Abstract:
A developer storage and delivery system for liquid electrophotography comprising:  
     (a) an insulting substrate with a first surface and a second surface;  
     (b) a plurality of discrete conductive heating elements mounted on said first surface; and  
     (c) a phase change developer having a melting point of at least 22° C.,  
     wherein said phase change developer is on the top surface of each of said discrete conductive heating elements, except that a small portion of both ends of each of said conductive heating elements is free of said phase change developer for conducting electricity.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates generally to a developer storage and delivery system, and more particularly concerns storing a phase change developer on a continuous web or on an endless belt and delivering the phase change developer to a liquid electrophotographic developing system.  
           [0003]    2. Background of the Art  
           [0004]    In electrophotography, a photoreceptor in the form of a plate, belt, or drum having an electrically insulating photoconductive element on an electrically conductive substrate is imaged by first uniformly electrostatically charging the surface of the photoconductive element, and then exposing the charged surface to a pattern of light. The light exposure selectively dissipates the charge in the illuminated areas, thereby forming a pattern of charged and uncharged areas (i.e., an electrostatic latent image). A liquid or dry developer is then deposited in either the charged or uncharged areas to create a toned image on the surface of the photoconductive element. The resulting visible image can be fixed to the photoreceptor surface or transferred to a surface of an intermediate transfer material or a suitable receiving medium such as sheets of material, including, for example, paper, polymer, transparency, metal, metal coated substrates, composites and the like. The imaging process can be repeated many times on the reusable photoconductive element.  
           [0005]    In some electrophotographic imaging systems, the latent images are formed and developed on top of one another in a common imaging region of the photoreceptor. The latent images can also be formed and developed in multiple passes of the photoreceptor around a continuous transport path (i.e., a multi-pass system). Alternatively, the latent images can be formed and developed in a single pass of the photoreceptor around the continuous transport path. A single-pass system enables the multi-color images to be assembled at extremely high speeds relative to the multi-pass system At each color development station, color developers are applied to the photoreceptor belt, for example, by electrically biased rotating developer rolls.  
           [0006]    Image developing methods can be classified into liquid type and dry type. The dry type method uses dry (e.g., powder) developers and the wet type method uses liquid developers.  
           [0007]    Dry developers are generally prepared by mixing and dispersing colorant particles and a charge director into a thermoplastic binder resin. This mixing and dispersing is followed by milling or micropulverization. The resulted developer often comprises a powder having particle sizes that are generally in the range of about 4 to 10 microns. If the fine powder of a dry developer is scattered, it poses an environmental problem because of its small particle size. Therefore, most dry developers are stored in a cartridge which is easily handled and disposed of. Furthermore, the stability of dry developer is usually much better than that of liquid developer.  
           [0008]    Liquid developers are usually prepared by dispersing colorant particles, a charge director, and a binder in an insulating liquid (i.e., a carrier or a vehicle). Liquid developer based imaging systems incorporate many features similar to those of dry developer based system. However, liquid developer particles are significantly smaller than dry developer particles. Because of their small particle size, ranging from 3 microns to submicron size, liquid developers are capable of producing very high resolution images. However, liquid developers have some drawbacks.  
           [0009]    The major drawbacks of liquid developers are (1) the emission of the liquid carrier from liquid developers to the environment during the drying and transfer process due to inefficient solvent recovery system; (2) the need and difficulty in disposing the waste liquids; (3) the inconvenience of using and handling of liquid developers; (4) and the aggregation and sedimentation instability of (the materials inside the developer are stable, both individually and in their association with other materials in the developer, e.g., non-reactive) liquid developers.  
           [0010]    While known liquid developers and processes are suitable for their intended purposes, a need remains for liquid developers and processes that reduce or substantially eliminate the above-mentioned drawbacks. Additionally, there is a need for liquid developers and processes that enable the formation of high quality images on a wide variety of substrates.  
           [0011]    There have been many attempts to solve some of the above-mentioned drawbacks of liquid developers and dry developers. For example, U.S. Pat. No. 5,075,735 to Tsuchiya et al. discloses a developer delivery system comprising stripes or bars of solid developer mounted across a belt. The stripes or bars of solid developer are caused to drop onto a heater by a cutter and then the solid developer is melted by the heater into liquid. The resulted liquid developer is then used to develop electrophotographic images.  
           [0012]    U.S. Pat. No. 5,815,780 to Boerger et al. discloses an apparatus for storing and delivering toner. The toner is stored on a belt in discretely sealed toner bubbles filled with toner. An extractor unit then causes toner bubbles to rupture, allowing the toner to fall into a developer housing to replenish the toner supply.  
           [0013]    U.S. Pat. No. 5,998,081 to Morrison et al. discloses a metallic web coated with a solid developer which is melted by an external conductive heating element. The melted developer is caused to form visible images by contacting with electrostatic latent images.  
         SUMMARY OF THE INVENTION  
         [0014]    This invention provides an improved developer storage and delivery system which eliminates or reduces the above-mentioned drawbacks of liquid developers and processes while providing high quality images on a wide variety of substrates.  
