Printing – Planographic – Lithographic plate making – and processes of making or using...
Reexamination Certificate
2002-01-15
2003-09-30
Funk, Stephen R. (Department: 2854)
Printing
Planographic
Lithographic plate making, and processes of making or using...
C101S457000
Reexamination Certificate
active
06626108
ABSTRACT:
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.
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. No. 5,493,971 discloses wet-plate constructions that extend the benefits of ablative laser imaging technology to traditional metal-based plates. Such plates remain the standard for most of the long-run printing industry due to their durability and ease of manufacture. As shown in
FIG. 1
, a lithographic printing construction
100
in accordance with the ′971 patent includes a grained-metal substrate
102
, a protective layer
104
that can also serve as an adhesion-promoting primer, and an ablatable oleophilic surface layer
106
. In operation, imagewise pulses from an imaging laser (typically emitting in the near-infrared, or “IR” spectral region) interact with the surface layer
106
, causing ablation thereof and, probably, inflicting some damage to the underlying protective layer
104
as well. The imaged plate
100
may then be subjected to a solvent that eliminates the exposed protective layer
104
, but which does no damage either to the surface layer
106
or the unexposed protective layer
104
lying thereunder. By using the laser to directly reveal only the protective layer and not the hydrophilic metal layer, the surface structure of the latter is fully preserved; the action of the solvent does no damage to this structure.
A related approach is disclosed in published PCT Application Nos. US99/01321 and US99/01396. A printing member in accordance with this approach, representatively illustrated at
200
in
FIG. 2
, has a grained metal substrate
202
, a hydrophilic layer
204
thereover, an ablatable layer
206
, and an oleophilic surface layer
208
. Surface layer
208
is transparent to imaging radiation, which is concentrated in layer
206
by virtue of that layer's intrinsic absorption characteristics and also due to layer
204
, which provides a thermal barrier that prevents heat loss into substrate
202
. As the plate is imaged, ablation debris is confined beneath surface layer
208
; and following imaging, those portions of surface layer
208
overlying imaged regions are readily removed. Because layer
204
is hydrophilic and survives the imaging process, it can serve the printing function normally performed by grained aluminum, namely, adsorption of fountain solution.
Both of these constructions rely on removal of the energy-absorbing layer to create an image feature. Exposure to laser radiation may, for example, cause ablation—i.e., catastrophic overheating-of the ablated layer in order to facilitate its removal. Accordingly, the laser pulse must transfer substantial energy to the absorbing layer. This means that even low-power lasers must be capable of very rapid response times, and imaging speeds (i.e., the laser pulse rate) must not be so fast as to preclude the requisite energy delivery by each imaging pulse.
DESCRIPTION OF THE INVENTION
Brief Summary of the Invention
The present invention obviates the need for substantial ablation as an imaging mechanism, combining the benefits of simple construction, the ability to utilize traditional metal base supports, and amenability to imaging with low-power lasers that need not impart ablation-inducing energy levels. In preferred embodiments, the invention utilizes a printing member having a topmost layer that is ink-receptive and a hydrophilic metal substrate. The topmost layer does not significantly absorb imaging radiation, but an intermediate layer disposed between the topmost layer and the metal substrate does absorb imaging radiation. In one version, in response to an imaging pulse, the absorbing layer debonds from the surface of the adjacent metal substrate; in another version, an interior split is formed within the absorbing layer, facilitating removal of the portion of that layer above the split. In neither case does the absorbing layer undergo substantial ablation.
It must be stressed that it is ordinarily impractical or even impossible to image, by ablation, constructions in which an absorbing layer directly overlies the metal substrate. This is because because the thick metal substrate acts as a heat sink, drawing laser energy needed to heat the absorbing layer to achieve imaging. Because ablation is not involved as an imaging mechanism in the present invention, however, this condition is avoided. Sufficient energy is concentrated in the upper portions of the absorbing-layer thickness to cause debonding notwithstanding heat transport into the metal substrate. It is also possible to create an absorber gradient within the absorbing layer, with the absorber concentration diminishing from the top of the layer to the bottom, so that the surface in contact with the metal substrate has very little absorber. This concentration gradient further discourages transfer of heat to the metal substrate while preserving sufficient overall absorption and heating to effect interfacial debonding. Indeed, some transfer of heat to the metal substrate (as well as to an overlying layer, when present) is desirable to avoid unintended ablation of the absorbing layer, which can result in production of unwanted volatile debris.
In use, the printing member is selectively exposed to laser radiation in an imagewise pattern. Where the printing member has received laser exposure—that is, where the substrate and absorbing layer have been detached from each other—remnants of the absorbing layer and the overlying layer (or layers) is readily removed by post-imaging cleaning (see, e.g., U.S. Pat. Nos. 5,540,150; 5,870,954; 5,755,158; and 5,148,746) to produce a finished printing place.
Accordingly, layers that would otherwise undergo complete destruction as a consequence of ablation imaging are retained in the present constructions, and serve as highly durable layers that participate in the printing process. Key to the present invention, then, is irreversible detachment between layers caused by heating, without ablation, of a radiation-absorptive layer, and an absorber concentration gradient that prevents excessive energy dissipation from the absorbing layer.
The plates of the present invention are “positive-working” in the sense that inherently ink-receptive areas receive laser output and are ultimately removed, revealing the hydrophilic layer that will reject ink duri
Funk Stephen R.
Presstek Inc.
Testa Hurwitz & Thibeault LLP
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