JP2006163267A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent the reverse surface of paper from being stained by a patch for density detection formed outside an image region in an image forming apparatus outputting an image having uniform glossiness and maintaining reproducibility and stability of tints and shades like a photograph. <P>SOLUTION: In an image forming apparatus which has a 1st image carrier and a 2nd image carrier and forms a patch for density detection outside an image region, the patch on the 2nd image carrier is covered with transparent toner when the density of the patch for density detection is detected on the 2nd image carrier. When the density of the patch is detected on the 1st image carrier, transparent toner is transferred onto the 2nd image carrier except the image region. In an image forming apparatus which has an image carrier and a transfer material carrier and forms a patch for density detection outside an image region on the image carrier to detect density, transparent toner is previously transferred onto the transfer material carrier outside the image region. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus such as an electrophotographic copying machine or a laser printer. More specifically, the gloss on the output paper is made uniform by using a transparent toner in addition to the chromatic toner and Bk toner, and the appearance and feeling of the photograph. In addition, the present invention relates to an image forming apparatus that realizes stable image quality of an output image by forming a toner image for density detection different from that for an output image and performing density correction based on the detection result. The present invention relates to a toner image forming method.

  The following are known as conventional image forming apparatuses.

  As a conventional image forming apparatus, as shown in FIG. 12, the photosensitive drum 1 is rotationally driven in the direction of arrow A at a predetermined peripheral speed (process speed). The peripheral surface of the photosensitive drum 1 is charged to a predetermined polarity and potential by the charging means 2 (primary charging). The laser beam scanner as the image exposure means 3 outputs on-off modulated laser light corresponding to image information input from an external device such as an image scanner (not shown) or a computer, and charges the photosensitive drum 1. The processing surface is subjected to scanning exposure. By this scanning exposure, an electrostatic latent image corresponding to target image information is formed on the surface of the photosensitive drum 1.

  Developers 4Y, 4M, 4C, and 4k containing toners of yellow (Y), magenta (M), cyan (C), and black (Bk) color components are arranged in the vicinity of the photosensitive drum 1 and are based on image information. The developing device 4 to be used is selected and the developer (toner) is developed on the surface of the photosensitive drum 1 so that the electrostatic latent image is visualized as a toner image. A reversal development method is used in which toner is attached to the exposed portion of the electrostatic latent image for development.

  An endless belt-shaped intermediate transfer belt 6 is placed opposite to the photosensitive drum 1, and each color unfixed toner image on the photosensitive drum 1 formed every rotation of the intermediate transfer belt 6 is intermediate-transferred by the primary transfer means 5. A full-color image in which four unfixed toner images are superimposed on the intermediate transfer belt is obtained by sequentially performing primary transfer electrostatically on the belt 6 sequentially. On the other hand, every time one rotation of the photosensitive drum 1 after the primary transfer, the surface of the photosensitive drum 1 is cleaned by the cleaning device 11 to remove the transfer residual toner and repeatedly enters an image forming process.

  The secondary transfer roller 9 can be brought into and out of contact with the intermediate transfer belt 6 and is not brought into contact with the intermediate transfer belt 6 while the unfixed toner image is overlaid. To form a contact nip to form a secondary transfer site.

  A recording material (hereinafter referred to as “paper”) 7 is accommodated in a paper supply cassette (not shown), and a paper supply roller (not shown) is driven based on a paper supply start signal so that the paper 7 in the paper supply cassette is The sheets are fed one by one, and the paper 7 is conveyed in the direction of arrow B by the registration rollers 8. Then, the conveyance of the paper 7 is controlled by the registration roller 8 so as to synchronize with the timing at which the leading end of the toner image on the intermediate transfer belt 6 reaches the secondary transfer site.

  The paper 7 introduced into the secondary transfer site is nipped and conveyed at the secondary transfer site, and at that time, a constant voltage bias (transfer bias) controlled to a predetermined level is applied to the transfer roller 9 from the secondary transfer bias applying means. The secondary transfer roller 9 is applied with a transfer bias having a polarity opposite to that of the toner, so that the full-color image of four colors superimposed on the intermediate transfer belt 6 at the transfer site is transferred to the paper 7 at a time. An unfixed toner image is formed.

  The paper 7 to which the toner image has been transferred at the transfer portion is separated and conveyed from the intermediate transfer belt 6, conveyed and introduced into a fixing device (not shown), and subjected to a heat and pressure fixing process of the toner image.

  On the other hand, the intermediate transfer 6 after the transfer separation is subjected to the image forming process after the transfer residual toner, paper dust, and the like are cleaned by the cleaning device 12.

  However, there is a problem in that the gloss (gloss) is different between a portion where the toner image fixed on the sheet is melted and heated and does not exist. This is especially important when competing in a marketplace where you are familiar with the look and feel of photography. A solution to this problem is typically the addition of a clear toner so that the clear toner has similar gloss properties to other toners.

  For example, as shown in FIG. 13, in addition to a developer containing yellow (Y), magenta (M), and cyan (C) and black (Bk) toners of chromatic color toner, it is made transparent by melting and fixing in the vicinity of the photosensitive drum 1. As an example, the developing unit 4E including the transparent toner is disposed, and the paper 7 is covered with the transparent toner 14 regardless of the location where the toner image 15 is not present as shown in FIG. As another example, as shown in FIG. 14 (b), the amount of the transparent toner 14 deposited on the paper 7 is adjusted according to the amount of accumulated toner on the paper 7 where the toner image is present and where it is not. The paper 7 is covered. In this way, the gloss of the entire surface of the output paper is prevented from becoming non-uniform so that an image output by an image forming apparatus such as a copying machine or a laser printer has a photographic appearance.

  In addition, when competing in a market familiar with the appearance and feel of photographs, the color and reproducibility and stability of the output image are also important. In other words, when a plurality of the same images are output, it is necessary that the density of the final image is the same from the initial image.

  Therefore, the following measures are taken.

  After the photosensitive drum 1 is charged by the charging unit 2, the exposure control unit 3 exposes the density control reference pattern so that it does not overlap the image corresponding to the image information input from the external device such as the image scanner / computer. An electrostatic latent image is formed, and this electrostatic latent image is developed by the developing device 4 to form a density control reference pattern toner image 16 (hereinafter referred to as a patch).

  The toner image of the patch 16 is transferred onto the intermediate transfer belt 6 by the primary transfer roller 5, the density is detected by the patch sensor 17, and the toner density on the photosensitive drum 1 is controlled based on the output signal of the patch sensor 17. The density of the toner image on the paper is made constant.

  When a plurality of images are output continuously, a patch 16 is formed at a position corresponding to the interval between sheets, and density correction is performed in parallel with the formation of the output image. In this way, when a plurality of the same images are output, control is performed so that the density of the first image to the last image looks the same. Further, between the sheets, the secondary transfer roller 9 is separated from the intermediate transfer belt 9 so that the patch 16 does not adhere, and the patch toner is collected by the cleaning device 12 together with the residual secondary transfer.

  However, in recent years, due to demands for improving productivity in the market, the toner image transport speed has increased, the intermediate transfer belt 6 and the secondary transfer roller 9 are separated from each other, and then the next image reaches the secondary transfer site. In addition, the time for contact with the intermediate transfer belt 6 again is insufficient, and the patch 16 adheres to the transfer roller 9, which causes a problem that the back side of the paper 7 to be conveyed next is stained.

  Therefore, as in Patent Document 1, when the unfixed toner image of the full-color image on the intermediate transfer belt 6 passes through the transfer portion together with the paper 7, a transfer bias (forward bias) is applied to the secondary transfer roller 9, and an intermediate Patch 16 on the transfer belt 6 When the toner passes through the secondary transfer site, a bias of reverse polarity to the transfer bias (reverse bias) is applied to the secondary transfer roller 9 to patch the secondary transfer roller 9 16 Toner was prevented from adhering.

