JP2011180343A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-speed, high-reliable and inexpensive image forming apparatus, which forms double-sided images in an image forming form of the so-called one-path form. <P>SOLUTION: The image forming apparatus includes: a first intermediate transfer body to which first toner images formed on a plurality of first image carriers are primarily transferred, and which is the intermediate transfer body for carrying the first toner image; and a second image carrier for carrying a second monochrome toner image. The first toner image carried on the intermediate transfer body is electrostatically transferred to a first surface of a recording medium, and the second toner image is directly electrostatically transferred to a second surface of the recording medium on a reverse side to the first surface on an upstream side or a downstream side of the intermediate transfer body in a recording medium conveying direction. The recording medium which carries the toner images on the first and second surfaces is fed to a fixing stage, to perform double-sided image fixing at a time, and a charge polarity of the first toner image carried on the intermediate transfer body is made to be different from a charge polarity of the second toner image. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an improvement in an image forming apparatus such as an electrophotographic printer, a facsimile, a copying machine, or a multifunction machine having at least two of these functions, and more specifically, forms images on both sides of a recording medium such as recording paper. The present invention relates to an improvement of an image forming apparatus capable of performing the above.

  In image forming apparatuses such as electrophotographic printers, facsimiles, and copiers, an image can be formed on both surfaces (front and back surfaces, or first and second surfaces) of a recording medium such as recording paper. Things are known. As a means for achieving this type of double-sided image, the first toner image on one side formed on the image carrier is temporarily transferred and fixed on the first side of the recording medium, and the recording medium is A method of passing through a reversing path or a switchback path and then reversing, then feeding again to the same image forming unit and transferring and fixing a second toner image on the second surface or back surface of the recording medium ( The two-pass method) has been commonly used.

  However, when double-sided image formation is performed by this type of two-pass method, since the first-side image is fixed on the recording medium by performing a fixing process, the recording medium is once subjected to heat and pressure. Therefore, when the second side image is to be formed due to this, the recording medium is curled or paper dust is adhered. When the recording medium is curled or has paper dust attached, the recording medium is switched in the conveying direction or is held between a pair of conveying rollers in order to rotate or reverse the recording medium in the image forming apparatus. When transporting a relatively complicated recording medium, it becomes easy to cause a transport jam, and there are many problems in ensuring the reliability of transporting the recording medium. In addition, image formation failure may occur due to transfer failure to the curled recording medium. Furthermore, after forming the first surface image and performing the fixing process, in order to obtain the second surface image, the second image is formed again using the same image forming unit, and the fixing process is performed again. Since double-sided image formation is performed, double-sided image formation simply takes twice as long as single-sided image formation, and the speed-up problem required of recent image forming apparatuses can also be addressed. There was also a problem that it was difficult.

  Therefore, a technique for obtaining double-sided image formation as described in Patent Document 1 and Patent Document 2 in a single fixing step is known. In the image forming apparatuses disclosed in Patent Document 1 and Patent Document 2 capable of forming double-sided images, a toner image of a first surface image is first formed on a plurality of image carriers, and this is formed on a first intermediate transfer belt. Once formed as a full-color toner image, the full-color toner image on the first intermediate transfer belt is secondarily transferred onto a second intermediate transfer belt arranged along the conveyance path of the recording medium. Next, a second surface image is formed on the first intermediate transfer belt, and the second surface image on the first intermediate transfer belt is secondarily transferred to a recording medium while the second surface image on the second intermediate transfer belt is transferred. One image is transported in close contact with the recording medium, and a second transfer unit such as a transfer charger facing the second intermediate transfer belt provided downstream of the secondary transfer nip portion in the transport direction is used for the second transfer. The first image that is in close contact with the recording medium on the intermediate transfer belt is subjected to tertiary transfer, and then the double-sided image is fixed to the recording medium by simultaneously applying heat and pressure from both sides of the recording medium with a fixing device. . That is, in the image forming apparatuses disclosed in Patent Document 1 and Patent Document 2, the secondary transfer nip portion onto the recording medium is transferred to the second surface of the second toner image on the second surface, and then transferred to the tertiary transfer portion. Thus, the so-called one-pass double-sided image formation is achieved by transferring the first toner image onto the first surface in the subsequent stage. Certainly, when such an image forming apparatus is used, there is no need to provide a reverse conveying path or a switchback conveying path, and it is possible to pass the recording medium through the fixing device when forming images on both sides of the recording medium. Since it is only once, not only the problem of transfer failure due to curling of the recording medium caused by the application of heat and pressure but also the problem of conveyance jam due to the curl and paper dust Resolve.

  However, even if such a configuration is adopted, since the same image forming unit is used twice to form both-side images, there is room for improvement in the problem of speeding up the image forming speed. is there. With regard to this speed-up problem, for example, it is desired to form a relatively simple double-sided image such that a full-color image such as a graphic is formed on one side and a single-color toner image containing only character information is formed on the other side. In particular, such double-sided image formation is frequently used in the form of a direct mail or a leaflet. As a result, many requests have been made. Even in the case of carrying out a relatively simple double-sided image formation in which only a single color toner image is formed on one side, the same image forming unit is used twice in the conventionally known one-pass format. Since double-sided images are formed, improvement in speed-up is particularly desired. In addition, since the same image forming unit, and hence the same image carrier, is used, the toner image charged with the same polarity is subjected to secondary transfer and tertiary transfer on both sides of the recording medium. As a result, the toner image on the second surface carried by the intermediate transfer member may cause toner scattering during the third transfer of the first surface image, thereby contaminating the inside of the image forming apparatus. There may be a problem of image defects due to scattered toner adhering to the recording medium due to the above, and there is a problem in the reliability of the image to be formed. Further, in these image forming apparatuses, since the secondary transfer belt on which the toner image to be tertiary transferred is once transferred needs to be conveyed to the tertiary transfer portion while being in close contact with the recording medium, the second endless second transfer belt is required. The secondary transfer belt needs to have a length corresponding to at least the entire surface of the recording medium, and there is a problem that the apparatus is large and expensive.

  SUMMARY OF THE INVENTION In view of the above-described conventional problems, an object of the present invention is to provide a high-speed, high-reliability, and low-cost image forming apparatus that forms a double-sided image using a so-called one-pass image forming method. To do.

In order to achieve the above object, the present invention provides:
A first intermediate transfer member to which a first toner image formed by a plurality of first image carriers is primarily transferred, and an intermediate transfer member for supporting the first toner image;
A second image carrier for carrying a monochrome second toner image;
A first transfer means facing the first intermediate transfer body, the first transfer means for electrostatically transferring the first toner image to a first surface of a recording medium;
A second transfer unit opposed to the second image carrier, wherein the second toner image is a recording medium opposite to the first surface on which the first toner image is electrostatically transferred; A second transfer means for electrostatic transfer to the second surface,
The first toner image carried on the intermediate transfer member is electrostatically transferred to the first surface of the recording medium by applying a transfer bias between the intermediate transfer member and the first transfer unit. And
By applying a transfer bias between the second image carrier and the second transfer unit, the second toner image is upstream or downstream of the intermediate transfer member in the recording medium conveyance direction. , Directly electrostatically transferred to the second surface of the recording medium,
An image forming apparatus in which a recording medium carrying a toner image on the first surface and the second surface is sent to a fixing step, and double-sided image fixing is performed at once,
An image forming apparatus is proposed in which the toner charge polarity of the first toner image carried on the intermediate transfer member is different from the toner charge polarity of the second toner image.

