JP2009271455A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To minimize toner-transfer-scattering in upstream of each transfer portion, and to suppress image quality deterioration due to the transfer-scattering, even in an image forming apparatus having a constitution that a toner image transfer process is repeated a plurality of times. <P>SOLUTION: The image forming apparatus has a constitution that the toner image transfer process is repeated a plurality of times so as to transfer the toner image to a recording medium. The image forming apparatus includes: a primary transfer belt 31 to which the toner image formed on each of a plurality of photoreceptor drums 22 is transferred; a secondary transfer belt 32 which is stretched to be laid between rollers 32a and 32b, and to which the toner image on the primary transfer belt 31 is transferred; a tertiary transfer roller 33 for transferring the toner image on the secondary transfer belt 32 to the recording medium 11. The image forming apparatus has a layout constitution that, at the first transfer portion where the roller 32a faces the primary transfer belt 31 across the secondary transfer belt 32, the secondary transfer belt 32 winds round along the primary transfer belt 31 in an upstream direction, further, at the second transfer portion where the roller 32b faces the tertiary transfer roller 33 through the secondary transfer belt 32, the secondary transfer belt 32 winds round along the tertiary transfer roller 33. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a printer or a copying machine using an electrophotographic system, and more particularly to an image forming apparatus that transfers a toner image to a recording medium by repeating a plurality of times of transfer.

  In recent years, among color image forming apparatuses using an electrophotographic method, as disclosed in Patent Document 1, a toner image formed on a photoreceptor is transferred to a recording medium by repeating a plurality of transfers. There is something. Specifically, the toner image formed on the photoreceptor is first transferred to the primary intermediate transfer member (primary transfer), then transferred to the secondary intermediate transfer member (secondary transfer), and then transferred to the recording medium ( (Tertiary transfer).

  In this way, the intermediate transfer method in which the transfer of the toner image is repeated a plurality of times can shorten the transport distance of the toner image from the photosensitive member to the recording medium, so that the image information is sent to the image forming apparatus. There is an advantage that the time to printout can be shortened.

JP 2002-372828 A

  However, a phenomenon called transfer scattering is likely to occur in the transfer of the toner image, which leads to a decrease in image quality. As one of the causes of the occurrence of transfer splattering, when a toner image on a photosensitive member is transferred onto an intermediate transfer member or a recording medium, a phenomenon that the toner flies from the photosensitive member upstream of the transfer portion is considered. Yes. This transfer splatter is preferably reduced as much as possible in order to greatly reduce the print quality of characters and lines. However, in the intermediate transfer method in which the transfer of the toner image described above is repeated a plurality of times, the time until the printout can be shortened, but the number of times of transfer increases, so that transfer scatter easily occurs.

  Accordingly, an object of the present invention is to suppress toner transfer scattering upstream of each transfer portion as much as possible and suppress deterioration in image quality due to transfer scattering even when the toner image transfer is repeated a plurality of times. .

  The present invention for achieving the above-mentioned object is an image forming apparatus for transferring a formed toner image to a recording medium by repeating a plurality of times of transfer, and an image carrier that carries and conveys a toner image; The toner image on the image carrier is temporarily transferred in contact with the image carrier, the intermediate transfer belt stretched by the first roller and the second roller, and the intermediate transfer belt through the recording medium. A transfer member that transfers the toner image of the intermediate transfer belt to a recording medium, and in the first transfer portion where the first roller and the image carrier face each other via the intermediate transfer belt, From the point A where the straight line L1 connecting the center of the image carrier and the center of the first roller intersects the circumference of the first roller, the straight line L1 and the first roller on the first transfer portion side The common outer tangent line L3 of the second roller intersects The image carrier is arranged with respect to the intermediate transfer belt stretched by the first roller and the second roller so that the point B is closer to the center of the image carrier, and the intermediate transfer belt is In the second transfer portion where the second roller and the transfer member face each other, a point where the straight line L2 connecting the center of the transfer member and the center of the second roller intersects with the circumference of the second roller From C, the point D where the straight line L2, the first roller on the second transfer portion side, and the common outer tangent line L4 of the second roller intersect is closer to the center of the transfer member. The transfer member is disposed on an intermediate transfer belt stretched between a roller and the second roller.

