Abstract:
This device for applying a moistening liquid to a paper web includes a liquid applicator roller ( 44, 46 ) for applying the moistening liquid ( 38 ) on the paper web ( 18 ). The liquid applicator roller includes an application surface ( 60 ), and is characterized in that the application surface ( 60 ) is formed of a material capable of creating a corona effect on the paper web by discharge of static electricity between the surface and the paper web ( 18 ). Application to paper web dryers of rotary offset presses.

Description:
This application is a 371 of PCT/EP2007/001536 filed 22 Feb. 2007. 
     The present invention relates to a device for applying a moistening liquid to a paper web, comprising a liquid applicator roller for applying the moistening liquid on the paper web at a liquid application zone, the liquid applicator roller comprising an application surface. 
     BACKGROUND 
     In the state of the art, devices are known for applying moistening liquid on paper webs. The moistening devices usually comprise an applicator roller that is used to transfer the moistening liquid from a liquid tank to the paper web. 
     As the application roller is in direct contact with the paper web carrying ink, ink build-up has been noticed on the application roller. 
     A number of measures have been taken in order to reduce this ink build-up on the applicator roller. For example the moistening liquid used is a mixture of silicone and water. 
     However, the known devices are still subject to ink build-up on the applicator roller. 
     SUMMARY OF THE INVENTION 
     Also, the amount of moistening liquid transferred from the applicator roller to the paper web needs to be as high as possible. 
     It is an object of the present invention to reduce the ink build-up on the applicator roller and to improve the transfer of liquid from the applicator roller to the paper web. 
     The present invention provides a device, characterized in that the device comprises means for creating a corona effect by discharge of static electricity on the paper web upstream of the liquid application zone. 
     Further improvements according to specific embodiments of the invention comprise the following features:
         the means for creating a corona effect comprise the application surface, this surface being formed of a material capable of creating the corona effect on the paper web between the application surface and the paper web;   the material is a plastics material;   the material contains Fluorine, in particular the material consists of Poly-tetra-fluorine-ethylene (PTFE);   the device is adapted to convey the paper web ( 18 ) in a paper web travel direction (D), and the device is adapted to rotationally drive the applicator roller in a roller direction against the paper web travel direction; and   that the liquid applicator roller comprises an outer layer defining the application surface and made of the material capable of creating the corona effect and an inner core made of a material that is different from the material of the outer layer.       

     Another object of the invention is a rotary printing press having at least one chill roll for a paper web adapted to cool a first side of the paper web, characterized in that it comprises a device for applying a moistening liquid to said first side of the paper web, and in that the device is a device as defined above. 
     The rotary printing press according to specific embodiments of the invention comprise the following features:
         the applicator roller of the device is arranged upstream of the chill roll in the paper web travel direction; and the applicator roller is arranged downstream of the chill roll in the paper web travel direction.       