In the present invention, the charging polarity of the plurality of first image carriers and the charging polarity of the second image carrier are the same, and the first image carrier is the first image carrier. Developed by using a reversal developing method in which the toner has a charge having the same polarity as the charged polarity of the body or a normal developing method in which the toner having a charge having a polarity opposite to the charged polarity is used, It is preferable that the image carrier is developed by a reversal development method in which the first image carrier is developed or a development method opposite to the regular development method.
Furthermore, in the present invention, the first toner image formed on the first image carrier is developed by the reversal development method, and the second toner formed on the second image carrier. More preferably, the image is developed by the regular development method.

  Furthermore, in the present invention, an endless conveyance / transfer belt that is wound around at least two conveyance / transfer belt rollers is provided along the conveyance path of the recording medium, and the at least two conveyance / transfer belt rollers include the intermediate transfer belt. A transfer roller for a transfer transfer belt facing one of the transfer member or the second image carrier, which is disposed on the downstream side when viewed in the transfer direction of the recording medium, wherein the transfer roller is the first transfer unit or the second image carrier. A transfer roller for a conveyance transfer belt provided in place of the transfer means, and a conveyance transfer belt which is disposed upstream of the conveyance transfer belt transfer roller as viewed in the recording medium conveyance path and is electrically floated. It is preferable to have a float roller for use.

  Furthermore, in the present invention, the first intermediate transfer member is provided on the second image carrier with a separation unit for separating the first transfer member from the first transfer unit or the transfer roller for the transfer belt. It is preferable to form an image using only the second toner image thus formed.

Furthermore, in the present invention, after the intermediate transfer member is separated from the first transfer unit or the transfer roller for the transfer roller by the separation unit, the intermediate transfer member is disposed inside the intermediate transfer member. It is preferable that at least one of the support rollers around which the belt is wound is displaced so that the belt tension of the intermediate transfer member is maintained.
Furthermore, in the present invention, after the intermediate transfer member is separated from the first transfer unit or the transfer roller for the transfer transfer belt by the separation unit, the first transfer unit or the transfer for transfer belt is used. It is preferable that a guide roller that is displaced so as to be in contact with the transfer roller and that guides conveyance of the recording medium is provided.

  Furthermore, in the present invention, one of the plurality of first image carriers is a third image carrier that carries a monochromatic toner image, and the plurality of first image carriers and the intermediate transfer member. And the intermediate transfer member is provided on the first surface side for electrostatic transfer to the first surface of the recording medium, and the intermediate transfer member is downstream of the third image carrier in the recording medium conveyance direction. A charging polarity of a first toner image carried on the plurality of first image carriers and a toner charging polarity of a third toner image carried on the third image carrier; It is also preferable to configure the second toner image carried on the second image carrier to have a different toner charging polarity.

  Furthermore, in the present invention, a second image carrier that carries the monochrome toner image is provided. A second surface of the recording medium is further provided with a plurality of fourth image carriers. A second intermediate transfer member for carrying four toner images is provided, and the toner charge polarity of the first toner image carried on the plurality of first image carriers and the third image carrier are carried on the third image carrier; The toner charge polarity of the third toner image, the toner charge polarity of the second toner image carried on the second image carrier, and the fourth toner image carried on the fourth image carrier. It is also preferable that the toner charging polarity is different.

Furthermore, in the present invention, it is preferable that the second toner image and / or the third toner image is a black toner.
Furthermore, in the present invention, the first intermediate transfer member and / or the second intermediate transfer member is an intermediate transfer belt stretched around at least two rollers, and the intermediate transfer belt is an elastic intermediate having elasticity. A transfer belt is more preferable.

  Furthermore, in the present invention, among the toners carried on the second image carrier and / or the third image carrier, the second image carrier and / or the third image carrier are not transferred to the recording medium. It is preferable to collect the toner remaining on the image carrier and reuse it again for development on the second image carrier and / or the third image carrier.

  Furthermore, in the present invention, it is preferable that a reversing conveyance path for reversing the recording medium on which both-side images are fixed in the fixing step is further provided.

  According to the image forming apparatus of the present invention, the first toner image formed by the plurality of first image carriers is a first intermediate transfer member to which the first toner image is primarily transferred, and the first toner image is carried. An intermediate transfer body for carrying out the process, a second image carrier for carrying a second toner image of a single color, and a first transfer means facing the first intermediate transfer body, A first transfer means for electrostatically transferring the toner image to the first surface of the recording medium, and a second transfer means facing the second image carrier, wherein the second toner image And a second transfer means for electrostatic transfer to a second surface of the recording medium opposite to the first surface on which the first toner image is electrostatically transferred. The first toner image carried on the transfer member is applied with a transfer bias between the intermediate transfer member and the first transfer unit. The second toner image is electrostatically transferred to the first surface of the recording medium, and the second image image is transferred to the second image carrier on the upstream side or the downstream side in the recording medium conveyance direction of the intermediate transfer member. Recording medium having a toner image carried on the first surface and the second surface by being electrostatically transferred directly to the second surface of the recording medium by applying a transfer bias to the transfer means Is sent to the fixing step and double-sided image fixing is performed at a time, so that the second image carrier for forming a monochromatic toner image to be formed on one side of the recording medium As a result of the toner image formed on the recording medium being directly transferred to the recording medium, not only can the image forming apparatus be reduced in size, but also in the downsized image forming apparatus, double-sided images on the recording medium can be achieved. Are formed simultaneously on each side of the recording medium. Transcribed, it becomes possible to increase the speed of image forming speed by image fixing at a time subsequent single fixing step. Furthermore, the image forming apparatus of the present invention is configured such that the toner charging polarity of the first toner image carried on the intermediate transfer member is different from the toner charging polarity of the second toner image. Therefore, the recording medium in a state where the first (or second) toner image is transferred due to the difference in charging polarity between the first toner image and the second toner image transferred to both surfaces of the recording medium. When transferring the second (or first) toner image to the toner, a force in the direction attracted to the recording medium acts on the toner transferred to the first (or second) recording medium. As a result, problems such as toner scattering can be effectively prevented. As a result, in combination with the effects described above, it is possible to provide a high-speed, high-reliability, and low-cost image forming apparatus. Become. Note that such an image forming apparatus forms a full-color image such as a graphic on one side and only character information on the other side, particularly when performing double-sided image formation on direct mail, leaflets, etc. When forming a double-sided image in such a form, a great effect can be obtained.

1 is a schematic cross-sectional view for illustrating a basic embodiment of an image forming apparatus according to the present invention. It is a schematic diagram for demonstrating the double-sided image transfer part of this invention. FIG. 5 is a schematic diagram for explaining another embodiment of the double-sided image transfer unit of the present invention, and shows an example in which a conveyance transfer belt facing the intermediate transfer member is provided. FIG. 6 is a schematic cross-sectional view for illustrating another embodiment of the image forming apparatus according to the present invention, and shows an example in which the first intermediate transfer member is separated from the first transfer unit. FIG. 10 is a schematic diagram for explaining still another embodiment of the double-sided image transfer unit of the present invention, in which one of the plurality of first image carriers is separately provided as a third image carrier. The example provided is shown. It is a schematic diagram for demonstrating another embodiment of the double-sided image transfer part of this invention, In the said figure, the example in which several 4th image carrier is further provided is shown. FIG. 9 is a schematic cross-sectional view for illustrating still another embodiment of the image forming apparatus according to the present invention, and shows an example in which a reverse conveyance path for reversing the recording medium is provided at the subsequent stage of the fixing device.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

  FIG. 1 is a schematic sectional view for illustrating a basic embodiment of an image forming apparatus according to the present invention. In the image forming apparatus shown in FIG. 1, an electrophotographic printer is illustrated as an example. . First, the basic configuration of the image forming apparatus of the present invention will be described with reference to FIG.

