U.S. patent number RE35,698 [Application Number 08/527,987] was granted by the patent office on 1997-12-23 for donor roll for scavengeless development in a xerographic apparatus.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to Thomas J. Behe, Grace T. Brewington, Jeffrey J. Folkins, Gerald T. Lioy, Joseph G. Schram, William H. Wayman.
United States Patent |
RE35,698 |
Behe , et al. |
December 23, 1997 |
Donor roll for scavengeless development in a xerographic
apparatus
Abstract
A donor roll for the conveyance of toner in a development system
for an electrophotographic printer includes an outer surface of
phenolic resin. The resin is doped to a suitable conductivity to
facilitate a discharge time constant thereon of less than 300
microseconds. The donor roll is used in conjunction with an
electrode structure as used in scavengeless development.
Inventors: |
Behe; Thomas J. (Webster,
NY), Folkins; Jeffrey J. (Rochester, NY), Lioy; Gerald
T. (Rochester, NY), Brewington; Grace T. (Fairport,
NY), Schram; Joseph G. (Liverpool, NY), Wayman; William
H. (Ontario, NY) |
Assignee: |
Xerox Corporation (Stamford,
CT)
|
Family
ID: |
25497610 |
Appl.
No.: |
08/527,987 |
Filed: |
September 14, 1995 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
955965 |
Oct 2, 1992 |
05245392 |
Sep 14, 1993 |
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Current U.S.
Class: |
399/286;
399/285 |
Current CPC
Class: |
G03G
15/0803 (20130101); G03G 15/0818 (20130101); G03G
2215/0643 (20130101) |
Current International
Class: |
G03G
15/08 (20060101); G03G 015/06 () |
Field of
Search: |
;355/245,259,247,249,261-263,265,215 ;118/653,647-651,654,661
;399/286,285 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0-458-603-B1 |
|
Nov 1991 |
|
EP |
|
56-22459 |
|
Mar 1981 |
|
JP |
|
56-123581 |
|
Sep 1981 |
|
JP |
|
57-118278 |
|
Jul 1982 |
|
JP |
|
58-16273 |
|
Jan 1983 |
|
JP |
|
58-220165 |
|
Dec 1983 |
|
JP |
|
59-7384 |
|
Jan 1984 |
|
JP |
|
59-51364 |
|
Mar 1984 |
|
JP |
|
60-17462 |
|
Jan 1985 |
|
JP |
|
60-17463 |
|
Jan 1985 |
|
JP |
|
60-17770 |
|
Jan 1985 |
|
JP |
|
60-17461 |
|
Jan 1985 |
|
JP |
|
61-13270 |
|
Jan 1986 |
|
JP |
|
61-140967 |
|
Jun 1986 |
|
JP |
|
61-148478 |
|
Jul 1986 |
|
JP |
|
61-194463 |
|
Aug 1986 |
|
JP |
|
61-254969 |
|
Nov 1986 |
|
JP |
|
61-284584 |
|
Dec 1986 |
|
JP |
|
62-47673 |
|
Mar 1987 |
|
JP |
|
62-247386 |
|
Oct 1987 |
|
JP |
|
63-92369 |
|
Jun 1988 |
|
JP |
|
63-307489 |
|
Dec 1988 |
|
JP |
|
64-34656 |
|
Mar 1989 |
|
JP |
|
1-102485 |
|
Apr 1989 |
|
JP |
|
1-138577 |
|
May 1989 |
|
JP |
|
1-142756 |
|
Jun 1989 |
|
JP |
|
1-142753 |
|
Jun 1989 |
|
JP |
|
1-142752 |
|
Jun 1989 |
|
JP |
|
1-142750 |
|
Jun 1989 |
|
JP |
|
1-142749 |
|
Jun 1989 |
|
JP |
|
1-142748 |
|
Jun 1989 |
|
JP |
|
1-144077 |
|
Jun 1989 |
|
JP |
|
1-142751 |
|
Jun 1989 |
|
JP |
|
1-142754 |
|
Jun 1989 |
|
JP |
|
1-142755 |
|
Jun 1989 |
|
JP |
|
1-142576 |
|
Jun 1989 |
|
JP |
|
1-142575 |
|
Jun 1989 |
|
JP |
|
1-142574 |
|
Jun 1989 |
|
JP |
|
1-144078 |
|
Jun 1989 |
|
JP |
|
1-255873 |
|
Oct 1989 |
|
JP |
|
1-257881 |
|
Oct 1989 |
|
JP |
|
4-162060 |
|
Jun 1992 |
|
JP |
|
4-220667 |
|
Aug 1992 |
|
JP |
|
4-240676 |
|
Aug 1992 |
|
JP |
|
Other References
Translation of Japanese Patent Application Laid-Open No. 4-255878;
Laid Open Date: Sep. 10, 1992; pp. 4-5 and one page of drawings,
Figs. 1 and 3-7. .
Translation of Japanese Patent Application Laid-Open No. 4-243279;
Laid Open Date: Aug. 31, 1992; pp. 6-12 and two pages of drawings,
Figs. 1-7. .
