Vacuum drums for printing, and duplex printers

Abstract

A vacuum drum assembly for a printing machine has a drum with an array of passageways distributed along its length and around its periphery to permit airflow from outside the drum to inside the drum in response to reduced air pressure inside the drum. An array of valves are movable between a closed position in which that valve restricts at least one of the passageways and an open position in which the restriction of the same passageways is reduced. When a partial area of the drum is wrapped with a sheet of material, at least some of the passageway valves adjacent the edges of that area are open, and the valves for the passageways not covered by the sheet and not adjacent the edges of that area are closed. The drum open area is regulated to be small in regions where there is no paper. Accordingly, the open area of the drum is adapted to the paper shape and size and the paper position on the drum, while minimizing the required suction flow. A duplex printing machine has two such vacuum drum assemblies with their drums parallel. The air pressure inside the drums is reduced and the drums are counter-rotated. Printing material is fed to the first drum and held on the first drum by vacuum and rotated therewith. A first print head prints on one side of the material. The material is released from the first drum in a direction towards the second drum and held on the second drum by vacuum and rotated therewith. A second print head then prints on the material on the second drum. The material is then released from the second drum.