Patent Publication Number: US-2005120893-A1

Title: Inking device for gravure printing cylinder

Description:
The invention relates to gravure printing machine comprising a device for applying ink onto a printing cylinder, including an ink supply system.  
      In gravure printing machines, ink is conventionally applied onto the surface of a rotating printing cylinder. To this end, the printing cylinder may pass through an ink trough. The printing cylinder with ink applied thereon then passes by a doctor unit where the polished surface of the printing cylinder is wiped clean prior to the proper printing process, so that ink will remain only in the engraved printing portions of the printing cylinder surface. Subsequently, the remaining ink is transferred onto a print medium which is pressed against the printing cylinder by means of a pressure roller. When, subsequently, the surface of the printing cylinder is again immersed into the ink trough, ink is applied once again. Instead of being immersed in an ink trough, the printing cylinder may also be supplied with ink from a roller which is immersed into the ink trough.  
      The known methods and devices for inking a gravure printing cylinder have in common, that the surface of the printing cylinder, after having performed the print operation and before it is inked once again, passes through a region where it is in contact with ambient air. In case of rapidly rotating printing cylinders, this leads to a layer of entrained air being built up on the surface oft the printing cylinder. When, subsequently, the printing cylinder surface is inked again, two undesired effects are observed. First, the air layer impedes the process of wetting the cylinder surface with ink, and, second, part of the air is mixed into the ink. Thus, when the printing cylinder passes through the ink trough, the entrained air layer is directly introduced into the ink in the ink trough, whereas, when the printing cylinder is inked by means of a roller, the entrained air is introduced, together with the ink, into the nip between the inking roller and the printing cylinder. The introduction of air into the ink can lead to foaming of the ink, and air may enter into the engraved portions on the surface of the inked printing cylinder. This will deteriorate the resulting printed image.  
      These problems become particularly virulent when the print quality and printing speed shall be increased further in modern printing machines.  
      It is an object of the invention to provide a gravure printing machine having a device for applying ink onto a printing cylinder, wherein the problems indicated above are mitigated.  
      According the invention, this object is achieved by a gravure printing machine of the type indicated above, in which a stripping device, that is arranged to strip-off the air layer entrained by the cylinder, is associated with the ink supply system in such a manner that the printing cylinder does not come into substantial contact with air in a region between the stripping device and the ink supply system.  
      Thus, the air entrained by the surface of the printing cylinder is stripped-off from the printing cylinder by means of the stripping device, before the printing cylinder surface is inked by the ink supply system. The surface of the printing cylinder is thereby prevented from coming into substantial contact with air in the region between the stripping device and the ink supply system, and this avoids that a new layer of entrained air is built up. In this way, there is no direct introduction of air into the ink that is applied onto the printing cylinder. The printing cylinder will receive only ink that has not been caused to foam by being mixed with air. Even when the rotary speed and the surface speed of the printing cylinder are increased, which promotes the tendency to entrain an air layer, the process of applying ink onto the surface of the printing cylinder will not be disturbed. The amount of ink being applied can be controlled by active supply of ink to the printing cylinder. As an alternative, the ink may be contained in an ink trough through which the surface of the printing cylinder passes through.  
      Useful embodiments of the invention are indicated in the dependent claims.  
      Preferably, the stripping device is arranged immediately upstream of the ink supply system. This is the easiest way to avoid a contact of the printing cylinder surface with air between the instants of passing the stripping device and passing the ink supply system. Moreover, the stripping device may advantageously be integrated with the ink supply system into one construction unit, so that these devices may be set against the printing cylinder together.  
      Preferably, the stripping device is a doctor blade assembly. The doctor blade may have a construction similar to that of known doctor blades in doctor units and may be set against the printing cylinder in a similar manner. Preferably, the ink supply system is arranged to supply ink onto the printing cylinder along the back side of the doctor blade. Then, the ink may be applied immediately after the entrained air layer has been stripped-off.  
      In a first embodiment, the ink supply system has an ink applying nozzle. The letter may be supplied with ink from an ink supply passage and may for example have a downwardly directed nozzle opening for creating an ink curtain flowing against the printing cylinder. More particularly, the ink supply nozzle may be arranged to direct the ink onto the back side of the doctor blade of the stripping device without being sputtered. Then, the ink curtain flows along the back side of the doctor blade.  
