JP5099192B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as a laser printer.

  As an electrophotographic color printer, a tandem color laser printer comprising a plurality of photoconductors arranged in parallel corresponding to yellow, magenta, cyan, and black toners and a conveyor belt facing each photoconductor It has been known.

  For example, four photosensitive drums (yellow, magenta, cyan, and black) arranged in parallel in the front-rear direction, a transfer belt that is in contact with each photosensitive drum and transports paper from the front to the back, and sandwiches the transfer belt A laser printer including a transfer roller facing each photosensitive drum has been proposed (for example, see Patent Document 1).

Japanese Patent Laid-Open No. 2006-98772

  However, in Patent Document 1 described above, when forming a monochrome image, only black toner is used. Therefore, each of the color (yellow, magenta, and cyan) photosensitive drums does not carry a toner image. It is in contact with the transfer belt.

  Therefore, after the toner image carried on the black photosensitive drum is once transferred to the paper, it is applied to the color photosensitive drum arranged behind the black photosensitive drum (downstream in the paper transport direction). When facing each other, the sheet may be reversely transferred from a sheet to a color photosensitive drum.

  In this case, the reversely transferred toner image is transferred again to the paper (reverse transfer ghost) at a position deviated from the original image after one rotation of the photosensitive drum.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus capable of suppressing the occurrence of reverse transfer ghost in a monochrome mode in a direct tandem type image forming apparatus.

In order to achieve the above object, the first invention includes a conveying belt that conveys a member to be transferred and a plurality of photoconductors that carry a developer image transferred to the member to be transferred, and forms a monochrome image. An image forming apparatus capable of switching between a monochrome mode and a color mode for forming a color image, wherein the plurality of photoconductors include a first photoconductor carrying a black developer image and a developer image other than black. A plurality of second photoconductors to be carried, a first transfer member facing the first photoconductor across the transport belt, and a second facing the second photoconductor across the transport belt. A bias is applied to the two transfer members, the contact / separation member that brings the at least one second transfer member into contact with or away from the conveyance belt, the first transfer member, and all the second transfer members. Bahia A applying means, said first photoreceptor, in transport direction of the transfer member is disposed upstream of the at least one of said second photosensitive body, all of the photosensitive member, the monochrome mode and In the color mode, the first contact member is in contact with the transport belt, and the contact / separation member faces at least the second photoconductor disposed adjacent to the downstream side of the first photoconductor in the transport direction. Two transfer members are brought into contact with the conveyor belt in the color mode, separated from the conveyor belt in the monochrome mode, and the bias applying unit is configured to transfer the first transfer member in the monochrome mode and the color mode, and A transfer bias is applied to all the second transfer members.

According to a second aspect of the present invention, there is provided a monochrome mode including a conveyance belt that conveys a member to be transferred and a plurality of photosensitive members that carry a developer image transferred to the member to be transferred, and a monochrome image that forms a monochrome image; And a plurality of photoconductors each including a first photoconductor carrying a black developer image and a plurality of second photoconductors carrying a developer image other than black. A first transfer member that opposes the first photoconductor across the conveyor belt, a second transfer member that opposes the second photoconductor across the conveyor belt, In the transporting direction of the transfer member, the charge-removing member disposed adjacent to the downstream side of each of the second transfer members and facing the transport belt and at least one second transfer member are in contact with the transport belt. Or release A contact / separation member to be moved, a first bias applying means for applying a transfer bias to the first transfer member and all the second transfer members, and a second bias applying means for applying a transfer bias to each of the charge eliminating members. The first photoconductor is disposed upstream of at least one second photoconductor in the transport direction of the transfer member, and all the photoconductors are in the monochrome mode and the color mode. The second transfer member is in contact with the transport belt, and the contact / separation member faces at least the second photoconductor disposed adjacent to the downstream side of the first photoconductor in the transport direction. Is brought into contact with the conveyor belt in the color mode and is separated from the conveyor belt in the monochrome mode. In the second mode, a transfer bias is applied to the first transfer member, and a transfer bias is not applied to the second transfer member separated from the conveying belt. It is characterized in that a transfer bias is applied to the charge eliminating member disposed adjacent to the downstream side of the second transfer member separated from the conveying belt.