  As shown in FIG. 1, in this image forming apparatus, as the first image forming unit 100, four drum-shaped image carriers 12 (Y, M, C, K) which are a plurality of first image carriers. ) Is disposed, and a toner image to be transferred is formed on the first surface (or surface) of the recording medium using the first image forming unit 100. Incidentally, yellow (Y), magenta (M), cyan (C) and black (K) toner images are formed on the four image carriers shown here. These image carriers 12 (Y, M, C, K) have the same configuration except for different toner colors, and charging members are provided around these image carriers 12 (Y, M, C, K). 10 (Y, M, C, K), a developing device 11 (Y, M, C, K), a cleaning device 13 (Y, M, C, K), a static elimination device (not shown), and the like are arranged. Note that these photoconductors 12 (Y, M, C, K) have the same configuration except for the color of the toner to be handled, and hence the following symbols (Y, M, C, K) representing colors. The first image forming unit 100 will be described by appropriately omitting.

  First, in the photoreceptor 12, for example, a surface layer of an organic semiconductor, which is a photoconductive substance, is coated on an aluminum cylinder having a diameter of 30 to 120 mm. An amorphous silicon surface layer may be coated. Further, it may be a belt shape instead of a drum shape. The charging member 10 is in pressure contact with the surface of the photoconductor 12, and the charging member 10 is driven to rotate as the photoconductor 12 is driven to rotate. In addition, a predetermined bias voltage is applied to the charging member 10 by a high voltage power source (not shown) so that the surface of the photoconductor 12 to be rotationally driven can be uniformly charged. The charging member 10 shown here employs a roller-like member that contacts the photoreceptor 12 as the first image carrier, but a non-contact type utilizing corona discharge or the like is employed. Is also possible.

  In the image forming apparatus shown in FIG. 1, a first exposure device 7 is provided in parallel below the four first photoconductors 12. The first exposure device 7 has appropriate optical components such as a light source, a polygon mirror, an f-θ lens, and a reflection mirror, and includes image information formed according to image data of each color toner image. Based on this, each photoconductor 12 charged by the charging member 10 is exposed, and an electrostatic latent image is formed on each photoconductor 12. In addition, it can replace with the exposure apparatus 7 of this structure, and can also employ | adopt the exposure means to irradiate LED light from an LED array. The electrostatic latent image formed on the photoconductor 12 using the exposure device 7 is developed and visualized by applying each color toner as it passes through the developing device 11 by the rotation of the photoconductor 12. The Note that toner bottles 20Y, 20M, 20C, and 20K filled with toners of yellow, magenta, cyan, and black are disposed above the image forming apparatus, and the toner bottles 20Y, 20M, and 20C are disposed. A predetermined replenishment amount of toner from each of the color developing devices 11Y, 11M, 11C, and 11K to the respective color developing devices 11Y, 11M, 11C, and 11K through a conveyance path (not shown) from 20K and 20K, and a second developing device 43, a third developing device, and a fourth developing device. The developing device is replenished.

  Further, an endless belt-like intermediate transfer belt 8 configured as a first intermediate transfer member 8 is disposed so as to face each of the photoreceptors 12 serving as the first toner image carrier. The intermediate transfer belt 8 shown in FIG. 1 is configured to be wound around a plurality of support rollers (for example, support rollers 15a, 15b, 15c, etc.), and one of these rollers 15a, 15b, 15c, etc. The intermediate transfer belt 8 is connected to a drive motor (not shown) as a drive source, and by driving the drive motor, the intermediate transfer belt 8 rotates in a counterclockwise direction in the drawing, and other support-rotatable support rollers are provided. It is driven and rotated. A primary transfer roller 9 is disposed on the back surface of the intermediate transfer belt 8 so as to face the photoreceptor 12 with the belt interposed therebetween. A primary transfer bias having a predetermined voltage (plus voltage in the illustrated example) is applied to the primary transfer roller 9 from a high voltage power source (not illustrated), and the developing device 11 causes a predetermined polarity (example illustrated in the figure) by the action of the primary transfer bias. In this case, a toner image visualized with toner charged to a negative polarity is primarily transferred to the intermediate transfer belt 8. The primary transfer residual toner that is left on the photoconductor 12 without being primarily transferred is removed by the cleaning device 13 in preparation for the next image forming operation by the photoconductor 12, and the toner on the photoconductor 12 is completely removed. Removed.

  Furthermore, in the illustrated image forming apparatus 1, a first toner image is electrostatically transferred to the first surface of the recording medium downstream of the primary transfer roller 9 and the like in the driving direction of the intermediate transfer belt 8. As the transfer means, a first secondary transfer roller 18 is provided. The first secondary transfer roller 18 is opposed to the support roller 15b with the intermediate transfer belt 8 serving as a first intermediate transfer member interposed therebetween, and the intermediate transfer belt is formed by the secondary transfer roller 18 and the support roller 15b. A first electrostatic transfer nip portion is formed via 8. In the illustrated example, a (secondary) transfer roller is employed as the first transfer unit. However, the present invention is not limited to this, and for example, a transfer belt stretched around a plurality of rollers. Alternatively, a transfer charger that performs transfer without contact with the intermediate transfer belt 8 that is an intermediate transfer member may be used. Therefore, the transfer roller 18 shown here is an example of a first transfer unit.

  In the image forming apparatus, the first paper feeding cassette 25 having the first paper feeding roller 28 and the second paper feeding cassette 26 having the second paper feeding roller 29 are arranged so as to overlap in the vertical direction. In each of the paper feed cassettes 25 and 26, a recording medium such as a sheet on which an image is to be formed is accommodated in a state where a plurality of recording media are stacked. Further, a manual feed tray 27 having a manual paper feed roller 30 is provided on the right side of the first paper feed cassette 25 in the drawing so as to extend from the side surface of the housing instead of the housing of the image forming apparatus main body. On top of this, a bundle of recording media can be placed. In addition to these paper feed cassettes and manual feed trays, a pair of registration rollers that perform registration of recording media by abutting when a recording medium on which an image is to be formed are conveyed from the paper feed cassette or the manual feed tray. In addition to 35 and the like, a fixing device 50 and a paper discharge roller pair 36 are provided in the subsequent stage such as the image forming unit 100.

  Further, in the image forming apparatus of the present invention, in order to achieve double-sided image formation, the intermediate transfer belt 8 which is a first intermediate transfer body for electrostatic transfer of the first toner image described above to a recording medium. And the second image carrier 40 as the second image forming unit 200 on the upstream side in the recording medium conveyance direction and opposite to the intermediate transfer belt 8 as viewed in the recording medium conveyance path. Is provided. The second image carrier 40 carries a second toner image, which is a monochromatic toner image, and directly electrostatically transfers the second toner image to the second surface (or back surface) of the recording medium. Is provided. The toner used in the second image carrier 40 is preferably black toner that is most commonly used as character information. Unlike the illustrated example, the first image is used. It may be provided on the downstream side in the recording medium conveyance direction when viewed from the forming unit 100.

  In the vicinity of the second image carrier 40, similarly to the photoconductor 12 as the first toner image carrier, a charging member (not shown), a developing device 41, a cleaning device 43, and a charge eliminating device (not shown). Etc.) are arranged. A charging member (not shown) for the second image carrier 40 is in pressure contact with the surface of the photoconductor 40, and a predetermined bias voltage is applied from a high voltage power source (not shown), and the surface of the photoconductor 40 to be rotationally driven is integrated. It can be charged like this. As the charging member 3, it is possible to adopt a method in which the charging member is directly pressed against the second photoconductor 40, or a non-contact type using corona discharge or the like. Is also possible.

