Image forming apparatus

Abstract

To improve image quality and prevent a decrease in machine lifespan. [Solution] The image forming apparatus 1 comprises a photoreceptor drum 4, an intermediate transfer belt 5 that moves in contact with the photoreceptor drum 4, and a primary transfer roller 31 that transfers the toner image of the photoreceptor drum 4 to the intermediate transfer belt 5. The first contact area 4S of the photoreceptor drum 4 with respect to the intermediate transfer belt 5 and the second contact area 1S of the primary transfer roller 31 with respect to the intermediate transfer belt 5 are overlapped, and in the direction in which the photoreceptor drum 4 and the primary transfer roller 31 are aligned with the intermediate transfer belt 5 in between, the apex of the primary transfer roller 31 that protrudes the most towards the photoreceptor drum 4 is driven into the photoreceptor drum 4 via the intermediate transfer belt 5, and the amount Kr of the apex of the primary transfer roller 31 driven into the photoreceptor drum 4 with respect to the amount of overlap is set to a predetermined appropriate range.

Description

Technical Field

[0001] The present invention relates to an image forming apparatus that forms an image by an electrophotographic method, and particularly relates to a technique for appropriately setting the positional relationship between a photoreceptor drum and a primary transfer roller.

Background Art

[0002] In an electrophotographic image forming apparatus, an electrostatic latent image is formed on the surface of a photoreceptor drum, toner is applied to the electrostatic latent image to form a toner image on the surface of the photoreceptor drum, and an endless intermediate transfer belt is pressed against the photoreceptor drum by a transfer roller to primarily transfer the toner image from the photoreceptor drum to the intermediate transfer belt, and further secondarily transfer the toner image from the intermediate transfer belt to a recording paper.

[0003] In the image forming apparatus described in Patent Document 1, the primary transfer roller is arranged such that the rotation center of the primary transfer roller is located downstream of the rotation center of the photosensitive drum with respect to the rotation direction of the intermediate transfer belt. The intermediate transfer belt has a base layer and a surface layer provided on the outer peripheral surface of the base layer. Further, when the surface resistivity measured from the outer peripheral surface side of the intermediate transfer belt is G and the surface resistivity measured from the inner peripheral surface side of the intermediate transfer belt is N, 0.75 ≦ N / G ≦ 1.2 is satisfied. Thereby, even in a configuration where the primary transfer roller is offset downstream with respect to the photoreceptor drum, the scattering of toner and the occurrence of discharge traces are suppressed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In Patent Document 1, the primary transfer roller is offset downstream of the photosensitive drum, so that the contact area of ​​the primary transfer roller with respect to the intermediate transfer belt is separated downstream from the contact area of ​​the photosensitive drum with respect to the intermediate transfer belt. As a result, the contact area of ​​the photosensitive drum with respect to the intermediate transfer belt is separated from the primary transfer roller. Consequently, the nip area is separated from the primary transfer roller. In this case, the pressure applied to the recording paper in the nip area tends to become unstable, and image defects (banding) occur due to the rotation of the intermediate transfer belt. Furthermore, if the spring load applied to the primary transfer roller is increased to avoid this, and the pressure acting from the primary transfer roller to the photosensitive drum via the intermediate transfer belt is increased, the pressure in the nip area increases, the damage to the photosensitive drum and intermediate transfer belt increases, and the machine life decreases.

[0006] This invention has been made in view of the above circumstances, and aims to improve image quality by suppressing banding and other issues associated with the rotation of the intermediate transfer belt, and to prevent a decrease in the mechanical lifespan of the photoreceptor drum and the intermediate transfer belt. [Means for solving the problem]

[0007] An image forming apparatus according to one aspect of the present invention comprises: a photoreceptor drum that carries an electrostatic latent image and develops the electrostatic latent image into a toner image by the application of toner; an intermediate transfer belt that moves in contact with the photoreceptor drum; and a primary transfer roller provided on the opposite side of the photoreceptor drum from the intermediate transfer belt, which presses the intermediate transfer belt against the photoreceptor drum to transfer the toner image of the photoreceptor drum from the photoreceptor drum to the intermediate transfer belt. The contact area of ​​the photoreceptor drum with respect to the intermediate transfer belt is designated as a first contact area, and the contact area of ​​the primary transfer roller with respect to the intermediate transfer belt is designated as a second contact area. In the direction of movement of the intermediate transfer belt, the second contact area and a part of the first contact area are overlapped, and in the direction in which the photoreceptor drum and the primary transfer roller are aligned with the intermediate transfer belt in between, the apex of the primary transfer roller that protrudes the most towards the photoreceptor drum is embedded towards the photoreceptor drum via the intermediate transfer belt. [Effects of the Invention]

[0008] According to the present invention, it is possible to suppress banding and other issues associated with the rotation of the intermediate transfer belt, thereby improving image quality and preventing a decrease in the mechanical lifespan of the photoreceptor drum and the intermediate transfer belt. [Brief explanation of the drawing]

[0009] [Figure 1] This is a cross-sectional view showing an image forming apparatus according to one embodiment of the present invention. [Figure 2] This is a side view showing the intermediate transfer unit and the like in the image forming apparatus of this embodiment. [Figure 3] This is a schematic diagram showing a magnified view of a set of primary transfer rollers, a photoreceptor drum, and an intermediate transfer belt in an intermediate transfer unit. [Figure 4] This is a magnified view of a portion of Figure 3, showing the first contact area of ​​the photoreceptor drum that contacts the intermediate transfer belt, the second contact area of ​​the primary transfer roller that contacts the intermediate transfer belt, and the overlapping area of ​​the second contact area that overlaps with the first contact area. [Figure 5] (A) shows the conditions for Experiment 1, and (B) is a table showing whether or not banding occurred in Experiment 1. [Figure 6] (A) is a table showing whether or not banding occurred in Experiment 2-1, and (B) is a table showing whether or not drum ghosting occurred in Experiment 2. [Figure 7] (A) is a table showing whether or not banding occurred in Experiment 2-2, and (B) is a table showing whether or not drum ghosting occurred in Experiment 2. [Figure 8] (A) is a table showing whether or not banding occurred in Experiment 2-3, and (B) is a table showing whether or not drum ghosting occurred in Experiment 2. [Figure 9] (A) is a table showing whether or not banding occurred in Experiment 2-4, and (B) is a table showing whether or not drum ghosting occurred in Experiment 2. [Figure 10] Figures (A) to (C) show drum ghosting that occurred on the surface of the photosensitive drum. [Figure 11] This graph shows the maximum pressure PM in the nip region of the primary transfer roller as a function of load N, when the offset amount F is set to 0 mm, 2.0 mm, 4.0 mm, and 6.0 mm. [Figure 12] This graph shows the Mottle index relative to the maximum pressure PM in the nip region when the offset amount F is set to 0 mm, 2.0 mm, 4.0 mm, and 6.0 mm. [Modes for carrying out the invention]

[0010] Hereinafter, an image forming apparatus according to an embodiment of the present invention will be described with reference to the drawings. Figure 1 is a cross-sectional view showing an image forming apparatus according to one embodiment of the present invention. This image forming apparatus 1 comprises an image reading unit 11 and an image forming unit 12.

[0011] The image reading unit 11 has an image sensor that optically reads the image of the document, and the analog output of this image sensor is converted into a digital signal to generate image data that represents the image of the document.

