Electrophotography – Control of electrophotography process – Control of transfer
Reexamination Certificate
2000-10-25
2002-08-13
Brase, Sandra (Department: 2852)
Electrophotography
Control of electrophotography process
Control of transfer
C399S049000
Reexamination Certificate
active
06434348
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus employing an electrophotographic system and, for example, to an image forming apparatus such as a copier, a printer, a facsimile machine and the like.
2. Related Background Art
FIG. 8
is a schematic sectional view of a conventional color image forming apparatus of electrophotographic type employing an intermediate transfer member. In such an image forming apparatus, the step of primarily transferring a toner image formed on a photosensitive member to the intermediate transfer member is repeated for a plurality of color toners, and the toner images on the intermediate transfer member in a secondary transfer step are collectively transferred to a transfer material such as a paper, so that a full color image can be obtained.
Now, an image forming process will be described.
The surface of a photosensitive member
101
as an image bearing member is uniformly charged by a primary charger
107
, and an image pattern corresponding to an original image is exposed by a laser exposure apparatus
105
to form an electrostatic latent image on the photosensitive member
101
. At the time of developing, a developing bias is applied to the developing sleeves of developing devices
108
,
109
to develope the electrostatic latent image on the photosensitive member into a toner image. A transfer bias is applied from a primary transfer roller
106
to an intermediate transfer member
102
, and in an primary transfer nip portion, the toner image on the photosensitive member is transferred to the intermediate transfer member
102
by an electric potential difference between the photosensitive member
101
and the intermediate transfer member
102
. By repeating a series of the above described steps for four color toners of yellow (Y), magenta (M), cyan(C) and black (K), a full color image is formed wherein toner images of four different colors are superimposed on top of each other on the intermediate transfer member
102
. Note that, after the primary transfer, the toner remaining on the photosensitive member is removed by a cleaning blade
110
and collected in a cleaning container.
Next, a secondary transfer belt
103
separated from the intermediate transfer member
102
during a series of the above described steps is brought into pressure contact with the intermediate transfer member
102
, and when a paper or the like as a transfer member P′ is passed through between the secondary transfer belt
103
and the intermediate transfer member
102
, a full color toner image is transferred from the intermediate transfer member
102
to the transfer member. Thereafter, the transfer member is conveyed to a fixing device in which the toner image is subjected to color mixing and at the same time fixed to the transfer member, thereby obtaining a full color image as a permanent image.
The secondary transfer residual toner remaining on the intermediate transfer member
102
is removed by a cleaning member (a blade)
104
and collected in a cleaning container.
However, in the above described configuration, there have been problems as follows.
Since a charged state of the photosensitive member and a charged amount per unit weight of the toner vary with the durability thereof and circumstances, an image density also varies with such a fluctuation. In order to avoid this, the method was employed wherein an image for detection (hereinafter referred to as a patch image) responding to a definite density signal is formed on the photosensitive member and the image density thereof is detected by a sensor and, on the basis of the detected signal, a charging bias inputted to the primary charger
107
, an exposure intensity by the exposure device
105
and the charged amount per unit weight of the toner are controlled. There has been known the method wherein the density of the above described patch image is detected on the photosensitive member or transferred to the intermediate transfer member and detected on the intermediate transfer member. Particularly in recent years, in keeping with the miniaturization of the apparatus, it is becoming difficult to obtain a space (a space for installing a sensor) for detecting the density on the photosensitive member and there are many cases where the method for detecting on the intermediate transfer member is adapted. However, in this case, since the density of the patch image formed on the photosensitive member is not directly detected, but detected after it is once transferred to the intermediate transfer member, a transfer efficiency of the patch image should be very high.
A transfer bias (a transfer voltage or a transfer electric current) inputted to the primary transfer roller
106
which is set at the time when a toner image of plural colors is multi-transferred from the photosensitive member to the intermediate transfer member is determined by several conditions. Among them, because a transfer efficiency and a re-transfer rate greatly contribute to density, hue or tone and the like, they are highly valued.
FIG. 2
is a graph showing a relationship between a transfer residual rate and a transfer electric current and a relationship between a re-transfer rate and the transfer electric current. The continuous line A represents the transfer residual rate. The broken line B represents the re-transfer rate. As shown in
FIG. 2
, when the above described transfer electric current is increased, the transfer efficiency (the primary transfer efficiency) is raised (a transfer residual rate A is decreased). However, the re-transfer rate B is also raised. Therefore, at the time when the transfer bias is set, both the transfer efficiency and the re-transfer rate may be preferably optimized.
Note that the transfer efficiency is represented by the proportion of the toner density (the transfer residual rate is represented by the proportion of the transfer residual toner density on the photosensitive member) on the intermediate transfer belt at the time when the sum of the toner density on the intermediate transfer belt subsequent to the primary transfer and the transfer residual toner density on the photosensitive drum is represented by 100. If the toner amount(density) of the toner image formed on the photosensitive member is represented by X and the toner amount (density) of the toner image transferred to the intermediate transfer member with this toner image on the photosensitive member transferred to the intermediate transfer member is represented by Y, the following expression is established:
(
Y/X
)×100(%).
Also, the transfer residual rate (which means the rate wherein a toner image is not transferred to the intermediate transfer member but has remaining on the photosensitive member and is contrary to the above described transfer efficiency) is represented by the following expression:
{(
X−Y
)/
X
}×100(%).
Also, the re-transfer rate is represented by the proportion of the density of the toner re-transferred to the photosensitive member at the time when the sum of the density of the toner image transferred to the intermediate transfer member and the density of the toner re-transferred (offset) to the photosensitive member when the toner image on the intermediate transfer member passes through the primary transfer portion for the next time is expressed by 100. The re-transfer rate is represented by the following expression if the amount of the toner re-transferred to the photosensitive side is expressed by Z:
(
Z/Y
)×100(%).
The above described transfer efficiency, transfer residual rate and re-transfer rate are derived from measuring by densitometer (a product name: 404, manufactured by X-rite Corporation) the density of each toner removed by Mylar tape and attached to CLC80 g/m
2
paper adapted by us as the standard paper for color.
However, in the case where a Dmax patch image (the Dmax referred to herein means the maximum density and the Dmax patch means a developed patch image as against the latent image which is Dmax as an ima
Brase Sandra
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
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