Electrophotography – Supplemental electrophotographic process – Exposure or charging
Reexamination Certificate
2001-06-27
2003-01-14
Pendegrass, Joan (Department: 2852)
Electrophotography
Supplemental electrophotographic process
Exposure or charging
C399S175000
Reexamination Certificate
active
06507719
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a transfer image forming apparatus using an electrophotographic process or electrostatic recording process and, more particularly, to an apparatus for forming an image by using a magnetic brush contact charging apparatus as a charging processing means.
2. Related Background Art
As a photosensitive body used in a transfer electrophotographic image forming apparatus, an organic photosensitive body, amorphous-silicon-based photosensitive body (to be referred to as an a-Si-based photosensitive body hereinafter), or the like is often used. The a-Si-based photosensitive body has high surface hardness, exhibits high sensitivity with respect to a semiconductor laser or the like, and suffers almost no deterioration due to repetitive use, and hence is used as an electrophotographic photosensitive body for a high-speed copying machine or laser beam printer (LBP).
As a charging processing means for a photosensitive body, a corona charging apparatus using electric discharge has been put into practice. However, since the a-Si-based photosensitive body has a relative dielectric constant as large as 11 to 12, which is larger than that of an organic photosensitive body, the capacitance is large. As a consequence, for example, the charging ability tends to deteriorate, and image deletion tends to occur due to latent image deletion caused by discharge.
In contrast to this, if the a-Si-based photosensitive body is charged by a contact charging member using a conductive roller, a fur brush, a magnet roller bearing magnetic particles, or the like, since the surface of the a-Si-based photosensitive body is formed by a layer of 10
9
to 10
14
&OHgr;cm, a charge potential almost equivalent to the DC component of a bias applied to the contact charging member can be obtained on the photosensitive body surface.
Such a charging method is called “injection charging” because the photosensitive body is charged by directly injecting charge into the body without using discharge. If this injection charging is used, completely ozoneless, low-power-consumption charging can be performed because the photosensitive body is charged without using any discharge phenomenon as in a case where a corona charging apparatus is used. This technique therefore has attracted a great deal of attention. In addition, this can prevent a deterioration in charging ability and image deletion. Furthermore, since the photosensitive body is charged at a potential near the applied voltage, potential control is facilitated.
In a contact charging apparatus of a magnetic brush scheme as a kind of injection charging scheme, conductive magnetic particles (carriers) are magnetically constrained as a magnetic brush on a magnet directly or on a sleeve incorporating a magnet, and the magnetic brush is brought into contact with the photosensitive body while being stopped or rotated. By applying a voltage to the magnetic brush, charging is started.
FIG. 2
is a cross-sectional view showing an example of the schematic arrangement of a magnetic brush contact charging apparatus
30
. The magnetic brush contact charging apparatus of this example is of a sleeve rotation type.
A rotation drum type electrophotographic photosensitive body
1
as a body to be charged is rotated/driven in the clockwise direction indicated by the arrow at a predetermined peripheral velocity (process speed), e.g., 150 mm/sec.
The charging apparatus
30
includes a housing
305
and a nonmagnetic sleeve (charging sleeve)
303
serving as a magnetic brush bearing member which is rotatably mounted in the housing
305
with its lower surface being exposed to the outside.
A magnet roller (permanent magnet roller)
302
serves as a magnetic field generating member, which is inserted in the nonmagnetic sleeve. This magnet roller
302
is a non-rotating fixed member supported on a fixed central shaft
306
. The two ends of the nonmagnetic sleeve
303
are rotatably and axially supported on the two end sides of the fixed central shaft
306
. The nonmagnetic sleeve
303
is coaxially rotated around the fixed magnet roller
302
at a predetermined peripheral velocity, e.g., 150 mm/sec, in the clockwise direction indicated by the arrow by a driving system (not shown), i.e., in the counter direction with respect to the photosensitive body
1
.
Charging magnetic particles (carriers) are stored in the housing
305
. A regulating blade
301
serves as a magnetic particle regulating means placed at the opening portion of the housing
305
at a predetermined distance from the nonmagnetic sleeve
303
. The charging magnetic particles
304
in the housing
305
are magnetically constrained and borne as a magnetic brush on the outer surface of the nonmagnetic sleeve
303
by the magnetic field generated by the magnet roller
302
in the sleeve. When the charging magnetic particles
304
are rotated/conveyed upon rotation of the nonmagnetic sleeve
303
and pass through the gap between the nonmagnetic sleeve
303
and the regulating blade
301
, the layer thickness is regulated to a predetermined value, and the particles are carried out as a magnetic brush
305
a
of the housing
305
.
The nonmagnetic sleeve
303
is placed to oppose the photosensitive body
1
through a gap smaller than the layer thickness of the magnetic brush
304
a
having undergone the above layer thickness regulation. Therefore, the magnetic brush
304
a
whose layer thickness is regulated by the regulating blade
301
and conveyed to the gap portion where the nonmagnetic sleeve
303
opposes the photosensitive body
1
upon rotation of the nonmagnetic sleeve
303
comes into contact with the surface of the photosensitive body
1
with a width, and moves in a direction opposite to the moving direction of the surface of the photosensitive body
1
to slide on the surface of the photosensitive body
1
. The nip portion width of a magnetic brush contact nip portion n (charged nip portion) is preferably adjusted to 1 to 10 mm.
The magnetic brush
305
a
passing through the gap portion where the nonmagnetic sleeve
303
opposes the photosensitive body
1
is conveyed back into the housing
305
upon rotation of the nonmagnetic sleeve
303
to be cyclically used.
When the nonmagnetic sleeve
303
is rotated, and a predetermined charging voltage is applied from a charging bias application power supply S to the nonmagnetic sleeve
303
, charge is applied from the charging magnetic particles constituting the magnetic brush
305
a
onto the photosensitive body
1
in the charging nip portion n, and the rotating photosensitive drum surface is contact-charged to a value near a potential corresponding to the applied charging voltage.
In the image forming apparatus using the above a-Si-based photosensitive body, an optical memory formed in image exposing operation causes a potential difference after the photosensitive body is charged. This appears as an image. As a means for solving this problem, an ante-charge exposing apparatus (pre-exposing step) for uniformly exposing the photosensitive body is generally placed between a cleaner and a charging member. With this arrangement, an optical memory of a preceding image on the photosensitive body is erased.
An optical memory in the image forming apparatus using the above a-Si-based photosensitive body will be described in more detail below.
When the a-Si-based photosensitive body is charged, and image exposure is performed, optical carriers are generated to lower the charging potential, thereby forming an electrostatic latent image. At this time, many dangling bonds (uncombined bonds) existing in the a-Si-based photosensitive body are set at a localized level to trap some optical carriers, retarding their migration or decreasing the recombination rate of optical carriers. In the image forming process, therefore, some optical carriers generated by exposure are released from the localized level at the same time an electric field is applied to the a-Si-based photosensitive body in charging operation in the next
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Pendegrass Joan
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