Automatic duplex printing apparatus

A duplex printer apparatus utilizing non-contact printing devices in the form of a Direct Electrostatic Printer (DEP). Simplex images are formed on one side of an intermediate in the form of a belt using one or more DEP printing devices. A substrate such as plain paper is superimposed on the images formed on the intermediate. Continued movement of the belt in its endless path effects passage of the substrate past different DEP printing devices for forming images on the other side of the substrate.

BACKGROUND OF THE INVENTION 
This invention relates to electrostatic printing devices and more 
particularly to duplex printing utilizing non-impact printing devices 
which utilize electronically addressable printheads for depositing 
developer in image configuration on plain paper substrates. 
Many methods for automatiac duplex printing in xerographic processors are 
known. There is the two pass method employed in the Xerox 4000.TM. and 
9400.TM. reproduction machines. That is, after the first side of copy 
sheets are imaged and fused, the sheets are collected in a duplex tray. 
Afther the last sheet in a set has been received in the duplex tray, the 
sheets are again passed through the xerographic processing stations. This 
time an image is transferred and fused onto the opposite side of each copy 
sheet having an image on the first side. 
In the Xerox 9700.TM. machine, the copy sheets also pass through the 
processing stations twice. However, they are not collected in a duplex 
tray. After the first image has been transferred and fused, the sheets 
pass through a stop and reverse mechanism (inverter). Then the sheets join 
in an interleafing fashion the stream of copy sheets to receive an image 
on the opposite side. 
There are some disadvantages with these systems, in particular for a given 
image throughput rate. For example, two passes through the fuser require 
more energy, and the fuser needs to operate at twice the speed. During the 
first pass through the fuser, the paper loses 50 percent of its moisture. 
This curls the paper and makes the second pass for duplexing difficult. 
Paper picks up oil on the first pass through the fuser, sometimes leading 
to image deletions on the second image and oil deposits on the 
photoreceptor. Jam rates during two-pass duplex operation are much greater 
than for simplex operation. In the first place, in a two-pass duplex 
system, the paper path is usually very long, and the paper has to 
negotiate all obstacles twice. Excessive paper curl is not only 
troublesome in the processor but also extremely difficult to handle in 
output stackers and finishing devices. 
In other prior art systems such as in U.S. Pat. No. 4,095,979means are 
shown for "immediate" or single-pass duplex copying or forming first and 
second images sequentially on a photoreceptor. The first image is 
transferred from the photoreceptor to the first side of a copy sheet. Then 
the sheet is stripped off the photoreceptor, inverted while the first 
image remains unfixed, and then the second image is transferred to the 
second side of the copy sheet. Both images are then fixed onto the copy 
sheet by a suitable fuser. This type of system can be described as a 
"single-pass" to the fuser. 
Other single-pass duplex printing methods use intermediate image carriers 
(belt or drum). The first and second images are sequentially formed on a 
photoreceptor. The first image is transferred to an intermediate image 
carrier. The copy sheet is then passed between the photoreceptor and the 
intermediate image carrier, simultaneously receiving first and second 
images. 
The duplex methods discussed above only utilize one photoreceptor. Other 
systems, e.g. U.S. Pat. No. 3,580,070 and 3,775,102 deal with "single-pass 
duplex" methods employing two photoreceptors and two exposure systems. 
First images are deposited on one photoreceptor and second images are 
deposited on the other photoreceptor. These systems are considered th 
eultimate duplex throughput systems since they produce twice the number of 
images of "two-pass duplex" systems at same process speed. These 
"single-pass duplex" systems, however, generally require web paper feed in 
which the copy is spooled up on a roll or cut into individual sheets after 
fusing. This unfortunately, introduces additional components and 
complexity into the system. 
U.S. Pat. No. 4,427,285 discloses a discrete copy sheet feed system rather 
than a web paper feed system. A two photoreceptor, "single-pass duplex" 
apparatus is disclosed wherein two images are formed, one on each 
photoreceptor and then transferred to opposite sides of the image 
receiving sheet. 
