JP5935483B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus including a charging roller that charges an image carrier.

  In general, in an image forming apparatus employing an electrophotographic system, a toner image based on image data is formed on the peripheral surface of an image carrier such as a photosensitive drum after charging. This toner image is transferred to a recording medium such as paper and then fixed on the recording medium, thereby completing a printed matter.

  The image carrier charging method mainly includes a non-contact charging method and a contact charging method. Non-contact methods include a corotron method and a scorotron method based on corona discharge. On the other hand, the contact charging method includes a roller charging method and a brush charging method based on spark discharge (also referred to as proximity discharge) in a minute gap.

  In the non-contact method, ozone is generated by corona discharge. Therefore, when the non-contact method is employed, the image forming apparatus needs an ozone exhaust system including a duct, a fan, and the like. As a result, the image forming apparatus is increased in cost and size.

  In contrast to the non-contact method, ozone is not generated in the contact method, so that the image forming apparatus does not require an exhaust system. Due to this merit, the contact method is often used in low-end printers. Furthermore, in recent years, the contact method has also been improved in charging stability, and is therefore being adopted in MFPs (Multifunction Peripherals) for high-speed offices.

  There is a roller charging method that has been put into practical use by the contact method. Since the charging roller discharges in the state of being close to the image carrier, it is easily affected by the surface condition (unevenness, dirt, material non-uniformity, etc.). Due to this influence, charging unevenness occurs in the image carrier, so that the charging roller is required to be driven to rotate at a constant speed with respect to the image carrier.

  In general, when the pressure contact force of the charging roller with respect to the image carrier is increased, the charging roller is favorably rotated with a small slip amount with respect to the rotation of the image carrier. In other words, followability is improved. For example, in Patent Document 1 below, the rotation centers of the photosensitive drum, the charging roller, and the sponge member are arranged on the same straight line, and the charging roller and the sponge member are urged toward the rotation center of the image carrier.

JP 2007-121544 A

  However, in the method described in Patent Document 1, the followability is improved by increasing the pressure contact force, so that the load on the image carrier by the charging roller increases. As a result, there is a problem that the amount of wear of the image carrier increases and the life thereof decreases.

  In addition, since the degree of adhesion of the charging roller to the image carrier increases, dirt or the like attached to the image carrier easily adheres to the surface of the charging roller. This adhesion of dirt causes discharge unevenness of the charging roller. Further, in order to cope with the higher speed, a larger pressure contact force is required, so that the discharge irregularity becomes more severe as the speed of the image forming apparatus is increased.

  SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an image forming apparatus capable of improving the followability while reducing the pressure contact force of the charging roller to the image carrier.

In order to achieve the above object, one aspect of the present invention is an image forming apparatus, an image carrier that rotates in a predetermined rotation direction, and the image carrier that is in contact with the image carrier. A charging roller that is driven by the rotation of the charging roller to charge the peripheral surface of the image carrier, an urging unit that urges the charging roller in a predetermined urging direction, and presses the charging roller to rotate. A cleaning roller for cleaning the charging roller, and the cleaning roller is 180 ° or more downstream in the rotation direction of the charging roller with reference to a contact position between the image carrier and the charging roller. The image bearing member is disposed at a position rotated by an angle of 270 ° or less, and the predetermined biasing direction is based on a direction from the rotation center of the charging roller toward the rotation center of the image carrier. It inclined to the downstream side of the direction of rotation of the Thus , the pressing direction of the cleaning roller is inclined to the downstream side in the rotation direction of the charging roller with reference to the direction from the rotation center of the cleaning roller to the rotation center of the charging roller .

  According to the above aspect, it is possible to improve the followability of rotation with the image carrier while reducing the pressing force of the charging roller to the image carrier.

