Image forming apparatus

Abstract

An 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 image carrier disposed on the most downstream side in the traveling direction of the intermediate transfer belt develops a black toner image. The rotation axis of at least one of the transfer rollers disposed upstream from the transfer roller disposed on the most downstream side in the traveling direction of the intermediate transfer belt is offset upstream from a position directly above the rotation axis of the corresponding image carrier.

Description

Technical Field

[0001] The present invention relates to an image forming device. Background Art

[0002] The conventional image forming apparatus comprises: a plurality of image bearing bodies, 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 bearing body. The exposure device exposes the surface of the image bearing body charged by the charging device to form an electrostatic latent image on the surface of the image bearing body. The developing device develops the electrostatic latent image formed on the surface of each image bearing body into a toner image of a different color. The intermediate transfer belt sequentially transfers the toner image formed on the image bearing body. The transfer roller is pressed against the image bearing body via the intermediate transfer belt.

[0003] The rotation axis of the transfer roller corresponding to the black toner disposed on the most downstream side of the intermediate transfer belt in the traveling direction is offset toward the downstream side from the rotation axis of the corresponding image carrier directly above. This can suppress the occurrence of discharge and reduce the scattering of black toner that is easily visible as contamination.

[0004] When the conventional technology is used, there is a possibility that defects may occur in the transferred images of colors other than black.

[0005] In view of the above problems, an object of the present invention is to provide an image forming apparatus capable of improving the image quality of a transferred image in primary transfer. Summary of the invention

[0006] In order to achieve the above-mentioned purpose, the first structure of the present invention is an image forming device, which comprises: a plurality of image bearing bodies, 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 bearing body. The exposure device exposes the surface of the image bearing body charged by the charging device to form an electrostatic latent image on the surface of the image bearing body. The developing device uses a developer containing a toner to develop the electrostatic latent image formed on the surface of the image bearing body into a toner image. The intermediate transfer belt sequentially transfers the toner image formed on the image bearing body. The transfer roller is pressed against the image bearing body via the intermediate transfer belt. The image bearing body arranged on the most downstream side in the travel direction of the intermediate transfer belt develops a black toner image. The rotation axis of at least one transfer roller arranged on the upstream side compared to the transfer roller arranged on the most downstream side in the travel direction of the intermediate transfer belt is offset more toward the upstream side than directly above the rotation axis of the corresponding image bearing body.

[0007] According to a first aspect of the present invention, there is provided an image forming apparatus capable of improving the image quality of a transfer image in primary transfer.

[0008] Other objects of the present invention and specific advantages obtainable by the present invention will become more apparent from the following description of the embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 1 is a schematic cross-sectional view showing the overall structure of an image forming apparatus 100 according to an embodiment of the present invention.

[0010] Figure 2 yes Figure 1 A partially enlarged view of the periphery of the image forming portion Pa in FIG.

[0011] Figure 3 1 is a side cross-sectional view of the intermediate transfer unit 30 included in the image forming apparatus 100 .

[0012] Figure 4 It is a partially enlarged view of the primary transfer roller 6 d and the secondary transfer nip portion N and its surroundings of the intermediate transfer unit 30 .

[0013] Figure 5 This is a block diagram showing an example of a control path of the image forming apparatus 100 .

[0014] Figure 6 It is a schematic cross-sectional view of the primary transfer rollers 6 a to 6 d and the photosensitive drums 1 a to 1 d .

[0015] Figure 7 It is a graph showing the relationship between the offset amount of the primary transfer roller 6 d and the image density.

[0016] Figure 8 This is a graph showing the relationship between the contact surface pressure between the primary transfer roller 6 d and the intermediate transfer belt 8 and the image density. DETAILED DESCRIPTION

[0017] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Figure 1 1 is a schematic diagram showing the structure of an image forming apparatus 100 according to an embodiment of the present invention. Figure 2 yes Figure 1 1 is an enlarged view of the vicinity of the image forming section Pa in FIG. The image forming sections Pb to Pd also have basically the same configuration, and thus their description is omitted.

