Electrostatic latent image development

Abstract: An image development method and apparatus in which image-wise charging of a toner layer (58) is accomplished by induced air breakdown electrical discharge such that free mobile ions are introduced in the vicinity of an electrostatic latent image coated with a layer of developing material. The latent image causes the free mobile ions to flow in an image-wise ion stream corresponding to the latent image, which, in turn, leads to image-wise charging of the toner layer (58), such that the toner layer (58) itself becomes the latent image carrier. The latent image carrying toner layer is subsequently developed and transferred to a copy substrate to produce an output document (70).

Description: The present invention relates generally to electrostatic latent image development, and, more particularly, concerns an apparatus and method for developing an electrostatic latent image having a layer of marking material coated thereon by selectively applying charge potential to the toner layer via air breakdown to create an image-wise charged toner layer capable of being developed and selectively separated for producing an output image corresponding thereto. Generally, processes for electrostatographic copying and printing are initiated by selectively charging and/or discharging a charge receptive imaging member in accordance with an original input document or an imaging signal, generating an electrostatic latent image on the imaging 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. However, JP-A-61-251867 discloses an imaging apparatus in which toner is deposited uniformly on a photosensitive body and uniformly charged. The photosensitive body is then irradiated with light of image information. An AC discharger selectively discharges the photosensitive body and toner layer to invert the charging polarity of the toner corresponding to the image information. The toner is then transferred to charge transfer paper of apposite polarity to form a toner image. The present invention contemplates an electrostatographic imaging process wherein an electrostatic latent image bearing member having a layer of marking material coated thereon is selectively charged in an imagewise manner to create a secondary latent image corresponding to the electrostatic latent image on the imaging member. Image-wise charging is accomplished by inducing the ionization of air via a phenomenon known as air breakdown for introducing free mobile ions in the vicinity of the electrostatic latent image coated with the layer of toner particles. The formation of electrostatic charge patterns by electrical discharges involves the phenomena of ionic conduction through gases. It is known that 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 the point of air breakdown. This discharge is usually accompanied by a visible spark. However, when the conductors are very close together (a few thousands of an inch) discharge can take place without sparking and electrical charges will be collected on a receiving surface during discharges. The latent image causes the free mobile ions to flow in an image-wise ion stream corresponding to the latent image. These ions, in turn, are captured by the marking material in the layer, leading to image-wise charging of the marking layer with the marking material layer itself becoming the latent image carrier. The latent image carrying toner layer is subsequently developed by selectively separating image areas of the toner layer and transferring the separated image to a copy substrate for producing an output document. In accordance with a first aspect of the present invention, an image developing apparatus comprises means for depositing a layer of marking particles on the imaging member; means for inducing air breakdown creating an electrical discharge in a vicinity of the layer of marking particles on the imaging member to selectively charge the layer of marking particles in response to the electrostatic latent image on the imaging member so as to create a second any electrostatic latent image in the layer of marking particles; and means for selectively separating portions of the layer of marking particles in accordance with the second any latent image for creating a developed image corresponding to the electrostatic latent image formed on the imaging member. In accordance with a second aspect of the present invention, an imaging apparatus comprises an imaging member for having an electrostatic latent image formed thereon, said imaging member having a surface capable of supporting toner particles; an imaging device for generating the electrostatic latent image on said imaging member, wherein the electrostatic latent image includes image areas defined by a first charge voltage and non-image areas defined by a second charge voltage distinguishable from the first charge voltage; and, an image development apparatus according to the first aspect wherein the means for depositing a layer of marking particles comprises a toner supply apparatus for depositing toner particles on the surface of said imaging member to form a toner layer thereon adjacent the electrostatic latent image on said imaging member wherein the means for introducing air breakdown comprises a biased member and wherein the means for selectively separating comprises a separator member. In accordance with a third aspect of the present invention an image development process comprises the steps of: depositing a layer of marking particles on the imaging member; inducing air breakdown for selectively charging the layer of marking particles in response to the electrostatic latent image to create a second any electrostatic latent image in the layer of marking particles corresponding to the electrostatic latent image on the imaging member; and, selectively separating portions of the layer of marking particles in accordance with the second any latent image for creating a developed image. In accordance with a fourth aspect of the present invention an imaging process comprises the steps of: generating