Image forming apparatus

Abstract

The image forming apparatus includes: a photosensitive drum; an intermediate transfer belt for moving in a state of being in contact with the photosensitive drum; and a primary transfer roller for transferring the toner image of the photosensitive drum from the photosensitive drum to the intermediate transfer belt. A part of a first contact region of the photosensitive drum with respect to the intermediate transfer belt and a part of a second contact region of the primary transfer roller with respect to the intermediate transfer belt are overlapped with each other, and an offset amount for the amount of the overlap is set within a predetermined appropriate range. The offset amount is a spacing distance of a rotation center of the primary transfer roller relative to a rotation center of the photosensitive drum in a moving direction of the intermediate transfer belt. According to the present invention, the image quality can be improved by suppressing strips and the like that accompany the rotation of the intermediate transfer belt, and the reduction in the machine life of the photosensitive drum and the intermediate transfer belt can be prevented.

Description

Technical Field

[0001] This invention relates to an image forming apparatus that forms an image by electrophotography, and more particularly to a technique for appropriately setting the positional relationship between a photosensitive drum and a primary transfer roller. Background Technology

[0002] In an electrophotographic image forming apparatus, an electrostatic latent image is formed on the surface of a photosensitive drum. Toner is applied to the electrostatic latent image, and a toner image is formed on the surface of the photosensitive drum. A seamless intermediate transfer belt is pushed onto the photosensitive drum by a transfer roller, so that the toner image is transferred from the photosensitive drum to the intermediate transfer belt in one step, and then the toner image is transferred from the intermediate transfer belt to the recording paper in a second step.

[0003] An image forming apparatus is known in which a primary transfer roller is configured such that, in the rotation direction of an intermediate transfer belt, its rotation center is located further downstream than the rotation center of a photosensitive drum. The intermediate transfer belt includes a base layer and a surface layer disposed on the outer peripheral surface of the base layer. Furthermore, when the surface resistivity measured from the outer peripheral surface of the intermediate transfer belt is defined as G, and the surface resistivity measured from the inner peripheral surface of the intermediate transfer belt is defined as N, the condition 0.75 ≤ N / G ≤ 1.2 is satisfied. Therefore, even with a structure in which the primary transfer roller is offset downstream relative to the photosensitive drum, toner scattering and the generation of discharge marks are suppressed. Summary of the Invention

[0004] Here, as an image forming apparatus, there exists an apparatus where, by shifting the primary transfer roller downstream relative to the photosensitive drum, the contact area between the primary transfer roller and the intermediate transfer belt moves further downstream than the contact area between the photosensitive drum and the intermediate transfer belt, thus separating the contact area between the photosensitive drum and the intermediate transfer belt from the primary transfer roller. Consequently, the clamping area separates from the primary transfer roller. In this case, the pressure applied to the intermediate transfer belt in the clamping area easily becomes unstable, resulting in image defects (striping) as the intermediate transfer belt rotates. Furthermore, to avoid this situation, when the spring load applied to the primary transfer roller is increased, increasing the pressure acting on the photosensitive drum from the primary transfer roller via the intermediate transfer belt, the pressure in the clamping area rises, leading to greater damage to the photosensitive drum and the intermediate transfer belt, and a reduction in machine life.

[0005] The present invention was made in view of the above circumstances, and its object is to suppress the strips and the like that that accompany the rotation of the intermediate transfer belt, improve image quality, and prevent the reduction of the machine life of the photosensitive drum and the intermediate transfer belt.

[0006] As one aspect of the present invention, a technique that further improves the above-described technique is proposed.

[0007] An image forming apparatus according to one aspect of the present invention includes: a photosensitive drum for carrying an electrostatic latent image, the electrostatic latent image being developed into a toner image by imparting toner; an intermediate transfer belt for moving along the rotational direction of the photosensitive drum while in contact with the photosensitive drum; and a primary transfer roller disposed on a side opposite to the photosensitive drum relative to the intermediate transfer belt, rotating along the moving direction of the intermediate transfer belt while the intermediate transfer belt is pressed against the photosensitive drum, thereby transferring the toner image of the photosensitive drum from the photosensitive drum to the intermediate transfer belt. When the contact area of ​​the photosensitive drum relative to the intermediate transfer belt is designated as a first contact area, and the contact area of ​​the primary transfer roller relative to the intermediate transfer belt is designated as a second contact area, the second contact area and a portion of the first contact area overlap each other in the moving direction of the intermediate transfer belt.

[0008] According to the present invention, it is possible to suppress the strips and the like that that accompany the rotation of the intermediate transfer belt, thereby improving image quality and preventing a reduction in the machine life of the photosensitive drum and the intermediate transfer belt. Attached Figure Description

[0009] Figure 1 This is a cross-sectional view illustrating an image forming apparatus according to one embodiment of the present invention.

[0010] Figure 2 This is a side view showing the intermediate transfer unit and the like in the image forming apparatus of this embodiment.

