Ion charging development system to deliver toner with low adhesion

Interdigitated electrodes on a donor roll enable uncharged toner to be picked up from a fluidized bed reservoir. This layer of toner is subsequently charged by exposure to a corona device and delivered to a development zone, where it is used to develop an electrostatic latent image. Residual toner on the donor is neutralized by exposure to a second corona device and then stripped for return to the fluidized bed by applying an AC voltage between adjacent donor electrodes. So-called ion charging of the toner is known to cause the particles to have low adhesion, allowing development with DC fields alone. Optionally, an AC voltage can also be applied to adjacent donor electrodes in the development zone to enhance particle release. In addition to providing a means to impart adequate flow to the toner in this single component development system, the fluidized bed reservoir, in conjunction with ion charging, also provides a means for blending dry powder toners to achieve custom color development.

BACKGROUND OF THE INVENTION 
This invention relates generally to a development apparatus for ionographic 
or electrophotographic imaging and printing apparatuses and machines, and 
more particularly is directed to a process of loading the surface of an 
interdigitated electroded donor roll with uncharged toner particles, 
subsequently corona charging the toner, and forming a toner cloud in a 
development zone. 
Generally, the process of electrophotographic printing includes charging a 
photoconductive member to a substantially uniform potential so as to 
sensitize the surface thereof. The charged portion of the photoconductive 
surface is exposed to a light image from either a scanning laser beam, an 
LED array or an original document being reproduced. By selectively 
discharging certain areas on the photoconductor, an electrostatic latent 
image is recorded on the photoconductive surface. This latent image is 
subsequently developed by charged toner particles supplied by the 
development sub-system. 
Powder development systems normally fall into two classes: two component, 
in which the developer material is comprised of magnetic carrier granules 
having toner particles adhering triboelectrically thereto and single 
component, which typically uses toner only. The development system 
disclosed herein is of the latter, or single component, type. Toner 
particles are attracted to the latent image forming a toner powder image 
on the photoconductive surface The toner powder image is subsequently 
transferred to a copy sheet, and finally, the toner powder image is heated 
to permanently fuse it to the copy sheet in image configuration. 
The adhesion of charged toner particles in large part determines the 
operating latitude of powder xerographic development systems. It has been 
found that triboelectrically charged toner has high electrostatic 
adhesion, due to non-uniform surface charge distributions and localized 
regions of high surface charge density on the toner particles. The high 
adhesion of tribo-charged toner severely restricts the operating latitude 
of powder development systems, particularly those in which a toner cloud 
is generated to develop the latent image. 
For powder xerography, the image quality requirements make it necessary to 
reduce the toner particle size to around 5 microns or less in diameter. 
For printers serving the color offset printing markets, the development 
system requires high quality, high speed and robust toner delivery. The 
ability to blend different color toners to achieve custom colors is 
another requirement. Unfortunately, traditional powder development systems 
based on triboelectric toner charging do not appear to have the operating 
latitude necessary to simultaneously satisfy all of these requirements. As 
will be demonstrated below, however, the use of an ion charging-based 
development system potentially enables the extended capabilities required 
for high quality production color printing with dry powder. 
The operating latitude of a powder xerographic development system is 
determined to a great degree by the ease with which toner particles are 
supplied to an electrostatic image. Placing charge on the particles, to 
enable movement and imagewise development via electric fields, is most 
often accomplished with triboelectricity. However, all development systems 
which use triboelectricity to charge toner, whether they be two component 
(toner and carrier) or monocomponent (toner only), have one feature in 
common: charges are distributed non-uniformly on the surface of the toner. 
This results in high electrostatic adhesion due to locally high surface 
charge densities on the particles. Toner adhesion, especially in the 
development step, is a key factor which limits performance by hindering 
toner release. As the toner particle size is reduced to enable higher 
image quality, the charge Q on a triboelectrically charged particle, and 
thus the removal force (F=QE) acting on the particle due to the 
development electric field E, will drop roughly in proportion to the 
particle surface area. On the other hand, the electrostatic adhesion 
forces for tribo-charged toner, which are dominated by charged regions on 
the particle at or near its points of contact with a surface, do not 
decrease as rapidly with decreasing size. This so-called "charge patch" 
effect makes smaller, tribo-charged particles much more difficult to 
develop and control. 
