Abstract:
A device for preventing contamination of a print sheet and the reduction in the effectiveness of a scorotron type charger by free toner and paper particles. The device includes a paper dust eliminator extending the length of a photosensitive member and a wiping mechanism at each end of the photosensitive member for removing free toner that has accumulated there. Further, a partition that separates the developing section from the charger can be provided to further reduce free toner from attaching to the charger.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to an image forming apparatus that forms an image by transferring a developing agent onto a sheet by electrophotography. 
     2. Description of Related Art 
     In a well-known developing device, development is made by electrostatically adhering the toner held on the surface of the developing roller to an electrostatic latent image formed on the surface of the photosensitive drum. In such a developing device, toner leakage often becomes a problem. Due to the toner leakage, the inside of the developing device is smeared with toner, resulting in improper printing, or getting a user&#39;s hand or clothing dirty. 
     Above all, this tendency becomes high when a non-magnetic one-component agent is used as the toner. Because the toner is mainly held on the surface of the developing roller with the intermolecular force only, even a slightest impact or inclination causes the toner to leak from the periphery of the developing roller. 
     Generally, sealing members made of a urethane sponge or a PET film are affixed inside the developing device to prevent the toner leakage. 
     However, to form a high-precision image, a polymerized toner, whose particles are very small in diameter and have a spherical shape with high fluidity, has been generally used in recent years. Because of the high fluidity, the toner leaks from the sealing members. 
     In most cases, the toner that leaks in such manner is charged insufficiently or uncharged. Therefore, it is apt to be adhered to end portions of the photosensitive member which are out of an area for the image forming. As a result, the toner adheres to an end portion of the sheet, staining the sheet. 
     Further, the leaked toner soars into a mist and adheres to a wire of the charging device, having a detrimental influence on the discharge. The result is improper charging or the production of electric noise, and the operation of the image forming apparatus becomes unstable. 
     SUMMARY OF THE INVENTION 
     The invention overcomes the above-mentioned problems and provides an image forming apparatus that produces no deleterious effects on the image quality and the operation of the apparatus in the case of toner leakage. 
     Because toner migrates toward the ends of the photosensitive drum it is necessary to prevent material or contaminants that accumulate there from being freed so they migrate to the wire of the charging device. To preclude this, covers are provided that extend between the ends of the photosensitive drum and the ends of the wire to prevent free toner from reaching the wire. Further, scraping members are provided in the developing cartridge at each end of the photosensitive drum. The scraper members extend from compartments within the developing cartridge and catch toner that is scraped from the photosensitive drum. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the invention will be described in detail with reference to the following figures wherein; 
     FIG. 1 is a sectional view schematically showing the structure of a laser beam printer; 
     FIG. 2 is a side view of a process cartridge; 
     FIG. 3 is a sectional view of the developing device; 
     FIG. 4 is a plan view of the photosensitive device and paper dust eliminator; 
     FIG. 5A is a perspective view showing grid covers and a paper dust eliminator; 
     FIG. 5B is an enlarged view of a part C circled in FIG. 5A; 
     FIG. 5C is a side elevation of the grid cover and the paper dust eliminator when viewed from the direction A of FIG. 5A; 
     FIG. 5D is an enlarged plan view showing how the grid cover is attached; 
     FIG. 6 shows removal of the process cartridge from the printer; 
     FIG. 7 shows removing the developing device from a processing cartridge; 
     FIG. 8 is a plan view showing the process cartridge when the developing device is removed therefrom; 
     FIGS. 9A and 9B are perspective views of toner chambers of the process cartridge; 
     FIG. 9C and 9D show drum wipers fixed to the toner chambers; 
     FIG. 10A is a sectional view of the process cartridge with the photosensitive drum mounted, taken along the plane of line  10 — 10  of FIG. 8; 
     FIG. 10B is an enlarged view of a part C circled in FIG. 10A; and 
     FIG. 11 shows a sectional side view of the process cartridge and a partition member therein. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A preferred embodiment of the invention will be described in detail with reference to the accompanying drawings. 
     FIG. 1 is a sectional view schematically showing the structure of a laser beam printer  1 . The laser beam printer  1  includes a feeder unit  3  for feeding paper on the bottom of a main case  2 . 
