Image formation device

Abstract

An image formation device (1) comprises a photosensitive drum (4), an intermediate transfer belt (5) that is moved in contact with the photosensitive drum (4), a primary transfer roller (31) that transfers a toner image on the photosensitive drum (4) from the photosensitive drum (4) to the intermediate transfer belt (5), and a spring (33) that biases the primary transfer roller (31) into pressure contact with the intermediate transfer belt (5). The photosensitive drum (4) and the primary transfer roller (31) are disposed in an offset manner in the moving direction of the intermediate transfer belt (5). The amount of the offset is 6.0-10 mm, and the load to bias the primary transfer roller (31) is set to 0.4-1.0 N.

Description

Image forming apparatus

[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 setting the positional relationship between a photoreceptor drum and a primary transfer roller.

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

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

[0004] Japanese Patent Application Laid-Open No. 2020-095227

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

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

[0007] An image forming apparatus according to one aspect of the present invention includes: a photoreceptor drum that carries an electrostatic latent image and develops the electrostatic latent image into a toner image by the application of toner; an intermediate transfer belt that moves in the rotational direction of the photoreceptor drum while in contact with the photoreceptor drum; a primary transfer roller provided on the opposite side of the photoreceptor drum from the intermediate transfer belt, which rotates in the direction of movement of the photoreceptor drum while pressing the intermediate transfer belt against the photoreceptor drum, and transfers the toner image of the photoreceptor drum from the photoreceptor drum to the intermediate transfer belt; and the 1 The device includes a biasing unit that biases the secondary transfer roller to press it against the intermediate transfer belt, wherein the photoreceptor drum and the primary transfer roller are arranged such that the rotation center of the primary transfer roller is spaced apart from the rotation center of the photoreceptor drum in the direction of movement of the intermediate transfer belt, the offset amount, which is the distance between the rotation center of the photoreceptor drum and the rotation center of the primary transfer roller, is set to 6.0 mm or more and 10 mm or less, and the load applied by the biasing unit to bias the primary transfer roller is set to 0.4 N or more and 1.0 N or less.

[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.

[0009] This is a cross-sectional view showing an image forming apparatus according to one embodiment of the present invention. This is a side view showing an intermediate transfer unit, etc., in the image forming apparatus of this embodiment. This is a schematic diagram showing an enlarged view of a set of primary transfer rollers, a photoreceptor drum, and an intermediate transfer belt in the intermediate transfer unit. This is a partially enlarged view 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 overlap area of ​​the second contact area that overlaps with the first contact area. This is a diagram showing examples of conditions applied to the image forming apparatus. This is a table showing the occurrence of transfer memory, gaps, density defects, and mottle under each condition when the offset amount between the photoreceptor drum and the primary transfer roller is set to 0 mm or more and 12 mm or less, and the load of the primary transfer roller is changed in steps from 0.1 N to 1.3 N or less. This is a table showing the occurrence of mottle numerically under each condition when the offset amount between the photoreceptor drum and the primary transfer roller is set to 0 mm or more and 12 mm or less, and the load of the primary transfer roller is changed in steps from 0.1 N to 1.3 N or less.

[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. Note that the backup rollers 251 and 252 are not shown in Figure 1.

[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 an image on the recording paper indicated by the above image data, 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 secondary 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 thermal compression, and then the recording paper P is discharged to the discharge tray 17 via the discharge roller 16.

[0014] Figure 2 is a side view of the intermediate transfer unit 20. Figure 2 shows the configuration of the intermediate transfer unit 20 as viewed from the side opposite to the viewing direction in Figure 1. As shown in Figure 2, the intermediate transfer unit 20 is provided with four primary transfer rollers 31, a drive roller 23, a tension roller 24, and two backup rollers 251 and 252 (not shown in Figure 1). An intermediate transfer belt 5 is stretched across the drive roller 23, the tension roller 24, and each backup roller 25, and each primary transfer roller 31 is pressed against the respective photoreceptor drum 4 via the intermediate transfer belt 5. In this state, when the drive roller 23 is rotated, the intermediate transfer belt 5 moves in a circular motion while contacting each photoreceptor drum 4, and the toner image of each color is transferred from each photoreceptor drum 4 to the intermediate transfer belt 5. When the drive roller 23 is rotated and the intermediate transfer belt 5 rotates endlessly due to circular motion, the tension roller 24 rotates in accordance with the movement of the intermediate transfer belt 5. Therefore, the tension roller 24 is also a driven roller that rotates in accordance with the movement of the intermediate transfer belt 5. The belt cleaning unit 18 removes toner remaining on the surface of the intermediate transfer belt 5. Each primary transfer roller 31 extends in a direction perpendicular to the direction of movement A of the intermediate transfer belt 5, that is, in the width direction of the intermediate transfer belt 5. The rotation axis 1x (Figure 3) of the intermediate transfer belt 5 also extends in the same width direction.

