Abstract:
A method for shifting a doctor blade in its longitudinal direction, the doctor blade being set against a cylinder of a printing machine, wherein the shift movement of the doctor blade is such that, for most of the time of the print operation, the shift velocity of the doctor blade is in a range from 0 to 0.1 m/s, and a printing machine having a doctor blade assembly, including a doctor blade arranged to be set against a cylinder, and a shift mechanism for shifting the doctor blade in its longitudinal direction, such that the shift mechanism is adapted to execute the method.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a method for shifting a doctor blade, that is set against a cylinder of a printing machine, in direction of the longitudinal axis of the doctor blade, as well as an apparatus for carrying out this method. 
   In printing machines, doctor blade assemblies are generally used for doctoring the surface of a rotating cylinder, e.g. the printing cylinder, by means of a doctor blade. In a gravure printing machine, for example, such a doctor device has the function to wipe the surface of t he gravure printing cylinder after it has been inked and before the proper print process takes place, so that printing ink will remain only in the printing, engraved portions of the printing cylinder surface. 
   In conventional printing machines, the doctor blades are generally formed by thin sheets of hardened or non-hardened spring steel. An edge of a doctor blade that is set against the surface of the print cylinder or anilox roller, is subject to considerable wear. In order to increase the lifetime of the doctor blade, the doctor blade is oscillated in its longitudinal direction, so that the wear of the doctor blade is distributed as far as possible. The frequency of such an oscillation may for example be 1 Hz or more, depending on the production speed, and the oscillation stroke may for example be as large as 40 mm. Although such an oscillation of the doctor blade distributes the locus of wear, it is still necessary relatively frequently to replace a worn doctor blade. This necessitates a stop of the printing machine. 
   In order to further reduce the wear of the doctor blade, doctor blades have been used, which have a ceramic coating on the side facing the printing cylinder. This increases the lifetime of the doctor blade to a multiple. 
   Applicant&#39;s European patent application EP 1 362 696 suggests to oscillate the doctor blade with a frequency in the acoustic or ultrasonic range. This measure serves to remove contaminations from the gravure printing cylinder. Here, the stroke of the vibration movement may be less than 0.2 mm. 
   On the one hand, a lateral oscillation of the doctor blade is desirable in order to avoid an uneven wear of the doctor blade, but on the other hand the oscillation may have a negative effect on the printed image, because differences in the speed of the doctor blade may result in slightly different amounts of ink in the engraved portions of the printing cylinder. With the result that ink is non-uniformly applied onto the medium. Especially the difference between a range where the doctor blade moves with an even speed, and a range where the direction of movement of the doctor blade is reversed, may be visible in the printed image. Since the frequency of the oscillation of the doctor blade is in many cases coupled to the rotary frequency of the printing cylinder, different motion patterns are used for different doctor blades, in quality multi colour printing, for example, in order to prevent the deviations in the individual colours from adding-up. However, in multi colour printing for particularly high quality, the different speeds, directions of movement and direction reversals accompanying the oscillation of the doctor blade may still have a negative effect on the printed image. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide a method for shifting a doctor blade set against a cylinder of a printing machine, wherein a high print quality can be achieved with a wear of the doctor blade as uniform and as small as possible, and to provide a printing machine for carrying out this method. 
   According to the invention, this object is achieved by a method as indicated above, wherein the shift movement of the doctor blade is such that, during most of the time of the print operation, the speed of the shift movement of the doctor blade is in a range from 0 to 0.1 m/s, preferably in a range between 0 and 0.01 m/s. The object is further achieved with a printing machine as indicated in claim  7 . 
   Preferably, the velocity of the shift movement of the doctor blade during most of the time of the print operation is in a range from 0 to 1 mm/s, more preferably in a range from 0 to 0.1 mm/s, and particularly preferred is a range from 0 to 0.01 mm/s. 
   The velocities indicated above for most of the time of the printing operation may in particular apply to time periods which correspond to 90%, more particularly 99% of the total time of the print operation. 
