JP2005037667A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which can prevent a reverse transfer phenomenon and contamination in a charging apparatus. <P>SOLUTION: The image forming apparatus 10 is equipped with: a plurality of image carrying bodies 26 carrying tonor images of the respective colors including at least black; a plurality of primary transfer bodies 34, 36 each of which touches a part of the plurality of image carrying bodies and onto which toner images on the image carrying bodies 26 are primarily transferred; a secondary transfer body 38 which entirely touches the plurality of primary transfer bodies 34, 36 and onto which the toner images on the primary transfer bodies 34, 36 are secondarily transferred; and a third transfer body 72 which touches the secondary transfer body 38 to thirdly transfer the toner image on the secondary transfer body 38. In this apparatus, a black toner having ≤7 μm average particle size is transferred in the downstream side of the primary transfer body 36 disposed in the downstream side with respect to the rotation direction of the secondary transfer body 38. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus using an electrophotographic system such as a copying machine, a printer, a facsimile machine, and a multifunction machine of these, and more particularly to a means for preventing contamination of a charging device.

  2. Description of the Related Art Conventionally, image forming apparatuses such as copying machines and printers using an electrophotographic system (electrostatic transfer system) are widely known. In such an image forming apparatus, the toner image is formed on the recording sheet by transferring the toner image onto the recording sheet by the final transfer member and fixing the toner image (for example, Patent Documents). 1).

  In such an image forming apparatus, the conventional toner has an average particle diameter of 7.5 μm and is usually negative (this is referred to as positive toner). However, in recent years, there is a tendency that the particle size of the toner is reduced, and the smaller the particle size of the toner, the more positive charge is injected by the transfer process and the property of becoming positive polarity (this is called reverse polarity toner). is doing. In particular, black toner uses carbon black as a coloring material, and therefore has conductivity and has a characteristic that positive charges are easily injected. Therefore, the tendency of the black toner to become a reverse polarity toner is remarkable.

  Therefore, as shown in FIG. 7, image forming units 94Y to 94K of No. 1: Yellow (Y), No. 2: Magenta (M), No. 3: Cyan (C), No. 4: Black (K) in order from the top. After the black toner is primarily transferred to the downstream intermediate transfer body 100 (referred to as intermediate transfer body 1 in FIG. 8) by the photoconductor 96K, cyan toner is transferred to the downstream side by the photoconductor 96C. When the primary transfer is performed on the downstream intermediate transfer body 100, the black toner that has been primarily transferred is reversely transferred (retra) to the photoconductor 96C side, and as shown in FIG. There has been a problem that the charging device (charger) 98C for charging the photoconductor 96C of (C) is soiled.

The black toner reversely transferred (retra) to the photoconductor 96C cannot be collected by the cleaning device 102. As a result, image defects such as fogging and ghosting due to charging failure are caused. In addition, the toner using silica as an external additive in order to improve transferability and fluidity also has a characteristic that silica absorbs moisture. Therefore, depending on the amount of external addition (when the amount is increased), There is a problem of accelerating such reverse transfer (retra) phenomenon.
JP 2001-75448 A

  In view of the above problems, an object of the present invention is to obtain an image forming apparatus capable of preventing the reverse transfer (retra) phenomenon from occurring and preventing the charging device (charger) from being soiled. To do.

  In order to achieve the above object, an image forming apparatus according to claim 1 according to the present invention includes a plurality of image carriers each carrying a toner image of each color including at least a black color, and the plurality of image carriers. A plurality of primary transfer members on which the toner images on the image carrier are primarily transferred, and all of the plurality of primary transfer members, so that the toner images on the primary transfer member are In an image forming apparatus comprising: a secondary transfer body to be transferred next; and a tertiary transfer body that is in contact with the secondary transfer body and tertiary-transfers a toner image on the secondary transfer body. A black toner having an average particle diameter of 7 μm or less is transferred to the downstream side of the primary transfer member provided on the downstream side in the rotation direction.

  According to the first aspect of the present invention, black toner having an average particle size of 7 μm or less is transferred to the downstream side of the primary transfer member provided on the downstream side in the rotation direction of the secondary transfer member. Here, the black toner having an average particle size of 7 μm or less is likely to be a reverse polarity toner, and thus is easily reversely transferred. However, the black toner is transferred on the downstream side of the primary transfer member, and thus disposed on the upstream side thereof. The image is not reversely transferred to the other color image carrier. Therefore, the trouble that the charging device of other colors gets dirty does not occur, and the image defect due to the charging failure does not occur.