[0012] The image forming unit 12 forms the image shown by the above image data on the recording paper, and includes an image forming unit 3M for magenta, an image forming unit 3C for cyan, an image forming unit 3Y for yellow, and an image forming unit 3Bk for black. In each of the image forming units 3M, 3C, 3Y, and 3Bk, the surface of the photoreceptor drum 4 is uniformly charged, the surface of the photoreceptor drum 4 is exposed to light to form an electrostatic latent image on the surface of the photoreceptor drum 4, the electrostatic latent image on the surface of the photoreceptor drum 4 is developed into a toner image, and the toner image on the surface of the photoreceptor drum 4 is transferred to the intermediate transfer belt 5 in the intermediate transfer unit 20. As a result, a color toner image is formed on the intermediate transfer belt 5. This color toner image is secondarily transferred to the recording paper P that has been transported from the paper feeding unit 14 through the transport path 8 in the nip area NP2 between the intermediate transfer belt 5 and the secondary transfer roller 6.

[0013] After this, the recording paper P is heated and pressurized in the fixing unit 15, fixing the toner image on the recording paper P by heat and pressure, and then the recording paper P is discharged to the discharge tray 17 via the discharge roller 16.

[0014] FIG. 2 is a side view showing the intermediate transfer unit 20. FIG. 2 shows the configuration of the intermediate transfer unit 20 viewed from the side opposite to the viewing direction in FIG. 1. As shown in FIG. 2, in the intermediate transfer unit 20, four primary transfer rollers 31, a drive roller 23, a tension roller 24, and two backup rollers 25 (not shown in FIG. 1) are provided. The intermediate transfer belt 5 is wound around the drive roller 23, the tension roller 24, and each backup roller 25, and each primary transfer roller 31 is pressed against each photosensitive drum 4 via the intermediate transfer belt 5. In this state, when the drive roller 23 is rotationally driven, the intermediate transfer belt 5 orbits while contacting each photosensitive drum 4, and toner images of each color are transferred from each photosensitive drum 4 to the intermediate transfer belt 5. The belt cleaning unit 18 removes the toner remaining on the surface of the intermediate transfer belt 5. Each primary transfer roller 31 extends in a direction orthogonal to the moving direction A of the intermediate transfer belt 5, that is, in the width direction of the intermediate transfer belt 5. The rotation axis 1x (FIG. 3) of the intermediate transfer belt 5 also extends in the width direction.

[0015] For example, the primary transfer roller 31 uses a conductive roller or the like using a conductive rubber component. The primary transfer roller 31 is composed of a columnar core metal made of SUS or iron or the like with a conductive layer provided on the outer peripheral surface. This conductive layer is formed of a rubber material (for example, NBR (nitrile rubber), EPDM (ethylene propylene diene rubber), epichlorohydrin rubber, etc.) in order to obtain a stable resistance value.

[0016] As shown in FIG. 2, the backup roller 25 is disposed before and after each photosensitive drum 4 of each color and the corresponding primary transfer roller 31 in the moving direction A. The backup roller 25 is, for example, a metal roller provided with a knurl.

[0017] Each primary transfer roller 31 has its rotating shaft supported by bearings 34 provided at both ends of the primary transfer roller 31. The rotating shaft of the primary transfer roller 31 is supported by the bearings 34 so as to be movable in the vertical direction. A stopper 32 is fixed at a position spaced upward from each bearing 34. A spring 33 is inserted in a compressed state between each bearing 34 and each stopper 32, and the bearing 34 of the primary transfer roller 31 is biased toward the intermediate transfer belt 5 by the biasing force of each spring 33. That is, the spring 33 is a compression spring. Thereby, the primary transfer roller 31 presses the intermediate transfer belt 5 and presses the photosensitive drum 4 via the intermediate transfer belt 5. Incidentally, the spring 33 corresponds to the biasing portion in the claims.

[0018] Below the intermediate transfer belt 5, a developing unit 26, a drum cleaning unit 27, a charging unit 28, etc. are provided for each photosensitive drum 4. Each photosensitive drum 4 is rotationally driven in the direction of the arrow. As the photosensitive drum 4 rotates, the surface of the photosensitive drum 4 is uniformly charged by the charging unit 28, and the surface of the photosensitive drum 4 is exposed by an exposure device (not shown), so that an electrostatic latent image is formed on the surface of the photosensitive drum 4. Toner is applied to the electrostatic latent image on the surface of the photosensitive drum 4 by the developing unit 26, and the electrostatic latent image is developed into a toner image. The toner image formed on the surface of the photosensitive drum 4 is primarily transferred to the intermediate transfer belt 5 by the primary transfer roller 31. Thereafter, the surface of the photosensitive drum 4 is discharged, and the residual toner on the surface of the photosensitive drum 4 is removed by the drum cleaning unit 27.

[0019] On the intermediate transfer belt 5, a color toner image formed by overlapping the toner images on the surfaces of the respective photosensitive drums 4 as described above is formed, and the color toner image is secondarily transferred from the intermediate transfer belt 5 to the recording paper P in the nip region NP2 between the secondary transfer roller 6 and the intermediate transfer belt 5.

[0020] In the image forming apparatus 1 of this embodiment, if a photoreceptor drum 4 and a primary transfer roller 31 that presses the photoreceptor drum 4 via an intermediate transfer belt 5 are considered as one set, then four sets of photoreceptor drums 4 and primary transfer rollers 31 are provided. In each set of photoreceptor drums 4 and primary transfer rollers 31, in the direction of movement A of the intermediate transfer belt 5, upstream of the secondary transfer roller 6, the downstream portion of the first contact area of ​​the photoreceptor drum 4 that contacts the intermediate transfer belt 5 and the upstream portion of the second contact area of ​​the primary transfer roller 31 that contacts the intermediate transfer belt 5 are arranged to overlap each other in the direction of movement A.

[0021] Furthermore, in the direction in which the photoreceptor drum 4 and the primary transfer roller 31 are aligned with the intermediate transfer belt 5 in between (up and down in Figure 2), the apex of the primary transfer roller 31 that protrudes most towards the photoreceptor drum 4 is positioned to be embedded into the photoreceptor drum 4 via the intermediate transfer belt 5. In this embodiment, the amount by which the apex of the primary transfer roller 31 is embedded into the photoreceptor drum 4 with respect to the overlap amount of the first contact area and the second contact area is set to a predetermined appropriate range.

[0022] Figure 3 is a schematic diagram showing an enlarged view of the intermediate transfer belt 5 and a pair of photoreceptor drums 4 and primary transfer rollers 31. Figure 4 is a partially enlarged view of Figure 3 showing the first contact area, the second contact area, and the overlapping area of ​​the second contact area that overlaps the first contact area. As shown in Figure 3, in the direction of movement A of the intermediate transfer belt 5, a pair of photoreceptor drums 4 and primary transfer rollers 31 are positioned upstream of the secondary transfer rollers 6 (not shown).