   When, during most of the time of the print operation, the doctor blade is shifted with a low velocity or is even held stationary, adverse effects of the movement of the doctor blade on the print quality can be reduced remarkably in comparison to a conventional, more rapid oscillation of the doctor blade. The smaller the velocity of the shift movement is, if it is not zero, the smaller is the effect on the printed image. Nevertheless, a slow but long lasting movement of the doctor blade or an intermittent movement of the doctor blade has the effect that, in the course of time, the doctor blade is shifted by a stroke of several millimetres, so that the wear of the doctor blade is distributed over the stroke length. 
   The invention avoids a non-uniform wear of the doctor blade and at the same time achieves a particularly high print quality, because the ink is applied very uniformly onto the print medium when the doctor blade is at rest or moves very slowly. 
   Useful embodiments of the invention are indicated in the dependent claims. 
   In a first embodiment of the method, the shift movement of the doctor blade is divided in terms of time into motion periods and rest periods, and the length of the rest periods are predominant during the print operation. In particular, the duration of the motion periods may be less than 10% of the duration of the rest periods, preferably less than 1%. It may even be significantly smaller. The duration of the rest periods should preferably be selected such that a noticeable, non-uniform wear of the doctor blade does not yet occur during the rest period. In this embodiment, the uniformity and hence the quality of the printed image is particularly high while the doctor blade is at rest. 
   In a printing machine, in which the printing medium is drawn from a coil and/or is wound onto a coil, it is known that a coil exchange operation during the continuing print operation leads to a temporary production of waste. This is the case for example, when a spliced section that has been formed in the web of the print medium during an automatic coil exchange passes through the print unit or when a web section is printed on, which must later be severed when a take-up roller is automatically exchanged. Therefore, it is preferable in the method according to the invention that the motion periods of the doctor blade are so timed that they coincide with the production of waste. The irregularities in the printed image that are caused by the movement of the doctor blade will then not impair the quality of the resulting print, because they are removed together with the waste, anyway. 
   Preferably, the shift movement of the doctor blade is automatically synchronised with the coil exchange operation. 
   In a second embodiment of the method, the shift movement is a periodic to-and-fro movement with a period of at least 10 seconds, preferably at least 1 minute, more preferably at least 10 minutes. Such a shift movement can be performed continuously during the print operation and will significantly reduce the deterioration of the printed image in comparison to a faster oscillation of the doctor blade. Provided that the doctor blade is sufficiently wear resistant, the period may be substantially longer and may amount to 1 hour, for example. In the second embodiment, the negative effect of the movement of the doctor blade on the printed image is also reduced significantly, and a non-uniform wear is nevertheless avoided. 
   In both embodiments of the method, the shift movement may be superposed with a vibration movement with a frequency of at least 1 Hz, preferably at least 5 Hz. This vibration movement may also be an acoustic oscillation or a supersonic oscillation. The vibration movement is preferably performed in a longitudinal direction of the doctor blade. Its purpose is among others to remove contaminations from the engraved portions of the cylinder or from the doctor blade, as has been described in applicant&#39;s European patent application EP 1 362 696. Especially in the second embodiment, however, the vibration movement has also the purpose to reduce differences in the effect of the doctor blade which occur when the doctor blade comes to rest when the direction of movement is reversed, due to the periodic to-and-fro movement. The movements of the doctor blade that are induced by the vibration are not considered as part of the “shift movement” in the meaning of the invention, and the limits for the velocity as indicated in claim  1  relate only to the shift movement as such, without taking the vibration into account. 
   Claim  7  relates to a printing machine with a doctor blade assembly having a doctor blade arranged to be set against a cylinder, and a shifting device for shifting the doctor blade in its longitudinal direction, wherein the shifting device is adapted to execute a method according to any of the claims  1  to  4 . The shifting device may be adapted for automatic or semi-automatic operation. The printing machine has the advantages that have been described in conjunction with the method. 
   In a preferred embodiment, the doctor blade assembly of the printing machine has an oscillator for generating a vibration movement of the doctor blade, and the shifting device and the oscillator are adapted to execute a method according to any of the claims  1  to  4 , wherein a vibration movement with a frequency of at least 1 Hz, preferably at least 5 Hz, is superposed to the shift movement. 
