Abstract:
A method of exchanging printing units at a running rotary printing press, wherein a first printing unit is withdrawn from the printing medium, and a second printing unit for printing the same image with the same register is set against the printing medium, includes the steps of applying to the printing cylinder of the second printing unit, prior to start of the printing operation, a register printing block elevated relative to the regular printing block; prior to changeover of the printing units, setting the second printing unit against the printing medium such that register marks print with the register printing block without an image printing with the regular printing block; detecting register marks and adjusting the second printing unit register by reference to the detected register mark; and with the register adjusted, exchanging the printing units by withdrawing the first printing unit and setting the second printing unit.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a method of exchanging a printing unit at a running rotary printing press, wherein at least one first printing unit is withdrawn from a printing medium and a second printing unit for printing the same image with the same register is set against the printing medium. 
   When certain kinds of printing media, such as fleece for baby diapers, are printed on a rotary printing press such as a flexographic printing press, the printing cylinders employed therein have only a short lifetime, because the printing plates are soiled relatively quickly by, e.g., dust particles that are released from the printing medium. For this reason, frequent interruptions of the operation for cleaning the printing plates on the printing cylinders are necessary. This causes a reduction of the productivity and, consequently, increased costs. 
   SUMMARY OF THE INVENTION 
   In view of this problem, the applicant has already shown, on the fair DRUPA 2004 from May 16, to May 19, 2004 in Düsseldorf, Germany, a flexographic printing press wherein, adjacent to a central impression cylinder, two sets of printing units are provided which can alternatingly be set against and withdrawn from the printing medium. When the printing plates on the printing cylinders of the first set have become soiled, a changeover to the second set is performed without having to interrupt the operation of the printing press. While the image to be printed is printed with the printing units of the second set, the printing cylinders of the first set can be withdrawn so far that they can be cleaned without any risk. When the printing cylinders of the second set have become soiled, the printing operation can thus be continued with the cleaned printing cylinders of the first set. In this way, expensive interruptions of the printing operation can be avoided. 
   However, in view of the further processing of the printing medium, it is required that the printed images that have been printed alternatingly with the printing units of the first and the second set are exactly in registry with one another. Closed-loop control systems have become known, which permit a largely automated feedback-control of the register, so that the colour separations of the printed image that are printed with different printing units can be superposed in registry. These control systems comprise a video camera which detects the position of register marks on the printing medium. Then, for the purpose of adjusting the longitudinal register, for example, the drive system for the printing cylinders is feedback-controlled on the basis of detected positions of the register marks until all printing units are exactly in registry. 
   In the printing press described above, the register must be re-adjusted after each exchange of the printing units. Since, however, this operation requires a certain amount of time, in case of a high-speed printing press running at a printing speed of, e.g., 600 to 1000 m/min, as much as 100 m or more of waste may be produced after each exchange of the printing units before the register has been adjusted again with sufficient accuracy. This implies not only a loss in productivity but has also the consequence that a part of the cost savings achieved by alternatingly printing with two sets of printing units is consumed by increased material costs. 
   It is an object of the invention to provide a method that permits a reduced consumption of material when a printing unit is exchanged while the printing press is running. 
   According to the invention, this object is achieved by a method comprising the following steps:
         before the printing operation starts, the printing cylinder of the second printing unit is provided with a register printing block that is elevated relative to the regular printing blocks,   before the printing units are exchanged, the second printing unit is shifted towards the printing medium to such an extent that register marks are printed with the register printing block, without a printed image being printed with the regular printing blocks,   the register marks are detected, and the register of the second printing unit is adjusted on the basis of the detected register marks,   when the register has been adjusted, the exchange of the printing units is performed by withdrawing the first printing unit and fully setting the second printing unit.       

   Thus, in this method, the feedback-adjustment of the register is performed already before the actual changeover of the printing units, so that the printing operation with the new printing unit of the new set of printing units can be continued in registry immediately after the changeover, without substantial amounts of waste being produced. 
