Image forming apparatus

Abstract

To reduce a potential unevenness in an axial direction of a photoconductor drum due to the difference in peripheral speed between the photoconductor drum and an electrifying roller.SOLUTION: An image forming apparatus comprises: a rotating photoreceptor; an electrifying roller that is in contact with the photoreceptor to rotate following the photoreceptor and performs contact electrification for the photoreceptor; and a balance roller that is in contact with the electrifying roller and presses the electrifying roller in a direction approaching the photoreceptor. The electrifying roller has a shaft core that is rotatably supported, and an elastic layer that is provided on an outer periphery of the shaft core and in contact with the photoreceptor. The electrifying roller has a shape in which the outer diameter of the elastic layer is increased from the ends toward the center in an axial direction, and the difference between the outer diameter at the ends and the outer diameter at the center of the elastic layer is 60 μm or less.SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] The present invention relates to an electrophotographic image forming apparatus applied to a copying machine, a laser printer, a facsimile machine, a printing apparatus, or a multifunction machine thereof.

Background Art

[0002] In an image forming apparatus employing the electrophotographic method, after charging the surface of a photosensitive drum, an electrostatic latent image is formed on the surface of the photosensitive drum by exposure, and toner is developed on the electrostatic latent image to be visualized as a toner image. Further, the toner image formed on the photosensitive drum is transferred to a recording material, and the transferred toner image is fixed to the recording material by heating or pressurization. In the image forming apparatus, an image is formed on the recording material in this manner.

[0003] As a configuration for charging the surface of the photosensitive drum, a charging roller is brought into contact with the photosensitive drum, and while applying a high voltage to the charging roller, the charging roller is rotationally driven relative to the photosensitive drum to charge the surface of the photosensitive drum.

[0004] Further, by pressing the charging roller toward the photosensitive drum, the charging roller is rotated while being uniformly and closely contacted with the photosensitive drum in the axial direction to realize high-quality image formation.

[0005] As a general conventional configuration, both ends of the axis of the charging roller are biased in the direction of the photosensitive drum by compression springs, and the charging roller is pressed against the photosensitive drum and rotated. Therefore, even if deflection occurs in the axial direction of the charging roller, in order to ensure a uniform contact width of the charging roller with the photosensitive drum, a device is made to form the outer diameter of the elastic layer of the charging roller into a so-called crown shape. Here, the crown shape is a shape in which the outer diameter of the elastic layer of the charging roller is thickened at a constant ratio from the end side in the axial direction to the central portion.

[0006] Furthermore, as another conventional configuration, Patent Document 1 discloses a configuration in which a balance roller that contacts the surface of the charging roller from the opposite side of the photosensitive drum presses the charging roller in the direction of the photosensitive drum, causing the charging roller to be in pressure contact with the photosensitive drum and rotate. The balance roller disclosed in Patent Document 1 has both ends pivotally supported at positions offset toward the photosensitive drum side from the axial center position when the balance roller contacts the charging roller that contacts the photosensitive drum, and presses the charging roller toward the photosensitive drum side over the entire axial length by its own elastic force.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] In the above-described general conventional configuration, since the outer diameter of the elastic layer of the charging roller is not a straight shape that is constant in the axial direction but the aforementioned crown shape, the peripheral speed difference between the charging roller and the photosensitive drum cannot be made uniform in the axial direction, and there is a possibility that potential unevenness may occur on the surface of the photosensitive drum.

[0009] Also, in the conventional configuration using the above-described balance roller, the balance roller presses the charging roller toward the photosensitive drum side over the entire axial length by the elastic force of the balance roller, forming a uniform contact width (hereinafter referred to as the contact nip width) between the photosensitive drum and the charging roller. At this time, even if the charging roller can be pressed by the balance roller and a uniform contact nip width can be formed between the charging roller and the photosensitive drum, if the shape of the charging roller is not appropriately set, there is a possibility that the peripheral speed difference between the charging roller and the photosensitive drum cannot be made uniform in the axial direction. Therefore, due to the peripheral speed difference between the photosensitive drum and the charging roller, there is a possibility that potential unevenness may occur on the surface of the photosensitive drum.

[0010] An object of the present invention is to reduce the potential unevenness in the axial direction of a photoreceptor drum due to the peripheral speed difference between the photoreceptor drum and the charging roller.

Means for Solving the Problems

[0011] A typical configuration of the present invention includes a rotating photoreceptor, a charging roller that contacts the photoreceptor and rotates driven by the photoreceptor to perform contact charging on the photoreceptor, and a balance roller that contacts the charging roller and presses the charging roller in a direction toward the photoreceptor. The charging roller has a shaft core that is rotatably supported, and an elastic layer provided on the outer periphery of the shaft core and contacting the photoreceptor. The charging roller has a shape in which the outer diameter of the elastic layer is made thicker from the axial end portion to the central portion, and the difference between the outer diameter of the end portion and the outer diameter of the central portion of the elastic layer is 60 μm or less. ri , The length L2 of the balance roller is provided in the range of L1 > L2 with respect to the length L1 of the elastic layer of the charging roller, and the diameter d2 of the balance roller is provided in the range of d1 ≥ d2 with respect to the diameter d1 of the axis of the charging roller. characterized by this.

Effects of the Invention

[0012] According to the present invention, it is possible to reduce the potential unevenness in the axial direction of the photoreceptor due to the peripheral speed difference between the photoreceptor and the charging roller.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

[0014] Hereinafter, with reference to the drawings, embodiments of the present invention will be exemplarily and detailedly described. However, the dimensions, materials, shapes, relative arrangements, etc. of the components described in the following examples should be appropriately changed according to the configuration of the apparatus to which the present invention is applied and various conditions, and are not intended to limit the scope of the present invention only thereto.

[0015] [Example 1] <Image Forming Apparatus> Using FIG. 1, the schematic configuration of the image forming apparatus will be described. FIG. 1 is an explanatory diagram of the configuration of the image forming apparatus.

[0016] As shown in FIG. 1, the image forming apparatus 100 is a tandem type intermediate transfer full-color printer in which the image forming units PY, PM, PC, and PK of yellow, magenta, cyan, and black are arranged along the intermediate transfer belt 90.

[0017] In the image forming unit PY, a yellow toner image is formed on the photosensitive drum 1Y and transferred to the intermediate transfer belt 90. In the image forming unit PM, a magenta toner image is formed on the photosensitive drum 1M and transferred to the intermediate transfer belt 90. In the image forming units PC and PK, a cyan toner image and a black toner image are respectively formed on the photosensitive drums 1C and 1K and transferred to the intermediate transfer belt 90.

[0018] The full-color toner image formed by overlapping the four-color toner images is conveyed to the secondary transfer unit 11 as the intermediate transfer belt 90 rotates and is secondarily transferred to the recording material 13. The recording material 13 is separated one by one from a recording material cassette (not shown) and conveyed to the registration roller 12. The registration roller 12 feeds the recording material 13 to the secondary transfer unit 11 in synchronization with the toner image on the intermediate transfer belt 90. The recording material 13 on which the full-color toner image has been secondarily transferred in the secondary transfer unit 11 is heated and pressurized by a fixing device (not shown) to fix the image on the surface and then discharged outside the apparatus.

[0019] The image forming units PY, PM, PC, and PK are substantially the same in configuration except that the colors of the toners used in the developing devices 4Y, 4M, 4C, and 4K are different. Hereinafter, the image forming unit PY will be described, and for the image forming units PM, PC, and PK, it is assumed that the Y at the end of the reference numerals of the constituent members of the image forming unit PY is read as M, C, and K for explanation.

[0020] <Image Forming Unit> With reference to FIG. 2, the schematic configuration of the image forming unit PY (PM, PC, PK) will be described. FIG. 2 is an explanatory diagram of the configuration of the image forming unit PY (PM, PC, PK).

[0021] As shown in FIG. 2, around the photosensitive drum 1Y of the image forming unit PY, a charging roller 2Y, an exposure device (laser scanner) 3Y, a developing device 4Y, a cleaning blade 7Y, and a primary transfer roller 9Y are arranged. Further, in the image forming unit PY, a balance roller 5Y that contacts the surface of the charging roller 2Y and presses the charging roller 2Y in the direction of the photosensitive drum 1Y, and a cleaning member 8Y that cleans the charging roller 2Y are arranged.

[0022] The photosensitive drum 1Y as a photoreceptor rotates at a predetermined speed. In this embodiment, the peripheral speed of the photosensitive drum is 320 mm / s.

[0023] <Primary transfer roller> The primary transfer roller 9Y used in Example 1 forms a primary transfer portion between the photosensitive drum 1Y and the intermediate transfer belt 90. A DC voltage of the opposite polarity to the toner is applied to the primary transfer roller 9Y, and constant voltage control is performed so that it becomes -12 μA during a solid white image, and the toner image carried on the photosensitive drum 1Y is primarily transferred to the intermediate transfer belt 90.

