IMAGE FORMING APPARATUS

Abstract

An image forming apparatus (1) includes image bearing members (31), transfer members (52), and a power supply section (57). The image bearing members (31) each bear one of toner images in different colors from one another. The transfer members (52) are located opposite to the image bearing members (31). The power supply section (57) charges the transfer members (52). Through the above, the toner images on the respective image bearing members (31) are transferred to a moving transfer target (51). The power supply section (57) includes a power supply device (58 or 58a) connected to at least two of the transfer members (52). The transfer members (52) connected to the power supply device (58 or 58a) are each located at a position shifted upstream or downstream of a corresponding one of the image bearing members (31) in a moving direction (X) of the transfer target (51).

Claims

1. An image forming apparatus capable of forming a color image by transferring toner images in respective colors different from one another such that the toner images are superimposed on one another, the image forming apparatus comprising: a plurality of image bearing members each capable of bearing one of the toner images in the respective colors; a plurality of transfer members each located opposite to a corresponding one of the plurality of image bearing members; and a power supply section capable of causing the toner images on the respective image bearing members to be transferred to a moving transfer target by charging the plurality of transfer members, wherein the power supply section includes a first power supply device connected to at least two transfer members of the plurality of transfer members, and the at least two transfer members of the plurality of transfer members connected to the first power supply device are each located at a position shifted upstream or downstream of a corresponding one of the image bearing members in a moving direction of the moving transfer target.

2. The image forming apparatus according to claim 1, further comprising a plurality of static eliminating devices each configured to eliminate static electricity from a corresponding one of the plurality of image bearing members, wherein a static eliminating device of the plurality of static eliminating devices that is located between adjacent image bearing members of the plurality of image bearing members irradiates with light respective image bearing members of the plurality of image bearing members located upstream and downstream in the moving direction of the moving transfer target.

3. The image forming apparatus according to claim 2, wherein the first power supply device is connected to at least two transfer members of the plurality of transfer members, the at least two transfer members including a transfer member located the most upstream among the plurality of transfer members in the moving direction of the moving transfer target.

4. The image forming apparatus according to claim 3, wherein the transfer member located the most upstream in the moving direction of the moving transfer target is shifted by a shift amount larger than shift amounts of the other transfer members.

5. The image forming apparatus according to claim 4, wherein the shift amounts of the other transfer members are the same.

6. The image forming apparatus according to claim 4, wherein the transfer members are shifted by shift amounts that decrease downstream in the moving direction of the moving transfer target.

7. The image forming apparatus according to claim 1, wherein the plurality of image bearing members each include a positively chargeable photosensitive member, and the image forming apparatus further includes a plurality of chargers each capable of charging a corresponding one of the photosensitive members to a positive potential.

8. The image forming apparatus according to claim 1, wherein the first power supply device is connected to all the plurality of transfer members.

9. The image forming apparatus according to claim 1, wherein the power supply section further includes a second power supply device connected to one transfer member of the plurality of transfer members.

10. The image forming apparatus according to claim 9, wherein the one transfer member connected to the second power supply device is located at a position shifted upstream or downstream of a corresponding one of the image bearing members in the moving direction of the moving transfer target.

11. The image forming apparatus according to claim 9, wherein the one transfer member connected to the second power supply device is located at the same position as a corresponding one of the image bearing members in the moving direction of the moving transfer target.

12. The image forming apparatus according to claim 9, wherein the second power supply device is connected to a transfer member of the plurality of transfer members that is located opposite to an image bearing member of the plurality of image bearing members that bears a toner image in black color.

13. The image forming apparatus according to claim 4, wherein the transfer members are each shifted by a shift amount of at least 3.0 mm

14. The image forming apparatus according to claim 13, wherein the transfer member located the most upstream in the moving direction of the moving transfer target is shifted by a shift amount of at least 6.0 mm

Description

BRIEF DESCRIPTION OF DRAWINGS

[0017] FIG. 1 is a vertical cross sectional view of an image forming apparatus according to an embodiment of the present invention.

[0018] FIG. 2 is an enlarged vertical cross sectional view of an image forming section and a transfer section according to the embodiment of the present invention.

[0019] FIG. 3 is a diagram illustrating a power supply system for primary transfer rollers according to the embodiment of the present invention.

[0020] FIG. 4 is a diagram illustrating another example of the power supply system for the primary transfer rollers according to the embodiment of the present invention.

[0021] FIG. 5 shows results of first through third examples of the present invention and first and second comparative examples.

[0022] FIG. 6 shows results of the third and fourth examples of the present invention.

[0023] FIG. 7 shows results of the first and third comparative examples.

DESCRIPTION OF EMBODIMENTS

[0024] The following describes an embodiment of the present invention with reference to the drawings. Note that in the drawings, elements that are the same or substantially equivalent are labelled using the same reference signs and explanation thereof is not repeated. The drawings schematically illustrate elements of configuration in order to facilitate understanding. Numerical values, materials of the elements of configuration and the like described in the following embodiment are merely examples that do not impart any specific limitations and may be altered in various ways so long as such alterations do not substantially deviate from effects of the present invention.

[0025] FIG. 1 is a vertical cross sectional view of an image forming apparatus according to the present embodiment. The image forming apparatus 1 of the present embodiment is a color image forming apparatus of an intermediate transfer belt type. The image forming apparatus 1 is capable of forming a color image (color toner image) by transferring toner images in respective colors of yellow (Y), cyan (C), magenta (M), and black (Bk) such that the toner images are superimposed on one another.

[0026] The image forming apparatus 1 includes a housing 2, an image forming section 3, an exposure device 4, a transfer section 5, a paper feed cassette 6, a paper feed section 7, a first sheet conveyance section 8, a fixing section 9, an exit tray 10, a manual feed tray 11, a paper feed roller 12, a second sheet conveyance section 13, a third sheet conveyance section 14, and toner supplying sections 15.