   Preferred embodiments of the method are indicated in the dependent claims. 
   The register printing block that is used for printing the register marks while the register is adjusted must be slightly elevated in comparison to the regular printing block of the printing cylinder in order to prevent the regular printing block, i.e. the effective surface of the printing cylinder, from coming into contact with the printing medium in this phase. Otherwise, the image printed with the first printing unit would be superposed by the image printed with the second printing unit. On the other hand, however, the register printing block must not compromise the printing of the regular printed image with the second printing unit after the changeover of the printing units. This can conveniently be achieved by forming the register printing block from a resilient material, so that it may be compressed when coming into engagement with the printing medium supported on the central impression cylinder, when the printing cylinder is fully set against the printing medium. 
   Preferably, the register printing block is made of an elastic material that can elastically restore its original configuration after it has been compressed by the central impression cylinder, and which may therefore be used again for printing register marks in subsequent instances of exchanging the printing units. Such a resilient or elastic register printing block facilitates also the inking of the printing cylinder with an anilox roller that has a uniform diameter over its entire length and rolls over both, the regular printing block and the register printing block in order to transfer ink onto the printing cylinder. As an alternative, it is possible, however, that the diameter of the anilox roller is reduced in a marginal portion of the web of the printing medium, where the register marks are formed, so that the slightly elevated register printing block will be inked but not compressed. 
   Optionally, the ink supply system may be configured such that the portion of the printing cylinder where the register printing block is formed is supplied with ink only during the adjustment operation prior to a changeover of the printing units. 
   Register marks that have been printed with the printing cylinder of the first printing unit may be used as reference for adjusting the register of the second printing unit. Optionally, these register marks may be formed by register printing blocks that serve for adjusting the register of the first printing unit in the next changeover (from the second printing unit back to the first one). 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment example will now be described in conjunction with the drawings, wherein: 
       FIG. 1  is a schematic view of a flexographic printing press in a first operating condition; 
       FIG. 2  shows the printing press of  FIG. 1  in an operating condition immediately before a changeover of printing units; 
       FIG. 3  shows the printing press in an operating condition after the changeover; 
       FIG. 4  shows a section of a web of a printing medium including a margin with register marks printed thereon: 
       FIGS. 5 and 6  are enlarged schematic views of a printing cylinder in a set position for printing register marks; and 
       FIG. 7  shows the printing cylinder of  FIGS. 5 and 6  in a set position for printing the actual image. 
   

   DETAILED DESCRIPTION 
     FIG. 1  schematically shows a flexographic printing press having a central impression cylinder (CI)  10  and a number of printing units ( 12 ,  14 ,  16 ,  18 ) arranged at the periphery thereof. As is conventional for flexographic printing presses, each printing unit comprises a printing cylinder and an anilox roller, which has not been shown, and an inking system for inking the anilox roller. 
   In the example shown, the printing units  16  and  18  form a first set with which a two-colour image can be printed onto a web  22  of a print medium running over the CI  10 . Accordingly, the printing cylinder  20  of each printing unit  16 ,  18  carries on its surface a printing block for a colour separation of the desired image, and the registers of the printing cylinders  20  are adjusted such that the colour separation images are superposed in registry with one another. 
   The printing units  12  and  14  form a second set of printing units that serve for printing the same image. Thus, the printing cylinder  20  of the printing unit  12  has the like printing block as the printing cylinder of the printing unit  16 , and the printing block in the printing unit  14  corresponds to that in the printing unit  18 . In  FIG. 1 , the printing units  12  and  14  of the second set are inactive, and their printing cylinders  20  are withdrawn from the peripheral surface of the CI  10 . 
