An image forming apparatus for forming an image on a recording sheet includes a developing device for supplying a developing agent to a surface of a photosensitive element. The developing device is generally configured to have a developing agent storage chamber and a developing chamber that can communicate with each other via a supplying port. A developing agent supplying roller and a developing roller are accommodated in the developing chamber.
The image forming apparatus, in some cases, has to dispose the developing device therein such that the developing agent storage chamber is located above the developing chamber, as disclosed, for example, in JP-A-2001-166556. The image forming apparatus disclosed in JP-A-2001-166556 adopts a horizontal tandem-type image forming system in which plural photosensitive drums are aligned in a horizontal direction and in parallel to one another to form a color image on a recording sheet by developing agents. The horizontal tandem-type image forming system suffers from a problem in that the length becomes long in a direction in which plural photosensitive drums are aligned.
To solve this problem, the developing devices which correspond, respectively, to the photosensitive drums have to reduce their dimensions in the direction in which the photosensitive drums are aligned. To this end, the developing agent storage chamber, which is normally located adjacent to the developing chamber, is located above the developing chamber as disclosed in JP-A-2001-166556.
The developing device disclosed in JP-A-2001-166556 can use gravity to downwardly move the developing agent from the developing agent storage chamber to the developing chamber, thereby supplying the developing agent to the feeding roller and the developing roller.
However, since the developing agent is only moved downwardly from the developing agent storage chamber to the developing chamber in a one-way direction, repetition of development results in increase in a ratio of deteriorated developing agent in the developing chamber. In particular, by fresh developing agent being additionally supplied and forced into the developing chamber downwardly from the developing agent storage chamber, the deteriorated developing agent is accumulated at longitudinal end portions of the developing roller in the developing chamber. The accumulated developing agent is likely to lower the quality of an image considerably at both end portions of the image, in particular, when a color image is formed.
Not only the horizontal tandem-type color laser printer but also a monochrome laser printer suffers from the same problem when a developing agent storage chamber is located above a developing chamber.
Since the problem of deterioration of a developing agent is caused by subjecting the developing agent to sliding friction, the problem commonly can occur over developing devices in which a developing agent is charged through friction charging. In the event that the developing agent is deteriorated, phenomena such as a reduction in fluidity due to the embedment of externally added agents which are externally added to the developing agent and a reduction in charging quantity due to a reduction in charging function are generated. This phenomena may cause rough and scattered printing and in worst case, background fogging, on an image formed on a recording sheet.
When a fresh and non-deteriorated developing agent is added to the deteriorated developing agent whose charging function is reduced, a charging failure is caused in the deteriorated developing agent depending upon conditions, and this may cause a situation in which the background fogging is worsened.