  According to the present invention, the intermediate transfer belt is wound around the image carrier upstream of the first transfer portion, and the intermediate transfer belt is wound around the transfer member also upstream of the second transfer portion. As a result, it is possible to minimize the transfer scattering of the toner image that occurs at the time of transfer of each transfer portion, and it is possible to suppress the deterioration of the image quality due to the transfer scattering.

  Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the drawings. However, the dimensions, materials, shapes, and relative arrangements of the components described in the following embodiments should be changed as appropriate according to the configuration of the apparatus to which the present invention is applied and various conditions. Therefore, unless specifically stated otherwise, the scope of the present invention is not intended to be limited thereto.

[First Embodiment]
First, an intermediate transfer type image forming apparatus that repeats transfer of a toner image a plurality of times will be described with reference to FIG. FIG. 1 is a structural sectional view showing a tertiary transfer type color image forming apparatus employing a primary transfer belt and a secondary transfer belt, which is an example of an electrophotographic color image forming apparatus.

  As shown in FIG. 1, the color image forming apparatus includes feeding units 21a and 21b, and photosensitive members (hereinafter referred to as photosensitive drums) 22Y, 22M, 22C, and 22K for the image forming units arranged in parallel for development colors. Yes. Further, the image forming unit is configured together with the photosensitive drum, and has charging means, developing means, and cleaning means as process means acting on the photosensitive drum. Specifically, the chargers 23Y, 23M, 23C, and 23K constituting the charging unit, the developing units 26Y, 26M, 26C, and 26K constituting the developing unit, and the cleaning units 27Y and 27M for cleaning the transfer residual toner on the photosensitive drum. , 27C, 27K. Further, the toner cartridges 25Y, 25M, 25C, and 25K, and toner supply flexible pipes (not shown) that supply toner from the toner cartridges to the corresponding color developing devices are provided. Further, a primary transfer belt 31, a secondary transfer belt 32, a tertiary transfer roller 33, and a fixing unit 50 are provided. The color image forming apparatus forms an electrostatic latent image on each photosensitive drum by exposure controlled by an image processing unit (not shown) based on an image signal, and develops the electrostatic latent image to form a single color toner image. . Then, this single color toner image is superimposed to form a multicolor toner image, and this multicolor toner image is transferred to the recording medium 11 by repeating a plurality of transfers. Then, the multicolor toner image on the recording medium 11 is fixed. Hereinafter, the operation of the color image forming apparatus will be described in detail.

  First, charging, exposure, and development will be described.

  The photosensitive drums 22Y, 22M, 22C, and 22K are configured by applying an organic optical transmission layer to the outer periphery of an aluminum cylinder, and are rotated counterclockwise when a driving force of a driving motor (not shown) is transmitted.

  As described above, as charging means, the four chargers 23Y, 23M for charging the photosensitive drums of yellow (Y), magenta (M), cyan (C), and black (K) for each image forming unit. 23C and 23K. Each charger uniformly charges the surface of each photosensitive drum. Here, the surface of the photosensitive drum was set to about −300V.

  Exposure to the photosensitive drums 22Y, 22M, 22C, and 22K is sent from the scanner units 24Y, 24M, 24C, and 24K, and electrostatic latent images are obtained by selectively exposing the surfaces of the photosensitive drums 22Y, 22M, 22C, and 22K. Is formed. Here, the exposed portion was set to be neutralized to about −60V.

  The toner can be selectively developed by the potential difference between the exposed portion and the non-exposed portion generated by the exposure described above. As described above, in order to visualize the electrostatic latent image, four developing units for developing yellow (Y), magenta (M), cyan (C), and black (K) are provided for each image forming unit. Development units 26Y, 26M, 26C, and 26K. Further, the transfer residual toner on each photosensitive drum is collected by the cleaning means 27Y, 27M, 27C, and 27K.

  Next, primary transfer will be described.

  The primary transfer belt 31 is an image carrier belt (image carrier) stretched by a plurality of rollers, and is an image carrier belt that contacts a plurality of photosensitive drums and temporarily transfers a toner image of each photosensitive drum. is there. Here, the primary transfer belt 31 is stretched by two rollers, a primary transfer belt driving roller 31a and a primary transfer belt driven roller 31b. Further, a cleaning unit 31 c for cleaning the secondary transfer residual toner on the primary transfer belt 31 is provided.

