GB2130745A - Plate making process for light- sensitive planographic plates - Google Patents

Abstract

The plate making process utilizes an original having an image on a semitransparent base. Imagewise exposure is carried out through the original which is placed on a light- sensitive layer of a light-sensitive planographic plate so as to bring the image in close contact with said light- sensitive layer. The printing plate produced provides excellent image quality and high printing endurance and can be obtained rapidly at a relatively low cost by avoiding the use of a plate making camera.

Description

SPECIFICATION Plate making process for light-sensitive planographic plates FIELD OF THE INVENTION The present invention relates to a process which comprises directly printing on a light-sensitive planographic plate without producing a lithographic film from an original film and producing a printing plate by automatic development, and particularly to a plate making process for rapidly producing a printing plate having a high quality at a low cost.

BACKGROUND OF THE INVENTION A conventional plate making process for light-sensitive planographic plates involves directly exposing to a light-sensitive planographic plate with a reflection or transmission stripped original film obtained from an original by means of a plate making camera or a laser plate making machine, etc. and producing a printing plate by development. However, when a plate making camera is used, there are many problems. For example, there is an increase in the number of steps caused by addition of an exposing step by the plate making camera, deterioration of image quality caused by the optical system of the plate making camera, increase in the number of steps caused by erasing operation of traces of composing on the stripped original film, and scaling up of equipment.Further, when the laser plate making machine is used, the equipment is expensive and large-sized and the number of steps is somewhat increased. Furthermore, the above described direct plate making process requires a lightsensitive planographic plate having a sufficiently high sensitivity.

SUMMARY OF THE INVENTION As a result of many studies, the present inventors have found that the above described disadvantages may be overcome by a plate making process which involves using an original having an image on a semitransparent base. The original is placed on a light-sensitive face of light-sensitive planographic plate (hereinafter, refer to as PS plate) so as to bring the image of said original in close contact with the light-sensitive face, imagewise exposing it, and thereafter carrying out conventional processing for the PS plate.

DETAILED DESCRIPTION OF THE INVENTION In the following, the present invention is illustrated in detail.

The present invention is characterised by a combination of the particular PS plate used, a method of making an original film or stripped original film used for imagewise exposure of the PS plate, and a method of exposing. Firstly, in the original film or stripped original film used, letters or images are provided on a semitransparent base. The term "semitransparent base" means a plastic film or paper in which the light intensity of the irradiating light is reduced by light scattering. An original in which an image is formed on a base such as a semitransparent film or paper, for example, an original film obtained by using thin phototype setting paper, may be used, or an original, obtained by providing an original film which has an image on a transparent film, on a semitransparent film or paper base may be used.The semitransparent base may have a light transmissivity of 95% to 5%, preferably 90% to 10%, though it depends on the sensitivity of the PS plate used. When the semitransparent base is used, traces of composing and scratches on the stripped original film do not appear. In case of a PS plate used in the present invention, particularly, a PS plate having a nonsilver light-sensitive layer and a silver halide light-sensitive layer which are provided on a base having a hydrophilic surface in this order, tone reproducibility of Gothic type letters or halftone dots is rather unexpectedly improved.

Secondly, a conventional PS plate may be used. It is preferable to use a positive type PS plate, particularly a PS plate having a sensitivity about 10 times higher than that of a conventional diazo lightsensitive layer. A PS plate having such a high sensitivity can be produced according to the technique described in, for example, British Patent 2,080,964, Research Disclosure No.13505(1975), No.14614 (1976) or No. 18183 or "Kobunshi" (1970) Vol. 19, No. 215, 102 to 109. Further, it is possible to use a printing plate described in "lnsatsu Zasshi" Vol.61 No. 6 (1978). The most suitable PS plate in the present invention is that which is produced by the process described in U.S. Patent 4,268,609 or 4,299,912 (incorporated herein by reference to disclose such a PS plate).This kind of PS plate is remarkably excellent due to its high sensitivity, excellent image reproducibility, high printing durability and easy printing operation, and high reproducibility of quality. This PS plate is a planographic plate having a layer construction wherein a nonsilver light-sensitive layer capable of forming an oleophilic image, for example, a layer composed of a composition containing an o-quinone diazide compound or a high molecular compound having the group

in the main chain or side chains of the polymer, and a high-speed light-sensitive layer containing silver halide light-sensitive particles are provided on a base having a hydrophilic surface, for example, an aluminium base having an anodized layer, in this order.

Thirdly, the method of exposing comprises placing the image provided on the semitransparent base so as to contact it closely or bring it adjacent as closely as possible with the light-sensitive layer of the PS plate while exposing it from the side of the semitransparent base. In this case, it is possible to automatically expose to light by a very simple apparatus without requiring the use of a plate making camera. Further, there is no deterioration of image quality caused by the optical system of the plate making camera.

Fourthly, production of a stripped original film used conventionally is not required. Namely, an original film obtained by using phototype setting paper of electronic computing phototype setting paper, drawings or letters drawn on tracing paper with an ink, an image typewritten on thin semitransparent paper by a word processer, or a printing paper image obtained by photographing the image formed on CRT (cathode-ray tube), etc. can be used as a stripped original film by laying out and arranging on the semitransparent base. More preferably, a partially transparent film-original can be arranged on the semitransparent base, too. Accordingly, the cost of materials such as lithographic film, etc. can be economized. Other advantages will become apparent to those skilled in the art from the examples.

