Abstract:
The present invention provides an image forming apparatus which comprises an image bearing member, a charging rotary member rotating while contacting with the image bearing member, and a slide friction sheet contacting with the charging rotary member along an axial direction of the charging rotary member for dispersing a contamination adhered to the charging rotary member.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to an image forming apparatus such as a copying machine and a printer etc, and more particularly to an image forming apparatus using a charging rotary body coming into contact with an image bearing body.  
           [0003]    2. Related Background Art  
           [0004]    An image forming apparatus such as a copying machine or a printer etc adopting an image forming method based on an electrophotographic system, has hitherto been constructed to form an image generally by a charging step of equally uniformly charging an image bearing body with electricity, a latent image forming step of writing an electrostatic latent image onto the image bearing body, a developing step of developing the electrostatic latent image with a toner, a transferring step of transferring the toner on the image bearing body onto a transfer material, a fixing step of fixing the toner on the transfer material, and a cleaning step of cleaning a residue such as transfer residual toner remaining on the image bearing body after the transferring step.  
           [0005]    In the construction described above, a disposal-toner container contains the toner collected by the cleaning step, executed after the transferring step, of cleaning the residue on the photosensitive drum such as the transfer residual toner remaining on the image bearing body.  
           [0006]    A charging unit for charging the image bearing body involves the use of a wide-spread contact charging system using a charging member brought into press-contact with the image bearing body.  
           [0007]    The contact charging system has advantages such as saving the electric power and generating just a small amount of ozone.  
           [0008]    What is predominant among the contact charging systems is a system using a charging roll in terms of a durability etc.  
           [0009]    Further, there has been in recent years proposed a cleanerless process for actualizing an extremely efficient image forming apparatus by attaining an omission of collecting the on-the-photosensitive-drum residue such as the transfer residual toner in the cleaning step described above, a down-size of the image forming apparatus and an omission of the maintenance such as disposing of the toner and so forth.  
           [0010]    In the cleanerless process, a developing device adheres the toner with a reversal developing to a portion where a surface potential is attenuated upon the photosensitive drum being exposed to the light, and collects the on-the-photosensitive-drum residue such as the transfer residual toner remaining-on non-exposed portions.  
           [0011]    More specifically, after the transferring step, the on-the-photosensitive-drum residue such as the transfer residual toner is, after passing through the charging step, collected by the developing device by power of static electricity due to a difference between the surface potential of the image bearing body and a developing bias (which is referred to as a back-contrast).  
           [0012]    [0012]FIG. 8 shows an example of the apparatus using the cleanerless process.  
           [0013]    Referring to FIG. 8, a charging roller  12  charges a photosensitive drum  1  classified as a rotary drum type electrophotographic photosensitive body serving as a charged body and an image bearing body as well.  
           [0014]    Further, the charging system involves the use of a method of applying a DC voltage of −1300 V to a core bar of the charging roller  12  and charging the photosensitive drum  1  in contact with the drum  1 , wherein a charging potential (a dark area potential) of the photosensitive drum  1  is set to −700 V.  
           [0015]    Referring again to FIG. 8, a laser beam  9  is emitted from an unillustrated exposing unit on the photosensitive drum  1  to form thereon an electrostatic latent image of image information.  
           [0016]    Subsequently, the latent image is developed with the toner by the developing roller  2  of the developing device. Simultaneously, in the present image forming apparatus incorporating no cleaning device, the developing roller  2  collects a residue  10  on the photosensitive drum  1  such as transfer residual toner remaining on the photosensitive drum  1 .  
           [0017]    The developed image is transferred onto a transfer material  5  led in between the photosensitive drum  1  and a transfer roller  4  at a proper timing from an unillustrated sheet supply unit.  
           [0018]    A layer thickness regulating member  3  composed of a urethane rubber or a metal plate, of which a proximal end is fitted to a developer container, comes into elastic-contact with the developing roller  2 , thus regulating a layer thickness of the toner on the surface of the developing roller  2  to a predetermined uniform value. An agitating member  7  provided in a toner hopper  6  serving as a container rotates, thereby supplying a predetermined quantity of toner to the developing roller  2 .  
           [0019]    The transfer material  5  passing through the transfer unit is separated from the photosensitive drum  1 , then conveyed to an unillustrated image transfer unit, and repeatedly used for forming the image.  
           [0020]    For collecting the residue  10  on the photosensitive drum  1  such as the transfer residual toner due to the back-contrast described above, it is required that the residue  10  on the photosensitive drum  1  such as the transfer residual toner charged to a positive polarity when in the transferring step, be charged to a negative polarity.  
