JP3658592B2 - Image forming apparatus - Google Patents

Description

The present invention is a copying machine, a facsimile, relates to an image forming equipment such as printers, more particularly, an intermediate transfer member capable of transferring the toner image on the image bearing member, an intermediate transfer member the toner image on said image bearing member those related to the image forming equipment that includes a transfer unit for transferring the above.

  Conventionally, as an image forming apparatus using such an intermediate transfer member, for example, toner images of different colors sequentially formed on an image carrier are sequentially transferred onto the intermediate transfer member and superimposed, 2. Description of the Related Art A color image forming apparatus is known that collectively transfers a transfer material to a transfer material such as paper.

  When performing image formation on both sides of the transfer material in an image forming apparatus equipped with the above intermediate transfer member, the transfer material once discharged out of the apparatus is set again in the manual feed tray or paper cassette, and the reverse / double-sided tray is optional. The method of mounting with can be considered. However, the method of setting the transfer material on the manual feed tray again has a problem that the operability is poor and the image is easily misplaced. In addition, the method using the above-described reversing / double-sided tray is expensive, and particularly when forming a full-color image, the amount of toner on the transfer material increases and the transfer material is likely to curl. It is easy to cause troubles such as jams. If these methods are used in a full color image forming apparatus, the oil contained in the transfer material once passed through the fixing device in order to use the oil in the fixing device, there is a risk of adverse effects on the intermediate transfer member. Is expensive.

  As an image forming apparatus provided with an intermediate transfer member that forms a multicolor image on both sides of a transfer material without using a reversing unit, a multicolor image has been conventionally used on the surface of the image carrier and the intermediate transfer member. The toner images are formed by superimposing the toner images, and the toner images are transferred onto a transfer material conveyed between the image carrier and the intermediate transfer member. However, since this type of image forming apparatus employs a non-contact developing method using a one-component developer to superimpose a toner image on an image carrier, an image such as poor halftone image reproducibility is used. There is a problem that the quality is inferior.

The present invention has been made in view of the above-described problems, and its object is to freely provide a high-quality multicolor image on the front surface, back surface, or both surfaces of the transfer material without providing a reversing means for the transfer material. it is to provide an image forming equipment which can be formed.

In order to achieve the above object, the invention of claim 1 includes a first intermediate transfer member capable of transferring a toner image on an image carrier, and a toner image on the image carrier on a first intermediate transfer member. a first transfer means for transferring the rolling Utsushizai can convey the and the second intermediate transfer member capable of transferring the toner image on the first intermediate transfer member, the toner on the first intermediate transfer member A second transfer means for transferring the image onto the surface of the transfer material conveyed by the second intermediate transfer member or onto the second intermediate transfer member; and a toner image on the second intermediate transfer member to the second intermediate transfer member. And a third transfer means for transferring to the back surface of the transfer material conveyed by the intermediate transfer member. As the second intermediate transfer member, an endless belt member stretched around a plurality of support rollers is used . as second transfer means, distribution inside of the belt member so as to alternate in the transfer to the transfer and the second intermediate transfer member to the transfer material One of the transfer roller provided that, the third transfer means is than the transfer roller, characterized in that arranged at a distance to the transfer material conveying direction downstream side.

In the present invention, when an image is formed only on the surface of the transfer material, a toner image corresponding to the normal image of the desired image is formed on the image carrier. This toner image is transferred to the first intermediate transfer member, and the toner image on the first intermediate transfer member is transferred to the surface of the transfer material conveyed by the second intermediate transfer member.
When an image is formed only on the back surface of the transfer material, a toner image corresponding to a mirror image of the desired image is formed on the image carrier. This toner image is transferred to the first intermediate transfer member and further transferred onto the second intermediate transfer member. Then, the toner image on the second intermediate transfer member is transferred onto the transfer material conveyed by the second intermediate transfer member.
When images are formed on both surfaces of the transfer material, a toner image corresponding to a mirror image of the desired image for the back surface formed on the image carrier is transferred to the second intermediate transfer member via the first intermediate transfer member. Transcribed above. Then, a toner image corresponding to the normal image of the desired image for the surface formed on the image carrier is transferred to the first intermediate transfer member. The toner images transferred to the first intermediate transfer member and the second intermediate transfer member are respectively transferred to the front and back surfaces of the transfer material conveyed by the second intermediate transfer member.

