JP2010032574A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus in which charging rollers for all colors are replaced nearly at the same time irrespective of their use efficiency, and also, which improves image quality of a full color image. <P>SOLUTION: In a tandem image forming apparatus 100, the charging rollers 2a to 2d, photoreceptor drums 1a to 1d and a cleaning member 21 for cleaning the charging rollers 2a to 2d are provided. Image formation is performed by varying the cleaning power of a brush 21c of the cleaning member 21 with respect to the charging roller 2d between a full color mode and a monochromatic mode. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image forming apparatus using an electrophotographic method, and more particularly to a charger for charging a photoreceptor.

  FIG. 9 shows a schematic diagram of a conventional tandem type color image forming apparatus (printer), and FIG. 10 shows a configuration around the charging roller and the photosensitive drum. In the main body of the image forming apparatus 100, four image forming portions Pa, Pb, Pc, and Pd are sequentially arranged from the upstream side in the transport direction (the right side in FIG. 9). These image forming portions Pa to Pd are provided corresponding to images of four different colors (magenta, cyan, yellow, and black), and magenta, cyan, and yellow are respectively subjected to charging, exposure, development, and transfer processes. And a black image are sequentially formed.

  Photosensitive drums (image carriers) 1a, 1b, 1c, and 1d that carry visible images (toner images) of the respective colors are disposed in the image forming portions Pa to Pd, and these photosensitive drums 1a. The toner image formed on 1d is conveyed by a conveying belt 50 that moves adjacent to each image forming unit while rotating counterclockwise (arrow direction) in FIG. 9 by a driving means (not shown). The image is sequentially transferred onto the paper P, and further fixed on the paper P in the fixing unit 7 and then discharged from the apparatus main body. An image forming process for each of the photosensitive drums 1a to 1d is executed while rotating the photosensitive drums 1a to 1d clockwise in FIG.

  The paper P to which the toner image is transferred is accommodated in a paper cassette 16 at the lower part of the apparatus, and is conveyed to the image forming portions Pa to Pd via the paper supply roller 12a and the registration roller 12b. A sheet made of dielectric resin is used for the conveyance belt 50, and a belt in which the flange portions are overlapped and joined to each other to form an endless shape, or a belt without a seam (seamless) is used. A cleaning blade 19 for removing toner adhering to the conveyance belt 50 is disposed on the upstream side of the driven roller 11.

  Next, the image forming units Pa to Pd will be described. There are charging rollers 2a, 2b, 2c and 2d for charging the photosensitive drums 1a to 1d and image information on the photosensitive drums 1a to 1d around and above the photosensitive drums 1a to 1d, which are rotatably arranged. LED heads 4a, 4b, 4c, and 4d for individually exposing, developing units 3a, 3b, 3c, and 3d that form toner images on the photosensitive drums 1a to 1d, and residual on the photosensitive drums 1a to 1d. Cleaning sections 5a, 5b, 5c and 5d for removing the developer (toner) are provided.

  When the start of image formation is input by the user, first, the surfaces of the photosensitive drums 1a to 1d are uniformly charged by the charging rollers 2a to 2d. The charging rollers 2a to 2d are chargers of a type that uniformly charges the drum surface by applying a voltage in contact with the surfaces of the photosensitive drums 1a to 1d, and in particular, a high resistance rubber roller is used. Many charging rollers are used.

  Next, light is irradiated by the LED heads 4a to 4d, and electrostatic latent images corresponding to image signals are formed on the respective photosensitive drums 1a to 1d. Each of the developing units 3a to 3d includes a developing roller (developer carrying member) disposed opposite to the photosensitive drums 1a to 1d, and each of magenta, cyan, yellow, and black toners is supplied by a replenishing device (not shown). A predetermined amount is filled. This toner is supplied onto the photosensitive drums 1a to 1d by the developing rollers of the developing units 3a to 3d, and electrostatically adheres to the electrostatic latent images formed by exposure from the LED heads 4a to 4d. A corresponding toner image is formed.