an electrostatic latent image on an imaging member having a surface capable of supporting toner particles, wherein the electrostatic latent image includes image areas defined by a first charge voltage and non-image areas defined by a second charge voltage distinguishable from the first charge voltage; and, carrying out an image development process in accordance with the third aspect of this invention, which the depositing of a layer of marking particles comprises depositing toner particles on the surface of said imaging member to form a toner layer thereon adjacent the electrostatic latent image on said imaging member, wherein the step of inducing air breakdown creates an electrical discharge in the vicinity of the toner layer on the latent image bearing imaging member, wherein the electrical discharge selectively delivers charged ions to the toner layer in response to the electrostatic latent image on said imaging member to form a secondary latent image in the toner layer having image and non-image areas corresponding to the electrostatic latent image on said imaging member, and wherein the selectively separating step includes transferring portions of the toner layer thereto in accordance with the secondary latent image in the toner layer to create a developed image corresponding to the electrostatic latent image formed on said imaging member. Embodiments of the present invention will now be described with reference to the accompanying drawings; in which:- FIG. 1 is a simple schematic illustration depicting a system and process for image-wise toner layer charging via air breakdown and image development; FIG. 2 is an exploded view illustrating image-wise charging of a toner layer via air breakdown, wherein a charged toner layer is selectively reverse charged in accordance with a latent image adjacent thereto; FIG. 3 is an exploded view illustrating image-wise toner layer charging via air breakdown, wherein a neutrally charged toner layer is selectively charged in an image-wise manner; and, FIG. 4 is an exploded view illustrating image-wise toner layer charging via air breakdown, and image separation using a singular biased roll member. Moving now to Fig. 1, an exemplary imaging apparatus capable of image-wise toner charging via air breakdown in accordance with the present invention is shown, comprising an assemblage of operatively associated image forming elements, including an imaging member 10 situated in contact with an image separating member 20 at an image separating nip 12 formed therebetween. Imaging member 10 includes an imaging surface of any type capable of having an electrostatic latent image formed thereon. An exemplary imaging member 10 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 imaging member, it will be understood that the present invention contemplates the use of various alternative embodiments for imaging member 10 as are well known in the art of electrostatographic printing, including, for example, but not limited to, non-photosensitive imaging members such as a dielectric charge retaining member of the type used in ionographic printing machines, or electroded substructures capable of generating charged latent images. Imaging member 10 is rotated, as indicated by arrow 11, 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, in the exemplary embodiment of Fig. 1, the photoconductive surface of imaging member 10 passes through a charging station, which may include a corona generating device 30 or any other charging apparatus for applying an electrostatic charge to the surface of the imaging member 10. The corona generating device 30 is provided for charging the photoconductive surface of imaging member 10 to a relatively high, substantially uniform potential. It will be understood that various charging devices, such as charge rollers, charge brushes and the like, as well as induction and semiconductive charge devices, among other devices which are well known in the art, may be incorporated into the charging station for applying a charge potential to the surface of the imaging member 10. After the imaging member 10 is brought to a substantially uniform charge potential, the charged surface thereof is advanced to an image exposure station, identified generally by reference numeral 40. The image exposure station projects a light image corresponding to the input image onto the charged photoconductive surface. In the case of an imaging system having a photosensitive imaging member, as currently described, the light image projected onto the surface of the imaging member 10 selectively dissipates the charge thereon for recording an electrostatic latent image on the photoconductive surface. The 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 input image informational areas. The image exposure station 40 may incorporate 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 imaging member 10. Alternatively, various other electronic devices known in the art may be utilized for generating an electronic information signal for creating the electrostatic latent image on the imaging member. It will be understood that the electrostatic latent image may be comprised of image and non-image areas that are defined areas having opposite charge polarities or by areas that merely have first and second distinguishable charge potential levels. In a typical electrostatographic printing process, after the electrostatic latent image is generated on the surface of the imaging member 10, the image would be developed into a visible image on the surface of the imaging member 10 by selectively attracting charged toner particles to areas of the latent image thereon. By contrast, in the present invention, a layer of charged or uncharged toner particles is deposited on the entire surface of the latent image bearing imaging member 10. To that end, a toner supply apparatus or applicator 50 is provided, as depicted in the exemplary embodiment of Fig. 1, whereby a layer of charged or uncharged toner particles (and possibly some carrier mechanism such as a liquid solvent) is