Abstract:
A method for preventing contamination of a lens assembly by charged particles on an image bearing surface in an electrophotographic printer includes providing a conductive electrode with an opening adjacent the lens assembly; charging the conductive electrode with a variable voltage power supply; and matching a voltage on the image bearing surface with the variable voltage power supply.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates in general to electrophotographic printing and in particular to preventing contamination of a lens assembly. 
       BACKGROUND OF THE INVENTION 
       [0002]    Printers are useful for producing printed images of a wide range of types. Printers print on receivers (or “imaging substrates”), such as pieces or sheets of paper or other planar media, glass, fabric, metal, or other objects. Printers typically operate using subtractive color: a substantially reflective receiver is overcoated image-wise with cyan (C), magenta (M), yellow (Y), black (K), and other colorants. Various schemes can be used to process images to be printed. Printers can operate by inkjet, electrophotography, and other processes. 
         [0003]    In the electrophotographic (EP) process, an electrostatic latent image is formed on a photoreceptor by uniformly charging the photoreceptor using a primary charger, e.g. corona or roller charger, and then optically discharging selected areas of the uniform charge to yield an electrostatic charge pattern corresponding to the desired image (a “latent image”). After the latent image is formed, charged toner particles are brought into the vicinity of the photoreceptor and are attracted to the latent image to develop the latent image into a visible image. Note that the visible image may not be visible to the naked eye depending on the composition of the toner particles, e.g., clear toner. 
         [0004]    After the latent image is developed into a visible image on the photoreceptor, a suitable receiver is brought into juxtaposition with the visible image. A suitable electric field is applied to transfer the toner particles of the visible image to the receiver to form the desired print image on the receiver. The receiver is then removed from its operative association with the photoreceptor and subjected to heat or pressure to permanently fix (“fuse”) the print image to the receiver. Plural print images, e.g., of separations of different colors, are overlaid on one receiver before fusing to form a multi-color print image on the receiver. 
         [0005]    The electrostatic transfer of the charged toner particles is rarely 100%, residual toner left on the photoreceptor can be as much as 10% of the developed image. This necessitates a cleaning step where a blade or brush mechanism mechanically removes the residual toner from the photoreceptor surface. However, this step may also not be 100% effective and small amounts of charged toner particles will remain on the photoreceptor as the photoreceptor cycles back to the beginning of another imaging sequence. As the residual toner on the photoreceptor passes under the primary charger it will accumulate more charge. See  FIG. 2 , reference  45 . Therefore, this charged toner will be more likely to contaminate surfaces near the photoreceptor under the influence of an electrostatic attractive force generated between that surface and the photoreceptor. 
         [0006]    One such area of concern for toner contamination is an LED printhead housing and lens located just after the primary charger and used to create the latent image. The housing is connected to electrical ground to create an electrostatic shield and minimize the electromagnetic interference (EMI) created by the printhead electronics. However, this grounded housing also creates a strong electric field that electrostatically attracts residual toner on the photoreceptor. See  FIG. 2 , references  46 A and  46 B. Residual toner attracted to the housing can end up contaminating the surface of the insulating lens located within an opening of the housing. This toner reduces the exposure efficiency of the printhead and, more importantly, creates a non-uniformity in the exposure that is difficult to compensate, resulting in objectionable artifacts in the print quality. 
         [0007]    U.S. Pat. No. 5,911,093 (Ohsawa) presents the problem of contamination of a corotron charger housing by residual toner on the photoreceptor as the photoreceptor passes by the corotron charger for the uniform charging of the photoreceptor process step. The contamination is prevented by applying a bias to the normally grounded charger housing. However, this solution has some drawbacks. It is well known that biasing the shell of a corotron charger effects the output of the charger. Also, biasing the charger shell can prevent contamination only because the shell itself is a conductor. The solution presented in U.S. Pat. No. 5,911,093 would not be feasible, for example, with a lens assembly made of an insulating glass or transparent plastic material. 
         [0008]    U.S. Pat. No. 4,697,914 (Hauser) discloses an electrode mounted on the housing of a development apparatus adjacent to an opening through which toner contained in the development station may escape and contaminate the photoreceptor due to a combination of aerodynamic and electrostatic forces. This electrode is electrically biased at a constant voltage, creating an electric field that prevents the toner from escaping through the opening, causing the toner to remain in the development station and not contaminate non-image areas of the photoreceptor. One or more constant voltage power supplies are added to provide this function. 