[0023] If the contact area of ​​the photoreceptor drum 4 with respect to the intermediate transfer belt 5 is defined as the first contact area 4S, and the contact area of ​​the primary transfer roller 31 with respect to the intermediate transfer belt 5 is defined as the second contact area 1S, then the downstream portion of the first contact area 4S of the photoreceptor drum 4 and the upstream portion of the second contact area 1S of the primary transfer roller 31 are overlapped. Here, overlap refers to the state in which a portion of the second contact area 1S and a portion of the first contact area 4S overlap each other in the direction of movement A. In Figures 3 and 4, the amount of the above overlap is indicated as "R".

[0024] In more detail, in the direction of movement A of the intermediate transfer belt 5, if we define the upstream end of the first contact area 4S as 4a and the downstream end as 4b, and the upstream end of the second contact area 1S as 1a and the downstream end as 1b, then the upstream end 1a of the second contact area 1S is positioned upstream of the downstream end 4b of the first contact area 4S. Furthermore, if we define the rotation center of the photoreceptor drum 4 as 4x and the rotation center of the primary transfer roller 31 as 1x, then in the direction of movement A of the intermediate transfer belt 5, the rotation center 1x of the primary transfer roller 31 is positioned downstream of the rotation center 4x of the photoreceptor drum 4. If the distance between the rotation center 4x and the rotation center 1x is the offset amount F, then the offset amount F is set to a value greater than "0". As a result, the downstream portion of the first contact area and the upstream portion of the second contact area of ​​the primary transfer roller 31 overlap.

[0025] Furthermore, as shown in Figure 3, if the range of the second contact area 1S of the primary transfer roller 31 that overlaps with the first contact area 4S of the photoreceptor drum 4 is defined as the overlap area R, then by setting the overlap area Rs of the overlap area R to less than the maximum value Vs of the overlap area Rs, a state is achieved in which parts of the second contact area 4S and the first contact area 1S overlap each other in the direction A of movement of the intermediate transfer belt 5. From this state, the vertex 1c of the primary transfer roller 31 that protrudes the most towards the photoreceptor drum 4 is embedded into the photoreceptor drum 4 via the intermediate transfer belt 5 with the appropriate amount of embedding as described above.

[0026] The overlap area Rs reaches its maximum value RM when the photoreceptor drum 4 and the primary transfer roller 31 are in contact with the same position on the intermediate transfer belt 5 (the same position where the front and back sides of the intermediate transfer belt 5 overlap) in the direction of movement A. Furthermore, as the primary transfer roller 31 moves away from that same position, the overlap area Rs becomes smaller and falls below the maximum value Vs.

[0027] The first contact area 4S presses the primary transfer roller 31 against the photoreceptor drum 4 via the intermediate transfer belt 5, sandwiching the intermediate transfer belt 5 between the photoreceptor drum 4 and the primary transfer roller 31. The first contact area 4S is a nip area NP1 that transfers the toner image from the photoreceptor drum 4 to the intermediate transfer belt 5.

[0028] Furthermore, the second contact area 1S is the area where the primary transfer roller 31, to which a transfer bias is applied, is pressed against the photoreceptor drum 4 via the intermediate transfer belt 5. The pressure of the intermediate transfer belt 5 on the photoreceptor drum 4 by the primary transfer roller 31 in the second contact area 1S promotes the transfer of the toner image from the photoreceptor drum 4 to the intermediate transfer belt 5.

[0029] When the downstream portion of the first contact area 4S (nip area NP1) of the photoreceptor drum 4 and the upstream portion of the second contact area 1S of the primary transfer roller 31 overlap, the pressure received from the primary transfer roller 31 in the nip area NP1 is less likely to fluctuate, the pressure applied to the intermediate transfer belt 5 is stable, and image defects (e.g., banding) associated with the rotation of the intermediate transfer belt 5 are less likely to occur. Furthermore, the pressure in the nip area NP1 can be efficiently increased without significantly increasing the load applied to the primary transfer roller 31 by the biasing force of the spring 33, and the pressure applied to the intermediate transfer belt 5 can be stabilized. As a result, the load on the photoreceptor drum 4 and the intermediate transfer belt 5 is reduced, and the machine life is extended.

[0030] Here, in the direction of movement A of the intermediate transfer belt 5, by setting the offset amount F, which is the distance between the rotation center of the primary transfer roller 31 and the rotation center of the photoreceptor drum 4 in the direction of movement A, with respect to the overlap amount when the second contact area 1S and the first contact area 4S are overlapped, within a predetermined appropriate range, it is possible to suppress banding and other issues associated with the rotation of the intermediate transfer belt 5, thereby improving image quality and more reliably preventing a decrease in the mechanical life of the photoreceptor drum 4 and the intermediate transfer belt 5.

[0031] However, when the downstream portion of the first contact area 4S (nip area NP1) of the photoreceptor drum 4 and the upstream portion of the second contact area 1S of the primary transfer roller 31 overlap, the larger the offset amount F, the smaller the overlap area Rs becomes, the lower and more unstable the pressure in the nip area NP1 becomes, and the transfer of the toner image from the photoreceptor drum 4 to the intermediate transfer belt 5 becomes unstable.

[0032] Therefore, in this embodiment, in the direction A of movement of the intermediate transfer belt 5, with a portion of the second contact area 4S and the first contact area 1S overlapping each other, the apex 1c of the primary transfer roller 31 is made to bite into the photoreceptor drum 4 side via the intermediate transfer belt 5 by the amount described above. This ensures that even when the offset amount F is large and the overlap area Rs is small, the load applied to the primary transfer roller 31 by the biasing force of the spring 33 is not excessively large, thereby securing the pressure necessary for toner transfer to the nip area NP1 and further stabilizing the transfer of the toner image from the photoreceptor drum 4 to the intermediate transfer belt 5. This further stabilizes the effects of reducing image defects, improving image quality, and extending the mechanical life of the photoreceptor drum 4 and the intermediate transfer belt.

[0033] In other words, in this embodiment, by providing the above-mentioned amount of penetration, the transfer of the toner image from the photoreceptor drum 4 to the intermediate transfer belt 5 is stable even when the overlap area Rs is narrow, thus increasing the design freedom for the structure of the photoreceptor drum 4, the primary transfer roller 31, and the intermediate transfer belt 5. These effects will be explained later by Experiments 1 and 2.

[0034] <Specific Examples of This Embodiment> In this example, the diameter of the photoreceptor drum 4 is set to φ30 mm, and the diameter of the primary transfer roller 31 is set to φ12 mm.

[0035] The overlap area Rs reaches its maximum value RM when the photoreceptor drum 4 and the primary transfer roller 31 are in contact at the same position on the intermediate transfer belt 5. When the apex 1c of the primary transfer roller 31 is driven more into the photoreceptor drum 4 side via the intermediate transfer belt 5, if the amount that the apex 1c of the primary transfer roller 31 is driven into the photoreceptor drum 4 side is defined as the amount of penetration Kr, then, as will be clear from Experiment 2 described later, by setting the overlap area Rs to be greater than 0% and 40% or less of the maximum value RM, and setting the appropriate range for the amount of penetration Kr to be greater than 0 mm and 0.5 mm or less, the image quality is improved and the machine life is extended as described above.

[0036] Alternatively, as will be clear from Experiment 2, which will be described later, when the overlap area Rs is set to be greater than 0% and less than or equal to 30% of the maximum value RM, and the apex 1c of the primary transfer roller 31 is made to bite into the photoreceptor drum 4 side via the intermediate transfer belt 5, by setting the appropriate range of the biting amount Kr of the apex 1c of the primary transfer roller 31 to a range greater than 0 mm and less than or equal to 1.0 mm, the effects of improved image quality and extended machine life can be reliably obtained as described above.

