Method for making a lithographic printing plate

Abstract: There is provided a method for making a lithographic printing plate from an original containing continuous tones comprising the steps of: screening said original to obtain screened data image-wise exposing a lithographic printing plate precursor according to said screened data, said lithographic printing plate precursor having a flexible support carrying a surface capable of being differentiated in ink accepting and ink repellant areas upon said image-wise exposure and an optional development step and optionally developing a thus obtained image-wise exposed lithographic printing plate precursor, characterized in that said screening is a frequency modulation screening.

Description: 1. Field of the invention. The present invention relates to a method for making a lithographic printing plate, in particular to a method wherein a lithographic printing plate precursor having a flexible support, e.g. a polyester film support is used. 2. Background of the invention. Lithographic printing is the process of printing from specially prepared surfaces, some areas of which are capable of accepting ink (oleophilic areas) whereas other areas will not accept ink (oleophobic areas). The oleophilic areas form the printing areas while the oleophobic areas form the background areas. Two basic types of lithographic printing plates are known. According to a first type, so called wet printing plates, both water or an aqueous dampening liquid and ink are applied to the plate surface that contains hydrophilic and hydrophobic areas. The hydrophilic areas will be soaked with water or the dampening liquid and are thereby rendered oleophobic while the hydrophobic areas will accept the ink. A second type of lithographic printing plates operates without the use of a dampening liquid and are called driographic printing plates. This type of printing plates comprise highly ink repellant areas and oleophilic areas. Generally the highly ink repellant areas are formed by a silicon layer. Lithographic printing plates can be prepared using a photosensitive lithographic printing plate precursor, also called imaging element. Such imaging element is exposed in accordance with the image data and is generally developed thereafter so that a differentiation results in ink accepting properties between the exposed and unexposed areas. Examples of photosensitive lithographic printing plate precursors are for example the silver salt diffusion transfer (hereinafter DTR) materials disclosed in EP-A-410500, EP-A-483415, EP-A-423399, imaging elements having a photosensitive layer containing diazonium salts or a diazo resin as described in e.g. EP-A-450199, imaging elements having a photosensitive layer containing a photopolymerizable composition as described in e.g. EP-A-502562, EP-A-491457, EP-A-503602, EP-A-471483 or DE-A-4102173. Alternatively a lithographic printing plate may be prepared from a heat mode recording material as a lithographic printing plate precursor. Upon application of a heat pattern in accordance with image data and optional development the surface of such heat mode recording material may be differentiated in ink accepting and ink repellant areas. The heat pattern may be caused by a direct heating source such as a thermal head but may also be caused by a light source as e.g. a laser. In the latter case the heat mode recording material will include a substance capable of converting the light into heat. Heat mode recording materials that can be used for making a lithographic printing plate precursor are described in e.g. EP-A-92201633, DE-A-2512038, FR-A-1.473.751, Research Disclosure 19201 of april 1980 or Research Disclosure 33303 of januari 1992. From the above it will be clear that lithographic printing is only capable of reproducing two tone values because the areas will accept ink or not. Thus lithographic printing is a so called binary process. In order to reproduce originals having continuously changing tone values by such process halftone screening techniques are applied. EP-A-0 571 010, which constitutes prior art within the meaning of Art. 54(3) EPC, discloses frequency modulation screening in combination with a digital film recorder, consisting of a scanning laser beam exposing a photosensitive material, which can be a photographic film from which later on a printing plate is prepared by means of photomechanical techniques. In COMPUTER GRAPHICS, vol. 25, no. 4, July 1991, pages 81-90, L. VELHO et al. "Digital Halftoning with Space Filling Curves" a new digital halftoning technique is disclosed, that uses space filling curves to generate aperiodic patterns of clustered dots, to reproduce continuous-tone picture on a specific graphic display device. The work presented in the paper addresses problems associated with laser printers, based on electrophotographic technology. The technique is primarily intended for medium to high resolution bi-level devices that cannot accommodate isolated black or white pixels. A 300 dpi laser printer is used as the graphics output device. In a commonly used halftone screening technique, the continuously changing tone values of the original are modulated with periodically changing tone values of a