U.S. patent number 3,901,189 [Application Number 05/437,122] was granted by the patent office on 1975-08-26 for magnetic brush developing apparatus.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to Lawrence J. Fraser.
United States Patent |
3,901,189 |
Fraser |
August 26, 1975 |
Magnetic brush developing apparatus
Abstract
An electrostatic type copier or reproduction machine having one
or more magnetic brush devices for developing the latent
electrostatic images formed on the copier photoconductive member,
such magnetic brush devices being grounded to permit use of
uncoated developer carrier without shorting the magnetic brush or
brushes. To suppress image background and enhance image development
in the absence of bias on the magnetic brushes, a pre-development
corotron is used to place a charge of opposite polarity on the
latent electrostatic images to reduce background charge.
Inventors: |
Fraser; Lawrence J. (Rochester,
NY) |
Assignee: |
Xerox Corporation (Stamford,
CT)
|
Family
ID: |
23735166 |
Appl.
No.: |
05/437,122 |
Filed: |
January 28, 1974 |
Current U.S.
Class: |
399/156 |
Current CPC
Class: |
G03G
15/09 (20130101) |
Current International
Class: |
G03G
15/09 (20060101); G03G 015/08 () |
Field of
Search: |
;118/637 ;117/17.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Stein; Mervin
Assistant Examiner: Salser; Douglas
Claims
What is claimed is:
1. In a xerographic development apparatus for developing the latent
electrostatic images formed on a uniformly charged photoconductive
member, the combination of:
developing means for developing said latent electrostatic images on
said photoconductive member, said developing means including a
developer mix comprised of uncoated conductive carrier and ink
particles, and at least one magnetic developing brush, said brush
being free of electrical bias to permit use of said uncoated
conductive carrier, and
means to suppress image background while avoiding biasing of said
magnetic brush, said suppressing means comprising a charge
producing device upstream of said magnetic developing brush adapted
to expose said photoconductive member to a charge opposite in
polarity to the charge on said photoconductive member before
development whereby to reduce image background charge levels.
2. The apparatus according to claim 1 including means to ground
said magnetic developing brush.
3. The apparatus according to claim 1 in which said charge
producing device is adapted to reverse the polarity of the charge
of said image background areas.
4. In a xerographic development apparatus adapted to use a
developer mix incorporating uncoated conductive carrier particles,
the combination of:
at least one developing member;
means to ground said developing member to obviate shorting of said
developing member by said conductive carrier, and
means to suppress the background areas of a latent electrostatic
image to enhance development thereof by said developing member
without placing a bias voltage on said developing member, said
background suppressing means comprising a charge producing device
upstream of said developing member adapted to reverse the polarity
of the charge comprising said image background areas.
5. In an electrostatic reproduction apparatus having a
photoconductive member on which latent electrostatic images of the
originals being reproduced are made, first charging means for
uniformly charging said member in preparation for imaging, means to
expose said charged member to a document being copied to form a
latent electrostatic image of the document thereon, means to
develop the latent image with a developer material consisting of a
mixture of bare carrier and toner particles, the developing means
including a housing for said mixture and at least one magnetic
brush, and transfer means for transferring the image developed on
said member to copy substrate material, the combination of:
means connecting said magnetic brush to a ground common to said
developer housing whereby a bridging circuit established by a
conglomeration of said carrier particles between said brush and
said housing will not change the developing characteristics of said
magnetic brush; and
second charging means upstream of said magnetic brush to generate a
charge opposite said first charging means, said second charging
means charge being adapted to reduce overall latent electrostatic
image charge so that the charge comprising image background areas
are reversed to suppress developing of said image background areas.
Description
This invention relates to a developing system for reproduction
machines, and more particularly, to an improved developing system
designed for use with uncoated developer carrier.
As will be understood by those familiar with electrostatic type
reproduction and copier machines, the efficacy of such copiers
depends in large measure upon the proper relative charge, i.e., the
electrostatic development field, being maintained between the
copier photoconductive member and the image developing material.
For, as appreciated, this electrostatic development field is relied
upon to attract the ink portion of the developing material,
commonly called toner, to the photoconductive member in conformance
with both the outline and density of the electrostatic latent image
on the photoconductive member. It will be understood that the
electrostatic image was previously formed through exposure of the
charged photoconductive member to a light image of the original
being copied.
Many copier machines employ one or more magnetic brushes to bring
the developing material, which is normally comprised of carrier and
toner, into operative relationship with the photoconductive member.