  According to another aspect of the present invention, there is provided an image forming apparatus comprising: a plurality of image carriers each carrying a toner image of each color including at least a black color; and a part of the plurality of image carriers being in contact with each other; A plurality of primary transfer members to which a toner image on a carrier is primarily transferred; a secondary transfer member in contact with all of the plurality of primary transfer members and a toner image on the primary transfer member is secondarily transferred; and And a tertiary transfer member that contacts the secondary transfer member and tertiary-transfers the toner image on the secondary transfer member, at least a primary provided on the downstream side in the rotation direction of the secondary transfer member. It is characterized in that black toner having an average particle diameter of 7 μm or less is not transferred upstream of the transfer body.

  According to the second aspect of the present invention, black toner having an average particle diameter of 7 μm or less is not transferred at least on the upstream side of the primary transfer member provided on the downstream side in the rotation direction of the secondary transfer member. Here, the black toner having an average particle size of 7 μm or less is likely to be a reverse polarity toner, and thus is easily reversely transferred. However, since the black toner is not transferred at least upstream of the primary transfer body, The color image carrier can be disposed, and the black toner is not reversely transferred to the image carrier. Therefore, the trouble that the charging device of other colors gets dirty does not occur, and the image defect due to the charging failure does not occur.

  According to a third aspect of the present invention, there is provided an image forming apparatus comprising: a plurality of image carriers each carrying at least a toner image of each color including a black color; and a part of the plurality of image carriers being in contact with each other; A plurality of primary transfer members to which a toner image on a carrier is primarily transferred; a secondary transfer member in contact with all of the plurality of primary transfer members and a toner image on the primary transfer member is secondarily transferred; and A primary transfer provided on the downstream side in the rotation direction of the secondary transfer body in an image forming apparatus comprising a tertiary transfer body that contacts the secondary transfer body and tertiary-transfers a toner image on the secondary transfer body. A black toner containing 0.7% by weight or more of an external additive of silica having an average particle diameter of 0.1 μm or more is transferred to the downstream side of the body.

  In the invention of claim 3, 0.7 wt% or more of an external additive of silica having an average particle size of 0.1 μm or more is included on the downstream side of the primary transfer body provided on the downstream side in the rotation direction of the secondary transfer body. The black toner thus transferred is transferred. Here, since silica has moisture absorption characteristics, the black toner containing it is easily reverse-transferred, but the black toner is transferred on the downstream side of the primary transfer body. The image is not reversely transferred to the other color image carrier. Therefore, the trouble that the charging device of other colors gets dirty does not occur, and the image defect due to the charging failure does not occur.

  According to a fourth aspect of the present invention, there is provided an image forming apparatus comprising: a plurality of image carriers each carrying a toner image of each color including at least a black color; and a part of the plurality of image carriers being in contact with each other; A plurality of primary transfer members to which a toner image on a carrier is primarily transferred; a secondary transfer member in contact with all of the plurality of primary transfer members and a toner image on the primary transfer member is secondarily transferred; and And a tertiary transfer member that contacts the secondary transfer member and tertiary-transfers the toner image on the secondary transfer member, at least a primary provided on the downstream side in the rotation direction of the secondary transfer member. A black toner containing 0.7% by weight or more of an external additive of silica having an average particle diameter of 0.1 μm or more is not transferred upstream of the transfer body.

  In the invention of claim 4, the external additive of silica having an average particle diameter of 0.1 μm or more is at least 0.7 wt% on the upstream side of the primary transfer body provided at least on the downstream side in the rotation direction of the secondary transfer body. The contained black toner is not transferred. Here, since silica has a hygroscopic property, black toner containing it is easily reverse-transferred, but since the black toner is not transferred at least upstream of the primary transfer body, In addition, an image carrier of another color can be provided, and the black toner is not reversely transferred to the image carrier. Therefore, the trouble that the charging device of other colors gets dirty does not occur, and the image defect due to the charging failure does not occur.

  The image forming apparatus according to claim 5 is in contact with four image carriers on which toner images of yellow, magenta, black, and cyan are carried, and two of the four image carriers. A first primary intermediate transfer member to which a toner image on the carrier is primarily transferred and a second two of the four image carriers that are in contact with each other, and a toner image on both image carriers is primarily transferred. A primary intermediate transfer member, a secondary intermediate transfer member in contact with the first and second primary intermediate transfer members, and a toner image on both primary intermediate transfer members is secondarily transferred; and the secondary intermediate transfer member; And a final transfer body for final transfer of a toner image on the secondary transfer body onto a recording medium, and a secondary intermediate transfer body with reference to a position where the final transfer body contacts the secondary intermediate transfer body The first primary intermediate transfer member on the upstream side in the rotation direction and the second primary intermediate transfer member on the downstream side. In the image forming apparatus provided with, on the downstream side of the second primary intermediate transfer member, the average particle diameter of 7μm or less of the black toner is characterized in that it is transcribed.