superimposed two-dimensional screen. The modulated tone values are then subject to a thresholding process wherein tone values above the treshold value will be reproduced and those below will not be reproduced. The process of tone-value modulation and thresholding results in a two-dimensional arrangement of equally spaced "screen dots" whose dimensions are proportional to the tone value of the original at that particular location. The number of screen dots per unit distance determines the screen frequency or screen ruling. This screening technique wherein the screen frequency is constant and inversely proportional to the halftone cell size and, hence, to the maximum density of the screen dot, is referred to as amplitude-modulation screening or autotypical screening. This technique can be implemented photo-mechanically or electronically. The photo-mechanical implementation involves an analog process wherein a screen of equally spaced dots is physically superimposed, in contact or in projection with the original. Screen dots are formed when this combination is photographically reproduced in a system wherein thresholding is achieved through the use of special photographic films and developing chemicals producing a very high photographic contrast resulting a sharp distinction between tone values above and below a certain level. The electronic implementation of autotypical screening is a digital process whereby the continuous tone values of the original are broken up into discrete tone-value levels, specified at discrete areal coordinates within the original image. Each tone value is compared with an electronic threshold level, and values above the threshold are reproduced while those below the threshold are not. Screen dots are formed when a specific pattern of threshold values is defined in a two-dimensional array corresponding to the size of a halftone cell, and this threshold pattern is periodically applied accross the image. It will further be clear that in order to reproduce a color image using lithographic printing it will be required to separated the image in three or more part-images corresponding to primary colors that when printed over each other yield the desired color at any place within the image. Each of these color separation has to be screened as described above. Supports that are commonly used for lithographic printing plates are metal supports such as e.g. aluminium and flexible supports such as e.g. paper or organic resin supports such as e.g. polyester. Metal supports are generally used for high quality printing and print jobs that require a large number of copies, typically around 100000. Lithographic printing plates having a flexible support are generally used for print jobs requiring a medium print quality and only a limited number of copies typically around 10000. An important problem that occurs with lithographic printing plates having a flexible support is that during startup of the printing process the plate tends to enlarge somewhat until an equilibrium state is reached. As a consequence the first copies will not be of acceptable quality and have to be disposed of. This problem is particular apparent when images having continuous tones in particular color images have to be reproduced. Further in the case of printing color images register faults may occur during the printing process. Such faults may limit the printing endurance of the plate and it can be understood that the risk that such a fault results in an unacceptable quality of the copy will increase when the screen ruling used for screening the color image increases. 3. Summary of invention. It is an object of the present invention to provide an improved method for making a lithographic printing plate having a flexible support. Further objects of the present invention will become clear from the description hereinafter. According to the present invention there is provided a method for making a lithographic printing plate from an original containing continuous tones comprising the steps of: screening said original to obtain screened data image-wise exposing a lithographic printing plate precursor according to said screened data, said lithographic printing plate precursor having a flexible support carrying a surface capable of being differentiated in ink accepting and ink repellant areas upon said image-wise exposure and an optional development step and optionally developing a thus obtained image-wise exposed lithographic printing plate precursor, characterized in that said screening is a frequency modulation screening. 4. Brief description of the drawings. The present invention is illustrated by way of example and without the intention to limit the invention thereto with the following drawings: Figure 1 shows a Hilbert curve before (a) and after randomization (b). Figure 2 shows the order of processing image pixels when the image is recursively subdivided into matrices. Figure 3 shows a schematic representation of a circuit for implementing a halftoning method according to the invention. 