  An exposure device (not shown) for writing an electrostatic latent image on the second image carrier 40 charged to a predetermined polarity by the charging member is also provided. Similarly to the exposure device 7 for forming the first toner image, the second exposure device for the second image carrier 40 is also configured to include an appropriate optical component as appropriate. The second image carrier 40 is exposed based on image information formed in accordance with image data of a single color toner image to be formed on the second surface of the medium. The electrostatic latent image formed on the second image carrier 40 is also given a monochromatic toner, preferably a black toner, when passing through the developing device 41 by the rotation of the second image carrier 40. To be developed. The transfer residual toner in the second image carrier remaining without being directly transferred to the conveyed recording medium is removed by the cleaning device 43, and then the second image carrier is also removed by a static eliminator (not shown). The charge on 40 is removed, and the next second surface image forming operation is prepared.

  Next, an image forming operation in the image forming apparatus shown in FIG. 1 will be described with reference to FIGS. FIG. 2 is a schematic diagram for explaining the double-sided image transfer portion of the present invention. Also in this image forming operation, the configuration in which a toner image is formed on each photosensitive member 12 as a plurality of first image carriers and the toner image is transferred to the intermediate transfer belt 8 is different only in the color of the toner image. Since all of them are substantially the same, the subscripts Y, M, C, and K are omitted as necessary.

  First, when the image forming apparatus receives a double-sided image forming operation command from a personal computer (not shown) or the like, the plurality of photoreceptors 12 as the first image carrier and the photoreceptor 40 as the second image carrier are described. The photosensitive drums 12 and 40 are irradiated with light from a static eliminator (not shown) to initialize the surface potential by rotating in a clockwise direction and a counterclockwise direction by a driving source (not shown). The surfaces of the first photosensitive member 12 and the second photosensitive member 40 that have been initialized with the surface potential are uniformly charged to a predetermined polarity by the first charging member 10 and a second charging member (not shown). Is done. Thereafter, in the illustrated embodiment, the surfaces of the charged first photoconductor 12 and second photoconductor 40 are irradiated with laser light from the first exposure device 7 or a second exposure device (not shown). As a result, an electrostatic latent image, which is monochromatic image information obtained by separating a desired full-color image into yellow, cyan, magenta, and black toner color information, is formed on each of the first photoconductors 12. On the other hand, a monochromatic electrostatic latent image to be formed on the second surface of the recording medium is formed on the second photoreceptor 40. When the electrostatic latent images formed on the first photosensitive member 12 and the second photosensitive member 40 pass through the developing device 11 and the developing device 41, respectively, toner of each color (developer) is applied to make the image visible. Visualized as a toner image.

  The intermediate transfer belt 8 as a first intermediate transfer member for carrying the first toner image is driven to run counterclockwise in the figure, but the primary transfer roller 9 includes a photosensitive member 12. A primary transfer voltage having a polarity opposite to the toner charging polarity of the toner image formed on each of the toner images is applied. As a result, a transfer electric field is formed between the photoreceptor 12 as the first toner image carrier and the intermediate transfer belt 8, and the toner images on the photoreceptor 12 are rotationally driven in synchronization with the photoreceptor 12. The primary transfer is electrostatically transferred onto the intermediate transfer belt 8. In this way, the color toner images that are primarily transferred are superimposed on the intermediate transfer belt 8 at sequential timing from the upstream side in the transport direction of the intermediate transfer belt 8, and transferred onto the first surface as a desired full-color image. A full color image to be formed is formed on the intermediate transfer belt 8.

  Here, in the present invention, the charging polarity of the toner carried on the plurality of photoconductors 12 for the first toner image and the charging of the toner carried on the photoconductor 40 for the second toner image. It is charged differently from the polarity. That is, for example, the charging polarity of the toner charged by the developing device 11 to be carried on the first photoconductor 12 is negative, and the toner to be carried on the second photoconductor 40 by the developing device 41. The charge polarity of the toner to be charged is configured to be positive (see FIG. 2).

  On the other hand, the recording medium on which an image is to be formed is stacked as a recording medium bundle on the two paper feed cassettes 25 and 26, the manual feed tray 27, and the like. The registration roller pair 35 is separated and fed one by one by the action of an appropriate conveying member such as the manual sheet feeding roller 30. At that time, the leading edge of the conveyed recording medium comes into contact with the nip portion of the registration roller pair 35 that has not yet started to rotate, and a so-called loop is formed to register the recording medium. Thereafter, the timing of the first toner image formed on the intermediate transfer belt 8 for the first toner image and the second toner image formed on the second image carrier 40 is determined, and the registration roller pair 35 In the example shown in FIG. 1, first, a second image carrier 40 and a second transfer roller 42 as a second transfer unit facing the image carrier 40 are firstly formed. The recording medium is fed to the second toner image electrostatic transfer nip portion. The second transfer unit is not limited to the transfer roller as in the case of the first transfer unit. For example, a transfer belt stretched around a plurality of rollers can be used. A transfer charger that performs transfer without contact with the image carrier 40 can also be used. Therefore, the transfer roller 42 shown here is also an example of the second transfer means. In the electrostatic transfer nip portion for the second toner image, for example, the transfer roller 42 serving as the second transfer unit has a negative transfer bias (in the illustrated example, a polarity opposite to the charged polarity of the second toner image). Transfer voltage) is applied, a transfer bias (transfer voltage) is applied between the transfer roller 42, which is the second transfer means, and the second image carrier 40, whereby the second image carrier. The second toner image formed on the photosensitive member 40 is directly electrostatically transferred to the second surface of the recording medium. Further, the recording medium is provided between an intermediate transfer belt 8 on which a full-color image of a first toner image is primarily transferred and a secondary transfer roller 18 as a first transfer unit facing the intermediate transfer belt 8. The first toner image is transferred to the electrostatic transfer nip portion. At this time, the rotation of the intermediate transfer belt 8 causes the recording medium on which the second toner image is electrostatically transferred at the second toner image electrostatic transfer nip portion to move the first toner at a desired position. The conveyance timing of the recording medium is combined so that the image can be electrostatically transferred to the recording medium.

  The first electrostatic transfer nip portion for toner image is formed by a support roller 15b and a secondary transfer roller 18 as a first transfer means opposed to the support roller 15b via an intermediate transfer belt 8. In the first toner image electrostatic transfer nip portion, for example, a positive transfer in the illustrated example having a polarity opposite to that of the toner image carried on the surface of the intermediate transfer belt 8 on the first secondary transfer roller 18. A bias (transfer voltage) is applied, whereby the first full color toner image primarily transferred to the intermediate transfer belt 8 is collectively electrostatically transferred to the recording medium. Thereafter, the recording medium on which the toner images on the first surface and the second surface are electrostatically transferred is further conveyed to a fixing device 50 constituted by a pair of fixing rollers 51 and the like, each of which is heated. Then, heat and pressure are applied to both sides of the heated fixing roller pair 51 to fix the double-sided toner image on the recording medium as a semi-permanent image. The recording medium on which the image is fixed on both sides is further conveyed and discharged to a recording medium discharge section such as a discharge tray via the discharge roller pair 36, and the image forming operation is completed. The residual toner remaining on the intermediate transfer belt 8 without being transferred at the first toner image electrostatic transfer nip portion where the secondary transfer roller 18 as the first transfer means is disposed is removed by the intermediate transfer belt cleaning. It is removed by the means 19 and collected in the waste toner box 300.

