Image forming apparatus maintaining clean back surface of recording material

1. An image forming apparatus comprising: an image forming unit having an image supporting body on which an electrostatic latent image is formed and a developing unit that develops the electrostatic latent image to a toner image;

a transfer roller that is disposed to face the image supporting body;

a circular transfer belt running in a predetermined running direction, the transfer belt being interposed between the image supporting body and the transfer roller at a transfer position and having a conveying surface onto which a recording material, to which the toner image is directly transferred at the transfer position, is placed;

a cleaning unit that can be switched between a cleaning state in which the cleaning unit removes a toner attached to the conveying surface and a non-cleaning state in which the cleaning unit does not remove the toner attached to the conveying surface;

a density detection unit that is disposed on a downstream side of the transfer position and an upstream side of a cleaning position of the cleaning unit in the running direction, the density detection unit detecting toner density on the conveying surface;

a calibration processing unit that executes a calibration process for calibrating toner density of the toner image formed on the image supporting body; and

a control unit that: in a state in which the calibration processing unit has executed the calibration process, instructs the image forming unit to adjust the toner density of the toner image formed on the image supporting body based on the toner density detected by the density detection unit; and

in a state in which the calibration processing unit has not executed the calibration process, switches the cleaning unit to the cleaning state or maintains the cleaning unit in the non-cleaning state based on the toner density detected by the density detection unit,

wherein the control unit switches the cleaning unit to the cleaning state in a case in which the toner density detected by the density detection unit is greater than a predetermined density.

2. The image forming apparatus according to claim 1 , wherein the control unit determines whether the toner density detected by the density detection unit is greater than the predetermined density or not, after elapse of a predetermined time period after completion of the calibration process by the calibration processing unit.

3. The image forming apparatus according to claim 1 , wherein the density detection unit detects the toner density of the transfer belt at a position not aligned with a position corresponding to an outer edge of a standard-size recording material in a width direction orthogonal to the running direction.

4. The image forming apparatus according to claim 1 , wherein:

the transfer belt transfers the toner image directly to the recording material by applying a voltage having a polarity reverse to a polarity of a voltage of toner attached to the electrostatic latent image to the transfer roller; and

the cleaning unit cleans the conveying surface by collecting residual toner attached to the conveying surface by an electrostatic force.

5. The image forming apparatus according to claim 1 , further comprising:

a paper path that conveys the recording material vertically from a lower side to an upper side;

a driving roller; and

a driven roller that is disposed obliquely above the driving roller,

wherein the transfer belt is stretched around the driving roller and the driven roller thereby composing a part of the paper path, and is disposed obliquely with respect to a perpendicular direction.