Electrophotography – Image formation – Transfer
Reexamination Certificate
2001-08-10
2003-05-06
Royer, William J. (Department: 2852)
Electrophotography
Image formation
Transfer
C399S322000, C399S324000
Reexamination Certificate
active
06560437
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer and a copier which adopts an electrophotographic system.
2. Related Background Art
Conventionally, an image forming apparatus adopting an electrophotographic system comprises: a latent image bearing body for bearing a latent image; developing means for visualizing the latent image as a developer image by supplying a developer to the latent image bearing body; transferring means for transferring the developer image onto a recording medium; and fixing means for fixing the developer image on the recording medium by heating and pressurizing the recording medium which bears the developer image.
Further, in such an image forming apparatus, there has been known and put into practice an image forming apparatus comprising residual charge elimination means which charge-eliminates the recording medium by applying a bias from a power source to the recording medium after the transfer of the developer image to the recording medium by the transferring means.
For example, in such an image forming apparatus, as shown in
FIG. 7
, as a method of separating a paper
114
, which is the recording medium after the toner image which is the developer image was transferred, from a photosensitive drum
103
which is the latent image bearing body, a method is generally adapted wherein, by applying a bias voltage of the polarity opposite to the bias applied (hereinafter, referred to as a residual charge elimination bias) to the transferring roller
107
which is transferring means, a charge on the paper
114
is charge-eliminated and an absorption force between the photosensitive drum
103
and the paper
114
is weakened.
A residual charge elimination needle
120
has a current-limiting resistor
121
having a resistance of 5 &OHgr; to 50 &OHgr; connected to a power source
122
in series to prevent a current leakage due to a high voltage. In the present example, in order to prevent the bias applied to the transferring roller
107
from leaking to the residual charge elimination needle
120
, a residual charge elimination needle holder
120
a
for shielding between the transferring roller
107
and the residual charge elimination needle
120
is installed. Note that, in general, the above described method of separating the paper
114
from the photosensitive drum
103
by the residual charge elimination needle
120
is used in combination with a method of abutting a separating claw (not shown) against the photosensitive drum
103
and forcing the paper to be stripped off after transfer in order to enhance separability of a thin paper from the photosensitive drum
103
.
In a fixing apparatus
117
provided for such an image forming apparatus, as shown in
FIG. 7
, the paper
114
which bears an unfixed toner image T is passed through a nip formed by a roller heated from the inside by a heater
133
which is heating means, thermal fixing means
131
which is a film-shaped fixing member and a pressure roller
132
which is a pressing body and pressure-welded to the thermal fixing means
131
, and heated and pressurized so that the unfixed toner image T is fixed on the paper
114
.
In recent years, in order to solve an offset problem in such a fixing apparatus
117
, there have been on the increase apparatuses having such a constitution that, on the surfaces of the thermal fixing means
131
and the pressure roller
132
, a potential difference is induced in such a direction as to press an unfixed toner image on the paper
114
against the paper
114
, the offset toward the thermal fixing means
131
.
For example, as for the fixing apparatus of the above described constitution, there are such apparatuses available wherein, by applying a bias (if a negative toner, −100 V to −2000 V) of the same polarity as the toner (unfixed toner) to the metal core of the fixing roller, a potential which repels the toner is induced on the surface, while, on the other hand, the pressure roller disperses an electrically conductive agent on a surface layer fluororesin layer and an elastic layer to make it as a medium resistive roller (having a surface resistance of 10
7
&OHgr; to 10
12
&OHgr;) and, by connecting a diode to the metal core, the potential difference with the fixing roller is maintained.
Or there are such apparatuses available, wherein, as shown in
FIG. 7
, the thermal fixing means
131
(for example, the fixing roller metal core) is grounded (of course, it may be applied with a bias of the same polarity as the toner), and the surface resistance of the pressure roller
132
is made a low resistive not more than 10
6
&OHgr;, and an electrode of an electrically conductive brush
134
and the like applied with a bias (hereinafter, referred to as a pressure bias) of the polarity reverse to the toner by a power source
136
is brought into contact with the surface of the pressure roller
132
, so that the potential difference with the thermal fixing means
131
is maintained.
This apparatus comprises a current-limiting resistor
135
of 100 M&OHgr; to 1000 M&OHgr; which controls a large electric current so as not to flow even if the pressure bias of high voltage continues to be applied and, compared to the current-limiting resistor
121
of the residual charge elimination bias, a high value resistor is used for the current-limiting resistor
135
.
In this way, by generating a potential difference in such a direction as to press the toner on the paper, there is no more to coat a surface lubricant such as a silicone oil and the like on the surface of the thermal fixing means as in the past or to make a cleaning member abut against the thermal fixing means or the pressure roller.
As a result, there are no more accidents such as an oil leakage and the like and a user's labor to periodically replace the cleaning member is eliminated.
FIG. 8
is a chart showing a timing of the residual charge elimination bias at the time when the paper
114
passes through on the residual charge elimination needle
120
with the position of the residual charge elimination needle
120
as a reference. Note that, in
FIG. 8
, an arrow mark is a top end of the paper
114
.
The residual charge elimination bias adequately eliminates the charge on the paper and applies a bias of −0.5 kV to −1.5 kV (hereinafter, referred to as a low level) from the top end of the paper to the halfway of the image so that the paper does not stick and pile by a guide member installed between the transferring portion and the fixing portion. While, at the paper rear end, it is preferable to strongly charge-eliminate the paper by a bias of −2 kV to −3 kV (hereinafter, referred to as high level) so that the paper does not jump up to allow the image to rub against the base of a cartridge.
By the way, usually, in the miniaturized apparatus, the paper is arranged so as to stretch across the transferring portion and the fixing portion and, when such an image forming apparatus as described above is sued under high humidity, because the paper is humidified under high humidifying circumstances, the potential of the pressure roller is lowered by a strong bias applied to the residual charge elimination needle so that the paper rear end does not jump up after the toner image is transferred on the paper. For this reason, there were some cases where a force to retain the unfixed toner image on the paper runs out and black spots phenomenon on a paper (hereinafter merely referred to as black spots) occurs at the fixing portion.
That is, in the case where the fixing apparatus is an apparatus wherein the surface resistance of the pressure roller is set not more than 10
6
&OHgr; and the bias of the polarity reverse to the toner is applied to the pressure roller, when a strong bias of the residual charge elimination needle is applied to the paper rear end as shown in
FIG. 9
, the potential of the humidified paper is lowered (B of FIG.
9
). Incidentally, if the paper rear end passes through the residual charge e
Kobaru Yasunari
Narahara Takashi
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Royer William J.
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