  The image forming apparatus of the present invention configured as described above implements double-sided image formation on the second side (or back side) of the recording medium using a single color toner image. For image formation, for example, when performing double-sided image formation on direct mail, flyers, etc., a full-color image such as a graphic is formed on one side, and only double-sided images are formed on the other side. Compared to conventional image forming apparatuses, it is not necessary to use a second intermediate transfer belt that carries the entire surface of the second toner image to be formed. Can be achieved. Further, in this image forming apparatus, when fixing a double-sided image on a recording medium, a double-sided image is simultaneously formed by the photoconductor 12 as the first image carrier and the photoconductor 40 as the second image carrier. Further, in fixing these images, it is only necessary to pass the recording medium once through the fixing device 50. Thus, not only can an image forming apparatus capable of forming double-sided images at a very high speed be provided, but also curling can be achieved. It is not necessary to reverse the conveyance and switch back of the recording medium that is generated or to which paper dust adheres, and it is possible to provide a highly reliable image forming apparatus that does not cause a problem of conveyance jam. Further, for example, as illustrated in FIG. 2, when the first toner image is electrostatically transferred after the second toner image is electrostatically transferred, the second toner image and the first toner are transferred. Since the charging polarity is different from that of the image, the second toner image previously electrostatically transferred to the recording medium is transferred to the first transfer means 18 in order to electrostatically transfer the first toner image. Even if a voltage (a positive voltage in the example shown in the figure) is applied, the voltage applied to the first transfer means 18 is repelled and comes into close contact with the recording medium. Therefore, it is possible to prevent the image forming apparatus from being contaminated. In addition, it is possible to prevent image defects caused by the scattered toner and to obtain a good image result. In the example described so far, the toner of the toner image on the side of the first secondary transfer roller 18 as an example of the first transfer unit and the second transfer roller 42 as an example of the second transfer unit. By applying a transfer bias different from the charging polarity, the toner image is electrostatically transferred to each surface of the recording medium. On the contrary, for example, the support facing the first transfer means 18 is supported. By applying a transfer bias having the same polarity as the charging polarity of the first toner image to the roller 15b, the repulsive force between the toner in the first toner image and the transfer bias having the same polarity as that of the first toner image is used. Thus, the toner image can be electrostatically transferred to the recording medium.

  Further, when the charging polarity of the first toner image is different from the charging polarity of the second toner image, the photosensitive member 12 as the first image carrier and the photosensitive member as the second image carrier are used. Although it is possible to use the photoconductor 40 as a photoconductor of a different type having a different charge polarity (for example, the photoconductor 12 is configured as a photoconductor that is positively charged, and the toner charge polarity of the first toner image is set to The photosensitive member 40 is configured as a photosensitive pair that is negatively charged while being negatively charged, and the toner charging polarity of the second toner image is made positive.), And has the same charging polarity as shown in FIG. Photoconductors 12 and 40 can also be used.

  When the photoconductors 12 and 40 are configured as photoconductors having the same charging polarity, the photoconductor 12 as the first image carrier is, for example, the charge polarity of the first image carrier. A reversal development system (so-called negative development system) developed using toner having the same polarity charge, or a regular development system (so-called positive development system) developed using toner having a charge opposite in polarity to the charged polarity. The photosensitive member 40, which is the second image carrier, may be configured to be developed by the reverse developing method. As described above, when the same photoconductor is used, the photoconductor 12 as the first image carrier and the photoconductor 40 as the second image carrier can be shared, and as a result, the image forming apparatus can be used. This is preferable because not only the configuration of the image forming apparatus is simplified but also a low-cost image forming apparatus including a running cost can be provided. Note that the positive development method referred to here means that, for example, the photosensitive member is negatively charged by a charging device, and then the background exposure is performed by the exposure device to form a positive electrostatic latent image, and toner fog does not occur in the background portion. This is a method of forming a toner image by adhering positively charged toner to a portion where a negative charge remains without exposure on the photosensitive member using a developing bias of about a level. A developing bias that exposes the image forming part of the charged photoreceptor to form a negative electrostatic latent image and provides a sufficient amount of toner without causing toner fog in the unexposed background part. To form a toner image by adhering negatively charged toner to a portion where the potential is attenuated from a predetermined negative potential. Both developing methods are well known to those skilled in the art. It is a scheme.

  Further, when the photosensitive member 12 and the photosensitive member 40 are used in common, as shown in FIG. 2, a reversal developing method (so-called negative developing method) is adopted as a developing method of the photosensitive member 12 as the first image carrier. The regular developing method (so-called positive developing method) is preferable for the developing method of the photoconductor 40 as the second image carrier. In the case of the positive development system, even if the photosensitive member potential increases after exposure, which may occur when the photosensitive member deteriorates, the background stain is less likely to occur, and further, the photosensitive member itself In the case where a defect such as a scratch occurs, the toner does not adhere to the portion, and the image defect is not noticeable. Therefore, the second image carrier 40 is required to obtain a monochrome image, particularly a character image. This is because it is preferable to use the positive development method on the side because characters can be expressed clearly and unnecessary dot images are not generated.

  So far, the basic embodiment of the image forming apparatus of the present invention has been described with reference to FIGS. 1 and 2, but the present invention is not limited to the configuration shown in FIG. For example, in the example shown in FIG. 1, the second image carrier 40 is disposed on the upstream side in the recording medium conveyance direction as viewed from the intermediate transfer member 8. The object of the present invention can be achieved even if the body 8 is arranged downstream in the conveying direction.

  Further, as described above, in the basic configuration of the present invention shown in FIGS. 1 and 2, the secondary transfer roller 18 is used as a first transfer means for transferring the first toner image to the recording medium. And the transfer roller 42 is used as the second transfer means to directly transfer the second toner image to the recording medium. However, the first transfer means or the second transfer means may be the first transfer means. Instead of the secondary transfer roller 18 and the second transfer roller 42, a transfer transfer belt may be employed. An example of this embodiment is schematically shown in FIG. 3, and another embodiment of the present invention will be described below with reference to FIG.

  In the example shown in FIG. 3, a conveyance transfer belt 70 is provided to face the intermediate transfer body 8 in the first image forming unit 100 disposed on the downstream side in the recording medium conveyance direction of the second image forming unit 200. An example is shown. The endless transport transfer belt 70 shown here is wound around at least two transport transfer belt rollers 71 and 72. Further, the endless transport transfer belt 70 is provided along the transport path of the recording medium. It has been. In the example shown in the drawing, the at least two transport transfer belt rollers 71 and 72 record more than the transport transfer belt roller 71 facing the intermediate transfer belt 8 that is the first intermediate transfer body, and the transport transfer roller 71. The float roller 72 is arranged on the upstream side as viewed in the medium conveyance path, and is electrically floated. Further, since the transfer transfer belt roller 71 is provided in place of the first secondary transfer roller 18 or the second transfer roller 42 described above, for example, a toner image on the intermediate transfer member 8 facing the roller 72 is provided. The transfer roller is also used as a transfer roller to which a secondary transfer bias opposite to that is applied, that is, as a transfer roller for a conveyance transfer belt.