   The printing press shown herein is particularly suited for printing onto webs  22  that consist of a material that causes a relatively rapid soiling of the printing blocks on the printing cylinders. Accordingly, when only a single set of printing units were present, the printing operation would have to be interrupted in intervals of, e.g., 15-20 minutes in order to clean the printing blocks. These intervals are considerably shorter than the intervals in which the coil of the web of the printing medium must be exchanged, and they therefore cause a significant reduction of the productivity of the printing press. In contrast, the printing press shown in  FIG. 1  permits, in principle, a non-interrupted operation wherein one and the same printed image is printed alternatingly with the printing units  16  and  18  of the first set and the printing units  12 ,  14  of the second set. For the printing units that are not active, i.e. the printing units  12  and  14  in  FIG. 1 , the printing blocks can be cleaned while the printing press is running. 
   In practice, the number of printing units of an individual set may be larger than 2 and may amount to 4 or 5, for example, depending on the number of colours to be employed. Since the tendency of the printing blocks to become soiled does not only depend on the material of the web  22  but also on the consistency of the ink employed and the structure of the respective image to be printed, it is also possible that the printing blocks in the different units have different lifetimes. For this reason, a mode of operation is conceivable wherein only individual pairs of printing units are operating alternatingly, while other colour separations of the image are respectively printed with only a single printing unit which may continuously remain active because the printing block thereof does not tend to become soiled so rapidly. 
   However, when printing alternatingly with different printing units, it must be assured that the registers of the printing cylinders that are alternatingly exchanged with one another are accurately aligned with one another, so that the printed image will always be printed in registry onto the web  22 , irrespective of which of the printing units has been used for printing. Otherwise, a further processing of the printed web in a manner specifically adapted to the register of the printed image, e.g. punching, crimping and the like, would not be possible or would at least be made considerably difficult. 
   Thus, when the changeover from the printing units  16 ,  18  of the set that is active in  FIG. 1  to the printing units  12 ,  14  of the second set shall be performed, it is not sufficient to drive the printing cylinders of the second set and to set them against the CI, but the register thereof, especially the longitudinal register, must be adjusted to that of the printing units  16 ,  18 . This process requires a certain amount of time, so that, in a high-speed printing press operating with a printing speed of 600 to 1000 m/min for example, 100 m or more of the web  22  may be printed with an unacceptable image (waste) before the register is adjusted correctly. For this reason, in the method proposed herein, the adjustment of the register is performed already prior to the changeover of the printing units, so that the production of waste is largely avoided. 
   According to  FIG. 2 , the printed image that has been printed onto the web  22  with the printing units  16 ,  18  of the first set is inspected with a camera  24 . The electronic image data are analysed in a control unit  26  in order to determine the actual register of the printed image on the web relative to a suitable reference. In particular, the control unit  26  determines a value for the longitudinal register of the printed image. 
   Each of the printing cylinders of the individual printing units have a separate drive unit, so that the angular position of each printing cylinder can be controlled individually. As an example,  FIG. 2  shows a motor  28  for driving the printing cylinder of the printing unit  12 . Now, in order to correctly adjust the register for the printing cylinders of the printing units  12  and  14 , the drive units of the printing cylinders must be controlled such that the printed image is formed in the correct position on the web. However, during this operation, the printing cylinders of the printing units  12  and  14  shall not yet print an image onto the web, because, prior to the changeover, the printed image is still printed with the printing units  16  and  18 . For this reason, as has been shown schematically in  FIG. 2 , the printing cylinders  20  of the printing units  12  and  14  each have a register printing block  30  which is elevated relative to the peripheral surface of the printing cylinder or, more exactly, the peripheral surface of the printing blocks that serve for printing the desired image. In  FIG. 2 , the printing cylinders  20  are set against the CI  10  only to such an extent that only the register printing blocks  30  but not the regular printing blocks may contact the web  22 . In  FIG. 2 , it is the register printing block  30  of the printing unit  12  that just contacts the web. 