  Then, after an electric field is applied to the conveying belt 50 at a predetermined transfer voltage, the magenta, cyan, yellow, and black toner images on the photosensitive drums 1a to 1d are conveyed to the conveying belt 50 by the transfer rollers 6a to 6d. Is transferred onto the sheet P. These four color images are formed with a predetermined positional relationship predetermined for forming a predetermined full-color image. Thereafter, the toner remaining on the surfaces of the photosensitive drums 1a to 1d is removed by the cleaning units 5a to 5d in preparation for the subsequent formation of a new electrostatic latent image. As shown in FIG. 10, the cleaning units 5a to 5d are provided with cleaning blades 5aa, 5ba, 5ca, and 5da, and residual toner is scraped off by the cleaning blades 5aa to 5da.

  The conveyance belt 50 is stretched between the driving roller 10 and the driven roller 11. When the conveyance belt 50 starts to rotate counterclockwise as the driving roller 10 is rotated by a driving motor (not shown), the paper P is loaded. The images are conveyed from the registration roller 12b to the image forming portions Pa to Pd at a predetermined timing, and the images are sequentially transferred onto the paper P at the nip portion with the conveying belt in each image forming portion, thereby forming a full color image. The paper P on which the toner image is transferred is conveyed to the fixing unit 7.

  The sheet P conveyed to the fixing unit 7 is heated and pressurized when passing through the nip portion (fixing nip portion) of the pair of fixing rollers 13 to fix the toner image on the surface of the sheet P, and a predetermined full-color image is formed. It is formed. The paper P on which the full color image is formed is discharged to the discharge tray 17 by the discharge roller 15. When a monochrome image is formed, only the image forming unit Pd is operated and image formation is performed in the same manner as described above.

  As described above, in the color image forming apparatus using the electrophotographic method, the recording medium transported to the transport belt 50 comes into contact with the photoreceptors 1a to 1d carrying magenta, cyan, yellow, and black toners. Thus, the toner images formed on the surface of the photosensitive member are sequentially transferred. In the full color mode for transferring a full color image, the charging rollers 2a to 2d charge all the photosensitive drums 1a to 1d for cyan, magenta, yellow and black to form toner images, but transfer a monochrome image. In the monochrome mode, only the black photosensitive drum 1d is charged by the charging roller 2d to form a toner image.

  However, in the general market, the ratio of monochrome image formation (printing) is still higher than full-color image formation (printing), and the black charging roller 2d is replaced by the other three-color charging rollers 2a to 2c. It is often charged more frequently than it is. As a result, dirt such as a toner external additive adheres to the surface of the charging roller 2d and the service life is shortened, so that the replacement timing is faster than the other charging rollers 2a to 2c.

  In such a case, since it is necessary to replace the other charging rollers 2a to 2c after replacing the charging roller 2d, the downtime is doubled compared to the case where the black and color charging rollers 2a to 2d are replaced at the same time. As a result, both the user and the serviceman are forced to bear a burden.

  For this reason, a method for extending the life of the black charging roller has been proposed. For example, Patent Document 1 discloses a tandem color image forming apparatus including a plurality of photoconductors, in which a charging unit for a photoconductor for recording a black electrostatic image is charged in a non-contact manner. By using a contact-type charging roll as the charging means for the photosensitive member that records electrostatic images of colors other than black, the photosensitive member is prevented from being worn by charging. A method for enabling the use of a photoreceptor and toner and reducing the cost is disclosed.

  However, in the method of Patent Document 1, a corotron charger is used as a charger for black. For this reason, although it is one of the four colors, there is a possibility that the ozone emission amount may increase by using a corotron (or scorotron) charger. In particular, when an amorphous silicon (a-Si) photoconductor is used, it is necessary to flow a large amount of charging current through the corotron charger, which may further increase ozone discharge.

  For example, when the charger and the photosensitive drum are unitized, the replacement of the unit is forced due to the life of the charger, so that it is necessary to extend the life of the charger. In particular, when the a-Si photosensitive drum is charged, the life of the charger tends to be shorter than when the organic photosensitive drum is charged. For this reason, a charging roller with a small ozone discharge amount is used, and a method for cleaning the surface of the charging roller is used.