  The primary transfer rollers 30Y, 30M, 30C, and 30K are disposed inside the primary transfer belt 31 so as to correspond to the photosensitive drums 22Y, 22M, 22C, and 22K with the primary transfer belt 31 interposed therebetween, and the primary transfer portions of the respective colors are arranged. Forming. Further, primary transfer auxiliary rollers 34Y, 34M, 34C, and 34K are arranged upstream of the primary transfer units, and the primary transfer belt 31 is wound around the photosensitive drums 22Y, 22M, 22C, and 22K upstream of the primary transfer units. It has a configuration.

For the primary transfer belt 31, use a resin film such as urethane resin, fluorine resin, nylon resin, polyimide resin, or a resin film in which resistance is adjusted by dispersing carbon or conductive powder in these resins. Can do. Alternatively, an elastomer sheet having a multi-layer structure in which a resin layer such as urethane resin, fluorine resin, nylon resin, polyimide resin or the like is formed as a release layer on the toner carrier surface side of a base layer sheet such as urethane rubber or NBR. Etc. can also be used. In the primary transfer belt 31 used here, carbon is dispersed in polyimide to adjust the surface resistivity ρs = 1 × 10 ¹² Ω / □ or less. Thereby, the electric charge added to the primary transfer belt 31 in the transfer process or the like can be attenuated without providing a special static elimination mechanism. The primary transfer belt 31 is a single-layer endless belt having a thickness of 100 μm.

  The measurement of the surface resistivity described above is based on JIS-K6911. After obtaining good contact between the electrode and the belt surface by using conductive rubber as an electrode, a high resistance resistance meter (R8340 manufactured by Advantest Corporation) is used. It measured using. The measurement conditions were applied voltage = 100 V and voltage application time = 30 s.

The primary transfer belt driving roller 31a used here is a roller composed of an aluminum cored bar and a hydrin rubber layer having a layer thickness of 3 mm. The resistance of the hydrin rubber is adjusted to 1 × 10 ⁶ Ω. Yes. The primary transfer belt driven roller 31b used here is an aluminum roller.

  The primary transfer belt driving roller 31a and the primary transfer belt driven roller 31b are set to have the same diameter, and here the diameter is 50 mm.

  The above-mentioned roller resistance value was measured using a high resistance resistance meter (R8340 manufactured by Advantest Co., Ltd.) while the roller to be measured was brought into contact with an aluminum cylinder having a diameter of 30 mm and rotated with respect to the aluminum cylinder. The measurement conditions were applied voltage = 100 V, voltage application time = 30 s, contact force = 9.8 N, and rotational peripheral speed = 117 mm / s.

  The primary transfer belt drive roller 31a is driven by a drive motor (not shown) according to the image forming operation, and the primary transfer belt 31 is rotated in the clockwise direction. The primary transfer rollers 30Y, 30M, 30C, and 30K are driven to rotate by the primary transfer belt 31. Then, a primary transfer bias is applied to each primary transfer roller by a primary transfer bias source (not shown), whereby each color toner image on each photosensitive drum is sequentially primary transferred onto the primary transfer belt 31. Here, a voltage of +400 V was applied as the primary transfer bias.

  Next, secondary transfer will be described.

  The secondary transfer belt 32 is an intermediate transfer belt (endless belt) that is in contact with the primary transfer belt 31 to temporarily transfer the toner image of the primary transfer belt 31 and is stretched between a first roller and a second roller. is there. Here, the secondary transfer belt 32 is stretched by a secondary transfer belt drive roller (first roller) 32a and a secondary transfer belt driven roller (second roller) 32b. Further, a cleaning unit 32 c for cleaning the tertiary transfer residual toner on the secondary transfer belt 32 is provided.

  The secondary transfer belt drive roller 32a is disposed inside the secondary transfer belt 32 so as to correspond to the primary transfer belt drive roller 31a with the primary transfer belt 31 and the secondary transfer belt 32 interposed therebetween. 1st transfer part) is formed. The secondary transfer belt driven roller 32b is disposed upstream of the secondary transfer portion.

