Abstract:
An apparatus and method for arranging the spacing of magnets in an electrostatogaphic printer to eliminate steaks due to the magnetic carrier that does not move uniformly across the photoconductor surface.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application has related subject matter to U.S. patent application Ser. No. 12/533,044, filed on Jul. 31, 2009 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to electrographic printers and apparatus thereof. More specifically, the invention is directed to an apparatus and method for arranging the spacing of magnets in an electrostatographic printer. 
       BACKGROUND OF THE INVENTION 
       [0003]    Electrographic printers and copiers utilizing developer comprising toner, carrier, and other components use a developer mixing apparatus and related processes for mixing the developer and toner used during the printing process. The term “electrographic printer” is intended to encompass electrophotographic printers and copiers that employ dry toner developed on an electrophotographic photoconductor element, as well as ionographic printers and copiers that do not rely upon an electrophotographic photoconductor. The electrographic apparatus often incorporates an electromagnetic brush station or similar development station, to develop the toner to a substrate (an imaging/photoconductive member bearing a latent image), after which the applied toner is transferred onto a sheet and fused thereon. 
         [0004]    A toner image may be formed on a photoconductor by the sequential steps of uniformly charging the photoconductor surface in a charging station using a corona charger or equivalent means, exposing the charged photoconductor to a pattern of light in an exposure station to form a latent electrostatic image, and toning the latent electrostatic image in a developer station to form a toner image on the photoconductor surface. The toner image may then be transferred in a transfer station directly to a receiver, e.g., a paper sheet, or it may first be transferred to an intermediate transfer member (ITM) and subsequently transferred to the receiver. The toned receiver is then moved to a fusing station where the toner image is fused to the receiver by heat and/or pressure. 
         [0005]    In electrostatographic copiers and printers, pigmented thermoplastic particles, commonly known as “toner,” are applied to latent electrostatic images to render such images visible. Often, the toner particles are mixed with and carried by somewhat larger particles of magnetic material, the resulting mix referred to as a two-component developer. During the mixing process, the magnetic carrier particles serve to triboelectrically charge the toner particles to a polarity opposite that of the carrier. In use, the development mix is advanced, typically by magnetic forces, from a sump to a position in which it contacts the latent charge image. 
         [0006]    The relatively strong electrostatic forces associated with the latent charge image operate to strip the toner from the carrier, causing the toner to remain with the charged image. Thus, it will be appreciated that, as multiple charge images are developed in this manner, toner particles are continuously depleted from the mix and a fresh supply of toner must be dispensed from time-to-time in order to maintain a desired image density. Usually, the fresh toner is supplied from a toner supply bottle mounted upside-down, i.e., with its mouth facing downward, at one end of the image-development apparatus. Under the force of gravity, toner accumulates at the bottle mouth, and a metering device, positioned adjacent the bottle mouth, operates to meter sufficient toner to the developer mix to compensate for the toner lost as a result of image development. Usually, the toner-metering device operates under the control of a toner concentration monitor that continuously senses the ratio of toner to carrier particles in the development mix. 
         [0007]    Development stations require replenishment of toner into the developer sump to replace toner that is deposited on the photoconductor or receiver. The toner and carrier must be mixed and transported to a position where the developer can be in contact with the latent charged image. If the magnetic carrier and toner are not uniformly distributed across the surface of the photoconductor, the printing process is compromised. 
         [0008]    The present invention corrects the problems of non-uniform magnetic carrier transport from the development station to the photoconductor surface as well as any other factors that contribute to non-uniform distribution of toner on the photoconductor that leads to streaking during printing. The apparatus and related methods described allow the printer to reduce or eliminate streaking associated with the prior art, in order to produce the high quality prints required by consumer demand, by utilizing the magnets more effectively to achieve more uniform toning. 
       SUMMARY OF THE INVENTION 
       [0009]    The invention is in the field of printing for electrographic printers. More specifically, the invention is directed to an apparatus and method for arranging the orientation of magnets in an electrostatographic printer 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a side elevational view, in cross-section, of a reproduction apparatus magnetic brush developer station according to this invention. 
           [0011]      FIG. 2  is an end view, partly in cross-section and on an enlarged scale, of the development roller of the magnetic brush development station of  FIG. 1 . 
           [0012]      FIG. 3  is a view, in perspective, of the bar magnets of  FIG. 1 . 
           [0013]      FIG. 4  is a view, in perspective, of the bar magnets arranged in a helical configuration. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]      FIG. 1  shows an electrostatic printer magnetic brush developer station, according to this invention, sometimes simply referred to as a developer station, designated generally by the numeral  10 . The development station housing  12  encloses a feed apparatus  14  and a powder conveyance device  16  and forms, in part, a reservoir of developer material  18  comprising a powder  20  and a magnetic carrier material  22 , hereafter referred to as magnetic carrier  22 . The magnetic carrier is described as a powder and could include one or more of magnetic carrier and magnetized toner including pigments (two component developers) as well as any other materials that are influenced by an electric and/or magnetic field. 