However, when the cleaning force is increased, there is a possibility that banding images and the like are generated by inducing rotation fluctuation of the charging roller. Thus, for example, Patent Document 2 discloses a rotatable cleaning member that has a contact portion and a non-contact portion in the circumferential direction, and that cleans the charging roll. By holding the opposite position and stopping, after the image formation is completed, the surface of the charging roll is cleaned by rotating it for a predetermined time during non-image forming rotation, thereby effectively improving the removal of foreign matter on the surface of the charging roll. A method for realizing high-quality and low-cost image formation for a long period of time is disclosed.
Japanese Patent No. 3587094 JP 2002-196568 A

  However, with the method of Patent Document 2, it is difficult to extend the life of the charging roller for a predetermined color. For this reason, for example, when the frequency of black image formation is high, the life of the black charging roller is shortened, and it is difficult to make the life of the charging roller almost the same for all colors.

  Furthermore, since the user may frequently print not only black but also a predetermined color, for example, the life of the charging roller that is frequently used is extended according to the usage situation such as the purpose and application of the user. There is a need. On the other hand, if the cleaning power of the charging roller for a predetermined color is increased, the dirt attached to the surface of the charging roller can be removed, but the image may be deteriorated. For this reason, it is necessary to optimize the setting of the cleaning power of the charging roller by taking various balances according to the specifications required for each apparatus main body.

  SUMMARY OF THE INVENTION In view of the above problems, the present invention provides an image forming apparatus capable of making the replacement timing substantially the same for all colors and improving the image quality of a full-color image even when there is a frequently used charging roller. With the goal.

  In order to achieve the above object, the present invention provides an image forming apparatus comprising a plurality of image forming units arranged in parallel, the image forming apparatus including an image carrier and a charging roller for charging the image carrier. A cleaning member for cleaning is provided, and the cleaning force of the cleaning member is variable according to the image to be formed.

  Further, the invention is characterized in that the cleaning member is arranged so that the cleaning member can bite into the charging roller, and the cleaning force is varied by varying a biting amount of the cleaning member. .

  According to the present invention, the cleaning member is disposed so as to be capable of reciprocating in the axial direction of the charging roller, and the cleaning force is varied by varying a reciprocating period of the cleaning member.

  According to the present invention, the cleaning member is composed of a rotating body that is rotatably supported, and the cleaning force is varied by varying the number of rotations of the cleaning member.

  According to the present invention, the image carrier includes a black image carrier for forming a black image and a color image carrier for forming an image other than black, and the charging roller includes the black image carrier. A black charging roller for charging the image bearing member and a color charging roller for charging the color image bearing member. The cleaning force of the cleaning member for cleaning the black charging roller is a full color image. It is characterized by being variable at the time of formation and at the time of monochrome image formation.

  According to the first configuration of the present invention, the cleaning force of the cleaning member with respect to the charging roller is changed according to the image to be formed, so that cleaning is performed when an image of a predetermined color (spot color) that is frequently used is formed. Since it is possible to weaken the force and increase the cleaning power when forming a full-color image, it is possible to delay the deterioration of the frequently used charging roller and bring its life closer to other charging rollers. Can be exchanged at approximately the same time.

  Further, it is possible to suppress a desired deterioration rate of the charging roller according to the use state of the user. Furthermore, since the cleaning power of all the charging rollers can be made substantially the same when forming a full-color image, the full-color image quality can be maintained at a high image quality. Therefore, even if there is a charging roller that is frequently used, it is possible to make the replacement timing substantially the same for all colors and to improve the image quality of a full-color image.

  According to the second configuration of the present invention, in the image forming apparatus having the first configuration, the cleaning member is disposed so as to be able to bite into the charging roller, and cleaning is performed by varying the biting amount of the cleaning member. The force was made variable. As a result, the amount of cleaning member biting into the charging roller that is frequently used can be varied, so that the replacement timing of the charging roller can be made more effective at almost the same time and the image quality of the full color image can be improved. It becomes possible.

  According to the third configuration of the present invention, in the image forming apparatus having the first or second configuration, the cleaning member is disposed so as to be capable of reciprocating in the axial direction of the charging roller, and the reciprocating cycle of the cleaning member is set. By changing it, the cleaning power was made variable. As a result, the reciprocating cycle of the cleaning member with respect to the charging roller that is frequently used can be varied, so that the replacement timing of the charging roller can be made more effective at almost the same time and the image quality of the full color image can be improved. It becomes possible.

