Method and system for manufacturing intaglio printing plates for the production of security papers

1. A method for manufacturing intaglio printing plates for the production of security papers, wherein a laser beam produced by a laser engraving unit is used to engrave intaglio printing patterns directly into a surface of a laser-engravable printing plate medium, wherein laser engraving of the printing plate medium is carried out layer by layer in several individual engraving steps performed one after the other in register so that the intaglio printing patterns are gradually engraved into the surface of the printing plate medium up to desired engraving depths, the surface of the engraved printing plate medium being cleaned from residues of the laser engraving process following and between each individual engraving step while the laser engraving unit is inoperative,

wherein the laser beam produced by the laser engraving unit is directed towards the surface of the printing plate medium, the laser engraving unit being moved during each individual engraving step from a start position to an end position over the surface of the printing plate medium, and wherein cleaning of the surface of the engraved printing plate medium is performed while the laser engraving unit, which is inoperative, is moved back from the end position to the start position.

2. The method according to claim 1 , wherein each individual engraving step results in a selective removal of a layer of material from the printing plate medium the thickness of which does not exceed a pre-selected maximum value.

3. The method according to claim 2 , wherein the pre-selected maximum value is of between 10 to 15 microns.

4. The method according to claim 1 , wherein at least part of the intaglio printing patterns are engraved to a depth of at least 80 microns.

5. The method according to claim 1 , wherein cleaning of the surface of the engraved printing plate medium following and between each individual engraving step is performed mechanically.

6. The method according to claim 5 , wherein cleaning of the surface of the engraved printing plate medium following and between each individual engraving step is performed by means of a rotating brush that is moved over the surface of the engraved printing plate medium following and between each individual engraving step.

7. The method according to claim 1 , wherein each engraving step involves a pixel-by-pixel engraving of the printing plate medium based on three-dimensional pixel data of an engraving depth-map of the intaglio printing patterns.

8. The method according to claim 1 , wherein the printing plate medium is mounted onto the circumference of a support cylinder which is rotated in front of the laser engraving unit, the laser engraving unit being movable parallel to the axis of rotation of the support cylinder, wherein the laser engraving unit is moved during each individual engraving step from the start position to the end position step by step along the axis of rotation of the support cylinder so as to process successive annular portions of the surface of the printing plate medium.

9. The method according to claim 1 , wherein engraving of the surface of the printing plate medium is first performed by starting with the deepest intaglio printing patterns and gradually adding shallower intaglio printing patterns during subsequent engraving steps.

10. The method according to claim 9 , wherein a focus of the laser beam is adjusted during at least one of the individual engraving steps.

11. The method according to claim 9 , wherein a focus of the laser beam is not adjusted during the individual engraving steps and wherein engraving of the surface of the printing plate medium is decomposed into a plurality of engraving phases, each comprising one or more individual engraving steps, whereby only part of the desired intaglio printing patterns are engraved during each engraving phase.

12. The method according to claim 1 , wherein an angle of incidence of the laser beam with respect to the surface of the printing plate medium remains constant throughout the engraving process.

13. The method according to claim 1 , wherein the printing plate medium is a metallic printing plate medium.

14. A laser engraving system for manufacturing intaglio printing plates for the production of security papers, comprising a support element for mounting a laser-engravable printing plate medium, a laser engraving unit producing a laser beam that is directed towards a surface of the printing plate medium for engraving intaglio printing patterns directly into the surface of the printing plate medium layer by layer in several individual engraving steps and a cleaning unit for cleaning the surface of the engraved printing plate medium and removing residues therefrom following and between each individual engraving step while the laser engraving unit is inoperative,

wherein the laser engraving unit is movable during each individual engraving step from a start position to an end position over the surface of the printing plate medium, and wherein the cleaning unit is operative while the laser engraving unit, which is inoperative, is moved back from the end position to the start position.

15. The laser engraving system according to claim 14 , wherein the printing plate medium is mounted onto the circumference of a support cylinder acting as the support element, which support cylinder is rotated in front of the laser engraving unit, which laser engraving unit is movable parallel to the axis of rotation of the support cylinder.

16. The laser engraving system according to claim 15 , wherein the laser engraving unit is movable during each individual engraving step from the start position to the end position step by step along the axis of rotation of the support cylinder so as to process successive annular portions of the surface of the printing plate medium.

17. The laser engraving system according to claim 14 , wherein the laser engraving unit and the cleaning unit are mounted on a common frame that is movable with respect to the surface of the printing plate medium.

18. The laser engraving system according to claim 14 , wherein the cleaning unit comprises a rotatable brush that can be brought into contact with the surface of the engraved printing plate medium to mechanically remove residues from the engraved printing plate medium.

19. The laser engraving system according to claim 14 , wherein the cleaning unit is designed to perform a mechanical cleaning of the surface of the engraved printing plate medium following and between each individual engraving step.

20. The laser engraving system according to claim 14 , wherein the laser engraving unit is controllable to selectively remove, during each individual engraving step, a layer of material from the printing plate medium, the thickness of which does not exceed a pre-selected maximum value.

21. The laser engraving system according to claim 20 , wherein the pre-selected maximum value is of between 10 to 15 microns.

22. The laser engraving system according to claim 14 , wherein the laser engraving unit is controllable to perform a pixel-by-pixel engraving of the printing plate medium based on three-dimensional pixel data of an engraving depth-map of the intaglio printing patterns.