This application is based on applications Nos. 47898/1999, 47899/1999, 68562/1999, 68563/1999 and 89609/1999 filed in Japan, the contents of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a developing apparatus used for developing an electrostatic latent image formed on an image carrying member in an image forming apparatus such as a copying machine or a printer, and more particularly, to a developing apparatus in which a developer carrying member for holding a developer and an image carrying member having an electrostatic latent image formed thereon are provided so as to be opposite to each other at a required distance and so adapted that an AC voltage is applied to the developer carrying member to exert an AC electric field between the developer carrying member and the image carrying member, and the developer held in the developer carrying member is supplied to the electrostatic latent image formed on the image carrying member to perform development, characterized in that the density of an image to be formed does not greatly vary even when a distance at which the developer carrying member and the image carrying member are opposite to each other varies, thereby obtaining a good image having a stable image density.
2. Description of the Related Art
In an image forming apparatus such as a copying machine or a printer, various developing apparatuses have been conventionally used for developing an electrostatic latent image formed on an image carrying member.
Known as such a developing apparatus have been a developing apparatus of a contact development type, in which a developer carrying member is provided so as to be brought into contact with an image carrying member, and a developer is introduced into a developing area in contact with the image carrying member by the developer carrying member to perform development, and a developing apparatus of a non-contact development type, in which a developer carrying member is provided so as to be opposite to an image carrying member at a required distance, a developer is introduced into a developing area opposite to the image carrying member by the developer carrying member, an AC voltage is applied to the developer carrying member to exert an AC electric field between the developer carrying member and the image carrying member, and therefore the developer is supplied to an electrostatic latent image formed on the image carrying member to perform development.
The developing apparatus of a contact development type is superior in reproducibility of an electrostatic latent image formed on the image carrying member because the developer is brought into contact with the image carrying member to perform development. However, there are some problems. For example, the developer also adheres to a non-image portion where no electrostatic latent image is formed, so that an image to be formed is fogged, and the surface of the image carrying member is worn by the developer carrying member.
On the other hand, in the case of the developing apparatus of a non-contact development type in which the developer carrying member is provided so as to be opposite to the image carrying member at a required distance, the above-mentioned problems in the developing apparatus of a contact development type are reduced.
In the case of the developing apparatus of a non-contact development type, however, the distance at which the image carrying member and the developer carrying member are opposite to each other may, in some cases, vary due to inferior forming precision, mounting precision, and so forth of the image carrying member and the developer carrying member. When the distance at which the developer carrying member and the image carrying member are opposite to each other is decreased, the AC electric field exerted between the developer carrying member and the image carrying member is strengthened, so that the density of an image to be formed is increased. On the other hand, when the distance at which the developer carrying member and the image carrying member are opposite to each other is increased, the AC electric field exerted between the developer carrying member and the image carrying member is weakened, so that the density of the image to be formed is decreased. Consequently, the density of the image to be formed becomes non-uniform, thereby making it possible to obtain a good image having a stable image density.