"A Design of Experiment Study of Plasma Sprayed Alumina-Titania
Coatings", T. J. Steeper et al., Proceedings of the International
Thermal Spray Conference & Exposition, Orlando, Florida, USA,
28 May-5, Jun., 1992,pp. 415-420. .
"A Taguchi Experimental Design Study of Plasma Sprayed
Alumina-Titania Coatings", T. J. Steeper et al., Proceedings of the
Fourth National Thermal Spray Conference, Pittsburgh, PA, USA 4-10
May 1991, pp. 13-20. .
"A Taguchi Design of Experiment Study of Plasma Sprayed Alumina
Coatings", T. J. Steeper et al., Proceedings of the 1993 National
Thermal Spray Conference, Anaheim, CA 7-11 Jun., 1993, pp. 31-36.
.
"Plasma Spray Coating Parameter Development", J. Walter et al.,
Thermal Spray Research and Applications, Proceedings of the Third
National Thermal Spray Conference, Long Beach, CA, USA, 20-25 May,
1990, pp. 729-742..
|
Primary Examiner: Smith; Matthew S.
Attorney, Agent or Firm: Oliff & Berridge
Claims
We claim:
1. An apparatus for developing an electrostatic latent image,
comprising:
a housing defining a chamber for storing a supply of developer
material therein;
a donor roll, having an outer surface .[.including phenolic
resin.]., mounted at least partially in the chamber of said
housing, said donor roll being adapted to advance developer
material to the latent image, with the discharge time constant of
the surface of the donor roll being less than 300 microseconds;
and
an electrode member positioned in the space between the latent
image and the donor roll, the electrode member being closely spaced
from the donor roll and being electrically biased to detach toner
particles from the donor roll so as to form a toner powder cloud in
the space between the electrode member and the latent image with
detached toner particles from the toner cloud developing the latent
image.
2. An apparatus as in claim 1, wherein the .[.phenolic resin.].
.Iadd.outer layer .Iaddend.of the donor roll is doped to a
conductivity greater than 10.sup.-8 (ohm-cm).sup.-1.
3. An apparatus as in claim 1, wherein the electrode member
includes a plurality of small diameter wires.
4. An apparatus as in claim 1, further comprising:
a transport roll mounted in the chamber of the housing and being
positioned adjacent the donor roll, the transport roll being
adapted to advance developer material to the donor roll; and
means for rotating the transport roll and the donor roll.
5. An apparatus as in claim 4, further comprising:
means for applying an alternating electric field between the donor
roll and the transport roll to assist in transferring at least a
portion of the developer material from the transport roll to the
donor roll.
6. An apparatus as in claim 5, wherein the applying means applies
an electrical field that alternates at a selected frequency ranging
between about 200 Hz and about 20 kHz with a voltage less than 400
V.sub.rms.
7. An apparatus for developing an electrostatic latent image,
comprising:
a housing defining a chamber for storing a supply of developer
material therein;
a donor roll mounted at least partially in the chamber of said
housing, said donor roll being adapted to advance developer
material to the latent image; and
an electrode member positioned in the space between the latent
image and the donor roll, the electrode member being closely spaced
from the donor roll and being electrically biased to detach toner
particles from the donor roll so as to form a toner powder cloud in
the space between the donor roll and the latent image with detached
toner particles from the toner cloud developing the latent
image;
wherein the electrical discharge time constant of the surface of
the donor roll is less than 300 microseconds.
8. An apparatus for developing an electrostatic latent image on a
charge-retentive surface, comprising:
a housing defining a chamber for storing a supply of developer
material therein; and
a donor roll mounted at least partially in the chamber of said
housing, said donor roll being adapted to advance developer
material to the latent image, including a conductive inner portion
and a substantially non-metallic outer layer, wherein the thickness
of the outer layer divided by the dielectric constant thereof is
less than the distance between the donor roll and the
charge-retentive surface.
9. An apparatus as in claim 8, wherein the thickness of the outer
layer divided by the dielectric constant thereof is less than the
average diameter of particles in the developer material to be
applied to the charge-retentive surface.
10. An apparatus as in claim 8, further comprising:
an electrode member including at least one wire positioned in the
space between the latent image and the donor roll, the electrode
member being closely spaced from the donor roll and being
electrically biased to detach toner particles from the donor roll
so as to form a toner powder cloud in the space between the donor
roll and the charge-retentive surface with detached toner particles
from the toner cloud developing the latent image; and
wherein the thickness of the outer layer divided by the dielectric
constant thereof is less than the diameter of the wire. .Iadd.
11. An apparatus according to claim 1, wherein said outer surface
of the donor roll includes phenolic resin. .Iaddend..Iadd.
12. A donor roll for use in a machine in which a voltage
differential is applied between a core of the donor roll and a
charging region adjacent to an outer surface of the donor roll, the
donor roll comprising:
a cylindrical core; and
a layer secured to said cylindrical core, wherein said layer is
formed of a semiconductive material selected to control an RC
circuit time constant relating to electrical response of the
semiconductive layer to the applied voltage differential.
.Iaddend..Iadd.13. A donor roll according to claim 12, wherein the
RC circuit time constant is less than 300 microseconds.
.Iaddend..Iadd.14. A donor roll according to claim 12, wherein said
layer comprises a phenolic material. .Iaddend..Iadd.15. A donor
roll according to claim 12, wherein said layer has a thickness of 1
to 25 mils. .Iaddend..Iadd.16. A donor roll according to claim 12,
wherein said layer has a thickness of 1 to 2 mm. .Iaddend..Iadd.17.
