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 from 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 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 A of movement of the intermediate transfer belt 5, is set to a predetermined appropriate range relative to the amount of overlap.

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 sheet.

[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, scattering of toner and generation 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 intermediate transfer belt in the nip area tends to become unstable, causing image defects (banding) 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 the rotational direction of the photoreceptor drum while 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 rotates in the direction of movement of the photoreceptor drum while pressing the intermediate transfer belt against the photoreceptor drum, and transfers the toner image of the photoreceptor drum from the photoreceptor drum to the intermediate transfer belt, wherein 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, and in the direction of movement of the intermediate transfer belt, a part of the second contact area and a part of the first contact area are overlapped with each other. [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, and (B) is a table showing whether or not drum ghosting occurred in Experiment 2. [Figure 7] Figures (A) to (C) show drum ghosting that occurred on the surface of the photosensitive drum. [Figure 8] 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 9] 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 the respective 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 the 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, for the primary transfer roller 31, a conductive roller using a conductive rubber component or the like is used. The primary transfer roller 31 is composed of a cylindrical 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 arranged 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 knurling.

[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, for each photosensitive drum 4, a developing unit 26, a drum cleaning unit 27, a charging unit 28, etc. are provided. 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] Figure 3 is a schematic diagram showing an enlarged view of the intermediate transfer belt 5 and the set of photoreceptor drums 4 and primary transfer rollers 31 in Figure 2. 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, the set of photoreceptor drums 4 and primary transfer rollers 31 are positioned upstream of the secondary transfer rollers 6 (not shown).

[0022] 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. In this case, 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 arranged to overlap. Overlap here refers to a state in which parts of the second contact area 1S and 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".

[0023] In detail, in the direction of movement A of the intermediate transfer belt 5, one upstream end of the first contact area 4S is designated as 4a and one downstream end as 4b, and one upstream end of the second contact area 1S is designated as 1a and one downstream end as 1b. At this time, the upstream end 1a of the second contact area 1S is positioned upstream of the downstream end 4b of the first contact area 4S. Also, the rotation center of the photoreceptor drum 4 is designated as 4x, and the rotation center of the primary transfer roller 31 is designated as 1x. At this time, 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. When the distance between the rotation center 4x and the rotation center 1x is defined as the offset amount F, the offset amount F is set to a value greater than "0". Therefore, the downstream portion of the first contact area and the upstream portion of the second contact area of ​​the primary transfer roller 31 overlap.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] Thus, in this embodiment, by overlapping the downstream portion of the first contact area 4S (nip area NP1) with the upstream portion of the second contact area 1S, it is possible to improve the reduction of image defects, image quality, and extension of the mechanical life of the photoreceptor drum 4 and intermediate transfer belt without increasing the spring load applied to the primary transfer roller 31.

[0028] Furthermore, in this embodiment, in order to further improve image quality by suppressing banding and other issues associated with the rotation of the intermediate transfer belt 5, and to more reliably obtain the effect of preventing a decrease in the mechanical life of the photoreceptor drum 4 and the intermediate transfer belt 5, the offset amount, 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 movement direction A of the intermediate transfer belt 5, is set to a predetermined appropriate range with respect to the overlap amount when the second contact area 1S and the first contact area 4S are overlapped in the movement direction A of the intermediate transfer belt 5.

[0029] These effects will be explained later by Experiments 1 and 2.

[0030] <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.

[0031] Furthermore, as will be clear from Experiment 2, which will be described later, when 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, as shown in Figure 3, is defined as the overlap area R, when the overlap area Rs of the overlap area R is set to exceed 0% and not exceeding 50% of the maximum value RM of the overlap area Rs, the predetermined appropriate range of the offset amount F is set to 4.0 mm or more and 6.0 mm or less.

[0032] As described above, overlap can suppress banding, but if the overlap is too large, the drum ghost performance may deteriorate. For further improvement in image quality, it is preferable to set the overlap area Rs and the offset amount F to a combination that falls within an appropriate range. By setting the overlap area Rs and the offset amount F within the above range, it is possible to achieve both suppression of banding and improvement of drum ghost performance.

[0033] Alternatively, as will be evident from Experiment 2, which will be discussed later, setting the overlap area Rs to be greater than 0% of the maximum value RM and less than or equal to 25%, and setting the predetermined appropriate range of the offset amount F to be between 3.0 mm and 6.0 mm, allows for a more reliable balance between suppressing banding and improving drum ghost performance.

[0034] Here, 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, the overlap area Rs reaches its maximum value RM. Also, the further the primary transfer roller 31 moves away from that same position, the smaller the overlap area Rs becomes.