5. Detailed description of the invention. Frequency modulation screening is a technique in which the continuously changing tone values of an original are reproduced by means of equally sized micro dots, the number of which is proportional to the tone value of the original image. The name frequency modulation refers to the fact that the number of micro dots per unit surface (the frequency) fluctuates in proportion to the tone value in that same area. It has been found that the number of copies that have to be disposed of at the start of the printing process can be reduced when using a frequency modulation screening for screening continuous tones in an image to be reproduced. It was further noted that a printing process using 3 or more printing plates corresponding to the color separation of a color image to be reproduced was less susceptible to register faults i.e. even if such an error occured it did not immediately lead to unacceptable quality of the following copies. The copies obtained were of high quality and comparable with qualities obtained when a high screen ruling was used in a method for imaging the printing plate precursors according to the prior art. A suitable frequency modulation screening technique for use in connection with the present invention is the well-known Error diffusion first described by Floyd and Steinberg "An adaptive algorithm for spatial grey scale" SID 75 Digest. Society for information display 1975, pp. 36-37. According to the error diffusion technique the image pixels of a continuous tone image are processed one after the other according to a predetermined path e.g. from left to right and top to bottom. The tone value of each image pixel is thereby compared with a threshold value which is generally the tone value half-way the tone scale e.g. 128 when the tones of the image-pixels range from 0 to 256. Depending on whether the tone value of the image pixel is above or below the threshold value a halftone dot will be set or not in the corresponding reproduction of the image pixel. The resulting error or weighted error, i.e. the difference between the reproduction value and actual value of the image pixel, is then added to the tone value of one or more neighbouring image pixels that are still unprocessed. Details about the error diffusion screening method may be found in the aforementioned reference or in US-P-5.175.804. A more preferred variant of frequency modulation screening for use in connection with the present invention is a method similar to the error diffusion with the exception that the order in which the image pixels are processed can be described by a space filling deterministic fractal curve or a randomized space filling curve. This type of frequency modulation screening comprises the following steps: selecting an unprocessed image pixel according to a space filling deterministic fractal curve or a randomized space filling curve and processing said unprocessed image pixel as follows: determining from the tone value of said unprocessed image pixel a reproduction value to be used for recording said image pixel on a recording medium e.g. a photographic film or lithographic printing plate precursor, calculating an error value on the basis of the difference between said tone value of said unprocessed image pixel and said reproduction value, said unprocessed image pixel thereby becoming a processed image pixel, adding said error value to the tone value of an unprocessed image pixel and replacing said tone value with the resulting sum or alternatively distributing said error value over two or more unprocessed image pixels by replacing the tone value of each of said unprocessed image pixels to which said error value will be distributed by the sum of the tone value of the unprocessed image pixel and part of said error, repeating the above steps until all image pixels are processed. A suitable deterministic fractal curve is for example the so called "Hilbert Curve" disclosed by Witten Ian H., and Radford M. Neal, "Using Peano Curves for Bilevel Display of Continuous-Tone Images", IEEE CG&A, May 1982, pp. 47-52. According to the most preferred embodiment of the present invention the order of processing the image pixels is ruled by a randomized space filling curve. With the term "randomized space filling curve" is meant that the processing of the image pixels follows basically a pre-determined curve that assures that each image pixel will be processed but which curve is randomized at a number of points so that patterns are avoided. Such randomized space filling curve can be obtained in different ways. For example the Hilbert Curve may be used as the pre-determined curve on which randomization is performed. A computer program that can be used to obtain a randomized Hilbert Curve is shown in annex 1. Figure 1 gives a visualization of a Hilbert Curve before and after randomization. The randomization of the Hilbert Curve may be carried out by following the curve and at every point of the curve deciding at random whether or not the curve will be permutated at the particular point. According to an alternative a randomized space filling curve may be obtained by dividing the image into matrices of image pixels. Within each of these matrices the image pixels are processed at random untill all image pixels are processed. The order in which the matrices are processed may then be selected at random or in a predetermined way. An alternative to