A donor roll according to claim 12, wherein said core comprises a
conductive material. .Iaddend..Iadd.18. A donor roll according to
claim 17, wherein said core comprises aluminum. .Iaddend..Iadd.19.
A donor roll according to claim 12, wherein a thickness of said
layer divided by a dielectric constant thereof is less than a
distance between the donor roll and the charging region.
.Iaddend..Iadd.20. A donor roll according to claim 12, wherein said
layer has a thickness less than 1.65 mm. .Iaddend..Iadd.21. A donor
roll according to claim 20, wherein said layer has a thickness less
than 0.33
mm. .Iaddend..Iadd.22. An apparatus for developing an electrostatic
latent image, comprising:
a housing defining a chamber for storing a supply of developer
material therein; and
a donor roll having a cylindrical core and a layer secured to said
cylindrical core and mounted at least partially in the chamber of
said housing, wherein said layer is formed of a semiconductive
material selected to control an RC circuit time constant relating
to electrical response of the semiconductive layer to the applied
voltage differential.
.Iaddend..Iadd.23. An apparatus according to claim 22, wherein the
RC circuit time constant of said layer of said donor roll is less
than 300 microseconds. .Iaddend..Iadd.24. An apparatus according to
claim 22, wherein said layer comprises a phenolic material.
.Iaddend..Iadd.25. An apparatus according to claim 22, wherein said
layer has a thickness of 1 to 25 mils. .Iaddend..Iadd.26. An
apparatus according to claim 22, wherein said layer has a thickness
of 1 to 2 mm. .Iaddend..Iadd.27. An apparatus according to claim
22, wherein said core comprises a conductive material.
.Iaddend..Iadd.28. An apparatus according to claim 27, wherein said
core comprises aluminum. .Iaddend..Iadd.29. An apparatus according
to claim 22, wherein a thickness of said layer divided by a
dielectric constant thereof is less than a distance between the
donor roll and the latent image. .Iaddend..Iadd.30. An apparatus
according to claim 22, further comprising an electrode member
positioned in the space between the latent image and the donor
roll, the electrode member being closely spaced from the donor roll
and being electrically biased to detach toner particles from the
donor roll so as to form a toner powder cloud in the space between
the electrode member and the latent image with detached toner
particles from
the toner cloud developing the latent image. .Iaddend..Iadd.31. An
apparatus according to claim 22, wherein said layer has a thickness
less than 1.65 mm. .Iaddend..Iadd.32. An apparatus according to
claim 31, wherein said layer has a thickness of less than 0.33
mm.
.Iaddend..Iadd. An electrophotographic printing machine of the type
having a developer unit adapted to develop with toner an
electrostatic latent image recorded on a photoconductive member,
wherein the developer unit comprises:
a housing defining a chamber for storing a supply of developer
material therein; and
a donor roll having a cylindrical core and a layer secured to said
cylindrical core and mounted at least partially in the chamber of
said housing, wherein said layer is formed of a semiconductive
material selected to control an RC circuit time constant relating
to electrical response to the semiconductive layer to the applied
voltage differential. .Iaddend..Iadd.34. A printing machine
according to claim 33, wherein the RC circuit time constant of said
layer of said donor roll is less than 300 microseconds.
.Iaddend..Iadd.35. A printing machine according to claim 33,
wherein said layer comprises a phenolic material.
.Iaddend..Iadd.36. A printing machine according to claim 33,
wherein said layer has a thickness of 1 to 25 mils.
.Iaddend..Iadd.37. A printing machine according to claim 33,
wherein said layer has a thickness of 1 to 2 mm. .Iaddend..Iadd.38.
A printing machine according to claim 33, wherein said core
comprises a conductive material. .Iaddend..Iadd.39. A printing
machine according to claim 38, wherein said core comprises
aluminum. .Iaddend..Iadd.40. A printing machine according to claim
33, wherein a thickness of said layer divided by a dielectric
constant thereof is less than a distance between the donor roll and
the latent image. .Iaddend..Iadd.41. A printing machine according
to claim 33, further comprising an electrode member positioned in a
space between the latent image and the donor roll, the electrode
member being closely spaced from the donor roll and being
electrically biased to detach toner particles from the donor roll
so as to form a toner powder cloud in the space between the
electrode member and the latent image with detached toner particles
from the toner cloud developing the
latent image. .Iaddend..Iadd.42. A printing machine according to
claim 33, wherein said layer has a thickness less than 1.65 mm.
.Iaddend..Iadd. . A printing machine according to claim 42, wherein
said
layer has a thickness less than 0.33 mm. .Iaddend..Iadd.44. A donor
roll of a scavengerless xerographic apparatus comprising:
a cylindrical core; and
a layer secured to said cylindrical core and formed of a
semiconductive material;
wherein a discharge time constant of said layer is selected to
discharge a predetermined portion of a charge on said layer within
a predetermined time period. .Iaddend.
Description
The present invention relates to developer apparatus for
electrophotographic printing. More specifically, the invention
relates to a donor roll as part of a scavengeless development
process.
