Overcoated transfer roller for transferring developed images from one surface to another

An apparatus which transfers a developed image from a charge retentive surface to a copy sheet. The apparatus includes a transfer roller that is overcoated with an insulator and charged by either a brush, roller or a blade. The biased transfer roller is brought into contact with the back of a copy sheet thereby transferring charges that migrate through the copy sheet to enable toner transfer from the photoconductive surface to the copy sheet. A metal blade can be used to simultaneously charge and clean the transfer roller.

This invention relates to an electrophotographic printing machine, and more 
particularly concerns an apparatus for transferring a developed image from 
a photoconductive surface to a copy sheet. 
In a conventional transfer station in xerography, a developed image of 
toner policies from the image developer material is transferred from a 
photoreceptor (the imaging surface) to a cut or roll fed copy sheet (the 
final image support surface), either directly or after an intermediate 
image transfer to an intermediate surface. Such image transfers are also 
required in other electrostatographic processing systems, such as 
electrophotoretic development. In "TESI" systems the intermediately 
transferred image may be an undeveloped latent electrostatic image. 
Transfer is most commonly achieved by applying electrostatic force fields 
in a transfer nip sufficient to overcome the forces holding the toner to 
its original support surface and to attract most of the toner to transfer 
over onto the contacting second surface. These transfer fields are 
generally provided in one of two ways, by ion emission from a transfer 
corona generator onto the back of the copy sheet, as in U.S. Pat. No. 
2,807,233, or by a D.C biased transfer roller or belt rolling along the 
back of the copy sheet. Examples of bias roller transfer systems are 
described in U.S. Pat. Nos. 2,807,233; 4,043,684; 3,267,840; 3,328,193; 
3,598,580; 3,625,146; 3,630,591; 3,691,993; 3,702,482; 3,684,364; 
3,781,105 and 3,847,478. A transfer system is shown in U.S. Pat. No. 
4,947,214 in which a blade presses a copy sheet into contact with at least 
the developed image on a photoconductive surface. 
The difficulties of successful image transfer are well known. In the 
pre-transfer (pre-nip) region, before the copy paper contacts the image, 
if the transfer fields are high, the image is susceptible to premature 
transfer across the air gap, leading to decreased resolution or fuzzy 
images Further, if there is ionization in the pre-nip air gap from high 
fields, it may lead to strobing or other image defects, loss of transfer 
efficiency, and a lower latitude of system operating parameters. Yet, in 
the directly adjacent nip region itself, the transfer field should be 
large as possible (greater than approximately 20 volts per micron) to 
achieve high transfer efficiency and stable transfer. In the next adjacent 
post-nip region, at the photoconductor/copy sheet separation area, if the 
transfer fields are too low, hollow characters may be generated. On the 
other hand, improper ionization in the post-nip region may cause image 
instability or copy sheet detacking problems. Variations in ambient 
conditions, copy paper, contaminants, etc. can all affect the necessary 
transfer parameters for simplex copying and especially for duplex copying. 
Biased transfer rollers (BTR) offer a positive pressure to a copy sheet 
which can resolve transfer problems mentioned above in addition to 
problems due to cockles, folds and perforation pucker. Most require 
tailor-made resistance which is necessary to prevent arcing should the 
rollers come into contact with the photoconductor or receptor. Toner that 
is accidentally attached to the surface of the BTR can transfer to the 
back side of the copy sheet which makes cleaning of the BTR necessary. It 
also appears that the size of the conventional BTR (about 2-3 inches or 
more in diameter) contributes to a large pre and post nip breakdown zone 
formed adjacent either side of the receptor-to-copy paper contact zone 
which can lead to decreased resolution or fuzzy images. 
In accordance with one aspect of the present invention, there is provided 
an apparatus for transferring a developed image from a photoconductive 
surface to a copy sheet. The apparatus includes a roller that is 
overcoated with an insulator that is made of a hard or soft material. The 
highly insulating surface is charged by a brush, roller or blade and is 
brought into contact with the back of a copy sheet thereby transferring 
charges that migrate through the paper to enable toner transfer from the 
photoconductive surface to the copy sheet. A blade cleans the insulating 
surface to prevent contamination of the back side of the copy sheet 
In another aspect of the present invention, there is provided an 
electrophotographic printing machine of the type in which a developed 
image is transferred from a photoconductive surface to a copy sheet at a 
transfer station. The improved printing machine includes a biased transfer 
roller that has a high voltage applied to its core and is covered by an 
insulator in order to achieve good transfer without air breakdown. The 
insulator is discharged by a grounded brush as it is cycled, however, the 
brush does not necessarily have to be grounded.