  A single color toner image may be transferred to the first intermediate transfer member and the second intermediate transfer member, or multiple color toner images may be transferred in multiple layers.

According to the present invention, it is not necessary to provide a reversing means for the transfer material, and unlike the case where a plurality of toner images are formed on the image carrier, a contact development method can be adopted, so that a higher There is an effect that a multicolor image with high image quality can be freely formed on the front surface, back surface, or both surfaces of the transfer material .

Hereinafter, an embodiment in which the present invention is applied to a color copying apparatus which is an image forming apparatus will be described. FIG. 1 is a schematic configuration diagram of the entire color copying apparatus according to the present embodiment, and FIG. 2 is a schematic configuration diagram of a main part of the entire copying apparatus.
First, an outline of a color image reading apparatus (hereinafter referred to as “color scanner”) 1 and a color image recording apparatus (hereinafter referred to as “color printer”) 2 constituting the color copying apparatus will be described with reference to FIG.

  The color scanner 1 forms an image of a document 3 on a color sensor 7 through an illumination lamp 4, a mirror group 5, and a lens 6, and the color image information of the document is, for example, blue (hereinafter referred to as B). , Green (hereinafter referred to as “G”), and red (hereinafter referred to as “R”) color separation light is read and converted into an electrical image signal. Then, based on the color separation image signal intensity levels of B, G, and R obtained by the color scanner 1, color conversion processing is performed by an image processing unit (not shown), and black (hereinafter referred to as Bk), cyan (Cyan, hereinafter referred to as C), magenta (Magenta, hereinafter referred to as M), and yellow (Yellow, hereinafter referred to as Y) color image data are obtained.

  In the color printer 2, the writing optical unit 8 converts the color image data from the color scanner 1 into an optical signal, performs optical writing corresponding to the original image, and electrostatically applies to the photoconductor 9 as an image carrier. A latent image is formed. The photosensitive member 9 rotates counterclockwise as indicated by an arrow. Around the photosensitive member 9, a photosensitive member cleaning unit (including a pre-cleaning static eliminator) 10, a static elimination lamp 11, a charger 12, a potential sensor 13, and a Bk developing unit are provided. 14, a C developing unit 15, an M developing unit 16, a Y developing unit 17, an optical sensor 18 for detecting a development density pattern, a first intermediate transfer belt 19 as a first intermediate transfer member, and a first transfer unit as a first transfer unit. A first intermediate transfer belt unit having one transfer bias roller 20a is disposed.

  Each of the above-described developing devices pumps and stirs the developing sleeve (14a, 15a, 16a, 17a) that rotates by bringing the ears of the developer into contact with the surface of the photoreceptor 9 in order to develop the electrostatic latent image. The developing paddles (14b, 15b, 16b, 17b) that rotate to achieve this, and the toner density sensors (14c, 15c, 16c, 17c) of the developer are included.

  In FIG. 2, the first intermediate transfer belt 19 is stretched around a drive roller 21, a first transfer bias roller 20a, and driven roller groups 20b and 20c, and is driven and controlled by a drive motor (not shown) as will be described later. The Further, a separation mechanism (not shown) that drives the first intermediate transfer belt 19 to be detachable from the photosensitive member 9 is provided.

  A belt cleaning unit 22 is provided at a predetermined position facing the surface of the first intermediate transfer belt 19. The unit 22 includes a brush roller 22a, a rubber blade 22b, and a contact / separation mechanism 22c from the belt. The contact / separation operation timing of the belt cleaning unit 22 is separated from the belt surface from the start of printing until the belt transfer at the rear end of the Y color (fourth color of the final color) image is finished, At the timing, the contact / separation mechanism 22c is brought into contact with the belt surface for cleaning.

