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 which is a metal roller. 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 arranged apart in the direction of movement of the intermediate transfer belt 5, and in the direction in which the photoreceptor drum 4 and the primary transfer roller 31 are aligned with respect to the intermediate transfer belt 5, the apex of the primary transfer roller 31 that protrudes the most towards the photoreceptor drum 4 is embedded into the photoreceptor drum 4 side via the intermediate transfer belt 5.

Description

Technical Field

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[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, 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 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, for example, 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, wherein the primary transfer roller is a metal roller, and 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, the second contact area and the first contact area are arranged so as not to overlap in the direction of movement of the intermediate transfer belt, 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 into the photoreceptor drum side 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] This is a diagram showing the experimental conditions. [Figure 6] (A) is a table showing whether or not banding occurred in Experiment 1-1, and (B) is a table showing whether or not drum ghosting occurred in Experiment 1. [Figure 7] (A) is a table showing whether or not banding occurred in Experiment 1-2, and (B) is a table showing whether or not drum ghosting occurred in Experiment 1. [Figure 8] (A) is a table showing whether or not banding occurred in Experiments 1-3, and (B) is a table showing whether or not drum ghosting occurred in Experiment 1. [Figure 9] (A) is a table showing whether or not banding occurred in Experiments 1-4, and (B) is a table showing whether or not drum ghosting occurred in Experiment 1. [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 driving 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 driving 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 driving 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] The primary transfer roller 31 is a metal roller made of SUM (free-cutting steel), SUS (stainless steel), or aluminum. However, the material of the primary transfer roller 31 is not intended to be limited to these. Further, the primary transfer roller 31 may be a metal roller having a surface treatment by an oxide film (for example, anodizing treatment, etc.), plating (for example, electroless nickel plating, etc.), or an insulating paint (for example, an acrylic resin or a polyurethane resin, etc.).

[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 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 is pushed toward the photoreceptor drum 4 side through 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 photoreceptor drum 4. Each photoreceptor drum 4 is rotationally driven in the direction of the arrow. As the photoreceptor drum 4 rotates, the surface of the photoreceptor drum 4 is uniformly charged by the charging unit 28, and the surface of the photoreceptor drum 4 is exposed by an exposure device (not shown), so that an electrostatic latent image is formed on the surface of the photoreceptor drum 4. Toner is applied to the electrostatic latent image on the surface of the photoreceptor 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 photoreceptor drum 4 is primarily transferred to the intermediate transfer belt 5 by the primary transfer roller 31. After that, the surface of the photoreceptor drum 4 is discharged, and the residual toner on the surface of the photoreceptor 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 photoreceptor 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 we consider a photoreceptor drum 4 and a primary transfer roller 31 facing the photoreceptor drum 4 via an intermediate transfer belt 5 as one set, then four sets of photoreceptor drums 4 and primary transfer rollers 31 are provided. Each set of photoreceptor drums 4 and primary transfer rollers 31 is positioned upstream of the secondary transfer rollers 6 in the direction A of movement of the intermediate transfer belt 5.

[0021] Figure 3 is a schematic diagram showing an enlarged view of the intermediate transfer belt 5, a set of photoreceptor drums 4, and primary transfer rollers 31. Figure 4 is a partially enlarged view of Figure 3 showing the first and second contact areas.

[0022] When 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, the second contact area 1S and the first contact area 4S are positioned so as not to overlap in the direction of movement of the intermediate transfer belt 5. In Figures 3 and 4, the distance between the second contact area 1S and the first contact area 4S in the direction of movement A is shown as "R".

[0023] Furthermore, in the direction (up and down in Figure 3) 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 positioned so that it is embedded into the photoreceptor drum 4 via the intermediate transfer belt 5.

[0024] Furthermore, in this embodiment, the amount by which the apex of the primary transfer roller 31 bites into the photoreceptor drum 4 side with respect to 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 intermediate transfer belt 5 movement direction A, is set to a predetermined appropriate range.

[0025] In more detail, in the direction of movement A of the intermediate transfer belt 5, if one upstream end of the first contact area 4S is 4a and one downstream end is 4b, and one upstream end of the second contact area 1S is 1a and one downstream end is 1b, then the upstream end 1a of the second contact area 1S is positioned downstream of the downstream end 4b of the first contact area 4S. Furthermore, if the rotation center of the photoreceptor drum 4 is 4x and the rotation center of the primary transfer roller 31 is 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, and 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".