[0027] The image forming section 3 includes four photosensitive drums 31 (image bearing members) corresponding to the respective colors of yellow, cyan, magenta, and black. The photosensitive drums 31 are each capable of bearing one of toner images in the respective colors different from one another. The photosensitive drums 31 each have a diameter φ of for example 30 mm The image forming section 3 is capable of forming the toner images in the respective colors of yellow, cyan, magenta, and black each on the circumferential surface of one of the four photosensitive drums.

[0028] Specifically, the image forming section 3 includes four development rollers 32 corresponding to the respective colors of yellow, cyan, magenta, and black. The development rollers 32 are each located opposite to a corresponding one of the photosensitive drums 31. The development rollers 32 supply toners of the respective colors to the respective photosensitive drums 31. Through the above, the photosensitive drums 31 bear the toner images in the respective colors.

[0029] The exposure device 4 is located below the four photosensitive drums 31. The exposure device 4 scans each photosensitive drum 31 corresponding to a color necessary to form an image with light (for example, laser beam) based on image data. As a result, an electrostatic latent image is formed on the photosensitive drum 31 scanned with the light. Thereafter, a toner (developer) is supplied from a corresponding one of the development rollers 32 to the photosensitive drum 31 on which the electrostatic latent image has been formed. Through the above, the electrostatic latent image is developed to form a toner image in the color necessary to form the image.

[0030] The transfer section 5 includes an endless intermediate transfer belt 51 (transfer target) and four primary transfer rollers 52 (transfer members) each located opposite to a corresponding one of the four photosensitive drums 31.

[0031] The intermediate transfer belt 51 includes a base layer formed from a resin and a coating layer covering a surface of the base layer. The thickness of the intermediate transfer belt 51 is about 80-120 μm and the thickness of the coating layer is about 10 μm. A thermoplastic resin is for example employable as a material of the base layer. Examples of employable thermoplastic resins include polyamide (PA) and polycarbonate (PC). Note that a thermosetting resin may be used as a material of the base layer of the intermediate transfer belt 51. Examples of employable thermosetting resins include polyimide (PI), polyamide alloy (PAA), and silicone resins. An insulating resin is used as a material of the coating layer. Examples of employable insulating resins include polycarbonate, acrylic resins, and fluorine-based resins.

[0032] The base layer of the intermediate transfer belt 51 contains electrically conductive particles such as carbon black and ionic conductive materials. The volume resistivity of the base layer is controlled to be from about 1.0×10.sup.8Ω.Math.cm to about 1.0×10.sup.11Ω.Math.cm at the time of application of a voltage of 250 V. The surface resistivity of the intermediate transfer belt 51 is controlled to be at least 1.0×10.sup.10Ω/sq at the time of application of a voltage of 250 V. The surface resistivity of the intermediate transfer belt 51 may be for example at least 1.0×10.sup.10Ω/sq and no greater than 1.0×10.sup.11Ω/sq at the time of application of a voltage of 250 V.

[0033] The primary transfer rollers 52 are elastic rollers each including a metal shaft such as an iron shaft and an elastic layer surrounding the metal shaft. The primary transfer rollers 52 each have a diameter φ of for example 12.0 mm The thickness of the elastic layer is for example about 3 mm An electrically conductive foamed elastic body containing electrically conductive particles such as carbon black and ionic conductive materials is for example employable as a material of the elastic layer. Examples of employable electrically conductive foamed elastic bodies include foamed EPDM obtained by foaming an ethylene-propylenediene rubber and foamed NBR obtained by foaming a nitrile rubber. The surface resistivity of each of the primary transfer rollers 52 is controlled to be at least 1.0×10.sup.6Ω/sq at the time of application of a voltage of 1000 V. The surface resistivity of each of the primary transfer rollers 52 may for example be at least 1.0×10.sup.6.8Ω/sq and no greater than 1.0×10.sup.78Ω/sq at the time of application of a voltage of 1000 V.

[0034] The intermediate transfer belt 51 is located above the four photosensitive drums 31. The primary transfer rollers 52 are each located inside of the intermediate transfer belt 51. The primary transfer rollers 52 are each located opposite to a corresponding one of the photosensitive drums 31 with the intermediate transfer belt 51 therebetween. The primary transfer rollers 52 are each pressed against the circumferential surface of a corresponding one of the photosensitive drums 31 with the intermediate transfer belt 51 therebetween. As a result, each of the primary transfer rollers 52 and a corresponding one of the photosensitive drums 31 form a primary transfer nip N1 therebetween.

[0035] The transfer section 5 further includes a drive roller 53, a driven roller 54, and a tension roller 55. The intermediate transfer belt 51 is stretched around the drive roller 53, the driven roller 54, and the tension roller 55. The tension roller 55 urges the intermediate transfer belt 51 outward from the inside of the intermediate transfer belt 51. The tension roller 55 gives a specific tension to the intermediate transfer belt 51. The intermediate transfer belt 51 rotates in a rotational direction X (counterclockwise direction in FIG. 1) in accompaniment to rotation of the drive roller 53.

[0036] The toner images formed (carried) on the circumferential surfaces of the respective photosensitive drums 31 are each transferred (primarily transferred) to the outer circumferential surface of the intermediate transfer belt 51 rotating in the rotational direction X at a corresponding one of the primary transfer nips N1. For example, in a situation in which toner images in a plurality of colors are necessary to form an image, the toner images are each formed on the circumferential surface of one of at least two photosensitive drums 31 of the four photosensitive drums 31. The toner images are transferred to the outer circumferential surface of the intermediate transfer belt 51 in order from upstream in the rotational direction X of the intermediate transfer belt 51 (the moving direction of the transfer target) along with rotation of the intermediate transfer belt 51 such that the toner images are superimposed on one another.

[0037] The transfer section 5 further includes a secondary transfer roller 56 located opposite to the drive roller 53. The secondary transfer roller 56 is pressed against the circumferential surface of the drive roller 53 with the intermediate transfer belt 51 therebetween. As a result, the secondary transfer roller 56 and the drive roller 53 form a secondary transfer nip N2 therebetween.