  According to the invention of claim 5, black toner having an average particle size of 7 μm or less is transferred downstream of the second primary intermediate transfer member. Here, the black toner having an average particle size of 7 μm or less is likely to be a reverse polarity toner, and thus is easily reversely transferred. However, since the black toner is transferred downstream of the second primary intermediate transfer member, The image is not reversely transferred to another color image carrier disposed on the side. Therefore, the trouble that the charging device of other colors gets dirty does not occur, and the image defect due to the charging failure does not occur.

  According to a sixth aspect of the present invention, there is provided an image forming apparatus comprising: four image carriers that carry yellow, magenta, black, and cyan toner images; and two of the four image carriers that are in contact with each other. A first primary intermediate transfer member to which a toner image on the carrier is primarily transferred and a second two of the four image carriers that are in contact with each other, and a toner image on both image carriers is primarily transferred. A primary intermediate transfer member, a secondary intermediate transfer member in contact with the first and second primary intermediate transfer members, and a toner image on both primary intermediate transfer members is secondarily transferred; and the secondary intermediate transfer member; And a final transfer body for final transfer of a toner image on the secondary transfer body onto a recording medium, and a secondary intermediate transfer body with reference to a position where the final transfer body contacts the secondary intermediate transfer body The first primary intermediate transfer member on the upstream side in the rotation direction and the second primary intermediate transfer member on the downstream side. In the image forming apparatus provided with, is characterized in that at least the upstream side of the first and second primary intermediate transfer member, the following black toner average particle size 7μm is not transferred.

  In the invention of claim 6, black toner having an average particle size of 7 μm or less is not transferred at least upstream of the first and second primary intermediate transfer members. Here, the black toner having an average particle size of 7 μm or less is likely to be a reverse polarity toner, and thus is easily transferred in reverse, but the black toner is not transferred at least upstream of the first and second primary intermediate transfer members. Therefore, an image carrier of another color can be disposed on the upstream side of each, and the black toner is not reversely transferred to the image carrier. Therefore, the trouble that the charging device of other colors gets dirty does not occur, and the image defect due to the charging failure does not occur.

  The image forming apparatus according to claim 7 is in contact with four image carriers on which toner images of yellow, magenta, black, and cyan are carried, and two of the four image carriers. A first primary intermediate transfer member to which a toner image on the carrier is primarily transferred and a second two of the four image carriers that are in contact with each other, and a toner image on both image carriers is primarily transferred. A primary intermediate transfer member, a secondary intermediate transfer member in contact with the first and second primary intermediate transfer members, and a toner image on both primary intermediate transfer members is secondarily transferred; and the secondary intermediate transfer member; And a final transfer body for final transfer of a toner image on the secondary transfer body onto a recording medium, and a secondary intermediate transfer body with reference to a position where the final transfer body contacts the secondary intermediate transfer body The first primary intermediate transfer member on the upstream side in the rotation direction and the second primary intermediate transfer member on the downstream side. In the image forming apparatus, a black toner containing 0.7% by weight or more of an external additive of silica having an average particle size of 0.1 μm or more is transferred downstream of the second primary intermediate transfer member. It is characterized by being.

  In the invention of claim 7, a black toner containing 0.7 wt% or more of an external additive of silica having an average particle size of 0.1 μm or more is transferred downstream of the second primary intermediate transfer member. Here, since silica has moisture absorption characteristics, the black toner containing it is easily reverse transferred, but the black toner is transferred downstream of the second primary intermediate transfer member. There is no reverse transfer to the image carrier of another color disposed on the upstream side. Therefore, the trouble that the charging device of other colors gets dirty does not occur, and the image defect due to the charging failure does not occur.

  The image forming apparatus according to claim 8 is in contact with four image carriers on which toner images of yellow, magenta, black, and cyan are carried, and two of the four image carriers, and both images A first primary intermediate transfer member to which a toner image on the carrier is primarily transferred and a second two of the four image carriers that are in contact with each other, and a toner image on both image carriers is primarily transferred. A primary intermediate transfer member, a secondary intermediate transfer member in contact with the first and second primary intermediate transfer members, and a toner image on both primary intermediate transfer members is secondarily transferred; and the secondary intermediate transfer member; And a final transfer body for final transfer of a toner image on the secondary transfer body onto a recording medium, and a secondary intermediate transfer body with reference to a position where the final transfer body contacts the secondary intermediate transfer body The first primary intermediate transfer member on the upstream side in the rotation direction and the second primary intermediate transfer member on the downstream side. In which an external additive of silica having an average particle size of 0.1 μm or more is contained at least 0.7 wt% at least upstream of the first and second primary intermediate transfer members. The color toner is not transferred.