   In a particularly preferred embodiment of the printing machine, the doctor blade has a ceramic coating on the side that is subject to wear. This permits a long duration of the rest period in the shift operation of the doctor blade or a periodic to-and-fro movement with a particularly long period, without causing uneven wear of the doctor blade. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention will now be explained in conjunction with the drawings, wherein: 
       FIG. 1  is a schematic view of a gravure printing machine; 
       FIG. 2  is a schematic section along the line II—II in  FIG. 1 ; 
       FIG. 3  is a schematic view of a portion of a gravure printing machine having a winder; 
       FIG. 4  is a diagram illustrating a shift operation with motion periods and rest periods; 
       FIG. 5  is a diagram of a periodic to-and-fro movement; 
       FIG. 6  is a schematic view of a gravure printing machine having a doctor blade assembly with oscillator; and 
       FIG. 7  is a diagram of phases of a periodic to-and-fro movement, to which a vibration movement is superposed. 
   

   DETAILED DESCRIPTION 
   The gravure machine shown in  FIG. 1  comprises a gravure printing cylinder  10 , an inking roller  12  and a pressure roller  14  which are rotatably supported between side members  16 ,  18  of a machine frame. 
   Drive means and adjusting means of the machine have not been shown, because they are not essential for understanding the invention. 
   The inking roller  12  has its top held in engagement with the gravure printing cylinder  10  and has its bottom immersed into an inking trough, as can be seen more clearly in  FIG. 2 . Thus, the inking roller  12  takes up ink from the inking trough  20  and transfers the ink onto the peripheral surface of the gravure printing cylinder  10  which rotates in the direction of an arrow shown in  FIG. 2  and transfers the ink onto a print medium web  22  which passes through between the pressure roller  14  and the printing cylinder and is pressed against the printing cylinder by the pressure roller. 
   A doctor blade assembly  24  is arranged at the gravure printing cylinder  10 . As is known per-se, this doctor blade assembly is formed by a doctor blade  26  mounted to a bracket  28 . The bracket is mounted on a shaft  30  which extends between two levers  32  and  34 . The lower ends of the levers  32 ,  34  are held on a rotatable shaft  36  by which the whole doctor blade assembly  24  may be pivoted against the periphery of the printing cylinder, so that the surface thereof is doctored with an edge of the doctor blade  26 . In this way, the ink that has been applied by the inking roller  12  is removed from the smooth, non-printing surface areas of the gravure printing cylinder  10 , so that the ink will only remain in the engraved portions which create the printed image on the web  22 . 
   Mounted to the lever  34  is a shift mechanism  38  with which the bracket  28  and hence the doctor blade  26  may be shifted relative to the levers  32 ,  34  in the direction of the longitudinal axis of the doctor blade, as is indicated by a double arrow in  FIG. 1 . Thus, the shift mechanism  38  is a transverse displacement mechanism which generates a movement of the edge of the doctor blade  26  relative to the gravure printing cylinder  10  in a direction transverse to the direction of transport of the web  22 , i.e. in axial direction of the printing cylinder. In place of the shift mechanism  38 , there may also be provided a shift mechanism which shifts the whole doctor blade assembly  24 , including the levers  32 ,  34 , in the indicated direction. 
     FIG. 2  shows a ceramic coating  40  on the bottom side of the doctor blade  26 , which coating reduces the wear of the doctor blade  26  resulting from contact with the rotating gravure printing cylinder  10 . 
   As is shown in  FIG. 3 , the web  22  having passed through the printing machine passes over deflection rollers  42  and a slack-removing roller  44  and is wound onto a coil  46 . As is well known in the art, the coil may be replaced automatically or manually by a new drum  48  which will then form a core of the next coil. The bearings for the deflection rollers  42  and the core  46  and associated equipment of the winder have not been shown here, because they are not essential for understanding the invention. 
     FIGS. 4 and 5  are sketches illustrating two types of shift movement of the doctor blade  26 , which may be created by the shift mechanism  38  of the doctor blade assembly  24 . The shift movement always takes place in the longitudinal direction of the doctor blade. What is shown is the position of the doctor blade  26  in the direction of the shift movement, which position varies in time. In order to clearly show the differences between the different motion patterns, the diagrams in  FIGS. 4 and 5  and also in  FIG. 7 , which will be described below, are not to scale. 
