Multicolor image-on-image forming machine using air breakdown charge and development (ABCD) Process

A multicolor image-on-image reproduction machine includes a main assembly, a controller, a movable image bearing member having a path of movement, and a plurality of air breakdown charge and development (ABCD) imaging units mounted along the path of movement for forming color separation toner images. Each (ABCD) imaging unit includes a photoreceptor having a photoconductive surface forming a toner image separation development nip with the movable image bearing member; a toner supply apparatus for applying a layer of toner onto the photoconductive surface; a charging device for uniformly charging the photoconductive surface through the layer of toner; and an exposure device connected to the controller for image-wise exposing of the photoconductive surface and the layer of toner to form therein image areas and background areas of a desired color separation image. The multicolor image-on-image reproduction machine then includes a bias source for biasing the image bearing member at the toner image separation development nip to a potential sufficient to cause air breakdown selective recharging of the background areas of the layer of toner, thereby enabling the image areas of the layer of toner to be separated and developed as a color separation toner image onto the movable image bearing member, and the background areas thereof to remain on the photoreceptor.

BACKGROUND OF THE INVENTION
 The present invention relates generally to electrostatic latent image
 development, and, more particularly, concerns a multicolor image-on-image
 reproduction machine using air breakdown charge and development (ABCD)
 process.
 Generally, processes for electrostatographic copying and printing are
 initiated by selectively charging and/or discharging a charge receptive
 image bearing member in accordance with an original input document or an
 imaging signal, generating an electrostatic latent image on the image
 bearing member. This latent image is subsequently developed into a visible
 image by a process in which charged developing material is deposited onto
 the surface of the latent image bearing member, wherein charged particles
 in the developing material adhere to image areas of the latent image. The
 developing material typically comprises carrier granules having toner
 particles adhering triboelectrically thereto, wherein the toner particles
 are electrostatically attracted from the carrier granules to the latent
 image areas to create a powder toner image on the image bearing member.
 Alternatively, the developing material may comprise a liquid developing
 material comprising a carrier liquid having pigmented marking particles
 (or so-called toner solids) charge director materials dissolved therein,
 wherein the liquid developing material is applied to the latent image
 bearing image bearing member with the marking particles being attracted to
 the image areas of the latent image to form a developed liquid image.
 Regardless of the type of developing material employed, the toner or
 marking particles of the developing material are uniformly charged and are
 electrostatically attracted to the latent image to form a visible
 developed image corresponding to the latent image on the image bearing
 member.
 The developed image is subsequently transferred, either directly or
 indirectly, from the image bearing member to a copy substrate, such as
 paper or the like, to produce a "hard copy" output document. In a final
 step, the image bearing member is cleaned to remove any charge and/or
 residual developing material therefrom in preparation for a subsequent
 image forming cycle.
 The above-described electrostatographic printing process is well known and
 has been implemented in various forms in the marketplace to facilitate,
 for example, so-called light lens copying of an original document, as well
 as for printing of electronically generated or digitally stored images
 where the electrostatic latent image is formed via a modulated laser beam.
 Analogous processes also exist in other electrostatic printing
 applications such as, for example, ionographic printing and reproduction
 where charge is deposited in image-wise configuration on a dielectric
 charge retentive surface (see, for example, U.S. Pat. No. 4,267,556 and
 4,885,220, among numerous other patents and publications), as well as
 other electrostatic printing systems wherein a charge carrying medium is
 adapted to carry an electrostatic latent image. It will be understood that
 the instant invention applies to all various types of electrostatic
 printing systems and is not intended to be limited by the manner in which
 the image is formed on the image bearing member or the nature of the
 latent image bearing member itself.
 As described hereinabove, the typical electrostatographic printing process
 includes a development step whereby developing material is physically
 transported into contact with the image bearing member so as to
 selectively adhere to the latent image areas thereon in an image-wise
 configuration. Development of the latent image is usually accomplished by
 electrical attraction of toner or marking particles to the image areas of
 the latent image. The development process is most effectively accomplished
 when the particles carry electrical charges opposite in polarity to the
 latent image charges, with the amount of toner or marking particles
 attracted to the latent image being proportional to the electrical field
 associated with the image areas. Some electrostatic imaging systems
 operate in a manner wherein the latent image includes charged image areas
 for attracting developer material (so-called charged area development
 (CAD), or "write white" systems), while other printing processes operate
 in a manner such that discharged areas attract developing material
 (so-called discharged area development (DAD), or "write black" systems).
