Rotatable knife roll

A rotatable knife roll for cutting out window openings in envelopes, comprising:

• a roll body having two face sides;
• at least one foil knife disposed on a part of the roll body and having a sharp and endless cutting edge closed within itself;
• a plurality of breakthroughs within said cutting edge;
• at least one suction-air control valve arranged on at least one face side of the roll body and being rotatable relative to said roll body, said suction-air control valve comprising:
• (a) at least one suction-air-control channel;
• and
• (b) at least one fresh-air channel;
• a plurality of suction apertures disposed on the knife roll, some of said apertures being connected to the breakthroughs, for temporarily retaining a letter envelope blank over a suction and transport angle zone α, and for retaining a piece of material cut out by the foil knife across a suction and transport angle zone β, said suction apertures being arranged in rows to which said suction and fresh air channels are allocated,
• at least one suction channel connected with the suction apertures and with the suction air control channel and fresh air channel, for conducting air between the suction-air control channel and the fresh-air channel and the suction apertures;
• and
• a 3/3-way valve associated with the suction channel upstream of the suction-air control valve, said 3/3-way valve having an open position for each of suction and transport angle zones α and β, and one common closing position for suction and transport angle zones α and β.