Abstract:
A method for reducing gloss variation in prints, in an electrostatic printing system having a fuser for fusing sheets having marking particles thereon, the method including transporting a first sheet to the fuser; transporting a second sheet to the fuser; and abutting a trailing edge of the first sheet to a leading edge of the second sheet prior to the trailing edge of the first sheet entering the fuser.

Description:
BACKGROUND 
       [0001]    The present disclosure relates generally to an electrostatographic or xerographic printing machine, and more particularly concerns a fixing device and a fixing method of forming an unfixed toner image of an image pattern corresponding to objective image information on a surface of a recording media. 
         [0002]    In a typical electrostatographic reproduction process machine, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is imagewise exposed in order to selectively dissipate charges thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules to the latent image forming a toner powder image on the photoconductive member. The toner powder image is then transferred from the photoconductive member to a copy sheet. The toner particles are heated at a thermal fusing apparatus at a desired operating temperature so as to fuse and permanently affix the powder image to the copy sheet having a certain gloss. In recent years, in particular, for a full-color image, a demand for an enhancement of image quality by making the image glossy has been increased. It is highly desirable to have printed images with uniform gloss throughout the entire sheet. 
         [0003]    Fuser gaps exist between consecutive sheets of paper (inter-document zone or IDZ). Applicants have found in a print job consisting of many sheets of paper, if the fusers are asynchronous with the page stream, IDZs create hot zones on the fuser which come around and create high temperature zones within the sheets of paper that come in contact with them. These step changes in the paper temperature and previously fused image contact with the fusers can lead to gloss variations (gloss non-uniformity) that are very obvious to the human eye due to the abrupt nature of the temperature change in the process direction. A conventional solution to this problem is to operate only at fuser temperatures where gloss is independent of temperature. This is not feasible when a lower gloss level is desired. 
         [0004]    It is desirable to have a simple apparatus construction which can generate images with high glossiness and is free from gloss non-uniformity. 
       SUMMARY 
       [0005]    There is provided a method for reducing gloss variation in prints, in an electrostatic printing system having a fuser for fusing sheets having marking particles thereon, the method including transporting a first sheet to the fuser; transporting a second sheet to the fuser; and abutting a trailing edge of the first sheet to a leading edge of the second sheet prior to the trailing edge of the first sheet entering the fuser. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Other features of the present invention will become apparent as the following description proceeds and upon reference to the drawings, in which: 
           [0007]      FIG. 1  is a schematic of an example of a digital imaging system, which can employ the media transport of the present disclosure. 
           [0008]      FIG. 2  is a schematic of an embodiment of a media transport of the present disclosure. 
           [0009]      FIGS. 3 and 4  show numerical simulation data of fuser temperature as a function of time at Nip exit and Nip entrance where one can see the benefits of employing the media transport of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    While the present invention will be described in connection with a preferred embodiment thereof, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. 
         [0011]    In as much as the art of electrophotographic printing is well known, the various processing stations employed in the  FIG. 1  printing machine will be shown hereinafter schematically and their operation described briefly with reference thereto. 
         [0012]      FIG. 1  is a partial schematic view of a digital imaging system, such as the digital imaging system of U.S. Pat. No. 6,505,832 which is hereby incorporated by reference. The imaging system is used to produce color output in a single pass of a photoreceptor belt. It will be understood, however, that it is not intended to limit the invention to the embodiment disclosed. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims, including a multiple pass color process system, a single or multiple pass highlight color system, and a black and white printing system. 
         [0013]    Referring to  FIG. 1 , an Output Management System  660  may supply printing jobs to the Print Controller  630 . Printing jobs may be submitted from the Output Management System Client  650  to the Output Management System  660 . A pixel counter  670  is incorporated into the Output Management System  660  to count the number of pixels to be imaged with toner on each sheet or page of the job, for each color. The pixel count information is stored in the Output Management System memory. The Output Management System  660  submits job control information, including the pixel count data, and the printing job to the Print Controller  630 . Job control information, including the pixel count data, and digital image data are communicated from the Print Controller  630  to the Controller  490 . 
         [0014]    The printing system preferably uses a charge retentive surface in the form of an Active Matrix (AMAT) photoreceptor belt  410  supported for movement in the direction indicated by arrow  412 , for advancing sequentially through the various xerographic process stations. The belt is entrained about a drive roller  414 , tension roller  416  and fixed roller  418  and the drive roller  414  is operatively connected to a drive motor  420  for effecting movement of the belt through the xerographic stations. A portion of belt  410  passes through charging station A where a corona generating device, indicated generally by the reference numeral  422 , charges the photoconductive surface of photoreceptor belt  410  to a relatively high, substantially uniform, preferably negative potential. 