 Image quality in electrostatographic printing applications may vary
 significantly due to numerous conditions affecting latent image formation
 as well as development, among various other factors. In particular, image
 development can be effected by charge levels, both in the latent image, as
 well as in the developing material. For example, when the charge on dry
 toner particles becomes significantly depleted, binding forces with the
 carrier also become depleted, causing an undesirable increase in image
 development, which, in turn, causes the development of the latent image to
 spread beyond the area defined thereby. Similarly, one problem affecting
 the control of image quality in ionographic devices involves a phenomenon
 known as "image blooming" resulting from the effect of previously
 deposited ions or charge on the path of subsequent ions directed to the
 charge retentive surface. This problem is particularly noticeable when
 printing characters and edges of solid areas, resulting in character
 defects, wherein blooming artifacts may include picture elements being
 displaced by 1-2 pixels in distance. Image blooming can also be caused by
 poor charge retention and/or charge migration in the electrostatic latent
 image on the latent image bearing member, a problem which is particularly
 prevalent in ionographic systems, wherein a focused beam ion source is
 utilized for image-wise charging of a dielectric latent image bearing
 member.
 The present invention more particularly, concerns a multicolor
 image-on-image reproduction machine advantageously using air breakdown
 charge and development (ABCD) process, and the following disclosures may
 be relevant to some aspects of the present invention. U.S. Pat. No.
 4,504,138 discloses a method of developing a latent electrostatic charge
 image formed on a photoconductor surface comprising the steps of applying
 a thin viscous layer of electrically charged toner particles to an
 applicator roller preferably by electrically assisted separation thereof
 from a liquid toner suspension, defining a restricted passage between the
 applicator roller and the photoconductor surface which approximates the
 thickness of the viscous layer, and transferring the toner particles from
 the applicator roller at the photoconductor surface due to the
 preferential adherence thereof to the photoconductor surface under the
 dominant influence of the electric field strength of the electrostatic
 latent image carried by the photoconductive surface, the quantity of toner
 particles transferred being proportional to the relative incremental field
 strength of the latent electrostatic image. An apparatus for carrying out
 the method of the invention is also disclosed, which includes an
 applicator roller mounted for rotation in a container for toner
 suspension, an electrode arranged adjacent the circumferential surface of
 the roller to define an electrodeposition chamber therebetween and
 electrical connections between the roller, the electrode and a voltage
 source to enable electrolytic separation of toner particles in the
 chamber, forming a thin highly viscous layer of concentrated toner
 particles on the roller.
 U.S. Pat. No. 5,387,760 discloses a wet development apparatus for use in a
 recording machine to develop a toner image corresponding to an
 electrostatic latent image on an electrostatic latent image carrier. The
 apparatus includes a development roller disposed in contact with or near
 the electrostatic latent image carrier and an application head for
 applying a uniform layer of the wet developer to the roller.
 U.S. Pat. No. 5,436,706 discloses an imaging apparatus including a first
 member having a first surface having formed thereon a latent electrostatic
 image, wherein the latent electrostatic image includes image regions at a
 first voltage and background regions at a second voltage. A second member
 charged to a third voltage intermediate the first and second voltages is
 also provided, having a second surface adapted for resilient engagement
 with the first surface. A third member is provided, adapted for resilient
 contact with the second surface in a transfer region. The imaging
 apparatus also includes an apparatus for supplying liquid toner to the
 transfer region thereby forming on the second surface a thin layer of
 liquid toner containing a relatively high concentration of charged toner
 particles, as well as an apparatus for developing the latent image by
 selective transferring portions of the layer of liquid toner from the
 second surface to the first surface.