         [0009]    It is possible to use air flow to prevent contamination of the housing and lens. However this solution has significant drawbacks such as added cost, higher acoustic noise, and design complexity, particularly for retrofitting into existing printers at customer sites. It is, therefore, desirable to provide a solution to the lens contamination problem that minimizes cost and design complexity. 
       SUMMARY OF THE INVENTION 
       [0010]    According to one embodiment of the present invention a method for preventing contamination of a lens assembly by charged particles on an image bearing surface in an electrophotographic printer includes providing a conductive electrode with an opening adjacent the lens assembly; charging the conductive electrode with a variable voltage power supply; and matching a voltage on the image bearing surface with the variable voltage power supply. 
         [0011]    The electrostatic attractive force may be minimized in one of two ways: a) for new printers the housing is not grounded but connected to the grid supply for the primary charger ( FIG. 3 ), b) for existing printers in the field a part is attached to the existing grounded housing—the part has a conductive electrode not contacting the housing and electrically connected to the grid supply for the primary charger ( FIG. 4 ). The surface potential of the photoreceptor is typically within 100V of the grid voltage so the electric field between a part connected to the grid supply and the photoreceptor surface is too small to provide a significant attractive force on the residual toner remaining on the photoreceptor surface, thereby keeping the housing and printhead lens free of toner contamination. The photoreceptor surface potential is part of the color process control system, and may vary between −250 and −850 volts. The grid voltage tracks this within 100 volts such that when the grid bias is connected to the housing the attractive field is low over the full range of process control. 
         [0012]    For the creation of the latent image, the printhead lens must be transparent so as to allow efficient transmission of light to the photoreceptor over a wide dynamic range. Adding a transparent biased electrode in the optical path of the lens would add significant cost. As a low cost alternative, the housing may be modified as described above and will have a geometry such that the bias electrode forms a slot in the plane of the lens or in a plane between the lens and the photoreceptor. Ideally the width of the slot has a similar dimension or smaller than the separation between the electrode and the photoreceptor. If the width of the slot is larger than the separation between the electrode and the photoreceptor, some contamination benefit may still exist though the efficacy of the method will be reduced. 
         [0013]    These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  provides an elevational cross-section showing portions of a typical electrophotographic printer. 
           [0015]      FIG. 2  provides a close up view of the electrophotographic subsystems that are most relevant to this invention with the writer housing grounded. 
           [0016]      FIG. 3  provides a close up view of the electrophotographic subsystems that are most relevant to this invention with the writer housing electrically connected to the primary charger grid power supply. 
           [0017]      FIG. 4  provides a close up view of the electrophotographic subsystems that are most relevant to this invention with an isolated electrode structure attached to the grounded writer housing electrically and electrically connected to the primary charger grid power supply. 
           [0018]      FIG. 5 a    shows a top view of a dielectric layer in the isolated electrode structure; 
           [0019]      FIG. 5 b    shows one embodiment of an electrode placed on top of the dielectric layer shown in  FIG. 5   a;    
           [0020]      FIG. 5 c    shows another embodiment of a pair of electrodes placed on top of the dielectric layer shown in  FIG. 5   a.    
           [0021]      FIG. 6  shows a perspective view of the isolated electrode structure attached to the grounded writer housing. 
           [0022]      FIG. 7  shows a cut away perspective view of the isolated electrode structure attached to the grounded writer housing. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    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 scope of the invention. 
         [0024]    The electrophotographic (EP) printing process can be embodied in devices including printers, copiers, scanners, and facsimiles, and analog or digital devices, all of which are referred to herein as “printers.” Electrostatographic printers such as electrophotographic printers that employ toner developed on an electrophotographic receiver can be used, as can ionographic printers and copiers that do not rely upon an electrophotographic receiver. Electrophotography and ionography are types of electrostatography (printing using electrostatic fields), which is a subset of electrography (printing using electric fields). 
         [0025]      FIG. 1  is an elevational cross-section showing portions of a typical electrophotographic printer  100 . Printer  100  is adapted to produce print images, such as single-color (monochrome), CMYK, or hexachrome (six-color) images, on a receiver (multicolor images are also known as “multi-component” images). Images can include text, graphics, photos, and other types of visual content. An embodiment involves printing using an electrophotographic print engine having six sets of single-color image-producing or -printing stations or modules arranged in tandem, but more or fewer than six colors can be combined to form a print image on a given receiver. Other electrophotographic writers or printer apparatus can also be included. Various components of printer  100  are shown as rollers; other configurations are also possible, including belts. 
