Abstract:
A method of printing comprising includes spreading and flattening a plastic film web before passing the plastic film web through a first printing unit; and spreading and flattening the plastic film web after the plastic film web exits the first printing unit and before passing the plastic film web through a second printing unit.

Description:
[0001]    This is a Divisional of U.S. patent application Ser. No. 13/914,000, filed Jun. 10, 2013 which claims priority to U.S. Provisional Patent Application No. 61/713,297 filed on Oct. 12, 2012, both of which are hereby incorporated by reference herein. 
     
    
       [0002]    The present invention relates generally to a method for printing presses and more specifically to a registration control in a printing press. 
       BACKGROUND 
       [0003]    U.S. Pat. No. 4,404,906 discloses using a bowed roller solely upstream of a first printing unit to expand a web laterally to counteract web fan-out from water absorption. Bowed rollers are intentionally not used upstream of the printing units other than the first printing unit. Instead, straight axis idler rollers are used upstream of the other printing units. The web is wrapped around the bowed roller at a wrap angle of between 60° and 180°. 
       SUMMARY OF THE INVENTION 
       [0004]    A printing press is provided. The printing press includes a first printing unit including a first plate cylinder, a first blanket cylinder and a first impression cylinder forming a first printing nip with the first blanket cylinder, a first bowed roller upstream of the first printing unit spreading a web before the web enters the first printing nip, a second printing unit downstream of the first printing unit including a second plate cylinder, a second blanket cylinder and a second impression cylinder forming a second printing nip with the second blanket cylinder, and a second bowed roller between the first printing and the second printing unit, the second bowed roller spreading the web before the web enters the second printing nip. 
         [0005]    A method printing is also provided. The method includes spreading and flattening a plastic film web before passing the plastic film web through a first printing unit and spreading and flattening the plastic film web after the plastic film web exits the first printing unit and before passing the plastic film web through a second printing unit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The present invention is described below by reference to the following drawings, in which: 
           [0007]      FIG. 1  shows a schematic side view of a printing press according to an embodiment of the present invention; 
           [0008]      FIG. 2  shows a plan view of a bowed roller of the printing press; and 
           [0009]      FIG. 3  shows a schematic side view of a printing unit of the printing press. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]      FIG. 1  shows a printing press  10  according to an embodiment of the present invention. Printing press  10  includes a first printing unit  12 , a second printing unit  14  downstream of first printing unit  12 , a third printing unit  16  downstream of second printing unit  14  and a fourth printing unit  18  downstream of third printing unit  16 . In a preferred embodiment, each printing unit  12 ,  14 ,  16 ,  18  is a vertically aligned printing unit including a plate cylinder  20 , a blanket cylinder  22  and an impression cylinder  24  forming a printing nip  26  for printing images on one side of a web  28  passing horizontally through printing nip  26 . Printing unit  12  is the first upstream printing unit of printing press  10  and receives web  28  from an unwind unit  30 . A bowed roller  32  is provided upstream of each printing unit  12 ,  14 ,  16 ,  18  for flattening and spreading web  28  before web  28  enters the respective printing nip  26 . Accordingly, one bowed roller  32  is provided between unwind unit  30  and first printing unit  12 , one bowed roller  32  is provided between printing units  12 ,  14 , one bowed roller  32  is provided between printing units  14 ,  16  and one bowed roller  32  is provided between printing units  16 ,  18 . In this embodiment, no other rolls, rollers or web guiding components are provided for contacting web  28  between printing units  12 ,  14 ,  16 ,  18  other than bowed rollers  32  upstream of printing units  14 ,  16 ,  18  and bowed rollers  32  only contact the non-printed side of web  28 , preventing the smearing of ink on the printed side of web  28 . In a preferred embodiment, web  28  is a plastic film, which may be formed of for example polyethylene terephthalate film or oriented polypropylene film. 
         [0011]    In this embodiment, printing press  10  is a four color printing press, with first printing unit  12  including an inking unit supplying black ink and printing black ink on web  28 , second printing unit  14  including an inking unit supplying yellow ink and printing yellow ink on web  28 , third printing unit  16  including an inking unit supplying magenta ink and printing magenta ink on web  28  and fourth printing unit  18  including an inking unit supplying cyan ink and printing cyan ink on web  28 . In other embodiments, the order of the ink may be varied or different colors of inks may be applied. Also, printing press  10  may include more or less than four printing units. 
