Abstract:
A method and apparatus for retracting a magnetic brush in an imaging device. The apparatus includes a housing containing carrier beads and toner particles; a magnetic structure at least partly disposed within the housing and generating at least one magnetic field; and an endless sleeve disposed around and rotatable about the magnetic structure, at least a portion of the sleeve extending from the housing. During toner development, the magnetic structure is in a first position for developing a magnetic brush of the carrier beads and toner particles along the portion of the sleeve extending from the housing which forms a developer nip with a photoconductor, and during a period of time when toner development is not to be performed, the magnetic structure is in a second position for retracting the magnetic brush by causing the carrier beads to be positioned against the portion of the endless sleeve extending from the housing forming the developer nip.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     None. 
     REFERENCE TO SEQUENTIAL LISTING, ETC 
     None. 
     BACKGROUND 
     1. Field of the Disclosure 
     The present disclosure relates generally to developing a magnetic brush for transferring toner to a photoconductive member, and particularly to selectively retracting the developed magnetic brush from contact with the photoconductor to substantially prevent background toner development. 
     2. Description of the Related Art 
     Electrophotography selectively develops toner to the photoconductor by discharging areas of the photoconductor that correspond to areas to be colored and leaving other areas charged so as to prevent toner development in those “white” areas. For negatively charged toner, these white areas are created by providing a photoconductor charge that is more negative than the developer bias. This difference between the photoconductor and developer biases is called the “white vector.” If the white vector is too small, then mechanical development takes place resulting in a haze of toner on areas of the photoconductor that should be white. Increasing the white vector produces a reduction in toner to the white areas, but if the white vector is increased too much there is another increase in toner developed into the white areas. This increased development is the result of high electric field strengths that can result in modifications to the toner charge. The white vector is adjusted to its desired value when the least amount of toner is developed into the white or background areas of the photoconductor. Toner developed into these white areas is called “background toner” and is wasted toner. This toner waste reduces the useful life of a toner cartridge since it represents an undesirable draw of toner for printing. 
     A typical laser printer will create an image on the page with approximately 0.4 mg/cm 2  of print. Background toner with an appropriately adjusted white vector can be about 0.001 mg/cm 2 . This may seem like an insignificant amount of waste toner, but since the areas of unprinted “white” are typically orders of magnitude larger than the printed areas, this waste toner has greater significance. For color laser printers, the cost to the customer of background toner when printing a white page can, in some circumstances, approach or exceed the cost of a sheet of paper. Most laser printers use white vector to limit development of background toner when the machine is running but not actually developing toner that goes to the printed page. White vector is an effective way to keep the white areas of prints white, but an improved method is needed to prevent toner waste when not printing. 
     SUMMARY 
     Example embodiments disclosed herein improve upon the shortcomings of existing dual component electrophotographic imaging devices and satisfy a significant need for reducing or otherwise eliminating background toner development. According to an example embodiment, there is shown a developer apparatus for an imaging system, including a housing containing a mixture of carrier beads and toner particles; a magnetic structure at least partly disposed within the housing and generating at least one magnetic field; and an endless sleeve disposed around and rotatable about the magnetic structure and extending from the housing. A portion of the endless sleeve extending from the housing forms a developer nip when the developer apparatus is operably coupled to an adjacent photoconductor. Toner is triboelectrically attracted to the surface of the carrier beads and the magnetic properties of the beads cause the beads to form structures called bead chains that are aligned with the direction of the magnetic field in their vicinity. During a printing operation in which toner is to be developed, the magnetic structure is in a first orientation for causing bead chains to “stand up” and extend outwardly from the portion of the endless sleeve, thereby forming a magnetic brush of the carrier beads and toner particles along the portion of the sleeve, for contacting with an adjacent photoconductor of the imaging system. However, during a period of time when toner development is not to be performed, the magnetic structure is in a second orientation which causes the bead chains to “lay down” and be positioned along the surface of the portion of the endless sleeve so as to be spaced apart from the adjacent photoconductor. As a result, toner particles are not positioned to contact the adjacent photoconductive member of the imaging system to be transferred thereto during times when toner development is not being performed. 