[0015] The primary transfer roller 31 is a metal roller made of SUM (free-cutting steel), SUS (stainless steel), or aluminum. However, this does not mean that the material of the primary transfer roller 31 is limited to these materials. In addition, the primary transfer roller 31 may be a metal roller that has been surface-treated with an oxide film (e.g., anodizing), plating (e.g., electroless nickel plating), or insulating paint (e.g., acrylic resin or polyurethane resin).

[0016] As shown in Figure 2, the backup rollers 251 and 252 are positioned in front of and behind the photoreceptor drum 4 of each color and the corresponding primary transfer roller 31 in the direction of movement A. In this embodiment, the backup rollers 251 and 252 are, for example, metal rollers provided with a plurality of grooves, i.e., knurling, extending in the direction of rotation axis on their circumferential surface. However, it is also possible that only one of the backup rollers 251 and 252 is made of a metal roller provided with the grooves.

[0017] The backup roller 251, as the first backup roller, is positioned upstream of the secondary transfer roller 6, which is located in the direction of movement of the intermediate transfer belt 5 and nips the drive roller 23 across the intermediate transfer belt 5. It is positioned downstream of the primary transfer roller 31 and the photoreceptor drum 4, which are located at the furthest downstream end, and applies tension from the inside of the endlessly rotating intermediate transfer belt 5.

[0018] The backup roller 252, as a second backup roller, is positioned downstream of the driven roller and upstream of the primary transfer roller 31 and photoreceptor drum 4, which are located at the uppermost position, in the direction of movement of the intermediate transfer belt 5, and applies tension from the inside of the endlessly rotating intermediate transfer belt 5.

[0019] Each primary transfer roller 31 has its rotation axis supported by bearings 34 provided at both ends of the primary transfer roller 31. The rotation axis of the primary transfer roller 31 is supported by the bearings 34 so that it can move vertically. 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 biasing force of each spring 33 biases the bearing 34 of the primary transfer roller 31 toward the intermediate transfer belt 5. In other words, the spring 33 is a compression spring. As a result, the primary transfer roller 31 presses against the intermediate transfer belt 5 and also presses against the photoreceptor drum 4 via the intermediate transfer belt 5. Note that the spring 33 corresponds to the biasing part in the claims. However, the primary transfer roller 31 may be biased toward the intermediate transfer belt 5 by a configuration other than biasing by the spring 33 described above. For example, the intermediate transfer unit 20 may be positioned such that the primary transfer roller 31 presses against the intermediate transfer belt 5 and presses against the photoreceptor drum 4, and the overall position of the intermediate transfer unit 20 relative to the photoreceptor drum 4 is adjusted to a predetermined value.

[0020] Below the intermediate transfer belt 5, each photoreceptor drum 4 is provided with a developing unit 26, a drum cleaning unit 27, and a charging unit 28. Each photoreceptor drum 4 is rotated in the direction of the arrow, and as the photoreceptor drum 4 rotates, the surface of the photoreceptor drum 4 is uniformly charged by the charging unit 28, the surface of the photoreceptor drum 4 is exposed by an exposure device (not shown), 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, the electrostatic latent image is developed into a toner image, and the toner image formed on the surface of the photoreceptor drum 4 is primary transferred to the intermediate transfer belt 5 by the primary transfer roller 31. After this, the surface of the photoreceptor drum 4 is de-staticized, and residual toner on the surface of the photoreceptor drum 4 is removed by the drum cleaning unit 27.

[0021] As described above, a color toner image is formed on the intermediate transfer belt 5 by superimposing the toner images on the surfaces of each photoreceptor drum 4, 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. In this embodiment, the secondary transfer roller 6 is positioned to nip with the drive roller 23 in the direction of movement A of the intermediate transfer belt 5, with the intermediate transfer belt 5 in between. However, alternatively, the secondary transfer roller 6 may be positioned to nip with the tension roller 24 in the direction of movement A of the intermediate transfer belt 5, with the intermediate transfer belt 5 in between.