[0026] The first contact area 4S is formed by the primary transfer roller 31 pressing the intermediate transfer belt 5 against the photoreceptor drum 4. 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.

[0027] Furthermore, the second contact area 1S is the area where the primary transfer roller 31, to which a transfer bias is applied, presses the intermediate transfer belt 5 against the photoreceptor drum 4. 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.

[0028] Current flows between the primary transfer roller 31, to which a transfer bias is applied, and the photoreceptor drum 4, which is positioned opposite it, via the intermediate transfer belt 5. At this time, a certain voltage is required between the primary transfer roller 31 and the photoreceptor drum 4 in order to transfer the toner image on the surface of the photoreceptor drum 4 to the intermediate transfer belt 5. However, if, for example, the first contact area 4S (nip area NP1) of the photoreceptor drum 4 and the second contact area 1S of the primary transfer roller 31 are positioned at the same location in the direction of movement A, the primary transfer roller 31 and the photoreceptor drum 4 will be in contact with the intermediate transfer belt 5 in between, and since the primary transfer roller 31 is a metal roller, the conductivity between the primary transfer roller 31 and the photoreceptor drum 4 via the intermediate transfer belt 5 will be good and the resistance will be low, making it difficult to generate a voltage between the primary transfer roller 31 and the photoreceptor drum 4 sufficient to transfer the toner image.

[0029] Therefore, in this embodiment, as described above, 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 spaced apart. However, the opposing primary transfer roller 31 and photoreceptor drum 4 are in contact with the intermediate transfer belt 5. By increasing the distance between the opposing primary transfer roller 31 and photoreceptor drum 4, the resistance between the primary transfer roller 31 and photoreceptor drum 4 when a transfer bias is applied is made greater than the resistance when the primary transfer roller 31 and photoreceptor drum 4 are in contact with the intermediate transfer belt 5 in between. That is, since current flows from the spaced-apart primary transfer roller 31 to the photoreceptor drum 4 via the intermediate transfer belt 5, the resistance generated by the intermediate transfer belt 5 becomes greater than the resistance when the primary transfer roller 31 and photoreceptor drum 4 are in contact with the intermediate transfer belt 5 in between. By utilizing the resistance generated by the intermediate transfer belt 5, a voltage sufficient to enable the transfer of the toner image is generated between the primary transfer roller 31 and the photoreceptor drum 4.

[0030] Furthermore, if 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 spaced apart, the surface of the intermediate transfer belt 5 running between the primary transfer roller 31 and the photoreceptor drum 4 may flutter, potentially hindering the transfer of the toner image. Therefore, in this embodiment, 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 most towards the photoreceptor drum 4 is positioned so that it is embedded in the photoreceptor drum 4 via the intermediate transfer belt 5. This configuration suppresses fluttering of the surface of the intermediate transfer belt 5 running between the primary transfer roller 31 and the photoreceptor drum 4.

[0031] Furthermore, when adopting this configuration that allows the primary transfer roller 31 to bite into the intermediate transfer belt 5, the pressure applied by the primary transfer roller 31 can be efficiently increased without significantly increasing the load applied to the primary transfer roller 31 by the biasing force of the spring 33. This stabilizes the pressure applied by the intermediate transfer belt 5 to the photoreceptor drum 4 in the nip region NP1. As a result, the load on the photoreceptor drum 4 and the intermediate transfer belt 5 is reduced, extending the machine's lifespan.

[0032] Furthermore, if the amount of penetration is excessive, the primary transfer roller 31 pushes the intermediate transfer belt 5 too far towards the photoreceptor drum 4, causing the intermediate transfer belt 5 to adhere excessively to the surface of the photoreceptor drum 4. The surface of the photoreceptor drum 4 is preferably a smooth, arc-shaped plane, but depending on the manufacturing precision of the photoreceptor drum 4, there may be some unevenness on its surface. In such cases, if the intermediate transfer belt 5 adheres too closely to the surface of the photoreceptor drum 4, the unevenness on the drum surface can deform the intermediate transfer belt 5 as it runs in contact with the photoreceptor drum 4, causing the surface of the intermediate transfer belt 5 to become uneven. When this occurs, the toner image on the photoreceptor drum 4 is not transferred to the intended position on the intermediate transfer belt 5, and does not overlap with the toner images of other colors transferred on the intermediate transfer belt 5 in the correct position, causing so-called color misalignment. In addition, the larger the offset amount F, the greater the decrease and instability of the pressure in the nip area NP1, making the transfer of the toner image from the photoreceptor drum 4 to the intermediate transfer belt 5 unstable.