While the present invention will hereinafter be described in connection 
with a preferred embodiment and method of use, it will be understood that 
it is not intended to limit the invention to that embodiment or method of 
use. 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. 
For a general understanding of the features of the present invention, 
reference is made to the drawings. In the drawings, like reference 
numerals have been used throughout to identify identical elements. FIG. 1 
schematically depicts an electrophotographic printing machine 
incorporating the features of the present invention therein. It will 
become evident from the following discussion that the apparatus of the 
present invention may be employed in a wide variety of electrostatographic 
printing machines and is not specifically limited in its application to 
the particular embodiment or method of use described herein. 
Referring now to FIG. 1 of the drawings, the electrophotographic printing 
machine employs a photoconductive belt 10. Preferably, the charge 
retentive photoconductive belt 10 is made from a photoconductive material 
coated on a ground layer, which, in turn, is coated on an anti-curl 
backing layer. The photoconductive material is made from a transport layer 
coated on a generator layer. The transport layer transports positive 
charges from the generator layer. The interface layer is coated on the 
ground layer. The transport layer contains small molecules of 
di-m-tolydiphenylbiphenyldiamine dispersed in a polycarbonate. The 
generation layer is made from trigonal selenium. The grounding layer is 
made from a titanium coated Mylar. The ground layer is very thin and 
allows light to pass therethrough. Other suitable photoconductive 
materials, ground layers, and anti-curl backing layers may also be 
employed. Belt 10 moves in the direction of arrow 12 to advance successive 
portions of the photoconductive surface sequentially through the various 
processing stations disposed about the path of movement thereof. Belt 10 
is entrained about stripping roller 14, tensioning roller 16, rollers 18, 
and drive roller 20. Stripping roller 14 and rollers 18 are mounted 
rotatably so as to rotate with belt 10. Tensioning roller 16 is 
resiliently urged against belt 10 to maintain belt 10 under the desired 
tension. Drive roller 20 is rotated by a motor coupled thereto by suitable 
means such as a belt drive. As roller 20 rotates, it advances belt 10 in 
the direction of arrow 12. 
Initially, a portion of photoconductive belt 10 passes through charging 
station A. At charging station A, two corona generating devices, indicated 
generally by the reference numerals 22 and 24 charge photoconductive belt 
10 to a relatively high, substantially uniform potential. Corona 
generating device 22 places all of the required charge on photoconductive 
belt 10. Corona generating device 24 acts as a leveling device, and fills 
in any areas missed by corona generating device 22. 
Next, the charged portion of photoconductive belt 10 is advanced through 
imaging station B. At imaging station B, a document handling unit, 
indicated generally by the reference numeral 26, is positioned over platen 
28 of the printing machine. Document handling unit 26 sequentially feeds 
documents from a stack of documents placed by the operator in the document 
stacking and holding tray. The original documents to be copied are loaded 
face up into the document tray on top of the document handling unit. A 
document feeder, located below the tray, feeds the bottom document in the 
stack to rollers. The rollers advance the document onto platen 28. When 
the original document is properly positioned on platen 28, a belt 
transport is lowered onto the platen with the original document being 
interposed between the platen and the belt transport. After imaging, the 
original document is returned to the document tray from platen 28 by 
either of two paths. If a simplex copy is being made or if this is the 
first pass of a duplex copy, the original document is returned to the 
document tray via the simplex path. If this is the inversion pass of a 
duplex copy, then the original document is returned to the document tray 
through the duplex path. Imaging of a document is achieved by two Xenon 
flash lamps 30 mounted in the optics cavity which illuminate the document 
on platen 28. Light rays reflected from the document are transmitted 
through lens 32. Lens 32 focuses the light image of the original document 
onto the charged portion of the photoconductive surface of belt 10 to 
selectively dissipate the charge thereon. This records an electrostatic 
latent image on photoconductive belt 10 which corresponds to the 
informational areas contained within the original document. Thereafter, 
photoconductive belt 10 advances the electrostatic latent image recorded 
thereon to development station C. 