[0022] In the image forming apparatus 1 of this embodiment, if a photoreceptor drum 4 and a primary transfer roller 31 that presses the photoreceptor drum 4 via an intermediate transfer belt 5 are considered as one set, then four sets of photoreceptor drums 4 and primary transfer rollers 31 are provided.

[0023] 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 regions.

[0024] 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 arranged 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".

[0025] 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.

[0026] 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.

[0027] 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".

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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 is 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. The resistance generated by the intermediate transfer belt 5 is used to generate a voltage between the primary transfer roller 31 and the photoreceptor drum 4 that is sufficient to transfer the toner image.

[0032] 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.

[0033] 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.

[0034] 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 happens, the toner image on the photoreceptor drum 4 is not transferred to the intended position on the intermediate transfer belt 5, and it does not overlap with the toner images of other colors transferred on the intermediate transfer belt 5 in the correct position, resulting in what is known as color misalignment. Furthermore, the larger the offset amount F, the greater the decrease and instability of the pressure in the nip region NP1, resulting in unstable transfer of the toner image from the photoreceptor drum 4 to the intermediate transfer belt 5.

[0035] 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.

[0036] <Specific Example of This Embodiment> The conditions in this example are as shown in Figure 5, and the explanation will be given using the case where the diameter of the photoreceptor drum 4 is φ30 mm and the diameter of the primary transfer roller 31 is φ12 mm as an example.

[0037] In this example, 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 / □, and the volume resistivity of the intermediate transfer belt 5 is 6.0E9 Ω·m. The tension of the intermediate transfer belt 5 is set to 25 N.

[0038] 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, based on the maximum value RM of the overlap area Rs. 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.

[0039] Furthermore, the transfer current It flowing between the primary transfer roller 31 and the photoreceptor drum 4 is set to -3.0 to -15.0 μA.

[0040] In this example, the offset amount F (unit: mm) and the load L (unit: N) applied to the primary transfer roller 31 are changed in stages to evaluate the occurrence of transfer memory, gaps, poor concentration, and mottling.

[0041] Here, transfer memory is the phenomenon where the previously transferred image remains on the surface of the photosensitive drum 4, and this remaining image is superimposed on the next image; this is also called drum ghosting. Missing parts are a form of transfer defect in which a portion of the transferred image is missing. Density defects refer to the phenomenon in which the image has a density different from its original density. Mottle indicates uneven toner density.

[0042] Figure 6 shows the occurrence of transfer memory, gaps, poor concentration, and mottling under each condition when the offset amount F is gradually changed from 0 mm to 12 mm and the primary load is gradually changed from 0.1 N to 1.3 N. Here, for transfer memory, ◎ indicates no occurrence at all, ○ indicates almost no occurrence, △ indicates visible, and × indicates very visible. For gaps and poor concentration, ◎ indicates no occurrence at all, ○ indicates almost no occurrence, △ indicates present, and × indicates frequent occurrence.

[0043] And, FIG. 7 numerically shows the occurrence status of mottle under each condition when the offset amount F is gradually changed to a value of 0 mm or more and 12 mm or less, and the primary load is gradually changed to a value of 0.1 N or more and 1.3 N or less. The lower the numerical value of mottle, the better. In FIG. 6 described above, for mottle, the numerical value is indicated as ◎ when it is 0.57 or less, ○ when it is 0.65 or less, △ when it is 0.8 or less, and × when it is greater than 0.8.

[0044] As shown in FIGS. 6 and 7, when the offset amount F is gradually changed to a value of 6.0 mm or more and 10 mm or less, and under each condition in the range A where the primary load L is 0.4 N or more and 1.0 N or less, for the transfer memory, missing, density defect, and the occurrence status of mottle, good results were obtained as compared with the case of other conditions.

[0045] And, when the offset amount F is gradually changed to a value of 7 mm or more and 9 mm or less, and under each condition in the range B where the primary load L is 0.5 N or more and 0.9 N or less, for the transfer memory, missing, density defect, and the occurrence status of mottle, even better results were obtained as compared with the case of other conditions.

[0046] From this, it can be understood that under the condition where the offset amount F and the primary load L are in the above range A, the adhesion force of the toner on the surface of the intermediate transfer belt 5 is reduced, the image quality is improved, the load on the photoreceptor drum 4 is reduced, the damage to the photoreceptor drum 4 and the intermediate transfer belt 5 is suppressed, and the effect of improving their lifetimes is obtained. Furthermore, under the condition where the offset amount F and the primary load L are in the above range B, the said effect is obtained even better.

