Patent Publication Number: US-6219501-B1

Title: Method and apparatus for toner cake delivery

Description:
This application is related to U.S. patent application Ser. No. 09/457,456, entitled “Method and Apparatus for Delivery of High Solids Content Toner Cake in a Contact Electrostatic Printing System”, filed Dec. 8, 1999, in the names of Chu-heng Liu, Weizhong Zhao, and Paul W. Morehouse, Jr., and U.S. patent application Ser. No. 09/525,344, entitled “Toner Cake Delivery System Having a Carrier Fluid Separation Surface”, filed Mar. 13, 2000, in the name of Chu-Heng Liu. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to electrostatic latent image development systems that operate using liquid developing material, and, more particularly, relates to a system for electrostatic development of a latent image, wherein the latent image is developed with use of a toner cake layer having a high solids content toner. 
     BACKGROUND OF THE INVENTION 
     Various methods of developing a latent image have been described in the art of electrophotographic printing and copying systems. A typical electrostatographic printing process includes a development step whereby a quantity of developing material is physically transported into the vicinity of a latent image bearing imaging member, with the marking particles (described herein as toner) in the developing material are caused to migrate via, e.g., electrical attraction, to the image areas of the latent image so as to selectively adhere to the imaging member in an image-wise configuration. 
     Of particular interest with respect to the present invention is the concept of forming a thin layer of liquid developing material on a first surface of a first member, wherein the layer has a high concentration of charged toner. The layer on the first member is brought into contact with an electrostatic latent image on a second surface of a second member, wherein development of the latent image occurs upon separation of the first and second surfaces, as a function of the electric field strength generated by the latent image. In this process, toner particle migration or electrophoresis is replaced by direct surface-to-surface transfer of a toner layer induced by image-wise fields. 
     For the purposes of the present description, the concept of latent image development via direct surface-to-surface transfer of a toner layer via image-wise fields will be identified generally as contact electrostatic printing (CEP). Exemplary patents which may describe certain general aspects of contact electrostatic printing, as well as specific apparatus therefor, may be found in U.S. Pat. Nos. 4,504,138; 5,436,706; 5,596,396; 5,610,694; and 5,619,313. 
     It is desirable that the aforementioned layer of liquid developing material be provided in a very thin and very uniform layer that exhibits a high proportion of solids, that is, having a high solids content. Even more desirable is such a layer exhibiting the following advantageous characteristics: a selectable, uniform thickness, preferably in the range of 3-10 microns; a high solids content, preferably in the range of 15 to 35 percent solids; and an accurately metered mass per unit area on the order of 0.1 mg per cm 2 . 
     The intuitive and conventional approach is to attempt the formation of such a layer by direct application of liquid developing material having a high solids content. However, due to the very complicated rheological behavior of a liquid developing material having the requisite high solids content, such direct application of a supply of such liquid developing material to a receiving member typically does not achieve a layer having the aforementioned desirable characteristics. For example, the resulting layer has been found to exhibit a variable thickness and a non-uniform mass per unit area, which renders the layer generally unsuitable for most contact electrostatic printing applications. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, there is provided an imaging system for effecting electrostatic printing of an image, wherein the imaging system includes at least one contact electrostatic printing engine operable in a novel fashion upon a copy substrate, wherein each contact electrostatic printing engine images and develops an electrostatic latent image representative of the image, and subsequently transfers the developed image to the copy substrate. 
     In accordance with another aspect of the present invention, a toner cake delivery apparatus may be constructed and operated in accordance with the contact electrostatic printing process to which the present invention is directed, wherein a secondary toner cake layer of high solids content and selectable, uniform thickness is created from a primary toner cake layer. This secondary toner cake layer is then highly advantageous for use in development of an electrostatic latent image. 
     In one embodiment of the present invention, a primary toner cake layer is created on a donor surface of a movable coating member and is subject to uniform charging to a first polarity. Next, the primary toner cake layer is exposed to a charging device whereupon it is subject to an induced charge of a second, opposing polarity to a selectable depth. As a result, the primary toner cake layer exhibits, when considered along its cross-sectional dimension, an inner layer having uniform charge of the first polarity and an outer layer having uniform charge of the second polarity. The primary toner cake layer is brought into pressure contact at a process nip with a receiving surface of movable receiving member. The donor surface and the receiving surface are subject to a electrical bias differential such that the outer layer is subject to a strong attraction to the receiving surface. Due to the bias differential and subsequent separation of the donor surface and the receiving surface, the primary toner cake layer is divided. The outer layer of the primary toner cake layer transfers to the receiving surface of the receiving member and the inner layer of the primary toner cake layer remains on the donor surface. At least one of the inner and outer layers is generally characterized as having a high solids content (e.g., approximately 10-50 percent solids, and preferably in the range of approximately 15 to 35 percent solids, or greater), and exhibits the additional advantageous characteristics of a uniform thickness, in the range of 1-15 microns, and an accurately metered mass per unit area on the order of 0.1 mg per cm 2 . 