Jumping development systems, in which toner is required to jump a gap to 
develop the electrostatic latent image, are capable of image quality which 
can be superior to in-contact systems, such as magnetic brush development. 
Unfortunately, they are also much more sensitive to toner adhesion. In 
fact, high toner adhesion has been identified as a major limitation in 
jumping development. Up to now, mechanical and/or electrical agitation of 
toner have been used to break these adhesion forces and allow toner to be 
released into a cloud for jumping development. This approach has had 
limited success, however. More agitation often releases more toner, but 
high adhesion due to triboelectric charging still dominates in toner cloud 
generation and causes unstable development. For full color printing system 
architectures in which the complete image is formed on the image bearing 
member, an increase in toner delivery rate produces a highly interactive 
toner cloud, which disturbs previously developed particles on the latent 
image. This erases many of the original benefits of jumping development 
for color xerographic printing for the so-called image-on-image (IOI) 
architecture. Again, as the toner size is reduced, the above limitations 
become even more acute due to increased toner adhesion. 
Given that charged particle adhesion is a major limiting factor in 
development with dry powder, it has been a goal to identify toner charging 
and delivery schemes which keep toner adhesion low. Clearly, the adhesion 
of the charged toner depends sensitively on the method used to charge the 
particles. Triboelectric charging is known to produce highly adhering 
particles. On the other hand, ion toner charging, which occurs when toner 
particles capture ions emitted by a nearby corona device, results in a 
more uniform deposition of charge on the particle's surface, and thus 
lowers the adhesion of the particles for a given charge level. 
It is well known that fluidizing reservoirs, commonly referred to as 
fluidized beds, provide a means for storing, mixing and transporting toner 
in certain single component development systems. Efficient means for 
fluidizing toner and charging the particles within the fluidized bed are 
disclosed in U.S. Pat. No. 4,777,106 and U.S. Pat. No. 5,532,100, which 
are hereby incorporated by reference. In these disclosures, corona devices 
are embedded in the fluidized toner for simultaneous toner charging and 
deposition onto a receiver roll. While the development system as described 
has been found satisfactory in some development applications, it leaves 
something to be desired in the way in applications requiring the blending 
of two or more dry powder toners to achieve custom color development. 
Also, it has been found in the above systems that there are frequently 
disturbances to the flow in the fluidized bed associated with charged 
particles in the high electric fields surrounding corona devices immersed 
in the reservoir. Finally, it is known that residual toner left on the 
donor roll after development contributes to non-uniformities in 
subsequently loaded toner layers, thereby leading to the so-called 
"ghosting" defect in printed images. 
Briefly, the present invention obviates the problems noted above by 
enabling a gentle toner handling system in which non-contact metering and 
particle charging on an electroded donor roll can be controlled 
independently to provide charged toner particles with low adhesion for 
xerographic development. The toner is initially extracted 
electrostatically from a fluidized bed and deposited as a net neutral 
layer on a donor member. This toner layer is subsequently charged with a 
DC or AC corona device and delivered to a latent image. This so-called ion 
charging produces a more uniform deposition of charge on the toner 
particles, resulting in significantly lowered particle adhesion. In 
addition, the ion charging process is independent of toner pigment, 
allowing mixtures of two of more different colored toners to be charged 
homogeneously. Residual toner on the donor is neutralized and returned to 
the fluidized bed toner reservoir during each complete cycle of the donor 
roll. 
There is also provided an apparatus for developing a latent image recorded 
on an imaging surface, comprising; a housing defining a reservoir storing 
a supply of developer material comprising toner; means for fluidizing said 
developer material in the chamber of said housing; a donor member, mounted 
partially in said chamber and spaced from the imaging surface, for 
transporting toner on an outer surface said donor member to a region 
opposed from the imaging surface, said toner donor member having a 
plurality of electrodes positioned near the outer surface of donor member; 
means for electrical biasing a portion of said electrode members on a 
region of said donor member positioned in close proximity to said 
fluidized toner so as to electrostatically load toner onto the region of 
the donor member; means for ion charging said toner loaded on the region 
of said donor member; means for electrical biasing said electrode members 
positioned in close proximity to said imaging member to detach toner from 
said region of said donor member as to form a toner cloud for developing 
the latent image; and means for discharging and removing residual toner on 
the region of said donor and returning said toner to the reservoir.

