U.S. patent number 3,651,784 [Application Number 04/838,779] was granted by the patent office on 1972-03-28 for low potential development electrode.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to Robert E. Hewitt.
United States Patent |
3,651,784 |
Hewitt |
March 28, 1972 |
LOW POTENTIAL DEVELOPMENT ELECTRODE
Abstract
A low potential electrode for use in a xerographic development
system. The electrode is positioned adjacent to the point of entry
of a moving image bearing photoconductive surface into a
development zone and has associated therewith means for redirecting
the flow of developer material moving between the electrode and the
photoconductive surface whereby the developer material impinges
against the photoconductive surface.
Inventors: |
Hewitt; Robert E. (Penfield,
NY) |
Assignee: |
Xerox Corporation (Rochester,
NY)
|
Family
ID: |
25278025 |
Appl.
No.: |
04/838,779 |
Filed: |
July 3, 1969 |
Current U.S.
Class: |
118/636;
399/294 |
Current CPC
Class: |
G03G
15/0801 (20130101) |
Current International
Class: |
G03G
15/08 (20060101); G03g 013/00 () |
Field of
Search: |
;118/636,637
;117/17.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Stein; Mervin
Assistant Examiner: Millstein; Leo
Claims
What is claimed is:
1. Apparatus for controlling the movement of two-component
developer material which is flowing through a development zone in
close proximity to a moving latent electrostatic image bearing
member, including:
an electrode positioned in the development zone in a closely spaced
parallel relationship with the latent electrostatic image bearing
member, said electrode being located in a region of the development
zone wherein a latent image being transported on the image bearing
member initially engages the developer material;
means, associated with said electrode, for directing the flow of
the developer material in substantially a parallel direction
relative to said electrode along a first portion thereof;
means, associated with said electrode, for redirecting the flow of
the developer material from being in substantially a parallel
direction relative to said electrode along the first portion
thereof to being substantially in a transverse direction relative
to said electrode along a second portion thereof, thereby effecting
the impingement of the developer material with the image bearing
member;
means for electrically biasing said electrode to a potential less
than any potential on the image bearing member; and
means for moving the image bearing member in a direction opposed to
the direction of movement of the developer material.
2. The apparatus of claim 1 wherein the said electrode is
maintained at a ground potential.
3. In a xerographic developing apparatus of the type wherein a
rotatable photosensitive drum is moved through a development zone
and in which a continuous flow of two-component developer material
is moved in close proximity with said drum surface, the apparatus
including
means to rotate the drum in opposition to the flow of developer
material,
an electrode positioned in the development zone in close parallel
relation to the moving drum surface adjacent to the start of said
development zone at which point the latent electrostatic image
moves into close proximity with said flow of two-component
developer material,
means to electrically bias said electrode to a potential less than
any potential on the drum surface, and
means associated with said electrode to redirect the flow of
developer material moving through the start of the development
region and said electrode between the drum into impingement against
the drum surface.
4. The apparatus of claim 3 wherein at least a portion of said
electrode is located below the horizontal centerline of said
drum.
5. The apparatus of claim 4 wherein said electrode is maintained at
a ground potential.
6. The apparatus of claim 4 wherein said electrode is maintained at
approximately 150 volts below any potential on the drum surface.
Description
This invention relates to xerographic development and, in
particular, to a development to produce rapid and efficient
development at the image entrance to a xerographic development
system.
More specifically, this invention relates to a low potential
development electrode having means associated therewith to redirect
a flow of developer material moving between the electrode and an
image bearing plate wherein the developer is impinged against the
plate surface whereby images are rapidly and efficiently developed
in opposed flow systems or in inverted development zones.
In the art of xerography, as originally disclosed by Carlson in
U.S. Pat. No. 2,287,691, a xerographic plate formed of a conductive
backing upon which is placed a conductive insulating material, is
charged uniformly and a surface of the plate exposed to a light
image of an original to be reproduced. The photoconductive coating
is caused to become conductive under the influence of the light
image so as to selectively dissipate the electrostatic charge found
thereon, thus producing an electrostatic latent image. The latent
image is made visible or developed by means of a variety of
pigmented resins which have been specifically developed for this
purpose. The pigmented resin material, or toner, is
electrostatically attracted to the latent image area on the
photoconductive surface in proportion to the amount of charge found
thereon. Areas of small charge concentration become areas of low
toner density while areas of greater charge concentration become
proportionally more dense. The fully developed image is generally
transferred to a final support material, as for example, paper, and
the image fixed thereto to form a permanent record of the
original.