transported onto the surface of the imaging member 10. The exemplary embodiment of Fig. 1 shows an illustrative toner applicator 50, wherein a housing 52 is adapted to accommodate a supply of toner particles 54 and any additional carrier material, if necessary. In an exemplary embodiment, the toner applicator 50 includes an applicator roller 56 which is rotated in a direction as indicated by arrow 57 to transport toner from housing 52 into contact with the surface of the imaging member 10, forming a substantially uniformly distributed layer of toner, or a so-called "toner cake", 58 thereon. The toner cake 58 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 imaging member 10 by merely providing adequate proximity and/or contact pressure between the applicator roller 56 and the imaging member 10. Alternatively, electrical biasing may be employed to assist in actively moving the toner particles onto the surface of the imaging member 10. Thus, in one exemplary embodiment, the applicator roller 56 can be coupled to an electrical biasing source 55 for implementing a so-called forward biasing scheme, wherein the toner applicator 56 is provided with an electrical bias of magnitude greater than both the image and non-image (background) areas of the electrostatic latent image on the imaging member 10, thereby creating electrical fields extending from the toner applicator roll 56 to the surface of the imaging member 10. These electrical fields cause toner particles to be transported to imaging member 10 for forming a substantially uniform layer of toner particles on the surface thereof. It will be understood that numerous other devices or apparatus may be utilized for depositing toner layer 58 on the surface of the imaging member 10, including various well known apparatus analogous to development devices used in conventional electrostatographic applications, such as, but not limited to: powder cloud systems which transport developing material to the imaging member by means of a gaseous medium such as air; brush systems which transport developing material to the imaging member by means of a brush or similar member; and cascade systems which transport developing material to the imaging member by means of a system for pouring or cascading the toner particles onto the surface of the imaging member. In addition, various systems directed toward the transportation of liquid developing material having toner particles immersed in a carrier liquid, can be incorporated into the present invention. Examples of such liquid transport system can include a fountain-type device as disclosed generally in US-A-5,519,473, or any other system capable of causing a flow of liquid developing material, including toner particles immersed in a liquid carrier medium, onto the surface of the imaging member. It is noted that, in the case of liquid developing materials, it is desirable that the toner cake formed on the surface of the imaging member 10 should be comprised of at least approximately 10% by weight toner solids, and preferably in the range of 15% - 35% by weight toner solids. With respect to the foregoing toner cake formation process and various apparatus therefor, it will be understood that the presence of the latent image on the imaging member may generate some fringe fields in areas of interface between image and non-image areas of the latent image. However, these fringe fields are minimal relative to the fields associated with conventional electrostatic latent image development such that, although some toner layer nonuniformity may result, the toner layer 58 deposited on the imaging member 10 surface can be characterized as having a substantially uniform density per mass area in both image and background areas of the latent image. In fact it is not a requirement of the invention that the toner layer be uniform or even substantially uniformly distributed on the surface of the imaging member 10, so long as the toner layer covers, at a minimum, the desired image areas of the latent image. In accordance with the present invention, after the toner layer 58 is formed on the surface of the electrostatic latent image bearing imaging member 10, the toner layer is charged in an image-wise manner by inducing ionization of the air in the vicinity of the toner layer on the electrostatic latent image bearing imaging member 10. Thus, in accordance with the present invention a biased roll member 60 is provided, situated adjacent the toner layer 58 on the imaging member 10, for introducing free mobile ions in the vicinity of the charged latent image to facilitate the formation of an image-wise ion stream extending from the roll member 60 to the latent image on the surface of the image bearing member 10, as will be described. The image-wise ion stream generates a secondary latent image in the toner layer 58 made up of oppositely charged toner particles in image configuration corresponding to the original latent image generated on the imaging member 10. The process of generating a secondary latent image in the toner cake layer will be described in greater detail with respect to Fig. 2, where an initially charged toner cake 58 is illustrated, for purposes of simplicity only, as a uniformly distributed layer of negatively charged toner particles having the thickness of a single toner particle. The toner cake resides on the surface of the imaging member 10 which is shown as being transported from left to right past the biased roll member 60. The primary function of the biased roll member 60 is to provide free mobile ions in the vicinity of the imaging member 10 having the toner layer 58 and latent image thereon. As previously noted, it is known that 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 the 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 roll member 60 and the toner layer 58. The present invention the exploitation of this phenomenon to induce image-wise charging. In operation, the biased roll member 60 is coupled to an electrical biasing source 63 capable of providing an appropriate voltage potential to