           [0015]    In a first aspect, the invention features a developer storage and delivery system for liquid electrophotography that includes:  
           [0016]    a conductive substrate with a first surface and a second surface;  
           [0017]    a plurality of discrete conductive heating elements mounted on said first surface; and  
           [0018]    a phase change developer having a melting point of at least 22° C., wherein said phase change developer is on the top surface of each of said conductive heating elements, except that a minor portion of the top surface of each of said conductive heating elements is free of said phase change developer. The term minor portion is used in its normal sense as less than 50% of the surface area directly over the conductive stripes is free of the phase change developer. It is preferred that this minor area be a small area, defined herein as less than 20% of the surface area over the conductive stripes or of the entire surface of the developer system, for example, 0.05 or 0.1% to 20%, 1 to 15%, 0.2 to 10%, 0.1 to 5% and 0.1 to 2% of the surface area over the conductive stripes or the developer system. The conductive stripes are for conducting electricity, preferably as part of a resistive heating element to heat the phase change developer. The developer is not necessarily conductive, and there must be at least a minor area and preferably a small area (as defined above) that is exposed to enable external electrical contact to connect the resistive heating element with an external power source.  
           [0019]    In a second aspect, the invention features a developer storage and delivery system for liquid electrophotography that includes:  
           [0020]    a conductive substrate with a first surface and a second surface;  
           [0021]    a plurality of discrete conductive heating elements mounted on said first surface; and  
           [0022]    a phase change developer having a melting point of at least 22° C.,  
           [0023]    wherein said phase change developer is a continuous layer on the top surface of each of said conductive heating elements and on said first surface free of said conductive heating elements, except that a small of the top surface of each of said conductive heating elements is free of said phase change developer for conducting electricity.  
           [0024]    In a third aspect, the invention features a developer storage and delivery system for liquid electrophotography that includes:  
           [0025]    an insulating substrate with a first surface and a second surface;  
           [0026]    a plurality of discrete conductive heating elements mounted on said first surface; and  
           [0027]    a phase change developer having a melting point of at least 22° C., wherein said phase change developer is placed on the top surface of each of said discrete conductive heating elements, except that a small portion of both ends of each of said conductive heating elements is free of said phase change developer for conducting electricity.  
           [0028]    In a fourth aspect, the invention features a developer storage and delivery system for liquid electrophotography that includes:  
           [0029]    an insulating substrate with a first surface and a second surface;  
           [0030]    a plurality of discrete conductive heating elements mounted on said first surface; and  
           [0031]    a phase change developer having a melting point of at least 22° C., wherein said phase change developer forms a layer that is preferably a continuous layer (This embodiment has a continuous developer layer on top of both the substrate and the heating elements, this structure shown primarily because it is easy to manufacture. This embodiment is shown in FIG. 2. Non-continuous coatings may comprise, for example only, porous, patterned, striped, etc. coatings. However, we prefer the developer to be continuous so that no area of the developer roll is uncovered) on the top surface of each of said conductive heating elements and on said first surface free of said conductive heating elements, except that a small portion of both ends of each of said conductive heating elements is free of said phase change developer for conducting electricity.  
           [0032]    The term “phase change developer” has an accepted meaning within the imaging art, however, some additional comments are useful in view of phenemic differences amongst mechanisms in this field. As the term indicates, the developer system is present as one physical phase under storage conditions (e.g., usually a solid) and transitions into another phase during development (usually a liquid phase), usually under the influence of heat or other directed energy sources. There are basically two preferred mechanisms in which these phase changes appear: a) complete conversion of the phase change developer layer from a solid to a liquid and b) release of a liquid from a phase change developer layer with a solid carrier in the phase change developer layer remaining as a solid during and after development. The first system operates by the entire layer softening to a point where the entire layer flows, carrying the active developer component to the charge distributed areas and depositing the developer composition on the appropriate areas where the charges attract the developer. In this case, the developer may be originally or finally in a solid phase or liquid phase within the phase change developer layer, but with the softened (flowable or liquefied) layer carrying the developer or allowing the developer to move over the surface of the layer having image-effecting charge distribution over its surface. The second system, where a liquid developer forms on the surface of the phase change developer carrying layer, usually maintains a solid carrying layer with a liquid developer provided on the surface of the carrier layer. This system may function, for example, by the developer having a lower softening point or even being present as a liquid (e.g., liquid/solid dispersion, liquid/solid emulsion) in the solid carrier layer. Upon activation or stimulation (e,g, by energy, such as heat), the developer composition will exude or otherwise emit from the surface of the solid carrier. This can occur by a number of different phenomena, and the practice of the invention is not limited to any specifically described phenomenon. For example, a phase change developer layer may be constructed by blending a developer composition that is solid at 22° C., which may be dispersed in a solid binder that is solid at 70° C., and the phase change developer composition coated on the imaging surface. Upon heating of the phase change developer layer to a temperature between 25° C. and 65° C., for example, especially where the developer composition is present at from 1 to 60% by weight of the phase change developer layer, the developer will soften or liquefy, and the developer composition will flow to the surface of the developer layer. The developer may be present as droplets and spread by physical action or may flow in sufficient volume to wet the surface of the developer layer and form a continuous layer of liquid. Thus, in the practice of the present invention, the phase change developer layer may be heated above room temperature and below or above the melt, softening or flow temperature of the carrier solid in the phase change developer layer.  
           [0033]    The concept of an ‘activation point’ or ‘activation temperature’ is particularly easily understood in the concept of the present invention. At room temperature, below the activation temperature, the phase change developer layer will not allow the developer to readily distribute over the differentially charged layer to form a pattern or latent image or image in response to the distribution of charges. When the activation temperature has been exceeded on the phase change developer layer, the developer becomes able to be distributed over the differentially charged layer to form a pattern or latent image or image in response to the distribution of charges. The activation point or activation temperature is therefore the temperature at which the phase change developer layer passes from a state in which the developer is electrophotographically inactive to a state where the developer is electrophotographically active, as the temperature increases.  