  According to the first aspect of the present invention, in the monochrome mode, at least the second transfer member corresponding to the second photoconductor disposed adjacent to the downstream side in the transport direction of the first photoconductor is in a state where the transfer bias is applied. , Separated from the conveyor belt.

  Therefore, the pressing force of the member to be transferred against the second photoconductor is reduced, and the developer image transferred to the member to be transferred is continuously carried on the member to be transferred by the transfer bias applied to the second transfer member. Can do.

  Accordingly, it is possible to suppress the developer image transferred to the transfer member from being reversely transferred to the second photoconductor.

  As a result, in the direct tandem image forming apparatus, the occurrence of reverse transfer ghost in the monochrome mode can be suppressed.

  According to the second aspect of the invention, in the monochrome mode, at least the second transfer member corresponding to the second photoconductor disposed adjacent to the downstream side in the transport direction of the first photoconductor is transported without applying a transfer bias. While being separated from the belt, a transfer bias is applied to the charge eliminating member disposed adjacent to the second transfer member.

  Therefore, the pressing force of the member to be transferred against the second photoconductor can be reduced, and the developer image transferred to the member to be transferred can be continuously carried on the member to be transferred by the transfer bias applied to the charge eliminating member. .

  Accordingly, it is possible to suppress the developer image transferred to the transfer member from being reversely transferred to the second photoconductor.

  As a result, in the direct tandem image forming apparatus, the occurrence of reverse transfer ghost in the monochrome mode can be suppressed.

1 is a side sectional view showing a color laser printer as an example of an image forming apparatus of the present invention. 4A and 4B are side views for explaining the operation of the transfer unit in the first embodiment, where FIG. 5A shows a color mode, and FIG. 5B shows a monochrome mode. It is a side view for demonstrating operation | movement of the transfer unit in 2nd Embodiment, Comprising: (a) shows the time of color mode, (b) shows the time of monochrome mode.

1. Overall Configuration of Color Laser Printer As shown in FIG. 1, a color laser printer 1 as an example of an image forming apparatus is a horizontal type direct tandem type color laser printer. The color laser printer 1 includes a sheet feeding unit 3 for feeding a sheet P as an example of a member to be transferred, and an image forming unit 4 for forming an image on the fed sheet P. And.
(1) Main Body Casing The main body casing 2 is formed in a box shape having a substantially rectangular shape in side view that accommodates the paper feeding unit 3 and the image forming unit 4, and a process unit 9 described later is attached to and detached from one side wall thereof. A front cover 5 is formed. The front cover 5 is swingably provided with respect to the main body casing 2 with the lower end portion as a fulcrum.

In the following description, the side on which the front cover 5 is provided (the left side in FIG. 1) is the front side, and the opposite side (the right side in FIG. 1) is the rear side. In addition, when the color laser printer 1 is viewed from the front side, the left and right reference is used. That is, the front side in FIG. 1 is the right side and the back side is the left side.
(2) Paper Feed Unit The paper feed unit 3 includes a paper feed tray 6 that accommodates the paper P. The paper feed tray 6 is detachably attached to the bottom of the main casing 2.

The sheets P stored in the sheet feed tray 6 are fed one by one toward the pair of registration rollers 7 arranged on the upper side of the front end of the sheet feed tray 6, and at a predetermined timing, the image forming unit 4 (between a photosensitive drum 12 (described later) and a conveyor belt 18 (described later)).
(3) Image Forming Unit The image forming unit 4 includes a scanner unit 8, a process unit 9, a transfer unit 10, and a fixing unit 11.
(3-1) Scanner Unit The scanner unit 8 is disposed in the upper part of the main body casing 2. As indicated by solid lines, the scanner unit 8 emits laser beams toward four photosensitive drums 12 (described later) based on the image data to expose the photosensitive drums 12 (described later).
(3-2) Process Unit (3-2-1) Configuration of Process Unit The process unit 9 is disposed below the scanner unit 8 and above the transfer unit 10, and is an example of a photoreceptor. A photosensitive drum 12, a scorotron charger 13 and a developing unit 14 are provided.

