JP5071302B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus.

  Conventionally, a toner image is primarily transferred by a photosensitive drum (image carrier) onto the surface of an endless transfer belt that is wound around and rotated by a plurality of rollers such as a drive roller, a driven roller, and a backup roller. An image forming apparatus that forms an image by secondary transfer of a toner image onto a sheet is known (for example, see Patent Document 1).

This image forming apparatus includes four photosensitive drums of yellow (Y), magenta (M), cyan (C), and black (BK), and a secondary transfer roller that presses the transfer belt toward the backup roller. When the transfer belt on which the toner image is primarily transferred passes through the backup roller portion, the toner image is secondarily transferred by passing the paper between the transfer belt and the secondary transfer roller. An image is formed on a sheet.
JP 2007-33938 A

  However, in the image forming apparatus configured as described above, when secondary transfer is performed on a sheet on which an image is to be formed, an impact may occur such that the rotational speed of the transfer belt, the secondary transfer roller, or the like temporarily varies. There is a problem that the pitch of image formation is disturbed with the impact, and pitch spots may occur in the image formed on the paper.

  An object of the present invention is to provide an image forming apparatus excellent in image forming capability.

In order to solve the above problems, an invention according to claim 1 is an image forming apparatus,
A plurality of image carriers that carry toner images;
A transfer belt on which the toner image of the image carrier is primarily transferred;
A drive roller on which the transfer belt is suspended and rotating the transfer belt;
A driven roller that is suspended by the transfer belt and rotates as the transfer belt rotates;
A pressed roller on which the transfer belt is suspended;
A secondary transfer roller that presses the transfer belt toward the pressed roller;
With
A color mode in which the plurality of image carriers are in contact with the transfer belt to primarily transfer toner images of a plurality of colors, and one of the plurality of image carriers is in contact with the transfer belt; A black toner image can be switched to a monochrome mode for primary transfer,
The secondary transfer roller is configured to reduce a pressing force when the toner image on the transfer belt is secondarily transferred to a sheet passing between the secondary transfer roller and the transfer belt in the monochrome mode. It is characterized by.

According to a second aspect of the present invention, in the image forming apparatus according to the first aspect,
The secondary transfer roller is switched so as to reduce the pressing force when the sheet is thicker than a predetermined thickness.

  According to the present invention, the secondary transfer roller of the image forming apparatus can switch the pressing force when the transfer belt toner image is secondarily transferred to a sheet passing between the secondary transfer roller and the transfer belt. Therefore, when performing image formation in monochrome mode where the tension of the transfer belt of the image forming apparatus is relatively low, the secondary transfer roller can be switched to reduce the pressing force of the secondary transfer roller. Fluctuations in the rotational speed of the pressed roller, secondary transfer roller, transfer belt, etc. can be further suppressed, and troubles related to pitch unevenness can be reduced, thereby improving the image forming ability. it can.

  In particular, when image formation is performed in the monochrome mode in the image forming apparatus and the sheet on which the image is formed is thicker than a predetermined thickness, switching is performed so as to reduce the pressing force of the secondary transfer roller. Thus, it is possible to further suppress fluctuations in the rotational speed of the pressed roller, the secondary transfer roller, the transfer belt, and the like during the secondary transfer, and it is possible to further reduce troubles related to pitch unevenness.

  Hereinafter, preferred embodiments for carrying out the present invention will be described with reference to the drawings. However, although various technically preferable limitations for implementing the present invention are given to the embodiments described below, the scope of the invention is not limited to the following embodiments and illustrated examples.

  FIG. 1 is a conceptual diagram of the image forming apparatus G.

  For example, as shown in FIG. 1, the image forming apparatus G includes a plurality of photoconductors 31 that are image carriers, and includes a yellow photoconductor 31Y, a magenta photoconductor 31M, a cyan photoconductor 31C, and a black photoconductor 31C. This is a so-called tandem color image forming apparatus that forms a full-color image by arranging the photosensitive drums 31K for the transfer belt 40 facing each other in a column.

  The image forming apparatus G includes an automatic document feeder ADF on the top thereof.