the roll member, sufficient to produce air breakdown in the vicinity of a latent image bearing imaging member. Preferably, the voltage applied to the roll 60 is maintained at a predetermined potential such that electrical discharge is induced only in a limited region where the surface of the roll member 60 and the imaging member 10 are in very close proximity and the voltage differential between the roll and the image and/or non-image areas of the 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 roll 60 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 on the imaging member. Alternatively, in another embodiment, the bias applied to the roll 60 will be sufficient to exceed the Paschen threshold with respect to both the image or non-image areas of the original latent image. The air breakdown induced in this situation will 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 roll 60 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 effected. Conversely, a bias of -1000 volts applied to roll 60, 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 alternative, a bias of approximately -600 volts applied to roll 60 will result in charge generation in the areas adjacent both image and non-image areas with ions flowing in opposite directions. This so-called 2-way air breakdown mode is illustrated in Fig. 2, wherein electrical discharge via air breakdown is induced in a pre-nip region immediately prior to a nip region created by contact between the imaging member 10 and the roll member 60. The electrical discharge causes electrostatic fields to develop between the roll member 60 and the imaging member 10 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 imaging member 10. The magnitude of the bias potential applied to the roll member 60 operates to control the image-wise ionization and the amount of charge and the charge uniformity applied to the imaging surface 10. Thus, in accordance with the example described above, 2-way air breakdown can be induced by applying a bias voltage to roll 60 which is sufficient to exceed the Paschen threshold with respect to both image and non-image areas of a latent image on an imaging member brought into the vicinity of the roll 60. Providing that this bias applied to roll 60 in a range intermediate to the potential associated with the image and non-image areas, will result in proper control of the direction of charge flow for creating the desired latent image in the toner layer. With respect to the process illustrated by Fig. 2, it will be seen that the function of the charging device 60 is to charge the toner layer 58 in an image-wise manner. This process will be described with respect to a negatively charged toner layer, although it will be understood that the process can also be implemented using a positively charged toner layer. In addition, the process of the present invention can also be implemented using an uncharged or neutral toner layer, as will be described in greater detail as the present description proceeds. In the case of a charged toner layer, the process of the present invention requires that, at a minimum, the air breakdown process provide ions having a charge opposite the toner layer charge polarity. In the case of a negatively charged toner layer 58, as shown in Fig. 2, the biased roll member 60 is provided with an energizing bias intermediate the potential of the image and non-image areas of the latent image on the imaging member 10 yet exceeding the Paschen threshold voltage so that positive ions will be generated and caused to flow in the direction of low potential areas of the latent image. Under certain circumstances, such as when the charge on the toner layer is sufficient to prevent charge reversal due to injected wrong sign charge, the energizing bias can be higher or lower than the bias of the image and non-image areas of the latent image. In addition, the energizing bias can be provided in the form of either a direct current (DC) electrical bias or an alternating current (AC) bias with or without a DC offset. Fig. 2 illustrates the effect of the field lines in the case of a roll member energized by a DC voltage intermediate the charge potential associated with image and non image areas of the latent image, represented by (+) and (-) signs, respectively, on the back side of the imaging member 10. As illustrated, positive ions flow from the roll member 60, in the direction of the field lines, while negative ions (electrons) flow in a direction opposite to the direction of the field lines. The positive ions generated in the vicinity of a positively charged area (relative to the roll member bias potential) of the latent image are repelled from the toner layer 58 while the positive ions in the vicinity of a negatively charged area (relative to the roll member bias potential) of the latent image are attracted to the toner layer 58, and captured thereby. Conversely, negative ions generated in the vicinity of a positively charged area (relative to the roll member bias potential) of the latent image are attracted to the imaging member 10 and absorbed into the negatively charged toner 58, thereby enhancing toner charge in that area, while the negative ions in the vicinity of a negatively charged areas (relative to the roll member bias potential) of the latent image are repelled by the toner layer. The free flowing ions generated by the air breakdown induced ionization in the pre-nip region are captured by toner layer 58 in a manner corresponding to the latent image on the imaging member, causing image-wise charging of the toner layer 58, and creating a secondary latent image within the toner layer 58 that is charged opposite in charge polarity to the charge of the original latent image. Under optimum conditions, the charge associated with the original latent image will be converted into the secondary latent image in the toner layer 58 and/or absorbed by the charging roll 60 such that the voltage differential between which defines image and non-image areas in the original electrostatic latent image becomes