[0011] Figure 3 This is an enlarged schematic diagram showing a set of primary transfer rollers and photosensitive drums, as well as an intermediate transfer belt, in the intermediate transfer unit.

[0012] Figure 4 yes Figure 3 The enlarged view shows the first contact area of ​​the photosensitive drum in contact with the intermediate transfer belt, the second contact area of ​​the primary transfer roller in contact with the intermediate transfer belt, and the overlapping area of ​​the second contact area that coincides with the first contact area.

[0013] Figure 5A The conditions for Experiment 1 are shown.

[0014] Figure 5B This is a table showing whether or not bands were generated in Experiment 1.

[0015] Figure 6A This is a table showing whether or not bands were generated in Experiment 2.

[0016] Figure 6B This is a table showing whether or not drum ghosting occurred in Experiment 2.

[0017] Figure 7 A to Figure 7 C is a diagram showing the ghosting effect produced on the surface of the photosensitive drum.

[0018] Figure 8 This is a graph showing the maximum pressure PM in the pinch area relative to the load N of the primary transfer roller when the offset F is set to 0mm, 2.0mm, 4.0mm, and 6.0mm.

[0019] Figure 9 This is a graph showing the maximum pressure PM of the indicator Mottle relative to the pinch area when the offset F is set to 0mm, 2.0mm, 4.0mm, and 6.0mm. Detailed Implementation

[0020] Hereinafter, the image forming apparatus according to embodiments of the present invention will be described with reference to the accompanying drawings. Figure 1 This is a cross-sectional view showing an image forming apparatus according to one embodiment of the present invention. The image forming apparatus 1 includes an image reading unit 11 and an image forming unit 12.

[0021] The image reading unit 11 has an image sensor that optically reads the image of the original document, converts the analog output of the image sensor into a digital signal, and generates image data representing the image of the original document.

[0022] Image forming unit 12 is an image forming unit that forms an image represented by the aforementioned image data onto recording paper. It 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 photosensitive drum 4 is uniformly charged, the surface of the photosensitive drum 4 is exposed, and an electrostatic latent image is formed on the surface of the photosensitive drum 4. The electrostatic latent image on the surface of the photosensitive drum 4 is developed into a toner image, and the toner image on the surface of the photosensitive drum 4 is transferred to the intermediate transfer belt 5 in the intermediate transfer unit 20. Thus, a colored toner image is formed on the intermediate transfer belt 5. This colored toner image is then transferred a second time in the pinch area NP2 between the intermediate transfer belt 5 and the secondary transfer roller 6 onto the recording paper P that is conveyed from the paper supply unit 14 through the transport path 8.

[0023] Then, the recording paper P is heated and pressurized by the fixing unit 15, and the toner image on the recording paper P is fixed by heat pressing. Then, the recording paper P is discharged into the discharge tray 17 by the discharge roller 16.

[0024] Figure 2 This is a side view showing the intermediate transfer unit 20. Figure 2 Showing from as and Figure 1The structure of the intermediate transfer unit 20 is visually confirmed from the side opposite to the visual confirmation direction. For example... Figure 2 As shown, the intermediate transfer unit 20 is equipped with four primary transfer rollers 31, a drive roller 23, a tension roller 24, and two support rollers 25. Figure 1 (Illustration omitted), an intermediate transfer belt 5 is mounted on the drive roller 23, tension roller 24, and each support roller 25, pressing each primary transfer roller 31 onto each photosensitive drum 4 via the intermediate transfer belt 5. In this state, when the drive roller 23 is rotated, the intermediate transfer belt 5 rotates and moves while in contact with each photosensitive drum 4, and the toner image of each color is transferred from each photosensitive drum 4 to the intermediate transfer belt 5. The belt cleaning section 18 removes the toner residue 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 of the intermediate transfer belt 5 is ( Figure 3 It also extends in that width direction.

[0025] For example, the primary transfer roller 31 may use a conductive roller or the like, which employs a conductive rubber component. The primary transfer roller 31 is constructed such that a conductive layer is provided on the outer circumferential surface of a cylindrical core metal made of SUS or iron. This conductive layer is formed of a rubber material (e.g., NBR (nitrile butadiene rubber), EPDM (ethylene propylene diene monomer rubber), epichlorohydrin rubber, etc.) to obtain a stable resistance value.

[0026] like Figure 2 As shown, in the direction of movement A, the support roller 25 is positioned before and after each color photosensitive drum 4 and its corresponding primary transfer roller 31. The support roller 25 is, for example, a metal roller with knurling.

[0027] The rotation axis of each primary transfer roller 31 is supported by bearings 34 located at both ends of the primary transfer roller 31. The rotation axis of the primary transfer roller 31 is supported by the bearings 34, allowing it to move freely in the vertical direction. A stop 32 is fixed at a position away from the bearings 34 upwards. A spring 33 is inserted in a compressed state between the bearings 34 and the stop 32, and the bearings 34 of the primary transfer roller 31 are forced towards the intermediate transfer belt 5 by the force of the springs 33. That is, the springs 33 are compression springs. Thus, the primary transfer roller 31 presses against the intermediate transfer belt 5, and via the intermediate transfer belt 5, presses against the photosensitive drum 4. It should be noted that the spring 33 corresponds to the force-applying part in the claims.