[0024] <Intermediate transfer belt> The intermediate transfer belt 90 is a belt member that is conveyed and driven in the direction of the arrow shown in FIG. 1. In this embodiment, an intermediate transfer belt made of polyether ether ketone having a volume resistivity ρv of 10^10 (Ω·cm) and a surface resistivity ρs of 10^8 (Ω) was used. The volume resistivity ρv of the intermediate transfer belt is preferably 10^8 (Ω·cm) to 10^12 (Ω·cm), and the surface resistivity ρs is preferably 10^8 (Ω) to 10^13 (Ω), and materials such as polyether ether ketone and polyimide are generally used.

[0025] <Configuration of charging roller> The charging roller 2Y contacts the photosensitive drum 1Y and rotates following the photosensitive drum 1Y to perform contact charging on the photosensitive drum 1Y. As shown in FIG. 3(a), the charging roller 2Y is composed of a mandrel 20Y as a rotatably supported axis and an elastic layer formed by coating the outer periphery of the mandrel 20Y and contacting the photosensitive drum 1Y. The elastic layer of the charging roller 2Y is a rubber roller and has a configuration of a base layer 21Y and a surface layer 22Y. The mandrel 20Y is made of iron plated with chromium, the base layer 21Y uses hydrin rubber, and the surface layer 22Y uses a material based on nylon resin. The surface shape of the charging roller is produced by mixing a paint containing nylon resin particles and coating it on the base layer. Note that the charging roller 2Y is not limited to the above materials and manufacturing methods, and for example, general rubber and resin materials can be used.

[0026] The charging roller 2Y uses a rubber roller with an outer diameter D2 of 12 mm, a volume resistivity of the rubber roller of 10^5 (Ω·cm), and a hardness of 66° (JIS-A). In this embodiment, the surface roughness of the charging roller 2Y is Rz = 15 μm, and the uneven interval Sm is 100 μm.

[0027] The surface roughness Rz and the uneven interval Sm are the average roughness and average interval of JIS B0601 (1994). The measurement is performed on the surface of the charging roller in the axial direction. Measuring instrument: Tokyo Seimitsu surface roughness meter Surfcorder 480 equivalent (contact type), measurement point: one point at the longitudinal center, vertical magnification: x2000, horizontal magnification: x50, measurement conditions: cut-off λc 0.8 mm, measurement length: 4.0 mm, feed rate: 0.3 mm / s.

[0028] Also, for the surface of the charging roller 2Y, in order to improve the contamination resistance to toner and external additives (toner additives) and to countermeasure minute abnormal discharges, a surface roughness Rz of 6 μm or more is generally used. However, in the charging roller 2Y with the aforementioned surface roughness, it has been found that there is a potential unevenness of about 10V to 15V. The measurement of this potential unevenness was performed using a measurement method such as a surface potential microscope (KFM: Kelvin Force Microscope, Hitachi, Ltd.) to measure the microscopic potential unevenness on the order of 10 μm.

[0029] Both ends of the core metal 20Y of the charging roller 2Y are movably supported in the direction of the photosensitive drum 1Y by guide members (not shown). The charging roller 2Y is not directly pressurized, but is pressed in the direction of the photosensitive drum 1Y by a balance roller 5Y described later.

[0030] <Schematic description of the crown shape of the charging roller> The crown shape of the charging roller will be described with reference to FIGS. 4(a) and 4(b). FIGS. 4(a) and 4(b) are schematic views showing the crown shape of the charging roller 2Y.

[0031] As shown in FIG. 4(a), in order to uniformly contact the charging roller 2Y with the photosensitive drum 1Y in the axial direction, the charging roller 2Y has a crown shape in which the outer diameter of the base layer (elastic layer) 21Y having the surface layer 22Y is thickened at a constant rate from the axial end portion to the central portion.

[0032] A method for measuring the crown amount 23 of the crown shape of the charging roller 2Y is shown below. Here, the crown amount 23 is the difference between the outer diameters D1 and D3 of the end portions of the elastic layer of the charging roller 2Y and the outer diameter D2 of the central portion.

[0033] With respect to the axial length L1 of the base layer (elastic layer) 21Y having the surface layer 22Y of the charging roller 2Y, the outer diameter at the position 10 mm from one axial end is D1, the outer diameter at the position 10 mm from the other end is D3, and the outer diameter at the central position is D2. Note that, as the positions of the outer diameters D1 and D3 of the elastic layer, the positions 10 mm from the end portions are exemplified, but the present invention is not limited thereto.

[0034] The outer diameter at each position of the charging roller 2Y is measured continuously using a laser length measuring machine (such as RSV-1560 manufactured by Tokyo Optoelectronics Industry Co., Ltd.) while fixing the mandrel and rotating the charging roller. Based on the measured values, the crown amount 23 is calculated by the following formula (A).

[0035] Crown amount 23 = (outer diameter D2) - {(outer diameter D1 + outer diameter D3) / 2} ··· Formula (A)

[0036] Note that, for example, even if the relationship of the outer diameters at each position of the charging roller 2Y is D1 = D2 = D3 and the charging roller 2Y has a straight shape as shown in FIG. 4(b), the crown amount 23 = 0, and it is included in the configuration of the crown shape of the charging roller 2Y of the present embodiment.

[0037] <Configuration of the balance roller> The balance roller 5Y contacts the charging roller 2Y and presses the charging roller 2Y in the direction toward the photosensitive drum 1Y. Both ends of the balance roller 5Y are movably supported in the direction of the photosensitive drum 1Y by guide members (not shown). Also, the balance roller 5Y is in contact with the charging roller 2Y, and a pressing force of a compression spring (not shown) is applied to both ends, pressing the charging roller 2Y in the direction of the photosensitive drum 1Y.

[0038] The axial length L2 of the balance roller 5Y is 360 [mm] (however, the constraint condition is L1 > L2), the diameter d2 is 6 [mm] (however, the constraint condition is d1 ≥ d2), and the balance roller 5Y is made of SUS material. In this embodiment, although the balance roller 5Y is not of a crown shape, it is also effective to make it into a crown shape in which the outer diameter of the balance roller 5Y gradually increases from the ends to the center at a certain ratio in the axial direction of the balance roller 5Y.

[0039] <Configuration of the cleaning member> The cleaning member 8Y is formed into a cylindrical shape by inserting a mandrel (φ6) coated with an adhesive into a sponge such as urethane and polishing it so that the outer diameter of the sponge becomes a predetermined diameter (for example, 10.7 mm). The sponge used here is urethane in the shape of a square bar, the sponge density is 70 kg / m 3 , the sponge hardness is 313.8 N, and the sponge cell density is 80 cells / 25 mm.

[0040] Also, in this embodiment, both ends of the cleaning member 8Y are rotatably supported by bearings. Further, the bearings are biased in the direction of the charging roller 2Y by a spring as biasing means via a spring receiving portion. In this embodiment, the cleaning member 8Y is pressed toward the charging roller 2Y with a force of 100 gf on one side and a total of 200 gf on both sides. Due to the force of this spring, the cleaning member 8Y is pressed against the charging roller 2Y, and a frictional force is generated. Due to this frictional force, the cleaning member 8Y rotates passively with respect to the charging roller 2Y.

[0041] <Arrangement of the cleaning member> As shown in FIG. 2, the cleaning member 8Y is in contact with the charging roller 2Y at a contact position described below. That is, the cleaning member 8Y is in contact with the charging roller 2Y at a position downstream of the contact portion between the charging roller 2Y and the balance roller 5Y with reference to the rotation direction of the charging roller 2Y. In addition, the cleaning member 8Y is in contact with the charging roller 2Y at a position upstream of the contact portion between the charging roller 2Y and the photosensitive drum 1Y with reference to the rotation direction of the charging roller 2Y. The reason for arranging the cleaning member 8Y downstream of the balance roller 5Y is that as a result of the study by the inventors of the present application, the deposits on the charging roller have decreased as compared with the configuration arranged upstream. Here, the deposits on the charging roller refer to toner and external additives. In the following description, the deposits refer to toner and external additives.

[0042] This can be considered as follows as the cleaning mechanism by the cleaning member. The deposits adhering to the charging roller 2Y are repelled by the cleaning member 8Y in contact with the charging roller 2Y, so that the adhesion force decreases due to a decrease in the contact area between the surface of the charging roller and the deposits. Then, the deposits with reduced adhesion force on the surface of the charging roller are considered to be returned to the photosensitive drum 1Y from the charging roller 2Y by an electric field at the contact timing with the next photosensitive drum 1Y and removed from the surface of the charging roller 2Y.