substantially or completely dissipated. In the above-described process, a charged toner layer 58 is situated on a latent image bearing imaging member 10, wherein the charged toner layer 58 is exposed to charged ions for selectively reversing the preexisting charge of the toner layer. Since the toner layer is initially charged, fringe fields, illustrated as field lines extending between image and non-image areas on the latent image can influence the charged toner cake. While the existence of these fringe fields may be advantageous if the fringe fields can be properly controlled, these fringe fields may manifest themselves as image quality defects in the final output document. Thus, the present invention contemplates an alternative embodiment to the image-wise toner layer charging process via air breakdown described hereinabove, wherein the fringe field effect may be substantially eliminated. In this alternative embodiment, the image-wise toner charging process of the present invention is carried out using a neutrally charged toner cake layer coated on the imaging member. In this case, roll member 60, or multiple roll members, present both negative and positive polarity ions to the toner layer in the vicinity of the latent image for oppositely charging regions of the toner layer corresponding to image and non image areas of the latent image. In an exemplary embodiment, an AC biasing source 63 is provided for energizing roll member 60 to provide ions of opposite polarity. Also, under appropriate conditions, as in the case of 2-way breakdown, as previously described, ions of both polarities can be generated. Alternatively, a combination of two independent roll members capable of providing opposite polarity ions can be used by biasing each roll member with independent, DC biasing sources. Image-wise toner charging of a neutrally charged toner cake leads to another alternative embodiment for the present invention which is illustrated in Fig. 3. In this embodiment, air breakdown is induced in both the pre-nip and post-nip regions to provide the opposite charge polarity ions required to appropriately image-wise charge the neutral toner layer. This concept can be enabled by a segmented bias roll member of the type well known in the art and disclosed generally in US-A-3,847,478. The segmented bias roll is provided with a plurality of discrete conductive electrodes 61, with each electrode being independently biased or energized via independent conductive shoe members 62 which are further coupled to independent biasing sources 63. In the embodiment illustrated in Fig. 3, the segmented bias roll member 62 is provided with a positive DC bias relative to the latent image in the pre-nip region and a negative DC bias relative to the latent image in the post-nip region. It will be recognized that the bias voltage applied to the roll member 60 is not required to be intermediate the potentials associated with the image and non-image areas of the original latent image on the imaging member. Rather, a voltage which causes air breakdown relative to only one of either the image or non-image areas may be applied to the roll member. Thus, in the exemplary embodiment of Fig. 3, the conductive shoes 62 are each independently driven by independent DC biasing sources 63 to induce image-wise air breakdown which generates oppositely charged ion streams in opposite directions. This embodiment operates in a manner similar to the embodiment of Fig. 2, wherein positive ions generated by air breakdown in the post-nip region in the vicinity of a positively charged area of the latent image are repelled by the underlying latent image, while the positive ions in the vicinity of negatively charged areas of the latent image are attracted to the imaging member 10 and captured by the neutrally charged toner layer. Conversely, negative ions generated in the pre and post nip regions between the member 60 and the imaging member 10 are absorbed by the neutral toner particles adjacent positively charged areas of the latent image, while negative ions in the vicinity of a negatively charged areas of the latent image are repelled by the latent image. Thus, the free flowing ions generated by the roll member 60 in the pre and post nip regions are selectively captured by toner layer 58 in accordance with the charge of the latent image areas on the imaging member 10. This process induces image-wise charging of the toner layer 58, creating a secondary latent image within toner layer 58 made up of image and background areas which are charged oppositely with respect to the charge of the original latent image on the imaging member 10. Once again, under optimum conditions, the charge of the original latent image is converted into the secondary latent image in the toner layer and/or absorbed by the roll member 60 such that the original electrostatic latent image is substantially or completely dissipated into the toner layer after the image-wise toner charging process is complete. It is noted that, after the secondary latent image is formed in the toner layer, the latent image bearing toner layer is advanced to the image separator 20. Thus, referring back to Fig. 1, image separator 20 may be provided in the form of a second biased roll member having a surface adjacent to the surface of the imaging member 10 and preferably contacting the toner layer 58 residing on image bearing member 10. An electrical biasing source is coupled to the image separator 20 to bias the image separator 20 so as to attract either image or non-image areas of the latent image formed in the toner layer 58 for simultaneously separating and developing the toner layer 58 into image and non-image portions. In the embodiment of Fig. 1, the image separator 20 is biased with a polarity opposite the charge polarity of the image areas in the toner layer 58 for attracting image areas therefrom, thereby producing a developed image made up of separated image and transferred portions of the toner cake on the surface of the image separator 20, while leaving background image byproduct on