DETAILED DESCRIPTION OF THE FIGURES 
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. 
In as much 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 44. 
Preferably the surface 12 is made from a selenium alloy. The substrate is 
preferably made from an aluminum alloy or a suitable photosensitive 
organic compound. The substrate is preferably made from a polyester film 
such as Mylar (a trademark of Dupont (UK) Ltd.) which has been coated with 
a thin layer of aluminum alloy which is electrically grounded. The belt is 
driven by means of motor 54 along a path defined by rollers 49, 50 and 52, 
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 48 charges surface 12 to a 
relatively high, substantially uniform, potential. A high voltage power 
supply 50 is coupled to device 48. 
Next, the charged portion of photoconductive surface 12 is advanced through 
exposure station B. At exposure station B, ROS 56 lays out the image in a 
series of horizontal scan lines with each line having a specified number 
of pixels per inch. The ROS includes a laser having a rotating polygon 
mirror block associated therewith. The ROS imagewise exposes the charged 
photoconductive surface 12. After the electrostatic latent image has been 
recorded on photoconductive surface 12, belt 10 advances the latent image 
to development station C as shown in FIG. 3. At development station C, a 
development system or developer unit 34, develops the latent image 
recorded on the photoconductive surface. The chamber in the developer 
housing stores a supply of developer material. The developer material may 
be a one component developer material consisting primarily of toner 
particles. The developer material may be a custom color consisting of two 
or more different colored dry powder toners. 
Again referring to FIG. 3, after the electrostatic latent image has been 
developed, belt 10 advances the developed image to transfer station D, at 
which a copy sheet 64 is advanced by roll 62 and guides 66 into contact 
with the developed image on belt 10. A corona generator 68 is used to 
spray ions on to the back of the sheet so as to attract the toner image 
from belt 10 to the sheet. As the belt turns around roller 49, 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 71 and a 
back-up roller 72. The sheet passes between fuser roller 71 and back-up 
roller 72 with the toner powder image contacting fuser roller 71. In this 
way, the toner powder image is permanently affixed to the sheet. After 
fusing, the sheet advances through chute 74 to catch tray 75 for 
subsequent removal from the printing machine by the operator. 
After the sheet is separated from photoconductive surface 12 of belt 10, 
the residual developer material adhering to photoconductive surface 12 is 
removed therefrom by a rotating fibrous brush 78 at cleaning station F 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 in greater detail, development system 34 includes a 
housing defining a reservoir 76 for storing and fluidizing a supply of 
toner therein. The bottom of this fluidizing reservoir is comprised of a 
porous plate 200, with pore size of 5 microns or less, which allows gas to 
flow from plenum 205 to reservoir 76 but contains the toner in the 
reservoir. Gas (air) is supplied to the plenum through an opening 210 
below the porous plate. The gas flow may be constant or may be modulated 
in time, enabling easier fluidization of the toner. As an additional aid 
to fluidizing the toner, the reservoir 76 may be vibrated (not shown). 
Although the toner in reservoir 76 exists in an approximately charge 
neutral state, it is known that the particles possess small amounts of 
negative or positive net charge. 
Donor structure 42, which may be in the form of a roll or a continuous 
belt, is comprised of at least two sets of closely spaced interdigitated 
electrodes 92 and 94, which are be covered by an electrically relaxable 
overcoat 70. One set of electrodes 92 is connected together (commons), 
while the other set 94 is addressable individually (actives). The surface 
of donor structure 42 is in contact with or near the surface of the 
fluidized toner bed in reservoir 76. By applying a DC bias 102 between 
adjacent sets of electrodes 92 and 94, via a conducting brush commutator 
105, fringe fields of approximately 0.2 to 0.3 volts/micron are 
established between the sets of electrodes in loading zone 207, enabling 
gentle and controllable loading of uncharged toner onto the surface of 
donor roll 42. 
