Image forming apparatus

Abstract

To provide an image forming apparatus that can improve the image quality of a transfer image in primary transfer.SOLUTION: An image forming apparatus comprises a plurality of image carriers, charging devices, an exposure device, developing devices, an intermediate transfer belt, and a plurality of transfer rollers. A black toner image is developed on the image carrier arranged on the most downstream side in the direction of travel of the intermediate transfer belt. A rotation shaft of at least one transfer roller arranged on the upstream side of the transfer roller arranged on the most downstream side in the direction of travel of the intermediate transfer belt, is offset to the upstream side relative to a position immediately above a rotation shaft of a corresponding image carrier.SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus. [Background technology]

[0002] A conventional image forming apparatus is disclosed in Patent Document 1. This image forming apparatus includes a plurality of image carriers, a charging device, an exposure device, a developing device, an intermediate transfer belt, and a plurality of transfer rollers. The charging device charges the surface of the image carrier. The exposure device exposes the surface of the image carrier charged by the charging device to light, forming an electrostatic latent image on the surface of the image carrier. The developing device develops the electrostatic latent image formed on the surface of each image carrier into a toner image of a different color. The toner images formed on the image carrier are sequentially transferred to the intermediate transfer belt. The transfer roller is pressed against the image carrier via the intermediate transfer belt.

[0003] The rotation axis of the transfer roller corresponding to the black toner arranged at the most downstream side in the direction of travel of the intermediate transfer belt is offset downstream from the rotation axis of the corresponding image carrier, thereby suppressing the occurrence of discharge and reducing the scattering of black toner, which tends to cause noticeable stains. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2008-304780 A Summary of the Invention [Problem to be solved by the invention]

[0005] In the configuration of Patent Document 1, there is a possibility that defects may occur in transferred images of colors other than black.

[0006] SUMMARY OF THE PRESENT DISCLOSURE In view of the above problems, an object of the present invention is to provide an image forming apparatus capable of improving the quality of a transferred image in primary transfer. [Means for solving the problem]

[0007] In order to achieve the above object, the first configuration of the present invention is an image forming apparatus including a plurality of image carriers, a charging device, an exposure device, a developing device, an intermediate transfer belt, and a plurality of transfer rollers. The charging device charges the surface of the image carrier. The exposure device exposes the surface of the image carrier charged by the charging device to form an electrostatic latent image on the surface of the image carrier. The developing device develops the electrostatic latent image formed on the surface of the image carrier into a toner image using a developer containing toner. The toner images formed on the image carrier are sequentially transferred to the intermediate transfer belt. The transfer roller is pressed against the image carrier via the intermediate transfer belt. A black toner image is developed on the image carrier arranged on the most downstream side in the traveling direction of the intermediate transfer belt. The rotation axis of at least one transfer roller arranged upstream of the transfer roller arranged on the most downstream side in the traveling direction of the intermediate transfer belt is offset upstream from directly above the rotation axis of the corresponding image carrier. Effect of the Invention

[0008] According to the first aspect of the present invention, it is possible to provide an image forming apparatus capable of improving the quality of a transferred image in the primary transfer. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic cross-sectional view showing an overall configuration of an image forming apparatus 100 according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a partially enlarged view of the image forming section Pa in FIG. [Diagram 3] A side cross-sectional view of an intermediate transfer unit 30 mounted in an image forming apparatus 100. [Figure 4] A partially enlarged view of the primary transfer roller 6d and the secondary transfer nip portion N of the intermediate transfer unit 30. [Diagram 5] A block diagram showing an example of a control path of the image forming apparatus 100. [Figure 6] Schematic cross-sectional view of primary transfer rollers 6a to 6d and photoconductor dryers 1a to 1d [Figure 7] Graph showing the relationship between the offset amount of the primary transfer roller 6d and image density [Figure 8] A graph showing the relationship between the contact surface pressure between the primary transfer roller 6d and the intermediate transfer belt 8 and image density. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. Fig. 1 is a schematic diagram showing the configuration of an image forming apparatus 100 according to one embodiment of the present invention. Fig. 2 is an enlarged view of the vicinity of an image forming section Pa in Fig. 1. Note that image forming sections Pb to Pd have basically the same configuration, so their description will be omitted.