[0033] Therefore, in this embodiment, the occurrence of such problems is further prevented by setting the amount by which the apex of the primary transfer roller 31 bites into the photoreceptor drum 4 side with respect to the offset amount F to a predetermined appropriate range.

[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. The primary transfer roller 31 is a metal roller.

[0035] As described above, by (i) arranging 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 at a distance apart, (ii) setting the offset amount F, which is the distance between the rotation center 1x of the primary transfer roller 31 and the rotation center 4x of the photoreceptor drum 4 in the direction of movement A of the intermediate transfer belt 5, to the range of |1.0|mm ≤ F|10.0|mm, and (iii) setting the appropriate range for the amount Kr that the apex 1c of the primary transfer roller 31 bites into the photoreceptor drum 4 side to the range of 0.1mm to 1.5mm, the image quality is improved and the machine life is extended.

[0036] Alternatively, by (i) arranging 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 at a distance apart, (ii) setting the offset amount F, which is the distance between the rotation center 1x of the primary transfer roller 31 and the rotation center 4x of the photoreceptor drum 4 in the direction of movement A of the intermediate transfer belt 5, to the range of |4.0|mm ≤ F|8.0|mm, and (iii) setting the appropriate range for the amount Kr that the apex 1c of the primary transfer roller 31 bites into the photoreceptor drum 4 side to the range of 0.1mm to 1.5mm, the above-mentioned effects of improved image quality and extended machine life can be reliably obtained.

[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 can be set appropriately in addition to (i), (ii), and (iii) shown above.

[0038] For example, (iv) when 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 0.6N or more and 3.0N or less. Also, (v) when 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 0.6N or more and 1.4N or less. Also, (vi) when an elastic belt is used as the intermediate transfer belt 5, it is preferable to set the load N applied to the primary transfer roller 31 by the biasing force of each spring 33 to 0.2N or more and 5.0N or less. This ensures that the pressing force per unit area of ​​the intermediate transfer belt 5 is set appropriately. 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.

[0039] Furthermore, in addition to the conditions (i), (ii), and (iii) shown above, and the conditions that combine (i), (ii), and (iii) with any of (iv) through (vi), any of the following may be used in appropriate combinations.

[0040] 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. The resin belt is, for example, a resin belt with a coating layer on its surface. 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.

[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.

[0044] <Experiment 1> The conditions for Experiment 1 are as shown in Figure 5, and similar to the specific example of 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 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.

[0047] 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.

[0048] 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.

[0049] In Experiment 1, four experiments, 1-1, 1-2, 1-3, and 1-4, were conducted by varying the amount of penetration Kr mentioned above. The results of Experiment 1-1 are shown in Tables H21 and H22 in Figures 6(A) and (B), the results of Experiment 1-2 are shown in Tables H31 and H32 in Figures 7(A) and (B), the results of Experiment 1-3 are shown in Tables H41 and H42 in Figures 8(A) and (B), and the results of Experiment 12-4 are shown in Tables H51 and H52 in Figures 9(A) and (B).

[0050] In Experiment 1-1, the penetration depth Kr was set to 0 mm; in Experiment 1-2, it was set to 0.5 mm; in Experiment 1-3, it was set to 1.0 mm; and in Experiment 1-4, it was set to 1.5 mm.

[0051] Furthermore, in each of experiments 1-1 to 1-4, 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.

[0052] In each of experiments 1-1 to 1-4, the offset amount F was gradually changed from 0mm, 1.0mm, 2.0mm, 3.0mm, 4.0mm, 5.0mm, 6.0mm, 7.0mm, and 8.0mm. For each offset amount F, the overlap rate Rr was also gradually changed from 100%, 75%, 50%, 25%, 0%, -25%, -50%, -75%, and -100%, and the occurrence of banding and drum ghosting was evaluated.

[0053] Overlap refers to the state in which the second contact area 1S and the first contact area 4S overlap each other in the direction of movement A. 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. The overlap ratio Rr is the ratio of the overlap area Rs to the maximum value RM, with the maximum value RM of the overlap area Rs as the reference value. When the overlap ratio Rr is a negative value, it indicates that the second contact area 1S and the first contact area 4S are separated by an amount equal to the overlap area Rs corresponding to the overlap ratio Rr.