  The photosensitive drums 12 are formed in a substantially cylindrical shape extending in the left-right direction, are provided along the left-right direction, and are arranged in parallel at four intervals in the front-rear direction. Specifically, from the front side to the rear side, a black photosensitive drum 12K as an example of a first photosensitive member, and a yellow photosensitive drum 12Y, a magenta photosensitive drum 12M, and a cyan photosensitive member as examples of three second photosensitive members. The drum 12C is sequentially arranged.

  The scorotron charger 13 is disposed on the upper rear side of the photosensitive drum 12 so as to face the photosensitive drum 12 with a gap.

  The developing unit 14 is disposed above the photosensitive drum 12 so as to correspond to each photosensitive drum 12. Specifically, a black developing unit 14K, a yellow developing unit 14Y, a magenta developing unit 14M, and a cyan developing unit 14C are sequentially arranged from the front side to the rear side.

  Each developing unit 14 includes a developing roller 15.

  The developing roller 15 is rotatably supported at the lower end of the developing unit 14 so as to be exposed from the rear side, and is opposed to and contacts the photosensitive drum 12 from the front upper side.

The developing unit 14 contains toner as an example of a developer corresponding to each color in the space above the developing roller 15.
(3-2-2) Developing Operation in Process Unit The toner in the developing unit 14 is carried on the surface of the developing roller 15 as the developing roller 15 rotates.

  On the other hand, the surface of the photosensitive drum 12 is uniformly charged by the scorotron charger 13 as the photosensitive drum 12 rotates, and then is scanned at high speed by a laser beam (see the broken line in FIG. 1) from the scanner unit 8. Exposed. As a result, an electrostatic latent image corresponding to the image to be formed on the paper P is formed on the surface of the photosensitive drum 12.

When the photosensitive drum 12 further rotates, the toner carried on the surface of the developing roller 15 is supplied to the electrostatic latent image formed on the surface of the photosensitive drum 12. As a result, the electrostatic latent image on the photosensitive drum 12 is visualized, and a toner image by reversal development is carried on the surface of the photosensitive drum 12.
(3-3) Transfer Unit The transfer unit 10 is arranged along the front-rear direction in the main body casing 2 above the paper feed unit 3 and below the process unit 9. The transfer unit 10 includes a driving roller 16, a driven roller 17, a conveyance belt 18, and four transfer rollers 19.

  The driving roller 16 and the driven roller 17 are arranged to face each other with a space in the front-rear direction.

  The conveyor belt 18 is wound around the driving roller 16 and the driven roller 17 so that the photosensitive belt 12 is opposed to the photosensitive drum 12 from the lower side and an upper portion thereof is in contact with the photosensitive drum 12. Further, the transport belt 18 is rotated by the drive roller 16 so that the upper part of the transport belt 18 that contacts each photosensitive drum 12 moves from the front side toward the rear side.

  Each transfer roller 19 is provided to face each photosensitive drum 12 with the upper portion of the transport belt 18 interposed therebetween. Specifically, a black transfer roller 19K as an example of a first transfer member facing the black photosensitive drum 12K, a yellow photosensitive drum 12Y, a magenta photosensitive drum 12M, and a cyan photosensitive drum 12C from the front side to the rear side. A yellow transfer roller 19Y, a magenta transfer roller 19M, and a cyan transfer roller 19C, which are examples of three second transfer members that face each other, are sequentially arranged.