     The invention relates also to a method of applying moistening liquid on a paper L  5  web, characterised in that it comprises the following steps:
         driving a paper web in a paper web travel direction,   rotating a liquid applicator roller,   applying a moistening liquid with the liquid applicator roller to the paper web in a liquid application zone and   creating a corona of discharge of static electricity in a zone of the paper web in a zone upstream of the liquid application zone.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be best understood in light of the following description of a specific, non-limiting embodiment of the invention. The description refers to the annexed drawings, which show: 
       On  FIG. 1  a schematic side view of a rotary printing press according to the invention, 
       On  FIG. 2  a part of the chill roll assembly of the press of  FIG. 1 , 
       On  FIG. 3  a schematic view of the moistening device of the press of  FIG. 1 , and 
       On  FIG. 4  an enlarged schematic side view of an applicator roller and a paper web of the press of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a web-fed offset printing press according to the invention, designated by the general reference  2 . 
     The printing press  2  comprises a splicer  4 , four printing units  6 , a web catcher  10 , a hot air dryer  12 , a chill roll assembly  14  and a folder  16 . 
     The rotary printing press  2  comprises also a paper web  18 , which travels through the press  2  along a paper web travel direction D from the splicer  4 , through the printing units  6 , the web catcher  10 , the hot air dryer  12 , and the chill roll assembly  14  to the folder  16 . 
     Part of the chill roll assembly  14  is depicted in  FIG. 2 . The chill roll assembly  14  comprises nine chill rolls of which six chill rolls  20 ,  22 ,  24 ,  26 ,  30  and  32  are visible on  FIG. 2 . 
     The chill roll assembly  14  comprises also a nip roll  33  cooperating with chill roll  26 . 
     As can be seen on  FIG. 3 , the printing press  2  is furthermore equipped with a moistening device  34  which is adapted to apply a moistening liquid  38  to the paper web  18 . The moistening device  34  comprises a liquid tank  36  which contains the moistening liquid  38 , which is in the present case a mixture of water and silicone. 
     Two applicator assemblies  40 ,  42  are adapted to apply the moistening liquid  38  to the paper web  18 . The applicator assembly  40  is adapted to apply the liquid to a first side of the paper web  18  and the applicator assembly  42  is adapted to apply liquid to a second side of the paper web  18 . 
     Each applicator assembly  40 ,  42  has an applicator roller  44 ,  46 , which is rotationally driven by a motor  48 ,  50  in an applicator roller direction AD. The direction AD is counter clockwise in  FIGS. 2 to 4 . Each applicator assembly  40 ,  42  has a moistening liquid bath  52 ,  54  which contains an exactly defined amount of moistening liquid  38  and in which the applicator roller  44 ,  46  is partially immerged. 
     The level of moistening liquid  38  in the moistening liquid baths  52 ,  54  with respect to the applicator rollers  44 ,  46  is regulated by a regulation mechanism  56 . The rotational speed of the applicator rollers  44 ,  46  is regulated by a motor regulation device  58 . 
       FIG. 4  depicts in a detailed manner the applicator roller  44  and the moistening liquid bath  52 . 
     The applicator roller  44  has an outer cylindrical application surface  60 , which forms an application surface for the moistening liquid  38  and which is made of a material adapted to produce a corona effect  70  on the paper web  18 . 
     The material of the outer surface  60  is for example a plastic having a fluorine component. In a preferred embodiment, the material of the outer surface  60  is Poly-tetra-fluorine-ethylene (PTFE). The material of this embodiment also has the advantage of a low adherence of the ink, so that ink build-up on the application surface  60  is eliminated. 
     The applicator roller  44  defines a horizontal contact zone  62  with a first side of the paper web  18 . 
     When driven by the motor  48 , the applicator roller  44  receives a layer  64  of moistening liquid from the bath  52  and conveys this liquid layer  64  to the contact zone  62 . The contact zone  62  is divided into a liquid application zone  62 A and a dry zone  62 B. In the liquid application zone  62 A the liquid layer  64  is applied to the paper web  18  forming a liquid film  66  on the web  18 . At the dry zone  62 B of the contact zone  62 , which is upstream with respect to the paper web travel direction D of the liquid application zone  62 A, the applicator roller  64  and the paper web  18  are dry. The friction between the paper web  18  and the applicator roller  60  in the dry zone  62 B of the contact zone  62  creates a discharge of static electricity between the applicator surface  60  and the paper web  18 . This discharge of static electricity creates a corona  70  that lowers the surface tension of the paper web  18  and improves therefore the transfer of the liquid film  64  from the roller  60  to the web  18 . Also, the corona  70  further lowers ink build-up. 
     The applicator roller  46  works in an identical manner as the applicator roller  44  with the only differences that the contact zone between the applicator roller  46  and the paper web  18  is vertical and that the contact zone is defined on a second side of the paper web  18 . 
     The applicator rollers  44 ,  46  according to the invention result in an evenly distributed liquid film  66  on the paper web  18 . The liquid film  64  is transferred in its entirety to the paper web  18  and the ink stays on the paper web. 
     According to another embodiment of the invention, the application surface  60  is made of a material other than PTFE, for example polyethylene. More generally speaking all other plastics material can be used to create the corona effect. 
     According to another embodiment of the invention, the application roller  44 ,  46  comprises an outer layer made of the PTFE material and an inner core made of a material different from the PTFE material. 
     According to another variant, means for creating the corona are arranged upstream of the liquid applicator roller in the paper web travel direction. In this case, the roller can be driven in the paper web travel direction and be of a material other than plastic.