One of the most widely used methods of developing a latent
electrostatic image is by means of the "cascade" technique in which
a two-component developer material is caused to flow over an image
bearing photoconductive surface to effect image development. It has
been found that when certain resin based toner materials are
brought into rubbing contact with triboelectrically remote granular
beads, commonly referred to as "carrier," an electrostatic charge
is generated between the two materials. The toner and carrier
materials are triboelectrically charged to opposite polarities and
the finer particulate toner material adheres to the carrier in a
charged state. As the toner ladened beads move in contact with the
photoconductive surface, the toner is electrostatically stripped
from the carrier and attracted into the image areas by the
relatively strong latent electrostatic image force fields.
In conventional downhill development, both the image bearing
photoconductive surface and the developer flow are arranged to move
in the same direction. The downwardly flowing carrier beads first
give up their toner material as explained above to develop the
latent electrostatic image. The toner deplete carrier beads which
have given up some of their toner in the development process become
electrostatically unbalanced and seeks to neutralize themselves by
attracting more charged toner material to their surfaces. As the
toner starved beads flow over the plate, they eventually overtake
developer image areas on the plate and perform a second important
function. The moving beads mechanically scrub and dislodge loosely
held toner from the photoconductive surface in the non-imaged areas
and electrostatically attract and hold the removed toner material
to the bead surface thereby effectively cleaning the plate
surface.
Downhill cascade development has enjoyed a wide commercial success
in automatic xerographic machines because of its inherent
advantages. Downhill cascade development, however, is dependent on
the xerographic plate being arranged to move in the same direction
as the developer flow and therefore seriously restricts the
configuration of the automatic apparatus in which it can be
utilized. Often times it is desirous in automatic xerography to
develop a latent electrostatic image on a moving photoconductive
surface that is inverted or moving in opposition to the flow of
developer material.
It is therefore a primary object of this invention to improve
xerographic development.
A further object is to improve xerographic development apparatus so
that rapid and efficient image development can be produced on a
photoconductive surface particularly a surface which is moving in
opposition to the developer flow or in an inverted development
zone.
These and other objects of the present invention are attained by
apparatus to control the flow of two-component developer material
moving through a development zone in contact with a moving latent
electrostatic image bearing photoconductive surface including an
electrode positioned in the development zone in close parallel
relation to the entrance to the development zone of the latent
electrostatic image being transported therethrough on the
photoconductive surface, means to electrically bias said electrode
to a charge lower than the electrostatic charge on the drum surface
in the non-imaged areas, means to interrupt the flow of developer
material moving between the electrode and the photoconductive
surface such that the developer material is impinged against the
photoconductive surface.
For a better understanding of the invention as well as other
objects and further features thereof, reference is had to the
following detailed description of the invention to be read in
conjunction with the accompanying drawings wherein:
FIG. 1 is a schematic side elevation of an automatic xerographic
machine utilizing the present invention;
FIG. 2 is an enlarged side elevation of the development apparatus
used in the xerographic reproducing machine shown in FIG. 1 with
parts broken away to better illustrate the construction
thereof;
FIG. 3 is a schematic view of the force fields associated with the
lower electrode shown in FIG. 2;
FIG. 4 is an enlarged partial sectional view of the developing
apparatus of FIG. 2 showing the notched portion of the main
developing electrode;
FIG. 5 is an enlarged partial sectional view of the developing
apparatus of FIG. 2 showing the entrance chute to the developer
zone and the cleanup electrode.
The general apparatus of the instant invention as shown herein
embodies in an automatic xerographic machine employing a
drum-shaped xerographic plate 10 comprising a photoconductive layer
which is placed upon a conductive backing. Drum 10 is mounted on
shaft 11 journaled in the machine frame (not shown) and is rotated
in the direction indicated by the arrow by means of motor 9 causing
the drum surface to sequentially pass through a plurality of
xerographic processing stations.