[0018] In the main body of the image forming apparatus 100 (here, a color printer), from the upstream side in the conveying direction (in Figure 1 The image forming units Pa, Pb, Pc, and Pd are arranged in sequence. These image forming units Pa to Pd are arranged corresponding to images of different four colors (yellow, magenta, cyan, and black), and form yellow, magenta, cyan, and black images in sequence through charging, exposure, development, and transfer processes.

[0019] The image forming units Pa to Pd are provided with photosensitive drums 1a, 1b, 1c, and 1d that carry visible images (toner images) of respective colors. Also, an intermediate transfer belt 8 is provided adjacent to each of the image forming units Pa to Pd. The intermediate transfer belt 8 is wound around a plurality of rollers including a driving roller 10 and a tension roller 11. Figure 1 In the middle, it rotates counterclockwise. Figure 2 As shown, around the photosensitive drum 1a, along the drum rotation direction ( Figure 2 In the clockwise direction of the image processing apparatus, a charging device 2a, a developing device 3a, a cleaning device 7a, and an anti-static lamp 20 are arranged, and a primary transfer roller 6a is arranged across the intermediate transfer belt 8.

[0020] The photosensitive drums 1a to 1d are 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 stacked on the surface of a cylindrical conductive substrate 19a made of aluminum as the photosensitive layer 19b.

[0021] The charging devices 2 a to 2 d include a charging roller 21 that contacts the photosensitive drum 1 a 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 .

[0022] The developing devices 3a to 3d are two-component developing devices having two stirring and conveying screws 25 and a developing roller 29, and are filled with a predetermined amount of two-component developer containing toners of yellow, magenta, cyan, and black and a magnetic carrier. A magnetic brush is formed on the surface of the developing roller 29 using the two-component developer, and when a developing voltage having the same polarity as the toner (positive polarity in this case) is applied to the developing roller 29, the magnetic brush is brought into contact with the surface of the photosensitive drum 1a to attach the toner, thereby forming a toner image. In addition, when the ratio of the toner in the two-component developer filled in each developing device 3a to 3d becomes lower than a predetermined value due to the formation of the toner image, the toner is replenished from the toner container 4a to 4d to each developing device 3a to 3d.

[0023] When image data is input from a host device such as a personal computer, first, the main motor 40 (see Figure 5 ) starts to rotate the photosensitive drums 1a to 1d. In addition, the belt drive motor 41 (see Figure 5 ) The intermediate transfer belt 8 is started to be rotated and driven. Then, the charging devices 2a to 2d are used to uniformly charge the surfaces of the photosensitive drums 1a to 1d to the same polarity as the toner (positive polarity in this case). Then, the exposure device 5 is used to irradiate light according to the image data, and an electrostatic latent image is formed on each photosensitive drum 1a to 1d, the charge of which is attenuated according to the image data. Furthermore, the developing devices 3a to 3d are used to supply toner to the photosensitive drums 1a to 1d and electrostatically adhere to the toner, thereby forming a toner image corresponding to the electrostatic latent image.

[0024] Then, a predetermined primary transfer electric field is applied between the primary transfer rollers 6a to 6d and the photosensitive drums 1a to 1d, thereby primary transferring the yellow, magenta, cyan, and black toner images on the photosensitive drums 1a to 1d to the intermediate transfer belt 8. After the primary transfer, the toner and the like remaining on the surface of the photosensitive drums 1a to 1d are removed by the cleaning devices 7a to 7d. After the primary transfer, the residual charge remaining on the surface of the photosensitive drums 1a to 1d is removed by the charge removal lamp 20.

[0025] The transfer paper S to which the toner image is transferred is stored in a paper cassette 16 disposed at the lower part of the image forming apparatus 100. The transfer paper S is conveyed to a nip portion (secondary transfer nip portion) between a secondary transfer roller 9 provided adjacent to the intermediate transfer belt 8 and the intermediate transfer belt 8 via a paper feed roller 12a and a registration roller pair 12b at a predetermined timing. The transfer paper S to which the toner image on the intermediate transfer belt 8 is secondarily transferred by the secondary transfer roller 9 is conveyed to the fixing unit 13.

