The present invention relates to a developing apparatus, applied to an electrophotographic device such as an electrophotographic copying machine, for developing an electrostatic latent image by a one-component developing agent.
Conventionally, developing techniques such as a cascade technique and a magnet brush technique are mainly used in electrophotographic devices such as an electrophotographic copying machine, a facsimile device, and a printer. However, since color recording is popular in recent years, a technique of developing in a noncontact state an image on a photosensitive body capable of superposing and developing an unfixed image has been studied. A basic arrangement of such a noncontact type developing technique is disclosed in U.S. Pat. Nos. 3,232,190, 3,866,574, and 3,893,418 or in G.B. Pat. No. 1,458,766. In these disclosed apparatuses, a developing roller is arranged close to a photosensitive drum at a small gap of about 5 to 500 .mu.m, and a thin uniform toner layer is formed on a cylindrical surface of the developing roller. A DC or AC voltage is applied between the developing roller and the photosensitive drum to produce a DC or AC electric field in the gap therebetween. As a result, toner particles fly and become adhered to an electrostatic latent image on the photosensitive drum where the electric field intensity is high. In a non-image portion, flight of toner particles is restricted, and in the AC electric field, toner particles return to the developing roller. Thus, toner is selectively adhered on the photosensitive drum to develop an electrostatic latent image. On the other hand, in U.S. Pat. No. 3,893,418, gradation reproducibility is selected by switching a frequency utilizing the fact that gradation of a developed image varies in accordance with a frequency of an applied AC voltage.
A most important point in these techniques is to uniformly form a toner layer on the developing roller and to uniformly charge the toner. In consideration of this situation, the present inventors have proposed improved techniques of forming a toner layer in Japanese Patent Disclosure (Kokai) No. 59-31979, or Japanese patent application Nos. 58-143340 and 58-161241. In these techniques, since an elastic blade is abutted against a developing roller to form a toner layer, a toner layer can be uniformly formed on a developing roller with a very small and simple arrangement.
When a negative electrostatic latent image is to be developed or a positive electrostatic latent image is to be reversely developed, a toner which is charged to have a positive polarity is used. In this case, in order to improve a frictional charging property with respect to a toner, a charge control agent or a resin is added to a toner. However, as for a positively charged toner, no such charge control agent or resin having good properties is present, and a charge amount of a toner layer formed on a developing roller is unstable. As a result, fogging of an image tends to occur, and a layer formation characteristic is degraded under humid conditions. Therefore, when a toner having a positive polarity is used, an electrophotographic characteristic is poor as compared with a case wherein a toner having a negative polarity is used. Although such a situation does not change in a two-component developing method, the above drawbacks are typical in a one-component developing method, so that a strong demand has arisen for a drastic countermeasure.