Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates
Reexamination Certificate
1996-09-18
2002-03-19
Angebranndt, Martin (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making printing plates
C430S200000, C430S201000, C430S331000, C101S457000
Reexamination Certificate
active
06358671
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to methods and compositions for cleaning lithographic printing members following digital imaging on- or off-press.
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. 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-repellent 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 disclose 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. Pat. Nos. 5,339,737 and 5,379,698, the entire disclosures of which are hereby incorporated by reference, disclose a variety of lithographic plate configurations for use with such imaging apparatus. In particular, the '698 patent discloses laser-imageable plates that utilize thin-metal ablation layers which, when exposed to an imaging pulse, are vaporized and/or melted even at relatively low power levels. The remaining unimaged layers are solid and durable, typically of polymeric or thicker metal composition, enabling the plates to withstand the rigors of commercial printing and exhibit adequate useful lifespans.
In one general embodiment, the plate construction includes a first, topmost layer chosen for its affinity for (or repulsion of) ink or an ink-abhesive fluid. Underlying the first layer is a thin metal layer, which ablates in response to imaging (e.g., infrared, or “IR”) radiation. A strong, durable substrate underlies the metal layer, and is characterized by an affinity for (or repulsion of) ink or an ink-abhesive fluid opposite to that of the first layer. Ablation of the absorbing second layer by an imaging pulse weakens the topmost layer as well. By disrupting its anchorage to an underlying layer, the topmost layer is rendered easily removable in a post-imaging cleaning step. This, once again, creates an image spot having an affinity for ink or an ink-abhesive fluid differing from that of the unexposed first layer.
A considerable advantage to these types of plates is avoidance of environmental contamination, since the products of ablation are confined within a sandwich structure; laser pulses destroy neither the topmost layer nor the substrate, so debris from the ablated imaging layer is retained therebetween. This is in contrast to various prior-art approaches, where the surface layer is fully burned off by laser etching; see, e.g., U.S. Pat. Nos. 4,054,094 and 4,214,249. In addition to avoiding airborne byproducts, plates based on sandwiched ablation layers can also be imaged at low power, since the ablation layer does not serve as a printing surface and therefore need not be thick to resist abrasion; a durable surface layer is generally thick and/or refractory, ablating only in response to significant energy input.
An accepted approach to cleaning involves subjecting the imaged plate to mechanical action, e.g., rubbing or wiping with a cloth, or the rotation of a brush (see U.S. Pat. No. 5,148,746). Mechanical action can occur under dry conditions or be accompanied by a cleaning fluid. In the latter case, the fluid assists in the cleaning process, reducing the amount and intensity of mechanical friction necessary to remove debris and, as a result, lessening the chance of damage to the intact top layer. The cleaning fluid is generally a non-solvent for that layer, once again in order to avoid damage to unimaged areas. In particular, dry plates utilize silicone top layers, which are permeable to various solvents and tend to “swell” under their influence, resulting in weakened anchorage to underlying layers and, consequently, reduced plate durability and performance. Unfortunately, the need to preserve the silicone layer can limit the overall degree of cleaning effectiveness. Without complete removal of silicone byproducts and other pyrolitic debris from imaged portions of the plate, the necessary affinity difference between ink-repellent and ink-accepting layers cannot be achieved.
In particular, inadequate post-image processing of a silicone-surfaced dry plate results in insufficient retention of ink by the ink-receptive (generally polyester) layer. Yet the source of this behavior is not easily identified; it does not arise merely from stubbornly adherent silicone fragments. Simple mechanical rubbing of the silicone layer, for example, reliably removes from the ink-accepting layer all debris visible even under magnification, and well before damage to the unimaged silicone areas might occur. Nonetheless, such plates still may print with the inferior quality associated with inadequate affinity for ink. And while ink acceptance is substantially improved through cleaning with a solvent, this process can degrade silicone anchorage to unimaged portions of the plate.
DESCRIPTION OF THE INVENTION
Brief Summary of the Invention
Study of the imaging process and its effect on certain types of plate constructions, particularly those containing thin-metal ablation layers below silicone top coatings, suggests that the observed printing deficiencies arise from subtle chemical and morphological changes induced by the imaging process. Plates based on thin-metal imaging layers require heating to substantially higher temperatures to undergo ablation than, for example, laser-imageable printing plates having self-oxidizing (e.g., nitrocellulose) ablation layers. Particularly when low-power imaging sources are used, the exposure time necessary for catastrophic heat buildup can be significant, affording opportunity for unwanted thermal reactions. For example, the low-power imaging pulse of a diode laser must persist for a minimum duration (usually 5-15 &mgr;sec) in order to heat a metal such as titanium beyond its melting point of 1680° C. The resulting thermal breakdown products combine both chemically and mechanically, so that non-solvent cleaning procedures cannot extract all traces of silicone material from the ink-receptive film surface. Moreover, intermixture of these breakdown products interferes with the otherwise natural formation of a textured surface on the film. The combined effect is to reduce the film's oleop
Lanphear Susan J.
Lewis Thomas E.
Angebranndt Martin
Presstek Inc.
Testa Hurwitz & Thibeault LLP
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