[0037] Alternatively, as will be clear from Experiment 2 described later, when the overlap area Rs is set to exceed 0% and not exceeding 20% ​​of the maximum value RM, and the apex 1c of the primary transfer roller 31 is driven into the photoreceptor drum 4 side via the intermediate transfer belt 5, setting the appropriate range for the amount Kr driven into the apex 1c of the primary transfer roller 31 to a range of over 0 mm and not exceeding 1.5 mm will reliably improve image quality and extend machine life as described above.

[0038] Furthermore, as will be clear from Experiment 2, which will be discussed later, if we focus solely on the offset amount F, we can obtain the effect of improved image quality and extended machine life when the offset amount F is set in the range of 0.5 mm ≤ F6.0 mm.

[0039] Preferably, as will become clear from Experiment 2 described later, it is best to set the offset amount F in the range of 3.0 mm ≤ F6.0 mm, which will more reliably improve image quality and extend the machine's lifespan.

[0040] Furthermore, the load N applied to the primary transfer roller 31 by the biasing force of the spring 33 is set according to the size of the recording paper, etc. As described above, the primary transfer roller 31 is biased toward the intermediate transfer belt 5 by the biasing force of the spring 33, and the primary transfer roller 31 is pressed against the photoreceptor drum 4 via the intermediate transfer belt 5. By adjusting the biasing force of the spring 33, the load N is set appropriately in addition to the overlap area Rs (an example of overlap amount) and the penetration amount Kr shown above.

[0041] For example, if the maximum size of the recording paper is a standard A3 size and the width of the intermediate transfer belt 5 is set according to the A3 size, it is preferable to set the load N applied to the primary transfer roller 31 by the biasing force of each spring 33 to be 0.6N or more and 3.0N or less. Also, if the maximum size of the recording paper is a standard A4 size and the width of the intermediate transfer belt 5 is set according to the A4 size, it is preferable to set the load N applied to the primary transfer roller 31 by the biasing force of each spring 33 to be 0.6N or more and 1.4N or less. This ensures that the pressing force per unit area of ​​the intermediate transfer belt 5 is set appropriately.

[0042] Furthermore, the combination of (i) overlap area Rs (an example of overlap amount) and penetration amount Kr shown above, or (ii) the above combination and load N, plus any of the following may be set as appropriate.

[0043] When an elastic belt is used as the intermediate transfer belt 5, the thickness of the intermediate transfer belt 5 is set to 30 μm or more and 400 μm or less. When a resin belt is used as the intermediate transfer belt 5, the thickness of the intermediate transfer belt 5 is set to 30 μm or more and 150 μm or less. An elastic belt is also called an intermediate transfer belt with an elastic layer, and is made by laminating multiple layers including an elastic layer. A resin belt is, for example, a resin belt with a coating layer on its surface.

[0044] Furthermore, the tension of the intermediate transfer belt 5 is set to be between 15N and 45N.

[0045] Furthermore, when a transfer bias is applied to the primary transfer roller 31, if the current flowing between the primary transfer roller 31 and the photoreceptor drum 4 is denoted as the transfer current It, the transfer current It is set to the range of │2.0μA│≦It│40.0μA│.

[0046] Preferably, the transfer current It is appropriately set within the range of -3.0 μA to -15.0 μA, depending on the dielectric constant of the photoreceptor drum 4, the type of toner, etc. <Experiment 1>

[0047] The conditions for Experiment 1 are as shown in Figure 5(A), and, similar to the specific example in the above embodiment, the diameter of the photoreceptor drum 4 is φ30 mm and the diameter of the primary transfer roller 31 is φ12 mm.

[0048] Furthermore, under the conditions of Experiment 1, the offset amount F from the rotation center 4x of the photoreceptor drum 4 to the rotation center 1x of the primary transfer roller 31 in the direction A of movement of the intermediate transfer belt 5 is set to 4.0 mm.

[0049] Furthermore, under the conditions of Experiment 1, the overlap area Rs of the overlap region R, which is the range of the second contact region 1S of the primary transfer roller 31 that overlaps with the first contact region 4S of the photoreceptor drum 4, is set to 25% of the maximum value RM of the overlap area Rs. The vertex 1c of the primary transfer roller 31 is then driven in more towards the photoreceptor drum 4 via the intermediate transfer belt 5, and the appropriate range for the amount Kr of the penetration of the vertex 1c of the primary transfer roller 31 is set to 0.5 mm.

[0050] Furthermore, under the conditions of Experiment 1, the intermediate transfer belt 5 is a resin belt with a thickness of 65 μm. The surface resistivity of the intermediate transfer belt 5 is 3.0E10 Ω / □ (ohms per square), and the volume resistivity of the intermediate transfer belt 5 is 6.0E9 Ω·m.

[0051] Furthermore, under the conditions of Experiment 1, the tension of the intermediate transfer belt 5 is set to 25N. The load applied to the primary transfer roller 31 by the spring 33 is set to 1.2N.

[0052] Furthermore, under the conditions of Experiment 1, the transfer current It flowing between the primary transfer roller 31 and the photoreceptor drum 4 was set to -3.0 to -15.0 μA.

[0053] Based on the conditions of Experiment 1, the electrostatic latent image on the surface of each photoreceptor drum 4 is developed to form the respective toner image on the surface of each photoreceptor drum 4, and the toner image on the surface of each photoreceptor drum 4 is primary transferred to the intermediate transfer belt 5 by each primary transfer roller 31. The results of Experiment 1 are shown in Table H11 of Figure 5(B). In Table H11 of Figure 5(B), "○" indicates no banding and "×" indicates banding. In Experiment 1, no banding (horizontal streaks) occurred in the color toner image formed on the intermediate transfer belt 5.

[0054] Furthermore, in Comparative Example 1 in Table H11, the offset amount F under the conditions of Experiment 1 is changed to 0, and the rotation center 4x of the photoreceptor drum 4 and the rotation center 1x of the primary transfer roller 31 are aligned in the direction of movement of the intermediate transfer belt 5. In this case, the photoreceptor drum 4 and the primary transfer roller 31 make linear contact with each other at the same position on the intermediate transfer belt 5 (the same position where the front and back sides of the intermediate transfer belt 5 overlap).

[0055] Furthermore, in Comparative Example 2 in Table H11, the offset amount F under the conditions of Experiment 1 is changed to a larger value, and the first contact area 4S of the photoreceptor drum 4 is separated from the second contact area 1S of the primary transfer roller 31. In this case, the overlap area Rs becomes 0.

[0056] As shown in Table H11 of Figure 5(B), in both Comparative Example 1 and Comparative Example 2, image defects (banding) occur in the color toner image formed on the intermediate transfer belt 5.

