Image forming apparatus including multiple photosensitive drums and multiple developing rollers

An image forming apparatus, comprising:

• a first photosensitive drum;
• a first developing roller configured to supply toner to the first photosensitive drum,
• the first developing roller being movable between:
• a first contact position where the first developing roller is in contact with the first photosensitive drum;
• and
• a first separated position where the first developing roller is separated from the first photosensitive drum;
• a first cam configured to rotate, thereby moving the first developing roller between the first contact position and the first separated position;
• a second photosensitive drum;
• a second developing roller configured to supply toner to the second photosensitive drum,
• the second developing roller being movable between:
• a second contact position where the second developing roller is in contact with the second photosensitive drum;
• and
• a second separated position where the second developing roller is separated from the second photosensitive drum;
• a second cam configured to rotate, thereby moving the second developing roller between the second contact position and the second separated position;
• a sheet tray in which sheets are accommodated;
• a supplying mechanism configured to supply the sheets from the sheet tray;
• a sheet sensor configured to detect passage of the sheet at an upstream position, in the sheet moving direction, with respect to the first and second photosensitive drums;
• and
• a controller,
• wherein, when the first cam is continuously rotating, the first developing roller reciprocally moves between the first contact position and the first separated position periodically at every first time period;
• wherein, when the second cam is continuously rotating, the second developing roller reciprocally moves between the second contact position and the second separated position periodically at every second time period;
• wherein the second time period is shorter than the first time period;
• wherein, in case where a first sheet and a subsequent second sheet are subjected to printing,
• the controller is configured to:
• determine a sheet interval between the first sheet and the second sheet based on a timing when the sheet sensor detects the first sheet and a timing when the sensor detects the second sheet;
• when the sheet interval is equal to or greater than a particular threshold interval, position both the first developing roller and the second developing roller to the first and second separated positions, respectively, after developing of images to be transferred onto the first sheet has been completed, and thereafter, position at least one of the first developing roller at the first contact position and the second developing roller at the second contact position, the at least one of the first developing roller and the second developing roller to be used for printing on the second sheet, and start developing images to be transferred on the second sheet;
• and
• when the sheet interval is less than the particular threshold interval, keep at least one of the first developing roller at the first contact position and the second developing roller at the second contact position to be used for printing on the second sheet, and start developing an image to be transferred onto the second sheet.