Claims
- 1. A method of imaging a lithographic plate having a printing surface comprising the steps of mounting said plate to the plate cylinder of a lithographic press having at least one plate cylinder, a corresponding number of blanket cylinders and an impression cylinder, exposing said printing surface to spark discharges between said plate and an electrode spaced close to said printing surface produced in response to picture signals representing an image, said spark discharges producing sufficient heat to physically change the affinity of said printing surface for printing liquid selected from the group consisting of water and ink at the points thereof exposed to said spark discharges thereby producing directly on said plate image spots suitable for inking and reproduction, moving said electrode and said plate relatively to effect a scan of said printing surface, and controlling the spark discharges to the plate in accordance with picture signals so that they occur at selected times in the scan thereby forming an array of said image spots on said printing surface which make the plate press ready.
- 2. The method defined in claim 1 wherein a high positive voltage is applied to said electrode to produce said spark discharges.
- 3. The method defined in claim 1 wherein a high negative voltage is applied to said electrode to produce said spark discharges.
- 4. The method defined in claim 1 and including the additional step of varying a spark discharge characteristic selected from the group consisting of spark number, spark duration and spark current to vary the sizes of the image spots produced by the discharges.
- 5. The method defined in claim 1 and including the additional step of saving the image formed on the plate by coating the surface of the plate with an energy curable ink and then illuminating the coating with electromagnetic energy.
- 6. The method defined in claim 5 wherein the portion of said surface coated with ink is the portion having an affinity for ink.
- 7. The method defined in claim 6 wherein said surface is selected from the group consisting of aluminum and chrome.
- 8. The method defined in claim 1 and including the additional steps of
- applying a first ink to the surface of the printing plate following said exposure thereof so as to develop the image on the plate surface; and
- transferring the developed image via the blanket cylinder to a recording medium carried by the impression cylinder.
- 9. The method defined in claim 8 and including the additional steps of
- exposing a second similar plate mounted on a plate cylinder of the press in the same fashion as is the first-mentioned plate in response to picture signals representing a second aspect of said image so that a second spot array is applied on press to the printing surface of the second plate;
- applying a second ink to said second plate surface following the exposure thereof so as to develop the image on said second plate surface; and
- transferring the developed image on the second plate surface via a blanket cylinder to the recording medium carried by the impression cylinder so that that image is in register with the image transferred to that medium from the first-mentioned plate.
- 10. The method defined in claim 9 wherein the first-mentioned and second plates are mounted successively in time to the same plate cylinder of the press.
- 11. The method defined in claim 9 wherein the first-mentioned and second plates are mounted to different plate cylinders of the press.
- 12. The method defined in claim 9 wherein different color inks are applied to the two printing plates.
- 13. The method defined in claim 1 and further including performing said mounting, exposing, moving and controlling steps to image at least two additional lithographic plates on a press having at least three plate cylinders using picture signals representing three color components of the same picture.
- 14. Apparatus for imaging a lithographic plate, said apparatus comprising means for supporting a lithographic plate; a spark discharge electrode; means for positioning said electrode close to the printing surface of a lithographic plate supported by said supporting means; means for moving said positioning means and plate relative to one another so that said electrode scans over the printing surface of said plate; means for applying a DC potential difference between the electrode and the plate in the form of pulses at selected points in the scan so that spatial spark discharges occur between said electrode and said plate which produce physical transformations of said plate printing surface structure at those points which change the affinity of those points for printing liquid selected from the group consisting of ink and water so as to directly produce image spots on said plate which make the plate press ready.
- 15. The apparatus defined in claim 14 wherein said electrode is a thin wire made of a refractory metal.
- 16. The apparatus defined in claim 15 wherein said wire has a reduced diameter end facing the plate.
- 17. The apparatus defined in claim 14 wherein said applying means include means for changing the polarity of said pulses.
- 18. The apparatus defined in claim 14 wherein said applying means include a transformer having primary and secondary windings; means for applying DC voltage pulses to said primary winding; and means for connecting said electrode and said plate in circuit with said secondary winding.
- 19. The apparatus defined in claim 18 wherein said pulse applying means comprise a DC voltage source, a resistor and a capacitor in series and means for controllably dumping the energy in said capacitor to said primary winding.
- 20. The apparatus defined in claim 18 and further including means for controlling the number of pulses at each selected point in the scan so as to vary the number of spark discharges at each selected point in the scan thereby varying the sizes of said spots.
- 21. The apparatus defined in claim 14 and further including means for varying the magnitude of the potential difference between said electrode and said plate so as to vary the sizes of said spots.
- 22. The apparatus defined in claim 14 and further including means for varying a spark discharge characteristic selected from the group consisting of spark number, spark duration and spark current to vary the sizes of said image spots.
- 23. The apparatus defined in claim 14 wherein said applying means include a high voltage switch and a capacitor-regenerating resistor.
- 24. The apparatus defined in claim 23 wherein said applying means include means for applying a bias voltage to said electrode to provide high voltage pulses to said electrode without requiring a high voltage rating on said switch.
- 25. The apparatus defined in claim 14 and further including a source of electromagnetic energy positioned opposite said imaged plate so as to illuminate the imaged plate surface for curing ink on said imaged plate surface.
- 26. The apparatus defined in claim 14 wherein said positioning means include a carriage supporting said electrode and a lead screw threaded through said carriage for moving said carriage parallel to said plate, and said moving means comprise means for rotating said lead screw and further including means for monitoring the position of said carriage along said plate and producing carriage position signals, and means for controlling said applying means so that spark discharges are produced at selected positions on said plate.
- 27. A method of imaging on a press including a plate cylinder, a printing plate having a printing surface whose structure gives that surface an affinity for a printing liquid selected from the group consisting of water and ink, said method comprising the steps of
- mounting said plate to the plate cylinder;
- without contacting said plate surface, exposing the plate surface to spark discharges between an electrode spaced close to said plate and selected points on said printing surface to produce physical transformations of the plate surface structure at those selected points which change the affinity of those surface points for the printing liquid;
- moving the electrode and the print cylinder relatively to effect a scan of the printing surface by the electrode, and
- controlling the spark discharges in accordance with picture signals representing an image so that they occur at selected times in the scan thereby directly producing on the printing surface an array of image spots which make the plate press ready.
- 28. The method defined in claim 27 including the additional step of varying a spark discharge characteristic selected from the group consisting of spark number, spark duration and spark current to vary the sizes of the image spots produced on the plate by the discharges.
- 29. The method defined in claim 27 and saving the image formed on the plate by
- coating the entire printing surface of the imaged plate with an ink-receptive liquid which adheres to said image spots;
- washing said liquid from the printing surface except at the image spots thereon, and curing the liquid remaining on the plate surface.
- 30. The method defined in claim 29 including the steps of forming the printing liquid of a material that is curable by electromagnetic energy, and exposing said remaining liquid to said energy.
RELATED APPLICATION
This application is a continuation-in-part of Ser. No. 07/234,475, now U.S. Pat. No. 4,911,075, and a division thereof in response to the restriction requirement mailed Mar. 30, 1989.
This invention relates to offset lithography. It relates more specifically to improved lithography plates and method and apparatus for imaging these plates.
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Divisions (1)
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Number |
Date |
Country |
Parent |
234475 |
Aug 1988 |
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