Patent Publication Number: US-8971767-B2

Title: Charge applying member and image forming apparatus with the same

Description:
INCORPORATION BY REFERENCE 
     This application claims priority to Japanese Patent Application No. 2012-160684 filed on Jul. 19, 2012, the entire contents of which are incorporated by reference herein. 
     BACKGROUND 
     The present disclosure relates to a charge applying member configured to apply an electric charge to a member to be charged for use in an image forming apparatus, such as a copier, a printer, a facsimile machine, or a multifunction peripheral having these functions, and an image forming apparatus with the charge applying member. 
     The image forming apparatus involves previously charging a photoconductor serving as an image carrier to a predetermined potential before forming an electrostatic latent image on the photoconductor. An example of a known charging device capable of charging the surface of the photoconductor is a contact type charging device which includes: a charging member including a conductive roller and a shaft passing through the axis of the conductive roller and having electric conductivity; a pair of bearings journaling both ends of the shaft; and a feeder member making contact with one end of the shaft and capable of applying a voltage to the conductive roller. The conductive roller has an elastic layer formed on the surface thereof and the voltage can be applied to the elastic layer. In the case of using the contact type charging device, the charging member and the photoconductor rotate relative to each other with the charging member held in contact with the photoconductor, so that the charging device applies an electric charge to the photoconductor. 
     If in this charging device rust forms at sliding interfaces between the shaft and the bearings or a contact site between the shaft and the feeder member, this may cause rotation failure or electric feeding failure, resulting in charging failure. For example, if a paper sheet is heated in order to fix a toner image transferred to the paper sheet and water contained in the paper sheet is evaporated in the charging device by the heating, so that the humidity inside the charging device is increased or if the charging member is exposed to high humidity for long periods, water may penetrate the conductive roller. Conductive rollers are usually made of a special material containing, for example, a highly acidic substance. Water having penetrated the conductive roller may force the highly acidic substance in the conductive roller to ooze out of it and the highly acidic substance may adhere to the surface of the shaft. If a highly acidic substance adheres to the shaft, rust will form on a contact portion of the shaft with the conductive roller, in which case it is difficult to stop the rust from developing from the contact portion with the conductive roller to sliding portions of the shaft with the bearings and a contact site of the shaft with the feeder member. 
     Relevant techniques for reducing charging failure due to rusting are known. In an example of the known techniques, a charging member includes a conductive roller and a shaft inserted into and through the conductive roller, wherein a rust inhibitor is applied to portions of the shaft facing both ends of the conductive roller. With this structure, even if rust forms on a portion of the shaft in contact with the conductive roller, the rust inhibitor makes the rust less likely to develop to the sliding portions with the bearings and the contact site with the feeder member. 
     SUMMARY 
     A technique further improved over the above known technique is herein proposed as one aspect of the present disclosure. 
     A charge applying member according to one aspect of the present disclosure is configured to apply an electric charge to a member to be charged for use in an image forming apparatus which is configured to develop an electrostatic latent image into a toner image and transfer the toner image to a transfer member. The charge applying member according to the one aspect of the present disclosure includes a conductive roller and a shaft. 
     The conductive roller includes a first region held in contact with a member to be charged and facing an image forming region of the member to be charged and a second region disposed next to a longitudinal end of the first region and facing a region of the member to be charged located outside the image forming region and is configured to apply an electric charge to the member to be charged. 
     The shaft is formed of a metallic member inserted into the conductive roller and is configured to be given a voltage from a power supply. 
     A region plated with a metal having a lower ionization tendency than the shaft is formed as a plated region on a portion of an outer periphery of the shaft corresponding to the first and second regions and a non-plated region is formed on a part of a portion of the outer periphery of the shaft corresponding to the second region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view schematically showing the structure of an image forming apparatus including a charge applying member according to an embodiment as one aspect of the present disclosure. 
