Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Patent
1993-10-26
1996-08-27
Reinhart, Mark J.
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
355 86, 247264, B41J 2435
Patent
active
055505740
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and an apparatus for exposing multi-pages original images onto a light sensitive offset plate or an intaglio plate by means of a laser exposure/burning process.
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 a negative of positive transparent films, as has so far been usual, limits, be 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 number of pixels, generally between 64 and 256 pixels 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 today 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 displacable 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 past 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, and 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 gun generally is mounted at 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 too 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 of the printing plate. Another problem is to, as far as possible, eliminate the vibrations which unevitably appear when the copying table and the laser unit move in relation to each other; vibrations, and even with a very little unexpected movement of the laser dot on the printing plate, for instance caused by a slight shaking of the laser gun, the dot will be unsharp on the printing plate, in particular since the laser gun normally projects between 64 and 256 pixels for creating each separate screen dot when exposing an optimum grey scale; printing plate, generally the base o
REFERENCES:
patent: 4052603 (1977-10-01), Karlson
patent: 4159991 (1979-04-01), Dillow
patent: 4993696 (1991-02-01), Furukawa et al.
patent: 5322268 (1994-06-01), Okutsu et al.
Misomex Aktiebolag
Reinhart Mark J.
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