     In an alternative to the foregoing embodiment, the charging device may be omitted and the applied bias differential is controlled so as to reverse the first polarity charges only to a selectable depth. As a result, the primary toner cake layer exhibits, when considered along its cross-sectional dimension, an inner layer having uniform charge of the first polarity and an outer layer having uniform charge of the second polarity. 
     In another alternative embodiment, the toner cake delivery apparatus accomplishes the aforementioned uniform charging to a first polarity by use of a supply of high solids content liquid developing material generally made up of toner particles immersed in a liquid carrier material and also typically including a charge director for providing a mechanism for producing an electrochemical reaction in the liquid developing material composition which generates the desired electrical charge on the toner particles. A high solids content liquid developing material applicator provides a relatively uniform, charge layer of high solids content liquid developing material onto the coating member, which may be provided in the form of a drum, web, belt, or similar coating member. 
     An embodiment of a contact electrostatic printing engine may be constructed to include the aforementioned toner cake layer delivery apparatus and an imaging member for receiving an electrostatic latent image. The imaging member acts as the aforementioned receiving member and thus includes an image bearing surface capable of receiving and supporting the aforementioned secondary toner cake layer. An image-wise exposure device is provided for generating the electrostatic latent image on the 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. The aforementioned toner cake delivery apparatus is then employed for depositing the secondary toner cake layer on the image bearing surface of the imaging member so as to form a layer of high solids content that is adjacent the electrostatic latent image on the imaging surface of the imaging member. A separator member is also provided for selectively separating portions of the secondary toner cake layer in accordance with the latent image, to create a developed image corresponding to the electrostatic latent image formed on the imaging member. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other aspects of the present invention will become apparent from the following description in conjunction with the accompanying drawings wherein like reference numerals have been used throughout to identify identical or similar elements. 
     FIG. 1 is a simplified schematic representation of a toner cake layer delivery apparatus constructed according to the present invention for use in a contact electrostatic printing (CEP) system; and 
     FIG. 2 is an elevational view schematically depicting an embodiment of a CEP engine constructed for imaging and development of an electrostatic latent image. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The present invention is directed to an electrostatic imaging system wherein latent image development is carried out via direct surface-to-surface transfer of a highly concentrated toner cake layer, utilizing image-wise electrostatic forces to separate the layer of toner cake into image and non-image regions, regardless of where the layer of toner cake is formed prior to image separation or how the image separating electrostatic forces are generated. Although the following description will describe, by example, at least one embodiment of a contact electrostatic printing engine, and related processes that incorporate a photosensitive imaging member, it will be understood that the present invention contemplates the use of various alternative contact electrostatic printing engines and respective imaging members as are 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 electrode substructures capable of generating charged latent images. 
     In a principal feature of the invention, the highly concentrated toner cake layer described herein is derived from a supply of high solids content liquid developing material. A primary toner cake layer is presented in the form of a layer of marking material that is supported on a donor surface which is brought into pressure contact with a receiving surface. The primary toner cake layer is separated into an inner layer that remains on the first surface and an outer layer that is transferred to the receiving surface. 
     Additionally, the contact electrostatic printing process of the present invention may include development of an electrostatic latent image on an image support using supply limited development techniques, i.e., the developing potential of the latent image is not typically exhausted after being initially developed. 
     Additionally, the contact electrostatic printing process of the present invention includes limited relative movement between toner particles during and after latent image development, wherein the high solids content of the toner cake layer prevents toner particles from moving relative to each other. 
     FIG. 1 is a simplified schematic representation of a toner cake layer delivery apparatus  10  constructed according to the present invention for use in an electrostatographic imaging system, such as a contact electrostatic printing (CEP) system. The contact electrostatic printing engine may be employed for imaging and developing a electrostatic latent image that corresponds to a desired image, wherein a layer of highly concentrated toner cake is used for development of the latent image, with subsequent separation and transfer of the developed image onto a copy substrate, thereby providing an output image on the copy substrate. 