For the purpose of the present invention, the several xerographic
processing stations positioned in the path of movement of drum 10
as shown in FIG. 1 may be described functionally as follows:
a charging station A, at which is positioned a corona generating
device 12 for placing a uniform positive electrostatic charge on
the photoconductive layer of the drum surface as the drum is driven
in the direction indicated;
an exposure station B, at which a light or radiation pattern of the
original document to be reproduced, which is supported on platen
14, is projected onto the drum surface thereby dissipating the
charge found thereon in the areas exposed so as to form a latent
electrostatic image of the original document;
a developing station C, having a housing generally designated 20,
in which a two-component developer material utilizing, in this
case, negatively charged toner particles is delivered to the
entrance of the development zone from where it is caused to flow in
opposition over an upwardly moving portion of the drum surface by
means of bucket conveyors system 27 thus enabling the toner
particles to contact and adhere to the electrostatic image on the
drum surface to form a developed powder image in image
configuration of the original document to be reproduced;
a transfer station D, at which the xerographic powder image is
electrostatically transferred from the drum surface to a sheet of
final support material by means of transfer corotron 24; and
a drum cleaning and discharge station E, at which the drum surface
is exposed to a cleaning corotron 39 and then contacted by means of
a doctor blade 41 to remove residual toner particles which may
remain thereon after the transfer operation and where the drum
surface is exposed to the source of illumination 43 to effect
substantially complete discharge of any residual electrostatic
charge remaining thereon.
Transfer corotron 24 located at station D sprays the backside of
the final support material with positive ions thus producing a
charge of sufficient magnitude on the back of the paper to attract
the toner from the drum surface to the final support material.
However, the positive charge which is sprayed over the non-imaged
background areas electrostatically tacks the support material to
the moving drum surface. A plurality of arcuate shaped stripper
fingers positioned subsequent to the transfer station are arranged
to lift the leading edge of the tacked support material from the
drum and direct the material upwardly. As the drum continues to
drive the support material forward, the finger strip the material
from the drum and guide it into contact with vacuum transport 26.
The support material, a portion of which is still electrostatically
tacked to the drum surface is caused to move along the vacuum
transport towards fuser assembly 30.
Under the influence of the rotating drum, the support material is
moved along the stationary vacuum transport 26 into the nip between
the upper fuser roller 31 and the lower fuser roll 32. The coacting
fuser rolls are arranged to apply a pressure driving force to the
sheet of support material positioned therebetween and to forward
the sheet at a synchronous speed with the rotation drum surface. A
radiant source of energy 33 extending transverse to the lower fuser
roll, is capable of rapidly transferring heat energy to the lower
fuser roll. The heat energy is stored on the surface of the roll
and is brought into thermal contact with the image bearing support
material as the lower roll is rotated in the direction indicated by
the arrow. Image fixing is accomplished in this xerographic
reproducing device by means of a combination of pressure and heat
energy which is transferred to the powder image as the support
material moves through the fuser roll assembly.
The copy, with the fixed image thereon, is transported through a
circular path of travel, generally referred to as 40, comprising a
series of pinch rolls arranged to either discharge a final support
material from the apparatus into catch tray 36 or to feed the
support material back into a second feed tray 35. Support material
stored in feed tray 35 are once again redirected through the
xerographic processing stations to form a second or duplex image on
the backside thereof.
It has long been known in the xerographic art that the developing
characteristics of a two-component material is substantially
altered when the material is caused to flow between a biased
electrode and an image bearing photoconductive surface during the
developing process. Although the exact reason for this change in
developability has heretofore not been clearly understood, the
results achieved were nevertheless clearly evident in the quality
of the copy produced.
Tests were conducted to determine what effect an electrode had on
the dynamic flow characteristics of a two-component developer
material during development. An extended stationary electrode was
placed in the developer housing of an automatic xerographic machine
and positioned in close parallel relation to a rotating drum
surface. The drum was xerographically imaged by conventional
xerographic techniques to produce a latent image thereon. The
electrode was biased to a potential similar in polarity to the
latent image and having a magnitude below the image potential but
above background potential. Broad solid areas were xerographically
imaged on the drum surface and then developed by passing a
continuous flow of developer material between the drum surface and
the electrode with the drum being moved in opposition to the
developer flow. It was noted that the electrode enhanced the solid
area image development. However, in all cases, the leading edge,
that is, the edge of the image which first passed through the
electroded region, had a washed out or underdeveloped appearance.
It was theorized that in the above environment, a directional force
field is established between the electrode and the background areas
on the drum surface which acts to force toner particles in the
developer flow towards the electroded side of the system. In other
words, a toner gradient is established in the developer flow when a
non-imaged area is in the electroded zone resulting in a heavy flow
concentration of toner moving along the electroded side of the
system. When a non-imaged area precedes a latent image into the
electroded region, insufficient toner is initially available at the
drum surface to fully develop the leading edge of the image.