           [0021]    It proved from an experiment that according to the method of applying only the DC voltage to the core bar of the charging roller  12  and thus charging the surface of the photosensitive drum  1  while coming into contact with the photosensitive drum  1 , the polarity of the residue  10  such as the transfer residual toner can be reversed from positive to negative to some extent during the charging step, however, a part of the residue  10  such as the transfer residual toner, which is insufficient in terms of reversion of its polarity, is ununiformly adhered (as a contaminant  11  adhered onto the charging roller) to the surface layer of the charging roller  12 . This conduces to a problem of causing a decline of charging uniformity.  
           [0022]    As described above, the contact charging system has the problem that the charging member is contaminated and declines in terms of its charging uniformity. This problem is serious in the cleanerless process.  
         SUMMARY OF THE INVENTION  
         [0023]    It is a primary object of the present invention to provide an image forming apparatus capable of preventing a charging member coming into contact with an image bearing body from being partially contaminated and effecting ununiform charging.  
           [0024]    It is another object of the present invention to provide an image forming apparatus capable of dispersing a contamination onto a charging rotary member by a slide friction sheet.  
           [0025]    It is still another object of the present invention to provide an image forming apparatus capable of reciprocating the slide friction sheet.  
           [0026]    Further objects of the present invention will be apparent in the following explanation. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    [0027]FIG. 1 is an explanatory sectional view showing a construction in a first embodiment;  
         [0028]    [0028]FIG. 2 is a view showing a portion vicinal to a slide friction member in the first embodiment;  
         [0029]    [0029]FIG. 3 is a graphic chart showing a relationship between the number of passing sheets and a quantity of contaminant adhered onto a charging roller;  
         [0030]    [0030]FIGS. 4A and 4B are graphic charts showing a profile of a charging potential in the first embodiment;  
         [0031]    [0031]FIG. 5 is an explanatory sectional view showing a construction in a second embodiment;  
         [0032]    [0032]FIG. 6 is a view showing a portion vicinal to the slide friction member in a second embodiment;  
         [0033]    [0033]FIGS. 7A, 7B,  7 C and  7 D are graphic charts showing a profile of the charging potential in the second embodiment; and  
         [0034]    [0034]FIG. 8 is an explanatory sectional view showing one example of an image forming apparatus. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]    Embodiments of the present invention will hereinafter be described with reference to the accompanying drawings.  
         [0036]    [0036]FIG. 1 is a sectional view showing a principal portion of an image forming apparatus in an embodiment of the present invention. Note that the same members as those in the example shown in FIG. 8 are marked with the like numerals, of which an explanation is omitted.  
         [0037]    The reference symbol PC designates a process cartridge defined as a unit attachable to and detachable from an apparatus body.  
         [0038]    The process cartridge PC is constructed as an integral unit including the electrophotographic photosensitive drum  1  classified as the image bearing body, the charging unit having the charging roller  8 , and the developing unit having the developing roller  2 .  
         [0039]    The image forming apparatus in first embodiment adopts, as in the example shown in FIG. 8, the cleanerless process of executing again the uniform charging without cleaning the residual toner after transferring the toner image.  
         [0040]    A developer in first embodiment is a non-magnetic one-component developer involving the use of a toner exhibiting advantages such as an excel transferability and causing a less quantity of abrasion of the photosensitive drum  1  because of having a high lubricating property when the cleaning member such as a blade and a fur brush etc cleans the residue  10  off the photosensitive drum  1  like the transfer residual toner remaining on the photosensitive drum  1  without being transferred, i.e., the toner of which a configuration is spherical and a surface is smooth.  
         [0041]    To be more specific, shape factors of the spherical toner used herein are given, wherein SF-1 is 100 to 180, and SF-2 is 100 to 140.  
         [0042]    One hundred of toner images are sampled at random by use of FE-SEM (S-800) made by Hitachi Ltd., and pieces of image information thereof are inputted to and analyzed by an image analyzer (Luze×3) made by Nicolet Japan Corporation through an interface. Then, Sf-1 and SF-2 are defined by values calculated in the formulae which follow:  
           SF -1=( M×LNG ) 2 /AREA×π/4×100  
           SF -2=( PERI ) 2 /AREA×π/4×100  
         [0043]    where AREA is the toner projection area, M×LNG is the absolute maximum length, and PERI is the peripheral length.  
         [0044]    The toner shape factor SF-1 indicates a degree of spherical shape, in which as the value becomes larger than 100, the shape becomes more undefined gradually from the sphere. SF-2 indicates a degree of ruggedness, wherein as the value becomes larger than 100, the ruggedness on the toner surface becomes more conspicuous.  