In magnetic brush development, the magnetic field generated by the
brush or brushes serves initially to attract the developing
material to the brush periphery. Thereafter, as the developing
material is brought into proximity with the photoconductive member
by the moving brush due to the charge relationships between brush,
developing material and photoconductive member, toner brush,
developing material and photoconductivemember, toner is attracted
from the brush to the photoconductive member in conformance with
the latent electrostatic image thereon. To enhance magnetic brush
development and suppress development of unwanted non-image areas,
the magnetic brush or brushes are normally biased to a
predetermined potential.
As noted, the developing material is composed of carrier particles
or beads and toner, the toner particles, due to the triboelectric
relationship between the carrier and toner materials, being
normally attracted to the carrier. It is this carrier bearing toner
that is brought by the developing mechanism, i.e., the aforesaid
magnetic brushes, into juxtaposition with the photoconductive
member to allow the electrostatic charges on the photoconductive
member to draw the toner from the carrier beads and onto the
photoconductive member in conformance with the latent electrostatic
image thereon.
While the aforesaid carrier may comprise various materials and
material compositions, in the case of magnetic developing systems,
the carrier material must have magnetic properties in order for the
magnetic brush or brushes to operate. However, this may give rise
to a shorting problem particularly where uncoated or bare carrier
beads are employed in conjunction with biased magnetic brushes
since the carrier beads may jam or conglomerate between a magnetic
brush and the developing material housing or the photoconductor
itself and thus provide a short circuit path to the brush. This
condition, which alters the operating bias on the magnetic brush or
brushes affected, impairs or destroys developing efficacy, and for
that reason may inhibit use of uncoated carrier in magnetic brush
developing systems, or for that matter, in other development
systems utilizing biased developing components.
It is therefore a principle object of the present invention to
provide a new and improved developing system for electrostatic type
reproduction machines which permits use of either coated or
uncoated developer carrier.
It is an object of the present invention to provide an
electrostatic copier with magnetic brush type developer usable with
bare conductive carrier in the developer mix.
It is a further object of the present invention to provide a
magnetic brush type developing apparatus for reproduction machines
incorporating means to reduce image background charge before
development.
It is an object of the present invention to provide an improved
electrostatic copier with a pre-development corotron to reduce
latent electrostatic image charges thereby avoiding the need to
place a bias on the copier developing mechanism.
It is an object of the present invention to provide an improved
magnetic brush developing arrangement which obviates the need to
utilize insulated operating components.
This invention relates to a xerographic development apparatus
adapted to permit the use of uncoated conductive carrier in the
developer mix used to develop the latent electrostatic images
formed on a uniformly charged photoconductive member, the
combination comprising, means to bring the developer mix including
carrier into operative development relationship with the latent
electrostatic image on the photoconductive member including at
least one magnetic developing brush, and means to suppress image
background to avoid developing image background areas, the image
suppressing means comprising a charge producing device adapted to
expose the photoconductive member to a charge opposite in polarity
to the charge on the photoconductive member whereby to reduce image
background charge levels.
Other objects and advantages of the present invention will be
apparent from the ensuing description and drawings in which:
FIG. 1 is a schematic sectional view of an exemplary electrostatic
type reproduction machine incorporating the image background
suppressing means of the present invention;
FIG. 2 is an enlarged side view of the magnetic brush developing
apparatus of the machine shown in FIG. 1 together with the image
suppressing means of the present invention;
FIG. 3 is an elevational view of the opposite side of the magnetic
brush developing apparatus shown in FIG. 2 illustrating the brush
drives; and
FIG. 4 is a top plan view of the magnetic brush developing
apparatus showing details of the brush biasing means.
For purposes of explanation, the invention is incorporated into an
exemplary copier/reproduction machine designated generally by the
numeral 5 and schematically illustrated in FIG. 1 of the
drawings.
A document 11 to be copied is placed upon a transparent support
platen 16 fixedly arranged in an illumination assembly, generally
indicated by the reference numeral 10, positioned at the left end
of the machine. Light rays from an illumination system are flashed
upon the document to produce image rays corresponding to the
informational areas. The image rays are projected by means of an
optical system onto the photosensitive surface of a xerographic
plate in the form of a flexible photoconductive belt 12 arranged on
a belt assembly, generally indicated by the reference numeral
14.