Such an image forming apparatus is conventionally known.
JP 7-253730 A

  However, a part of the patch 16 toner that comes into contact with the surface of the secondary transfer roller 9 adheres to the secondary transfer roller 9 due to the boundary force, and adheres to the back of the paper 7 that has been transported again, resulting in back soiling. There was a case.

  Therefore, it is conceivable to scrape off the patch 16 toner adhering to the transfer roller 9 by contacting a cleaning blade or scraper. However, since the secondary transfer roller 9 has elasticity in order to form a stable contact nip at the secondary transfer site and obtain a stable transfer electric field at the transfer site, an elastic cleaning blade or the like is stable. The blade cannot be abutted and the blade vibrates slightly, or if a scraper such as resin is abutted, the surface of the transfer roller 6 is scraped, and the contact force equivalent to the initial value cannot be maintained. A part of the adhered toner of the patch 16 could not be scraped off and rubbed through, and eventually the back side of the paper 7 was soiled.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus that does not recognize the stability of the image density, the productivity improvement, and the stain on the back of the paper.

  The image forming apparatus according to claim 1, wherein an image carrier that carries at least one charged toner image and a transparent toner image, and a transfer material sandwiched between the image carrier and the image carrier on the image carrier. In the image forming apparatus, the image area on the image carrier having transfer means for transferring the toner image to the transfer material and the density detection means for detecting the color toner density formed outside the image area on the image carrier. The transparent toner is placed on the color toner formed outside.

  An image forming apparatus according to a second aspect relates to the first aspect, wherein the image carrier and the transfer unit do not sandwich the transfer material outside the image area on the image carrier and the image carrier and the transfer unit. A bias having the same polarity as that of the color toner is applied to the transfer unit at a position corresponding to a position where only the toner is in direct contact and when the outside of the image area on the image carrier contacts the transfer unit. It is characterized by that.

  The image forming apparatus according to claim 3, comprising at least one first image carrier that carries at least one charged color toner image and a transparent toner image, and the first image carrier. A primary transfer means for sandwiching the second image carrier and primarily transferring the toner image on the first image carrier to the second image carrier, and the second image carrier, and a transfer material Secondary transfer means for transferring the toner image on the second image carrier to the transfer material, and detecting the density of the color toner image formed outside the image area on the first image carrier In the image forming apparatus having the density detecting means, the transparent toner is primarily transferred outside the image area on the second image carrier prior to the primary transfer of the color toner image.

  The image forming apparatus according to a fourth aspect relates to the third aspect, wherein the second image carrier and the second image carrier are outside the image area on the first image carrier and the second image carrier. The transfer means does not sandwich the transfer material and corresponds to a position where only the second image carrier and only the second transfer means are in direct contact with each other, and the first image carrier and the first image carrier. A bias having the same polarity as that of the color toner is applied to the first transfer unit when the outside of the image area on the second image carrier contacts.

  6. The image forming apparatus according to claim 5, wherein at least one image carrier that carries at least one charged color toner image and a transparent toner image and a transfer material are sandwiched between the image carrier and the image carrier. In the image forming apparatus comprising: a transfer member that transfers a toner image on an image carrier to the transfer material; and a density detection unit that detects the color toner density formed outside the image area on the image carrier. Prior to transfer of an image to the transfer material, the transparent toner is transferred to a position of a transfer member that directly contacts the outside of the image area on the image carrier.

  The image forming apparatus according to claim 6 relates to claim 5, wherein the image carrier and the transfer member do not sandwich the transfer material outside the image area on the image carrier, and only the image carrier and the transfer unit. A bias having the same polarity as that of the color toner is applied to the transfer member at a position corresponding to the position where the toner is in direct contact and when the outside of the image area on the image carrier contacts the transfer member. Features.

  According to the image forming apparatus of the first aspect, since the transfer unit and the color toner patch for density detection are not in direct contact, the color toner does not adhere to the transfer unit due to the boundary force or van der Waals force. The transfer means that contacts the back surface of the transfer material prevents the back surface of the transfer material from being stained with color toner.

  According to the image forming apparatus of the second aspect, with respect to the first aspect, there is a color toner patch for density detection at a position corresponding to the space between the sheets where no transfer material is present, and the same polarity as the toner between the sheets. By applying a bias, it is possible to prevent the color toner patch from stably adhering to the transfer means without hindering the transfer of the toner image to the transfer material.

  According to the image forming apparatus of claim 3, since the second image carrier and the color toner patch for density detection are not in direct contact with each other, the boundary force or van der Waals force is applied on the second image carrier. The second transfer means that does not adhere to the color toner patch, and that the second transfer means does not directly contact the color toner patch for density detection, and the second transfer means that contacts the back face of the transfer material is the back face of the transfer material. Not to get dirty.

  According to an image forming apparatus according to a fourth aspect, there is provided a color toner patch for density detection outside the image area on the first image carrier corresponding to the position between the sheets where no transfer material is present. And applying a bias of the same polarity as the toner when the second image carrier corresponding to the position between the papers where no transfer material is present contacts the first transfer means, so that the second of the toner image The color toner patch can be stably prevented from adhering to the second image carrier without hindering the primary transfer to the image carrier.

  According to the image forming apparatus of claim 5, since the transfer member and the color toner patch for density detection are not in direct contact, the color toner patch does not adhere to the transfer member due to the boundary force or van der Waals force. The transfer member in contact with the back surface of the transfer material is prevented from contaminating the back surface of the transfer material.

  According to the image forming apparatus of the sixth aspect, with respect to the fifth aspect, there is a color toner patch for density detection outside the image area on the first image carrier corresponding to the position between the paper where no transfer material is present. In addition, by applying a bias having the same polarity as that of the toner to the transfer member between the papers, the color toner patch can be prevented from stably adhering to the transfer member without hindering the transfer of the toner image to the transfer material. Can do.

  Hereinafter, the present invention will be described in detail based on embodiments shown in the accompanying drawings.

<Example 1>
FIG. 1 shows an image forming apparatus according to the first embodiment.

  Reference numeral 1 denotes a photosensitive drum (latent image carrier), which rotates in the direction of arrow A, and its surface is uniformly charged by the charging device 2. Reference numeral 3 denotes an exposure apparatus that performs exposure based on image information. An electrostatic latent image corresponding to the image information is formed on the photosensitive drum 1 by a known electrophotographic process.

  The developing devices 4Y, 4M, 4C, 4k, and 4E contain yellow (Y), magenta (M), cyan (C) and black (k) chromatic toners, and transparent toner, respectively. The aforementioned electrostatic latent image is developed by these developing devices 4Y, 4M, 4C, 4k, and 4E, and a toner image is formed on the surface of the photosensitive drum 1. A reversal development method is used in which toner is attached to the exposed portion of the electrostatic latent image for development.

  Reference numeral 6 denotes an intermediate transfer belt disposed so as to be in contact with the surface of the photosensitive drum 1, and is stretched around a plurality of stretching rollers 18 to 23 so as to rotate in the direction of arrow G. It has become. In this embodiment, the tension roller 20 is a tension roller that controls the tension of the intermediate transfer belt 6 to be constant, the tension roller 22 is a driving roller for the intermediate transfer belt 6, and the tension roller 21 is for secondary transfer. The opposite roller.

  In this embodiment, as the intermediate transfer belt 6, an appropriate amount of carbon black is contained as an antistatic agent in resins such as polyimide, polycarbonate, polyester, polypropylene, polyethylene terephthalate, acrylic, vinyl chloride, or various rubbers, A volume resistivity of 1E + 8 to 1E + 13 [Ω · cm] and a thickness of 0.07 to 0.1 [mm] are used.

  An endless belt-shaped intermediate transfer belt 6 is disposed opposite the photosensitive drum 1, and each color unfixed toner image on the photosensitive drum 1 formed every rotation of the intermediate transfer belt 6 is intermediately transferred by the primary transfer roller 5. A full-color image in which four unfixed toner images are superimposed on the intermediate transfer belt is obtained by sequentially performing primary transfer electrostatically on the belt 6 sequentially. Next, based on the image information, the transparent toner is developed on the photosensitive drum 1 so that the height of the toner layer of the output image on the intermediate transfer belt is uniform, and is superimposed on the unfixed full-color image on the intermediate transfer belt 6. Primary transfer. At this time, the transparent toner is not developed from the photosensitive drum 1 in that area so that the transparent toner is not primarily transferred onto the intermediate transfer belt 6 at a portion not corresponding to the sheet.