[0038] The paper feed cassette 6 is located below the exposure device 4. The paper feed cassette 6 is capable of accommodating a plurality of sheets S (recording medium). The sheets S are for example paper.

[0039] The paper feed section 7 picks up one of the sheets S accommodated in the paper feed cassette 6 and feeds the sheet S to the most upstream part of the first sheet conveyance section 8. Specifically, the paper feed section 7 includes a pickup roller 71 and a paper feed roller pair 72. The pickup roller 71 is located above an end of the paper feed cassette 6. The pickup roller 71 picks up the sheet S from the paper feed cassette 6. The paper feed roller pair 72 feeds the sheet S to the most upstream part of the first sheet conveyance section 8. The paper feed roller pair 72 feeds one sheet S at a time to the first sheet conveyance section 8.

[0040] The first sheet conveyance section 8 conveys the sheet S to the secondary transfer nip N2. Through the above, toner images are transferred to the sheet S at the secondary transfer nip N2. Specifically, the first sheet conveyance section 8 includes a registration roller pair 81 located upstream of the secondary transfer nip N2. The registration roller pair 81 controls a timing at which the sheet S passes through the secondary transfer nip N2.

[0041] The first sheet conveyance section 8 conveys the sheet S to which the toner images have been transferred to the exit tray 10 via the fixing section 9. The exit tray 10 is provided on the top face of the housing 2.

[0042] The fixing section 9 includes a pressure member 91 and a heating member 92. The pressure member 91 and the heating member 92 apply pressure and heat to the sheet S, whereby the unfixed toner images are fixed to the sheet S.

[0043] The manual feed tray 11 is attached to a side wall of the housing 2. A plurality of sheets S can be placed on the manual feed tray 11. The paper feed roller 12 is located on the base end side of the manual feed tray 11. The paper feed roller 12 feeds a sheet S on the manual feed tray 11 to the most upstream part of the second sheet conveyance section 13. The second sheet conveyance section 13 joins the first sheet conveyance section 8 at a position upstream of the registration roller pair 81. The second sheet conveyance section 13 conveys the sheet S to the first sheet conveyance section 8.

[0044] The upstream end of the third sheet conveyance section 14 is connected to the first sheet conveyance section 8 at a position downstream of the fixing section 9, and the downstream end of the third sheet conveyance section 14 is connected to the first sheet conveyance section 8 at a position upstream of the registration roller pair 81. The third sheet conveyance section 14 conveys a sheet S to a position of the first sheet conveyance section 8 upstream of the registration roller pair 81 after toner images are fixed to a surface of the sheet S by the fixing section 9 during duplex printing. The third sheet conveyance section 14 conveys the sheet S such that the sheet S is reversed to transfer toner images to the other surface of the sheet S.

[0045] The four toner supplying sections 15 corresponding to the respective colors of yellow, cyan, magenta, and black are located above the intermediate transfer belt 51. The toner supplying sections 15 each contain a toner of one of the respective colors and supply the toners to the image forming section 3.

[0046] The following describes the image forming section 3 and the transfer section 5 in detail with reference to FIGS. 2 and 3. In FIGS. 2 and 3, letters “y”, “c”, “m”, and “bk” are appended to reference numerals of elements such as the photosensitive drums 31 corresponding to the yellow (Y), cyan (C), magenta (M), and black (Bk) colors, respectively.

[0047] FIG. 2 is an enlarged vertical cross sectional view of the image forming section 3 and the transfer section 5. As illustrated in FIG. 2, the image forming section 3 includes charging rollers 33y, 33c, 33m, 33bk (chargers), static eliminating devices 34y, 34c, 34m, 34bk (static eliminating sections), and cleaning blades 35y, 35c, 35m, 35bk (cleaning sections) in addition to the photosensitive drums 31y, 31c, 31m, 31bk (image bearing members), and the development rollers 32y, 32c, 32m, 32bk (development sections). The charging rollers 33y, 33c, 33m, and 33bk are each located opposite to the circumferential surface of a corresponding one of the photosensitive drums 31y, 31c, 31m, and 31bk. The static eliminating devices 34y, 34c, 34m, and 34bk are each located opposite to the circumferential surface of a corresponding one of the photosensitive drums 31y, 31c, 31m, and 31bk. The cleaning blades 35y, 35c, 35m, and 35bk are each located opposite to the circumferential surface of a corresponding one of the photosensitive drums 31y, 31c, 31m, and 31bk. The photosensitive drums 31y, 31c, 31m, and 31bk each have a photosensitive layer and rotate in a rotation direction R (clockwise direction in FIG. 2). The charging roller 33y, the development roller 32y, the static eliminating device 34y, and the cleaning blade 35 are arranged in the noted order in the rotation direction R of the corresponding photosensitive drum 31y. Likewise, the charging rollers 33c, 33m, 33bk, the development rollers 32c, 32m, 32bk, the static eliminating devices 34c, 34m, 34bk and the cleaning blades 35c, 35m, 35bk are each arranged in the rotation direction R of a corresponding one of the photosensitive drums 31c, 31m, 31bk in the noted order.

[0048] The charging rollers 33y, 33c, 33m, and 33bk each charge a corresponding one of the photosensitive drums 31y, 31c, 31m, and 31bk. The charging rollers 33y, 33c, 33m, and 33bk in the present embodiment are positive DC charging rollers. That is, the charging rollers 33y, 33c, 33m, and 33bk each apply a positive direct current voltage to a corresponding one of the photosensitive drums 31y, 31c, 31m, and 31bk. Through the above, the surfaces of the photosensitive drums 31y, 31c, 31m, and 31bk (surfaces of the photosensitive layers) are each charged to a positive potential. The surface potentials of the photosensitive drums 31y, 31c, 31m, and 31bk can be for example from about 350 V to about 600 V.

