JP5051289B2 - 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-described object, the present invention provides a monochrome mode that includes a conveyance belt that conveys a member to be transferred and a plurality of photosensitive members that carry developer images transferred to the member to be transferred, and forms a monochrome image. And a color mode for forming a color image, wherein the plurality of photoconductors carry a first photoconductor carrying a black developer image and a developer image other than black. A plurality of second photoconductors, a first transfer member facing the first photoconductor across the transport belt, and a second transfer facing the second photoconductor across the transport belt A member, a first biasing member provided corresponding to the first transfer member, and biasing the first transfer member toward the first photoconductor, and provided corresponding to each of the second transfer members. Corresponding to each of the second transfer members A plurality of second urging members that urge toward the second photosensitive member, and the urging force of each of the second urging members include a first urging force set in the color mode and a monochrome mode. Changing means for changing to a second urging force set to be smaller than the first urging force, wherein the first photoconductor is at least one of the second photoconductors in the transport direction of the member to be transferred. The urging force of the second transfer member corresponding to the second photoconductor disposed adjacent to the downstream side in the transport direction of the first photoconductor is disposed at the upstream side. It is characterized by changing.

  According to the present invention, in the monochrome mode, at least the urging force of the second urging member corresponding to the second photoconductor disposed adjacent to the downstream side in the transport direction of the first photoconductor is set in the color mode. It can be changed to a second urging force that is smaller than the first urging force.

  For this reason, in the monochrome mode, it is possible to reduce the pressing force of the second transfer member corresponding to the second photoconductor adjacently disposed on the downstream side in the transport direction of the first photoconductor to the second photoconductor.

  Thereby, it is possible to prevent the developer image transferred from the first photosensitive member to the transfer member from being reversely transferred to the second photosensitive member.

  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. It is a front view for demonstrating operation | movement of the transfer unit in 2nd Embodiment, (a) shows the time in color mode, (b) shows the time in 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.

  The transfer unit 10 includes a compression spring 31 and a linear cam mechanism 30 as an example of a changing unit.

  One compression spring 31 is provided between the transfer roller shaft 34 of each transfer roller 19 and each corresponding support member 32 (described later). Specifically, the black side compression spring 31A as an example of a first urging member provided corresponding to the black transfer roller 19K and the second urging member provided as an example corresponding to each color transfer roller 19 are provided. Each collar side compression spring 31B is provided.

  Each compression spring 31 has one end connected to both ends in the left-right direction of the corresponding transfer roller shaft 34 and the other end connected to the upper end of a corresponding support member 32 (described later). The spring constants of the compression springs 31 are all set to be the same.

  Thereby, each compression spring 31 elastically presses the corresponding transfer roller 19 toward the corresponding photosensitive drum 12.

  The linear cam mechanism 30 includes a support member 32 and a linear cam 33.

  The support member 32 is formed in a substantially U shape in a side view with its lower side open, and is arranged one by one at a distance below the left and right ends of the transfer roller shaft 34 corresponding to each transfer roller 19. Yes. Specifically, the black support member 32A provided corresponding to the black transfer roller 19K, and the respective colors provided corresponding to the color transfer rollers 19 (the yellow transfer roller 19Y, the magenta transfer roller 19M, and the cyan transfer roller 19C). A side support member 32B is provided. Further, a support member-side inclined surface 36 that is inclined upward as it goes forward is formed at the rear upper end portion of the support member 32.

  Further, the color side support member 32 </ b> B is close to the frame (not shown) of the transfer unit 10 in the vicinity of the transfer roller shaft 34 (see FIG. 2A), and is separated from the transfer roller shaft 34. (Refer to FIG. 2B), it is slidably supported up and down. Further, the collar side support member 32B is always urged by an urging means (not shown) so as to be disposed at a close position.

  The black-side support member 32A is fixed to the frame (not shown) of the transfer unit 10.

  The linear cam 33 is formed in a substantially bowl shape extending in the front-rear direction between each color transfer roller 19 and each color-side support member 32B. The linear cam 33 is provided with three displacement portions 37.

  The displacement portions 37 are arranged in parallel at intervals in the front-rear direction so as to correspond to the color-side support members 32B. Each displacement portion 37 is formed in a substantially trapezoidal shape in side view that bulges downward from the lower surface of the linear motion cam 33. Each displacement portion 37 is formed with a cam-side inclined surface 38 that is inclined upward toward the front at the front end portion thereof, and the lower surface thereof is continuously rearward from the rear end portion 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 a frame (not shown) of the transfer unit 10 so that each displacement portion 37 is disposed on the rear side of the corresponding color side support member 32B. (See FIG. 2 (a)).

