Electrophotography – Image formation – Development
Reexamination Certificate
2001-07-31
2003-01-28
Ngo, Hoang (Department: 2852)
Electrophotography
Image formation
Development
C399S267000, C399S272000, C399S265000, C399S266000
Reexamination Certificate
active
06512909
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus such as copying machine, printer, facsimile, and combination of these using electrophotographic process, and more particularly concerns an image forming apparatus adopting a non-contact development method, wherein two component developer utilizing a magnetic carrier to charge non-magnetic toner is used, only charged toner is held on a developer roll(donor roll), and the electrostatic latent image on an electrostatic latent image carrier(hereafter referred to as photoreceptor) is developed by allowing the toner to jump thereto.
2. Description of the Related Art
Non-contact development methods have been studied as a means for developing with single component developer. Through the years, they have been studied for high speed image forming apparatuses, for example, for color superimposing on one drum in which a plurality of color images are formed sequentially thereon.
By color superimposing on one drum, it is possible to form a color image with little color drift by accurately superposing toner on a photoreceptor, so the methods have received attention as a technical art suitable for high quality color image forming.
An example of conventional non-contact development is disclosed in U.S. Pat. No. 3,866,574. According to the disclosure, a thin layer of non-magnetic toner is formed on a donor roll(developer roll), the roll being positioned not contacting with the photoreceptor and the toner being allowed to jump to the latent image on the photoreceptor by applying alternating voltage.
Another example of conventional non-contact developing method is disclosed in U.S. Pat. No. 3,929,098. In the disclosure, a development apparatus wherein two component developer is advanced to a donor roll using a magnetic roll and the toner is transported on to the donor roll to form a thin layer of the toner thereon is described.
In this example, two component developer is adopted, and although the formation of thin layer on the donor roll is possible, the removal of the toner on the donor roll is difficult when the charge potential of toner is high, so a strong alternating voltage is necessary to be applied for the removal of the toner. But, since the strong alternating voltage unsettles the thin layer of toner on the photoreceptor, it is not suited for color superimposition.
With the conventional arts mentioned above, the deviation of potential after exposure which is the potential of the photoreceptor right after the exposure varies largely relying on the environmental conditions. As a result, a high surface potential of the photoreceptor is required, and the potential of development electric field has been inevitably set high.
Further, the control of toner charge is complicated, and a high bias voltage(development voltage) is required for the developer roll.
Therefore, consumed region and not-consumed region of toner are developed on the developer roll, potential difference between the toner adhered to the roll and that freshly supplied thereon is easily produced resulting in a hysteresis phenomenon(memory phenomenon), that is, a phenomenon in which the ghost image of the preceding image development appears superposed on the present image.
Still further, since negatively chargeable toner is generally used in the conventional arts, there has been a tendency that, when the toner is repeatedly exposed to a high electric field, the potential difference between the charge potential of the toner in a development region and that of the toner in a non-development region increases, especially in low temperature, low humidity environment. As a result, the tendency has been toward occurrence of conspicuous ghost image.
To prevent the occurrence of hysteresis phenomenon, an apparatus having a member for scraping the developed toner on the developer roll and a device for recovering scraped toner, is disclosed in Japanese Unexamined Patent Publication No.11-231652. But, the provision of the scraping member induces strong physical and electrical stress in the toner, which causes the deterioration of the toner.
Further, a so-called powder cloud development process is proposed in Japanese Unexamined Patent Publication No.3-113474. This powder cloud development enables color superimposition without unsettling the developed toner by providing an auxiliary electrode composed of wires between a donor roll and a photoreceptor and applying a weak alternating voltage to the auxiliary electrode. But, with this art, the auxiliary electrode is liable to be contaminated, and there is a tendency that image degradation occurs when the wires vibrate.
Theoretical investigation made on the formation of thin layer of toner on a developer roll in a touchdown development using two component developer is described in the Journal of Institute of Electrophotography, vol.19, No.2(1981), pp.44-51.
But, with the touchdown development, the replacement of the remaining developed toner with supply toner is not easy, and a selective development phenomenon might occur resulting in a low development performance.
Further, in Japanese Unexamined Patent Publication No.7-72733, a method of stabilizing toner charge by recovering the toner on a developer roll onto a magnetic roll through reversing the polarity of the potential difference between the developer roll and the magnetic roll during the intermediate zone between copying of one sheet and subsequent one, is described. But, when the polarity of the potential difference is inverted, the charge of the toner changes and so-called “fog” might occur.
Further, in said Journal of Institute of Electrophotography is disclosed a drawing shown in
FIG. 10
in which toner supply capability &thgr; is defined as &thgr;=n·(Vm/Vd), and is described that the toner supply capability can be increased by increasing the peripheral speed Vm of the magnetic brush of the magnetic roll relative to the peripheral velocity Vd of the developer roll.
According to U.S. Pat. No. 5,063,875, the peripheral speed of the magnetic roll is set to 2 to 5 times faster than that of the developer roll, and according to Japanese Unexamined Patent Publication No.11-231652, the speed ratio is set to 2 to 3.
However, there have been problems that, when the peripheral speed of the magnetic roll is increased relative to that of the developer roll, the rotation torque of the magnetic roll rotating with magnetically attracting carrier thereon increases, deterioration of the carrier is accelerated by collisions of carrier granules thmselves, the toner impinges on the regulating blade for regulating the height of magnetic brush and scatters resulting in lessened transported amount of toner to the developer roll side and w increased agitation of toner which increases Q/M of toner(the charge of toner per unit mass). As result, the electric adhesion force of the toner to the developer roll is increased resulting in decreased toner quantity jumping onto the photoreceptor. Accordingly, sufficient image density can not be obtained.
In particular, in a color image forming apparatus in which a plurality of colors are superimposed by arranging a plurality of photoreceptors and development devices in sequence in the transfer direction of recording medium(recording sheet or intermediate transfer member), there is a time lag between the time the image transfer start position on the recording medium reaches the preceding photoreceptor and the time the same reaches the succeeding photoreceptor for color superimposition.
Although to compose the apparatus so that the start position of each color superimposition coincides, is possible by retarding the mechanical driving of the succeeding photoreceptor and development device, construction and control becomes complicated. In addition, in the case of a high speed apparatus, there is a time lag until the specified speed is reached, and a high-level technique is required to allow the photoreceptor and developer roll to reach the specified speed in a short time.
To eliminate the time lag until
Ochiai Eiji
Ozawa Yoshio
Sakata Shoichi
Hogan & Hartson LLP
Kyocera Corporation
Ngo Hoang
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