  Similarly to the primary transfer belt 31, the secondary transfer belt 32 adjusts the resistance by dispersing a resin film such as urethane resin, fluorine resin, nylon resin, polyimide resin, or carbon or conductive powder in these resins. Resin film can be used. Or an elastomer sheet having a multi-layer structure in which a resin layer such as a urethane resin, a fluorine resin, a nylon resin, or a polyimide resin is formed as a release layer on the toner carrier surface side of a base sheet such as urethane rubber or NBR Can also be used.

In the secondary transfer belt 32 used here, carbon is dispersed in polyimide to adjust the surface resistivity ρs = 1 × 10 ¹² Ω / □ or less to a medium resistance. Thereby, the electric charge added to the primary transfer belt 31 in the transfer process or the like can be attenuated without providing a special static elimination mechanism. The secondary transfer belt 32 is also a single-layer endless belt having a thickness of 100 μm.

  The measurement of the surface resistivity described above is based on JIS-K6911. After obtaining good contact between the electrode and the belt surface by using conductive rubber as an electrode, a high resistance resistance meter (R8340 manufactured by Advantest Corporation) is used. It measured using. The measurement conditions were applied voltage = 100 V and voltage application time = 30 s.

The secondary transfer belt driving roller 32a used here is a roller composed of an aluminum cored bar and a hydrin rubber layer having a layer thickness of 3 mm. The resistance of the hydrin rubber is adjusted to 1 × 10 ⁶ Ω. It is said. The secondary transfer belt driven roller 32b used here is an aluminum roller.

  The secondary transfer belt drive roller 32a and the secondary transfer belt driven roller 32b are set to have the same diameter, and here the diameter is 25 mm.

  The above-mentioned roller resistance value was measured using a high resistance resistance meter (R8340 manufactured by Advantest Co., Ltd.) while the roller to be measured was brought into contact with an aluminum cylinder having a diameter of 30 mm and rotated with respect to the aluminum cylinder. The measurement conditions were applied voltage = 100 V, voltage application time = 30 s, contact force = 9.8 N, and rotational peripheral speed = 117 mm / s.

  The secondary transfer belt driving roller 32a is driven according to the image forming operation, and the secondary transfer belt 32 is rotated in the counterclockwise direction. By providing a potential difference between the primary transfer belt drive roller 31a and the secondary transfer belt drive roller 32a by a secondary transfer bias source (not shown), the toner image formed on the primary transfer belt 31 is subjected to secondary transfer. Secondary transfer onto the belt 32 is performed. In the secondary transfer portion, the toner remaining on the primary transfer belt 31 without being subjected to the secondary transfer is collected by the cleaning unit 31c.

  Here, the primary transfer belt drive roller 31a is grounded, and a voltage of +1000 V is applied to the secondary transfer belt drive roller 32a, whereby a potential difference is generated between the primary transfer belt drive roller 31a and the secondary transfer belt drive roller 32a. Provided.

  Next, tertiary transfer will be described.

  The tertiary transfer roller 33 is a transfer member that contacts the secondary transfer belt 32 via a recording medium and transfers the toner image on the secondary transfer belt 32 to the recording medium. Here, the tertiary transfer roller 33 is disposed outside the secondary transfer belt 32 so as to correspond to the secondary transfer belt driven roller 32b with the secondary transfer belt 32 interposed therebetween, and forms a tertiary transfer portion. Therefore, the secondary transfer belt driving roller 32a is positioned upstream of the tertiary transfer portion.

The tertiary transfer roller 33 used here is a sponge roller composed of an aluminum cored bar having a diameter of 15 mm and a foamed hydrin rubber layer having a thickness of 5 mm. The resistance of the hydrin rubber is adjusted to 1 × 10. ⁸ Ω. The hardness of the tertiary transfer roller 33 is 35 ° (ASKER-C).

  The above-mentioned roller resistance value was measured using a high resistance resistance meter (R8340 manufactured by Advantest Co., Ltd.) while the roller to be measured was brought into contact with an aluminum cylinder having a diameter of 30 mm and rotated with respect to the aluminum cylinder. The measurement conditions were applied voltage = 100 V, voltage application time = 30 s, contact force = 9.8 N, and rotational peripheral speed = 117 mm / s.