At this time, the cam-side inclined surface 38 of each displacement portion 37 is opposed to the support member-side inclined surface 36 of the corresponding collar-side support member 32B from the rear.
(2) Energizing force changing operation of compression spring When each displacement portion 37 of the linear cam 33 is disposed behind the corresponding collar side support member 32B, as shown in FIG. Reference numeral 33 does not press the collar side support member 32B, and the collar side support member 32B is disposed in the proximity position by the urging force of the urging means (not shown).

  At this time, the collar-side compression spring 31B is compressed by the corresponding collar-side support member 32B and is in the first compressed state. Accordingly, the color side compression spring 31B urges the corresponding transfer roller 19 toward the photosensitive drum 12 with the first urging force.

  Then, when the linear motion cam 33 is slid toward the front side by a driving means (not shown) in the main casing 2, the cam side inclined surface 38 of each displacement portion 37 becomes the support member side of the corresponding collar side support member 32 </ b> B. It contacts the inclined surface 36 from the rear side.

  When the linear cam 33 is further moved to the front side, the support member side inclined surface 36 of each collar side support member 32 </ b> B is relatively lower than the cam side inclined surface 38 of the corresponding displacement portion 37. So that it is slid to the lower side by the cam side inclined surface 38 of the corresponding displacement portion 37. As a result, the collar side support member 32B is moved downward from the proximity position toward the separation position.

Then, as shown in FIG. 2B, the collar side compression spring 31 </ b> B is in the second compressed state restored from the first compressed state, and has a second urging force that is smaller than the first urging force. The corresponding transfer roller 19 is urged toward the photosensitive drum 12. The second urging force is set to be the same in all the collar side compression springs 31B.
(3) Power Supply to Transfer Roller The transfer roller shaft 34 of each transfer roller 19 is electrically connected to a power source 41 provided in the main body casing 2 via a transfer bias line 42.

Each transfer roller 19 is connected to each transfer roller 19 via each transfer bias line 42 regardless of whether 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 from the power source.
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, the collar-side support member 32B is disposed in the proximity position.

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 32B is pressed downward by the linear motion cam 33 and moved from the proximity position toward the separation position.

  As a result, the color side compression spring 31B biases the corresponding transfer roller 19 toward the photosensitive drum 12 with a second biasing force smaller than the first biasing force.

  That is, when the color laser printer 1 is switched to the monochrome mode, the pressing force of the color transfer roller 19 to the photosensitive drum 12 is reduced.

  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, 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, as described above, the pressure of the color transfer rollers 19 to the photosensitive drum 12 is reduced. A transfer bias is applied to each color transfer roller 19.

  Therefore, the black toner image transferred to the paper P is less likely to adhere to the surface of the photosensitive drum 12 due to the relatively reduced pressing force of the transfer roller 19 to the photosensitive drum 12, and the transfer bias causes the black toner image of the paper P to be adhered. Supported on the surface.

  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 this color laser printer 1, as shown in FIG. 2, in the monochrome mode, the biasing force of the color side compression spring 31B is smaller than the first biasing force set in the color mode. The urging force can be changed to 2.

  Therefore, in the monochrome mode, the pressing force of the color transfer rollers 19 (the yellow transfer roller 19Y, the magenta transfer roller 19M, and the cyan transfer roller 19C) to the photosensitive drum 12 can be reduced.

  Thereby, it is possible to prevent the toner image transferred from the black photosensitive drum 12 </ b> K to the paper P from being 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 in front of the photosensitive drums 12 for all 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 support member 32 and the linear motion cam 33 change the urging force of the transfer rollers 19 for all the colors.

  Therefore, the pressing force of the transfer rollers 19 of all colors to the photosensitive drum 12 can be reduced uniformly.

As a result, the occurrence of reverse transfer ghost in the monochrome mode can be more reliably suppressed.
(4) According to the color laser printer 1, the urging force of the color side compression spring 31B is all set to be the same in the monochrome mode.

  Therefore, the biasing force of the collar side compression spring 31B can be changed to the same biasing force at a time by the movement of the linear cam 33 without providing a changing means for each collar side compression spring 31B.

As a result, the urging force of the collar side compression spring 31B can be uniformly changed with a simple configuration as compared with the configuration in which the changing means is provided for each collar side compression spring 31B.
(5) According to the color laser printer 1, as shown in FIG. 2, transfer bias is applied to all transfer rollers 19 in the monochrome mode and the color mode.

  Therefore, not only in the color mode but also in the monochrome mode, the toner image can be carried on the paper P by the transfer bias of each transfer roller 19 when the paper P faces the color photosensitive drums 12.