  According to the fourth configuration of the present invention, in the image forming apparatus having any one of the first to third configurations, the cleaning member is configured by a rotating body that is rotatably supported, and the cleaning member By changing the number of rotations, the cleaning force was made variable. As a result, the rotation speed of the cleaning member with respect to the charging roller that is frequently used can be varied, so that the replacement timing of the charging roller can be made more effective at almost the same time and the image quality of the full color image can be improved. It becomes possible.

  According to a fifth configuration of the present invention, in the image forming apparatus having any one of the first to fourth configurations, the image carrier is composed of a black image carrier for forming a black image, and a black image carrier. A color image bearing member for forming an image other than the above, and a charging roller comprising a black charging roller for charging the black image bearing member and a color charging roller for charging the color image bearing member. The cleaning force of the cleaning member for cleaning the black charging roller is variable between full-color image formation and monochrome image formation.

  This is more effective because it suppresses the deterioration rate of the black charging roller, which is generally frequently used in the market, and can maintain a full color image with higher image quality.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic side view showing a configuration around an image forming unit and a charging roller of a tandem color image forming apparatus according to a first embodiment of the present invention, and FIG. 2 is a diagram of a black charging roller and a photosensitive drum. FIG. 3 is a schematic perspective view of the periphery as viewed from the left side of FIG. 1, and FIG. Portions common to FIGS. 9 and 10 are denoted by the same reference numerals and description thereof is omitted. In FIG. 1, the cleaning blades 5aa to 5da are omitted.

  As shown in FIG. 1, from the upstream side, a photosensitive drum 1a on which a cyan image is formed, a photosensitive drum 1b on which a magenta image is formed, a photosensitive drum 1c on which a yellow image is formed, and the outermost drum. On the downstream side, a photosensitive drum 1d on which a black image is formed is rotatably arranged. Further, the conveying belt 50 is stretched around the driving roller 10 and the driven roller 11, and is driven by the driving roller 10, so that the cyan photosensitive drum 1a, the magenta photosensitive drum 1b, the yellow photosensitive drum 1c, The black photosensitive drum 1d and the driven roller 11 are sequentially contacted and circulated counterclockwise.

Above the photosensitive drums 1a to 1d, charging rollers 2a to 2d that are rotatably contacted with the surfaces of the drums and charge the respective surfaces are disposed. As the charging rollers 2a to 2d, for example, a solid type such as a conductive rubber roller using epichlorohydrin rubber having a resistance value of 10 ⁵ to 10 ⁶ Ω and a surface roughness R _Z = 10 μm can be suitably used. It is not limited to this. In addition, for example, a sponge type in which a foam rubber roller is covered with a tube, a conductive brush, or the like can be used. The charging rollers 2a to 2d are supported so as to be driven by the apparatus main body, and are pressed against the photosensitive drums 1a to 1d with a predetermined nip pressure.

  As shown in FIG. 2, a cleaning member 21 is disposed above the charging rollers 2a to 2d so as to come into contact with the surfaces of the rollers and clean the surfaces. Since the configuration of the cleaning member 21 is the same for each color, the cleaning member 21 for cleaning the black charging roller 2d will be described.

  As shown in FIG. 2, the cleaning member 21 includes a support shaft 21a disposed substantially parallel to the charging roller 2d, and a brush 21c disposed along the lower surface of the support shaft 21a and contacting the surface of the charging roller 2d. Brush portion 21b. The brush 21c is made of, for example, a resin material such as conductive nylon, and has a hardness that can bite into the surface of the charging roller 21d.

  The support shaft 21a is urged downward from above by a coil spring (not shown). Further, both left and right end portions of the support shaft 21a are supported from below by a swinging member 23 capable of moving the cleaning member 21 up and down. The swing member 24 is fixed to a swing shaft 25 connected to a cam or the like (not shown), and can be swung up and down together with the swing shaft 25 by being driven by the cam or the like.

  As the swing member 24 swings, the vertical arrangement of the cleaning member 21 can be set. As a result, as shown in FIG. 3, the amount of biting D in the axial direction from the surface to the charging roller 2d of the brush 21c can be varied. The swing member 24 can be swung by a solenoid or the like in addition to the cam, and the means is not particularly limited.

  The cleaning member 21 having the above-described configuration is disposed on the charging rollers 2a to 2c in addition to the charging roller 2d. In the full-color mode for forming a full-color image, the amount of biting D of the brush 21c with respect to the charging roller 2d is set substantially the same as that of the other three-color charging rollers 2a to 2c, so that the cleaning power between the four colors is substantially reduced. It can be made uniform and a high quality image can be formed.