         FIG. 2  is a cross-sectional view showing a charging member for use in the image forming apparatus. 
         FIG. 3  is a plan view showing the charging member for use in the image forming apparatus. 
         FIG. 4  is a view schematically showing a transfer roller pressed against a photoconductor with a conveying belt. 
         FIG. 5  is a cross-sectional view showing a transfer roller pressed against a photoconductor and a paper sheet. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, a description will be given of a charge applying member and an image forming apparatus according to an embodiment as one aspect of the present disclosure with reference to the drawings. 
       FIG. 1  is a view schematically showing the structure of an image forming apparatus including a charge applying member according to an embodiment as one aspect of the present disclosure. The image forming apparatus  1  is a tandem color printer, wherein an organic photoconductor photosensitive material (OPC photosensitive material) is used as a photosensitive material forming photosensitive layers of rotatable photoconductors  11   a ,  11   b ,  11   c ,  11   d  and the photoconductors  11   a ,  11   b ,  11   c ,  11   d  are disposed for different colors of magenta, cyan, yellow, and black, respectively. The photoconductors  11   a  to  11   d  may be amorphous silicon photoconductors. Arranged around each photoconductor  11   a  to  11   d  are a developing device  2   a  to  2   d , an exposure unit  12 , a charging device  13   a  to  13   d , and a cleaning device  14   a  to  14   d.    
     The developing devices  2   a  to  2   d  are disposed to the right of and facing the photoconductors  11   a  to  11   d , respectively, and configured to supply toner to the photoconductors  11   a  to  11   d , respectively. The charging devices  13   a  to  13   d  are disposed upstream of the developing devices  2   a  to  2   d  in the direction of rotation of the photoconductors  11   a  to  11   d  and facing the surfaces of the photoconductors  11   a  to  11   d , respectively, and configured to charge the surfaces of the photoconductors  11   a  to  11   d , respectively. 
     The exposure unit  12  is disposed below the developing devices  2   a  to  2   d  and configured to scan-expose each photoconductor  11   a  to  11   d  based on image data, including characters and/or pictures, input from a personal computer or the like to an image input section (not shown). The exposure unit  12  includes a laser light source and a polygon mirror and also includes reflecting mirrors and lenses associated with the photoconductors  11   a  to  11   d . Laser light emitted from the laser light source is applied, via the polygon mirror, the reflecting mirrors, and the lenses, to the surfaces of the photoconductors  11   a  to  11   d  from downstream of the charging devices  13   a  to  13   d  in the direction of rotation of the photoconductors. The laser light thus applied causes electrostatic latent images to be formed on the respective surfaces of the photoconductors  11   a  to  11   d  and the electrostatic latent images are developed into toner images by the associated developing devices  2   a  to  2   d.    
     An endless intermediate transfer belt  17  is mounted around a tension roller  6 , a drive roller  25 , and a driven roller  27 . The drive roller  25  is driven into rotation by an unshown motor and the intermediate transfer belt  17  is driven to cycle by the rotation of the drive roller  25 . 
     The photoconductors  11   a  to  11   d  are arranged under and in contact with the intermediate transfer belt  17  to lie next to each other along a direction of conveyance (the direction of the arrow in  FIG. 1 ). Primary transfer rollers  26   a  to  26   d  each having a shaft and a roll face the associated photoconductors  11   a  to  11   d , respectively, with the intermediate transfer belt  17  in between and are pressed against the intermediate transfer belt  17  to thus form a primary transfer section. In the primary transfer section, the primary transfer rollers  26   a  to  26   d  to which a transfer bias has been applied cause the respective toner images on the photoconductors  11   a  to  11   d  to be sequentially transferred to the intermediate transfer belt  17  at their respective predetermined timings with the rotation of the intermediate transfer belt  17 . Thus, the toner images of four colors including magenta, cyan, yellow, and black are superimposed on the surface of the intermediate transfer belt  17  to form a single toner image thereon. 
     A secondary transfer roller  34  faces the drive roller  25  with the intermediate transfer belt  17  in between and is pressed against the intermediate transfer belt  17  to thus form a secondary transfer section. In the secondary transfer section, the secondary transfer roller  34  to which a transfer bias has been applied causes the toner image on the surface of the intermediate transfer belt  17  to be transferred to a paper sheet P. After the transfer, a belt cleaning device  31  cleans residual toner on the intermediate transfer belt  17 . 
