Abstract:
A printing press with a doctor blade device ( 24 ), which has a doctor blade ( 26 ), which can be placed against a cylinder ( 10 ), wherein the doctor blade ( 26 ) is caused to oscillate with a frequency of at least 1 Hz by an oscillator ( 38 ).

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a printing press with a doctor blade device, which has a doctor blade, which can be placed against a cylinder. 
     In printing presses, doctor blade devices are generally used to scrape the surface of a rotating cylinder, for example, of the printing cylinder, with the help of a doctor blade. For example, in a gravure printing press, such a doctor blade device has the function of scraping clean the smoothly polished surface of the gravure printing cylinder after the inking and before the actual printing process, so that the printing ink remains only in the printing, recessed regions of the printing cylinder surface. It may, however, happen that dust particles, dried residues of ink or other dirt particles build up in front of the edge of the doctor blade and then reach the printing recesses of the gravure cylinder and affect the printed image. In order to solve this problem, it is known that a transverse motion, transversely to the direction of rotation, that is, in the axial direction, be imparted to the doctor blade. The transverse component of the relative motion between the doctor blade and the printing cylinder then contributes to removing impurities from the recesses or detaching them from the doctor blade, so that they are washed away with the printing ink and do not reach the recesses of the printing cylinder at all. 
     Usually, the doctor blade is moved with constant speed in the axial direction. However, the direction of motion must be reversed from time to time, typically after a few seconds, so that the doctor blade carries out only a limited travel, for example, of a few millimeters or centimeters. The greater this travel, the further does the doctor blade have to protrude over the ends of the printing cylinder. A relatively expensive mechanism is required to drive and control the transverse motion of the doctor blade. In addition, the overhang of the doctor blade and its transverse motion cause printing ink to drip from the ends of the doctor blade. These drips have to be collected with expensive collecting equipment. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a printing press with a doctor blade device, with which, with a simplified construction, a high printing quality can be achieved. 
     Pursuant to the invention, this objective is accomplished by an oscillator, which oscillates the doctor blade with a frequency of at least 1 Hz. 
     Due to the comparatively high frequency of the oscillator, a significantly shorter travel of the doctor blade is achieved at the same average velocity, so that the doctor blade has to protrude less wide over the cylinder. Since the movement of the doctor blade is a forced oscillation with a short travel, the control of the course of the movement is simplified significantly and the suspension or guidance for the doctor blade can also be significantly simplified structurally. In addition, since the acceleration of the doctor blade is proportional to the square of the frequency, significantly higher accelerations are achieved with the inventive device because of the high frequency. Accordingly, a higher acceleration is imparted to the dirt particles by the doctor blade, so that these particles are detached more efficiently. 
     Advantageous developments of the invention arise out of the dependent claims. 
     Preferably, the oscillator is formed by an acoustic oscillator, which generates oscillations with the desired frequency in the infrasound range (1 to 15 Hz), in the audible range (15 Hz to 20 kHz) or the ultrasound range, which is particularly preferred. The acoustic oscillator can be structurally very simple and can be formed, for example, by a conventional, electromagnetic transducer, a piezoelectric transducer or the like. 
     The oscillator may produce oscillations in the longitudinal direction of the doctor blade and/or in the direction perpendicular to the plane of the doctor blade. Since, in the final analysis, only the relative motion matters, it is also conceivable to have the oscillator act on the cylinder. 
     Due to an asymmetric (for example, sawtooth-like) oscillation pattern, a transport effect can be achieved similar to that of a vibrating conveyor, so that the dirt particles can be removed even more efficiently. For the same purpose, it is also possible to cause the doctor blade to oscillate elliptically. 
     The higher the selected frequency of the oscillator, the smaller, in general, is the travel, which can be selected. If the travel is sufficiently small, the necessary mobility of the doctor blade can be achieved on the basis of the inherent elasticity of the mechanical suspension or of the doctor blade alone, so that expensive mechanisms for the mobile support and guidance of the doctor blade are no longer required. 
     If the travel of the doctor blade is selected to be smaller than the resolution capability of the human eye, of, for example of 0.2 mm or less, it is also ensured that the vibration of the doctor blade does not leave any visible traces in the printed image. 