     FIG. 1 accordingly illustrates a toner cake layer delivery apparatus  10 , wherein a primary toner cake layer of high solids content is created and then split to form a secondary toner cake layer having desirable characteristics for use in image development, such as a consistent, selectable thickness and density. The secondary toner cake layer may be delivered to a suitable latent image bearing imaging member (as will be described below), such that a developed image is created by separating and selectively transferring portions of the secondary toner cake layer in correspondence with the image and non-image regions of the latent image. 
     The primary and secondary toner cake layers described herein, having a high solids content, may be distinguished from a toner cake layer having low solids content, according to the following criteria. An approximately 1-10 percent solids content is considered to be characteristic of a low solids content toner cake layer; an approximately 10-50 percent solids content, or greater, and preferably on the order of approximately 15 to 35 percent solids, is considered to be characteristic of the desired high solids content toner cake layer. The secondary toner cake layer described herein also preferably exhibits the additional advantageous characteristics of a uniform thickness, selectable from the range of approximately 1-15 microns, and an accurately metered mass per unit area of approximately 0.1 mg per cm 2 . 
     The toner cake layer delivery apparatus  10  includes a supply  11  of high solids content liquid developing material from which a high solids content liquid developing material applicator  12  obtains a sufficient amount of high solids content liquid developing material to apply a relatively uniform layer  13  of high solids content liquid developing material onto a donor surface  15  of a movable coating member  14 . In the illustrated embodiment, the coating member  14  is provided in the form of a cylindrical roll; however, alternative embodiments include, for example, a movable belt, reciprocating plate, and the like. For the purposes of this description, a high solids content liquid developing material is generally characterized as having a solids content that is on the order of approximately 15 to 35 percent solids. 
     The toner cake layer delivery apparatus  10  optionally includes a metering device, which, depending on its mode of operation and position with respect to the donor surface  15 , may be operated to reduce the high solids content liquid developing material layer  13  to exhibit a selectively metered thickness and thus provide a primary toner cake layer  16 . For example, an optional metering roll  17 , when situated in close proximity to the donor surface  15 , provides a shear force against the high solids content liquid developing material layer  13 , thereby providing a controlled thickness of the primary toner cake layer  16 . The excess material eventually falls away from the metering roll  17  and may be transported to the supply  11  for reuse. 
     After application of the high solids content liquid developing material layer  13  to the coating member  14  and concurrent rotation of the coating member  14 , the high solids content liquid developing material layer  13  is then subject to charging to a first polarity by a first charging device  34 . (Alternatively, if the high solids content liquid developing material is already chemically charged to the first polarity, the charging device  34  may be employed to enhance the desired charge, or simply omitted if further charging is not necessary.) Subsequent engagement with the metering roll  17  adjusts the toner solid concentration of the high solids content liquid developing material layer  13 . The thickness of the primary toner cake layer  16  may be considered to be in the range of two to ten times the thickness of the desired secondary toner cake layer; for example, the primary toner cake layer  16  may exhibit a thickness in the range of 6-100 microns. 
     Next, a second charging device  36  induces, to a selectable depth, a sufficient charge of a second, opposing polarity (i.e., opposite to that of the first polarity) in the primary toner cake layer  16 . A power supply and control circuit  55 , operable in response to a secondary toner cake layer density sensor  32 , determines the appropriate charge levels to be provided by the first and second charging devices  34 ,  36  as well as a bias voltage provided to the surface  15  of the coating member  14 . Suitable embodiments of the sensor  32  include optical density sensors and the like. 
     Continued rotation of the coating member  14  then brings the toner cake layer  16  into engagement with a receiving surface  21  of a receiving member  20 . According to the influence of an electrical bias differential between the receiving surface  21  and the donor surface  15 , the charge per unit mass in the primary toner cake layer  16 , and the distribution of the first and second polarity charges in the primary toner cake layer  16  (one or more of which are established by the power supply and control circuit  55 ), the primary toner cake layer  16  splits into an outer layer  16 B and an inner layer  16 A, at least one of which exhibits a controlled, selectable thickness and surface uniformity. Thus, at a consistent and selectable depth of the primary toner cake layer  16 , an outer portion of the primary toner cake layer  16  is defined by its strong attraction to the surface  21 ; this outer portion splits from the remainder of the primary toner cake layer  16  so as to form the outer layer  16 B. Meanwhile, the remainder of the primary toner cake layer  16  is strongly attracted to the surface  15  of the coating member  14  in such a way as to form the inner layer  16 A on the surface  15  of the coating member  14 . 
