Abstract:
A plate cylinder has a cylinder body, a first image area at one end of the cylinder body, the first image area having a first average diameter, a second image area at the other end of the cylinder body having a second average diameter, and a third image area between the first and second image areas. The third image area has a third average diameter larger than the first and second average diameters. A print unit, web offset printing press and a plate bender are also provided.

Description:
BACKGROUND INFORMATION  
       [0001]     The present invention relates generally to offset printing and more specifically to a plate cylinder for an offset printing press and related plate bender.  
         [0002]     U.S. Pat. No. 4,487,122 discloses a system for compensating for roll deflection to provide contact pressure across the width of a web disposed between a pair of counter rollers. U.S. Pat. No. 5,094,163 discloses a rotogravure press with electrically conductive layers. U.S. Pat. No. 5,913,266 and U.S. Patent Publication No. 2003/0061955 disclose plate cylinders, and are hereby incorporated by reference herein. U.S. Pat. Nos. 6,283,027 and 6,105,498 disclose varying profile blankets, including printing blankets with concave and convex profiles. A concave blanket cylinder is also disclosed.  
         [0003]     U.S. Pat. No. 5,987,949 discloses a plate bender and is hereby incorporated by reference herein.  
       BRIEF SUMMARY OF THE INVENTION  
       [0004]     Especially with multi-image width printing presses, deflection at the middle of the plate and blanket cylinders can provide problems with printing a web. For example, the middle of the web may be printed too lightly.  
         [0005]     U.S. patent application Ser. No. 10/617,639, filed Jul. 11, 2003 and hereby incorporated by reference herein, discloses a printing blanket comprising a carrier sleeve layer having at least one axially convex surface when disposed on a blanket cylinder and a print layer disposed over the carrier sleeve layer. In this application, the convexity of the carrier sleeve layer may be provided, for example, by having the carrier sleeve layer have a uniform inner diameter and a convex outer diameter. The carrier sleeve layer itself is thus thicker in an axial middle than at the ends. Alternately, the carrier sleeve can be of uniform thickness, and the blanket cylinder or a shim may provide the surface convexity.  
         [0006]     Crowning of blankets or a blanket cylinder to have a larger diameter can result to fan-in, i.e. a lateral squeezing together of the web and thus the print, during printing.  
         [0007]     The present invention provides a plate cylinder comprising a cylinder body, a first image area at one end of the cylinder body, the first image area having a first average diameter, a second image area at the other end of the cylinder body having a second average diameter, and a third image area between the first and second image areas, the third image area having a third average diameter, the third average diameter being larger than the first and second average diameters.  
         [0008]     By having a larger diameter image area in the middle of the plate cylinder, print pressure or squeeze can be increased at the center of the web.  
         [0009]     Each image area may comprise two side-by side images, so that the plate cylinder is six images wide. Preferably, each image area supports a single image in the circumferential direction. A one-by-six plate cylinder thus may be provided. However, any ratio of one-by-three or greater plate cylinder may be provided, for example a two-by-six plate cylinder. Ratios of one-by-four or greater are preferred, as increased widths create more need for increased print pressure. A ratio of one-to-six is possible, and a ratio of one-to-six or greater may be most preferable.  
         [0010]     One or more printing plates may be attached at each image area.  
         [0011]     The first and second image areas may have a constant diameter, as may the third image area. However, the image areas may be of non-constant diameter, for example crowned or conically shaped to provide the different average diameters.  
         [0012]     The cylinder body may have a larger diameter middle section so that the third average diameter is larger than the end section.  
         [0013]     Alternatively, packing may be applied to increase the diameter of a printing plate or plates in the third image area. As another alternative, the plates used in the third image area may be thicker than plates used in the first and second image areas. In these cases, the cylinder body may have a uniform diameter.  
         [0014]     An offset printing press comprising the plate cylinder and a blanket cylinder is also provided. Preferably, the blanket cylinder has an effective blanket diameter which is uniform to reduce fan-in.  
         [0015]     A plate bender for printing plates for use on the plate cylinder of the present invention is also provided, the plate bender including a movable section for altering the effective post-bending length of the plate as a function of an intended axial position of the plate on the plate cylinder.  
         [0016]     The present invention also provides a plate bender for printing plates having a movable section skewable with respect to the plate. The movable section may be curved to fit with a crowned plate cylinder.  
         [0017]     Plate cylinder as defined herein includes a directly imaged cylinder where images are written directly on the outer surface of the cylinder, for example by a laser. While the printing plates preferably are flat lithographic printing plates, for certain embodiments, the printing plates may also be tubular in nature. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The present invention will be further described with respect the following Figures, in which:  
         [0019]      FIG. 1  shows schematically a first embodiment of the printing press of the present invention with a plate cylinder having a larger diameter cylinder body middle image area;  
         [0020]      FIGS. 2 and 3  show an alternate plate cylinders with a uniform diameter cylinder body,  FIG. 2  having packing underneath the middle image area and  FIG. 3  having thicker plates in the middle image area;  
         [0021]      FIGS. 4 and 5  show crowned and conical plate cylinder image areas, respectively;  
         [0022]      FIG. 6  shows a schematic top view of a plate bender according to the present invention; and  
         [0023]      FIGS. 7 and 8  show two partial side views of the plate bender. 