[0049] The static eliminating devices 34y, 34c, 34m, and 34bk are each located downstream of a corresponding one of the primary transfer nips N1 in the rotation direction R of the photosensitive drums 31y, 31c, 31m, and 31bk. The static eliminating devices 34y, 34c, 34m, and 34bk irradiate the circumferential surfaces of the photosensitive drums 31y, 31c, 31m, and 31bk with static elimination light. That is, the static eliminating device 34y irradiates with the static elimination light, the circumferential surface of the photosensitive drum 31y located upstream of the static eliminating device 34y in the rotational direction X of the intermediate transfer belt 51. Likewise, the static eliminating devices 34c, 34m, and 34bk irradiate with the static elimination light, the circumferential surfaces of the photosensitive drums 31c, 31m, and 31bk respectively, which are located upstream of the static eliminating devices 34c, 34m, and 34bk respectively in the rotational direction X of the intermediate transfer belt 51. Through the above, post-transfer static elimination is performed on the photosensitive drums 31y, 31c, 31m, and 31bk. That is, static electricity is eliminated (charges are removed) from the circumferential surfaces of the photosensitive drums 31y, 31c, 31m, and 31bk after the primary transfer.

[0050] The static eliminating device 34y is located between the adjacent photosensitive drums 31y and 31c. The static eliminating device 34c is located between the adjacent photosensitive drums 31c and 31m. The static eliminating device 34m is located between the adjacent photosensitive drums 31m and 31bk. The static eliminating device 34bk is located downstream of the photosensitive drum 31bk in the rotational direction X of the intermediate transfer belt 51. That is, the static eliminating device 34bk is located the most downstream among the static eliminating devices 34y, 34c, 34m, and 34bk in the rotational direction X of the intermediate transfer belt 51. The static eliminating device 34y located upstream of the static eliminating device 34bk is capable of further irradiating with light, the photosensitive drum 31c located downstream of the static eliminating device 34y in the rotational direction X of the intermediate transfer belt 51. Likewise, the static eliminating devices 34c and 34m located upstream of the static eliminating device 34bk is capable of further irradiating with light, the photosensitive drums 31m and 31bk respectively, which are located downstream of the static eliminating devices 34c and 34m respectively in the rotational direction X of the intermediate transfer belt 51. Through the above, pre-transfer static elimination is performed on the photosensitive drums 31c, 31m, and 31bk. That is, static electricity is eliminated from the circumferential surfaces of the photosensitive drums 31c, 31m, and 31bk before the primary transfer (the photosensitive drums 31c, 31m, and 31bk bearing the toner images). The pre-transfer static elimination reduces a potential difference between an imaged portion (portion bearing a toner image) and a non-imaged portion (portion bearing no toner image) on the circumferential surface of each of the photosensitive drums 31c, 31m, and 31bk. Through the above, occurrence of the transfer memory is prevented.

[0051] Edges of the cleaning blades 35y, 35c, 35m, and 35bk are each in contact with the circumferential surface of a corresponding one of the photosensitive drums 31y, 31c, 31m, and 31bk. Through the above, toners remaining on the circumferential surfaces of the photosensitive drums 31y, 31c, 31m, and 31bk after the primary transfer can be removed. Specifically, the cleaning blades 35y, 35c, 35m, and 35bk scrape the residual toners.

[0052] The primary transfer rollers 52y, 52c, 52m, and 52bk are each displaced (shifted) downstream of a position right above a corresponding one of the photosensitive drums 31y, 31c, 31m, and 31bk in the rotational direction X (moving direction) of the intermediate transfer belt 51. Specifically, the central axis of each of the primary transfer rollers 52y, 52c, 52m, and 52bk is displaced downstream of the central axis of a corresponding one of the photosensitive drums 31y, 31c, 31m, and 31bk in the rotational direction X of the intermediate transfer belt 51.

[0053] FIG. 3 is a diagram illustrating a power supply system for the four primary transfer rollers 52y, 52c, 52m, and 52bk. As illustrated in FIG. 3, the transfer section 5 further includes a power supply section 57 connected to the four primary transfer rollers 52y, 52c, 52m, and 52bk. The power supply section 57 is capable of charging each of the primary transfer rollers 52y, 52c, 52m, and 52bk. The power supply section 57 in the present embodiment includes a constant voltage source 58 (first power supply device) connected to the four primary transfer rollers 52y, 52c, 52m, and 52bk. The constant voltage source 58 applies a bias voltage (transfer voltage) to each of the primary transfer rollers 52y, 52c, 52m, and 52bk at the time of the primary transfer. As a result, the primary transfer rollers 52y, 52c, 52m, and 52bk are charged. A potential difference (transfer field) between a surface potential of each of the photosensitive drums 31y, 31c, 31m, and 31bk and a surface potential of a corresponding one of the primary transfer rollers 52y, 52c, 52m, and 52bk causes the primary transfer of the toner images from the circumferential surfaces of the respective photosensitive drums 31y, 31c, 31m, and 31bk to the outer circumferential surface of the rotating intermediate transfer belt 51 (transfer target). The constant voltage source 58 in the present embodiment generates a negative bias voltage. The bias voltage is for example −1600 V.

[0054] At the time of the primary transfer, a negative current flows into each of the photosensitive drums 31y, 31c, 31m, and 31bk from a corresponding one of the primary transfer rollers 52y, 52c, 52m, and 52bk through the intermediate transfer belt 51. That is, a current flows into each of the primary transfer rollers 52y, 52c, 52m, and 52bk from a corresponding one of the photosensitive drums 31y, 31c, 31m, and 31bk.

[0055] The primary transfer rollers 52y, 52c, 52m, and 52bk in the present embodiment are each displaced (shifted) downstream of a corresponding one of the photosensitive drums 31y, 31c, 31m, and 31bk in the rotational direction X of the intermediate transfer belt 51. The displacement of the primary transfer rollers 52y, 52c, 52m, and 52bk results in reduction of the area of each primary transfer nip N1. As a result, as compared with a situation in which the primary transfer rollers are each located right above a corresponding one of the photosensitive drums (i.e., situation in which the central axis of each primary transfer roller is aligned with the central axis of a corresponding photosensitive drum in the rotational direction of the intermediate transfer belt), values of currents flowing into the photosensitive drums 31y, 31c, 31m, and 31bk from the respective primary transfer rollers 52y, 52c, 52m, and 52bk are reduced even in a configuration in which the single constant voltage source 58 gives a potential to each of the primary transfer rollers 52y, 52c, 52m, and 52bk. Further, through the above, the values of the currents flowing into the photosensitive drums 31y, 31c, 31m, and 31bk are equalized.