[0035] 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 2.0 mm ≤ F6.0 mm.

[0036] Preferably, as will be evident from Experiment 2 described later, it is best to set the offset amount F in the range of 3.0 mm ≤ F5.0 mm, which will more reliably improve image quality and extend the machine's lifespan.

[0037] 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. Therefore, 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 offset amount F shown above.

[0038] 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.

[0039] Furthermore, the combination of (i) overlap area Rs (an example of overlap amount) and offset amount F shown above, or (ii) the above combination and load N, plus any of the following may be set as appropriate.

[0040] 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.

[0041] Furthermore, the tension of the intermediate transfer belt 5 is set to be between 15N and 45N.

[0042] 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│.

[0043] 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>

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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).

[0052] 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.

[0053] 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.

[0054] 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 overlap as described above, and the offset amount F is set 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, ensuring the pressure necessary for toner transfer to the nip area NP1, preventing banding, and extending the mechanical life of the photoreceptor drum 4 and the intermediate transfer belt. <Experiment 2>

[0055] In Experiment 2, the offset amount F and overlap area Rs were changed in stages to evaluate the occurrence of banding and drum ghosting. Drum ghosting is a phenomenon in which the previously transferred image remains on the surface of the photoreceptor drum 4, and this remaining image is superimposed on the next image; it is also called transfer memory.

[0056] Under the conditions of Experiment 2, 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.

[0057] Furthermore, under the conditions of Experiment 2, 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.

[0058] Furthermore, under the conditions of Experiment 2, the tension of the intermediate transfer belt 5 was set to 25N, similar to Experiment 1.

[0059] Furthermore, under the conditions of Experiment 2, 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.

[0060] On the other hand, in Experiment 2, as shown in Tables H21 and H22 in Figures 6(A) and (B), the offset amount F was changed in stages from 0 mm, 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, and for each offset amount F, the overlap rate Rr was changed in stages from 100%, 75%, 50%, 25%, and 0%, and the occurrence of banding and drum ghosting was evaluated. "〇" indicates no banding or no drum ghosting, "△" indicates reduced banding or reduced drum ghosting, and "×" indicates banding or drum ghosting.

[0061] 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.

[0062] For offset amounts F = 0mm, 1.0mm, 2.0mm, 3.0mm, 4.0mm, 5.0mm, 6.0mm, 7.0mm, and 8.0mm, 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.

[0063] Tables H21 and H22 in Figures 6(A) and (B) evaluate banding and ghosting when the offset amount F is set to 0 mm and the overlap rate Rr is 100%. 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 overlap area R of the second contact area 1S that overlaps the first contact area 4S becomes linear. Since the overlap area Rs cannot be changed, only an overlap rate Rr of 100% is listed.

[0064] Furthermore, Tables H21 and H22 evaluate banding and ghosting when the offset amount F is set to 1.0 mm and the overlap rate Rr is 100% and 75%, respectively. When the offset amount F is set to 1.0 mm, the photoreceptor drum 4 and the primary transfer roller 31 are either in contact with the same position on the intermediate transfer belt 5, or the primary transfer roller 31 is only slightly away from that same position, so the overlap area Rs can be slightly changed, and the overlap rates Rr are listed as 100% and 75%.

[0065] Furthermore, Tables H21 and H22 evaluate banding and ghosting when the offset amount F is set to 2.0 mm and the overlap rate Rr is 100%, 75%, and 50%. When the offset amount F is set to 2.0 mm, the photoreceptor drum 4 and the primary transfer roller 31 either contact the same position on the intermediate transfer belt 5, or the primary transfer roller 31 can be moved further away from that same position than when the offset amount F = 1.0 mm, so the overlap area Rs changes, and the overlap rates Rr are listed as 100%, 75%, and 50%.

[0066] Tables H21 and H22 evaluate banding and ghosting when the offset amount F is set to 3.0 mm, 4.0 mm, 5.0 mm, 6.0 mm, 7.0 mm, or 8.0 mm, and the overlap rate Rr is 100%, 75%, 50%, 25%, and 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 either contact the same position on the intermediate transfer belt 5, or the primary transfer roller 31 can be moved significantly away from that same position, so the overlap area Rs changes significantly, and the overlap rate Rr is listed as 100%, 75%, 50%, 25%, and 0%.

[0067] As is clear from Table H21, when the offset amount F is 0 mm or more and 6.0 mm or less, and the overlap rate Rr exceeds 0%, banding is reduced ("△") or not occurred ("〇").

[0068] Furthermore, as is clear from Table H22, when the offset amount F is between 2.0 mm and 8.0 mm, and the overlap rate Rr is 50% or less, drum ghosting is reduced ("△") or absent ("〇").