[0028] Below the intermediate transfer belt 5, each photosensitive drum 4 is provided with a developing section 26, a drum cleaning section 27, and a charging section 28. Each photosensitive drum 4 is driven to rotate in the direction of the arrow. As the photosensitive drum 4 rotates, its surface is uniformly charged by the charging section 28. The surface of the photosensitive drum 4 is exposed by an exposure device (not shown), forming an electrostatic latent image on its surface. The developing section 26 applies toner to the electrostatic latent image on the surface of the photosensitive drum 4, and the electrostatic latent image is developed into a toner image. The toner image formed on the surface of the photosensitive drum 4 is transferred to the intermediate transfer belt 5 in one pass by the primary transfer roller 31. Afterward, the surface of the photosensitive drum 4 is de-electrified, and any residual toner on the surface of the photosensitive drum 4 is removed by the drum cleaning section 27.

[0029] As described above, a colored toner image is formed on the intermediate transfer belt 5 by superimposing the toner images on the surfaces of each photosensitive drum 4. The colored toner image is transferred from the intermediate transfer belt 5 to the recording paper P in the pinch area NP2 between the secondary transfer roller 6 and the intermediate transfer belt 5.

[0030] In the image forming apparatus 1 of this embodiment, when a photosensitive drum 4 and a primary transfer roller 31 that presses the photosensitive drum 4 via an intermediate transfer belt 5 are configured as a group, four groups of photosensitive drums 4 and primary transfer rollers 31 are provided. In any group consisting of photosensitive drums 4 and primary transfer rollers 31, in the moving direction A of the intermediate transfer belt 5, upstream of the secondary transfer roller 6, the downstream portion of the first contact area of ​​the photosensitive 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 moving direction A.

[0031] Figure 3 It is to zoom in and show Figure 2 A schematic diagram of the intermediate transfer belt 5, a set of photosensitive drums 4, and the primary transfer roller 31. Figure 4 yes Figure 3 A magnified view of a portion shows the first contact area, the second contact area, and the overlapping area of ​​the second contact area that coincides with the first contact area. For example... Figure 3 As shown, in the moving direction A of the intermediate transfer belt 5, a set of photosensitive drums 4 and primary transfer rollers 31 are arranged on the upstream side of the secondary transfer roller 6 (not shown).

[0032] The contact area between the photosensitive drum 4 and the intermediate transfer belt 5 is designated as the first contact area 4S, and the contact area between the primary transfer roller 31 and the intermediate transfer belt 5 is designated as the second contact area 1S. At this time, the downstream portion of the first contact area 4S of the photosensitive drum 4 overlaps with the upstream portion of the second contact area 1S of the primary transfer roller 31. It should be noted that, here, overlap refers to the state where a portion of the second contact area 1S and the first contact area 4S are superimposed on each other in the movement direction A. Figure 3 as well as Figure 4 In the diagram, the amount of overlap mentioned above is denoted as "R".

[0033] Specifically, in the moving direction A of the intermediate transfer belt 5, one end of the first contact area 4S on the upstream side is designated as 4a, and one end on the downstream side is designated as 4b. Similarly, one end of the second contact area 1S on the upstream side is designated as 1a, and one end on the downstream side is designated as 1b. At this time, the upstream end 1a of the second contact area 1S is positioned further upstream than the downstream end 4b of the first contact area 4S. Furthermore, the rotation center of the photosensitive 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 moving direction A of the intermediate transfer belt 5, the rotation center 1x of the primary transfer roller 31 is further downstream than the rotation center 4x of the photosensitive drum 4. When the distance between the rotation center 4x and the rotation center 1x is defined as the offset F, the offset F is set to a value exceeding "0". Therefore, the downstream portion of the first contact area overlaps with the upstream portion of the second contact area of ​​the primary transfer roller 31.

[0034] The first contact area 4S presses the primary transfer roller 31 onto the photosensitive drum 4 via the intermediate transfer belt 5, thereby clamping the intermediate transfer belt 5 between the photosensitive drum 4 and the primary transfer roller 31. The first contact area 4S is the pinching area NP1 used to transfer the toner image of the photosensitive drum 4 to the intermediate transfer belt 5.

[0035] Furthermore, the second contact area 1S is the area where the primary transfer roller 31, which is subjected to a transfer bias, is pressed onto the photosensitive drum 4 via the intermediate transfer belt 5. By pressing the intermediate transfer belt 5 onto the photosensitive drum 4 using the primary transfer roller 31 in the second contact area 1S, the toner image is transferred from the photosensitive drum 4 to the intermediate transfer belt 5.