         [0012]    As web  28  passes over each bowed roller  32  at a wrap angle θ, bowed roller  32  spreads and flattens web  28 . It has been discovered that providing a bowed roller  32  upstream of each printing unit  12 ,  14 ,  16 ,  18  to consistently spread and flatten web  28  prior to printing with multiple printing units is key to good registration and blur free printing. U.S. Pat. No. 4,404,906 discloses that fan-out is caused solely at the first printing unit by corrugation or fluting experienced by web as the web leaves the feed roll, and that after the first color is printed on the web, the web is no longer corrugated. It has been discovered that this is not the case where the web speed of the printing press is varied between print jobs or during a print job. When such web speed variations occur, particularly when using thin plastic films of for example between 0.5 and 1.1 mils thick, fluting or corrugation is experienced between each printing unit, causing lateral registration problems and blurring. Printing at high web speeds has caused downstream printing units to print wide due to web fan-in, and the print grows wider with increasing web speeds. This is typically the opposite to web fan-out. Providing a bowed roller  32  upstream of each printing unit  12 ,  14 ,  16 ,  18  allows web  28  to be stable and axially tensioned as web  28  passes through each printing unit  12 ,  14 ,  16 ,  18 . This helps to maintain the lateral registration of web  28  for each printing unit  12 ,  14 ,  16 ,  18  for a broad range of web speeds and helps prevent blurring, particularly when web  28  is a plastic film web formed of for example polyethylene terephthalate film or oriented polypropylene film and the plastic film web is printed with higher tack ink. Higher tack ink as used herein is an ink having a tack of approximately 14 to 16, as measurement by an inkometer. An example of a high tack ink is 7600/7800 Duracure ink. The high ink tack, together with thin films and lower web tensions, causes the web to follow the blankets according to the heavier ink coverage areas. This is already non-uniform because the images are rarely perfectly consistent and the uneven web follow causes the web to form out of plane troughs. These out of plane troughs can also change with press speed and are the cause of the misregistration and blurred images between printing units. 
         [0013]    In a preferred embodiment, each bowed roller  32  is positioned the same distance a upstream of the respective printing unit  12 ,  14 ,  16 ,  18 . Accordingly, the distance a between the first bowed roller  32  and the first printing nip  26  of printing unit  12 , the distance a between the second bowed roller  32  and the second printing nip  26  of printing unit  14 , the distance a between the third bowed roller  32  and the third printing nip  26  of printing unit  16  and the distance a between the fourth bowed roller  32  and the fourth printing nip  26  of printing unit  18  are all equal. Also, in a preferred embodiment, each bowed roller  32  is positioned the same distance b downstream of the previous printing unit  12 ,  14 ,  16 . Accordingly, the distance b between the second bowed roller  32  and the first printing nip  26  of printing unit  12 , the distance b between the third bowed roller  32  and the second printing nip  26  of printing unit  14  and the distance b between the fourth bowed roller  32  and the third printing nip  26  of printing unit  16  are all equal. 
         [0014]      FIG. 2  shows an embodiment of one of bowed rollers  32 . As shown in  FIG. 2 , each bowed roller  32  may have an outer surface formed of an elastomeric covering  34  and may include a plurality of circumferentially aligned grooves  36  formed therein. Grooves  36  may help prevent air entrapment at higher speeds of web  28 . In alternative embodiments, other groove patterns may be provided elastomeric covering  34 , for example axial or spiral grooves. As used herein, the term “bowed roller” refers to a roller having a central axis  38  that is bowed with respect to a straight axis  40  passing through the centers of the ends of roller  32 . In preferred embodiments of the present invention, bowed rollers  32  are bowed such that a distance d between central axis  38  and straight axis  40  is between 0.05 and 0.4 inches at a peak  42  in the axial middle of bowed roller  32 . Such bowing causes peak  42  to protrude radially distance d past a front side edge  44 . Accordingly, bowed rollers  32  are considered as being bowed by distance d, which is between 0.05 and 0.4 inches in preferred embodiments. In one preferred embodiment, bowed rollers  32  may be bowed by a distance d of 0.12 inches. 
         [0015]      FIG. 3  shows an embodiment of printing unit  12 , which is configured in substantially the same manner as printing units  14 ,  16 ,  18 , the only difference being the color of ink being printed.  FIG. 3  illustrates the wrap angle θ of web  28  around the upper surface of bowed roller  32 . Unlike the 60° and 180° wrap angles of U.S. Pat. No. 4,404,906, in preferred embodiments of the present invention, wrap angle θ is between 5° and 45° for each bowed roller  32  to effectively prevent images printed by printing units  12 ,  14 ,  16 ,  18  from being misaligned with respect to each other. In one preferred embodiment, the wrap angle θ of bowed roll  32  is the same for each printing unit  12 ,  14 ,  16 ,  18 .  FIG. 3  shows peak  42  of bowed roller  32  protruding toward printing unit  12  in the direction web  28  travels past a front side edge  44  by distance d. 
         [0016]    In the preceding specification, the invention has been described with reference to specific exemplary embodiments and examples thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of invention as set forth in the claims that follow. The specification and drawings are accordingly to be regarded in an illustrative manner rather than a restrictive sense.