1 is a schematic diagram illustrating a main part of an image forming apparatus according to a first embodiment of the present invention. It is an enlarged view of the imaging unit shown in FIG. (A) is a schematic diagram of the image carrier, the charging roller, and the cleaning roller shown in FIG. 2, (B) is a schematic diagram showing the detailed configuration of the urging means, and (C) is the same diagram. It is a schematic diagram which shows the urging | biasing direction by the urging means of (B). (A) is a schematic diagram of an image carrier, a charging roller, and a cleaning roller of the first comparative example, (B) is a schematic diagram showing a detailed configuration of an urging means, and (C) is the same. It is a schematic diagram which shows the urging | biasing direction by the urging means of a figure (B). (A), (B) and (C) are schematic views showing the charging roller of the second, third and fourth embodiments. (A), (B) and (C) are schematic views showing charging rollers of second, third and fourth comparative examples. It is a schematic diagram which shows an inclination angle. (A) is a schematic diagram which shows the arrangement position of the cleaning roller which concerns on this embodiment, (B)-(D) is a schematic diagram which shows the specific example of the arrangement position of the cleaning roller. (A) And (B) is a schematic diagram which shows the arrangement position of the cleaning roller which concerns on a comparative example. It is a schematic diagram which illustrates the pressing direction of a cleaning roller.

(Introduction)
Hereinafter, the image forming apparatus according to each embodiment of the present invention will be described. First, in each figure, the X axis indicates the left-right direction (lateral direction) of the image forming apparatus, the Y axis indicates the front-rear direction (depth direction) of the image forming apparatus, and the Z axis indicates the height direction of the image forming apparatus. .

  In addition, alphabetic small letters a, b, c, and d described after the reference numerals are subscripts representing yellow (Y), magenta (M), cyan (C), and black (Bk). For example, the photosensitive drum 21a means the yellow photosensitive drum 21. Reference numerals without subscripts a to d mean that they are for Y, M, C, and Bk colors. For example, the photosensitive drum 21 means the photosensitive drum 21 for each color.

(First embodiment)
Next, the schematic configuration and operation of the image forming apparatus according to the first embodiment will be described with reference to FIGS. First, in FIG. 1, the image forming apparatus is, for example, a tandem type full-color MFP (Multifunction Peripheral) that employs an electrophotographic system.

  The image forming apparatus includes an intermediate transfer belt 11 at a substantially central portion inside the main body. The intermediate transfer belt 11 is wound around the outer periphery of the roller 12, the tension roller 13, and the guide roller 14, and is driven to rotate in the counterclockwise direction indicated by the dotted arrow α. A cleaning unit 15 for the intermediate transfer belt 11 is provided at the left end of the intermediate transfer belt 11.

  Immediately below the intermediate transfer belt 11, the imaging units 2a to 2d are arranged in this order from left to right. As shown in an enlarged view in FIG. 2, the imaging unit 2 has a photosensitive drum 21 as an image carrier. The imaging unit 2 further includes a charging roller 22, a developing device 24, and a cleaning unit 26 in order along the rotation direction (the direction indicated by the dotted arrow β) around the photosensitive drum 21. A charging roller cleaning roller 28 is provided on the peripheral surface of the charging roller 22. A laser scanning optical system (not shown) is disposed below the imaging unit 2.

  Refer to FIG. 1 again. A primary transfer roller 30 is provided at a position facing each photosensitive drum 21 across the intermediate transfer belt 11. Further, the secondary transfer roller 16 is pressed against the roller 12 with the intermediate transfer belt 11 interposed therebetween. As a result, a nip as a secondary transfer region 17 is formed between the secondary transfer roller 16 and the intermediate transfer belt 11.

  In the image forming apparatus, a sheet feeding cassette (not shown) is detachably disposed on the lower stage of the main body. A recording medium (typically, paper) loaded in the paper cassette is pulled out one by one by a rotating paper feed roller and sent out to a transport path indicated by a dotted arrow γ (hereinafter referred to as a transport path γ). It is.

  The conveyance path γ extends from the paper feed cassette to the paper discharge tray (not shown) through the timing roller pair 18, the secondary transfer region 17, the fixing unit (not shown), and the like. The paper fed from the paper feed cassette is transported to the timing roller pair 18 where it is temporarily stopped. The paper once stopped is sent to the secondary transfer region 17 at a predetermined timing by the rotation control of the timing roller pair 18.

  Next, a schematic operation of the image forming apparatus will be described. In the image forming apparatus, the charging roller 22 charges the rotating photosensitive drum 21. In the laser scanning optical system, Y, M, C, and K light emitting elements are driven by digital image signals of corresponding colors. In response to this, a light beam B modulated by the digital image signal is emitted from each light emitting element.

