1. Field of the Invention
The present invention relates to a developing device used in an image forming apparatus such as an electrophotographic copier, a printer, a facsimile, and a composite apparatus having functions of those devices, and to an image forming apparatus provided with the developing device. In particular, the present invention relates to a developing device which replenishes a two-component developer including toner and carrier and discharges surplus developer and to an image forming apparatus provided with the developing device.
2. Description of Related Art
In image forming apparatuses, an electrostatic latent image formed on an image carrier including a photosensitive member and the like is developed by a developing device and visualized as a toner image. Examples of the developing device include one employing a two-component developing method in which a two-component developer is used. The developing device of this type includes a developing container in which a two-component developer including toner and carrier is stored, and there are arranged a developing roller for supplying the developer to the image carrier and a stirring member for supplying the developer in the developing container to the developing roller while stirring and conveying the developer.
In the developing device, the toner is consumed by a developing operation; meanwhile, the carrier remains in the developing device without being consumed. Accordingly, the carrier stirred together with the toner in the developing container is deteriorated in proportion to a stirring frequency. As a result, charging performance of the carrier with respect to the toner is gradually deteriorated.
In this context, there has been well-known a developing device according to a first related art in which deterioration of charging performance is suppressed by replenishment of developer including carrier into a developing container and discharge of surplus developer.
In the developing device according to the first related art, two stirring members each including a rotary shaft and a helical blade helically formed about an outer periphery of the rotary shaft are arranged in parallel with each other in respective conveyance paths. A partition portion is provided between the conveyance paths, and communication portions for exchanging developer are provided to both end portions of the partition portion. A developer discharge port is provided on a downstream side of the conveyance path with respect to a developer conveying direction. Between the stirring member and the developer discharge port, a reverse helical blade helically formed in a direction reverse to that of the helical blade of each of the stirring members is provided as a discharge regulating portion integrally with the rotary shaft. With this structure, when being replenished into the developing container, developer is conveyed to the downstream side of the conveyance path while being stirred by rotation of the stirring members. When being rotated in the same direction as that of the stirring members, the reverse helical blade imparts a conveyance force in a direction reverse to the developer conveying direction due to the stirring members to the developer. The developer is retained by the conveyance force in the reverse direction on the downstream side of the conveyance path and increased in height. As a result, surplus developer climbs over the reverse helical blade (discharge regulating portion) so as to move to the developer discharge port, with the result of being discharged outside.
However, in the developing device according to the first related art, even when new developer is not replenished, the developer conveyed by the helical blade of each of the stirring members moves to the downstream side of the conveyance path in an undulating manner in conformity with an outer periphery of the helical blade, with the result of colliding against the discharge regulating portion. When the developer collides against the discharge regulating portion, a height of the developer with respect to an outer periphery of the discharge regulating portion varies from each other in accordance with axial positions of the helical blade with respect to the discharge regulating portion. When the developer collides against the discharge regulating portion at a position at which the developer has height, the developer climbs over the discharge regulating portion so as to move into the developer discharge port by an impact of the collision. As a result, the developer is excessively discharged, which may lead to a risk of instability of the developer amount in the developing container. In particular, in an apparatus performing high-speed image formation, the stirring members are rotated at high speed together with the photosensitive member, and hence there prominently occurs an inconvenience of excessive discharge of the developer.