(1) Field of the Invention
The present invention relates to a developing unit which is used in an image forming apparatus for forming images based on electrophotography and which develops a static latent image into a developed image by supplying the developer to the photosensitive member by way of a developer support having a developer magnetically attracted to the peripheral surface thereof.
(2) Description of the Prior Art
An image forming apparatus for forming images based on electrophotography has a developing unit which supplies a developer to the surface of the photosensitive member after it has been exposed to light of an image. The developer supplied from the developing unit develops the static latent image which has been formed on the surface of the photosensitive member by the photo-conducting effect into a developer image. For example, a typical developing unit which develops the static latent image using a dual-component developer consisting of a toner powder of thermoplastic resin and a magnetic carrier powder is comprised of a cylindrical developing sleeve that incorporates fixed magnets and rotates opposing the surface of the photosensitive member and an agitator for agitating the dual component developer stored therein.
Inside the developing unit, the toner which has been agitated by the agitator electrostatically adheres to the surface of the carrier while the carrier having the toner electrostatically adhering on the surface thereof is mainly magnetically attracted to the surface of the developing sleeve under the magnetic fields of the fixed magnets. The carrier attracting the toner thereon is conveyed by the rotation of the developing sleeve to the developing position where the sleeve opposes the surface of photosensitive member. When the toner on the surface of the developing sleeve faces the static latent image on the surface of the photosensitive member, it transfers from the carrier to the static latent image on the surface of the photosensitive member while the carrier alone remains attracted to the surface of the developing sleeve and returns into the developing unit. Accordingly, the adherence of the toner becomes irregular in the axial direction of the developing sleeve after the sleeve has passed by the surface of the photosensitive member. If this situation is left as it is, it will produce density unevenness in the image causing degradation of image quality in subsequent image formation.
In general, in a developing unit, two magnetic poles of a like polarity are arranged adjacent to each other as the fixed magnets incorporated inside the developing sleeve, on the downstream side of the developing position with respect to the rotational direction of the developing sleeve so that a repulsive magnetic field may be generated between these two magnetic poles to temporarily remove the dual-component developer from the developing sleeve. Then the dual-component developer thus removed from the developing sleeve is agitated by an agitator arranged in the repulsive magnetic field to make the toner concentration uniform, in the axial direction of the developing sleeve. Thereafter the dual-component developer is re-attracted to the surface of the developing sleeve by the magnetic pole which is positioned on the more downstream side.
However, it is difficult to completely remove the dual-component developer from the surface of the developing sleeve by only the action of the repulsive magnetic field which is generated by the fixed magnets inserted within the developing sleeve, so some part of the dual-component developer having passed through the developing position cannot be removed from the surface of the developing sleeve and may be conveyed again to the developing position for the subsequent image formation. In such a case, it is impossible to completely negate image density unevenness due to irregularity of the toner concentration. A dual-component developer adheres to the surface of the developing sleeve not only by magnetic attraction but also by electrostatic attraction. For example, if a dual-component developer is composed of a small-sized toner having a mean particle diameter of about 4 to 10 .mu.m and a small-sized carrier having a mean particle diameter of about 20 to 50 .mu.m, the static charge per unit weight of the toner or the carrier becomes relatively high so that the electrostatic attraction of the developer to the surface of the developing sleeve becomes stronger, thus further making it difficult to remove the dual-component developer from the surface of the developing sleeve.
When as in the above way the dual-component developer having passed through the developing position cannot be removed from the developing sleeve and is repeatedly used for image formation, it is impossible to avoid degradation of image quality due to image density unevenness. In particular, the reproducibility of the image during image formation at a high resolution using a micro-toner will be deteriorated. Further, the carrier which has remained attracted to the surface of the developing sleeve and hence has continuously received mechanical loads, so that the dual-component developer becomes degraded at an early stage, which poses problems of a further lowering of the image density and fogging in the image.
To deal with these problems, in a conventional developing unit, for example, Japanese Patent Application Laid-Open Hei 6 No.194962 has offered a configuration in which a magnetic member is arranged in the repulsive magnetic field formed by the fixed magnets within the conveyance path of the dual-component developer attracted to the surface of the developing sleeve so that it is magnetized by magnetic induction of the fixed magnets, whereby the dual-component developer is fully removed from the surface of the developer support by using a magnetic brush formed between the developer support and the magnetic member.
Another method has also been proposed in which a cleaning member is arranged so as to abut the surface of the developing sleeve in the repulsive magnetic field formed by the fixed magnets within the conveyance path of the dual-component developer attracted to the surface of the developing sleeve in order to mechanically remove the dual-component developer from the surface of the developing sleeve.
However, when a magnetic member or cleaning member is laid out in the conveyance path of the dual-component developer, they will hinder the normal flow state of the dual-component developer in the developing unit. So it becomes impossible to uniformly attract an adequate amount of the dual-component developer to the surface of the developing sleeve, thus defeating the object and contributing to disadvantages such as degradation of image quality, etc. In particular, in the configuration in which a cleaning member abutting the surface of the developing sleeve removes the dual-component developer from the surface of the developing sleeve, the dual-component toner receives heavy mechanical loads, which not only causes toner aggregation and early degradation of the dual-component developer but also a considerable increase of the driving torque of the developing sleeve and/or frictional heat due to rubbing between the developing sleeve and the cleaning member.
Since in all the conventional configurations it is assumed that a repulsive magnetic field generated by the fixed magnets inserted within the developing sleeve is used in order to remove the dual-component developer from the surface of the developing sleeve, if the developing sleeve is made smaller as to its diameter with the demand for making the image forming apparatus compact, high precision is needed for setting the polarizing positions of the fixed magnets, resulting in increased cost.
These problems occur not only in developing units using a dual-component developer containing a toner and a carrier but also in a developing unit which magnetically holds a mono-component developer of a magnetic toner on a developer support and supplies the photosensitive member with it.