[0047] Also, from the results shown in FIGS. 6 and 7, it can be understood that good results for the above effect are also obtained when (i) the offset amount F is in the range of 7 mm or more and 9 mm or less and the primary load L is in the range of 0.4 N or more and 1.0 N or less, or (ii) the offset amount F is in the range of 6 mm or more and 10 mm or less and the primary load L is in the range of 0.5 N or more and 0.9 N or less.

[0048] Furthermore, in the image forming apparatus 1 according to the present embodiment, since the second contact region 1S and the first contact region 4S are configured not to overlap, the above effect can be surely obtained.

[0049] Furthermore, in the image forming apparatus 1 according to the present embodiment, backup rollers 251 and 252 are provided. Since at least one of the backup rollers 251 and 252 is a metal roller provided with a plurality of grooves (knurling) extending in the rotation axis direction on the peripheral surface, the above effect can be surely obtained.

[0050] Thus, according to the present embodiment, it is possible to suppress banding and the like accompanying the rotation of the intermediate transfer belt, improve the image quality, and prevent a decrease in the mechanical life of the photoreceptor drum and the intermediate transfer belt.

[0051] In addition, the configuration described using FIGS. 1 to 7 is merely one embodiment of the present invention, and is not intended to limit the present invention to such a configuration.

Claims

1. The apparatus comprises: a photoreceptor drum that carries an electrostatic latent image, the electrostatic latent image being developed into a toner image by the application of toner; an intermediate transfer belt that moves in the rotational direction of the photoreceptor drum while in contact with the photoreceptor drum; and a primary transfer roller provided on the opposite side of the photoreceptor drum from the intermediate transfer belt, which rotates in the direction of movement of the photoreceptor drum while pressing the intermediate transfer belt against the photoreceptor drum, thereby transferring the toner image of the photoreceptor drum from the photoreceptor drum to the intermediate transfer belt, wherein the photoreceptor drum and the primary transfer roller are arranged such that the rotational center of the primary transfer roller is spaced apart from the rotational center of the photoreceptor drum in the direction of movement of the intermediate transfer belt, and the offset amount, which is the distance between the rotational center of the photoreceptor drum and the rotational center of the primary transfer roller, is 6.0 mm or more and 10 mm or less. An image forming apparatus in which the primary transfer roller is arranged in a state in which it is biased and pressed against the intermediate transfer belt, and the load that biases the primary transfer roller is set to 0.4 N or more and 1.0 N or less.

2. The image forming apparatus according to claim 1, wherein the offset amount is 7 mm or more and 9 mm or less.

3. The image forming apparatus according to claim 1, wherein the load on which the primary transfer roller is biased is set to 0.5 N or more and 0.9 N or less.

4. The image forming apparatus according to claim 1, wherein the offset amount is set to 7 mm or more and 9 mm or less, and the load on which the primary transfer roller is biased is set to 0.5 N or more and 0.9 N or less.

5. The intermediate transfer belt is in an endless form and is stretched between the drive roller and the driven roller, and rotates endlessly by the rotational force applied from the drive roller. A plurality of primary transfer rollers and photoreceptor drums are provided at a position between the driven roller and the drive roller. A first backup roller is provided upstream of the secondary transfer roller, which is located at a position where it nips the intermediate transfer belt with the drive roller or the driven roller in the direction of movement of the intermediate transfer belt, and downstream of the primary transfer roller and photoreceptor drum, which are located at the downstream end, and applies tension from the inside of the endlessly rotating intermediate transfer belt. A second backup roller is provided downstream of the driven roller, which is located upstream of the primary transfer roller and photoreceptor drum, which are located at the upstream end, and applies tension from the inside of the endlessly rotating intermediate transfer belt. The image forming apparatus according to claim 1, wherein at least one of the first tension roller and the second tension roller is a metal roller having a plurality of grooves on its circumferential surface that extend in the direction of the rotation axis.

6. The image forming apparatus according to any one of claims 1 to 5, further comprising a biasing unit that biases the primary transfer roller and presses it against the intermediate transfer belt, wherein the primary transfer roller is biased by the biasing unit.

7. An image forming apparatus according to any one of claims 1 to 5, wherein the contact area of ​​the photoreceptor drum with respect to the intermediate transfer belt is defined as a first contact area, and the contact area of ​​the primary transfer roller with respect to the intermediate transfer belt is defined as a second contact area, and 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.

8. The image forming apparatus according to claim 7, wherein, 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 most towards the photoreceptor drum is embedded into the photoreceptor drum via the intermediate transfer belt.

Images