Tri-level background suppression scheme using an AC scorotron with front erase

Deposition of background toner particles during tri-level image transfer from an imaging member to a final substrate is minimized. A front erase step is used prior to pretransfer corona treatment in order to reduce the background voltage level to substantially the residual voltage level of the photoreceptor imaging member used in the imaging process. When the image is subjected to pretransfer corona positively charged black and color background toner particles become negative and wrong sign black toner becomes more negative. Using negative transfer corona effects transfer of toner forming the black and color images and inhibits transfer of background toner.

BACKGROUND OF THE INVENTION 
This invention relates generally to tri-level imaging and more particularly 
to a method and apparatus for reducing the amount of background toner 
particles deposited on a final substrate during the transfer of a 
tri-level image from a charge retentive surface to a substrate such as 
plain paper. 
In the practice of conventional xerography, it is the general procedure to 
form electrostatic latent images on a xerographic surface by first 
uniformly charging a charge retentive surface such as a photoreceptor. 
Only the imaging area of the photoreceptor is uniformly charged. The image 
area does not extend across the entire width of the photoreceptor. 
Accordingly, the edges of the photoreceptor are not charged. The charged 
area is selectively dissipated in accordance with a pattern of activating 
radiation corresponding to original images. The selective dissipation of 
the charge leaves a latent charge pattern on the imaging surface 
corresponding to the areas not exposed by radiation. 
This charge pattern is made visible by developing it with toner by passing 
the photoreceptor past a single developer housing. The toner is generally 
a colored powder which adheres to the charge pattern by electrostatic 
attraction. The developed image is then fixed to the imaging surface or is 
transferred to a receiving substrate such as plain paper to which it is 
fixed by suitable fusing techniques. 
In tri-level, highlight color imaging, unlike conventional xerography, the 
image area contains three voltage levels which correspond to two image 
areas and to a background voltage area. One of the image areas corresponds 
to non-discharged (i.e. charged) areas of the photoreceptor while the 
other image areas correspond to discharged areas of the photoreceptor. 
The concept of tri-level, highlight color xerography is described in U.S. 
Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach 
teaches the use of tri-level xerography as a means to achieve single-pass 
highlight color imaging. As disclosed therein the charge pattern is 
developed with toner particles of first and second colors. The toner 
particles of one of the colors are positively charged and the toner 
particles of the other color are negatively charged. In one embodiment, 
the toner particles are supplied by a developer which comprises a mixture 
of triboelectrically relatively positive and relatively negative carrier 
beads. The carrier beads support, respectively, the relatively negative 
and relatively positive toner particles. Such a developer is generally 
supplied to the charge pattern by cascading it across the imaging surface 
supporting the charge pattern. In another embodiment, the toner particles 
are presented to the charge pattern by a pair of magnetic brushes. Each 
brush supplies a toner of one color and one charge. In yet another 
embodiment, the development systems are biased to about the background 
voltage. Such biasing results in a developed image of improved color 
sharpness. 
In highlight color xerography as taught by Gundlach, the xerographic 
contrast on the charge retentive surface or photoreceptor is divided 
three, rather than two, ways as is the case in conventional xerography. 
The photoreceptor is charged, typically to 900 v. It is exposed imagewise, 
such that one image corresponding to charged image areas (which are 
subsequently developed by charged-area development, i.e. CAD) stays at the 
full photoreceptor potential (V.sub.cad or V.sub.ddp, shown in FIG. 1a). 
The other image is exposed to discharge the photoreceptor to its residual 
potential, i.e. V.sub.dad or V.sub.c (typically 100 v) which corresponds 
to discharged area images that are subsequently developed by 
discharged-area development (DAD) and the background areas exposed such as 
to reduce the photoreceptor potential to halfway between the V.sub.cad and 
V.sub.dad potentials, (typically 500 v) and is referred to as V.sub.white 
or V.sub.w. The CAD developer is typically biased about 100 v (V.sub.bb, 
shown in FIG. 1b) closer to V.sub.cad than V.sub.white (about 600 v), and 
the DAD developer system is biased about 100 v (V.sub.cb, shown in FIG. 
1b) closer to V.sub.dad than V.sub.white (about 400 v). 
After development of the tri-level image is complete, a pre-transfer step 
must be performed in order to make all of the toner on the photoreceptor 
(both colors) common in polarity so that conventional transfer methods can 
be utilized. For sake of illustration, it is assumed that a pre-transfer 
device is operating in the positive mode, and that transfer is operating 
negatively. When the developed tri-level image is exposed to a positive 
pre-transfer dicorotron, the negative charge on the color toner changes to 
positive, making it common in sign with the black toner. This enables 
transfer of the developed image to paper using negative corona. However, 
because the low charged and/or wrong sign toner present in the background 
areas is also exposed to the pre-transfer dicorotron, it also becomes 
positive (or more positive in the case of the wrong-sign color 
background). As a result, the background toner also tends to transfer to 
paper, which results in visible background on the fused tri-level prints. 
