Patent Publication Number: US-2011052293-A1

Title: Post image transfer finishing in liquid electro-photographic printing

Description:
The present application is a continuation of co-pending PCT/US2008/061115 filed on Apr. 22, 2008 by David S. Vejtasa, Clayton L. Holstun and Forest Sun Patton and entitled POST IMAGE TRANSFER FINISHING IN LIQUID ELECTRO-PHOTOGRAPHIC PRINTING, the full disclosure of which is hereby incorporated by reference 
    
    
     BACKGROUND 
     Liquid electro-photographic (LEP) printing, sometimes also referred to as liquid electrostatic printing, uses liquid toner to form images on paper or other print medium. LEP is often used for large scale commercial printing. The basic LEP printing process involves placing a uniform electrostatic charge on a photoconductor, the photoconductive surface on a rotating drum for example, and exposing the photoconductor to light in the pattern of the desired printed image to dissipate the charge on the areas of the photoconductor exposed to the light. The resulting latent electrostatic image on the photoconductor is developed by applying a thin layer of liquid toner to the photoconductor. Liquid toner generally consists of charged toner particles dispersed in a carrier liquid. The charged toner particles adhere to the discharged areas on the photoconductor (discharged area development DAD) or to the charged areas (charged area development CAD), depending on the charge of the toner particles, to form the desired toner image on the photoconductor. The toner image is transferred from the photoconductor to an intermediate transfer member and then from the intermediate transfer member to the paper or other print medium. 
     In some LEP printers, the intermediate transfer member includes a removable, replaceable “blanket” wrapped around a rotating drum. The toner image on the surface of the photoconductive drum is transferred to the surface of the blanket as the surface of the blanket rotates against the surface of the photoconductive drum. The toner image on the blanket is then transferred to the paper as the paper is pressed against the blanket while passing through a nip between the intermediate transfer member/drum and a pressure roller. When the same image is printed repeatedly, many times in succession before printing a new image, as is often the case in commercial printing when hundreds or thousands of pages may be printed with the same image, the blanket may retain or “remember” the old image when a new image is printed due to differential wear of the outer, release layer on the intermediate transfer member blanket. If the blanket&#39;s memory of the old image is sufficiently strong, the old image may be visible in the new image. This undesirable phenomenon, which is referred to as gloss memory, may become more pronounced as the blanket ages until, eventually, the blanket must be replaced due to a gloss memory “failure.” 
    
    
     
       DRAWINGS 
         FIG. 1  is a block diagram illustrating the basic components an LEP print engine according to an embodiment of the disclosure. 
         FIG. 2  is a perspective view illustrating an LEP printer constructed according to one embodiment of the disclosure. 
         FIG. 3  is perspective view illustrating in more detail the print engine in the printer shown in  FIG. 3 . 
         FIG. 4  is a flow chart illustrating an LEP printing method according to an embodiment of the disclosure. 
     
    
    