[0036] When the downstream portion of the first contact area 4S (pinch area NP1) of the photosensitive drum 4 overlaps with the upstream portion of the second contact area 1S of the primary transfer roller 31, the pressure received from the primary transfer roller 31 in the pinch area NP1 is less prone to fluctuation, the pressure applied to the intermediate transfer belt 5 is stable, and image defects (e.g., striping) accompanying the rotation of the intermediate transfer belt 5 are less likely to occur. Furthermore, the pressure in the pinch area NP1 can be increased efficiently, and the pressure applied to the intermediate transfer belt 5 can be stabilized without significantly increasing the load applied to the primary transfer roller 31 by the force of the spring 33. Therefore, the load on the photosensitive drum 4 and the intermediate transfer belt 5 is reduced, and the machine life is extended.

[0037] Thus, in this embodiment, by overlapping the downstream portion of the first contact area 4S (pinch area NP1) with the upstream portion of the second contact area 1S, even without increasing the spring load applied to the primary transfer roller 31, it is possible to improve the reduction of image defects, image quality, and extend the machine life of the photosensitive drum 4 and the intermediate transfer belt.

[0038] Furthermore, in this embodiment, in order to more reliably obtain the effect of suppressing the strips that accompany the rotation of the intermediate transfer belt 5 to improve image quality and prevent the reduction of the machine life of the photosensitive drum 4 and the intermediate transfer belt 5, the offset amount for the amount of overlap is set within a predetermined appropriate range. The amount of overlap is the amount of overlap when the second contact area 1S and the first contact area 4S overlap in the moving direction A of the intermediate transfer belt 5. The offset amount is the distance between the rotation center of the primary transfer roller 31 and the rotation center of the photosensitive drum 4 in the moving direction A.

[0039] These effects are illustrated in Experiments 1 and 2, which are described later.

[0040] <Specific Examples of This Implementation Method>

[0041] In this example, the diameter of the photosensitive drum 4 is set to φ30mm, and the diameter of the primary transfer roller 31 is set to φ12mm.

[0042] Furthermore, it is evident from Experiment 2 described later that when... Figure 3 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 photosensitive drum 4 is set as the overlapping area R, when the overlapping area Rs of the overlapping area R is set to be more than 0% and less than 50% of the maximum value RM of the overlapping area Rs, the above-determined appropriate range of the offset F is set to more than 4.0 mm and less than 6.0 mm.

[0043] As mentioned above, banding can be suppressed by setting overlap, but if the overlap is too large, there is a risk of degrading drum ghosting performance. To further improve image quality, it is preferable to set the overlap area Rs and the offset F to a combination of appropriate ranges. By setting the overlap area Rs and the offset F to the above ranges, a balance can be achieved between banding suppression and improved drum ghosting performance.

[0044] Alternatively, it is evident from Experiment 2 described later that when the overlap area Rs is set to be more than 0% and less than 25% of the maximum value RM, and the predetermined appropriate range of the offset F is set to be more than 3.0 mm and less than 6.0 mm, it is possible to more reliably achieve a balance between strip suppression and improved drum ghosting performance.

[0045] Here, when the photosensitive drum 4 and the primary transfer roller 31 contact the intermediate transfer belt 5 at the same position (the same position where the front and back sides of the intermediate transfer belt 5 overlap) in the moving direction A, the overlap area Rs is at its maximum value RM. In addition, the further away the primary transfer roller 31 is from this same position, the narrower the overlap area Rs becomes.

[0046] Furthermore, it is evident from Experiment 2 described later that if only the offset F is considered, setting the offset F within the range of 2.0mm ≤ F ≤ 6.0mm can achieve the effect of improving image quality and extending machine life.

[0047] Preferably, as can be seen from Experiment 2 described later, setting the offset F within the range of 3.0mm≦F≦5.0mm can more reliably achieve the effect of improving image quality and extending machine life.

[0048] Furthermore, the load N applied to the primary transfer roller 31 by the force of the spring 33 is set according to the size of the recording paper, etc. As described above, since the primary transfer roller 31 is pressed against the photosensitive drum 4 by the force of the spring 33 towards the intermediate transfer belt 5, the load N is appropriately set in addition to the overlap area Rs (an example of overlap amount) and offset F shown above by adjusting the force of the spring 33.

[0049] For example, when the maximum size of the recording paper is standard A3, and the width of the intermediate transfer belt 5 is set according to the A3 size, it is preferable to set the load N pressed on the primary transfer roller 31 by the force of each spring 33 to be more than 0.6N and less than 3.0N. Furthermore, when the maximum size of the recording paper is standard A4, and the width of the intermediate transfer belt 5 is set according to the A4 size, it is preferable to set the load N pressed on the primary transfer roller 31 by the force of each spring 33 to be more than 0.6N and less than 1.4N. This ensures that the pressing force per unit area of ​​the intermediate transfer belt 5 is neither too high nor too low.

[0050] Furthermore, in addition to the combination of (i) overlapping area Rs (an example of overlapping amount) and offset F shown above, or (ii) the combination and the load N mentioned above, any of the following can be appropriately combined.