However, the image potential soon becomes the dominant force in the
system and the toner is then attracted from the electrode to the
drum resulting in a shifting of the toner gradient. Because the
photoconductor was moving in opposition to the developer flow
during these tests, the time to complete the toner gradient shift
was clearly reflected by an underdevelopment along the leading edge
of the images. Once the shift was completed, however, a good solid
area development resulted.
To prove that a toner gradient was established in the flow of
two-component developer material by a development electrode and
that this gradient was responsive to changes in the electrostatic
field characteristics of the system, the above experiment was
repeated with two relatively broad latent images being
xerographically imaged longitudinally across the drum surface.
Initially, the two imaged areas were offset from each other at some
distance so that a relatively long non-imaged history was
experienced by the development system as the images followed each
other through the electroded zone. With the relatively long
non-imaged history separating the two images, the lead edge effect
on both images was evident. The non-imaged history between images
was progressively shortened by moving the latent electrostatic
images closer together. A point finally was reached wherein the
second image through the electroded region no longer had evidence
of an underdeveloped lead edge thereon. The lead edge of the first
image through the electroded zone, however, clearly showed signs of
underdevelopment. It was evident that the non-imaged history
between the images had been shortened so that insufficient time was
provided to allow the gradient to shift from the drum side of the
system. Sufficient toner thus was still available at the drum
surface to develop the leading edge of the second image through the
electroded region.
From these tests it was concluded that a toner gradient could be
established in a two-component developer flow moving through an
electroded development zone. Furthermore, the developability of the
system could be controlled by positioning the toner gradient in or
out of contact with the photosensitive plate to be developed.
Apparatus is herein disclosed in which the above findings are
utilized to produce a development system in which complete control
is afforded over a two-component developer material as the material
moves through a xerographic development zone. Although the present
apparatus is illustrated in an opposed flow system, it should be
clear, because of the complete control afforded over the developer
material, that the present apparatus is equally well adapted for
use in a wide variety of machine environment and is in no way
restricted to use in the particular embodiment. It should be
further evident, that the present apparatus can be utilized in any
number of two-component development systems to develop image upon
xerographic plates which are supported in any number of positions
inverted or otherwise.
Referring now specifically to FIG. 2 through 5, the apparatus of
the present development system basically comprises a series of
conductive control members separated by insulating blocks which are
supported in close parallel relation to a moving xerographic drum
surface so as to form a continuous enclosed flow path therebetween.
This flow path is herein referred to as the development zone and is
numerically designated 51. Positioned at the upper entrance to the
enclosed development zone is an entrance chute, generally
referenced 60, through which a continuous flow of two-component
developer material is introduced into the development zone. A
series of conductive control members form the backwall of the
development zone 51 and, as will be explained in greater detail
below, function to control the distribution of developer material
in the flow stream during development. The front wall of the
development zone is described by the upwardly moving drum surface.
It should be noted that in this particular development apparatus,
the drum surface is moving upwardly in opposition to the downwardly
moving developer flow stream. This particular flow relationship
between developer and plate is directly opposite that utilized in
most conventional cascade development as disclosed by Walkup in
U.S. Pat. No. 2,618,551 in that therefore, the carrier beads do not
operate in the classical sense to first give up toner during the
development process and then, when partial denuded, scavange the
weakly held background development from the non-imaged areas.
The conductive control members, and the insulating blocks
separating these members, are mounted on a non-conductive rigid
support frame 50 and the frame affixed to the sidewalls of the
developer housing 20. An opening is provided in one end wall of the
housing through which the rotating drum surface is allowed to pass
in close proximity to the conductive control members supported
therein. The control members and the insulating blocks both extend
horizontally across the drum surface and have end seals (not shown)
provided which ride in contact with the extreme ends of the drum
surface to enclose the development zone 51.
Two-component developer material is transported from a storage and
mixing area in the sump 55 of the developer housing into an
entrance chute 61 by means of a conveyor 27 (FIG. 1). The conveyor
is made-up of a series of horizontally extended elongated buckets
56 affixed to an endless belt which passes over pulley assemblies
57 and 58. As the buckets are transported in the direction
indicated through the developer sump area, the buckets become
loaded with developer material. The continuous movement of the
buckets through the developer mix sufficiently agitates the
developer mix to produce triboelectric charging of the materials.
The loaded bucket, upon leaving the sump area, are raised to the
top of the developer housing where they are discharged into
entrance chute 60 thus supplying a continuous flow of material to
the development zone.