[0043] Next, as a result of examining the configuration in which the balance roller 5Y is arranged on the downstream side of the cleaning member 8Y with reference to the rotation direction of the charging roller 2Y, the following can be considered. When the balance roller 5Y is arranged on the downstream side of the cleaning member 8Y, the adhesion of the deposits attached to the charging roller 2Y to the charging roller 2Y decreases due to the rubbing of the cleaning member 8Y. Among the deposits with reduced adhesion, some deposits transfer from the charging roller 2Y to the balance roller 5Y and adhere to the balance roller 5Y. On the other hand, some deposits remaining on the charging roller 2Y side without transferring are pressed at the contact portion with the balance roller 5Y, so they adhere firmly to the charging roller 2Y. Therefore, some deposits firmly attached to the charging roller 2Y are less likely to be returned to the photosensitive drum 1Y by an electric field from the charging roller 2Y at the contact timing with the next photosensitive drum 1Y, and it is considered that they will rotate with the charging roller 2Y (and the balance roller 5Y).

[0044] For this reason, by arranging the cleaning member 8Y on the downstream side of the balance roller 5Y, it is considered that the deposits on the charging roller are reduced compared to the case where it is arranged on the upstream side of the balance roller 5Y.

[0045] <Crossing angle θ between the photosensitive drum and the charging roller> As shown in FIG. 3(b), the crossing angle θ between the charging roller 2Y and the photosensitive drum 1Y is the deviation angle between the rotation axis CL1 that is the rotation center of the photosensitive drum 1Y and the rotation axis CL2 that is the rotation center of the charging roller 2Y when viewed from above. In other words, the crossing angle θ is the crossing angle at which the rotation axis CL1 of the photosensitive drum 1Y and the rotation axis CL2 of the charging roller 2Y cross when the photosensitive drum 1Y and the charging roller 2Y are simultaneously viewed in the direction of pressing from the charging roller 2Y to the photosensitive drum 1Y.

[0046] <Crossing angle θ′ between the charging roller and the balance roller> Also, as shown in FIG. 3(c), the crossing angle θ′ between the balance roller 5Y and the charging roller 2Y is the deviation angle between the rotation axis CL2 of the charging roller 2Y and the rotation axis CL5 that is the rotation center of the balance roller 5Y when viewed from above. In other words, the crossing angle θ′ is the crossing angle at which the rotation axis CL2 of the charging roller 2Y and the rotation axis CL5 of the balance roller 5Y cross when the charging roller 2Y and the balance roller 5Y are simultaneously viewed in the direction of pressing from the balance roller 5Y to the charging roller 2Y.

[0047] <Relationship between the crossing angle θ and the crossing angle θ′> As an advantage of providing the crossing angle θ and the crossing angle θ′, when the member on the pressing side is a member with a large difference in pressing force between the central portion and the end portion in the axial direction, such as when pressing from the core metals at both ends, there is an effect that the contact pressure in the axial direction can be made uniform compared to the case where the crossing angle is not provided.

[0048] However, in view of the configuration of this embodiment, it is preferable to make the longitudinal pressing force and the contact state from the charging roller 2Y to the photosensitive drum 1Y uniform. That is, the crossing angle θ is preferably small. More specifically, the crossing angle θ is preferably <0.03°.

[0049] On the other hand, regarding the pressing from the balance roller 5Y to the charging roller 2Y, it is not necessary to make the longitudinal pressing force uniform more than the pressing from the charging roller 2Y to the photosensitive drum 1Y, which causes uneven potential in the longitudinal direction. Furthermore, since the balance roller 5Y is a member that directly contacts the charging roller 2Y, a configuration may be considered in which the pressing to the central portion in the longitudinal direction of the charging roller 2Y provided as an image is avoided and the end portion is pressed. From the above background, the crossing angle θ′ between the balance roller 5Y and the charging roller 2Y is allowed to be larger than the crossing angle θ between the photosensitive drum 1Y and the charging roller 2Y.

[0050] Therefore, in this embodiment, the intersection angle θ between the photosensitive drum 1Y and the charging roller 2Y is set to be smaller than the intersection angle θ′ between the charging roller 2Y and the balance roller 5Y (θ < θ′). By adopting such a configuration, the longitudinal pressure and the contact state from the charging roller 2Y to the photosensitive drum 1Y can be made uniform, so that the potential unevenness in the longitudinal direction (axial direction) of the photosensitive drum can be suppressed to a small level. On the other hand, there is an effect that the longitudinal pressure from the balance roller 5Y to the charging roller 2Y can be made uniform regardless of the pressing method. More specifically, it is preferable that |θ′| - |θ| < 0.03°.

[0051] <Arrangement angle of balance roller> As shown in FIG. 3(a), the rotation center C5 of the balance roller 5Y is arranged by shifting it by an angle θb to the upstream side in the rotation direction of the charging roller 2Y. Here, the angle θb is the angle formed by the straight line S1 and the straight line S2. The straight line S1 is a straight line connecting the rotation center C1 of the photosensitive drum 1Y and the rotation center C2 of the charging roller 2Y, and the straight line S2 is a straight line connecting the rotation center C2 of the charging roller 2Y and the rotation center C5 of the balance roller 5Y.

[0052] In the arrangement configuration where the balance roller 5Y is not shifted by the angle θb, when the charging roller 2Y rotates following the photosensitive drum 1Y, it has been found that the charging roller 2Y is deflected in the downstream direction of the rotation direction of the photosensitive drum 1Y due to the forces received from the photosensitive drum 1Y and the balance roller 5Y.

[0053] By shifting the rotation center C1 of the balance roller 5Y by the angle θb as in this embodiment, it is possible to suppress the charging roller 2Y from being deflected in the downstream direction of the rotation direction of the photosensitive drum 1Y, and the potential unevenness of the photosensitive drum 1Y can be suppressed to a small level. Considering that the balance roller 5Y presses the charging roller 2Y toward the photosensitive drum 1Y, the angle θb is preferably set in the range of 0.06° < θb < 30°.

[0054] <Schematic description of the pressure application configuration of the charging roller> Next, the pressure application configurations of the charging rollers in Examples 1-a, 1-b, and 1-c will be described in comparison with Comparative Examples 1 to 4.

[0055] Example 1-a has a charging roller 2Y with a smaller crown amount compared to Examples 1-b and 1-c described later. Specifically, for a charging roller 2Y with a crown amount of 20 μm, pressure is applied not directly to the mandrel 20Y of the charging roller 2Y, but to both ends B1 and B2 of the balance roller 5Y that contacts the charging roller 2Y. The mandrel 20Y of the charging roller 2Y is guided in the direction of the photosensitive drum 1Y and is not directly pressurized. The balance roller 5Y is supported by a bearing portion of POM (polyacetal resin), and by springs attached to the bearing portion, the charging roller 2Y is pressurized with a pressing force of 600 gf on one side and a total of 1200 gf on both sides, and the charging roller 2Y is brought into contact with the photosensitive drum 1. The contact state of the charging roller 2Y with a crown amount of 20 μm in Example 1-a is shown in Fig. 5(a).

[0056] In Example 1-b, for a charging roller 2Y with a crown amount of 40 μm, pressure is applied not directly to the mandrel 20Y of the charging roller 2Y, but to both ends B1 and B2 of the balance roller 5Y that contacts the charging roller 2Y. The pressing force on the balance roller 5Y is the same as in Example 1-a.

[0057] In Example 1-c, for a charging roller 2Y with a crown amount of 60 μm, pressure is applied not directly to the mandrel 20Y of the charging roller 2Y, but to both ends B1 and B2 of the balance roller 5Y that contacts the charging roller 2Y. The pressing force on the balance roller 5Y is the same as in Example 1-a. The contact state of the charging roller 2Y with a crown amount of 60 μm in Example 1-c is shown in Fig. 5(b).

[0058] Comparative Example 1 has a charging roller 2Y with a larger crown amount compared to Examples 1-a, 1-b, and 1-c. Specifically, for a charging roller 2Y with a crown amount of 80 μm, pressure is applied not directly to the mandrel 20Y of the charging roller 2Y, but to both ends B1 and B2 of the balance roller 5Y that contacts the charging roller 2Y. The pressing force on the balance roller 5Y is the same as in Example 1-a. The contact state of the charging roller 2Y with a crown amount of 80 μm in Comparative Example 1 is shown in Fig. 5(c).

[0059] In Comparative Examples 2 to 4, for the charging rollers 2Y with crown amounts of 40, 60, and 80 μm respectively, pressure is directly applied to the mandrel 20Y of the charging roller 2Y. The charging roller 2Y has mandrels 20Y at both ends supported by bearing parts, and is pressed with a pressing force of 600 gf on one side and a total of 1200 gf on both sides by springs attached to the bearing parts, and is brought into contact with the photosensitive drum 1Y. The contact state of the charging roller 2Y with a crown amount of 40 μm in Comparative Example 2 is shown in Fig. 5(d), the contact state of the charging roller 2Y with a crown amount of 60 μm in Comparative Example 3 is shown in Fig. 5(e), and the contact state of the charging roller 2Y with a crown amount of 80 μm in Comparative Example 4 is shown in Fig. 5(f).

