Electrostatic image forming apparatus utilizing dual chargers to clean transfer belt

An image forming apparatus comprising:

• an image bearing member for bearing a toner image;
• a rotatable intermediary endless belt;
• a plurality of stretching rollers for stretching said intermediary endless belt;
• a primary transfer device, which forms a primary transfer nip between said image bearing member and said intermediary endless belt, for primary-transferring the toner image from said image bearing member onto said intermediary endless belt at the primary transfer nip;
• a collecting device for collecting the toner on said image bearing member;
• a secondary transfer device for secondary-transferring the toner image from said intermediary endless belt onto a transfer material;
• a first charging device, movable between a first contact position in which said first charging device is contacted to said intermediary endless belt and a first separation position in which said first charging device is separated from said intermediary endless belt, for electrically charging residual toner remaining on said intermediary endless belt at the first contact position;
• a second charging device, movable between a second contact position in which said second charging device is contacted to said intermediary endless belt and a second separation position in which said second charging device is separated from said intermediary endless belt, for electrically charging the residual toner remaining on said intermediary endless belt at the second contact position, wherein said second charging device is provided downstream of said first charging device with respect to a rotational direction of said intermediary endless belt;
• and
• a switching unit for switching positions of said first charging device relative to said intermediary endless belt and positions of said second charging device relative to said intermediary endless belt,
• wherein said image forming apparatus is capable of executing a collecting mode in which the residual toner electrically charged by said first and second charging devices is moved from said intermediary endless belt to said image bearing member in at the primary transfer nip and then is collected by said collecting device,
• wherein said switching unit moves, when said first and second charging devices are to be moved from the first and second separation positions to the first and second contact positions, said first charging device from the first separation position to the first contact position at a timing when said first charging device opposes a non-image transfer area adjacent to an image transfer area and then moves said second charging device from the second separation position to the second contact position at a timing when said second charging device opposes the non-image transfer area,
• wherein said switching unit moves, when said first and second charging devices are to be moved from the first and second contact positions to the first and second separation positions, said first charging device from the first contact position to the first separation position at a timing when said first charging device opposes the non-image transfer area and then moves said second charging device from the second contact position to the second separation position at a timing when said second charging device opposes the non-image transfer area,
• wherein one of the stretching rollers is an opposing roller which opposes said first and second charging devices through said intermediary endless belt, and
• wherein said switching unit is a cam provided at the opposing roller.