     A paper feed cassette  32  capable of containing paper sheets P is disposed in a lower portion of the interior of the image forming apparatus  1 . A stack tray  35  is disposed to the right of the paper feed cassette  32  and configured to supply manually fed paper sheets. A first paper conveyance path  33  is disposed to the left of the paper feed cassette  32  and configured to convey a paper sheet P taken from the paper feed cassette  32  to the secondary transfer section on the intermediate transfer belt  17 . Furthermore, a second paper conveyance path  36  is disposed to the left of the stack tray  35  and configured to convey a paper sheet taken from the stack tray  35  to the secondary transfer section. A fixing section  18  and a third paper conveyance path  39  are disposed in an upper left portion of the interior of the image forming apparatus  1 . The fixing section  18  performs a fixing process for the paper sheet P on which the toner image has been formed. The third paper conveyance path  39  conveys the paper sheet P subjected to the fixing process to a paper output section  37 . 
     The paper feed cassette  32  is configured to allow paper sheets P to be reloaded therein by pulling it out to the outside of the apparatus (toward the front side of the paper plane of  FIG. 1 ). The paper sheets P contained therein can be fed out one by one to the first paper conveyance path  33  by a pickup roller  33   b  and a handling roller  33   a.    
     The first paper conveyance path  33  and the second paper conveyance path  36  join together before a registration roller pair  33   c . The registration roller pair  33   c  conveys the paper sheet P to the second transfer section in synchronization of the timing of an image forming operation on the intermediate transfer belt  17  with that of a paper feed operation. The paper sheet P conveyed to the secondary transfer section is subjected to secondary transfer of the toner image on the intermediate transfer belt  17  by the secondary transfer roller  34  to which a transfer bias has been applied, and then conveyed to the fixing section  18 . 
     The fixing section  18  includes a fixing belt configured to be heated by a heater or the like, a fixing roller in contact with the inner surface of the fixing belt, a pressure roller disposed facing and pressed against the fixing roller with the fixing belt in between, and so on. The fixing section  18  performs a fixing process by heating and pressing the paper sheet P to which the toner image has been transferred. After the toner image is fixed in the fixing section  18 , the paper sheet P, if necessary, is reversed by a fourth paper conveyance path  40 , subjected to secondary transfer of a toner image on the other side of the paper sheet by the secondary transfer roller  34 , and subjected to a fixing process in the fixing section  18 . The paper sheet on which the toner image has been fixed passes through the third paper conveyance path  39  and is then output to the paper output section  37  by an output roller pair  19   a.    
       FIGS. 2 and 3  are views showing the structure of a charging device for use in the above image forming apparatus  1 , wherein  FIG. 2  is a cross-sectional view of the charging device and  FIG. 3  is a plan view thereof. The following description is given of the structure and operation of the charging device  13   a  associated with the photoconductor  11   a  shown in  FIG. 1 . The structures and operations of the charging devices  13   b  to  13   d  are the same as those of the charging device  13   a  and, therefore, the explanation thereof is omitted. Furthermore, letters a to d representing the individual charging devices and photoconductors for different colors are also omitted and, if necessary, these individual components are described with the letters a to d annexed to the reference numerals. 
     As shown in  FIG. 2 , the charging device  13  employs a contact charging system in which a charging member  41  serving as the charge applying member is held in contact with the surface of the photoconductor  11  serving as the member to be charged to charge the photoconductor  11 . 
     The charging member  41  includes a conductive roller  42  held in contact with the surface of the photoconductor  11  and configured to apply an electric charge to the photoconductor  11 , a shaft  43  inserted into and integrated with the conductive roller  42 , and plated regions Ma covering the surface of the shaft  43 . 
     The conductive roller  42  is made of an elastic material, such as Hydrin rubber containing a chlorine-based additive, pressed against the photoconductor  11  by an urging member  45 , such as a compression coil spring, and formed of a rubber roller having a superior electric conductivity of 10 7  Ωcm or less. Therefore, the conductive roller  42  is driven into rotation with the rotation of the photoconductor  11  and given a predetermined voltage from a power supply  46  to be described later through the shaft  43  to charge the photoconductor  11  over the entire circumference. 