Then, the paper P fed from the paper feeding unit 3 is transported by the transport belt 18 from the front side to the rear side so as to sequentially pass through the transfer positions where the photosensitive drums 12 and the transfer rollers 19 face each other. Is done. During the conveyance, the color toner images carried on the respective photosensitive drums 12 are sequentially transferred onto the paper P to form a color image.
(3-4) Fixing Unit The fixing unit 11 includes a heating roller 20 and a pressure roller 21 that faces the heating roller 20 and is disposed on the rear side of the transfer unit 10. In the transfer unit 10, the color image transferred to the paper P is thermally fixed to the paper P by being heated and pressed while the paper P passes between the heating roller 20 and the pressure roller 21. .
(4) Paper discharge The paper P on which the toner image has been fixed is conveyed by each paper discharge roller 22 so as to pass through a U-turn path (not shown), and is discharged onto a paper discharge tray 23 formed on the upper side of the scanner unit 8. Is done.
2. Details of Transfer Unit (1) Configuration of Transfer Unit As described above, the transfer unit 10 in the first embodiment includes a conveyor belt 18 that contacts each photosensitive drum 12 from below, and a conveyor belt 18 as shown in FIG. And transfer rollers 19 facing the photosensitive drums 12.

  The conveyor belt 18 is formed endlessly from an elastic material having conductivity.

  Each transfer roller 19 includes a transfer roller shaft 34 and a roller portion 35.

  The transfer roller shaft 34 is formed in a rod shape extending left and right from a conductive material such as metal.

  The roller portion 35 is formed in a substantially cylindrical shape so as to cover the surface of the transfer roller shaft 34 from a conductive elastic material or the like.

  Further, the transfer unit 10 includes a linear cam mechanism 30 as an example of a contact / separation member.

  The linear cam mechanism 30 includes a support member 32, a compression spring 31, and a linear cam 33.

  The support members 32 are formed in a substantially rectangular shape when viewed from the side, and are provided one at each of the left and right ends of the transfer roller shaft 34 of each transfer roller 19. Specifically, the black side support member 32A corresponding to the black transfer roller 19K and the color side support members 32B corresponding to the color transfer rollers 19 (the yellow transfer roller 19Y, the magenta transfer roller 19M, and the cyan transfer roller 19C) are provided. Is provided. Further, a support member-side inclined surface 29 that is inclined upward as it goes forward is formed at the rear upper end portion of the support member 32. Further, a transfer roller shaft insertion hole 36 is formed through each support member 32 along the left-right direction, and the transfer roller shaft 34 is inserted into the transfer roller shaft insertion hole 36 so as to be relatively rotatable. . Thus, each support member 32 supports the corresponding transfer roller shaft 34.

  The color side support member 32B is supported by the frame 28 of the transfer unit 10 so as to be slidable up and down.

  As a result, the color-side support member 32B is slid upward, and a contact position (see FIG. 2A) where the color transfer rollers 19 are brought into contact with the transport belt 18, and is slid downward to transfer each color. The roller 19 is moved to a separation position (see FIG. 2B) that separates the roller 19 from the conveyance belt 18.

  The support member 32 corresponding to the black transfer roller 19K is fixed to the frame 28 of the transfer unit 10.

  One compression spring 31 is provided between each support member 32 and the frame 28 of the transfer unit 10.

  Each compression spring 31 has one end connected to the lower end portion of the corresponding support member 32 and the other end connected to the frame 28 of the transfer unit 10.

  Accordingly, each compression spring 31 elastically presses the transfer roller 19 toward the photosensitive drum 12 via the support member 32.

  The linear cam 33 is formed in a substantially bowl-like shape extending in the front-rear direction between the conveyance belt 18 and each color-side support member 32B. The linear cam 33 is provided with three displacement portions 37.

  Each displacement portion 37 is formed in a substantially trapezoidal shape in side view that bulges downward from the lower surface of the linear cam 33, and is arranged in parallel at intervals in the front-rear direction so as to correspond to each collar-side support member 32B. Has been placed. Each displacement portion 37 is formed with a cam-side inclined surface 38 that is inclined upward toward the front at its front end, and its lower surface is continuously rearward from the rear end of the cam-side inclined surface 38. It is formed as a flat surface 39 extending toward the surface.