[0054] On the other hand, under the conditions of Experiment 1-1, as shown in Tables H21 and H22 in Figures 6(A) and (B), the penetration amount Kr was set to 0 mm, and the overlap rate Rr was changed stepwise for each offset amount F at each stage to evaluate the occurrence of banding and drum ghosting.

[0055] Note that in Figures 6(A)(B) to 9(A)(B), the conditions for which no numerical values ​​are listed were not included in the evaluation for this experiment.

[0056] In Figures 6(A)(B) to 9(A)(B), 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 is not evaluated.

[0057] In Experiment 1-1, Tables H21 and H22 (insertion amount Kr=0mm) in Figures 6(A) and (B) show that when the offset amount F is set to 1.0mm to 10.0mm and the overlap rate Rr is set to 0% to -100%, each stage is evaluated as follows: for banding, "×" indicates that it occurs; for drum ghosting, "○" indicates that it does not occur when the overlap rate Rr is 0%, "×" indicates that it occurs when the overlap rate Rr is -25% to -75%, and "○" indicates that it does not occur when the overlap rate Rr is -100%.

[0058] Next, under the conditions of Experiment 1-2, as shown in Tables H31 and H32 in Figures 7(A) and (B), the penetration amount Kr was set to 0.5 mm, and the overlap rate Rr was changed stepwise for each offset amount F at each stage to evaluate the occurrence of banding and drum ghosting.

[0059] In Tables H31 and H32 of Figures 7(A) and (B), for each offset amount F = 3.0 mm to 4.0 mm, when the overlap rate Rr is 0% to -100%, the evaluation is "○" indicating no banding and "○" indicating no drum ghosting. Also, at the offset amount F = 5.0 mm, when the overlap rate Rr is 0% to -25%, the evaluation is "○" indicating no banding and "○" indicating no drum ghosting.

[0060] Next, under the conditions of Experiment 1-3, as shown in Tables H41 and H42 in Figures 8(A) and (B), the penetration amount Kr was set to 1.0 mm, and the overlap rate Rr was changed stepwise for each offset amount F at each stage to evaluate the occurrence of banding and drum ghosting.

[0061] In Tables H41 and H42 of Figures 8(A) and (B), for each offset amount F = 3.0 mm to 8.0 mm, when the overlap rate Rr is 0% to -100%, the evaluation for banding is "○" indicating no banding or "△" indicating reduced banding, and "○" indicating no drum ghosting. Furthermore, for each offset amount F = 7.0 mm to 8.0 mm, when the overlap rate Rr is 0% to -100%, the evaluation for banding is "○" indicating no banding or "△" indicating reduced banding, and "○" indicating no drum ghosting can be confirmed.

[0062] Next, under the conditions of Experiment 1-4, as shown in Tables H51 and H52 in Figures 9(A) and (B), the penetration amount Kr was set to 1.5 mm, and the overlap rate Rr was changed stepwise for each offset amount F at each stage to evaluate the occurrence of banding and drum ghosting.

[0063] In Tables H51 and H52 of Figures 9(A) and (B), for each offset amount F = 3.0 mm to 8.0 mm, when the overlap rate Rr is 0% to -100%, the evaluation for banding is "○" indicating no banding, "△" indicating reduced banding, and "○" indicating no drum ghosting. Furthermore, at the offset amount F = 8.0 mm, the evaluation for banding reduction and drum ghosting absence is "○" regardless of the overlap rate Rr being 0% to -100%.

[0064] Comparing the tables shown in Figures 6(A)(B) to 9(A)(B) above, it can be seen that setting the penetration amount Kr = 1.0 mm to 1.5 mm, the offset amount F = 3.0 mm to 8.0 mm, and the overlap rate Rr = 0% to -100% is suitable for suppressing banding and drum ghosting. Furthermore, when the penetration amount Kr = 0.5 mm, it can be seen that setting the offset amount F = 3.0 mm to 4.0 mm and the overlap rate Rr = 0% to -100%, or setting the offset amount F = 5.0 mm and the overlap rate Rr = 0% to -25%, is suitable for suppressing banding and drum ghosting. From these findings, it can be assumed that the occurrence or absence of banding and drum ghosting is mainly due to the penetration amount Kr and the offset amount F.

