Abstract: A device and a method for improving the quality of prints using a printing device and system for printing sheets whereby, for each sheet, at least one register mark per color printing unit of the multi-color printing machine is produced, assigned to said sheet and defined with respect to its position, including in duplex printing.

Abstract: The object of the invention is to achieve a correctly positioned print in duplex-printing mode. A method for the detection of marks (1, 1?, 1?) by a sensor array (10) for a printing machine has been provided, whereby the marks (1) on the first (recto) printing side (5) of a sheet (3) are detected, the sheet is turned over and shifted in a direction transverse to the transport direction, and the marks (1?) on a second (verso) printing side (6) of said sheet are detected. Furthermore, a printing machine is provided with an alignment device (40) for shifting a sheet (3) in a direction transverse to the transport direction for the detection of marks (1?) on the second printing side, said marks being offset relative to the marks (1) on the first printing side, after said sheet (3) has been turned over.

Abstract: Cleaning of contamination in a printer, especially contamination by the use of a fixing aid, in which the contamination is determined by evaluating the register of a print of the printing machine. In the case that the register error is larger than a given amount, a cleaning run is performed. A control device is provided for evaluation of the register for determining contamination of the printer, especially by a fixing aid.