         [0015]    The development roller  24 , also referred to as a toning roller  24 , is mounted within the development station housing  12 . The development roller  24 , which includes a core magnet  26 , is shown in  FIG. 1  as a fourteen-pole core magnet rotating counterclockwise inside a rotating shell  28 . The development roller delivers a required quantity of developer material; including the powder  20  and the magnetic carrier  22 , from the reservoir to the development zone (area neighboring  24  between the development roller  24  and the photoconductor  30 ). In prior art these magnets are arranged in a fixed manner so that all the magnets are uniformly arranged around the circumference of the developer roller perpendicular to the process direction. In the invention described below, the arrangement and spacing of the magnets is varied according to the specific needs of the printer for that paper and/or developer as well as other relevant process and environmental conditions that influence printing quality, including streaking on the prints. The core magnet  26  and the shell  28  can have many other suitable relative rotations as is known in the art. 
         [0016]      FIG. 2  shows a part of the development station  24  that employs a hard magnetic carrier, also known as a developer that uses a ferrite or other material that is resistant to demagnetization. In other embodiments a soft magnetic carrier could be used. The development station transports the developer through the action of magnets, such as rotating magnets. As shown in  FIG. 2 , rotating magnets cause the magnetic pole flips which in turn causes carrier chains to form and flip, thus moving the developer around the circumference in an “in track” direction in relation to the development roller  28 . This results in no appreciable transverse (cross track) flow of developer on the roller surface  36 , parallel to the roller axis  34 . High speed video observations and laser measurements of developer thickness uniformity down the length of the development roller show that any thickness non-uniformities, whether due to uneven developer-feed-roller application or due to the propensity of hard magnetic carrier materials to clump unevenly, persist during the development process, thereby engendering uneven toning on the latent image. The measurement of developer is an average bulk velocity which is discussed in U.S. Ser. No. 12/533,044, filed Jul. 31, 2009, which is hereby incorporated by reference. An uneven developer mass density (on the development roller) produces optical density streaks on the printed image, which are unacceptable. Since uneven clumping of the developer is especially characteristic of hard ferrite magnetics, this enhances the problems. 
         [0017]    This invention creates transverse motion of the developer to time-average the uneven mass density over the development zone, which extends a short distance, in one case about 0.375″ in the in-track (process) direction and by making the effective lateral transport of the developer occur in a short enough time period, this time averages the mass density. The current embodiment shown in  FIG. 3  has bar magnets  32  that are arranged parallel to the development roller axis  34 . This arrangement produces developer flow around the surface of the roller  36  perpendicular to the roller axis  34 . The lack of transverse developer motion on the development roller allows a non-uniform, circumferential mass density to perpetuate in place, producing optical density streaks in the toned image, when using this current magnet configuration. 
         [0018]      FIG. 4  shows one embodiment of this invention including a development roller  24  with a shell  28  containing two or more rotating magnets arranged to minimize the streaking. These magnets can rotate either separately from the shell or in synch with the shell. The developer can include hard or soft carrier particles. It can be part of a single or dual component system. In this embodiment the magnets  40   a - f  are arranged in a helical manner within the shell  28  to receive developer from a developer reservoir. A controller for controlling the rotation speed of the two or more rotating magnets in relation to the photoconductor speed such that the powder is moved to the photoconductor as needed, and more specifically, in a manner that includes a cross track direction, is provided. Those skilled the art understand that these could be two or more magnets as required for effective transport of the powder. 
         [0019]    The magnets  40   a - f  are wrapped around a developer roller core  42  in a predefined arrangement, such as the predefined helix angle of inclination A shown in  FIG. 4  in order to promote developer flow in both in the “In-Track” and “Cross-Track” direction as required to give the uniformity desired. The amount of “Cross-Track” developer displacement is proportional to the helix angle A. This geometry creates the movement of developer in the cross track as well as the in track direction as needed for uniformity. Actual measurements have confirmed these results. The measurements come from affixing magnets at various helix angles and observing the movement of the developer in the cross track direction as well as the in track direction. The cross track movement of developer was increased or decreased by changing the angle and rotational speed of the magnets. 
         [0020]      FIG. 4  shows 6 magnets but more or less could be used based on the priority requirement, including the print speed. The angle of inclination A of the helix in this embodiments between the magnets in  FIG. 4  is about 20 degrees but could be varied from 10-55 degrees as needed. The quality of the images formed on the image-forming element depends on the number and spacing of these magnets. The number, strength and rotational speed of the magnets can be variable and depend on the particular imaging application. 
         [0021]    The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.