As seen best in FIG. 4, the belt 12 comprises a photoconductive
layer or surface 6 normally selenium, which is the light receiving
surface and imaging medium for the apparatus, on a conductive
backing or substrate 7. Normally, photoconductive surface 6 is
somewhat less in dimension than that of substrate 7 leaving
uncoated side edge portions 8. The photoconductive surface 6 of the
belt 12 is made photosensitive by a previous step of uniformly
charging the same by means of a corona generating device or
corotron 13. Power for corotron 13 is provided by a suitable power
source.
The belt is journaled for continuous movement upon three rollers
20, 21 and 22 positioned with their axes in parallel. The
photoconductive belt assembly 14 is slidably mounted upon two
support shafts 23 and 24, with the roller 22 rotatably supported on
the shaft 23 which is secured to the frame of the apparatus and is
rotatably driven by a suitable motor and drive assembly (not shown)
in the direction of the arrow at a constant rate. During exposure
of the belt 12, the reflected light image of such original document
positioned on the platen is flashed on the surface of the belt to
produce an electrostatic latent image thereon at exposure station
27.
As the belt surface continues its movement, the electrostatic image
passes through a developing station 28 in which there is positioned
a magnetic brush developing apparatus, generally indicated by the
reference numeral 30, and which provides development of the
electrostatic image by means of multiple brushes as the same moves
through the development zone, as more fully hereinafter
described.
The developed electrostatic image is transported by the belt to a
transfer station 29 whereat a sheet of copy paper or transfer
member is moved between a transfer roller and the belt at a speed
in synchronism with the moving belt in order to accomplish transfer
of the developed image solely by an electrical bias on the transfer
roll. There is provided at this station a sheet transport
mechanism, generally indicated at 17, adapted to transport sheets
of paper from a paper handling mechanism, generally indicated by
the reference numeral 18, to the developed image on the belt at the
station 29.
After the sheet is stripped from belt 12, it is conveyed into a
fuser assembly, generally indicated by the reference numeral 19,
wherein the developed and transferred xerographic powder image on
the sheet material is permanently affixed thereto. After fusing,
the finished copy is discharged from the apparatus at a suitable
point for collection externally of the apparatus. The toner
particles remaining as residue on the developed images, background
particles and those particles otherwise not transferred are carried
by the belt 12 to a cleaning apparatus 26 positioned on the rim of
the belt between rollers 20 and 22 adjacent a pre-clean corona
generator 25. Further details regarding the structure of the belt
assembly 14 and its relationship with the machine and support
therefor may be found in U.S. Pat. No. 3,730,623, issued May 1,
1973, to W. E. Jordan, and assigned to the same assignee.
Referring to FIGS. 2 through 4, there is illustrated the magnetic
brush developing assembly 30 which comprises a series of rollers 36
rotatably supported in frame end plates 32, 34. The rollers 36 are
comprised of a cylindrical sleeve 46 of a roughened surface formed
of a nonpermeable material and extending almost the length of the
housing of the developing apparatus 30. End sleeves 48 are shrunk
fit on each cylindrical sleeve 46 adjacent the ends thereof. One
end of the sleeve 46 is closed by a cap 50 which supports a roller
drive shaft 52 in coaxial alignment with the sleeve 46. The other
end of the sleeve 46 is closed by a cap 54 having an orifice 56
through which extends shaft 58 of the internal bar magnets 61.
Suitable bearing means 60 are provided to permit the sleeve 46 to
rotate relative to shaft 58.
The roller drive shafts 52 are suitably mounted in bearings in end
plate 32 and carry on their projecting ends drive sprockets 96.
Referring specifically to FIGS. 2 and 3, the magnet roller assembly
30 is disposed within a housing, generally indicated as 66, having
a generally rectangular cross section and a length extending beyond
the width of the photoconductive belt 12. Housing 66 is
substantially closed except for an opening opposite photoconductive
belt 12 whereat development of the latent image on belt 12 is
effected. Housing 66 in effect serves as a container for developing
mix or material 75 comprising carrier beads of magnetizable
material and colored electrostatic toner particles which adhere
thereto.
Magnetic brush rollers 36 are grounded through a wiper 70, wiper 70
being in electrical contact with magnet shafts 58. Wiper 70 extends
along and is supported by side plate 34 to form an electrical path
from each of the rollers 36 to ground line 71.
During development, the rollers 36 are rotated in unison in the
same direction from a suitable drive source via sprockets 96, the
internal bar magnets 61 remaining stationary. The brush bristles
produced by the influence of the magnetic field emanating from the
bar magnets 61 acting upon the magnetizable carrier beads in the
developing material will form on the upper region of the sleeve 46
between the rollers 36 and the surface of the selenium belt 12.