[0057] From the results of Experiment 1, Comparative Example 1, and Comparative Example 2, when the downstream portion of the first contact area 4S (nip area NP1) of the photoreceptor drum 4 and the upstream portion of the second contact area 1S of the primary transfer roller 31 are overlapped as described above, and the apex 1c of the primary transfer roller 31 is made to bite into the photoreceptor drum 4 side via the intermediate transfer belt 5 as described above, fluctuations in the pressure received from the primary transfer roller 31 in the nip area NP1 can be sufficiently suppressed without increasing the spring load applied to the primary transfer roller 31, the pressure necessary for toner transfer can be secured in the nip area NP1, banding does not occur, and the mechanical life of the photoreceptor drum 4 and the intermediate transfer belt is extended. <Experiment 2>

[0058] Experiment 2 is divided into four experiments: Experiment 2-1, Experiment 2-2, Experiment 2-3, and Experiment 2-4. The results of Experiment 2-1 are shown in Tables H21 and H22 in Figures 6(A) and (B), the results of Experiment 2-2 are shown in Tables H31 and H32 in Figures 7(A) and (B), the results of Experiment 2-3 are shown in Tables H41 and H42 in Figures 8(A) and (B), and the results of Experiment 2-4 are shown in Tables H51 and H52 in Figures 9(A) and (B).

[0059] In Experiment 2-1, the penetration depth Kr was set to 0 mm; in Experiment 2-2, it was set to 0.5 mm; in Experiment 2-3, it was set to 1.0 mm; and in Experiment 2-4, it was set to 1.5 mm.

[0060] Furthermore, in each of experiments 2-1 to 2-4, the offset amount F was changed in stages, and the overlap area Rs was changed in stages for each offset amount F to evaluate the presence or absence of banding and drum ghosting. "○" indicates no banding or no drum ghosting, "△" indicates reduced banding or reduced drum ghosting, and "×" indicates banding or drum ghosting. Drum ghosting is a phenomenon in which the previously transferred image remains on the surface of the photosensitive drum 4, and this remaining image is superimposed on the next image; it is also called transfer memory.

[0061] Under the conditions of each experiment 2-1 to 2-4, the diameter of the photoreceptor drum 4 was set to φ30 mm and the diameter of the primary transfer roller 31 was set to φ12 mm, similar to experiment 1 above.

[0062] Furthermore, under the conditions of each experiment 2-1 to 2-4, the intermediate transfer belt 5 is a resin belt with a thickness of 65 μm, similar to experiment 1 above. The surface resistivity of the intermediate transfer belt 5 is 3.0E10 Ω / □, and the volume resistivity of the intermediate transfer belt 5 is 6.0E9 Ω·m.

[0063] Furthermore, under the conditions of each experiment 2-1 to 2-4, the tension of the intermediate transfer belt 5 was set to 25N, similar to experiment 1 above.

[0064] Furthermore, under the conditions of each experiment 2-1 to 2-4, the transfer current It flowing between the primary transfer roller 31 and the photoreceptor drum 4 was set to -3.0 to -15.0 μA, similar to experiment 1 above.

[0065] On the other hand, under the conditions of Experiment 2-1, as shown in Tables H21 and H22 in Figures 6(A) and (B), the indentation amount Kr was set to 0 mm, and the offset amount F was changed in stages to 1.0 mm, 2.0 mm, 3.0 mm, 4.0 mm, 5.0 mm, 6.0 mm, 7.0 mm, and 8.0 mm. For each offset amount F, the overlap rate Rr was changed in stages to 75%, 50%, 25%, and 0%, and the occurrence of banding and drum ghosting was evaluated.

[0066] The overlap rate Rr is the ratio of the overlap area Rs to the maximum value RM, with RM being the reference value for the overlap area Rs. The primary transfer roller 31 changes in accordance with the load N pressing against the intermediate transfer belt 5, i.e., the biasing force of the spring 33. When the load N due to the biasing force of the spring 33 is increased, and the primary transfer roller 31 is pressed against the photoreceptor drum 4 via the intermediate transfer belt 5 and comes closest to it, the photoreceptor drum 4 and the primary transfer roller 31 come into contact at the same position on the intermediate transfer belt 5 (the same position where the front and back sides of the intermediate transfer belt 5 overlap), and the overlap area Rs of the overlap area R of the second contact area 1S that overlaps with the first contact area 4S becomes the maximum value RM, and the overlap rate Rr becomes 100%. Conversely, as the load N due to the biasing force of the spring 33 decreases, the primary transfer roller 31 moves further away from the intermediate transfer belt 5, the overlap area Rs becomes narrower, and the overlap rate Rr decreases.

[0067] For offset amounts F = 1.0 mm, 2.0 mm, 3.0 mm, 4.0 mm, 5.0 mm, 6.0 mm, 7.0 mm, and 8.0 mm, when the load N is increased and the photoreceptor drum 4 and the primary transfer roller 31 come into contact with the same position on the intermediate transfer belt 5, the overlap area Rs reaches its maximum value RM, and the overlap rate Rr becomes 100%. Conversely, as the load N decreases, the primary transfer roller 31 moves away from that same position, the overlap area Rs becomes smaller, and the overlap rate Rr decreases.

[0068] In Tables H21 and H22 of Figures 6(A) and (B), for any offset amount F = 1.0 mm, 2.0 mm, 3.0 mm, 4.0 mm, 5.0 mm, 6.0 mm, 7.0 mm, or 8.0 mm, when the overlap rate Rr is 100%, the photoreceptor drum 4 and the primary transfer roller 31 are in contact with the same position on the intermediate transfer belt 5, and the penetration amount Kr is 0 mm and cannot be changed. Therefore, the presence or absence of banding and drum ghosting has not been evaluated.

[0069] Furthermore, when the offset amount F is set to 0 mm, the rotation center 1x of the primary transfer roller 31 is located directly above the rotation center 4x of the photoreceptor drum 4, and the penetration amount Kr is 0 mm and cannot be changed, so the presence or absence of banding and drum ghosting cannot be evaluated.

[0070] In Tables H21 and H22 of Figures 6(A) and (B), since the penetration amount Kr = 0 mm, even when setting any combination of offset amount F = 1.0 mm, 2.0 mm, 3.0 mm, 4.0 mm, 5.0 mm, 6.0 mm, 7.0 mm, or 8.0 mm and overlap rate Rr = 75%, 50%, 25%, or 0%, the vertex 1c of the primary transfer roller 31 is not penetrated by the photoreceptor drum 4 side via the intermediate transfer belt 5.

[0071] In Tables H21 and H22 of Figures 6(A) and (B), when the offset amount F is set to 1.0 mm and the overlap rate Rr is set to 75%, the evaluation is "○" for no banding and "×" for drum ghosting.

[0072] Furthermore, in Tables H21 and H22 of Figures 6(A) and (B), when the offset amount F is set to 2.0 mm and the overlap rate Rr is set to 75% or 50%, the evaluation is "○" for no banding and "△" for reduced drum ghosting.

[0073] Furthermore, in Tables H21 and H22 of Figures 6(A) and (B), the presence or absence of banding and ghosting is evaluated for each combination of offset amount F = 3.0 mm, 4.0 mm, 5.0 mm, 6.0 mm, 7.0 mm, or 8.0 mm and overlap rate Rr = 75%, 50%, 25%, or 0%. When the offset amount F is set to 3.0 mm to 7.0 mm and the overlap rate Rr is set to 75%, 50%, or 25%, the evaluation is "No banding" (○) or "Reduced banding" (△). Similarly, when the offset amount F is set to 8.0 mm and the overlap rate Rr is set to 75%, the evaluation is "No banding" (○). Furthermore, when the offset amount F is set to 3.0mm to 8.0mm and the overlap rate Rr is set to 75%, the drum ghost occurrence is evaluated as "△" (meaning reduced occurrence). When the offset amount F is set to 3.0mm to 8.0mm and the overlap rate Rr is set to 50%, 25%, or 0%, the drum ghost occurrence is evaluated as "△" (meaning reduced occurrence) or "〇" (meaning no occurrence).