  In response to the image forming operation, the tertiary transfer roller 33 is rotated clockwise by a drive motor (not shown). The recording medium 11 is conveyed from the feeding unit 21 to the tertiary transfer unit and nipped by the tertiary transfer unit. By providing a potential difference between the tertiary transfer roller 33 and the secondary transfer belt driven roller 32b by a tertiary transfer bias source (not shown), the toner image formed on the secondary transfer belt 32 is transferred onto the recording medium 11. And tertiary transfer. In the tertiary transfer portion, the tertiary transfer residual toner remaining on the secondary transfer belt 32 without being tertiary transferred is collected by the cleaning unit 32c.

  Here, the secondary transfer belt driven roller 32b is grounded and a voltage of +1500 V is applied to the tertiary transfer roller 33, thereby providing a potential difference between the tertiary transfer roller 33 and the secondary transfer belt driven roller 32b.

  The recording medium 11 after the third transfer is conveyed to the fixing unit 50 and is melted and fixed by a fixing roller 51 to be heated and a pressure roller 52 for bringing the recording medium 11 into pressure contact with the fixing roller 51. The recording medium 11 after fixing is discharged to a discharge tray at the top of the apparatus, and the image forming operation is completed.

  Here, as a comparative example, a conventional secondary transfer type image forming apparatus will be briefly described with reference to FIG. A description of the same parts as those of the above-described embodiment will be omitted. In the secondary transfer type image forming apparatus shown in FIG. 2, the processes of charging, exposure, development, and primary transfer are the same as those of the present embodiment described above. Then, the toner image formed by the primary transfer sequentially on the primary transfer belt 31 is secondarily transferred onto the recording medium 11 by the secondary transfer portion in contact with the secondary transfer roller 32. Note that the rotation directions of the photosensitive drum and the intermediate transfer belt are different from those in the tertiary transfer system. As indicated by a one-dot broken line in the figure, the paper passes once on the driven roller 31b and then is sent to the secondary transfer portion on the driving roller 31a. That is, it takes a long time for the primary transferred toner image to be transferred to the recording medium 11, and the time until the printout is increased.

  FIG. 3 shows another comparative example of a conventional secondary transfer type image forming apparatus. A description of the same parts as those of the above-described embodiment will be omitted. Each process is the same as that of the secondary transfer type image forming apparatus described above, but an exposure unit and a photosensitive drum are arranged below the intermediate transfer belt. In this case, as indicated by a dashed line in the figure, the toner image formed on the intermediate transfer belt is immediately sent to the secondary transfer portion on the drive roller, so that the primary transferred toner image is transferred to the recording medium 11. The time until transcription is short. However, the toner tends to fall to the lower part of the apparatus due to gravity, and the exposure unit is soiled and image defects are likely to occur.

  On the other hand, the image forming apparatus of the tertiary transfer system can send the toner image immediately from the secondary transfer portion to the tertiary transfer portion by reducing the size of the secondary transfer body (the belt 32 and the rollers 32a and 32b). Therefore, the time until the primary transferred toner image is transferred to the recording medium 11 can be sufficiently shortened. Further, since the exposure unit can be disposed on the upper side of the intermediate transfer belt, image defects due to contamination of the exposure unit are unlikely to occur. However, since the transfer process is performed three times, it is disadvantageous for transfer scattering.

  Here, one of the causes of the transfer scattering will be described with reference to FIG. FIG. 4 is an enlarged cross-sectional view of a main part showing an enlarged view of one primary transfer portion, in which 22 is a photosensitive drum, 30 is a primary transfer roller, and 31 is a primary transfer belt.

  As one of the causes of the transfer splatter, as shown in FIG. 4, the toner jumps due to the electric field formed upstream of the transfer portion, and the toner is transferred to a position T2 different from the position T1 to be originally transferred. Can be considered. In the figure, T1 indicates the position of the toner that is originally transferred at the transfer portion, and T2 indicates the position of the toner that has jumped due to the electric field upstream of the transfer portion. As a method of suppressing transfer scattering due to toner flying upstream of the transfer portion, a configuration may be adopted in which the electric field upstream of the transfer portion is weakened. For example, there is a method in which a toner image is electrostatically transferred by applying a bias at the transfer unit after physically holding the toner upstream of the transfer unit.

  First, a means for suppressing transfer scattering at the primary transfer portion in the present embodiment will be described.