[0051] When using an elastic tape as the intermediate transfer tape 5, the thickness of the intermediate transfer tape 5 is set to 30 μm or more and 400 μm or less. When using a resin tape as the intermediate transfer tape 5, the thickness of the intermediate transfer tape 5 is set to 30 μm or more and 150 μm or less. The elastic tape, also known as an intermediate transfer tape with an elastic layer, is an elastic tape formed by stacking multiple layers including an elastic layer. The resin tape is, for example, a resin tape with a coating on its surface.

[0052] In addition, the tension of the intermediate transfer belt 5 is set to 15N or more and 45N or less.

[0053] In addition, when the current flowing between the primary transfer roller 31 and the photosensitive drum 4 when a transfer bias voltage is applied to the primary transfer roller 31 is defined as the transfer current It, the transfer current It is set within the range of |2.0μA|≦It≦|40.0μA|.

[0054] Preferably, the transfer current It is set appropriately within the range of -3.0μA to -15.0μA, depending on the dielectric constant of the photosensitive drum 4 and the type of toner.

[0055] <Experiment 1>

[0056] The conditions for Experiment 1 are as follows: Figure 5A As shown, similarly to the specific example of the above embodiment, the diameter of the photosensitive drum 4 is set to φ30mm, and the diameter of the primary transfer roller 31 is set to φ12mm.

[0057] In addition, under the conditions of Experiment 1, the offset F of the intermediate transfer belt 5 in the moving direction A, from the rotation center 4x of the photosensitive drum 4 to the rotation center 1x of the primary transfer roller 31, was set to 4.0 mm.

[0058] In addition, under the conditions of Experiment 1, the overlapping area Rs of the overlapping region R is set to 25% of the maximum value RM of the overlapping area Rs. The overlapping region R is the range of the second contact area 1S of the primary transfer roller 31 that coincides with the first contact area 4S of the photosensitive drum 4.

[0059] Furthermore, under the conditions of Experiment 1, the intermediate transfer tape 5 was a resin tape with a thickness of 65 μm. The surface resistivity of the intermediate transfer tape 5 was 3.0E10Ω / □ (ohms per square), and the volume resistivity of the intermediate transfer tape 5 was 6.0E9Ω·m.

[0060] In addition, under the conditions of Experiment 1, the tension of the intermediate transfer belt 5 was set to 25 N. The load applied to the primary transfer roller 31 by the spring 33 was set to 1.2 N.

[0061] In addition, under the conditions of Experiment 1, the transfer current It flowing between the primary transfer roller 31 and the photosensitive drum 4 was set to -3.0 to -15.0 μA.

[0062] Based on the conditions of Experiment 1, the electrostatic latent images on the surfaces of each photosensitive drum 4 are developed, forming their respective toner images on the surfaces of each photosensitive drum 4. These toner images are then transferred in one pass to the intermediate transfer belt 5 via primary transfer rollers 31. The results of Experiment 1 are as follows: Figure 5B As shown in Table H11. Figure 5B In Table H11, “〇” indicates that no banding was generated, and “×” indicates that banding was generated. In Experiment 1, no banding (horizontal stripes) was generated in the colored toner image formed on the intermediate transfer belt 5.

[0063] Furthermore, in Comparative Example 1 in Table H11, the offset F under the conditions of Experiment 1 was changed to 0, so that the rotation center 4x of the photosensitive drum 4 and the rotation center 1x of the primary transfer roller 31 were aligned in the moving direction of the intermediate transfer belt 5. In this case, the photosensitive drum 4 and the primary transfer roller 31 were in linear contact with each other at the same position on the intermediate transfer belt 5 (at the same position where the front and back sides of the intermediate transfer belt 5 are superimposed).

[0064] Furthermore, in Comparative Example 2 in Table H11, the offset F under the conditions of Experiment 1 was increased, causing the first contact area 4S of the photosensitive drum 4 to move away from the second contact area 1S of the primary transfer roller 31. In this case, the overlap area Rs is 0.

[0065] As shown in Table H11 of Figure 5(B), in either Comparative Example 1 or Comparative Example 2, an image defect (striping) is generated in the colored toner image formed on the intermediate transfer belt 5.

[0066] Based on the results of Experiment 1, Comparative Example 1, and Comparative Example 2, when the downstream portion of the first contact area 4S (pinch area NP1) of the photosensitive drum 4 and the upstream portion of the second contact area 1S of the primary transfer roller 31 are overlapped as described above, and the offset F is set as described above, even without increasing the spring load applied to the primary transfer roller 31, the variation in pressure received from the primary transfer roller 31 in the pinch area NP1 is sufficiently suppressed. This ensures that the pressure required for toner transfer in the pinch area NP1 does not produce streaks, thus extending the machine life of the photosensitive drum 4 and the intermediate transfer belt.

[0067] <Experiment 2>

[0068] In Experiment 2, the presence or absence of stripes and drum ghosting was evaluated by periodically changing the offset F and the overlap area Rs. Drum ghosting refers to the phenomenon where the image from the previous transfer remains on the surface of the photosensitive drum 4, and this remaining image is superimposed on the next image; it is also known as transfer memory.