As a result, the toner image transferred to the paper P can be further suppressed from being transferred to the color photosensitive drum 12.
5. Second Embodiment In the first embodiment described above, the urging force of each compression spring 31 against the transfer roller 19 is changed by changing the compression state of each compression spring 31 by the linear motion cam mechanism 30.

On the other hand, in the second embodiment, as shown in FIGS. 3 and 4, an urging unit 53 having a pair of compression springs 51 and an eccentric cam mechanism 50 are provided. The biasing force to the transfer roller 19 is changed by bringing the compression spring 51 into contact with or away from the transfer roller shaft 34. In the second embodiment, the same members as those in the first embodiment are denoted by the same reference numerals, and the description thereof is omitted.
(1) Configuration of Transfer Unit The transfer unit 10 of the second embodiment includes an urging unit 53 and an eccentric cam mechanism 50 as an example of a changing unit.

  One urging unit 53 is disposed below each of the left and right ends of the transfer roller shaft 34 corresponding to each transfer roller 19. Specifically, the black side urging unit 53A as an example of a first urging member provided corresponding to the black transfer roller 19K and the second urging member provided as an example corresponding to the color transfer roller 19 are provided. A color side urging unit 53B is provided.

  Each urging unit 53 includes a support member 55 and a pair of compression springs 51.

  The support member 55 is formed in a substantially U shape in a side view with the lower side opened, and is disposed below the transfer roller shaft 34 with a gap.

  Both compression springs 51 are adjacent to each other on the left and right. In other words, two compression springs 51 are disposed at each of the left and right ends of the transfer roller shaft 34 (see FIG. 4).

  The compression spring 51 on the inner side in the left-right direction has one end connected to the left-right end of the transfer roller shaft 34 and the other end connected to the upper end of the support member 55.

  In addition, one end of the compression spring 51 on the outer side in the left-right direction of the collar-side urging unit 53B is connected to the lower surface of a pressing member 54 (described later), and the other end is connected to the upper end of the support member 55. In addition, one end of the compression spring 51 on the outer side in the left-right direction of the black side biasing unit 53A is connected to the left-right end of the transfer roller shaft 34 of the black transfer roller 19K, and the other end is the upper end of the corresponding support member 55. It is connected to the.

  In each urging unit 53, the spring constants of the compression springs 51 on the inner side in the left-right direction are all set to be the same, and the spring constants of the compression springs 51 on the outer side in the left-right direction are all set to be the same. That is, the urging forces of the urging units 53 are all set to be the same.

  One eccentric cam mechanism 50 is provided at each of the left and right ends of each color transfer roller 19, and includes a pressing member 54 and an eccentric cam 52.

  The pressing member 54 is provided on the upper side of the compression spring 51 on the outer side in the left-right direction between the support member 55 and the transfer roller shaft 34 and is formed in a substantially bowl shape extending in the front-rear direction. The pressing member 54 is rotatably supported by a frame (not shown) of the transfer unit 10 at the front end. The pressing member 54 is always urged counterclockwise as viewed from the right side by the urging force of the compression spring 51 on the outer side in the left-right direction, and is in contact with the transfer roller shaft 34 from below.

  The eccentric cam 52 is disposed on the upper side of the rear end portion of the pressing member 54 and is formed in a substantially elliptical shape in a side view. The eccentric cam 52 is always arranged so as to extend in the front-rear direction (see FIG. 3A), and is rotatably supported by a frame (not shown) of the transfer unit 10 at the front end.

Further, when the eccentric cam 52 is disposed along the front-rear direction, the eccentric cam 52 is spaced apart from the rear end portion of the pressing member 54 in the vertical direction (see FIG. 3A) and extends along the vertical direction. When it is pivoted so as to hang down, it is arranged so as to come into contact with the rear end of the pressing member 54 (see FIG. 3B).
(2) Biasing force changing operation of the urging unit The eccentric cam 52, as described above, is arranged along the front-rear direction as shown in FIGS. 3 (a) and 4 (a). It is spaced apart from the upper side of the rear end. The pressing member 54 is in contact with the transfer roller shaft 34 from below by the urging force of the compression spring 51 on the outer side in the left-right direction.

  At this time, the collar side urging unit 53B transfers the urging force of both the compression springs 51 (that is, the resultant force of the urging force of the compression spring 51 on the outer side in the left-right direction and the urging force of the compression spring 51 on the inner side in the left-right direction). The roller 19 is urged toward the photosensitive drum 12. The urging force of the collar side urging unit 53B at this time is defined as a first urging force. The black side urging unit 53A also urges the transfer roller 19 toward the photosensitive drum 12 by the first urging force, similarly to the color side urging unit 53B.