the surface of the imaging member 10. Alternatively, the image separator 20 can be provided with an electrical bias having a polarity appropriate for attracting non-image areas away from the imaging member 10, thereby maintaining toner portions corresponding to image areas on the surface of the imaging member, yielding a developed image thereon, while selectively separating and transferring non-image or background areas to the image separator 20. After the developed image is created, either on the surface of the imaging member 10 or on the surface of the imaging separator 20, the developed image may then be transferred to a copy substrate 70 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. Alternatively, a pressure transfer system may be employed which may include a heating and/or chemical application device for assisting in the pressure transfer and fixing of the developed image on the output copy substrate 70. In yet another alternative, image transfer can be accomplished via surface energy differentials wherein the surface energy between the image and the member supporting the image prior to transfer is lower than the surface energy between the image and the substrate 70, inducing transfer thereto. In a preferred embodiment, as shown in Fig. 1, the image is transferred to a copy substrate via a heated pressure roll 80, 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. Since the art of electrostatographic printing is well known, it is noted that several concepts for transfer and/or fusing which could be beneficially used in combination with the image-wise charging system of the present invention have been disclosed in the relevant patent literature. In a final step in the process the background image byproduct on either the imaging member 10 or the image separator 20 is removed from the surface thereof in order to clean the surface in preparation for a subsequent imaging cycle. Fig. 1 illustrates a simple blade cleaning apparatus 90 for scraping the imaging member 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. In a preferred embodiment the removed toner associated with the background image is transported to a toner sump or other reclaim vessel so that the waste toner can be recycled and used again to produce the toner cake in subsequent imaging cycles. Once again, it is noted that several concepts for cleaning and toner reclaim which could be beneficially used in combination with the image-wise charging system of the present invention have been disclosed in the relevant patent literature. It will be understood that the apparatus and processes described hereinabove represent only a few of the numerous system variants that could be implemented in the practice of the present invention. One particular variant printing system incorporating the teaching of the present invention will be described with respect to Fig. 4, wherein a negatively charged toner cake is provided on the surface of an imaging member 10. The negatively charged toner layer deposited on the imaging member 10 is advanced directly to image separator 20 which is electrically biased to perform the same function as biased roll member 60. Thus, in this embodiment, the image separator roll is biased sufficiently for inducing air breakdown in the pre-nip region to cause image-wise charging of the toner layer 58 in a manner similar to that described with respect to the pre-nip region shown in Fig. 3. Thereafter, the image and non-image areas of the image-wise charged toner layer are separated in the post-nip region in a manner as previously described with respect to image separator 20. It will be understood that the process of this embodiment may be implemented via the application of an electrical bias to separator 20 using a single biasing source as shown in Fig. 1, or using a dual biasing source/segmented bias roll scheme as described with respect to Fig. 3. In an exemplary embodiment illustrating the practice of the present invention in accordance with the embodiment of Fig. 4, a photoreceptive member is initially charged to -500 volts and thereafter selectively discharged to 0 volts for producing an electrostatic latent image thereon. Negatively charged toner particles immersed in a liquid carrier medium applied to the surface of the photoreceptive member to form a negatively charged, high solid content, toner layer thereon. The Paschen threshold in this case is 600 volts. The image separator is biased to +500 volts, wherein air breakdown occurring only in the areas where the original charge potential of -500 volts remains on the photoreceptive member causes positive ions to be attracted to the photoreceptive member. These positive ions are captured in the toner layer to change that portion of the toner layer to a positively charged latent image area. Thereafter, the +500 volts applied to the image separator operates to attract negatively charged portions of the latent image in the post nip region so as to develop the latent image associated with the toner layer by selectively separating portions thereof from the imaging member. Since the latent image on the imaging member dissipates as a function of the air breakdown process, no air breakdown occurs in the post nip region where image separation occurs. The foregoing process has been demonstrated to produce very high resolution images with substantially undeveloped background image development. In review, the present invention provides a novel image development method and apparatus, whereby image-wise charging is accomplished by air breakdown such that free mobile ions are introduced in the vicinity of an electrostatic latent image coated with a layer of developing material. The latent image causes the free mobile ions to flow in an image-wise ion stream corresponding to the latent image, which, in turn, leads to image-wise charging of the toner layer, such that the toner layer itself becomes the latent image carrier. The latent image carrying toner layer is subsequently developed and transferred to a copy substrate to produce an output document.

