Lithographic printing press for processless plate

Abstract

An imaged plate with an oleophilic resin coating non-ionically adhered on a hydrophilic substrate can be directly processed on-press by the application of disruptive mechanical tension forces to remove the unimaged areas as undissolved particles, using the tack of the ink on-press.

Description

RELATED APPLICATIONS[0001]This application is a divisional of U.S. application Ser. No. 12 / 586,764 filed Sep. 28, 2009 for “Non-Chemical Development of Printing Plates”, which is a continuation-in-part of U.S. application Ser. No. 11 / 493,183 filed Jul. 26, 2006 for “Imagable Printing Plate for On-Press Development”, which claims priority under 35 U.S.C. §119(e) from U.S. Provisional Application No. 60 / 704,140 filed Jul. 29, 2005, for “Imagable Printing Plate for On-Press Development”, and benefit under 35 U.S.C. §120 is also claimed from U.S. application Ser. No. 11 / 821,721 filed Jun. 25, 2007 for “Water Spray Development of Planographic Plates” and U.S. application Ser. No. 12 / 215,124 filed Jun. 25, 2008 for “Heated Water Spray Processor”. The complete disclosures of these applications are hereby incorporated by reference.BACKGROUND OF THE INVENTION[0002]The present invention relates to imagable lithographic plates for printing.[0003]Plates of interest have a solvent-soluble, radia...

AI Technical Summary

Benefits of technology

[0014]The most evident advantage is that no separate developing equipment or step is required between the imager and the press. A second significant advantage, whether or not the plate is passed through a pre-press water processor, is that there is little or no chemical treatment required of the waste stream associated with developing the plate. A third significant advantage is that because dissolution of the polymer resin is not relied upon for processing the plate, higher molecular weight resins can be used in the imagable coating, thereby producing more durable oleophilic areas and longer plate life on press.

[0016]First, the imagable coating as initially applied and dried on the substrate, has a relatively low degree of adhesion to the substrate. This is preferably achieved by using a substrate having a grained, positively charged (anionic) hydrophilic surface to which the coating mildly adheres non-ionically as a result of drying. Such substrate can be a grained aluminum sheet treated with silicate or other known hydrophilizing agents. Drying produces a mild degree of polymerization such that the bottom surface of the coating mechanically interengages and thus adheres to the irregularities in the grained surface of the substrate, and the body of the coating achieves sufficient cohesion to permit further handling, shipment, and imaging of the plates. For on-press development, the cohesion of the dried, unimaged coating is greater than its adhesion to the substrate and its tack or adhesion to the ink and blanket roll is greater than its adhesion to the substrate, but the adhesion of the imaged coating to the substrate is greater than its adhesion to the ink.

[0019]Practitioners in this field had no reason to investigate or optimize the difference in adhesion of non-aqueous photopolymerizable resins as a basis for non-chemical, and especially mechanical, removal of the nonimage areas. Because it was the established practice that nonimage areas of the imaged plate could be substantially completely dissolved by the non aqueous developer solution, the main objective for improving coatings has been to increase the adhesion, cohesion, and durability of the imaged areas and thereby enable the plate to better withstand the rigors of the printing press. Any desired relationship between the imaged and unimaged areas was based on relative solubility, not relative mechanical adhesion, to minimize incidental dissolution of any of the exposed surface the imaged areas while the developer solution dissolved substantially all of the non image areas.

[0020]With the present invention, several techniques are available for facilitating or increasing the speed of the removal of the unimaged areas in solidus, i.e., without dissolution.

[0022]According to another technique, the plates are heated after imaging to increase the difference in cohesion and adhesion of the coating to the substrate as between the imaged and unimaged areas, such that a greater force can be applied to the plates to dislodge only the unimaged areas. In particular, a thermally imagable negative working plate can be exposed to heat for a short period of time after imaging, whereby the imaged portions become more stable and tougher, while the portions of the coating that are to be removed are not significantly affected. The heating step preferably, but not necessarily, immediately follows the imaging step, but can be at a different location from the imaging step.

[0024]In a further preference, the water soluble top coat conventionally used to protect photosensitive (PS) coatings is washed off the PS coating after imaging (and after any subsequent heating step) and the plates stored temporarily until mounted on press. The top coat is typically a water soluble film former (such as PVOH) that prevents atmospheric oxygen from diffusing into the coating and quenching the free radicals necessary for inducing polymerization. The removal of this topcoat has been found to substantially immunize the imaged coating from further polymerization in the unimaged areas due to ambient light. Thus, the plates need not be handled in yellow or other special light between imaging and mounting on press.

Problems solved by technology

During start-up, when the blanket compresses against the inked-imaged plate surfaces, the high-tack adhesion of the ink to the blanket exceed the adhesion of the unimaged areas of the plate, and the cohesion of the unimaged areas of the plate also exceed the adhesion to the plate, so the fractured non-image particles are deposited on the paper web by the blanket and eventually end up in the initial start-up paper waste.

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