      Preferably, the stripping device and the ink supply system are arranged within an angular range of the printing cylinder of less that 90°, preferably less than 60°, behind the pressure roller where the proper printing process takes place. If possible, this angular range should be even smaller. Preferably, the stripping device and the ink supply system are located as close as possible to the pressure roller, so that the range, in which the printing cylinder is in contact with air between the pressure roller and the stripping device is as small as possible. This is not only advantageous for suppressing the build-up of the entrained air layer, but also shortens the time in which the surface of the printing cylinder my run dry.  
      In another embodiment, the ink supply system comprises a container filled with ink and open towards the printing cylinder, and the printing cylinder comes into contact with the ink in the container. For example, the ink supply system may comprise an upwardly open ink trough into which the printing cylinder immerses.  
      Preferably, the stripping device is adapted to be set against the printing cylinder in a position below the level of ink in the container. For example, the stripping device may be a doctor blade assembly. In this case, preferably, the doctor blade is capable of being engaged with its edge against the printing cylinder in a condition, where the edge is below the level of ink in the container. By means of the stripping device, the air layer entrained by the printing cylinder is stripped-off, and, immediately thereafter, the printing cylinder is in contact with the ink. In this way, the ink may reliably be applied even at high rotary speed of the printing cylinder, without causing ink that has been mixed with air to be applied onto the printing cylinder. The ink will be mixed with the air stripped-off from the printing cylinder only, if at all, in a limited region upstream of the stripping device. By appropriately designing the stripping device and the container, it can be assured that the air will escape from this region towards the ink surface. 
    
    
      Preferred embodiment examples of the invention will now be explained in conjunction with the drawings, wherein:  
       FIG. 1  is a schematic view of a part of a gravure printing machine according to a first embodiment; and  
       FIG. 2  shows another embodiment. 
    
    
      The gravure printing machine shown in  FIG. 1  comprises a printing cylinder  10  and a pressure roller  12  which are rotatably supported in a machine frame. The frame and associated drive and adjusting means have not been shown here, because they are not essential for understanding the invention.  
      The gravure printing cylinder  10  is in contact with a stripping device  14 . At the locus of contact, ink  18  is applied onto the surface of the printing cylinder by means of an ink supply system  16 . The printing cylinder  10  rotates in a direction indicated by an arrow and thereby carries along the ink  18  that has been applied to its surface. A surplus of ink  18  may drop into a collecting trough  20  arranged beneath the printing cylinder  10 . The printing cylinder is doctored in a doctor unit  22  which, in a known manner, is formed by a doctor blade  24  mounted to a bracket  26 . The edge of the doctor blade  24  set against the surface of the printing cylinder  10  removes the ink from the smooth, non-printing surface portions of the printing cylinder  10 , so that the ink will only remain in the engraved portions which are to create a printed image on a print medium web  28 . The web  28  is pressed against the printing cylinder  10  by means of the pressure roller  12 .  
      Between the pressure roller  12  and the stripping device  14 , the gravure printing cylinder  10  passes through an angular range a which, in the example shown, amounts to approximately 60°. Because of the high rotary speed of the gravure printing cylinder  10 , its surface velocity may for example be as high as 10 m/s. While the surface of the printing cylinder  10  passes through the angular range α between the pressure roller  12  and the stripping device  14 , an entrained air layer is built up on its surface. This layer is stripped-off from the surface of the printing cylinder  10  by means of the stripping device  14  which is formed by a doctor blade assembly having a doctor blade  32  mounted to a bracket  30 . Similar to what is known for doctor units, the doctor blade assembly may be fixed to levers with which it may be pivoted against the periphery of the printing cylinder  10 , so that the surface thereof is engaged by the edge of the doctor blade  32 . In this way, the doctor blade  32  is capable of reliably stripping-off the layer of air that has been entrained by the surface of the printing cylinder  10 . The doctor blade  32  is inclined relative to the surface of the printing cylinder  10  in the direction of rotation of the latter.  