[0060] Also, for space saving and cost reduction, the charging roller 2Y has a mandrel 20Y with a diameter d1 = 6 - 8 mm, an elastic layer (rubber roller) with a thickness of 1 - 3 mm, a SUS material with a longitudinal elastic modulus Young's modulus E = 206000, the longitudinal width L1 of the elastic layer (rubber roller) = 320 mm, the shaft length support span L2 = 350 mm, and a total load P [kgf] of about 1, and is configured in relation to the charging roller with a maximum deflection amount δmax > 50 μm or more and the load.

[0061] The maximum deflection amount δmax is calculated by the following formula (B). This is a formula for calculating the deflection amount of a general beam with both ends supported in the longitudinal direction and an evenly distributed load applied.

[0062] Maximum deflection amount δmax [mm] = 5 × p × L^4 / 384 × E × I ··· Formula (B)

[0063] Note that the parameters of the above formula (B) are as follows. p [N / mm]: Evenly distributed load = (F1 + F2) / L2, L [mm]: Roller length L2, E [MPa]: Young's modulus (structural steel: 206000), I [mm^4]: Second moment of cross section = Roller diameter d1^4 × pi PI / 8.

[0064] The charging roller 2Y has a DC bias of -1300 V applied to it. Here, the charging method using a DC bias has a greater effect of suppressing potential unevenness on the photosensitive drum, so this method is used. However, it is not limited to this, and it is also effective in a method where an AC bias is superimposed on the DC bias.

[0065] <Mechanism of peripheral speed difference and potential unevenness> As a new finding of the present invention, it has been found that simply equalizing the contact width (hereinafter referred to as the contact nip width) between the photosensitive drum and the charging roller in the axial direction cannot completely suppress the potential unevenness on the surface of the photosensitive drum. Therefore, a mechanism will be explained in which the speed difference (peripheral speed difference) ΔV between the surface speed Vr at the axial end of the charging roller, the surface speed Vc at the axial center, and the surface speed Vdr of the photosensitive drum is the main cause of potential unevenness.

[0066] <Micro-potential unevenness and potential unevenness> In order to maintain high durability, the surface roughness Rz of the charging roller 2Y is 6 μm, and there is potential unevenness ΔVd of about 15 V on the photosensitive drum 1Y with a resolution of the order of 10 μm size. It has been found that there is potential unevenness of about ΔVd = 30 V at Rz = 15 μm. Also, this potential unevenness is reflected as a coarseness in the image within the drum surface, but it has been found that it is difficult to be recognized as a color tone. On the other hand, potential unevenness on the order of several millimeters reflected in the halftone image within the drum surface can be measured using a surface potential measuring instrument (MODEL 344, manufactured by trek) or the like.

[0067] <Peripheral speed difference ΔV between the charging roller and the photosensitive drum> The case of Example 1-b will be described regarding the speed difference (hereinafter referred to as the peripheral speed difference) ΔV between the charging roller 2Y and the photosensitive drum 1Y.

[0068] As shown in Fig. 5(a), the charging roller 2Y has a crown shape (the outer diameter D2 at the center is thick, and the outer diameters D1 and D3 at the ends are thin). Therefore, as shown in Fig. 6, in the axial direction, when the mandrel 20Y of the charging roller 2Y has the same angular velocity ω, the surface speed (peripheral speed) Vc at the center (outer diameter D2) of the charging roller 2Y is different from the surface speed (peripheral speed) Vr at the ends (outer diameters D1 and D3). Specifically, at the center (outer diameter D2) of the charging roller 2Y, since the surface speed Vc of the charging roller is 319 mm / sec with respect to the surface speed of the photosensitive drum of 320 mm / s, the peripheral speed difference ΔV is as small as about 1 mm / sec.

[0069] This peripheral speed difference ΔV is obtained by measuring the speeds of the surface of the charging roller and the surface of the photosensitive drum using a laser Doppler velocimeter (LV-20Z manufactured by Canon), and using ΔV = the surface speed Vr of the charging roller - the surface speed Vdr of the photosensitive drum. Similarly, at the ends (outer diameters D1 and D3) of the charging roller 2Y, since the surface speed Vr of the charging roller is 314 mm / sec with respect to the surface speed of the photosensitive drum of 320 mm / s, the peripheral speed difference ΔV is 6 mm / sec, indicating that the peripheral speed difference is larger compared to the center.

[0070] <Mechanism of potential unevenness> As described above, when there is a peripheral speed difference ΔV of about 6 mm / s between the charging roller 2Y and the photosensitive drum at the ends of the charging roller 2Y, the overlap of the minute potential unevenness generated in the first rotation of the photosensitive drum and the discharge unevenness during recharging by the charging roller 2Y after the second rotation of the photosensitive drum is likely to occur. Therefore, when the peripheral speed difference ΔV with respect to the photosensitive drum is large at the ends of the charging roller 2Y, it is found that due to the overlap of the aforementioned minute potential unevenness and the discharge unevenness of recharging, potential unevenness of the order of several millimeters in size that is averaged occurs on the surface of the photosensitive drum. As a result, it is found that the potential unevenness at the ends of the charging roller increases, changing the latent image potential and the developing density, thus affecting the color variation within the drum surface.

[0071] Also, it is known that this peripheral speed difference makes it easier for the toner and external additive on the photosensitive drum to adhere to the charging roller. Therefore, this peripheral speed difference also has an impact on the amount of toner and external additive adhering to the charging roller due to paper passing durability. Thus, it can be understood that there is a synergistic effect on both the potential unevenness due to the overlap of minute potential unevenness and the potential unevenness due to deposits on the charging roller.

[0072] From the above, it can be said that it is important not only to equalize the axial contact nip width between the charging roller and the photosensitive drum, but also to suppress the peripheral speed difference between the axial end and the center of the charging roller with respect to the photosensitive drum.

[0073] <Verification and Evaluation of Example 1> In the verification experiment, the high-voltage power supply of the image forming apparatus described in this example was modified to a Canon copier (product name: image RUNNER ADVANCE C9280), and the image forming unit with the above-described configuration was incorporated. Printing with an image ratio of 10% was performed for 100,000 sheets of paper passing, the color unevenness (potential unevenness factor) within the drum surface and the photosensitive drum potential were measured, and also the contamination amount of the charging roller was measured for Si by fluorescent X-ray, and a determination was made. Table 1 shows the results of Example 1-a, 1-b, 1-c and Comparative Examples 1 to 4. In Table 1, the color unevenness is the potential unevenness of the photosensitive drum measured, and was determined as the color unevenness within the drum surface from the measured potential unevenness, with good being "○", slightly bad being "△", and bad being "×". Also, the contamination amount of the charging roller is the adhesion amount of the toner and external additive adhering to the surface of the charging roller, with a small adhesion amount being "○", slightly large being "△", and large being "×".

[0074]

Table 1

[0075] In the configuration of this embodiment (Embodiment 1-a), the axial color unevenness (potential unevenness of about 5 V) from the initial stage was a quite good result. This is because the pressure in the axial direction is uniform due to the pressure applied to the charging roller 2Y by the balance roller 5Y, and the crown amount of the charging roller is as small as 20 μm. Therefore, the peripheral speed difference ΔV is uniform in the axial direction. Furthermore, it can be seen that the amount of contamination of the charging roller is also small due to the small peripheral speed difference ΔV, and the adhesion of toner and external additives to the charging roller is suppressed. As a result, in the configuration of this embodiment, both the color unevenness due to potential unevenness and the amount of deposits on the charging roller are good, and stable and good images can be output from the initial stage to after paper passage.

[0076] In Embodiment 1-b, the crown amount is larger at 40 μm compared to Embodiment 1-a. However, within this range, Embodiment 1-b is similar to Embodiment 1-a in that both the color unevenness due to potential unevenness and the amount of deposits on the charging roller are good, and stable and good images can be output from the initial stage to after paper passage.

[0077] In Embodiment 1-c, due to the large crown amount of 60 μm, the axial color unevenness (potential unevenness of about 10 V) from the initial stage was slightly poor. Also, the amount of deposits on the charging roller 2Y after paper passage tended to be slightly more. This is presumably because, in the axial direction, although the pressure unevenness in the axial direction is small due to the pressure applied by the balance roller 5Y, unevenness remains in the peripheral speed difference due to the crown amount of the charging roller itself. As a result, due to the peripheral speed difference, the amount of deposits on the charging roller has also deteriorated, and the image is slightly defective compared to Embodiment 1-a both at the initial stage and after paper passage.

[0078] In Comparative Example 1, the crown amount is even larger at 80 μm. The axial color unevenness (potential unevenness of about 40 V) from the beginning is even worse compared to Examples 1-3 and is not at an acceptable level as color unevenness. This is presumably because at the axial ends, the contact nip width of the charging roller is not sufficient, resulting in charging defects. Also, due to the crown shape of the charging roller, there is an outer diameter difference between the central part and the ends in the longitudinal direction, and the peripheral speed difference becomes large, so the amount of deposits on the charging roller has also deteriorated. The amount of toner and external additives adhering to the charging roller increases with the passage of paper, and the potential unevenness after paper passage is even larger.

