last, are known and are available from a number of manufacturers. The invention also relates to a method of printing multiple colors on both sides of sheets. 
     German Published, Non-Prosecuted Patent Application DE 42 13 024 A1 proposes varnishing both sides of the printed sheets in one machine run, i.e. in line, in presses of the type indicated above. For that purpose, a varnishing unit and a dryer are provided upstream of the reversing device in the printing press. Thus, the first side of the sheet is varnished and dried before the sheet is reversed and printed on the second side. 
     In that press, the first side of the sheet is varnished before the sheet passes the printing nips of the perfecting units. Consequently, the shine effect created on the sheet surface as a result of the varnishing operation is reduced: the more perfecting units or printing nips that are present, the less shine remains. In addition, problems occur in particular when the varnish applied to the first side of the sheet has not dried completely in the drying device. In that case, the surfaces of impression cylinders in the perfecting unit, which are optimized for contact with oil-based offset printing units, may become soiled by residual varnish, mixed with paper dust etc., which means that they will have to be cleaned in a time-consuming process. 
     In another type of known printing press, described in European Patent Application EP 0 976 555 A1, corresponding to U.S. Pat. No. 6,338,299, for example, the sheet is printed with offset printing inks alternatingly by printing units that are disposed above and below the path of paper travel. Subsequently, the sheet is varnished on both sides by varnishing units respectively disposed above and below the path of paper travel. Apart from the fact that that type of printing press is not very common because access to the printing units below the path of paper travel is difficult, those presses have the following problem: when the last color is printed onto the second side, the sheet has already passed all of the printing nips. In comparison with a press that prints first on one side and then on the other side completely, the printed image is widened to a greater extent (fan-out effect) because the sheet passes through twice the number of printing nips, and it is impossible to compensate for that effect by register adjustment. 
     In addition to in-line varnishing units, there are separate, stand-alone varnishing devices known in the art to varnish both sides of printed sheets. As is described in European Patent Application EP 1 117 487 A1, corresponding to U.S. Pat. No. 6,718,908, such varnishing devices may also include reversing devices. However, a disadvantage of stand-alone varnishing devices is that the sheets must first be formed into a pile and then be fed to the varnishing device individually. When they are fed to the varnishing device, the sheets coming from the delivery pile of the printing press have already been powdered, a fact which may be detrimental to the shine effect created by the varnishing operation and may otherwise cause difficulties. In addition, stand-alone varnishing devices require more space and more equipment than an in-line construction because two feeders and two deliveries are necessary. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a sheet-fed offset printing press and a method of printing multiple colors on both sides of sheets, which overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and which produce high-quality sheets that are printed and varnished on both sides in an in-line process. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a sheet-fed offset printing press for double-sided multi-color printing on sheets, preferably sheets of paper. The printing press comprises a first row of in-line printing units for printing on a first side of the sheets, a reversing device disposed downstream of the first row of in-line printing units, a second row of in-line printing units, disposed downstream of the reversing device, for printing on a second side of the sheets, at least two varnishing units disposed downstream of the second row of in-line printing units. The at least two varnishing units have varnishing blanket cylinders causing the front sides and the back sides of the sheets passing through to be varnished. A further reversing device is fitted between the at least two varnishing units. 
     With the objects of the invention in view, there is also provided a method of printing multiple colors on both sides of the sheets, preferably sheets of paper. The method comprises:
         a) individually feeding sheets in a sheet pile to a number of in-line printing units and printing on a front side of the sheets;   b) then turning the sheets having been printed in the in-line printing units and printing multiple colors on a back side of the sheets in a row of further in-line printing units;   c) then applying one or more coats of varnish to the printed back side of the sheets;   d) then turning the sheets and subsequently varnishing the front side of the sheets having been printed first; and   e) then delivering the sheets having been printed and varnished on both sides to a pile or for further processing.       

     Thus, according to the invention, the sheets are initially printed on both sides before they are varnished, and the sheet is turned between the varnishing of the front side and the varnishing of the back side. In a sheet-fed printing press of such a construction, it is possible to produce sheets that are varnished on both sides in an in-line process without the difficulties explained in the introduction. A greater degree of glossiness can thus be achieved on both sides of the sheet, and the cylinder surfaces and cylinder jackets in the perfecting unit can be optimized for contact with oil-based ink without the necessity of special cleaning operations. 