  On the other hand, in the monochrome mode for forming a monochrome image, the biting amount D of the brush 21c with respect to the charging roller 2d is made larger than that in the full color mode, so that the surface of the charging roller 2d can be effectively cleaned. Life can be extended.

  In this case, if the cleaning power is increased, banding image quality may be generated by inducing rotation fluctuation of the charging roller 2d, and the image quality is lower than that in the full color mode, but the monochrome mode requires higher image quality than the full color mode. Therefore, it is sufficient that a sufficient image quality can be formed practically. For example, an image of a practically sufficient level may be formed with a text image.

  As a result, even when the frequency of monochrome image formation is high, the cleaning force of the cleaning member 21 on the charging roller 2d can be sufficiently exerted to extend the life of the charging roller 2d, and the charging roller 2a can be used between four colors. The lifetime of ˜2d can be made substantially the same. Therefore, even if there is a charging roller that is frequently used, it is possible to make the replacement timing substantially the same for all colors and to improve the image quality of full-color images.

  Further, as described above, if the biting amount D of the brush 21c is increased, the life of the charging roller 2d in the monochrome mode can be easily extended, but there is a possibility that a practically sufficient image quality cannot be obtained. Further, if the biting amount D is reduced, it is possible to prevent deterioration in image quality in the monochrome mode, but it may be difficult to sufficiently extend the life of the charging roller 2d. Therefore, for example, the biting amount D can be appropriately set in consideration of these.

  FIG. 4 is a plan view showing a cleaning member used in the tandem color image forming apparatus according to the second embodiment of the present invention. As shown in FIG. 4, the outer peripheral surface of an eccentric cam 26 driven by a motor (not shown) is in contact with the left end surface of the support shaft 21 a of the cleaning member 21. Further, the support shaft 21a is biased to the left side of the drawing by a biasing member such as a tension spring (not shown).

  Thus, when the eccentric cam 26 rotates about the rotation shaft 26a, the support shaft 21a can reciprocate in the axial direction, and the reciprocating cycle of the cleaning member 21 can be varied by varying the rotational speed of the eccentric cam 26. can do. The same cleaning member 21 is disposed for the charging rollers 2a to 2c. Since other configurations are completely the same as those of the first embodiment, description thereof is omitted.

  According to the present embodiment, the cycle of the reciprocating motion of the cleaning member 21 that cleans the charging roller 2d can be varied, whereby the cleaning force of the cleaning member 21 can be varied. In the full-color mode, the reciprocating cycle is set to be substantially the same as that of the cleaning members 21 for the other three-color charging rollers 2a to 2c, and the cleaning power is set to a level sufficient to improve the image quality. In the monochrome mode, the reciprocation cycle is shortened and the cleaning power is increased compared to the full color mode, thereby improving the image quality of the full color image and extending the life of the charging roller 2d.

  Similar to the above, the reciprocating cycle can be set as appropriate in consideration of the image quality in the monochrome mode and the life extension of the charging roller 2d. In the present embodiment, since the reciprocating cycle of the cleaning member 21 can be varied and the amount of biting D can be varied, the cleaning force of the charging roller 2d can be varied more effectively. However, it is of course possible to make the bite amount D substantially constant in the monochrome mode and the full color mode and to change only the cycle of the reciprocating motion.

  FIG. 5 is a perspective view showing a cleaning member and a charging roller used in a tandem type color image forming apparatus according to a third embodiment of the present invention, and FIG. 6 is a plan view seen from the direction A in FIG. 7 is a plan view seen from the axial direction of FIG. Portions common to those in FIGS. 1 to 4 are denoted by common reference numerals and description thereof is omitted.

  As shown in FIGS. 5-7, the cleaning member 21 is comprised from the brush roller which has the elongate column-shaped spindle 21a and the brush part 21b by which the brush 21c was arrange | positioned on the surface of the spindle 21a. ing. The support shaft 21a is rotatably supported by the bearing member 28 together with the rotation shaft 2da of the charging roller 2d.