         [0045]    A method capable of manufacturing the toner may include, in addition to the manufacturing method based on a so-called pulverizing method, if within the above range of the shape factor, a method of directly generating the toner by use of a suspension polymerization method disclosed in Japanese Patent Application Laid-Open Nos.36-10231 and 59-53856, a dispersion polymerization method of directly generating the toner by using an aqueous organic solvent in which a polymer obtained as being soluble in a monomer is insoluble, and an emulsion polymerization method represented by a soap-free polymerization method of generating the toner by direct polymerization under an existence of a water soluble polarity polymerization initiator.  
         [0046]    In accordance with first embodiment, colored suspension particles having a weight average particle diameter of 7 μm are manufactured by use of the suspension polymerization method under a normal pressure or under pressurization in which the toner shape factor SF-1 can be easily controlled to 100 to 180, and SF-2 to 100 to 140, and a particulate toner exhibiting a sharp particle size distribution and having a particle diameter of 4 to 8 μm is comparatively easily obtained, wherein styrene and n-butyl acrylate are used as monomers, a metal compound salicylate is used as a charge control agent, saturated polyester is used as a polarity resin, and a coloring agent is added.  
         [0047]    Then, hydrophobic silica of 1.5 wt % is added, thus manufacturing the negative polar toner exhibiting the excel transferability and a small quantity of abrasion when cleaning the photosensitive drum  1  as described above.  
         [0048]    Note that the (magnetic/non-magnetic) pulverized toner can be used as a developer in first embodiment. The rotary drum-type electrophotographic photosensitive body (the photosensitive drum  1 ) is used as the charged body (the image bearing body).  
         [0049]    The charging roller  8  including the elastic material layer is used as the charging device. The charging system involves the use of the method of charging the photosensitive drum  1  while coming into contact therewith by applying the DC voltage of −1300 V to the core bar of the charging roller  8 , wherein the charging potential (the dark area potential) of the photosensitive drum  1  is set to −700 V.  
         [0050]    Further, a sheet-like slide-friction member  81  disposed in close proximity to the charging roller  8  is used as the device for uniformly dispersing by slide friction the residue  10  on the photosensitive drum  1  such as the transfer residual toner etc adhered ununiformly to the surface layer of the charging roller  8 . The slide friction member  81  is fixed directly to a support member  80  provided on a box body of the process cartridge PC.  
         [0051]    This slide friction sheet  81  is composed of a resinous film of which a main component is polyimide having a thickness of 50 μm.  
         [0052]    Then, the slide friction sheet  81  exclusive of its edges comes into contact with the charging roller  8 , wherein a nip having a width of approximately 0.5 mm is formed.  
         [0053]    According to the experiment, as shown in a graphic chart in FIG. 3, in the case of using the method of charging the photosensitive drum  1  while coming into contact with the drum  1  by applying the DC voltage to the core bar of the charging roller  8 , if a density of the on-the-photosensitive-drum residue  10  such as the transfer residual toner etc for one periphery of the photosensitive drum  1 , is 0.05 (mg/cm 2 ) or under, a density (mg/cm 2 ) of the adhered-to-the-charging roller contaminant  11  adhered to the surface layer of the charging roller  8  is saturated. Referring gain to FIG. 3, the axis of abscissas indicates the number of sheets passing through, and the axis of ordinates indicates the quantity of contaminant adhered to the charging roller.  
         [0054]    Referring back to FIG. 1, a laser beam  9  defined as an exposure light beam emitted by an unillustrated exposure device impinges upon the photosensitive drum  1  to form thereon an electrostatic latent image of image information. An electric potential (a bright area potential) of the surface of the photosensitive drum  1  at the exposure portion is set to −120 V.  
         [0055]    Subsequently, this latent image is developed (at a developing bias is −350 V) with the toner by the developing roller  2  of the developing device disposed coming into contact with or in close proximity to the photosensitive drum  1 . The developed image is led to a transferring unit between the photosensitive drum  1  and the transferring roller  4  at a proper timing from an unillustrated sheet supply unit, and is transferred onto the transfer material  5 .  
         [0056]    The transfer material  5  passing through the transferring unit is separated from the photosensitive drum  1  and conveyed to an unillustrated image fixing unit.  
         [0057]    The on-the-photosensitive-drum residue  10  such as the transfer residual toner etc is ununiformly adhered to the surface layer of the charging roller  8  in accordance with an image pattern etc, thus turning out to be the contaminant  11  adhered to the charging roller. The contaminant  11  adhered to the charging roller is, however, uniformly dispersed by the sheet-like slide friction member disposed in close proximity to the charging roller  8 , and adhered uniformly in the peripheral direction to the surface layer of the charging roller  8 .  