[0069] Under the conditions of Experiment 2, similar to Experiment 1 above, the diameter of the photosensitive drum 4 was set to φ30mm, and the diameter of the primary transfer roller 31 was set to φ12mm.

[0070] Furthermore, under the conditions of Experiment 2, similar to Experiment 1 above, the intermediate transfer tape 5 was a resin tape with a thickness of 65 μm. The surface resistivity of the intermediate transfer tape 5 was 3.0E10Ω / □, and the volume resistivity of the intermediate transfer tape 5 was 6.0E9Ω·m.

[0071] In addition, under the conditions of Experiment 2, the tension of the intermediate transfer belt 5 was set to 25N, similar to that of Experiment 1.

[0072] In addition, under the conditions of Experiment 2, similar to Experiment 1 above, the transfer current It flowing between the primary transfer roller 31 and the photosensitive drum 4 was set to -3.0 to -15.0 μA.

[0073] On the other hand, in Experiment 2, such as Figure 6A , Figure 6B As shown in Tables H21 and H22, the offset F was changed in stages to 0mm, 1.0mm, 2.0mm, 3.0mm, 4.0mm, 5.0mm, 6.0mm, 7.0mm, and 8.0mm. For each offset F, the overlap ratio Rr was changed in stages to 100%, 75%, 50%, 25%, and 0% to evaluate the generation of stripes and ghosting. "〇" indicates that no stripes or ghosting were generated, "△" indicates that the generation of stripes or ghosting was reduced, and "×" indicates that stripes or ghosting were generated.

[0074] The overlap ratio Rr is based on the maximum value RM of the overlap area Rs, representing the ratio of the overlap area Rs to the maximum value RM. The load N of the primary transfer roller 31 pressing the intermediate transfer belt 5, i.e., the force of the spring 33, changes accordingly. When the load N caused by the force of the spring 33 increases, and the primary transfer roller 31 is pushed onto the photosensitive drum 4 via the intermediate transfer belt 5 and gets closest to the photosensitive drum 4, the photosensitive drum 4 and the primary transfer roller 31 contact the intermediate transfer belt 5 at the same position (the same position where the front and back sides of the intermediate transfer belt 5 overlap), and the overlap area Rs of the second contact area 1S that coincides with the first contact area 4S is the maximum value RM, and the overlap ratio Rr is 100%. In addition, the smaller the load N caused by the force of the spring 33, the farther the primary transfer roller 31 is from the intermediate transfer belt 5, the narrower the overlap area Rs, and the smaller the overlap ratio Rr.

[0075] For any offset F = 0mm, 1.0mm, 2.0mm, 3.0mm, 4.0mm, 5.0mm, 6.0mm, 7.0mm, or 8.0mm, when the load N increases and the photosensitive drum 4 and the primary transfer roller 31 contact the same position with the intermediate transfer belt 5, the overlap area Rs is at its maximum value RM, and the overlap rate Rr is 100%. In addition, the smaller the load N, the further the primary transfer roller 31 is from the same position, the narrower the overlap area Rs, and the smaller the overlap rate Rr.

[0076] exist Figure 6A , Figure 6B In Tables H21 and H22, the stripes and ghosting are evaluated when the offset F is set to 0 mm and the overlap rate Rr is 100%. When the offset 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 photosensitive drum 4. The overlap area R of the second contact area 1S, which coincides with the first contact area 4S, is linear, and the overlap area Rs cannot be changed. Therefore, only the overlap rate Rr of 100% is recorded.

[0077] Furthermore, in Tables H21 and H22, the stripe and ghosting were evaluated when the offset F was set to 1.0 mm and the overlap ratio Rr was 100% and 75%. When the offset F was set to 1.0 mm, since the photosensitive drum 4 and the primary transfer roller 31 were in contact with the intermediate transfer belt 5 at the same position, or the primary transfer roller 31 was only slightly away from that same position, the overlap area Rs could be slightly varied, and overlap ratios of 100% and 75% were recorded.

[0078] Furthermore, in Tables H21 and H22, the stripe and ghosting were evaluated when the offset F was set to 2.0 mm and the overlap ratio Rr was 100%, 75%, and 50%. When the offset F was set to 2.0 mm, the photosensitive drum 4 and the primary transfer roller 31 contacted the same position with the intermediate transfer belt 5, or the primary transfer roller 31 was made to be further away from the same position than when the offset F = 1.0 mm. Therefore, the overlap area Rs changed, and the overlap ratio Rr was recorded as 100%, 75%, and 50%.