  The linear cam 33 is slidably supported back and forth on the frame 28 of the transfer unit 10 so that each displacement portion 37 is disposed behind the corresponding color side support member 32B (FIG. 2A). )reference).

At this time, the cam side inclined surface 38 of each displacement part 37 is opposed to the support member side inclined surface 29 of the corresponding collar side support member 32B from the rear side.
(2) Contacting / separating operation of the transfer roller When each displacement portion 37 of the linear motion cam 33 is disposed on the rear side of the corresponding collar side support member 32B, as shown in FIG. Does not press each color support member 32 </ b> B, and the transfer roller 19 is arranged at the contact position by the urging force of the compression spring 31. Thus, each color transfer roller 19 is in contact with the conveyance belt 18.

  Then, when the linear motion cam 33 is slid toward the front side by a driving means (not shown) in the main body casing 2, the cam side inclined surface 38 of each displacement portion 37 is supported by the corresponding color side support member 32B. It abuts on the side inclined surface 29 from the rear side.

When the linear cam 33 is further moved to the front side, the support member side inclined surface 29 of each collar side support member 32B is slid rearward and downward relative to the corresponding displacement portion 37. The cam-side inclined surface 38 of the corresponding displacement portion 37 is pressed downward. As a result, each color-side support member 32B is moved downward, and each color transfer roller 19 is moved from the contact position to the separation position.
(3) Power Supply to Transfer Unit The main casing 2 includes a power source 41 and a transfer bias line 42 as an example of a bias applying unit.

  The power source 41 is provided in the main body casing 2.

  The transfer bias line 42 is electrically connected to the power source 41 and the transfer roller shaft 34 of each transfer roller 19.

Each transfer roller 19 has a transfer bias line 42 through the transfer bias line 42 when the color laser printer 1 is in the color mode (see FIG. 2A) or the monochrome mode (see FIG. 2B). A transfer bias is applied.
3. Image Forming Operation (1) Image Forming Operation in Color Mode When the color laser printer 1 is in the color mode, as shown in FIG. 2 (a), the linear cam 33 is retracted to the rear side.

  At this time, each collar-side support member 32 </ b> B is slid upward by the urging force of the compression spring 31 and is disposed at the contact position. As a result, the conveyance belt 18 and the color transfer rollers 19 are in contact with each other.

As described above, the paper P is transported from the front side to the rear side by the transport belt 18 so as to sequentially pass through the transfer positions where the photosensitive drums 12 and the transfer rollers 19 face each other. During the conveyance, the toner images of the respective colors carried on the respective photosensitive drums 12 are sequentially transferred onto the paper P by the transfer bias, thereby forming a color image.
(2) Image Forming Operation in Monochrome Mode When the color laser printer 1 is switched from the color mode to the monochrome mode, the linear cam 33 is slid forward by the drive means (not shown) in the main body casing 2. .

  Then, as described above, the collar side support member 32 </ b> B is pressed downward by the linear motion cam 33. As a result, the color transfer rollers 19 are separated from the transport belt 18.

  As described above, the paper P is transported from the front side to the rear side by the transport belt 18 so as to sequentially pass through the transfer positions where the photosensitive drums 12 and the transfer rollers 19 face each other.

  Then, first, when the paper P is opposed to the black photosensitive drum 12K, the black toner image carried on the black photosensitive drum 12K is transferred to the paper P.

  Thereafter, the paper P is sequentially passed between the color photosensitive drums 12 on which the toner images are not carried and the corresponding transfer rollers 19.

  At this time, the color transfer rollers 19 are separated from the transport belt 18 as described above. That is, the black toner image transferred to the paper P is not pressed against the color photosensitive drums 12 by the color transfer rollers 19. A transfer bias is applied to each color transfer roller 19.