     The feeder unit  3  comprises a paper pressing plate  10  that is urged upward by a spring (not shown), a paper feed roller  11 , and a frictionally separating member  14  that is pressed against the paper feed roller  11 . A sheet of paper stacked on the paper pressing plate  10  is pressed up so as to make contact with the paper feed roller  11 . When the paper feed roller  11  is rotated in the direction of the arrow shown in FIG. 1 in a predetermined timed sequence, only the upper most sheet is separated and fed from between the paper feed roller  11  and the frictionally separating member  14 . 
     A pair of resist rollers  12 ,  13  are rotatably supported downstream with respect to a paper feed path in which the paper is fed. The resist rollers  12 ,  13  convey the paper to a transfer position formed by a photosensitive drum  20  and a transfer roller  21  in the predetermined timed sequence. 
     The photosensitive drum  20  includes an organic photosensitive member mainly composed of polycarbonate which is to be positively charged. Specifically, the photosensitive drum  20  is formed of a cylindrical aluminum sleeve as a main body and a hollow drum on the outer circumference thereof. On the hollow drum, a light conductive layer with a thickness of approximately 20 μm is formed from resin-dispersed polycarbonate. The photosensitive drum  20  is rotatably supported on the main case  2  with the cylindrical sleeve being grounded, and rotationally driven by a driving mechanism (not shown) in the direction of the arrow. 
     A charging device  30  is of the scorotron type and discharges a corona from a tungsten wire. As shown in FIG. 3, the charging device  30  includes a wire  31 , a shield  32  surrounding the wire  31 , and a grid electrode  33  provided so as to face the wire  31 . The shield  32  is integral with the grid electrode  33 . A holding portion  34  that holds the shield  32  is provided with an opening B. A charge-capturing electrode  90  is provided on the shield  32  so that it covers the opening B. One end of the charge-capturing electrode  90  extends toward a paper dust eliminator  80 . The charge-capturing electrode  90  directly receives ions discharged from the wire  31 , and becomes charged. 
     A laser scanner unit  40  includes a laser generator (not shown) that generates a laser beam L for forming an electrostatic latent image on the photosensitive drum  20 , a polygon mirror (pentahedral mirror)  41  that is rotationally driven, a pair of lenses  42 ,  45 , and reflection mirrors  43 ,  44 ,  46 . 
     A toner chamber  52  is formed within a case  51  of the developing device  50 . The toner chamber  52  accommodates an agitator  53 , which is universally pivotable around a rotating shaft  54 . The toner collected in the toner chamber  52  is a nonmagnetic, one-component developing agent that is to be positively charged. Each particle of the toner has a toner base particle of 6-10 μm in diameter and 8 μm on the average. The toner base particle is formed by adding a known coloring agent, such as carbon black, and a charge control agent, such as nigrosine, triphenylmethane, quanterary ammonium salt, to a styrene acrylic resin that is spherically formed by suspension polymerization. Silica, as an additive, is further added to the surface of the toner base particle. 
     Formed on the side of the photosensitive drum  20  is a developing chamber  55  in which a developing process is made through an opening A in the toner chamber  52 . In the developing chamber  55 , a toner supply roller  56  and a developing roller  57  are rotatably supported. The toner supply roller  56  supplies the toner, which is conveyed to the developing chamber  55  through the opening A, to the developing roller  57 . The developing roller  57  carries the toner on a surface thereof. The toner on the developing roller  57  is regulated to a predetermined thickness by a layer thickness-regulating blade  58 , which is an elastically thin metal sheet with a contact portion made of silicone rubber at a tip thereof. 
     The transfer roller  21  is rotatably supported and made from an electrically conductive foamed elastic material, such as a silicone rubber and a urethane rubber. When a voltage is applied to the transfer roller  21 , a toner image on the photosensitive drum  20  is surely transferred onto the paper. 
     A process cartridge  2   a  includes the photosensitive drum  20 , the charging device  30 , the developing device  50 , and the transfer roller  21  within a frame thereof. The process cartridge  2   a  is provided detachably to the laser printer  1 . The developing device  50  is designed as a developing cartridge detachable from the process cartridge  2   a.    
     The fixing unit  70  is provided further downstream with respect to the paper feed path where the paper is fed and passed between the resist rollers  12 ,  13  and between the photosensitive drum  20  and the transfer roller  21 . The fixing unit  70  includes a heat roller  71  and a pressing roller  72 . The toner image transferred onto the paper is heated and pressed while it is conveyed between the heat roller  71  and the pressing roller  72 , and then the toner image is fixed on the paper. 