  The documents D placed on the document placement table 103 of the automatic document feeder ADF are separated one by one, sent to the document conveyance path, and conveyed by the conveyance drum 102.

  The image of the document D being conveyed is read by the document reading unit 1 at the document image reading position RP. The document D that has been read is discharged to the document discharge tray 107 by the document discharge roller 105 or the like.

  The image forming apparatus G includes a document reading unit 1, exposure units 2Y, 2M, 2C, and 2K, image forming units 3Y, 3M, 3C, and 3K, an intermediate transfer unit 4, a fixing unit 5, a reverse discharge unit 6, and a refeed. The unit 7, the sheet feeding unit 8, the control unit C, and the like are housed in one housing.

  The document reading unit 1 irradiates the image of the document with the lamp L at the document reading position RP, guides the reflected light by the first mirror unit 11, the second mirror unit 12, the lens 13, etc. Form an image on the surface.

  The image signal photoelectrically converted by the image pickup device CCD is subjected to processing such as A / D conversion, shading correction, and compression in the image reading control unit 14 and is stored as image data in the memory of the control unit C.

  The image data stored in the memory of the control unit C is subjected to appropriate image processing according to conditions set by the user, and output image data is generated.

The exposure units 2Y, 2M, 2C, and 2K include a laser light source, a polygon mirror, a plurality of lenses, and the like, and generate a laser beam.
The exposure units 2Y, 2M, 2C, and 2K are photosensitive elements that are constituent elements of the image forming units 3Y, 3M, 3C, and 3K corresponding to the output information output based on the output image data sent from the control unit C. The surfaces of the bodies 31Y, 31M, 31C, and 31K are scanned and exposed with a laser beam. By this laser beam scanning exposure, latent images are formed on the photoreceptors 31Y, 31M, 31C, and 31K charged by the main charging portions 32Y, 32M, 32C, and 32K.

  The yellow image forming unit 3Y includes a photoconductor 31Y and a main charging unit 32Y, a developing unit 33Y, a primary transfer roller 34Y, and a cleaning unit 35Y disposed around the photoconductor 31Y. The same applies to the image forming units 3M, 3C, and 3K corresponding to magenta, cyan, and black.

The latent images on the photoconductors 31Y, 31M, 31C, and 31K are visualized by development by the corresponding developing units 33Y, 33M, 33C, and 33K, and toner images are formed on the photoconductors 31.
The photoreceptor 31 may be a drum-shaped image carrier or a belt-shaped image carrier. The photoconductor 31 of this embodiment is a drum.

  The toner images formed and carried on the photoreceptors 31Y, 31M, 31C, and 31K are placed at predetermined positions on the transfer belt 40, which is an intermediate transfer body, by the primary transfer rollers 34Y, 34M, 34C, and 34K of the intermediate transfer unit 4. Sequential transfer and primary transfer are performed.

  Residual toner is removed from the surface of each photoreceptor 31 after the transfer of the toner image by the cleaning units 35Y, 35M, 35C, and 35K.

The intermediate transfer unit 4 includes a transfer belt 40, a driving roller 41, a driven roller 42, a backup roller 43 that is a pressed roller, a secondary transfer roller 44, a tension roller 45, primary transfer rollers 34Y, 34M, 34C, and 34K. ing.
The transfer belt 40 is an endless belt that is suspended and supported by a driving roller 41, a tension roller 45, a driven roller 42, a backup roller 43, and the like, and rotates clockwise in the drawing as the driving roller 41 is driven. Is done.

  The toner image transferred onto the transfer belt 40 is conveyed from the trays PG 1, PG 2, PG 3 of the paper feed unit 8 or the paper feed device S 1, and timed by the paper feed roller 81, and the backup roller 43 of the transfer belt 40. Secondary transfer is performed by the secondary transfer roller 44 on the paper P sent to the portion.

  After the transfer of the toner image onto the paper P, the surface of the transfer belt 40 is cleaned by the belt cleaning unit 40a and used for the next image transfer.

  On the other hand, the paper P on which the toner image has been transferred is sent to the fixing unit 5 and heated under pressure, whereby the toner image is fixed on the paper P and an image is formed.

