Patent Publication Number: US-2011076053-A1

Title: Image forming apparatus and image forming method

Description:
This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Application No. 2009-225430 filed on 29 Sep. 2009, the entire content of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus and an image forming method. 
     2. Related Art 
     In an image forming apparatus using xerography, a charging device (charging portion) having a charging roller charges a photoreceptor drum (photoreceptor) and an exposure portion irradiates with light (exposes to light) a surface of the photoreceptor drum thus charged, thereby forming an electrostatic latent image (latent image) on the surface of the photoreceptor drum. In addition, a developing device (developing portion) supports toner (developer), and when a developing bias voltage is applied, the toner attaches to the electrostatic latent image to form a toner image (developed image). The toner image is primarily transferred to a transfer surface of an intermediate transfer body, and then secondarily transferred to and fixed onto paper (recording medium). 
     Here, a minute amount of toner may remain on the surface of the photoreceptor drum without being transferred to the paper and needs to be removed before subsequent image formation. The surface of the photoreceptor drum is cleaned using a cleaning blade. However, abrasion by cleaning of the surface greatly influences the lifetime of the photoreceptor drum. Given this, a configuration aiming at prolonging the lifetime of the photoreceptor drum is disclosed. 
     Hereinafter, a length in a rotational axis direction of a region to be charged (in other words, a width of the charging roller) is referred to as a charging width, a length in the rotational axis direction of a region in which development is performed (in other words, a width of the developing roller) is referred to as a developing width, and a length in the rotational axis direction of a region corresponding to a region through which the paper passes is referred to as a paper feeding width. A region outside of the paper feeding width is a range in which an image is not formed. This is because, in the region outside of the paper feeding width, the surface of the photoreceptor drum does not contact the paper, even if it is the transfer surface. In order to form a toner image also in right and left edges of the paper, the developing width needs to be greater than the paper feeding width. The charging width is set to be greater than the developing width. This is because, if the charging width is smaller than the developing width, toner would continue to attach to a region outside of the charging width. 
     In other words, by applying the developing bias voltage, although toner (including an external additive) is directed toward the region outside of the paper feeding width and inside of the developing width of the photoreceptor drum, the region corresponds to a region inside of the charging width. As a result, the toner itself does not attach to the surface of the photoreceptor drum and returns to the developing device. On the other hand, the external additive included in the toner is detached from the toner and left in the region. Thereafter, only the external additive is transferred from the surface of the photoreceptor drum to the transfer surface of the intermediate transfer body. 
     Here, cleaning of the transfer surface in a state where only the external additive has been transferred thereto is difficult with a cleaning apparatus of the intermediate transfer body. In addition, due to a slight difference in linear velocity between the intermediate transfer body and the photoreceptor drum, the region outside of the paper feeding width and inside of the developing width on the surface of the photoreceptor drum, which has contacted the transfer surface of the intermediate transfer body, is abraded with the external additive. As a result, there is a problem of reduced lifetime of the photoreceptor drum due to the abrasion of the region on the photoreceptor drum. 
     Accordingly, the abovementioned conventional art has a problem of abrasion of the surface of the photoreceptor drum by the transfer surface of the intermediate transfer body. 
     SUMMARY OF THE INVENTION 
     Given this, the present invention is made in view of the abovementioned problems to provide an image forming device that prevents abrasion of a surface of a photoreceptor drum by a transfer surface of an intermediate transfer body. 
     An image forming apparatus according to an aspect of the present invention includes: a photoreceptor; a charging portion; an exposure portion; a developing portion; a transfer body; a cleaning portion; and a control portion. The charging portion charges the photoreceptor. The exposure portion forms an electrostatic latent image on the photoreceptor by exposing the photoreceptor that has been charged. The developing portion forms a developed image on the photoreceptor by developing the electrostatic latent image thus formed by a developer containing an external additive. The developed image thus formed is primarily transferred to the transfer body and the transfer body secondarily transfers the developed image thus primarily transferred to a predetermined recording medium. The cleaning portion cleans the transfer body after the developed image is secondarily transferred to the recording medium. The control portion performs control to: form a cleaning developed image on the photoreceptor outside of a paper feeding width and inside of a developing width in a rotational axis direction of the photoreceptor, primarily transfer the cleaning developed image to the transfer body, and make the cleaning portion clean the cleaning developed image thus primarily transferred. 