[0026] The transfer paper S conveyed to the fixing section 13 is heated and pressed by the fixing roller pair 13a, so that the toner image is fixed on the surface of the transfer paper S, thereby forming a predetermined full-color image. The transfer paper S with the full-color image formed thereon is discharged to the discharge tray 17 by the discharge roller pair 15 directly (or after being distributed to the reverse conveying path 18 by the branch section 14 and having images formed on both sides).

[0027] Figure 3 1 is a side cross-sectional view of the intermediate transfer unit 30 installed in the image forming apparatus 100. Figure 3 As shown, the intermediate transfer unit 30 includes an intermediate transfer belt 8 stretched between a driving roller 10 and a tension roller 11 , primary transfer rollers 6 a to 6 d in contact with the photosensitive drums 1 a to 1 d via the intermediate transfer belt 8 , and a pressure switching roller 34 .

[0028] The intermediate transfer belt 8 is a resin belt mainly composed of polyimide resin, and has a thickness of, for example, 40 to 100 [μm] and a Young's modulus of elasticity of 3000 to 6000 [MPa].

[0029] The driving roller 10 and the tension roller 11 are respectively arranged on the downstream side and the upstream side with respect to the traveling direction of the conveying surface (lower surface) of the intermediate transfer belt 8. A belt cleaning unit 37 (see FIG. 1 ) is arranged at a position opposite to the tension roller 11. Figure 1 ), which is used to remove the residual toner on the surface of the intermediate transfer belt. A secondary transfer roller 9 is pressed against the driving roller 10 via the intermediate transfer belt 8, forming a secondary transfer roller nip N.

[0030] The intermediate transfer unit 30 is provided with a roller approaching and separating mechanism 35. The roller approaching and separating mechanism 35 includes: a pair of supporting members (not shown) and a driving unit (not shown). The pair of supporting members supports 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 can be perpendicular to the traveling direction of the intermediate transfer belt 8 (along the same direction as the intermediate transfer belt 8). Figure 3 The drive unit moves the primary transfer rollers 6a to 6d and the pressure switching roller 34 in a reciprocating manner in the vertical direction. The proximity separation mechanism 35 can switch the four primary transfer rollers 6a to 6d to the following modes: Figure 1 ), 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 retracting mode in which all four primary transfer rollers 6a to 6d are separated from the photosensitive drums 1a to 1d.

[0031] Figure 4 FIG. 2 is a partial enlarged view of the primary transfer roller 6d and the secondary transfer roller nip N of the intermediate transfer unit 30. Figure 4 The primary transfer and secondary transfer of the toner image are described. Figure 4 In the description, positively charged toner having a positive (plus) charging polarity is described.

[0032] like Figure 4 As shown, the primary transfer rollers 6a to 6d are connected to a primary transfer voltage power supply 54a. The driving roller 10 (secondary transfer opposing roller) is connected to a secondary transfer voltage power supply 54b. Figure 5 ) receives an image forming command, an electrostatic latent image is formed on the surface of the photosensitive drum 1a to 1d, and the developing device 3a to 3d (refer to Figure 2 ) supplies toner T to form a toner image. The toner image formed on the photosensitive drums 1a to 1d moves to the primary transfer nip portions Na to Nd due to the rotation of the photosensitive drums 1a to 1d (see Figure 6 ).

[0033] The primary transfer rollers 6a to 6d are applied with a negative polarity (negative) primary transfer voltage by the primary transfer voltage power supply 54a. As a result, the toner images on the photosensitive drums 1a to 1d are attracted to the primary transfer rollers 6a to 6d in the primary transfer roller nips Na to Nd and are primarily transferred to the intermediate transfer belt 8. The toner images primarily transferred to the intermediate transfer belt 8 move to the secondary transfer roller nip N due to the rotation of the intermediate transfer belt 8.