  The light beam B is applied to the peripheral surface of the corresponding photosensitive drum 21 via a polygon mirror, a scanning lens, or the like, and an electrostatic latent image of each color is formed. Each developing device 24 supplies corresponding color toner to the peripheral surface of the rotating photosensitive drum 21 by a built-in developing roller. As a result, the electrostatic latent image formed on the peripheral surface of the photosensitive drum 21 corresponding to each developing device 24 is developed to form a corresponding color toner image.

  The toner images of Y, M, C, and K are primarily transferred so as to overlap on the rotationally driven intermediate transfer belt 11 by the action of the corresponding primary transfer roller 30. As a result, a composite toner image is formed on the intermediate transfer belt 11. The composite toner image is conveyed toward the secondary transfer region 17 by the rotational drive of the intermediate transfer belt 11.

  Further, the sheet sent to the transport path γ is sent to the secondary transfer area 17 by the timing roller pair 18 in accordance with the timing when the composite toner image reaches the secondary transfer area 17. In addition, a voltage having a polarity opposite to the charging polarity of the toner is applied to the secondary transfer roller 16. As a result, the composite toner image is secondarily transferred from the intermediate transfer belt 11 to the paper in the secondary transfer region 17. The sheet on which the composite toner image has been transferred is conveyed to a fixing unit through a conveyance path γ and subjected to a fixing process, and then discharged from a pair of discharge rollers to a discharge tray.

  Next, with reference to FIGS. 3A to 3C, the roller charging method, which is a feature of this embodiment, will be described. 3A to 3C, more specifically, the photosensitive drum 21 extends in the vertical direction of the image forming apparatus, and a rotation axis parallel to the Y axis at the time of image formation. It rotates around A21 in the direction indicated by the dotted arrow β (that is, clockwise).

  The charging roller 22 extends in the vertical direction of the image forming apparatus and is disposed so as to contact the peripheral surface of the photosensitive drum 21. The charging roller 22 is rotated in the direction indicated by the dotted arrow δ (that is, counterclockwise) around the rotation axis A22 parallel to the Y axis following the rotation of the photosensitive drum 21. Further, in the charging roller 22, the metal rotation shaft A22 is connected to a power supply circuit.

  The cleaning roller 28 extends in the vertical direction of the image forming apparatus and is disposed so as to contact the peripheral surface of the charging roller 22. The cleaning roller 28 is rotated in the opposite direction to the charging roller 22 (in the same direction as the photosensitive drum 21) around the rotation axis A28 parallel to the Y axis, following the rotation of the charging roller 22.

  At the time of image formation, when a current or voltage is applied to the rotation axis A22 by a power supply circuit, a spark discharge is generated in a minute gap near these nips due to a potential difference between the photosensitive drum 21 and the charging roller 22, thereby The charging roller 22 uniformly charges the peripheral surface of the photosensitive drum 21 to a predetermined potential. Further, the cleaning roller 28 removes deposits (dirt or the like) on the peripheral surface of the rotating charging roller 22.

  The above charging roller 22 is urged in the urging direction ε by the urging means 32 shown in FIG. 3B and presses the photosensitive drum 21. Specifically, the charging roller 22 is rotatably supported by two bearings 32 a and 32 b that constitute the urging means 32. The charging roller 22 is housed in the guide 32c together with the bearings 32a and 32b, and is urged in the urging direction ε by the spring 32d from the rear.

  Here, as shown in FIG. 3C, when the direction from the center of the rotation axis A22 of the charging roller 22 to the center of the rotation axis A21 of the photosensitive drum 21 is a direction ζ, the biasing direction ε is The biasing means 32 is designed so as to be inclined downstream of the rotation direction β of the photosensitive drum 21 with respect to the direction ζ. With this configuration, it is possible to improve the followability with the photosensitive drum 21 while reducing the pressing force Fa of the charging roller 22 to the photosensitive drum 21.

  A specific operation of the above configuration will be described with reference to FIG. Since the charging roller 22 rotates following the photosensitive drum 21, a tangential force (hereinafter referred to as a driven rotational force) F ′ is applied to the charging roller 22 at a contact position with the photosensitive drum 21. In addition to this, in the present embodiment, the biasing means 32 applies a force F in the biasing direction ε to the charging roller 22. The force F can be broken down into a force Fa (hereinafter referred to as a pressure contact force) Fa directed to the rotation center of the photosensitive drum 21 and a force Fb in the same direction as the force F ′ (hereinafter referred to as a rotation assist force).