[0011] Four image forming units Pa, Pb, Pc, and Pd are arranged in sequence from the upstream side in the transport direction (the left side in FIG. 1) inside the main body of image forming apparatus 100 (here, a color printer). These image forming units Pa to Pd are provided corresponding to images of four different colors (yellow, magenta, cyan, and black), and sequentially form images of yellow, magenta, cyan, and black through the processes of charging, exposing, developing, and transferring, respectively.

[0012] Each of the image forming stations Pa to Pd is provided with photoconductor drums 1a, 1b, 1c, and 1d that carry a visible image (toner image) of each color. In addition, an intermediate transfer belt 8 that is wound around a plurality of rollers including a drive roller 10 and a tension roller 11 and rotates in a counterclockwise direction in FIG. 1 is provided adjacent to each of the image forming stations Pa to Pd. As shown in FIG. 2, a charging device 2a, a developing device 3a, a cleaning device 7a, and a static elimination lamp 20 are disposed around the photoconductor drum 1a in the drum rotation direction (clockwise direction in FIG. 2), and a primary transfer roller 6a is disposed with the intermediate transfer belt 8 sandwiched therebetween.

[0013] The photosensitive drums 1a to 1d are each composed of a conductive substrate 19a and a photosensitive layer 19b formed on the surface of the conductive substrate 19a. In this embodiment, an amorphous silicon photosensitive layer is laminated as the photosensitive layer 19b on the surface of the cylindrical conductive substrate 19a made of aluminum.

[0014] The charging devices 2a to 2d each have a charging roller 21 that contacts the photosensitive drum 1a and applies a charging voltage (DC voltage+AC voltage) to the drum surface, and a charging cleaning roller 24 for cleaning the charging roller 21.

[0015] The developing devices 3a to 3d are of a two-component development type having two stirring and conveying screws 25 and a developing roller 29, and are filled with a predetermined amount of two-component developer containing yellow, magenta, cyan, and black toner and a magnetic carrier. A magnetic brush is formed on the surface of the developing roller 29 using the two-component developer, and the magnetic brush is brought into contact with the surface of the photosensitive drum 1a while a developing voltage of the same polarity as the toner (positive polarity in this case) is applied to the developing roller 29, thereby adhering the toner and forming a toner image. When the ratio of toner in the two-component developer filled in each of the developing devices 3a to 3d falls below a specified value due to the formation of the toner image, toner is replenished from the toner containers 4a to 4d to each of the developing devices 3a to 3d.

[0016] When image data is input from a host device such as a personal computer, first, the main motor 40 (see FIG. 5) starts rotating the photoconductor drums 1a-1d. In addition, the belt drive motor 41 (see FIG. 5) starts rotating the intermediate transfer belt 8. Next, the charging devices 2a-2d uniformly charge the surfaces of the photoconductor drums 1a-1d to the same polarity as the toner (positive polarity in this case). Next, the exposure device 5 irradiates light according to the image data, and electrostatic latent images with attenuated charge are formed on the photoconductor drums 1a-1d according to the image data. Then, the developing devices 3a-3d supply toner onto the photoconductor drums 1a-1d, and the toner adheres electrostatically to form toner images according to the electrostatic latent images.

[0017] Then, a predetermined primary transfer electric field is applied between the primary transfer rollers 6a-6d and the photoconductor drums 1a-1d by the primary transfer rollers 6a-6d, whereby the yellow, magenta, cyan and black toner images on the photoconductor drums 1a-1d are primarily transferred onto the intermediate transfer belt 8. Toner and the like remaining on the surfaces of the photoconductor drums 1a-1d after the primary transfer are removed by cleaning devices 7a-7d. Residual charges remaining on the surfaces of the photoconductor drums 1a-1d after the primary transfer are removed by a discharge lamp 20.

[0018] The transfer paper S onto which the toner image is transferred is stored in a paper cassette 16 arranged at the bottom inside the image forming apparatus 100. The transfer paper S is transported at a predetermined timing via a paper feed roller 12a and a pair of registration rollers 12b to a nip portion (secondary transfer nip portion) between the intermediate transfer belt 8 and a secondary transfer roller 9 provided adjacent to the intermediate transfer belt 8. The transfer paper S onto which the toner image on the intermediate transfer belt 8 has been secondarily transferred by the secondary transfer roller 9 is transported to a fixing unit 13.