  According to the invention of claim 8, there is provided a black toner containing 0.7% by weight or more of an external additive of silica having an average particle size of 0.1 μm or more at least upstream of the first and second primary intermediate transfer members. Not transcribed. Here, since silica has moisture absorption characteristics, black toner containing it is easily reverse-transferred, but the black toner is at least upstream of the first and second primary intermediate transfer members. Since the image is not transferred, an image carrier of another color can be disposed on the upstream side of each, and the black toner is not reversely transferred to the image carrier. Therefore, the trouble that the charging device of other colors gets dirty does not occur, and the image defect due to the charging failure does not occur.

  As described above, according to the present invention, since the black toner is not reversely transferred to the photoconductors of other colors, there is no problem that stains the charging devices of other colors. Therefore, image defects due to poor charging do not occur.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS The best mode for carrying out the present invention will be described below in detail based on the embodiments shown in the drawings. FIG. 1 is a schematic side view showing a tandem type full color printer as an image forming apparatus according to the present invention, and FIG. 2 is a schematic side view showing a main part of the full color printer shown in FIG. The full color printer 10 is roughly composed of an image forming unit 12, an intermediate transfer device 14, a final transfer unit 16, a fixing device 18, and a paper feeding unit 20.

  The image forming unit 12 includes four image forming units 22Y, 22M, 22K, and 22C for yellow (Y), magenta (M), black (K), and cyan (C) in order from the top, and an exposure device 24. It consists of and. Each of the image forming units 22Y to 22C includes four photosensitive drums (image bearing members) 26Y, 26M, 26K, and 26C, and charging rolls (contact type charging members) 28Y and 28M that respectively contact the photosensitive drums 26Y to 26C. 28K, 28C, developing devices 30Y, 30M, 30K, 30C facing the photosensitive drums 26Y-26C, respectively, and photosensitive brush rolls 32Y, 32M, 32K, 32C contacting the photosensitive drums 26Y-26C, respectively. It is configured.

  Here, with respect to the arrangement of each member around the photosensitive drum 26, the charging roll 28, the developing device 30, and the first to be described later are arranged around the photosensitive drum 26 from the upstream side to the downstream side in the rotation direction of the photosensitive drum 26. Upstream or downstream intermediate transfer rolls 34, 36 and a photosensitive brush roll 32 are arranged in this order.

  The photosensitive drums 26Y to 26C are applied with a DC voltage of about −900 V by the charging rolls 28Y to 28C, and are thereby uniformly charged to about −420V. When the electrostatic latent image is written by the exposure device 24, the surface potential is neutralized to about -70V.

The developing devices 30Y to 30C are magnetic brush contact type two-component developing type developing devices including a developing roll, a developer amount regulating member, a developer conveying member, and an auger for conveying and stirring the developer. Incidentally, amount of the developer conveyed to the developing portion is regulated by the developer amount regulating member is about ^{30g / m 2 ~45g / m 2} , the charge amount of the toner present on the developing roll to the lever is approximately - It is about 40 μC / g to −50 μC / g. The developing devices 30Y to 30C are developed by applying an AC + DC developing voltage. The developing voltage is 4 kHz for AC, 1.3 kVpp, and about −350 V for DC.

  The intermediate transfer device 14 includes a first upstream intermediate transfer roll (first upstream intermediate transfer member) 34 that contacts the photosensitive drums 26Y and 26M, and a first downstream side that contacts the photosensitive drums 26K and 26C. An intermediate transfer roll (first downstream intermediate transfer body) 36, a second intermediate transfer roll 38 (second intermediate transfer body) in contact with the two first intermediate transfer rolls 34, 36, and a second The toner sensor 40 is provided for optically detecting the presence and density of the toner image on the intermediate transfer roll 38 in a non-contact manner. Based on the detection result of the toner sensor 40, each charging bias, each developing bias, the amount of toner replenished in each developing device 30, and the like are controlled.

  Further, the first upstream intermediate transfer roll 34 includes a first upstream cleaning device 42. The first upstream cleaning device 42 includes a metal (stainless) cleaning roll 44 (cleaning body) that contacts the first upstream intermediate transfer roll 34, a cleaning blade 46 that contacts the cleaning roll 44, In the vicinity of the upstream side in the rotation direction of the first upstream intermediate transfer roll 34 relative to the cleaning roll 44, the brush roll 48 that contacts the intermediate transfer roll 34, and these cleaning roll 44, cleaning blade 46, and brush roll 48 are accommodated. A cleaner housing (container) 50 is provided.