           [0034]    The developer storage and delivery system of the present invention will be described primarily with respect to electrophotographic office printing; however, it is to be understood that these developers are not so limited in their utility and may also be employed in other imaging processes, other printing processes, or other developer transfer processes, such as high speed printing presses, photocopying apparatus, microfilm reproduction devices, facsimile printing, ink jet printers, instrument recording devices, and the like.  
         DETAILED DESCRIPTION OF THE INVENTION 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]    The foregoing advantages, construction and operation of the present invention will become more readily apparent from the following description and accompanying drawings in which:  
         [0036]    [0036]FIG. 1 is a diagrammatic illustration of a developer storage and delivery system wherein a phase change developer is placed on top of discrete conductive heating elements.  
         [0037]    [0037]FIG. 2 is a diagrammatic illustration of a developer storage and delivery system wherein a continuous coating of a phase change developer is placed on top of both a conductive substrate and discrete conductive heating elements.  
         [0038]    [0038]FIG. 3 is a diagrammatic illustration of a developer storage and delivery system wherein stripes of conductive heating element are placed on an insulated substrate, with optional electrical leads in contact with each end of the stripes, and no phase change developer is shown.  
         [0039]    [0039]FIG. 4 is a diagrammatic illustration of a basic liquid electrophotographic process in which the present invention has utility and an apparatus for performing that process.  
         [0040]    [0040]FIG. 5 is a diagrammatic illustration of an apparatus and a method for producing a multi-colored image in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0041]    Liquid electrophotography is a technology which produces or reproduces an image on paper or other desired receiving material. Liquid electrophotography uses liquid developers which may be black or which may be of different colors for the purpose of plating solid colored material onto a surface in a well-controlled and image-wise manner to create the desired prints. Typically, a colored image is constructed of four image planes. The first three image planes are constructed with a liquid developer in each of the three subtractive primary printing colors, yellow, cyan and magenta. The fourth image plane uses black developer.  
         [0042]    In this invention, instead of conventional liquid developers, phase change developer storage and delivery systems are used. FIG. 1 and FIG. 2 illustrate two aspects of this invention. The phase change developer storage and delivery system comprises conductive substrate  101  in the form of a continuous web or an endless belt or loop. The phase change developer storage and delivery system also comprises phase change developer  104  which is positioned, provided or placed on top of discrete conductive heating elements  102 . Conductive heating elements  102  may be in the form of a coating, etched pattern, a stripe, a bar, an embedded element, or any other useful forms or shapes. Phase change developer  104  may be in the form of discrete stripes, bars, or coatings placing on top of conductive heating elements  102 , as shown in FIG. 1, or in the form of a continuous coating placing on top of both conductive heating elements  102  and conductive substrate  101 , as shown in FIG. 2. Phase change developer  104  may be applied on conductive heating elements  102  by gravure coating, roll coating, curtain coating, extrusion, lamination, spraying, or other coating techniques. The coating of the phase change developer  102  may be assisted with ultrasound, electrical field or magnetic field.  
         [0043]    The components described above are all conventional in the art and any suitable combination of materials for conductive substrate  101 , conductive heating elements  102  and phase change developer  104  may be employed in the phase change developer storage and delivery system of the invention A particularly preferred phase change developer system is described in copending application Attorney&#39;s docket No. 456.008US1, filed the same date as this Application and titled “PHASE CHANGE DEVELOPER FOR LIQUID ELECTROPHOTOGRAPHY.” 
         [0044]    Conductive heating elements  102  are either perpendicular or skewed at an angle to the edges of substrate  101 . External electrical contact  103  is used to pass a current through each of conductive heating elements  102 . Therefore, good conductivity between external electrical contact  103  and discrete conductive heating elements  102  is needed and may be provided by keeping a small portion of the top surface of each of conductive heating elements  102  free of phase change developer  104 . When a current is passed from electrical contact  103  through each of conductive heating elements  102  one by one, phase change developer  104  on each of conductive heating elements  102  is melted and turned into liquid state one by one. The developer and/or individual components within the developer may be melted to become flowable and/or active in the developing process. The phase change developer storage and delivery system may be run continuously or be indexed.  
         [0045]    [0045]FIG. 3 shows another aspect of this invention. The phase change developer is not shown in FIG. 3. However, it should be placed on top of conductive heating elements  102 . Although the preferred embodiment of the invention has been described as continuous, this preference is primarily due to the ease of manufacture of the coating (e.g., see FIG. 2 for an example of a preferred method of manufacturing a system according to the invention) not to essential functional activities. Because the coating is softened or melted, porous or discontinuous coatings will flow and may form essentially continuous developer layers as flowable or liquid materials, Additionally, the continuous nature of the coating does not mean that the coating has to form a coating over the entire surface of the sheet or element. Conductive heating elements  102  are placed on an electrically insulating substrate  105 . Optionally, conductive contacts  106  are used to pass current through each of conductive heating elements  102  one by one by contacting electrical contacts  103 . The phase change developer storage and delivery system may be run continuously or be indexed. When a current is applied to conductive heating elements  102 , the phase change developer is melted and turned into liquid state that may be used subsequently in a liquid electrophotography process. The components described in FIG. 3 are all conventional in the art and any suitable combination of materials for insulating substrate  105 , conductive heating elements  102 , conductive contacts  106 , and the phase change developer may be employed in the phase change developer storage and delivery system of the invention.  