In the well-known process of electrophotographic printing, a charge
retentive surface, typically known as a photoreceptor, is
electrostatically charged, and then exposed to a light pattern of
an original image to selectively discharge the surface in
accordance therewith. The resulting pattern of charged and
discharged areas on the photoreceptor form an electrostatic charge
pattern, known as a latent image, conforming to the original image.
The latent image is developed by contacting it with a finely
divided electrostatically attractable powder known as "toner."
Toner is held on the image areas by the electrostatic charge on the
photoreceptor surface. Thus, a toner image is produced in
conformity with a light image of the original being reproduced. The
toner image may then be transferred to a substrate or support
member (e.g., paper), and the image affixed thereto to form a
permanent record of the image to be reproduced. Subsequent to
development, excess toner left on the charge retentive surface is
cleaned from the surface. The process is useful for light lens
copying from an original or printing electronically generated or
stored originals such as with a raster output scanner (ROS), where
a charged surface may be imagewise discharged in a variety of
ways.
In the process of electrophotographic printing, the step of
conveying toner to the latent image on the photoreceptor is known
as "development." The object of effective development of a latent
image on the photoreceptor is to convey toner particles to the
latent image at a controlled rate so that the toner particles
effectively adhere electrostatically to the charged areas on the
latent image. A commonly used technique for development is the use
of a two-component developer material, which comprises, in addition
to the toner particles which are intended to adhere to the
photoreceptor, a quantity of magnetic carrier beads. The toner
particles adhere triboelectrically to the relatively large carrier
beads, which are typically made of steel. When the developer
material is placed in a magnetic field, the carrier beads with the
toner particles thereon form what is known as a magnetic brush,
wherein the carrier beads form relatively long chains which
resemble the fibers of a brush. This magnetic brush is typically
created by means of a "developer roll." The developer roll is
typically in the form of a cylindrical sleeve rotating around a
fixed assembly of permanent magnets. The carrier beads form chains
extending from the surface of the developer roll, and the toner
particles are electrostatically attracted to the chains of carrier
beads. When the magnetic brush is introduced into a development
zone adjacent the electrostatic latent image on a photoreceptor,
the electrostatic charge on the photoreceptor will cause the toner
particles to be pulled off the carrier beads and onto the
photoreceptor. Another known development technique involves a
single-component developer, that is, a developer which consists
entirely of toner. In a common type of single-component system,
each toner particle has both an electrostatic charge (to enable the
particles to adhere to the photoreceptor) and magnetic properties
(to allow the particles to be magnetically conveyed to the
photoreceptor). Instead of using magnetic carrier beads to form a
magnetic brush, the magnetized toner particles are caused to adhere
directly to a developer roll. In the development zone adjacent the
electrostatic latent image on a photoreceptor, the electrostatic
charge on the photoreceptor will cause the toner particles to be
pulled off the developer roll and onto the photoreceptor.
An important variation to the general principle of development is
the concept of "scavengeless" development. The purpose and function
of scavengeless development are described more fully in, for
example, U.S. Pat. No. 4,868,600 to Hays et al., U.S. Pat. No.
4,984,019 to Folkins, U.S. Pat. No. 5,010,367 to Hays, or U.S. Pat
No. 5,063,875 to Folkins et al. In a scavengeless development
system, toner is conveyed to the photoreceptor by means of AC
electric fields supplied by self-spaced electrode structures,
commonly in the form of wires extending across the photoreceptor,
positioned within the nip between a donor roll and photoreceptor.
Because there is no physical contact between the development
apparatus and the photoreceptor, scavengeless development is useful
for devices in which different types of toner are supplied onto the
same photoreceptor, as in "tri-level" or "recharge, expose, and
develop" highlight or image-on-image color xerography.
A typical "hybrid" scavengeless development apparatus includes,
within a developer housing, a transport roll, a donor roll, and an
electrode structure. The transport roll operates in a manner
similar to a developer roll, but instead of conveying toner
directly to the photoreceptor, conveys toner to a donor roll
disposed between the transport roll and the photoreceptor. The
transport roll is electrically biased relative to the donor roll,
so that the toner particles are attracted from the transport roll
to the donor roll. The donor roll further conveys toner particles
from the transport roll toward the photoreceptor. In the nip
between the donor roll and the photoreceptor are the wires forming
the electrode structure. During development of the latent image on
the photoreceptor, the electrode wires are AC-biased relative to
the donor roll to detach toner therefrom so as to form a toner
powder cloud in the gap between the donor roll and the
photoreceptor. The latent image on the photoreceptor attracts toner
particles from the powder cloud, forming a toner powder image
thereon.
Another variation on scavengeless development is single-component
scavengeless development, also known as scavengeless SCD. In
scavengeless SCD, the donor roll and the electrode structure create
a toner powder cloud in the same manner as the above-described
scavengeless development, but instead of using a magnetic brush to
convey toner particles from the toner supply in the developer
housing to the donor roll, a portion of the donor roll is exposed
directly to a supply of single-component developer, which is pure
toner. Scavengeless SCD provides the same advantages as the basic
case of hydrid scavengeless development, and is useful in
situations where the size, weight, or power consumption of the
apparatus is of particular concern.