     An image forming method according to an aspect of the present invention includes steps of: 
     charging a photoreceptor; 
     forming an electrostatic latent image on the photoreceptor by exposing the photoreceptor that is charged; 
     forming a developed image on the photoreceptor by developing the electrostatic latent image thus formed by a developer containing an external additive; primarily transferring the developed image thus formed to a transfer body; 
     secondarily transferring the developed image thus primarily transferred to a predetermined recording medium; 
     cleaning the transfer body after the developed image is secondarily transferred to the recording medium; 
     forming a cleaning developed image on the photoreceptor outside of a paper feeding width and inside of a developing width in a rotational axis direction of the photoreceptor; primarily transferring the cleaning developed image thus formed to the transfer body; and 
     making the cleaning portion clean the cleaning developed image thus primarily transferred. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view schematically showing a configuration of a printer according to the present embodiment; 
         FIG. 2  is a block diagram of the printer of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of the vicinity of an image forming unit of  FIG. 1 ; 
         FIG. 4  is an explanatory diagram of the widths of a photoreceptor drum, an intermediate transfer belt, a charging roller, and a developing roller of  FIG. 3 ; and 
         FIG. 5  is an explanatory diagram of a cleaning toner image to be transferred to the intermediate transfer belt of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred embodiment of the present invention is described hereinafter with reference to the drawings. 
       FIG. 1  schematically shows a configuration of a printer  1  that can perform color printing as an example of the image forming apparatus. A cross section shown in  FIG. 1  is taken from a left side face of the printer  1 . Accordingly, a front face of the printer  1  is shown on a right side and a back face thereof is shown on a left side in  FIG. 1 . 
     As shown in  FIG. 1 , an ejected paper tray  36  is provided on an upper side of an apparatus main body  2  of the printer  1 . A plurality of operation keys for various operations by a user and a front cover  5  on which a display for displaying a variety of information is provided in the vicinity of the ejected paper tray  36 . 
     In addition, a paper feeding cassette  4  is disposed on a lower side of the apparatus main body  2 . In a storage portion  40  thereof, sheets of paper as an example of the recording medium are stored in a stacked state. On an upper right side of the storage portion  40  in  FIG. 1 , a roller  46  is provided. 
     The paper is fed toward an upper right side of the paper feeding cassette  4  in  FIG. 1 . Subsequently, the paper thus fed is conveyed upward inside the apparatus main body  2 , along a front face of the printer  1 . 
     In addition, the paper feeding cassette  4  is configured to be slidable toward the front face of the printer  1 , in other words toward a right side in  FIG. 1 . In such a state of the paper feeding cassette  4  having been slid outwards, replenishment and exchange of the paper in the storage portion  40  is possible. 
     Inside of the apparatus main body  2 , a conveyance roller  10 , a resist roller  14 , an image forming portion  16  and a transfer portion  30  are disposed in this order on a downstream side of the paper feeding cassette  4  in a paper conveying direction. 
     In the image forming portion  16 , four image forming units  17  are disposed in parallel. 
     In each image forming unit  17 , a photoreceptor drum (photoreceptor)  18  is provided (see  FIGS. 1 and 3 ). The photoreceptor drum  18  is rotatably disposed. The photoreceptor drum  18  is driven clockwise in  FIGS. 1 and 3  by a drive motor (not shown). 
     The photoreceptor drum  18  of the present embodiment is formed to be 30 mm in diameter, for example. The photoreceptor drum  18  is an OPC drum having an organic layer on a surface thereof. 
