Electrophotography – Image formation – Transfer
Reexamination Certificate
2001-05-09
2002-10-22
Lee, Susan S. Y. (Department: 2852)
Electrophotography
Image formation
Transfer
Reexamination Certificate
active
06470166
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a color-image forming apparatus equipped with a photosensitive drum and a transfer belt that forms an image by overlapping several pieces of image information through electro-photography and other technologies.
BACKGROUND OF THE INVENTION
In the prior-art color-image forming apparatus applying electro-photography, an image has been typically formed through the procedures below:
i) a charger charges a photosensitive drum serving as an image-carrier;
ii) with acceptance of laser radiation according to image information, the photosensitive drum forms an electrostatic latent image thereon;
iii) a developing unit develops the electrostatic latent image into a visible toner image; then
iv) the visible toner image is transferred onto a sheet of paper or other sheet-type materials.
Responding to the needs for color image, many kinds of tandem type color-image forming apparatuses have been developed so far.
A typical tandem type apparatus has plural image-carriers—each carrier is responsible for carrying cyan-, magenta-, yellow-, and preferably black-images. The individual four images are formed on their respective carriers in the series of image-forming processes described above. All of the separately carried images are overlapped at a proper position of each carrier and transferred onto a recording material to form a full-color image.
In another tandem type color-image forming apparatus, the toner images formed on the respective image-carriers are temporarily transferred onto an inter-stage transfer member one upon another. After that, the overlapped full-color toner image is transferred onto a recording material in one operation.
Such a tandem type apparatus contributes to high-speed image forming due to its structure in which each color image has its specific image-forming section.
FIG. 20
shows a schematic diagram of the prior-art color-image forming apparatus. In the apparatus shown in
FIG. 20
, image-forming units
41
,
42
,
43
, and
44
form toner images colored in black (K), cyan (C), magenta (M), and yellow (Y), respectively. According to an image signal from an exposure unit
45
, photosensitive drums
41
a,
42
a,
43
a,
and
44
a
disposed in image-forming units
41
,
42
,
43
, and
44
, respectively, form each electrostatic latent image thereon. Inter-stage transfer belt
46
has a closed-loop structure. Transfer unit
47
transfers the respective toner images formed on drums
41
a,
42
a,
43
a,
and
44
a
onto belt
46
. Fixing unit
48
finally transfers the toner image from belt
46
to sheet P fed from paper cassette
49
and fixes the image into place.
Here will be described the inner structure of image-forming units
41
through
44
in some detail. In image-forming unit
41
for black (K), for example, a laser beam from exposure unit
45
creates an electrostatic latent image over the peripheral surface of photosensitive drum
41
a.
Developing roller
41
c
applies toner onto the latent image formed on drum
41
a
to obtain a visible image. Cleaner
41
d
cleans out the residual toner on drum
41
a
after the toner image has been transferred onto belt
46
. The procedure described above is performed in other units
42
through
44
.
In such structured color-image forming apparatus, here will be described, for example, how the latent image carrying black component is formed on the drum.
Firstly, Charger
41
b
evenly charges over photosensitive drum
41
a
responsible for black. Then, according to image information from the host computer (not shown), exposure unit
45
applies laser light onto drum
41
a
to create a latent image. A thin layer of toner on developing roller
41
c
allows the latent image to be visible as a black-toner image. Having contact with drum
41
a,
inter-stage transfer belt
46
travels in the direction indicated by the arrow A shown in FIG.
20
. The visible black-toner image is transferred onto belt
46
through the application of transferring pressure from the inside of belt
46
to transfer unit
47
.
In the meantime, the latent image of the cyan component is created and then developed into a visible cyan-toner image with the help of cyan-toner layered on developing roller
42
c.
The cyan-toner image is transferred to belt
46
where the transfer process for the black-toner image has just been provided, thereby the two images are overlapped with each other.
In the same manner, the magenta- and the yellow-toner images are formed and overlapped with one after another. Thus, belt
46
carries the four-color overlapped image thereon.
The full-color toner image on belt
46
is finally transferred by transfer roller
50
onto sheet P coming from paper cassette
49
. After that, sheet P travels through fixing unit
48
to have the toner image fixed thereon, and goes out in the direction B shown in FIG.
20
.
After the toner image has been transferred onto sheet P, the residual toner on belt
46
is cleared out by belt cleaner
51
.
According to the prior-art apparatus, however, its structure—the photosensitive drum contacting with the inter-stage transfer belt at all times—can raise a problem. If there is a difference in speed between the rotation of the drum and the running of the transfer belt, damage or wear can occur on the surfaces rubbing against each other, which may result in degradation in image quality or loss of life.
A suggestion that the drive timing of the drum should agree with that of the belt may be a remedy for the problem described above. However, initiating the operation of the two units with exact same drive-timing is practically impossible due to time-lags of the driving systems—delay in response of a motor, gears, and an actuator—of the drum and the belt. Therefore, minute abrasion will persist in such a situation.
Besides, electric current required to drive a motor sees its peak just at the beginning of rotation. Therefore, a surge of power would be the result if such driving devices started their operations in unison.
Furthermore, even in the case that only one color, for example, monochrome (usually, black) print is required, other image-forming units—units for cyan, magenta, and yellow—also have to work with an “idle” printing motion. This wasteful motion produces friction between the members forming the photosensitive drum, the developing roller, and the cleaner, thereby shortening their useful life.
In the event of an interruption of the printing due to paper jamming or other malfunctions, the toner image left of the drum and the inter-stage transfer belt has to be cleaned out for the next printing. Being typical of the structure of the tandem type apparatus, the transfer belt requires a much longer time to travel one rotation than the drum needs to rotate one turn. That is, the drum unnecessarily has to have several turns while the belt travels at least one rotation. This structural inconvenience also causes friction between parts forming the image-forming units, thereby reducing their longevity.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a color-image forming apparatus having a structure that minimizes damage in the photosensitive drum and the inter-stage transfer belt due to friction occurring between them.
The apparatus of the present invention includes an exposure unit, plural image-forming units, and an endless inter-stage transfer belt.
The image-forming units are organized in a tandem type arrangement, and each contains: i) it's own photosensitive drum on which an electrostatic latent image is formed by the exposure unit; and ii) it's own developing unit for forming a toner image by applying toner onto the latent image. In addition, each unit has toner of a different color. Looped over plural rollers in its traveling path, the endless inter-stage transfer belt rotates to run along the direction of the arrangement of the image-forming units. A full-color toner image is formed on the belt as it runs, thereby overlapping the toner images formed on the drums upon another.
In the process, the belt comes
Mizoguchi Yoshihiro
Nagai Yuji
Ogata Kazuhiro
Shimotoso Tadashi
Yoshihara Takashi
Lee Susan S. Y.
Wenderoth , Lind & Ponack, L.L.P.
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