At development station C, a magnetic brush developer unit, indicated 
generally by the reference numeral 34, has three developer rolls, 
indicated generally by the reference numerals 36 and 38. A paddle wheel 42 
picks up developer material and delivers it to the developer rolls. When 
developer material reaches rolls 36 and 38, it is magnetically split 
between the rolls with half of the developer material being delivered to 
each roll. Photoconductive belt 10 is partially wrapped about rolls 36 and 
38 to form extended development zones. Magnetic roll 44 is a carrier 
granule removal device adapted to remove any carrier granules adhering to 
belt 10. Thus, rolls 36 and 38 advance developer material into contact 
with the electrostatic latent image. The latent image attracts toner 
particles from the carrier granules of the developer material to form a 
toner powder image on the photoconductive surface of belt 10. Belt 10 then 
advances the toner powder image to transfer station D. 
At transfer station D, a copy sheet is moved into contact with the toner 
powder image. The copy sheet is frequently wrinkled. First, 
photoconductive belt 10 is exposed to a pre-transfer light from a lamp 
(not shown) to reduce the attraction between photoconductive belt 10 and 
the toner powder image. The copy sheet is advanced along the sheet path 
and pressed into contact with the toner powder image on photoconductive 
surface 10 by a biased transfer apparatus, indicated generally by the 
reference numeral 45. Biased transfer apparatus 45 includes a roller which 
presses the copy sheet into contact with the toner powder image developed 
on photoconductive belt 10. This substantially eliminates any spaces 
between the copy sheet and the toner powder image. The continuous pressing 
of the sheet into contact with the toner powder image at the transfer 
station insures that the copy sheet is substantially wrinkle free at the 
transfer station. Corona generated by the biased transfer roll charges the 
copy sheet to the proper magnitude and polarity so that the copy sheet is 
tacked to photoconductive belt 10 and the toner powder image attracted 
from the photoconductive belt to the copy sheet. In this way, the copy 
sheet moves with photoconductive belt 10, in the direction of arrow 12. 
Further details of this apparatus will be described hereinafter with 
reference to FIG. 2. 
After transfer, the beam strength of the copy sheet provides the detachment 
forces enabling detack from belt 10. Conveyor 50, positioned to receive 
the copy sheet, advances it to fusing station E. 
Fusing station E includes a fuser assembly, indicated generally by the 
reference numeral 52, which permanently affixes the transferred toner 
powder image to the copy sheet. Preferably, fuser assembly 52 includes a 
heated fuser roller 54 and a pressure roller 56 with the powder image on 
the copy sheet contacting fuser roller 54. The pressure roller is cammed 
against the fuser roller to provide the necessary pressure to fix the 
toner powder image to the copy sheet. The fuser roll is internally heated 
by a quartz lamp. Release agent, stored in a reservoir, is pumped to a 
metering roll. A trim blade trims off the excess release agent. The 
release agent is transferred to a donor roll and then to the fuser roll. 
After fusing, the copy sheets are fed through a decurler 58. Decurler 58 
bends the copy sheet in one direction to put a known curl in the copy 
sheet and then bends it in the opposite direction to remove that curl. 
Forwarding roller pairs 60 then advance the sheet to duplex turn roll 62. 
Duplex solenoid gate 64 guides the sheet to the finishing station F or to 
duplex tray 66. In the finishing station, the copy sheets are collected in 
sets with the copy sheets of each set being stapled or glued together. 
Alternatively, duplex solenoid gate 64 diverts the sheet into duplex tray 
66. The duplex tray 66 provides an intermediate or buffer storage for 
those sheets that have been printed on one side and on which an image will 
be subsequently printed on the second, opposed side thereof, i.e. the 
sheets being duplexed. The sheets are stacked in duplex tray 66 face down 
on top of one another in the order in which they are copied. 
In order to complete duplex copying, the simplex sheets in tray 66 are fed, 
in seriatim, by bottom feeder 68 from tray 66 back to transfer station D 
via conveyor 70, and rollers 72, for transfer of the toner powder image to 
the opposed sides of the copy sheets. Inasmuch as successive bottom sheets 
are fed from duplex tray 66, the proper or clean side of the copy sheet is 
positioned in contact with belt 10 at transfer station D so that the toner 
powder image is transferred thereto. The duplex sheet is then fed through 
the same path as the simplex sheet to be advanced to finishing station F. 
Copy sheets are fed to transfer station D from the secondary tray 74. 
Secondary tray 74 includes an elevator driven by a bidirectional AC motor. 
Its controller has the ability to drive the tray up or down. When the tray 
is in the down position, stacks of copy sheets are loaded thereon or 
unloaded therefrom. In the up position, successive copy sheets may be fed 
therefrom by sheet feeder 76. Sheet feeder 76 is a friction retard feeder 
utilizing a feed belt and take-away rolls to advance successive copy 
sheets to transport 70 which advances the sheets to rolls 72 and then to 
transfer station D. 