[0074] Next, under the conditions of Experiment 2-2, as shown in Tables H31 and H32 in Figures 7(A) and (B), the indentation amount Kr was set to 0.5 mm, and the offset amount F was gradually changed to 1.0 mm, 2.0 mm, 3.0 mm, 4.0 mm, 5.0 mm, 6.0 mm, 7.0 mm, and 8.0 mm. For each offset amount F, the overlap rate Rr was gradually changed to 75%, 50%, 25%, and 0%, and the occurrence of banding and drum ghosting was evaluated.

[0075] In Tables H31 and H32 of Figures 7(A) and (B), for any of the offset amounts F = 1.0 mm, 2.0 mm, 3.0 mm, 4.0 mm, 5.0 mm, 6.0 mm, 7.0 mm, and 8.0 mm, when the overlap rate Rr is 100%, the photoreceptor drum 4 and the primary transfer roller 31 are in contact with the same position on the intermediate transfer belt 5, and the penetration amount Kr is 0 mm and cannot be changed. Therefore, the presence or absence of banding and drum ghosting has not been evaluated.

[0076] Furthermore, when the offset amount F is set to 0 mm, the rotation center 1x of the primary transfer roller 31 is located directly above the rotation center 4x of the photoreceptor drum 4, and the penetration amount Kr is 0 mm and cannot be changed, so the presence or absence of banding and drum ghosting cannot be evaluated.

[0077] Tables H31 and H32 in Figures 7(A) and (B) evaluate the presence or absence of banding and ghosting when the offset amount F is set to 1.0 mm and the overlap rate Rr is set to 75%. When the offset amount F is set to 1.0 mm, the photoreceptor drum 4 and the primary transfer roller 31 do not contact the same position on the intermediate transfer belt 5, and the primary transfer roller 31 is slightly separated from that same position. As a result, the apex 1c of the primary transfer roller 31 can be pushed 0.5 mm into the photoreceptor drum 4 side via the intermediate transfer belt 5 from the state where the overlap rate Rr was set to 75%. The presence or absence of banding ("○") and the presence or absence of drum ghosting ("×") when pushed in by 0.5 mm are evaluated.

[0078] Tables H31 and H32 in Figures 7(A) and (B) evaluate the presence or absence of banding and ghosting when the offset amount F is set to 2.0 mm and the overlap rate Rr is set to 75% and 50%, respectively. When the offset amount F is set to 2.0 mm, the photoreceptor drum 4 and the primary transfer roller 31 do not contact the same position on the intermediate transfer belt 5, and the primary transfer roller 31 moves away from that same position. As a result, the apex 1c of the primary transfer roller 31 can be pushed 0.5 mm into the photoreceptor drum 4 side via the intermediate transfer belt 5 from the state where the overlap rate Rr was set to 75% or 50%. When the overlap rate Rr was set to 75% and the roller was pushed in by 0.5 mm, it was evaluated as no banding occurring ("○") and drum ghosting occurring ("×"). When the overlap rate Rr was set to 50% and the roller was pushed in by 0.5 mm, it was evaluated as no banding occurring ("○") and drum ghosting being reduced ("△").

[0079] Tables H31 and H32 in Figures 7(A) and (B) evaluate the presence or absence of banding and ghosting for each combination of offset amount F = 3.0 mm, 4.0 mm, 5.0 mm, 6.0 mm, 7.0 mm, or 8.0 mm and overlap rate Rr = 75%, 50%, 25%, or 0%. When the offset amount F is set to 3.0 mm to 8.0 mm, the photoreceptor drum 4 and the primary transfer roller 31 do not contact the same position on the intermediate transfer belt 5, and the primary transfer roller 31 is moved significantly away from that same position. As a result, the apex 1c of the primary transfer roller 31 can be pushed 0.5 mm into the photoreceptor drum 4 side via the intermediate transfer belt 5 compared to when the overlap rate Rr was set to 75%, 50%, 25%, or 0%. If the overlap rate Rr is set to 75% and then increased by 0.5mm, the evaluation will be "○" for no banding, "×" for drum ghosting, or "△" for reduced banding. Similarly, if the overlap rate Rr is set to 50% and then increased by 0.5mm, the evaluation will be "○" for no banding, "△" for reduced banding, or "×" for banding, and "△" for reduced drum ghosting or no banding. Furthermore, if the overlap rate Rr is set to 25% or 0% and then increased by 0.5mm, the evaluation will be "○" for no banding, "△" for reduced banding, or "×" for banding, and "○" for no drum ghosting.

[0080] Next, under the conditions of Experiment 2-3, as shown in Tables H41 and H42 in Figures 8(A) and (B), the indentation amount Kr = 1.0 mm, and the offset amount F was changed in stages to 1.0 mm, 2.0 mm, 3.0 mm, 4.0 mm, 5.0 mm, 6.0 mm, 7.0 mm, and 8.0 mm. For each offset amount F, the overlap rate Rr was changed in stages to 75%, 50%, 25%, and 0%, and the occurrence of banding and drum ghosting was evaluated.

[0081] In Tables H41 and H42 of Figures 8(A) and (B), similar to Tables H31 and H32 of Figures 7(A) and (B), the presence or absence of banding and drum ghosting is not evaluated for overlap rate Rr=100% and offset amount F=0mm.

[0082] In Tables H41 and H42 of Figures 8(A) and (B), when the offset amount F is set to 1.0 mm and the overlap rate Rr is set to 75%, and the apex 1c of the primary transfer roller 31 is pushed 1.0 mm into the photoreceptor drum 4 side via the intermediate transfer belt 5, the absence of banding is evaluated as "○" and the presence of drum ghosting is evaluated as "×".

[0083] Furthermore, in Tables H41 and H42 of Figures 8(A) and (B), when the offset amount F is set to 2.0 mm and the overlap rate Rr is set to 75% or 50%, and the apex 1c of the primary transfer roller 31 is moved 1.0 mm towards the photoreceptor drum 4 via the intermediate transfer belt 5, the absence of banding is evaluated as "○", and the presence of drum ghosting is evaluated as "×", or reduction of drum ghosting as "△".

[0084] Furthermore, in Tables H41 and H42 of Figures 8(A) and (B), the presence or absence of banding and ghosting is evaluated for each combination of offset amount F = 3.0 mm, 4.0 mm, 5.0 mm, 6.0 mm, 7.0 mm, or 8.0 mm and overlap rate Rr = 75%, 50%, 25%, or 0%. For any of the offset amounts F = 3.0 mm, 4.0 mm, 5.0 mm, 6.0 mm, 7.0 mm, or 8.0 mm, when the apex 1c of the primary transfer roller 31 is moved 1.0 mm towards the photoreceptor drum 4 side via the intermediate transfer belt 5 from a state where the overlap rate Rr was set to 75%, 50%, 25%, or 0%, the presence or absence of banding is evaluated as "○" (no banding) or "△" (reduced banding), and the presence or absence of drum ghosting is evaluated as "×" (due to drum ghosting), "△" (reduced banding), or "○" (no banding).