     In addition, an exposure portion  15  is provided between the photoreceptor drum  18  and the paper feeding cassette  4 . Laser light is emitted from the exposure portion  15  toward each photoreceptor drum  18 . In addition, as shown in  FIGS. 1 and 3 , a charging device (charging portion)  20 , a developing device (developing portion)  24 , an intermediate transfer roller  13  and a cleaning portion  50  are provided at arbitrary positions on the periphery of each photoreceptor drum  18 . 
     The charging device  20  is positioned in a lower portion of the image forming unit  17 , as shown in  FIG. 3 . The charging device  20  includes a charging roller  21  and a friction roller  22 . The photoreceptor drum  18  contacts the charging roller  21 . The friction roller  22  is provided with a brush to abrade and clean a surface of the charging roller  21  by friction. The charging device  20  charges the surface of the photoreceptor drum  18 . The charging roller  21  is, for example, made of epichlorohydrin rubber and formed to be 12 mm in diameter. 
     In addition, the developing device  24  is disposed to the left side of the image forming unit  17 . The developing device  24  has a developing roller  25  that faces the photoreceptor drum  18 . 
     The developing roller  25  is driven counterclockwise in  FIG. 3  by a drive motor (not shown). It should be noted that a gap control roller  26  is provided in both ends of the developing roller  25  ( FIG. 4 ). The gap control roller  26  rotates according to rotation of the photoreceptor drum  18  and sets a gap between the developing roller  25  and the photoreceptor drum  18 . 
     The image forming portion  16  includes an intermediate transfer belt (transfer body)  12  made of rubber. The intermediate transfer belt  12  is disposed above each photoreceptor drum  18 . Between the intermediate transfer belt  12  and the ejected paper tray  36 , four toner containers  23  are disposed ( FIG. 1 ). The toner containers  23  are disposed in an order of magenta, cyan, yellow, and black from a back side to a front side of the printer  1 . The container for black ink is configured to be the largest in capacity. 
     The transfer portion  30  is provided with a transfer roller  31 . The transfer roller  31  is configured to be able to be in pressure-contact against the intermediate transfer belt  12  from an obliquely lower side. 
     In addition, the intermediate transfer belt  12  and the transfer roller  31  form a nip portion for transferring a toner image (developed image), which is formed by the toner (developer) supplied from the four toner containers  23 , to the paper. 
     In addition, on a downstream side of the transfer portion  30  in the paper conveying direction, a fixing portion  32 , an ejection branch portion  34  and the ejected paper tray  36  are disposed in this order. 
     In the present embodiment, a duplex printing paper path  38  is formed between the transfer portion  30  and a manual feeding tray  3 . The duplex printing paper path  38  branches off from the ejection branch portion  34  on a front face side of the apparatus main body  2 , and extends downward to connect to an upstream side of the resist roller  14 . 
     Here, a minute amount of external additive (titanium oxide, silica, alumina and the like) is added to the toner of the present embodiment. As shown in  FIG. 3 , the above-mentioned cleaning portion  50  is provided with a housing  51  having an opening directed toward the photoreceptor drum  18 , on a downstream side of a transfer position of the intermediate transfer roller  13  in a rotational direction of the photoreceptor drum  18 . The cleaning portion  50  includes a cleaning blade  52  and a toner collection portion  80  at appropriate positions in the housing  51 . 
     More specifically, the cleaning blade  52  is composed of a zinc steel plate main body fixed in a lower end of the housing  51  and a blade portion made of polyurethane rubber adhered to the main body. An edge of the blade portion of the cleaning blade  52  extends along a rotational axis of the photoreceptor drum  18 , at a position lower than the rotational axis of the photoreceptor drum  18 . The edge contacts the surface of the photoreceptor drum  18  and scrapes residual toner including the external additive and a discharge product that are attached to the surface of the photoreceptor drum  18 . 
     The residual toner and the like scraped from the surface of the photoreceptor drum  18  by the cleaning blade  52  are collected into the toner collection portion  80 . 
