Lithographic printing plate and method for producing the same

Abstract

 An improved lithographic printing plate includes a substrate layer (2) of a nonconducting, hydrophobic material, an intermediate film (3) of conducting, hydrophilic material such as aluminum and a top protective film (5) of relatively hard hydrophilic dielectric material, such as aluminum oxide. A printing image is formed in the lithographic printing plate by an electroerosion process wherein erosion electrodes (7) are pulsed with voltage to break down the dielectric film (5) in areas adjacent to the erosion electrodes (7) and to evaporate or otherwise remove corresponding portions of the conducting film (3), thereby creating holes (8) that extend through the dielectric (5) and conducting (3) films and that expose portions of the surface of the underlying hydrophobic substrate (2).

Description

Technical Field

[0001] The invention relates to lithographic printing plates and, more particularly, to a printing plate having a protective dielectric film that improves the wear characteristics of the plate and that enhances the electroerosion of an adjacent conducting layer of the plate.

Background Art

[0002] Lithographic printing plates are employed to print a particular image in ink on sheets of a recording medium, for example paper. The lithographic printing process is dependent upon the immiscibility of grease and water and, more particularly, upon the tendency of one substance to retain a greasy, image-forming material and a complementary substance to retain an aqueous dampening fluid.

[0003] A lithographic printing plate or offset master typically includes an imaging area comprised of oleophilic or hydrophobic material and a non-image area comprised of oleophobic or hydrophilic material. A greasy material is applied to the hydrophobic image area of the plate and the entire surface of the plate is then moistened with an aqueous solution. The image area will tend to repel the water and the non-image area will tend to retain the water and, thus, upon a subsequent application of greasy ink, the image portion retains the ink whereas the moistened non-image area repels it. The ink on the image area may then be transferred to the surface of a material on which the image is reproduced, for example paper or cloth, through an intermediary offset or blanket cylinder. The printing plate may be used in the above-described printing process to print many sheets of paper or cloth before chemical or physical wear of the imaging or non-imaging area of the plate results in an unacceptable degradation in the clarity of the printed image.

[0004] In order to extend the print lifetime of a lithographic printing plate, it is necessary to utilize imaging and non-imaging materials that are resistant to chemical and physical wear. In the U.S. Patent to Chu, "Process of Electrolytically Anodizing a Mechanically Grained Aluminum Base and Article Made Thereby", No. 3,891,516, issued June 24, 1975, a more durable lithographic printing plate is disclosed. The printing plate includes an aluminum base plate to which is anodized a layer of aluminum oxide. The layer of aluminum oxide covers the entire surface of the aluminum plate and thereby provides a hydrophilic surface that is resistant to abrasion, wear and erosion. A layer of photoresist is applied over the aluminum oxide and is etched by a wet chemical development process to provide a hydrophobic printing surface.

[0005] Although the lithographic printing plate of Chu has an increased resistance to wear and corrosion at its hydrophilic non-image surface, the plate is still subject to wear at its photoresist, hydrophobic printing surface. Also, the printing plate of Chu must be constructed by the relatively complicated, time-consuming and expensive process of photographic exposure and wet chemical development.

[0006] A relatively simple and cost-efficient electroerosion process has been developed to form image and non-image areas on printing plates from digitally coded information, thereby avoiding the time consuming photographic process of Chu. In known electroerosion processes, a printing plate is provided with a nonconducting hydrophobic substrate, for example a polyester material sold under the trademark MYLAR that is covered, for example by an 800 angstrom film of a hydrophilic material such as aluminum. An image is formed in the plate by electrically eroding a plurality of holes in the aluminum film and thereby exposing the surface of the MYLAR substrate at each hole. The image that is to be printed is, of course,-formed by the pattern of the holes in the aluminum.

[0007] A hole is formed in the aluminum layer by moving an erosion electrode adjacent to a point on the surface of the aluminum layer and applying a voltage pulse to the electrode so that a spot on the aluminum is rapidly heated and a corresponding portion of the aluminum is evaporated or otherwise removed from the substrate. Thereafter, the erosion electrode is moved to the next printing position and the electrical erosion process is repeated. In practice, a line of erosion electrodes is scanned across the aluminum surface of a printing plate and particular electrodes in the line of electrodes are energized to form holes in accordance with digitally coded image information. See for example U.S. patent 3,483,027 Reitzerfeld.

