Laser-imageable printing members and methods for wet lithographi

Details Laser-imageable printing members and methods for wet lithographi Laser-imageable printing members and methods for wet lithographi

1994-04-13

1996-02-27

Funk, Stephen

Printing

Planographic

Lithographic printing plates

101457, 101459, 101467, B41N 108

Patent

active

054939712

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to so-called "wet" lithographic printing plate constructions that may be imaged on- or off-press using digitally controlled laser output.
2. Description of the Related Art
Traditional techniques of introducing a printed image onto a recording material include letterpress, flexographic and gravure printing and offset lithography. All of these printing methods require a printing member, usually loaded onto or integral with a plate cylinder of a rotary press for efficiency, to transfer ink in the pattern of the image. In letterpress and flexographic printing, the image pattern is represented on the printing member in the form of raised areas that accept ink and transfer it onto the recording medium by impression; flexographic systems, which utilize elastomeric surfaces, have received more widespread acceptance due to the broad variety of compatible substrates and the ability to run with fluid inks. Gravure printing cylinders, in contrast to raised-surface systems, contain series of wells or indentations that accept ink for deposit onto the recording medium; excess ink must be removed from the cylinder by a doctor blade or similar device prior to contact between the cylinder and the recording medium.
In the case of offset lithography, the image is present on a plate or mat as a pattern of ink-accepting (oleophilic) and ink-repellent (oleophobic) surface areas. In a dry printing system, the plate is simply inked and the image transferred onto a recording material; the plate first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening (or "fountain") solution to the plate prior to or in conjunction with inking. The ink-repellent fountain solution prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
If a press is to print in more than one color, a separate printing plate corresponding to each color is required, each such plate usually being made photographically as described below. In addition to preparing the appropriate plates for the different colors, the operator must mount the plates properly on the plate cylinders of the press, and coordinate the positions of the cylinders so that the color components printed by the different cylinders will be in register on the printed copies. Each set of cylinders associated with a particular color on a press is usually referred to as a printing station.
In most conventional presses, the printing stations are arranged in a straight or "in-line" configuration. Each such station typically includes an impression cylinder, a blanket cylinder, a plate cylinder and the necessary ink (and, in wet systems, dampening) assemblies. The recording material is transferred among the print stations sequentially, each station applying a different ink color to the material to produce a composite multi-color image. Another configuration, described in U.S. Pat. No. 4,936,211, relies on a central impression cylinder that carries a sheet of recording material past each print station, eliminating the need for mechanical transfer of the medium to each print station.
With either type of press, the recording medium can be supplied to the print stations in the form of cut sheets or a continuous "web" of material. The number of print stations on a press depends on the type of document to be printed. For mass copying of text or simple monochrome line-art, a single print station may suffice. To achieve full tonal rendition of more complex monochrome images, it is customary to employ a "duotone" appro

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