Image forming apparatus with control of voltage application...

Electrophotography – Control of electrophotography process – Control of transfer

Reexamination Certificate

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Details

C399S302000

Reexamination Certificate

active

06330407

ABSTRACT:

FIELD OF THE INVENTION AND RELATED ART
The present invention relates to an image forming apparatus employing an electrophotographic system, an electrostatic recording system, or the like. In particular, it relates to an image forming apparatus comprising an intermediary transfer member, in addition to those systems mentioned above.
As an image forming apparatus capable of forming an image with no color aberration, an image forming apparatus with an intermediary transfer member has been proposed.
FIG. 12
illustrates the general structure of such an image forming apparatus. A photosensitive drum
101
which is being driven in the direction indicated by an arrow mark is first uniformly charged on its peripheral surface by a charge roller
102
. Next, the charged surface is exposed to a laser beam
103
, which is moved in a manner to scan the peripheral surface of the photosensitive drum
101
while being turned on or off on the basis of image formation data. As a result an electrostatic latent image is formed on the photosensitive drum
101
. This electrostatic latent image is developed (visualized) by a developing apparatus
104
in which a plurality of developing devices
104
a,
104
b,
104
c,
and
104
d
are rotatively disposed so that their position can be switched. Each of these developing devices comprises a development sleeve. Black toner as the toner of the first color is contained in the first development device
104
a;
magenta color toner as the toner of the second color, in the second developing device
104
b;
cyan color toner as the toner of the third color, in the third developing device
104
c,
and yellow color toner as the toner of the fourth color is contained in the fourth developing device
104
d.
The aforementioned electrostatic latent image is developed (visualized) by the first developing device
104
a
containing black toner as the first toner. The normal polarity of the toner is negative. The first toner image, that is, the image created by developing the electrostatic latent image with the use of the first toner, is electrostatically transferred (primary transfer), at the first transfer point
106
a,
onto the surface of an intermediary transfer belt
105
which is being rotatively driven in the direction indicated by an arrow mark, with the surface of the intermediary transfer belt
105
squarely facing the peripheral surface of the photosensitive drum
101
. After the primary transfer, a small amount of the first transfer residual toner, that is, the toner remaining on the peripheral surface of the photosensitive drum
101
after the first transfer, is removed by a cleaning apparatus
107
.
Next, the above described process is repeated three more times. As a result, toner images are transferred in layers onto the surface of the intermediary transfer belt
105
. More specifically, the second toner image created by developing an electrostatic image by the magenta toner, the third toner image created by developing an electrostatic latent image by the cyan toner, the fourth toner image created by developing an electrostatic latent image by the yellow toner, are consecutively transferred in layers onto the surface of the intermediary transfer belt
105
.
Thereafter, a transfer roller
108
for the secondary transfer, which is kept separated from the surface of the intermediary transfer belt
105
when not in action, is placed in contact with the surface of the intermediary transfer belt
105
, and is rotatively driven. The interface between the photosensitive drum
101
and the transfer roller
108
for the secondary transfer forms a secondary transfer point (station)
106
b,
at which the toner images on the surface of the intermediary transfer belt
105
are transferred (secondary transfer) all at once onto the surface of a piece of transfer medium P conveyed to the second transfer point
106
b
with predetermined timing. This transfer medium P is conveyed to an unillustrated fixing apparatus, in which the full-color image composed of four color toner images is fixed. Thereafter, the recording medium is discharged out of the image forming apparatus.
In the case of an image forming apparatus in which the places of a plurality of the aforementioned developing devices must be switched by the rotary moving means
4
for each development step, the provision of the time t for switching the developing device is necessary for each color, and this time t affects the size of the intermediary transfer belt
5
in the following manner.
That is, the intermediary transfer belt is being rotatively driven at a predetermined velocity even during the developing device switching time t, and therefore, the peripheral length L of the intermediary transfer belt must include the margin V−t for switching the developing devices. Thus, when the length of the largest piece of recording medium usable with the image forming apparatus is L
p
, the following relationship must be satisfied.
L−L
p
≧V−t
  (1)
Provided, for example, that the developing device switching time t is 1,200 milliseconds, and recording medium size is A4 (210 mm in width×297 mm in length)/letter size 215.9 mm in width×279.4 in length), in order to print a full-color image at a processing speed V, the intermediary transfer belt must be longer in peripheral length than 441 (mm)=297 (mm)+1,200 (msec)×120 (mm/sec). However, the condition expressed by the formula (1) is such a condition that is required only for forming a full-color image by placing in layers a plurality of toner images. In other words, when a monochrome image is formed, the aforementioned relation does not need to be satisfied because the formation of a monochrome image does not require the developing device switching time t. Therefore, the size of a monochrome image can be as large as the peripheral length of the intermediary transfer belt
5
can afford. For example, in the case of an image forming apparatus capable of forming an image which is 215.9 mm long in terms of the rotational direction of the intermediary transfer belt, that is, accommodating a piece of recording medium as long as 215.9 mm, it can form a monochromatic image as large as the printable surface size of a legal size recording medium (215.9 mm in width×355.6 in length), that is, the longest sheet of recording medium among the cut sheets of known regulation sizes, but cannot form a full-color image as large as a legal size monochrome image, which makes the apparatus odd in terms of specification, and confuses the user. Further, if an attempt is made to solve this problem by employing an intermediary transfer belt which makes it possible to form a full-color image matching the legal size, the image formation apparatus size becomes rather large, which inevitably leads to cost increase.
Japanese Laid-Open Patent Application No. 225,520/1995 discloses an image forming process, according to which, when it is necessary to form a full-color image on a large piece of recording medium, the intermediary transfer belt is idled one full rotation, instead of transferring (primary transfer) the toner image of the second toner onto the recording medium immediately after the toner image of the first color is transferred (primary transfer) onto the surface of the intermediary transfer belt, so that during this idling period, the developing device for the first color is switched with the developing device for the second color. Then, the toner image of the second color is transferred onto the surface of the intermediary transfer belt. In other words, the intermediary transfer belt is rotated seven times overall to finish transferring a full-color image onto the recording medium. Further, in order to prevent the so-called reverse transfer phenomenon, that is, a problematic phenomenon that the toner which has been transferred (primary transfer) onto the surface of an intermediary transfer belt is transferred back onto the peripheral surface of a photosensitive drum, the amount of the power given to the cornea type charging dev

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