In any type of scavengeless development apparatus, one of the most
important elements is the donor roll which conveys toner particles
to the wires forming the electrode structure in the nip between the
donor roll and the photoreceptor. Broadly speaking, a donor roll
can be defined as any roll in which pure toner particles are
intended to adhere to the surface thereof. In order to function in
a commercially-practical embodiment of scavengeless development, a
donor roll must meet certain requirements. In general, a donor roll
should include a conductive core and define a partially conductive
surface, so that the toner particles may adhere electrostatically
to the surface in a reasonably controllable fashion. In hybrid
scavengeless development, the donor roll provides an electrostatic
"intermediate" between the photoreceptor and the transport roll.
The provision of this intermediate and the scavengeless nip is to
prevent unwanted interactions between the development system and
the photoreceptor, in particular with a pre-developed latent image
already on the photoreceptor before the latent image in question is
developed. This lack of interaction makes scavengeless development
preferably in situations where a single photoreceptor is developed
numerous times in a single process, such as in color or in
highlight-color xerography.
The donor roll must further have desirable wear properties so the
surface thereof will not be abraded by adjacent surfaces within the
apparatus, such as the magnetic brush of a transport roll. Further,
the surface of the donor roll should be without anomalies such as
pin holes, which may be created in the course of the manufacturing
process for the donor roll. The reason that this such small surface
imperfections must be avoided is that any such imperfections,
whether pinholes created in the manufacturing process or abrasions
made in the course of use, is that such imperfections can result in
electrostatic "hot spots" caused by arcing in the vicinity of such
structural imperfections. Ultimately, the most important
requirement of the donor roll can be summarized by the phase
"uniform conductivity;" the surface of the donor roll must be
partially conductive relative to a more conductive core, and this
partial conductivity on the surface should be uniform through the
entire surface area. Other physical properties of the donor roll,
such as the mechanical adhesion of toner particles, are also
important, but are generally not as quantifiable in designing
development apparatus. In addition, the range of conductivity for
the service of a donor roll should be well chosen to maximize the
efficiency of a donor roll in view of any number of designed
parameters, such as energy consumption, mechanical control and the
discharge time-constant of the surface.
U.S. Pat. No. 3,950,089 discloses a development apparatus in which
a surface for the direct conveyance of electrically-conductive
toner comprises a dielectric sheath of a thickness of 1-25 mils,
having a resistivity of 10.sup.7 to 10.sup.9 ohm-cm.
U.S. Pat. No. 4,034,709 discloses a development apparatus in which
a surface for the direct conveyance of toner comprises
styrene-butadiene, of a resistivity of 10.sup.2 to 10.sup.6
ohm-cm.
U.S. Pat. No. 4,774,541 discloses a development apparatus in which
a surface for the direct conveyance of toner is doped with carbon
black to a conductivity of 10.sup.-6 to 10.sup.-10 1/ohm-cm.
In the prior art, there are numerous instances in which the
physical properties of phenolic resin are exploited for various
purposes relating to development of electrostatic latent images.
U.S. Pat. No. 4,827,305 discloses a development apparatus in which
a transport roll includes restricting rollers mounted on the ends
thereon which are made of phenolic resin.
U.S. Pat. No. 4,989,044 discloses another single-component
developer system in which the sleeve of a transport roll has an
outer coating layer made of a resin material, such as phenolic
resin, in which electrically conductive fine particles are
disbursed.
U.S. Pat. No. 4,990,963 discloses a transport roll made of a resin
solution of 17% phenol resin.
U.S. Pat. No. 5,054,419 discloses a developing apparatus wherein a
cylindrical bar is used to meter the amount of toner on the surface
of a transport roll. In one embodiment, the cylindrical bar is made
of phenolic resin.
U.S. Pat. No. 5,099,285 discloses experiments with a transport roll
for single-component development in which the outer layer is in one
embodiment made of phenolic plastic.
According to the present invention, there is provided an apparatus
for developing an electrostatic latent image. A housing defines a
chamber for storing a supply of developer material therein. A donor
roll, having an outer surface comprising phenolic resin, is mounted
at least partially in the chamber of said housing, to advance
developer material to the latent image. An electrode member is
positioned in the space between the latent image and the donor
roll, closely spaced from the donor roll and electrically biased to
detach toner particles from the donor roll so as to form a toner
powder cloud in the space between the electrode member and the
latent image with detached toner particles from the toner cloud
developing the latent image.
According to one preferred embodiment of the present invention, the
phenolic resin comprising the outer surface of the donor roll has a
discharge time constant of less than 300 microseconds. Further, the
surface is doped to conductivity of greater than 10.sup.-8 (ohm
cm).sup.-1.
According to another aspect of the present invention, there is
provided a donor roll having a non-metallic outer layer wherein the
thickness of the outer layer divided by the dielectric constant
thereof is less than certain other parameters of the development
system.
In the drawings:
FIG. 1 is a simplified elevational view of a hybrid scavengeless
development station, incorporating a donor roll according to the
present invention;
FIG. 2 is a simplified elevational view of a single component
scavengeless development station, incorporating a donor roll
according to the present invention; and
FIG. 3 is a simplified elevational view of an electrophotographic
printing apparatus in which the present invention may be
embodied.