     As mentioned, it is contemplated that sufficient electrical bias is applied so as to establish a bias differential between the receiving surface  21  and the donor surface  15 . Furthermore, it is contemplated that certain embodiments may employ an electrical bias differential consisting of the combination of a DC voltage and an AC signal. The resulting modulated bias differential fluidizes the portion of the primary toner cake layer  16  present in the gap between the coating member  14  and the receiving member  20 , thus enhancing the separation of the inner and outer layers  16 A,  16 B. 
     With regard to an alternative to the illustrated embodiment, one may observe that a strong electrical field is present at the gap between the coating member  14  and the receiving member  20 ; however, the intensity of the electric field decreases according to the radial distance from the receiving member  20 . Accordingly, with sufficient control of the selected bias voltage differential and the uniform charge of first polarity established in the primary toner cake layer  16 , the illustrated embodiment may be operated without use of the charging device  36 . In this alternative mode of operation, the bias voltage differential is selected to penetrate the primary toner cake layer  16  only to a selectable depth, so as to establish the aforementioned inner and outer layers  16 A,  16 B. 
     If the high solids content liquid developing material layer  13  is supplied by the supply  11  in a neutral (uncharged) state, the first charging device  34  is operated to charge the layer  13  prior to its transformation to the primary toner cake layer  16 . If the high solids content liquid developing material layer  13  is supplied by the supply  11  in a charged state by use of any variety of chemical charging techniques, the first charging device  34  may be omitted, as known in the art. The coating member  14  can be biased using the biasing and control unit  55  to enhance or control the quality of the high solids content liquid developing material layer  13 . 
     Still further rotation of the receiving member  20  and the coating member  14  exposes each of the newly formed inner and outer layers  16 A,  16 B. In the illustrated embodiment, the outer layer  16 B is contemplated as being the most amenable to use as the desired secondary toner cake layer, whereas the inner layer  16 A is removed from the coating member  14  by a cleaning unit  18 . Conservation of some or all of the inner layer  16 A thus removed by the cleaning unit  18  is contemplated for subsequent provision to the supply  11 . 
     Alternatively, in an instance wherein the inner layer  16 A is selected as the desired secondary toner cake layer, continued rotation of the coating member  14  allows the cleaning unit  18  to be employed for removing any remnants of the inner layer  16 A situated on the surface  15  after such a development step. In this instance, the outer layer  16 B may be similarly removed from the surface  21  by an appropriate cleaning unit (not shown) and conserved, e.g., to replenish the supply  11 . 
     A variety of devices or apparatus may be utilized as the applicator  12  for applying the high solids content material layer  13  to the surface of the coating member  14 , such as, but not limited to, known systems directed toward the transportation of liquid developing material having toner particles immersed in a carrier liquid, including various apparatus used in conventional lithographic printing applications as well as traditional liquid electrostatographic applications. For example, the applicator  12  can include a fountain-type device as disclosed generally in commonly assigned U.S. Pat. No. 5,519,473 (incorporated by reference herein). A reverse roll member may also be provided, wherein the function of the reverse roll member can be two-fold: for metering a portion of the liquid carrier away from the liquid developing material as it is applied to the surface of the coating member  14 ; and/or for electrostatically pushing (via a suitable biasing source, not shown) the liquid developing material toward the surface of the coating member  14 . Additionally embodiments of the applicator  12  include the following: a slot die, an extrusion member, a slide, a liquid developing material curtain, a gravure roll, a forward roll, a squeegee roll, a blade apparatus, a foam roller or belt, a wired rod, a screen coater, or a shoe. 
     One may appreciate that although the coating member  14  of FIG. 1 is shown and described herein in the form of a cylinder, the coating member  14  may alternatively be provided in other forms, such as in the form of a continuous flexible belt. 
     FIG. 2 is an elevational view schematically depicting a first embodiment of contact electrostatic printing (CEP) engine  100  constructed for imaging and development of a component electrostatic latent image, with advantageous use of a toner cake layer delivery apparatus  150  constructed according to the apparatus  10  of FIG. 1, wherein a secondary toner cake layer is created and delivered to a latent image-bearing surface for use in development of the latent image. 