  Therefore, the black toner image transferred to the paper P is carried on the surface of the paper P by the transfer bias without being attached to the surface of each color photosensitive drum 12.

  That is, the black toner image transferred to the paper P is prevented from being reversely transferred to the color photosensitive drums 12.

  Thus, a monochrome image is formed on the paper P without generating a reverse transfer ghost in the monochrome mode.

When the color mode is switched, the linear cam 33 is slid again toward the rear side by a driving means (not shown) in the main casing 2.
4). Operation and Effect (1) According to the color laser printer 1, as shown in FIG. 2B, in the monochrome mode, the color transfer rollers 19 are separated from the transport belt 18 with a transfer bias applied. Let

  Therefore, the pressing force of the paper P against the color photosensitive drums 12 is reduced, and the toner image transferred onto the paper P is continuously carried on the paper P by the transfer bias applied to the color transfer rollers 19. Can do.

  Thereby, it is possible to prevent the toner image transferred to the paper P from being reversely transferred to the color photosensitive drums 12.

As a result, in the direct tandem color laser printer 1, the occurrence of reverse transfer ghost in the monochrome mode can be suppressed.
(2) Also, according to the color laser printer 1, as shown in FIG. 2, the black photosensitive drum 12K is disposed on the front side of the photosensitive drums 12 for all the colors.

  For this reason, in the color mode, it is possible to superimpose a color toner image on a black toner image in order to develop more colors (for example, to overlay yellow on a black background and develop brown) Etc.)

As a result, various colors can be realized in the color mode.
(3) According to the color laser printer 1, as shown in FIG. 2, the linear cam mechanism 30 separates all the color transfer rollers 19 from the photosensitive drum 12.

Therefore, the occurrence of reverse transfer ghost in the monochrome mode can be more reliably suppressed.
5. Second Embodiment In the first embodiment described above, in the monochrome mode, each color transfer roller 19 is separated from the photosensitive drum 12 with a transfer bias applied.

On the other hand, in the second embodiment, as shown in FIG. 3B, in the monochrome mode, each color transfer roller 19 is separated from the photosensitive drum 12 in a state where no transfer bias is applied, and the color-side charge removal is performed. A transfer bias is applied to the member 51B. In the second embodiment, the same members as those in the first embodiment described above are denoted by the same reference numerals, and the description thereof is omitted.
(1) Configuration of Transfer Unit The transfer unit 10 includes a charge removal member 51.

The charge removal member 51 is formed in a substantially flat plate shape extending in the left-right direction from a conductive material such as metal, and is disposed adjacent to the rear side of each transfer roller 19. Specifically, a black-side charge removal member 51A disposed adjacent to the black transfer roller 19K and a color-side charge removal member 51B disposed adjacent to each color transfer roller 19 are sequentially arranged. Further, the static elimination member 51 is disposed to face the transport belt 18 with a slight space therebetween.
(2) Power supply to transfer unit The main casing 2 includes a bias line 52 electrically connected to each transfer roller 19 and each color-side charge removal member 51B, and an earth line 53 that grounds all the charge removal members 51; The first switch 57 is provided.

  The bias line 52 is electrically connected to the power source 41, and includes a transfer roller side bias line 54 as an example of a first bias applying unit and a neutralizing member side bias line 55 as an example of a second bias applying unit. I have.

  Four transfer roller side bias lines 54 are provided corresponding to each transfer roller 19, and are electrically connected to each transfer roller 19. Specifically, a black transfer roller side bias line 54A connected to the black transfer roller 19K and each color transfer roller side bias line 54B connected to each color transfer roller 19 are provided.

  A second switch 56 is provided in the middle of the color transfer roller side bias line 54B.

  The second switch 56 energizes the color transfer roller side bias line 54B to supply power from the power source 41 to the transfer roller 19 (see FIG. 3A), and energizes the color transfer roller side bias line 54B. Is switched to an OFF state (see FIG. 3B) in which power supply from the power source 41 to the transfer roller 19 is canceled.