         [0046]    The typical process involved in liquid electrophotography can be illustrated with respect to a single color by reference to FIG. 4. Light sensitive photoreceptor  10  is arranged on or near the surface of a mechanical carrier such as drum  12 . Photoreceptor  10  can be in the form of a belt or loop mounting on the outer surface of the drum. Photoreceptor  10  can also be coated on the outer surface of drum  12 . The mechanical carrier could, of course, be a belt or other movable support object. Drum  12  rotates in the clockwise direction of FIG. 4 moving a given location of photoreceptor  10  past various stationary components which perform an operation relative to photoreceptor  10  or an image formed on drum  12 .  
         [0047]    Of course, other mechanical arrangements could be used which provide relative movement between a given location on the surface of photoreceptor  10  and various components which operate on or in relation to photoreceptor  10 . For example, photoreceptor  10  could be stationary while the various components move past photoreceptor  10  or some combination of movement between both photoreceptor  10  and the various components could be facilitated. It is only important that there be relative movement between photoreceptor  10  and the other components. As this description refers to photoreceptor  10  being in a certain position or passing a certain position, it is to be recognized and understood that what is being referred to is a particular spot or location on photoreceptor  10  which has a certain position or passes a certain position relative to the components operating on photoreceptor  10 .  
         [0048]    In FIG. 4, as drum  12  rotates, photoreceptor  10  moves past erase lamp  14 . When photoreceptor  10  passes under erase lamp  14 , radiation  16  from erase lamp  14  impinges on the surface of photoreceptor  10  causing any residual charge remaining on the surface of photoreceptor  10  to “bleed” away. Thus, the surface charge distribution of the surface of photoreceptor  10  as it exits erase lamp  14  is quite uniform and nearly zero depending upon the photoreceptor.  
         [0049]    As drum  12  continues to rotate and photoreceptor  10  next passes under a charging device  18 , such as a roll corona, a uniform positive or negative charge is imposed upon the surface of photoreceptor  10 . This prepares the surface of photoreceptor  10  for an image-wise exposure to radiation by laser imaging device  20  as drum  12  continues to rotate. Wherever radiation from laser imaging device  20  impinges on the surface of photoreceptor  10 , the surface charge of photoreceptor  10  is reduced significantly while areas on the surface of photoreceptor  10  which do not receive radiation are not appreciably discharged. Areas of the surface of photoreceptor  10  which receive some radiation are discharged to a degree that corresponds to the amount of radiation received. This results in the surface of photoreceptor  10  having a surface charge distribution which is proportional to the desired image information imparted by laser imaging device  20  when the surface of photoreceptor  10  exits from under laser imaging device  20 .  
         [0050]    As drum  12  continues to rotate, the surface of photoreceptor  10  passes by developer storage and delivery system  22 . Developer storage and delivery system  22  are illustrated in FIGS.  1 - 3  above. A liquid developer is obtained by melting or liquefying the phase change developer in developer storage and delivery system  22  and then is applied to the surface of image-wise charged photoreceptor  10  in the presence of a positive or negative electric field which is established by placing developer roll  26  near the surface of photoreceptor  10  and imposing a bias voltage on developer roll  26 . The positive or negative electric field may also be established by placing a grounded developer roll  26  near the surface of photoreceptor  10  and imposing a bias voltage on photoreceptor  10 .  
         [0051]    The liquid developer consists of positively or negatively charged “solid” developer particles of the desired color for the portion of the image being printed. The “solid” material in the developer, under force from the established electric field, migrates to and plates upon the surface of photoreceptor  10  in areas where the surface voltage is less than the bias voltage of developer roll  26 . The “solid” material in the developer will migrate to and plate upon the developer roll in areas where surface voltage of photoreceptor  10  is greater than the bias voltage of developer roll  26 . Excess developer not sufficiently plated to either the surface of photoreceptor  10  or to developer roll  26  is removed.  
         [0052]    The image developed on photoreceptor  10  is then transferred, either indirectly by way of transfer rollers  38  and  40 , as illustrated in FIG. 4, or preferably directly to the receiving medium  36  to be printed. Typically, heat and pressure are utilized to fuse the image to receiving medium  36 . The resultant “print” is a hard copy manifestation of the image information written by laser imaging device  22  and is of a single color, the color represented by liquid developer  24 .  
         [0053]    While photoreceptor  10 , drum  12 , erase lamp  14 , charging device  18 , laser imaging device  20 , developer storage and delivery system  22 , developer roll  26 , and transfer rollers  38  and  40  have been only diagrammatically illustrated in FIG. 4 and only generally described with relation thereto, it is to be recognized and understood that these components are generally well known in the art of electrophotography and the exact material and construction of these elements is a matter of design choice which is also well understood in the art.  
         [0054]    It is possible, of course, to make prints containing many colors rather than one single color. The basic liquid electrophotography process and apparatus described in FIG. 4 can be used by repeating the process described above for one color, a number of times wherein each repetition may image-wise expose a separate primary color plane, e.g., cyan, magenta, yellow or black, and each developer storage and delivery system  22  may be of a separate primary printing color corresponding to the image-wise exposed color plane. Superposition of four such color planes may be achieved with good registration onto the surface of photoreceptor  10  without transferring any of the color planes until all have been formed. Subsequent simultaneous transfer of all of these four color planes to a suitable receiving medium  36  may yield a quality color print.  