     The shaft  43  is made of an electrically conductive material, for example, an iron material which is likely to rust but relatively inexpensive, such as SUM, and supplied with electricity from one axial end by the power supply  46 . Furthermore, the shaft  43  is fixedly press-fitted into a hollow part of the conductive roller  42  to form an integral structure with the conductive roller  42 . A voltage is applied through the shaft  43  to the conductive roller  42 , so that the surface of the photoconductor  11  in contact with the conductive roller  42  is charged. Although the conductive roller  42  and the shaft  43  may be integrally fixed to each other in the above manner, they may be bonded together by an electrically conductive adhesive. 
     The plated regions Ma are regions plated with a metal having a lower ionization tendency than the shaft  43  and thus prevent corrosion of the shaft  43 . For example, if the shaft  43  is made of an iron material, such as SUM, the plated regions Ma are regions plated with a lower ionization tendency than iron, such as electroless nickel, silver, copper or tin. In view of the prevention of corrosion of the shaft  43 , the electric conductivity of the plated regions Ma, and the cost of the plating material, nickel plating is superior. Therefore, the plated region Ma is preferably regions plated with nickel. 
     As shown in  FIG. 3 , the surface of the shaft  43  is plated. Specifically, the conductive roller  42  is fixed on the shaft  43 , unshown bearings are disposed at both ends of the conductive roller  42 , and a connecting conductor (not shown) from the power supply  46  (see  FIG. 2 ) is disposed in contact with one end of the shaft  43 . The conductive roller  42  includes: a first region La facing an image forming region of the photoconductor  11  (see  FIG. 2 ) for forming an image; and second regions Lb disposed next to both longitudinal ends of the first region La and facing regions of the photoconductor  11  located outside the image forming region. Furthermore, the plated regions Ma are formed on surface portions of the shaft  43  corresponding to the first region La and the second regions Lb, sliding portions of the shaft  43  with the bearings, and a portion of the shaft  43  connected to the power supply  46 . 
     On the other hand, non-plated regions Mb are formed on part of the surface portions of the shaft  43  corresponding to the second regions Lb. Each non-plated region Mb is formed, over the entire circumference of the shaft  43 , with a relatively small width in the axial direction of the shaft  43 , such as 0.5 mm. In the non-plated regions Mb, the surface of the shaft  43  (i.e., the bare surface of the shaft material) faces the surface of the conductive roller  42  fixed on the shaft  43 . 
     With the above structure, if the humidity inside the image forming apparatus  1  is increased or the charging member  41  is exposed to high humidity for long periods, so that water penetrates the conductive roller  42  to make the surface of the shaft  43  susceptible to rust, rust will form concentrically on the non-plated regions Mb formed on the portions of the shaft  43  corresponding to the second regions Lb and form less on the portion of the shaft  43  corresponding to the first region La. Therefore, the first region La facing the image forming region of the photoconductor  11  is not affected by rust, so that the conductive roller  42  can be uniformly given a voltage from the power supply  46  through the shaft  43  and the image forming region of the photoconductor  11  can be uniformly charged. 
     For example, in the technique for reducing charging failure due to rusting by applying a rust inhibitor to the portions of the shaft facing both ends of the conductive roller, rust will form on the surface portion of the shaft in contact with the conductive roller but may not necessarily form evenly on it. Therefore, rusted sites on the shaft show large electric resistance owing to the rust and the electric resistance varies between the rusted sites and non-rusted sites on the shaft. Thus, an electric charge given from the charging member to the photoconductor becomes unsteady to make it difficult to uniformly charge the photoconductor. 
     In contrast, in the embodiment given by the present disclosure, rust is concentrated on the non-plated regions Mb formed on part of the shaft portions corresponding to the second regions Lb, so that the shaft portion corresponding to the first region La is less rusted. Therefore, even if rust forms on the non-plated regions Mb facing the regions of the photoconductor outside the image forming region, it is less likely that the portion of the shaft  43  facing the image forming region of the photoconductor  11  will cause variations in electric resistance owing to the presence of rusted sites and non-rusted sites. Thus, even in an environment likely to cause the shaft  43  to get rusted, charging failure of the photoconductor  11  due to rusting of the shaft  43  can be reduced, so that the image forming region of the photoconductor  11  can be uniformly charged. 