   Thus, in the operating condition shown in  FIG. 2 , the printing units  12  and  14  of the second set are used only for printing register marks onto the web  22  on which the desired image and possibly further register marks will then be printed by means of the printing units  16  and  18  of the first set. The register marks formed with the printing units  12  and  14  are detected by the camera  24 , and by reference to these register marks, the control unit  26  can determine the actual value of the longitudinal register of the printing cylinders  12  and  14  and compare it to a target value which is given by the actual longitudinal register of the image printed with the printing units  16 ,  18 . Then, the control unit  26  controls the motor  28  for the printing cylinder  20  so as to advance or delay the printing cylinder until the actual value for the longitudinal register coincides with the target value. In this way, the longitudinal register of the printing units  12  and  14  is precisely adjusted to the longitudinal register of the printing units  16  and  18 . Optionally, the side register of the printing units  12  and  14  may be adjusted or checked and possibly corrected at the same time. 
   It is only when the register has been precisely adjusted in this way that the actual changeover is performed by fully setting the printing cylinders of the printing units  12  and  14  against the CI  10  and simultaneously withdrawing the printing cylinders of the printing units  16  and  18  from the CI, as has been shown in  FIG. 3 . Now, the printing cylinders  20  of the printing units  12  and  14  roll over the web  22  with their peripheral surface, i.e. with their printing blocks, so that the image is now printed with these printing cylinders. The elevated register printing blocks  30  are under these conditions radially pressed back into the respective printing cylinder so that they do not hamper the printing operation. 
   In the condition shown in  FIG. 3 , the printing cylinders of the printing units  16 ,  18  of the first set may be cleaned, so that, later, when the printing cylinders of the printing units  12  and  14  of the second set have become soiled, it is possible to switch back again to the printing units  16 ,  18  of the first set. Then, prior to this new changeover, the register of the printing units  16  and  18  is adjusted in a procedure analogous to the one shown in  FIG. 2 . 
     FIG. 4  shows a section of the web  22  onto which an image  32  has been printed with the printing units  16 ,  18  of the first set. In addition, register marks  36  have been printed on a margin  34  of the web by means of these printing units (preferably at least one register mark per printing unit of the first set). The register marks  36  are detected by the camera  24  and are electronically processed in the control unit  26 . In this process, the colour dot forming the register mark is measured, and the exact co-ordinates of the “center of gravity”thereof are calculated. Then, these co-ordinates serve as a reference for adjusting the register, in particular the longitudinal register, of the printing units  12 ,  14  of the second set. Then, on the basis of this reference and the known position of the register printing blocks  30  relative to the respective image, the control unit  26  calculates a target position  38  for the register mark  40  that is printed with the register printing block  30  of the printing unit  12  and  14 , respectively, in the condition shown in  FIG. 2 . Then, the drive unit for the respective printing cylinder is controlled such that the longitudinal position of the register mark  40  on the web is shifted until it coincides with the target position  38 . Subsequently, the actual changeover is performed, corresponding to a transition from the condition shown in  FIG. 2  to the condition shown in  FIG. 3 . 
   In  FIGS. 5 ,  6  and  7 , the printing cylinder of the printing unit  12  has been shown in an enlarged scale. A printing block  42  serving for printing the desired image has been indicated on the peripheral surface of the printing cylinder.  FIGS. 5 and 6  show the position of the printing cylinder  20  at the time when the register is adjusted. As can be seen especially in  FIG. 5 , the printing block  42  does not yet contact the web  22  in this position. Only the register printing block  30  can contact the web  22  and can print one or more register marks as has been shown in  FIG. 6 . 
   The register printing block  30  is made of an elastically compressible material. When the printing cylinder  20  is fully set against the CI, so that the printing block  42  rolls over the web  22  and prints an image onto the web, as shown in  FIG. 7 , the register printing block  30 , each time it reaches the position shown in  FIG. 7  where it faces the web, can be compressed by the CI to such an extent that it retreats into the surface of the printing block  42 . After this, it will always restore its original posture, so that it may again be used for printing the register marks  40  during the next but one changeover of the printing units.