U.S. Pat. No. 4,714,939 discloses an electrographic reproduction apparatus, 
of the single-pass type, capable of producing simplex or duplex copies on 
a receiver sheet traveling in a continuous direction along a path. The 
reproduction apparatus comprises a first dielectric member movable along a 
first path, a portion of such first path being tangent to and on one side 
of the sheet travel path. Transferable images, corresponding to 
information to be reproduced, are sequentially formed on such first 
member. A second member is movable along a second path. One portion of 
such second path is tangent to the sheet travel path on the opposite side 
from the first path and another portion of the second path, spaced from 
such one portion, is located to position the second member in image 
transfer relation to the first dielectric member. An electrostatic field, 
reversible in its effective direction, is utilized to transfer a 
transferable image from the first dielectric member to the second member 
at the portion of the second path where the first and second members are 
in image transfer relation and transfer such image from the second member 
to one side of a receiver sheet traveling along its travel path at the 
location where the position of the first path is tangent to the sheet 
travel path and for producing a duplex copy, a second image is transferred 
from the first dielectric member to the opposite side of such receiver 
sheet at the location where the portion of the first path is tangent to 
the sheet travel path. 
U.S. Pat. No. 32,422 discloses a method and apparatus for producing duplex 
copies. First and second unfixed images are transferred to opposite sides 
of a copy sheet before fixing of either image to the copy sheet. The first 
and second unfixed images may be electroscopic images sequentially formed 
on a photoconductor by electrophotographic techniques. The first unfixed 
electroscopic image is transferred from the photoconductor to a first side 
of a copy sheet, the sheet is inverted while the first image thereon 
remains unfixed, the second unfixed electroscopic image is transferred to 
the second side of the copy sheet, the copy sheet with the first and 
second unfixed images thereon is then transported to a fixing station. 
Another technique involving the use of only one photoconductor, utilizes an 
intermediate image transfer member to receive the first image formed on 
the photoconductor before transfer to a final support medium. The 
intermediate transfer member as disclosed in U.S. Pat. No. 3,671,118 and 
3,697,170 is such a belt. 
Of the various electrostatic printing techniques, the most familiar and 
widely utilized is that of xerography wherein latent electrostatic images 
formed on a charge retentive surface are developed by a suitable toner 
material to render the images visible, the images being subsequently 
transferred to plain paper. 
A lesser known form of electrostatic printing is one that has come to be 
known as Direct Electrostatic Printing (DEP). This form of printing 
differs from the aforementioned xerographic form, in that, the toner or 
developing material is deposited directly onto a plain (i.e. not specially 
treated) substrate in image configuration. This type of printing device is 
disclosed in U.S. Pat. No. 3,689,935 issued Sept. 5, 1972 to Gerald L. 
Pressman et al. In general, this type of printing device uses 
electrostatic fields associated with addressable electrodes for allowing 
passage of developer material through selected apertures in a printhead 
structure. Additionally, electrostatic fields are used for attracting 
developer material to an imaging substrate in image configuration. 
Pressman et al disclose an electrostatic line printer incorporating a 
multilayered particle modulator or printhead comprising a layer of 
insulating material, a continuous layer of conducting material on one side 
of the insulating layer and a segmented layer of conducting material on 
the other side of the insulating layer. At least one row of apertures is 
formed through the multilayered particle modulator. Each segment of the 
segmented layer of the conductive material is formed around a portion of 
an aperture and is insulatively isolated from every other segment of the 
segmented conductive layer. Selected potentials are applied to each of the 
segments of the segmented conductive layers which a fixed potential is 
applied to the continuous conductive layer. An overall applied field 
projects charged particles through the row of apertures of the particle 
modulator and the density of the particle stream is modulated according to 
the pattern of potentials applied to the segments of the segmented 
conductive layer. The modulated stream of charged particles impinge upon a 
print-receiving medium interposed in the modulated particle stream and 
translated relative to the particle modulator to provide line-by-line scan 
printing. In the Pressman et al device the supply of the toner to the 
control member is not uniformly effected and irregularities are liable to 
occur in the image on the image receiving member. High-speed recording is 
difficult and moreover, the openings in the printhead are liable to be 
clogged by the toner. 