  Since the rotation assisting force Fb is added to the driven rotational force F ′ when the charging roller 22 rotates, the followability of the charging roller 22 with respect to the photosensitive drum 21 is improved. As a result, charging unevenness in the photosensitive drum 21 can be suppressed. In the present embodiment, the driven rotational force F ′ itself is not increased, but the followability is improved by the rotation assisting force Fb. In other words, in the present embodiment, it is not necessary to apply a large pressing force Fa to the photosensitive drum 21. Therefore, it is possible to keep the pressure contact Fa small, thereby reducing the amount of wear of the photosensitive drum 21 and extending its life.

  Here, FIGS. 4A to 4C show the arrangement relationship, the configuration of the urging means, and the urging direction of the first comparative example (that is, Patent Document 1) for comparison with the present embodiment. . As shown in FIGS. 4B and 4C, the first comparative example is different from the first embodiment in that the biasing direction ε of the charging roller 402 is the direction of the rotation center of the photosensitive drum 401. It is only a point. In this configuration, as shown in FIG. 4C, the urging force F becomes the pressure contact force Fa, so that a large pressure contact force Fa can be applied to the photosensitive drum 401. As a result, the followability of the charging roller 402 is improved. On the other hand, the load on the photosensitive drum 401 is increased by the large pressure contact force Fa. As a result, there is a problem that the amount of wear of the photosensitive drum 401 increases and the life thereof decreases. In addition, since the adhesion between the charging roller 402 and the photosensitive drum 401 is improved, dirt or the like attached to the photosensitive drum 401 is easily attached to the surface of the charging roller 402. The adhesion of dirt or the like causes discharge unevenness of the charging roller 402.

(Other embodiments)
5A, 5B, and 5C show the charging roller of the second, third, and fourth embodiments. In the second, third, and fourth embodiments, compared to the first embodiment, the charging roller 21 and the cleaning roller 28 are 90 °, 180 °, and 90 ° counterclockwise around the rotation axis A21 of the photosensitive drum 21. It is different in that it is provided at a position of 270 °. These embodiments also have the same effects as the first embodiment. As described above, since the installation position of the charging roller 21 and the like is not substantially restricted, the degree of freedom in designing the image forming apparatus is increased, and downsizing is facilitated.

  6A, 6B, and 6C show the charging roller, the cleaning roller, and the urging means of the second, third, and fourth comparative examples. Compared with the first comparative example (see FIG. 4A), the second, third, and fourth comparative examples are configured such that the charging roller, the cleaning roller, and the urging means are centered on the rotation axis of the photosensitive drum. It differs in that it is provided at 90 °, 180 ° and 270 ° counterclockwise.

This inventor evaluated the technical effect of 1st embodiment and 3rd embodiment, contrasting with a 1st comparative example and a 3rd comparative example. The outline of the evaluation method is the following steps (1) to (3).
(1) Drive a modified version of the imaging unit in FIG. 2 to develop a halftone image (2) Print out the halftone image developed in (1) above on A4 size paper (3) (2) Sensory evaluation of degree of occurrence of charging unevenness due to slip in printed matter

  Occurrence is shown in four levels. Specifically, when the degree of occurrence of charging unevenness is 5% or less, ◎ is given as the highest evaluation level. Also, if it is more than 5% and 10% or less, the next evaluation level ○ is more than 10% and if it is 30% or less, the third evaluation level Δ is more than 30%. Level x is attached.

  Specific charging conditions are as follows. First, a charging roller having an outer diameter of 12 mm, an axial diameter of 8 mm, and an elastic layer of 2 mm is used. The material of the elastic layer is epichlorohydrin rubber. In addition, a resistance adjustment layer in which roughness particles are mixed is formed on the surface layer of the elastic layer. A charging bias voltage obtained by superimposing an AC bias on a DC bias is applied to the charging roller by constant voltage control. This charging bias voltage is a sine wave having a voltage center of −500 V, a peak-to-peak voltage of 1500 V, and a frequency of 1 kHz.

