Apparatus for clamping and holding a printing plate on an exposure drum

#The invention pertains to the field of electronic reproduction technology and relates to an apparatus for clamping and holding a printing plate on the surface of an exposure drum in a printing plate exposer that records a printing original onto a printing plate.

#In reproduction technology, printing originals are produced for printed pages, which printing originals contain all the elements to be printed such as text, graphics, and images. For color printing, a separate printing original is produced for each printing ink, the separate printing original containing all the elements that are printed in the respective color. For four-color printing, they are the print colors cyan, magenta, yellow, and black (CMYK). The printing originals are usually recorded onto printing plates in a printing plate exposer, and the printing plates are, then, used to print large runs of the printed pages in a printing press. As an alternative, the printing originals can be transferred directly to a digital printing press as digital data. There, the printing original data are, then, exposed onto printing plates, for example, using an exposure unit that is integrated into the printing press, before the printing of the edition begins immediately thereafter.

#In the recording apparatuses that are used in electronic reproduction technology to expose printing plates, a laser beam is generated, for example, by a laser diode, shaped by optical measures and focused onto the printing plate and deflected over the printing plate point by point and line by line by a deflection system. To increase the exposure speed, a bundle of laser beams can also be generated, and a plurality of image lines are exposed simultaneously each time they sweep across the printing plate. The printing plate can be situated on an exposure drum (external drum exposer), in a cylindrical recess (internal drum exposer), or on a flat surface (flatbed exposer). In an external drum exposer, for example, the printing plate to be exposed is fitted onto a rotatably mounted exposure drum. While the exposure drum rotates, an exposure head is moved axially along the drum at a relatively short distance. The exposure head focuses one or more laser beams onto the drum surface, the laser beams sweeping across the drum surface in the form of a tight helix. As such, one or more image lines is/are exposed during each revolution of the drum.

#FIG. 1 is a diagrammatic, perspective view of a basic construction of an external drum exposer according to the invention;

#FIG. 2 is a fragmentary, enlarged cross-sectional view of a clamping apparatus according to the invention for a printing plate;

#FIG. 3 is a diagrammatic, perspective view of an exposure drum according to the invention having suction grooves and suction channels;

#Referring now to the figures of the drawings in detail and first, particularly to FIG. 1 thereof, there is shown the basic construction of an external drum exposer. An exposure drum 1 is rotatably mounted and can have a uniform rotational movement imparted to it in the direction of the rotation arrow 2 using a non-illustrated rotational drive. An unexposed, rectangular printing plate 3 is clamped onto the exposure drum 1, which printing plate 3 has a front side 4, a left-hand lateral edge 5, a right-hand lateral edge 6, and a rear edge 7. The printing plate 3 is clamped such that its front edge 4 touches contact pins 8 that are connected fixedly to the exposure drum 1 and that protrude beyond the surface of the exposure drum 1. Moreover, a plurality of clamping bars 9 press the front edge 4 firmly onto the surface of the exposure drum 1 and, as a result, fix the front edge 4 of the printing plate 3 in position. The printing plate 3 is held over its entire area on the drum surface by a vacuum device that attracts the printing plate 3 by suction through suction grooves (not illustrated in FIG. 1) in the drum surface so that the printing plate 3 is not detached by the centrifugal forces during rotation. Additionally, the rear edge 7 of the printing plate 3 is fixed in position using terminating elements 10.

#An exposure head 11 is moved axially along the exposure drum 1 at a relatively short distance, while the exposure drum 1 rotates. The exposure head 11 focuses one or more laser beams 12 onto the drum surface, the laser beams 12 sweeping across the drum surface in the form of helixes that are close to one another. As such, one or more image lines is/are exposed onto the printing plate 3 in the circumferential direction x during each revolution of the drum. The exposure head 11 is moved in the feed direction y by a feed spindle 13 to which it is connected with a form-fitting connection and which is set moving rotationally using a feed drive 14.

#FIG. 2 shows a preferred embodiment of the clamping apparatus according to the invention for the front edge 4 of the printing plate 3 in a cross-sectional illustration through the drum surface. The printing plate 3 is pressed onto the exposure drum 1 by a clamping bar 9. The clamping bar 9 is connected to a pull rod 20 that is guided through a guide bushing 21 into the interior of the exposure drum. The pull rod 20 terminates at its lower end with a disk 22. A compression spring 23 is mounted between the guide bushing 21 and the disk 22. The compression spring 23 exerts a force on the disk 22 in the direction of the arrow A and, as a result pulls, the pull rod 20 and the clamping bar 9 in the direction of the arrow A. As a result, the printing plate 3 is clamped fixedly. A plurality of clamping bars 9 are disposed in the axial direction of the exposure drum 1 (FIG. 1), each clamping bar 9 contributing to the fixed clamping of the printing plate 3 by a compression spring 23 and a pull rod 20.