  Three neutralizing member side bias lines 55 are provided corresponding to each color side neutralizing member 51B, and are electrically connected to each color side neutralizing member 51B via a first switch 57.

  The earth lines 53 are grounded, and four earth lines 53 are provided corresponding to the respective static eliminating members 51. Specifically, a black-side ground line 53A connected to the black-side static elimination member 51A and each color-side ground line 53B connected to each color-side static elimination member 51B via the first switch 57 are provided. ing.

  Three first switches 57 are provided corresponding to each color-side charge removal member 51B, and are connected to the color-side charge removal member 51B, and are selectively connected to the color transfer roller-side bias line 54B and the color-side ground line 53B. It is connected to the.

  That is, the first switch 57 connects the color-side charge removal member 51B and the color-side ground line 53B and also disconnects the color-side charge removal member 51B and the color transfer roller-side bias line 54B (see FIG. 3 ( a)) and the color side neutralizing member 51B and the color transfer roller side bias line 54B, and the bias side for releasing the connection between the color side static eliminating member 51B and the color side ground line 53B (FIG. 3B). To see).

  When the color laser printer 1 is in the color mode, as shown in FIG. 3A, the first switch 57 is switched to the ground side, and the second switch 56 is turned on.

  As a result, the color-side static elimination member 51B is grounded via the color-side earth line 53B, and a transfer bias is applied to each color transfer roller 19 via the color transfer roller-side bias line 54B. .

  The black-side static elimination member 51A is also grounded via the black-side ground line 53A, and a transfer bias is also applied to the black transfer roller 19K via the black transfer roller-side bias line 54A.

  That is, when the color laser printer 1 is in the color mode, all the charge removal members 51 are grounded, and the transfer bias is applied to all the transfer rollers 19.

  When the color laser printer 1 is switched from the color mode to the monochrome mode, as shown in FIG. 3B, the first switch 57 is switched to the bias side, and the second switch 56 is turned off.

  As a result, a transfer bias is applied to the color-side charge removal member 51B via the charge-removal member-side bias line 55, and no transfer bias is applied to each color transfer roller 19.

  The black-side static elimination member 51A is grounded via the black-side ground line 53A, and a transfer bias is applied to the black transfer roller 19K via the black transfer roller-side bias line 54A.

That is, when the color laser printer 1 is in the monochrome mode, the application of the transfer bias to the color transfer rollers 19 among the transfer rollers 19 is canceled, and the color-side charge removal member 51B among the charge removal members 51 is removed. A transfer bias is applied.
(3) Effects of Second Embodiment According to the color laser printer 1 of the second embodiment, as shown in FIG. 3B, in the monochrome mode, a transfer bias is applied to each color transfer roller 19. Without being separated from the conveying belt 18, and a transfer bias is applied to each color-side neutralizing member 51 </ b> B disposed adjacent to the rear side of each color transfer roller 19.

  Therefore, the pressing force of the paper P against the color photosensitive drums 12 can be reduced, and the toner image transferred onto the paper P can be continuously carried on the paper P by the transfer bias applied to the color-side charge removal member 51B. it can.

  Thereby, it is possible to prevent the toner image transferred to the paper P from being reversely transferred to the color photosensitive drum 12.

  As a result, in the direct tandem color laser printer 1, it is possible to suppress the occurrence of reverse transfer ghost in the monochrome mode.

Further, according to the color laser printer 1 of the second embodiment, the same operational effects as those of the color laser printer 1 of the first embodiment described above can be obtained.
6). Other Embodiments In each of the above-described embodiments, the color transfer rollers 19 are all separated from the corresponding photosensitive drums 12, but only the yellow transfer roller 19Y can be separated from the yellow photosensitive drum 12Y.