U.S. Pat. No. 4,491,855 issued on Jan. 1, 1985 in the name of Fuji et al 
discloses a method and apparatus utilizing a controller having a plurality 
of openings or slit-like openings to control the passage of charged 
particles and to record a visible image of charged particles directly on 
an image receiving member. Specifically, disclosed therein is an improved 
device for supplying the charged particles to a control electrode that has 
allegedly made high-speed and stable recording possible. The improvement 
in Fuji et al lies in that the charged particles are supported on a 
supporting member and an alternating electric field is applied between the 
supporting member and the control electrode. Fuji et al purports to 
obviate at least some of the problems noted above with respect to Pressman 
et al. Thus, Fuji et al alleges that their device makes it possible to 
sufficiently supply the charged particles to the control electrode without 
scattering them. 
U.S. Pat. No. 4,568,955 issued on Feb. 4, 1986 to Hosoya et al discloses a 
recording apparatus wherein a visible image based on image information is 
formed on an ordinary sheet by a developer. The recording apparatus 
comprises a developing roller spaced at a predetermined distance from and 
facing the ordinary sheet and carrying the developer thereon. It further 
comprises a plurality of addressable recording electrodes and 
corresponding signal sources connected thereto for attracting the 
developer on the developing roller to the ordinary sheet by generating an 
electric field between the ordinary sheet and the developing roller 
according to the image information. A plurality of mutually insulated 
electrodes are provided on the developing roller and extend therefrom in 
one direction. A.C. and D.C. voltage sources are connected to the 
electrodes, for generating alternating electric fringte fields between 
adjacent ones of the electrodes to cause oscillations of the developer 
positioned between the adjacent electrodes along electric lines of force 
therebetween to thereby liberate the developeer from the developing 
roller. 
U.S. Pat. No. 4,912,489 discloses a Direct Electrostatic Printing device 
comprising a printhead structure comprising a shield electrode structure 
and a control electrode structure supported by an insulative support 
member. The printhead structure is positioned such that the control 
electrode is opposite the toner supply. Wrong sign toner accumulates on 
the control electrode. 
Single-pass duplex printing systems can offer significant reduction in 
paper handling complexity and greatly increase duplex output productivity. 
With low cost, high reliability marking approaches such as DEP printers, 
the addition of a second marking stage to allow single-pass duplex 
printing is justified because the extra machine complexity normally added 
for high throughput duplex (duplex path hardware, timing gates, timing 
sensors, etc.) would be eliminated. 
BRIEF DESCRIPTION OF THE INVENTION 
Briefly, the present invention utilizes the direct marking aspects of DEP 
in a unique combination. DEP toner imaging to an intermediate belt with 
subsequent transfer to a paper sheet provides one image side while toner 
imaging directly to the paper sheet provides the other image side of the 
duplex print. The intermediate belt acts both a DEP image receiver and as 
an electrostatic paper transport through the direct-to-paper DEP imaging 
system. The sytem for providing electrostatic tacking of the paper to the 
belt also provides the electrostatic fields for transfer of the 
intermediate image. 
Thus, a first image is formed on an intermediate such as a belt moved in an 
endless path. The belt is first moved through a first imaging station 
containing one or more direct printing devices, for example, DEP (Direct 
Electrostatic Printer) printers. Simplex images are formed on the 
intermediate belt at the first imaging station. A cust sheet of plain 
paper is then superimposed over the simplex image and electrostatically 
adhered to the belt. Continued movement of the intermediate belt and the 
sheet of plain paper in synchronism causes the other side of the sheet of 
paper to pass through a second imaging station containing one or more 
direct imaging devices. Images are formed on the other side of the cut 
sheet.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION 
Disclosed in FIG. 1 is an embodiment of a Direct Electrostatic Printing 
(DEP) apparatus 10 incorporated the invention. 
The printing apparatus 10 includes a developer delivery or conveying system 
generallly indicated by reference character 12, a printhead structure 14 
and a backing electrode structure 16. 
As disclosed herein, the developer delivery system 12 comprises a donor 
roll structure. The donor roll structure which is preferably coated with 
Teflon-S (Trademark of E. I. duPont) is spaced from the printhead 
approximately 0.003 to 0.015 inch. Teflon-S is a tetrafluoroethylene 
fluorocarbon polymer that is loaded with carbon black. Alternately, 
developer delivery system 12 may comprise any other suitable device know 
in the art. For example, it may comprise a Toner Cloud Development 
(T.C.D.) system of the type disclosed in U.S. Pat. No. 4,647,179. It may 
also comprise a belt. The primary purpose of the delivery system is to 
effect delivery of toner particles 18 to the printhead structure 14. 