Claims: An image development apparatus for developing an electrostatic latent image formed on an imaging member (10, comprising: means (50) for depositing a layer of marking particles (58) on the imaging member (10); means (60) for inducing air breakdown creating an electrical discharge in a vicinity of the layer of marking particles (58) on the imaging member (10) to selectively charge the layer of marking particles (58) in response to the electrostatic latent image on the imaging member (10) so as to create a secondary electrostatic latent image in the layer of marking particles (58); and means (20,60) for selectively separating portions of the layer of marking particles (58) in accordance with the secondary latent image for creating a developed image corresponding to the electrostatic latent image formed on the imaging member (10). An imaging apparatus, comprising: an imaging member (10 for having an electrostatic latent image formed thereon, said imaging member (10) having a surface capable of supporting toner particles (58); an imaging device (40) for generating the electrostatic latent image on said imaging member (10), wherein the electrostatic latent image includes image areas defined by a first charge voltage and non-image areas defined by a second charge voltage distinguishable from the first charge voltage; and, an image development apparatus according to claim 1, wherein the means (50) for depositing a layer of marking particles comprises a toner supply apparatus (52,54,56) for depositing toner particles on the surface of said imaging member (10) to form a toner layer (58) thereon adjacent the electrostatic latent image on said imaging member (10), wherein the means for introducing air breakdown comprises a biased member (60) and, wherein the means for selectively separating comprises a separator member (20). An imaging apparatus according to claim 2, wherein said toner supply apparatus (50) is adapted to deposit a layer of uncharged toner particles (58) on the surface of said imaging member (10) or is adapted to deposit a layer of electrically charged toner particles (58) on the surface of said imaging member (10). An imaging apparatus according to claim 2 or 3, wherein said toner supply apparatus (50) is adapted to accommodate liquid developing material including toner particles immersed in a liquid carrier medium and having a toner solids percentage by weight of at least approximately 10% and preferably between approximately 15% and 35%. An imaging apparatus according to any one of the preceding claims, wherein the means for selectively separating comprises a separator member (20) adapted to attract toner layer image or non-image areas associated with the secondary latent image away from the imaging member (10) so as to maintain toner layer non-image or image areas, respectively, associated with the secondary latent image on the surface of the imaging member (10), the separator member (20) including a peripheral surface and an electrical biasing source coupled to said peripheral surface, for contacting the toner layer (58) to electrically attract selected portions thereof away from the imaging member. An imaging apparatus according to claim 5, further including a cleaning apparatus (90) for removing toner layer non-image areas associated with the secondary latent image from the surface of said imaging member (10). An image development process for developing an electrostatic latent image formed on an imaging member (10), comprising the steps of: depositing a layer of marking particles (58) on the imaging member (10); inducing air breakdown for selectively charging the layer of marking particles (58) in response to the electrostatic latent image to create a secondary electrostatic latent image in the layer of marking particles corresponding to the electrostatic latent image on the imaging member; and, selectively separating portions of the layer of marking particles (58) in accordance with the secondary latent image for creating a developed image. An imaging process, comprising the steps of: generating an electrostatic latent image on an imaging member (10) having a surface capable of supporting toner particles (58), wherein the electrostatic latent image includes image areas defined by a first charge voltage and non-image areas defined by a second charge voltage distinguishable from the first charge voltage; and, carrying out an image development process according to claim 7, wherein the depositing of a layer of marking particles comprises depositing toner particles (58) on the surface of said imaging member (10) to form a toner layer thereon adjacent the electrostatic latent image on said imaging member (10), wherein the step of inducing air breakdown creates an electrical discharge in the vicinity of the toner layer (58) on the latent image bearing imaging member, wherein the electrical discharge selectively delivers charged ions to the toner layer in response to the electrostatic latent image on said imaging member (10) to form a secondary latent image in the toner layer (58) having image and non-image areas corresponding to the electrostatic latent image on said imaging member (10), and wherein the selectively separating step includes transferring portions of the toner layer (58) thereto in accordance with the secondary latent image in the toner layer (58) to create a developed image corresponding to the electrostatic latent image formed on said imaging member (10).

Inventors: LIU CHU-HENG, ZHAO WEIZHONG, LIU, CHU-HENG, ZHAO, WEIZHONG
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1998
Main classification: G03G15
IPC codes: G03G15