Image forming apparatus

An image forming apparatus comprising:

• a photosensitive drum which is rotatable;
• a charging unit that rotates in contact with the photosensitive drum, and charges a surface of the photosensitive drum;
• an exposing unit that exposes the surface of the photosensitive drum charged by the charging unit;
• a developing unit that supplies toner containing an external additive which has a charging polarity that is an opposite polarity of a charging polarity of the toner to the surface of the photosensitive drum exposed by the exposing unit;
• a transfer unit that transfers the toner, which has been supplied to the photosensitive drum by the developing unit, to a recording material;
• and
• a controller that controls the charging unit, the developing unit and the transfer unit,
• wherein, after the toner supplied to the surface of the photosensitive drum is transferred to the recording material, the developing unit is configured to collect toner remaining on the surface of the photosensitive drum,
• wherein the controller is configured to perform:
• an image forming operation to form an image on the recording material;
• and an external additive coating operation to transfer the external additive or the toner containing the external additive from the developing unit to the photosensitive drum, and transfer the external additive from the photosensitive drum to the charging unit, and
• wherein the controller is configured not to perform exposure by the exposing unit in the external additive coating operation, and configured to control voltages so as to satisfy:
• Vrg
• >
• Vdg
• ⁢
• 2
• Vdg
• ⁢
• 1
• >
• Vcg
• Vback_g
• >
• Vback_p
• Vtg
• >
• Vdg
• ⁢
• 3
• Vtg
• -
• Vdg
• ⁢
• 3
• <
• Vtth
• ,
• where V1>V2 represents a potential relationship in which an electrostatic force, directed from a position of a potential V1 to a position of a potential V2, acts on particles having a same charging polarity as a charging polarity of the external additive,
• Vcg represents a charging voltage in the external additive coating operation,
• Vdg1 represents a surface potential of the photosensitive drum in the external additive coating operation,
• Vdg2 represents a surface potential of the photosensitive drum in a developing portion facing the developing unit in the external additive coating operation,
• Vdg3 represents a surface potential of the photosensitive drum in a transfer portion facing the transfer unit in the external additive coating operation,
• Vrg represents a developing voltage to be applied to the developing unit in the external additive coating operation,
• Vtg represents a transfer voltage to be applied to the transfer unit in the external additive coating operation,
• Vtth represents a threshold of a potential difference with which discharge is generated between the transfer unit and the photosensitive drum,
• Vback_g represents a potential difference between the developing voltage and the surface potential of the photosensitive drum in the developing portion in the external additive coating operation, and
• Vback_p represents a potential difference between the developing voltage and the surface potential of the photosensitive drum in the developing portion in the image forming operation.