Using a composite type PS plate having a nonsilver light-sensitive layer and a silver halide lightsensitive layer (for example, described in U.S. Patent, 4,268,609, etc.) which is a particularly preferred embodiment of the present invention, plate making can be automatically carried out by the process described in, for example, U.S. Patent 4,299,912 or 4,141 733 (incorporated herein by reference to disclose such a plate making method) by means of an automatic development apparatus. Namely, after imagewise exposure, silver halide is developed (first development) and treated at once with a fixing solution. It is then exposed to active rays containing ultraviolet rays and, thereafter, sent to the second development.In this stage, the nonimage part is dissolved and the silver halide containing high-speed light-sensitive layer is removed, and the silver used is nearly completely recovered. Thus a printing plate having an oleophilic image on the hydrophilic surface of the base is obtained. Usually, in the second development, a conventional developing solution of the PS plate, for example, an aqueous solution of alkali silicate described in U.S. Patent 4,259,434 (incorporated herein by reference to disclose such a solution) in case that the nonsilver light-sensitive layer is composed of an o-quinonediazide compound, is used. In the first development, an aqueous solution described in British Patent 2,069,1 64A is used.

The aqueous solution had a composition of 700 g of water, 140 g of sodium sulfite, 40 g of hydroquinone, 40 g of potassium carbonate, 7 g of sodium hydroxide, 55 g of diethylene glycol, 15 g of potassium bromide, 2.5 g of 4-methyl-1 -phenyl-3-pyrazolidone and water to make total amount 11 and was used after being diluted with water in a ratio of 1/1, i.e., 1 part the developer composition to 1 part water by volume.

EXAMPLE 1 A 2S aluminum plate which had been mechanically grained by the method described in Japanese Patent Application (OPI) No. 33911/73 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") was dipped in a 2% aqueous solution of sodium hydroxide maintained at 400C for 1 minute to etch part of the surface thereof. After washing with water, the plate was dipped in a sulfuric acid-choromic acid mixed solution for about 1 minute to expose the surface of pure aluminum. The plate was then placed in 20% sulfuric acid maintained at 300C and was subjected to anodic oxidation at direct current voltage of 1.5 V and a current density of 3 Alum2 for 2 minutes. The thus-treated plate was washed with water and dried.

A light-sensitive solution having the composition hereinafter described was then coated on the above-treated aluminum plate in a dry amount of about 2 g/cm2 using a roll coater to provide a nonsilver light-sensitive layer.

Composition of Non-Silver Light-Sensitive Solution Amount (g) Na phthoquinone- ,2-diazido(2)-5-sulfonic acid ester of acetone-pyrogallol resin (prepared by the method described in Example 1 or U.S.

Patent 3,635,709) 2.5 Cresol-formaldehyde resin 5.0 Methyl ethyl ketone 75 Cyclohexanone 60 A light-sensitive solution having the composition hereinafter described was then coated thereon in a dry amount of about 5.2 g/m2 and dried with hot air having a final temperature of 11 00C.

Thus PS plate was prepared.

Composition of Direct-Positive Light-Sensitive Silver Halide Emulsion Solution Emulsion prepared by dissolving 20 g of a copolymer of benzyl methacrylate and methyacrylic acid (benzyl methacrylate: 70% by weight; methacrylic acid: 30% by weight; molecular weight: about 10,000) in 277 g of ethyl acetate and dispersing the resulting solution in 556 mi of a 10% aqueous gelatin solution by using as a dispersant 10 g of sodium nonylbenzene-sulfonate 1,470 g Direct positive light-sensitive silver halide raw emulsion prepared as described hereinafter 1,000 g Solution (1/250 mol) of pinakryptol yellow in a 1::1 w/w solution of water and methanol 144 ml Aqueous solution (1%) of glyoxal 8 ml Preparation of Direct Positive Light-Sensitive Silver Halide Raw Emulsion To a first solution (prepared by dissolving 10 g of inert gelatin and 5 ml of a 1 N solution of sodium chloride in 500 ml by heating at 600 C) were gradually added a second solution (prepared by dissolving 100 g of silver nitrate in 500 ml of water by heating at 600 C) and a third solution (prepared by dissolving 23 g of sodium chloride and 23 of potassium bromide in 150 ml of water, further adding 50 mg of potassium hexachloroirridinate (VI), K2lrCl6, thereto, and maintaining the resulting mixture at 600C) with stirring over a period of 20 minutes.Thereafter, 1 5 ml of a 0.2 N potassium iodide solution was added and, after the temperature was lowered to 1 70C, the resulting mass was washed with water.