     In an example embodiment, the magnetic structure is rotated between the first and second positions by a motor, solenoid or the like. In another example embodiment, the magnetic structure is substantially freely rotatable about a shaft and is coupled to the endless sleeve via attractive forces with the carrier beads on the sleeve such that the magnetic structure rotates with the endless sleeve between the first and second positions. In this embodiment, the developer apparatus further includes an extension member having a first end connected to the shaft and a distal end. The extension member rotates with the shaft and thereby with the magnetic structure. The developer apparatus further includes a pair of stop members extending from or within the housing in proximity to the distal end of the extension member. The stop members are positioned to contact the distal end of the extension member and thereby serve to limit the amount of rotation of the magnetic structure to be between the first and second positions when the magnetic structure moves with the endless sleeve. 
     In yet another example embodiment, the magnetic structure is fixed. Following completion of toner development during a printing operation, the endless sleeve is rotated in a direction opposite to the direction of rotation during toner development, until the portion of the endless sleeve has little to no beads and toner particles that are disposed adjacent the photoconductive member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-mentioned and other features and advantages of the disclosed embodiments, and the manner of attaining them, will become more apparent and will be better understood by reference to the following description of the disclosed embodiments in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a side elevational view of an improved imaging device according to an example embodiment; 
         FIG. 2  is a cross sectional view of the developer unit of  FIG. 1  according to an example embodiment; 
         FIG. 3  is a perspective view of an end portion of a portion of the developer unit of  FIG. 2 ; and 
         FIGS. 4 and 5  are simplified side views of the developer unit of  FIG. 2  during printing and non-printing operations, respectively, showing magnetic fields generated thereby. 
     
    
    
     DETAILED DESCRIPTION 
     It is to be understood that the present disclosure is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The present disclosure is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless limited otherwise, the terms “connected,” “coupled,” and “mounted,” and variations thereof herein are used broadly and encompass direct and indirect connections, couplings, and mountings. In addition, the terms “connected” and “coupled” and variations thereof are not restricted to physical or mechanical connections or couplings. 
     Terms such as “first”, “second”, and the like, are used to describe various elements, regions, sections, etc. and are not intended to be limiting. Further, the terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. 
     Furthermore, and as described in subsequent paragraphs, the specific configurations illustrated in the drawings are intended to exemplify embodiments of the disclosure and that other alternative configurations are possible. 
     Reference will now be made in detail to the example embodiments, as illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts. 
       FIG. 1  illustrates a color image forming device  100  according to an example embodiment. Image forming device  100  includes a first toner transfer area  102  having four developer units  104  that substantially extend from one end of image forming device  100  to an opposed end thereof. Developer units  104  are disposed along an intermediate transfer member (ITM)  106 . Each developer unit  104  holds a different color toner mixed with carrier beads. The developer units  104  may be aligned in order relative to the direction of the ITM  106  indicated by the arrows in  FIG. 1 , with the yellow developer unit  104 Y being the most upstream, followed by cyan developer unit  104 C, magenta developer unit  104 M, and black developer unit  104 K being the most downstream along ITM  106 . 
     Each developer unit  104  is operably connected to a toner reservoir  108  for receiving toner for use in a printing operation. Each toner reservoir  108 K,  108 M,  108 C and  108 Y is controlled to supply toner as needed to its corresponding developer unit  104 . Each developer unit  104  is associated with a distinct photoconductive member  110  that receives toner therefrom during toner development to form a toned image thereon. Each photoconductive member  110  is paired with a transfer member  112  for use in transferring toner to ITM  106  at first transfer area  102 . 