 U.S. Pat. No. 5,619,313 discloses a method and apparatus for simultaneously
 developing and transferring a liquid toner image. The method includes the
 steps of moving a photoreceptor including a charge bearing surface having
 a first electrical potential, applying a uniform layer of charge having a
 second electrical potential onto the charge bearing surface, and
 image-wise dissipating charge from selected portions on the charge bearing
 surface to form a latent image electrostatically, such that the
 charge-dissipated portions of the charge bearing surface have the first
 electrical potential of the charge bearing surface. The method also
 includes the steps of moving an member biased to a third electrical
 potential that lies between said first and said second potentials, into a
 nip forming relationship with the moving image bearing member to form a
 process nip. The method further includes the step of introducing charged
 liquid toner having a fourth electrical potential into the process nip,
 such that the liquid toner sandwiched within the nip simultaneously
 develops image portions of the latent image onto the member, and
 background portions of the latent image onto the charge bearing surface of
 the photoreceptor.
 SUMMARY OF THE INVENTION
 In accordance with one aspect of the present invention, there is provided a
 multicolor image-on-image reproduction machine that includes a main
 assembly, a controller, a movable image bearing member having a path of
 movement, and a plurality of air breakdown charge and development (ABCD)
 imaging units mounted along the path of movement for forming color
 separation toner images. Each (ABCD) imaging unit includes a photoreceptor
 having a photoconductive surface forming a toner image separation
 development nip with the movable image bearing member; a toner supply
 apparatus for applying a layer of toner onto the photoconductive surface;
 a charging device for uniformly charging the photoconductive surface
 through the layer of toner; and an exposure device connected to the
 controller for image-wise exposing of the photoconductive surface and the
 layer of toner to form therein image areas and background areas of a
 desired color separation image. The multicolor image-on-image reproduction
 machine then includes a bias source for biasing the image bearing member
 at the toner image separation development nip to a potential sufficient to
 cause air breakdown selective recharging of the background areas of the
 layer of toner, thereby enabling the image areas of the layer of toner to
 be separated and developed as a color separation toner image onto the
 movable image bearing member, and the background areas thereof to remain
 on the photoreceptor.

DETAILED DESCRIPTION OF THE INVENTION
 While the present invention will be described in terms of an illustrative
 embodiment or embodiments, it will be understood that the invention is
 adaptable to a variety of copying and printing applications, such that the
 present invention is not necessarily limited to the particular embodiment
 or embodiments shown and described herein. On the contrary, the following
 description 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.
 The present invention relates generally to electrostatic latent image
 development, and, more particularly, concerns a multicolor image-on-image
 reproduction machine using air breakdown charge and development (ABCD)
 process. An Air breakdown charge and Development (ABCD) process as
 disclosed for example in commonly assigned U.S. Pat. No. 5,937,243, issued
 Aug. 10, 1999 to Liu et al, involves the formation of a desired final
 toner image from a layer of marking material coated onto an image bearing
 member. This is achieved by selectively applying charges to the layer of
 marking material via air breakdown so as to create an image-wise charged
 marking material layer having image areas and background areas. The
 image-wise charged marking material layer is thus capable of being
 selectively separated image-wise, into background areas, and image areas
 comprising the desired final toner image.