  Further, a second intermediate transfer belt as a second intermediate transfer body capable of transferring a toner image on the first intermediate transfer belt 19 so as to face a portion of the first intermediate transfer belt 19 wound around the driving roller 21. 23, and a second intermediate transfer belt unit having a second transfer bias roller 23a as a second transfer means. The second intermediate transfer belt 23 opposes the transfer sheet 24 to the drive roller 21 of the first intermediate transfer belt 19 by a plurality of support rollers such as a second transfer bias roller 23a, a drive roller 23b, and driven rollers 23c and d. It is stretched so that it can be directly conveyed from the printing unit to a fixing device 28 described later. By stretching in this way, when the image is formed on the back surface (lower surface in the drawing) of the transfer paper 24, the back surface of the transfer paper 24 contacts the second intermediate transfer belt 23 to which the toner image is transferred. However, the toner image on the back surface of the transfer paper 24 is disturbed because the transfer paper with the toner image transferred to the back surface is directly guided to the fixing device without coming into contact with other conveying members. It is settled without.

  In addition, each of the rollers is arranged so that the stretched portion between the second transfer bias roller 23 a and the driven roller 23 d of the second intermediate transfer belt 23 is pressed against the driving roller 21 via the first intermediate transfer belt 19. It is installed. As described above, since the stretched portion of the second intermediate transfer belt 23 is configured to be softly pressed against the driving roller 21 via the first intermediate transfer belt 19, the transfer roller is interposed via the first intermediate transfer belt 19. Therefore, it is possible to prevent the character part from being lost as compared with the case where the driving roller 21 is pressed with a higher pressing force. Further, the transfer sheet is linearly conveyed at the pressure contact position of the second intermediate transfer belt 23, and the size of the electrical resistance of the first intermediate transfer belt 19 and the second intermediate transfer belt 23 is defined as described later, thereby transferring the transfer paper. Since the effect of adsorbing the paper 24 to the second intermediate transfer belt 23 is obtained, the separation property of the transfer paper from the first intermediate transfer belt 19 is improved, and a static elimination needle for assisting the separation of the transfer paper as in the past. There is no need to provide a separate auxiliary means.

  Further, the toner image on the second intermediate transfer belt 23 is transferred to the back surface of the transfer paper 24 so as to face a portion wound around the driven roller 23b of the second intermediate transfer belt 23 on the fixing device side to be described later. A transfer charger 23e is provided as third transfer means. In this way, by transferring the toner image to the back surface of the transfer paper 24 immediately before the fixing device, for example, the toner on the back surface of the transfer paper 24 once transferred as in the middle of the second intermediate transfer belt 23. The image is not disturbed by the second intermediate transfer belt 23. A belt cleaning unit 23f using a cleaning blade is provided at a portion of the second intermediate transfer belt 23 that is wound around the driven roller 23c.

The first intermediate transfer belt 19 has a thickness of about 150 μm, a surface resistivity of 1 × 10 ⁷ to 1 × 10 ¹² Ω, and a volume, in which carbon is dispersed in a resin material (for example, polycarbonate, fluorine resin, etc.). A resin sheet having a specific resistivity of 1 × 10 ⁸ to 1 × 10 ¹⁴ Ωcm can be used. The upper limit of the resistivity is set so that the electric charge does not remain more than necessary, thereby eliminating the need for a static eliminating means for eliminating the residual electric charge. The lower limit of the resistivity is set such that a predetermined transfer electric field is formed without transfer charge flowing into the surrounding earth member. Moreover, the material whose surface specific resistivity is larger than the volume specific resistivity of the resin sheet is selected, which makes it difficult for dielectric breakdown to occur.

  The resin sheet is less stretched than a rubber material and is suitable for superimposing multi-color toner images that require high-precision alignment. It becomes necessary. FIG. 3 shows a typical deviation regulating means. A deviation regulating member 19 a made of a rubber material is attached to the inner end of the first intermediate transfer belt 19 in the width direction, and a roller 20 b that supports the belt 19. This is regulated at the end in the axial direction.

The second intermediate transfer belt 23 has a thickness on the surface layer of a base rubber 230 such as urethane or chloroprene rubber having a thickness t = 0.5 to 0.6 mm, as shown in the sectional view of FIG. What gave the fluorine coat 231 of (delta) = 10-30 micrometers can be used. A belt using such a rubber material is suitable for a case where a plurality of toner images do not need to be superimposed with high accuracy because the elongation is larger than that of a resin sheet. In the case of a rubber material, as shown in FIG. 4 (b), the belt 23 can be protruded from the end of the row 23b, and the elasticity of the belt 23 can be used to regulate the deviation of the belt. There is no need to provide a means of restricting slippage. Further, in the case of a rubber material, the margin for slip at the time of rotational driving is large. Further, the surface specific resistance of the base rubber of the belt 23 is about 1 × 10 ⁷ to 1 × 10 ¹² Ω, and the surface specific resistivity on the fluorine coat is 1 × 10 ⁸ to 1 × 10 ¹³ Ω. . By using the belt having such a resistivity, there is an advantage that the transfer charge can be reduced and the cost can be reduced by eliminating the charge eliminating means as in the case of the first intermediate transfer belt 19.