     During color image formation, the surface of each photoconductive member  110  is charged to a specified voltage. At least one laser beam LB from a printhead  130  is directed to the surface of each photoconductive member  110  and discharges those areas it contacts to form a latent image thereon. The developer unit  104  then supplies toner to photoconductive member  110  to form a toner image thereon. The toner is attracted to the areas of the surface of photoconductive member  110  that are discharged by the corresponding laser beam LB from the printhead  130 . 
     ITM  106  is disposed adjacent to each developer unit  104 . In this embodiment, ITM  106  is formed as an endless belt disposed about a drive roller and other rollers. During image forming operations, ITM  106  moves past photoconductive members  110  in a clockwise direction as viewed in  FIG. 1 . One or more of photoconductive members  110  applies its toner image in its respective color to ITM  106 . For monochrome images, a toner image is applied from a single photoconductive member  110 K. For multi-color images, toner images are applied from two or more photoconductive members  110 . In one embodiment, a positive voltage field formed in part by transfer member  112  attracts the toner image from the associated photoconductive member  110  to the surface of a moving ITM  106 . 
     ITM  106  rotates and collects the one or more toner images from the one or more developer units  104  and then conveys the one or more toner images to a media sheet at a second transfer area  114 . Second transfer area  114  includes a second transfer nip formed between at least one back-up roller  116  and a second transfer roller  118 . 
     Fuser assembly  120  is disposed downstream of second transfer area  114  and receives media sheets with the unfused toner images superposed thereon. In general terms, fuser assembly  120  applies heat and pressure to the media sheets in order to fuse toner thereto. After leaving fuser assembly  120 , a media sheet is either deposited into output media area  122  or enters duplex media path  124  for transport to second transfer area  114  for imaging on a second surface of the media sheet. 
     Image forming device  100  is depicted in  FIG. 1  as a color laser printer in which toner is transferred to a media sheet in a two step operation. Alternatively, image forming device  100  may be a color laser printer in which toner is transferred to a media sheet in a single step process—from photoconductive members  110  directly to a media sheet. In another alternative embodiment, image forming device  100  may be a monochrome laser printer which utilizes only a single developer unit  104  and photoconductive member  110  for depositing black toner to media sheets. Further, image forming device  100  may be part of a multi-function product having, among other things, an image scanner for scanning printed sheets. 
     Image forming device  100  further includes a controller  140  and memory  142  communicatively coupled thereto. Though not shown in  FIG. 1 , controller  140  may be coupled to components and modules in image forming device  100  for controlling same. For instance, controller  140  may be coupled to toner reservoirs  108 , developer units  104 , photoconductive members  110 , fuser  120  and/or printhead  130 . It is understood that controller  140  may be implemented as any number of controllers and/or processors for suitably controlling image forming device  100  to perform, among other functions, printing operations. 
     The toner in developer unit  104  is charged to an appropriate amount to facilitate the correct amount of development on the surface of photoconductive member  110 . A dual component development system includes a developer mix containing a portion of polymeric resin based toner, and magnetic carrier beads. Typically the magnetic carrier beads will have a polymeric coating constructed of a triboelectrically different resin than the toner. When the toner is mixed with the carrier, the toner will charge to one polarity, while the carrier coating will charge to the opposite polarity. At this point, the toner will adhere to the oppositely charged carrier beads. In the example embodiments described herein, image forming device  100  utilizes a dual component development system. 
       FIGS. 2 and 3  illustrate a developer unit  104  in association with a corresponding photoconductive member  110 . Developer unit  104  includes a housing  201  having a chamber in which toner, deposited from a toner reservoir  108 , is mixed with the carrier beads. In an example embodiment, dual augers  202  are used to mix the toner and carrier beads. In this embodiment, a first auger  202 A may be used to mix toner and carrier beads by moving them in a first direction and a second auger  202 B may be used to mix the toner and beads by moving them in the direction opposite to the first direction. It is understood that developer unit  104  may utilized other mechanisms for suitably mixing the toner and carrier beads. 
