Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates
Utility Patent
1999-01-21
2001-01-02
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making printing plates
C430S273100, C430S272100, C101S467000
Utility Patent
active
06168903
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to digital printing methods and materials, and more particularly to imaging of lithographic printing-plate constructions on- or off-press using digitally controlled laser output.
2. Description of the Related Art
In offset lithography, a printable image is present on a printing member as a pattern of ink-accepting (oleophilic) and ink-rejecting (oleophobic) surface areas. Once applied to these areas, ink can be efficiently transferred to a recording medium in the imagewise pattern with substantial fidelity. Dry printing systems utilize printing members whose ink-repellent portions are sufficiently phobic to ink as to permit its direct application. Ink applied uniformly to the printing member is transferred to the recording medium only in the imagewise pattern. Ordinarily, the printing member 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 inking. The fountain solution prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
Traditional platemaking processes tend to be time-consuming and require facilities and equipment adequate to support the necessary chemistry. To circumvent these shortcomings, practitioners have developed a number of electronic alternatives to plate imaging. With these systems, digitally controlled devices alter the ink-receptivity of blank plates in a pattern representative of the image to be printed. U.S. Pat. Nos. 5,339,737, 5,783,364, and 5,807,658, the entire disclosures of which are hereby incorporated by reference, disclose a variety of lithographic plate configurations for use with imaging apparatus that operate by laser discharge. These include wet plates as described above and dry plates to which ink is applied directly. The plates may be imaged on a stand-alone platemaker or directly on-press.
Laser-imageable materials may be imaged by pulses of near-infrared (near-IR) light from inexpensive solid-state lasers. Such materials typically exhibit a nonlinear response to near-IR exposure, namely, a relatively sharp imaging-fluence threshold for short-duration laser pulses but essentially no response to ambient light. A longstanding goal of plate designers is to reduce the threshold laser fluence necessary to produce an imaging response while maintaining desirable properties such as durability, manufacturability, and internal compatibility.
One strategy frequently proposed in connection with photothermal materials is incorporation of energetic (e.g., self-oxidizing) compositions, which, in effect, contribute chemical energy to the imaging process. For example, the '737 patent mentioned above discloses nitrocellulose layers that undergo energetic chemical decomposition in response to heating. Unfortunately, these materials have not been shown to reduce the fluence thresholds necessary for imaging. Instead, they are either employed as essentially interchangeable alternatives to non-energetic materials, or as propellant layers in transfer-type materials (see, e.g., U.S. Pat. Nos. 5,308,737, 5,278,023, 5,156,938, and 5,171,650).
DESCRIPTION OF THE INVENTION
Brief Summary of the Invention
It has been found, surprisingly, that the combination of a thermally activated gas-forming composition with a material that strongly absorbs imaging radiation, used as co-active imaging layers in a laser-imageable construction, results in substantial enlargement of the area affected by a laser pulse (as compared with constructions utilizing as imaging layers either component alone). The result is considerable reduction in the fluence necessary to create an image spot of a given size.
A printing member in accordance with the present invention includes a solid substrate, gas-producing and radiation-absorptive layers over the substrate, and a topmost layer that contrasts with the substrate in terms of lithographic affinity. The order in which the gas-producing and radiation-absorptive layers appear depends on the mode of imaging—that is, whether laser radiation is applied through the topmost layer or through the substrate. In operation, exposure of the radiation-absorptive layer to laser light causes this layer to become intensely hot. This, in turn, activates the gas-producing layer, causing rapid evolution and expansion of gaseous decomposition products. The gases stretch the overlying topmost layer to create a bubble over the exposure region, where the imaging layers have been destroyed. If this process is sufficiently explosive, the neck of the bubble expands beyond the diameter of the incident laser beam, tearing the topmost layer and the underlying imaging layers away from the substrate outside the exposed region. The entire affected area is easily removed during a post-imaging cleaning process, resulting in an image spot larger than the incident beam diameter. Furthermore, because the decomposition gases are retained within the bubble, there is no danger of environmental contamination.
Post-imaging cleaning can be accomplished either manually (by dry rubbing or rubbing with a cleaning liquid, as described in U.S. Pat. No. 5,540,150) or using a contact cleaning device (e.g., a rotating brush as described in U.S. Pat. No. 5,148,746) or other suitable means (e.g., as set forth in U.S. Pat. No. 5,755,158).
It should be stressed that, as used herein, the term “plate” or “member” refers to any type of printing member or surface capable of recording an image defined by regions exhibiting differential affinities for ink and/or dampening fluid; suitable configurations include the traditional planar or curved lithographic plates that are mounted on the plate cylinder of a printing press, but can also include seamless cylinders (e.g., the roll surface of a plate cylinder), an endless belt, or other arrangement.
Furthermore, the term “hydrophilic” is herein used in the printing sense to connote a surface affinity for a fluid which prevents ink from adhering thereto. Such fluids include water, aqueous and non-aqueous dampening liquids, the non-ink phase of single-fluid ink systems. Thus, a hydrophilic surface in accordance herewith exhibits preferential affinity for any of these materials relative to oil-based materials.
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Cassidy Kenneth R.
D'Amato Richard J.
Lewis Thomas E.
Baxter Janet
Gilliam Barbara
Presstek Inc.
Testa Hurwitz & Thibeault LLP
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