US Patent 7390133 Duplex printer including print unit shifting and web shifting mechanisms

After the front surface of a web is printed by a first-side printing unit, the web is shifted in the direction of its width by a direction-changing unit, flipped over by rollers, and transported to a second-side printing unit, where the back surface of the web is printed. A sensor detects a displacement between the second-side printing unit and the web or marks printed on the web. According to the amount and direction of displacement detected by the sensor, a printing unit body is shifted in the direction of the web width, and the web traveling as opposed to the first-side printing unit is shifted in the direction of its width by an edge guide.