  In the image forming apparatus configured as described above, when performing double-sided image formation, the positively charged toner which is the second toner image electrostatically transferred first on the upstream side when viewed in the conveyance direction of the recording medium is electrically The transfer roller 71 for the transfer roller and the support roller 15b in a state where the positively charged toner is sandwiched between the recording medium and the transfer transfer belt 70. To the first electrostatic transfer nip portion for toner image. That is, the first toner image transferred to the recording medium is sandwiched between the conveyance transfer belt 70 and the recording medium and pressed against the recording medium, while the first toner image is transferred to the recording medium. It is electrostatically transferred to the first surface. Therefore, when the first toner image is electrostatically transferred to the recording medium, the second toner image previously transferred is sandwiched between the conveyance transfer belt 70 and the recording medium, so that the toner scattering is more effective. In addition, the toner image to be formed can be effectively prevented from being disturbed. In the illustrated example, a conveyance transfer belt 70 is provided instead of the secondary transfer roller 18 as a first transfer unit facing the intermediate transfer belt 8 carrying the first toner image. Then, without being limited thereto, for example, when the photoreceptor 40 as the second image carrier is provided on the downstream side as viewed in the recording medium conveyance direction from the intermediate transfer belt 8, A transfer transfer belt 70 can be provided in place of the transfer roller 42 as the second transfer means for the second image carrier. That is, in the embodiment configured as described above, an endless conveyance transfer belt 70 that is wound around at least two conveyance transfer belt rollers 71 and 72 is provided along the conveyance path of the recording medium. The transfer belt rollers 71 and 72 are transport transfer belt transfer rollers 71 facing one side of the intermediate transfer member 8 or the second image carrier 40 disposed on the downstream side in the recording medium transport direction. The transfer roller for transfer belt 71 provided in place of the first transfer means 18 or the second transfer means 42 described above, and the transfer roller 71 for transfer transfer belt as viewed in the recording medium transfer path. It is only necessary to have a float roller 72 for a transfer transfer belt that is disposed on the upstream side and functions as a float roller that is electrically floated. By applying a transfer bias between the transfer roller 71 for the transfer belt and the first intermediate transfer member 8 or the second image carrier 40, the first or second toner image is electrostatically charged. Configured to be transcribed. Note that even in the example shown here, for example, the transfer roller having the same polarity as the charging polarity of the toner image on the intermediate transfer member 8 is applied to the support roller 15b, and the repulsive force with the toner is used. It is also possible to electrostatically transfer an image to a recording medium.

  As yet another embodiment, as indicated by the white arrow in FIG. 4, a support roller 15 b that faces the secondary transfer roller 18 that is the first transfer means and supports the first intermediate transfer body 8. May be provided so that the intermediate transfer member 8 can be separated from the first transfer unit 18. In this configuration, for example, when only a character image is formed on the second surface of the recording medium, it is only necessary to form a monochromatic toner image using the photoconductor 40 as the second image carrier. Therefore, the intermediate transfer member 8 is retracted and the first photosensitive member 12 and the intermediate transfer member 8 are not operated, whereby the intermediate transfer member 8 and the respective photosensitive members 12 serving as the first image carrier are deteriorated. It becomes possible to prevent. Although not shown, when the transfer roller 71 for the conveyance transfer belt is used instead of the secondary transfer roller 18 as the first transfer unit as shown in FIG. 3, the intermediate transfer member It is the transfer roller 71 for the transfer transfer belt that is retracted. Hereinafter, the relationship between the secondary transfer roller 18 serving as the first transfer unit and the separation unit will be described using the illustrated example, but this relationship is naturally replaced by the transfer roller 71 for the transfer transfer belt. Is.

  Further, with respect to the separation means for retracting the support roller 15b from the first secondary transfer roller 18 as the first transfer means and separating the intermediate transfer body 8 from the first secondary transfer roller 18, A link mechanism that is conventionally known and well known to those skilled in the art, or a dedicated solenoid that operates in a direction in which the support roller 15b is separated from the first secondary transfer roller 18 may be used.

  In addition to this, the support roller 15b is retracted, and at the same time, a support roller 25 that is one of the support rollers that support the intermediate transfer belt 8 from the inside of the intermediate transfer belt 8 that is the first intermediate transfer member is provided. By operating or displacing, the belt tension or belt tension of the intermediate transfer belt 8 may be maintained substantially equal to that before the support roller 15b is retracted. This can be achieved by making the retraction amount of the support roller 15b and the displacement amount of the support roller 25 substantially equal. With this configuration, it is possible to prevent the belt tension of the intermediate transfer belt 8 from changing as the support roller 15b is retracted, and to extend the life of the intermediate transfer belt 8. Is preferred. Still further, as shown in FIG. 3, the drive guide roller 23 that contacts the outside of the intermediate transfer belt 8 and guides the drive of the intermediate transfer belt 8 is transferred to the transfer roller 18 that is the first transfer means. It may be configured to abut and serve as a guide roller 23 that guides the conveyance of the recording medium. With this configuration, the guide roller 23 and the first secondary transfer roller 18 are moved after the support roller 15b is retracted from the first secondary transfer roller 18 in order to separate the intermediate transfer belt 8. It is possible to prevent the recording medium from being unstable at the first electrostatic transfer nip portion for toner image, so that the recording medium can be prevented from becoming unstable. In the illustrated example, the drive guide roller 23 is used as the guide roller 23, but a dedicated guide roller that contacts the first secondary transfer roller 18 when the intermediate transfer belt 8 is retracted is provided. However, the object of the present invention can be achieved. The operations of the support roller 25 and the drive guide roller 23 can also be achieved by using a conventionally known link mechanism or a dedicated solenoid.

  Furthermore, in the embodiment shown in FIG. 1, among the toner images carried on the photoconductor 40 carrying a monochromatic toner image, the image on the photoconductor 40 is not electrostatically transferred to the second surface of the recording medium. The second transfer toner remaining on the image carrier remaining in the toner is recovered in a waste toner recovery box 300 via a residual toner recovery transport path. For example, as shown in FIG. A second secondary transfer residual toner recycling path 301 is provided for conveying the transfer residual toner on the second image carrier, and the transfer residual toner is supplied to the photosensitive member 40 again through the developing device 41. Can be reused (so-called toner recycling).

  Next, still another embodiment of the present invention will be described with reference to FIG. FIG. 5 is a schematic diagram for explaining still another embodiment of the double-sided image transfer unit of the present invention, in which the image carrier as one of the plurality of first image carriers 12 is shown. An example in which the body 12K is provided separately as the third image carrier 80 is shown. Note that, like the first image carrier 12, a charging member, an exposure device, a developing device, a cleaning device, and the like are provided around the third image carrier 80. FIG. In order to deepen the image, only the third image carrier 80 and the intermediate transfer belt 8 as the first intermediate transfer member are described in addition to the first image carrier 12 and the second image carrier 40. It should be noted that the other components are not shown except for the fixing roller pair 51 that is heated and pressed against each other.

  In the image forming apparatus having the configuration as shown in FIG. 5, the image carrier 12K, which is one of the plurality of first image carriers 12 described above, converts the monochrome toner image to the first surface of the recording medium. As the third image carrier 80 for transferring to the second image carrier, the plurality of other first image carriers 12 and the intermediate transfer member 8 are provided separately and independently. The photoreceptor 80 as the third image carrier is provided upstream from the plurality of first image carriers 12 in the recording medium conveyance direction and the plurality of first image carriers 12. Arranged on the side. Similarly to the first plurality of image carriers 12, the third image carrier 80 is appropriately exposed from an exposure device (not shown) to form an electrostatic latent image, and then not shown. By passing through the third developing device, toner is applied to form a toner image. At this time, the toner charging polarity of the third toner image carried on the third image carrier 80, The toner charging polarity of the first toner image carried on one of the plurality of image carriers 12 is configured to have the same charging polarity and is carried on the second image carrier 40. The second toner image is configured to be different from the toner charging polarity. In the example shown in FIG. 5 as well, the toner image formed on the photoconductor 80 as the third image carrier is the third image carrier 80 and the second photoconductor 80 facing the photoconductor 80. By applying a transfer bias between three transfer rollers (not shown), the image is electrostatically transferred to the recording medium.