the above method of dividing the image into matrices is the recursively division of the image into smaller matrices untill the size of a matrix reaches an image pixel. At every subdivision into smaller submatrices a random ordering of processing the matrices is assigned to every submatrix. Annex 2 shows a computer program that can be used to carry out this process and figure 2 shows the resulting order in which image pixels are processed in this case. It will be clear that this method works well for square image but imposes problems for other images. To overcome this problem the original image may be padded with zeors along its longest side untill a square is obtained. In a second approach, a path may be calculated with the size of the longest rectangular image. The points of the path that do not belong to the image may then be skipped during processing. Figure 3 shows a circuit to perform a frequency modulation screening in combination with a binary recording device, e.g. an image-setter. First the different building blocks of this circuit are described, later on its operation will be explained. Block (20) is a memory block containing the contone pixel values of an image. Typically these are 8 bit values, organized as N lines with M columns. Block (30) is a memory block with the same lay out as block (20), in which the the halftoned pixel values will be stored. In the case of a binary recording device, every halftoned pixel word has a length of 1 bit. Block (80) is a device capable of image-wise exposing a substrate e.g. a photographic film or a lithographic printing plate precursor using the information in block (30) on a substrate e.g. a photographic film or a lithographic printing plate precursor. Block (70) is an arithmetic unit capable of calculating the sum of the pixelvalue P(i,j) and the error E at the output of a delay register (60). The conversion of a contone pixel value into a halftoned pixel value takes place in block (40). This conversion may be based on a thresholding operation: if the contone value at point (i,j) is below the value of 128, a value "0" is stored in the halftone memory, otherwise a "1" is stored. Block (50) contains an arithmetic unit that is capable to calculate the error between the original contone value, and the halftoned pixel value, and to store it in the delay register (60). Block (8) is a counter that sequences the processing of the N*M pixels of the image. Block (10) is LUT with N*M entries (one for every image pixel), and a UNIQUE combination of a row and column address that corresponds with one pixel position in the image. Block (5) is a clock. The table of block (10) thus holds the order in which the image pixels will be processed. This table may be calculated according to one of the methods described above. The operation of the diagram is now explained. At every clock pulse, the counter (8) is incremented, and a new pair of coordinates (i(n),j(n)) is obtained from block (10). These coordinates are used as address values to the pixel memory (20), to obtain a contone pixel value P(i(n),j(n)). This pixel value is immediately added to the error E(i(n-1),j(n-1)), that was stored in register (60) after the previous halftone step, and the sum of both is compared to the threshold value (41) in block (40). The outcome of the thresholding operation determines the value H(i(n),j(n)) that will be written into the halftone pixel memory at position (i(n),j(n)). At the same time a new error E(i(n),j(n)) is calculated from the difference between P(i(n),j(n)) and H(i(n),j(n)), and stored in the delay register (60). The circuit is initialized by setting the counter (8) to 1, the error to 128, and the operation is terminated when the counter reaches the level N*M. After that, the halftone memory (30) is read out line by line, column by column, and its contents are recorded on a substrate by the recorder (80). According to a variant of the above circuit the error that is obtained from the difference between the contone pixel and the halftoned pixel value, may, instead of being diffused only to the next pixel in the order of processing, diffused to more than one of the unprocessed pixels. Instead of using the error of one pixel, one may also use an average error of a number of pixels. In case of a color image, the above described screening process is performed on each of the color separations of the image. Preferably the color image is separated in its Yellow, Magenta, Cyan and Black components. Each of these components may then be screened according to the present invention and used to image-wise expose four lithographic printing plate precursors. Four lithographic printing plates, one for each color separation, will thus be obtained. The color separations can then be printed over each other in register in a lithographic printing machine using the four plates. Image-wise exposure in accordance with the present invention may proceed by a scan-wise exposure by means of e.g. a laser or LED directly on the printing plate precursor (so called computer to plate) or it may be performed by first exposing an intermediate photographic film of high contrast, generally a high contrast silver halide film, and then using the imaged photographic film as