Erase unit for ion deposition web-fed print engine

An erase unit for an ion deposition web-fed print engine includes a plenum extending the width of the electrostatic image on the image cylinder and defining an ionization chamber with the image cylinder surface. The plenum has first and second electrodes separated by a dielectric. Upon application of a time-varying potential having a frequency of 0.2 to 50 mHz across the electrodes, ionization occurs, causing an ion current flowing in relation to the image surface until the image surface and the biasing electrode are at the same potential, thus erasing any residual electrostatic image on the cylindrical surface. By providing a DC biasing voltage, the residual electrostatic image may be erased when equalization occurs, with the image cylinder maintaining a pre-charged potential.

BACKGROUND AND SUMMARY OF THE INVENTION 
The present invention relates to an ion deposition web-fed print engine 
having a novel and improved erase unit for removing residual electrostatic 
potential of an image remaining on the engine's image cylinder after the 
toner developed latent image has been transferred to a substrate. 
Ion deposition printers conventionally transpose or transform 
computer-generated signals, such as word processing signals, for image 
printing on a substrate, for example, paper. More particularly, an ion 
deposition print engine typically includes an image cylinder mounted in 
opposition to an impression cylinder, with the substrate, i.e., a web of 
paper, passing between the image and impression cylinders. The image 
cylinder includes a dielectric layer which receives an electrostatic image 
from an ion cartridge. The cartridge is driven electronically from the 
computer or word processing system. The electrostatic image imposed on the 
image cylinder is contacted with toner from a supply. At the nip between 
the image and impression cylinders, the toner is transferred to the 
substrate, i.e., the paper, in the identical form of the electrostatic 
image on the image cylinder and fused to the substrate. Further rotation 
of the image cylinder causes it to pass a multi-component cleaning 
station, which physically removes residual solid particulate matter (i.e., 
toner). The image cylinder finally passes in opposition to an erase unit, 
which removes any residual electrostatic potential of the image on the 
image cylinder surface, whereby a fresh electrostatic image may be placed 
on the dielectric layer by the ion cartridge. The process is then repeated 
with the same or different images. The present invention is particularly 
concerned with a novel and improved erase unit for the ion deposition 
print engine. 
Presently known erase units for ion deposition print engines use a 
high-density ion current generator to erase the latent residual 
electrostatic image remaining on the image cylinder after transfer of the 
image to the substrate. One such known erase unit comprises a central 
glass rod with four individual glass-coated erase wires mounted 90.degree. 
apart around the central glass rod and wrapped with a spiral-coiled screen 
wire. To erase the residual electrostatic potential remaining on the image 
cylinder, the erase wire is activated by application of high voltage RF 
energy. This causes atmospheric breakdown and ionization on the surface of 
the glass-coated erase wire at the junctions of the spiral screen wire. 
The resultant pool of ions, both positive and negative, migrate to the 
residual electrostatic image areas on the drum surface as a result of the 
net electrical field present between the screen wire and the residual 
electrostatic images. 
While acceptable in that configuration, the above-described erase unit has 
certain limitations. For example, the life of the erase unit is somewhat 
limited. When one of the erase wires is no longer effective, the unit is 
rotated 90.degree. to bring the adjacent wire into close proximity with 
the drum. A disadvantage with this type of erase unit is the downtime 
involved in order to displace the next wire into position. Also, the 
glass-coated wire with the spiral wire wrapping is prone to contamination 
and readily and easily damaged. If contaminated, the erase unit is 
substantially non-recoverable. Further, there is a limitation in the 
voltage range for pre-charging the image cylinder. Still further, the 
operation of this known erase unit is in ambient conditions. This makes it 
prone to unusual and undesirable deposition of ionic compounds, 
particularly in ammonia and amine-laden atmospheres. Moreover, the 
operation is at relatively low frequency, thus limiting overall output. 
According to the present invention, there is provided a novel and improved 
erase unit for an ion deposition print engine which minimizes or 
eliminates the foregoing and other problems associated with prior erase 
units for similar type print engines. Particularly, the present invention 
provides front and rear, or first and second, electrodes and a circuit for 
providing a time-varying potential across the electrodes. The first 
electrode may form the base of a plenum into which inert gas, preferably 
argon, is provided for generating positive and negative ions within the 
plenum adjacent the image surface containing the residual electrostatic 
potential in response to the creation of an electric field within the 
plenum. The second, or front electrode, also called the biasing electrode, 
is disposed within the plenum and separated from the first, or rear, 
electrode by a dielectric, for example, formed of glass. Side and end 
walls are also provided to further define the plenum whereby the region 
within the plenum filled with the argon (inert) gas lies in contact with 
the image cylinder. When the circuit is activated, positive and negative 
ions are generated adjacent the second, or bias, wire and the electric 
field between the bias wire and the image drum surface provides the 
driving force for those ions of appropriate polarity to migrate to the 
cylinder. The ions created within the plenum are also under the influence 
of the electric field created by the second electrode and the image 
cylinder assembly by a DC biasing voltage. That field is a function of the 
residual image cylinder voltage and the erase bias on the second electrode 
and the distance between the second electrode and the image surface. As 
long as there is a difference between the residual image cylinder voltage 
and the erase bias on the second electrode, a net ion migration to the 
image cylinder surface occurs. As the image cylinder voltage reaches the 
value of the erase bias by the charging or discharging of the net ionic 
migration, the ion current will stop. Thus, in a pure eraser application, 
the bias or second electrode wire is held near a ground potential to 
produce a zero volt condition on the image drum. It is, however, also 
important in certain applications to adjust a pre-biasing potential to a 
specific level for use with other parts of the imaging and development 
process. Thus, the erase bias potential can be set to a specific level 
necessary for another part of the process and the image cylinder will be 
charged or discharged to that desired level. That is, by driving the 
second wire with the DC bias, the residual image potential on the drum is 
erased and brought to a biased condition with a surface voltage matching 
that of the bias wire. 