[0034] The secondary transfer voltage power supply 54b applies a positive polarity (positive) secondary transfer voltage to the driving roller 10. As a result, the toner image on the intermediate transfer belt 8 is conveyed to the secondary transfer roller nip portion N, and is transferred to the transfer paper S passing through the secondary transfer roller nip portion N by the potential difference between the driving roller 10 to which the secondary transfer voltage is applied and the secondary transfer roller 9 which is grounded (earth).

[0035] Next, the control path of the image forming apparatus 100 of the present invention will be described. Figure 5 1 is a block diagram showing an example of a control path used in the image forming apparatus 100 of the present invention. In addition, since the image forming apparatus 100 is used and various controls are performed on each device unit, the control path of the image forming apparatus 100 as a whole is complicated. Therefore, the part of the control path necessary for implementing the present invention is mainly described here.

[0036] The control unit 90 includes at least a CPU (Central Processing Unit) 91 as a central processing unit, a ROM (Read Only Memory) 92 which is a storage unit dedicated to reading, a RAM (Random Access Memory) 93 which is a storage unit that can read and write, a temporary storage unit 94 which temporarily stores image data, etc., a counter 95 which counts the cumulative number of printed sheets, and a plurality of (here, two) I / Fs (interfaces) 96 which send control signals to various devices in the image forming apparatus 100 or receive input signals from the operation unit 60. In addition, the control unit 90 can be arranged at any position inside the main body of the image forming apparatus 100.

[0037] The ROM 92 stores a control program for the image forming apparatus 100 and data such as numerical values ​​required for control that do not change when the image forming apparatus 100 is used. The RAM 93 stores necessary data generated during the control of the image forming apparatus 100 and data temporarily required for the control of the image forming apparatus 100.

[0038] In addition, the control unit 90 sends control signals to various parts and devices in the image forming apparatus 100 from the CPU 91 through the I / F 96. In addition, signals indicating the status thereof or input signals are sent from various parts and devices to the CPU 91 through the I / F 96. Examples of the various parts and devices 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.

[0039] 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 through the image input unit 50 is converted into a digital signal and then output to the temporary storage unit 94 via the I / F 96 .

[0040] The voltage control circuit 51 is connected to a charging voltage power supply 52, a developing voltage power supply 53, and a transfer voltage power supply 54, and operates the above-mentioned power supplies according to the output signal from the control unit 90. The above-mentioned power supplies are based on the control signal from the voltage control circuit 51. The charging voltage power supply 52 applies a charging voltage to the charging roller 21 in the charging device 2a~2d. The developing voltage power supply 53 applies a developing voltage formed by superimposing a developing DC voltage and a developing AC voltage to the developing roller 29 in the developing device 3a~3d. The transfer voltage power supply 54 includes: a primary transfer voltage power supply 54a for applying a prescribed primary transfer voltage to the primary transfer roller 6a~6d, and a secondary transfer voltage power supply 54b for applying a prescribed secondary transfer voltage to the driving roller 10 (both refer to Figure 4 ).

[0041] The operation unit 60 is provided with a liquid crystal display unit 61 and an LED 62 for displaying various states. The user stops the image formation by operating the stop / clear button of the operation unit 60, and returns various settings of the image forming apparatus 100 to the default state by operating the reset button. The liquid crystal display unit 61 displays the state of the image forming apparatus 100, or displays the image formation status and the number of copies to be printed. Various settings of the image forming apparatus 100 are performed by the printer driver of the personal computer.

[0042] Figure 6 This is a schematic cross-sectional view of the primary transfer roller 6d and the photosensitive drums 1a to 1d, showing the positional relationship among the photosensitive drums 1a to 1d, the primary transfer rollers 6a to 6d, and the intermediate transfer belt 8. The upstream side of the conveying surface (lower surface) of the intermediate transfer belt 8 in the traveling direction is set as +X, and the downstream side is set as −X.

[0043] The rotation shafts 61a to 61c of the primary transfer rollers 6a to 6c disposed at the upstream side -X relative to the primary transfer roller 6d are offset more toward the upstream side +X than 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 offset toward the upstream side +X compared to the perpendicular line passing through the axis centers of the rotation shafts 101a to 101c of the photosensitive drums 1a to 1c.