The thickness of the deposited toner layer can be controlled by the DC bias 
102 between the sets of interdigitated electrodes 92 and 94. This 
microfield loading scheme takes advantage of the native toner charge 
distribution of the particles in the fluidized bed reservoir 76, which has 
some small width about zero charge. The combination of the fluidized bed 
reservoir, which presents essentially free uncharged toner particles to 
the donor, with the localized fields at the donor surface allows the 
slight net charges on the particles (both positive and negative) to be 
used to pick up toner onto the donor 42. 
As the donor 42 rotates in the direction of arrow 68, the layer of 
uncharged toner on its surface is brought under corona charging device 
300, where the toner is charged to an average Q/M ratio of from -30 to -50 
microCoulombs/gram. Corona device 300 may be in the form of an AC or DC 
charging device (e.g. scorotron). As donor 42 is rotated further in the 
direction indicated by arrow 68, the now charged toner layer is moved into 
development zone 310, defined by the gap between donor 42 and the surface 
of the photoreceptor bet 10. Toner is released from the surface of the 
donor 42, forming a toner cloud 112, and imagewise develops the 
electrostatic latent image 14 on photoreceptor 10. 
The separation of the toner loading and toner charging steps, as described 
here, is highly advantageous, allowing independent control over the amount 
of the thickness of the uncharged toner layers as well as the charge level 
and charge distribution of the toner particles after exposure to charging 
device 300. As mentioned previously, it has been found that the charging 
of toner layers on the donor after loading onto a donor avoids 
difficulties associated with placing the charging device in the fluidized 
bed of toner. In previous disclosures, it has been found that corona 
devices embedded in the fluidized toner necessarily generate high electric 
fields which exert strong forces on even slightly charged toner particles, 
causing violent instabilities in the toner bed. These instabilities cause 
non-uniformities in the deposited toner layers which must be eliminated 
before the toner is developed to an image. The separate charging of the 
toner in layers, as described here, may sacrifice some of the charge 
uniformity on the particles that is possible when charging is performed by 
immersing a corona device in the fluidized bed. However, charge 
spectrograph data and developability experiments suggest that any 
differences between the two methods, either in charging uniformity or 
particle adhesion, are small; charging in layers retains the general low 
adhesion benefits of ion charging. 
Due to the gentle loading of toner in loading zone 207 and ion charging by 
corona device 300, which both act to keep toner adhesion to donor 42 low, 
the charged toner in development zone 310 is capable of releasing from the 
donor solely due to the DC electric field in the development zone. This DC 
field is provided by both the DC bias of from 0 to 1000 volts from power 
supply 108, applied to both sets of electrodes 92 and 94 via commutator 
107 (similar to commutator 105), and the latent image 14 on photoconductor 
10. To provide enhanced toner release, which enables higher toner delivery 
rates and increased development speed, an AC bias can be applied between 
adjacent sets of donor electrodes 92 and 94 in development zone 310. In 
FIG. 1, this AC bias is supplied by power supply 104 via commutator 107. 
When the AC fringe field is applied to a toner layer via an electrode 
structure in close proximity to the toner layer, the time-dependent 
electrostatic force acting on the charged toner momentarily breaks the 
adhesive bond to cause toner detachment and the enhancement of the powder 
cloud 112. The enhancement in developed toner mass from this optional use 
of AC during development has been measured to be approximately 20%. 
Further rotation of donor 42 brings any residual (un-developed) toner on 
the donor roll under AC corona device 400, where it is brought to a charge 
neutral state, removed from the donor and returned to the fluidized bed 
reservoir 76. Stripping of toner is facilitated by applying an AC bias 
between the sets of electrodes 92 and 94 via commutator 115. 
Alternatively, a blade (not shown) may be used to remove the toner from 
the donor 42. Complete stripping ensures erasure of all history of 
previous development and loading on the donor, eliminating the possibility 
of "ghosting". In addition, the return of unused toner in a charge neutral 
state maintains a steady native charge distribution in the fluidized bed, 
minimizing fluctuations in layer thickness during the initial loading step 
which may result from a significant net charge on the toner in the 
reservoir. 
As successive electrostatic latent images are developed, the toner 
particles within the chamber 76 are depleted to an undesirable level. A 
toner dispenser (not shown) stores a supply of toner particles. The toner 
dispenser is in communication with chamber 76 of housing 44. As the level 
of toner particles in the chamber is decreased, fresh toner particles are 
furnished from the toner dispenser. In this manner, a substantially 
constant amount of toner particles are in the fluidizing reservoir of the 
developer housing. 