  The eccentric cam 52 is rotated clockwise as viewed from the right side by drive means (not shown) in the main body casing 2. Then, the eccentric cam 52 is brought into contact with the rear end portion of the pressing member 54 from above and presses the rear end portion of the pressing member 54 downward against the urging force of the compression spring 51 on the outer side in the left-right direction. To do.

  Then, the pressing member 54 is rotated clockwise as viewed from the right side against the urging force of the compression spring 51 on the outer side in the left-right direction, and is separated downward from the transfer roller shaft 34.

  As a result, as shown in FIGS. 3B and 4B, the urging of the compression spring 51 on the outer side in the left-right direction to the transfer roller shaft 34 is released, and the collar-side urging unit 53B is moved to the inner side in the left-right direction. The transfer roller 19 is urged toward the photosensitive drum 12 only by the urging force of the compression spring 51. That is, the color side urging unit 53B urges the corresponding transfer roller 19 toward the photosensitive drum 12 with a second urging force smaller than the first urging force.

Also in the color laser printer 1 of the second embodiment, the same operational effects as those of the first embodiment can be obtained.
6). Third Embodiment In each of the above-described embodiments, the urging force of the black side compression spring 31A or the black side urging unit 53A is not changed between the color mode and the monochrome mode, but in the third embodiment, in the monochrome mode. The urging force of the black side compression spring 31A or the black side urging unit 53A is changed to be smaller than the urging force in the color mode. Note that in the third embodiment, members similar to those in the above-described embodiments are given the same reference numerals, and descriptions thereof are omitted.

  For example, the linear cam mechanism 30 or the eccentric cam mechanism 50 is configured to correspond to all the transfer rollers 19, and in the monochrome mode, the urging force of all the compression springs 31 or the urging units 53 is applied in the color mode. Change to be smaller than power.

  According to the color laser printer 1 of the third embodiment, in the monochrome mode, not only the pressing force of each color transfer roller 19 against the color photosensitive drum 12 but also the pressing force of the black transfer roller 19K against the black photosensitive drum 12K is reduced. Is done.

  Therefore, excessive transfer of toner from the black photosensitive drum 12K to the paper P due to the pressing force of the black transfer roller 19K can be suppressed.

  This further suppresses the reverse transfer of the toner image on the paper P to each color photosensitive drum 12 when the paper P to which the black toner image is transferred faces the color photosensitive drum 12. Can do.

  As a result, the occurrence of reverse transfer ghost in the monochrome mode can be further suppressed.

Also, in the color laser printer 1 of the third embodiment, the same operational effects as those of the above-described embodiments can be obtained.
7). Other Embodiments In the above-described embodiments, the urging force of all the color side compression springs 31B or the color side urging units 53B is changed. It is also possible to change only the power.

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

  Therefore, it is possible to suppress the occurrence of reverse transfer ghost in the monochrome mode by reducing the pressing force of the yellow transfer roller 19Y to the yellow photosensitive drum 12Y and suppressing the occurrence of reverse transfer ghost in the yellow photosensitive drum 12Y. .

DESCRIPTION OF SYMBOLS 1 Color laser printer 12 Photosensitive drum 18 Conveying belt 19 Transfer roller 30 Linear motion cam mechanism 31A Black side compression spring 31B Color side compression spring 50 Eccentric cam mechanism 53A Black side biasing unit 53B Color side biasing unit P Paper

Claims (6)

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 first biasing member provided corresponding to the first transfer member and biasing the first transfer member toward the first photosensitive member;
A plurality of second urging members provided corresponding to the respective second transfer members and urging each of the second transfer members toward the corresponding second photoconductor;
The urging force of each second urging member is changed to a first urging force set in the color mode and a second urging force set smaller than the first urging force in the monochrome mode. And changing means to
The first photoconductor is disposed upstream of at least one second photoconductor in the conveying direction of the transfer target member,
The changing unit changes at least an urging force of the second transfer member corresponding to the second photoconductor disposed adjacent to the downstream side in the transport direction of the first photoconductor. Image forming apparatus.   2. The image forming apparatus according to claim 1, wherein the first photoconductor is disposed upstream of all the second photoconductors in the transport direction.   The image forming apparatus according to claim 1, wherein the changing unit changes a biasing force of all the second transfer members.   4. The image forming apparatus according to claim 1, wherein the urging forces of the second urging members are all set to be the same in the monochrome mode. 5.   5. The biasing force of the first biasing member in the monochrome mode is set to be smaller than the biasing force of the first biasing member in the color mode. 6. The image forming apparatus described in the item.   The transfer bias is applied to the first transfer member and all the second transfer members in the monochrome mode and the color mode, respectively. The image forming apparatus described in 1.

Images