[0085] Next, under the conditions of Experiment 2-4, as shown in Tables H51 and H52 in Figures 9(A) and (B), the indentation amount Kr was set to 1.5 mm, and the offset amount F was gradually changed to 1.0 mm, 2.0 mm, 3.0 mm, 4.0 mm, 5.0 mm, 6.0 mm, 7.0 mm, and 8.0 mm. For each offset amount F, the overlap rate Rr was gradually changed to 75%, 50%, 25%, and 0%, and the occurrence of banding and drum ghosting was evaluated.

[0086] In Tables H51 and H52 of Figures 9(A) and (B), similar to Tables H41 and H42 of Figures 8(A) and (B), the presence or absence of banding and drum ghosting is not evaluated for overlap rate Rr=100% and offset amount F=0mm.

[0087] In Tables H51 and H52 of Figures 9(A) and (B), when the offset amount F is set to 1.0 mm and the overlap rate Rr is set to 75%, and the apex 1c of the primary transfer roller 31 is pushed 1.5 mm into the photoreceptor drum 4 side via the intermediate transfer belt 5, the occurrence of banding is evaluated as "○" (no banding) and the occurrence of drum ghosting is evaluated as "×" (drum ghosting).

[0088] Furthermore, in Tables H51 and H52 of Figures 9(A) and (B), when the offset amount F is set to 2.0 mm and the overlap rate Rr is set to 75% or 50%, and the apex 1c of the primary transfer roller 31 is driven 1.5 mm into the photoreceptor drum 4 side via the intermediate transfer belt 5, the absence of banding is evaluated as "○", and the presence of drum ghosting is evaluated as "×", or reduction of drum ghosting as "△".

[0089] Furthermore, in Tables H51 and H52 of Figures 9(A) and (B), the presence or absence of banding and ghosting is evaluated for each combination of offset amount F = 3.0 mm, 4.0 mm, 5.0 mm, 6.0 mm, 7.0 mm, or 8.0 mm and overlap rate R = 75%, 50%, 25%, or 0%. For any of the offset amounts F = 3.0 mm, 4.0 mm, 5.0 mm, 6.0 mm, 7.0 mm, or 8.0 mm, when the apex 1c of the primary transfer roller 31 is moved 1.5 mm towards the photoreceptor drum 4 side via the intermediate transfer belt 5 from a state where the overlap rate Rr was set to 75%, 50%, 25%, or 0%, the presence or absence of banding is evaluated as "○" (no banding) or "△" (reduced banding), and the presence or absence of drum ghosting is evaluated as "×" (due to drum ghosting), "△" (reduced banding), or "○" (no banding).

[0090] Here, comparing Tables H51 and H52 in Figures 9(A) and (B), we focus on the overlap rate Rr at which the presence or absence of banding is "○" or "△" and the presence or absence of drum ghosting is "○" or "△". Similarly, comparing Tables H31 and H32 in Figures 7(A) and (B), we focus on the overlap rate Rr at which the presence or absence of banding is "○" or "△" and the presence or absence of drum ghosting is "○" or "△". When the apex 1c of the primary transfer roller 31 is made to bite into the surface from a state where the overlap rate Rr is set to greater than 0% and 50% or less, or preferably greater than 0% and 40% or less, it can be seen that it is best to set the offset amount F in the range of 3.0 mm to 7.0 mm and the biting amount Kr in the range of greater than 0 mm and 0.5 mm or less.

[0091] Furthermore, by comparing Tables H21 and H22 in Figures 6(A) and (B), and focusing on the overlap rate Rr at which the presence or absence of banding is "○" or "△" and the presence or absence of drum ghosting is "○" or "△", and by comparing Tables H41 and H42 in Figures 8(A) and (B), and focusing on the overlap rate Rr at which the presence or absence of banding is "○" or "△" and the presence or absence of drum ghosting is "○" or "△", it can be seen that when the apex 1c of the primary transfer roller 31 is made to bite into the surface from a state where the overlap rate Rr is set to greater than 0% and 50% or less, or preferably greater than 0% and 30% or less, it is good to set the offset amount F in the range of 3.0 mm to 7.0 mm and the biting amount Kr in the range of greater than 0 mm and 1.0 mm or less.

[0092] Furthermore, by comparing Tables H21 and H22 in Figures 6(A) and (B), and focusing on the overlap rate Rr at which the presence or absence of banding is "○" or "△" and the presence or absence of drum ghosting is "○" or "△", and by comparing Tables H51 and H52 in Figures 9(A) and (B), and focusing on the overlap rate Rr at which the presence or absence of banding is "○" or "△" and the presence or absence of drum ghosting is "○" or "△", it can be seen that when the apex 1c of the primary transfer roller 31 is made to bite into the overlap rate Rr set to greater than 0% and 25% or less, or preferably greater than 0% and 20% or less, it is good to set the offset amount F in the range of 3.0 mm to 7.0 mm and the biting amount Kr in the range of greater than 0 mm and 1.5 mm or less.

[0093] By appropriately setting the biting amount Kr according to the overlap rate Rr and offset amount F in this way, it is possible to determine whether or not banding occurs ("○" or "△") and whether or not drum ghosting occurs ("○" or "△").

[0094] Figure 10(A) shows an image of drum ghosting on the surface of the photoreceptor drum 4 when the offset amount F is 0 mm, and it can be seen that drum ghosting is clearly occurring. Figure 10(B) shows an image of drum ghosting on the surface of the photoreceptor drum 4 when the offset amount F is 4.0 mm and the overlap rate Rr is 50% or more, and it can be seen that the drum ghosting has been reduced. Figure 10(C) shows an image of drum ghosting on the surface of the photoreceptor drum 4 when the offset amount F is 4.0 mm and the overlap rate Rr is 25% or less, and it can be seen that the drum ghosting has been eliminated.

[0095] Figure 11 is a graph showing the maximum pressure PM for a given load N when the apex 1c of the primary transfer roller 31 is bitten into the photoreceptor drum 4 side via the intermediate transfer belt 5, with the horizontal axis showing the load N of the primary transfer roller 31 due to the biasing force of the spring 33, and the vertical axis showing the maximum pressure PM in the nip region NP1. The offset amount F is set to 0 mm, 2.0 mm, 4.0 mm, and 6.0 mm, and the overlap rate Rr is set to a range greater than 0% and less than or equal to 25%, and the biting amount Kr is set to a range greater than 0 mm and less than or equal to 0.5 mm. As is clear from the graph in Figure 11, the maximum pressure PM in the nip region NP1 decreases as the offset amount F increases. This suggests that the pressure is distributed throughout the entire nip region NP1. As a result, the load on the photoreceptor drum 4 is reduced, damage to the photoreceptor drum 4 and the intermediate transfer belt 5 is suppressed, and their lifespan is improved.

[0096] Figure 12 is a graph showing the maximum pressure PM in the nip region NP1 on the horizontal axis and the Mottle index, which quantifies image graininess, on the vertical axis, with the Mottle index relative to the maximum pressure PM when the offset amount F is set to 0 mm, 2.0 mm, 4.0 mm, and 6.0 mm. The lower the Mottle index, the better the image quality, so it is preferable to set the Mottle index lower than 1. As is clear from the graph in Figure 12, if the maximum pressure PM in the nip region NP1 is 0.15 or less, the Mottle index can be set lower than 1. This is thought to be because the adhesion force of the toner on the surface of the intermediate transfer belt 5 is reduced, improving image quality.