The developer preferably comprises any suitable insulative non-magnetic 
toner/carrier combination having Aerosil (Trademark of Degussa, Inc.) 
contained therein in an amount equal to 1/2% by weight and also having 
zinc stearate contained therein in an amount equal to 1% by weight. The 
toner 18 may be charged positively or negatively. Purposes of this 
disclosure it is assumed that the toner is negatively charged. 
The printhead structure 14 comprises a layered member including an 
electrically insulative base member 20 which may be fabricated from a 
polyimide film approximately 0.001 inch thick. The base member may be clad 
on the one side thereof with a continuous conductive electrode structure 
or shield 22 of aluminum which is approximately one micron thick. The 
opposite side of the base member 20 may carry a segmented conductive 
control electrode structure 24 thereon which is fabricated from aluminum. 
The printhead structure 14 is positioned in the printing device such that 
the shield electrode structure 22 faces the donor roll structure 12. 
A plurality of holes or apertures 26 (only one of which is shown) 
approximately 0.007 inch in diameter are provided in the layered member in 
a pattern suitable for use in recording information. The apertures form an 
electrode array of individually addressable electrodes. A preferred 
aperture array is disclosed in U.S. Pat. No. 4,860,036, incorporated 
herein by reference. The '036 patent was granted to Fred W. Schmidlin on 
Aug. 22, 1989. 
Movement of the charged toner to the printhead structure is effected 
through the application of a DC biased AC peak voltage of about 550 volts 
with a DC bias of +40 volts. This bias is provided via voltage source 13. 
With a voltage applied to shield in accordance with the present invention 
and zero volts applied to an addressable electrode, toner 18 is propelled 
through the aperture associated with that electrode. The apertures extends 
through the base 20 and the conductive layers 22 and 24. 
With a negative 350 volts applied to an addressable electrode via voltage 
source 15, toner is prevented from being propelled through the aperture. 
Image intensity can be varied by adjusting the voltage on the control 
electrodes between 0 and minus 350 volts. Addressing of the individual 
electrodes can be effected in any well known manner know in the art of 
printing using electronically addressable printing elements. 
The addressing of the electrodes is synchronized with the arrival of a copy 
substrate or image receiver 28 adjacent the apertures. A suitable 
substrate sensor (not shown) is used for detection of the copy substrate 
28. The output signal from the sensor is transmitted to a controller (not 
shown) to initiate addressing of the appropriate control electrodes. 
The electrode or shoe 16 preferably has an arcuate shape but as will be 
appareciated, the present invention is not limited by such a 
configuration. The shoe 16 which is positioned on the opposite side of the 
plain paper copy substrate 28 from the printhead deflects the recording 
substrate in order to provide and extended area of contact between the 
medium and the shoe. 
The substrate or recording medium 28 may comprise cut sheets of paper fed 
from a supply tray (not shown). The sheets of paper are spaced from the 
printhead 12 a distance in the order of 0.0005 to 0.030 inch as they pass 
therebetween. The sheets 58 are transported in contact with the shoe 16 
via edge transport roll pairs 100. 
During printing the shoe 16 is electrically biased to a DC potential of 
approximately +300 volts via a DC voltage source 30 for the purpose of 
attracting the toner particles moved through the apertures. 
A pulsed DC or DC biased AC voltage is applied to the shield electrode 
structure 22 via voltage source 32. The voltage applied to the shield 
electrode structure is at the same frequency as the AC voltage applied to 
the toner supply but is approximately 180.degree. out of phase therewith. 
The pulsed DC voltage is negative to coincide with the positive cycle of 
the AC voltage applied to the donor roll thereby establishing an 
electrostatic field about the shield electrode structure. Thus, the 
voltage applied to the shield electrode structure reduces the fringe field 
between the shield and control electrodes and increases the field between 
the toner supply and the shield. This causes wrong sign toner to be 
attracted to the shield electrode structure which is on the toner supply 
side of the printer rather than to the control electrode side of the 
printer. The natural AC jumping of toner occurring between the toner 
supply device and the shield electrode structure prevents buildup of toner 
particles around the printhead apertures. Thus, the present 
materials/process requirement of very low wrong sing toner for Direct 
Electrostatic Printing are relieved. 
