There are several printing methods, and planographic printing method is predominated at present. The planographic printing method is a method in which, employing a planographic printing plate with ink receptive image portions and ink repulsive non-image portions present at substantially the same plane, printing is carried out by supplying ink only to the image portions, and transfer the ink to material to be printed such as paper. That is, planographic printing method employs the difference of ink receptivity between image portions and non-image portions. In order to prepare such a planographic printing plate, a presensitized planographic printing plate (hereinafter referred to also as PS plate) is used. As described in T. Yonezawa, "PS Ban Gairon", pp. 18-81, Insatsu Gakkai Shuppanbu (1993), the PS plate herein referred to implies the following:
The PS plate implies a presensitized planographic printing plate comprising a hydrophilic aluminum support and provided thereon, a lipophilic light sensitive layer. The planographic printing plate is prepared by remaining the light sensitive layer at image portions and removing the light sensitive layer at non-image portions to expose the hydrophilic aluminum surface capable of forming a dampening water layer and repelling ink, according to lithography. There are a conventional PS plate, from which a planographic printing plate having a dampening water layer as an ink repelling layer is prepared, and a waterless PS plate, from which a waterless planographic printing plate having a silicone rubber layer as an ink repelling layer is prepared.
In the conventional PS plate, a grained aluminum plate is ordinarily employed as a support. The surface of the aluminum plate is required to have a water holding property and an excellent adhesion to a lipophilic light sensitive layer, such that the light sensitive layer is not removed from the surface during printing. Therefore, the surface of the aluminum plate is ordinarily grained, and optionally subjected to surface treatment such as anodizing treatment to improve both water holding property and adhesion to a lipophilic light sensitive layer. The conventional PS plate is widely employed.
The conventional PS plate provides a practical excellent printing plate. However, with conventional printing plates, which employ a grained aluminum plate as the support, it is difficult to control the balance between ink and a dampening water during printing, which is likely to cause printing failure such as stain occurrence and ink emulsification according to printing conditions. Further, such printing plates occasionally result in lowering of small dot reproduction due to halation, which is caused by the surface-roughened aluminum plate.
There are two plate making processes in the printing field, one is a direct plate making process of preparing a direct printing plate by directly forming an image necessary to print on a plate, and the other an indirect plate making process of preparing an indirect printing plate by forming an image on an intermediate medium such as a drum, and then transferring the formed image to a plate.
As desk top publishing (DTP) systems progress or computer to plate (CPT) plate making process develops, the direct or indirect plate making process has been increasingly used. As one embodiment of the direct plate making processes, there is a method comprising the steps of imagewise transferring a lipophilic transfer layer to the roughened surface of a support to prepare a printing plate, however, the roughened surface occasionally causes problems in that the transferred small dots have fringes around the periphery, which result in print images of poor quality.
As one embodiment of the indirect plate making processes, there is a method of preparing an off-set master for a printing plate. The off-set master has an image receiving layer comprised of zinc oxide and a binder on a paper support or a plastic film support. Toner images are formed on the image receiving layer employing a copy machine or a laser printer to prepare an off-set printing plate. The off-set printing plate has lipophilic, toner image portions and non-image portions without toner images, which are made to be hydrophilic by an etching solution containing, for example, potassium cyanide.
The off-set master requires etching before printing, whereby non-image portions of the master are made to be hydrophilic. However, the etching treatment still has a problem in that it is insufficient to make the non-image portions hydrophilic, and the master is likely to produce stains at the non-image portions.
As is described above, a support for a planographic printing plate having satisfactory properties has not yet been obtained.