[0056] Thus, the values of the currents flowing into the photosensitive drums 31y, 31c, 31m and 31bk are reduced and equalized according to the present embodiment in a configuration in which the number of power supply devices (the constant voltage source 58 in the present embodiment) is smaller than the number of the primary transfer rollers 52y, 52c, 52m, and 52bk. Therefore, occurrence of the transfer memory and insufficient density are prevented resulting in prevention of deterioration of image quality. Further, the power supply section 57 in the present embodiment includes a power supply device (the constant voltage source 58 in the present embodiment) connected to at least two primary transfer rollers (the primary transfer rollers 52y, 52c, 52m, and 52bk in the present embodiment) of the primary transfer rollers 52y, 52c, 52m, and 52bk. Therefore, the image forming apparatus 1 is simplified and downsized by setting the number of power supply devices (the constant voltage source in the present embodiment) smaller than the number of the primary transfer rollers.

[0057] Further, in a situation in which the primary transfer rollers are each located right above a corresponding one of the photosensitive drums, currents flowing from the primary transfer rollers into the photosensitive drums flow in the direction of thickness of the intermediate transfer belt. Therefore, the currents flowing into the photosensitive drums are influenced by the volume resistivity of the intermediate transfer belt. As a result, values of the currents flowing into the photosensitive drums may vary due to variation of the thickness of the intermediate transfer belt (variation of the volume resistivity). Particularly in a situation in which a thermoplastic resin is used as the material of the elastic layer of the intermediate transfer belt, the values of the currents flowing into the photosensitive drums tend to vary due to large variation of the thickness of the intermediate transfer belt (variation of the volume resistivity).

[0058] By contrast, the primary transfer rollers 52y, 52c, 52m, and 52bk in the present embodiment are displaced (shifted). Therefore, currents tend to flow along the surface of the intermediate transfer belt 51 into the photosensitive drums 31y, 31c, 31m, and 31bk. As a result, values of the currents flowing into the photosensitive drums 31y, 31c, 31m, and 31bk are less influenced by the volume resistivity of the intermediate transfer belt 51 having large variation and more influenced by the surface resistivity of the intermediate transfer belt 51 having small variation. Therefore, the values of the currents flowing into the photosensitive drums 31y, 31c, 31m, and 31bk are reduced more stably and equalized.

[0059] Further, a positive DC charging roller method is employed in the present embodiment as a method for charging the photosensitive drums 31y, 31c, 31m, and 31bk. The transfer memory tends to occur in such a configuration. However, the values of the currents flowing into the photosensitive drums 31y, 31c, 31m, and 31bk are reduced in the present embodiment due to the displacement (shifting) of the primary transfer rollers 52y, 52c, 52m, and 52bk. Therefore, even in a configuration in which the positive DC charging roller method is employed, the values of the currents flowing into the photosensitive drums 31y, 31c, 31m, and 31bk are reduced and equalized. Further, the pre-transfer static elimination is performed on the photosensitive drums 31c, 31m, and 31bk in the present embodiment. Through the above, occurrence of the transfer memory is further prevented.

[0060] In the present embodiment, the pre-transfer static elimination is performed on the photosensitive drums 31c, 31m, and 31bk other than the photosensitive drum 31y that is located the most upstream in the rotational direction X of the intermediate transfer belt 51. In such a configuration, a surface potential of the photosensitive drum 31y may become higher than surface potentials of the other photosensitive drums 31c, 31m, and 31bk and a value of a current flowing into the photosensitive drum 31y may become larger than values of currents flowing into the other photosensitive drums 31c, 31m, and 31bk. However, the value of the current flowing into the photosensitive drum 31y is reduced in the present embodiment due to the displacement (shifting) of the primary transfer roller 52y. Therefore, even in the configuration in which the pre-transfer static elimination is performed on the photosensitive drums 31c, 31m, and 31bk other than the photosensitive drum 31y, the values of the currents flowing into the photosensitive drums 31y, 31c, 31m, and 31bk are reduced and equalized.

[0061] Further, variation in thickness may arise among the photosensitive layers of the photosensitive drums 31y, 31c, 31m, and 31bk due to exchange of the photosensitive drums 31y, 31c, 31m, and 31bk. For example, in a situation in which only one of the photosensitive drums 31y, 31c, 31m, and 31bk (a photosensitive drum for one color) has not been exchanged, the thickness of the photosensitive layer of the unexchanged photosensitive drum is smaller than the thicknesses of the photosensitive layers of the other photosensitive drums. In such a situation, a value of a current flowing into the unexchanged photosensitive drum may become larger than values of currents flowing into the other photosensitive drums. However, the values of the currents flowing into the photosensitive drums 31y, 31c, 31m, and 31bk are reduced in the present embodiment due to the displacement (shifting) of the primary transfer rollers 52y, 52c, 52m, and 52bk. Therefore, the values of the currents flowing into the photosensitive drums 31y, 31c, 31m, and 31bk are reduced and equalized even when there is variation in thickness among the photosensitive layers of the photosensitive drums 31y, 31c, 31m, and 31bk.