[0079] In Comparative Example 2, the crown amount of the charging roller is 40 μm, which is the same as that of Example 1-b. However, it has a configuration in which pressure is directly applied to the mandrels at both ends of the charging roller instead of pressing via a balance roller. In Comparative Example 2, the axial color unevenness (potential unevenness of about 40 V) from the beginning was quite bad. This is because at the central part in the axial direction, the contact nip width of the charging roller is not sufficient. Also, due to the abnormal potential fluctuation, the amount of deposits on the charging roller after paper passage also tended to be extremely large. The amount of toner and external additives adhering to the charging roller increases with the passage of paper, and the potential unevenness after paper passage is even larger.

[0080] In Comparative Example 3, due to the crown shape of the charging roller, the contact nip width of the charging roller is uniform in the longitudinal direction, and the initial axial color unevenness (potential unevenness of about 5 V) was a quite good result. This is because the amount of lift at the central part due to pressurization at both ends of the charging roller and the crown amount are balanced. However, with the passage of paper, the result was that the amount of deposits on the charging roller was slightly more compared to this example. This is because due to the outer diameter difference caused by the crown shape of the charging roller, a peripheral speed difference occurs between the axial ends and the central part of the charging roller, and the amount of toner and external additives adhering to the charging roller at the ends is also slightly more, and the image after paper passage is slightly defective compared to this example.

[0081] In Comparative Example 4, since the crown amount is too large, the axial color unevenness (potential unevenness of about 40 V) from the initial stage has deteriorated and is not at an acceptable level as color unevenness. This is presumably because, at the axial ends of the charging roller, the contact nip width of the charging roller is not sufficient, resulting in charging defects. Also, due to the crown shape of the charging roller, an outer diameter difference occurs between the central part and the ends in the longitudinal direction of the charging roller, increasing the peripheral speed difference. As a result, the adhesion amounts of toner and external additives to the charging roller have also deteriorated. The adhesion amount to the charging roller increases with the passage of paper, and the potential unevenness after the passage of paper becomes even larger.

[0082] From the above, according to this embodiment, a charging roller with a crown amount 23 set to 60 μm or less by the balance roller 5Y is pressed against the photosensitive drum. More preferably, a charging roller with a crown amount 23 set to 40 μm or less by the balance roller 5Y is pressed against the photosensitive drum. Thereby, the contact nip width and the peripheral speed difference between the charging roller and the photosensitive drum can be made uniform in the axial direction, the axial potential unevenness of the photosensitive drum can be reduced, and the color unevenness within the drum surface can be suppressed.

[0083] [Example 2] Example 2 will be described with reference to FIGS. 7 and 8. In Example 2, unlike Example 1, the axial length of the balance roller is shorter than that of the charging roller. Since the other configurations are the same as those in Example 1 described above, the same reference numerals are given to the same configurations and the description thereof is omitted. Hereinafter, the description will focus on the differences from Example 1.

[0084] [Schematic description of charging nip formation in Example 2] FIG. 7(a) is a schematic diagram showing the pressure contact state between the balance roller 5Y and the charging roller 2Y, and the pressure contact state between the charging roller 2Y and the photosensitive drum 1Y.

[0085] In the configuration shown in Fig. 7(a), the charging roller 2Y is sandwiched in series between the balance roller 5Y and the photosensitive drum 1Y. The axial length L2 of the balance roller 5Y is configured to be longer than the axial length L1 of the base layer (elastic layer) 21Y of the charging roller 2Y.

[0086] Both ends of the balance roller 5Y and the core metal 20Y of the charging roller 2Y are movable in the direction of the photosensitive drum 1Y by guide members (not shown), and pressing forces F1 and F2 of compression springs (not shown) are applied to both ends of the balance roller 5Y.

[0087] Thereby, the charging roller 2Y is pressed against the photosensitive drum 1Y over the entire axial length of the balance roller 5Y, and the contact nip is formed by the close contact between the photosensitive drum 1Y and the base layer (elastic layer) 21Y having the surface layer 22Y of the charging roller 2Y.

[0088] At this time, in order to form a uniform contact nip width in the axial direction of the charging roller 2Y, the deflection amount of the balance roller 5Y must be made smaller than the deflection amount of the core metal 20Y of the charging roller 2Y. Therefore, it is necessary to configure the diameter d2 of the balance roller 5Y to be larger than the diameter d1 of the core metal 20Y of the charging roller 2Y, increase the bending rigidity of the balance roller 5Y, and reduce the deflection amount.

[0089] For this reason, in the configuration shown in Fig. 7(a), the diameter of the balance roller 5Y increases, and there has been a concern about the increase in the size of the apparatus.

[0090] Fig. 7(b) is a schematic view showing the contact state between the balance roller 5Y and the charging roller 2Y and the pressure contact state between the charging roller 2Y and the photosensitive drum 1Y in Example 2.

[0091] In the configuration of Example 2 shown in Fig. 7(b), for the sake of facilitating comparison with the configuration shown in Fig. 7(a), the material of the balance roller 5Y is described with the overall configuration simplified as structural steel. However, it is not limited thereto, and a configuration in which an elastic layer is provided on the outer periphery of the axial core (core metal) of the structural steel may be used.

[0092] As shown in FIG. 7(b), in this embodiment, the contact order of the balance roller 5Y, the charging roller 2Y, and the photosensitive drum 1Y is the same as the configuration shown in FIG. 7(a). However, in this embodiment, within an appropriate range where a uniform contact nip width can be formed in the axial direction of the charging roller 2Y, the axial length L2 of the balance roller 5Y is configured to be shorter than the axial length L1 of the base layer (elastic layer) 21Y of the charging roller 2Y. In addition, the diameter d2 of the balance roller 5Y is configured to be thinner than the diameter d1 of the core metal 20Y of the charging roller 2Y.

[0093] Also, the maximum deflection amount δmax of the balance roller 5Y is calculated by the formula (B) described in Example 1.

[0094] As described above, in order to form a uniform contact nip width in the axial direction of the charging roller 2Y, it is necessary to reduce the deflection amount of the balance roller 5Y. Therefore, from the formula (B), even when the diameter d2 of the balance roller 5Y is configured to be thinner than the diameter d1 of the core metal 20Y of the charging roller 2Y, the length L2 of the balance roller 5Y is configured to be shorter. Thereby, the deflection amount of the balance roller 5Y can be reduced.

[0095] <Simulation Conditions for Charging Nip Width in Example 2> With each parameter of this embodiment shown in FIG. 7(b), a simulation was performed using the non-linear structural analysis tool Abaqus for the range of the length L2 of the balance roller 5Y capable of forming a uniform contact nip width in the axial direction under the following simulation conditions. However, the photosensitive drum 1Y was set to a rigid body with a diameter of 30 mm.

[0096] The parameters of the charging roller 2Y are as follows. The length L1 of the base layer (elastic layer) 21Y of the charging roller 2Y is 347 [mm], and the crown amount 23 of the base layer (elastic layer) 21Y is 0 (straight shape). Also, the diameter d1 of the core bar 20Y satisfies 12.0 ≥ the diameter d1 of the core bar 20Y [mm] ≥ 8.0. Further, the Young's modulus E of the core bar 20Y is 206000 [MPa]. Also, the thickness T of the base layer (elastic layer) 21Y satisfies 3.0 ≥ the thickness T of the base layer (elastic layer) 21Y [mm] ≥ 1.5. Moreover, the Young's modulus E of the base layer (elastic layer) 21Y is 1.8 [MPa].

[0097] The parameters of the balance roller 5Y are as follows. The length L2 [mm] of the balance roller 5Y (where the constraint condition is L1 > L2). The diameter d2 of the balance roller 5Y satisfies 12.0 ≥ the diameter d2 [mm] ≥ 4.0 (where the constraint condition is d1 ≥ d2). The pressing forces F1, F2 of the compression springs that bias the balance roller 5Y satisfy 800 ≥ the pressing forces F1 = F2 of the compression springs [gf] ≥ 200. The Young's modulus E of the balance roller 5Y is 206000 [MPa].

[0098] <Simulation result> Figure 8 is a graph of the simulation results showing the range of the length L2 of the balance roller 5Y capable of forming a uniform contact nip width in the axial direction in this embodiment. In Figure 8, the vertical axis represents the length L2 of the balance roller 5Y, and the horizontal axis represents the diameter d2 of the balance roller 5Y.

[0099] Also, y = 13.5x + 172, y = 13.5x + 164, y = 13.5x + 156 shown in Figure 8 indicate that even when the diameter of the axis (core bar 20Y) of the charging roller 2Y changes, the slopes representing the relationship between the axial length L2 and the diameter d2 of the balance roller 5Y are the same.

[0100] As shown in FIG. 8, when the diameter d2 of the balance roller 5Y is d2 ≧ 4.0 mm, even if the crown amount 23 of the base layer (elastic layer) 21Y of the charging roller 2Y is 23 = 0 (straight shape), the diameter d1 of the core metal 20Y of the charging roller 2Y is d1 = 12.0 mm (solid line in the figure), 10.0 mm (dashed line in the figure), and 8.0 mm (dotted chain line in the figure), it shows that it is possible to form a contact nip width that is uniform in the axial direction.