[0008] A disadvantage of known electroerosion printing processes is that the metallized plastic printing plates have a relatively short print lifetime. The lifetime is limited both by the relative softness and low resistance to abrasion and corrosion of aluminum, or other common lithographic metals suited to the electroerosion process, and by the small thickness of these metals that can be eroded electrically. Accordingly, typical lithographic plates having an aluminum film of less than 800 angstroms may be expected to produce a few hundred prints before physical wear of the aluminum surface causes non-printing regions of the plate to ink and to print.

[0009] The print life of lithographic plates may be increased somewhat by using a thicker metal film. However, with a thicker film, more electrical power must be applied to the printing electrodes to form a hole that extends to the substrate. As.a practical matter, the aluminum film of prior art plates has not exceeded 1000 angstroms, due both to limitations in the amount of power that may be applied by a printing electrode, and the fact that the high thermal conductivity of the metal films results in spot welding of the electrode to the substrate.

[0010] A further disadvantage of electroerosion systems is that the surface of the metal film of a plate is often burnished or scratched by the printing electrodes as the electrodes move over the surface of the plate. The burnishing or scratching is particularly damaging if the printing electrodes are pressed against the surface of the metal with excessive force. If the metal is scratched, the normally non-printing metal surface of the plate will produce an objectionable gray or lined background for a printed image.

[0011] Accordingly, it is an object of the invention to provide a lithographic printing. plate upon which an image may be formed by the electroerosion process in an energy- efficient manner.

[0012] A further object of the invention is to provide such a printing plate that is resistant to burnishing or scratching and to chemical or physical wear and that has a correspondingly extended print lifetime..

[0013] Another object of the invention is to provide a method for producing a lithoaraohic printing plate that has increased durability and that is suitable for energy-efficient imaging by an electroerosion process.

[0014] These and other objects of the invention will become apparent from a review of the detailed specification which follows and a consideration of the accompanying drawing.

Disclosure of the Invention

[0015] In order to achieve the objects of the invention and to overcome the problems of the prior'art, the lithographic printing plate, according to the invention, includes a substrate of nonconductive hydrophobic material, for example a polyester such as is sold under the trademark MYLAR and a first film of conducting hydrophilic material, for example aluminum.

[0016] A second film of hydrophilic, dielectric material, for example aluminum oxide (A1₂0₃) is provided to protect the aluminum film from scratching or burnishing and to extend the print life of the printing plate. The dielectric also enhances the erosion of spots of aluminum in response to voltage pulses.

[0017] The lithographic printing plate of the invention is made by depositing a layer of aluminum over the hydrophobic substrate by appropriate means, such as electron beam evaporation, sputtering or resistance evaporation. The protective layer of aluminum oxide may be applied by known thin film techniques, such as electron beam evaporation, sputtering or anodizing.

Brief Description of the Drawing

[0018] The drawing illustrates a perspective view in partial section, not to scale, of a lithographic printing plate in accordance with the invention and associated electroerosion imaging apparatus.

Best Mode for Carrying out the Invention

[0019] The remaining portion of this specification will describe preferred embodiments of the invention when read in conjunction with the attached drawing in which like reference characters identify identical apparatus.

[0020] The drawing illustrates a perspective view in partial section of a lithographic printing plate 1 in accordance with the invention and an associated electroerosion printing apparatus. The printing plate has been drawn out of scale in order to facilitate an understanding of the invention. The lithographic printing plate of the invention has a nonconducting, hydrophobic substrate 2 made of, for example, a polyethylene terephthalate such as is'sold under the trademark MYLAR or a polyimide such as is sold under the trademark KAPTON. A first film 3 of conducting hydrophilic material made of, for example, aluminum is formed on the substrate 2 by electron beam evaporation. Electron beam evaporation techniques are well-known to the art and, therefore, it will be understood by those skilled in the art how such techniques may be employed to deposit a layer of aluminum on the substrate.

[0021] In a preferred embodiment of the invention, the aluminum film 3 is evaporated on the substrate 2 to a depth of approximately 2000 angstroms, a thickness substantially in excess of the typical thickness of 1000 angstroms or less for corresponding conductive films of prior art lithographic printing plates. However, as a practical matter, the aluminum film may have a thickness at least within the range of 1000 to 3000 angstroms, without departing from the invention.

[0022] A second film 5 of relatively hard, hydrophilic dielectric material, for example aluminum oxide (A1₂0₃) is disposed over the aluminum film 3. The aluminum oxide film may be applied by sputtering, electron beam evaporation or anodizing techniques that are well-known to the art. In the preferred embodiment of the invention, approximately ₅₀₀ angstroms of aluminum oxide is deposited over the film 3 of aluminum by electron beam evaporation. Since the aluminum oxide is a relatively hard material and, in particular, is much harder than the aluminum, the plate constructed in accordance with the invention has a substantially increased durability and toughness and is, therefore, more resistant to physical or chemical wear.