a mask for exposing a lithographic printing plate precursor to a conventional light source in a camera exposure or contact exposure. Suitable devices for scan-wise exposure of a lithographic printing plate precursor are e.g. Cathode Ray Tubes, LED's or lasers. Most preferably used devices are lasers, the particular type of laser and power being dependent on the type of printing plate precursor. Generally a lithographic printing plate precursor based on a silver halide photosensitive layer will require less powerful lasers while heat mode recording materials will generally require powerful lasers. Examples of lasers that can be used in connection with the present invention are e.g. He/Ne lasers, Argon ion lasers, semiconductor lasers, YAG lasers e.g. Nd-YAG lasers etc.. Flexible supports suitable for use in connection with the present invention are e.g. paper, paper coated on one or both sides with an organic resin such as e.g. a polyethylene resin, organic resin supports such as e.g. polyester, polycarbonate, polyvinyl chloride, polystyrene, cellulose esters such as cellulose triacetate etc.. The method of the present invention can be used with lithographic printing plate precursors having a surface that can be differentiated upon image-wise exposure and an optional development step. Examples of printing plate precursors that can be used in connection with the present invention are printing plate precursors having a photosensitive layer or a heat mode recording layer. A particular suitable printing plate precursor or imaging element is a so called mono-sheet DTR material. The mono-sheet DTR material comprises on a flexible support in the order given a silver halide emulsion layer and an image receiving layer containing physical development nuclei e.g. a heavy metal sulphide as e.g. PdS. The image receiving layer is preferably free of binder or contains a hydrophilic binder in amount of not more than 30% by weight. Subsequent to image-wise exposure the mono-sheet DTR material is developed using an alkaline processing liquid in the presence of developing agents e.g. of the hydroquinone type and/or pyrazolidone type and a silver halide solvent such as e.g. a thiocyanate. Subsequently the plate surface is neutralized with a neutralizing liquid. Details about the consitution of this type of mono-sheet DTR material and suitable processing liquids can be found in e.g. EP-A-474922, EP-A-423399, US-P-4.501.811 and US-P-4.784.933. Lithographic printing plate precursors of this type are marketed by Agfa-Gevaert NV under the name of SUPERMASTER and SETPRINT. These type of printing plate precursors can be exposed in a camera or a laser or LED containing device. Examples of HeNe laser containing exposure units are the image-setters LINOTRONIC 300, marketed by LINOTYPE-HELL Co, and Selectset 5000/7000, marketed by Miles Inc.. An image-setter provided with an Ar ion laser that can be used is LS 210, marketed by Dr-Ing RUDOLF HELL GmbH. Exposure units provided with a laserdiode that can be used are LINOTRONIC 200, marketed by LINOTYPE-HELL Co, and ACCUSET, marketed by Miles Inc.. An other type of imaging element suitable for use in connection with the present invention is one comprising on a flexible support having a hydrophilic surface or being coated with a hydrophilic layer a photosensitive layer containing a diazo resin, diazonium salt or a photopolymerizable composition. Printing plate precursors having such a photosensitive layer are disclosed in EP-A-450199, EP-A-502562, EP-A-487343, EP-A-491457, EP-A-503602, EP-A-471483, DE-A-4102173, Japanese patent application laid open to public inspection number 244050/90 etc.. Subsequent to the exposure these printing plate precursors are developed using plain water, a developing liquid being generally a mixture of water and one or more organic solvents or some of them may be developed using a delamination foil. An imaging element suitable for use in connection with the present invention and that can be used to yield driographic printing plates is disclosed in e.g. EP-A-475384, EP-A-482653, EP-A-484917 etc.. It is also possible to use imaging elements having a heat mode recording layer. Such heat mode recording layer is a layer containing a substance that is capable of converting light into heat. Examples of heat mode recording layers are e.g. vacuum or vapour deposited Bismuth or Aluminium layers, layers containing infra-red dyes or pigments, layers containing carbon black etc.. Suitable heat mode recording materials for use in connection with the present invention are described in e.g. DE-A-2512038, Research Disclosure 19201 of april 1980, Research Disclosure 33303 of januari 1992, EP-A-92201633 or FR-A-1.473.751. The latter two heat mode recording materials do not require a developing step or can be developed by simply cleaning the heat mode recording material with e.g. a dry cotton pad. A particular interesting heat mode recording material that can be used in connection with the present invention comprises on a flexible support a heat mode recording layer and hardened hydrophilic surface layer having a thickness of not more than 3µm. After image-wise exposure, preferably through said hardened hydrophilic surface layer, in accordance