While the present invention will be described in connection with a
preferred embodiment thereof, it will be understood that it is not
intended to limit the invention to that embodiment. On the
contrary, it is intended to cover all alternatives, modifications,
and equivalents as may be included within the spirit and scope of
the invention as defined by the appended claims.
Inasmuch as the art of electrophotographic printing is well known,
the various processing stations employed in the FIG. 3 printing
machine will be shown hereinafter schematically and their operation
described briefly with reference thereto.
Referring initially to FIG. 3, there is shown an illustrative
electrophotographic printing machine incorporating the development
apparatus of the present invention therein. The printing machine
incorporates a photoreceptor 10 in the form of a belt having a
photoconductive surface layer 12 on an electroconductive substrate
14. Preferably the surface 12 is made from a selenium alloy. The
substrate 14 is preferably made from an aluminum alloy which is
electrically grounded. The belt is driven by means of motor 24
along a path defined by rollers 18, 20 and 22, the direction of
movement being counter-clockwise as viewed and as shown by arrow
16. Initially a portion of the belt 10 passes through a charge
station A at which a corona generator 26 charges surface 12 to a
relatively high, substantially uniform, potential. A high voltage
power supply 28 is coupled to device 26. After charging, the
charged area of surface 12 is passed to exposure station B.
At exposure station B, an original document 30 is placed face down
upon a transparent platen 32. Lamps 34 flash light rays onto
original document 30. The light rays reflected from original
document 30 are transmitted through lens 36 to form a light image
thereof. Lens 36 focuses this light image onto the charged portion
of photoconductive surface 12 to selectively dissipate the charge
thereon. This records an electrostatic latent image on
photoconductive surface 12 which corresponds to the informational
areas contained within original document 30.
After the electrostatic latent image has been recorded on
photoconductive surface 12, belt 10 advances the latent image to
development station C. At development station C, a development
system, develops the latent image recorded on the photoconductive
surface. Preferably, development system includes a donor roller 40
and electrode wires positioned in the gap between the donor roll
and photoconductive belt. Electrode wires 41 are electrically
biased relative to donor roll 40 to detach toner therefrom so as to
form a toner powder cloud in the gap between the donor roll and
photoconductive surface. The latent image attracts toner particles
from the toner powder cloud forming a toner powder image thereon.
Donor roll 40 is mounted, at least partially, in the chamber of
developer housing 38. The chamber in developer housing 38 stores a
supply of developer material. The developer material is a two
component developer material of at least magnetic carrier granules
having toner particles adhering triboelectrically thereto. A
transport roller disposed interiorly of the chamber of housing 38
conveys the developer material to the donor roller. The transport
roller is electrically biased relative to the donor roller so that
the toner particles are attracted from the transport roller to the
donor roller. The development apparatus will be discussed
hereinafter, in grater detail, with reference to FIG. 1.
After the electrostatic latent image has been developed, belt 10
advances the developed image to transfer station D, at which a copy
sheet 54 is advanced by roll 52 and guides 56 into contact with the
developed image on belt 10. A corona generator 58 is used to spray
ions on to the back of the sheet so as to attract the toner image
from belt 10 the sheet. As the belt turns a round roller 18, the
sheet is stripped therefrom with the toner image thereon.
After transfer, the sheet is advanced by a conveyor (not shown) to
fusing station E. Fusing station E includes a heated fuser roller
64 and a back-up roller 66. The sheet passes between fuser roller
64 and back-up roller 66 with the toner powder image contacting
fuser roller 64. In this way, the toner powder image is permanently
affixed to the sheet. After fusing, the sheet advances through
chute 70 to catch tray 72 for subsequent removal from the printing
machine by the operator.
After the sheet is separated from photoconductive surface 12 of
belt 10, the residual toner particles adhering to photoconductive
surface 12 are removed therefrom by a rotatably mounted fibrous
brush 74 in contact with photoconductive surface 12. Subsequent to
cleaning, a discharge lamp (not shown) floods photoconductive
surface 12 with light to dissipate any residual electrostatic
charge remaining thereon prior to the charging thereof for the next
successive imaging cycle.
It is believed that the foregoing description is sufficient for
purposes of the present application to illustrate the general
operation of an electrophotographic printing machine incorporating
the development apparatus of the present invention therein.
Referring now to FIG. 1, there is shown development system 38 in
grater detail. Housing 38 defines a chamber for storing a supply of
developer material 47 therein. Positioned in the bottom of housing
38 is a horizontal auger which distributes developer material
uniformly along the length of transport roll 46, so that the
lowermost part of roll 46 is always immersed in a body of developer
material.
Transport roll 46 comprises a stationary multi-polar magnet 48
having a closely spaced sleeve 50 of non-magnetic material,
preferably aluminum, designed to be rotated about the magnetic core
48 in a direction indicated by the arrow. Because the developer
material includes magnetic carrier granules, the effect of the
sleeve rotating through stationary magnetic fields is to cause
developer material to be attracted to the exterior of the sleeve. A
doctor blade 62 is used to limit the radial depth of developer
remaining adherent to sleeve 50 as it rotates to the nip 68 between
transport roll 46 and donor roll 40. The donor roll is kept at a
specific voltage, by a DC power supply 76, to attract a thin layer
of toner particles transport roll 46 in nip 68 to the surface of
donor roll 40. Either the whole of the donor roll 40, or at least a
peripheral layer thereof, is preferably of material which has low
electrical conductivity, as will be explained in detail below. The
material must be conductive enough to prevent any build-up of
electric charge with time, and yet its conductivity must be low
enough to form a blocking layer to prevent shorting or arcing of
the magnetic brush to the donor roll.
