The present invention relates to a method of producing a directly imageable waterless planographic printing plate directly using a laser beam, particularly a method of producing a directly imageable waterless planographic printing plate which allows printing without using dampening water.
A process of producing an offset printing plate directly from an original without using a plate making film, the so-called directly imageable process (also called direct process) begins to be used also in the fields of general offset printing and gravure printing, as well as in the small-scale printing industry, because of such features as simplicity requiring little skill, speediness in obtaining a printing plate in a short time, and rationality of selectability adapted to desired quality and cost from a variety of systems. Especially recently, in response to the rapid progress of output systems such as prepress systems, image setters and laser printers, new types of various planographic printing plates for the direct process are being developed.
The processing methods for these planographic printing plates include methods of irradiating with a laser beam, methods of writing by a thermal head, methods of partially applying a voltage by a pin electrode, methods of forming an ink repellent layer or an ink acceptable layer by ink jet, etc.
Among them, the methods of using a laser beam are more excellent than the other methods in resolution and processing speed, and are diverse.
The planographic printing plates using a laser beam include the photon mode type based on photo reaction and the heat mode type in which thermal reaction is caused by light-heat conversion. The heat mode type can be advantageously handled in a daylight room, and its usefulness is being reviewed recently because of the sudden progress of semiconductor lasers used as the light sources.
For producing a planographic printing plate in the heat mode, particularly a waterless planographic printing plate not requiring the use of dampening water for printing, the following methods have been proposed, but they involve various problems.
For example, U.S. Pat. Nos. 5,339,737, 5,353,705, EP 0580393, etc. propose methods for forming a printing plate based on the ablation of the heat sensitive layer. However, these methods have such problems that a high laser energy is necessary for ablating the heat sensitive layer, that a byproduct such as gas is produced by ablation, being likely to adversely affect the optical system, that the residue of ablation must be removed, and that it is difficult to reproduce very small halftone dots.
For producing a printing plate using a developer, the following methods are proposed.
U.S. Pat. No. 5,378,580 proposes to rub the plate surface for development using a solvent (IPA) not swelling the ink repellent layer after laser beam irradiation. However, the use of IPA is unpreferable in view of environmental sanitation.
U.S. Pat. No. 5,849,464 proposes to give a liquid with a surface tension of 25 to 50 dyn/cm.sup.3 (250 to 500 .mu.N/cm.sup.2) for rubbing after laser beam irradiation. However, since rubbing is effected with the ink repellent layer swollen, the ink repellent layer is likely to be flawed disadvantageously.
Japanese Patent Laid-Open (Kokai) No. Heill-198335, etc. propose on the removal of the ink repellent layer at the laser beam irradiated area in the presence of, for example, water or water containing a surfactant after laser beam irradiation. However, these methods have such problems that though halftone dots larger than a certain size can be reproduced, very small halftone dots can be reproduced only unstably, and that the ink acceptance of halftone dots is low.
Problems of the conventional waterless planographic printing plates as described above include that it is difficult to reproduce very small halftone dots and that the grooves formed by laser irradiation into which ink is to be accepted, hereinafter called the image ditch cells are deepened.
If the image ditch cells are deepened, the ink is built up to make the prints felt like being coarse. Furthermore, in rotary offset printing, a longer oven must be used, or the printing speed must be lowered to evaporate off the ink solvent. As described above, deep image ditch cells give many disadvantages in the printing process.