     DESCRIPTION 
     Embodiments of the disclosure were developed in an effort to extend the useful life of the blanket on an intermediate transfer member in an LEP printer by reducing the effects of gloss memory. It has been discovered that altering the surface texture of the printed toner image to a more uniform texture can extend the life of the intermediate transfer member blanket by masking the onset of a gloss memory failure. One technique for altering the surface texture of the printed image is to simultaneously apply heat and pressure to the printed image. It has been observed that such post image transfer finishing operations also may be used to improve adhesion between the toner layer/image and the print media and to selectively improve or change gloss levels. 
       FIG. 1  is a block diagram illustrating the basic components of an LEP print engine  10  according to one embodiment of the disclosure. Referring to  FIG. 1 , in print engine  10  a uniform electrostatic charge is applied to a photoconductive surface, the outer surface of a photoconductor drum  12  for example, by a scorotron or other suitable charging device  14 . The photoconductor  12  used for LEP printing is commonly referred to as a photo imaging plate (PIP). A scanning laser or other suitable photo imaging device  16  exposes selected areas on photoconductor  12  to light in the pattern of the desired printed image to dissipate the charge on the areas of photoconductor  12  exposed to the light. In discharge area development (DAD), for example, the discharged areas on photoconductor  12  form an electrostatic image which corresponds to the image to be printed. This electrostatic image is said to be a “latent” image because it has not yet been developed into a toner image. A thin layer of liquid toner is applied to the patterned photoconductor  12  using a developer roller  18 . Developer roller  18  represents generally a typically complex developer unit, often referred to as a binary ink developer (BID), that supplies ink to a small roller that rotates against photoconductor  12 . Hence, the developer unit is depicted generally in  FIG. 1  by a developer roller  18 . 
     The latent image on photoconductor  12  is developed through the application of the liquid toner which adheres to the discharged areas of photoconductor  12  in a uniform layer of toner on photoconductor  12 , developing the latent electrostatic image into a toner image. The toner image is transferred from photoconductor  12  to an intermediate transfer drum/member (ITM)  20  and then from intermediate transfer member  20  to paper or other print medium  22  as paper  22  passes through a nip  23  between intermediate transfer member  20  and a pressure roller  24 . Print medium  22 , which is also referred to as paper  22 , represents generally any suitable print medium and may be delivered to print engine  10  as a continuous web dispensed from a roll or as individual sheets. Pressure roller  24  is commonly referred to as an impression cylinder (IMP). An LED lamp or other suitable discharging device  26  removes residual charge from photoconductor  12  and toner residue is removed at a cleaning station  28  in preparation for developing the next image or for applying the next toner color plane. Components  12 - 28  of print engine  10  are conventional components whose structure and operation is well known to those skilled in the art of LEP printing. 
     Intermediate transfer member  20  typically will include a removable, replaceable blanket wrapped around a drum. The comparatively soft, compliant blanket is heated to drive off most of the carrier fluid component of the liquid toner and, in one example process using Hewlett-Packard Co. ElectroInk® liquid toner, the toner dries to an approximately 90% solid layer about 1 μm thick before being transferred to paper  22 . The thin layer of toner is then transferred, and simultaneously fused, to paper  22  through the application of pressure at the nip  23  between transfer member  20  and impression cylinder  24 . Unlike electro-photographic printers that use dry toner, in which the dry toner particles are transferred to the paper and then fused at a downstream fuser in a separate operation, the liquid toner used in LEP printers is transferred and fused to the paper at the same time in a single operation. Thus, a subsequent fusing operation has not heretofore been deemed necessary or desirable, and is not used, in conventional LEP printers. 
     After the toner layer/image has been transferred and fused to paper  22  at nip  23 , paper  22  passes through a finishing station  30 . In the embodiment shown, finishing station  30  includes a heated finishing roller  32  and a mating pressure roller  34 . Heat and pressure are applied to the toner layer on paper  22  as paper  22  passes through a nip  36  between rollers  32  and  34 . Finishing station  30  might also be characterized as a re-finishing station in the sense that, in a conventional LEP print engine, the image is “finished” after it is transferred from the intermediate transfer member to the paper. Thus, the image may be said to be “re-finished” in the new LEP print engine  10  through the application of heat and pressure at nip  36 . Selectively applying heat and pressure at finishing station  30  re-fixes memories of an old image that may be present in the toner layer on paper  22  so that the old image is not visible in the new image, as described in more detail below. This refinishing operation may also be used to adjust the gloss level of the toner layer and develop better adhesion of the toner layer to paper  22 . 
     In an LEP print engine, when the same image is repeatedly applied to and then transferred from the intermediate transfer member many times in succession, the surface texture of that area of the blanket to which the image is applied may change, becoming different from the surface texture on other areas of the blanket. Thus, the blanket retains or “remembers” this image as variations in the surface texture of the blanket. Since the surface of the toner image in LEP is in direct contact with the surface of the intermediate transfer member blanket, differential surface textures on the blanket left from printing the old image may appear on the surface of the new toner image. This phenomenon is called gloss memory. When the new toner image is transferred to the paper, light will reflect differently off areas with different surface textures and, hence, the old image may be visible in the new image. LEP toners are so thin, however, that the toner image reflects the underlying surface roughness of the paper and the gloss level of the toner image is determined by the paper as well as the surface texture of the toner image. Nevertheless, the application of heat and pressure to the toner image after it has been transferred to the paper will still reduce or eliminate the differences in surface texture manifesting as gloss memory by altering the surface texture of the toner image, giving the top surface of the toner image on the paper a more uniform texture that reflects light more consistently. 
