Underscreen cleaning

A cleaning system 49 for cleaning a printing screen (PS, fig.4) within a printing machine comprises a cleaning head 3, a drive means 56, a vacuum source 50, a flow line 52 and a flow control means 51. The drive means performs a cleaning stroke in which the cleaning head is moved across a surface of the printing screen. The vacuum source generates a vacuum or reduced pressure. A flow line extends between the vacuum source and the cleaning head, for drawing air from the cleaning head to the vacuum source and thus transfer the vacuum or reduced pressure generated by the vacuum source to the cleaning head to generate a suction force at the cleaning head. A flow control means adjusts the suction force generated at the cleaning head in dependence of print parameters determined in association with the print operation. The print parameters may be determined from Gerber-type data associated with a pattern of apertures formed in the printing screen. The print parameters may be determined by an inspection system operative to inspect a printed workpiece printed using the printing screen during the print operation and may be indicative of the print quality of the printed workpiece.