Applicants have used electric field detachment to measure charged particle 
adhesion for both tribo-charged and ion charged toners. In these studies, 
an electric field is applied to move charged toner from a donor to a 
receiver. The receiver is equipped with an optical sensor to detect the 
amount of toner developed as a function of applied field, giving a direct 
measure of the adhesion of the particles on the donor. The advantages of 
using ion charged toner can be seen in the experimental electric field 
detachment data of FIG. 2. Ion charged toner particles develop to the 
receiver far more easily and completely than identical triboelectrically 
charged particles with approximately the same total charge. The average 
charge to mass ratios for both toner samples was approximately -20 
microCoulombs/gram. This is direct evidence of the dramatically reduced 
adhesion possible with ion charged toner from an invention as described 
above. 
It has been found that toner charging by exposure to corona in the manner 
just described is also advantageous because the resulting particle charge 
is, to a great degree, independent of the material properties of the 
pigment contained in the toner. This is not the case, for example, with 
triboelectric charging, which is known to be highly dependent on the type 
and quantity of pigment in the toner. The pigment independence of ion 
charging, combined with the use of a fluidized bed as a toner reservoir, 
enables the blending of two or more dry powder toners of different colors 
to achieve custom color development. Since, in the present invention, the 
charge distribution of the neutral toner in the fluidized bed influences 
the fringe field loading onto the donor, it is desirable in the case of a 
blend of toners that the charge distributions of the different 
constituents overlap to a significant degree. In practice, it has been 
found that this condition is easy to satisfy with the proper pigment and 
external additive choices. 
It should be evident by one skilled in the art that the single color 
printing process described above can be modified to allow xerographic 
printing of more than one color. For example, tandem printing architecture 
is one such modification, in which each color has its own complete marking 
station, including photoconductor, exposure device, and development, 
transfer and cleaning subsystems. After development of the electrostatic 
latent image, the color separations are transferred to a medium, which 
could be paper or some intermediate belt, where the full color image is 
successively built up. Another example, image-on-image (IOI) mode of 
printing is another possible architecture, in which the full image, made 
up of the two or more color separations, is built up on a single 
photoconductor and later transferred to paper in a single transfer step. 
The IOI architecture is the less forgiving of the two architectures, as it 
demands that each successive development step not disturb the previous 
toner image on the photoconductor. Development systems which possess these 
qualities are often termed scavengeless. 
Due to the low adhesion of ion charged toner and the easier release of such 
toner from a development system such as described above, ion chargingbased 
development is expected to be scavengeless in nature, and thus highly 
desirable for IOI printing. Low toner adhesion from ion charging also has 
other benefits, which apply to both the tandem as well as the IOI 
architectures, such as the ability to deliver small particles for high 
quality images and the possibility of higher toner delivery rates to 
enable higher speeds. As mentioned previously, the ability to blend toners 
for custom color is yet another important attribute of ion charging-based 
development systems. The ability to perform custom color development, 
resulting from the pigment independence of ion charging, benefits both 
tandem and IOI xerographic printing. 
An additional advantage of the present invention that it allows for 
movement of toner with electrical forces only, enabled by a donor with 
individually addressable electrodes. Reduced mechanical contact with the 
toner, as a result of the absence of carrier beads for charging and the 
abandonment of metering and charging blades in the current proposal, 
enables longer toner life. This is especially important during operation 
with low toner throughput (low area coverage documents, for example), 
where toner residence times in the development system can be long. In 
addition, failure of the charging system due to degradation of the 
triboelectric charging member (ie, carrier or charging blades) is avoided. 
In summary, there is provided a development system of the present invention 
that utilizes independently controlled non-contact metering and ion 
charging of toner. The resulting toner delivery system is designed to 
produce charged, low adhesion toner and present it gently to an 
electrostatic latent image in the form of a toner cloud. 
Other embodiments and modifications of the present invention may occur to 
those skilled in the art subsequent to a review of the information 
presented herein; these embodiments and modifications, as well as 
equivalents thereof, are also included within the scope of this invention.