Image forming apparatus which prevents toner images from sticking to each other

An image forming apparatus comprising:

• an image forming unit configured to form toner images onto a recording sheet based on image data;
• a fixing device configured to thermally fix the toner image onto the recording sheet;
• a stack tray which a recording sheet conveyed from the fixing device is discharged to and the recording sheet discharged is stacked on;
• and
• a controller configured to compute an amount of a toner on a first surface of a first recording sheet to be stacked on the stack tray and an amount of a toner on a second surface of a second recording sheet to be stacked on the stack tray subsequent to the first recording sheet when a two-sided printing mode in which images are formed on two sides of a recording sheet is set, the second surface of the second recording sheet coming into contact with the first surface of the first recording sheet on the stack tray, and to control a recording sheet discharge interval between discharging the first recording sheet onto the stack tray and discharging the second recording sheet onto the stack tray based on a combination of computed amounts of toners on the first surface and on the second surface,
• wherein, in order to determine the recording sheet discharge interval, the controller compares a total value of the amount of the toner on the first surface of the first recording sheet and the amount of the toner on the second surface of the second recording sheet with a second threshold value in a case where the amount of the toner on the second surface of the second recording sheet is equal to or smaller than a first threshold value, and the controller compares the total value with a third threshold value smaller than the second threshold value in a case where the amount of the toner on the second surface of the second recording sheet is larger than the first threshold value.