     A pair of conveying rollers  73  are provided downstream from the fixing unit  70  in the paper feed path. A pair of paper discharge rollers  74  and a discharged paper tray  75  are provided further downstream from the conveying rollers  73 . 
     The paper dust eliminator  80  provided adjacent to the photosensitive drum  20  comprises a holder  83 , a urethane sheet  82  supported to an upper portion of the holder  83  at a base end thereof and covered with the nonwoven cloth  81  at an edge thereof, a brush-like member  86  catching paper dust, and a conductive plate  84  supporting the brush-like member  86  and being mounted on the holder  83 . 
     The adopted nonwoven cloth  81  is produced from a random arrangement of fibers intertwined with each other. This is because the cloth has highly flexible fibers and preferably catches minute pieces of paper dust among the fibers. Polyester or polyamide material is used. The nonwoven cloth  81  is impregnated with textile oil. As the textile oil, mineral oil or synthetic oil is used. The nonwoven cloth  81  is formed so that its longitudinal length is substantially equal to that of the photosensitive drum  20 . A double-sided tape is affixed to the edge of the urethane sheet  82  as a base material in the sheet form. 
     The urethane sheet  82  is made of a urethane rubber and its harness is 92 degHs (JIS K-6301). The urethane sheet  82  is positioned so that its edge is pressed into contact with the photosensitive drum  20 . 
     The holder  83  is formed so as to have the same length as the photosensitive drum  20  in the longitudinal direction. Both ends of the holder  83  in the longitudinal direction are secured to a frame supporting the photosensitive drum  20  using screws (not shown). A paper dust chamber  88  is formed inside the holder  83 . It has an opening opposite the photosensitive drum  20 . A urethane film  87  is attached to a lower portion of the holder  83  at its base end using the double-sided tape, and makes contact with the photosensitive drum  20  at its free end. The urethane film  87  helps to prevent paper dust from falling out of the chamber  88  in the holder  83 . 
     The brush-like member  86  is a sheet on which fibers are transplanted. As shown in FIG. 3, it is provided on the conductive plate  84  made of aluminum. The plate  84  is mounted on the holder  83 , and the urethane sheet  82  is affixed to the plate  84 . 
     The plate  84  and the charge capturing electrode  90  are secured using a screw  85  at their ends. The charge-capturing electrode  90 , the plate  84 , and the brush-like member  86  are continuously electrically charged. As the charge capturing electrode  90  becomes charged by directly receiving ions discharged from the wire  31 , the plate  84  and the brush-like member  86 , which are electrically continuous with the charge capturing electrode  90 , also become charged. In other words, voltages can be applied to the brush-like member  86  without the need for a power source only for it. 
     As shown in FIG. 3, grid covers  100  are provided as partitioning members on an upper position of the paper dust eliminator  80 . The grid covers  100  are made of an insulating resin member, such as PET. They are formed in such a size that they cover the nonwoven cloth  81  as shown in FIG. 4 and a part of the grid electrode  33 , as shown in FIG. 3, at both ends of the photosensitive drum  20 . However, the positions of the grid covers  100  are adjusted so that they do not cover the wire  31  at their edges on the side of the grid electrode  33 . Therefore, the grid covers  100  have no detrimental effect on the charging process of the photosensitive drum  20  by the charging device  30 . 
     The grid covers  100  are affixed to the urethane sheet  82  to which the nonwoven cloth  81  is attached by using an adhesive or double-sided tape, as shown in FIGS. 5A to  5 D. FIG. 5A is a perspective view showing the grid covers  100  and the paper dust eliminator  80 . FIG. 5B is an enlarged view of a part C circled in FIG.  5 A. FIG. 5C is a side elevation of the grid cover  100  and the paper dust eliminator  80  when viewed from the direction A of FIG.  5 A. FIG. 5D is an enlarged plan view showing how the gird cover is attached. 
     The grid covers  100  are bent with a predetermined angle to an affixing portion as shown in FIGS. 5A to  5 D. As shown in FIG. 3, they are structured so as to press into contact with the grid electrode  33 . They are also structured so that a predetermined clearance is formed between each of the grid covers  100  and the nonwoven cloth  81  when the paper dust eliminator  80  is mounted to the process cartridge  2   a . Accordingly, no detrimental effect is given to the contact status of the nonwoven cloth  81  on the photosensitive drum  20 . 