Copy sheets may also be fed to transfer station D from the auxiliary tray 
78. The auxiliary tray 78 includes an elevator driven by a bidirectional 
AC motor. Its controller has the ability to drive the tray up or down. 
When the tray is in the down position, stacks of copy sheets are loaded 
thereon or unloaded therefrom. In the up position, successive copy sheets 
may be fed therefrom by sheet feeder 80. Sheet feeder 80 is a friction 
retard feeder utilizing a feed belt and take-away rolls to advance 
successive copy sheets to transport 70 which advances the sheets to rolls 
72 and then to transfer station D. 
Secondary tray 74 and auxiliary tray 78 are secondary sources of copy 
sheets. A high capacity feeder, indicated generally by the reference 
numeral 82, is the primary source of copy sheets. High capacity feeder 82 
includes a tray 84 supported on an elevator 86. The elevator is driven by 
a bidirectional motor to move the tray up or down. In the up position, the 
copy sheets are advanced from the tray to transfer station D. A vacuum 
feed belt 88 feeds successive uppermost sheets from the stack to a take 
away roll 90 and rolls 92. The take-away roll 90 and rolls 92 guide the 
sheet onto transport 93. Transport 93 and roll 95 advance the sheet to 
rolls 72 which, in turn, move the sheet into the transfer zone at transfer 
station D 
Invariably, after the copy sheet is separated from photoconductive belt 10, 
some residual particles remain adhering thereto. After transfer, 
photoconductive belt 10 passes beneath corona generating device 94 which 
charges the residual toner particles to the proper polarity. Thereafter, 
the pre-charge erase lamp (not shown), located inside photoconductive belt 
10, discharges the photoconductive belt in preparation for the next 
charging cycle. Residual particles are removed from the photoconductive 
surface at cleaning station G. Cleaning station G includes an electrically 
biased cleaner brush 96 and two de-toning rolls 98 and 100, i.e. waste and 
reclaim de-toning rolls. The reclaim roll is electrically biased 
negatively relative to the cleaner roll so as to remove toner particles 
therefrom. The waste roll is electrically biased positively relative to 
the reclaim roll so as to remove paper debris and wrong sign toner 
particles. The toner particles on the reclaim roll are scraped off and 
deposited in a reclaim auger (not shown), where it is transported out of 
the rear of cleaning station G. 
The various machine functions are regulated by a controller. The controller 
is preferably a programmable microprocessor which controls all of the 
machine functions hereinbefore described. The controller provides a 
comparison count of the copy sheets, the number of documents being 
recirculated, the number of copy sheets selected by the operator, time 
delays, jam corrections, etc. The control of all of the exemplary systems 
heretofore described may be accomplished by conventional control switch 
inputs from the printing machine consoles selected by the operator. 
Conventional sheet path sensors or switches may be utilized to keep track 
of the position of the documents and the copy sheets. In addition, the 
controller regulates the various positions of the gates depending upon the 
mode of operation selected. 
Referring now to FIGS. 2 and 3, there is shown an elevational view further 
illustrating the features of the present invention. As shown thereat, bias 
transfer roller apparatus 45 has a highly insulating overcoating 47 placed 
on top of grounded transfer roller core 46. The bias transfer roller is a 
two piece member consisting of a support 46 and an insulating overcoating 
47 on the support that is thin and can range, for example, from about 
0.0001 inches to about 0.02 inches in thickness. The insulating 
overcoating can be a relatively soft dielectric film, such as, yellow 
beeswax, carnauba wax, low molecular weight plastics (e.g., polyethelyne, 
polycarbonate), mylar or a very hard material, such as, anodized aluminum, 
ceramic, high molecular weight plastics, varnishes or glass. BTR Roller 45 
is small in comparison to conventional BTRs and has a diameter of between 
about 1/16 to about 3/4 inch in diameter, preferably about 1/2 inch in 
diameter thereby reducing the effect in distance over which image 
degradation can occur in the pre and post nip zones. The roller is contact 
charged by brush 48 in FIG. 2 and a blade 49 in FIG. 3 and rotates in the 
direction of arrow 7. In FIG. 3, a metal cleaning blade is employed to 
remove any toner 8 accidentally contacting roller 45. The cleaning feature 
of FIG. 2 can be accomplished by biased brush charging member 48 although 
additional vacuum cleaning of the brush may be used to remove particles, 
if desired. It is also contemplated that a single metal blade can perform 
both the charging and cleaning functions, such as, the cleaning blade 6 in 
FIG. 3. Roller 45 contacts the back of copy sheet 81 transferring charges 
that migrate through the copy sheet to enable transfer. Tests have shown 
that only a thousand volts of charge to the transfer roller will enable 
transfer. This gives rise to a novel feature of the present invention in 
that a limited number of charges are deposited on the surface of the 
insulator at the charging station. If the roller should contact the 
photoconductive surface, no sustained arc is possible since there is no 
reservoir of charge to maintain the arc, therefore, no damage to the 
photoconductive surface takes place. The roller 45 to photoconductive 
surface 10 rotation may be synchronous or asynchronous which produces an 
asymmetrical charge distribution that is beneficial in some applications. 