  In the embodiment as shown in FIG. 5, when the full color toner image is formed on the first surface of the recording medium and the single color toner image is formed on the second surface as described above, On the surface side, first, a monochromatic toner image is formed on the third image carrier 80, electrostatically transferred to a recording medium, and then the toner images formed by the first other plurality of image carriers 12 are transferred. A primary transfer is performed on the first intermediate transfer belt 8, and this primary transfer image is superposed on the previously formed third toner image on the recording medium to obtain a full color toner image. At that time, since the third single-color toner image and the first toner image are formed of toner having the same charging polarity, for example, electrostatic charge is applied to the secondary transfer roller 18 serving as the first transfer unit. Even if the transfer bias is applied, the toner does not scatter. Further, on the second surface side opposite to the electrostatic transfer of the toner image on the first surface to the recording medium, a monochrome toner image is electrostatically transferred to the second surface as in the above-described embodiment. In this case, the toner charge polarity of the first toner image and the third toner image is configured to be different from the toner charge polarity of the second toner image. Even when the toner is electrostatically transferred to a recording medium, the problem of toner scattering does not occur as described above, and the object of the present invention can be achieved. Further, when a monochromatic toner image is formed on both sides, the first image carrier 12 and the intermediate transfer member 8 are simply moved by operating the third image carrier 80 and the second image carrier 40. Since it is not necessary to operate the first image carrier 12 and the intermediate transfer member 8, it is possible to extend the life of the first image carrier 12 and the intermediate transfer member 8.

  As described above, the embodiment shown in FIG. 5 has a relatively low frequency of full-color image formation, but is a first image carrier for users who frequently form single-color images on both sides of a recording medium. 12 and the first intermediate transfer body 8 can be extended in service life, and by not operating these image carrier 12 and intermediate transfer body 8, it is very advantageous because the image forming speed can be increased. Note that the toner carried on the photoconductor 80 as the third image carrier is black as is the case with the photoconductor 40 as the second image carrier in order to form a single color toner image such as characters. A toner is preferred.

  Furthermore, in the present embodiment, the third image carrier 80 is provided not only independently and separately from the plurality of other first image carriers 12 but also from the first intermediate transfer member 8. Is also arranged upstream of the recording medium conveyance direction. By arranging in this way, no other toner color from the first intermediate transfer member 8 is mixed into the third image carrier 80, so that it is the same as the second image carrier 40 described above. By providing a third image carrier transfer residual toner recycling path that continues from the third image carrier cleaning device to the third developing device, the second image carrier remaining on the photoconductor 80 as the third image carrier is provided. It is possible to reuse (so-called toner recycling) the third image carrier residual toner.

  A further embodiment of the present invention is then shown in FIG. FIG. 6 is a schematic diagram showing still another embodiment of the image forming unit of the present invention. In FIG. 6, a recording medium provided with a second image carrier 40 that carries a monochromatic toner image is provided. A plurality of fourth image carriers 90 (Y, M, K) are provided on the second surface side, and a fourth image carrier 90 (Y, M, K) formed by the fourth image carriers 90 (Y, M, K). An embodiment is shown in which a second intermediate transfer member 98 is provided on which a toner image is primarily transferred and carries the superimposed toner image. In FIG. 6, in order to deepen the understanding of the invention as in FIG. 5 described above, a plurality of first image carriers, that is, a photoconductor 12, a first intermediate transfer member 8, and a second image carrier. In addition to the photoconductor 40 and the fixing roller pair 51, only a plurality of image carriers 90 (Y, M, C) and a second intermediate transfer belt 98 as a fourth image carrier are shown.

  In the image forming apparatus shown in FIG. 6, a charging member and a developing device are also provided around the plurality of fourth image carriers 90 (Y, M, K), similarly to the first image carrier 12 described above. A cleaning device, a charge removal device, and the like, and a fourth exposure device for creating an electrostatic latent image on the fourth image carrier, and a second intermediate transfer belt 98. A fourth primary transfer roller is also arranged, and a fourth toner image superimposed on the second intermediate transfer belt 98 can be formed in the same manner as the first image carrier. ing. A fourth transfer roller, which is a fourth transfer means for electrostatically transferring the fourth toner image formed on the intermediate transfer belt 98 to a recording medium, also faces the second intermediate transfer belt 98. When the transfer bias is applied between the fourth transfer means and the second intermediate transfer belt 98, the fourth toner image is statically applied to the second surface of the recording medium. In this case, the second toner image formed on the second image carrier 40 disposed upstream of the second intermediate transfer belt 98 in the recording medium conveyance direction is transferred. The toner images are superposed to form a desired full-color toner image. Further, the toner charge polarity of the fourth toner image carried on the fourth image carrier 90 (Y, M, K) is the same as that of the second toner image carried on the second image carrier 40. While being the same as the charging polarity, the toner charging polarity of the first toner images carried on the plurality of first image carriers 12 is different. Although not shown, the third image carrier 80 may be independent from the plurality of first image carriers 12 as shown in FIG. At that time, the toner charge polarity of the third toner image carried on the third image carrier 80 is the same as the toner charge polarity of the toner image carried on the first image carrier 12. The toner charge polarity of the toner image carried on the fourth image carrier 90 is different.

  When configured as shown in FIG. 6, unlike the embodiments described so far, it is possible to cope with the case where a double-sided full-color image is formed. The toner charge polarity of the toner images carried on the first image carrier 12 and the third image carrier 80 for forming the image on the surface, and the second image for forming the image on the second surface, respectively. Since the toner images carried on the carrier 40 and the fourth image carrier 90 are configured to have different toner charging polarities, the problem of toner scattering does not occur as described above. Since the fixing process is performed only once, it is preferable because the problem of conveyance jam due to a recording medium curl or paper dust does not occur. In the embodiment shown in FIG. 6, when forming a full-color image such as a graphic on one side and forming a double-sided image on the other side only in character information, for example, This can be dealt with by not operating the fourth image carrier 90 and the second intermediate transfer member 98.

  Then yet another embodiment is shown in FIG. The embodiment shown in FIG. 7 shows an example of an embodiment in which a reversal conveyance path 500 for reversing a recording medium that has undergone the double-sided image fixing process in the fixing device 50 is further provided.

  In the embodiment shown in FIG. 7, after the recording medium on which the double-sided image is electrostatically transferred in the various embodiments described above is subjected to the fixing process using the fixing roller pair 51 in the fixing device 50, The recording medium is reversed and discharged. In the illustrated example, unless the recording medium is introduced into the reverse conveyance path 500, the image is displayed with the first surface facing down and the second surface facing up. When the sheet is discharged to the outside of the forming apparatus, it is discharged to the outside of the image forming apparatus with the first surface facing up and the second surface facing down. The reversal of the recording medium using the reversal conveyance path 500 is well known to those skilled in the art.

  FIG. 7 illustrates an example of the reverse conveyance path 500. In the illustrated example, switching from the normal discharge direction to the reverse conveyance path 500 is seen in the recording medium conveyance direction rather than the fixing device 50. The switching claw 501 is arranged on the downstream side, and the switching claw 501 receives a driving force from a driving source such as a solenoid (not shown) to change the conveyance direction of the recording medium to the normal discharge direction side or reverse conveyance. It is configured to be displaceable so that it can be switched to the path 500 side. Here, in the state of the switching claw 501 shown in FIG. 7, the switching claw 501 is positioned so as to guide the recording medium to the reverse conveyance path 500 side, and as indicated by the arrow in the figure, The recording medium is introduced to the reverse conveyance path 500 side, and is sandwiched between appropriate reverse conveyance roller pairs arranged in the reverse conveyance path 500, receives the rotational driving force of these reverse conveyance roller pairs, and further reversely conveys. It is made to enter into the road 500. Next, by using a sensor or the like (not shown), for example, when it is detected that the trailing edge of the recording medium has exited the fixing roller pair 51 in the fixing device 50, the reverse conveyance that has conveyed the recording medium into the reverse conveyance path 500 is performed. While the driving of the roller pair is stopped, the reverse conveying roller pair rotates in the reverse direction, and the recording medium that has entered the reverse conveying path 500 is discharged from the reverse conveying path 500. The recording medium that has exited the reverse conveyance path 500 is again guided by the switching claw 501 and the guide plate, conveyed to the discharge roller pair 36, and discharged to the outside of the image forming apparatus through the discharge roller pair 36. .