     The plating process, inclusive of the non-plated regions Mb, is implemented by applying a liquid plating masking agent (subjecting a masking treatment) to the portions of the shaft  43  corresponding to the non-plated regions Mb and then immersing the shaft  43  into a plating bath. Alternatively, the non-plated regions Mb may be formed by first plating the entire shaft  43  and then stripping the plating on predetermined surface portions of the shaft  43  with a scriber or the like. Instead of providing the non-plated regions Mb at the portions of the shaft  43  corresponding to both the second regions Lb, a single non-plated region Mb may be provided to correspond to one of the second regions Lb. Furthermore, instead of providing the non-plated region Mb over the entire circumference of the shaft  43 , the non-plated region Mb may be formed on part of the circumference of the shaft  43 , such as a half the circumference. 
     In the above embodiment, an example has been described in which the member to be charged is applied to the photoconductor  11  and the charge applying member is applied to the charging member  41 . However, the present disclosure is not limited to this combination. For example, in  FIG. 1 , the charge applying member may be the primary transfer roller  26  (transfer roller) pressed against the photoconductor  11  with the intermediate transfer belt  17  in between and configured to transfer a toner image carried on the photoconductor  11  to the intermediate transfer belt  17  by the application of a voltage and the member to be charged may be the intermediate transfer belt  17 . In this case, the formation of rust on the shaft portion corresponding to the first region La of the primary transfer roller  26  facing the image forming region of the intermediate transfer belt  17  is reduced, so that the roll of the primary transfer roller  26  can be uniformly given a voltage from the power supply through the shaft and the image forming region of the intermediate transfer belt  17  can be uniformly charged. 
     Alternatively, the charge applying member may be the secondary transfer roller  34  (transfer roller) configured to transfer a toner image carried on the intermediate transfer belt  17  to a paper sheet P serving as a recording medium by the application of a voltage. Still alternatively, shown in  FIG. 4 , the charge applying member may be a transfer roller  104  pressed against a photoconductor  101  with a conveying belt  102  carrying a paper sheet  103  in between and configured to transfer a toner image formed on the photoconductor  101  to the paper sheet  103  on the conveying belt  102  by the application of a voltage. Still alternatively, shown in  FIG. 5 , the charge applying member may be a transfer roller  203  pressed against a photoconductor  201  to form a nip therebetween and configured to transfer a toner image carried on the photoconductor  201  to a paper sheet  202  conveyed to the nip by the application of a voltage. Also in these cases, the same effects as in the above embodiment can be performed. 
     EXAMPLES 
     The following is a description of Examples 1 and 2 in which the details of the embodiment as one aspect of the present disclosure are specified and Comparative Examples 1 and 2. However, the present disclosure is not limited to the following examples only. 
     Test 1 and Test 2 were conducted using Examples 1 and 2 in which non-plated regions Mb were provided on the shaft  43  of the charging member  41  and Comparative Examples 1 and 2 in which no non-plated regions Mb were provided on the shaft  43 . Test 1 was conducted to evaluate the number of rusted sites formed on the shaft  43  and Test 2 was conducted to evaluate the number of rusted sites formed on the shaft  43  and the quality of the resultant image. 
     The conductive roller  42  of the charging member  41  used in Tests 1 and 2 was made of Hydrin rubber containing a chlorine-based additive and having an electric conductivity of 10 6  Ωm, the shaft  43  was formed of a drawn material of SUM  22 , and the surface of the shaft  43  was plated with 3 to 6 μm thick electroless nickel. 
     The charging member  41  used in Example 1 and Comparative Example 1 for Test 1 was one in which the conductive roller  42  and the shaft  43  were integrally fixed to each other. On the other hand, the charging member  41  used in Example 2 and Comparative Example 2 for Test 1 was one in which the conductive roller  42  and the shaft  43  were bonded together by a hydrophobic adhesive. In Examples 1 and 2, 0.5 mm wide, full-circumference non-plated regions Mb were formed on surface portions of the shaft  43  corresponding to the second regions Lb located at both ends of the conductive roller  42 . In Comparative Examples 1 and 2, no non-plated region Mb was formed. 
     In Test 1, each of the above charging members  41  was immersed into 4% brine for three hours, the conductive roller  42  was then removed from the charging member  41 , and the number of rusted sites on the surface portion of the shaft  43  corresponding to the first region La was visually measured. The results are shown in TABLE 1. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Number of Rusted Sites 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Example 1 
                 0 
               