         [0055]    While the above described liquid electrophotography process is suitable for construction of a multi-colored image, the process is somewhat slow because photoreceptor  10  should repeat the entire sequence for each color of the typical four color colored image. When the above process is performed for a particular color, e.g., cyan, laser imaging device  20  causes areas receiving radiation to at least partially discharged to create a surface charge distribution pattern of the surface of photoreceptor  10  which represents the portion of the image to be reproduced representing that particular color, e.g., cyan. After development by developer storage and delivery system  22 , the surface charge distribution of photoreceptor  10  is still quite variable (assuming at least some pattern to the image to be reproduced) and too low to be subsequently imaged. Photoreceptor  10  then should be erased to make the surface charge distribution uniform and should be again charged to provide a sufficient surface charge to allow a subsequent development process to plate liquid developer upon developed areas of photoreceptor  10 .  
         [0056]    While not required by all embodiments of the present invention, FIG. 5 diagrammatically illustrates an apparatus  42  and a method for producing a multicolored image. Photoreceptor  10  is mechanically supported by belt  44  which rotates in a clockwise direction around rollers  46  and  48 . Photoreceptor  10  is first conventionally erased with erase lamp  14 . Any residual charge left on photoreceptor  10  after the preceding cycle is preferably removed by erase lamp  14  and then conventionally charged using charging device  18 , such procedures being well known in the art. Laser imaging device  50 , similar to laser imaging device  20  illustrated in FIG. 4, exposes the surface of photoreceptor  10  to radiation in an image-wise pattern corresponding to a first color plane of the image to be reproduced.  
         [0057]    With the surface of photoreceptor so image-wise charged, charged pigment particles in a first phase change developer in developer storage and delivery system  54  corresponding to the first color plane will migrate to and plate upon the surface of photoreceptor  10  in areas where the surface voltage of photoreceptor  10  is less than the bias of developer roll  56  associated with developer storage and delivery system  54 . The charge neutrality of the first phase change developer in its liquid phase is maintained by negatively (or positively) charged counter ions that balance the positively (or negatively) charged pigment particles. Counter ions are deposited on the surface of photoreceptor  10  in areas where the surface voltage is greater than the bias voltage of developer roll  56  associated with developer storage and delivery system  54 .  
         [0058]    At this stage, photoreceptor  10  contains on its surface an image-wise distribution of plated “solids” of liquid phase change developer in accordance with a first color plane. The surface charge distribution of photoreceptor  10  has also been recharged with plated developer particles as well as with transparent counter ions from liquid phase change developer both being governed by the image-wise discharge of photoreceptor  10  due to laser imaging device  50 . Thus, at this stage the surface charge of photoreceptor  10  is also quite uniform. Although not all of the original surface charge of photoreceptor may have been obtained, a substantial portion of the previous surface charge of photoreceptor has been recaptured. Although photoreceptor  10  is now ready to be processed for the next color plane of the image after such recharging, it is preferable to recharge photoreceptor  10  with a corona (not shown in FIG. 5) before the next step.  
         [0059]    As belt  44  continues to rotate, photoreceptor  10  next is image-wise exposed to radiation from laser imaging device  58  corresponding to a second color plane. Note that this process occurs during a single revolution of photoreceptor  10  by belt  44  and without the necessity of photoreceptor  10  being subjected to erase subsequent to exposure to laser imaging device  50  and developer storage and delivery system  54  corresponding to a first color plane. The remaining charge on the surface of photoreceptor  10  is subjected to radiation corresponding to a second color plane. This produces an image-wise distribution of surface charge on photoreceptor  10  corresponding to the second color plane of the image.  
         [0060]    The second color plane of the image is then developed by developer storage and delivery system  62  containing a second phase change developer. Although the second phase change developer in its liquid phase contains “solid” color pigments consistent with the second color plane, the liquid stage of the phase change developer also contains substantially transparent counter ions which, although they may have differing chemical compositions than substantially transparent counter ions of the first liquid developer in developer storage and delivery system  54 , still are substantially transparent and oppositely charged to the “solid” color pigments. Developer roll  64  provides a bias voltage to allow “solid” color pigments of liquid developer  62  create a pattern of “solid” color pigments on the surface of photoreceptor  10  corresponding to the second color plane. The transparent counter ions also substantially recharge photoreceptor  10  and make the surface charge distribution of photoreceptor  10  substantially uniform. Preferably, the uniformity of the surface charge distribution on photoreceptor  10  is further improved by corona charging.  
         [0061]    A third color plane of the image to be reproduced is deposited on the surface of photoreceptor  10  is similar fashion using laser imaging device  66  and developer storage and delivery system  70  containing a third phase change developer using developer roll  72 .  
         [0062]    Similarly, a fourth color plane is deposited upon photoreceptor  10  using laser imaging device  74  and developer storage and delivery system  78  containing a fourth phase change developer using developer roll  80 .  
         [0063]    The completed four color image is then transferred, either indirectly by way of transfer rollers  38  and  40 , as illustrated in FIG. 5, or preferably directly to the receiving medium  36  to be printed. Typically, heat and/or pressure are utilized to fix the image to receiving medium  36 . The resultant “print” is a hard copy manifestation of the four color image.  
         [0064]    With proper selection of charging voltages, photoreceptor capacity and phase change developer, this process may be repeated an indeterminate number of times to produce a multi-colored image having an indeterminate number of color planes. Although the process and apparatus has been described above for conventional four color images, the process and apparatus are suitable for multi-color images having two or more color planes.  