In the duplex printing system illustrated in FIG. 2 and generally indicated 
by the reference character 39, the electrode shoe 16 is replaced by an 
intermediate belt 41 which is entrained about a plurality of rollers 40, 
42, and 44, the latter of which is operatively connected to a drive motor 
46. The intermediate belt serves both as a DEP image receiver and an 
electrostatic paper transport. A plurality of DEP devices 50 and 52 
similar to the one illustrated in FIG. 1 are positioned adjacent the 
intermediate belt and serve to form toner images on the belt. A single DEP 
device may be utilized to form monochrome images but when more than one 
DEP device is employed each of the devices utilizes toners having 
different physical properties. For example, the toners may be different 
colors or they may be the same color, in which case one may be 
non-magnetic while the other is magnetic. The formation of images with a 
plurality of different color toners may result in highlight color images 
or process color images in accordance with the way in which the system is 
utilized. Operation of the DEP devices as well as the movement of the 
intermediate belt are controlled via an Electronic Sub-System (ESS) 54. 
The ESS comprises electronic components and logic circuitry which are 
standard in the art for timing the actuations of the DEP devices as well 
establishing which of the apertures of the printhead structure will pass 
toner particles and which ones will not. 
Once toner images are formed on the intermediate belt, a sheet of plain 
paper 34 is fed from a supply tray 56 into contact with images on the 
belt. A pair of rollers 58 and 60 are provided for electrostatically 
tacking the paper sheet and also providing electrostatic fields for 
effecting transfer of the images formed on the belt to the sheet of paper. 
To this end, the rollers 58 and 60 are electrically biased via a power 
source 62. The pressure rollers mechanically maintain intimate contact of 
the paper with the intermediate near the region of paper charging so that 
high electrostatic pressures can be achieved even with some paper 
distortion. It can be appreciated by those skilled in the art that there 
are many ways to achieve desired intimate paper contact near the region of 
paper charging. For example, a biased pressure roller can be used for the 
mechanical approach and for a portion of the paper charging, and a corona 
charging device close to this mechanical pressure device can be used to 
help generate the desired charging current density to the paper for 
electrostatic transport and to assist the toner transfer from the 
intermediate belt. The Xerox 9700.TM. machine shows an example of one 
possible embodiment of a bias roll with a corotron assist transfer system. 
As another example, a pressure blade near a transfer corotron is used in 
the Xerox 5090.TM. product. 
It can be appreciated that a conventional electrostatic transfer design, 
for example a corona device without mechanical pressure assist devices, 
can be utilized in many applications where very little paper distortion is 
present. In fact, this invention helps enable this because paper 
distortion that typically will be present in the duplex imaging step of 
conventional multi-pass duplex systems, due to previous heating of the 
paper and fixing of the toner image in the simplex imaging step, will not 
be present with the single pass duplex process of this invention. It can 
also be appreciated that some systems may wish to allow wide amounts of 
paper distortion to be used with this invention, for example due to 
certain types of manufactured papers. In these cases, mechanical gripping 
assist such as gripper fingers or vacuum on the paper edges can be used 
with electrostatic forces to assist paper transport. 
Reliable electrostatic tacking and transfer with high relative humidity 
conditioned papers requires careful attention in the paper charging design 
and also careful selection of the electrical properties of the surface of 
the intermediate belt. Lateral conduction along the paper from the paper 
charging region to nearby conductive members, such as lead-in baffles, 
that touch the paper during lead-in and lead-out of the paper is one 
problem that can cause significant loss of paper charge. To those skilled 
in the art, this is typically solved in electrostatic transfer systems by 
applying a suitable potential, opposite in polarity to the toner charge 
polarity, on the lead-in and lead-out baffles to allow sufficient charging 
of the laterally conductive high relative humidity conditioned papers. In 
some cases this can be adequately done by self biasing of the baffles with 
resistors or diodes connected between the electrically isolated baffles 
and ground reference for the machine. In this case, the lateral current 
flow along the papers under high relative humidity conditions causes the 
baffle bias. The bias baffle approaches work very well for electrostatic 
transfer and electrostatic tacking if the toner image is being transferred 
from relatively insulating materials such as a photoconductor surface in 
the dark. 