[0023] As shown in the drawing, a printing image is formed in the lithographic printing plate 1 by moving a plurality of electrodes 7 over the plate and energizing particular electrodes to form corresponding holes 8 in the aluminum and the aluminum oxide so that the underlying surface of the polyester substrate is exposed at each hole. In operation, a broad area electrode 9 is placed in conductive contact with the aluminum film 3 of the printing plate, for example by pressing the electrode 9 against an area of the printing plate at which the aluminum oxide has been removed and the aluminum has been exposed. A control apparatus 11 then operates a scanning mechanism to scan the erosion electrodes 7 across the aluminum oxide surface of the printing plate and to energize particular erosion electrodes 7 with voltage pulses, for example of from 10-100 volts and 1 msec to 1 µsec duration, in accordance with a digital image pattern that is stored in the control apparatus. The control apparatus is not a part of the present invention and, therefore, is not disclosed in detail. However, electroerosion scanners are known to the art and are commercially available.

[0024] When the control apparatus 11 energizes a particular erosion electrode 7 with a voltage pulse, the energy of the electrical pulse is passed to an area of the aluminum oxide film that is immediately adjacent to the electrode. The voltage pulse is sufficiently large to break down the aluminum oxide and to cause a heating current I to flow from the printing electrode 7 to the broad area electrode 9, through the aluminum film. The concentrated current in the area of the aluminum film adjacent to the point of application of the voltage pulse causes a hole to be evaporated in the aluminum. In experimental tests, an erosion pulse of 50 volts and 200 microseconds duration was sufficient to erode a hole extending to the MYLAR substrate in a printing plate having a 2000 angstrom film of aluminum and associated 500 angstrom film of aluminum oxide, in accordance with the invention. However, when a pulse of the same magnitude was applied to a prior art printing plate having only a _MYLAR substrate and an aluminum film, only approximately 800 angstroms of aluminum was eroded.

[0025] It is theorized that the greater penetration for the printing plate of the invention is due to the.fact that the dielectric layer of aluminum oxide acts as a capacitor that initially stores energy as a voltage pulse is applied and that releases the stored energy when the dielectric film breaks down. The release of the stored energy apparently adds to the heat that is normally produced by the erosion current I and, therefore, more aluminum is evaporated.

[0026] A capacitive breakdown scheme has been employed in the U..S. Patent to Reis, "Electrosensitive Recording", No. 3,299,433, issued January 17, 1967_i to heat a surface recording medium that changes color in response to applied heat._' However, the capacitive breakdown that is disclosed in the Reis patent is not directed to an electroerosion process wherein a hole is formed in a dielectric film and an underlying aluminum film.

[0027] It has also been experimentally determined that a prior art nrinting plate having a polyester substrate, such as MYLAR and an 800 angstrom aluminum layer is subject to scratching and burnishing on the exposed aluminum film when the erosion electrodes 7 contact the plate with pressures required by the electroerosion process while scanning the plate. Thus, the contact pressure of the erosion electrodes must be closely monitored in prior art electroerosion systems in order to avoid such undesirable burnishing or scratching. However, a printing plate constructed in accordance with the invention, having a 2000 angstrom aluminum film and an associated 500 angstrom aluminum oxide film, is resistant to scratching and burnishing, with only moderate attention being given to the contact pressure of the erosion electrodes. In addition, the above prior art printing plate was able to print no more than 500 copies before significant image degradation occurred due to wearing of the imaging surfaces of the plate, while the above plate constructed in accordance with the invention was used to print 10,000 copies, with, apparently, no signs of wear.

[0028] It should be understood that, since the printing plate of the invention is not subject to scratching and burnishing by the erosion electrodes 7, only routine attention need be given to electrode pressure during the process of forming an image on the plate. However, prior art plates are subject to objectionable scratching and burnishing by the erosion electrodes and, therefore, during the imaging process', particular care must be taken to avoid excessive pressure of the erosion electrodes.

[0029] Accordingly, the lithographic printing plate of the invention is less difficult to produce than prior art printing plates, since less care need be taken in the imaging process. In addition, the hard layer of aluminum oxide on the printing plate provides an extended operational lifetime that is many times greater than has heretofore been achieved. Moreover, the dielectric film increases the energy efficiency of the erosion process and, therefore, substantially thicker layers of metal may be eroded, thereby extending the print lifetime of the printing plate even further.

