Duplex image-forming apparatus

Claim 1

A duplex image forming apparatus adapted to form a first toner image on a first side of a transfer sheet and a second toner image on a second side of a transfer sheet, said apparatus comprising: an initial image bearing member; an image forming unit, including an exposure unit and a developing unit, adapted to form said first toner image and said second toner image on said initial image bearing member, said image forming unit starting to form said second toner image at timing (P); an intermediate image bearing member provided with a reference position; a first transfer unit adapted to transfer said second toner image from said initial image bearing member to said intermediate image bearing member; said image forming unit starting formation of said first toner image at timing (Q); a sheet feeder adapted to feed said transfer sheet toward said first transfer unit, said sheet feeder starting to feed at time (R); said first transfer unit adapted to transfer said first toner image onto said first side of said transfer sheet; a second transfer unit adapted to transfer said second toner image from said intermediate image bearing member onto said second side of said transfer sheet; a fixing unit to fix said first toner image on said first side of said transfer sheet and said second toner image on said second side of said transfer sheet simultaneously; a detector to detect said reference position on said intermediate image bearing member; and a control for said timings (P), (Q), and (R) based on detection of said reference position, whereby a leading edge of said first toner image on said first side of said transfer sheet and a leading edge of said second toner image on said second side of said transfer sheet coincide with a leading edge of said transfer sheet.