It is well known in the prior art to subject a developed image on a charge 
retentive surface to corona discharge prior to image transfer for various 
reasons. For, example, U.S. Pat. No. 3,444,369 issued on May 13, 1969 
relates to a method and apparatus for the reduction of background in 
transferred xerographic copy. A developed toner image on a photoconductive 
surface is subjected to a low level corona discharge of a polarity 
opposite the charge on the toner particles overlying the image areas. The 
corona discharge adjacent the image areas will be repelled by the like 
sign, but highly charged image areas of the photoconductive surface to 
thereby render the image area toner unaffected. The corona discharge 
adjacent the non-image areas of the photoconductive surface will not be 
repelled and will thus convert the toner overlying the non-image areas to 
a polarity opposite that on the image area toner particles. This will 
permit the electrostatic transfer of the image area toner, but will tend 
to suppress the transfer of the non-image area toner to a backing sheet. 
It is also known to utilize light exposure and corona discharge prior to 
image transfer as shown in U.S. Pat. No. 4,506,971. In this device the 
light exposure occurs prior to the corona exposure. As stated therein, 
blurred images are minimized or eliminated in a xerographic reproduction 
prior to transfer by first exposing the image to light to at least 
substantially discharge the background around the image and to reduce the 
charge on the photoreceptor holding the image thereto. Secondly, a charge 
of opposite polarity of the charged photoreceptor is deposited onto the 
image and photoreceptor. This, as stated, produces a very stable image for 
transfer since a very strong holding force is produced to hold the image 
in place as the image enters the transfer station. 
U.S. Pat. No. 3,784,300 issued on Jan. 8, 1974 relates to a copying 
apparatus with a pre-transfer station including a pre-transfer corotron 
and lamp arranged such that the light exposure of the photoreceptor is 
subsequent and not simultaneous with the pre-transfer corona charging. 
U.S. Pat. No. 4,205,322 issued on May 27, 1980 relates to an electrostatic 
recording apparatus in which a toner image consisting of toner particles 
of at least two different kinds and of different polarities is efficiently 
and reliably transferred to a recording medium such as an ordinary sheet 
of paper. The toner particles having different polarities are all 
converted into those having one polarity and after such conversion the 
toner image (with its two kinds of particles) is electrostatically 
transferred to the recording medium, the transfer involving both kinds of 
particles at the same time. 
U.S. Pat. No. 5,038,177 granted to Parker et al on Aug. 6, 1991 describes 
that balanced, efficient corona transfer for both the charged area image 
and the discharged area image of a developed tri-level image is obtained 
by the provision of a selective pretransfer charge corona device in 
combination with a pretransfer discharge lamp. While improved transfer 
over prior art devices is obtained using a pretransfer lamp prior to 
pretransfer charging the preferred embodiment of the invention utilizes a 
pretransfer lamp before and in coincidence with pretransfer charging. In 
this patent the pretransfer lamp is positioned adjacent the side of the 
photoreceptor opposite the toner images for controlling the magnitude and 
distribution of pre-transfer current so that disproportionately more 
charge is added to the part of composite tri-level image that must have 
its polarity reversed compared to elsewhere on the image. 
U.S. patent application Ser. No. 08/179,176 filed in the name of Pietrowski 
et al on Jan. 10, 1994 discloses pre pretransfer treatment for multiple 
toner images for increasing the operating latitude for 
pretransfer/transfer. In one embodiment of the invention, a pre 
pretransfer corona device is used to drive the tribos of two multiple 
toner images toward each other prior to pretransfer. A single constant 
current corona discharge device is used in that embodiment. Subsequent 
pretransfer treatment serves to reduce the delta tribo between the two 
images thereby providing an operating latitude of 3 micro coul/g. 