     Developer unit  104  may further include a magnetic structure  204  and an endless sleeve  206  which is disposed about magnetic structure  204 . Magnetic structure  204  generally serves to attract the mixture of toner and carrier beads onto endless sleeve  206  due to magnetic forces acting on the carrier beads. Magnetic structure  204  may be constructed from a permanent magnetic material containing ferrite, and magnetized to produce a multiplicity of magnetic poles  204 A positioned around magnetic structure  204 . Magnetic poles  204 A are spaced apart from each other to control the strength and direction of the magnetic field outside the surface of endless sleeve  206 , and to thereby direct the formation of bead chains at different stages of the development process. Magnetic structure  204  may further include a shaft (not shown in  FIG. 2 ) extending longitudinally therethrough. 
     As shown in  FIG. 2 , a portion of each of magnetic structure  204  and sleeve  206  extends from an opening in housing  201  so as to be positioned adjacent photoconductive member  110 . Sleeve  206  may be constructed from a non-magnetic material such as aluminum or the like. Sleeve  206  may be substantially entirely disposed about magnetic structure  204 . In an example embodiment, sleeve  206  is rotatable about magnetic structure  204 . A motor  208  ( FIG. 1 ) may be coupled to rotate sleeve  206  about magnetic structure  204  as controlled by controller  140 . Motor  208  may be mechanically coupled to sleeve  206  using coupling mechanisms known in the art. In an example embodiment, sleeve  206  of each developer unit  104  may be controlled by a distinct motor  208 . Alternatively, a single motor  208  may be coupled to and rotate sleeve  206  of more than one developer unit  104 . 
     During toner development, sleeve  206  is rotated in a forward direction (clockwise as shown in  FIG. 2 ) so that carrier beads having toner particles adhered thereto cling to sleeve  206  due to magnetic forces acting on the carrier beads from magnetic structure  204 . As sleeve  206  is rotated relative to magnetic structure  204  and the magnetic forces generated thereby, the magnetic carrier bead chains move in an alternating manner from substantially laying down and disposed against sleeve  206  to standing up and extending outwardly therefrom so as to form a magnetic brush. A trim bar  210  regulates the length of the outwardly extending carrier chains on sleeve  206 . When sleeve  206  is further rotated so that the carrier beads are in the developer nip N adjacent photoconductive member  110 , the carrier beads again form chains extending outwardly from sleeve  206 . As the carrier chains forming the magnetic brush make contact with photoconductive member  110  in developer nip N, toner particles detach from their carrier beads due to the charge of the latent image on photoconductive member  100  and move to the discharged areas of photoconductive member  110 . Continued clockwise rotation of sleeve  206  results in the carrier beads separating from sleeve  206  due to a reduction in magnetic forces from magnetic structure  204  acting on the carrier beads. The separated carrier beads are then mixed with toner by augers  202  to begin again the toner development process. 
     Example embodiments provide a reduction in background toner development without carrier bead loss. Background toner development is reduced by any of a number of mechanisms. In an example embodiment, magnetic structure  204  is rotated in a reverse direction during times when toner development is not to occur so that the carrier chains at developer nip N are disposed substantially against and/or tangent to sleeve  206  instead of being arranged in chains of carrier beads extending outwardly therefrom. The amount of rotation of magnetic structure  204  may be between about 20 degrees and about 40 degrees, such as about 30 degrees. Without the presence of erect, outwardly extending carrier chains in developer nip N to contact photoconductive member  110 , toner is unable to move thereto. In this way, there are two angular orientations or positions of magnetic structure  204 —a first angular orientation in which magnetic structure  204  is positioned to cause chains of carrier beads to extend outwardly from sleeve  206  in developer nip N during times when toner is to be developed, and a second angular orientation in which magnetic structure  204  is positioned to cause the carrier beads to lie substantially flat against sleeve  206  in developer nip N when toner development is not desired to occur. 
