Electrophotography – Image formation – Development
Reexamination Certificate
2001-04-06
2002-11-26
Pendegrass, Joan (Department: 2852)
Electrophotography
Image formation
Development
C399S270000, C430S122520
Reexamination Certificate
active
06487384
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an image forming apparatus using electrophotography and in particular to an image forming apparatus comprising a developing unit using a magnetic developer.
2. Description of the Related Art
A one-path color print system for printing two colors or more in one cycle is available as one system of electrophotography. As shown in JP-A-48-37148, the following developing method is available: A photosensitive body is charged and then the light exposure amount is changed in three levels of no exposure, weak exposure, and strong exposure in response to color information of an image on exposure to light, whereby electrostatic images at three levels of a high-level area, a low-level area, and an intermediate-level area therebetween as surface potential on the photosensitive body are formed and the high-level area is normally developed and the low-level area is inversely developed for forming two types of toner images on the photosensitive body. Toners different in charge polarity can be used as the two types of toners. A two-color image can be formed by using toners different in color as in the example in the related art. The description to follow assumes a two-color developing method for forming a two-color image.
As the electrostatic image developing method, a method using a dual-component developer of a mixture of toner of colored particles and carrier of magnetic particles is available. To develop the high-level area, a developing bias voltage between the high-level surface potential and the intermediate-level surface potential is applied to a developing electrode of a developing unit and on the other hand, to develop the low-level area, a developing bias voltage between the low-level surface potential and the intermediate-level surface potential is applied to the developing electrode of the developing unit, whereby the toners are developed in the high-level area and the low-level area.
FIG. 2
shows a schematic configuration of a printer using a system wherein three values of potential on a photosensitive body are provided, described in JP-A-48-37148 as one of the one-path color print systems. Throughout the specification, the system is called potential division developing system.
FIG. 3
is a potential model chart of the potential division developing system. The system is a system for printing in two colors of black and any other color than black, such as red, blue, or green. In the description to follow, any other color than black is red and the first developing is normal developing in a red developer and the second developing is inverse developing in a black developer.
A photosensitive body
1
having photoconductivity is charged by a charger
2
and then exposure responsive to image information is executed by an exposure unit
3
. A black image portion is exposed to strong light so that the photosensitive potential becomes the minimum as potential VL, a print area in red is not exposed to light as potential VH, and a background portion, namely, a white paper portion is exposed to weak light as potential VM of about a half the potential VH, whereby three-value potentials are formed. Next, a first developing unit
4
a
develops the red image portion. The developing unit
4
a
contains a developing roller
30
a
having a rotatable sleeve roller
31
a
on the outer periphery of a magnet
32
a
internally fixed. The sleeve roller
31
a
is electrically connected to a power supply and a bias voltage Vb
1
is applied so that |VH|>|Vb
1
|≦|VM|. A dual-component developer
20
a
of a mixture of a red toner
21
a
in the first developing unit
4
a
and a first carrier
25
a
for charging the red toner
21
a
to an opposite polarity to the charge polarity of the photosensitive body
1
by friction with the red toner
21
a
at an appropriate ratio is magnetically attracted to the surface of the sleeve roller
31
a
and is transported to a developing area
40
a
with rotation of the sleeve roller
31
a
and scrubs the surface of the photosensitive body
1
. The red toner
21
a
is deposited on the red image area by an electric field formed by an electrostatic latent image formed on the photosensitive body
1
and the bias voltage Vb
1
applied to the sleeve roller
31
a,
and developing is executed.
Next, a second developing unit
4
b
develops the black image portion. A black developer
20
b
used here is a mixture of a black toner
21
b
and a second carrier
25
b
for charging the black toner
21
b
to the same polarity as the charge polarity of the photosensitive body
1
by friction with the black toner
21
b
at an appropriate mix ratio. A bias voltage Vb
2
applied to the sleeve roller
31
b
is set so that |VM|>|Vb
2
|>|VL|, and the black image portion is developed.
An image is thus formed in the black and red toners different in charge polarity. Thus, in the state, if an attempt is made to apply a corona of a single positive or negative polarity and transfer in a transfer unit
7
, the toner of the same polarity as the applied corona repels and is not transferred to a sheet member
8
. Therefore, before the toner is transferred, the polarity of the toner needs to be matched with the polarity opposite to that of the applied corona in the transfer unit
7
, and a before-transfer charger
6
is installed. The toner with the polarity matched by the before-transfer charger
6
is transferred efficiently to the sheet member
8
by the transfer unit
7
. After the toner is transferred, the sheet member
8
passes through a fuser
9
and the toner is fixed onto the sheet member
8
.
The remaining toner or a deposit of paper powder, etc., on the photosensitive body
1
after transfer section passage is removed and collected from the photosensitive body
1
by a cleaning unit
11
.
The potential division developing system for performing two-color print according to the described process has very excellent features that it can execute two-color print at the same speed as monochrome print, that the costs are low because of one exposure unit, and that a position shift in a two-color image does not occur.
However, in the developing methods using a dual-component developer as well as the above-described two-color developing method, the carrier is developed in the potential area opposite to the potential area in which toner is developed with respect to the developing bias voltage; this is a problem. The reason is that in the dual-component developer, the toner and the carrier are mixed, whereby they are charged in opposite polarities to each other. For example, in the above-described two-color developing method, as shown in
FIG. 3
, to develop the high-level area charged negatively, the developing bias voltage Vb
1
between the high-level surface potential VH and the intermediate-level surface potential VM is applied to the developing electrode of the developing unit, whereby the toner charged to the positive polarity is developed in the high-level area. On the other hand, the carrier is charged to the negative polarity and thus is developed in the low-level area where the surface potential is lower than the developing bias Vb
1
. The carrier is magnetic particles and the developing electrode of the developing unit generally is a developing roll having an internal magnet wherein an external metal cylinder rotates, and the carrier is made to stay by the magnetic force of the internal magnet so that it is not developed to the photosensitive body. However, it is hard to completely make the carrier stay, and slight carrier developing occurs.
A phenomenon in which the carrier is developed as mentioned above generally is called carrier deposition, and also raises a problem of causing a toner image transfer failure to occur in the transfer step of a post-step of developing in one-color image formation in the related art. As a measure against the carrier deposition, after developing, a carrier collection roll having an internal ma
Ishii Masayoshi
Kato Hisayoshi
Kobayashi Shin'ya
Mabuchi Hiroyuki
Mitsuya Teruaki
Hitachi Koki Co,. Ltd.
Pendegrass Joan
Sughrue & Mion, PLLC
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