  Similarly, the first downstream intermediate transfer roll 36 includes a first downstream cleaning device 52. The first downstream cleaning device 52 includes a metal (stainless) cleaning roll (cleaning body) 54 that contacts the first downstream intermediate transfer roll 36, a cleaning blade 56 that contacts the cleaning roll 54, In the vicinity of the upstream side in the rotation direction of the first downstream intermediate transfer roll 36 relative to the cleaning roll 54, the brush roll 58 that contacts the intermediate transfer roll 36, the cleaning roll 54, the cleaning blade 56, and the brush roll 58 are accommodated. A cleaner housing (container) 60 is provided.

  Further, the second intermediate transfer roll 38 includes a second cleaning device 62. The second cleaning device 62 includes a metal (stainless steel) cleaning roll (cleaning body) 64 that contacts the second intermediate transfer roll 38, a cleaning blade 66 that contacts the cleaning roll 64, and a cleaning roll 64. Also, a brush roll 68 that contacts the intermediate transfer roll 38 in the vicinity of the downstream side in the rotation direction of the second intermediate transfer roll 38, and a cleaner housing (container) 70 that accommodates the cleaning roll 64, the cleaning blade 66, and the brush roll 68 are provided. I have.

  Here, the arrangement of the peripheral members in the first upstream intermediate transfer roll 34 is centered on the first upstream intermediate transfer roll 34 and the upstream side in the rotation direction of the first upstream intermediate transfer roll 34 from the upstream side. The photosensitive drum 26M, the photosensitive drum 26Y, the second intermediate transfer roll 38, the brush roll 48, and the cleaning roll 44 are sequentially arranged.

  The arrangement of the peripheral members in the first downstream intermediate transfer roll 36 is centered on the first downstream intermediate transfer roll 36 from the upstream side to the downstream side in the rotation direction of the first downstream intermediate transfer roll 36. The photosensitive drum 26C, the photosensitive drum 26K, the second intermediate transfer roll 38, the brush roll 58, and the cleaning roll 54 are disposed in this order.

  Further, the arrangement of the peripheral members in the second intermediate transfer roll 38 is such that the first upstream side is centered on the second intermediate transfer roll 38 from the upstream side to the downstream side in the rotation direction of the second intermediate transfer roll 38. An intermediate transfer roll 34, a first downstream intermediate transfer roll 36, a toner sensor 40, a final transfer roll 72, a cleaning roll 64, and a brush roll 68, which will be described later, are arranged in this order.

  The first intermediate transfer rolls 34 and 36 are formed by providing a silicon rubber layer on a metal pipe and further coating a high release layer thereon. The optimum value of the surface potential required to transfer the toner image from the photoreceptors 26Y to 26C to the first intermediate transfer rolls 34 and 36 is determined by the type and conveyance speed of the recording paper S, the charged state of the toner, It is set by the ambient temperature, humidity, etc. Here, the constant voltage control method is employed, and when the recording paper S is plain paper and the conveyance speed is full speed, the optimum value is set to +400 V in a normal temperature and normal humidity environment (see FIG. 3).

  Similar to the first intermediate transfer rolls 34 and 36, the second intermediate transfer roll 38 is formed by providing a silicon rubber layer on a metal pipe and further coating a high release layer thereon. The optimum value of the surface potential necessary for transferring the toner image from the first intermediate transfer rolls 34 and 36 to the second intermediate transfer roll 38 is determined based on the type, transport speed, and toner charge of the recording paper S. It is set according to the state, ambient temperature, humidity, and the like. Here, the constant voltage control method is employed, and when the recording paper S is plain paper and the transport speed is full speed, the optimum value is +850 V in a normal temperature and normal humidity environment (see FIG. 3).

  The final transfer roll 72 constituting the final transfer portion 16 is formed by providing a urethane rubber layer on a metal pipe and further applying a resin coating thereon. In order to transfer the toner image from the second intermediate transfer roll 38 onto the recording paper S, the optimum value of the transfer voltage applied to the final transfer roll 72 is determined based on the type of the recording paper S, the conveyance speed, and the toner. It is set according to the charging state, ambient temperature, humidity and the like. Usually, the transfer voltage applied to the final transfer roll 72 is in the range of about +1200 V to +4200 V. Here, the constant voltage control method is adopted, and the optimum value is about +2000 V in an environment of normal temperature and humidity.

  Unlike the first intermediate transfer rolls 34 and 36 and the second intermediate transfer roll 38, the final transfer roll 72 is configured such that a cleaning roll (cleaning body) is not in contact therewith. Further, the optimum value of the cleaning voltage to be applied to the cleaning rolls 44, 54, 64 and the brush rolls 48, 58, 68 of the cleaning devices 42, 52, 62 also depends on the type and conveyance speed of the recording paper S, the charged state of the toner, It is set by the ambient temperature, humidity, etc.