  On the other hand, the surface of the photosensitive drum 1 is cleaned by the cleaning device 11 for the transfer residual toner for each rotation of the photosensitive drum 1 after the primary transfer, and the image forming process is repeated. The primary transfer roller 5 described above is disposed on the back side of the intermediate transfer belt 6 at the primary transfer position of the intermediate transfer belt 6 facing the photosensitive drum 1. The toner image on the photosensitive drum 1 is primarily transferred onto the intermediate transfer belt 6 by applying a positive polarity primary transfer bias of reverse polarity. Reference numeral 11 denotes a drum cleaning device 11 for removing the toner remaining on the photosensitive drum 1 after the primary transfer.

  Further, in the present embodiment, the secondary transfer roller 9 disposed in pressure contact with the toner image carrying surface side of the intermediate transfer belt 6 is disposed at the secondary transfer position of the intermediate transfer belt 6 facing the paper 7 conveyance path. A tension roller 21 is provided on the back surface side of the transfer belt 6 and forms a counter electrode of the secondary transfer roller 9 and is grounded with the toner. The secondary transfer roller 9 is applied with a bias having a polarity opposite to that of the toner. Further, a belt cleaner 12 for removing toner remaining on the intermediate transfer belt 6 after the secondary transfer is provided on the downstream side of the secondary transfer position.

  Further, in the present embodiment, the paper 7 is sent to the secondary transfer position at a predetermined timing after being temporarily stopped by the registration roller 8, and the paper 7 after the secondary transfer is not shown. The toner is conveyed to a fixing device (not shown) by the conveying member, and the toner is melted and fixed on the paper.

  Further, after the photosensitive drum 1 is charged by the charging unit 2, the density control reference pattern does not overlap with the image corresponding to the image information input from an external device such as an image scanner / computer by the exposure of the exposure unit 3. For example, the electrostatic latent image is formed at a position corresponding to the space between the sheets, and the electrostatic latent image is developed by the developing devices 4Y, 4M, 4C, and 4k, and is then used as a density control reference pattern toner image (hereinafter referred to as a patch). Called 16). This is primarily transferred onto the intermediate transfer belt 6 and the density is detected by the patch sensor 17 disposed opposite the stretching roller 19, and the exposure amount of the exposure means 3 is adjusted based on the output signal of the patch sensor 17. The toner density on the photosensitive drum 1 is corrected and controlled to be constant.

  On the other hand, the electrostatic latent image for transparent toner formed on the photosensitive drum 1 is developed by the developing device 4E so that each color patch 16 whose density has been detected overlaps the entire surface of the patch 16 when passing through the primary transfer portion again. (Transparent toner is not primarily transferred to a location corresponding to the outside of the output image area as described above, but it is specially applied only to this location). As a result, the patch toner on the outermost layer on the intermediate transfer belt 6 becomes a transparent toner instead of a chromatic toner or Bk toner before passing through the secondary transfer portion. As the amount of the transparent toner used in the area corresponding to the space between the papers, the same amount as the maximum weight per unit area of the chromatic color or Bk toner used in the output image was used.

  For example, as shown in FIG. 2, first, an output image and a toner image of the patch 16 are formed by primary transfer. In this state, the density of the patch 16 is detected. Thereafter, the transparent toner is primarily transferred so as to cover the patch 16 during the primary transfer of the transparent toner. As a result, the patch toner on the outermost layer on the intermediate transfer belt 6 becomes a transparent toner, not a chromatic toner or Bk toner, before passing through the secondary transfer portion.

  The secondary transfer roller 9 of this embodiment is made of an elastic layer of an ion conductive solid rubber (NBR rubber) and a cored bar, has an outer diameter of 24 mm, a roller surface roughness Ra = 5.0 (μm) or less, and a resistance value. N / N (23 ° C., 50% RH) measurement A transfer roller of 1E + 6 to 1E + 8Ω with 2 kV applied was used.

  In the present embodiment, as shown in FIG. 3, the paper 7 is conveyed by the registration rollers 8 so that the leading edge of the toner image of the output image on the intermediate transfer belt 6 reaches the secondary transfer portion. In this case, a constant voltage bias (transfer bias) controlled to a predetermined level is applied to the transfer roller 9 from the secondary transfer bias applying means. Four colors superimposed on the intermediate transfer belt 6 at the transfer site by applying a transfer bias of +20 to +6000 V opposite in polarity to the toner to the secondary transfer roller 9 and flowing a secondary transfer current of +10 to +50 μA The full-color image is transferred onto the paper 7 at once, and a full-color unfixed toner image is formed on the paper. In addition, a constant voltage bias (reverse polarity with respect to the transfer bias) -10 to -1000 V controlled in a predetermined manner so as to synchronize with the timing at which the region on the intermediate transfer belt 6 corresponding to the space reaches the secondary transfer portion Applied, a current of 0 to -10 μA flows through the secondary transfer site. This suppresses the chromatic color patch toner and the transparent toner or the Bk patch toner and the transparent toner in the region on the intermediate transfer belt 6 corresponding to the position between the papers from adhering to the secondary transfer roller 9. .

  The surface layer of the transparent toner between the papers that directly contact the secondary transfer roller 9 has a boundary force due to the amount of electric charge of the paper itself, or a van der Waals force with the surface of the secondary transfer roller 9, etc. It will stick to. However, even if the second transfer of the output image conveyed next in that state is performed and the transparent toner adheres to the back of the paper 7, it is not recognized as the back dirt because it is a transparent toner.

  In this embodiment, with the configuration as described above, the toner adhering to the secondary transfer roller 9 is made transparent toner so that dirt on the back of the paper is not recognized.

<Example 2>
FIG. 4 shows an image forming apparatus according to the second embodiment.

  Reference numerals 1Y, 1M, 1C, 1k, and 1E denote photosensitive drums (latent image carriers) that rotate in the direction of arrow A, and their surfaces are uniformly charged by charging devices 2Y, 2M, 2C, 2k, and 2E. The Reference numerals 3Y, 3M, 3C, 3k, and 3E denote exposure apparatuses that perform exposure based on image information. An electrostatic latent image corresponding to image information is formed on the photosensitive drums 1Y, 1M, 1C, 1k, and 1E by a known electrophotographic process.

  The developing devices 4Y, 4M, 4C, 4k, and 4E contain yellow (Y), magenta (M), cyan (C) and black (k) chromatic toners, and transparent toner, respectively. The aforementioned electrostatic latent images are developed by these developing devices 4Y, 4M, 4C, 4k, and 4E, and toner images are formed on the respective photosensitive drums 1Y, 1M, 1C, 1k, and 1E. A reversal development method is used in which toner is attached to the exposed portion of the electrostatic latent image for development.

  Reference numeral 6 denotes an intermediate transfer belt disposed so as to be in contact with the surface of the photosensitive drum 1. The intermediate transfer belt is stretched around a plurality of stretching rollers 20, 21, 22, 24 and rotated in the direction of arrow G. It comes to move. In this embodiment, the tension roller 20 is a tension roller that controls the tension of the intermediate transfer belt 6 to be constant, the tension roller 22 is a driving roller for the intermediate transfer belt 6, and the tension roller 21 is for secondary transfer. The opposite roller.

  In this embodiment, as the intermediate transfer belt 6, an appropriate amount of carbon black is contained as an antistatic agent in resins such as polyimide, polycarbonate, polyester, polypropylene, polyethylene terephthalate, acrylic, vinyl chloride, or various rubbers, A volume resistivity of 1E + 8 to 1E + 13 [Ω · cm] and a thickness of 0.07 to 0.1 [mm] are used.