  As shown in FIG. 1, the primary transfer portion in the present embodiment is transferred by a bias applied to the primary transfer roller 30 after the toner is physically held upstream of the primary transfer portion by the primary transfer auxiliary roller 34. An electric field is formed. That is, the occurrence of transfer scattering at the primary transfer portion can be suppressed to a low level.

  Next, means for suppressing transfer scattering in the secondary transfer unit and the tertiary transfer unit in the present embodiment will be described.

  FIG. 5 is a cross-sectional view of the main part showing the positional relationship between the rollers of the secondary transfer unit and the tertiary transfer unit of the present embodiment. Point A in FIG. 5 indicates a point where a straight line L1 connecting the center of the primary transfer belt drive roller 31a and the center of the secondary transfer belt drive roller 32a intersects with the circumference of the secondary transfer belt drive roller 32a. A point B in FIG. 5 indicates a line L1 connecting the center of the primary transfer belt drive roller 31a and the center of the secondary transfer belt drive roller 32a and the secondary transfer belt drive on the secondary transfer unit side (first transfer unit side). This is the point where the roller 32a and the common outer tangent line L3 of the secondary transfer belt driven roller 32b intersect. Comparing point A and point B, point B is closer to the center of primary transfer belt drive roller 31a than point A. With such an arrangement, the secondary transfer belt 32 is wound around the primary transfer belt drive roller 31a via the primary transfer belt 31 upstream of the secondary transfer portion. That is, the secondary transfer belt driven roller 32b serves as a tertiary transfer counter roller, and also serves to physically hold the primary transfer belt 31 and the secondary transfer belt 32 upstream of the secondary transfer portion. Therefore, since the toner image formed on the primary transfer belt 31 is electrostatically transferred after being physically held upstream of the secondary transfer portion, the occurrence of transfer scattering in the secondary transfer portion is suppressed to a low level. it can.

  A point C in FIG. 5 indicates a point where a straight line L2 connecting the center of the tertiary transfer roller 33 and the center of the secondary transfer belt driven roller 32b intersects with the circumference of the secondary transfer belt driven roller 32b. A point D in FIG. 5 indicates a straight line L2 connecting the center of the tertiary transfer roller 33 and the center of the secondary transfer belt driven roller 32b, and the secondary transfer belt drive roller 32a on the tertiary transfer portion side (second transfer portion side). The common outer tangent line L4 of the secondary transfer belt driven roller 32b intersects. Comparing point C and point D, point D is closer to the center of the tertiary transfer roller 33 than point C. With this arrangement, the secondary transfer belt 32 is wound around the tertiary transfer roller 33 upstream of the tertiary transfer portion. That is, the secondary transfer belt drive roller 32a functions as a secondary transfer roller, and physically holds the recording medium 11 and the secondary transfer belt 32 fed to the tertiary transfer unit upstream of the tertiary transfer unit. Also plays a role. Therefore, since the toner image formed on the secondary transfer belt 32 is electrostatically transferred after being physically held upstream of the tertiary transfer portion, it is possible to suppress the occurrence of transfer scattering in the tertiary transfer portion. .

  In this way, the secondary transfer belt 32 is wound around the primary transfer belt drive roller 31a (primary transfer belt 31) upstream of the secondary transfer portion, and the secondary transfer belt 32 also contacts the tertiary transfer roller 33 upstream of the tertiary transfer portion. It becomes the arrangement configuration which winds. As a result, it is possible to minimize the transfer scattering of the toner image that occurs at the time of transfer of each transfer portion, and it is possible to suppress the deterioration of the image quality due to the transfer scattering.

  In this embodiment, the configuration in which rollers having the same diameter are used as the stretching rollers of the secondary transfer belt is exemplified. However, the present invention is not limited to this. By using the arrangement described in the embodiment, the scattering of the transfer can be suppressed.

[Second Embodiment]
Next, a second embodiment will be described. The same components as those in the first embodiment described above are denoted by the same reference numerals, and the description thereof is omitted. In the present embodiment, the configurations of the secondary transfer bias and the tertiary transfer bias are different from those of the first embodiment described above.

