Electrophotography – Control of electrophotography process – Having temperature or humidity detection
Reexamination Certificate
2000-09-22
2002-03-05
Beatty, Robert (Department: 2852)
Electrophotography
Control of electrophotography process
Having temperature or humidity detection
C399S062000
Reexamination Certificate
active
06353716
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application is related to Japanese Patent Applications No. Hei 11(1999)-268055 filed on Sep. 22, 1999 and Hei 11(1999)-268056 filed on Sep. 22, 1999, whose priorities are claimed under 35 USC §119, the disclosure of which is incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic type copying machine, laser beam printer, facsimile machine or the like which uses two-component developing agent comprising toner and carriers, and particularly to an image forming apparatus which can correct the specific concentration of toner in accordance with variation of humidity to stably form an image.
2. Description of the Related Art
A conventional image forming apparatus is equipped with a photosensitive drum for carrying an electrostatic latent image thereon, which is disposed in the housing of the apparatus. The surface of the photosensitive drum is uniformly electrified by an electrifying unit, and then an electrostatic latent image is formed on the surface of the photosensitive drum thus electrified. The electrostatic latent image formed on the surface of the photosensitive drum is developed with toner by a developing unit to form a toner image. The toner image is transferred onto a sheet serving as a transfer medium fed to a transfer unit.
Subsequently, the sheet is peeled off from the photosensitive drum by a peeling unit and then the toner image is fixed onto the sheet by a fixing unit. In such a conventional image forming apparatus, two-component developing agent comprising toner and carriers is used as developing agent of the developing unit. The toner is formed by dispersing colored pigment into resin, and the carriers are composed of ferromagnetic particles, for example, iron powder or the like. In the two-component developing agent comprising the toner and the carriers thus formed, the weight ratio (weight percentage) of the toner is equal to several %, and the toner of the developing agent is wasted every time an electrostatic latent image on the photosensitive drum is developed. Accordingly, the weight ratio of the toner in the developing agent, that is, the specific concentration of the toner must be kept in a predetermined range.
FIG. 7
is a block diagram showing a circuit for controlling the specific concentration of toner in the conventional image forming apparatus, and
FIGS. 8 and 9
are graphs showing the output characteristics of an auto toner control sensor (hereinafter referred to as “ATC sensor”) for the toner specific concentration.
As shown in
FIG. 7
, the toner specific concentration of the developing agent in a developing unit
101
is detected by a magnetic type ATC sensor
102
, and CPU
103
controls toner supply from a toner cartridge
104
in accordance with detection information of the ATC sensor
102
.
FIG. 8
shows the relationship between the toner specific concentration and the output value of the ATC sensor
102
. When reduction of the toner specific concentration of the developing agent in the developing unit
101
is detected on the basis of the output value of the ATC sensor
102
by the CPU
103
, CPU
103
drives a toner cartridge driving motor
106
to supplement toner into the developing unit through the toner cartridge
104
. As a result of the toner supplement, the toner specific concentration increases, and the output voltage from the ATC sensor
102
is reduced. For example when the output voltage of the ATC sensor
102
is reduced to be less than a reference value V
0
shown in
FIG. 8
(for example, 2V), CPU
103
stops the driving motor
106
to stop the toner supplement from the toner cartridge
104
, whereby the toner specific concentration of the developing agent in the developing unit
101
is kept to 4 wt % (reference value V
0
), for example.
The two-component developing agent in the developing unit
101
is liable to be influenced by humidity. Therefore, if no correction is carried out on the toner specific concentration in accordance with variation of humidity, the toner specific concentration increases as the atmosphere is more humid. On the other hand, the toner specific concentration is reduced as the atmosphere is less humid. This phenomenon occurs because the ATC sensor
102
detects the magnetic flux density to control the toner specific concentration. For example, when the atmosphere is more humid, the electrification amount of the developing agent is lowered, and the developing agent is kept more tight as a whole. Therefore, the ATC sensor
102
judges that the density of the carriers is high, that is, the toner specific concentration is low, and thus instructs to start the toner supplement.
On the other hand, when the atmosphere is less humid, the electrification amount of the developing agent is increased, and the developing agent is kept swollen. Therefore, the ATC sensor
102
judges that the density of the carriers is low, that is, the toner specific concentration is high, and thus instructs to stop the toner supply.
That is, the characteristic curve indicating the output of the ATC sensor of
FIG. 8
is varied like a characteristic curve with respect to the variation of humidity as shown in
FIG. 9. A
stable image can not be obtained without correcting the above phenomenon
Consequently, when the atmosphere is kept in a high humidity state, the toner specific concentration of the developing agent rises up or the electrification amount is lowered. Therefore, toner scattering or background fogging (which means increase of background density and thus the background looks dirty) or defacement of images occurs, so that the image quality is lowered. On the other hand, when the atmosphere is kept in a low humidity state, the toner specific concentration of the developing agent is reduced or the electrification amount is increased. Accordingly, the image density is lowered, and for example, blurring of characters occurs, so that the image quality is lowered.
Furthermore, the developing agent is deteriorated with time lapse due to increase of the copy amount (copy frequency) and also increase of the developing agent stirring time, and due to the deterioration of the developing agent, the toner specific concentration rises up or the electrification amount is lowered as compared with the toner specific concentration at the initial stage of the developing agent, that is, when the device is set up or the developing agent has been just exchanged. Therefore, toner scattering or background fogging, or image defacement occurs, and thus the image quality is lowered.
In order to solve the problem, according to an image forming apparatus disclosed in Japanese Unexamined Patent Publication No. Hei 4(1992)-12380, a humidity sensor
105
is provided as shown in FIG.
7
. If the humidity (relative humidity) is 70% or more, the toner supplement amount is controlled by a controller
103
a
so that the toner specific concentration of the developing agent in the developing unit
1
is equal to a first reference value (for example, 3.5 wt %). On the other hand, if the humidity is below 40% or less, the toner supplement amount is controlled by the controller
103
a
so that the toner specific concentration of the developing agent in the developing unit
1
is equal to a second reference value (for example, 4.5 wt %) higher than the first reference value.
FIG. 10
shows the operating characteristic of the humidity sensor
105
in the image forming apparatus.
In the image forming apparatus as described above, a control reference value (target value) indicated by a line C of
FIG. 10
is provided. That is, under the humidity of 40% or less, the toner specific concentration is controlled to be equal to 4.5 wt %, and under the humidity of 70% or more, the toner specific concentration is controlled to be equal to 3.5 wt %.
However, actually, the toner specific concentration is shifted as indicated by A-line or B-line of
FIG. 10
with variation of the humidity. That is, in
F
Kawamoto Hiroji
Maeda Yasutaka
Matsuoka Tomoe
Narimatsu Masayasu
Sakagami Yuka
Beatty Robert
Sharp Kabushiki Kaisha
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