     More specifically, the toner collection portion  80  has a screw  88  in the vicinity of the bottom face of the housing  51 . The screw  88  is disposed on a right side of the cleaning blade  52  in  FIG. 3 . The screw  88  extends in a rotational axis direction of the photoreceptor drum  18 . The tip end of the screw  88  is connected to a drive motor (not shown). When the drive motor operates, the residual toner and the like inside the housing  51  are collected into a collection container via the screw  88 . 
     On the other hand, the abovementioned intermediate transfer belt  12  is cleaned by the cleaning portion  70 . 
     The cleaning portion  70  of the present embodiment is provided on the intermediate transfer belt  12  on an opposite side to the pressure-contact position between the intermediate transfer belt  12  and the transfer roller  31  ( FIG. 1 ). 
     More specifically, the cleaning portion  70  is provided on an upstream side in a running direction of the intermediate transfer belt  12  of the image forming unit  17  for magenta, which is disposed on a back side of the printer  1 . The cleaning portion  70  includes a cleaning roller, a blade and the like. 
     The cleaning roller is formed of a brush made of electrically conductive nylon. The cleaning roller is rotated by a drive motor (not shown), and contacts and cleans the transfer surface of the abovementioned intermediate transfer belt  12  made of rubber. 
     The cleaning roller thus removes the residual toner including the external additive adhered to the transfer surface of the intermediate transfer belt  12 , paper dust generated from the paper and the like, thereby cleaning the transfer surface of the intermediate transfer belt  12 . In addition, the blade scrapes the toner and the like by contacting the cleaning roller. The toner that is scraped away from the cleaning roller is collected into another collection container by using, for example, a feeding roller. 
     Incidentally, in the image forming unit  17  of the present embodiment, as shown in  FIG. 4 , when rotational axes are aligned, a drum width W 1  that is a width of the photoreceptor drum  18 , an intermediate transfer belt width W 2  that is a width of the intermediate transfer belt  12 , a charging width W 3  that is a width of the charging roller  21  (i.e. a length in a rotational axis direction of a region to be charged on the surface of the photoreceptor drum  18 ), a developing width W 4  that is a width of the developing roller  25  (i.e. a length in the rotational axis direction of a region in which development is performed on the surface of the photoreceptor drum  18 ), and a paper feeding width W 5  that is a length in the rotational axis direction of a region corresponding to a region through which the paper passes are not equal. 
     More specifically, as shown in  FIG. 4  illustrating a relationship between the widths of the components, the drum width W 1  shown by dashed lines in  FIG. 4  is configured to be greater than or equal to the intermediate transfer belt width W 2  shown by solid lines in  FIG. 4 . 
     In addition, the intermediate transfer belt width W 2  is configured to be greater than or equal to the charging width W 3  shown by two-dot dashed lines in  FIG. 4 . Furthermore, the charging width W 3  is configured to be greater than or equal to the developing width W 4  shown by solid lines in  FIG. 4 . This is because, if the charging width W 3  is smaller than the developing width W 4 , toner would continue to attach to a region outside of the charging width W 3 . 
     Moreover, the developing width W 4  is always configured to be greater than the paper feeding width W 5  shown by dashed-dotted lines in  FIG. 4 . This is for forming a toner image also in right and left edges of the paper. 
     In other words, on the surface of the photoreceptor drum  18 , a region outside of the paper feeding width W 5  in a rotational axis direction of the photoreceptor drum  18  does not contact the paper, even if it is the transfer surface of the intermediate transfer belt  12 . Therefore, an image is not formed in the region. 
     However, by applying a developing bias voltage, the toner including the external additive is directed also toward a region inside of the developing width W 4  shown by the solid lines in  FIG. 4  (a non-image region  76  marked by diagonal lines in  FIG. 4 ), which is a region outside of the paper feeding width W 5  shown by the dashed-dotted lines. 
     Among the toner directed toward the non-image region  76 , the toner itself does not attach to the surface of the photoreceptor drum  18  and returns to the developing device  24 . This is because the non-image region  76  corresponds to the region inside of the charging width W 3  shown by the two-dot dashed lines in  FIG. 4 . 