  The paper P that has undergone the fixing process by the fixing unit 5 is conveyed by the reverse paper discharge unit 6 and discharged onto the paper discharge tray 61. When the paper P is turned upside down and discharged, the paper P is once guided downward by the paper discharge guide 62, and the paper reversing roller 63 holds the rear end of the paper P, and then the paper P is reversed and conveyed. The paper is guided to the paper discharge roller 64 by the paper guide 62 and discharged.

  When image formation is also performed on the back surface of the paper P, the paper P on which the toner image on the front surface has been fixed is transported to the refeed unit 7 below by the paper discharge guide 62, and the refeed reverse roller After the rear end is sandwiched by 71, the paper P is reversed by reverse feeding and sent to the refeed conveyance path 72 for image formation on the back surface.

  Next, the intermediate transfer unit 4 of the image forming apparatus G will be described.

For example, as shown in FIG. 2, the intermediate transfer unit 4 includes a drive roller 41, a tension roller 45, a driven roller 42, and a backup roller 43 arranged in the clockwise order in the figure. The transfer belt 40 is suspended from the roller 45, the driven roller 42, and the backup roller 43. Note that primary transfer rollers 34Y, 34M, 34C, and 34K are arranged in the clockwise order in the drawing between the tension roller 45 and the driven roller 42, and the transfer belt 40 is sandwiched between the primary transfer rollers 34. Photoconductors 31Y, 31M, 31C, and 31K are provided at the positions, respectively.
Further, a secondary transfer roller 44 is provided at a position corresponding to the backup roller 43 with the transfer belt 40 interposed therebetween, and the secondary transfer roller 44 presses the transfer belt 40 toward the backup roller 43.

The driving roller 41 is rotationally driven by the driving force transmitted from the driving source, and rotates the transfer belt 40 clockwise in the drawing.
The driven roller 42 rotates with the rotation of the transfer belt 40 so as to assist the rotation of the transfer belt 40.

The driven roller 42 includes a primary transfer region in which the photoreceptors 31Y, 31M, 31C, and 31K are in contact with the transfer belt 40, and a secondary transfer roller 44 in contact with the transfer belt 40 on the downstream side of the primary transfer region. It arrange | positions at the 1st area | region located between transfer areas.
The driving roller 41 is downstream of the secondary transfer region where the secondary transfer roller 44 contacts the transfer belt 40 and upstream of the primary transfer region where the photoconductors 31Y, 31M, 31C and 31K are in contact with the transfer belt 40. Is disposed in the second region located at A tension roller 45 is disposed on the downstream side of the drive roller 41 in the second region.

The secondary transfer roller 44 is rotatably supported by a roller support 440 as shown in FIGS. 2, 3 (a), 3 (b), and 4, for example. It is pivotally supported by 440a.
The roller support 440 includes a first pressure-bonding lever 441, a second pressure-bonding lever 442, a first roller 443, a second roller 444, a first transfer spring 445, a second transfer spring 446, a guide member 447, a support body 448, and the like. I have. In addition, in the roller support 440 of the secondary transfer roller 44 shown in FIG. 3, the first pressure-bonding lever 441 is shown in a partially broken view so that the first roller 443 and the second roller 444 can be seen. .

The cam portion 46 is rotated by the cam rotation shaft 46 a and the stop position is switched so as to contact either one of the first roller 443 and the second roller 444.
For example, when an image is formed on general paper P such as plain paper in the image forming apparatus G, an operation for selecting plain paper is performed in an operation unit (not shown) provided on the outer surface side of the casing of the apparatus. Then, the cam portion 46 is switched to an arrangement that abuts against the first roller 443 corresponding to the standard pressure bonding mode.
Further, when the image forming apparatus G performs monochrome printing and forms an image on a sheet P that has a thickness of 200 [g / m ² ] or more and is handled as a thick sheet, for example, on the outer surface side of the casing of the apparatus When an operation for selecting monochrome printing and cardboard is performed in the operation unit (not shown) provided in the cam unit 46, the cam unit 46 is switched to an arrangement in contact with the second roller 444 corresponding to the low pressure bonding mode.