     In addition, the grid covers  100  are provided at positions corresponding to both ends  20   a  of the photosensitive drum  20  which are out of the image forming area, as shown in FIG.  4 . Sealing members  112  are pressed into contact with the developing roller  57  at both ends, also out of the image forming area, to prevent the toner leakage. 
     FIG. 8 is a plan view showing the process cartridge  2   a  when the developing device  50  is removed therefrom. The photosensitive drum  20  is indicated by a double dashed chain line. 
     As shown in FIG. 8, toner chambers  60 ,  61  are formed at places corresponding to both ends of the photosensitive drum  20 . The toner chambers  60 ,  61  are a quadrangle surrounded by walls on all sides as shown in FIGS. 9A and 9B. They are formed integrally with the process cartridge  2   a.    
     Drum wipers  62 ,  63  of FIGS. 9C and 9D are affixed to the shaded areas of the toner chambers  60 ,  61  of FIGS. 9A and 9B respectively by double-sided tape. 
     The drum wipers  62 ,  63  are made of the PET film, however, they can be made of a resin film such as a urethane film. 
     When the drum wipers  62 ,  63  are affixed to the toner chambers  60 ,  61 , and the photosensitive drum  20  is mounted on the process cartridge  2   a , the drum wipers  62 ,  63  are pressed into contact with the peripheral surfaces the photosensitive drum  20  at both ends, as shown in FIG.  10 A. FIG. 10A is a sectional view of the process cartridge  2   a  with the photosensitive drum  20  mounted, taken along the plane of line  10 — 10  of FIG.  8 . 
     When the photosensitive drum  20  is mounted, as shown in FIG. 10B, the drum wiper  62  makes contact with the photosensitive drum  20  in directions opposite to one another. This is because the toner can not be effectively removed if the drum wiper  62  makes contact with the photosensitive drum  20  only in the same direction, as indicated with a double dashed chain line. 
     The drum wipers  62 ,  63  are provided at both ends of the photosensitive drum  20 , out of the image forming area, and which also correspond to both ends of the developing roller  57 . The places at which the drum wipers  62 ,  63  are provided are chosen based on a determination the toner leakage is apt to occur from both ends of the developing roller  57  of the developing device  50  and the toner transferred onto the photosensitive drum  20  is easily collected on both ends of the photosensitive drum  20 . 
     The operation of the laser beam printer I will now be described. 
     First, the surface of the photosensitive drum  20  is charged uniformly by the charging device  30 . When it is irradiated by the laser beam L, that is modulated according to image information from the scanner unit  40 , an electrostatic latent image corresponding to the image information is formed on the surface of the photosensitive drum  20 . The toner is adhered onto the electrostatic latent image in the developing device  50 , making the electrostatic latent image visible. The visible image formed on the surface of the photosensitive drum  20  is conveyed to the transfer position along with the rotation of the photosensitive drum  20 . At the transfer position, the paper is supplied via the paper feed roller  11  and the resist rollers  12 ,  13 . The transfer bias applied by the transfer roller  21  allows the visible image on the surface of the photosensitive drum  20  to be transferred onto the paper. 
     The paper onto which the visible image has been transferred is conveyed to the fixing unit  70 . The paper is sandwiched between the heat roller  71  and the pressing roller  72  to pass through therebetween, and the visible image is fixed onto the paper by the pressure and the heat. The paper onto which the visible image has been fixed is ejected to the paper tray  75  on the upper position of the laser beam printer  1  via the conveying rollers  73  and the paper discharge rollers  74 , and this completes the image formation. 
     The toner remaining on the surface of the photosensitive drum  20  after the transfer is collected to the developing device  50 , and reused for the developing. In this manner, the laser beam printer  1  adopts so-called cleaner-less developing method. Therefore, no container for waste toner scraped from the surface of the photosensitive drum  20  is needed. This enables a reducation in the size of the laser beam printer  1  and for the effective use of the toner. 
     The paper dust is accumulated on the photosensitive drum  20  after the transfer. However, big fibrous paper dust is caught by the brush-like member  86 , and minute dust is eliminated by the nonwoven cloth  81 . 