  As other parameters, the process speed Vp, the urging force F, and the inclination angle θ are changed. The process speed Vp is a parameter indicating the followability of the charging roller with respect to the photosensitive drum. The faster the process speed Vp, the greater the moment of the photosensitive drum by the drive motor than the frictional force between the charging roller and the photosensitive drum. Therefore, the follower rotation of the charging roller with respect to the photosensitive drum is reduced. In this evaluation, evaluation was performed for a speed region from 100 mm / s to 300 mm / s.

  The urging force F is a parameter related to the pressure contact force Fa to the photosensitive drum and the rotation assist force to the charging roller. The total pressure due to the spring load is 10N or 15N. Note that, for example, the first embodiment and the third embodiment have different forces due to their own weights of the charging roller and the urging means, and it is necessary to consider this, but this evaluation is based on the urging force F applied to the charging roller. Based on the total pressure.

  Further, as shown in FIG. 7, the inclination angle θ is an angle of the biasing direction ε with respect to the direction ζ from the rotation center of the charging roller 22 toward the rotation center of the photosensitive drum 21. This inclination angle θ is also a parameter related to the pressure contact force Fa and the rotation assist force, as in the case of the urging force F. The inclination angle θ was evaluated at four levels within a range of 5 ° to 45 °. The evaluation was limited to 45 ° because the rotation assist force Fb (= F × sin θ) exceeds the pressure contact force Fa (= F × cos θ) when the urging force F is disassembled as shown in FIG. is there. More specifically, since the press contact force Fa contributes the most to the followability, the press contact force Fa needs to be greater than the rotation assist force Fb. When the pressing force Fa decreases, the followability also decreases. In addition, due to structural constraints such as the outer diameter of the charging roller 22 and the photosensitive drum 21, the evaluation was limited to 45 °.

Table 1 shows the evaluation results under the above conditions.

  From the evaluation results shown in Table 1, it can be seen that the first and third embodiments have an effect of suppressing the occurrence of charging unevenness due to slip compared to the first and third comparative examples. In particular, in the high-speed region where the followability is lowered, it has been found that the first and third embodiments have a small degree of occurrence of charging unevenness due to slip, and a good effect can be obtained. In other words, the first and third embodiments have been found to be compatible with high-speed image forming apparatuses.

  Moreover, it can be seen from the evaluation results in Table 1 that the third embodiment can suppress the occurrence of charging unevenness due to slip. That is, it is preferable from the viewpoint of followability that the urging force F applied to the charging roller is increased by the weight of the charging roller and the urging means.

  Further, when the same urging force F is applied to the charging rollers of the first embodiment and the first comparative example, the first embodiment is less likely to cause uneven charging due to slip and has better followability. Further, with respect to the inclination angle θ, the rotation assisting force Fb increases as it approaches 45 °. Therefore, the degree of occurrence of charging unevenness due to slip is small, and the followability is improved.

The applicant of the present application also evaluated the influence of the durability of the first to fourth embodiments. For comparison, the same evaluation was performed for the first comparative example to the fourth comparative example. The outline of the evaluation method is the following steps (1) to (3).
(1) A modified version of the imaging unit shown in FIG. 2 is driven and repeatedly developed and endured. (2) After the end of (1) above, a halftone image is printed out on A3 size paper. (3) Above (2) Sensory evaluation of the degree of charging unevenness caused by dirt on the charging roller

  Occurrence is shown in four levels. Specifically, when the density fluctuation is 3% or less in A3 size paper, ◎ is given as the highest evaluation level. Also, if it is more than 3% and less than 5%, the next evaluation level ○ is more than 5%, and if it is less than 10%, the third evaluation level △ is more than 10% and the lowest evaluation level Level x is attached.

  The durability condition in the above (1) is about 40 hours at a system speed Vp of 100 mm / s. The specific charging conditions are the same as described above. Further, the urging force F is 15N and 20N, and the inclination angle θ is only 45 °.

Table 2 shows the evaluation results based on the above conditions.

