Conventionally, in the image forming apparatus using an electrophotographic method, a single-component developing method, which uses only a toner as a developer and a two component developing method using a toner and a carrier have been known as development methods of an electrostatic latent image formed on an image carrying member. The single-component developing method generally has good image quality. In general, since a thin layer of toner is formed by a regulating plate urged against a toner carrying member and a toner carrying member, it has advantages in simplification of the apparatus, miniaturization and cost reduction. However, the single-component developing method has a shortcoming that a toner is easily degraded by strong stress of a regulation section for charging toner, and the charge-receiving property of a toner is thus rapidly deteriorated. Therefore, a service life of a developing apparatus becomes comparatively short.
Since in the two component developing method, a toner is charged by triboelectric charging by mixing the toner with carrier, it has an advantage that stress given to the toner is small. However, it has problems that a magnetic brush formed of carrier contacts and affects an electrostatic latent image, and carrier adheres on a background of an image.
A hybrid development method, which makes use of the strong points of the single-component developing method and the two component developing method, is proposed in Japanese Patent Application Publication No. H03-113474. A developer is made of the two-component developer containing a carrier and a toner, and the toner is charged by being stirred within a developer container. This developer is conveyed by the developer carrying member (magnet roller), and the toner is adhered onto the surface of the toner carrying member by an electric field. With the toner on the toner carrying member, an electrostatic latent image on an image carrying member is developed. In this hybrid development method, the toner is charged by being mixed with the carrier, and since only the toner performs development, high definition and endurance are both attainable.
However, the hybrid development method has a problem called a development history (ghost), which is a phenomenon that an afterimage of residual toner, which was not used for development, on the toner carrying member appears with a density difference after the following development process.
As a method of solving the problem in this hybrid development method, Japanese Patent Application Publication No. H10-319708 discloses, an apparatus, which is provided with a toner-collecting developer-carrying member for collecting residual toner on the toner carrying member in addition to a toner-supplying developer-carrying member for supplying the toner to the toner carrying member. In this apparatus, the toner-supplying developer-carrying member and the toner-collecting developer-carrying member are closely arranged. And after supplying the toner to the toner carrying member from the toner-supplying developer-carrying member, the developer, in which the toner density fell, is delivered to the toner-collecting developer-carrying member. The toner-collecting developer-carrying member collects the residual toner on the toner carrying member effectively by using this developer (carrier). AS described above, the residual toner is effectively collected from the developer carrying member, and then the developer carrying member is supplied with the toner to be used for the next development, thereby solving the problem of the development history.
However, in the apparatus disclosed by Japanese Patent Application Publication No. H10-319708, the developer (carrier) exists between the toner-supplying developer-carrying member and the toner-collecting developer-carrying member, and there is a problem that leakage current occurs through this developer when under a high-humidity environment. When a large leakage current occurs, toner supply to the toner carrying member and the residual toner collection from the toner carrying member cannot be normally performed, and a problem that a stable image cannot be formed will arise.
In view of the above problems, an object of the present invention is to provide a developing apparatus and an image forming apparatus, which control occurrence of leakage current under a high-humidity environment, which are capable of forming a stable and high definition image while employing a hybrid development method and having a toner-collecting developer-carrying member.