     On the other hand, among the toner directed toward the non-image region  76 , the external additive is detached from the toner and left in the non-image region  76 . Thereafter, only the external additive is transferred from the surface of the photoreceptor drum  18  to the transfer surface of the intermediate transfer belt  12 , and the non-image region  76  of the photoreceptor drum  18 , which contacts the transfer surface a subsequent time, is abraded by the external additive. 
     In addition, the non-image region  76  that is abraded by the external additive is formed on all the photoreceptor drums  18  for magenta, cyan, yellow, and black. 
     Given this, a new toner image is formed in the non-image region  76  in the present embodiment. This is performed in response to a drive signal from a controller (control portion)  90  shown in  FIG. 2 . 
     More specifically, the controller  90  includes a cleaning determination portion  92  and a cleaning performing portion  93 . The cleaning determination portion  92  determines whether or not having reached a predetermined number of sheets (for example, 500 sheets) for which there is concern of abrasion by the external additive. The cleaning determination portion  92  outputs a result of the determination to the cleaning performing portion  93 . 
     For example, in a case where the cleaning determination portion  92  determines that printing of 120 sheets of paper has finished, the cleaning performing portion  93  does not output a drive signal to the image forming unit  17  since the concern of abrasion by the external additive is small. 
     On the contrary, in a case where the cleaning determination portion  92  determines that printing of 500 sheets of paper has finished, for example, the cleaning performing portion  93  of the present embodiment outputs a drive signal to the image forming unit  17  for black and the cleaning portion  70 . 
     More specifically, in the image forming unit  17  for black, the charging roller  21  thereof charges the surface of the photoreceptor drum  18 . The exposure portion  15  irradiates the non-image region  76  of the photoreceptor drum  18  with laser light (exposes the photoreceptor drum  18 ) at a timing at which the toner image for printing on paper is not formed on the surface of the photoreceptor drum  18 , i.e. between an image for the present sheet of paper and an image for the next sheet of paper. In other words, the exposure portion  15  extends an irradiation range of the laser light to the region outside of the paper feeding width W 5  shown by the dashed-dotted lines in  FIG. 4 . Thereafter, the image forming unit  17  for black forms an electrostatic latent image (a cleaning electrostatic latent image) on the surface of the photoreceptor drum  18  for a solid image (a toner image formed on the entire non-image region  76 ). 
     Subsequently, the toner from the developing roller  25  is attached to the electrostatic latent image for the solid image formed on the surface of the photoreceptor drum  18 . In addition, a bias voltage that directs the toner image from the photoreceptor drum  18  to the intermediate transfer belt  12  is applied to the intermediate transfer roller  13 . As a result, on the transfer surface of the intermediate transfer belt  12 , between the image for the present sheet of paper and the image for the next sheet of paper (a range of an interpaper region L in  FIG. 5  on the transfer surface of the intermediate transfer belt  12 ), two cleaning toner images (cleaning electrostatic latent images)  78  are transferred. 
     On the other hand, a bias voltage of the reverse polarity to a bias voltage that directs the toner image from the intermediate transfer belt  12  to the transfer roller  31  is applied to the transfer roller  31  of the transfer portion  30 . 
     As a result, the cleaning toner images  78  reach the cleaning portion  70  without moving toward the transfer roller  31 . Thereafter, the cleaning toner images  78  are cleaned by the brush-shaped cleaning roller in the cleaning portion  70 . It should be noted that the cleaning roller is formed with a width greater than the developing width W 4  shown by solid lines in  FIG. 5 . 
     As shown in  FIG. 1 , when the printer  1  performs printing, the paper is fed from the paper feeding cassette  4  in a state of being separated one by one by the roller  46 . The paper thus fed reaches the resist roller  14 . The resist roller  14  waits for a timing for transfer of the toner image formed in the image forming portion  16  and feeds the paper to the transfer portion  30  at a predetermined feed timing, while correcting a skew of the paper. 