         [0026]    Referring to  FIG. 1 , printer  100  is an electrophotographic printing apparatus having a number of tandemly-arranged electrophotographic image-forming printing modules  31 ,  32 ,  33 ,  34 ,  35 ,  36 , also known as electrophotographic imaging subsystems. Each printing module  31 ,  32 ,  33 ,  34 ,  35 ,  36  produces a single-color toner image for transfer using a respective transfer subsystem  50  (for clarity, only one is labeled) to a receiver  42  successively moved through the modules. Receiver  42  is transported from supply unit  40 , which can include active feeding subsystems as known in the art, into printer  100 . In various embodiments, the visible image can be transferred directly from an imaging roller to a receiver  42 , or from an imaging roller to one or more transfer roller(s) or belt(s) in sequence in transfer subsystem  50 , and thence to receiver  42 . Receiver  42  is, for example, a selected section of a web of, or a cut sheet of, planar media such as paper or transparency film. 
         [0027]    Each printing module  31 ,  32 ,  33 ,  34 ,  35 ,  36  includes various components. For clarity, these are only shown in printing module  32 . Around photoreceptor  25  are arranged, ordered by the direction of rotation of photoreceptor  25 , primary charger  21 , exposure subsystem  22 , and toning station  23 . 
         [0028]    In the EP process, an electrostatic latent image is formed on photoreceptor  25  by uniformly charging photoreceptor  25  and then discharging selected areas of the uniform charge to yield an electrostatic charge pattern corresponding to the desired image (a “latent image”). Primary charger  21  produces a uniform electrostatic charge on photoreceptor  25  or its surface. Exposure subsystem  22  selectively image-wise discharges photoreceptor  25  to produce a latent image. Exposure subsystem  22  can include a laser and raster optical scanner (ROS), one or more LEDs, or a linear LED array. 
         [0029]    After the latent image is formed, charged toner particles are brought into the vicinity of photoreceptor  25  by toning station  23  and are attracted to the latent image to develop the latent image into a visible image. Note that the visible image may not be visible to the naked eye depending on the composition of the toner particles (e.g. clear toner). Toning station  23  can also be referred to as a development station. Toner can be applied to either the charged or discharged parts of the latent image. 
         [0030]    After the latent image is developed into a visible image on photoreceptor  25 , a suitable receiver  42  is brought into juxtaposition with the visible image. In transfer subsystem  50 , a suitable electric field is applied to transfer the toner particles of the visible image to receiver  42  to form the desired print image  48  on the receiver, as shown on receiver  42 A. 
         [0031]    The imaging process is typically repeated many times with reusable photoreceptors  25 . To prepare the photoreceptor for reuse after transferring the toner image to the transfer subsystem  50 , a cleaning and regeneration subsystem  24  is provided. The cleaning station can include a blade cleaner or a fiber brush cleaner. Regeneration of the photoreceptor can include charging and exposure functions and is optional. 
         [0032]    Receiver  42 A is then removed from its operative association with photoreceptor  25  and subjected to heat or pressure to permanently fix (“fuse”) print image  48  to receiver  42 A. Plural print images, e.g. of separations of different colors, are overlaid on one receiver before fusing to form a multi-color print image  48  on receiver  42 A. Receiver  42 A is shown after passing through printing module  36 . Print image  48  on receiver  42 A includes unfused toner particles. 
         [0033]    Subsequent to transfer of the respective print images  48 , overlaid in registration, one from each of the respective printing modules  31 ,  32 ,  33 ,  34 ,  35 ,  36 , receiver  42 A is advanced to a fuser  60 , i.e. a fusing or fixing assembly, to fuse print image  48  to receiver  42 A. Transport web  81  transports the print-image-carrying receivers (e.g.,  42 A) to fuser  60 , which fixes the toner particles to the respective receivers  42 A by the application of heat and pressure. The receivers  42 A are serially de-tacked from transport web  81  to permit them to feed cleanly into fuser  60 . Transport web  81  is then reconditioned for reuse at cleaning station  86  by cleaning and neutralizing the charges on the opposed surfaces of the transport web  81 . A mechanical cleaning station (not shown) for scraping or vacuuming toner off transport web  81  can also be used independently or with cleaning station  86 . The mechanical cleaning station can be disposed along transport web  81  before or after cleaning station  86  in the direction of rotation of transport web  81 . 
