Photocopying – Projection printing and copying cameras – Step and repeat
Patent
1993-10-26
1995-07-18
Gellner, M. L.
Photocopying
Projection printing and copying cameras
Step and repeat
347263, G03B 2710
Patent
active
054346460
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and an apparatus for exposing a multi-pages original images onto a light sensitive material like a light sensitive offset plate or an intaglio plate by means of a laser exposure/burning process.
In the following the expression "pages" does not only relate to pages to be printed but also a series of pictures, tables and many other images which are to be exposed separately onto a special part of a light sensitive material. It should also be noted that the above mentioned light sensitive plates are only one example, out of many other possible examples, of light sensitive materials, and that the method and the apparatus may as well be used for exposing of images onto films and onto any kinds of materials having a light sensitive layer.
Lately some print makers have started to use laser light beams for exposing light sensitive offset plates or intaglio plates. The advantages of using laser is, among other things, that it is possible concerning text, sketches and half-tone pictures etc. without the need of using negative of positive transparent films, as has so far been necessary, limits, De determined and controlled, in particular the dot creating a half-tone picture in the print and which corresponds to the screen dot in conventional screen copying processes, and this is possible in that the laser light builds up the screen dot from a large riumber of pixies, generally between 64 and 256 pixies per screen dot, image on the printing plate can be varied within wide limits.
Like in conventional copying processes the laser light provides an activation of the illuminated part of the printing plate so that, in the succeeding developing process, said activated printing plate parts become ink carrying if the printing plate is of the positive type, or non-ink carrying respectively if the printing plate is of the negative type.
There are to-day three main methods of laser exposing printing plates, namely exposing a plate lying on a flat bed, the so called flat bed method, exposing a plate mounted inside a cylindrical drum whereby the exposing is made from inside the curved plate, which process is called internal drum method, and exposing of a plate mounted exteriorly on a cylindrical drum, whereby the exposure is made from outside, which process is called external drum method.
Each one of said three methods is disadvantageous in some respects: exposure step by means of a sweeping laser light beam. Generally the laser apparatus including the so called "laser gun" is stationary, whereas the printing plate is displaceable on an exposure table in relation to the laser unit. During the exposure the laser unit emits an oscillating light beam which hits the plate while said plate is slowly moving passed the laser unit until the entire plate has been exposed. Alternatively it is possible to keep the exposure table still standing and to allow the laser unit to wander over the plate during the exposure. In this type of the flat bed copying machine the distance from the laser gun or the lens system thereof to the side edges of the printing plate is substantially longer than to the central parts of the plate. This has as an effect that the sharpness and the shape of the laser dot exposed on the printing plate varies depending on where the laser beam hits the plate, For reducing, to some extent, the problem with this variation in dot sharpness and shape the laser gungenerally is mounted.on a relative high distance above the printing plate, and-for equalizing the always existing unavoidable variations in dot sharpness and shape in this type of copying machine the laser gun generally is adjusted so that the sharpness of the laser dot is located slightly to high at the edges of the plate and slightly to low at the central parts of the plate. As a consequence an ideal dot is located somewhere between the centre and the edges of the plate. Depending on the said machine design the laser copying machine is high and clumsy, and the laser burning is relatively uneven. when comparing different parts o
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Gellner M. L.
Malley Daniel P.
Misomex Aktiebolag
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