Transport roll 46 is biased by both a DC voltage source 78 and an
AC voltage source 80. The effect of the DC electrical field is to
enhance the attraction of developer material to sleeve 50. It is
believed that the effect of the AC electrical field applied along
the transport roll in nip 68 is to loosen the toner particles from
their adhesive and triboelectric bonds to the carrier particles. AC
voltage source 80 can be applied either to the transport roll as
shown in FIG. 1, or directly to the donor roll in series with
supply 76.
It has been found that a value of up to 200 V.sub.rms is sufficient
for the output of source 80 for the desired level of reload
efficiency of toner particles to be achieved. The actual value can
be adjusted empirically: in theory it could be any value up to a
voltage of about 400 V.sub.rms. The source should be at a frequency
of about 2 kHz. If the frequency is too low, e.g. less than 200 Hz,
banding will appear on the copies. If the frequency is too high,
e.g. more than 15 kHz, the system would probably work but the
electronics may become expensive because of capacitive loading
losses.
Electrode wires 41 are disposed in the space between the belt 10
and donor roller 40. A pair of electrode wires are shown extending
in a direction substantially parallel to the longitudinal axis of
the donor roll 40. The electrode wires are made from of one or more
thin (i.e. 50 to 100 .mu.m diameter) steel wires which are closely
spaced from donor roller 40. The distance between the wires and the
donor roll 40 is approximately 25 .mu.m or the thickness of the
toner layer formed on the donor roll 40. The wires are self-spaced
from the donor roller by the thickness of the toner on the donor
roller. To this end the extremities of the wires supported by the
tops of end bearing blocks also support the donor roller for
rotation. The wire extremities are attached so that they are
slightly below a tangent to the surface, including toner layer, of
the donor structure. Mounting the wires in such a manner makes them
insensitive to roll runout due to their self-spacing. An
alternating electrical bias is applied to the electrode wires by an
AC voltage source 84. The applied AC establishes an alternating
electrostatic field between the wires and the donor roller which is
effective in detaching toner from the surface of the donor roller
and forming a toner cloud about the wires, the height of the cloud
being such as not to be substantially in contact with the belt
10.
At the region where the photoconductive belt 10 passes closest to
donor roll 40, a stationary shoe 82 bears on the inner surface of
the belt. The position of the shoe relative to the donor roll
establishes the spacing between the donor roll and the belt. The
position of the shoe is adjustable and it is positioned so that the
spacing between the donor roll and photoconductive belt is
preferably about 0.4 mm.
Another factor which has been found to be of importance is the
speed with which the sleeve 50 is rotated relative to the speed of
rotation of donor roll 40. In practice both would be driven by the
same motor, but a gear train would be included in the drive system
so that sleeve 50 is driven at a significantly faster surface
velocity than is donor roll 40. A transport roll:donor roll speed
ratio of 3:1 has been found to be particularly advantageous, and
even higher relative speeds might be used in some embodiments of
the invention. In other embodiments the speed ratio may be as low
as 2:1.
FIG. 2 is a simplified plan view of a single-component scavengeless
development station. The specific design of the single-component
station in FIG. 2 is generally disclosed in U.S. Pat. No.
5,128,723, assigned to the assignee of the present application. In
FIGS. 1 and 2, like reference numerals indicate like elements. As
in the hybrid system of FIG. 1, the single-component system
includes a donor roll 40 and electrode wires 41, but the donor roll
40 picks up toner to convey to the photoreceptor 10 directly from a
supply of pure toner in the housing 38. In the single-component
system of FIG. 2, there is no transport roll 46 and therefore no
carrier beads are used in the developer. The specific design of the
developer station in FIG. 2 may include special items useful in
single-component developing, such as a charging rod 78 or
electrically biased toner mover 94, the precise function of which
is described in the above-reference patent.
According to the present invention, and referring to either FIGS. 1
or 2, the outer surface 42 of donor roll 40 is made from a
self-supporting cylinder of phenolic resin, preferably of the type
manufactured by Tokai Rubber Industries of Japan, particularly of
the "LGC" and "GCS'" formulations which are proprietary to that
manufacturer. When this outer roll of phenolic is used, the core of
donor roll 40 is intended to be of a conventional conductive
material, such as aluminum. This phenolic resin is extruded in a
self-supporting tube, doped to obtain a preselected conductivity,
and, if necessary, ground down through techniques well-known in the
art to assume the desired precise dimensions for a particular
development apparatus. In one embodiment of the present invention,
the intended wall thickness of the phenolic cylinder forming outer
surface 42 is between 1 and 2 mm, on a donor roll 40 having a total
outer diameter of approximately 25 mm; this thickness represents a
compromise between concerns of mechanical stability and cost. It
has been found that this phenolic resin is particularly suited for
the design parameters of a donor roll in scavengeless development,
either of the magnetic brush or single-component variety. Because
the self-supporting tube of phenolic resin may be made with
relatively thick walls, the thickness of the walls can be exploited
to ensure that surface anomalies such as craters or pin holes are
kept to a minimum. Phenolic resin has been shown to be a suitably
hard substance which has presented no significant abrasion problems
when placed within moving contact with a magnetic brush for an
extended period. And, once again, because phenolic resin is
relatively easily worked, it is possible to grind down such a
cylinder to a small extent to ensure precise dimensions.