         [0034]    Fuser  60  includes a heated fusing roller  62  and an opposing pressure roller  64  that form a fusing nip  66  therebetween. In an embodiment, fuser  60  also includes a release fluid application substation  68  that applies release fluid, e.g. silicone oil, to fusing roller  62 . Alternatively, wax-containing toner can be used without applying release fluid to fusing roller  62 . Other embodiments of fusers, both contact and non-contact, can be employed. For example, solvent fixing uses solvents to soften the toner particles so they bond with the receiver  42 . Photoflash fusing uses short bursts of high-frequency electromagnetic radiation (e.g. ultraviolet light) to melt the toner. Radiant fixing uses lower-frequency electromagnetic radiation (e.g. infrared light) to more slowly melt the toner. Microwave fixing uses electromagnetic radiation in the microwave range to heat the receivers (primarily), thereby causing the toner particles to melt by heat conduction, so that the toner is fixed to the receiver  42 . 
         [0035]    The receivers (e.g., receiver  42 B) carrying the fused image (e.g., fused image  49 ) are transported in a series from the fuser  60  along a path either to a remote output tray  69 , or back to printing modules  31 ,  32 ,  33 ,  34 ,  35 ,  36  to create an image on the backside of the receiver (e.g., receiver  42 B), i.e. to form a duplex print. Receivers (e.g., receiver  42 B) can also be transported to any suitable output accessory. For example, an auxiliary fuser or glossing assembly can provide a clear-toner overcoat. Printer  100  can also include multiple fusers  60  to support applications such as overprinting, as known in the art. 
         [0036]    In various embodiments, between fuser  60  and output tray  69 , receiver  42 B passes through finisher  70 . Finisher  70  performs various media-handling operations, such as folding, stapling, saddle-stitching, collating, and binding. 
         [0037]    Printer  100  includes main printer apparatus logic and control unit (LCU)  99 , which receives input signals from the various sensors associated with printer  100  and sends control signals to the components of printer  100 . LCU  99  can include a microprocessor incorporating suitable look-up tables and control software executable by the LCU  99 . It can also include a field-programmable gate array (FPGA), programmable logic device (PLD), microcontroller, or other digital control system. LCU  99  can include memory for storing control software and data. Sensors associated with the fusing assembly provide appropriate signals to the LCU  99 . In response to the sensors, the LCU  99  issues command and control signals that adjust the heat or pressure within fusing nip  66  and other operating parameters of fuser  60  for receivers. This permits printer  100  to print on receivers of various thicknesses and surface finishes, such as glossy or matte. 
         [0038]    Image data for writing by printer  100  can be processed by a raster image processor (RIP; not shown), which can include a color separation screen generator or generators. The output of the RIP can be stored in frame or line buffers for transmission of the color separation print data to each of respective LED writers, e.g. for black (K), yellow (Y), magenta (M), cyan (C), and red (R), respectively. The RIP or color separation screen generator can be a part of printer  100  or remote therefrom. 
         [0039]    Various parameters of the components of a printing module (e.g., printing module  32 ) can be selected to control the operation of printer  100 . In an embodiment, primary charger  21  is a corona charger including a grid between the corona wires (not shown) and photoreceptor  25 . Voltage source  21   b  applies a voltage to grid  21   a  (shown in  FIG. 2 ) to control charging of photoreceptor  25 . In an embodiment, a voltage bias is applied to toning station  23  to control the electric field, and thus the rate of toner transfer, from toning station  23  to photoreceptor  25 . In an embodiment, a voltage is applied to a conductive base layer of photoreceptor  25  before development, that is, before toner is applied to photoreceptor  25  by toning station  23 . The applied voltage to the photoreceptor can be zero; the base layer can be grounded. This also provides control over the rate of toner deposition during development. In an embodiment, the exposure applied by exposure subsystem  22  to photoreceptor  25  is controlled by LCU  99  to produce a latent image corresponding to the desired print image. All of these parameters can be changed, as described below. 
         [0040]    Further details regarding printer  100  are provided in U.S. Pat. No. 6,608,641 (Alexandrovich et al.) and in U.S. Publication No. 2006/0133870 (Ng et al.), the disclosures of which are incorporated herein by reference. 
         [0041]      FIG. 2  provides a close up view of the electrophotographic subsystems that are most relevant to this embodiment of the invention. A photoreceptor  25  passes by a cleaning station  24 , removing most but not all of untransferred toner  44 . Subsequently, photoreceptor  25  passes under primary charger  21 , charging both photoreceptor  25  and residual toner  45 . Then photoreceptor  25  passes under LED printhead (with lens)  12  having an electrically grounded housing  14 , resulting in the attraction of some residual toner  46   a  and  46   b  to both the LED printhead (with lens)  12  and grounded housing  14 . This results in diminishing the performance of the LED printhead and negatively impacting the quality of the latent image. 