  FIG. 6 is an enlarged view of the secondary transfer unit and the tertiary transfer unit in the present embodiment. In this embodiment, as the secondary transfer bias, a voltage of −1000 V is applied to the primary transfer belt drive roller 31a, and the secondary transfer belt drive roller 32a is grounded. Thereby, a potential difference is provided between the primary transfer belt drive roller 31a and the secondary transfer belt drive roller 32a. As a tertiary transfer bias, a voltage of −1500 V is applied to the secondary transfer belt driven roller 32b, and the tertiary transfer roller 33 is grounded. Thereby, a potential difference is provided between the tertiary transfer roller 33 and the secondary transfer belt driven roller 32b.

  That is, a potential difference is set between the secondary transfer belt driven roller 32 b and the tertiary transfer roller 33 in a direction in which the toner image on the secondary transfer belt 32 is electrostatically transferred to the tertiary transfer roller 33. On the other hand, a potential difference is set between the secondary transfer belt drive roller 32 a and the tertiary transfer roller 33 in such a direction that the toner image on the secondary transfer belt 32 is not electrostatically transferred to the tertiary transfer roller 33. The potential difference between the transfer portions is the same as that in the first embodiment described above. Therefore, toner flying upstream of the secondary transfer portion is not only physically suppressed but also electrostatically suppressed.

  In addition, the secondary transfer belt in this embodiment is adjusted to a resistance value that does not charge up. For this reason, the effect of suppressing the transfer scattering due to the potential difference between the primary transfer belt driving roller 31a and the secondary transfer belt driven roller 32b is more reliable.

  As described above, according to the present embodiment, it is possible to further suppress the occurrence of transfer scattering during secondary transfer.

[Third Embodiment]
Next, a third embodiment will be described. The same components as those in the first embodiment described above are denoted by the same reference numerals, and the description thereof is omitted. In the present embodiment, the configurations of the secondary transfer bias and the tertiary transfer bias are different from those in the first and second embodiments described above.

  FIG. 7 is an enlarged view of the secondary transfer unit and the tertiary transfer unit in the present embodiment. In the present embodiment, as the secondary transfer bias, the primary transfer belt drive roller 31a is grounded, and a voltage of +1000 V is applied to the secondary transfer belt drive roller 32a. Thereby, a potential difference is provided between the primary transfer belt drive roller 31a and the secondary transfer belt drive roller 32a. As a tertiary transfer bias, a voltage of −1500 V is applied to the secondary transfer belt driven roller 32b, and the tertiary transfer roller 33 is grounded. Thereby, a potential difference is provided between the tertiary transfer roller 33 and the secondary transfer belt driven roller 32b.

  That is, the toner image on the primary transfer belt 31 is electrostatically transferred to the secondary transfer belt 32 between the secondary transfer belt drive roller 32a and the primary transfer belt drive roller 31a (primary transfer belt 31). The potential difference is set. On the other hand, a potential difference between the secondary transfer belt driven roller 32b and the primary transfer belt drive roller 31a (primary transfer belt 31) in a direction in which the toner image on the primary transfer belt 31 is not electrostatically transferred to the secondary transfer belt 32. Is set to Therefore, toner flying upstream of the secondary transfer portion is not only physically suppressed but also electrostatically suppressed. A potential difference in a direction in which the toner image on the secondary transfer belt 32 is not electrostatically transferred to the tertiary transfer roller 33 is provided between the secondary transfer belt driving roller 32 a and the tertiary transfer roller 33. Therefore, toner flying upstream of the tertiary transfer portion is not only physically suppressed but also electrostatically suppressed.

  In addition, the secondary transfer belt in this embodiment is adjusted to a resistance value that does not charge up. For this reason, the effect of suppressing the secondary transfer scattering due to the potential difference between the primary transfer belt driving roller 31a and the secondary transfer belt driven roller 32b is more reliable. Further, the effect of suppressing the tertiary transfer scattering due to the potential difference between the secondary transfer belt driving roller 32a and the tertiary transfer roller 33 is further ensured.

  As described above, according to the present embodiment, it is possible to further suppress the occurrence of transfer scattering during secondary transfer and tertiary transfer.

[Other Embodiments]
In the embodiment described above, the biaxially stretched primary transfer belt is exemplified as the image carrier on which multicolor toner images are sequentially formed. However, the primary transfer belt is not limited to biaxial, and may be multiaxial. The same effect can be obtained even when a primary intermediate transfer drum is used as the image carrier. Similar effects can also be obtained in a multiple development system in which the image carrier is a photoconductor.