     At higher frequencies, at which, in spite of the high velocity of sound in solids (about 5,000 m/s), the sound wavelength becomes smaller than the length of the doctor blade, it may be appropriate to modulate the oscillation frequency or to superimpose several anharmonic frequencies, so that permanent oscillation nodes do not develop over the length of the doctor blade. Alternatively, such oscillation nodes can also be suppressed by attenuating the sound waves at the end of the doctor blade, opposite to the oscillator, so that, essentially, reflections are not formed and a moving wave is obtained instead of a standing one. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following, examples are explained in greater detail by means of the drawing, in which 
     FIG. 1 shows a diagrammatic view of a gravure printing press, 
     FIG. 2 shows a diagrammatic section along the line II—II in FIG.  1  and 
     FIG. 3 shows a printing cylinder with a chambered doctor blade. 
    
    
     DETAILED DESCRIPTION 
     The gravure printing press, shown in FIG. 1, comprises a gravure cylinder  10 , an inking roller  12  and an impression roller  14 , which are mounted rotatably between side parts  16 ,  18  of a machine frame. The associated driving and adjusting devices are not shown here, since they are not essential for understanding the invention. 
     The top of the inking roller  12  is in contact with the gravure cylinder  10  and, with its bottom side, dips into the ink pan  20 , as can be seen clearly in FIG.  2 . The inking roller  12  takes up printing ink from the ink pan  20  and transfers it to the peripheral surface of the gravure cylinder  10 , which rotates in the direction of the arrow in FIG.  2  and transfers printing ink to a printing substrate sheet  22 , which passes between the impression roller  14  and the gravure cylinder and is pressed by the impression roller against the gravure cylinder. 
     A doctor blade device  24  is assigned to the gravure cylinder  10 . This doctor blade device is formed in a known manner by a doctor blade  26 , which is fastened at a holder  28 . The holder  28  is mounted on a shaft  30 , which extends between two levers  32 ,  34 . The lower ends of the levers  34  rest on a rotatable shaft  36 , with which the whole of the doctor blade device  24  can be swiveled against the periphery of the gravure cylinder  10 , so that the surface of the latter can be scraped with the blade of the doctor blade  26 . In this way, the printing ink, taken up by the inking roller  12 , is removed from the smooth, non-printing surface regions of the gravure cylinder  10 , so that the printing ink remains behind only in the recessed regions, which produce the printed image on the printing substrate sheet  22 . 
     On the shaft  30 , between the lever  34  and the holding device  28 , an oscillator  38  is inserted, which causes the holder  28 , and with that, also the doctor blade  26 , to oscillate at a high frequency in the direction parallel to the axis of the gravure cylinder  10 , as indicated by the double arrow in FIG.  1 . The oscillator  38  thus forms a transversely thrusting device, which produces a movement of the blade of the doctor blade  26 , relative to the gravure cylinder  10  in the direction transversely to the running direction of the printing substrate sheet  22 , that is, in the axial direction of the gravure cylinder. When the blade of the doctor blade  26  strikes dirt particles, the latter are detached by the oscillation of the doctor blade and removed with the printing ink. In this way, it is prevented that the printed image is affected by dirt particles permanently remaining in the recesses of the gravure cylinder. 
     The frequency of the oscillator  38  is, for example, in the ultrasonic range, so that the transverse oscillations of the doctor blade  26  have the shape of moving ultrasonic waves, which expand towards the opposite end of the doctor blade and are absorbed there by an absorber  40 , which is inserted between the lever  32  and the holding device  28 . 
     In the example shown, the lever  34 , at which the oscillator  38  is supported directly, is constructed particularly rigidly so that the ultrasonic oscillations are coupled effectively to the holding device  28  and the doctor blade  26 . 
     Since the transverse oscillations of the doctor blade are produced only with the help of the oscillator  38 , expensive driving mechanisms and mechanisms for producing and controlling the transverse motion of the doctor blade are not required. Transverse oscillations can also be produced effectively, even if the holding device  28  is fastened rigidly to the shaft  30 . 
     As a modified example, FIG. 3 shows a gravure cylinder  10  with a chambered doctor blade  42 . The doctor blade  26  is disposed in the interior of the chambered doctor blade and is caused to oscillate predominantly in the direction perpendicular to the plane of the doctor blade by the oscillator  38 , as indicated by a double arrow in FIG.  3 . The oscillator  38  can be disposed outside of the chambered doctor blade, so that the oscillations are transferred to the doctor blade. 
     The invention can be used not only for gravure printing presses, but also for other printing presses. For example, in the case of a flexographic printing press, if the chambered doctor blade  42  is disposed at the periphery of an anilox roller, the inner doctor blade  26  would be omitted and, instead, the doctor blade  26 ′, which terminates the doctor blade at the end, which is the rear end in the direction of rotation of the anilox roller, would be caused to oscillate.