     On the other hand, an input port  91  in  FIG. 2  is configured to be able to receive image data for printing from the outside. The image data is data of various images such as letters, symbols, figures, signs, diagrams and patterns. The controller  90  controls emission of light and the like based on the data. 
     More specifically, an eraser lamp  19  is turned on for each photoreceptor drum  18  ( FIG. 3 ). Thereafter, the charging device  20  charges the surface of each photoreceptor drum  18 . Next, the exposure portion  15  irradiates the surface of the photoreceptor drum  18  with laser light. An electrostatic latent image is thus formed on the surface of each photoreceptor drum  18 . Toner images of each color are formed from the electrostatic latent image. 
     Each toner image is secondarily transferred to the paper in the transfer portion  30 . It should be noted that the residual toner on the surface of the photoreceptor drum  18  is removed by the cleaning portion  50 . In addition, the residual toner on the surface of the intermediate transfer belt  12  is removed by the abovementioned cleaning portion  70 . 
     Subsequently, the paper is fed toward the fixing portion  32  in a state of supporting an unfixed toner image. In the fixing portion  32 , the toner image is fixed by heating and pressurizing. Thereafter, the paper fed from the fixing portion  32  is ejected to the ejected paper tray  36  via an ejection roller  35 . The paper ejected to the ejected paper tray  36  is stacked in a height direction in the ejected paper tray  36 . 
     Unlike this single-side printing, in duplex printing, a conveying direction of the paper ejected from the fixing portion  32  is switched at the ejection branch portion  34 . In other words, the paper printed on one side is returned into the apparatus main body  2  and conveyed to the duplex printing paper path  38 . Next, the paper is fed toward the upstream side of the resist roller  14 . Then, the paper is fed again toward the transfer portion  30 . As a result, a toner image is transferred to an unprinted side of the paper. 
     Incidentally, the cleaning determination portion  92  and the cleaning performing portion  93  of the above-mentioned embodiment control the image forming unit  17  for black based on the number of printed sheets, such that the cleaning toner image  78  is formed on the photoreceptor drum  18 . As a result, the cleaning toner image  78  is formed by the image forming unit  17  for black. However, the cleaning toner image  78  can be formed either by any of the image forming units  17  for magenta, cyan and yellow, or sequentially by these image forming units  17 . 
     Alternatively, in addition to the control based on the number of printed sheets, the cleaning determination portion  92  and the cleaning performing portion  93  can perform control based on a result of measurement of the transfer surface of the intermediate transfer belt  12  by a color density sensor. 
     In addition, in a case where the cleaning determination portion  92  determines that a printing job is terminated, the cleaning performing portion  93  can output a drive signal to the image forming unit  17  and the cleaning portion  70 . In other words, in addition to a timing between sheets of paper, a timing after termination of a printing job also corresponds to a case of not forming the toner image for printing on the paper on the surface of the photoreceptor drum  18 . 
     As described above, according to the present embodiment, in the plurality of image forming units  17 , toner images developed by the toners of respective colors from the latent images formed on the surface of each of the photoreceptor drums  18  are respectively formed on the surface thereof. The intermediate transfer belt  12  overlaps these respective toner images on the transfer surface (primary transfer), and then transfers the toner image onto the paper (secondary transfer). Thereafter, the cleaning portion  70  cleans the toner remaining on the transfer surface. The transfer surface thus cleaned is used for the next primary transfer. 
     Here, even when the developing bias voltage is applied, the toner itself in the non-image region  76  in  FIG. 4  (a region outside of the paper feeding width W 5  and inside of the developing width W 4 ) does not attach to the surface of the photoreceptor drum  18 . 
     On the other hand, the external additive is detached from the toner and left in the non-image region  76 . Only the external additive is transferred from the surface of the photoreceptor drum  18  to the transfer surface of the intermediate transfer belt  12 . This phenomenon can be determined from a white band appearing on the transfer surface along a circumferential direction thereof. 
     In a region with the white band, abrasion performance is high. As a result, the region with the white band abrades the non-image region  76  of the photoreceptor drum  18  more than the region inside of the paper feeding width W 5 . 