This takes the form of a "magnetic blanket" extending continuously
over all of the brushes of rollers 36 for the entire width of the
development zone 28 wherein the material is disposed or available
to some degree for developing purposes. Further details regarding
the formation and effect of the "magnetic blanket" are described in
U.S. Pat. No. 3,640,248, issued Feb. 8, 1972, to W. B. Nielander,
and assigned to the same assignee.
Rollers 36 cooperate with the electrostatic charge on belt 12 to
establish an electrostatic development field in the developing area
28. The aforesaid electrostatic development field serves to attract
toner particles to photoconductor belt 12 in conformance with the
latent electrostatic image on belt 12. As will be understood by
those skilled in the art, the aforesaid electrostatic development
field is essential if the latent images on belt 12 are to be
developed, and must be exactly controlled if optimum development of
the latent image on belt 12 is to be attained. Accordingly, any
uncontrolled change in the aforesaid electrostatic development
field may degrade the image developed to a point where it is less
than satisfactory.
The developing material 75 normally comprises a carrier, usually
small metal beads, and toner or ink particles. The carrier beads
and toner are chosen for their triboelectric relationship. To
enhance the aforedescribed electrostatic development prior practice
has been to bias the magnetic brush rollers 36 to a certain preset
potential level. In that circumstance, the metal carrier beads were
normally coated or encased in a suitable insulating release
material, which, so long as the coating remained intact and was not
worn off, kept the bead insulated. Further, to guard against
shorting of the magnetic brush rollers in these biased brush
systems, the end or mounting sleeves 48 were normally made from
insulating materials to prevent shorting of the brush rollers 36
with the uncoated sides 8 of belt 12. And, in a similar vein, other
components such as the drive sprockets 96 were made from insulating
material.
However, in some applications, it is desirable to use bare or
uncoated metal beads. Should carrier beads of this latter type jam
or compress together between a magnetic brush roller 36 and the
housing 66 or between a roller 36 and the uncoated portion, such as
sides 8, of belt 12, a current or ground path may be formed,
shorting out the brush and upsetting the electrostatic development
field. In addition, metal contaminants may be present in the
developing material and may short out one or more of the biased
brush rollers 36.
To assist in establishing the necessary electrostatic development
field without biasing the magnetic brush rollers 36, a second
corotron 80 is provided between the exposure and development
stations 27, 28, respectively. Corotron 80 is of a polarity
opposite that of charging corotron 13, and is preferably of a
constant current type. One suitable corotron is disclosed in U.S.
Pat. No. 2,868,989, issued Jan. 13, 1959, to A. C. Haacke. Corotron
80 is driven by a suitable operating circuit, such as that shown in
aforecited U.S. Pat. No. 2,868,989, the driving circuit for
corotron 80 being designed to provide preferably a corotron current
approximately one-fourth that of charge corotron 13.
In a typical reproduction or copying machine such as the machine 5
illustrated, charge corotron 13 may establish a charge of
approximately 800 volts positive on the photoconductive belt 12.
Following imaging at exposure station 27, the charge defining the
latent electrostatic image will vary, with image background areas
(the fully exposed portions of the image) being approximately 100
volts positive. Heretofore, in systems where the magnetic brush
rollers 36 are themselves biased, a typical bias might be between
160 to 200 volts positive in order to suppress, i.e. inhibit
development of, image background areas.
In the present invention wherein the several magnetic brush rollers
36 are not biased but are instead grounded, corotron 80 is set to
produce a charge of approximately 200 volts negative. This opposite
charge serves to reduce the overall charge of the latent
electrostatic image by approximately 200 volts with the result that
image background becomes approximately 100 volts negative. This
negative background voltage inhibits, i.e. suppresses, the
attraction of toner particles thereto to prevent unwanted
development of the image background areas.
While a multiple magnetic brush development system has been shown
and described, it will be understood that a single magnetic brush
may be used. And in cascade development systems employing one or
more development electrodes such as shown in U.S. Pat. No.
3,670,700, the development electrodes may be grounded and a
pre-development corotron such as corotron 80 used to inhibit
development of image background areas while allowing uncoated
carrier beads to be used.
While the invention has been described with reference to the
structure disclosed, it is not confined to the details set forth,
but is intended to cover such modifications or changes as may come
within the scope of the following claims.
* * * * *