  Further, the support shaft 21a is driven to rotate independently of the charging roller 2d by a motor or the like (not shown), and the brush 21c cleans the surface of the charging roller 21 by rotating the support shaft 21a. It has become. The rotation direction of the cleaning member 21 is set to be a driven direction with respect to the charging roller 2d (see FIG. 7).

  A disc-like cam 30 that is curved and deformed in the axial direction is provided at the left end of the support shaft 21a. The disc-shaped cam 30 is formed by being slightly inclined with respect to the boss portion 30b having an oval hole 30a that is non-rotatably fitted to the support shaft 21a from the direction perpendicular to the axis. In addition, a bottomed square box-like case member 31 into which the disc-shaped cam 30 is fitted is provided, and a part of the outer peripheral portion of the disc-shaped cam 30 is disposed on both sides in the axial direction near the bottom of the case member 31. A pair of protrusions 31a sandwiched between the two are formed.

  The case member 31 is fixed to the bearing member 28 so as not to move in the axial direction. Thus, when the cleaning member 21 is driven to rotate, the disk-shaped cam 30 of the cleaning member 21 is guided by the pair of protrusions 31a of the case member 31, and the cleaning member 21 can be reciprocated in the axial direction. Further, the rotational speed and the reciprocation cycle of the cleaning member 21 can be varied by varying the rotational speed of the motor. The same cleaning member 21 is disposed for the charging rollers 2a to 2c.

  According to this embodiment, the rotation speed of the cleaning member 21 of the charging roller 2d is set in the same manner as the other three colors in the full color mode, for example, and in the monochrome mode, the rotation speed is set larger than that in the full color mode. In the monochrome mode, the cleaning power is weakened to improve the image quality. In the monochrome mode, the cleaning power is increased to extend the life of the charging roller 2d. As described above, the number of rotations can be set as appropriate in consideration of the image quality in the monochrome mode and the viewpoint of extending the life of the charging roller 2d.

  In the present embodiment, since the rotational speed of the cleaning member 21 is varied and the reciprocating motion is performed in the axial direction, the reciprocation cycle can be varied, and the cleaning force of the charging roller 2d can be varied more effectively. can do. However, the disk-shaped cam 30 is disposed substantially parallel to the direction perpendicular to the axis instead of being inclined with respect to the direction perpendicular to the axis, so that the arrangement of the cleaning member 21 in the axis direction is substantially constant and only the rotational speed is variable. Of course, it is also possible to adopt a configuration. Other configurations are the same as those in the first embodiment, and a description thereof will be omitted.

  Note that, for example, a swing member 24 similar to that of the first embodiment may be disposed on the rotation shaft 2da of the charging roller 2d, and the biting amount D (see FIG. 3) of the brush 21c may be varied. With this configuration, the biting amount D, the reciprocation cycle, and the rotation speed can be varied, so that the cleaning power can be varied in the monochrome mode and the full color mode more appropriately and appropriately.

  FIG. 8 is a schematic configuration diagram of a tandem type color image forming apparatus according to the fourth embodiment of the present invention. In the present embodiment, instead of the conveyance belt 50, the intermediate transfer belt 8 stretched by the driving roller 10, the driven roller 11, and the tension roller 23 is used, and the toner images formed on the photosensitive drums 1a to 1d are transferred. After sequentially transferring (primary transfer) onto the intermediate transfer belt 8 by the transfer rollers 6a to 6d, the transfer is performed at once (secondary transfer) onto the paper P by the secondary transfer roller 9.

  Further, the image forming portions Pa to Pd are arranged below the intermediate transfer belt 8, and electrostatic latent images are formed on the photosensitive drums 1a to 1d using the exposure unit 4 instead of the LEDs 4a to 4d. Other configurations are the same as those in the first to third embodiments, and thus description thereof is omitted.

  In addition, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, the amount of biting, the reciprocation cycle, and the number of rotations described above may be varied individually or in combination, and are not particularly limited. Further, the amount of biting, the reciprocation period, and the number of rotations are merely examples for changing the cleaning force, and other configurations may be used as long as the cleaning force can be changed.

  In the above embodiment, the cleaning force of the cleaning member with respect to one charging roller (black charging roller) is changed. However, in the case where, for example, a two-color image is frequently formed, the two colors are supported. It is also possible to vary the cleaning force of the charging roller. In the above-described embodiment, the a-Si photosensitive drums 1a to 1d are used. However, the present invention is not particularly limited to this, and the same effect can be obtained even when an organic photosensitive drum is used. be able to.