  An endless belt-like intermediate transfer belt 6 is disposed opposite to the photosensitive drum 1, and an unfixed toner image on the photosensitive drum 1Y is electrostatically primary transferred onto the intermediate transfer belt 6 by the primary transfer roller 5Y, and then The unfixed toner images on the photosensitive drums 1M, 1C, and 1k are sequentially primary-transferred so as to be superimposed, thereby obtaining a full color image in which the four color unfixed toner images are superimposed on the intermediate transfer belt. Next, based on the image information, the transparent toner is developed on the photosensitive drum 1E so that the height of the toner layer of the output image on the intermediate transfer belt is uniform, and is superimposed on the unfixed full-color image on the intermediate transfer belt 6. Primary transfer. At this time, the transparent toner is not developed on the photosensitive drum 1E on the photosensitive drum 1E so that the transparent toner is not primarily transferred onto the intermediate transfer belt 6 at a location not corresponding to the area corresponding to the sheet.

  On the other hand, the surface of the photosensitive drum 1 is cleaned by the cleaning device 11 for the transfer residual toner for each rotation of the photosensitive drum 1 after the primary transfer, and the image forming process is repeated. The primary transfer rollers 5Y, 5M, 5C, 5K, and 5E are arranged on the back side of the intermediate transfer belt 6 at the primary transfer positions of the intermediate transfer belt 6 facing the photosensitive drums 1Y, 1M, 1C, 1k, and 1E. The toner image on the photosensitive drum 1 is transferred onto the intermediate transfer belt 6 by primary transfer bias applied to the primary transfer roller 5 with a positive polarity primary transfer bias opposite to the toner charging polarity. It has come to be. Reference numerals 11Y, 11M, 11C, 11K, and 11E denote drum cleaning devices 11 that remove toner remaining on the photosensitive drum 1 after the primary transfer.

  Further, in the present embodiment, the secondary transfer roller 9 disposed in pressure contact with the toner image carrying surface side of the intermediate transfer belt 6 is disposed at the secondary transfer position of the intermediate transfer belt 6 facing the paper 7 conveyance path. A tension roller 21 is provided on the back surface side of the transfer belt 6 and forms a counter electrode of the secondary transfer roller 9 and is grounded with the toner. The secondary transfer roller 9 is applied with a bias having a polarity opposite to that of the toner. Further, a belt cleaner 12 for removing toner remaining on the intermediate transfer belt 6 after the secondary transfer is provided on the downstream side of the secondary transfer position.

  Further, in the present embodiment, the paper 7 is sent to the secondary transfer position at a predetermined timing after being temporarily stopped by the registration roller 8, and the paper 7 after the secondary transfer is not shown. The toner is conveyed to a fixing device (not shown) by the conveying member, and the toner is melted and fixed on the paper.

  Further, after the photosensitive drum 1 is charged by the charging unit 2, the density control reference pattern does not overlap with the image corresponding to the image information input from an external device such as an image scanner / computer by the exposure of the exposure unit 3. For example, the electrostatic latent image is formed at a position corresponding to the space between the sheets, and the electrostatic latent image is developed by the developing devices 4Y, 4M, 4C, and 4k, and is then used as a density control reference pattern toner image (hereinafter referred to as a patch). Called 16). This is primarily transferred onto the intermediate transfer belt 6 and the density is detected by the patch sensor 17 disposed opposite the stretching roller 19, and the exposure amount of the exposure means 3 is adjusted based on the output signal of the patch sensor 17. The toner density on the photosensitive drum 1 is corrected and controlled to be constant.

  On the other hand, the electrostatic latent image for transparent toner formed on the photosensitive drum 1 is developed by the developing device 4E, and each color patch 16 whose density is detected passes through the primary transfer roller 5E which is the transparent toner primary transfer portion. At this time, the primary transfer is performed so as to overlap the entire surface of the patch 16 (described above, the transparent toner is not primarily transferred to a portion corresponding to the outside of the output image area, but this is specially performed only on this portion). As a result, the patch toner on the outermost layer on the intermediate transfer belt 6 becomes a transparent toner instead of a chromatic toner or Bk toner before passing through the secondary transfer portion. As the amount of the transparent toner used in the area corresponding to the space between the papers, the same amount as the maximum weight per unit area of the chromatic color or Bk toner used in the output image was used.

  For example, as shown in FIG. 2, first, an output image and a toner image of the patch 16 are formed by primary transfer. In this state, the density of the patch 16 is detected. Thereafter, the transparent toner is primarily transferred so as to cover the patch 16 during the primary transfer of the transparent toner. As a result, the patch toner on the outermost layer on the intermediate transfer belt 6 becomes a transparent toner, not a chromatic toner or Bk toner, before passing through the secondary transfer portion.

  The secondary transfer roller 9 of this embodiment is made of an elastic layer of an ion conductive solid rubber (NBR rubber) and a cored bar, has an outer diameter of 24 mm, a roller surface roughness Ra = 5.0 (μm) or less, and a resistance value. N / N (23 ° C., 50% RH) measurement A transfer roller of 1E + 6 to 1E + 8Ω with 2 kV applied was used.

  In the present embodiment, as shown in FIG. 3, the paper 7 is conveyed by the registration rollers 8 so that the leading edge of the toner image of the output image on the intermediate transfer belt 6 reaches the secondary transfer portion. In this case, a constant voltage bias (transfer bias) controlled to a predetermined level is applied to the transfer roller 9 from the secondary transfer bias applying means. Four colors superimposed on the intermediate transfer belt 6 at the transfer site by applying a transfer bias of +20 to +6000 V opposite in polarity to the toner to the secondary transfer roller 9 and flowing a secondary transfer current of +10 to +50 μA The full color image is transferred to the paper 7 together with the transparent toner to form a full color unfixed toner image on the paper. In addition, a constant voltage bias (reverse polarity with respect to the transfer bias) -10 to -1000 V controlled in a predetermined manner so as to synchronize with the timing at which the region on the intermediate transfer belt 6 corresponding to the space reaches the secondary transfer portion Applied, a current of 0 to -10 μA flows through the secondary transfer site. This suppresses the chromatic color patch toner and the transparent toner or the Bk patch toner and the transparent toner in the region on the intermediate transfer belt 6 corresponding to the position between the papers from adhering to the secondary transfer roller 9. .

  The surface layer of the transparent toner between the papers that directly contact the secondary transfer roller 9 has a boundary force due to the amount of electric charge of the paper itself, or a van der Waals force with the surface of the secondary transfer roller 9, etc. It will stick to. However, even if the second transfer of the output image conveyed next in that state is performed and the transparent toner adheres to the back of the paper 7, it is not recognized as the back dirt because it is a transparent toner.

  In this embodiment, with the configuration as described above, the toner adhering to the secondary transfer roller 9 is made transparent toner so that dirt on the back of the paper is not recognized.

<Example 3>
FIG. 5 shows an image forming apparatus according to the third embodiment.

  Reference numeral 1 denotes a photosensitive drum (latent image carrier), which rotates in the direction of arrow A, and its surface is uniformly charged by the charging device 2. Reference numeral 3 denotes an exposure apparatus that performs exposure based on image information. An electrostatic latent image corresponding to the image information is formed on the photosensitive drum 1 by a known electrophotographic process.

  The developing devices 4Y, 4M, 4C, 4k, and 4E contain yellow (Y), magenta (M), cyan (C) and black (k) chromatic toners, and transparent toner, respectively. The aforementioned electrostatic latent image is developed by these developing devices 4Y, 4M, 4C, 4k, and 4E, and a toner image is formed on the surface of the photosensitive drum 1. A reversal development method is used in which toner is attached to the exposed portion of the electrostatic latent image for development.

  Reference numeral 6 denotes an intermediate transfer belt disposed so as to be in contact with the surface of the photosensitive drum 1, and is stretched around a plurality of stretching rollers 18 to 23 so as to rotate in the direction of arrow G. It has become. In this embodiment, the tension roller 20 is a tension roller that controls the tension of the intermediate transfer belt 6 to be constant, the tension roller 22 is a driving roller for the intermediate transfer belt 6, and the tension roller 21 is for secondary transfer. The opposite roller.

