An image forming apparatus, comprising:
- a development device, including
- a development casing having a first transport chamber and a second transport chamber disposed in parallel to each other, a first partition wall for separating the first transport chamber and the second transport chamber along longitudinal direction thereof, a communication part for allowing the first transport chamber and the second transport chamber to communicate with each other at each end side of the first partition wall, a developer replenishment port for replenishing developer containing magnetic carrier and toner, and a developer discharge part disposed at a downstream side end of the second transport chamber so as to discharge excessive developer,
- a developer carrying member supported by the development casing in a rotatable manner, so as to carry the developer in the second transport chamber on surface thereof,
- a first stirring transport member having a first rotation shaft disposed in the first transport chamber, and a first transport impeller formed on an outer circumferential surface of the first rotation shaft, so as to stir and transport the developer in the first transport chamber in a first direction, when the first rotation shaft rotates positively,
- a second stirring transport member having a second rotation shaft disposed in the second transport chamber, and a second transport impeller formed on an outer circumferential surface of the second rotation shaft, so as to stir and transport the developer in the second transport chamber in a second direction opposite to the first direction, when the second rotation shaft rotates positively,
- a discharge impeller formed on a downstream side of the second transport impeller in the second direction, so as to transport the developer in the same direction as the second transport impeller, and to discharge the developer through the developer discharge part, when the second rotation shaft rotates positively, and
- a reverse transport impeller formed between the second transport impeller and the discharge impeller, so as to apply a transporting force to the developer in the second transport chamber in the opposite direction to the second transport impeller and the discharge impeller, when the second rotation shaft rotates positively:
- a toner storage part for storing the developer to be replenished to the development device:
- a drive unit connected to the second rotation shaft so as to rotate the second rotation shaft n positive and reverse directions;
- a control unit connected to the drive unit so as to control the second rotation shaft to rotate positively, while controlling the same to rotate reversely by a predetermined rotation amount at a predetermined time interval;
- an image carrying member on which an electrostatic latent image is formed:
- and
- a toner concentration detection sensor capable of detecting toner concentration in the developer in the development casing, wherein
- when the second rotation shaft rotates positively or reversely, a developer transporting force of the reverse transport impeller is larger than that of the second transport impeller and the discharge impeller, and
- the control unit is capable of performing a developer discharge mode, in which the first rotation shaft and the second rotation shaft are rotated reversely during a non image formation period, so that the developer around the reverse transport impeller is discharged through the developer discharge part, and
- when the toner concentration detected by the toner concentration detection sensor is larger than a predetermined value, the control unit shortens an execution interval between successive executions of the developer discharge mode.