[0019] The transfer paper S transported to the fixing section 13 is heated and pressurized by the fixing roller pair 13a, so that the toner image is fixed onto the surface of the transfer paper S, and a predetermined full-color image is formed. The transfer paper S on which the full-color image has been formed is discharged onto the discharge tray 17 by the discharge roller pair 15 as is (or after being diverted to the reversing conveyance path 18 by the branching section 14 and images are formed on both sides).

[0020] Fig. 3 is a side cross-sectional view of intermediate transfer unit 30 mounted on image forming apparatus 100. As shown in Fig. 3, intermediate transfer unit 30 has intermediate transfer belt 8 stretched between drive roller 10 and tension roller 11, primary transfer rollers 6a-6d in contact with photoconductor drums 1a-1d via intermediate transfer belt 8, and pressure switching roller 34.

[0021] The intermediate transfer belt 8 is a resin belt whose main component is polyimide resin, and has a thickness of, for example, 40 to 100 μm and a Young's modulus of 3000 to 6000 MPa.

[0022] The drive roller 10 and tension roller 11 are respectively disposed downstream and upstream in the traveling direction of the transport surface (lower surface) of the intermediate transfer belt 8. A belt cleaning unit 37 for removing toner remaining on the surface of the intermediate transfer belt 8 is disposed in a position facing the tension roller 11 (see FIG. 1). A secondary transfer roller 9 is pressed against the drive roller 10 via the intermediate transfer belt 8, forming a secondary transfer nip N.

[0023] The intermediate transfer unit 30 includes a roller contact / separation mechanism 35. The roller contact / separation mechanism 35 has a pair of support members (not shown) and a drive unit (not shown). The pair of support members support both ends of the rotation shafts of the primary transfer rollers 6a to 6d and the pressure switching roller 34 so that they can rotate and move perpendicularly to the traveling direction of the intermediate transfer belt 8 (vertical direction in FIG. 3). The drive unit moves the primary transfer rollers 6a to 6d and the pressure switching roller 34 back and forth in the vertical direction. The roller contact / separation mechanism 35 is switchable between a color mode in which the four primary transfer rollers 6a to 6d are pressed against the photosensitive drums 1a to 1d (see FIG. 1) via the intermediate transfer belt 8, a monochrome mode in which only the primary transfer roller 6d is pressed against the photosensitive drum 1d via the intermediate transfer belt 8, and a retraction mode in which all of the four primary transfer rollers 6a to 6d are separated from the photosensitive drums 1a to 1d.

[0024] Fig. 4 is a partially enlarged view of the periphery of the primary transfer roller 6d and the secondary transfer nip portion N of the intermediate transfer unit 30. The primary transfer and secondary transfer of the toner image will be described with reference to Fig. 4. Note that Fig. 4 describes a positively charged toner having a positive charge polarity.

[0025] As shown in Fig. 4, a primary transfer voltage power supply 54a is connected to the primary transfer rollers 6a to 6d. A secondary transfer voltage power supply 54b is connected to the drive roller 10 (secondary transfer opposing roller). When the control unit 90 (see Fig. 5) receives an image formation command, an electrostatic latent image is formed on the surface of the photosensitive drums 1a to 1d, and a toner image is formed by supplying toner T by the developing devices 3a to 3d (see Fig. 2). The toner images formed on the photosensitive drums 1a to 1d move to the primary transfer nip portions Na to Nd by the rotation of the photosensitive drums 1a to 1d (see Fig. 6).

[0026] A primary transfer voltage of negative polarity (minus) is applied to the primary transfer rollers 6a-6d by a primary transfer voltage power supply 54a. As a result, the toner images on the photosensitive drums 1a-1d are attracted to the primary transfer rollers 6a-6d at the primary transfer nips Na-Nd and are primarily transferred onto the intermediate transfer belt 8. The toner images primarily transferred onto the intermediate transfer belt 8 move to the secondary transfer nip N by the rotation of the intermediate transfer belt 8.

[0027] A positive polarity (plus) secondary transfer voltage is applied to the drive roller 10 by the secondary transfer voltage power supply 54b. As a result, the toner image on the intermediate transfer belt 8 is transported to the secondary transfer nip portion N and transferred to the transfer paper S passing through the secondary transfer nip portion N due to the potential difference between the drive roller 10 to which the secondary transfer voltage is applied and the secondary transfer roller 9 which is grounded (earthed).

