METHOD TO REGISTER COLORS IN IN-TRACK DIRECTION ON A MULTICOLOR WEB PRINTING MACHINE

1 . Method for printing a multi-color image on a printing material web, said web is moved in a transport direction along a plurality of printing units for respectively printing one color separation of the multi-color image, said method comprising the following steps:

generating a line clock for the printing unit being the first one in transport direction of the printing material web, as a function of a transport speed of the printing material web;

printing a plurality of synchronization marks with the printing unit that is the first in the direction of movement of the printing material web, said synchronization marks are located in a region of the printing material web outside the region of the multi-color image, and are printed in a prespecified clock that is derived from a partial set of the line clock;

detecting the synchronization marks in a position along the direction of movement of the printing material web, said position is located between the first and the second printing units; and

generating a line clock for the printing unit being the second one in the direction of movement of the printing material web, based on the detection of the synchronization marks.

2 . Method as in claim 1 , wherein the synchronization marks are respectively detected between two successive printing units, and a line clock is generated for the rear one of the two printing units based on the detection of the synchronization marks.

3 . Method as in claim 1 , wherein the synchronization marks are continuously printed in a prespecified clock.

4 . Method as in claim 1 , wherein the line clock for the second printing unit is interpolated between the detection of successive synchronization marks in order to achieve a desired resolution in transport direction.

5 . Method as in claim 1 , wherein the position of the detection of the synchronization marks is substantially closer to the second printing unit than to the first printing unit.

6 . Method as in Clam 1 , wherein the detection of the synchronization marks is directly used as the trigger for the line clock.

7 . Method as in claim 1 , wherein an error detection is provided, said error detection recognizing when a synchronization mark was not detected and appropriately adapting the line clock for the second/following printing unit.

8 . Method as in claim 1 , wherein the first printing unit additionally prints image start marks that can be distinguished from the synchronization marks, and wherein the image start marks are detected between the first and the second/following printing unit(s) so as to signal an image start.

9 . Method as in claim 8 , wherein the image start marks are different regarding their length in the direction of movement of the printing material web from the synchronization marks, and the image start marks and the synchronization marks are detected by the same detector.

10 . Method as in claim 8 , wherein the image start marks are different regarding their length transversely to the direction of movement of the printing material web from the synchronization marks, and the image start marks and the synchronization marks are detected by different detectors.

11 . Method as in claim 8 , wherein the image start marks act, at the same time, as synchronization marks.

12 . Method as in claim 1 , wherein the respective printing units apply ink to the printing material web for the printing of color separations.