         [0015]    Next, the charged portion of photoconductive surface is advanced through an imaging/exposure station B. At imaging/exposure station B, a controller, indicated generally by reference numeral  490 , receives the image signals from Print Controller  630  representing the desired output image and processes these signals to convert them to signals transmitted to a laser based output scanning device, which causes the charge retentive surface to be discharged in accordance with the output from the scanning device. Preferably the scanning device is a laser Raster Output Scanner (ROS)  424 . Alternatively, the ROS  424  could be replaced by other xerographic exposure devices such as LED arrays. 
         [0016]    The photoreceptor belt  410 , which is initially charged to a voltage V 0 , undergoes dark decay to a level equal to about −500 volts. When exposed at the exposure station B, it is discharged to a level equal to about −50 volts. Thus after exposure, the photoreceptor belt  410  contains a monopolar voltage profile of high and low voltages, the former corresponding to charged areas and the latter corresponding to discharged or developed areas. 
         [0017]    At a first development station C, developer structure, indicated generally by the reference numeral  432  utilizing a hybrid development system, the developer roller, better known as the donor roller, is powered by two developer fields (potentials across an air gap). The first field is the AC field which is used for toner cloud generation. The second field is the DC developer field which is used to control the amount of developed toner mass on the photoreceptor belt  410 . The toner cloud causes charged toner particles to be attracted to the electrostatic latent image. Appropriate developer biasing is accomplished via a power supply. This type of system is a non-contact type in which only toner particles (black, for example) are attracted to the latent image and there is no mechanical contact between the photoreceptor belt  410  and a toner delivery device to disturb a previously developed, but unfixed, image. A toner concentration sensor  200  senses the toner concentration in the developer structure  432 . 
         [0018]    The developed but unfixed image is then transported past a second charging device  436  where the photoreceptor belt  410  and previously developed toner image areas are recharged to a predetermined level. 
         [0019]    A second exposure/imaging is performed by device  438  which comprises a laser based output structure is utilized for selectively discharging the photoreceptor belt  410  on toned areas and/or bare areas, pursuant to the image to be developed with the second color toner. At this point, the photoreceptor belt  410  contains toned and untoned areas at relatively high voltage levels, and toned and untoned areas at relatively low voltage levels. These low voltage areas represent image areas which are developed using discharged area development (DAD). To this end, a negatively charged, developer material  440  comprising color toner is employed. The toner, which by way of example may be yellow, is contained in a developer housing structure  442  disposed at a second developer station D and is presented to the latent images on the photoreceptor belt  410  by way of a second developer system. A power supply (not shown) serves to electrically bias the developer structure to a level effective to develop the discharged image areas with negatively charged yellow toner particles. Further, a toner concentration sensor  200  senses the toner concentration in the developer housing structure  442 . 
         [0020]    The above procedure is repeated for a third image for a third suitable color toner such as magenta (station E) and for a fourth image and suitable color toner such as cyan (station F). The exposure control scheme described below may be utilized for these subsequent imaging steps. In this manner a full color composite toner image is developed on the photoreceptor belt  410 . In addition, a mass sensor  110  measures developed mass per unit area. Although only one mass sensor  110  is shown in  FIG. 1 , there may be more than one mass sensor  110 . 
         [0021]    To the extent to which some toner charge is totally neutralized, or the polarity reversed, thereby causing the composite image developed on the photoreceptor belt  410  to consist of both positive and negative toner, a negative pre-transfer dicorotron member  450  is provided to condition the toner for effective transfer to a substrate using positive corona discharge. 
         [0022]    Subsequent to image development a sheet of support material  452  is moved into contact with the toner images at transfer station G. The sheet of support material  452  is advanced to transfer station G by a sheet feeding apparatus  500 , described in detail below. The sheet of support material  452  is then brought into contact with photoconductive surface of photoreceptor belt  410  in a timed sequence so that the toner powder image developed thereon contacts the advancing sheet of support material  452  at transfer station G. 
         [0023]    Transfer station G includes a transfer dicorotron  454  which sprays positive ions onto the backside of sheet  452 . This attracts the negatively charged toner powder images from the photoreceptor belt  410  to sheet  452 . A detack dicorotron  456  is provided for facilitating stripping of the sheets from the photoreceptor belt  410 . 