  For example, the optimum value of the cleaning rolls 44 and 54 of the first cleaning devices 42 and 52 is +200 V in a normal temperature and humidity environment, and the optimum value of the cleaning roll 64 of the second cleaning device 62 is normal temperature. Under normal humidity environment, it is +1200 V (see FIG. 3).

  In the fixing device 18, a heating roll 74 and a pressure roll 76 are pressed to form a fixing nip. A halogen lamp (not shown) as a heat source is disposed in the heating roll 74, and is configured to heat the surface of the heating roll 74 to a predetermined fixing temperature during fixing. A fixing discharge roll pair 78 is disposed downstream of the fixing nip in the conveyance direction of the recording paper S.

  The paper feed unit 20 is formed along a conveyance path (indicated by a dotted line) P of the recording paper S from the paper feed tray 80 to the discharge tray 82. A plurality of recording sheets S are accommodated in the paper feed tray 80, and from the paper feed tray 80 to the downstream side of the transport path P, a roll pair 84 and a transport roll pair 86 including a pickup roll 84 </ b> A and a retard roll 84 </ b> B. A resist roll pair 88 and a discharge roll pair 90 (via the final transfer roll 72 and the fixing device 18) are arranged in this order.

  The full color printer 10 having such a configuration operates as follows. First, yellow, magenta, black, and cyan toner images are formed on the photosensitive drums 26Y to 26C by the image forming units 22Y to 22C, respectively. That is, the surface of each photosensitive drum 26 is uniformly charged by the charging roll 28, and a laser beam L corresponding to an output image is irradiated from the exposure device 24 onto the surface of the photosensitive drum 26 after charging, and the exposed portion. An electrostatic latent image is formed on the photosensitive drum 26 due to the potential difference from the unexposed portion. The developing device 30 selectively applies toner to the electrostatic latent image, and a toner image is formed on the photosensitive drum 26.

  First, the magenta toner image is primarily transferred from the magenta photosensitive drum 26M to the first upstream intermediate transfer roll 34. Next, the yellow toner image is primarily transferred from the yellow photosensitive drum 26 </ b> Y to the first upstream intermediate transfer roll 34 and is superimposed on the magenta toner image. On the other hand, similarly, a cyan toner image is primarily transferred from the cyan photosensitive drum 26C to the first downstream intermediate transfer roll 36. Next, the black toner image is primarily transferred from the black photosensitive drum 26 </ b> K to the first downstream intermediate transfer roll 36.

  The primary-transferred magenta and yellow toner images are secondarily transferred to the second intermediate transfer roll 38. On the other hand, the primary transferred cyan and black toner images are also secondarily transferred to the second intermediate transfer roll 38, where the magenta and yellow toner images, the cyan toner image previously transferred, Are superimposed on each other to form a full-color toner image on the second intermediate transfer roll 38.

  The secondary transfer full color toner image and black toner image reach the nip portion between the second intermediate transfer roll 38 and the final transfer roll 72. In synchronization with the timing, the recording sheet S as a recording sheet is conveyed from the registration roll pair 88 (see FIG. 1) to the nip portion, and the full-color toner image and the black toner image are tertiary-transferred onto the recording sheet S (final transfer). )

  The recording sheet S then passes through the nip portion between the heating roll 74 and the pressure roll 76 of the fixing device 18 (see FIG. 1). At that time, the full color toner image and the black toner image are fixed on the recording paper S by the action of heat and pressure applied from both rolls 74 and 76 to form a permanent image. Further, thereafter, the recording paper S is discharged to the discharge tray 82 by the discharge roll pair 90, and the full color image formation is completed.

  As described above, an image is formed (printed) on the recording paper S. The average particle size of the toner used here, particularly the black toner, is 7 μm or less (preferably in order to improve the character transferability) Is 5.5 μm to 6.5 μm). Therefore, this black toner is likely to be a reverse polarity toner, but as shown in FIG. 3, the photosensitive unit 22 of each color has the first: yellow (Y), second: magenta (M), 3 No .: Black (K) and No. 4 Cyan (C) are arranged in this order, and the magenta toner and the yellow toner are primarily transferred in order to the first upstream intermediate transfer roll 34, and the first downstream intermediate transfer roll In FIG. 36, cyan toner and black toner are primarily transferred in order. Therefore, the black toner primarily transferred to the first downstream intermediate transfer roll 36 is not reversely transferred (retraduced) to the cyan photosensitive drum 26C.

  In addition, since the first downstream intermediate transfer roll 36 to which the cyan toner and the black toner are sequentially transferred in order is provided with the first downstream cleaning device 52 as described above, The black toner and cyan toner remaining without being secondarily transferred to the intermediate transfer roll 38 are suitably removed by the cleaning device 52.