By using a system of the foregoing described type, there is provided an 
improved apparatus demonstrating higher density ionic output based on the 
use of inert gas, affording higher frequency RF energy and an improved 
configuration of the bias wire, resulting in an erasing operation at 
higher print speeds and a more efficient eraser mechanism. Additionally, 
the image cylinder may be pre-charged to a wide range of DC surface 
voltages by biasing the bias wire and creating a net electric field 
between the wire and the cylinder. Further, the erase unit hereof is 
substantially insensitive to harmful gases in the ambient environment and 
creates an equal and uniform output along its length due to its simple 
construction and the use of the inert gas environment. Still further, the 
improved eraser unit hereof affords greater operational longevity in 
comparison with the previously described eraser units because of the 
insensitivity of the materials used to degradation over time and the 
robust nature of the plasma-generating components, hence achieving less 
sensitivity to contamination and affording the capability of cleaning the 
unit should it become contaminated. 
In a preferred embodiment according to the present invention, there is 
provided an electrostatic ion deposition printer including an 
electrostatic print head for forming an electrostatic image, an image 
cylinder rotatable about an axis and having a dielectric layer for 
receiving the electrostatic image and means for transferring the image to 
a substrate, an erase unit for removing residual electrostatic potential 
of the image remaining on the image cylinder after the image has been 
transferred to the substrate, comprising first and second electrodes 
disposed adjacent a surface of the image cylinder at a location in 
opposition thereto and to the residual electrostatic potential remaining 
on the image cylinder, a dielectric disposed between the first and second 
electrodes and means for introducing a gas in a region adjacent the second 
electrode and between the dielectric and the image cylinder surface. 
Circuit means provide a time varying potential across the electrodes to 
ionize the gas in the region and enable substantial equalization of the 
residual potential on the image cylinder surface and the potential on the 
second electrode. 
In a further preferred embodiment according to the present invention, there 
is provided an electrostatic ion deposition printer including an 
electrostatic print head for forming an electrostatic image, an image 
cylinder rotatable about an axis and having a dielectric layer for 
receiving the electrostatic image, means for transferring the image to a 
substrate and an erase unit, including first and second electrodes 
disposed adjacent a surface of the image cylinder at a location in 
opposition thereto and to the residual electrostatic potential remaining 
on the image cylinder and a dielectric disposed between the first and 
second electrodes, a method for removing residual electrostatic potential 
remaining on the image cylinder after the image has been transferred to 
the substrate, comprising the steps of introducing a gas in a region 
adjacent the second electrode and between the dielectric and the image 
cylinder surface and providing a time varying potential across the 
electrodes to ionize the gas in the region and enable substantial 
equalization of the residual potential on the image cylinder surface and 
the potential on the second electrode. 
Accordingly, it is a primary object of the present invention to provide a 
novel and improved erase unit for an ion deposition web-fed print engine. 
These and further objects and advantages of the present invention will 
become more apparent upon reference to the following specification, 
appended claims and drawings.

DETAILED DESCRIPTION OF THE DRAWING FIGURES 
Reference will now be made in detail to a present preferred embodiment of 
the invention, an example of which is illustrated in the accompanying 
drawings. 
Referring now to FIG. 1, there is illustrated a portion of an ion 
deposition web-fed print engine, generally designated 10, and which 
includes an image cylinder 12 for printing an image on a substrate S, in 
this case, a web of paper passing over rolls, one of the rolls being 
illustrated at 14. As the paper passes through the nip between the 
pressure roll 16 and image cylinder 12, an electrostatic image is formed 
on the image cylinder 12 in a conventional manner by means of a print head 
18. The electrostatic image on image cylinder 12 is developed by the 
application of toner at 20 received from a supply 22. The toner is 
transferred to the substrate, i.e, the paper S, at the nip of the image 
cylinder 12 and pressure cylinder 16. Untransferred residual toner and 
other contaminants are removed from the image cylinder by a cleaning unit 
17. Any residual electrostatic potential remaining on the image cylinder 
12 is removed by an erase unit 24 before the image cylinder lies once 
again in opposition to the print head for receiving another electrostatic 
image. The erase unit 24 of the present invention is illustrated in FIGS. 
2-4. 
