Patent ID: 9684260
Date: 2017-06-20
CPC Classifications: G03G

Claim:
1. An image forming apparatus comprising: a photosensitive member; a charging device supplied with a DC bias and configured to electrically charge said photosensitive member; a developing device configured to develop, into a toner image, an electrostatic latent image formed on said photosensitive member charged by said charging device; an intermediary transfer member onto which the toner image is transferred from said photosensitive member; a transfer roller configured to transfer the toner image from said photosensitive member onto said intermediary transfer member; a transfer bias applying device configured to apply a bias to said transfer roller; a current detecting sensor configured to detect a current flowing through said transfer roller; and a controller configured to control a transfer bias applied to said transfer roller during image formation on the basis of a transfer bias which is determined, correspondingly to a bias causing a predetermined target current to flow through said transfer roller, on the basis of a predetermined test bias applied to said transfer roller during non-image formation and a value of a current flowing through said transfer roller under application of the predetermined test bias, wherein when the determined transfer bias exceeds a predetermined upper limit, said controller sets the transfer bias applied to said transfer roller during the image formation at the upper limit, and wherein in a same environmental condition, in a first case that the transfer bias applied to said transfer roller during the image formation is set at a value less than the predetermined upper limit, said controller executes an operation in a first mode in which an absolute value of a charging bias applied to said charging device is set at a first bias, and in a second case that the transfer bias applied to said transfer roller during the image formation is set at the predetermined upper limit and that a current-voltage characteristic of said transfer roller is such that value of a current flowing through said transfer roller under application of the upper limit transfer bias to said transfer roller is not more than the predetermined target current, said controller executes an operation in a second mode in which the absolute value of the charging bias applied to said charging device is set at a second bias larger than the first bias.