Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates
Reexamination Certificate
1999-03-18
2001-03-27
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making printing plates
C430S303000, C430S271100, C101S470000, C101S465000, C101S458000, C101S459000, C101S460000
Reexamination Certificate
active
06207349
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to digital printing apparatus and methods, 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-rejecting 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. Typically, 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 ink-abhesive fountain solution prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
To circumvent the cumbersome photographic development, plate-mounting and plate-registration operations that typify traditional printing technologies, practitioners have developed electronic alternatives that store the imagewise pattern in digital form and impress the pattern directly onto the plate. Plate-imaging devices amenable to computer control include various forms of lasers. For example, U.S. Pat. Nos. 5,351,617 and 5,385,092 (the entire disclosures of which are hereby incorporated by reference) describe an ablative recording system that uses low-power laser discharges to remove, in an imagewise pattern, one or more layers of a lithographic printing blank, thereby creating a ready-to-ink printing member without the need for photographic development. In accordance with those systems, laser output is guided from the diode to the printing surface and focused onto that surface (or, desirably, onto the layer most susceptible to laser ablation, which will generally lie beneath the surface layer).
U.S. Ser. Nos. 08/700,287 and 08/756,267, the entire disclosures of which are hereby incorporated by reference, describe a variety of lithographic plate configurations for use with such imaging apparatus. In general, the plate constructions include an inorganic layer (i.e., a metal, combination of metals, or a metal
on-metal compound) situated on an organic polymeric layer. The inorganic layer ablates in response to imaging (e.g., infrared, or “IR”) radiation. In one approach, the inorganic layer represents the topmost surface of the plate and accepts fountain solution, while the underlying polymeric layer accepts ink. In another approach, the inorganic layer serves only a radiation-absorption (rather than a lithographic) function, with the underlying layer accepting ink and an overlying layer either rejecting ink or accepting fountain solution. Ablation of the inorganic layer by an imaging pulse generally weakens the topmost layer as well, and this, combined with disruption of its anchorage (due to disappearance of the ablated inorganic layer), renders the topmost layer easily removable in a post-imaging cleaning step. With either of these two approaches, application of an imaging pulse to a point on the plate ultimately creates an image spot having an affinity for ink or an ink-abhesive fluid differing from that of unexposed areas, the pattern of such spots forming a lithographic plate image.
These types of plates can pose manufacturing challenges, as well as performance limitations, owing to the abrupt transition between an inorganic layer and an organic, polymeric layer. The divergent physical and chemical characteristics of such distinct layers can compromise their anchorage to one another—a critical performance requirement—as well as the durability of the inorganic layer. For example, because inorganic and organic materials typically have very different coefficients of thermal expansion and elastic moduli, even perfectly adhered inorganic layers may undergo failure (e.g., fracturing) due to temperature variations or the stress of plate manipulation and use. The different responses of two adjacent layers to an external condition can easily cause damage that would not occur in either layer by itself.
To improve interlayer anchorage, polymeric layers may be selected (or applied as intermediate coatings) based on chemical compatibility with inorganic material. A polymeric layer may also be pretreated (e.g., through plasma exposure) to modify the surface for greater interfacial compatibility with a subsequently applied inorganic layer. These approaches, however, have limited utility in addressing the effects of transition between fundamentally different materials.
DESCRIPTION OF THE INVENTION
Brief Summary of the Invention
The present invention reduces the abruptness of interfacial transition by altering the effective properties of the organic layer (to which the inorganic layer is applied) by incorporating an inorganic component within the matrix of the organic layer. In a first aspect, the invention comprises a method of fabricating a lithographic printing plate having adjacent organic and inorganic layers. A first layer comprising a curable polymer is softened, and an inorganic material—compatible with or, in some cases, compositionally identical to—the soon-to-be-applied inorganic layer is deposited onto a surface of the softened polymer. The inorganic material overspreads the surface and integrates within the soft polymeric layer; at this point, it may be desirable to assist the migration of the inorganic material into the polymer (e.g., by charging the inorganic material and applying an opposite charge to a conductor underlying the polymer). The polymer is then cured to immobilize the integrated deposition material, thereby forming a composite, and the desired inorganic layer is applied over the deposited inorganic material (and any exposed portions of the polymer). This second inorganic layer, and possibly the previously deposited inorganic material as well, is subject to ablative removal by exposure to laser radiation. The second inorganic layer and the organic/inorganic composite have different affinities for ink and/or an ink-abhesive fluid. The inorganic layer may, for example, be a metallic inorganic material as disclosed in the '287 and '267 applications. Despite the introduction of such an inorganic material within the matrix of the polymer, the natural affinity characteristics (e.g., oleophilicity) of the polymer may be retained. For example, while the inorganic phase may have a pronounced effect on the stiffness and heat-transport properties of the composite, thereby enhancing physical compatibility with a pure inorganic layer, it may not significantly affect surface energy (so that the composite retains the the affinity for ink and/or an ink-abhesive fluid that characterized the original polymer).
The deposition material may fully cover the surface of the polymeric material, forming a continuous layer thereover, or may instead form an intermittent pattern over the surface. In the former case, imaging radiation may remove both the second inorganic layer and the the deposition material from the polymer to expose the surface of the composite.
The polymer is generally chosen both for its lithographic affinity characteristics and also for its ability to be cured into a rigid, three-dimensional structure that permanently immobilizes the inorganic deposition material. Not suitable for the present invention are polymeric materials that ex
Baxter Janet
Cesari and McKenna LLP
Lee Sin J.
Presstek Inc.
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