 Referring first to FIG. 1, there is illustrated a tandem multicolor
 reproduction machine shown generally as 500. As shown, the tandem
 multicolor reproduction machine 500 includes a plurality of (ABCD) imaging
 units 100, 200, 300, 400 that each include respectively a photoreceptor
 member 112, 212, 312, 412, and that each employ a process of image-wise
 toner layer charging via an air breakdown charge and development process
 to form a color separation toner image on the photoreceptor. Each color
 separation toner image is then developed in registration onto a biased
 image bearing member 502, where it is conditioned by an image stabilizing
 and conditioning device 504 in accordance with the present invention
 Referring now to FIGS. 1 and 2, each (ABCD) imaging unit 100, 200, 300, 400
 as shown comprises an assemblage of operatively associated image forming
 elements, including a photoreceptor 112, 212, 312, 412 situated in contact
 with a biased image bearing member 502 at an image separating and transfer
 nip 512, 522, 532, 542 formed therebetween. Photoreceptor 112, 212, 312,
 412 includes an imaging surface of any type capable of having an
 electrostatic latent image formed thereon. Photoreceptor 112, 212, 312,
 412 may include a typical photoconductor or other photoreceptive component
 of the type known to those of skill in the art in electrophotography,
 wherein a surface layer having photoconductive properties is supported on
 a conductive support substrate. Although the following description will
 describe by example a system and process in accordance with the present
 invention incorporating a photoconductive photoreceptor, it will be
 understood that the present invention contemplates the use of various
 alternative embodiments for photoreceptor 112, 212, 312, 412 as are well
 known in the art of electrostatographic printing, including, for example,
 but not limited to, non-photosensitive photoreceptors such as a dielectric
 charge retaining member of the type used in ionographic printing machines,
 or electroded substructures capable of generating charged latent images.
 Photoreceptor 112, 212, 312, 412 is rotated, as indicated by arrow 111, so
 as to transport the surface thereof in a process direction for
 implementing a series of image forming steps in a manner similar to
 typical electrostatographic printing processes. Initially, the
 photconductive surface of photoreceptor 112, 212, 312, 412 through a
 coating station where a layer of charged or uncharged toner particles is
 deposited by a toner supply apparatus on the surface of the photoreceptor
 112, 212, 312, 412. To that end, a toner supply apparatus or applicator
 150, 250, 350, 450 is provided, as depicted in detail in FIG. 2, whereby a
 layer of charged or uncharged toner particles (and possibly some carrier
 mechanism such as a liquid solvent) is applied onto the surface of the
 photoreceptor 112, 212, 312, 412. The toner supply apparatus 150, 250,
 350, 450 may include an applicator roller 156 (biased by a source 155)
 which is rotated in a direction as indicated by arrow 157 to apply a
 substantially and uniformly distributed layer of toner, or a so-called
 "toner cake", 158 onto the surface of the photoreceptor 112, 212, 312,
 412. A shown, the toner supply and applicator apparatus 150, 250, 350, 450
 also includes a housing 152 that is adapted to accommodate a supply of
 toner particles 154 and any additional carrier material, if necessary. As
 shown, the applicator roller 156 is rotated in a direction as indicated by
 arrow 157 to transport toner from housing 152 into contact with the
 surface of the photoreceptor thus producing the toner "cake" or toner
 layer 158.
 The toner cake 158 described above can be created in various ways. For
 example, depending on the materials utilized in the printing process, as
 well as other process parameters such as process speed and the like, a
 layer of toner particles having sufficient thickness, preferably on the
 order of between 2 and 15 microns and more preferably between 3 and 8
 microns, may be formed on the surface of the photoreceptor 112, 212, 312,
 412 by merely providing adequate proximity and/or contact pressure between
 the applicator roller 156 and the photoreceptor 112, 212, 312, 412.
 Alternatively, electrical biasing may be employed to assist in actively
 moving the toner particles onto the surface of the photoreceptor 112, 212,
 312, 412.
 After the toner "cake" or layer 158 is formed on the surface of the
 photoreceptor 112, 212, 312, 412, it passes through a charging station,
 which as shown includes a corona generating device 130 or any other
 charging apparatus for applying a uniform layer of electrostatic charge to
 the toner cake or layer 158. The corona generating device 130 charges the
 toner cake or layer 158 to a relatively high and substantially uniform
 potential.
 After the toner cake or layer 158 is brought to a substantially uniform
 charge potential, it is advanced to an image exposure station, including
 an exposure device identified generally by reference numeral 140, 240,
 340, 440. At the exposure station, the exposure device 140, 240, 340, 440,
 uniformly exposes the charged toner cake or layer 158 to a laser based
 input and/or output source that is controlled by an electronic subsystem
 (ESS) controller 15. The ESS 15, for example, is the main multi-tasking
 processor for operating and controlling all of the other subsystems of the
 multicolor tandem machine 500, and the toner image forming operations of
 each imaging unit.