[0044] In addition, the rotation axis 61d of the primary transfer roller 6d disposed at the most downstream side +X in the traveling direction of the intermediate transfer belt 8 is offset toward the downstream side -X from directly above the rotation axis 101d of the corresponding photosensitive drum 1d. More specifically, the axis center of the rotation axis 61d of the primary transfer roller 6d is offset toward the downstream side -X compared to the perpendicular line passing through the axis center of the rotation axis 101d of the photosensitive drum 1d. In addition, in the present embodiment, the rotation axis 61d of the primary transfer roller 6d is offset toward the downstream side -X from directly above the rotation axis 101d of the corresponding photosensitive drum 1d, but it may be disposed directly above the rotation axis 101d of the photosensitive drum 1d.

[0045] In color printing, brightness and transmittance decrease in the order of yellow, magenta, cyan, and black. On the other hand, visual confirmation decreases in the order of black, cyan, magenta, and yellow. Black toner with high visual confirmation is contaminated when the toner scatters.

[0046] In the present embodiment, the primary transfer roller 6 d corresponding to the black toner having lower brightness and transmittance and higher visibility than the yellow toner, magenta toner, and cyan toner is offset toward the downstream side −X with respect to the photosensitive drum 1 d .

[0047] During the primary transfer, discharge is likely to occur at the downstream side -X of the primary transfer roller nip Nd between the photosensitive drum 1d and the intermediate transfer belt 8. At this time, by biasing the primary transfer roller 6d toward the downstream side -X relative to the photosensitive drum 1d, the occurrence of discharge can be suppressed. As a result, the scattering of black toner, which is easily visible as contamination, can be reduced, the occurrence of messy text can be suppressed, and the color development and color reproducibility of the transferred image can be improved. In addition, by reducing the scattering of toner, the consumption of toner can be reduced.

[0048] In addition, in this embodiment, the primary transfer rollers 6a to 6c are biased toward the upstream side +X relative to the photosensitive drums 1a to 1c, and it is difficult to suppress the occurrence of discharge. On the other hand, as the bias width toward the upstream side +X increases, when printing with halftone (dots) as a dot aggregate, the image density of the transferred image increases.

[0049] Compared with black toner, yellow toner, magenta toner, and cyan toner, which have higher brightness and transmittance and lower visual confirmation, are less visible when they are scattered than black toner. Therefore, the primary transfer rollers 6a to 6c are biased to the upstream side +X relative to the photosensitive drums 1a to 1c. Although this may cause text scrambling, it can improve the image density of yellow, magenta, and cyan. As a result, the image quality of the primary transfer image can be improved for colors other than black.

[0050] In addition, the bias amounts W1 to W3 of each primary transfer roller 6a to 6c relative to the photosensitive drums 1a to 1c are different. In the present embodiment, the bias amount is set larger for the colorant with higher brightness and transmittance and lower visual confirmation. Specifically, the bias amount W1 corresponding to the yellow colorant is the largest. In addition, the bias amount W3 corresponding to the cyan colorant is the smallest. Thus, for colors with lower visual confirmation and where contamination is not easily visible, the bias amount can be set larger to increase the image density. On the other hand, for colors with lower visual confirmation and where contamination is easily visible, the bias amount can be set smaller to reduce the scattering of the colorant. Thus, for colors with lower visual confirmation and where contamination is easily visible, the consumption of the colorant caused by scattering can be reduced, and the color rendering 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 bias amount.

[0051] In addition, as the contact surface pressure between the primary transfer rollers 6a to 6d and the intermediate transfer belt 8 is increased, the occurrence of messy text can be suppressed, and the color rendering and color reproducibility of the transferred image can be improved. Specifically, as the contact surface pressure is increased, the aggregation force of the toners becomes larger. As a result, the scattering of the toners can be reduced in the tiny discharge area generated near the primary transfer roller gap Na to Nd. That is, when the contact surface pressure is increased, the same effect as when the primary transfer roller 6d is biased toward the downstream side -X relative to the photosensitive drum 1d can be obtained.