When a plurality of toner images are sequentially superimposed on the second intermediate transfer belt 23 with high precision alignment, it is preferable to use a resin sheet having a small elongation as in the case of the first intermediate transfer belt. In this case, there is an advantage that the surface releasability of the second intermediate transfer belt 23 can be prevented from being deteriorated and the life can be extended.
Further, in order to satisfactorily separate the transfer sheet from the first intermediate transfer belt 19, it is preferable that the electric resistance of the second intermediate transfer belt 23 is larger than that of the first intermediate transfer belt 19.

  A fixing device 28 having a pair of upper and lower fixing rollers 280a and 280b is provided at a position where the transfer sheet separated from the second intermediate transfer unit is conveyed, as shown in the enlarged view of FIG. A layer (for example, an RTV silicone rubber layer) of a material having a high releasability and a small surface roughness is formed on the cored bar portion of each fixing roller 280a, b, and heaters 281a, b have. In this way, the surface portion is made of a material having a high releasability and a small surface roughness and excellent smoothness, so that an image with a color toner having a low softening temperature can be transferred onto the transfer paper in order to improve color reproducibility. Even when the toner image is formed on the transfer paper, good color toner images can be fixed on both sides of the transfer paper without causing toner offset to the fixing rollers 280a and 280b. The surface roughness of the fixing rollers 280a and 280b is preferably 4 [mu] m or less, more preferably about 2 [mu] m in terms of JIS 10-point average Rz.

  Further, around the upper and lower fixing rollers 280a and 280b, cleaning members 282a and b, oil supply members 283a and b, peeling claws 284a and b, temperature detection members 285a and b, an overtemperature preventing member 286a and b, and the like are provided. It has been. In the fixing device 28 of the present embodiment, the upper and lower fixing rollers 280a and 280b are composed of the same parts, and various members provided around the respective rollers are made common to reduce the cost.

  The color printer 2 is provided with a paper feed roller 25 and a registration roller 26 for feeding the transfer paper 24 at a predetermined timing. Further, as a control means for controlling the image forming process, a system control board (not shown) including, for example, a CPU, a ROM, a RAM, and an I / O interface is provided, and this system control board has an image mode selection means for selecting an image mode. An operation unit having a mode selection button, a belt drive motor, a solenoid in a belt contact / separation mechanism, various drive devices such as a developing sleeve motor, various detection devices, and the like are connected.

  The transfer paper cassettes 30, 31, 32, and 33 store various sizes of transfer paper, and the registration rollers 26 are timed from the size paper storage cassette designated on the operation panel (not shown). Is fed and transported. Reference numeral 34 denotes a manual paper feed tray such as OHP paper or cardboard.

Next, the toner image forming process and the transfer process in the color image forming apparatus having the above configuration will be described in the case of the Bk, C, M, and Y forming order (however, the toner image forming order is not limited to this). Absent).
When the copying operation is started, reading of the Bk image data is started at a predetermined timing by the color scanner 1, and based on this image data, the photosensitive member 9 uniformly charged by the charger 12 is irradiated with a laser beam. Optical writing and latent image formation begin (hereinafter, an electrostatic latent image based on Bk image data is referred to as a Bk latent image. C, M, and Y are respectively referred to as a C latent image, an M latent image, and a Y latent image). Before the latent image leading edge reaches the developing position of the Bk developing device 14 so that development can be performed from the leading edge of the Bk latent image, the developing sleeve 14a is started to rotate, and the Bk latent image is negatively charged with Bk toner. develop. Thereafter, the developing operation of the Bk latent image area is continued, and when the trailing end of the latent image has passed the Bk developing position, the development inoperative state is set. This is completed at least before the leading edge of the C latent image by the next C image data arrives.