BRIEF SUMMARY OF THE INVENTION 
Briefly, the present invention reduces the amount of background toner 
transfer from a tri-level imaging surface or photoreceptor to a final 
substrate such as plain paper. To this end, an imaging surface containing 
a tri-level image is exposed to a well collimated light. The surface 
containing the image is exposed to the collimated light to thereby reduce 
the background voltage part of the tri-level image to approximately the 
residual voltage level of the photoreceptor imaging surface. Subsequent to 
light exposure of the tri-level image, the image is subjected to a 
substantially constant voltage scorotron which causes the polarity of one 
of the images to become the same as the other image. The charge on the 
background particles becomes negative or more negative in the case of 
wrong sign black toner.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION 
As shown in FIG. 3, a printing machine incorporating the invention may 
utilize a charge retentive member in the form of a photoconductive belt 10 
consisting of a photoconductive surface and an electrically conductive, 
light transmissive substrate and mounted for movement past a charging 
station A, an exposure station B, developer station C, transfer station D 
and cleaning station F. Belt 10 moves in the direction of arrow 16 to 
advance successive portions thereof sequentially through the various 
processing stations disposed about the path of movement thereof. Belt 10 
is entrained about a plurality of rollers 18, 20 and 22. Roller 20 is used 
as a drive roller while the rollers 18 and 22 serve to tension the belt 10 
and effect substrate stripping from the belt 10, respectively. A motor 23 
rotates roller 20 to advance belt 10 in the direction of arrow 16. Roller 
20 is coupled to motor by suitable means such as a belt drive. 
As can be seen by further reference to FIG. 3, initially successive 
portions of belt 10 pass through charging station A. At charging station 
A, a corona discharge device such as a scorotron, corotron or dicorotron 
indicated generally by the reference numeral 24, charges the belt 10 to a 
selectively high uniform positive or negative potential, V.sub.0. Any 
suitable control, well known in the art, may be employed for controlling 
the corona discharge device 24. 
Next, the charged portions of the photoreceptor surface are advanced 
through exposure station B. At exposure station B, the uniformly charged 
photoreceptor or charge retentive surface 10 is exposed to a laser based 
output scanning device 25 which causes the charge retentive surface to be 
discharged in accordance with the output from the scanning device. 
Preferably the scanning device is a three level laser Raster Output 
Scanner (ROS). The resulting photoreceptor contains both charged-area 
images and discharged-area images as well as background areas intermediate 
the charged and discharged areas. 
The photoreceptor, which is initially charged to a voltage V.sub.0, 
undergoes dark decay to a level V.sub.ddp equal to about 900 volts. When 
exposed at the exposure station B it is discharged to V.sub.Color (DAD) 
equal to about -300 volts in the highlight (i.e. color other than black) 
color parts of the image. See FIG. 1a. The photoreceptor is also 
discharged to V.sub.w equal to -400 volts imagewise in the background 
(white) image areas. The photoreceptor is discharged to V.sub.Black(CAD) 
equal to -500 volts. After passing through the exposure station, the 
photoreceptor contains charged areas and discharged areas which 
corresponding to two images and to charged edges outside of the image 
areas. 
At development station C, a development system, indicated generally by the 
reference numeral 30 advances developer materials into contact with the 
electrostatic latent images. The development system 30 comprises first and 
second developer apparatuses 32 and 34. The developer apparatus 32 
comprises a housing containing a pair of magnetic brush rollers 35 and 36. 
The rollers advance developer material 40 into contact with the 
photoreceptor for developing the discharged-area images. The developer 
material 40 by way of example contains negatively charged red toner. 
Electrical biasing is accomplished via power supply 41 electrically 
connected to developer apparatus 32. A DC bias of approximately -350 volts 
is applied to the rollers 35 and 36 via the power supply 41. 
The developer apparatus 34 comprises a housing containing a pair of 
magnetic brush rolls 37 and 38. The rollers advance developer material 42 
into contact with the photoreceptor for developing the charged-area 
images. The developer material 42 by way of example contains positively 
charged black toner for developing the charged-area images. Appropriate 
electrical biasing is accomplished via power supply 43 electrically 
connected to developer apparatus 34. A suitable DC bias of approximately 
-450 volts is applied to the rollers 37 and 38 via the bias power supply 
43. 
Because the composite image developed on the photoreceptor consists of both 
positive and negative toner, a front erase lamp 48 and a positive 
pre-transfer corona discharge member 56 are provided to condition the 
toner for effective transfer to a substrate with minimal transfer of 
background toner particles. Negative corona discharge is utilized. 
A sheet of support material 58 is moved into contact with the toner image 
at transfer station D. The sheet of support material is advanced to 
transfer station D by conventional sheet feeding apparatus, not shown. 
Preferably, the sheet feeding apparatus includes a feed roll contacting 
the uppermost sheet of a stack copy sheets. Feed rolls, not shown, rotate 
so as to advance the uppermost sheet from stack into a chute which directs 
the advancing sheet of support material into contact with photoconductive 
surface of belt 10 in a timed sequence so that the toner powder image 
developed thereon contacts the advancing sheet of support material at 
transfer station D. 
Transfer station D includes a corona generating device 60 which sprays ions 
of a suitable polarity onto the backside of sheet 58. This attracts the 
charged toner powder images from the belt 10 to sheet 58. After transfer, 
the sheet continues to move, in the direction of arrow 62, onto a conveyor 
(not shown) which advances the sheet to fusing station E. 
