Electrophotography – Image formation – Transfer
Reexamination Certificate
2000-09-08
2002-08-13
Brase, Sandra (Department: 2852)
Electrophotography
Image formation
Transfer
C399S310000
Reexamination Certificate
active
06434356
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates generally to a recording apparatus such as a printer, a copying machine, or a facsimile machine which is designed to hold a transfer medium such as a printing sheet in a nip between an image carrier such as a photosensitive medium and a transfer member such as a transfer roller and apply a constant current to the transfer member to transfer visible images such as toner images formed on the image carrier to the transfer medium electrostatically.
2. Background Art
In recent years, as laser printers become increasingly more prevalent, there are increasing needs for compact structure and high-speed operation. Increased concerns about resources and environmental problems also require effective use of paper. From this point of view, printers are required to be capable of transferring images to both face and back of a print sheet. Further, with an increase in user convenience, there is an increasing need for printing images on various types of print mediums.
FIG. 6
shows one example of conventional printers.
The printer includes a brush electrifier
102
, an exposure device
103
, a developing device
104
, a transfer device
105
, a cleaner
106
, a fixing device
107
, a paper feeder
108
, a paper ejector
109
, and a reversing mechanism
110
. The brush electrifier
102
has a rotary brush disposed in contact with a photosensitive drum
101
and rotates and applies the voltage to the rotary brush to electrify the photosensitive drum
101
. The exposure device
103
activates a semiconductor laser in a given emission pattern to exposure the surface of the photosensitive drum
101
to laser beams to form an electrostatic latent image. The developing device
104
is of a non-magnetic component contact type and has disposed therein toner made of solid ink powder. The developing device
104
electrifies the toner and transport it to the surface of the photosensitive drum
101
to develop the electrostatic latent image. The transfer device
105
has a transfer roller
105
which forms a nip between itself and the photosensitive drum
101
through which transfer paper passes to transfer the toner image electrostatically. The cleaner
106
scrapes remaining toner off the photosensitive drum
101
after the image transferring operation. The fixing device
107
has a heat roller which fixes the toner image on the transfer paper with heat and pressure. The paper feeder
108
feeds the transfer paper to the transfer device
105
. The paper ejector
109
ejects the transfer paper out of the fixing device
107
. The reversing mechanism
110
turns over the transfer paper for transferring images on the back of the transfer paper.
In the image transferring operation, the photosensitive drum
101
is rotated at a given process speed. The brush electrifier
102
charges the surface of the photosensitive drum
101
at a black potential (several hundreds of minus volts).
Next, the semiconductor laser exposure device
103
is activated to emit light in a transfer pattern to form a desired latent image on the photosensitive drum
101
. Specifically, charges are produced on laser-exposed portions of the photosensitive drum
101
so that the potential thereof drops to a level called a brightness potential, usually several tens of minus volts.
The toner charged negatively by the developing device
104
is applied on the photosensitive drum
101
through the developing roller to form a toner image. The developing roller made up of a stainless shaft covered with conductive rubber. The electrostatic transfer of the toner to the latent image to form the visible toner image is achieved by pressing the developing roller against the photosensitive drum
101
and applying several hundreds minus volts to the shaft of the developing roller to produce a strong electric field oriented to the latent image.
The photosensitive drum
101
and the transfer roller
105
a
of the transfer device
105
are, as described above, pressed on each other to form a nip (also referred to as a transfer nip below). The toner image moves to the transfer nip with rotation of the photosensitive drum
101
. The transfer paper is transported from the paper feeder
108
to the transfer nip. A given current is supplied from a constant current source (not shown) to the transfer roller
105
a
to form the electric field between the transfer roller
105
a
and the photosensitive drum
101
so that the toner image is transferred electrostatically onto the transfer paper from the photosensitive drum
101
.
The transfer roller
105
a
is made of a stainless shaft covered with a conductive foam such as a rubber resin) having a preselected surface-to-shaft resistance and a preselected hardness. Specifically, the transfer roller
105
a
is made of a flexible elastic member which has the preselected hardness at least on the surface thereof in order to increase an area of contact with the photosensitive drum
101
. The transfer roller
105
a
is forced into constant engagement with the photosensitive drum
101
.
The toner on the photosensitive drum
101
are charged negatively, therefore the polarity of the transfer current is positive. For instance, in a case where a print sheet of a certain size is transported in a lengthwise direction thereof to transfer a toner image thereto, the transfer current required for proper transfer is determined as a function of the process speed and the width of the print sheet and controlled constantly.
The toner remaining on the photosensitive drum
101
is removed by a urethane rubber-made cleaning blade installed in the cleaner
106
to clean the surface of the photosensitive drum
101
for the next operation.
The toner image on the print sheet is transported to the fixing device
107
and fixed by the heat and pressure. The print sheet is then ejected by the paper ejector
109
.
Upon initiation of a back transfer mode of operation, the transfer paper is transported again to the transfer device
105
from the paper ejector
109
through the reversing mechanism
110
. After an toner image is transferred onto the back of the transfer paper and fixed by the fixing device
107
, the transfer paper is ejected by the paper ejector
109
.
Such an image transferring apparatus using a transfer member like the transfer roller
105
a
has a drawback in that transferred image defects such as discharge-caused marks, lack of image density, or toner spots may arise during transfer of an image to the back of transfer paper or when the transfer paper is changed in type.
Some of transfer mediums have a greater change in electric resistance ranging over four figures depending upon a change in environmental condition. For example, thick paper containing cotton has usually a surface electrical resistance (i.e., sheet resistivity) of the order of 10
9
&OHgr;/□ and a volume resistivity of the order of 10
8
&OHgr;/cm at high temperature and high humidity (e.g., 35° C., 80% RH(Relative Humidity)), but they decrease to 10
13
&OHgr;/□ and 10
12
&OHgr;cm, respectively, at low temperature and low humidity (e.g., 5° C., 10% RH).
The problem of electrical resistance may occur during transfer of an image to the back of the transfer paper. This is because an image is transferred again to the transfer paper which has once passed through the fixing device
107
. Specifically, some of transfer mediums require increasing the fixing temperature in order to provide a higher degree of fixing, thereby resulting in a great change in water content of the transfer mediums after the transfer of the image. This causes both the surface electrical resistance and the volume resistivity to increase in a few figures. Specifically, they will be 10
14
&OHgr;/□ and 10
14
&OHgr;cm, respectively, at low temperature and low humidity (e.g., 5° C., 10% RH).
A constant current control system may be used for power supply to the image transferring apparatus. In a case where a change in resistance of the transfer medium is relatively small, the constant cur
Hanzawa Hisashi
Ishijima Hiroyasu
Kamaji Hideki
Sato Kunihiko
Armstrong Westerman & Hattori, LLP
Brase Sandra
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