BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an image forming apparatus by using an electrophotographic system.

Description of the Related Art

In an image forming apparatus that forms an image on a recording material using an electrophotographic system such as a copier, a printer, and a facsimile machine, a configuration including a developing apparatus for visualizing an electrostatic latent image with nonmagnetic one-component toner has been known. As such a developing apparatus, there has been known one including a developing roller serving as a developer bearing member that bears and transports toner and a supply roller serving as a developer supply member that is arranged around the developing roller and supplies the toner to the developing roller. In the developing apparatus, the toner is supplied to the developing roller while being friction-charged by the mechanical rubbing between the supply roller and the developing roller. The supplied toner is controlled to have a certain thickness on the developing roller by a developer control member, and then transported to a developing region representing the adjacent region between the developing roller and a photosensitive drum serving as an electrostatic latent image bearing member to visualize an electrostatic latent image as a toner image.

Residual toner on the developing roller (hereinafter called “development residual toner”) that has not been used for developing in the developing region is scraped from the developing roller by the mechanical rubbing between the supply roller and the developing roller at the contact portion between the developing roller and the supply roller. At the same time, the toner is supplied from the supply roller to the developing roller. On the other hand, the scraped toner is mixed with toner inside and around the supply roller. Moreover, there has been generally used a method for applying a bias for generating the potential difference between a developing roller and a supply roller to supply toner from the supply roller to the developing roller and collect the toner from the developing roller with an electrostatic force (Japanese Patent Application Laid-open No. H9-15976). In Japanese Patent Application Laid-open No. H9-15976, there has been proposed a method for performing control to apply a bias for collecting the toner on an intermediate roller corresponding the developing roller during an image forming period and apply a bias for forming a toner layer on the intermediate roller during a non-image forming period.

Meanwhile, for an image formed by a series of image forming operations, an image and density intended by a user need to be output. In addition, in a full-color image generated by a plurality of image forming stations, the reproducibility of a tinge becomes important. Therefore, for the purpose of increasing the selection range of a tinge, there has been generally used a method for changing the rotation speed of a developing roller to change the peripheral velocity ratio of the developing roller with respect to a photosensitive drum (Japanese Patent Application Laid-open No. H8-227222). Hereinafter, an image forming operation in which the amount of supplied toner per unit area from a developing roller to a photosensitive drum is increased to increase density or color gamut will be called “high-density mode.”

SUMMARY OF THE INVENTION

However, when the high-density mode is performed in a configuration as shown in Japanese Patent Application Laid-open No. H9-15976, there is a likelihood that an uneven density image in a supply roller cycle (hereinafter called an “uneven density image”) occurs particularly at the rear end of an image. Such an uneven density image is likely to occur when an elastic sponge is used as the material of a supply roller, i.e., when a configuration includes a supply roller configured to be capable of retaining toner at fine irregularities on the sponge front surface made of a foaming body layer. The amount of the toner necessary for image formation is increased in the high-density mode. For this reason, if the toner is continuously supplied from a supply roller to a developing roller by a bias during the image formation, the toner inside the supply roller (the toner retained by the supply toner) is exhausted. Since the toner is unevenly supplied from the supply roller to the developing roller at this time, a toner layer thickness on the developing roller also become uneven, which results in an uneven density image.

The present invention has an object of providing a technology by which it is possible to reduce the occurrence of an uneven density image when an image forming operation to increase the amount of toner necessary for image formation per unit area is performed.

In order to achieve the above object, an image forming apparatus according to the present invention includes:

a developer bearing member that develops an electrostatic image with a developer, the electrostatic image being formed on an image bearing member; and

a supply member that is arranged in contact with the developer bearing member and supplies the developer to the developer bearing member,

wherein the image forming apparatus is capable of performing

a first image forming operation in which an image is formed at a first peripheral velocity ratio representing a ratio of a peripheral velocity of the developer bearing member to a peripheral velocity of the image bearing member and

a second image forming operation in which an image is formed at a second peripheral velocity ratio, which is greater than the first peripheral velocity ratio, and

a developing bias applied to the developer bearing member and a supply bias applied to the supply member are set such that a urging force in the second image forming operation becomes smaller than that in the first image forming operation, the urging force causing the developer at a contact portion between the developer bearing member and the supply member to move from the supply member to the developer bearing member, by a potential difference between the developing bias and the supply bias.

In order to achieve the above object, an image forming apparatus according to the present invention includes:

a rotatable developer bearing member that develops an electrostatic image with a developer, the electrostatic image being formed on an image bearing member;

a rotatable supply member that is arranged in contact with the developer bearing member and supplies the developer to the developer bearing member; and

a driving portion that rotates the image bearing member and the developer bearing member,

wherein the image forming apparatus is capable of performing

a first image forming operation in which an image is formed at a first peripheral velocity ratio representing a ratio of a peripheral velocity of the developer bearing member to a peripheral velocity of the image bearing member and

a second image forming operation in which an image is formed at a second peripheral velocity ratio, which is greater than the first peripheral velocity ratio, and

a developing bias applied to the developer bearing member and a supply bias applied to the supply member are set such that a urging direction of a urging force in the second image forming operation becomes opposite to that in the first image forming operation, the urging force causing the developer at a contact portion between the developer bearing member and the supply member to move between the developer bearing member and the supply member, by a potential difference between the developing bias and the supply bias.

According to the present invention, it is possible to reduce the occurrence of an uneven density image when an image forming operation to increase the amount of toner necessary for image formation per unit area is performed.

Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