         [0024]    After transfer, the sheet of support material  452  continues to move, in the direction of arrow  458 , onto a conveyor  600  and  601  of the present disclosure which advances the sheet to fusing station H. Fusing station H includes a fuser assembly, indicated generally by the reference numeral  460 , which permanently affixes the transferred powder image to sheet  452 . Preferably, fuser assembly  460  comprises a heated fuser roller  462  and a backup or pressure roller  464 . Sheet  452  passes between fuser roller  462  and backup roller  464  with the toner powder image contacting fuser roller  462 . In this manner, the toner powder images are permanently affixed to sheet  452 . After fusing, a chute, not shown, guides the advancing sheet  452  to a catch tray, stacker, finisher or other output device (not shown), for subsequent removal from the printing machine by the operator. 
         [0025]    After the sheet of support material  452  is separated from photoconductive surface of photoreceptor belt  410 , the residual toner particles carried by the non-image areas on the photoconductive surface are removed therefrom. These particles are removed at cleaning station I using a cleaning brush or plural brush structure contained in a housing  466 . The cleaning brushes  468  are engaged after the composite toner image is transferred to a sheet. 
         [0026]    Controller  490  regulates the various printer functions. The controller  490  is preferably a programmable controller, which controls printer functions hereinbefore described. The controller  490  may provide a comparison count of the copy sheets, the number of documents being recirculated, the number of copy sheets selected by the operator, time delays, jam corrections, etc. The control of all of the exemplary systems heretofore described may be accomplished by conventional control switch inputs from the printing machine consoles selected by an operator. Conventional sheet path sensors or switches may be utilized to keep track of the position of the document and the copy sheets. 
         [0027]    It is believed that the foregoing description is sufficient for purposes of the present application to illustrate the general operation of an electrophotographic printing machine incorporating the development apparatus of the present disclosure therein. 
         [0028]    Now focusing on the embodiments of the present disclosure referring to  FIG. 2 . 
         [0029]    Paper transport modules  600  and  601  preferably uses belt  102  supported for movement in the direction indicated by arrows. The belt of paper transport modules  600  is entrained about a drive roller  112 , tension roller  108  and fixed rollers  104  and  106 , the drive roller  112  being operatively connected to a drive motor. The belt of paper transport module  601  is entrained about a drive roller  112 , tension roller  108  and fixed rollers  104  and  106 , the drive roller  112  being operatively connected to a drive motor. 
         [0030]    Controller  100  controls the operation of paper transport modules  600  and  601 . In one mode of operation controller  100  controls paper transport modules  600  and  601  so that a first sheet  56  to the fuser is transported and a second sheet  55  to the fuser is transported with the trailing edge of the first sheet  56  abutting the leading edge of the second sheet  55  prior to the trailing edge of the first sheet  56  entering the fuser. 
         [0031]    Controller  100  may employ a sensor  115  for sensing trailing edge of the first sheet  56  and leading edge of the second sheet  55  whereupon controller  100  varies the speed of the second sheet  55  and/or the speed of the first sheet  56 , in response to sensor  115 , so that the trailing edge of the first sheet  56  and the leading edge of the second sheet  55  abut each other prior to the trailing edge of the first sheet  56  entering the fuser. 
         [0032]    Controller  100  controls the sheet transported on paper transport modules  600  and  601  so that the trailing edge of the first sheet and the leading edge of the second sheet have a predefined overlap, where preferably this predefined overlap is between 1 mm to 5 mm. 
         [0033]    To facilitate a slight overlap with the sheet already in the fuser, a short vacuum plate  130  is located just before the fuser to deflect the trail edge of the sheet in the fuser and enable this overlap without stubbing. With most media (and if designed with adequate flow), this vacuum plate  130  could be left in a stationary position. For the stiffest media this plate could optionally be rotated up to contact the media which assists in sheet acquisition, then moved down slightly to allow sheet overlap. Vacuum plate  130  operation is controlled by controller  100 . 
         [0034]    Principles of the present invention were tested using a thermal simulation model of the preheat transport module of the type of the present disclosure.  FIGS. 3 and 4  illustrate results thereof. The figures show time traces of the fuser roll temperature at the exit of the nip and at the entrance of the nip respectively.  FIGS. 3 and 4  compare the fuser roll temperature for three cases: (a)Inter-document zone length=103 mm, (b) Inter-document zone length=5 mm, (c) Inter-document zone length=−5 mm (overlap).  FIGS. 3 and 4  clearly show that overlapping is a very effective way of eliminating temperature variations on the fuser surface, which in turn will result in less gloss variation on prints. 
         [0035]    It is, therefore, apparent that there has been provided in accordance with the present invention a paper transport module that fully satisfies the aims and advantages hereinbefore set forth. While this invention has been described in conjunction with a specific embodiment thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims. 
         [0036]    It will be appreciated that various embodiments of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims. Unless specifically recited in a claim, steps or components of claims should not be implied or imported from the specification or any other claims as to any particular order, number, position, size, shape, angle, color, or material.