  Then, the Bk toner image formed on the photoconductor 9 is transferred to the surface of the first intermediate transfer belt 19 driven at the same speed as the photoconductor 9 (hereinafter, toner image transfer from the photoconductor 9 to the intermediate transfer belt is performed. "Belt transfer"). This belt transfer is performed by applying a predetermined bias voltage (positive voltage in this example) to the first transfer bias roller 20a in a state where the photosensitive member 9 and the first intermediate transfer belt 19 are in contact with each other.

  Here, on the photoconductor 9 side, the process proceeds to the C process after the Bk process, but reading of C image data by the color scanner 1 starts from a predetermined timing, and a C latent image is formed by laser beam writing using the image data. The C developing unit 15 starts rotating the developing sleeve 15a after the rear end of the previous Bk latent image has passed with respect to the developing position and before the front end of the C latent image reaches, Develop with C toner. Thereafter, development of the C latent image area is continued, and when the rear end portion of the latent image passes, the development inoperative state is set in the same manner as in the previous Bk developing device. This is also completed before the leading edge of the next M latent image arrives. In each of the M and Y processes, the image data reading, latent image forming, and developing operations are performed in the same manner as the Bk and C processes described above.

  As described above, the Bk, C, M, and Y toner images sequentially formed on the photosensitive member 9 are sequentially aligned on the same surface and transferred to the first intermediate transfer belt 19 to form a four-color toner image. To do. Thereafter, the four-color superimposed toner image on the first intermediate transfer belt 19 is applied to the second transfer bias roller 23a in a state where the first intermediate transfer belt 19 is in contact with the transfer paper 24 or the second intermediate transfer belt 23. By applying a bias voltage (a positive voltage in this example), the image is collectively transferred onto the surface of the transfer paper 24 or the second intermediate transfer belt 23. Here, when the four-color superimposed toner image is collectively transferred onto the second intermediate transfer belt 23, the toner image on the second intermediate transfer belt 23 is further transferred by the positive transfer discharge by the transfer charger 23e. It is transferred to the back surface of the paper 24.

  Here, as described above, the first intermediate transfer belt 19 and the second intermediate transfer belt 23 are made to have a medium resistance, so that it is possible to eliminate the need for static eliminating means. Needs to be neutralized by dropping it to ground.

  Further, when toner images having the same polarity are formed on both intermediate transfer belts 19 and 23, the polarity of the toner image on one belt is changed, and simultaneous transfer (one is pushed in) by the second bias transfer roller 23a. However, in the case of the configuration of the color image forming apparatus of this embodiment, the span from the simultaneous transfer position to the separation position where the transfer paper 24 is separated from the second intermediate transfer belt 23 is increased. Since the belt potential at the separation position is substantially 0 V due to the self-static effect of the belt 23, there is a high possibility that a transfer failure will occur. Further, as the charge used for transfer, a charge having a polarity opposite to that of the toner is generally used, and the push-type transfer method using a charge having the same polarity as that of the toner has a small transferability margin. For this reason, in the present embodiment, the transfer of the toner image from the second intermediate transfer belt 23 to the transfer paper 24 is a transfer having a polarity opposite to the polarity of the toner using the transfer charger 23e as the third transfer means. A method using electric charge is employed.

  As shown in FIGS. 6A to 6C, for example, when a four-color toner image is formed on the first intermediate transfer belt 19 in the order of Bk, C, M, and Y, the toner images of the respective colors. When the amount of toner is 1, the amount of toner image on the second intermediate transfer belt 23 transferred at a transfer rate of 90% is 0.9 for only Bk. When the toner image is further transferred onto the transfer paper 24, the amount of Y toner image is now 0.9. That is, only the toner in the lowermost layer and the uppermost layer reduces the amount of toner as a transfer residue. Therefore, in this embodiment, when forming images on both sides of the transfer paper 24 as will be described later, not only the lowermost toner on the first intermediate transfer belt 19 but also the uppermost toner is increased in advance. By setting the amount of toner, the amount of toner is balanced, and the density and hue of the copy images on the front and back surfaces of the transfer paper 24 can be made equal. Note that the toner amount in each color toner image can be easily controlled by changing a known writing light amount or developing bias voltage.

  As described above, the transfer paper 24 on which the four-color superimposed toner images are collectively transferred from at least one of the front and back surfaces from the surfaces of the intermediate transfer belts 19 and 23 is conveyed to the fixing device 28 and controlled to a predetermined temperature. The toner images are melted and fixed by the fixing rollers 280a and 280b, and are carried out to the copy tray 29 to obtain a full color copy.