[0028] Next, the control path of the image forming apparatus 100 of the present invention will be described. Fig. 5 is a block diagram showing an example of the control path used in the image forming apparatus 100 of the present invention. Note that, since various controls are performed on each part of the image forming apparatus 100 when the image forming apparatus 100 is used, the control path of the entire image forming apparatus 100 becomes complicated. Therefore, the following description will focus on the parts of the control path that are necessary for implementing the present invention.

[0029] The control unit 90 includes at least a CPU (Central Processing Unit) 91 as a central processing unit, a ROM (Read Only Memory) 92 as a read-only memory, a RAM (Random Access Memory) 93 as a readable and writable memory, a temporary memory 94 for temporarily storing image data and the like, a counter 95 for accumulating and counting the number of printed sheets, and a plurality of (here, two) I / Fs (interfaces) 96 for transmitting control signals to each device in the image forming apparatus 100 and receiving input signals from the operation unit 60. The control unit 90 can be disposed at any location inside the main body of the image forming apparatus 100.

[0030] The ROM 92 stores data such as a control program for the image forming apparatus 100, numerical values ​​necessary for control, and data that will not be changed during use of the image forming apparatus 100. The RAM 93 stores necessary data generated during the control of the image forming apparatus 100, data temporarily required for the control of the image forming apparatus 100, and the like.

[0031] Furthermore, the control unit 90 transmits control signals from the CPU 91 through the I / F 96 to each portion and device in the image forming apparatus 100. Furthermore, signals indicating the state and input signals are transmitted from each portion and device to the CPU 91 through the I / F 96. Examples of each portion and device controlled by the control unit 90 include image forming units Pa to Pd, exposure device 4, primary transfer rollers (transfer rollers) 6a to 6d, secondary transfer roller 9, main motor 40, belt drive motor 41, image input unit 50, voltage control circuit 51, operation unit 60, etc.

[0032] The image input unit 50 is a receiving unit that receives image data transmitted from a personal computer or the like to the image forming apparatus 100. The image signal input from the image input unit 50 is converted into a digital signal and then sent to the temporary storage unit 94 via the I / F 96.

[0033] The voltage control circuit 51 is connected to the charging voltage power supply 52, the developing voltage power supply 53, and the transfer voltage power supply 54, and operates each of these power supplies according to an output signal from the control unit 90. Each of these power supplies operates according to a control signal from the voltage control circuit 51, with the charging voltage power supply 52 applying a charging voltage to the charging roller 21 in the charging devices 2a to 2d. The developing voltage power supply 53 applies a developing voltage, which is a developing DC voltage superimposed on a developing AC voltage, to the developing roller 29 in the developing devices 3a to 3d. The transfer voltage power supply 54 has a primary transfer voltage power supply 54a that applies a predetermined primary transfer voltage to the primary transfer rollers 6a to 6d, and a secondary transfer voltage power supply 54b that applies a predetermined secondary transfer voltage to the drive roller 10 (see FIG. 4 for both).

[0034] The operation unit 60 is provided with a liquid crystal display unit 61 and an LED 62 that indicates various states, and the user operates the stop / clear button of the operation unit 60 to stop image formation, and operates the reset button to reset various settings of the image forming apparatus 100 to their default states. The liquid crystal display unit 61 indicates the state of the image forming apparatus 100, and displays the image formation status and the number of copies to be printed. Various settings of the image forming apparatus 100 are made from a printer driver on the personal computer.

[0035] 6 is a schematic cross-sectional view of the primary transfer roller 6d and the photoconductor drums 1a-1d, and shows the positional relationship between the photoconductor drums 1a-1d, the primary transfer rollers 6a-6d, and the intermediate transfer belt 8. The upstream side of the traveling direction of the transport surface (lower surface) of the intermediate transfer belt 8 is +X, and the downstream side is -X.

[0036] The rotation shafts 61a to 61c of the primary transfer rollers 6a to 6c, which are arranged on the upstream side -X of the primary transfer roller 6d, are offset upstream +X from directly above the rotation shafts 101a to 101c of the corresponding photosensitive drums 1a to 1c. More specifically, the axis centers of the rotation shafts 61a to 61c of the primary transfer rollers 6a to 6c are positioned shifted upstream +X from a perpendicular line passing through the axis centers of the rotation shafts 101a to 101c of the photosensitive drums 1a to 1c.