[0101] Therefore, not only the contact nip width in the axial direction is made uniform, but also by configuring the crown amount 23 of the base layer (elastic layer) 21Y of the charging roller 2Y of this embodiment to be 23 = 0 (straight shape), it becomes easy to make the peripheral speed difference ΔV between the charging roller 2Y and the photosensitive drum 1Y uniform in the axial direction, and the surface of the photosensitive drum 1Y can be charged uniformly.

[0102] Also, with respect to the length L1 of the base layer (elastic layer) 21Y of the charging roller 2Y, the length L2 of the balance roller 5Y is provided in the range of L1 > L2, and with respect to the diameter d1 of the core metal 20Y of the charging roller 2Y, the diameter d2 of the balance roller 5Y is provided in the range of d1 ≧ d2. Thereby, the problems of increasing the diameter of the balance roller 5Y and increasing the size of the apparatus can be solved.

[0103] Furthermore, at the diameter d1 of the core metal 20Y of each charging roller 2Y shown in FIG. 8, the length L2 of the balance roller 5Y capable of forming a contact nip width that is uniform in the axial direction is calculated and defined by the following formula (C).

[0104] L2 [mm] = L1 / 347 × {13.5 × d2 + (156 + 4 × (d1 - 8))} ± (d2 - 2) ··· Formula (C)

[0105] However, the length L1 of the base layer (elastic layer) 21Y of the charging roller 2Y is L1 ≧ 200 mm, and the diameter d1 of the core metal 20Y is d1 ≧ the diameter d2 of the balance roller 5Y ≧ 4.0 mm.

[0106] According to this embodiment, even for the balance roller 5Y that is shorter than the charging roller 2Y, in the charging electric field region near the upstream of the charging nip, the shape ranges of the charging roller 2Y and the balance roller 5Y are set so as not to cause a peripheral speed difference between the surface of the photosensitive drum 1Y and the surface of the charging roller 2Y. That is, as described above, the shape of the elastic layer of the charging roller 2Y, the relationship between the length L1 of the elastic layer of the charging roller 2Y and the length L2 of the balance roller 5Y, and the relationship between the diameter d1 of the core metal 20Y of the charging roller 2Y and the diameter d2 of the balance roller 5Y are set. Thereby, it is possible to form a contact nip width that is uniform in the axial direction of the charging roller 2Y, achieve the peripheral speed uniformity in the axial direction of the charging roller while miniaturizing the device, and uniformly charge the surface of the photosensitive drum 1Y.

[0107] In addition, the values of the photosensitive drum 1Y, the charging roller 2Y, and the balance roller 5Y described here are examples, and the present invention is not limited to these.

[0108] 〔Example 3〕 Example 3 will be described with reference to FIG. 9. In the case of this embodiment, the configuration inside the balance roller 5Y in contact with the charging roller 2Y is different from that in the case of Example 1. Since the other configurations and operations are the same as those of Example 1 described above, the same reference numerals are given to the same configurations and the description is omitted. Hereinafter, the differences from Example 1 will be mainly described.

[0109] <Explanation of the balance roller in Example 3>

[0110] FIGS. 9(a) and 9(b) are perspective views of the end portion of the balance roller 5Y as seen obliquely from the side. Stainless steel is used as the material of the balance roller 5Y. FIG. 9(a) is a view showing a solid balance roller 5Y-A in which the inside of the balance roller 5Y is entirely made of stainless steel. FIG. 9(b) is a view showing a hollow balance roller 5Y-B having a space inside the balance roller 5Y and having a configuration in which stainless steel is partially arranged to support the axis.

[0111] The balance roller 5Y and the charging roller 2Y are always in contact with each other. In accordance with the stop and drive of the charging roller 2Y, the balance roller 5Y also stops and drives. Along with its drive and stop, the surface of the charging roller 2Y wears out. The surface wear of the charging roller 2Y is related to the pressurization conditions of the balance roller 5Y, and it has been found that in particular, the weight of the balance roller 5Y is related. The surface wear of the charging roller 2Y is considered to be caused by two types of wear: friction in contact with the balance roller 5Y during driving and wear during stop. In particular, it is considered that the difference in the moment of inertia due to the weight of the balance roller 5Y affects the wear during stop.

[0112] In addition, the surface wear of the charging roller 2Y can be determined by measuring the surface roughness Rz of the charging roller 2Y. Since the method for measuring the surface roughness Rz of the charging roller 2Y was described in the above-described Example 1, it is omitted here.

[0113] Comparative Example A and Comparative Example B using the solid balance roller 5Y-A shown in Fig. 9(a) will be exemplified, and Example 3 using the hollow balance roller 5Y-B shown in Fig. 9(b) will be described. Comparative Example A is the solid balance roller 5Y-A shown in Fig. 9(a), with a diameter of Φ8.0 mm and a weight of about 150 g. Comparative Example B is the solid balance roller 5Y-A shown in Fig. 9(a), with a diameter of Φ10.0 mm and a weight of about 230 g. This example is the hollow balance roller 5Y-B shown in Fig. 9(b), with a diameter of Φ10.0 mm and a weight of 100 g. Comparative verification was performed using these balance rollers. An initial surface roughness Rz of 7.0 μm was used for the charging roller 2Y.

[0114] <Verification and Evaluation of Example 3> In the verification experiment, the high-voltage power supply of a Canon copier (product name: image RUNNER ADVANCE C9280) was modified for the image forming apparatus described in this example, and an image forming unit having the above-described balance roller was incorporated. Printing at an image ratio of 10% was performed for 100,000 sheets of paper passing, and the wear (surface roughness Rz) of the surface of the charging roller 2Y was measured.

[0115] In Comparative Example A, the surface roughness of the charging roller 2Y had decreased to 1.7 μm, and abnormal images occurred because the surface roughness of the charging roller 2Y decreased after exceeding about 70,000 sheets.

[0116] In Comparative Example B, the surface roughness of the charging roller 2Y had decreased to 0.9 μm, and abnormal images occurred because the surface roughness of the charging roller 2Y decreased after exceeding about 40,000 sheets.

[0117] In this Example, the surface roughness of the charging roller 2Y had only decreased to 6.1 μm, and no abnormal images occurred up to 100,000 sheets.

[0118] From the above, according to this Example, by using the hollow balance roller 5Y, the surface roughness of the charging roller 2Y can be maintained over a long period, and the occurrence of abnormal images can be prevented.

[0119] 〔Example 4〕 Example 4 will be described with reference to FIG. 10. In this Example, the hardness of the balance roller 5Y is defined such that the relationship between the contact nip width N1 between the charging roller 2Y and the photosensitive drum 1Y and the contact nip width N2 between the balance roller 5Y and the charging roller 2Y is N1 ≦ N2. Since the other configurations and operations are the same as those of Example 1 described above, the same reference numerals are given to the same configurations and the description thereof is omitted.

[0120] FIG. 10 is a cross-sectional view showing the contact states of the photosensitive drum 1Y, the charging roller 2Y, and the balance roller 5Y according to Example 4 and the respective contact nip widths N1 and N2.

[0121] Generally, even if the charging roller 2Y has not been used before initial use or has not been used for a long period of time, the material and pressing force are set so that it does not plastically deform at the contact portion with the photosensitive drum 1Y, or even if it does plastically deform, it is within the range where it does not appear as potential unevenness. To suppress plastic deformation, it is effective to increase the hardness of the charging roller 2Y. However, when the hardness of the charging roller 2Y increases, the charging roller 2Y becomes less likely to deform, so the contact nip width N1 with the photosensitive drum 1Y becomes narrower, and the peak value of the contact pressure locally increases. Then, since the amount of toner and external additives adhering to the charging roller 2Y tends to increase, the hardness of the charging roller 2Y is set to such an extent that deformation does not appear in the image at the design stage and the influence of the adhesion amount can also be suppressed.

[0122] Here, since the balance roller 5Y generally has a smaller diameter than the charging roller 2Y, its curvature is large, and the contact nip width N2 with the charging roller 2Y is likely to be small. When the hardness of the balance roller 5Y is high and the contact nip width N2 with the charging roller 2Y is small, even if the plastic deformation of the charging roller 2Y at the contact surface with the photosensitive drum 1Y is within the allowable range, the deformation of the charging roller 2Y may appear as an image defect due to plastic deformation at the contact surface with the balance roller 5Y.

