Image forming apparatus featuring dual polarity, multiple timing bias applying means

Abstract

An image formation apparatus includes: an image formation unit for forming toner images on a photosensitive drum; a transfer belt for transporting sheets of a transfer medium; an electrostatic transfer unit transferring toner images on the photosensitive drum onto the sheets; a cleaning unit for cleaning residual matter including toner form the photosensitive drum; a charger for applying a bias of a polarity which is inverse to a normal polarity of the toner, to the transfer unit; and a control unit for controlling operations of the charger. The control unit controls the charger such that a transfer bias is applied in sheet spacing between consecutive sheets, so that there is no flow of current to edge portions at the leading and trailing edges of the sheets, that sheet spacing transfer off time is one photosensitive drum cycle or less, that the transfer bias current is set to zero within leading and trailing sheet margins, and that the image is transferred onto the transfer medium in an image region.