     The illustrated CEP engine  100  is adapted for operation with respect to a copy substrate  175  carried on a substrate transfer path  170 . The engine  100  is preferably associated with a respective pressure roller  180  for establishing at least a basic contact transfer, electrostatic transfer, or transfixing of the developed image to the copy substrate  175 . An optional fuser assembly (not shown) may be provided for full or final fusing of the developed image when necessary. The engine  100  includes the toner cake layer delivery apparatus  10  of FIG. 1 for delivery of a secondary toner cake layer  158 B to an imaging member  210  and thereafter a nip  112  is created between the imaging member  210  and an image separation member  220 . The secondary toner cake layer  158 B, having a high solids content as described hereinabove, is brought into pressure contact with the surface of the image separation member  210 , as will be described in detail below, whereby the secondary toner cake layer  158 B is separated into image and non-image segments. Image development occurs as a function of surface-to-surface transfer of an assemblage or aggregate of particles making up a particular section of the secondary toner cake layer as opposed to electrostatic attraction of individual toner particles dispersed in a carrier liquid. 
     The CEP engine  100  thus comprises a first movable member in the form of the imaging member  210  which includes an imaging surface of any type capable of having an electrostatic latent image formed thereon. An exemplary imaging member  210  may include a typical photoconductor or other photoreceptive component of the type known to those of skill in the art of electrophotography, wherein a surface  114  having photoconductive properties is supported on a conductive support substrate. 
     Imaging member  210  and image separation member  220  are rotated so as to transport the surfaces thereof in a process direction  111  for implementing a series of image forming steps. It will be understood that, while imaging member  210  is shown and described herein in the form of a drum, the imaging member may alternatively be provided in the form of a continuous flexible belt which is entrained over a series of rollers, and is movable in the same direction as shown. 
     Initially, in the exemplary embodiment of FIG. 2, the photoconductive surface  114  of imaging member  210  passes through a charging station, which may include a corona generating device  130  or any other charging apparatus for applying an electrostatic charge to the photoconductive surface  114  of the imaging member  110 . The corona generating device  130  is provided for charging the photoconductive surface  114  to a relatively high, substantially uniform electrical charge potential. It will be understood that various charging devices, such as charge rollers, charge brushes and the like, as well as inductive and semiconductive charge devices, among other devices which are well known in the art, may be utilized at the charging station for applying a charge potential to the surface of the imaging member  210 . 
     After the imaging member  210  is brought to a substantially uniform charge potential, the charged photoconductive surface  114  is advanced to an image exposure station, identified generally by reference numeral  140 . The image exposure station projects a light image corresponding to the desired component image onto the charged photoconductive surface  114 . The light image projected onto the surface of the imaging member  210  selectively dissipates the charge thereon for recording an electrostatic latent image on the photoconductive surface  114 , wherein the electrostatic latent image comprises, in image configuration corresponding to the input image information, image areas defined by a first charge voltage potential and non-image areas defined by a second charge voltage potential. The image exposure station  140  may incorporate various optical image projection and formation components as are known in the art, and may include various well known light lens apparatus or digital scanning systems for forming and projecting an image from an original input document onto the imaging member  210 . Alteratively, various other electronic devices available in the art may be utilized for generating electronic information to create 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 by regions having opposite charge polarities or by regions having distinguishable first and second voltage potentials which are of the same charge polarity. 
     A toner cake layer delivery apparatus  150  is constructed as described hereinabove. The toner cake layer delivery apparatus  150  includes therein a feed line or reservoir adapted to provide a supply of high solids content liquid developing material, generally made up of toner particles immersed in a liquid carrier material and also typically includes a charge director for providing a mechanism for producing an electrochemical reaction in the liquid developing material composition which generates the desired electrical charge on the toner particles. 
     A coating member  156  is rotated in a direction as indicated by arrow  157  for installing the secondary toner cake layer  158 B onto the surface of the imaging member  210 . The uniformly distributed secondary toner cake layer  158 B is made up of densely packed toner particles in a small percentage of liquid carrier. Depending on the materials utilized in the liquid developing material composition, as well as other process parameters related to the printing system, such as process speed and the like, a secondary toner cake layer having consistent, selectable thickness, preferably between 2 and 15 microns and more preferably on the order of 5 microns or less, is formed on the surface  114  of the imaging member  210  as described hereinabove. A biasing and control unit  155  is coupled to the coating member  156  to assist this controlled formation of the secondary toner cake layer  158 B. 
