A printer assembly (10) adapted for use with a print head comprising a platen (14) operable to move in a first plane; and a vacuum source in fluid communication with the platen (14) to provide a vacuum communication path to draw air through the platen (14) such that a sheet of media arranged on the platen (14) is biased towards it, characterised by a movable aperture (52) in the vacuum communication path between the vacuum source and the platen (14), the movable aperture (52) being arranged to move with the platen (14).