  The surface of the photoconductor 9 after the belt transfer is cleaned by a photoconductor cleaning unit 10 (pre-cleaning static eliminator 10a, brush roller 10b, rubber blade 10c), and the static eliminator 11 removes the surface uniformly.

  At the time of repeat copy, the operation of the color scanner 1 and the image formation on the photoconductor 9 are the second Bk (first color) image process at a predetermined timing following the first Y (fourth color) image process. Proceed to In addition, the first intermediate transfer belt 19 has the second Bk toner in the region where the surface is cleaned by the cleaning unit 22 following the overall transfer process of the first four-color superimposed image to the transfer paper. The image is transferred to the belt. Thereafter, the operation is the same as that of the first sheet.

  The copy mode for obtaining four full colors has been described above. In the three-color copy mode and the two-color copy mode, the same operation as described above is performed for the designated color and the number of times. In the case of the single color copy mode, the copying operation is performed with only the developing device for that color being in the developing operation (agent spikelet) state until the predetermined number of sheets is completed.

  Next, an image forming operation when each image forming mode (front surface single color mode, back surface single color mode, front surface multicolor mode, back surface multicolor mode) is selected from the operation unit will be briefly described.

[Surface monochromatic mode]
A positive toner image is formed on the photoreceptor 9, and is transferred onto the first intermediate transfer belt 19 by the first bias transfer roller 23a to which a predetermined transfer voltage is applied, thereby forming a mirror toner image. Then, a second bias transfer roller 23a in which a predetermined transfer voltage is applied to the surface of the transfer paper 24 sent from the registration roller 26 with the timing of the mirror image on the first intermediate transfer belt 19 is applied. To form a normal toner image. This normal toner image is fixed on the surface of the transfer paper 24 by the fixing device 28 as a final image.
In this image forming mode, the second intermediate transfer belt 23 is used as a transfer conveyance belt that conveys the transfer paper 24 to the fixing device 28, and the transfer charger 23e is controlled to be inactive or to operate as a separation charger.

[Back side monochrome mode]
A mirror image toner image is formed on the photoconductor 9 and is transferred onto the first intermediate transfer belt 19 by the first bias transfer roller 23a to which a predetermined transfer voltage is applied to form a normal toner image. Then, the positive toner image on the first intermediate transfer belt 19 is transferred onto the second intermediate transfer belt 23 by the second bias transfer roller 23a to which a predetermined transfer voltage is applied to form a mirror toner image. To do. Then, the mirror image toner image on the second intermediate transfer belt 23 is transferred to the back surface of the transfer paper 24 sent from the registration roller 26 at the timing by the transfer charger 23e to which a predetermined transfer voltage is applied. A normal toner image is formed. This normal toner image is fixed on the surface of the transfer paper 24 by the fixing device 28 as a final image.
In this image forming mode, the second bias transfer roller 23a is controlled so as not to operate or to operate with the same polarity as the toner when the toner image is transferred to the back surface of the transfer paper 24.

[Surface multicolor mode (example of 4 colors full color)]
A positive Bk toner image is formed on the photoreceptor 9, and is transferred onto the first intermediate transfer belt 19 by a first bias transfer roller 23a to which a predetermined transfer voltage is applied, thereby forming a mirror Bk toner image. Then, the photoconductor 9 and the first intermediate transfer belt 19 are driven with high accuracy, and the C toner image is transferred by aligning the position on the Bk toner image, and further the M toner image and the Y toner image are transferred. A four-color full-color toner image as a mirror image is formed on the transfer belt 19. A second bias transfer roller in which a predetermined transfer voltage is applied to the surface of the transfer paper 24 sent from the registration roller 26 to the mirror image of the four-color full-color toner image of the mirror image on the first intermediate transfer belt 19. 23a is transferred to form a normal four-color full-color toner image. This normal toner image is fixed on the surface of the transfer paper 24 by the fixing device 28 as a final image.
In this image forming mode, the second intermediate transfer belt 23 is used as a transfer conveyance belt that conveys the transfer paper 24 to the fixing device 28, and the transfer charger 23e is controlled to be inactive or to operate as a separation charger.