     After the secondary toner cake layer  158 B is formed on the imaging member  210 , the secondary toner cake layer  158 B is brought into pressure contact with the image separation member  220  by transporting the secondary toner cake layer  158 B through a process nip  112  formed by the operative engagement of the imaging member  210  and the image separation member  220 . The toner cake layer  158 B has a solid-like property in the process nip  112  such that the presence of hydrodynamic lift occurring in the nip, as disclosed in some prior art references noted hereinabove, is not applicable to the concepts of the present invention. 
     One objective of the engine  100  illustrated in FIG. 1 is to place the secondary toner cake layer  158 B under pressure in the process nip  112 ; accordingly, it may desirable to provide either the imaging member  210  or the imaging member  110  in the form of a conformable member for permitting the surface of one member to correspond in form or character to the opposing surface in the nip region. When the imaging member  210  bearing the toner cake layer  158 B is engaged with the image separation member  220 , the secondary toner cake layer  158 B is substantially uniformly distributed within the nip created therebetween such that toner particle motion and/or liquid flow is negligible with no distortion being present or induced between the toner particles in the secondary toner cake layer  158 B. 
     It will be understood that the presence of the latent image on the imaging member  210  may generate some fringe fields in areas of interface between image and non-image areas of the latent image. However, compared to conventional development, the present invention will substantially eliminate fringe field related image defects due to the solid-like property of the secondary toner cake layer  158 B at the entrance of the nip. 
     An electrical biasing source  122  is coupled to the image separation member  220  for applying an electrical bias thereto so as to generate electrostatic fields between the surface of the image separation member  220  and the image or non-image areas on the surface of the imaging member  210 . These electrostatic fields generate fields in opposite directions, either toward the surface of the imaging member  210  or towards the surface of the image separation member  220  in accordance with image and non-image portions of the latent image. Moreover, these fields cause the separation of the image and non-image areas of the secondary toner cake layer  158 B upon continued rotation of the image separation member  220  and the imaging member  210  at the nip exit, thereby simultaneously separating and developing the secondary toner cake layer  158 B into image and non-image portions on the opposed surfaces of the imaging member  210  and the image separation member  220 . Development occurs with substantially reduced movement of the toner particles. The development can therefore be implemented at an increased rate to allow high speed processing and improved printing throughput rates. In the illustrated embodiment, the imaging member  210  is provided with an electrical bias appropriate for attracting non-image areas while repelling image areas toward the image separation member  220 , thereby maintaining toner portions corresponding to image areas on the surface of the image separation member  220 , yielding a developed image on the image separation member  220 . 
     The thickness of the toner cake layer  158 B in the process nip  112 , and therefore the process nip gap between the imaging member  210  and the image separation member  220 , is preferably less than 15 microns and more preferably less than 5 microns. The secondary toner cake layer  158 B can have a thickness of about 1 micron and still produce acceptable print quality. A process nip gap of less than 5 microns enables development of images of greater than 800 dots per inch (dpi). 
     The secondary toner cake layer  158 B is exposed to at least two very different and opposed stress forces as it is transported through of the process nip  112 . As the secondary toner cake layer  158 B enters the process nip  112  and travels therethrough, compressive stress forces are generated and exerted upon the secondary toner cake layer  158 B. Thereafter, as the secondary toner cake layer exits the process nip  112  and is separated into image and background areas on the opposed surfaces of the imaging member  210  and the image separation member  220 , tensile stress forces are generated and exerted upon the secondary toner cake layer  158 B. 
     Image quality is partially dependent on the ability of the secondary toner cake layer  158 B, and in particular, the toner particles therein, to maintain their integrity as an assemblage of toner particles such that lateral movement of the toner particles is prevented when the liquid developing material layer is exposed to compression stress forces, thereby allowing the toner particles to maintain their initial distribution and density levels as the secondary toner cake layer  158 B enters the nip  112 , and further allowing the toner particles of the liquid developing secondary toner cake layer  158 B to sustain an image pattern as it passes through the nip. At the exit, the toner patch in the image area will stay with one surface and the toner patch in the background area will stay with another surface according to the image-wise electrical field. In addition, image quality is further dependent on the ability of the toner particles in the secondary toner cake layer  158 B to break sharply along the image-background boundary where the electrostatic force is substantially zero. Thus, it is desired for the secondary toner cake layer  158 B to attain a shear tensile yield stress which is substantially lower than the stress induced by the electric fields at the exit of the nip  112  for preventing image quality degradation when the liquid developing material layer is exposed to tensile stress forces at the nip exit while separating into image and non-image regions on opposed surfaces. 