  In other words, the first downstream cleaning device 52 is provided with the brush roll 58 and the cleaning roll 54 regardless of whether the black toner remains the positive toner or the reverse polarity toner. By controlling the applied voltage to form an appropriate potential gradient, both the positive toner and the reverse toner can be collected and removed. Therefore, as shown in FIGS. 4 and 5, there is no problem that stains the cyan charging roll (charger) 28C, and no image defect such as fogging or ghosting due to charging failure occurs. The intermediate transfer body 1 in FIGS. 4 and 5 is the first downstream intermediate transfer roll 36.

  In addition, as shown in FIG. 6, another first intermediate transfer roll is provided between the first upstream intermediate transfer roll 34 and the first downstream intermediate transfer roll 36 so that the first intermediate intermediate roll is provided. The present invention also includes an image forming apparatus that is configured as a transfer roll 35 and is configured such that only black toner is primarily transferred to the first downstream intermediate transfer roll 36.

  That is, in this case, for example, only yellow toner is primarily transferred to the first upstream intermediate transfer roll 34, and magenta toner and cyan toner are primarily transferred to the first central intermediate transfer roll 35. First, yellow (Y), second: magenta (M), third: cyan (C), fourth: black (K) are arranged in this order, but the first downstream intermediate transfer roll Since only the black toner is transferred to 36, the black toner that has been primarily transferred to the photosensitive drums 26Y, 26M, and 26C of other colors is not reversely transferred (retra). That is, in this case as well, there is no problem that stains the charging rolls 28Y, 28M, and 28C, and image defects such as fogging and ghosting due to charging defects do not occur.

  In order to improve character transferability and fluidity, black toner may contain 0.7% by weight or more of an external additive of silica having an average particle size of 0.1 μm or more. However, silica has a moisture absorption property. Therefore, the reverse transcription (retra) phenomenon as described above is likely to occur. Therefore, in such a case as well, it is desirable to configure in the same manner as described above, and according to this, the reverse transfer (retra) phenomenon can be effectively prevented.

Schematic side view of a full-color printer according to the present invention Schematic side view of essential parts of a full-color printer according to the present invention Operation explanatory diagram of a full-color printer according to the present invention Explanatory drawing showing the black toner remaining on the black photosensitive drum Explanatory drawing showing cyan toner retraced on black photosensitive drum Operation explanatory diagram of a full color printer of another embodiment according to the present invention Operation explanatory diagram of a conventional full color printer Explanatory drawing showing black toner retreated on a cyan photosensitive drum

Explanation of symbols

10 Printer (image forming device)
22 Image forming unit 26 Photosensitive drum (image carrier)
34 First upstream intermediate transfer roll (primary transfer member / first primary intermediate transfer member)
36 First downstream intermediate transfer roll (primary transfer member / second primary intermediate transfer member)
38 Second intermediate transfer roll (secondary transfer member / secondary intermediate transfer member)
72 Final transfer roll (tertiary transfer body / final transfer body)

Claims (8)

