Electrophotography – Diagnostics – Consumable
Reexamination Certificate
2000-06-21
2002-01-08
Grainger, Quana M. (Department: 2852)
Electrophotography
Diagnostics
Consumable
C399S258000
Reexamination Certificate
active
06337957
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to image forming apparatuses such as copying machines, printers, facsimiles, etc. and a developing device for use in such image forming apparatuses. The present invention more particularly relates to a developing device and an image forming apparatus using the developing device that uses a two component developer including a toner and a magnetic carrier and that includes a self toner density control device to control the toner density of the developer on a developer bearing member so as to be within a fixed range without using a complex toner replenishing device.
2. Discussion of the Background
An example of a developing device that controls the density of toner in the developer on a developer bearing member without using a complex toner replenishing device is one in which the quantity of developer carried on and conveyed by a developer bearing member, that has an internally disposed magnetic field generating device, is regulated by a developer regulating member. The developer scraped off by the developer regulating member is put in a developer accommodating unit, and toner accommodated in a toner accommodating unit is supplied to the developer carried on the developer bearing member through a toner supplying opening, which is adjacent to the developer accommodating unit upstream in a developer conveying direction and which opposes a surface of the developer bearing member (see e.g. Japanese Laid-open Patent publication No. 9-22178). In this developing device, the toner contacting the developer carried on and conveyed by the developer bearing member at the toner supplying opening is taken into the conveyed developer at the interface between the conveyed developer and the developer in the developer accommodating unit. Once the toner density of the developer on the developer bearing member reaches a certain level, the volume of the developer accommodated in the developer accommodating unit starts to swell and thereby the developer in the developer accommodating unit extends so as to cover the toner supplying opening. As a result, the toner is stopped from being taken into the developer on the developer bearing member from the toner accommodating unit at the toner supplying opening. Thus, the toner density of the developer on the developer bearing member is controlled to be within a fixed range.
Also, a developing device is known in which a second developer regulating member is provided between the developer accommodating unit and the toner supplying opening of the toner accommodating unit so as to have a predetermined gap relative to a surface of a developer bearing member (see e.g. Japanese Laid-open Patent No. 9-197833). In this developing device, as the toner density of the developer on the developer bearing member increases, the thickness of a developer layer on the developer bearing member increases, and the increased portion of the developer layer is regulated by the second developer regulating member to pass the second developer regulating member. The developer thus regulated by the second developer regulating member covers the toner supplying opening located adjacent to and upstream of the second developer regulating member in the developer conveying direction, and thereby the toner is stopped from being taken into the developer being carried and conveyed by the developer bearing member. As a result, the toner density of the developer on the developer bearing member is controlled to be within a fixed range.
Such developing devices as noted above that control the toner density of the developer on a developer bearing member to be within a certain range (such a control is hereinafter referred to as a self toner density control) have an advantage in that a toner supplying unit can be simplified, because a toner density sensor is not required, and further a stirring device such as a paddle screw for stirring the developer can be eliminated. In addition, compared to a two component developing method, less magnetic carrier is required. Therefore, torque of the apparatus can be greatly reduced. From these points, the apparatus can be made compact and the cost of the apparatus reduced. Furthermore, because the toner in the toner accommodating unit can be directly supplied to the developer on the developer bearing member passing a developing area, the toner is promptly supplied to a part of the developer on the developer bearing member, where the toner density has been reduced by passing through the developing area. Accordingly, the developing devices can develop a solid image without decreasing the image density.
However, in such developing devices with a self toner density control capability, background soiling and decrease of image density occasionally occur with the lapse of time. Also, uneven image density sometimes occurs in a widthwise direction of an image, which direction is perpendicular to the direction in which the developer on the developer bearing member is conveyed (hereinafter referred to as the image width direction). The inventors of the present invention have analyzed the causes of the decrease in the image quality, and have found that the followings are some of the possible causes.
First, background soiling and decrease of image density with the lapse of time might be caused by an early deterioration of developer due to insufficient replacement of the developer on a developer bearing member with the developer in a developer accommodating unit. That is, if the developer carried on the developer bearing member is conveyed without being peeled off the developer bearing member at all, the developer on the developer bearing member is hardly replaced with the developer in the developer accommodating unit. If such replacement of the developer is not made, stress to the magnetic carrier in the developer carried on the developer bearing member is increased, which may lead to a deterioration of the magnetic carrier earlier than the magnetic carrier in the developer accommodated in the developer accommodating unit. If the magnetic carrier on the developer bearing member deteriorates earlier than the magnetic carrier in the developer in the developer accommodating unit, even when the developer as a whole in the developing device is not deteriorated so much, the charging capability of the magnetic carrier on the developer bearing member, that acts on a formation of a toner image, might be decreased. The decrease of the charging capability of the developer on the developer bearing member causes background soiling or decrease in the image density, thereby decreasing the image quality. Accordingly, the developer is forced to be replaced earlier than desired.
If a replacement of the developer on a developer bearing member with the developer in a developer accommodating unit is not sufficiently performed, inferior toner charging may also result. Namely, when such a replacement of the developer is not made, the toner in an amount corresponding to a consumed amount of the toner on the developer bearing member, which has been supplied onto the developer bearing member, must be mixed thereupon with the developer so as to be charged. However, when the surface moving speed of the developer bearing member is relatively fast (e.g., several hundred rpm when the developer bearing member is a roller), the toner can not be sufficiently charged in time for development, thereby causing an inferior charging of the toner. In particular, when much toner has been consumed at some parts of the developer bearing member, decrease of image density remarkably appears in such parts compared to other parts, thereby leading to a decrease of the image quality.
Secondly, uneven image density in the width direction of the developer bearing member (i.e., in the image width direction) might be caused by uneven toner charging in the width direction of the developer bearing member. When new toner is supplied to a part of the developer where toner has been consumed, supplying of the new toner might be delayed or the supplying qu
Imamura Tsuyoshi
Ishibashi Mikio
Katoh Shunji
Kawahara Shin-ichi
Ohjimi Tokuya
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