There is no electrical connection between the transfer roller nip and the 
contact charging station, and therefore, the limited number of charges 
deposited on the surface of the roller 45 at the charging station are 
carried to the transfer station, thereby transferring some of these 
limited number of charges that migrate through the copy sheet to enable 
transfer of the developed image to the copy sheet. Thus, the contact 
charging means of the present invention charges the transfer roller by 
contact therewith without generating and maintaining current in 
pre-transfer, transfer nip or post-transfer nip regions. Another way 
biased transfer roller 45 can be used is spaced from photoconductive 
surface 10. This non-contact method can have sufficient surface charge to 
produce ionization in the gap between the photoconductive surface and the 
biased transfer roller. This will also supply a charge limited transfer 
current. 
An alternative embodiment of the transfer apparatus of the present 
invention is shown in FIG. 4 and includes the apparatus of FIG. 2 with a 
positive DC bias to core member 46 of transfer roller 45 of approximately 
2 kV. By applying a DC bias to the transfer roller and grounding brush 48, 
some negative charges deposit on the outer insulator surface 47. These 
opposite sign charges affect the transfer field, such that the transfer 
field is between conductive roller 45 and the ground plane of 
photoconductor 10 or alternatively, the opposite sign charge produces a 
bias transfer roller surface which is more positive relative to the ground 
plane of photoconductor 10. This induces charge separation within the copy 
sheet 81 with negative charges forming on the bias transfer roller side, 
and positive charges forming on the photoconductor side. These positive 
charges assist in forming fields to provide transfer. By discharging the 
photoconductive surface 10 prior to transfer, with light or AC charges, 
the field holding the toner to the photoconductive surface 10 will be 
reduced thereby allowing lower transfer voltage/fields to be used. In a 
test of this system in a Xerox.RTM. 3100 machine with a transfer roller 
having a 3 mil (0.003 inches) mylar overcoat and a conductive discharging 
brush, excellent transfer was obtained with a positive 2000 volts applied 
to the transfer roller. No pre or post nip breakdown was observed for 
cockle, folded or perforated copy sheets. The alternative embodiment of 
the present invention shown in FIG. 5 is the same as that shown in FIG. 2 
except that biased brush 48 of FIG. 2 had been replaced with biased roller 
40. The apparatus of FIG. 2 is included here with grounded core member 46 
of transfer roller 45. By applying a DC bias to the charging roller 40 and 
grounding core member 46, some positive charges deposit on the outer 
insulator surface 47. These opposite sign charges affect the transfer 
field, such that the transfer field is between conductive roller 45 and 
the ground plane of photoconductor 10. 
In recapitulation, the transfer apparatus of the present invention includes 
a transfer roller that is overcoated with an insulator and charged by 
either a brush, roller or a blade. In all of the above embodiments, the 
transfer roller is preferably one shaft with a thin overcoating. The shaft 
can extend right out to the bearings and drive of the printer. The 
transfer core member or shaft 46 can be solid or a hollow cylinder, e.g., 
a tube and must be conductive. The biased roller is brought into contact 
with the back of a copy sheet thereby transferring charges that migrate 
through the copy sheet to enable toner transfer to the copy sheet. A metal 
blade can be used for both charging and cleaning the transfer roller, if 
desired. 
It is, therefore, evident that there has been provided, in accordance with 
the present invention, an apparatus that fully satisfies the aims and 
advantages hereinbefore set forth. While this invention has been described 
in conjunction with a preferred embodiment and method of use, it is 
evident that many alternatives, modifications, and variations will be 
apparent to those skilled in the art. Accordingly, it is intended to 
embrace all such alternatives, modifications and variations as fall within 
the spirit and broad scope of the appended claims.