  In this embodiment, as the intermediate transfer belt 6, an appropriate amount of carbon black is contained as an antistatic agent in resins such as polyimide, polycarbonate, polyester, polypropylene, polyethylene terephthalate, acrylic, vinyl chloride, or various rubbers, A volume resistivity of 1E + 8 to 1E + 13 [Ω · cm] and a thickness of 0.07 to 0.1 [mm] are used.

  An endless belt-shaped intermediate transfer belt 6 is disposed opposite the photosensitive drum 1, and each color unfixed toner image on the photosensitive drum 1 formed every rotation of the intermediate transfer belt 6 is intermediately transferred by the primary transfer roller 5. A full-color image in which four unfixed toner images are superimposed on the intermediate transfer belt is obtained by sequentially performing primary transfer electrostatically on the belt 6 sequentially. However, the transparent toner is primarily transferred to the intermediate transfer belt 6 before the unfixed toner image of the full color image is primarily transferred to the intermediate transfer belt 6. At that time, the transparent toner is developed on the photosensitive drum 1 based on the image information so that the height of the toner layer of the output image on the intermediate transfer belt including the chromatic toner and the Bk toner is uniform, and the primary transfer is performed. Then, an unfixed full-color image that is primarily transferred is superimposed on the intermediate transfer belt 6. At this time, the transparent toner is not developed from the photosensitive drum 1 in that region so that the transparent toner is not primarily transferred onto the intermediate transfer belt 6 in a portion not corresponding to the region corresponding to the sheet.

  On the other hand, the surface of the photosensitive drum 1 is cleaned by the cleaning device 11 for the transfer residual toner for each rotation of the photosensitive drum 1 after the primary transfer, and the image forming process is repeated. The primary transfer roller 5 is made of an elastic layer of an ion conductive sponge rubber (NBR rubber) and a cored bar. The outer diameter is 16 to 20 mm, the resistance value is N / N (23 ° C., 50% RH), and 500 V is applied. 1E + 6 to 1E + 8Ω transfer roller was used. The intermediate transfer belt 6 is disposed on the back surface side of the intermediate transfer belt 6 at a primary transfer position facing the photosensitive drum 1. The primary transfer roller 5 has a positive polarity opposite to the charging polarity of the toner. By applying a primary transfer bias of +50 to +4000 V, a current of +10 to +60 μA is supplied to the primary transfer portion so that the toner image on the photosensitive drum 1 is primarily transferred onto the intermediate transfer belt 6. It has become. Reference numeral 11 denotes a drum cleaning device 11 for removing the toner remaining on the photosensitive drum 1 after the primary transfer.

  Further, in the present embodiment, the secondary transfer roller 9 disposed in pressure contact with the toner image carrying surface side of the intermediate transfer belt 6 is disposed at the secondary transfer position of the intermediate transfer belt 6 facing the paper 7 conveyance path. A tension roller 21 is provided on the back surface side of the transfer belt 6 and forms a counter electrode of the secondary transfer roller 9 and is grounded with the toner. The secondary transfer roller 9 is applied with a bias having a polarity opposite to that of the toner. Further, a belt cleaner 12 for removing toner remaining on the intermediate transfer belt 6 after the secondary transfer is provided on the downstream side of the secondary transfer position.

  Further, in the present embodiment, the paper 7 is sent to the secondary transfer position at a predetermined timing after being temporarily stopped by the registration roller 8, and the paper 7 after the secondary transfer is not shown. The toner is conveyed to a fixing device (not shown) by the conveying member, and the toner is melted and fixed on the paper.

  Further, after the photosensitive drum 1 is charged by the charging unit 2, the density control reference pattern does not overlap with the image corresponding to the image information input from an external device such as an image scanner / computer by the exposure of the exposure unit 3. For example, the electrostatic latent image is formed at a position corresponding to the space between the sheets, and the electrostatic latent image is developed by the developing devices 4Y, 4M, 4C, and 4k, and is then used as a density control reference pattern toner image (hereinafter referred to as a patch). Called 16). The density is detected by the patch sensor 17, and the exposure amount of the exposure means 3 is adjusted based on the output signal of the patch sensor 17, so that the toner density on the photosensitive drum 1 is corrected and controlled to be constant.

  However, prior to the formation of the patch 16 of chromatic color toner or Bk toner, the electrostatic latent image for transparent toner formed on the photosensitive drum 1 is developed by the developing device 4E, and an intermediate transfer belt corresponding to the gap between the sheets 6 Primary transfer is performed at the upper position (in the above description, the transparent toner is not primarily transferred to a position corresponding to the outside of the output image area. As a result, when the chromatic color toner or the Bk toner patch 16 on the photosensitive drum 1 corresponding to the position between the paper passes through the primary transfer portion, it does not come in direct contact with the intermediate transfer belt 6 but directly contacts it. What is present is a transparent toner that has been primarily transferred to the area between the sheets in advance. As the amount of the transparent toner used in the area corresponding to the space between the papers, the same amount as the maximum weight per unit area of the chromatic color or Bk toner used in the output image was used.

  For example, as shown in FIG. 6, first, a transparent toner image for output image and between paper is formed on the intermediate transfer belt 6 by primary transfer. Next, primary transfer is performed so that the chromatic color or Bk toner image of the output image formed on the photosensitive drum 1 is superimposed on the transparent toner that has been previously primary transferred. Next, the toner image of the patch 16 formed in the region on the photosensitive drum 1 corresponding to the position between the sheets and whose density is detected by the patch sensor 17 passes through the primary transfer portion. At that time, the transparent toner has already been primarily transferred to the area on the intermediate transfer belt 6 corresponding to the space between the sheets. As a result, it is not the chromatic color toner or the Bk toner patch that is in direct contact with the intermediate transfer belt 6 at a position corresponding to the space between the papers at the primary transfer site.

By the way, in the embodiment, as shown in FIG. 7, a constant voltage bias (predetermined control) is performed so as to synchronize with the timing when the region of the photosensitive drum 1 corresponding to the sheet reaches the primary transfer portion. The chromatic color in the region on the photosensitive drum 1 corresponding to the position between the papers, by applying -100 to -1500 V and applying a current of -5 to -30 μA to the primary transfer site. Alternatively, the Bk patch 16 toner is prevented from adhering to the intermediate transfer belt 6. In addition, the transparent toner that has already been primarily transferred is electrostatically returned to the photosensitive drum 1 and the amount of the transparent toner corresponding to the gap between the papers adhering to the intermediate transfer belt 6 is also reduced. The transfer roller 9 is made of an elastic layer of an ion conductive solid rubber (NBR rubber) and a cored bar, has an outer diameter of 24 mm, a roller surface roughness Ra = 5.0 (μm) or less, and a resistance value of N / N (23 ° C.). , 50% RH) measurement, and a transfer roller of 1E + 6 to 1E + 8Ω with 2 kV applied.

  In this embodiment, the conveyance of the paper 7 is controlled by the registration roller 8 so as to synchronize with the timing at which the leading edge of the toner image of the output image on the intermediate transfer belt 6 reaches the secondary transfer portion. A constant voltage bias (transfer bias) controlled to a predetermined level is applied to the transfer roller 9 from the secondary transfer bias applying means. Four colors superimposed on the intermediate transfer belt 6 at the transfer site by applying a transfer bias of +20 to +6000 V opposite in polarity to the toner to the secondary transfer roller 9 and flowing a secondary transfer current of +10 to +50 μA The full-color image is transferred to the paper 7 together with the transparent toner, and a full-color unfixed toner image is formed on the paper.