 The image exposure device 140, 240, 340, 440 thus projects a light image
 corresponding to the color separation image onto the charged
 photoconductive surface through the toner cake or layer 158. The light
 image projected thus, selectively dissipates the charge thereon for
 recording a primary electrostatic latent image therein. The primary
 electrostatic latent image comprises image areas defined by a first charge
 voltage and non-image areas defined by a second charge voltage in image
 configuration corresponding to the color separation image informational
 areas. The image exposure device 140, 240, 340, 440 may comprise anyone of
 various optical image formation and projection components as are known in
 the art, and may include various well known light lens apparatus or
 digital scanning system for forming and projecting an image from an
 original input document onto the photoreceptor 112, 212, 312, 412.
 After the toner "cake" or layer 158 is image-wise exposed as such, it is
 then moved to the image separating and transfer nip 512, 522, 532, 542. As
 noted above, the image separating and transfer nip 512, 522, 532, 542 is
 formed therebetween the photoreceptor 112, 212, 312, 412 and the biased
 image bearing member 502. At the image separating and development nip, the
 exposed cake or layer 158 is recharged in an image-wise manner by inducing
 ionization of the air in the vicinity of the toner layer 158. To that end,
 the biased image bearing member 502 is provided, situated adjacent the
 toner layer 158, for introducing free mobile ions in the vicinity of the
 primary latent image in order to facilitate the formation of an image-wise
 ion stream extending from the image bearing member 502 to the primary
 latent image on the surface of the photoreceptor 112, 212, 312, 412. The
 image-wise ion stream generates a secondary latent image in the toner
 layer 158 made up of oppositely charged toner particles in image
 configuration corresponding to the primary latent image on the
 photoreceptor 112, 212, 312, 412.
 The process of generating a secondary latent image in the toner cake layer
 158 is described in greater detail in U.S. Pat. No. 5,937,243 as cited
 above, and relevant portions of which are incorporated herein by
 reference. As described therein, the charged toner cake for example can be
 a uniformly distributed layer of negatively charged toner particles having
 the thickness of a single layer or multiple layers of toner particles. The
 toner cake resides on the surface of the photoreceptor is transported past
 a biased member. The primary function of the biased member is to provide
 free mobile ions in the vicinity of the photoreceptor having the toner
 layer and primary latent image thereon. As it is known, when two
 conductors are held near each other with a voltage applied between the
 two, electrical discharge will occur as the voltage is increased to a
 point of air breakdown. Thus, at a critical point, a discharge current is
 created in the air gap between the conductors. This point is commonly
 known as the Paschen threshold voltage. When the conductors are very close
 together (a few thousandths of an inch) discharge can take place without
 sparking, such that a discharge current will be caused to flow across a
 gap between the biased member and the toner cake or layer. This phenomenon
 thus is used to induce image-wise charging, and hence a secondary latent
 image in the toner cake or layer 158.
 As shown, the image bearing member 502 is biased at the nip 512, 522, 532,
 542 by an electrical biasing source 563 capable of providing an
 appropriate voltage potential to the biased image bearing member 502,
 sufficient to produce image-wise air breakdown in the vicinity of a latent
 image bearing surface of the toner cake or layer 158. Preferably, the
 voltage applied to the biased image bearing member 502 is maintained at a
 predetermined potential such that electrical discharge is induced only in
 a limited region where the surface of the image bearing member 502 and the
 photoreceptor 112, 212, 312, 412 are in very close proximity and the
 voltage differential between the biased image bearing member 502 and the
 non-image areas of the primary latent image exceed the Paschen threshold
 voltage.