         [0065]    Charging device  18  is may be a charged roll or a scorotron type corona charging device. Charging device  18  has high voltage surfaces (not shown) coupled to a suitable positive high voltage source. The high voltage surfaces of charging device  18  are on or near the surface of photoreceptor  10  and are coupled to an adjustable positive voltage supply (not shown) to obtain an suitable positive surface voltage on photoreceptor  10 . Of course, connection to a positive voltage is required for a positive charging photoreceptor  10 . Alternatively, a negatively charging photoreceptor  10  using negative voltages would also be operable. The principles are the same for a negative charging photoreceptor  10 .  
         [0066]    Laser imaging device  50  imparts image information associated with a first color plane of the image, laser imaging device  58  imparts image information associated with a second color plane of the image, laser imaging device  66  imparts image information associated with a third color plane of the image and laser imaging device  74  imparts image information associated with a fourth color plane of the image. Although each of laser imaging devices  50 ,  58 ,  66  and  74  are associated with a separate color of the image and operate in the sequence as described above with reference to FIG. 5, for convenience they are described together below.  
         [0067]    Laser imaging devices  50 ,  58 ,  66  and  74  include a suitable high intensity electromagnetic radiation. The radiation may be a single beam or an array of beams. The array of beams may be generated by a LED (light emitting diode) array. The individual beams in such an array may be individually modulated. The radiation impinges, for example, on photoreceptor  10  as a line scan generally perpendicular to the direction of movement of photoreceptor  10  and at a fixed position relative to charging device  18 .  
         [0068]    The radiation scans and exposes photoreceptor  10  preferably while maintaining exact synchronism with the movement of photoreceptor  10 . The image-wise exposure causes the surface charge of photoreceptor  10  to be reduced significantly wherever the radiation impinges. Areas of the surface of photoreceptor  10  where the radiation does not impinge are not appreciably discharged. Therefore, when photoreceptor  10  exits from under the radiation, its surface charge distribution is proportional to the desired image information.  
         [0069]    The radiation (a single beam or array of beams) from laser imaging devices  50 ,  58 ,  66  and  74  is modulated conventionally in response to image signals for any single color plane information from a suitable source such as a computer memory, communication channel, or the like. The mechanism through which the radiation from laser imaging devices is manipulated to reach photoreceptor  10  is also conventional.  
         [0070]    Developer storage and delivery system  54  develops the first color plane of the image, developer storage and delivery system  62  develops the second color plane of the image, developer storage and delivery system  70  develops the third color plane of the image and developer storage and delivery system  78  develops the fourth color plane of the image. Although each of developer storage and delivery systems  54 ,  62 ,  70  and  78  are associated with a separate color of the image and operate in the sequence as described above with reference to FIG. 5, for convenience they are described together below.  
         [0071]    The phase change developers should have a melting point of at least about 22° C., more preferably at least about 30° C., and most preferably at least about 40° C. The phase change developers may comprise a colorant, a carrier, a binder resin, and optionally other additives, such as a charge director and an adjuvant.  
         [0072]    The carrier may be selected from a wide variety of natural and synthetic, organic and inorganic materials that are known in the art, but the carrier preferably has a Kauri-Butanol number less than 30. The carrier is typically chemically stable under a variety of conditions and electrically insulating. Electrically insulating refers to a material having a low dielectric constant and a high electrical resistivity. Preferably, the carrier has a dielectric constant of less than 5, more preferably less than 3. Electrical resistivities of carrier are typically greater than 10 9  Ohm-cm, more preferably greater than 10 10  Ohm-cm. The carrier preferably is also relatively nonviscous in its liquid state at the operating temperature to allow movement of the charged particles during development. In addition, the carrier should be chemically inert with respect to the materials or equipment used in the liquid electrophotographic process, particularly the photoreceptor and its release surface.  
         [0073]    A number of classes of organic materials meet some or many of the requirements outlined above. Non-limiting examples of suitable carrier include aliphatic hydrocarbons or paraffins (n-pentane, hexane, heptane and the like), cycloaliphatic hydrocarbons (cyclopentane, cyclohexane and the like), aromatic hydrocarbons (benzene, toluene, xylene and the like), halogenated hydrocarbon solvents (chlorinated alkanes, fluorinated alkanes, chlorofluorocarbons, and the like), silicone oils and waxes, vegetable oils and waxes, animal oils and waxes, petroleum waxes, mineral waxes, synthetic wax, such as Fischer-Tropsch wax, polyethylene wax, 12-hydroxystearic acid amide, stearic acid amide, phthalic anhydride imide, and blends of these materials. Preferred carriers include branched paraffinic blends such as Norpar™  18  (available from Exxon Corporation, N.J.), vegetable waxes, animal waxes, petroleum waxes, silicone waxes, and synthetic waxes.  
         [0074]    The roles of the binder resin are to be the vehicle for the pigments or dyes, to provide colloidal stability, and to aid fixing of the final image. The binder resin should contain charging sites or be able to incorporate materials that have charging sites. Furthermore, the binder resin should have a melting point above 22° C., more preferably above 30° C., and most preferably above 40° C. Non-limiting examples of suitable binder resin are crystalline polymers or copolymers derived from side-chain crystallizable and main-chain crystallizable polymerizable monomers, oligomers or polymers with melting transitions above 22° C. Suitable crystalline polymeric binder resins include homopolymers or copolymers of alkyl acrylates where the alkyl chain contains more than 13 carbon atoms (e.g., tetradecyl acrylate, pentadecyl acrylate, hexadecyl acrylate, heptadecyl acrylate, octadecyl acrylate, behenyl acrylate, etc); alkyl methacrylates wherein the alkyl chain contains more than 17 carbon atoms; ethylene; propylene; and acrylamide. Other suitable crystalline polymeric binder resins with melting points above 22° C. are derived from aryl acrylates and methacrylates; high molecular weight alpha olefins; linear or branched long chain alkyl vinyl ethers or vinyl esters; long chain alkyl isocyanates; unsaturated long chain polyesters, polysiloxanes and polysilanes; amino functional silicone waxes; polymerizable natural waxes, polymerizable synthetic waxes, and other similar type materials known to those skilled in the art.  