[0062] The following describes amounts Ly, Lc, Lm, and Lbk of displacement (shifting) of each of the primary transfer rollers 52y, 52c, 52m, and 52bk from a corresponding one of the photosensitive drums 31y, 31c, 31m, and 31bk (hereinafter, an amount of displacement of each primary transfer roller will be referred to as “a displacement amount”) with reference to FIG. 2. In the present embodiment, the pre-transfer static elimination is performed on the photosensitive drums 31c, 31m, and 31bk other than the photosensitive drum 31y. In such a configuration, a surface potential of the photosensitive drum 31y may become higher than surface potentials of the other photosensitive drums 31c, 31m, and 31bk and a value of a current flowing into the photosensitive drum 31y may become larger than values of currents flowing into the other photosensitive drums 31c, 31m, and 31bk. Therefore, a displacement amount Ly (shift amount) of the primary transfer roller 52y is preferably set to be larger than displacement amounts Lc, Lm, and Lbk (shift amounts) of the other primary transfer rollers 52c, 52m, and 52bk. By setting the displacement amounts Ly, Lc, Lm, and Lbk as above, the values of the currents flowing into the photosensitive drums 31y, 31c, 31m, and 31bk are equalized.

[0063] The displacement amounts Ly, Lc, Lm, and Lbk of the primary transfer rollers 52y, 52c, 52m, and 52bk are determined based on a relationship between the values of the currents flowing into the photosensitive drums 31y, 31c, 31m, and 31bk and a value of the bias voltage (I-V characteristic). That is, the displacement amounts Ly, Lc, Lm, and Lbk are determined such that the values of the currents flowing into the photosensitive drums 31y, 31c, 31m, and 31bk are equalized for a value of the bias voltage to be used.

[0064] The displacement amounts Ly, Lc, Lm, and Lbk are preferably set according to the following conditions (a) to (f). The values of the currents flowing into the photosensitive drums 31y, 31c, 31m, and 31bk are reduced and equalized according to the conditions (a) to (f).

[0065] (a) A displacement amount is reduced as the surface resistivity of the intermediate transfer belt becomes larger.

[0066] (b) A displacement amount is increased as the diameter of the photosensitive drums becomes larger.

[0067] (c) A displacement amount is increased as a surface potential of a photosensitive drum becomes higher.

[0068] (d) A displacement amount is reduced as the thickness of the intermediate transfer belt becomes larger.

[0069] (e) A displacement amount is reduced as the surface resistivity of the primary transfer rollers becomes larger.

[0070] (f) A displacement amount is increased as the diameter of the primary transfer rollers becomes larger.

[0071] In the present embodiment, the displacement amounts Ly, Lc, Lm, and Lbk are preferably set to be at least 3.0 mm Through the above, the values of the currents flowing into the photosensitive drums 31y, 31c, 31m, and 31bk are reduced and equalized. For example, the displacement amount Ly may be set at 6.0 mm and the displacement amounts Lc, Lm, and Lbk may be set at 4.0 mm. The displacement amount Ly is the displacement amount of the primary transfer roller 52y that is located the most upstream among the primary transfer rollers 52y, 52c, 52m, and 52bk in the rotational direction X of the intermediate transfer belt 51. The displacement amounts Lc, Lm, and Lbk are the displacement amounts of the primary transfer rollers 52c, 52m, and 52bk that are located downstream of the primary transfer roller 52y in the rotational direction X of the intermediate transfer belt 51.

[0072] The displacement amounts Lc, Lm, and Lbk of the primary transfer rollers 52c, 52m, and 52bk need not be necessarily the same. When a color image is formed, a thickness of toner images (color toner images) on the intermediate transfer belt 51 typically increases downstream in the rotational direction X of the intermediate transfer belt 51. Therefore, currents flowing into the photosensitive drums 31c, 31m, and 31bk are preferably larger than currents flowing into the adjacent upstream photosensitive drums 31y, 31c, and 31m, respectively. Therefore, the displacement amounts Ly, Lc, Lm, and Lbk of the primary transfer rollers 52y, 52c, 52m, and 52bk may be set so as to decrease downstream in the rotational direction X of the intermediate transfer belt 51. Through the above, values of currents flowing into the photosensitive drums 31y, 31c, 31m, and 31bk increase downstream in the rotational direction X of the intermediate transfer belt 51.

[0073] Through the above, the displacement amounts Ly, Lc, Lm, and Lbk of the primary transfer rollers 52y, 52c, 52m, and 52bk have been described. As described above, the displacement amounts Ly, Lc, Lm, and Lbk in the present embodiment preferably satisfy a relationship represented by Formula (1) given below.

Ly>Lc≧Lm≧Lbk   (1)

[0074] Although the present embodiment has been described for a configuration in which the single constant voltage source 58 applies the bias voltage to the four primary transfer rollers 52y, 52c, 52m, and 52bk, the present invention is not limited to this configuration. The present invention is applicable to a configuration in which bias voltages are applied to the primary transfer rollers through constant voltage sources (power supply devices) fewer than the primary transfer rollers. The present invention is for example applicable to an image forming apparatus 1 including two constant voltage sources 58a and 58b as illustrated in FIG. 4.

[0075] FIG. 4 is a diagram illustrating another example of the power supply system for the four primary transfer rollers 52y, 52c, 52m, and 52bk. The transfer section 5 in the example illustrated in FIG. 4 includes a first constant voltage source 58a (first power supply device) and a second constant voltage source 58b (second power supply device). The first constant voltage source 58a is connected to at least two primary transfer rollers (three primary transfer rollers 52y, 52c, and 52m in the example illustrated in FIG. 4) of the four primary transfer rollers 52y, 52c, 52m, and 52bk (a plurality of transfer members) and the second constant voltage source 58b is connected to the other primary transfer roller (the primary transfer roller 52bk in the example illustrated in FIG. 4). That is, the first constant voltage source 58a applies a bias voltage to the three primary transfer rollers 52y, 52c, and 52m of the four primary transfer rollers 52y, 52c, 52m, and 52bk and the second constant voltage source 58b applies a bias voltage to the one primary transfer roller 52bk.

[0076] The four primary transfer rollers 52y, 52c, 52m, and 52bk are all displaced in the example illustrated in FIG. 4. Therefore, effects similar to those achieved by the image forming apparatus 1 described above with reference to FIGS. 1 to 3 can be achieved by adjusting the displacement amounts Ly, Lc, Lm, and Lbk of the primary transfer rollers 52y, 52c, 52m, and 52bk as in the image forming apparatus 1 described above with reference to FIGS. 1 to 3.