A key parameter for the outer surface of the donor roll according
to the present invention is the discharge time constant thereof.
The time constant for discharge is the amount of time that 63% of a
given charge on the surface of the donor roll will be dissipated.
As such a time time constant is, as is well-known in electrical
engineering, a function of the resistance and capacitance of the
device in question, it follows that two key parameters for the
composition of the phenolic resin are its conductivity (which
relates to resistance) and its dielectric constant (which relates
to capacitance). Conductivity of the specific additives in the
phenolic resin is a factor in its overall conductivity. Among
conductive agents which may be used to obtain a desired
conductivity are carbon black or graphite, or a partially
conductive substance such as tin oxide or other metal oxide. The
reported dielectric constant for the "GCS'" phenolic is 33. These
parameters relate to the time constant by the relationship
where
.tau..sub.d =donor roll time constant(in seconds)
.epsilon..sub.o =free space permitivity constant
=0.885.times.10.sup.-14 sec/(cm-ohm)
K.sub.d =Phenolic coating dielectric constant (no units)
.sigma..sub.d =Phenolic coating electrical conductivity (in
1/(cm-.OMEGA.)).
As can be seen by the foregoing, the physical attribute of the
phenolic that can be most easily controlled to obtain a desired
time constant is the conductivity of the phenolic, which can be
influenced by the proper concentration and selection of additives.
For the application of a donor roll of the present invention to
hybrid scavengeless development with a magnetic brush transfer
roll, it has been calculated that the most desired range for this
time constant is from 1 to 300 microseconds, with a further
preferred range of 30 to 70 microseconds.
It should be emphasized that this range of desired optimal
discharge time constants is substantially different from previously
preferred ranges in the scavengeless context, particularly those
associated with anodized-aluminum donor rolls. In U.S. Pat. No.
5,063,875, assigned to the assignee of the present invention, for
example, the surface conductivity of a preferred anodized-aluminum
donor roll is 10.sup.-11 (ohm-cm).sup.-1. Considering that the
K.sub.d of anodized aluminum is 9, the resulting .tau..sub.d is 80
microseconds, which is long compared with residence times in the
donor-photoreceptor nip as well as the interface between the
magnetic brush and the donor roll. With a phenolic donor roll
having a discharge time constant in the preferred range according
to the present invention, a suitable range of conductivities is
between 10.sup.-8 and 10.sup.-6 (ohm-cm).sup.-1.
Although the above-described embodiment permits numerous advantages
for a practical development system, the preferred ranges of certain
of the physical properties thereof are selected according to
numerous, and occasionally conflicting, design parameters. These
design constraints are particularly apparent in the hybrid case
described above, wherein the donor roll is "loaded" with toner by
the magnetic brush of the transport roll. Among these design
constraints are: the surface charge relaxation (i.e., the discharge
time constant) of the donor roll; magnetic brush development
relaxation, which relates to the ability of the magnetic brush to
transport a maximum amount of toner to the donor roll across the
nip therebetween, and which is a function of the ability of
electric fields between the transport roll and the donor roll to
collapse quickly as the surface thereof moves away from the nip; AC
frequency relaxation, which relates to the conductivity or
insulative properties of the donor roll relative to the AC in the
electrode wires and affects the field intensification near the
wires; and the image development response of the photoreceptor,
which generally states that, for uniform field strength between the
donor roll and photoreceptor, the dielectric charges on the donor
roll must be able to relax faster than the photoreceptor image
voltages can change.
Taking all of these constraints and others into account, it has
been found that certain of these constraints can be met by a proper
selection of material for the outer surface 42 of donor roll 41,
particularly as regards the thickness s.sub.d of the dielectric
layer forming outer surface 42 and the actual dielectric constant
k.sub.d thereof. One or more such constraints for practical
applications of scavengeless development may be met simply by
providing a ratio s.sub.d /K.sub.d of dielectric thickness over
dielectric constant within certain ranges. (As dielectric constant
has no units, the units of the ratio are in length.) The use of
this ratio to meet some of the above design constraints may be
summarized as follows:
if s.sub.d /K.sub.d <<electrode wire diameter (.apprxeq.50
.mu.m) then the conductivity constraints for the AC frequency
relaxation can be neglected.
<<air gap between donor roll and photoreceptor (.apprxeq.300
.mu.m) then the photoreceptor image development response
constraints can be neglected.
<<toner particle diameter (.apprxeq.10 .mu.m) then the
magnetic brush development relaxation requirement can be
neglected.
While this invention has been described in conjunction with various
embodiments, it is evident that many alternatives, modifications,
and variations will be apparent to those skilled in the art.
Accordingly, it is intended to embrace all such alternatives,
modifications, and variations as fall within the spirit and broad
scope of the appended claims.
* * * * *