[0069] Figure 7(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 7(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 7(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.

[0070] Comparing Table H21 and Table H22, and focusing on the overlap rate Rr at which banding is present or absent ("○" or "△") and drum ghosting is present or absent ("○" or "△"), it can be seen that when the offset amount F is set to 4.0 mm or more and 6.0 mm or less, and the overlap rate Rr is set to greater than 0% and 50% or less, banding is reduced ("△") or absent ("○"), and drum ghosting is absent.

[0071] Alternatively, when the offset amount F is set to 3.0 mm or more and 6.0 mm or less, and the overlap rate Rr is set to greater than 0% and 25% or less, it can be seen that the occurrence of banding is reduced ("△") or eliminated ("〇"), and drum ghosting is eliminated.

[0072] Figure 8 is a graph showing the maximum pressure PM for a given load N, where the horizontal axis represents the load N of the primary transfer roller due to the biasing force of the spring 33, and the vertical axis represents the maximum pressure PM in the nip region NP1, when the offset amount F is set to 0 mm, 2.0 mm, 4.0 mm, or 6.0 mm, and the overlap rate Rr is set to a range greater than 0% and less than or equal to 25%. As is clear from the graph in Figure 8, as the offset amount F increases, the maximum pressure PM in the nip region NP1 decreases. This suggests that the pressure is distributed across 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.

[0073] Figure 9 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 9, 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.

[0074] Referring to Figure 8, 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 2.0 mm, 4.0 mm, and 6.0 mm.

[0075] From this, it can be seen that when the offset amount F is set to 2.0 mm or more and 6.0 mm or less, and further considering the results of Experiment 2 shown in Figures 6(A) and (B), when the offset amount F is set to 3.0 mm or more and 5.0 mm or less, the adhesion force of toner on the surface of the intermediate transfer belt 5 is reduced, improving image quality, reducing the load on the photoreceptor drum 4, suppressing damage to the photoreceptor drum 4 and the intermediate transfer belt 5, and improving their lifespan.

[0076] 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 overlap ratio Rr.

[0077] Furthermore, the configuration described using Figures 1 to 9 is merely one embodiment of the present invention, and is not intended to limit the present invention to this configuration. [Explanation of Symbols]

[0078] 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 in the rotational direction of the photoreceptor drum while in contact with the photoreceptor drum, A primary transfer roller is provided on the side opposite to the photoreceptor drum with respect to the intermediate transfer belt, and rotates in the direction of movement of the photoreceptor drum while pressing the intermediate transfer belt against the photoreceptor drum, thereby transferring the toner image on the photoreceptor drum from the photoreceptor drum to the intermediate transfer belt. An image forming apparatus in which, 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, a portion of the second contact area and a portion of the first contact area are overlapped in the direction of movement of the intermediate transfer belt.

2. The image forming apparatus according to claim 1, wherein the amount of offset, 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 predetermined appropriate range with respect to the amount of overlap.

3. The image forming apparatus according to claim 2, wherein the predetermined appropriate range of the offset amount is set to a range of 2.0 mm or more and 6.0 mm or less.

4. The amount of overlap is 50% or less of the maximum amount of overlap when the photoreceptor drum and the primary transfer roller contact the intermediate transfer belt at the same position in the direction of movement of the intermediate transfer belt. The image forming apparatus according to claim 3, wherein the predetermined appropriate range of the offset amount is set to 4.0 mm or more and 6.0 mm or less.

5. The amount of overlap is set to 25% or less of the maximum value of the overlap amount when the photoreceptor drum and the primary transfer roller contact the intermediate transfer belt at the same position in the direction of movement of the intermediate transfer belt. The image forming apparatus according to claim 3, wherein the predetermined appropriate range of the offset amount is set to 3.0 mm or more and 6.0 mm or less.

6. The image forming apparatus according to any one of claims 1 to 5, 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.

7. The image forming apparatus according to any one of claims 1 to 5, wherein, in the direction of movement of the intermediate transfer belt, the rotation center of the primary transfer roller is spaced upstream from the rotation center of the photoreceptor drum.

8. 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 5, wherein the transfer current It is set to the range of │2.0 μA│ ≤ It│40.0 μA│.

9. 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 5, 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.

10. 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 claim 9, 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.

11. Furthermore, the image forming apparatus according to any one of claims 1 to 5, 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.

12. Furthermore, the image forming apparatus according to any one of claims 1 to 5, wherein the tension of the intermediate transfer belt is set to 15 N or more and 45 N or less.

Images