[Back side multicolor mode (example of 4 colors full color)]
A mirror Bk toner image is formed on the photoconductor 9, and is transferred onto the first intermediate transfer belt 19 by the first bias transfer roller 23a to which a predetermined transfer voltage is applied, thereby forming a positive Bk toner image. Then, the photoconductor 9 and the first intermediate transfer belt 19 are driven with high accuracy, and the C toner image is transferred by aligning the position on the Bk toner image, and further the M toner image and the Y toner image are transferred. A regular four-color full-color toner image is formed on the transfer belt 19. Then, the four-color full-color toner image of the normal image on the first intermediate transfer belt 19 is transferred onto the second intermediate transfer belt 23 by the second bias transfer roller 23a to which a predetermined transfer voltage is applied. A full color toner image is formed. Then, the four-color full-color toner image of the mirror image on the second intermediate transfer belt 23 is transferred to the back surface of the transfer paper 24 sent from the registration roller 26 at a timing, and a transfer charger 23e to which a predetermined transfer voltage is applied. To form a regular four-color full-color toner image. This regular four-color full-color toner image is fixed on the surface of the transfer paper 24 by the fixing device 28 as a final image.
In this image forming mode, the second bias transfer roller 23a is controlled so as not to operate or to operate with the same polarity as the toner when the toner image is transferred to the back surface of the transfer paper 24.

  When the double-side mode in which image formation is performed on both sides of the transfer paper 24 is selected, the transfer paper is controlled by combining the operations in the above four modes regardless of whether the front and back sides are monochromatic or multi-colored. Images can be formed on both the front and back surfaces of 24.

  As described above, according to the present embodiment, the contact developing method can be adopted as compared with the case where a plurality of toner images are superimposed and formed on the photoconductor 9 without providing a reversing unit. A multi-color image with high image quality can be freely formed on the front surface, back surface, or both surfaces of the transfer paper 24.

  Note that, as the first, second, and third transfer means in the above embodiment, a transfer roller and a transfer charger can be appropriately selected and used. Here, when the transfer charger is used as the second transfer means, the second intermediate transfer belt is different from the case where the transfer roller is used even if it is arranged directly below the drive roller 21 of the first intermediate transfer belt 19. 23 and the first intermediate transfer belt 19 do not increase in pressure contact force.

1 is a front view illustrating a schematic configuration of a color image forming apparatus according to an embodiment. FIG. 3 is an enlarged view around a photoconductor and an intermediate transfer belt of the color image forming apparatus. FIG. 3 is an explanatory diagram of a first intermediate transfer belt of the color image forming apparatus. FIG. 4A is a partial cross-sectional view of a second intermediate transfer belt of the color image forming apparatus. FIG. 6B is an explanatory diagram of the second intermediate transfer belt. FIG. 2 is a front view illustrating a schematic configuration of a fixing device of the color image forming apparatus. (A)-(c) is explanatory drawing of the 4 color toner image on a 1st intermediate transfer belt, a 2nd intermediate transfer belt, and a transfer paper.

Explanation of symbols

9 Photoconductor 19 First intermediate transfer belt 20a First bias transfer roller 23 Second intermediate transfer belt 23a Second bias transfer roller 23e Transfer charger 24 Transfer paper 28 Fixing device 280a, b Fixing roller

Claims (1)

A first intermediate transfer member capable of transferring a toner image on the image carrier;
A first transfer means for transferring a toner image on the image carrier onto a first intermediate transfer member;
A second intermediate transfer member capable of transporting a transfer material and capable of transferring a toner image on the first intermediate transfer member;
A second transfer means for transferring the toner image on the first intermediate transfer member onto the surface of the transfer material conveyed by the second intermediate transfer member or onto the second intermediate transfer member;
A third transfer means for transferring the toner image on the second intermediate transfer member to the back surface of the transfer material conveyed by the second intermediate transfer member;
As the second intermediate transfer member, an endless belt member stretched over a plurality of support rollers is used.
As the second transfer means, there is provided one transfer roller disposed inside the belt member so as to be used for both the transfer to the transfer material and the transfer to the second intermediate transfer member ,
3. The image forming apparatus according to claim 3, wherein the third transfer unit is disposed at a position farther downstream than the transfer roller in the transfer material conveyance direction .

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