[0097] Referring to Figure 11, it can be seen that in order to keep the maximum pressure PM in the nip region NP1 below 0.15, it is necessary to appropriately adjust the load N applied to the primary transfer roller 31 by the biasing force of the spring 33 according to the size of the recording paper (width of the intermediate transfer belt 5), as described above, and then set the offset amount F to 0 mm, 2.0 mm, 4.0 mm, and 6.0 mm.

[0098] From this, it can be seen that when the apex 1c of the primary transfer roller 31 is driven into the photoreceptor drum 4 side via the intermediate transfer belt 5 from a state in which the second contact area 4S and a part of the first contact area 1S are overlapped, for example, when the overlap area Rs is set to less than the maximum value Vs, the offset amount F is set to 0.5 mm or more and 6.0 mm or less. Furthermore, considering the results of experiments 2-2 to 2-4 shown in Figures 6(A), (B) to 8(A), (B), it can be seen that when the offset amount F is set to the range of 3.0 mm ≤ F6.0 mm, the adhesion force of the toner on the surface of the intermediate transfer belt 5 is reduced, the image quality is improved, the load on the photoreceptor drum 4 is reduced, damage to the photoreceptor drum 4 and the intermediate transfer belt 5 is suppressed, and their lifespan is improved.

[0099] In the above embodiment, in the direction of movement A of the intermediate transfer belt 5, one upstream end 1a of the second contact area 1S is positioned upstream of one downstream end 4b of the first contact area 4S, and the center 1x of the primary transfer roller 31 is spaced downstream of the center 4x of the photoreceptor drum 4. However, instead, one downstream end 1b of the second contact area 1S may be positioned downstream of one upstream end 4a of the first contact area 4S, and the center 1x of the primary transfer roller 31 may be spaced upstream of the center 4x of the photoreceptor drum 4, causing the upstream portion of the first contact area 4S (nip area NP1) of the photoreceptor drum 4 and the downstream portion of the second contact area 1S of the primary transfer roller 31 to overlap. In this case as well, similar to the above embodiment, even without increasing the spring load applied to the primary transfer roller 31, image defects are less likely to occur, and the mechanical life of the photoreceptor drum 4 and the intermediate transfer belt can be extended. Furthermore, similar to Experiments 1 and 2, the effect can be reliably achieved by appropriately setting the offset amount F, overlap area Rs, and penetration amount Kr.

[0100] Furthermore, the configuration described using Figures 1 to 12 is merely one embodiment of the present invention, and is not intended to limit the present invention to this configuration. [Explanation of Symbols]

[0101] 1. Image forming apparatus 11 Image reading unit 12 Image forming unit 20 Intermediate Transfer Unit 31 Primary Transfer Roller 4. Photoconductor drum 5. Intermediate transfer belt 6. Secondary transfer roller 33 Springs

Claims

1. A photoreceptor drum that carries an electrostatic latent image, and in which the electrostatic latent image is developed into a toner image by the application of toner, An intermediate transfer belt that moves while in contact with the photosensitive drum, A primary transfer roller is provided on the side opposite to the photoreceptor drum with respect to the intermediate transfer belt, and presses the intermediate transfer belt against the photoreceptor drum to transfer the toner image from the photoreceptor drum to the intermediate transfer belt. When the contact area of ​​the photoreceptor drum with respect to the intermediate transfer belt is defined as the first contact area, and the contact area of ​​the primary transfer roller with respect to the intermediate transfer belt is defined as the second contact area, in the direction of movement of the intermediate transfer belt, the second contact area and a part of the first contact area are made to overlap each other. An image forming apparatus in which, in a direction in which the photoreceptor drum and the primary transfer roller are aligned with the intermediate transfer belt in between, the apex of the primary transfer roller that protrudes most towards the photoreceptor drum is embedded into the photoreceptor drum via the intermediate transfer belt.

2. The image forming apparatus according to claim 1, wherein the amount by which the apex of the primary transfer roller penetrates the photoreceptor drum side with respect to the amount of overlap is set to a predetermined appropriate range.

3. The image forming apparatus according to claim 2, wherein the amount of overlap is set to 40% or less of the maximum value of the amount of overlap, and the appropriate range of the amount of penetration is set to a range greater than 0 mm and less than or equal to 0.5 mm.

4. The image forming apparatus according to claim 2, wherein the amount of overlap is set to 30% or less of the maximum value of the amount of overlap, and the appropriate range of the amount of penetration is set to a range of more than 0 mm and 1.0 mm or less.

5. The image forming apparatus according to claim 2, wherein the amount of overlap is set to 20% or less of the maximum value of the amount of overlap, and the appropriate range of the amount of penetration is set to a range of more than 0 mm and 1.5 mm or less.

6. Furthermore, the image forming apparatus according to claim 1, wherein the offset amount F, which is the distance between the rotation center of the primary transfer roller and the rotation center of the photoreceptor drum in the direction of movement of the intermediate transfer belt, is set to a range of 0.5 mm ≤ F6.0 mm.

7. Furthermore, the image forming apparatus according to claim 1, wherein the offset amount F, which is the distance between the rotation center of the primary transfer roller and the rotation center of the photoreceptor drum in the direction of movement of the intermediate transfer belt, is set to a range of 3.0 mm ≤ F6.0 mm.

8. The image forming apparatus according to any one of claims 1 to 7, wherein, in the direction of movement of the intermediate transfer belt, the rotation center of the primary transfer roller is spaced downstream from the rotation center of the photoreceptor drum.

9. The image forming apparatus according to any one of claims 1 to 7, wherein, in the direction of movement of the intermediate transfer belt, the rotation center of the primary transfer roller is spaced upstream of the rotation center of the photoreceptor drum.

10. Furthermore, if the current flowing between the primary transfer roller and the photoreceptor drum when a transfer bias is applied to the primary transfer roller is denoted as the transfer current It, The image forming apparatus according to any one of claims 1 to 7, wherein the transfer current It is set to the range of │2.0 μA│ ≤ It│40.0 μA│.

11. The system further includes a biasing unit that biases the primary transfer roller to press it against the intermediate transfer belt, The image forming apparatus according to any one of claims 1 to 7, wherein the load applied by the biasing unit to the primary transfer roller is set to be 0.6 N or more and 3.0 N or less.

12. The system further includes a biasing unit that biases the primary transfer roller to press it against the intermediate transfer belt, The image forming apparatus according to any one of claims 1 to 7, wherein the load applied to the primary transfer roller by the biasing unit is set to be 0.6 N or more and 1.4 N or less.

13. Furthermore, the image forming apparatus according to any one of claims 1 to 7, wherein when an elastic belt is used as the intermediate transfer belt, the thickness of the intermediate transfer belt is set to 30 μm or more and 400 μm or less, and when a resin belt is used as the intermediate transfer belt, the thickness of the intermediate transfer belt is set to 30 μm or more and 150 μm or less.

14. Furthermore, the image forming apparatus according to any one of claims 1 to 7, wherein the tension of the intermediate transfer belt is set to 15 N or more and 45 N or less.

Images