Image forming apparatus which switches from color to monochrome mode with charging and developing bias reduction

An image forming apparatus configured to form an image on a sheet, comprising:

• a first photosensitive drum;
• a first developing roller movable between a contact position at which the first developing roller contacts the first photosensitive drum and a separation position which is apart from the contact position;
• a first charger configured to charge the first photosensitive drum;
• a second photosensitive drum;
• a second developing roller;
• a second charger configured to charge the second photosensitive drum;
• a moving mechanism configured to move the first developing roller between the contact position and the separation position;
• and
• a controller configured to switch a printing mode between
• a color mode in which the first and second developing rollers are at contact positions at which the first and second developing rollers contact the first and second photosensitive drums, respectively, and
• a monochrome mode in which the first developing roller is at the separation position and the second developing roller is at the contact position,
• wherein the controller is configured to:
• determine whether an image forming in the color mode is performed on a first sheet and then an image forming in the monochrome mode is successively performed on a second sheet;
• and
• in response to determining that the image forming in the color mode is performed on the first sheet and then the image forming in the monochrome mode is successively performed on the second sheet,
• perform charging bias reduction processing and developing bias reduction processing during a first inter-sheet period after the first sheet passes the first photosensitive drum and before the second sheet reaches the first photosensitive drum,
• the charging bias reduction processing being processing of gradually reducing a charging bias applied to the first charger from a first charging bias to a second charging bias,
• the developing bias reduction processing being processing of gradually reducing a developing bias applied to the first developing roller from a first developing bias to a second developing bias;
• and
• perform separation processing such that an execution period of the separation processing overlaps at least one of an execution period of the charging bias reduction processing and an execution period of the developing bias reduction processing, the separation processing being processing of controlling the moving mechanism to move the first developing roller from the contact position to the separation position.