     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 sheet-fed offset printing press and a method of printing multiple colors on both sides of 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. 1A  is a fragmentary, diagrammatic, longitudinal-sectional view of a straight-printing portion in an in-line sheet-fed offset printing press; 
         FIG. 1B  is a fragmentary, longitudinal-sectional view of a perfecting portion in the in-line sheet-fed offset printing press, which is to be placed to the left of  FIG. 1A ; 
         FIG. 2  is a fragmentary, longitudinal-sectional view of a second exemplary embodiment of the perfecting portion of the printing press as shown in  FIG. 1B ; 
         FIG. 3  is a fragmentary, longitudinal-sectional view of a third exemplary embodiment of the perfecting portion of the printing press as shown in  FIG. 1B ; and 
         FIG. 4  is a fragmentary, longitudinal-sectional view of an end of the press following the last printing unit of a fourth exemplary embodiment of the perfecting portion of the printing press as shown in  FIG. 1B . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the figures of the drawings in detail and first, particularly, to  FIG. 1A  thereof, there is seen a first portion of an offset printing press  1  of in-line construction, including a feeder  2  holding a pile  3  of unprinted paper and four printing units  7   a - d  for the four process colors. The four printing units  7   a  to  7   d  form a straight-printing portion of the press  1 , i.e. the portion that prints the front or first side of the sheets. The fourth printing unit  7   d  is followed by a first reversing device  4  that operates in accordance with the three-drum reversing principle. The reversing device  4  is formed of a feed drum  4   a , a storage drum  4   b , and a reversing drum  4   c . A second portion of the press, i.e. the portion in which the back or second side of the sheets is printed, is shown in  FIG. 1B . The drum  4   c  is supported in side frames  18   a  of a first perfecting unit  8   a  following the reversing device  4 . The reversed sheet is transferred to an impression cylinder  108   a  of the first perfecting unit  8   a . Four perfecting units  8   a - d  are followed by a first varnishing unit  9   a  of the chambered doctor-blade type, i.e. the varnishing unit  9   a  includes a screen cell roller  19   a  and a chambered doctor blade  20   a  containing aqueous dispersion varnish. Reference numeral  22   a  designates what is referred to as a “screen roller star”, which includes three further screen rollers with cells of different sizes. These three further screen rollers can be exchanged for the screen roller  19   a  to determine the amount of varnish to be applied. The entire surface of the second side of the sheet is coated with an aqueous dispersion varnish in the varnishing unit  9   a  on an impression cylinder  109   a.    
     The varnishing unit  9   a  is followed by a drying tower  10   a . The second side of the passing sheet is dried in the region of a cylinder  110  by hot air and IR light in the drying tower. 
     Downstream of the drying tower  10   a , as viewed in the direction of sheet travel, there is a second reversing device  14  that is of substantially identical construction to the first reversing device  4 . The reversing device  14  likewise includes three drums  14   a, b , and  c . In this case, the reversing drum  14   c  is supported in side walls  119   b  of a second varnishing unit  9   b  following the reversing device  14  and transfers the sheet to an impression cylinder  109   b  of the varnishing unit  9   b . The varnishing unit  9   b  is of the same type as the varnishing unit  9   a  and is likewise used to coat the entire surface of the first side of the sheet with an aqueous dispersion varnish. 
     The varnishing unit  9   b  is followed by a delivery  5  of the printing press. The delivery  5  includes revolving gripper bars driven by a chain conveyor  15 . These gripper bars  16  take over the sheets that have been varnished in the unit  9   b  and guide them through dryer sections  11   a, b, c  and  d , where the first side of the sheets is likewise dried by IR light and/or hot air to harden the dispersion varnish. The sheets, which have been varnished on both sides in this way, are then deposited on a sheet pile  6  in the delivery  5 . 
     While the sheets are transported through the printing units  7   a - d  and  8   a - d , the printed sheets do not come into contact with varnish. Surfaces of sheet-guiding impression cylinders  108   a - d  in the printing units  8   a - d  and guide plates of the transfer devices disposed between the printing units  8   a - d  may thus be coated with ink-repellent layers that are adapted to or optimized in terms of the properties of the oil-based offset inks. The viscous dispersion varnish that causes soiling is not introduced until the end of the press, when the process of printing with offset printing ink is completed. Thus, compared to configurations wherein the varnishing unit is located upstream of the first reversing device  4 , the useful life of the cylinder jackets and the intervals between cleaning operations can be increased to a considerable extent. 
     The reversing device  4  is convertible, i.e. it can be switched between straight or front-side printing mode and perfecting mode as described e.g. in German Published Patent Application DE 41 31 273 A1. The same applies to the reversing device  14 . Due to this variability, the printing press  1  can print a wide variety of different jobs. If both reversing devices  4  and  14  are switched to straight printing, a number of special or spot colors can be printed on the first side of the sheets in the printing units  8  of the perfecting module. The printing unit  8   d  can additionally apply a transparent, oil-based dull varnish onto certain areas in the printed image, which will then be coated with two layers of a high-gloss dispersion varnish on top of each other in the varnishing units  9   a  and  9   b . In this manner, high-quality paper board containers printed on one side may be produced, for example for packaging. 
     Once it has been switched to the straight printing mode, in particular the convertible reversing unit  14  disposed between the two varnishing units  9   a  and  9   b  also offers the possibility of applying a dispersion varnish as a primer in the first varnishing unit  9   a  to cover or “seal off” the offset inks underneath to avoid direct contact and thus chemical reactions between the offset inks and the high-gloss UV varnishes that will be printed in the second varnishing unit. 
     Another possibility is the application of gold varnish, for example in the first varnishing unit  9   a , and the subsequent coating of the entire surface of the printed sheet with a protective varnish in the second varnishing unit  9   b.    