The developer material delivered into the entrance chute is
introduced into the development zone 51 where it is allowed to flow
downwardly under the influence of gravity in opposition to the
upwardly moving photoconductive plate surface. The behavior of the
developer material, as it passes through the development zone, is
closely and automatically controlled by the control member to
develop the plate surface and clean unwanted background which
results in the development of extremely clear, clean, xerographic
images. Because of the unique sensitivity of the present system, a
wide variety of images such as line copy, solid area, half toned
images or any combination thereof can be processed without changing
the electrical or mechanical parameters of the present system. As
the developer material leaves the development zone, it is
intercepted by a pickoff baffle 62 which is mounted in close
proximity to the drum surface in the lower portion of the developer
housing. The intercepted developer material is redirected down an
incline chute 65 back into sump area 55 where it is stored and
recharged preparitory to being reused in the xerographic developing
process. Also positioned immediately below the pickoff baffle is a
developer housing seal 66 adapted to coact with the moving drum
surface to prevent any developer material which might migrate into
this area from escaping from the developer housing.
After the plate is charged and exposed, the latent image is
transported upwardly on the drum surface through the bottom opening
provided into development zone 51. It should be noted that in this
embodiment, the point of entry of the latent electrostatic image is
also the point at which developer material is leaving the
development zone. However, as will become apparent from the
discussion below, the developer material in this start of
developing region is, because of the systems unique characteristic
properly charged and in condition to produce complete image
development in a short period of time. In fact, a slightly
over-developed condition is produced in this start of development
region. More toner then is required for image development at the
drum surface at this time which results in some background being
developed. However, excessive background development is not harmful
in the present apparatus because the background is efficiently and
effectively cleaned from the plate surface as the plate moves
through the development zone.
A latent electrostatic image produced on a photoconductive surface,
such as a selenium drum, characteristically has an electrostatic
field charge pattern which is extremely strong and dense along the
edges or outer fringes. However, the density and strength of the
force field components, particularly the components perpendicular
to the plate surface, progressively diminish as you move away from
the edge areas. During development, the stronger and more dense
field components associated with the fringe areas reach out and
pull oppositely charged toner particles. However, the weaker and
less dense components associated with the large interior solid
areas cannot effectively or rapidly capture toner particles and
therefore these areas generally appear washed out due to
underdevelopment. A low potential development electrode 70, as
illustrated in FIGS. 2 and 3 is placed in close proximity to the
moving plate surface in the start of development region. The term
"low potential" as herein used applies broadly to any potential
which is lower than the potential found in the non-imaged areas on
the drum surface and the term is broad enough to include a grounded
electrode, an electrode biased to a polarity opposite to that on
the drum surface, or even a floating electrode. A drastic change is
noted in the developability of the system, particularly in regard
to solid and half-toned image areas, when a low potential electrode
is brought into close proximity to an underdeveloped latent
electrostatic image on the plate surface. The electrode causes the
field components normally associated with the weaker interior force
fields to become strengthened and more dense. By controlling the
electrical potential applied to the electrode, these fields
components can be made directional to force the charged toner
particles moving in the flow stream against the image bearing drum
surface.
Electrode 70 is connected to a suitable biasing source 96 through
means of wiring 113 and electrical terminal 76. The electrode is
placed at a potential below the potential found in the non-imaged,
or exposed areas on the drum surface. A force field is established
in this inverted portion of the development zone which acts to
force the toner in the flow stream upwardly against the drum
surface. The electrode, because it is biased below the background
potential, not only strengthens the force fields associated with
the solid image areas but also strengthens the force fields
associated with the exposed non-imaged areas so that a relatively
strong field exists across the entire drum surface in this
electroded region. This results in an extremely heavy concentration
of toner being made available at the drum surface at the start of
image development. This concentration is illustrated by the heavy
dark area in the flow stream as shown in FIG. 3. It should be
noted, that the placement of the heavy concentration of toner at
the drum surface in this inverted region is controlled by the
electrode. The force of gravity, although present, is negated by
the force field so that the toner in the stream flows in contact
with the inverted drum surface.
To further enhance early image development in the present
apparatus, the toner particles moving though the low potential
electroded region are first dislodged or freed from the carrier
beads so that the particles can be more readily acted upon and
controlled by the force field. Dislodgement of the toner is
accomplished by impacting the toner ladened carrier beads against
the drum surface as the low potential electrode region. A chamfer
79 is cut in the leading edge of electrode 70. The flow of
developer material, because it is moving through an inverted
region, is at this time moving in supporting contact with the
electroded side of the system and therefore falls into flowing
contact with the undercut chamfer which is shaped to redirect the
flow upwardly into contact with the drum surface. Upon striking the
drum, toner particles are jarred loose from the carrier beads and a
powder cloud of free toner material is formed in this start of
development region. The free toner which is still moving along
under the influence of the flow stream, is readily transported to
the drum side of the system by the directional electrostatic force
fields so that a toner gradient is now created in the flow with a
heavy concentration of toner being made available at the drum
surface. Although a notch or chamfer is herein used to redirect the
developer flow, any suitable means for directing developer material
into contact with the drum surface without seriously impeding the
developer flow could be similarly utilized.