 In one preferred embodiment, which will be known as "one-way breakdown", it
 is contemplated that the bias applied to the biased image bearing member
 502 is sufficient to exceed the Paschen threshold voltage only with
 respect to either one of the image or non-image areas of the original
 latent image in the toner cake on the photoreceptor. Alternatively, in
 another embodiment, the bias applied to the biased image bearing member
 502 will be sufficient to exceed the Paschen threshold with respect to
 both the image or non-image areas of the primary latent image. The air
 breakdown induced in this case can be caused to occur in a manner such
 that field lines are generated in opposite directions with respect to the
 image and non-image areas. For example, in the case where the Paschen
 threshold voltage is about 400 volts, and the image and non-image areas
 have voltage potentials of about 0 and 1200 volts respectively, a bias
 potential applied to the biased image bearing member 502 of approximately
 -200 volts will result in air breakdown that generates charges only in the
 region of the non-image areas such that the toner particles adjacent to
 this region will be affected. Conversely, a bias of -1000 volts applied to
 biased image bearing member 502, for example, will result in charge
 generation in the region of the image area of the latent image, with ions
 flowing in the opposite direction.
 In yet another example, a bias of approximately -600 volts applied to the
 biased image bearing member 502 will result in charge generation in the
 areas adjacent to both image and non-image areas with ions flowing in
 opposite directions. This so-called 2-way air breakdown mode occurs where
 electrical discharge via air breakdown is induced in a pre-nip region
 immediately prior to a nip region created by contact between the
 photoreceptor 112, 212, 312, 412 and the image bearing member 502. The
 electrical discharge causes electrostatic fields to develop between the
 image bearing member 502 and the photoreceptor 112, 212, 312, 412 in the
 pre-nip region. In turn, the force of these fields causes the air to
 become ionized, generating free mobile ions which are directed toward the
 photoreceptor 112, 212, 312, 412. In a preferred embodiment, as
 illustrated in FIG. 1, a "one-way" ABCD is implemented such that only the
 background areas 74 are subjected to air breakdown and charge reversal.
 After the secondary latent image is formed in the toner layer 158, the
 latent image bearing toner cake or layer 158 is moved completely through
 the image separating and transfer nip, 512, 522, 532, 542. Thus, referring
 back to FIG. 1, image separating and transfer nip, 512, 522, 532, 542 as
 mentioned above is formed by the photoreceptor 112, 212, 312, 412, and the
 biased image bearing member 502 having a surface adjacent to the surface
 of the photoreceptor 112, 212, 312, 412, and preferably contacting the
 toner layer 158 residing on photoreceptor 112, 212, 312, 412. The
 electrical biasing source 563 coupled to the biased image bearing member
 502 also biases the image separating and transfer nip, 512, 522, 532, 542
 so as to attract the image areas of the toner layer 158, thereby
 simultaneously separating and developing the toner layer 158 into image
 areas 172 (FIG. 2), and non-image areas 174. The polarity of the bias
 source 563 is such as to bias the image bearing member 502 (at the image
 separating and transfer nip, 512, 522, 532, 542) for attracting image
 areas 172 from the toner cake or layer 158. This results in image
 development by which image areas 172 of the toner cake 158 are separated
 and developed onto the surface of the biased image bearing member 502,
 while leaving background image areas 174 on the surface of the
 photoreceptor 112, 212, 312, 412.
 In a final step on each imaging unit 100, 200, 300400, the background areas
 174 left on the photoreceptor after image transfer to the mage bearing
 member 502 is either recycled into the toner supply apparatus (FIG. 1) or
 removed from the surface thereof by a cleaning unit 190 (FIG. 2) in order
 to clean the surface in preparation for a subsequent imaging cycle. FIG. 2
 illustrates a simple blade cleaning apparatus for scraping the
 photoreceptor surface as is well known in the art. Alternative embodiments
 may include a brush or roller member for removing toner from the surface
 on which it resides.
 Referring in particular to FIG. 1, after the image areas 172 from each of
 the imaging units 100, 200, 300, 400, for example imaging unit 100, are
 developed as above onto the biased image bearing member 502 as a color
 separation toner image, it is conditioned and stabilized by an image
 stabilization device 504 as shown prior to the development and transfer of
 a the subsequent color separation toner image by the next imaging unit. As
 shown, image stabilization device 504 comprises a preferably heated
 pressure roller 506, and charging unit 508. The pressure roller 506 is
 made suitable for contacting the image areas or toner image 172 on the
 image bearing member 502 in order to increase toner layer strength by
 taking out carrier liquid from the toner image. Heat from the heated
 pressure roller 506 operates to increase toner layer strength by fusing or
 partially fusing the toner image on the image bearing member 502. The
 charging unit 508 for example is a corona device, and preferably has the
 same polarity as the polarity of the charge on the toner forming the image
 areas 172.