  In addition, a constant voltage bias (reverse polarity with respect to the transfer bias) -10 to -1000 V controlled in a predetermined manner so as to synchronize with the timing at which the region on the intermediate transfer belt 6 corresponding to the space reaches the secondary transfer portion Applied, a current of 0 to -10 μA flows through the secondary transfer site. As a result, the transparent toner adhering to the region on the intermediate transfer belt 6 corresponding to the position between the sheets is prevented from adhering to the secondary transfer roller 9. However, the surface layer of the transparent toner between the papers that are in direct contact with the secondary transfer roller 9 is subjected to secondary transfer due to the boundary force due to the amount of electric charge itself or the van der Waals force with the surface of the secondary transfer roller 9 etc. It will stick to the roller 9. However, even if the second transfer of the output image conveyed next in that state is performed and the transparent toner adheres to the back of the paper 7, it is not recognized as the back dirt because it is a transparent toner.

  In this embodiment, with the configuration as described above, the toner adhering to the secondary transfer roller 9 is made transparent toner so that dirt on the back of the paper is not recognized.

<Example 4>
FIG. 8 shows an image forming apparatus according to the fourth embodiment.

  Reference numerals 1Y, 1M, 1C, 1k, and 1E denote photosensitive drums (latent image carriers) that rotate in the direction of arrow A, and their surfaces are uniformly charged by charging devices 2Y, 2M, 2C, 2k, and 2E. The Reference numerals 3Y, 3M, 3C, 3k, and 3E denote exposure apparatuses that perform exposure based on image information. An electrostatic latent image corresponding to image information is formed on the photosensitive drums 1Y, 1M, 1C, 1k, and 1E by a known electrophotographic process.

  The developing devices 4Y, 4M, 4C, 4k, and 4E contain yellow (Y), magenta (M), cyan (C) and black (k) chromatic toners, and transparent toner, respectively. The aforementioned electrostatic latent images are developed by these developing devices 4Y, 4M, 4C, 4k, and 4E, and toner images are formed on the respective photosensitive drums 1Y, 1M, 1C, 1k, and 1E. A reversal development method is used in which toner is attached to the exposed portion of the electrostatic latent image for development.

  Reference numeral 6 denotes an intermediate transfer belt disposed so as to be in contact with the surface of the photosensitive drum 1. The intermediate transfer belt is stretched around a plurality of stretching rollers 20, 21, 22, 24 and rotated in the direction of arrow G. It comes to move. In this embodiment, the tension roller 20 is a tension roller that controls the tension of the intermediate transfer belt 6 to be constant, the tension roller 22 is a driving roller for the intermediate transfer belt 6, and the tension roller 21 is for secondary transfer. The opposite roller.

  In this embodiment, as the intermediate transfer belt 6, an appropriate amount of carbon black is contained as an antistatic agent in resins such as polyimide, polycarbonate, polyester, polypropylene, polyethylene terephthalate, acrylic, vinyl chloride, or various rubbers, A volume resistivity of 1E + 8 to 1E + 13 [Ω · cm] and a thickness of 0.07 to 0.1 [mm] are used.

  An endless belt-like intermediate transfer belt 6 is disposed opposite to the photosensitive drum 1, and an unfixed toner image on the photosensitive drum 1Y is electrostatically primary transferred onto the intermediate transfer belt 6 by the primary transfer roller 5Y, and then The unfixed toner images on the photosensitive drums 1M, 1C, and 1k are sequentially primary-transferred so as to be superimposed, thereby obtaining a full color image in which the four color unfixed toner images are superimposed on the intermediate transfer belt. However, the transparent toner is primarily transferred to the intermediate transfer belt 6 before the unfixed toner image of the full color image is primarily transferred to the intermediate transfer belt 6. At that time, the transparent toner is developed on the photosensitive drum 1 based on the image information so that the height of the toner layer of the output image on the intermediate transfer belt including the chromatic toner and the Bk toner is uniform, and the primary transfer is performed. Then, an unfixed full-color image that is primarily transferred is superimposed on the intermediate transfer belt 6. At this time, the transparent toner is not developed from the photosensitive drum 1 in that region so that the transparent toner is not primarily transferred onto the intermediate transfer belt 6 in a portion not corresponding to the region corresponding to the sheet.

  On the other hand, the surface of the photosensitive drum 1 is cleaned by the cleaning device 11 for the transfer residual toner for each rotation of the photosensitive drum 1 after the primary transfer, and the image forming process is repeated. The primary transfer rollers 5Y, 5M, 5C, 5K, and 5E are made of an elastic layer of an ion conductive sponge rubber (NBR rubber) and a cored bar, and have an outer diameter of 16 to 20 mm, a resistance value of N / N (23 ° C., 50% RH), 1E + 6 to 1E + 8Ω transfer roller with 500V applied. In the primary transfer position of the intermediate transfer belt 6 facing the photosensitive drums 1Y, 1M, 1C, 1k, and 1E, the intermediate transfer belt 6 is disposed on the back surface side of the intermediate transfer belt 6. By applying a primary transfer bias of +50 to +4000 V with a positive polarity opposite to the charging polarity, a current of +10 to +60 μA is caused to flow through the primary transfer portion, and the toner image on the photosensitive drum 1 is transferred to the intermediate transfer belt 6. Primary transfer is performed on the top. Reference numeral 11 denotes a drum cleaning device 11 for removing the toner remaining on the photosensitive drum 1 after the primary transfer.

  Further, in the present embodiment, the secondary transfer roller 9 disposed in pressure contact with the toner image carrying surface side of the intermediate transfer belt 6 is disposed at the secondary transfer position of the intermediate transfer belt 6 facing the paper 7 conveyance path. A tension roller 21 is provided on the back surface side of the transfer belt 6 and forms a counter electrode of the secondary transfer roller 9 and is grounded with the toner. The secondary transfer roller 9 is applied with a bias having a polarity opposite to that of the toner. Further, a belt cleaner 12 for removing toner remaining on the intermediate transfer belt 6 after the secondary transfer is provided on the downstream side of the secondary transfer position.

  Further, in the present embodiment, the paper 7 is sent to the secondary transfer position at a predetermined timing after being temporarily stopped by the registration roller 8, and the paper 7 after the secondary transfer is not shown. The toner is conveyed to a fixing device (not shown) by the conveying member, and the toner is melted and fixed on the paper.

  Further, after the photosensitive drum 1 is charged by the charging unit 2, the density control reference pattern does not overlap with the image corresponding to the image information input from an external device such as an image scanner / computer by the exposure of the exposure unit 3. For example, the electrostatic latent image is formed at a position corresponding to the space between the sheets, and the electrostatic latent image is developed by the developing devices 4Y, 4M, 4C, and 4k, and is then used as a density control reference pattern toner image (hereinafter referred to as a patch). Called 16). The density is detected by the patch sensor 17, and the exposure amount of the exposure means 3 is adjusted based on the output signal of the patch sensor 17, so that the toner density on the photosensitive drum 1 is corrected and controlled to be constant.

  However, prior to the formation of the patch 16 of chromatic color toner or Bk toner, the electrostatic latent image for transparent toner formed on the photosensitive drum 1 is developed by the developing device 4E, and an intermediate transfer belt corresponding to the gap between the sheets 6 Primary transfer is performed at the upper position (in the above description, the transparent toner is not primarily transferred to a position corresponding to the outside of the output image area. As a result, when the chromatic color toner or the Bk toner patch 16 on the photosensitive drum 1 corresponding to the position between the paper passes through the primary transfer portion, it does not come in direct contact with the intermediate transfer belt 6 but directly contacts it. What is present is a transparent toner that has been primarily transferred to the area between the sheets in advance. As the amount of the transparent toner used in the area corresponding to the space between the papers, the same amount as the maximum weight per unit area of the chromatic color or Bk toner used in the output image was used.

  For example, as shown in FIG. 6, first, a transparent toner image for output image and between paper is formed on the intermediate transfer belt 6 by primary transfer. Next, primary transfer is performed so that the chromatic color or Bk toner image of the output image formed on the photosensitive drum 1 is superimposed on the transparent toner that has been previously primary transferred. Next, the toner image of the patch 16 formed in the region on the photosensitive drum 1 corresponding to the position between the sheets and whose density is detected by the patch sensor 17 passes through the primary transfer portion. At that time, the transparent toner has already been primarily transferred to the area on the intermediate transfer belt 6 corresponding to the space between the sheets. As a result, it is not the chromatic color toner or the Bk toner patch that is in direct contact with the intermediate transfer belt 6 at a position corresponding to the space between the papers at the primary transfer site.