Fusing station E includes a fuser assembly, indicated generally by the 
reference numeral 64, which permanently affixes the transferred powder 
image to sheet 58. Preferably, fuser assembly 64 comprises a heated fuser 
roller 66 and a backup roller 68. Sheet 58 passes between fuser roller 66 
and backup roller 68 with the toner powder image contacting fuser roller 
66. In this manner, the toner powder image is permanently affixed to sheet 
58. After fusing, a chute, not shown, guides the advancing sheet 58 to a 
catch tray, also not shown, for subsequent removal from the printing 
machine by the operator. 
After the sheet of support material is separated from photoconductive 
surface of belt 10, the residual toner particles carried by the non-image 
areas on the photoconductive surface are removed therefrom. These 
particles are removed at cleaning station F. A magnetic brush cleaner 
housing is disposed at the cleaner station F. The cleaner apparatus 
comprises a conventional magnetic brush roll structure for causing carrier 
particles in the cleaner housing to form a brush-like orientation relative 
to the roll structure and the charge retentive surface. It also includes a 
pair of detoning rolls for removing the residual toner from the brush. 
Subsequent to cleaning, a discharge lamp (not shown) floods the 
photoconductive surface with light to dissipate any residual electrostatic 
charge remaining prior to the charging thereof for the successive imaging 
cycle. 
After development of the tri-level latent image is complete, typical 
post-development voltages on the photoreceptor in the Charged Area, 
(V.sub.CAD), Discharged Area, (V.sub.DAD), and white areas (V.sub.WHITE) 
are as follows: 
V.sub.CAD (Post-DEV)=-500 V 
V.sub.DAD (Post-DEV)=-300 V 
V.sub.WHITE (Post-DEV)=-400 V 
In accordance with the invention, when the developed, two-color image is 
exposed to the front erase pre-transfer discharge lamp 48, the following 
occurs. Because very little toner exists in the background areas 
(V.sub.WHITE), the pre-transfer light discharges these background areas to 
approximately the residual potential of the photoreceptor, which for a 
commercially available active matrix photoreceptor, is typically about -50 
volts. The light from the lamp does not have a large effect on the post 
development voltages in either the CAD black or DAD color areas, because 
these areas are developed with toner which should block light from getting 
to the photoreceptor. In order to minimize the possibility that light 
might pass through the more translucent color toner (red, blue, or green), 
pre-transfer light that has a wavelength (.lambda.) that would be absorbed 
by the color toner (i.e. use blue light for red toner) may be employed. 
The light from the front erase pre-transfer lamp 48 is preferably a well 
collimated light to avoid discharging the photoreceptor near the edges of 
the developed black and color image areas, which may be especially 
critical for fine lines and halftone patterns. FIG. 2 depicts he fully 
developed Tri-level image voltage profile both before and after exposure 
to the front erase pre-transfer lamp 48. 
After exposure to the front erase pre-transfer lamp 48, the developed 
tri-level image is exposed to the AC scorotron 56 of the type disclosed in 
U.S. Pat. No. 4,591,713 granted to gundlach et al on May 27, 1986. The 
scorotron 56 comprises an insulative housing 72, a plurality of coronode 
wires 74 and a control screen or grid 76. The control grid 76 is biased 
with a steady state DC bias that is somewhat negative, in the order of 
-100 V to -150 V, given the electrostatics shown in FIGS. 2a and 2. The 
coronode wires 74 of the scorotron have a high voltage AC potential 
(sine-wave) source 78 applied to them that is sufficient in amplitude to 
generate both positive and negative charges during ionization. The 
frequency of the applied AC is high enough, in the order of 1-5 Khz, so as 
not to cause visible strobing on the prints. Because the post-development 
V.sub.CAD and V.sub.DAD (-500 V and -300 V, respectively), are both more 
negative than the scorotron control grid the positive charges from the 
scorotron flow to these image areas until their surface potential 
approaches the scorotron grid voltage. This flow of positive charge 
changes the negatively charged color toner to positive charge, without 
greatly increasing the positive charge already present on the black toner. 
Because the V.sub.WHITE regions of the photoreceptor are more positive 
than the scorotron control grid due to the front erase treatment using the 
lamp 48, the negative charges from the scorotron flow to the toner located 
in these areas until the surface potential approaches that of the control 
grid. As a result, the charge on the black and color toner comprising 
background become negative (or more negative in the case of wrong sign 
black), which significantly reduces the likelihood that background toner 
particles on the imaging surface will transfer to paper when using 
negative transfer current of the device 60. While an AC scorotron has been 
disclosed it will be appreciated that a DC device may also be utilized.