The secondary transfer roller 44 presses the transfer belt 40 toward the backup roller 43 by the pressing force of the first transfer spring 445 and the second transfer spring 446 in the standard press-bonding mode.
Specifically, as shown in FIG. 3A, when the cam portion 46 comes into contact with the first roller 443, both the first pressure-bonding lever 441 and the second pressure-bonding lever 442 are operated to perform the first transfer. The spring 445 and the second transfer spring 446 are pressed, and the pressing force by the two springs is transmitted to the support body 448 via the guide member 447, and the support body 448 rotates around the support shaft 440a. Thus, the secondary transfer roller 44 applies a pressing force by the first transfer spring 445 and the second transfer spring 446 to the transfer belt 40.
The spring load of the first transfer spring 445 is 27.5 ± 5% [N], and the spring load of the second transfer spring 446 is 13.5 ± 5% [N]. The load is about 41 [N].

Further, the secondary transfer roller 44 presses the transfer belt 40 toward the backup roller 43 by the pressing force of only the first transfer spring 445 in the low pressure bonding mode.
Specifically, as shown in FIG. 3B, as the cam portion 46 comes into contact with the second roller 444, only the second pressure-bonding lever 442 is operated to press only the first transfer spring 445. The pressing force by the one spring is transmitted to the support body 448 via the guide member 447, and the support body 448 rotates about the support shaft 440a, so that the secondary transfer roller 44 is transferred to the transfer belt 40. A pressing force is applied by the first transfer spring 445.
Since the spring load in the low pressure bonding mode is about 27.5 [N] due to the first transfer spring 445 alone, it is about 30% lower than in the standard pressure bonding mode. That is, the pressing force that the secondary transfer roller 44 acts on the transfer belt 40 in the low pressure bonding mode is about 30% lower than that in the standard pressure bonding mode.

  That is, in the image forming apparatus G, when an image is formed on a sheet P having a general thickness such as plain paper, the pressing force of 100% by the first transfer spring 445 and the second transfer spring 446 is applied in the standard press-bonding mode. The secondary transfer roller 44 is operated to perform secondary transfer. On the other hand, when monochrome printing is performed and an image is formed on thick paper P that is thicker than a predetermined thickness, the secondary transfer roller is pressed by about 30% by the first transfer spring 445 only in the low pressure bonding mode. 44 is activated to perform secondary transfer.

  Next, the reason why the driven roller 42 is arranged in the first area and the driving roller 41 is arranged in the second area in the image forming apparatus G will be described.

In the image forming apparatus G, when the sheet P is passed between the backup roller 43 and the secondary transfer roller 44 via the transfer belt 40 and the secondary transfer is performed, the gap between the secondary transfer roller 44 and the backup roller 43 is determined. The rotational speeds of the backup roller 43, the transfer belt 40, and the secondary transfer roller 44 are temporarily changed so that an impact is generated in the vicinity of the backup roller 43 by the thickness of the paper P at the timing when the paper P passes. It is known that
As the rotational speed varies, the pitch of image formation is disturbed, and pitch irregularities may occur in the image formed on the paper P. In order to reduce troubles related to such pitch irregularities, the driving roller The arrangement of 41 and the driven roller 42 was verified.

In the conventional image forming apparatus, the driving roller is disposed in the first area, and the driven roller is disposed in the second area. That is, the driving roller 42 and the driven roller 41 of the image forming apparatus G of the present invention are arranged in reverse.
Data obtained by measuring fluctuations in the rotation speed of the backup roller during image formation in this conventional image forming apparatus are shown in FIGS. 5 (a) and 6 (a). FIG. 5A shows variation data at the time of monochrome printing, and FIG. 6A shows variation data at the time of color printing.
In contrast, FIG. 5B and FIG. 6B show data obtained by measuring fluctuations in the rotational speed of the backup roller during image formation in the image forming apparatus G of the present invention. FIG. 5B shows variation data at the time of monochrome printing, and FIG. 6B shows variation data at the time of color printing.

