Image forming apparatus having a remaining developer circulation mechanism

An image forming apparatus comprising:

• a plurality of photosensitive members;
• at least one retaining member positioned in contact with a corresponding photosensitive member and configured to temporarily retain remaining developer remaining on this photosensitive member;
• a belt arranged opposite to the plurality of photosensitive members;
• a cleaning device positioned in contact with the belt and configured to collect developer on the belt;
• a remaining developer collecting unit configured to cause the remaining developer retained by the retaining member to be transferred onto the belt via the photosensitive member and thereafter to be collected by the cleaning device;
• a plurality of developing units;
• and
• a plurality of developer cartridges disposed above and detachably mounted to corresponding developing units,
• wherein each of the plurality of developing units comprises:
• a first housing having a first opening and a second opening;
• a development chamber formed in the first housing;
• a developing roller disposed in the development chamber and positioned in contact with a corresponding photosensitive member to supply developer to the photosensitive member:
• a supply carrier mechanism arranged in the development chamber and configured to carry developer in a direction from the first opening to the second opening and to supply developer to the developing roller;
• and
• a first shutter configured to be movable to open and close the first opening, and
• wherein each of the plurality of developer cartridges comprises:
• a second housing having a third opening and a fourth opening, the third opening being formed in a position facing the first opening and the fourth opening being formed in a position facing the second opening;
• a developer storage chamber formed in the second housing and configured to store developer, wherein the first and third openings provide a path through which the developer is able to flow from the developer storage chamber to the development chamber of the developing unit, and wherein the second and fourth openings provide a path through which the developer is able to flow from the development chamber to the developer storage chamber;
• a feed out carrier mechanism arranged in the developer storage chamber and configured to carry developer in a direction from the fourth opening to the third opening and to feed developer out from the developer storage chamber and into the development chamber;
• a collection carrier mechanism positioned in proximity to the fourth opening and configured to carry developer from the development chamber to the developer storage chamber;
• and
• a second shutter configured to be movable to open and close the third opening in synchronization with the first shutter.