Method of registering different colours in flexography, and flexographic printer comprising a device for implementing said method

A method of registering various colours in flexography, implemented by means of a flexographic printer of the type that comprises at least one rotating supporting drum, which supports the material to be printed, and at least first and second printing groups which include some respective printing rollers with the same known printing length, with said first and second printing groups configured, arranged and selectively driven to change between a printing position, in which said first and/or second printing roller are in contact with said material to be printed on the cited supporting drum, and an inactive position, in which the first and/or second printing roller are separated from the material to be printed, and where the first and second printing rollers are driven by at least one driving group regulated by at least one controller comprising:

• a) placing the first printing group at said printing position and the second printing group at said inactive position and print at least one first mark on the material to be printed by means of the first printing roller of the first printing group;
• b) detecting, by means of an optical sensor, located downstream from the first and second printing groups, the position of said first mark on the material to be printed, and generating a first position signal representative of the longitudinal and transversal positions of the first mark within said printing length;
• c) placing the first printing group at said inactive position and the second printing group at said printing position and print at least one second mark, separated from said first mark, on the material to be printed by means of the second printing roller of the second printing group;
• d) detecting, by means of said optical sensor, the position of said second mark on the material to be printed and generating a second position signal representative of the longitudinal and transversal positions of the second mark within said printing length;
• and one of:
• e1) generating, by means of said controller, first and second adjustment signals in function of a comparison between each of said first and second position signals and a pre-established position signal;
• and
• f1) adjusting, by means of said at least one driving group, the angular and axial positions of the first and second printing rollers based on said first and second adjustment signals;
• or
• e2) generating, by means of said controller, an adjustment signal in function of a comparison between the second position signal and the first position signal, which is taken as reference;
• and
• f2) adjusting, by means of said at least one driving group, the angular and axial positions of the second printing roller based on said adjustment signal in accordance with the detected position of the first mark within said printing length.