  By the way, in the embodiment, as shown in FIG. 7, a constant voltage bias (predetermined control) is performed so as to synchronize with the timing when the region of the photosensitive drum 1 corresponding to the sheet reaches the primary transfer portion. The chromatic color in the region on the photosensitive drum 1 corresponding to the position between the papers, by applying -100 to -1500 V and applying a current of -5 to -30 μA to the primary transfer site. Alternatively, the Bk patch 16 toner is prevented from adhering to the intermediate transfer belt 6. Further, the transparent toner that has already been primarily transferred is electrostatically returned to the photosensitive drum 1 and the amount of the transparent toner corresponding to the gap between the papers attached to the intermediate transfer belt 6 is also reduced.

  The secondary transfer roller 9 of this embodiment is made of an elastic layer of an ion conductive solid rubber (NBR rubber) and a cored bar, has an outer diameter of 24 mm, a roller surface roughness Ra = 5.0 (μm) or less, and a resistance value. N / N (23 ° C., 50% RH) measurement A transfer roller of 1E + 6 to 1E + 8Ω with 2 kV applied was used.

  In this embodiment, the conveyance of the paper 7 is controlled by the registration roller 8 so as to synchronize with the timing at which the leading edge of the toner image of the output image on the intermediate transfer belt 6 reaches the secondary transfer portion. A constant voltage bias (transfer bias) controlled to a predetermined level is applied to the transfer roller 9 from the secondary transfer bias applying means. Four colors superimposed on the intermediate transfer belt 6 at the transfer site by applying a transfer bias of +20 to +6000 V opposite in polarity to the toner to the secondary transfer roller 9 and flowing a secondary transfer current of +10 to +50 μA The full-color image is transferred to the paper 7 together with the transparent toner, and a full-color unfixed toner image is formed on the paper.

  In addition, a constant voltage bias (reverse polarity with respect to the transfer bias) -10 to -1000 V controlled in a predetermined manner so as to synchronize with the timing at which the region on the intermediate transfer belt 6 corresponding to the space reaches the secondary transfer portion Applied, a current of 0 to -10 μA flows through the secondary transfer site. As a result, the transparent toner adhering to the region on the intermediate transfer belt 6 corresponding to the position between the sheets is prevented from adhering to the secondary transfer roller 9. However, the surface layer of the transparent toner between the papers that are in direct contact with the secondary transfer roller 9 is subjected to secondary transfer due to the boundary force due to the amount of electric charge itself or the van der Waals force with the surface of the secondary transfer roller 9 etc. It will stick to the roller 9. However, even if the second transfer of the output image conveyed next in that state is performed and the transparent toner adheres to the back of the paper 7, it is not recognized as the back dirt because it is a transparent toner.

  In this embodiment, with the configuration as described above, the toner adhering to the secondary transfer roller 9 is made transparent toner so that dirt on the back of the paper is not recognized.

<Example 5>
FIG. 9 shows an image forming apparatus according to the fourth embodiment.

  Reference numerals 1Y, 1M, 1C, 1k, and 1E denote photosensitive drums (latent image carriers) that rotate in the direction of arrow A, and their surfaces are uniformly charged by charging devices 2Y, 2M, 2C, 2k, and 2E. The Reference numerals 3Y, 3M, 3C, 3k, and 3E denote exposure apparatuses that perform exposure based on image information. An electrostatic latent image corresponding to image information is formed on the photosensitive drums 1Y, 1M, 1C, 1k, and 1E by a known electrophotographic process.

  The developing devices 4Y, 4M, 4C, 4k, and 4E contain yellow (Y), magenta (M), cyan (C) and black (k) chromatic toners, and transparent toner, respectively. The aforementioned electrostatic latent images are developed by these developing devices 4Y, 4M, 4C, 4k, and 4E, and toner images are formed on the respective photosensitive drums 1Y, 1M, 1C, 1k, and 1E. A reversal development method is used in which toner is attached to the exposed portion of the electrostatic latent image for development.

  Reference numeral 25 denotes a transfer belt 25 which is a transfer material carrier disposed so as to be in contact with the surface of the photosensitive drum 1, and is stretched around a plurality of stretching rollers 20 and 22 in the direction of arrow G. To turn to. In the present embodiment, the tension roller 20 is a tension roller that controls the tension of the transfer belt 25 to be constant, and the tension roller 22 is a driving roller for the transfer belt 25.

  In the present embodiment, the transfer belt 25 includes a suitable amount of carbon black as a resistance adjuster in a resin such as polyimide, polycarbonate, polyester, polypropylene, polyethylene terephthalate, acrylic, vinyl chloride, or various rubbers, and its volume. The resistivity is 1E + 11 to 1E + 15 [Ω · cm] and the thickness is 0.07 to 0.1 [mm].

  Further, in the present embodiment, after the positioning is temporarily stopped by the registration roller 8, the paper 7 is sent to the transfer belt 25 sandwiched between the stretching roller 20 and the suction roller 26 at a predetermined timing. At this time, a bias +100 to +1000 V having a polarity opposite to that of the toner is applied to the suction roller to electrostatically attract the paper 7 to the transfer belt 25, and the paper 7 is conveyed by the transfer belt 25.

  The transfer belt 25 is disposed opposite to the photosensitive drum 1, and an unfixed toner image on the photosensitive drum 1Y is electrostatically transferred to the paper 7 conveyed by the transfer belt 25 by the primary transfer roller 5Y, and then superimposed. As described above, the unfixed toner images on the photosensitive drums 1M, 1C, and 1k are sequentially transferred to the paper 7, and a full-color image in which the four unfixed toner images are superimposed on the paper 7 carried on the transfer belt 25 is obtained. However, before transferring the unfixed toner image of the full-color image onto the paper 7, the transparent toner is transferred onto the paper 7 and the position of the transfer belt 25 corresponding to the space between the papers. At that time, the transparent toner is developed on the photosensitive drum 1 and transferred to the paper 7 based on the image information so that the height of the toner layer of the output image on the paper 7 including the chromatic toner and Bk toner is uniform. After that, an unfixed full-color image to be copied onto the paper 7 is overlapped. At this time, the transparent toner is not developed from the photosensitive drum 1 in that area so that the transparent toner is not transferred onto the transfer belt 6 in an area not corresponding to the sheet (excluding the gap between the sheets).

  On the other hand, for each rotation of the photosensitive drum 1 after the transfer, the surface of the photosensitive drum 1 is cleaned by the cleaning device 11 to remove the transfer residual toner, and the image forming process is repeated.

  The primary transfer rollers 5Y, 5M, 5C, 5K, and 5E are made of an elastic layer of an ion conductive sponge rubber (NBR rubber) and a cored bar, and have an outer diameter of 16 to 20 mm, a resistance value of N / N (23 ° C., 50% RH), 1E + 6 to 1E + 8Ω transfer roller with 500V applied. At the transfer position of the transfer belt 25 facing the photosensitive drums 1Y, 1M, 1C, 1k, 1E, the transfer belt 25 is disposed on the back surface side. By applying a positive polarity primary transfer bias of +100 to +4000 V, a current of +10 to +20 μA is supplied to the primary transfer portion, and the toner image on the photosensitive drum 1 is transferred to the transfer belt 25 or the transfer belt 25. It is designed to be transferred to the carried paper 7. Reference numeral 11 denotes a drum cleaning device 11 for removing the toner remaining on the photosensitive drum 1 after the primary transfer.

