Electrophotography – Control of electrophotography process – Of plural processes
Reexamination Certificate
2000-02-08
2002-04-23
Chen, Sophia S. (Department: 2852)
Electrophotography
Control of electrophotography process
Of plural processes
C399S045000, C399S068000
Reexamination Certificate
active
06377763
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image formation apparatus which performs a density control operation.
2. Related Background Art
Conventionally, various methods such as an electrophotographic method, a thermal transfer method, an inkjet method and the like are applied as color image formation methods to a color image formation apparatus. In recent years, the image formation apparatus which applies the electrophotographic method has been widely used, because the electrophotographic method is excellent in the points of high speed performance, high image quality performance and silence performance as compared with other methods. It should be noted that the electrophotographic method includes various kinds of methods. Here, an example of a color image formation apparatus which applies a tandem method especially excellent in the high speed performance will be described.
FIG. 11
is a schematic diagram showing the color image formation apparatus which applies the tandem method. In the image formation apparatus of the tandem method, a toner image formation unit
8
K for black, a toner image formation unit
8
M for magenta, a toner image formation unit
8
C for cyan and a toner image formation unit
8
Y for yellow are independently disposed. An image-transferred material such as an ordinary sheet, an OHP (overhead projector) sheet or the like on which a toner image is to be transferred is carried by a carrying belt
7
which is extended by a driving roller
5
and subdriving roller
6
, and passes the four toner image formation units sequentially. Every time the image-transferred material
1
passes the toner image formation unit, the toner image of the corresponding color is formed or overlaid on the material
1
. Finally, a full-color image is formed on the image-transferred material
1
.
Hereinafter, the color image formation apparatus which applies the tandem method will be explained in detail. Since the operations of the respective toner image formation units
8
K,
8
M,
8
C and
8
K are substantially the same, hereinafter the operation of only the toner image formation unit
8
M will be described as an example of the representative. In the unit
8
M, first, a photosensitive drum
9
which is rotatively driven in the direction indicated by the arrow is uniformly electrified at −600V by an electrifier
10
(hereinafter such a potential is called an electrification potential), and a latent image corresponding to a magenta image is formed on the drum
9
by a scan beam of a laser exposure optical system
11
or the like. A potential of the latent image formed by exposure with the scan beam is about −200V (hereinafter called an exposure portion potential). On the other hand, a magenta toner which has a certain quantity and was electrified to have a negative polarity is supplied onto a development roller
12
, and a development bias is applied to the roller
12
. Either a DC bias or a bias obtained by overlaying an AC bias on the DC bias can be used as the development bias. Even in case of either, by setting the DC component of the bias to have an appropriate value between the electrification potential and the exposure portion potential, it is possible to perform the development in which the toner is selectively adhered to the latent image on the photosensitive drum.
The magenta toner image thus formed on the photosensitive drum
9
is electrostatically transferred onto the image-transferred material
1
carried at the speed substantially the same as the rotation speed of the drum
9
, by a positive-polarity transfer bias applied to a transfer roller
13
.
Similarly, the above processing is performed in the toner image formation units
8
C,
8
Y and
8
K respectively, whereby the toner image of four colors is formed on the image-transferred material
1
. Next, the toner image is meltingly fixed to the image-transferred material
1
by a fixing unit
15
, and the material
1
is then discharged from the apparatus.
On the other hand, in the color image formation apparatus, if the density of each color and a halftone gradation characteristic change due to change of available environment, change by long-term use and the like, a color tone of an output image changes. Thus, some image density control means have been often provided to prevent such a problem. Conventionally, in the image density control, a density control sequence described as follows is performed after a power supply is turned on, after a sleep (afterheat) state is released, after images of a certain number are output, and the like. Thus, it is possible to always obtain the stable output image. Hereinafter, an example of the image density control sequence will be described.
First, a toner image (i.e., a test patch) having a specific pattern is formed on the photosensitive drum
9
or the carrying belt, and the density of the formed patch is detected by a density sensor
20
. As the test patch, a quadrangle pattern of 15 mm×15 mm is often used. As shown in
FIG. 12
, the density sensor
20
is mainly composed of a light emission element
21
such as an LED or the like, and a light reception element
22
such as a photodiode or the like. Thus, infrared light is irradiated on a pattern P by the light emission element
21
, and the diffused light from the pattern P can be detected by the light reception element
22
. The reason why the diffused light, rather than the regular reflection light, is used for the detection is that the diffused light is not influenced easily by the gap of an optical axis, the surface state of the background on which the test patch is formed, and the like. Since the reflection light detected by the light reception element
22
one-to-one correlates to the toner image density, the toner image density can be resultingly detected by the density sensor
20
.
The image density is controlled on the basis of an image formation condition including the electrification potential of the photosensitive drum, the laser exposure quantity, the development bias and the like. The halftone gradation characteristic is controlled based on an image data conversion table. Thus, the plural patches are formed by stepwise changing such the image formation condition and the image data conversion table. The densities of these patches are detected by the density sensor
20
, and the optimum value of the image formation condition is derived from the detected results.
As above, the density of the tentatively formed toner image is detected, and the detected result is fed back, thereby always obtaining the stable image.
Further, in the color image formation apparatus which applies the electrophotographic method, generally, there is the apparatus for which a low-speed fixing mode (or a low speed mode) is prepared to cope with the image-transferred material such as the OHP sheet, a board or the like. In the low speed fixing mode, since the fixing is performed at the speed lower than the ordinary fixing speed, a fixing time can be prolonged. Thus, even if the OHP sheet is used as the image-transferred material, the toner can be sufficiently melted, thereby increasing permeability. Besides, even if the board having a large heat capacity is used, thereby securing satisfactory fixability.
On the other hand, like the above color image formation apparatus, if the distance between the final transfer position (i.e., at the toner image formation unit
8
K) and the nip position of the fixing roller is shorter than the length of the image-transferred material, it is impossible to lower the operation speed of only the fixing unit in the low speed mode. Namely, it is necessary to lower the entire speed (processing speed) of the image formation including carrying speed of the image-transferred material, rotation speed of the photosensitive drum, electrification speed, development speed and transfer speed.
Further, there is the color image formation apparatus for which a high resolution mode to decrease the processing speed is prepared so as to increase the image density in
Canon Kabushiki Kaisha
Chen Sophia S.
Fitzpatrick ,Cella, Harper & Scinto
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