The next subsequent conductive member positioned in relation to the
direction of drum rotation is main development electrode 71. As
previously noted, the main development electrode is electrically
isolated from the low potential electrode by means of a dielectric
block 75. A suitable biasing source 97 is connected to the main
development electrode by means of wiring 114 and electrical
terminal 81. The two adjacent electrodes 70, 71 are of
substantially equal thickness. However, the dielectric block is
constructed to be of lesser thickness so that a pocket or void is
created on the backside of the system in the inverted zone. The
developer material in the flow stream readily falls into the void
and flows into contact with chamfer 79.
It has been found that the developability of the latent
electrostatic images in the low potential electrode region is
increased in direct proportion to the number of bead contacts that
can be made against the drum surface as well as the velocity at
which the beads strike the drum. The angle at which the chamfer
directs the beads into contact with the drum surface is therefore
optimized in this area both to increase the number of bead contacts
and the velocity of bead impact. It should be clear that his
optimum angle will vary as the position of the lower potential
electrode is changed in relation to the moving photoconductive
surface and is not necessarily the same in all development
systems.
The main development electrode is placed at an electrical bias such
that the electrode has a charge polarity similar to the charge
polarity placed on the drum surface and being of a magnitude less
than that of the charged image potential but greater than that of
the background potential thereon. By placing the main development
electrode as a potential somewhere between the image and the
background potentials the electrode acts as a self-regulating
device capable of enhancing image development at the same time
producing cleaning or scavanging of random background development
from the drum surface.
When an initially developed image passes from the low potential
electrode region into the main development electrode region, strong
force fields associated with the imaged areas predominate and a
directional force field is established tending to move the toner in
the flow stream into contact with the image bearing drum surface
where it is most needed to enhance and complete image development.
On the other hand, when a non-imaged area moves into the main
development electrode region stronger force field components
associated with the development electrode act to attract toner
particles towards the charged electrode member. A heavy
concentration of toner is therefore found on the backside of the
system in these areas and relatively toner deplete carrier beads
are presented to the drum surface by the flow stream. The toner
deplete carrier beads contacting the drum both mechanically scrub
and electrostatically attract loosely held toner from these
background areas. The toner removed by the beads either migrates
towards the backside of the system or is electrostatically bonded
to the bead surface. In any event, the toner material so removed is
captured in the flow system and carried along therewith away from
the cleaned surface area.
It can be seen, the main development electrode is capable of
reacting to the presence or absence of an image moving through the
main development electrode region and acts more or less as a switch
to shift the toner gradient within the flow stream in response to
the drum condition. The two-component developer material in the
flow stream is thus utilized in this region to either clean
unwanted background from the drum surface or to further enhance
development of the latent electrostatic images. In an opposed
system as herein described, the leading edge of the electrostatic
image is the trigger which starts the toner gradient to shift in
response to the change in condition on the drum surface. As
previously noted, a time lag in the response of the system is
experienced as the toner gradient shifts from one side of the
system to the other resulting in underdevelopment of the leading
edge of the image. This time delay in the electrical response of
the system is mechanically overcome in the present apparatus so
that the lead edge effect is eliminated.
Referring now specifically to FIG. 4, a notch or groove 85 is
placed in the inverted portion of the main development electrode
71. Developer material moving down the inverted portion of the main
development region falls into the void created by the groove. The
bottom surface of the groove is shaped so as to once again redirect
the main flow stream of developer material upwardly into impinging
contact with the moving drum surface. When the main development
electrode is acting as a cleaning device, this impingement of the
carrier beads against the non-imaged areas aids in mechanically
scrubbing or cleaning of residual toner therefrom. Similarly, the
impingement of developer material against a developed image area on
the drum surface aids in the development process by first
physically transporting toner from the backside of the system into
contact with the image areas thus overcoming any time delay
associated with the toner gradient shift and secondly by creating a
toner powder cloud of free toner in and about the image areas where
the free toner particles can be readily attracted into the image
areas.