[0052] In this embodiment, the contact surface pressure between the primary transfer roller 6d corresponding to the 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 the yellow toner, the magenta toner, and the cyan toner and the intermediate transfer belt 8. As a result, the scattering of the black toner can be further reduced, and the occurrence of the messy text phenomenon can be further suppressed. Therefore, the color development and color reproducibility of the transferred image can be further improved. In addition, by reducing the scattering of the black toner, the consumption of the black toner can be further reduced.

[0053] On the other hand, the image density when printing with halftone (halftone) is improved as the contact surface pressure between the primary transfer rollers 6a to 6d and the intermediate transfer belt 8 is increased. That is, when the contact surface pressure is reduced, the same effect as when the primary transfer rollers 6a to 6c are biased toward the upstream side +X with respect to the photosensitive drums 1a to 1c can be obtained.

[0054] In the present embodiment, the contact surface pressures of the primary transfer rollers 6a to 6c and the intermediate transfer belt 8 are different. Specifically, the contact surface pressure corresponding to the color with higher brightness and transmittance and lower visual confirmation is set to be larger. Specifically, the contact surface pressure corresponding to the yellow toner is the smallest. In addition, the contact surface pressure corresponding to the cyan toner is the largest. Thus, for colors with lower visual confirmation and pollution that is not easily visible, the contact surface pressure can be set smaller to increase the image density. On the other hand, for colors with lower visual confirmation and pollution that is easily visible, the contact surface pressure can be set larger to further reduce the scattering of the toner. Thus, for colors with lower visual confirmation and pollution that is easily visible, the consumption of toner caused by scattering can be reduced, and the color rendering 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.

[0055] Next, the relationship between the offset of the primary transfer roller and the image density is described. In addition, the relationship between the contact surface pressure and the image density when printing with halftone (dot) is described. As a test machine, a Figure 1 In the intermediate transfer type image forming apparatus 100 (manufactured by Kyocera Document Solutions Co., Ltd.) shown in FIG, the image density was measured by changing the bias amount and contact surface pressure of the primary transfer roller 6 d.

[0056] At this time, the intermediate transfer belt 8 is made of polyimide resin, and both sides of the intermediate transfer belt 8 are stretched by springs as a tension for stretching the intermediate transfer belt 8 .

[0057] The primary transfer roller 6d is a sponge roller made of EPDM. The load of the primary transfer roller 6d can be changed by providing springs on both sides of the axial direction. The primary transfer current flowing to the primary transfer roller 6d is set to -15 to -50 [μA]. In addition, the development bias is set to AC500 to 1400 [V], Vdc80 to 250 [V].

[0058] The photosensitive drum 1 d used was an amorphous silicon photosensitive drum having an amorphous silicon layer as the photosensitive layer 19 b and an OPC photosensitive drum (manufactured by Kyocera Document Solutions, Ltd.) having a positively charged single-layer OPC photosensitive layer as the photosensitive layer 19 b . Positively chargeable toner was used as the toner.

[0059] (Relationship between offset amount and image density)

[0060] The relationship between the offset of the primary transfer roller 6d and the image density was verified. As a test method, the offset W4 of the primary transfer roller 6d was changed in seven stages: -2 [mm], -1 [mm], 0 [mm], +1 [mm], +2 [mm], +3 [mm], and +4 [mm]. When printing with halftone (dots), the black image density was measured. The results were Figure 7 In addition, when the offset amount is negative, the axis of the primary transfer roller 6d is offset to the downstream side -X compared to the perpendicular line passing through the axis of the rotating shaft 101dc of the photosensitive drum 1d. In addition, when the offset amount is positive, the axis of the primary transfer roller 6d is offset to the upstream side +X compared to the perpendicular line passing through the axis of the rotating shaft 101dc of the photosensitive drum 1d.