     In one example configuration for finishing station  30 , suitable for re-finishing an ElectroInk® toner layer approximately 1 μm thick on paper  22 , finishing roller  32  is heated to an exterior surface temperature in the range of 160° C. to 190° C. while exerting a pressure on paper  22  at nip  36  in the range of 60 kPa to 90 kPa at a paper feed rate of approximately 1 m per second. It is expected that these operating parameters for finishing station  30  will be sufficient to suppress the effects of gloss memory that might otherwise be visible in the new image. Other configurations are possible. It may be desirable to vary the temperature, pressure and/or feed rate at finishing station  30  to achieve different degrees of gloss memory suppression and/or to help produce a desired gloss on the printed image. 
     Also, the surface texture of finishing roller  32  may be specifically designed to help produce a desired gloss on the printed image—matte, medium, or a high gloss photo finish, for example. 
       FIG. 2  illustrates one embodiment of an LEP printer  40  implementing a print engine  42  with a post image transfer finishing station  44 .  FIG. 3  is a detail view of print engine  42 . Printer  40  generally reflects the configuration of an HP Indigo® Press 5500 LEP printing press adapted to implement one embodiment of a print engine that includes a post image transfer finishing station. Referring to  FIGS. 2 and 3 , printer  40  includes a paper feed unit  46  with multiple paper input trays  48 ,  50 , and  52 . Sheets of paper are fed from stacks  48 - 50  across a feed bridge  54  to print engine  42  from which they emerge as printed sheets  54  conveyed along a discharge paper path  56  to an output stacker  58 . Although not shown, various operations may be performed along discharge path  56  including, for example, ILD (in-line densitometer) color calibration and adjustment and sheet routing to a proof tray. Printed sheets  54  may be routed back through print engine  42  via a duplex conveyor  60  at the urging of a so-called exit guide perfector  62  configured to selectively move sheets  54  out to discharge path  56  or back through duplex conveyor  60 . 
     Print engine  42  includes a scorotron charging device  64  located adjacent to a photoconductor  66  for applying a uniform electric charge to photoconductor  66 . A photo imaging device  68  exposes selected areas on photoconductor  66  to light in the pattern of the desired printed image. A thin layer of liquid toner is applied to the patterned photoconductor  66  through one or more of a series of developer units  70  to develop the latent image on photoconductor  66  into a toner image. Each developer unit  70  moves ink from an internal reservoir  72  to a developer roller  74  that rotates against photoconductor  66 . Each developer unit  70  usually applies a different color ink from a corresponding series of toner supply cans  76 . The toner held in each supply can  76  is typically about 20% solids, having the consistency of toothpaste. The paste-like toner is diluted to about 2% solids in dilution tanks  78  before it is pumped to a developer unit  70  and applied to photoconductor  66 . 
     The toner image is transferred from photoconductor  66  to the outside surface a replaceable blanket  80  on an intermediate transfer member  82 . The toner image is then transferred and fused to the paper as the paper passes through the nip between intermediate transfer member  82  and a pressure roller  84 . An LED lamp or other suitable discharging device  86  removes residual charge from photoconductor  66 . Toner residue is removed at a cleaning station  88  in preparation for developing the next image or applying the next toner color plane. Volatile fumes generated as the toner carrier fluid evaporates off intermediate transfer member blanket  80  are evacuated through a suction hood  90 . 
     After the toner layer/image has been transferred and fused to the paper, the paper passes through finishing station  44 . In the embodiment shown, finishing station  44  includes three finishing units  94 ,  96 , and  98 . Each finishing unit  94 - 98  includes a heated finishing roller  100  and a mating pressure roller  102  that are selectively engaged individually (one at a time) to re-finish a printed sheet  54  according to a desired gloss level. The surface texture of each finishing roller  110  in each unit  94 - 98  and the operating temperatures and pressures for each unit  94 - 98  may be configured to achieve the desired gloss level, for example a matte gloss for finishing unit  94 , a medium gloss for finishing unit  96 , and a high gloss, photo finish for finishing unit  98 . 
     Referring now to  FIG. 4 , a flowchart illustrating a method  104 , in an exemplary embodiment of the disclosure, for reducing the effects of gloss memory in LEP printing. Referring to  FIG. 4 , a latent electrostatic image is formed on a photoconductor (step  106 ). The latent image may be formed, as described above, by charging the photoconductor to a uniform level and then exposing the charged photoconductor to light in a pattern of a desired printed image to discharge the exposed areas of the photoconductor. The latent image is developed into a toner image (step  108 ) by applying a thin layer of liquid toner to the photoconductor. Charged toner particles adhere to the discharged areas, for example, of the photoconductor to form the desired toner image. The toner image on the photoconductor is transferred from the photoconductor to an intermediate transfer member (step  110 ) and then from the intermediate transfer member to a paper or other sheet or roll/web print medium and simultaneously fused to the paper (step  112 ). Then, the surface texture of the toner image on the print medium is altered to a more uniform texture (step  114 ) by, for example, simultaneously applying heat and pressure to the printed image. 
     The example embodiments shown in the figures and described above illustrate but do not limit the disclosure. Other forms, details, and embodiments may be made and implemented. Therefore, the foregoing description should not be construed to limit the scope of the disclosure, which is defined in the following claims.