     The rotation of magnetic structure  204  may be effectuated by a motor, solenoid or the like so as to rotate magnetic structure  204  between the first and second angular orientations. In this embodiment, controller  140  may control the motor for rotating magnetic structure  204  to the first angular orientation during toner development and to the second angular orientation at times when toner development is not intended to occur. Alternatively, magnetic structure  204  is configured to freely rotate only between the first and second angular orientations. In particular, the magnetic forces between rotatable magnetic structure  204  and the carrier beads clinging to sleeve  204  are sufficient to cause magnetic structure  204  to rotate in either direction with the rotation of sleeve  206 . By limiting the amount by which magnetic structure  204  may freely rotate with sleeve  206 , magnetic structure  204  may be positioned at the first angular orientation during forward (clockwise in  FIG. 2 ) rotation of sleeve  206 , such as during toner development onto the photoconductive member  110 , and at the second angular orientation after a reverse (counterclockwise) rotation of sleeve  206 , such as during times when toner development is not desired to occur. 
     Developer unit  104  may utilize a stop mechanism for limiting rotation of magnetic structure  204  between the above-described first and second angular orientations. With respect to  FIGS. 4 and 5 , which show magnetic structure  204  in the first and second angular orientations, respectively, and also to  FIG. 3 , developer unit  104  may include an extension member  400  having a first end which is secured to shaft  402  of magnetic structure  204  so as to rotate therewith, and a distal second end. In addition, developer unit  104  may include a pair of stop members  404 . Stop members  404  may be positioned along housing  201  of developer unit  104 , such as along an inner surface thereof. Stop member  404 A may be positioned along housing  201  so as to limit an extent of forward (clockwise in  FIG. 4 ) rotation of magnetic structure  204 , and stop member  404 B may be positioned along housing  201  so as to limit an extent of reverse (counterclockwise in  FIG. 5 ) rotation of magnetic structure  204 . The extent of forward rotation corresponds to the first angular orientation for proper toner development, and the extent of reverse rotation corresponds to the second angular orientation for preventing unwanted toner development. Specifically, as sleeve  206  is rotated in the forward direction, such as during toner development, extension member  400  contacts stop member  404 A and is prevented from further rotation, thereby preventing magnetic structure  204  from further forward rotation with sleeve  206 . Conversely, as sleeve  206  is rotated in the reverse direction, such as when toner development is not to occur, extension member  400  contacts stop member  404 B and is prevented from further rotation, thereby preventing magnetic structure  204  from further reverse rotation with sleeve  206 . In this way, magnetic structure  204  may be properly positioned for toner development and when toner is not desired to be developed. 
     In the above-described example embodiment, the angular distance between the first angular orientation and the second angular orientation may be about half the distance between the magnetic pole associated with toner development and the magnetic pole immediately forward (in a clockwise direction) associated with transport of carrier beads following toner development. For example, the angular distance may be between about 20 degrees and about 40 degrees, and particularly between about 25 degrees and about 35 degrees, such as about 30 degrees. 
     Instead of stop members  404  being attached along housing  201  of develop unit  104 , alternatively stop members  404  may be attached to the frame or the like of image forming device  100 . In this case, extension member  400  may extend at least partly externally to housing  201  so as to contact stop members  404 . 
     The above-described example embodiments described the use of rotating magnetic structure  204  between the first and second angular orientations. In another example embodiment, magnetic structure  204  is fixed in the first angular orientation and sleeve  206  is rotated in the reverse (counterclockwise, as shown in  FIGS. 3 &amp; 5 ) by an amount during times when toner development is not to occur. In this case, motor  208  rotates sleeve  206  in the reverse direction until there are no carrier beads contacting the photoconductive member  110 . In some systems utilizing an ITM, such as ITM  106 , the amount of reverse rotation of sleeve  206  may be between about 100 degrees and about 200 degrees, such as about 180 degrees. 
     The foregoing description of several methods and an embodiment of the invention have been presented for purposes of illustration. It is not intended to be exhaustive or to limit the invention to the precise steps and/or forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be defined by the claims appended hereto.