[0077] Further, in the example illustrated in FIG. 4, the second constant voltage source 58b applies the bias voltage to the primary transfer roller 52bk that is located the most downstream among the four primary transfer rollers 52y, 52c, 52m, and 52bk in the rotational direction X of the intermediate transfer belt 51. Therefore, a toner image in black can be formed without applying the bias voltage to the primary transfer rollers 52y, 52c, and 52m other than the primary transfer roller 52bk through the first constant voltage source 58a. Accordingly, power consumption at the time of formation of the toner image in black only can be reduced.

[0078] Further, in the example illustrated in FIG. 4, the bias voltage is applied to the primary transfer roller 52bk through a power supply device (the second constant voltage source 58b) different from that applies the bias voltage to the other three primary transfer rollers 52y, 52c, and 52m. Therefore, a value of a current flowing into the photosensitive drum 31bk corresponding to the primary transfer roller 52bk is controllable through the second constant voltage source 58b. Accordingly, the primary transfer roller 52bk may be located right above the photosensitive drum 31bk without displacement (shifting). Alternatively, the primary transfer roller 52bk may be displaced to control the value of the current flowing into the photosensitive drum 31bk through the displacement amount Lbk of the primary transfer roller 52bk and the second constant voltage source 58b.

[0079] In the image forming apparatus 1 described above with reference to FIGS. 1 to 3, the pre-transfer static elimination is performed on the photosensitive drums 31c, 31m, and 31bk other than the photosensitive drum 31y that is located the most upstream in the rotational direction X of the intermediate transfer belt 51. Meanwhile, in the configuration illustrated in FIG. 4, the first constant voltage source 58a applies the bias voltage to the three primary transfer rollers 52y, 52c, and 52m. The three primary transfer rollers 52y, 52c, and 52m include the primary transfer roller 52y corresponding to the photosensitive drum 31y to which the pre-transfer static elimination is not performed. Therefore, in the configuration illustrated in FIG. 4, the displacement amounts Ly, Lc, and Lm of the primary transfer rollers 52y, 52c, and 52m preferably satisfy a relationship represented by Formula (2) given below as in the image forming apparatus 1 described above with reference to FIGS. 1 to 3.

Ly>Lc≧Lm   (2)

[0080] Through the above, the embodiment of the present invention has been described with reference to the drawings. It should be noted that the present invention is not limited to the above embodiment and is practicable in various manners within the scope not departing from the gist of the present invention.

[0081] For example, in the above-described embodiment of the present invention, the photosensitive drums 31 are each charged to a positive potential. However, the present invention is not limited to such a configuration. The photosensitive drums 31 may each be charged to a negative potential. In this case, the primary transfer rollers 52 are each charged to a positive potential.

[0082] In the above-described embodiment of the present invention, the photosensitive drums 31 are charged by a roller method. However, the present invention is not limited to such a configuration. For example, the photosensitive drums 31 may be charged by a belt method.

[0083] In the above-described embodiment of the present invention, the photosensitive drums 31 are each charged by a direct current voltage. However, the present invention is not limited to such a configuration. The photosensitive drums 31 may each be charged by a voltage obtained by superimposing an alternating current voltage on a direct current voltage.

[0084] In the above-described embodiment of the present invention, the photosensitive drums 31 are charged by proximity discharge. However, the present invention is not limited to such a configuration. For example, the photosensitive drums 31 may be charged by a scorotron method.

[0085] In the above-described embodiment of the present invention, the photosensitive drums 31 each include a positively chargeable single-layer organic photosensitive member. However, the present invention is not limited to such a configuration. The photosensitive drums 31 may each include a negatively chargeable organic photosensitive member. Alternatively, the photosensitive drums 31 may each include an inorganic photosensitive member. Also, the photosensitive layers of the photosensitive drums 31 may each have a multi-layer structure.

[0086] In the above-described embodiment of the present invention, the central axis of each of the primary transfer rollers 52 is shifted (displaced) downstream of the central axis of a corresponding one of the photosensitive drums 31 in the rotational direction X (moving direction) of the intermediate transfer belt 51. However, the primary transfer rollers 52 may each be displaced upstream. Also, it is not required that all the primary transfer rollers 52 are displaced in the same direction. That is, there may be both a primary transfer roller 52 that is shifted downstream of the central axis of a corresponding one of the photosensitive drums 31 and another primary transfer roller 52 that is shifted upstream of the central axis of a corresponding one of the photosensitive drums 31 in the rotational direction X of the intermediate transfer belt 51.

[0087] In the above-described embodiment of the present invention, the single constant voltage source 58 or the two constant voltage sources 58a and 58b is/are used as the power supply device(s) for charging the four primary transfer rollers 52. However, the present invention is not limited to such a configuration. No specific limitations are placed on the number of the constant voltage sources (power supply devices) as long as the number is fewer than the number of the primary transfer rollers.

[0088] In the above-described embodiment of the present invention, the image forming apparatus 1 includes the first constant voltage source 58a connected to the three primary transfer rollers 52 and the second constant voltage source 58b connected to the one primary transfer roller 52. However, the present invention is not limited to such a configuration. For example, two constant voltage sources (power supply devices) may each be connected to a plurality of primary transfer rollers. Also, in a configuration in which a plurality of constant voltage sources (power supply devices) are used, no specific limitations are placed on connection destinations of the respective constant voltage sources (power supply devices).

[0089] In the above-described embodiment of the present invention, the constant voltage sources (constant voltage sources 58, 58a, and 58b) are used as the power supply devices for charging the four primary transfer rollers 52. However, the present invention is not limited to such a configuration. The power supply sources may be constant current sources.

[0090] Various alterations other than those described above may be made within the scope not departing from the gist of the present invention.

EXAMPLES

[0091] The following describes examples of the present invention. However, the present invention is not limited to the following examples.