The mass was melted by increasing the temperature to 600C and the PAg was adjusted to 4.0 using AgNO3 and KBr, and 10-5 mol per mol of AgX of gold chloride were then added. After adjustment of the pH to 10, the mass was aged for 10 minutes and then the pH was adjusted to 6.5 using citric acid. After the temperature was lowered to 1 70C, the mass was washed with water. Then, a mixed liquid of molten sodium chloride and molten potassium bromide was added to adjust the PAg to 7.0. Then, a fourth solution (prepared by dissolving 75 g of inert gelatin in 300 ml of water) was added to obtain a silver halide emulsion. The mean grain diameter of the above-obtained silver halide emulsion was 0.15 micron, and substantially all grains ware normal tetragonal system grains having a (100) plane.

To a semitransparent plastic film (having a light transmissivity of about 74%), a letter original obtained by using available phototype setting paper (the light transmissivity of the nonimage part was about 34%) and an original of an image of halftone dots having 133 lines/inch were applied to obtain a stripped original film.

On the light-sensitive layer of the PS plate, the stripped original film was placed closely so as to bring the face having the letters and images (reverse images with the left side set over the right) in contact with the light-sensitive layer, and it was exposed to light from the back of the original at 30 luxes for 10 seconds by means of an exposing apparatus (using a 100 v tungsten lamp as a light source). Thereafter, it was subjected to development using a specially produced automatic developing apparatus. That is, each plate was then passed through Developer (I) having the composition as described hereinafter at 320C for 20 seconds, and Fixer (I) having the composition as described hereinafter at 200C for 10 seconds.Subsequently, each plate was passed through an ultraviolet exposure zone, where three reflector type mercury lamps were placed, over a period of 1 5 seconds, and then through a washing zone where each plate was rubbed with a brush in water maintained at 40 to 450 C. Each plate was then introduced through a squeeze roller into Developer (II) (having the composition as described hereinafter) at 300C for 30 seconds to apply a 140 Bé aqueous gum arabic solution, whereby a lithographic printing plate No. 1 was obtained.

Composition of Developer (I) Water 700 ml Metol 3.0 g Sodium sulfite 45.0 g Hydroquinone 12.0g Sodium carbonate (monohydrate) 80.0 g Potassium bromide 2 g Water to make 1,000 ml This was used after being diluted with water in a ratio of 1/2, i.e., 1 part developer composition (I) to 2 parts water by volume.

Composition of Developer (II) JIS No. 1 sodium silicate 100 g Sodium metasilicate 50 g Pure water 1,800 ml Composition of Fixer (I) Water 700 ml Ammonium thiosulfate 224 g Sodium sulfite 20 g Water to make 1,000 ml On the other hand, the same letter-original and reflection original of an image of halftone dots having 133 lines/inch were applied to a white base paper to produce a stripped film original film. Using this original, it was exposed to light for 30 seconds at an iris of F: 11 by means of a plate making camera (Artgraphica type 601, produced by Dainippon Screen Co.) at the same magnification so as to have the same exposure. It was then subjected to the same treatment as described above to produce a printing plate A.

Printing was carried out using a printer: Heidel KOR to obtain prints. In the prints obtained by the printing plate No. 1, reproduction of the letters (Gothic type, Chinese characters) and that of the halftone dot image were remarkably excellent as compared with those in the printing plate A.

Printing plate used No. 1 A (comparison) Reproduction of Gothic Excellent Slight "veiling" *1 type letters (grade 5) Reproduction of Chinese Excellent Slight "break" *2 characters (grade 5) Edge sharpness Good Slight "roughness" t3 Reproduction of half-tone 2-98% 8-90% dot image (133 lines/inch) *1 "Veiling was observed in, for example, the letter " B ". "Veiling means that a letter is not identified because fat lines and thin lines of the letter are not distinguished.

*2 "Break was observed in, for example, the letter 2 "# ", the letter 3 " " showing "ffi" due to an omission of fine lines.

*3 "Roughness" means that edge lines are not straight, but show zigzag lines.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

Claims (7)

1. A plate making process which comprises a step wherein exposure is carried out through an original having an image on a semitransparent base, the original being placed on a light-sensitive layer of a light-sensitive planographic plate so as to bring the image in close contact with said light-sensitive layer.

2. A plate making process as claimed in Claim 1, wherein the light-sensitive layer of said lightsensitive planographic plate has a sensitivity of about 10 times greater than that of a light-sensitive planographic plate prepared utilizing diazo resin.

3. A plate making process as claimed in claim 1, wherein the semitransparent base has a light transmittivity in the range of 95% to 5%.

4. A plate making process using a light-sensitive planographic plate having a base with a hydrophilic surface on which is provided a non-silver light-sensitive layer capable of forming an oleophilic image and on the non-silver layer, a high speed light-sensitive layer containing silver halide light-sensitive particles, which comprises placing an original having an image on a semitransparent paper base or film base on the high speed light-sensitive layer surface, imagewise exposing through the original which is in close contact with the high speed light-sensitive layer, and subsequently carrying out development and fixation of the high-speed light-sensitive layer and exposure and development of the non-silver light-sensitive layer.

5. A plate making process as claimed in claim 1 substantially as described in the Example herein.

6. A planographic printing plate produced by a plate making process as claimed in any one of claims 1 to 5.

7. A print produced from a printing plate as claimed in claim 6.