[0123] Therefore, in this embodiment, the hardness of the balance roller 5Y is appropriately controlled. That is, the hardness of the balance roller 5Y is defined so that the relationship between the contact nip width N1 between the charging roller 2Y and the photosensitive drum 1Y and the contact nip width N2 between the balance roller 5Y and the charging roller 2Y satisfies N1≦N2. Thereby, the plastic deformation between the charging roller 2Y and the balance roller 5Y will not be greater than the plastic deformation between the charging roller 2Y and the photosensitive drum 1Y. Then, as long as the charging roller 2Y is designed so that the plastic deformation between it and the photosensitive drum 1Y does not appear as an image, the plastic deformation between the charging roller 2Y and the balance roller 5Y will not appear as an image, and good images can be output even before initial use or even if there is a period when it has not been used for a long time.

[0124] 〔Example 5〕 Example 5 will be described with reference to FIG. 11. In this example, the cleaning member 8Y is different from the configuration of Example 1 in that it is in contact with both the charging roller 2Y and the balance roller 5Y. Since the other configurations and operations are the same as those of Example 1 described above, the same reference numerals are given to the same configurations and the description thereof is omitted. Hereinafter, the description will focus on the differences from Example 1.

[0125] <Contact relationship between cleaning member and balance roller> In Example 5, as shown in FIG. 11, the cleaning member 8Y is arranged so as to be in contact with both the balance roller 5Y and the charging roller 2Y. Both ends of the cleaning member 8Y are rotatably supported by bearings. Further, the bearings are biased in the direction of the charging roller 2Y by a spring as a biasing means via a spring receiving portion. In this example, the cleaning member 8Y was pressed with a pressure of 100 gf on one side and a total of 200 gf on both sides in the direction from the cleaning member 8Y to the charging roller 2Y. By the force of this spring, the cleaning member 8Y is pressed against the charging roller 2Y, and a frictional force is generated. Due to this frictional force, the cleaning member 8Y rotates passively with respect to the charging roller 2Y.

[0126] Here, passive rotation is defined as the ratio Vb / Va of the peripheral speed Vb of the cleaning member 8Y to the peripheral speed Va of the charging roller 2Y being 90% or more and 100% or less. Regarding each speed, it was measured using the laser Doppler velocimeter (LV-20Z manufactured by Canon) described above.

[0127] The balance roller 5Y receives the toner and external additives attached to the surface of the photosensitive drum 1Y via the charging roller 2Y. Usually, the toner and external additives attached to this balance roller 5Y are reattached to the charging roller 2Y at the timing of contacting the charging roller 2Y again. Therefore, in this example, the cleaning member 8Y is brought into contact not only with the charging roller 2Y but also with the balance roller 5Y. As a result, the cleaning member 8Y can collect the deposits attached to the balance roller 5Y without reattaching them to the charging roller 2Y.

[0128] The cleaning member 8Y rotates in a driven manner with respect to the charging roller 2Y, rather than the balance roller 5Y. Thereby, wear of the particles contained in the surface layer of the charging roller 2Y described above can be suppressed, and charging defects generated by wear of the surface layer particles and reattachment of toner or external additives to the charging roller 2Y can be suppressed.

[0129] Here, as an example, the cleaning member 8Y is biased in the direction of the charging roller 2Y, but it is not limited thereto. As long as the cleaning member 8Y rotates in a driven manner with respect to the charging roller 2Y, the direction in which the cleaning member 8Y is biased may be shifted toward the axial center side of the balance roller 5Y. Further, the cleaning member 8Y may be rotated in a driven manner by adjusting the intrusion amount by pressing contact instead of spring biasing.

[0130] This time, as comparative examples, as shown in FIG. 11, as the pressing direction of the cleaning member 8Y, those obtained by changing the angle θc to 10, 20, and 30° and pressing were prepared as Comparative Examples 51, 52, and 53, respectively. As shown in FIG. 11, as the pressing direction of the cleaning member 8Y, the direction toward the rotation center C2 of the charging roller 2Y is set to an angle of 0°. The pressing direction of this cleaning member 8Y is on the straight line S3 connecting the rotation center C8 of the cleaning member 8Y and the rotation center C2 of the charging roller 2Y. Then, from this angle of 0°, the pressing direction of the cleaning member 8Y is changed to 10, 20, and 30° as the angle θc in the direction toward the rotation center C5 side of the balance roller 5Y. This angle θc is the angle formed by the straight line S3 connecting the rotation center C8 of the cleaning member 8Y and the rotation center C2 of the charging roller 2Y and the arrow indicating the pressing direction from the rotation center C8 of the cleaning member 8Y. As the pressing method of the cleaning member 8Y in each comparative example, similar to the cleaning member 8Y of this embodiment, the bearing is pressed by a spring as an urging means via a spring receiving portion at 100 gf on one side and 200 gf in total on both sides.

[0131] <Verification and Evaluation of Example 5> In the verification experiment, the high-voltage power supply of the image forming apparatus described in this embodiment was modified to a Canon copier (product name: image RUNNER ADVANCE C9280), and an image forming unit configured to include the cleaning member arranged as described above was incorporated. The peripheral speed ratio was measured using a laser Doppler velocimeter (Canon LV-20Z), and "〇" was marked on those that were driven to rotate and "×" on those that were not driven to rotate. Printing with an image ratio of 10% was performed for 100,000 sheets of paper passing through. The color unevenness in the plane (potential unevenness factor) and the photosensitive drum potential were measured, and the contamination amount of the charging roller 2Y was measured for Si by fluorescent X-ray, and a determination was made. Regarding the evaluation criteria, Example 1 was followed. Table 2 shows the results of Example 5 and Comparative Examples 51 to 53.

[0132]

Table 2

[0133] In Comparative Example 51, since the cleaning member 8Y was rotating at a peripheral speed ratio of 95% with respect to the charging roller 2Y, it was determined that it was driven to rotate. The contamination amount of the charging roller 2Y was also good. That is, the adhesion amount of toner and external additives to the charging roller 2Y was also small and good.

[0134] In Comparative Example 52, since the cleaning member 8Y was rotating at a peripheral speed ratio of 80% with respect to the charging roller 2Y, it was determined that it was not driven to rotate. Since it was not driven to rotate, the surface layer particles of the charging roller 2Y were worn, and the contamination amount of the charging roller 2Y also resulted in a bad outcome.

[0135] In Comparative Example 53, the cleaning member 8Y hardly rotated with respect to the charging roller 2Y and could not be measured. The surface layer particles of the charging roller 2Y were worn, and the contamination amount of the charging roller 2Y also resulted in a very bad outcome.

[0136] In Example 5 which is this example, since the cleaning member 8Y was rotating at a peripheral speed ratio of 95% with respect to the charging roller 2Y, it was determined that it was driven to rotate. The contamination amount of the charging roller 2Y was also good. That is, the adhesion amount of toner and external additives to the charging roller 2Y was also small and good.

[0137] From the above results, when the cleaning member 8Y is brought into contact with both the charging roller 2Y and the balance roller 5Y, it is important that the cleaning member 8Y is rotationally driven by the charging roller 2Y.

[0138] From the above, according to this embodiment, by rotationally driving the cleaning member 8Y in a driven manner with respect to the charging roller 2Y, it is possible to suppress contamination due to the adhesion of toner and external additives to the charging roller 2Y.

[0139] 〔Example 6〕 Example 6 will be described with reference to FIG. 12. FIGS. 12(a) and 12(b) are schematic cross-sectional views of the image forming unit. The image forming apparatus of this embodiment is different from the configuration of Example 1 in that it detachably includes a cartridge, and that a balance roller is provided in the image forming apparatus. Since other configurations are the same as those of Example 1 described above, the same reference numerals are given to the same configurations and the description thereof is omitted. Hereinafter, the description will focus on the differences from Example 1.

[0140] <Installation of the balance roller in Example 6> FIG. 12(a) is a diagram showing an image forming unit in an image forming apparatus according to a comparative example. The image forming apparatus shown in FIG. 12(a) detachably includes a cartridge 14Y (14M, 14C, 14K). The cartridge 14Y integrally includes a photosensitive drum 1Y, a charging roller 2Y as a process means acting on the photosensitive drum 1Y, a developing device 4Y, and a cleaning blade 7Y. Further, the cartridge 14Y integrally includes the balance roller 5Y and the cleaning member 8Y. That is, the image forming apparatus shown in FIG. 12(a) is an image forming apparatus of a type in which a cartridge 14Y having a photosensitive drum 1Y, a charging roller 2Y, and a balance roller 5Y is detachably mounted.

[0141] Figure 12(b) is a diagram showing an image forming unit in the image forming apparatus according to the present embodiment. As shown in Figure 12(b), the image forming apparatus according to the present embodiment detachably includes a cartridge 14Y (14M, 14C, 14K). The cartridge 14Y integrally includes a photosensitive drum 1Y, a charging roller 2Y as a process means acting on the photosensitive drum, a developing device 4Y, and a cleaning blade 7Y. Further, the cartridge 14Y integrally includes the cleaning member 8Y. The balance roller 5Y is provided on a support member 51 of a main body frame member 50 of the image forming apparatus. That is, in the image forming apparatus according to the present embodiment, a cartridge 14Y having a photosensitive drum 1Y and a charging roller 2Y is detachably mounted, and the balance roller 5Y is provided in the image forming apparatus.