Examples 1 to 3 and Comparative Examples 1 and 2

[0092] In the first through third examples and the first and second comparative examples, positively chargeable single-layer organic photosensitive drums having a diameter φ of 30 mm, primary transfer rollers having a diameter φ of 120 mm, and an intermediate transfer belt having a thickness of 120 μm were used. Carbon was dispersed in an elastic material of the primary transfer rollers to impart a conductive property to the elastic material of the primary transfer rollers. Similarly, carbon was dispersed in the intermediate transfer belt to impart a conductive property to the intermediate transfer belt. Photosensitive layers of the photosensitive drums had a thickness of 15 μm. The photosensitive drums were charged by the positive DC charging roller method such that the photosensitive drums had a surface potential of 500 V. The primary transfer rollers had a surface resistivity of 1.0×10.sup.7Ω/sq at the time of application of a voltage of 1000 V. The intermediate transfer belt had a surface resistivity of 1.0×10.sup.10Ω/sq at the time of application of a voltage of 250 V. Under the above conditions, a bias voltage was applied to the primary transfer rollers and values of currents flowing into the photosensitive drums were measured. The values of the currents flowing into the photosensitive drums were measured at points of connection between a constant voltage source and the primary transfer rollers.

[0093] In the first example, a value of a current flowing into the photosensitive drum was measured by setting a displacement amount of the primary transfer roller at 3.0 mm That is, the value of the current flowing into the photosensitive drum was measured by shifting the position of the primary transfer roller by 3.0 mm with respect to the photosensitive drum. In the second example, a value of a current flowing into the photosensitive drum was measured by setting a displacement amount of the primary transfer roller at 4.0 mm That is, the value of the current flowing into the photosensitive drum was measured by shifting the position of the primary transfer roller by 4.0 mm with respect to the photosensitive drum. In the third example, a value of a current flowing into the photosensitive drum was measured by setting a displacement amount of the primary transfer roller at 6.0 mm That is, the value of the current flowing into the photosensitive drum was measured by shifting the position of the primary transfer roller by 6.0 mm with respect to the photosensitive drum. In the first comparative example, a value of a current flowing into the photosensitive drum was measured without displacing (shifting) the primary transfer roller. That is, the value of the current flowing into the photosensitive drum was measured by setting a displacement amount of the primary transfer roller at 0.0 mm. In the second comparative example, a value of a current flowing into the photosensitive drum was measured by setting a displacement amount of the primary transfer roller at 2.0 mm That is, the value of the current flowing into the photosensitive drum was measured by shifting the position of the primary transfer roller by 2.0 mm with respect to the photosensitive drum. FIG. 5 shows measurement results of the first through third examples and the first and second comparative examples.

[0094] FIG. 5 shows graphs (I-V characteristics) obtained by plotting values of currents (-μA) flowing into the photosensitive drums with respect to values of the bias voltage (-V). In FIG. 5, the vertical axis represents the values of the currents (-μA) flowing into the photosensitive drums and the horizontal axis represents the values of the bias voltage (-V).

[0095] As shown in FIG. 5, around a value of “−1600 V” of the bias voltage that is necessary for the primary transfer, values of the currents decreased in situations in which the displacement amounts of the primary transfer rollers were at least 3.0 mm It was found from the results in FIG. 5 that in a situation in which displacement amounts of the primary transfer rollers 52y, 52c, 52m, and 52bk are set at “6.0 mm”, “4.0 mm”, “4.0 mm”, and “4.0 mm”, respectively, currents of “7.0 pA”, “8.0 pA”, “8.0 pA”, and “8.0 pA” flow into the photosensitive drums 31y, 31c, 31m, and 31bk, respectively, around the value of “−1600 V” of the bias voltage that is necessary for the primary transfer.

Example 4 and Comparative example 3

[0096] In the fourth example and the third comparative example, photosensitive layers of photosensitive drums had a thickness of 32 μm. Values of currents flowing into the photosensitive drums were measured under the same conditions as the third example and the first comparative example other than the thickness of the photosensitive layers of the photosensitive drums. That is, in the fourth example, a value of a current flowing into the photosensitive drum was measured by setting a displacement amount of the primary transfer roller at 6.0 mm, as in the third example. In the third comparative example, a value of a current flowing into the photosensitive drum was measured without displacing (shifting) the primary transfer roller, as in the first comparative example. FIG. 6 shows measurement results of the fourth example together with the measurement results of the third example. FIG. 7 shows measurement results of the third comparative example together with the measurement results of the first comparative example.

[0097] FIGS. 6 and 7 show graphs (I-V characteristics) obtained by plotting values of currents (- pA) flowing into the photosensitive drums with respect to values of the bias voltage (-V).

[0098] In FIGS. 6 and 7, the vertical axis represents the values of the currents (-μA) flowing into the photosensitive drums and the horizontal axis represents the values of the bias voltage (-V). As shown in FIG. 7, in situations in which the primary transfer rollers were not displaced, values of currents flowing into the photosensitive drums had large variation due to variation in the thickness of the photosensitive layers included in the photosensitive drums. By contrast, as shown in FIG. 6, in situations in which the primary transfer rollers were displaced, values of currents flowing into the photosensitive drums had no variation due to variation in the thickness of the photosensitive layers included in the photosensitive drums around the value of “−1600 V” of the bias voltage that is necessary for the primary transfer. Although the values of the currents varied when an absolute value of the bias voltage was greater than “2250 V”, the variation was small. Through the above, it was found that values of currents flowing into the photosensitive drums 31y, 31c, 31m, and 31bk are equalized by displacing the primary transfer rollers 52y, 52c, 52m, and 52bk even when there is variation in thickness among the photosensitive layers of the photosensitive drums 31y, 31c, 31m, and 31bk.

INDUSTRIAL APPLICABILITY

[0099] The present invention can be suitably applicable to image forming apparatuses such as a copier, a printer, a facsimile machine, and a multifunction peripheral.