[0061] (Relationship between contact surface pressure and image density)

[0062] The relationship between the contact surface pressure between the photosensitive drum 1d and the intermediate transfer belt 8 and the image density was verified. As a test method, the contact surface pressure between the photosensitive drum 1d and the intermediate transfer belt 8 was changed in three stages: 1 [N], 5 [N], and 8 [N]. When printing was performed with halftone (dots), the black image density was measured. The results were Figure 8 is shown in the diagram.

[0063] Based on the above relationship, it was confirmed that as the bias amount toward the upstream side +X increased, the image density when printing with halftone (dots) increased. It was also confirmed that as the contact surface pressure increased, the image density when printing with halftone (dots) decreased. It was thus confirmed that the balance between the optimal reproducibility and image density of the transferred image can be adjusted based on the bias amount and the contact surface pressure.

[0064] In addition, the present invention is not limited to the above-mentioned embodiments, and various modifications can be made without departing from the scope of the present invention. Figure 1 The tandem color printer shown above can also be applied to various image forming apparatuses such as color copiers and color multifunction machines, which use an intermediate transfer method of primarily transferring a toner image formed on a photosensitive drum to an intermediate transfer belt.

[0065] In addition, in the present embodiment, the rotation axes of the primary transfer rollers 6a to 6c arranged toward the upstream side +X than the primary transfer roller 6d are offset toward the upstream side +X more than 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 toward the upstream side +X more than directly above the rotation axis of the corresponding photosensitive drums 1a to 1c.

[0066] In the present embodiment, the image forming sections Pa to Pd are arranged in the order of yellow, magenta, cyan, and black toward the downstream side -X. However, the order of yellow, magenta, and cyan is not particularly limited as long as black is arranged on the most downstream side.

[0067] The present invention can be used in an image forming apparatus of an intermediate transfer method in which a toner image formed on an image carrier such as a photosensitive drum is transferred to an intermediate transfer belt. By using the present invention, it is possible to provide an image forming apparatus that can prevent transfer failure caused by discharge, and can suppress the reduction of secondary transfer performance even for paper with a rough surface, and can form high-quality images for a long period of time.

Claims

1. An image forming device, comprising: Multiple image carriers; a charging device for charging the surface of the image carrier; an exposure device for exposing the surface of the image carrier charged by the charging device 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 for sequentially transferring the toner images formed on the image carrier; a plurality of transfer rollers, which are pressed against the image carrier via the intermediate transfer belt; The image carrier disposed on the most downstream side in the traveling direction of the intermediate transfer belt develops the black toner image. The rotation axis of at least one of the transfer rollers disposed upstream of the transfer roller disposed most downstream in the traveling direction of the intermediate transfer belt is offset upstream from directly above the rotation axis of the corresponding image carrier.

2. The image forming apparatus according to claim 1, wherein: The rotation axis of the transfer roller disposed on the most downstream side in the traveling direction of the intermediate transfer belt is offset directly above the rotation axis of the corresponding image carrier or offset further downstream than directly above.

3. The image forming apparatus according to claim 1 or 2, characterized in that: The transfer rollers disposed on the upstream side of the transfer roller disposed on the most downstream side in the traveling direction of the intermediate transfer belt have different offset amounts with respect to the image carrier.

4. The image forming apparatus according to claim 1 or 2, wherein: The contact surface pressure between the transfer roller arranged on the most downstream side in the traveling direction of the intermediate transfer belt and the intermediate transfer belt is greater than the contact surface pressure between the transfer roller arranged on the upstream side and the intermediate transfer belt.

5. The image forming apparatus according to claim 4, wherein: The contact surfaces of the transfer rollers disposed upstream of the transfer roller disposed on the most downstream side in the traveling direction of the intermediate transfer belt have different pressures with respect to the intermediate transfer belt.

6. The image forming apparatus according to claim 1 or 2, characterized in that: The image carriers arranged upstream of the image carrier arranged on the most downstream side in the traveling direction of the intermediate transfer belt develop the yellow toner image, the magenta toner image, and the cyan toner image in order from the upstream side to the downstream side.

Images