Printing – Planographic – Lithographic printing plates
Reexamination Certificate
1999-09-03
2001-08-28
Funk, Stephen R. (Department: 2854)
Printing
Planographic
Lithographic printing plates
C101S458000, C101S467000
Reexamination Certificate
active
06279476
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to digital printing apparatus and methods, and ore particularly to imaging of lithographic printing-plate constructions on- or off-press using digitally controlled laser output.
BACKGROUND OF THE INVENTION
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. 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 fluid to the plate prior to inking. The dampening fluid 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. 5,783,364 and 5,807,658, 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 dampening fluid, while the underlying polymeric layer accepts ink. Application of an imaging pulse to a point on the plate ultimately creates an image spot having an affinity for a dampening fluid differing from that of unexposed areas, the pattern of such spots forming a lithographic plate image.
These types of plates can exhibit performance limitations, particularly after high numbers of impressions, owing to the abrupt transition between a hard inorganic layer and a soft 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 replaces the conventional polymeric ink-accepting layer with a hard, inorganic material that exhibits sufficient flexibility (at the deposition thicknesses envisioned) to accommodate the flexing and bending required of lithographic printing plates. This plate layer may overlie a relatively heavy, metal plate substrate or support (although, again, one flexible enough to accommodate plate mounting and use), resulting in a structure whose permanent layers all share the physical properties of inorganic materials.
The plates may also be provided with a protective layer that serves a variety of beneficial functions, including protection against handling and environmental damage and extension of plate shelf life, but which also is removed during the printing make-ready process.
In general, the plate constructions of the present invention include a durable hydrophilic layer; a hard, inorganic, oleophilic layer; and a substrate which, as noted above, may itself be metal. If a metal substrate is employed, an overlying layer provides sufficient thermal insulation to prevent substantial dissipation of heat—which is necessary to achieve ablation—into the substrate. Accordingly, in this context, the degree of thermal insulation is adequate if the imaging power (at a given pulse width) necessary for ablation is comparable to that used in connection with plates having thermally insulating (e.g., polyester) substrates.
The plates of the present invention can be either “positive-working” or “negative-working.” In positive-working versions, areas that are inherently ink-receptive receive laser output and are removed, revealing the hydrophilic layer that will reject ink during printing; in other words, the “image area” is selectively removed to reveal the “background.” In negative-working versions, areas that are inherently hydrophilic are removed to reveal an underlying ink-receptive image layer.
In all disclosed embodiments, the applied inorganic layers are desirably hard and adequately flexible at application thicknesses, and resist degradation by solvents typically used during press operation; in addition, all layers other than the ablation layer are thermally stable (exhibiting, for example, high melting points that resist degradation by imaging radiation), and reflect or at least do not absorb imaging radiation. In the case of oleophilic layers employed in negative-working embodiments, certain ceramic materials (as defined below) are suitable, and an intermediate tying layer may be used to anchor a hydrophilic ceramic layer to the oleophilic layer. In positive-working embodiments, the oleophilic layers may be refractory compounds doped with oleophilic material. An advantage to the latter embodiments is the ability to apply traditional means of correction following imaging and/or physical damage.
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 fountain solution; 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 used in the printing sense to connote a surface affinity for a fluid which prevents ink from adhering thereto. Such fluids include water for conventional ink systems, aqueous and non-aqueous dampening liquids, and 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.
REFERENCES:
patent: Re. 35512 (1997-05-01), Nowak et al.
patent: 3996057 (1976-12-01), Kawaziri et al.
patent: 4150623 (1979-04-01), Hamilton
patent: 5339737 (1994-08-01), Lewis et al.
patent: 5783364 (1998-
Funk Stephen R.
Presstek Inc.
Testa Hurwitz & Thibeault LLP
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