  The reverse transfer ghost in the monochrome mode is likely to occur mainly in the yellow photosensitive drum 12Y adjacent to the rear side of the black photosensitive drum 12K, and the magenta photosensitive drum 12M and the cyan photosensitive drum disposed immediately behind the yellow photosensitive drum 12Y. In the drum 12C, the frequency of occurrence is low.

  Therefore, the occurrence of the reverse transfer ghost in the monochrome mode can be suppressed by separating the yellow transfer roller 19Y from the yellow photosensitive drum 12Y and suppressing the occurrence of the reverse transfer ghost in the yellow photosensitive drum 12Y.

DESCRIPTION OF SYMBOLS 1 Color laser printer 12 Photosensitive drum 18 Conveyance belt 19 Transfer roller 30 Linear motion cam mechanism 42 Transfer bias line 51 Static elimination member 54 Transfer roller side bias line 55 Static elimination member side bias line P Paper

Claims (4)

Switch between monochrome mode for forming monochrome images and color mode for forming color images, equipped with a conveyor belt that transports the member to be transferred and a plurality of photoconductors that carry developer images transferred to the member to be transferred. An image forming apparatus comprising:
The plurality of photoconductors include a first photoconductor that carries a black developer image, and a plurality of second photoconductors that carry a developer image other than black,
A first transfer member facing the first photoconductor across the conveyor belt;
A second transfer member opposed to each of the second photosensitive members with the conveyance belt interposed therebetween;
A contact / separation member for contacting or separating at least one second transfer member with respect to the transport belt;
A bias applying means for applying a bias to the first transfer member and all the second transfer members;
The first photoconductor is disposed upstream of at least one second photoconductor in the conveying direction of the transfer target member,
All the photoreceptors are in contact with the conveyor belt in the monochrome mode and the color mode,
The contact / separation member contacts at least the second transfer member opposed to the second photoconductor disposed adjacent to the downstream side in the transport direction of the first photoconductor in contact with the transport belt in the color mode. And separated from the conveyor belt in the monochrome mode,
The image forming apparatus, wherein the bias applying unit applies a transfer bias to the first transfer member and all the second transfer members in the monochrome mode and the color mode. Switch between monochrome mode for forming monochrome images and color mode for forming color images, equipped with a conveyor belt that transports the member to be transferred and a plurality of photoconductors that carry developer images transferred to the member to be transferred. An image forming apparatus comprising:
The plurality of photoconductors include a first photoconductor that carries a black developer image, and a plurality of second photoconductors that carry a developer image other than black,
A first transfer member facing the first photoconductor across the conveyor belt;
A second transfer member opposed to each of the second photosensitive members with the conveyance belt interposed therebetween;
A static elimination member disposed adjacent to each downstream side of each of the second transfer members in the conveyance direction of the transferred member, and facing the conveyance belt;
A contact / separation member for contacting or separating at least one second transfer member with respect to the transport belt;
First bias applying means for applying a transfer bias to the first transfer member and all the second transfer members;
Second bias applying means for applying a transfer bias to each of the charge eliminating members,
The first photoconductor is disposed upstream of at least one second photoconductor in the conveying direction of the transfer target member,
All the photoreceptors are in contact with the conveyor belt in the monochrome mode and the color mode,
The contact / separation member contacts at least the second transfer member opposed to the second photoconductor disposed adjacent to the downstream side in the transport direction of the first photoconductor in contact with the transport belt in the color mode. And separated from the conveyor belt in the monochrome mode,
The first bias applying unit applies a transfer bias to the first transfer member in the monochrome mode, and does not apply a transfer bias to the second transfer member separated from the conveyance belt.
In the monochrome mode, the second bias applying unit applies a transfer bias to the charge removal member disposed adjacent to the downstream side of the second transfer member that is separated from the conveyance belt. Forming equipment.   3. The image forming apparatus according to claim 1, wherein the first photoconductor is disposed upstream of all the second photoconductors in the transport direction. 4.   4. The image forming apparatus according to claim 1, wherein the contact / separation member contacts and separates all the second transfer members. 5.

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