     Once switched to the straight printing mode, it is additionally possible to apply a clear varnish to the printed sheet in the first varnishing unit  9   a  and to subsequently dry the clear varnish before metallic varnishes are applied in the second varnishing unit  9   b . This may be done to prevent the offset inks printed on the sheet from marking the varnishing plate in the varnishing unit. 
     In the exemplary embodiment shown in  FIG. 2 , the perfecting portion of the printing press described above with reference to  FIGS. 1A and 1B  has been modified. In  FIG. 2 , the drying tower  10   a  of  FIG. 1B  has been eliminated, which means that the sheet exiting the varnishing unit  9   a  is turned over in the reversing device  14  while it is still wet. Downstream of the second varnishing unit  9   b , in which the first side is varnished, the sheet, which has now been varnished on both sides, passes through four dryer modules  21   a - h , in which the sheet is dried from both sides, i.e. from above and from below, by IR light and hot air. For this purpose, dryers  21   e  to  h , which are integrated into the sheet guiding system of the delivery  5 , may be used, as described in German Published, Non-Prosecuted Patent Application DE 10 2005 042 956 A1. 
     Since the separate drying tower is dispensed with, the press is shorter and requires less floor space than the press described with reference to the exemplary embodiment shown in  FIG. 1B . 
     In the exemplary embodiment of  FIG. 3 , the last printing unit  8   d  of the perfecting portion of the printing press  1  is followed by four varnishing units. The first varnishing unit  9   a  is of the screen roller type and applies an effect varnish onto the second side that is to be varnished. The effect varnish is dried in the following drying tower  10   a . The latter is followed by a varnishing unit  29   a , for example for UV varnish, in which the entire surface of the second side of the sheet is coated with a high-gloss transparent protection varnish. The reversing device  14  is then followed by an identical configuration of a varnishing unit  9   b  for effect varnish, a drying tower  10   b , and a varnishing unit  29   b , which applies the high-gloss protection varnish to the first side of the sheet. Intermediate-level dryers  31   a  and  31   b  disposed directly above impression cylinders  129   a  and  129   b  of the varnishing units  29   a  and  29   b  harden the UV varnish by UV light of the appropriate wavelength. In this embodiment, the dryer sections in the delivery  5  can be dispensed with. 
     When the reversing device  14  of this machine is converted from the perfecting mode to the straight printing mode, either the front side or the back side, depending on the setting of the first reversing device  4 , may be printed with four different layers of varnish on top of each other. Thus, highly specialized high-quality printed products can be produced, for example by printing a dispersion varnish in the first varnishing unit to cover and seal off the entire surface of the sheet that has been printed with offset ink, then gold varnish onto the dispersion varnish, and subsequently, in the third varnishing unit, a dull varnish to create a dull finish. The dull varnish, like the gold varnish, may only be applied to certain areas of the image (spot varnishing). In the fourth varnishing unit, the entire sheet is subsequently coated with a high-gloss UV varnish. 
     In the exemplary embodiment illustrated in  FIG. 4 , the last printing unit  8   d  of the perfecting portion is followed by a first varnishing unit  39   a  of the fountain-roller type. An applicator roller  49   a  of this varnishing unit  39   a  applies an aqueous dispersion varnish onto the second side of the sheet. An impression cylinder  59   a  of the varnishing unit  39   a  is followed by a transport drum  124   a , which also acts as a feeder drum of a reversing device  24 . An IR dryer  125   a , which applies IR light and hot air to the sheet resting on the drum  124   a  or rather its concave inner side, is disposed inside the drum  124   a . The drum  124   a  is constructed as a frame structure so that the radiation and, if applicable, the air used for the drying operation, can reach the surface of the sheet. 
     The drum  124   a  is then followed by a storage drum  24   b  and a reversing drum  24   c  of the reversing device  24 , which is followed by a second varnishing unit  39   b  of the fountain-roller type having side walls  139   b  supporting the reversing drum  24   c . A varnish applicator cylinder  49   b  of the latter varnishing unit now varnishes the first side of the sheets on an impression cylinder  59   b . This varnishing unit  39   b  is then followed by the delivery  5 , having gripper bars which take over the sheets that have now been varnished on both sides. The dryer modules  21   a - h  in the delivery  5  act on both sides of the sheets to dry the first side and to expel any residual wetness in the layer of varnish of the second side of the sheet. 
     The machine described in terms of this exemplary embodiment requires less floor space because the number of sheet-guiding cylinders can be reduced since the sheets are dried from inside the cylinder  124   a.    
     In addition to the exemplary embodiments described herein, further modifications and variations are possible. Depending on the type of varnish that is used, it is possible to use a varnishing unit with a fountain roller instead of a varnishing unit with a chambered doctor blade. Moreover, it is of course possible to provide additional printing units for printing, for example, two spot or special colors on each side of the sheets rather than to have only four printing units for the four process colors both in the straight printing portion and in the perfecting portion of the press.