      On the back side of the doctor blade  32 , the ink  18  is applied onto the printing cylinder  10  by means of the ink supply system  16 . The ink supply system  16  is mounted to the bracket  30  for the stripping device  14 . It comprises an ink distributing chamber  34  to which ink is continuously supplied through a supply passage  36 . On the downstream side, the ink distributing chamber  34  merges into an ink applying nozzle  38  which, as the ink distributing chamber  34 , extends along the doctor blade  32 . By means of the ink applying nozzle  38 , the ink  18  is guided to the back side of the doctor blade  32  without being sputtered, and the ink then hits the surface of the printing cylinder  10  as an ink curtain. In this way, an introduction of air into the ink  18  can reliably be prevented, even at very high surface speeds of the printing cylinder  10 . The ink  18  will therefore form a homogeneous ink layer on the printing cylinder  10 . This is promoted by the fact that the ink is supplied as soon as possible after the print operation at the web  28 , so that, advantageously, it takes a long time until the doctor unit  22  is reached. Consequently, the angular range a should preferably be small. To this end, the doctor blade  32  may be inclined relative to a vertical line by an angle β towards the ink supply system  16 , as in the example shown, so that the ink curtain  40  is located below the back side of the doctor blade  32 . Since the bracket  30  is tilted to the right side in the drawing, the edge of the doctor blade  32  can be brought closer to the pressure roller  12 .  
       FIG. 2  shows another embodiment of the gravure printing machine shown in  FIG. 1 . Here, the ink is applied onto the gravure printing cylinder  10  not by means of an ink applying nozzle, but the surface of the gravure printing cylinder  10  passes through an ink supply system configured as an ink trough  42  filled with ink  18 . The stripping device  14  formed by the doctor blade assembly having the doctor blade  32  fixed to the bracket  30  is here arranged inside of the ink trough  42 . The stripping device  14  is mounted to a holding structure  44  that is fixed at the ink trough  42  by fixtures permitting quick detachment.  
      For example, the holding structure  44  may comprise a plate at each end of the stripping device  14 , bounding the space above the stripping device  14  in lateral direction (before and behind the plane of the drawing).  
      The doctor blade  32  is adapted to be set against the periphery of the printing cylinder  10  in such a position that the surface of the printing cylinder is engaged by the edge of the doctor blade  32 . The edge of the doctor blade is located below the level  46  of ink in the ink trough  42 . The stripping device  14  is arranged on the side of the printing cylinder  10  on which the surface of the latter enters into the ink  18 . Thus, the doctor blade  32  is capable of reliably stripping-off a layer of air that has been entrained by the surface of the printing cylinder  10 . The air that has been introduced into the ink  18  in this way is located above the stripping device  14  and will rise to the surface of the ink  18 . Thus, the ink  18  in the other regions of the ink trough  42  can be prevented from being mixed with air.  
      In  FIG. 2 , phantom lines indicate a gravure printing cylinder  10 a the periphery of which is significantly smaller than that of the printing cylinder  10 . This corresponds to a significantly smaller printing length. The ink trough  42  is height-adjustable, as has been indicated by a double arrow, for being adapted to the size of the printing cylinder. The position of the stripping device  14  relative to the ink trough  42  may be made adjustable by suitably configuring the holding structure  44 . Optionally, the holding structure  44  may also be removed from the ink trough  42  and may be fixed thereon in a different position, or it may be moved together with the ink trough  42 .  
      In a modification of the illustrated embodiment, the stripping device  14  may also comprise a separator  48  which has been shown in dashed lines. This separator may for example be a grid or mesh which prevents the air that has been introduced into the ink  18  above the doctor blade  32  from spreading further. As shown, the separator  48  may extend from the surface of the doctor blade assembly to above the ink level or, as an alternative, it may extend from the doctor blade assembly to an edge of the ink trough  42 . Optionally, the separator  48  may also be a barrier, which is impenetrable for the ink.  
      The illustrated embodiments serve only as examples for a printing machine according to the invention. The stripping device  14  may also have a configuration other than a doctor blade assembly, as far as there is provided a member which can be set against the printing cylinder  10  and is capable of reliably stripping-off the entrained air layer. Further, additional equipment may be provided between the stripping device  14  and the ink supply system or between the ink supply system and the doctor unit  22 , such as, for example, another ink supply system.  
      The ink  18  is not limited to printing ink in the proper sense of the word. Instead, the invention is also applicable to other liquids with different viscosities, that are to be applied onto a printing cylinder.