  Comparing the data in FIG. 5 (a) and the data in FIG. 5 (b), which are the fluctuation data at the time of monochrome printing, the data in FIG. 5 (b) has a smaller degree of fluctuation in the rotation speed, and at the time of color printing. Comparing the data in FIG. 6 (a) and the data in FIG. 6 (b), the data in FIG. 6 (b) has a smaller degree of fluctuation in rotational speed. That is, in the image forming apparatus G of the present invention in which the driven roller 42 is disposed in the first area and the driving roller 41 is disposed in the second area, the fluctuation in the rotational speed of the backup roller 43 during secondary transfer is smaller. It can be said that the image forming apparatus G has a configuration in which one element that causes pitch spots is reduced.

This is because the drive roller 41 is disposed in the second region downstream of the secondary transfer region where the transfer belt 40 and the secondary transfer roller 44 are in contact with each other. Since the tension of the transfer belt 40 in the transfer region becomes relatively high and is not easily affected by an impact or the like, the sheet P passes between the backup roller 43 and the secondary transfer roller 44 via the transfer belt 40. This is because the fluctuation during the process is suppressed.
On the other hand, in the conventional image forming apparatus, since the driving roller is disposed in the first area upstream of the secondary transfer area where the transfer belt 40 and the secondary transfer roller 44 are in contact with each other, the driving roller is on the downstream side of the driving roller. Further, since the tension of the transfer belt 40 in the secondary transfer region becomes relatively low, the impact when the paper P passes between the backup roller 43 and the secondary transfer roller 44 via the transfer belt 40 is affected. This is because it is likely to occur and the fluctuation is likely to increase.

As described above, when the driven roller 42 is arranged in the first area and the driving roller 41 is arranged in the second area, fluctuations in the rotation speed of the backup roller 43, the transfer belt 40, and the like during the secondary transfer are suppressed. Therefore, the configuration of this roller arrangement was applied to the image forming apparatus G.
Thus, the image forming apparatus G in which the driven roller 42 is disposed in the first region and the driving roller 41 is disposed in the second region can reduce troubles related to pitch unevenness.

  Next, the reason for switching to the low pressure bonding mode when monochrome printing is performed in the image forming apparatus G and the thickness of the paper P on which an image is to be formed is greater than a predetermined thickness will be described.

  In the image forming apparatus G in which the driven roller 42 is disposed in the first area and the driving roller 41 is disposed in the second area, data obtained by measuring fluctuations in the rotation speed of the backup roller during image formation in which monochrome printing is performed in the monochrome mode. 5B is compared with the fluctuation data in FIG. 6B, which is data obtained by measuring fluctuations in the rotational speed of the backup roller during image formation when color printing is performed in the color mode. It can be seen that the degree of fluctuation in the rotational speed is smaller in the data of FIG.

  In color printing with the color mode selected, as shown in FIG. 2, the four primary transfer rollers 34Y, 34M, 34C, and 34K are in contact with the transfer belt 40, and the primary transfer area of the transfer belt 40 The four photoconductors 31Y, 31M, 31C, and 31K are in contact with each other to primarily transfer toner images of four colors (yellow, magenta, cyan, and black), whereas in monochrome printing in which the monochrome mode is selected, FIG. 3, the three primary transfer rollers 34 </ b> Y, 34 </ b> M, 34 </ b> C are separated from the transfer belt 40, and the one primary transfer roller 34 </ b> K is in contact with the transfer belt 40. Since the black toner image is primarily transferred by contact with one photosensitive member 31K, the photosensitive member 3 is transferred to the transfer belt 40 during color printing. And the tension on the transfer belt 40 becomes higher, and as a result, fluctuation when the paper P passes between the backup roller 43 and the secondary transfer roller 44 via the transfer belt 40 is suppressed. Because it will be.

When color printing is performed in the image forming apparatus G, the tension of the transfer belt 40 becomes relatively high compared to when monochrome printing is performed, and the transfer belt 40 is less affected by impact and the like. Therefore, since fluctuations when the paper P passes between the backup roller 43 and the secondary transfer roller 44 are suppressed, the occurrence of troubles related to pitch unevenness is relatively small.
On the other hand, when performing monochrome printing, since the tension of the transfer belt 40 is relatively low, the backup roller 43 and the transfer belt 40 are likely to fluctuate with the passage of the paper P during the secondary transfer. Then, when the thickness of the paper P on which the image is formed is thicker than a predetermined thickness, the fluctuation is suppressed by switching to the low pressure bonding mode.