Referring now to those drawing figures, there is illustrated an erase unit 
24 in radial opposition to the image cylinder 12. The erase unit 24 
includes, as best illustrated in FIG. 3, an elongated plenum 26, which 
extends parallel to the axis of rotation of the image cylinder 12 a 
distance at least equal to the transverse extent of the image on the 
cylinder 12. The plenum 26 is comprised of a back wall, not shown, side 
walls 30, and a rear wall formed of dielectric material 34. The side and 
end walls 30 and 32 are preferably formed of glass. The rear wall includes 
a first, or rear, electrode 29 of a pair of electrodes comprising first 
and second electrodes 29 and 31. Electrode 29 comprises a metal strip 
extending along the rear face of dielectric 34. Thus, the first electrode 
29 extends between the side walls 30 and end walls 32 and is spaced a 
further distance from the surface of the image cylinder 12 than the second 
electrode 31. The second electrode 31, that is, the bias electrode, is 
disposed within the plenum and separated from the first electrode 29 by a 
dielectric 34. The second or bias electrode 31 lies within the plenum 32 
on the inside of dielectric 34 and between the opposite side and end walls 
30 and 32. 
The plenum is designed to confine an inert gas, preferably argon, in the 
region of the second or bias electrode 31 using the dielectric 34 and the 
side and end walls 30 and 32, respectively, as the gas confining elements. 
The side and end walls, of course, terminate at their distal ends in close 
proximity to but spaced from the image cylinder surface. To maintain a 
supply of the inert gas within the plenum, and in accordance with the 
present invention, the second or bias electrode 31 is provided in hollow 
tubular form and has one end connected to a supply of argon gas 36 (FIG. 
4). The tube 31 is supported by the dielectric, to which it is secured by 
spaced mechanical clips 37. As illustrated, the electrode 31 extends the 
full length of the plenum and has a plurality of apertures 38 spaced 
longitudinally one from the other along the length of the electrode 31 and 
along opposite sides thereof. Consequently, gas supplied from source 36 
flows into one end of the electrode 31 and through the apertures 38 into 
the region adjacent the second electrode within the plenum for contact 
with the image surface of image cylinder 12. 
Referring now to FIG. 2, there is provided a circuit for providing a 
high-frequency time-varying potential of about 0.2 to 50 mHz across the 
electrodes 29 and 31 to ionize the gas within the plenum. For this 
purpose, a suitable AC source 40 is coupled to the first electrode 29. The 
AC source 40 is also connected to the second or bias electrode 31. A DC 
bias voltage may also be applied to the second electrode from a source 42 
to create an electric field between the second or biasing electrode 31 and 
the image cylinder 12. 
In operation, the image cylinder 12 rotates past the print head 18, where 
it receives the latent electrostatic image, which is developed on the drum 
surface as it rotates past the toner supply unit. The image is then 
transferred to the substrate S at the nip of the image cylinder and 
pressure roll 16. After removing residual toner at cleaning unit 17, 
further rotation of the image cylinder brings the portion of the cylinder 
containing any residual electrostatic image in opposition to the erase 
unit 24. 
By applying high-frequency, about 0.2 to 50 mHz high-voltage from the AC 
source to the electrodes 29 and 31, the inert argon gas within the plenum 
is excited to generate both positive and negative ions, particularly in 
the areas of high electric field gradients near the second or biasing 
electrode 31 and the surface of the dielectric insulator 34. The ions in 
that volume are also influenced by the electric field created between the 
second or biasing electrode 31 and the image cylinder surface by the DC 
biasing voltage 42. It will be appreciated that the electric field is a 
function of the residual image cylinder voltage, the erase bias applied on 
the second or bias electrode 31, and the distance between the bias 
electrode 31 and the image cylinder surface. Provided there is a 
difference between the residual image cylinder voltage and the erase bias 
on the second or bias electrode 31, there will be a net ion migration to 
the image cylinder surface. As the image cylinder voltage or potential 
reaches the potential of the erase bias by the charging or discharging 
from the net ionic migration, the ion current will stop. Thus, the 
resulting electric field between the bias electrode 31 and the image drum 
surface provides the driving force for those ions of appropriate polarity 
to migrate to the cylinder surface. Where it is desired to maintain the 
image cylinder potential at zero, the bias electrode is maintained near or 
at a ground potential to produce a zero volt condition on the image drum. 
Consequently, any and all residual charges on the image cylinder will be 
discharged to a zero potential. If it is desirable to use other image 
cylinder charge levels to adjust a pre-biasing potential to a specific 
level for use with other parts of the imaging and development process, the 
second or bias wire 31 may be driven by the DC power supply 42. In that 
instance, the ion flow will continue until there is substantial 
equalization of the residual potential on the image cylinder surface and 
the potential on the second electrode 31. Once that equalization is 
obtained, the drum image is erased and remains in a biased condition, with 
a surface voltage matching that of the second electrode. 
While the invention has been described in connection with what is presently 
considered to be the most practical and preferred embodiment, it is to be 
understood that the invention is not to be limited to the disclosed 
embodiment, but on the contrary, is intended to cover various 
modifications and equivalent arrangements included within the spirit and 
scope of the appended claims.