[0079] Furthermore, in Tables H21 and H22, the offset F was set to 3.0 mm, 4.0 mm, 5.0 mm, 6.0 mm, 7.0 mm, or 8.0 mm, and the striping and ghosting were evaluated when the overlap rate Rr was 100%, 75%, 50%, 25%, and 0%. When the offset F was set to 3.0 mm to 8.0 mm, the photosensitive drum 4 and the primary transfer roller 31 were in contact with the same position as the intermediate transfer belt 5, or the primary transfer roller 31 was able to be significantly moved away from the same position, thus the overlap area Rs changed significantly, and overlap rates Rr of 100%, 75%, 50%, 25%, and 0% were recorded.

[0080] It is evident from Table H21 that when the offset F is 0 mm or less than 6 mm and the overlap rate Rr exceeds 0%, a "△" is generated for strip reduction or no "〇" is generated.

[0081] Furthermore, as clearly shown in Table H22, when the offset F is 2.0 mm or more and 8.0 mm or less, and the overlap rate Rr is 50% or less, a "△" is generated to reduce drum ghosting or no "〇" is generated.

[0082] Figure 7 Image A shows the ghosting of the surface of the photosensitive drum 4 when the offset F is 0 mm, and it can be seen that the ghosting is obviously produced. Figure 7 Image B shows the ghosting of the surface of the photosensitive drum 4 when the offset F is 4.0 mm and the overlap Rr is 50% or more. It can be seen that the ghosting is reduced. Figure 7 C is an image showing the ghosting of the surface of the photosensitive drum 4 when the offset F is 4.0 mm and the overlap Rr is less than 25%, and it can be seen that the ghosting is eliminated.

[0083] Comparing Tables H21 and H22, when focusing on the overlap rate Rr of whether or not stripes are generated as “0” or “△” and whether or not drum ghosting is generated as “0” or “△”, when the offset F is set to 4.0 mm or more and 6.0 mm or less, and the overlap rate Rr is set to more than 0% and less than 50%, it can be seen that the stripes become less likely to generate “△” or not generate “0”, and the drum ghosting is not generated.

[0084] Alternatively, when the offset F is set to 3.0 mm or more and 6.0 mm or less, and the overlap rate Rr is set to more than 0% and less than 25%, it can be seen that the stripe becomes less visible and produces "△" or no "〇", and the drum ghosting does not occur.

[0085] Figure 8 The horizontal axis shows the load N of the primary transfer roller caused by the force of spring 33, and the vertical axis shows the maximum pressure PM in the pinch area NP1. It is a graph showing the maximum pressure PM relative to the load N when the offset F is set to 0mm, 2.0mm, 4.0mm, or 6.0mm, and the overlap ratio Rr is set in the range exceeding 0% and below 25%. From Figure 8 The chart clearly shows that as the offset F increases, the maximum pressure PM in the pinch area NP1 decreases. This indicates that the pressure is distributed throughout the pinch area NP1. Consequently, the load on the photosensitive drum 4 is reduced, damage to the photosensitive drum 4 and the intermediate transfer belt 5 is suppressed, and their lifespan is improved.

[0086] Figure 9 The horizontal axis shows the maximum pressure PM in the pinch area NP1, and the vertical axis shows the Mottle index, which quantifies the image graininess. It's a graph showing the Mottle index relative to the maximum pressure PM when the offset F is set to 0mm, 2.0mm, 4.0mm, and 6.0mm. Since a lower Mottle index indicates better image quality, it is preferable to set the Mottle index to below 1. Figure 9 As is evident in the chart, if the maximum pressure PM in the pinch area NP1 is below 0.15, the Mottle index can be set below 1. This is believed to be due to reduced adhesion of the toner to the surface of the intermediate transfer belt 5, thus improving image quality.

[0087] Reference Figure 8 It is known that in order to set the maximum pressure PM in the pinch area NP1 to below 0.15, it is necessary to adjust the load N applied to the primary transfer roller 31 by the 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 F to 2.0mm, 4.0mm, and 6.0mm.

[0088] Therefore, when the offset F is set to be greater than 2.0mm and less than 6.0mm, further consideration is needed. Figure 6A , Figure 6B The results of Experiment 2 show that when the offset F is set to 3.0 mm or more and 5.0 mm or less, the adhesion of the toner to the surface of the intermediate transfer belt 5 is reduced, the image quality is improved, the load on the photosensitive drum 4 is reduced, damage to the photosensitive drum 4 and the intermediate transfer belt 5 is suppressed, and their lifespan is increased.

[0089] It should be noted that in the above embodiment, in the moving direction A of the intermediate transfer belt 5, one end 1a of the upstream side of the second contact area 1S is positioned further upstream than one end 4b of the downstream side of the first contact area 4S, so that the center 1x of the primary transfer roller 31 is further downstream than the center 4x of the photosensitive drum 4. However, alternatively, one end 1b of the downstream side of the second contact area 1S can be positioned further downstream than one end 4a of the upstream side of the first contact area 4S, so that the center 1x of the primary transfer roller 31 is further upstream than the center 4x of the photosensitive drum 4, and the upstream portion of the first contact area 4S (pinch area NP1) of the photosensitive drum 4 overlaps with the downstream portion of the second contact area 1S of the primary transfer roller 31. In this case, similarly to the above embodiment, even without increasing the spring load applied to the primary transfer roller 31, it is possible to obtain the effect of reducing image defects and extending the machine life of the photosensitive drum 4 and the intermediate transfer belt. In addition, similar to Experiments 1 and 2, this effect can be reliably achieved by appropriately setting the offset F, overlap area Rs, and overlap rate Rr.