         [0075]    Suitable crystalline polymeric binder resins can be also an organosol composed of a high molecular weight (co)polymeric graft stabilizer (shell) covalently bonded to an insoluble, thermoplastic (co)polymeric core. The graft stabilizer includes a crystallizable polymeric moiety that is capable of independently and reversibly crystallizing at or above 22° C. The graft stabilizer includes a polymerizable organic compound or mire of polymerizable organic compounds of which at least one is a polymerizable crystallizable compound (PCC). Suitable PCC&#39;s include side-chain crystallizable and main-chain crystallizable polymerizable monomers, oligomers or polymers with melting transitions above 22° C. Suitable PCC&#39;s include alkylacrylates where the alkyl chain contains more than 13 carbon atoms (e.g., tetradecylacrylate, pentadecylacrylate, hexadecylacrylate, heptadecylacrylate, octadecylacrylate, etc); alkylmethacrylates wherein the alkyl chain contains more than 17 carbon atoms, ethylene; propylene; and acrylamide. Other suitable PCCs with melting points above 22° C. include aryl acrylates and methacrylates; high molecular weight alpha olefins; linear or branched long chain alkyl vinyl ethers or vinyl esters; long chain alkyl isocyanates; unsaturated long chain polyesters, polysiloxanes and polysilanes; amino functional silicone waxes; polymerizable natural waxes, polymerizable synthetic waxes, and other similar type materials known to those skilled in the art.  
         [0076]    Useful colorants are well known in the art and include materials such as dyes, stains, and pigments. Preferred colorants are pigments that may be incorporated into the polymer binder resin, are nominally insoluble in and nonreactive with the carrier, and are useful and effective in making visible the latent electrostatic image. Non-limiting examples of typically suitable colorants include: phthalocyanine blue (C.I. Pigment Blue 15:1, 15:2, 15:3 and 15:4), monoarylide yellow (C.I. Pigment Yellow 1, 3, 65, 73 and 74), diarylide yellow (C.I. Pigment Yellow 12, 13, 14, 17 and 83), arylamide (Hansa) yellow (C.I. Pigment Yellow 10, 97, 105, 138 and 111), azo red (C.I. Pigment Red 3, 17, 22, 23, 38, 48:1, 48:2, 52:1, 81, 81:4 and 179), quinacridone magenta (C.I. Pigment Red 122, 202 and 209) and black pigments such as finely divided carbon (Cabot Monarch 120, Cabot Regal 300R, Cabot Regal 350R, Vulcan X72) and the like.  
         [0077]    The optimal weight ratio of binder resin to colorant in the developer particles is on the order of 1/1 to 20/1, preferably between 3/1 and 10/1 and most preferably between 5/1 and 8/1. The total dispersed material in the carrier typically represents 0.5 to 70 weight percent, preferably between 5 and 50 weight percent, most preferably between 10 and 40 weight percent of the total developer composition.  
         [0078]    An electrophotographic phase change developer may be formulated by incorporating a charge control agent into the phase change developer. The charge control agent, also known as a charge director, provides improved uniform charge polarity of the developer particles. The charge director may be incorporated into the developer particles using a variety of methods, such as chemically reacting the charge director with the developer particle, chemically or physically adsorbing the charge director onto the developer particle (binder resin or pigment), or chelating the charge director to a functional group incorporated into the developer particle. A preferred method is attachment via a functional group built into the graft stabilizer. The charge director acts to impart an electrical charge of selected polarity onto the developer particles. Any number of charge directors described in the art may be used. For example, the charge director may be introduced in the form of metal salts consisting of polyvalent metal ions and organic anions as the counterion. Non-limiting examples of suitable metal ions include Ba(II), Ca(II), Mn(II), Zn(II), Zr(IV), Cu(II), Al(III), Cr(III), Fe(II), Fe(III), Sb(III), Bi(III), Co(II), La(III), Pb(II), Mg(II), Mo(III), Ni(II), Ag(I), Sr(II), Sn(IV), V(V), Y(III), and Ti(IV). Non-limiting examples of suitable organic anions include carboxylates or sulfonates derived from aliphatic or aromatic carboxylic or sulfonic acids, preferably aliphatic fatty acids such as stearic acid, behenic acid, neodecanoic acid, diisopropylsalicylic acid, octanoic acid, abietic acid, naphthenic acid, octanoic acid, lauric acid, tallic acid, and the like. Preferred positive charge directors are the metallic carboxylates (soaps) described in U.S. Pat. No. 3,411,936, incorporated herein by reference, which include alkaline earth- and heavy-metallic salts of fatty acids containing at least 6-7 carbons and cyclic aliphatic acids including naphthenic acid; more preferred are polyvalent metal soaps of zirconium and aluminum; most preferred is the zirconium soap of octanoic acid (Zirconium HEX-CEM from Mooney Chemicals, Cleveland, Ohio).  