[0037] Further, the rotation shaft 61d of the primary transfer roller 6d arranged on the most downstream side +X in the traveling direction of the intermediate transfer belt 8 is offset downstream -X from directly above the rotation shaft 101d of the corresponding photosensitive drum 1d. More specifically, the axis of the rotation shaft 61d of the primary transfer roller 6d is shifted downstream -X from a perpendicular line passing through the axis of the rotation shaft 101d of the photosensitive drum 1d. Note that, in this embodiment, the rotation shaft 61d of the primary transfer roller 6d is offset downstream -X from directly above the rotation shaft 101d of the corresponding photosensitive drum 1d, but may be arranged directly above the rotation shaft 101d of the photosensitive drum 1d.

[0038] In color printing, the colors yellow, magenta, cyan, and black have the highest brightness and transparency, respectively. On the other hand, the colors black, cyan, magenta, and yellow have the highest visibility, respectively. Black toner, which has high visibility, tends to stain easily when the toner scatters.

[0039] In this embodiment, the primary transfer roller 6d corresponding to black toner, which has lower lightness and transparency and higher visibility than yellow toner, magenta toner, and cyan toner, is offset to the downstream side −X with respect to the photosensitive drum 1d.

[0040] During primary transfer, discharge is likely to occur on the downstream side -X of the primary transfer nip Nd between the photosensitive drum 1d and the intermediate transfer belt 8. At this time, the occurrence of discharge can be suppressed by offsetting the primary transfer roller 6d to the downstream side -X with respect to the photosensitive drum 1d. This reduces scattering of black toner, which is prone to conspicuous stains, suppresses the occurrence of character scattering, and improves the color development and color reproducibility of the transferred image. In addition, by reducing toner scattering, the amount of toner consumed can be reduced.

[0041] In this embodiment, the primary transfer rollers 6a to 6c are offset to the upstream side +X with respect to the photoconductor drums 1a to 1c, making it difficult to suppress the occurrence of the discharge phenomenon. On the other hand, as the offset width to the upstream side +X is increased, the image density of the transferred image increases when printing with halftone (halftone dots), which is a collection of dots.

[0042] Yellow toner, magenta toner, and cyan toner, which have higher brightness and transparency and lower visibility than black toner, are less noticeable when the toner scatters than black toner. As a result, by offsetting the primary transfer rollers 6a to 6c to the upstream side +X with respect to the photosensitive drums 1a to 1c, the character scattering phenomenon occurs, but the image densities of yellow, magenta, and cyan can be increased. As a result, the image quality of the transferred image in the primary transfer can be improved for colors other than black.

[0043] In addition, the primary transfer rollers 6a to 6c have different offset amounts W1 to W3 with respect to the photosensitive drums 1a to 1c. In this embodiment, the offset amount is set larger for toners with high brightness and transparency and low visibility. Specifically, the offset amount W1 corresponding to yellow toner is the largest. Also, the offset amount W3 corresponding to cyan toner is the smallest. As a result, for colors with low visibility and low noticeable dirt, the offset amount can be set large to increase image density. On the other hand, for colors with low visibility and high noticeable dirt, the offset amount can be set small to reduce toner scattering. As a result, for colors with low visibility and high noticeable dirt, the toner consumption due to scattering can be reduced and the color development and color reproducibility of the transferred image can be improved. Therefore, the balance between the optimal reproducibility and image density of the transferred image can be adjusted based on the offset amount.

[0044] Furthermore, as the contact surface pressure between the primary transfer rollers 6a-6d and the intermediate transfer belt 8 increases, the occurrence of the character scattering phenomenon can be suppressed, and the color development and color reproducibility of the transferred image can be improved. Specifically, as the contact surface pressure increases, the cohesive force between the toner particles increases. This makes it possible to reduce toner scattering in the minute discharge regions that occur in the vicinity of the primary transfer nips Na-Nd. In other words, when the contact surface pressure is increased, the same effect can be obtained as when the primary transfer roller 6d is offset downstream -X with respect to the photoconductor drum 1d.