At least a plurality of image carriers each carrying a toner image of each color including at least a black color, and a plurality of image carriers that are in contact with each other, and a plurality of toner images on the image carrier that are primarily transferred A secondary transfer body that is in contact with the primary transfer body, all of the plurality of primary transfer bodies, and a toner image on the primary transfer body is secondarily transferred, and is in contact with the secondary transfer body, on the secondary transfer body; An image forming apparatus comprising: a tertiary transfer member that performs third transfer of the toner image;
An image forming apparatus, wherein black toner having an average particle size of 7 μm or less is transferred to a downstream side of a primary transfer body provided on a downstream side in the rotation direction of the secondary transfer body. At least a plurality of image carriers each carrying a toner image of each color including at least a black color, and a plurality of image carriers that are in contact with each other, and a plurality of toner images on the image carrier that are primarily transferred A secondary transfer body that is in contact with the primary transfer body, all of the plurality of primary transfer bodies, and a toner image on the primary transfer body is secondarily transferred, and is in contact with the secondary transfer body, on the secondary transfer body; An image forming apparatus comprising: a third transfer member that performs third transfer of the toner image;
An image forming apparatus, wherein black toner having an average particle size of 7 μm or less is not transferred at least upstream of a primary transfer member provided downstream of the secondary transfer member in the rotation direction. At least a plurality of image carriers each carrying a toner image of each color including at least a black color, and a plurality of image carriers that are in contact with each other, and a plurality of toner images on the image carrier that are primarily transferred A secondary transfer body that contacts the primary transfer body, all of the plurality of primary transfer bodies, and a toner image on the primary transfer body is secondarily transferred, and contacts the secondary transfer body, on the secondary transfer body. An image forming apparatus comprising: a tertiary transfer member that performs third transfer of the toner image;
A black toner containing 0.7% by weight or more of an external additive of silica having an average particle diameter of 0.1 μm or more is provided downstream of the primary transfer member provided on the downstream side in the rotation direction of the secondary transfer member. An image forming apparatus that is transferred. At least a plurality of image carriers each carrying a toner image of each color including at least a black color, and a plurality of image carriers that are in contact with each other, and a plurality of toner images on the image carrier that are primarily transferred A secondary transfer body that is in contact with the primary transfer body, all of the plurality of primary transfer bodies, and a toner image on the primary transfer body is secondarily transferred, and is in contact with the secondary transfer body, on the secondary transfer body; An image forming apparatus comprising: a tertiary transfer member that performs third transfer of the toner image;
A black toner containing 0.7 wt% or more of an external additive of silica having an average particle size of 0.1 μm or more at least on the upstream side of the primary transfer body provided on the downstream side in the rotation direction of the secondary transfer body Is not transferred, an image forming apparatus. Four image carriers that carry toner images of yellow, magenta, black, and cyan, and two of the four image carriers, and the toner images on both image carriers are primarily transferred. A first intermediate transfer member, a second primary intermediate transfer member that is in contact with the remaining two of the four image carriers and to which a toner image on both image carriers is primarily transferred, and the first and second A secondary intermediate transfer body in which the toner images on both primary intermediate transfer bodies are in contact with the primary intermediate transfer body and secondary transferred, and a toner image on the secondary transfer body is recorded in contact with the secondary intermediate transfer body. And a first primary intermediate transfer body on the upstream side in the rotational direction of the secondary intermediate transfer body with reference to a position where the final transfer body contacts the secondary intermediate transfer body. However, in the image forming apparatus provided with the second primary intermediate transfer member on the downstream side,
An image forming apparatus, wherein black toner having an average particle size of 7 μm or less is transferred downstream of the second primary intermediate transfer member. Four image carriers that carry toner images of yellow, magenta, black, and cyan, and two of the four image carriers, and the toner images on both image carriers are primarily transferred. A primary intermediate transfer member, a second primary intermediate transfer member that is in contact with the remaining two of the four image carriers and to which a toner image on both image carriers is primarily transferred, and the first and second A secondary intermediate transfer body in contact with the primary intermediate transfer body and the toner images on both primary intermediate transfer bodies are secondary transferred, and a toner image on the secondary transfer body in contact with the secondary intermediate transfer body And a first primary intermediate transfer body on the upstream side in the rotational direction of the secondary intermediate transfer body with reference to a position where the final transfer body contacts the secondary intermediate transfer body. In the image forming apparatus provided with the second primary intermediate transfer member on the downstream side,
An image forming apparatus, wherein black toner having an average particle diameter of 7 μm or less is not transferred at least upstream of the first and second primary intermediate transfer members. Four image carriers that carry toner images of yellow, magenta, black, and cyan, and two of the four image carriers, and the toner images on both image carriers are primarily transferred. A primary intermediate transfer member, a second primary intermediate transfer member that is in contact with the remaining two of the four image carriers and to which a toner image on both image carriers is primarily transferred, and the first and second A secondary intermediate transfer body in which the toner images on both primary intermediate transfer bodies are in contact with the primary intermediate transfer body and secondary transferred, and a toner image on the secondary transfer body is recorded in contact with the secondary intermediate transfer body. And a first primary intermediate transfer body on the upstream side in the rotational direction of the secondary intermediate transfer body with reference to a position where the final transfer body contacts the secondary intermediate transfer body. However, in the image forming apparatus provided with the second primary intermediate transfer member on the downstream side,
An image forming method wherein black toner containing 0.7% by weight or more of an external additive of silica having an average particle size of 0.1 μm or more is transferred downstream of the second primary intermediate transfer member. apparatus. Four image carriers that carry toner images of yellow, magenta, black, and cyan, and two of the four image carriers, and the toner images on both image carriers are primarily transferred. A primary intermediate transfer member, a second primary intermediate transfer member that is in contact with the remaining two of the four image carriers and to which a toner image on both image carriers is primarily transferred, and the first and second A secondary intermediate transfer body in which the toner images on both primary intermediate transfer bodies are in contact with the primary intermediate transfer body and secondary transferred, and a toner image on the secondary transfer body is recorded in contact with the secondary intermediate transfer body. And a first primary intermediate transfer body on the upstream side in the rotational direction of the secondary intermediate transfer body with reference to a position where the final transfer body contacts the secondary intermediate transfer body. However, in the image forming apparatus provided with the second primary intermediate transfer member on the downstream side,
A black toner containing 0.7% by weight or more of an external additive of silica having an average particle size of 0.1 μm or more is not transferred at least upstream of the first and second primary intermediate transfer members. Image forming apparatus.

Images