  As shown in the evaluation results of Table 2, when the urging force F is 20 N, the evaluation level is X in all the comparative examples, whereas the evaluation level is ◯ or Δ in all the embodiments. The reason for the occurrence of intense noise in the half-tone image in all the comparative examples is considered to be that dirt such as toner on the photosensitive drum due to durability adheres to the charging roller, resulting in uneven charging. . On the other hand, in all the embodiments, since the pressure contact force Fa of the charging roller to the photosensitive drum is small, the charging roller is difficult to get dirty, and as a result, it is considered that a good evaluation level was obtained.

  The inventor of the present application measured the wear amount of the photosensitive drum of the first embodiment and that of the first comparative example after the endurance. The amount of wear per rotation of the photosensitive drum 100 k was 1.2 μm in the first comparative example, but was 0.6 μm, which was reduced by about 50% in the first embodiment. Here, if the pressure contact force Fa of the charging roller in the first comparative example is 1, that of the first embodiment is about 0.7, which is about 30% reduction. According to the first embodiment, it can be seen that the wear amount is reduced more than the reduction of the pressing force Fa of the charging roller. The decrease in the amount of wear is considered to be due to a decrease in friction between the photosensitive drum and the charging roller due to a decrease in the pressure contact force Fa of the charging roller.

(Configuration of the cleaning roller of the first to fourth embodiments)
The cleaning roller 28 has, for example, an outer diameter of 10 mm, a shaft having a diameter of 6 mm, and a foam layer having a thickness of 2 mm around the shaft. The cleaning roller 28 is in contact with the peripheral surface of the charging roller 22 and rotates following the rotation of the charging roller 22. As shown in FIG. 3B, the cleaning roller 28 is supported by the same bearings 32 a and 32 b as the charging roller 22 so as to be pressed in the direction of the rotation center of the charging roller 22. In addition, the cleaning roller 28 may be supported by a bearing different from the bearings 32a and 32b.

  Depending on the position of the cleaning roller 28 with respect to the charging roller 22 and the pressing force that the cleaning roller 28 applies to the charging roller 22, the pressing force Fa and the rotation assisting force Fb that the charging roller 22 applies to the photosensitive drum 21 vary. Here, as shown in FIG. 8A, a line connecting the rotation center of the cleaning roller 28 and the rotation center of the charging roller 22 is a center line L1, and the rotation of the charging roller 22 is perpendicular to the center line L1. A line passing through the center is defined as a center line L2. At this time, the cleaning roller 28 rotates around the charging roller 22 within a range of 180 ° or more and 270 ° or less on the downstream side in the rotation direction of the charging roller 22 with reference to the contact position between the photosensitive drum 21 and the charging roller 22. It is preferable to contact the surface.

  At the contact position as described above, the cleaning roller 28 presses in the direction of the rotation center of the charging roller 22. As shown in FIG. 8B, the pressing force F ″ of the cleaning roller 28 is equal to the force Fa ″ in the same direction as the pressure contact force Fa (hereinafter referred to as pressure contact assist force) and the rotation assist force Fb in the same direction. Force Fb ″ (hereinafter referred to as second rotation assisting force) Fb ″. The pressure assisting force Fa ″ and the second rotation assisting force Fb ″ can further improve the followability of the charging roller 22. It becomes possible to suppress uneven charging of the photosensitive drum 21.

  As shown in FIG. 8C, when the cleaning roller 28 is in contact with the charging roller 22 at a position 270 ° from the contact position between the photosensitive drum 21 and the charging roller 22, the pressing of the cleaning roller 28 is performed. It is more preferable because all of the pressure can be used for the second auxiliary rotation force Fb ″.

  Further, as shown in FIG. 8D, when the cleaning roller 28 is in contact with the charging roller 22 at a position 180 ° from the contact position between the photosensitive drum 21 and the charging roller 22, the pressing of the cleaning roller 28 is performed. It is possible to use all of the pressure for the pressure assisting force Fa ″.

  Here, FIGS. 9A and 9B show the cleaning roller 403 provided outside the range of 180 ° or more and 270 ° or less on the downstream side in the rotation direction of the charging roller 402.

  In FIG. 9A, the cleaning roller 403 is provided at a position less than 180 °, and presses the charging roller 402 toward the rotation center of the charging roller 402. Since this pressing force F ″ includes a component opposite to the rotation assisting force Fb, the rotation assisting force Fb is weakened. As a result, the followability is reduced.