[0045] In this embodiment, the contact surface pressure between the primary transfer roller 6d corresponding to black toner and the intermediate transfer belt 8 is greater than the contact surface pressure between the primary transfer rollers 6a to 6c corresponding to yellow toner, magenta toner, and cyan toner and the intermediate transfer belt 8. This makes it possible to further reduce the scattering of black toner and further suppress the occurrence of the character scattering phenomenon. Therefore, it is possible to further improve the color development and color reproducibility of the transferred image. In addition, by reducing the scattering of black toner, the consumption of black toner can be further reduced.

[0046] On the other hand, the image density increases when printing in halftone (halftone dots) as the contact surface pressure between the primary transfer rollers 6a to 6d and the intermediate transfer belt 8 increases. In other words, when the contact surface pressure is reduced, the same effect can be obtained as when the primary transfer rollers 6a to 6c are offset upstream +X with respect to the photoconductor drums 1a to 1c.

[0047] In this embodiment, the contact surface pressures between the primary transfer rollers 6a to 6c and the intermediate transfer belt 8 are different from each other. Specifically, the contact surface pressure corresponding to a color with high brightness and high transparency and low visibility is set to be larger. Specifically, the contact surface pressure corresponding to the yellow toner is the smallest. Also, the contact surface pressure corresponding to the cyan toner is the largest. As a result, for a color with low visibility and low staining, the contact surface pressure can be set to be small to increase the image density. On the other hand, for a color with low visibility and high staining, the contact surface pressure can be set to be large to further reduce toner scattering. As a result, for a color with low visibility and high staining, the amount of toner consumed due to scattering can be reduced and the color development and color reproducibility of the transferred image can be improved. Therefore, the balance between the optimal reproducibility and image density of the transferred image can be adjusted based on the contact surface pressure.

[0048] Next, the relationship between the offset amount of the primary transfer roller and image density will be described. Also, the relationship between the contact surface pressure and image density when printing in halftone (halftone dot) will be described. As a test machine, an intermediate transfer type image forming apparatus 100 (manufactured by Kyocera Document Solutions Inc.) as shown in FIG. 1 was used, and the image density was measured by changing the offset amount and contact surface pressure of the primary transfer roller 6d.

[0049] At this time, the intermediate transfer belt 8 was made of polyimide resin, and was stretched on both sides by springs as a stretching tension for the intermediate transfer belt 8.

[0050] The primary transfer roller 6d is an EPDM sponge roller, and the load of the primary transfer roller 6d is variable by providing springs on both sides of the axial direction. The primary transfer current flowing through the primary transfer roller 6d is -15 to -50 [μA]. The developing bias is AC500 to 1400 [V], Vdc80 to 250 [V].

[0051] The photoconductor drum 1d used was an amorphous silicon photoconductor drum having an amorphous silicon layer as the photoconductor layer 19b, and an OPC photoconductor drum (manufactured by Kyocera Document Solutions, Inc.) having a positively charged single-layer OPC photoconductor layer as the photoconductor layer 19b. Positively charged toner was used.

[0052] (Relationship between offset amount and image density) The relationship between the offset amount of the primary transfer roller 6d and the image density was verified. As a test method, the offset amount W4 of the primary transfer roller 6d was changed to seven levels, namely -2 [mm], -1 [mm], 0 [mm], +1 [mm], +2 [mm], +3 [mm], and +4 [mm], and the black image density was measured when printing in halftone (halftone dot). The results are shown in the graph of FIG. 7. When the offset amount is negative, the axis of the primary transfer roller 6d is shifted downstream to -X from the perpendicular line passing through the axis of the rotation shaft 101dc of the photosensitive drum 1d. When the offset amount is positive, the axis of the primary transfer roller 6d is shifted upstream to +X from the perpendicular line passing through the axis of the rotation shaft 101dc of the photosensitive drum 1d.

[0053] (Relationship between contact pressure and image density) The relationship between the contact surface pressure between the photoconductor drum 1d and the intermediate transfer belt 8 and the image density was verified. The test method was to change the contact surface pressure between the photoconductor drum 1d and the intermediate transfer belt 8 to three levels: 1 [N], 5 [N], and 8 [N], and measure the black image density when printing in halftone (halftone dot). The results are shown in the graph in Figure 8.

[0054] From the above relationship, it was confirmed that as the offset amount to the upstream side +X increases, the image density increases when printed in halftone (dot) colors. It was also confirmed that as the contact surface pressure increases, the image density decreases when printed in halftone (dot) colors. This confirmed that the balance between the optimal reproducibility of the transferred image and the image density can be adjusted based on the offset amount and the contact surface pressure.

