Electrophotography – Image formation – Transfer
Reexamination Certificate
2002-09-05
2004-03-02
Ngo, Hoang (Department: 2852)
Electrophotography
Image formation
Transfer
C399S055000, C399S056000, C399S046000, C399S302000
Reexamination Certificate
active
06701116
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a quadruple-tandem type image forming apparatus employed for an electrophotogaphic process copying machine, a printer or the like.
2. Related Art Statement
FIG. 1
is a sectional view of a general quadruple-tandem type image forming apparatus, and printing operation of a color image will be explained with reference to FIG.
1
. Here, as photosensitive body drums
103
a
~
103
d
, OPC (Organic Photo Conductor) is employed.
The image forming steps in an image forming unit al will be explained. First, the surface of a photosensitive body drum
103
a
is uniformly charged in minus (−) by a scorothoron charger
105
a
. An exposure device not shown is disposed downstream of the scorothoron charger
105
a
, and exposure in conformity with image information is applied to the charged photosensitive body
103
a
to thereby form an electrostatic latent image. A 2-component developing unit
109
a
for encasing a yellow developer is disposed downstream of the exposure device, an electrostatic latent image is reversal-developed by a yellow toner, and a toner image is formed on the photosensitive body
103
a.
On the other hand, a sheet of paper P as a transfer material are supplied from a paper cassette not shown and conveyed, and paper P is then transported onto a conveying belt
111
by an aligning roller
114
adjusting to a timing at which a toner image is formed on the photosensitive body
103
a
. A bias voltage (+) reversed in polarity to a charge polarity of a toner is applied by a DC power supply
127
a
to a transfer roller
123
a
, and a toner image on the photosensitive body
103
a
is transferred onto paper P by a transfer electric field formed between the photosensitive body
103
a
and the transfer roller
123
a.
Here, a partial toner (a residual transfer toner) remained on the photosensitive body
103
a
without being transferred completely on paper P is cleaned by a photosensitive body cleaner
120
a
, and is sent as a waste toner to a waste toner box not shown. The cleaned surface of the photosensitive body
103
a
is exposed by an eliminator
121
a
and thereby eliminated, after which the steps of charge, exposure and development are repeated.
Also in image forming units b
1
, c
1
and d
1
, a toner image is formed by the process similar to that mentioned above adjusting to a timing at which the toner image is formed in the image forming unit a
1
, and toner images of magenta, cyan, and black formed on the photosensitive bodies
103
b
,
103
c
, and
103
d
of the image forming units b
1
, c
1
and d
1
are also sequentially transferred onto paper P conveyed by the conveying belt
111
. Generally, there is no uniformity in relation between an charge potential of a photosensitive body of each image forming unit, an charge quantity of toner, and a development potential.
A fixing unit not shown for fixing a toner on paper P is disposed downstream of the conveying belt
111
, and paper P is caused to pass through the fixing unit to obtain a fixed image.
As described above, in the transfer step, a toner on the photosensitive body is transferred to a transfer material by a transfer electric field generated between the photosensitive body and the transfer roller, but there poses a problem in that where the transfer electric field is large, there occurs a so-called reversal transfer phenomenon in which the toner once transferred to the transfer material is returned to the photosensitive body again.
As the procedure for suppressing such a reversal transfer phenomenon as described, there has been proposed a procedure described in Japanese Patent Application Laid-Open No. 209232/2001 in which the charge quantity of toner downstream in a moving direction is set to be lower than the charge quantity of toner upstream, and a transfer bias voltage is set to be lower closer to the downstream. However, even if the transfer bias voltage is the same, a tendency of the reversal transfer is changed by a surface potential of the photosensitive body, as illustrated in the embodiment described later, from which it is said that generally, a grade need not be provided relative to the charge quantity of toner or the transfer bias voltage, and further, there is a disadvantage that a variation in reproducibility or gradient of dots of a color toner occurs by the mere provision of a grade relative to the charge quantity of toner.
Further, there has been also proposed a procedure described in Japanese Patent Application Laid-Open No. 209232/2001 in which a development potential in toner image forming means downstream is set to be lower than that upstream, and |VL−Vbias| is made smaller closer to downstream to thereby make electrostatic attracting force exerted between a toner of reversal polarity and the photosensitive body small. However, also in this respect, as illustrated in the embodiment described later, a development contrast or a development quantity of toner is not directly related to the reversal transfer, and in addition, there is a problem also that a variation in reproducibility or gradient of dots of a color toner occurs by the mere provision of a grade relative to the development potential.
With respect to such a reversal transfer phenomenon as described, the inventor thought that it resulted from a Pachen-discharge generated in the vicinity of a transfer region because of the fact that where a difference between a potential (normally, an earth potential) at the back of the photosensitive body or an charge potential of the surface of the photosensitive body and the effective value of a transfer bias voltage is large, the above-described phenomenon occurs often, and that an charge quantity of toner on the transfer material after having passed through the transfer region increases than that prior to the passage whereas an charge quantity of the reversal transfer toner considerably lowers (becoming+polarity).
Therefore, preferably, the transfer electric field is controlled so as not to generate the Pachen-discharge in order to suppress the reversal transfer. As means for controlling the transfer electric field, there have been generally known a method for controlling the transfer bias voltage itself, and a method for, before transferring a toner on the photosensitive body to a transfer material, exposing it to lower a potential of the surface of the photosensitive body (elimination before transfer) to lower a transfer electric field. However, by merely setting the transfer bias voltage to be lower, the reversal transfer can be reduced but the residual transfer increases, and the transfer efficiency is lowered, posting a problem in reproducibility of an image. Further, in case of elimination before transfer, the reversal transfer can be reduced, but there poses a problem that when a charge on the photosensitive body disappears, Coulomb repulsion force caused by toners is affected to increase dusts on the image.
Setting an charge potential of the photosensitive body lower is also one means for suppressing the reversal transfer, but there is a problem that when the charge potential of the photosensitive body is set to be lower, an image concentration lowers or a reproducibility or gradient of dots lowers also according to the charge quantity of toner or development bias voltage.
In a so-called photosensitive body cleanerless system in which a cleaner for cleaning a toner on the photosensitive body is not disposed but cleaning is carried out simultaneously with development by a developing unit, a toner remained on the photosensitive body without being transferred to a transfer material (a residual transfer toner) can be recovered into the developing unit, thus providing the merit that a waste toner can be reduced, and the service life of the photosensitive body extends, whereas there is a great problem that when the reversal transfer phenomenon occurs simultaneously in a plurality of colors of toners, toners are mixed in color within the developing unit, and such a method as
Kabushiki Kaisha Toshiba
Ngo Hoang
LandOfFree
Image forming apparatus does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Image forming apparatus, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Image forming apparatus will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3192335