[0030] Although aluminum and aluminum oxide were employed to respectively form the first conducting film and second protective layer for a preferred embodiment of the invention, other materials may be employed without departing.from the spirit of the invention. For example, tungsten may be used in place of aluminun and hafnium oxide (HfO₂) or Schott glass may be used in place of the aluminum oxide.

[0031] An improved lithographic printing plate may also be made in accordance with the invention by depositing an aluminum film over a polyester substrate in the above-described manner and then depositing a chrome film over. the aluminum film and an aluminum oxide (A1₂₀₃) film or chrome oxide (Cr₂0₃) film over the chrome film. For such an embodiment of the invention, the combined chrome and aluminum films behave in much the same fashion as the aluminum film of the embodiment of the drawing.

[0032] It should be appreciated that means other than electron beam evaporation may be employed to deposit a conducting film on a polyester substrate, for example some conductors may be deposited by electroless deposition or sputtering.

[0033] It should also be appreciated that the dielectric film of the invention may be used in conjunction with an aluminum layer of typical thickness, for example 800 angstroms, and the voltage and/or duration of the erosion pulses of the. erosion electrodes may then be reduced, to conserve energy in the process of making the printing plate. Moreover, it should be understood that the cited examples of erosion electrode voltage and pulse duration and of the thickness of the aluminum and aluminum oxide films are not intended to limit the scope of the invention. Other magnitudes of electrode pulses and thicknesses of conducting and dielectric film may be used without departing from the spirit of the invention.

[0034] It has been observed in this regard that variation of the pulse voltage and duration affects a predictable modification of the size and shape of the eroded hole. Increasing the pulse voltage produces an overall enlargement of the hole in the direction of relative travel of the erosion electrode. Moreover, the size of the hole tends to follow, subject to the above-mentioned influences, the cross-sectional size of the erosion electrode. It is thus apparent that control of the size of the eroded hole, and consequently of the resolution of the eroded image, is provided by the specification of the erosion electrode dimensions, the pulse voltage and the pulse duration. The capability of producing "half-tone" images, in which varying shades of grey are produced by varying spot size rather than by varying spot density, is thus seen to reside in the above-described process.

[0035] The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the claims rather than by the foregoing description, and all changes which come within the meaning and range of the equivalents of the claims are therefore intended to be embraced therein.

Claims

1. Lithographic printing plate, comprising an electrically nonconducting substrate and an electrically conducting film disposed on said substrate, characterized by a dielectric film (5) disposed on a printing portion of said conducting film (3) for protecting said printing portion of the conducting film (3) and for breaking down in response to at least one electrical pulse of a particular voltage and duration applied by means of an electroerosion electrode (7) to said dielectric film (5) thereby electrically eroding said conducting film (3) at said point of application to form an aperture (8) extending through said conducting film (3) and said dielectric film (5) to expose said nonconducting substrate (2).

2. Printing plate according to claim 1, characterized in that said dielectric film (5) and said electrically conducting film (3) are hydrophilic.

3. Printing plate according to claim 1, characterized in that said conducting film (3) is made of aluminum.

4. Printing plate according to claim 1, characterized in that said conducting film (3) is made of tungsten.

5. Printing plate according to claim 1, characterized in that said dielectric film (3) is made of aluminum oxide A1₂⁰₃.

6. Printing plate according to claim 1, characterized in that said conducting film (3) includes a lower layer of aluminum and an upper layer of chrome and that said dielectric film (5) is made of chrome oxide Cr₂⁰₃.

7. Printing plate according to claim 1, characterized in that said conducting film (3) includes a lower layer of aluminum and an upper layer of chrome and that said dielectric film (5) is made of aluminum oxide Al₂O₃.

8. Printing plate according to claim 1, characterized in that the dielectric film (5) has a hardness in excess of the hardness of said conducting film (3).

9. Printing plate according to claim 1, characterized in that said particular voltage is from 10 to 100 volts, said particular duration is from 1 microsecond to 1 millisecond, the thickness of said conducting film (3) is from 1000 to 3000 angstroms and the thickness of said dielectric film (5) is from 100 to 800 angstroms.

10. Method for producing a lithographic printing plate comprising the steps of:

depositing an electrically conducting hydrophilic film (3) on a nonconducting hydrophobic substrate (2), depositing a hydrophilic dielectric film (5) on said electrically conducting film (3), and applying electrical voltage pulses to the surface of the dielectric film (5) to break down the dielectric film (5) at the point of application and to erode an adjacent portion of the conducting film (3) to expose a portion of the underlying surface of the nonconducting hydrophobic substrate (2).

Drawing