The bias baffle approach will not work acceptably if the resistivity of the 
intermediate surface is too low because the paper charge can then conduct 
along the paper to the local microscopic contact area regions between the 
paper and intermediate surface. This can drive the net paper charge, the 
electrostatic field for transfer and the electrostatic pressure between 
the paper and intermediate to very low levels during the contact dwell 
time of the paper and intermediate. To prevent this, the resistivity of 
the surface of the intermediate belt 41 must be chosen to be sufficiently 
insulating. Any material on the surface of the intermediate having a decay 
time for charge dissipation that is longer than the contact dwell time of 
the paper on the intermediate, which is typically in the 0.25 to 5.0 
second range for most configurations envisioned to be used with this 
invention, can be used to prevent this problem. It can be appreciated by 
those familiar to the art that there are numerous insulating or 
semi-conductive materials with sufficiently high resistivity to meet this 
criteria, such as mylar, tedlar, kynar, polycarbonate as typical examples. 
The sufficiently high resistivity surface layer will be on top of a support 
base in the preferred embodiment for the intermediate. The base can be a 
conductive substrate such as Ni or stainless steel, or it can for example 
be a metalized plastic base such as aluminized mylar with the aluminized 
layer facing the thin top surface layer. Electrical connection for the 
conductive portion of the base layer provides the necessary bias reference 
for the paper charging, toner transfer to the paper, and for the DEP 
imaging process. Most photoconductive belt structures can be used as the 
intermediate for this invention and these have an advantage that light can 
be used to dissipate cyclic charge buildup on the intermediate for process 
stability during continuous cyclic imaging. An example of a 
photoconductive belt structure that can be used with this invention is the 
type of structure used in xerographic products such as in the Xerox 
1075.TM.. If a photoconductor is not used, cyclic charge buildup can be 
prevented by conditioning of the surface of the intermediate belt with, 
for example, corona devices, as is well known to those skilled in the art. 
Continued movement of the intermediate belt together with the paper sheet 
tacked thereto effects movement of the sheet of paper past another pair of 
DEP devices 62 and 64 which are utilized for forming different color 
images on the side of a sheet of paper opposite to the one containing the 
images transferred thereto from the intermediate belt. 
The potential difference between the conductive substrate of the 
intermediate belt and the DEP heads 62 and 64 will generally need to be 
adjusted differently than the values chosen for the DEP heads 50 and 52 to 
compensate for the effect on the fields near the DEP head caused by the 
charge on the paper. The effect of the charge on the paper on the fields 
near the DEP heads 62 and 64 will depend, for example, on the thickness, 
the electrical properties of the paper used and on the environmental 
conditions. If the paper properties are known or measured in the machine, 
and if the environmental conditions are known, the DEP head voltages can 
be automatically adjusted for proper imaging conditions. Automatic 
compensation for environmental and paper property variations can also be 
achieved by sensing the voltage above the charged paper on the 
intermediate just prior to the DEP imaging head. This can be done using a 
type of sensor 65 generally known to those in the art as an electrostatic 
voltmeter. An average value obtained from such a measurement can then be 
used in a standard way to those skilled in the art to automatically update 
the voltage conditions on the DEP heads 62 and 64 for optimum imaging for 
the given paper or environmental conditions, such updating being 
accomplished through signals generated by the sensor 65 and processed via 
the ESS 54. It is known by those skilled in the art that the measured 
voltage above the paper acts just like an equivalent applied substrate 
voltage of equal value to the measured voltage above the paper. Thus the 
updated DEP head voltage conditions can be simply referenced to the 
measured voltage above the paper. 
A corona discharge device 66 positioned adjacent the roller 40 can be used 
to assist paper separation from the intermediate belt and also to reduce 
the chance of toner disturbance during the separation process, as is well 
known to those skilled in the art of detack in typical transfer systems. 
A duplex fuser indicated by reference character 68 includes a heated fuser 
roll 70 and a heated pressure roll 72 cooperating to form a nip 74 through 
which the image receiver sheets pass. An image receiver with fused images 
is deposited into a paper catch tray 76.