Here again, both developability and cleaning have been found to be
directly proportional to the number of bead impacts that can be
maintained and the velocity at which the beads strike the drum
surface. The shape of the notch or groove 85 in the main
development electrode therefore is designed to both optimize the
number of bead impacts and the velocity at which the beads strike
the drum surface. Although a notch is herein disclosed, it should
again be made clear that any suitable means capable of directing
the flow of developer material into impingement against the drum
surface without impeding the developer flow can be utilized.
The next subsequent electrode in relation to the direction of drum
travel is a cleanup electrode 72. As illustrated in FIG. 2, the
cleanup electrode is positioned in the upper part of the
development zone adjacent to the opening through which fresh
developer material is first introduced into the development zone.
The cleanup electrode is physically positioned adjacent to the main
development electrode and electrically isolated therefrom by means
of a dielectric block 92. The cleanup electrode functions primarily
to establish an extremely high directional force field capable of
attracting toner material to the electroded side of the system to
control the movement of free or weakly held toner particles through
the upper part of the development zone. The cleanup electrode
further functions to condition the carrier beads moving in contact
with the plate in this region to clean unwanted background from the
plate surface.
The cleanup electrode is connected to a suitable bias source 98 by
means of wire 115 and electrical terminal 95. The biasing source
functions to place the cleanup electrode at an extremely high
potential of a polarity similar to the polarity found in the
charged image areas on the drum surface. An extremely strong
directional force field is produced in this upper region of a
strength sufficient to force free or weakly held toner particles
away from the drum surface. Any free toner material found in the
region of the cleanup electrode region is therefore moved under
controlled conditions along the backside of the system. The charge
field established in this region is of sufficient strength to also
strip some of the toner, particularly weakly attracted toner from
the beads flowing in contact with the drum and move this toner
toward the back or electroded side of the development zone. With a
preponderence of the toner material concentrated in the backside of
the flow stream, beads moving in contact with the drum surface are
relatively deplete of toner particles and therefore can more
readily function to scrub and electrostatically attract weakly held
background development from the drum surface. As can be seen, the
cleanup electrode produces a twofold effect to prevent background
development from leaving the development zone. First, the free
toner and weakly held toner is forced away from the plate surface
and secondly carrier beads at the plate surface are conditioned to
mechanically and electrostatically clean background from the plate.
By determining the image charge found on the photoconductive
surface, and by placing an electrical charge on the cleanup
electrode substantially above this charge, that is, somewhere in
the range of between 150-700 volts above image potential, the above
noted results can be effectively obtained.
It should be apparent now that the movement of developer material
into the active development zone in the present apparatus must be
controlled both electrostatically and mechanically in order to
suppress and control the formation of unwanted powder clouds in
this introductory region. As illustrated in FIG. 5, the developer
entrance chute 60 is primarily formed by a horizontally extended
inclined conductive baffle 73 and a horizontally extended arcuate
shaped shield 74. Although not shown, both ends of the chute
between the baffle and the shield are closed by means of an
insulating material wherein the chute is capable of supporting
therein a quantity of two-component developer material. The
inclined baffle 73 has a downwardly extended leg 86 thereon which
turns at a relatively gentle radius into the development zone and
extends downward therein to form a portion of the backwall of the
development system. The shield 74, which is formed of a conductive
material, is supported in a relatively horizontal position upon an
insulating support 90. The lower portion of the shield terminates
in a lip 87 which is positioned adjacent to the turned portion of
the inclined baffle plate between the baffle and the
photoconductor. Lip 87 is complimentary to the curvature of the
turned portion of leg 86 on the lower baffle and cooperates
therewith to form an entrance throat 61 extending horizontally
across the width of the development zone. The chute is basically
funneled shaped tapering down from a relatively wide mouth to a
restricted throat 61. The throat opening is substantially equal in
width to the distance maintained between the drum surface and the
control electrodes so that a similar volume rate of flow is
supported in the introductory zone as is maintained in the active
development zone. In operation, the buckets deliver a continuous
supply of developer material into the funnel shaped entrance chute.
Although this developer material is normally delivered into the
entrance chute at a relatively high velocity, the velocity of the
developer material is initially throttled in the chute before it is
delivered through the throat entrance into the active development
region. The initial reduction in the developer flow velocity
combined with the gentle flow path presented to the developer
material results in a significant reduction in mechanical agitation
of the two-component developer material thus suppressing the
tendency of the material to form powder clouds in and about the
introductory region to the development zone.
