1. Field of the Invention
The present invention relate to a two-color image forming apparatus.
2. Description of the Prior Art
Generally, a two-color image forming apparatus employs a process of sequentially performing two-color developments. When the second color development is conducted, it is important from the viewpoint of the picture quality that the second color development is conducted without disturbing a visible image of the first color which has been formed.
The means proposed for the above purpose are as follows:
(1) The direction of transportation for the magnetic brush in the magnetic brush developing apparatus of a fixed magnetic roll and rotating sleeve type for performing the second color development is made the same as the moving direction of a photosensitive body around the development section and made similar in speed to the rotation of the photosensitive body, thereby causing brushing or frictional power of the magnetic brush against the photoconductive body surface weaker than the brushing power of the magnetic brush for the first color.
(2) The density of the magnetic flux of the main pole of the development of the magnetic roll in the developing apparatus for second color is made smaller than that of the magnetic roll of the development apparatus for the first color.
(3) The space between the magnetic roll of the developing apparatus for the second color and the surface of the photoconductive body is made longer than that between the magnetic roll of the developing apparatus for the first color and the surface of the photoconductive body.
(4) Two-component developing agents including an insulating toner are used for the development of the second color and the magnetic roll and sleeve are rotated in the same direction.
However, according to the means (1)-(3), the development condition is deteriorated, a sufficient density of the color cannot be obtained and the picture quality of the second color is lowered. According to the means (2), the magnetic power is weakened and a carrier attachment to the photoconductive body occur easily. Further, according to the means (4), the development agent is transported onto the sleeve, being subjected to a self rotation and the transportation of the developer can be performed in a gentle manner. Therefore, the visible image of the first color is not disturbed and the means (4) is effective for the development of the second color. However, the effectiveness of the transportation is not sufficient and the magnitude of the carrier is limited. Therefore, when the potential difference of the electrostatic latent images between positive and negative polarities corresponding to each color is determined large, the carrier attachment easily occurs, especially around the edge section. The means for causing the magnetic roll and sleeve to rotate in the same direction necessitates a high accuracy of a parts position matching as the allowance of the blade gap and the development gap is small and scatters a development or a developer to a large extent, for the transportation directions of the developing agent on the sleeve are opposite to from two layers of the transportation.
As to the problem that a carrier of the development agent is attatched to the visible image of the other color, it is proposed that the electric resistivity of a sleeve upon the first color development is made different from that upon the second color development and that the volume electric resistivity and the particle radius of a magnetic carrier of a development agent are different between the first color development and the second color development. However, the actual performance of the above proposals does not produce desired effects.