[0142] In the configuration shown in Figure 12(a), due to the force pressing the balance roller 5Y, the charging roller 2Y and the balance roller 5Y are in contact for a long time. Therefore, problems may occur due to the charging roller 2Y and the balance roller 5Y being in contact for a long time.

[0143] On the other hand, the configuration shown in Figure 12(b) is a configuration in which the balance roller 5Y is supported by a support member 51 provided on the main body frame member 50 of the image forming apparatus. And, for this image forming apparatus, a cartridge 14Y having a photosensitive drum 1Y and a charging roller 2Y is detachably mounted. According to this configuration, when the cartridge 14Y having the photosensitive drum 1Y and the charging roller 2Y is removed from the image forming apparatus, the balance roller 5Y on the apparatus side does not contact the charging roller 2Y on the cartridge 14Y side. Therefore, problems due to the charging roller 2Y and the balance roller 5Y being in contact for a long time do not occur.

[0144] Further, a configuration may be provided with a mechanism for bringing the balance roller 5Y into contact with or separating it from the charging roller 2Y. For example, a detachable mechanism that can be attached to and detached from the image forming apparatus may be provided on the support member 51 shown in FIG. 12(b), and the balance roller 5Y may be configured to contact or separate from the charging roller 2Y. Specifically, during image formation, the support member 51 is attached to the image forming apparatus, and the balance roller 5Y is brought into contact with the charging roller 2Y with a predetermined pressure. On the other hand, during non-image formation or when the image forming apparatus is not in use, the support member 51 is removed from the image forming apparatus using the detachable mechanism, thereby separating the balance roller 5Y from the charging roller 2Y. Alternatively, a moving mechanism may be provided on the support member 51, and the support member 51 is moved by the moving mechanism to bring the balance roller 5Y into contact with or separate from the charging roller 2Y. Even with such a configuration, it is possible to prevent problems caused by the charging roller 2Y and the balance roller 5Y being in contact for a long time.

[0145] In addition, in the image forming apparatus according to the above-described first to fifth embodiments, a configuration provided with a mechanism for bringing the balance roller 5Y into contact with or separating it from the charging roller 2Y can also obtain the same effect.

[0146] From the above, according to this embodiment, by configuring the balance roller 5Y to be able to contact or separate from the charging roller 2Y, it is possible to prevent problems caused by the charging roller and the balance roller 5Y being in contact for a long time.

[0147] In the above-described embodiments, four image forming units are used, but the number of units used is not limited and may be appropriately set according to need.

[0148] In the above-described embodiments, a printer was exemplified as the image forming apparatus, but the present invention is not limited thereto. For example, other image forming apparatuses such as copiers, facsimile machines, or multifunction machines that combine these functions may also be used. Further, an image forming apparatus was exemplified in which an intermediate transfer body is used, toner images of respective colors are sequentially superimposed and transferred onto the intermediate transfer body, and the toner image carried on the intermediate transfer body is collectively transferred onto a recording material. However, the present invention is not limited thereto. An image forming apparatus that uses a recording material carrier and sequentially superimposes and transfers toner images of respective colors onto the recording material carried on the recording material carrier may also be used. By applying the present invention to these image forming apparatuses, similar effects can be obtained.

Explanation of Reference Numerals

[0149] C1, C2, C5, C8 … Rotation centers CL1, CL2, CL5 … Axis of rotation PY, PM, PC, PK … Image forming units 1Y, 1M, 1C, 1K … Photoconductive drums 2Y, 2M, 2C, 2K … Charging rollers 5Y, 5M, 5C, 5K … Balance rollers 8Y, 8M, 8C, 8K … Cleaning members 20Y … Shaft 21Y … Base layer 22Y … Surface layer 100 … Image forming apparatus

Claims

1. A rotating photoreceptor, a charging roller that contacts the photoreceptor and rotates following the photoreceptor to perform contact charging on the photoreceptor, a balance roller that contacts the charging roller and presses the charging roller in a direction toward the photoreceptor, characterized by comprising: the charging roller has a shaft core rotatably supported, and an elastic layer provided on an outer periphery of the shaft core and contacting the photoreceptor, the charging roller has a shape in which an outer diameter of the elastic layer becomes thicker from an axial end portion to a central portion, and a difference between the outer diameter of the end portion and the outer diameter of the central portion of the elastic layer is 60 μm or less, the length L2 of the balance roller is in a range of L1 > L2 with respect to the length L1 of the elastic layer of the charging roller, and the diameter d2 of the balance roller is in a range of d1 ≥ d2 with respect to the diameter d1 of the shaft core of the charging roller. An image forming apparatus characterized by this.

2. the diameter d1 of the shaft core of the charging roller is in a range of 12.0 ≥ d1 [mm] ≥ 8.0, the diameter d2 of the balance roller is in a range of 12.0 ≥ d2 [mm] ≥ 4.0, the length L2 of the balance roller is calculated by the following formula: L2 [mm] = L1 / 347 × {13.5 × d2 + (156 + 4 × (d1 - 8))} ± (d2 - 2) The image forming apparatus according to claim 1, characterized by this.

3. The balance roller is provided in a shape in which an outer diameter of the balance roller becomes thicker from an axial end portion to a central portion. The image forming apparatus according to claim 1 or 2, characterized by this.

4. The balance roller is hollow. The image forming apparatus according to any one of claims 1 to 3, characterized by this.

5. The hardness of the balance roller is defined such that a relationship between a nip width N1 between the charging roller and the photoreceptor and a nip width N2 between the charging roller and the balance roller is N1 ≤ N2. The image forming apparatus according to any one of claims 1 to 4, characterized by this.

6. A cleaning member for cleaning the charging roller is provided, and a contact position of the cleaning member and the charging roller is on a downstream side with respect to a contact portion of the charging roller and the balance roller and on an upstream side with respect to a contact portion of the charging roller and the photoreceptor, based on a rotation direction of the charging roller. The image forming apparatus according to any one of claims 1 to 5, characterized by this.

7. The cleaning member abuts on both the charging roller and the balance roller. The image forming apparatus according to claim 6, wherein the image forming apparatus is characterized in that

8. The image forming apparatus is an image forming apparatus detachably provided with a cartridge, The cartridge includes the photosensitive member and the charging roller, The balance roller is provided in the image forming apparatus. The image forming apparatus according to any one of claims 1 to 7, wherein

9. A mechanism for bringing the balance roller into contact with or separating the balance roller from the charging roller is provided. The image forming apparatus according to any one of claims 1 to 8, wherein

10. Only a DC bias is applied to the charging roller. The image forming apparatus according to any one of claims 1 to 9, wherein

11. A rotating photosensitive member; A charging roller that contacts the photosensitive member and rotates following the photosensitive member to perform contact charging on the photosensitive member; A balance roller that contacts the charging roller and presses the charging roller in a direction toward the photosensitive member; Comprising The charging roller has a shaft core rotatably supported and an elastic layer provided on an outer periphery of the shaft core and contacting the photosensitive member. The charging roller has a shape in which an outer diameter of the elastic layer becomes thicker from an axial end portion to a central portion, and a difference between the outer diameter of the end portion of the elastic layer and the outer diameter of the central portion is 60 μm or less. The balance roller is hollow. The image forming apparatus, wherein

12. A rotating photosensitive member; A charging roller that contacts the photosensitive member and rotates following the photosensitive member to perform contact charging on the photosensitive member; A balance roller that contacts the charging roller and presses the charging roller in a direction toward the photosensitive member; Comprising The charging roller has a shaft core rotatably supported and an elastic layer provided on an outer periphery of the shaft core and contacting the photosensitive member. The charging roller has a shape in which an outer diameter of the elastic layer becomes thicker from an axial end portion to a central portion, and a difference between the outer diameter of the end portion of the elastic layer and the outer diameter of the central portion is 60 μm or less. The hardness of the balance roller is defined such that a relationship between a nip width N1 between the charging roller and the photosensitive member and a nip width N2 between the charging roller and the balance roller satisfies N1 ≦ N2. The image forming apparatus, wherein

13. A rotating photosensitive member; A charging roller that contacts the photoreceptor and rotates following the photoreceptor to perform contact charging on the photoreceptor; A balance roller that contacts the charging roller and presses the charging roller in a direction toward the photoreceptor; Comprising; The charging roller has a shaft core rotatably supported and an elastic layer provided on the outer periphery of the shaft core and contacting the photoreceptor; The charging roller has a shape in which the outer diameter of the elastic layer is thickened from the axial end portion to the central portion, and the difference between the outer diameter of the end portion and the outer diameter of the central portion of the elastic layer is 60 μm or less; An image forming apparatus comprising a cleaning member for cleaning the charging roller, wherein a contact position between the cleaning member and the charging roller is on the downstream side of a contact portion between the charging roller and the balance roller and on the upstream side of a contact portion between the charging roller and the photoreceptor, based on the rotation direction of the charging roller.

Images