  Further, in the present embodiment, the paper 7 carrying the unfixed toner and the transparent toner of the full color image is conveyed to the stretching roller 22 and is separated from the transfer belt 25 with the assistance of the separation claw 27. The separated paper 7 is transported to a fixing device (not shown) by a transport member (not shown), and the toner is melted and fixed on the paper. The separated transfer belt 25 is neutralized by a neutralizing brush 28. The neutralizing brush 28 is a fur brush roller with a hair length of 5 mm, a cored bar diameter of 8 mm, an outer diameter of 18 mm, and a resistance value of 1E + 6 to 1E + 8Ω with 100 V applied when measuring N / N (23 ° C., 50% RH). It was used. The neutralizing brush 28 and the stretching roller 22 are grounded.

  Further, after the photosensitive drum 1 is charged by the charging unit 2, the density control reference pattern does not overlap with the image corresponding to the image information input from an external device such as an image scanner / computer by the exposure of the exposure unit 3. For example, the electrostatic latent image is formed at a position corresponding to the space between the sheets, and the electrostatic latent image is developed by the developing devices 4Y, 4M, 4C, and 4k, and the density control reference pattern toner image (hereinafter referred to as a patch). Called 16). The density is detected by the patch sensor 17, and the exposure amount of the exposure means 3 is adjusted based on the output signal of the patch sensor 17, so that the toner density on the photosensitive drum 1 is corrected and controlled to be constant.

  However, prior to the formation of the patch 16 such as chromatic toner or Bk toner, the electrostatic latent image for transparent toner formed on the photosensitive drum 1 is developed by the developing device 4E, and the transfer belt 25 corresponding to the gap between the sheets is developed. The image is transferred to the upper position (described above, the transparent toner is not transferred to a position corresponding to the outside of the output image area. As a result, when the patch 16 of chromatic toner or Bk toner on the photosensitive drum 1 corresponding to the position between the paper passes through the transfer portion, it does not come into direct contact with the transfer belt 25, but is in direct contact. The transparent toner previously transferred to the area between the sheets is obtained. The amount of the transparent toner used between the papers was the same as the maximum weight per unit area of the chromatic color or Bk toner used for the output image.

  For example, as shown in FIG. 10, first, a transparent toner image for the output image and between paper is formed on the transfer belt 25. Next, the chromatic color or Bk toner image of the output image formed on the photosensitive drum 1 is transferred so as to be superimposed on the previously transferred transparent toner. Next, the toner image of the patch 16 formed in an area on the photosensitive drum 1 corresponding to the position between the sheets and whose density is detected by the patch sensor 17 passes through the transfer portion. At that time, the transparent toner has already been transferred to the area on the transfer belt 25 between the sheets. As a result, it is not the chromatic toner or the Bk toner patch that is in direct contact with the transfer belt 25 between the paper at the transfer site, but the transparent toner.

  By the way, in the present embodiment, as shown in FIG. 11, a constant voltage bias (predetermined control is performed so as to synchronize with the timing when the region of the photosensitive drum 1 corresponding to the space between the sheets reaches the primary transfer portion. (The reverse polarity to the transfer bias) -100 to -4500V is applied so that a current of -5 to -20μA flows to the primary transfer site, so that the chromatic color in the area on the photosensitive drum 1 corresponding to the position between the paper Alternatively, Bk patch 16 toner is prevented from adhering to the transfer belt 25. Further, the transparent toner on the transfer belt 25 that has already been transferred is electrostatically transferred to the photosensitive drum 1 and the amount of the transparent toner adhering to the transfer belt 25 is reduced. Even if the surface layer of the transparent toner between the paper in contact with the transfer belt 25 passes through the transfer portion of the final color Bk, the boundary force due to the amount of electric charge that it has and the van der Waals force with the surface of the secondary transfer roller 9 For example, the transfer belt 25 remains attached. However, in that state, it passes through the part of the suction roller 26 again, carries the paper 7, the transparent toner contacts the back of the paper 7, and the transparent toner adheres to the back of the paper 7. Is not recognized.

  In this embodiment, with the configuration as described above, the toner adhering to the secondary transfer roller 9 is made transparent toner so that dirt on the back of the paper is not recognized.

1 is a diagram illustrating Example 1. FIG. It is a figure explaining Example 1 and Example 2. FIG. It is a figure explaining Example 1 and Example 2. FIG. FIG. 6 is a diagram illustrating Example 2. FIG. 6 is a diagram for explaining a third embodiment. It is a figure explaining Example 3 and Example 4. FIG. It is a figure explaining Example 3 and Example 4. FIG. FIG. 10 is a diagram for explaining a fourth embodiment. FIG. 10 is a diagram for explaining a fifth embodiment. FIG. 10 is a diagram for explaining a fifth embodiment. FIG. 10 is a diagram for explaining a fifth embodiment. It is a figure explaining a technical background. It is a figure explaining a technical background. It is a figure explaining a technical background.

Explanation of symbols

1 Photosensitive drum
2 Primary charger
3 Exposure equipment
4 Developer
5 Primary transfer roller
6 Intermediate transfer belt
7 Transfer material / paper / paper
8 Registration roller
9 Secondary transfer roller
11 Drum cleaning device
12 Belt cleaning device
14 Transparent toner
15 Toner image of output image
16 Density detection patch
17 Patch sensor for patch detection
18 Belt tension roller
19 Belt tension roller
20 Belt tension roller
21 Belt tension roller (opposing roller for secondary transfer)
22 Belt tension roller (drive roller)
23 Belt tension roller
24 Belt tension roller
25 Transfer belt
26 Suction roller
27 Separating nails
28 Static elimination roller

Claims (6)

  An image carrier that carries at least one charged color toner image and a transparent toner image, and a transfer unit that holds the transfer material between the image carrier and transfers the toner image on the image carrier to the transfer material. And an image forming apparatus having density detecting means for detecting the density of the color toner formed outside the image area on the image carrier, the color toner formed on the color toner formed outside the image area on the image carrier. An image forming apparatus comprising a transparent toner. The outside of the image area on the image carrier is a portion corresponding to a position where the image carrier and the transfer unit do not sandwich the transfer material and only the image carrier and the transfer unit are in direct contact with each other, and 2. The image forming apparatus according to claim 1, wherein a bias having the same polarity as that of the color toner is applied to the transfer unit when the outside of the image area on the image carrier contacts the transfer unit.   At least one or more first image carriers that carry at least one charged color toner image and transparent toner image, and the first image carrier, and a second image carrier that sandwich the second image carrier. A transfer material is sandwiched between a primary transfer means for primary transfer of a toner image on one image carrier to the second image carrier and the second image carrier, and the second image carrier is placed on the second image carrier. An image forming apparatus comprising: a secondary transfer unit that transfers the toner image to the transfer material; and a density detection unit that detects a density of the color toner image formed outside the image area on the first image carrier. An image forming apparatus wherein primary transfer of transparent toner is performed outside an image area on the second image carrier prior to primary transfer of the color toner image.   On the first image carrier and outside the image area on the second image carrier, the second image carrier and the second transfer means do not hold the transfer material between the second image and the second image carrier. At a position corresponding to a position where only the carrier and only the second transfer means are in direct contact, and when the first image carrier and the outside of the image area on the second image carrier are in contact, 4. The image forming apparatus according to claim 3, wherein a bias having the same polarity as that of the color toner is applied to the first transfer unit.   At least one image carrier that carries at least one charged color toner image and transparent toner image, and a transfer material sandwiched between the image carrier, and the toner image on the image carrier is transferred to the transfer material In an image forming apparatus having a transfer member that transfers to a transfer material and a density detection unit that detects the density of the color toner formed outside the image area on the image carrier, prior to transfer of the color toner image to the transfer material An image forming apparatus, wherein the transparent toner is transferred to a portion of a transfer member that directly contacts an outside of an image area on the image carrier.   Outside the image area on the image carrier is a location corresponding to a position where the image carrier and the transfer member do not sandwich the transfer material and only the image carrier and the transfer means are in direct contact with each other, and 6. The image forming apparatus according to claim 5, wherein a bias having the same polarity as that of the color toner is applied to the transfer member when the outside of the image area on the image carrier contacts the transfer member.

Images