 In accordance with the present invention, the charging device 130 for each
 imaging unit 100, 200, 300, 400 charges the layer of toner 158 to a
 polarity that is opposite that of the bias source 563 for biasing the
 image bearing member 502. On the other hand, charging unit 508 of the
 image stabilization device 504 charges the color separation toner image
 172 to the same polarity as that of the charging device 130 of each the
 imaging units.
 Such advantageous effects of heat can also be obtained without contact
 using a radiant heat source to increase toner layer strength by
 crosslinking polymer chains of toner particles forming the toner image on
 the image bearing member. In any case, the image stabilization device 504
 thus conditions and stabilizes the color separation toner image so that
 minimum disturbances thereof will occur at the next image separation
 development nip. It also prevents color contamination at such next image
 separation development nip, as well as enhances the toner layer
 cohesiveness by increasing the solid concentration partially coalescing
 the toner particles. The image stabilization device 504 is additionally
 preferable in order to avoid any back transfer of the toner image already
 on the image bearing member 502 to the next photoreceptor, for example,
 due to wrong sign toner.
 In accordance with the present invention, the value of the bias source 563
 on the image bearing member 502 is preferably always outside the range of
 a bias on the photoreceptor of each imaging unit. In the machine 500, the
 bias source 563 at each image separating and transfer nip could be the
 same or equal for each imaging unit, and hence for each different color
 separation toner image being developed.
 After each of the imaging units 100, 200, 300, 400 has formed and developed
 a color separation toner image to form a multicolor composite image on the
 surface of the biased image bearing member 502 as above, the multicolor
 composite image may then be transferred to a copy substrate 70. As shown
 such transfer may be via any means known in the art, which may include an
 electrostatic transfer apparatus including a corona generating device of
 the type previously described or a biased transfer roll. In a preferred
 embodiment, as shown in FIG. 1, the image is transferred to a copy
 substrate 70 via a heated pressure roll 510, whereby pressure and heat are
 simultaneously applied to the image to simultaneously transfer and fuse
 the image to the copy substrate 70. It will be understood that separate
 transfer and fusing systems may be provided, wherein the fusing or
 so-called fixing system may operate using heat (by any means such as
 radiation, convection, conduction, induction, etc.), or other known
 fixation process which may include the introduction of a chemical fixing
 agent.
 In the present invention, the full or multicolor composite toner image is
 built up directly on a biased image bearing member 502 as opposed to a
 conventional intermediate transfer member. This advantageously enables
 easily holding the image electrostatically on the image bearing member
 502, thus preventing degradation or smearing of the previous image in the
 next development nip.
 As can be seen, there has been provided a multicolor image-on-image
 reproduction machine that includes a main assembly, a controller, a
 movable image bearing member having a path of movement, and a plurality of
 air breakdown charge and development (ABCD) imaging units mounted along
 the path of movement for forming color separation toner images. Each
 (ABCD) imaging unit includes a photoreceptor having a photoconductive
 surface forming a toner image separation development nip with the movable
 image bearing member; a toner supply apparatus for applying a layer of
 toner onto the photoconductive surface; a charging device for uniformly
 charging the photoconductive surface through the layer of toner; and an
 exposure device connected to the controller for image-wise exposing of the
 photoconductive surface and the layer of toner to form therein image areas
 and background areas of a desired color separation image. The multicolor
 image-on-image reproduction machine then includes a bias source for
 biasing the image bearing member at the toner image separation development
 nip to a potential sufficient to cause air breakdown selective recharging
 of the background areas of the layer of toner, thereby enabling the image
 areas of the layer of toner to be separated and developed as a color
 separation toner image onto the movable image bearing member, and the
 background areas thereof to remain on the photoreceptor.
 It will be understood that the machine and processes described hereinabove
 represent only a few of the numerous system variants that could be
 implemented in the practice of the present invention.