     The toner particles are required to migrate a relatively small distance, therefore allowing for increased process speeds. The toner particles are generally required to migrate less than one half the width or gap of the process nip  112 . As a result of the small toner migration, the image areas and background are interspersed due to each extending from the respective surfaces of the imaging member  110  and imaging member  210  more than one half of the gap of the process nip  112 . The thickness of each of the toner layers of the background and the image area are therefore greater than one half the gap of the process nip  112 . The spaces in the process nip from which the toner migrates continue to be occupied by carrier fluid. As a result of the relatively small toner migration, the toner layer of the background and the toner layer of the developed image are in substantial contact. As a result, there is edge to edge contact of the opposed toner layers in the process nip  112 . 
     The developed image and background are separated at the exit of the process nip  112 . The interspersed and contacting developed image area and background toner layers break or snap cleanly at the edge to edge contact. The clean breaking of the edge to edge contact provides for improved edge definition of the developed image relative to prior development systems. 
     In the illustrated embodiment, continued rotation of the image separation member  220  allows the developed image to be transferred from the surface of the image separation member  220  to a copy substrate  175  carried on the substrate transfer path  170 . 
     In the above described embodiments of a CEP engine, the developed image transfer step may be effected via selectable means known in the art, and in some embodiments may be effected in accordance with the registration requirements of a composite color image, such as an electrostatic transfer apparatus including a corona generating device or a biased transfer roll. In yet another alterative, 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 copy substrate, inducing transfer thereto. 
     A pressure transfer roll system may be employed to tack the developed image to the copy substrate  175 ; this system may include a heating and/or chemical application device for assisting in the pressure transfer and fixing of the developed image on the copy substrate  175 . The developed image may be transferred to a copy substrate  175  via a heated pressure roll  180 , whereby pressure and heat are simultaneously applied to the developed image to simultaneously transfer and at least partially fuse (e.g., transfuse) the developed image to the copy substrate  175 . 
     Any background image is removed in preparation for a subsequent imaging cycle. A simple blade cleaning apparatus  190  may be employed as is known in the art. Alternative embodiments may include a brush or roller member for removing toner from the surface on which it resides. The removed toner may be transported to a toner sump or other conservation vessel so that the waste toner can be recycled and used again in subsequent imaging cycles. 
     Additional details of the construction and operation of the illustrated embodiment of a CEP engine, and of further embodiments of a CEP engine, and variations thereof, may be found in commonly-assigned U.S. Pat. Nos. 5,826,147 and 5,937,243, the disclosures of which are incorporated herein by reference. 
     The liquid carrier medium utilized in the low solids content developing material may be selected from a wide variety of materials, including, but not limited to, any of several hydrocarbon liquids conventionally employed for liquid development processes, including hydrocarbons, such as high purity alkanes having from about 6 to about 14 carbon atoms, such as Norpar® 12, Norpar® 13, and Norpar® 15, and including isoparaffinic hydrocarbons such as Isopar® G, H, L, and N, available from Exxon Corporation. Other examples of materials suitable for use as a liquid carrier include Amsco® 460 Solvent, Amsco® OMS, available from American Mineral Spirits Company, Soltro®, available from Phillips Petroleum Company, Pagasol®, available from Mobil Oil Corporation, Shellsol®, available from Shell Oil Company, and the like. isoparaffinic hydrocarbons provide a preferred liquid media, since they are colorless, environmentally safe. These particular hydrocarbons may also possess a sufficiently high vapor pressure so that a thin film of the liquid evaporates from the contacting surface within seconds at ambient temperatures. 
     The secondary toner cake layer  158 B achieves high enough yield stress to substantially eliminate lateral movement of the toner particles in the secondary toner cake layer  158 B when exposed to compression stresses generated at the nip  112 , while also having sufficiently low yield stress to permit the toner layer to act as a liquid in the presence of tensile stress forces present in the vicinity of the exit of the nip. Further definition of operational parameters for such optimization of the contact electrostatic printing process, via preselecting materials having a particular yield stress and/or selectively varying the yield stress of a given liquid developing material, may be determined by those skilled in the art so as to preselect the materials making up the liquid developing material, the toner particle concentration of the liquid developing material, and the electrical field strength generated between the biased applicator on one surface and the electrostatic latent image on a second surface. 