         [0058]    As a result, a charging uniformity in the peripheral direction on the surface of the photosensitive drum  1  is enhanced as shown in graphic charts in FIGS. 4A and 4B. FIG. 4A is a graphic chart showing a state of the charging potential on the photosensitive drum, corresponding to a cycle of the charging roller in the reference example shown in FIG. 8. FIG. 4B is a graphic chart showing a state of the charging potential on the photosensitive drum, corresponding to a cycle of the charging roller in the first embodiment.  
         [0059]    Further, a spot diameter of the laser beam  9  which actualizes 600 dpi is 75 to 90 μm. It proved from an experiment that there is no image disorder due to a light interception if a quantity of the on-the-photosensitive-drum residue  10  such as the transfer residual toner etc for one periphery of the photosensitive drum  1  is under 0.1 (mg/cm 2 ).  
         [0060]    After the exposure, the on-the-photosensitive-drum residue  10  such as the transfer residual toner with the negative polarity, is electrically adhered to the developing roller  2  by a back contrast (350 V), and collected into the developing device. There is reused the on-the-photosensitive-drum- 1  residue  10  such as the transfer residual toner etc which has been collected by the developing roller  2 .  
         [0061]    As discussed above, in the first embodiment, the contaminant  11  adhered to the charging roller, into which the transfer residual toner adhered ununiformly in the peripheral direction to the charging roller  8  has changed, is dispersed by the slide friction against the simply constructed slide friction member  81 , whereby it is feasible to obtain the uniform charging potential of the charging roller  8  and to uniformly charge the photosensitive drum  1 .  
         [0062]    Further, the slide friction sheet  81  in the first embodiment is structured so that the sheet surface, particularly the surface excluding its edges, comes into surface contact with the charging roller  8 .  
         [0063]    Hence, there is no step of cleaning the contaminations by the toner and sheet powdery matters on the charging roller  8 .  
         [0064]    When cleaning the surface of the charging roller  8 , there must be a large attacking force upon the charging roller, however, an influence on the charging roller is small and a durable life-span of the charging roller is long in the first embodiment.  
         [0065]    (Second Embodiment)  
         [0066]    A second embodiment of the present invention will hereinafter be discussed with reference to the accompanying drawings.  
         [0067]    [0067]FIG. 5 is an explanatory sectional view showing a construction of a principal portion of the image forming apparatus to which the present invention is applied. FIG. 6 is a view showing a portion vicinal to a slide friction member  82  as viewed in the direction  6 - 6  in FIG. 5.  
         [0068]    The developer used in the second embodiment is, as in the first embodiment, a non-magnetic one component developer, of which the spherical toner shape factor SF-1 is 100 to 180 and SF-2 is 100 to 140.  
         [0069]    Further, the same components as those in the first embodiment are marked with the like numerals, the explanation of which is omitted. The photosensitive drum  1 , the developing roller  2  and the charging roller  8  have the same constructions.  
         [0070]    In the second embodiment, however, a moving mechanism for moving the slide friction member in reciprocation with respect to the charging roller  8  in parallel to the axial direction of the photosensitive drum  1 , includes a drive member  84  and a drive support member  83  reciprocated in an arrow direction in FIG. 6 by the drive member  84 , herein the slide friction member  82  is supported by the drive support member  83 .  
         [0071]    The residue  10  on the photosensitive drum  1  such as the transfer residual toner etc is ununiformly adhered to the surface layer of the charging roller  8  in accordance with an image pattern etc, thus turning out to be the contaminant  11  adhered to the charging roller. The adhered-to-the-charging-roller contaminant  11  is, however, uniformly dispersed by the sheet-like slide friction member  82  disposed in close proximity to the charging roller  8  and reciprocated in the axial direction of the photosensitive drum  1 , and adhered uniformly in the peripheral and longitudinal directions to the surface layer of the charging roller  8 .  
         [0072]    As a result, the charging uniformity in the peripheral and longitudinal directions on the surface of the photosensitive drum  1  is enhanced as shown in FIGS. 7A to  7 D. FIGS. 7A and 7B show charging potentials at points A and B spaced away in the axial direction of the photosensitive drum  1  in FIG. 8. FIGS. 7C and 7D show charging potentials at the points A and B (see FIG. 6) spaced away in the axial direction of the photosensitive drum  1  in the second embodiment.  
         [0073]    As described above, in the second embodiment, the contaminant  11  adhered to the charging roller, into which the transfer residual toner adhered ununiformly in the axial direction as well as in the peripheral direction to the charging roller  8  has changed, is dispersed more effectively by the slide friction against the simply constructed moving slide friction member  82 , whereby it is feasible to obtain the uniform charging potential.  
         [0074]    As discussed above, according to the present invention, a long-term durability can be attained because of giving no large attacking force to the charging roller, and it is possible to prevent the charging potential from being ununiform due to a partially contaminated surface of the charging roller.