     FIG. 4  illustrates a shift operation of the doctor blade  26 , which operation is divided into motion periods  50  and rest periods  52 . Here, the length of the rest periods  52  is at least 100 times as large as the length of the motion periods  50 . During a motion period  50 , the doctor blade  26  is shifted by a certain amount in its longitudinal direction. In  FIG. 4 , two motion periods  50  have been shown, in which the doctor blade  26  is shifted in the like direction and by the like amount of 1 mm, for example. After the doctor blade  26  has been shifted to a predetermined position in several motion periods  50 , the direction of the shift movement is reversed. The distance, by which the doctor blade is shifted in the further motion periods, may be varied. 
   Unlike the example shown in  FIG. 4 , the distance by which the doctor blade is shifted in one motion period  50  may also be varied when the direction of the movement is not changed. In order to make the wear of the doctor lade  26  as uniform as possible, it is preferable that the distances by which the doctor blade is shifted are varied such that a position which the doctor blade has once assumed will be assumed again only after a time period as long as possible. 
   The motion periods  50  are coincident in time with the operations of exchanging the coil  46 . One coil  46  is wound during each rest period  52 . A sensor, which has not been shown, detects the radius of the coil  46  and triggers the motion period  50  when the coil needs to be exchanged. The coil exchange itself is then triggered with such a time delay that, in the course of the coil exchange, the web will be severed approximately at a position which has been printed during the motion period  50 . If the movement of the doctor blade  26  has deteriorated the printed image, the defect will consequently be located in a section of the web which forms either the end of the old coil or the start of the new coil and will later be removed as waste, anyway. 
     FIG. 5  illustrates a motion pattern according to another embodiment. Here, the motion pattern is a periodic to-and-fro movement with a period of 1 hour. The stroke of the oscillation amounts to ±20 mm. for example. The velocity with which the doctor blade  26  is shifted is accordingly in a range from 0 to 0.04 mm/s. This very low velocity will affect the printed image only very little, if at all. Other oscillation patterns are also conceivable, for example, a saw-tooth oscillation. 
   A further improvement of the printed image can be achieved by means of a vibration movement which is superposed to the shift movement of the doctor blade  26 , as will be explained below. 
     FIG. 6  shows a gravure printing machine which has been modified in comparison to  FIG. 1  in that an oscillator  54  has been inserted on the shaft  30  between the shift mechanism  38  and the bracket  28 , the oscillator inducing high-frequency oscillations of the bracket  28  and hence the doctor blade  26  in the direction in parallel with the axis of the gravure printing cylinder  10 , i.e. in parallel with the shift direction of the doctor blade  26 . Thus the oscillator  54  forms another transverse displacement mechanism causing a vibration movement of the edge of the doctor blade  26 , which vibration movement is superposed with the shift movement. The frequency of the vibration is in the ultrasonic range, so that the vibration oscillations of the doctor blade have the form of running ultrasonic waves, which propagate towards the opposite end of the doctor blade and are there absorbed by a damper  56  interposed between the lever  32  and the bracket  28 . 
   However, the frequency may also be significantly smaller, and the oscillator  54  may be an acoustic oscillator, for example. Even a still smaller frequency of the vibration movement is possible. The stroke of the vibration movement may be very small and may for example amount to less than 0.2 mm. 
     FIG. 7  illustrates two phases of the shift movement of the doctor blade  26  according to  FIG. 5 , to which a vibration movement is superposed. In a phase  58  of the shift movement, the average position of the doctor blade  26  is gradually changed, while the doctor blade  26  simultaneously performs a vibration with a substantially smaller stroke about the average position. In a second phase  60 , the shift movement changes direction. Here, the superposed vibration has the effect that a stationary condition of the doctor blade  26  does not occur, not even temporarily, at the reversal point of the shift movement. In comparison to the shift movement without superposed vibration, as shown in  FIG. 5 , the superposed vibration shown in  FIG. 7  permits to further improve the print quality or, for a like quality, permits a higher shift velocity of the doctor blade. 
   The above embodiment examples relate to a gravure printing machine. However, the invention is applicable not only to gravure printing machines but also to other printing machines.