A duplex printing apparatus, comprising:
- a printing unit, which includes a plate cylinder around which a duplex master is wrapped, the duplex master including two images of a first engraved image and a second engrave image formed next to each other along a direction of rotation of the plate cylinder, and a pressing unit which is attached or detached with respect to the plate cylinder in a relative manner;
- a sheet feeding unit which feeds a sheet toward the printing unit;
- a sheet discharging unit which discharges a printed sheet printed by the printing unit;
- a sheet re-feeding unit which temporarily holds a surface-printed sheet, the front side of which is formed with a printed image by the printing unit so as to correspond to the first engraved image or the second engraved image, thereafter sends the surface-printed sheet to the pressing unit, and turns the surface-printed sheet by rotating the pressing unit to re-feed the surface-printed sheet to the printing unit;
- a switching unit which guides the sheet, which has passed through the printing unit, to the sheet re-feeding unit or the sheet discharging unit;
- and
- a control unit configured to control drive units of the sheet feeding unit, printing unit, sheet discharging unit, sheet re-feeding unit and switching unit respectively, so that printing speed increases from a low speed to a set speed when duplex printing is started,
- wherein, at the time of duplex printing, a first sheet is fed to the printing unit by the sheet feeding unit to perform, on the front side of the first sheet, surface printing corresponding to either one of the first engraved image and the second engrave image, thus obtained first surface-printed sheet on which printing is performed is guided to the sheet re-feeding unit by the switching unit, thereafter a second sheet is fed to the printing unit by the sheet feeding unit to perform, on the front side of the second sheet, surface printing corresponding to either one of the first engraved image and the second engraved image, and to re-feed the first surface-printed sheet to the printing unit by the sheet re-feeding unit to perform, on the reverse side of the first surface-printed sheet, reverse printing corresponding to the other one of the first engraved image and the second engraved image, and thus obtained first duplex-printed sheet is guided to the sheet discharging unit by the switching unit, while thus obtained second surface-printed sheet is guided to the sheet re-feeding unit,
- wherein the sheet re-feeding unit includes a sheet receiving board which catches a leading edge of the surface-printed sheet guided by the switching unit to an auxiliary tray,
- wherein the control unit is configured to control the drive units of the sheet feeding unit, printing unit, sheet discharging unit, sheet re-feeding unit and switching unit respectively, such that the printing speed increases in each simplex printing performed in duplex printing, and
- wherein the plate cylinder performs idling rotation every time when performing simplex printing of duplex printing until the printing speed increases from a low speed to a set speed and, during the idling rotation of the plate cylinder, the printing speed is increased by one speed level.