  In FIG. 9B, the cleaning roller 403 is provided at a position exceeding 270 °, and presses the charging roller 402 toward the rotation center of the charging roller 402. In this case, since the pressing force F ″ includes a component in the same direction as the rotation assisting force Fb, it is possible to improve the followability. However, the force F ″ includes a component in the opposite direction to the pressing force Fa. Since the pressure contact force Fa itself is weakened, this is contrary to the basic concept of the present embodiment (the charging roller presses the photosensitive drum). Further, in the case of FIG. 9B, there is a structural problem that the cleaning roller 403 easily comes into contact with the photosensitive drum 401.

(First example of cleaning roller pressing direction)
The cleaning roller 28 preferably presses the charging roller 22 in the direction of the rotation center of the charging roller 22. As a result, the pressing force of the cleaning roller 28 can be efficiently transmitted to the charging roller 22. Furthermore, since the followability of the cleaning roller 28 with respect to the charging roller 22 is improved, the charging roller 22 can be reliably cleaned.

  Here, for example, when the shaft of the cleaning roller 28 is supported by the bearings 32a and 32b, the distance between the shaft of the cleaning roller 28 and the shaft of the charging roller 22 is a fixed value. The charging roller 22 is pressed in the direction of the rotation center 22. In addition, when the bearing of the charging roller 22 and the bearing of the cleaning roller 28 are separate, the charging roller 22 can be pressed in the direction of the rotation center of the charging roller 22, but may be pressed in a different direction. It becomes possible.

(Second example of pressing direction of cleaning roller)
In addition to the above, the pressing direction of the cleaning roller 28 may be as shown in FIG. Specifically, in FIG. 10, the direction from the rotation center of the cleaning roller 28 to the rotation center of the photosensitive drum 21 through the rotation center of the charging roller 22 is defined as a direction ζ. In this case, the pressing direction η is designed to be inclined downstream of the rotation direction δ of the charging roller 22 with respect to the direction ζ. With this configuration, the followability of the cleaning roller 28 with respect to the charging roller 22 can be improved.

(Appendix)
In the above description, the image forming apparatus is described as being a tandem type, but is not limited thereto, and may be a cycle type. Further, the image forming apparatus may be capable of only monochrome printing.

  The outer diameter and material of the charging roller, and the charging bias condition are not limited to those described above, and appropriate values and materials may be selected as appropriate. For example, the material of the elastic layer of the charging roller is mainly composed of one selected from epichlorohydrin rubber, ethylene propylene rubber, nitrile rubber, chloroprene rubber, urethane rubber, silicon rubber, or a mixture of two or more. Rubber composition to be used. Among these, as the material for the elastic layer, a material mainly composed of epichlorohydrin rubber is desirable.

  In the above, the image forming apparatus includes the cleaning roller 28 as a preferable example. However, instead of the cleaning roller 28, a cleaning brush, a non-rotating type cleaning pad, or a cleaning film may be provided.

  The image forming apparatus according to the present invention can improve the followability of rotation with the photosensitive drum while reducing the pressing force of the charging roller to the photosensitive drum, and includes an MFP (Multifunction Peripheral) and a laser printer. Useful for copying machines, facsimiles, etc.

21 Photosensitive drum 22 Charging roller 32 Energizing means β Rotating direction of photoconductive drum ε Energizing direction

Claims (1)

An image carrier that rotates in a predetermined rotation direction;
A charging roller that is in contact with the image carrier and is driven to rotate by rotation of the image carrier to charge the peripheral surface of the image carrier;
Biasing means for biasing the charging roller in a predetermined biasing direction;
A cleaning roller that presses the charging roller and cleans the charging roller by rotating;
With
The cleaning roller is disposed at a position rotated by an angle of 180 ° or more and 270 ° or less on the downstream side in the rotation direction of the charging roller with reference to a contact position between the image carrier and the charging roller.
The predetermined biasing direction, based on the direction towards the center of rotation of said image bearing member from the center of rotation of the charging roller is inclined to the downstream side in the rotation direction of the image carrier,
The pressing direction of the cleaning roller is inclined to the downstream side in the rotation direction of the charging roller with reference to the direction from the rotation center of the cleaning roller to the rotation center of the charging roller.
Image forming apparatus.

Images