     Above all, applied to the brush-like member  86  is the voltage delivered by the charge capturing electrode  90  that becomes charged by a discharge from the wire  31  of the charging device  30 . As a result, the paper dust removing performance is extremely high. The paper dust becomes charged in a polarity opposite to that of the toner by the bias whose polarity is opposite to the toner charged during the transfer. On the other hand, the polarity of the voltage applied to the charging device  30  is the same as that of the toner. Thus, the charge-capturing electrode  90  and the brush-like member  86  have the same polarity as the toner, enabling the collection of the paper dust. 
     As described above, the laser beam printer  1  of the embodiment can apply the appropriate voltage to the brush-like member  86  without the need for a power source only for it. As a result, the manufacturing costs can be significantly reduced. 
     As such a printing operation proceeds, the toner collected in the developing device  50  is consumed. In this case, the process cartridge  2   a  in use is taken out of the front of the laser beam printer  1 , as shown in FIG. 6, and the developing device  50  is removed from the process cartridge  2   a , as shown in FIG. 7. A new developing device  50  is placed in the process cartridge  2   a , and then the process cartridge  2   a  is inserted from the front into the laser beam printer  1 . Thus, anybody can supply the toner easily without getting his or her hands dirty. Further, if the photosensitive drum  20  deteriorates, similarly as with the above case, the process cartridge  2   a  is taken out, and a new process cartridge  2   a  is installed in the laser beam printer  1 . 
     However, if a non-experienced user installs the developing device  50  as described above, oscillation and impact are given to the developing device  50 , and the toner leaks from the sealing members  112  on both ends of the developing roller  57 . Consequently, the toner undesirably remains adhered in lines, such as indicated by the dotted lines of FIG.  4 . Because the toner is not given a sufficient or normal frictional electric charge, it is transferred onto the ends  20   a  of the photosensitive drum  20  which are out of image forming area. 
     As described above, the drum wipers  62 ,  63  are positioned between a developing area where the developing roller  57  and the photosensitive drum  20  face each other and a transfer area where the photosensitive drum  20  and the transfer roller  21  face each other. If the toner should leak from the developing device  50  and transfer to the photosensitive drum  20 , the transferred toner is wiped off by the drum wipers  62 ,  63 , and dropped to the toner chambers  60 ,  61  before it is transferred onto the paper. Thus, in the transfer area, the toner that is not charged or insufficiently charged does not adhere to the ends  20   a  of the photosensitive drum  20  which are out of the image forming area, and a clean transfer is made. 
     As above, even if the toner leaks from the developing device  50 , the toner on the photosensitive drum  20  can be wiped off by the drum wipers  62 ,  63 . Therefore, smudges on a side of the paper are prevented. 
     The toner that the drum wipers  62 ,  63  can not wipe off or the toner that adheres out of the range of the drum wipers  62 ,  63  remains on the ends  20   a  of the photosensitive drum  20  and accumulates at the contact portion between the nonwoven cloth  81  and the photosensitive drum  20 , or passes through the contact portion. 
     The toner accumulating or passing through the contact portion as above spatters from the contact portion to the outside. 
     However, the contact portion is covered with the grid covers  100 , and a part of the grid electrode  33  is also covered with the grid covers  100 . If the toner spatters, it does not enter the inside of the charging device  30  or adhere to the wire  31 . As a result, abnormal discharge caused by the toner does not occur in the charging device  30 , and improper printing and noise occurrence are prevented. 
     Furthermore, a partition film  64  formed extending in the longitudinal direction of the photosensitive drum  20  can be disposed between the developing device  50  and the charging device  30  as shown in FIG.  11 . 
     The partition film  64  is made of the PET. When the partition film  64  is provided, in the unlikely event that the toner leaks from the developing device  50 , the toner is stopped at the partition film  64 , and does not reach the charging device  30 . Therefore, it does not adhere to the wire  31 . 
     As a result, improper charging by the charging device  30  does not occur, and electrical noise is not produced., Accordingly, the operation of the laser beam printer  1  becomes stable. 
     As an electrode causing a corona discharge in the charging device  30 , the wire electrode is adopted. However, an acicular electrode or a saw-tooth electrode can be adopted. 
     The invention can be applied to the charging of not only the photosensitive drum  20  but also an intermediate transfer member or paper in a color image forming apparatus. 
     The grid covers  100  made of the PET sheets are affixed, but they can be formed so that they are integral with the frame of the process cartridge  2   a.    
     The invention is not restrictive to details of the illustrated embodiment, but may be otherwise embodied with various changes or modifications in control of each portion of the apparatus, without departing from the principle of the invention.