     Exemplary marking material colors in the respective high solids content liquid developing materials are selectable as known in the art, e.g., cyan, magenta, yellow, and black; however, other component colors may be employed. It is contemplated that a contact electrostatic printing system would employ at least one of the illustrated CEP engines. Furthermore, the liquid developing material may be distinguishable according to one or more physical characteristics in addition to, or other than, the color of the marking material, and nonetheless such engines are encompassed by the present invention. 
     The toner solids, or so-called marking material particles, can comprise any particulate material that is compatible with the liquid carrier medium, such as those contained in the liquid developing materials disclosed in, for example, U.S. Pat. Nos. 3,729,419; 3,841,893; 3,968,044; 4,476,210; 4,707,429; 4,762,764; 4,794,651; and 5,451,483, among others. Preferably, the marking material particles should have an average particle diameter ranging from about 0.2 to about 10 microns, and most preferably between about 0.5 and about 2 microns. The marking material particles can consist solely of pigment particles, or may comprise a resin and a pigment; a resin and a dye; or a resin, a pigment, and a dye or resin alone. 
     Suitable resins include poly(ethyl acrylate-co-vinyl pyrrolidone), poly(N-vinyl-2-pyrrolidone), and the like, including, for example Elvax®, and/or Nucrel®, available from E. I. DuPont de Nemours &amp; Co. of Wilmington, Del. Suitable dyes include Orasol Blue 2GLN, Red G, Yellow 2GLN, Blue GN, Blue BLN, Black CN, Brown CR, all available from Ciba-Geigy, Inc., Mississauga, Ontario, Morfast Blue 100, Red 101, Red 104, Yellow 102, Black 101, Black 108, all available from Morton Chemical Company, Ajax, Ontario, Bismark Brown R (Aldrich), Neolan Blue (Ciba-Geigy), Savinyl Yellow RLS, Black RLS, Red 3GLS, Pink GBLS, and the like, all available from Sandoz Company, Mississauga, Ontario, among other manufacturers; as well as the numerous pigments listed and illustrated in U.S. Pat. Nos. 5,223,368; 5,484,670, the disclosures of which are totally incorporated herein by reference. Dyes generally are present in an amount of from about 5 to about 30 percent by weight of the toner particle, although other amounts may be present provided that the objectives of the present invention are achieved. 
     Suitable pigment materials include carbon blacks such as Microlith® CT, available from BASF, Printex® 140 V, available from Degussa, Raven® 5250 and Raven® 5720, available from Columbian Chemicals Company. Pigment materials may be colored, and may include magenta pigments such as Hostaperm Pink E (American Hoechst Corporation) and Lithol Scarlet (BASF), yellow pigments such as Diarylide Yellow (Dominion Color Company), cyan pigments such as Sudan Blue OS (BASF); as well as the numerous pigments listed and illustrated in U.S. Pat. Nos. 5,223,368; 5,484,670, the disclosures of which are incorporated herein by reference. Generally, any pigment material is suitable provided that it consists of small particles that combine well with any polymeric material also included in the developer composition. Pigment particles are generally present in amounts of from about 5 to about 60 percent by weight of the toner particles, and preferably from about 10 to about 30 percent by weight. 
     As previously indicated, in addition to the liquid carrier vehicle and marking material particles which typically make up the liquid developer materials, a charge director (sometimes referred to as a charge control additive) is also provided for facilitating and maintaining a uniform charge on the marking particles in the operative solution of the liquid developing material by imparting an electrical charge of selected polarity (positive or negative) to the marking particles. Examples of suitable charge director compounds include lecithin, available from Fisher Inc.; OLOA 1200, a polyisobutylene succinimide, available from Chevron Chemical Company; basic barium petronate, available from Witco Inc.; zirconium octoate, available from Nuodex; as well as various forms of aluminum stearate; salts of calcium, manganese, magnesium and zinc; heptanoic acid; salts of barium, aluminum, cobalt, manganese, zinc, cerium, and zirconium octoates and the like. The charge control additive may be present in an amount of from about 0.01 to about 3 percent by weight of solids, and preferably from about 0.02 to about 0.05 percent by weight of solids of the developer composition.