with the present invention the plate may be used directly on a printing press using a dampening liquid or the plate surface may first be cleaned with a dry cotton pad. Printing plates of high quality are obtained. A further alternative heat mode recording material that can be used in connection with the present invention comprises on a flexible support, preferably polyester, a heat mode recording layer and a highly ink repellant layer as e.g. a silicon layer. Such heat mode recording layer is preferably exposed through the support and afterwards it may be developed by rubbing the plate surface. The present is further illustrated by the following example without however the intention to limit the invention thereto. EXAMPLE A commercially available silver salt diffusion transfer lithographic printing plate (Supermaster OP-LL, available from Agfa-Gevaert NV) was image-wise exposed with screened cyan, magenta, yellow and black separations of a color image and subsequently developed using the processing liquids G260 (alkaline liquid containing silver halide solvent) and G360 (neutralization liquid) each available from Agfa-Gevaert NV. Two sets of 4 such printing plates were prepared. The first set (comparitive) was obtained by using a autotypical screen of 120l/cm for each of the color separations. The second set was obtained by screening each of the color separations according to a frequency modulation screening wherein the image pixels were processed in a random order by recursively subdividing the image of the color separation into matrices until the size of a matrix matched the size of an image pixel. The error for each image pixel was added to the tone value of the next image pixel processed. Each set of 4 plates was used on a printing machine using Hartmann Irocart inks. The dampening liquid used was an aqueous solution containing 5% by weight of a fountain concentrate G671c commercially available from Agfa-Gevaert NV and 15% by weight of isopropanol. It was found that with the comparitive set (autotypical screening) 450 copies had to be printed before a stable color reproduction was obtained, whereas in case of frequency modulation screening only 250 copies had to be printed before a stable color reproduction was obtained.

Claims: A method for making a lithographic printing plate from an original containing continuous tones comprising the steps of: screening said original to obtain screened data image-wise exposing a lithographic printing plate precursor according to said screened data, said lithographic printing plate precursor having a flexible support carrying a surface capable of being differentiated in ink accepting and ink repellant areas upon said image-wise exposure and an optional development step and optionally developing a thus obtained image-wise exposed lithographic printing plate precursor, characterized in that said screening is a frequency modulation screening. A method according to claim 1 wherein said frequency modulation screening proceeds according to the following steps: selecting an unprocessed image pixel according to a space filling deterministic fractal curve or a randomized space filling curve and processing said unprocessed image pixel as follows: determining from the tone value of said unprocessed image pixel a reproduction value to be used for recording said image pixel on a recording medium, calculating an error value on the basis of the difference between said tone value of said unprocessed image pixel and said reproduction value, said unprocessed image pixel thereby becoming a processed image pixel, adding said error value to the tone value of an unprocessed image pixel and replacing said tone value with the resulting sum or alternatively distributing said error value over two or more unprocessed image pixels by replacing the tone value of each of said unprocessed image pixels to which said error value will be distributed by the sum of the tone value of the unprocessed image pixel and part of said error, repeating the above steps untill all image pixels are processed. A method according to claim 2 wherein said original having continuous tones is subdivided in matrices of unprocessed image pixels and all of said image pixels within a matrix is processed before a subsequent matrix is processed. A method according to any of the above claims wherein said lithographic printing plate precursor contains a photosensitive layer. A method according to claim 1, 2 or 3, wherein said lithographic printing plate precursor contains a heat mode recording layer containing a substance capable of converting light into heat. A method according to any of the above claims wherein lithographic printing plate precursor contains a silver halide emulsion layer and an image receiving layer containing physical development nuclei and wherein subsequent to said image-wise exposure said lithographic printing plate is developed using an alkaline processing liquid in the presence of developing agent(s) and silver halide solvent(s).

Inventors: DELABASTITA PAUL, VAN HUNSEL JOHAN, DELABASTITA, PAUL, VAN HUNSEL, JOHAN
Applicants: AGFA GEVAERT NV, AGFA-GEVAERT NAAMLOZE VENNOOTSCHAP
Application year: 1993
Main classification: H04N1
IPC codes: G03F7, G03G13, H04N1