  The recording medium that has passed through the reversal conveyance path 500 in this way is reversed outside and discharged to the outside of the apparatus. If the recording medium is reversed in this way, for example, only the second image carrier 40 is used. When a monochrome image is formed on one side, the page order can be aligned by discharging the recording medium on which the image is formed with the second side of the recording medium facing down.

  Finally, various embodiments of the present invention have been described so far. The intermediate transfer belt 8 as the first intermediate transfer member and / or the second intermediate transfer belt 98 as the second intermediate transfer member are An elastic intermediate transfer belt having elasticity in order to cope with high image quality demanded in recent years, or to effectively prevent deterioration in image quality such as blurring even with a paper type having a large surface roughness It is preferable to be configured as The elastic intermediate transfer belt is formed by forming a conductive elastic layer made of urethane rubber, silicon rubber or the like on a base layer and covering the surface layer with a fluororesin as a protective layer. Generally known. As the base material of the conductive elastic layer, silicon rubber, acrylonitrile butadiene rubber (NBR), hydrogenated acrylonitrile butadiene rubber (H-NBR), chloroprene rubber (CR), ethylene-propylene-diene-methylene (EPDM), Urethane rubber or the like is used, and the material for the conductive protective layer is particularly limited as long as it can achieve the objectives of reducing frictional resistance, stability to electrical characteristics environment, and improving residual toner cleaning properties by reducing surface roughness. Although not fluoropolymer polymers such as polytetrafluoroethylene (PTFE), a copolymer of tetrafluoroethylene and perfluoroalkyl vinyl ether (PFA), and polyvinylidene fluoride (PVdF), alcohol-soluble nylon, silicone resin, Silane coupler, C It can be used a coating material dissolved or dispersed in the emulsion or organic solvent tan resin. These protective layers can be provided by applying by a method such as dip coating, spray coating, electrostatic coating, or roll coating.

  INDUSTRIAL APPLICABILITY The present invention is preferably used for an image forming apparatus such as an electrophotographic printer, a facsimile machine, a copying machine, or a multi-function machine having at least two functions for performing double-sided image formation in a so-called one-pass format. Can do.

7 First exposure device 8 First intermediate transfer member 9 First primary transfer roller 11 First developing device 12 First image carrier 18 First transfer means 50 Fixing device 51 Fixing roller pair 40 Second Image carrier 41 Second developing device 42 Second transfer means 80 Third image carrier 90 Fourth image carrier 98 Second intermediate transfer member

JP 2005-242169 A JP 2006-3679 A

Claims (13)

A first intermediate transfer member to which a first toner image formed by a plurality of first image carriers is primarily transferred, and an intermediate transfer member for supporting the first toner image;
A second image carrier for carrying a monochrome second toner image;
A first transfer means facing the first intermediate transfer body, the first transfer means for electrostatically transferring the first toner image to a first surface of a recording medium;
A second transfer unit opposed to the second image carrier, wherein the second toner image is a recording medium opposite to the first surface on which the first toner image is electrostatically transferred; A second transfer means for electrostatic transfer to the second surface,
The first toner image carried on the intermediate transfer member is electrostatically transferred to the first surface of the recording medium by applying a transfer bias between the intermediate transfer member and the first transfer unit. And
By applying a transfer bias between the second image carrier and the second transfer unit, the second toner image is upstream or downstream of the intermediate transfer member in the recording medium conveyance direction. , Directly electrostatically transferred to the second surface of the recording medium,
An image forming apparatus in which a recording medium carrying a toner image on the first surface and the second surface is sent to a fixing step, and double-sided image fixing is performed at once,
An image forming apparatus, wherein a toner charging polarity of a first toner image carried on the intermediate transfer member is different from a toner charging polarity of the second toner image. The charging polarity of the plurality of first image carriers and the charging polarity of the second image carrier are the same,
The first image carrier may be a reversal development method that is developed using a toner having the same polarity as the charge polarity of the first image carrier, or a toner having a charge opposite in polarity to the charge polarity. Developed with a regular development method developed using,
The second image carrier is configured to be developed by a reversal development method in which the first image carrier is developed or a development method opposite to the regular development method. The image forming apparatus according to claim 1.   The first toner image formed on the first image carrier is developed by the reversal development method, and the second toner image formed on the second image carrier is the regular development method. The image forming apparatus according to claim 2, wherein the image forming apparatus is developed by the process. An endless conveyance transfer belt wound around at least two conveyance transfer belt rollers is provided along the conveyance path of the recording medium;
The at least two transport transfer belt rollers are transport transfer belt transfer rollers facing one of the intermediate transfer body and the second image carrier, which is disposed on the downstream side in the recording medium transport direction. A transfer roller for the transfer belt provided in place of the first transfer unit or the second transfer unit, and an upstream side of the transfer roller for the transfer transfer belt as viewed in the recording medium transfer path. The image forming apparatus according to claim 1, further comprising a float roller for a conveyance transfer belt that is electrically floated.   A second toner image formed on the second image carrier, comprising a separating unit for separating the first intermediate transfer member from the first transfer unit or the transfer roller for the transfer belt; The image forming apparatus according to claim 1, wherein the image formation is performed only by using the image forming apparatus.   After the intermediate transfer member is separated from the first transfer unit or the transfer roller for the transfer transfer belt by the separation unit, the intermediate transfer member is supported inside the intermediate transfer member. The image forming apparatus according to claim 5, wherein at least one of the rollers is displaced to maintain a belt tension of the intermediate transfer member.   After the intermediate transfer member has been separated from the first transfer unit or the transfer roller for the transfer belt by the separating unit, the intermediate transfer member contacts the first transfer unit or the transfer roller for the transfer belt. 7. The image forming apparatus according to claim 5, further comprising a guide roller that is displaced and that guides conveyance of the recording medium. One of the plurality of first image carriers is a third image carrier that carries a monochromatic toner image, and is separate from the plurality of first image carriers and the intermediate transfer member and the intermediate transfer The body is provided on the first surface side for electrostatic transfer to the first surface of the recording medium;
The intermediate transfer member is disposed downstream of the third image carrier in the recording medium conveyance direction;
The charge polarity of the first toner image carried on the plurality of first image carriers, the toner charge polarity of the third toner image carried on the third image carrier, and the second image carrier The image forming apparatus according to claim 1, wherein the second toner image carried on the body is configured to have a different toner charging polarity. On the second surface side of the recording medium on which the second image carrier that carries the monochrome toner image is provided, a fourth toner image formed by a plurality of fourth image carriers is further carried. A second intermediate transfer member is provided,
The toner charge polarity of the first toner image carried on the plurality of first image carriers, the toner charge polarity of the third toner image carried on the third image carrier, and the second image The toner charging polarity of the second toner image carried on the carrier and the toner charging polarity of the fourth toner image carried on the fourth image carrier are different from each other. Item 9. The image forming apparatus according to any one of Items 1 to 8.   The image forming apparatus according to claim 1, wherein the second toner image and / or the third toner image is a black toner.   The first intermediate transfer member and / or the second intermediate transfer member is an intermediate transfer belt stretched around at least two rollers, and the intermediate transfer belt is an elastic intermediate transfer belt having elasticity. The image forming apparatus according to claim 1.   Of the toner carried on the second image carrier and / or the third image carrier, the toner remains on the second image carrier and / or the third image carrier without being transferred to the recording medium. The image forming apparatus according to claim 1, wherein the toner is collected and reused for development on the second image carrier and / or the third image carrier again. apparatus.   The image forming apparatus according to claim 1, further comprising a reversing conveyance path for reversing the recording medium on which the images on both sides are fixed in the fixing step.

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