As described above, when secondary transfer is performed in the image forming apparatus G, at the timing when the paper P passes between the secondary transfer roller 44 and the backup roller 43, the thickness of the paper P is equal to the thickness of the backup roller 43. The rotational speed of the backup roller 43 or the like may temporarily fluctuate so that an impact is generated in the vicinity.
As the thickness of the paper P is thicker, an impact that the paper P is caught between the rollers is more likely to occur. Therefore, when the paper P is thicker than a predetermined thickness, the pressing force by the secondary transfer roller 44 is reduced. The paper P is made easy to pass between the rollers. For example, when the predetermined thickness is 200 [g / m ² ], when the image is formed on thick paper having a thickness of 200 [g / m ² ] or more in the monochrome mode, the pressing by the secondary transfer roller 44 is performed. The low pressure bonding mode in which the pressure is reduced by about 30% is selected to reduce the pitch unevenness so that the paper P easily passes between the secondary transfer roller 44 and the backup roller 43. Note that when performing image formation on plain paper of less than 200 [g / m ² ], the standard press-bonding mode is selected.

  In each of the standard pressure bonding mode and the low pressure bonding mode, the degree of pitch disturbance when forming an image on cardboard was verified.

FIG. 7A shows data relating to pitch disturbance during image formation in which secondary transfer of monochrome printing is performed on a thick paper (paper P) having a thickness of 200 [g / m ² ] in the standard pressure bonding mode. FIG. 7B shows data relating to pitch disturbance during image formation in which secondary transfer of monochrome printing is performed in the low pressure bonding mode.
Comparing the data in FIG. 7A and the data in FIG. 7B, it can be seen that the data in FIG. 7B has less pitch fluctuation and disturbance. In other words, if the paper P on which the image is to be formed is thicker than a predetermined thickness, the pitch transfer during image formation is reduced by performing the secondary transfer in the low pressure bonding mode in which the pressing force by the secondary transfer roller 44 is reduced. can do.

  As described above, when the paper P on which the image is formed is thicker than the predetermined thickness and monochrome printing is performed, the low pressure bonding mode is selected, and the pressing force by the secondary transfer roller 44 is switched to be reduced by about 30%. By performing the secondary transfer, it is possible to suppress the impact generated in the vicinity of the backup roller 43 and reduce the pitch unevenness.

The number of flywheels of the black photosensitive member 31K in the image forming apparatus G is eight, which is twice the thickness of the conventional four. Pitch spots can be further reduced by doubling the thickness of the flywheel of the black photoreceptor 31K.
Further, regarding the driving of the photoconductor 31, it is possible to switch between executing (ON) and not executing (OFF) the feedforward control. However, when the feedforward control is turned OFF, the pitch unevenness can be further reduced. .

  As described above, in the intermediate transfer unit 4 of the image forming apparatus G, the primary transfer region where the photoconductors 31Y, 31M, 31C, and 31K are in contact with the transfer belt 40, and the transfer belt 40 on the downstream side of the primary transfer region. A first area driven roller 42 is disposed between the secondary transfer area 44 and the secondary transfer area in contact with the secondary transfer roller 44, and is positioned downstream of the secondary transfer area and upstream of the primary transfer area. By adopting a configuration in which the driving roller 41 is arranged in the second region, fluctuations in the rotational speed of the backup roller 43, the transfer belt 40, etc. during secondary transfer can be suppressed, and troubles related to pitch unevenness can be reduced. it can.

  In particular, the pressing force of the secondary transfer roller 44 pressing the transfer belt 40 toward the backup roller 43 can be switched between the standard pressure-bonding mode and the low pressure-bonding mode, and at the time of performing monochrome printing in the image forming apparatus G. If the paper P on which the image is formed is thicker than a predetermined thickness, the low pressure bonding mode is selected and the secondary transfer is performed by reducing the pressing force by the secondary transfer roller 44 by about 30%. Further, it is possible to further reduce pitch spots that are likely to occur during monochrome printing.