               
                   
                 Example 2 
                 0 
               
               
                   
                 Comparative Example 1 
                 14 
               
               
                   
                 Comparative Example 2 
                 8 
               
               
                   
                   
               
            
           
         
       
     
     As shown in TABLE 1, in Comparative Examples 1 and 2, a large number of rusted sites were formed on the surface portion of the shaft  43  corresponding to the first region La. In Examples 1 and 2, rust formed on the non-plated regions Mb but no rust formed on the surface portion of the shaft  43  corresponding to the first region La, resulting in good results. 
     Also for Test 2, like Test 1, the charging member  41  used in Example 1 and Comparative Example 1 was one in which the conductive roller  42  and the shaft  43  were integrally fixed to each other, and the charging member  41  used in Example 2 and Comparative Example 2 was one in which the conductive roller  42  and the shaft  43  were bonded together by a hydrophobic adhesive. In Examples 1 and 2, 0.5 mm wide, full-circumference non-plated regions Mb were formed on surface portions of the shaft  43  corresponding to the second regions Lb. On the other hand, in Comparative Examples 1 and 2, no non-plated region Mb was formed. 
     In Test 2, each of the above charging members  41  was allowed to stand in a hot and humid environment of 50° C. and 90% humidity for 14 hours, then subjected to electric conduction with 1000V AC voltage at 10 kHz for 7 days, then assembled in the image forming apparatus  1 , and evaluated for the quality of the resultant image. After the image quality evaluation, the conductive roller  42  was removed from the charging member  41  and the number of rusted sites on the surface portion of the shaft  43  corresponding to the first region La was visually measured. The results are shown in TABLE 2. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Number of Rusted Sites 
                 Image Quality 
               
               
                   
               
             
            
               
                 Example 1 
                 0 
                 good 
               
               
                 Example 2 
                 0 
                 good 
               
               
                 Comparative Example 1 
                 5 
                 no good 
               
               
                 Comparative Example 2 
                 8 
                 no good 
               
               
                   
               
            
           
         
       
     
     As shown in TABLE 2, in Comparative Examples 1 and 2, rust formed on the surface portion of the shaft  43  corresponding to the first region La. In Examples 1 and 2, rust formed on the non-plated regions Mb but no rust formed on the surface portion of the shaft  43  corresponding to the first region La, resulting in good results. Furthermore, in Comparative Examples 1 and 2, rust on the surface portion corresponding to the first region La caused uneven electric resistivity on the charging member  41  and thus nonuniform charging of the photoconductor  11 , resulting in repetitive pattern-like image failure on a gray background. In Examples 1 and 2, good images were obtained. 
     The charge applying member according to the embodiment as one aspect of the present disclosure is applicable to a charge applying member configured to apply an electric charge to a member to be charged for use in an image forming apparatus, such as a copier, a printer, a facsimile machine, or a multifunction peripheral having these functions. 
     Various modifications and alterations of this disclosure will be apparent to those skilled in the art without departing from the scope and spirit of this disclosure, and it should be understood that this disclosure is not limited to the illustrative embodiments set forth herein.