[0055] Additionally, the present invention is not limited to the above-described embodiment, and various modifications are possible without departing from the spirit of the present invention. For example, the present invention is not limited to the tandem color printer as shown in Fig. 1, but can be applied to various image forming apparatuses that use an intermediate transfer method in which a toner image formed on a photoconductor drum is primarily transferred onto an intermediate transfer belt, such as a color copier or a color multifunction machine.

[0056] In addition, in this embodiment, the rotation axes of the primary transfer rollers 6a to 6c, which are arranged on the upstream side +X of the primary transfer roller 6d, are offset upstream +X from directly above the rotation axes of the corresponding photosensitive drums 1a to 1c, but it is sufficient that the rotation axis of at least one of the primary transfer rollers 6a to 6c is offset upstream +X from directly above the rotation axis of the corresponding photosensitive drums 1a to 1c.

[0057] In addition, in this embodiment, the image forming units Pa to Pd are arranged in the order of yellow, magenta, cyan, and black toward the downstream side -X, but the order of yellow, magenta, and cyan is not particularly limited as long as black is arranged on the most downstream side. [Industrial Applicability]

[0058] The present invention can be used in an image forming apparatus of an intermediate transfer type, which transfers a toner image formed on an image carrier such as a photosensitive drum onto an intermediate transfer belt. By using the present invention, it is possible to provide an image forming apparatus that can prevent transfer failure due to discharge and suppress deterioration of secondary transferability even for paper with a rough surface, thereby forming high-quality images for a long period of time. [Explanation of symbols]

[0059] Pa~Pd Image forming section 1a to 1d Photoconductor drum (image carrier) 2a~2d Charging device 3a~3d developing device 5 Exposure equipment 6a~6d Primary transfer roller (transfer roller) 8 Intermediate transfer belt 9 Secondary transfer roller 10 Drive roller 11 Tension roller 19a Conductive substrate 19b Photosensitive layer 30 Intermediate transfer unit 100 Image forming device

Claims

1. A plurality of image carriers; a charging device for charging a surface of the image carrier; an exposure device that exposes the surface of the image carrier charged by the charging device to light to form an electrostatic latent image on the surface of the image carrier; a developing device for developing the electrostatic latent image formed on the surface of the image carrier into a toner image; an intermediate transfer belt onto which the toner images formed on the image carrier are sequentially transferred; a plurality of transfer rollers that are pressed against the image carrier via the intermediate transfer belt; In an image forming apparatus comprising: The image carrier disposed on the most downstream side in the direction of travel of the intermediate transfer belt has the black toner image developed thereon, An image forming apparatus characterized in that the rotation axis of at least one of the transfer rollers arranged upstream of the transfer roller arranged at the most downstream side in the direction of travel of the intermediate transfer belt is offset upstream from directly above the rotation axis of the corresponding image carrier.

2. 2. The image forming apparatus according to claim 1, wherein the rotation axis of the transfer roller arranged at the most downstream side in the direction of travel of the intermediate transfer belt is directly above the rotation axis of the corresponding image carrier or is offset downstream from directly above it.

3. 3. The image forming apparatus according to claim 1, wherein the transfer rollers arranged upstream of the transfer roller arranged on the most downstream side in the direction of travel of the intermediate transfer belt have different offset amounts with respect to the image carrier.

4. 3. The image forming apparatus according to claim 1, wherein the contact surface pressure between the intermediate transfer belt and the transfer roller arranged at the most downstream side in the direction of travel of the intermediate transfer belt is greater than the contact surface pressure between the intermediate transfer belt and the transfer roller arranged upstream of the transfer roller arranged at the most downstream side.

5. 5. The image forming apparatus according to claim 4, wherein the contact surface pressures between the intermediate transfer belt and each of the transfer rollers arranged upstream of the transfer roller arranged on the most downstream side in the direction of travel of the intermediate transfer belt are different from each other.

6. 3. The image forming apparatus according to claim 1, wherein the image carriers arranged upstream of the image carrier arranged at the most downstream side in the direction of travel of the intermediate transfer belt are developed in order from the upstream side to the downstream side with the yellow toner image, the magenta toner image and the cyan toner image.

Images