  In the above-described embodiment, four image forming units are used. However, the number of used image forming units is not limited, and may be set as necessary.

  In the above-described embodiments, the printer is exemplified as the image forming apparatus. However, the present invention is not limited to this, and other image forming apparatuses such as a copying machine and a facsimile machine, or a combination of these functions. Other image forming apparatuses such as multifunction peripherals may also be used. If the intermediate transfer method repeats the toner image transfer three times, the same effect can be obtained by applying the present invention to the other image forming apparatuses described above.

1 is a sectional view of a color image forming apparatus according to a first embodiment. Cross-sectional view of a secondary transfer type color image forming apparatus as a comparative example Cross-sectional view of a secondary transfer type color image forming apparatus as another comparative example Cross-sectional view of the main part showing the state of transfer splattering upstream of the transfer unit FIG. 3 is a cross-sectional view of the main part showing the positional relationship between the rollers of the secondary transfer unit and the tertiary transfer unit in the first embodiment. Enlarged view of the secondary transfer unit and the tertiary transfer unit in the second embodiment Enlarged view of the secondary transfer section and the tertiary transfer section in the third embodiment

Explanation of symbols

A, B, C, D ... Points L1, L2 ... Straight lines L3, L4 ... Common outer tangent line 11 ... Recording medium 22Y, 22M, 22C, 22K, 22T ... Photosensitive drum 31 ... Primary transfer belt 31a ... Primary transfer belt drive roller 31b ... primary transfer belt driven roller 32 ... secondary transfer belt 32a ... secondary transfer belt drive roller 32b ... secondary transfer belt driven roller 33 ... tertiary transfer roller 34 ... primary transfer auxiliary roller

Claims (6)

An image forming apparatus for transferring a formed toner image to a recording medium by repeating a plurality of transfers,
An image carrier that carries and conveys a toner image;
An intermediate transfer belt that is in contact with the image carrier and to which a toner image of the image carrier is temporarily transferred and is stretched by a first roller and a second roller;
A transfer member that contacts the intermediate transfer belt via a recording medium and transfers the toner image of the intermediate transfer belt to the recording medium,
In a first transfer portion where the first roller and the image carrier are opposed to each other via the intermediate transfer belt, a straight line L1 connecting the center of the image carrier and the center of the first roller and the first roller The point B where the straight line L1 and the first roller on the first transfer portion side and the common outer tangent line L3 of the second roller intersect is closer to the center of the image carrier than the point A where the circumference of The image carrier is arranged with respect to the intermediate transfer belt stretched by the first roller and the second roller,
In a second transfer portion where the second roller and the transfer member face each other via the intermediate transfer belt, a straight line L2 connecting the center of the transfer member and the center of the second roller and the circle of the second roller From the point C where the circumference intersects, the point D where the straight line L2, the first roller on the second transfer portion side, and the common outer tangent line L4 of the second roller intersect is closer to the center of the transfer member. In addition, the transfer member is disposed on an intermediate transfer belt stretched between the first roller and the second roller.   The image carrier is composed of a plurality of rollers and an image carrier belt stretched by the plurality of rollers, and one of the plurality of rollers faces the first roller. The image forming apparatus according to 1.   The image forming apparatus according to claim 2, wherein the image carrying belt is an image carrying belt that contacts a plurality of photoconductors and temporarily transfers a toner image of each photoconductor.   The difference between the second roller and the transfer member is set to a potential difference in a direction in which the toner image on the intermediate transfer belt is electrostatically transferred to the transfer member, and between the first roller and the transfer member. 4. The image forming apparatus according to claim 1, wherein the toner image on the intermediate transfer belt is set to a potential difference in a direction in which the toner image is not electrostatically transferred to the transfer member. 5.   Between the first roller and the image carrier, a potential difference in a direction in which the toner image of the image carrier is electrostatically transferred to the intermediate transfer belt is set. The second roller and the image carrier 5. Is set to a potential difference in a direction in which the toner image on the image carrier is not electrostatically transferred to the intermediate transfer belt. Image forming apparatus. The image forming apparatus according to claim 1, wherein a surface resistivity of the intermediate transfer belt is 1 × 10 ¹² Ω / □ or less.

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