[0065] Figure 10(A) shows drum ghosting on the surface of the photoreceptor drum 4 when the penetration amount Kr is 0 mm and the offset amount F is 0 mm, and it can be seen that drum ghosting is clearly occurring. Figure 10(B) shows drum ghosting on the surface of the photoreceptor drum 4 when the penetration amount Kr is 1.5 mm, 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 drum ghosting on the surface of the photoreceptor drum 4 when the penetration amount Kr is 1.0 mm, 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.

[0066] 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 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, 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.

[0067] 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.

[0068] Referring to Figure 12, it can be seen that in order to reduce the maximum pressure PM in the nip region NP1 to 0.15 or less, the load N applied to the primary transfer roller 31 by the biasing force of the spring 33 should be appropriately adjusted according to the size of the recording paper (width of the intermediate transfer belt 5), as described above, and that setting the offset amount F to 0 mm, 2.0 mm, 4.0 mm, or 6.0 mm is good for obtaining good results for the motor.

[0069] From this, if the second contact area 4S and the first contact area 1S are not overlapped, and the apex 1c of the primary transfer roller 31, which is made of metal rollers, is made to bite into the photoreceptor drum 4 side via the intermediate transfer belt 5, then, considering the results of experiments 1-2 to 1-4 shown in Figures 6(A), (B) to 9(A), (B), setting the biting amount Kr = 0.5 mm to 1.5 mm and the offset amount F = 3.0 mm to 8.0 mm (3.0 mm to 5.0 mm is optimal) effectively suppresses banding and drum ghosting. From this, it can be assumed that, theoretically, setting the biting amount Kr = 0.1 mm to 1.5 mm and the offset amount F = 1.0 mm to 10.0 mm (4.0 mm to 8.0 mm is even more preferable) contributes to suppressing banding and drum ghosting.

[0070] In the first embodiment described above, in the direction of movement A of the intermediate transfer belt 5, one upstream end 1a of the second contact area 1S is positioned downstream 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, as a further second embodiment, one downstream end 1b of the second contact area 1S is positioned upstream of one upstream end 4a of the first contact area 4S, and the center 1x of the primary transfer roller 31 is spaced upstream of the center 4x of the photoreceptor drum 4, thereby ensuring that the first contact area 4S (nip area NP1) of the photoreceptor drum 4 and the second contact area 1S of the primary transfer roller 31 are spaced apart and do not overlap. In this second embodiment, as with the first embodiment, even without increasing the spring load applied to the primary transfer roller 31, it is possible to obtain effects such as reduced image defects and extended mechanical life of the photoreceptor drum 4 and intermediate transfer belt. Furthermore, as with the above embodiment, these effects can be reliably achieved by appropriately setting the offset amount F and the biting amount Kr. When considering this second embodiment, the direction in which the primary transfer roller 31 and the photoreceptor drum 4 are offset is the opposite of that in the first embodiment in the direction of movement of the intermediate transfer belt 5. For this reason, the preferred offset amount F shown above is in the range of |1.0|mm ≤ F|10.0|mm, or more preferably, in the range of |3.0|mm ≤ F|8.0|mm, throughout the first and second embodiments.

[0071] 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]

[0072] 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. The primary transfer roller is a metal roller. 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, the second contact area and the first contact area are arranged so as not to overlap in the direction of movement of the intermediate transfer belt. 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 bites into the photoreceptor drum side with respect to the offset amount, 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.

3. The image forming apparatus according to claim 2, wherein the predetermined appropriate range of the amount of penetration is set to a range of 0.1 mm or more and 1.5 mm or less.

4. Furthermore, the image forming apparatus according to claim 3, 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 the range of |1.0| mm ≤ F|10.0| mm.

5. Furthermore, the image forming apparatus according to claim 4, 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 the range of |4.0| mm ≤ F|8.0| mm.

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 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 1.4 N or less.

11. The aforementioned intermediate transfer belt is an elastic belt having an elastic layer, 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 to the primary transfer roller by the biasing unit is set to be 0.2 N or more and 5.0 N or less.

12. 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.

13. 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.

14. The image forming apparatus according to any one of claims 1 to 5, wherein the primary transfer roller is a metal roller that has been surface-treated with an oxide film, plating, or insulating paint.

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