     In the present embodiment, the controller  90  forms the cleaning toner image  78  on the non-image region  76 , thereby applying the toner to a region where only the external additive is present. The external additive can be cleaned by the cleaning portion  70  when the toner itself is present. Thereafter, the controller  90  transfers (primarily transfers) the cleaning toner image  78  onto the transfer surface of the intermediate transfer belt  12 . The controller  90  makes the cleaning portion  70  clean the cleaning toner image  78  without secondarily transferring onto the paper. 
     Accordingly, the surface of the photoreceptor drum  18  is not easily abraded by the external additive. This can realize prolonging of the lifetime of the photoreceptor drum  18 . As a result, superior image formation can be performed for an extended period of time and the reliability of the printer  1  can be improved. 
     In addition, the charging device  20  of contact charging type does not generate much ozone or nitrogen oxide compared to the corona discharge type charging device, thereby improving image quality. However, even in the non-image region  76  of the photoreceptor drum  18 , the charging roller  21  applies voltage to the photoreceptor drum  18  by directly contacting the surface thereof. 
     In other words, leak occurs in the non-image region  76  due to intolerance to the voltage. Therefore, an electric current leaks outwards and a central portion of the surface of the photoreceptor drum  18  is not charged. This results in irregularity in an image such as a black band in a horizontal direction. However, by forming the abovementioned cleaning toner image  78 , leaks can be avoided even when using the charging device  20  of contact charging type. 
     In addition, the surface of the photoreceptor drum  18 , which is an OPC drum, is particularly easily abraded. Accordingly, abrasion by the external additive may greatly influence the photoreceptor drum  18 . However, by using the abovementioned cleaning toner image  78 , the characteristics of the photoreceptor drum  18  can be maintained for an extended period of time and a remarkable effect can be provided. 
     Furthermore, by using the intermediate transfer belt  12  made of rubber, drop off of letters during transfer can be prevented. Since the toner itself is applied to the region where only the external additive is present, as described above, there is no risk of adhesion of the external additive. 
     In addition, since irradiation from the exposure portion  15  is extended to the region outside of the paper feeding width W 5 , a latent image for the cleaning toner image  78  can be accurately formed. 
     The present invention is not limited to the abovementioned embodiment and can be modified in various ways without departing from the scope of the claims. 
     For example, the cleaning toner image  78  for the subsequent time can be formed in an interpaper region that is different from that of the present time. More specifically, by consecutively forming the cleaning toner image  78  between an X th  sheet and an x+1 th  sheet this time, between the x+1 th  sheet and an x+2 th  sheet next time and the like, the cleaning toner image  78  moves in a running direction of the intermediate transfer belt  12 . In such a way, the cleaning toner image  78  can be formed on the entire circumference of the intermediate transfer belt  12 . In other words, since the above-mentioned white band appearing on the transfer surface along the circumferential direction thereof can be entirely covered, abrasion of the photoreceptor drum  18  can be avoided more reliably. 
     In addition, in the above embodiment, the cleaning toner image  78  is formed only in the non-image region  76 . However, in the region inside of the paper feeding width W 5  shown by the dotted-dashed lines in  FIG. 4 , belt-shaped toner images may be consecutively formed along the rotational axis direction of the photoreceptor drum  18 , in addition to the cleaning toner images  78 . 
     Furthermore, in the above embodiment, the toner image is transferred to the intermediate transfer belt  12  above the photoreceptor drum  18 ; however, the toner image can be transferred to the intermediate transfer belt  12  below the photoreceptor drum  18 . 
     Moreover, in the above embodiment, the printer  1  is exemplified as the image forming apparatus. However, the image forming apparatus according to the present invention can naturally be applied to a multi-functional printer, a copy machine, a facsimile machine and the like. 
     In any of these cases, as in the abovementioned case, an effect of avoiding abrasion of the surface of the photoreceptor drum  18  by the transfer surface of the intermediate transfer belt  12  can be exerted.