         [0079]    The preferred charge direction levels for a given phase change developer formulation will depend upon a number of factors, including the composition of the graft stabilizer and organosol, the molecular weight of the organosol, the particle size of the organosol, the core/shell ratio of the graft stabilizer, the pigment used in making the developer, and the ratio of binder resin to pigment. In addition, preferred charge direction levels will also depend upon the nature of the electrophotographic imaging process, particularly the design of the developing hardware and photoconductive element. Those skilled in the art, however, know how to adjust the level of charge direction based on the listed parameters to achieve the desired results for their particular application.  
         [0080]    The useful conductivity range of a phase change developer is from about 10 to 1200 picomho-cm −1 . High conductivities generally indicate inefficient association of the charges on the developer particles and is seen in the low relationship between current density and developer deposited during development. Low conductivities indicate little or no charging of the developer particles and lead to very low development rates. The use of charge director compounds to ensure sufficient charge associated with each particle is a common practice. There has, in recent times, been a realization that even with the use of charge directors there can be much unwanted charge situated on charged species in solution in the carrier. Such unwanted charge produces inefficiency, instability and inconsistency in the development.  
         [0081]    Any number of methods may be used for effecting particle size reduction of the pigment in preparation of the phase change developers. Some suitable methods include high shear homogenization, ball-milling, attritor milling, high energy bead(sand) milling or other means known in the art. The operating temperature during particle size reduction is above the melting point of the crystalline polymeric binder resin. The resulted phase change developer is either cooled to room temperature to form a solid which optionally may be turned into a powder by pulverizing; sprayed to form droplets which then are cooled to form a powder; transferred to a mold and then cooled to form a shaped solid; or coated on a substrate and then cooled to form a coated web with a layer of the phase change developer. If the melting point of the phase transfer developer is less than 22° C., the phase transfer developer will not be solid at room temperature. If the melting point of the phase transfer developer is greater than 40° C., image splitting may occur. The viscosity of the phase transfer developer has been characterized in certain steps of the process as in the range of 10 to 100 pascal second. If the viscosity of the phase transfer developer is less than 10 pascal second, the phase transfer developer becomes too soft, and the viscosity of the phase transfer developer is greater than 100 pascal second, it will need a high temperature to melt the phase transfer developer. Where the viscosity of the phase transfer developer has been characterized in other steps as in the range of 0.001 to 0.01 pascal second, if the viscosity of the phase transfer developer is less than 0.001 pascal second, the liquid phase transfer developer will become too thin to be transferred on the developer, and the viscosity of the phase transfer developer is greater than 0.01 pascal second, the mobility of the liquid phase transfer developer will be too low for effective development of toned images.  
         [0082]    Two modes of development are known in the art, namely deposition of liquid developer  52 ,  60 ,  68  and  76  in exposed areas of photoreceptor  10  and, alternatively, deposition of liquid developer  52 ,  60 ,  68  and  76  in unexposed regions. The former mode of imaging can improve formation of halftone dots while maintaining uniform density and low background densities. Although the invention has been described using a discharge development system whereby the positively charged liquid developer is deposited on the surface of photoreceptor  10  in areas discharged by the radiation, it is to be recognized and understood that an imaging system in which the opposite is true is also contemplated by this invention. Development is accomplished by using a uniform electric field produced by developer roll  56 ,  64 ,  72  and  80  spaced near the surface of photoreceptor  10 .  
         [0083]    A thin, uniform layer of liquid developer is established on a rotating, cylindrical developer roll  56 ,  64 ,  72  and  80 . A bias voltage is applied to the developer roll intermediate to the unexposed surface potential of photoreceptor  10  and the exposed surface potential level of photoreceptor  10 . The voltage is adjusted to obtain the required maximum density level and tone reproduction scale for halftone dots without any background being deposited. Developer roll  56 ,  64 ,  72  and  80  is brought into proximity with the surface of photoreceptor  10  immediately before the latent image formed on the surface of photoreceptor  10  passes beneath the developer roll  56 ,  64 ,  72  and  80 . The bias voltage on developer roll  56 ,  64 ,  72  and  80  forces the charged pigment particles, which are mobile in the electric field, to develop the latent image. The charged “solid” particles in liquid developer will migrate to and plate upon the surface of photoreceptor  10  in areas where the surface charge of photoreceptor  10  is less than the bias voltage of developer roll  56 ,  64 ,  72  and  80 . The charge neutrality of liquid developer is maintained by oppositely-charged substantially transparent counter ions which balance the charge of the positively charged developer particles. Counter ions are deposited on the surface photoreceptor  10  in areas where the surface voltage of photoreceptor  10  is greater than the developer roll bias voltage.  
         [0084]    The toner images plated on the surface of organophotoreceptor  10  is further dried by drying mechanism  34 . Drying mechanism  34  may be passive, may utilize active air blowers blowing hot air  90 , or may be other active devices such as rollers or IP lamp. In a preferred embodiment, drying mechanism is passive such that most of the carrier fluid is absorbed by the receiving medium.  
         [0085]    Photoreceptor  10  may be in the form of a belt or a drum. Photoreceptor  10  may be an organophotoreceptor as described in a previous filed U.S. patent application (Ser. No. 60/242517), which is incorporated herein by reference. Photoreceptor  10  may also be an inorganic photoreceptor containing at least an inorganic photosensitive material known in the art, such as alpha-silicon and chalcogenide glasses.