  As described above, when monochrome printing is performed and the sheet P on which an image is formed is thicker than a predetermined thickness, the low pressure mode is selected and the pressing force by the secondary transfer roller 44 is reduced by about 30%. It can be said that the image forming apparatus G that can reduce the pitch unevenness by performing the secondary transfer is excellent in image forming capability.

  In the above-described embodiment, the image forming apparatus G in which the plurality of photoconductors 31Y, 31M, 31C, and 31K are arranged in the vertical direction facing the transfer belt 40 has been described as an example. For example, an image forming apparatus in which a plurality of photoconductors 31Y, 31M, 31C, and 31K face the transfer belt 40 and are arranged in the lateral direction may be used.

  In addition, it is needless to say that other specific detailed structures can be appropriately changed.

1 is a conceptual diagram illustrating an image forming apparatus according to the present invention. FIG. 3 is an enlarged view showing an intermediate transfer portion of the image forming apparatus. It is explanatory drawing which shows the secondary transfer roller and roller support body of an intermediate transfer part, and has shown the state (a) of the standard press bonding mode, and the state (b) of the low press bonding mode. It is explanatory drawing which looked at the transfer spring part of the roller support body in the expansion-contraction direction of a spring. It is explanatory drawing which shows the fluctuation | variation data of the rotational speed of the backup roller at the time of monochrome printing, The data (a) in the conventional structure by which the drive roller and the 2nd area follower roller are arrange | positioned in the 1st area | region, and 2nd area | region FIG. 6 shows data (b) in the configuration of the present invention in which the driving roller and the follower roller are arranged in the first region. It is explanatory drawing which shows the fluctuation | variation data of the rotational speed of the backup roller at the time of color printing, the data (a) in the conventional structure by which the drive roller and the 2nd area follower roller are arrange | positioned in the 1st area, and the 2nd area FIG. 6 shows data (b) in the configuration of the present invention in which the driving roller and the follower roller are arranged in the first region. It is explanatory drawing which shows the pitch disturbance at the time of image formation with respect to a thick paper, When the secondary transfer is performed in the data (a) regarding the pitch disturbance when the secondary transfer is performed in the standard pressure bonding mode, and the low pressure bonding mode. The data (b) regarding the pitch disturbance is shown.

Explanation of symbols

3 (3Y, 3M, 3C, 3K) Image forming unit 31 (31Y, 31M, 31C, 31K) Photoconductor (image carrier)
4 Intermediate Transfer Section 40 Transfer Belt 41 Drive Roller 42 Followed Roller 43 Backup Roller (Pressed Roller)
44 Secondary transfer roller 440 Roller support 441 First press lever 442 Second press lever 443 First roller 444 Second roller 445 First transfer spring 446 Second transfer spring 447 Guide member 448 Support body 45 Tension roller 46 Cam portion 46a Cam rotation shaft G Image forming apparatus

Claims (2)

A plurality of image carriers that carry toner images;
A transfer belt on which the toner image of the image carrier is primarily transferred;
A drive roller on which the transfer belt is suspended and rotating the transfer belt;
A driven roller that is suspended by the transfer belt and rotates as the transfer belt rotates;
A pressed roller on which the transfer belt is suspended;
A secondary transfer roller that presses the transfer belt toward the pressed roller;
With
A color mode in which the plurality of image carriers are in contact with the transfer belt to primarily transfer toner images of a plurality of colors, and one of the plurality of image carriers is in contact with the transfer belt; A black toner image can be switched to a monochrome mode for primary transfer,
The secondary transfer roller is configured to reduce a pressing force when the toner image on the transfer belt is secondarily transferred to a sheet passing between the secondary transfer roller and the transfer belt in the monochrome mode. An image forming apparatus.   The image forming apparatus according to claim 1, wherein the secondary transfer roller is switched so as to reduce the pressing force when the sheet is thicker than a predetermined thickness.

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