     
    
     DETAILED DESCRIPTION  
       [0024]      FIG. 1  shows schematically a print unit  5  of an exemplary offset lithographic web printing press  1  with a blanket cylinder  10  having a uniform radius outer surface  11  for printing a web  8 . A compressible blanket  20  fits over blanket cylinder  10 , for example by sliding axially. The blanket or blanket cylinder may have a uniform or profiled outer surface.  
         [0025]     A one-by-six plate cylinder  22  may have an image area  24  having two side-by-side images E, F, an image area  28  having two side-by-side images A, B and a middle image area  26  having two side-by-side images C, D, all of which are transferred via the blanket cylinder  20  to the web  8  as shown. The plate cylinder body, which may be made for example of steel and have axially-extending plate lock-up gaps  32 , has a larger diameter at image area  26 . This can be achieved for example through milling the cylinder body at areas  24 ,  28  to reduce the diameter. Six flat lithographic printing plates may be attached to the respective image areas via gaps  32 , the plates having images A, B, C, D, E, F respectively. Alternately the images can be directly imaged on the plate cylinder.  
         [0026]     Circumferential and/or lateral registration for each plate or image area may be provided, for example via a mechanism as disclosed in U.S. Pat. No. 6,559,908, which is hereby incorporated by reference herein.  
         [0027]     A similar blanket  40  and plate cylinder  42  may be provided for the opposite side of web  8 .  
         [0028]     The diameter difference is exaggerated for clarity in  FIG. 1 . The larger diameter of image area  26  increases print pressure at the center of blankets  20 ,  40 . Although some part of images B, E close to images C, D face reduced pressure due to the raised nature of the area  26 , the increased diameter of section  26  may be selected so that the compressible nature of blanket  20  and print pressure variation of the entire width provide adequate print pressure for all of images B and E. It is also noted that non-printed lateral areas typically extend between the image areas A, B, C, D, E, F.  
         [0029]      FIG. 2  shows an alternate embodiment of a plate cylinder  50  having a plate cylinder body  52  with a uniform outer diameter. Plates  54 ,  55 ,  58  and  59  all have a same diameter while plates  56 ,  57  have a larger effective diameter when placed on cylinder body  52  via packing materials  51 ,  53 . MYLAR for example in tape or sheet form may be used as a packing material. The plates may be directly imaged on the cylinder  50 .  
         [0030]      FIG. 3  shows an alternate embodiment of a plate cylinder  60  having a plate cylinder body  62  with a uniform outer diameter. Plates  64 ,  65 ,  66 ,  67 ,  68  and  69  all have a same inner diameter while plates  66  and  67  have a larger outer diameter than plates  64 ,  65 ,  68 ,  69  due to an increased thickness. The plates  64 ,  65 ,  66 ,  67 ,  68 ,  69 , while preferably flat, could also be manufactured as tubes and slid axially onto the plate cylinder body  62 . The plates may be directly imaged on the cylinder  50 .  
         [0031]      FIG. 4  shows a crowned plate cylinder  70 , the crowning being exaggerated for clarity. Plates  74 ,  75 ,  76 ,  77 ,  78  and  79  can be attached to the plate cylinder, although direct imaging is also possible.  
         [0032]      FIG. 5  shows a conically-sectioned plate cylinder, the conical sections being angled in an exaggerated manner for clarity. Image areas  84 ,  85 ,  86 ,  86 ,  88  and  89  may be provided for example by plates attached to the image areas or by direct imaging.  
         [0033]     The crowning or conical shape may be achieved for example through milling.  
         [0034]      FIG. 6  shows a plate bender  90  according to the present invention. Images may be imaged in a prepress imaging station  92  on the plates for the plate cylinder  22 , and the images may all have a similar size.  
         [0035]     Through prepress  92 , the desired axial location for each plate on the plate cylinder  22  is known, i.e. whether the plate is to print image A, B, C, D, E or F for one color.  
         [0036]     A processor  94  then controls the plate bender  90  so that a distance DI is altered. Pistons  96 ,  98  for example can move a base bar  110  with respect to a plate base  100  via tracks  102 ,  104 . As shown in exaggerated fashion in  FIGS. 7 and 8 , for a plate carrying image C the distance DI is larger than for the plate carrying image A, which is on a smaller diameter section of plate  22 . The axially-extending lock-up gaps  32  in the plate cylinder  22  hold the bent section of the plate and thus fasten the plates to the plate cylinder  22 . An overhead anvil may be provided to bend the plate edge about the base bar  110 .  
         [0037]     For the embodiment of  FIG. 5 , the plate bender  90  may have an extra base bar opposite the base bar  110  and also controlled by the processor  94 . Hinges  106 ,  108  may be provided between track  102 ,  104  and the base bar  110  to permit a skewing of the base bar  110  with respect to base  100  to create a skew angle SA, which can correspond to the conical angle for a section  84 ,  85 ,  86 ,  87  or  88  on plate cylinder  80 .  
         [0038]     For the embodiment of  FIG. 4 , the base bar  110  and the bar opposite base  100  may be rounded at the bend edge to match the crowning curvature.  
         [0039]     “Crowned” as defined herein means to have an outer surface curved convexly with respect to a cylinder rotational axis.