The inclined baffle 73, which makes up the lower wall of the
entrance chute 60 is placed at a relatively high potential by means
of a suitable biasing source 99 acting through wire 116 and
terminal 88. The lower baffle is placed at an electrical bias
having the same polarity as the charged polarity of the images on
the drum surface, however, the magnitude of the charge is
substantially greater than the image charge potential and the chute
electrically isolated from the cleanup electrode by means of a
dielectric block 93. On the other hand, the shield 74 is placed at
a relatively low potential, a potential lower than the entrance
chute potential, by means of biasing source 100 acting through wire
117 and terminal 89. Here again, the term low potential is used
broadly as described above. By maintaining the shield at a
relatively low potential while holding the baffle at a relatively
high potential, extremely strong force field is created in the
introductory region capable of attracting and/or forcing charged
toner particles moving through this region to the backside of the
system. It has been found that the reinforced force field created
in the entrance chute establishes a toner gradient in the flow of
developer material prior to the material coming in contact with the
drum surface.
Again, because the dependent leg 86 on the lower portion of lower
baffle 73 turns gently into the development zone and because the
velocity of the developer material passing through the entrance
chute is substantially throttled, little mechanical agitation of
the toner is experienced in this introductory region thereby
suppressing the tendency of powder clouds from being formed.
Furthermore, the magnitude and strength to which the two baffle
plates are biased creates an extraordinarily strong directional
force field in the introductory region tending to attract any free
and loosely held toner material in this region away from the
photoconductive surface. These two effects combine to insure that
little or no random toner particles migrate toward the plate
surface as the material is introduced into the active development
zone.
An optimum combination for the system as herein disclosed is one in
which the conductive electroded members are spaced between the
0.070 and 0.080 inches from the drum surface and the low potential
electrode biased at a substantially ground potential while holding
the main development electrode at between 50 and 150 volts above
the background voltage on the plate surface. It has been found that
a triangular shaped groove in the main development electrode have
an included angle of approximately 120 degrees will give good
results when positioned 10.degree. to 20.degree. below the
horizontal centerline of the drum. Furthermore, because the
response produced by the main development electrode is slower than
that produced by the low potential electrode, it has been found
that the main development electrode should be from three to four
times longer in relation to the direction of developer flow than
the low potential electrode. To further obtain optimum results it
is desirous to place the cleanup electrode at a potential between
150 and 700 volts above the image potential found on the charge
plate surface while the conductive shield and baffle comprising the
entrance chute are funneled down towards each other so that they
form a throat which is between 0.065 and 0.800 inches wide at the
introductory region to the development zone and the inclined baffle
biased to a potential between 300 and 700 volts above the image
potential found on the plate surface and the shield placed at
approximately a ground potential.
It should be clear from the prior discussion that the development
system herein described is a dynamic flow system. That is, the
toner gradient which is established in the flow stream and which is
directed towards or away from a moving drum surface is not
necessarily captured and held by the electrode but is in fact
carried along under the influence of the carrier beads in the flow
stream. It should be therefore obvious that the electrodes do not
become collectors of loose toner material but merely function as a
means of controlling the concentration of toner in the flow as it
moves through the development zone. By varying the intensity and
direction of the component force fields created in various parts of
the development zone and by redirecting the flow of developer
material at predetermined points to impinge the material against
the photoconductive surface, the developability of the system is
enhanced and controlled so that a quality xerographic image is
produced. The system as herein disclosed is not gravity dependent
and therefore is not limited to use in any specific machine
configuration. That is, the electrostatic and mechanical control of
the two-component developer flow is equally well adapted for use in
any development system utilizing a flow of two-component developer
material as a developing mechanism.
For purposes of explanatory convenience, references may have been
made in this disclosure to positively charged carrier materials and
negatively charged toner particles. It should be understood that a
description of this specific nature of the potentials involved on
the respective conductive members is not intended to limit this
invention to this specific relationship. It would be possible to
utilize carrier and toner materials having vastly different
relationships in regard to their triboelectric characteristics.
This of course would result in the requiring of similar changes in
the relationship of the biased charges found on the various
conductive members. All references to positive or negative charges
in this disclosure are therefore to be considered as defining a
relationship to oppositely charged bodies which may be either
positive or negative as long as this relationship of like or
dissimilar charges is maintained.
While this invention has been disclosed with references to the
structure described herein, it is not to be confined to the details
as set forth, and this application is to cover all modifications
and changes which may come with the scope of the following
claims.
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