[0090] Additionally, using Figures 1 to 9 The structure described is merely one embodiment of the present invention and does not imply that the present invention is limited to this structure.

Claims

1. An image forming apparatus, characterized in that, include: A photosensitive drum is used to carry an electrostatic latent image, which is developed into a toner image by being coated with toner. The intermediate transfer belt is used to move along the rotation direction of the photosensitive drum while in contact with it. as well as A primary transfer roller, positioned on the opposite side of the photosensitive drum relative to the intermediate transfer belt, rotates along the moving direction of the intermediate transfer belt while the intermediate transfer belt is pressed against the photosensitive drum, thereby transferring the toner image from the photosensitive drum to the intermediate transfer belt. When the contact area between the photosensitive drum and the intermediate transfer belt is designated as the first contact area, and the contact area between the primary transfer roller and the intermediate transfer belt is designated as the second contact area, the second contact area and a portion of the first contact area overlap each other in the moving direction of the intermediate transfer belt.

2. The image forming apparatus according to claim 1, characterized in that, The offset for the amount of overlap is set within a predetermined appropriate range, the offset being the distance between the rotation center of the primary transfer roller and the rotation center of the photosensitive drum in the direction of movement of the intermediate transfer belt.

3. The image forming apparatus according to claim 2, characterized in that, The predetermined appropriate range of the offset is set within the range of 2.0 mm and 6.0 mm.

4. The image forming apparatus according to claim 3, characterized in that, The amount of overlap is less than 50% of the maximum value of the overlap when the photosensitive drum and the primary transfer roller are in contact with the intermediate transfer belt at the same position in the direction of movement of the intermediate transfer belt. The predetermined appropriate range for the offset is set to be above 4.0 mm and below 6.0 mm.

5. The image forming apparatus according to claim 3, characterized in that, The amount of overlap is set to be less than 25% of the maximum value of the overlap when the photosensitive drum and the primary transfer roller are in contact with the intermediate transfer belt at the same position in the direction of movement of the intermediate transfer belt. The predetermined appropriate range for the offset is set to be above 3.0 mm and below 6.0 mm.

6. The image forming apparatus according to any one of claims 1 to 5, characterized in that, In the direction of movement of the intermediate transfer belt, the rotation center of the primary transfer roller is moved downstream relative to the rotation center of the photosensitive drum.

7. The image forming apparatus according to any one of claims 1 to 5, characterized in that, In the direction of movement of the intermediate transfer belt, the rotation center of the primary transfer roller is moved upstream relative to the rotation center of the photosensitive drum.

8. The image forming apparatus according to any one of claims 1 to 5, characterized in that, In the moving direction of the intermediate transfer belt, one end of the upstream side of the second contact area is positioned further upstream than one end of the downstream side of the first contact area, so that the rotation center of the primary transfer roller is further downstream than the rotation center of the photosensitive drum.

9. The image forming apparatus according to any one of claims 1 to 5, characterized in that, In the moving direction of the intermediate transfer belt, one end of the downstream side of the second contact area is positioned further downstream than one end of the upstream side of the first contact area, so that the rotation center of the primary transfer roller is further upstream than the rotation center of the photosensitive drum.

10. The image forming apparatus according to any one of claims 1 to 5, characterized in that, Furthermore, when the current flowing between the primary transfer roller and the photosensitive drum when a transfer bias voltage is applied to the primary transfer roller is defined as the transfer current It, The transfer current It is set within the range of |2.0μA|≦It≦|40.0μA|.

11. The image forming apparatus according to any one of claims 1 to 5, characterized in that, It also includes a force-applying part for applying force to the primary transfer roller to make the primary transfer roller make pressure contact with the intermediate transfer belt. The load applied to the primary transfer roller by the force-applying part is set to be 0.6N or more and 3.0N or less.

12. The image forming apparatus according to claim 11, characterized in that, It also includes a force-applying part for applying force to the primary transfer roller to make the primary transfer roller make pressure contact with the intermediate transfer belt. The load applied to the primary transfer roller by the force-applying part is set to be 0.6N or more and 1.4N or less.

13. The image forming apparatus according to any one of claims 1 to 5, characterized in that, Furthermore, when using an elastic strip as the intermediate transfer strip, the thickness of the intermediate transfer strip is set to 30 μm or more and 400 μm or less; when using a resin strip as the intermediate transfer strip, the thickness of the intermediate transfer strip is set to 30 μm or more and 150 μm or less.

14. The image forming apparatus according to any one of claims 1 to 5, characterized in that, Furthermore, the tension of the intermediate transfer belt is set to be above 15N and below 45N.

Images