         [0042]      FIG. 3  provides a close up view of the electrophotographic subsystems that are most relevant to this embodiment of the invention with the writer housing electrically connected to the primary charger grid power supply. Similar to the process in  FIG. 2 , after passing by cleaning station  24  and primary charger  21 , the photoreceptor  25  has a charged surface as well as some charged residual toner  45 . However, unlike the configuration in  FIG. 2 , housing  14  is now electrically connected to voltage source  21   b,  in common with grid  21   a . Consequently, residual toner  45  is not attracted to either housing  14  or to LED printhead (with lens)  12  and remains on photoreceptor  25 . This embodiment of the invention is suitable for new printers. 
         [0043]    In another embodiment, suitable for retrofitting into existing printers at customer sites, an isolated electrode structure needs to be placed onto the surface of housing  14  or otherwise attached to LED printhead (with lens)  12  so as to cover grounded housing  14  and straddle the printhead lens.  FIG. 4  provides a close up view of the electrophotographic subsystems that are most relevant to this embodiment with an isolated electrode structure attached to the grounded writer housing electrically and electrically connected to the primary charger grid power supply. Similar to the process in  FIG. 2 , after passing by cleaning station  24  and primary charger  21 , the photoreceptor  25  has a charged surface as well as some charged residual toner  45 . However, unlike the configuration in  FIG. 2 , housing  14  has an isolated electrode structure  16  covering the surface facing photoreceptor  25 . Mounted on dielectric layer  17  is isolated electrode  18  now electrically connected to voltage source  21   b,  in common with grid  21   a . Consequently, residual toner  45  is not attracted to either isolated electrode  18  covering housing  14  or LED printhead (with lens)  12  and remains on photoreceptor  25 . 
         [0044]      FIG. 5 a    shows a top view of dielectric layer  17  to be place on top of a grounded housing.  FIG. 5 b    shows one embodiment in which electrode  18  consists of one part (upper) electrode  18   a  which is placed on top of dielectric layer  16 .  FIG. 5 c    shows a second embodiment in which electrode  18  consists of two part (lower) electrodes  18   b  which are placed on top of the dielectric layer shown in  FIG. 5   a.    
         [0045]    Insulating materials that may be used for dielectric layer  17  include, but are not limited to, plastic films such as polyester terephthalate (PET), polyethylene, Teflon, nylon, acetal, polycarbonate, and Delrin, 
         [0046]    Conducting materials that may be used for electrode  18  or upper electrode  18   a  and lower electrode  18   b  include, but are not limited to, metals such as steel, copper, nickel, aluminum, or conductive plastics such as carbon loaded epoxies or conductive EPDM. 
         [0047]    Methods of affixing isolated electrode structure  16  to housing  14  include, but are not limited to, adhering with a magnet, glue, double-sided tape, or other adhesive, or fastening with clips. 
         [0048]      FIG. 6  shows a perspective view of the LED housing with a cutout (shown) for the printhead with a selfoc lens (hidden from view). A dielectric layer  17  and an affixed biased electrode  18  with a cutout for printhead lens is shown. The isolated electrode structure (parts  17  and  18 ) may clip on to the edge of cutout for original housing. 
         [0049]      FIG. 7  shows a cut away perspective view of the LED housing with the cutout for the printhead and the LED printhead with a selfoc lens  12  now in view. The dielectric layer  17  and the affixed biased electrode  18  with a cutout for printhead lens is shown relative to the housing  14  of the printhead assembly. 
         [0050]    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 scope of the invention. 
       PARTS LIST 
       [0000]    
       
           12  LED printhead (with lens) 
           14  housing 
           16  isolated electrode structure 
           17  dielectric layer 
           18  isolated electrode 
           18   a  one part (upper) electrode 
           18   b  two part (lower) electrode 
           21  primary charger 
           21   a  grid 
           21   b  voltage source 
           22  exposure subsystem 
           23  toning station 
           24  cleaning station 
           25  photoreceptor 
           31  printing module 
           32  printing module 
           33  printing module 
           34  printing module 
           35  printing module 
           36  printing module 
           40  supply unit 
           42  receiver 
           42 A receiver 
           42 B receiver 
           44  untransferred toner 
           45  residual toner 
           46 A residual toner 
           46 B residual toner 
           48  print image 
           49  fused image 
           50  transfer subsystem 
           60  fuser 
           62  fusing roller 
           64  pressure roller 
           66  fusing nip 
           68  release fluid application substation 
           69  output tray 
           70  finisher 
           81  transport web 
           86  cleaning station 
           99  logic and control unit (LCU) 
           100  printer