  Further, the charging rollers 2a to 2d, the photosensitive drums 1a to 1d, and the like can be unitized with each color. In the above embodiment, as an example, a tandem color printer having a plurality of image forming units has been described. However, the present invention is not limited to this, and the present invention is also applicable to image forming apparatuses such as copiers and facsimiles. be able to.

  Hereinafter, although an example and a comparative example explain the present invention still more concretely, the present invention is not limited to this example.

  Using the tandem type color image forming apparatus according to the first embodiment of the present invention shown in FIG. 1, the amount of biting of the brush 21c of the cleaning member 21 with respect to the black charging roller 2d is changed according to the following conditions, and the monochrome image and full color are displayed. An image was formed.

  In the monochrome mode, a text image (character image) with a printing rate of 5% was formed, and in the color mode, a halftone image with a printing rate of 5% was formed, and the image quality of each image was evaluated. In addition, a running test was performed in a normal temperature / humidity environment (20 ° C., 60% RH) with a printing ratio (monochrome / full color) of a monochrome image and a full color image of 2/6.

  For both monochrome images and full-color images, a document for image evaluation was printed at the beginning of printing, and then a document for image evaluation was printed at regular intervals, and image evaluation was performed for image quality and durability. The evaluation criteria for the image quality of a monochrome image are “◯” when the character of the monochrome image is clear at each sampling time, “△” when the character is not clear, and “X” when the character is clearly not clear. It was.

  The evaluation criteria for the image quality of a full-color image are “◯” when there is no density unevenness in the halftone image, “Δ” when density unevenness is recognized, and “when the density unevenness is noticeable”. × ”. In addition, the evaluation standard for durability is “○” when no image abnormalities such as fogging and streaks due to contamination of the charging rollers 2a to 2d are observed after printing 500,000 sheets, and charging is performed before printing 500,000 sheets. A case where an image abnormality such as fogging due to dirt on the rollers 2a to 2d, generation of streaks, or the like was recognized was indicated as “x”.

<Comparative Example 1>
Image formation was performed with the biting amount D of the cleaning member 21 with respect to the charging roller 2d set to 0.5 mm in both the color mode and the monochrome mode (image forming conditions in which the black cleaning force was increased in both modes). The amount of biting D of the cleaning member 21 with respect to the charging rollers 2a to 2c was also set to 0.5 mm.

<Comparative example 2>
Image formation was performed with the biting amount D of the cleaning member 21 with respect to the charging roller 2d set to 0.3 mm in both the color mode and the monochrome mode (image forming conditions in which the black cleaning power was weakened in both modes). The amount of biting D of the cleaning member 21 with respect to the charging rollers 2a to 2c was also set to 0.3 mm.

<Example>
Image formation was performed with the amount D of the cleaning member 21 with respect to the charging roller 2d set to 0.3 mm in the color mode and 0.5 mm in the monochrome mode (image formation in which the black cleaning power is weak in the color mode and strong in the monochrome mode). conditions). The amount of biting D of the cleaning member 21 with respect to the charging rollers 2a to 2c was 0.3 mm.

  As shown in Table 1, in the comparative example 1, in the image quality evaluation, although the characters of the monochrome image were not slightly clear, they could be read sufficiently, and the character image satisfies the quality level that can sufficiently withstand practicality. It was. Further, even in the durability evaluation, even after printing 500,000 sheets, no abnormal image such as fogging or streaks due to the contamination of the charging rollers 2a to 2d was observed. However, in the image quality evaluation of a full-color image, image density unevenness is recognized, and a full-color image that requires high-accuracy quality does not satisfy a quality level that can sufficiently withstand practicality.

  In Comparative Example 2, in the image quality evaluation, the characters of the monochrome image were clearly read and the density unevenness of the full color image was not recognized, but this was caused by the contamination of the charging roller 2d before reaching 500,000 prints. Image abnormalities such as fog and streaks were observed. As a result, it was found that if the cleaning power of the charging roller 2d, which is used more frequently than other colors, is weak, the life of the charging roller 2d is shortened.

  On the other hand, in the example, in the image quality evaluation of the monochrome image, although the character was not clear, the character image could be read sufficiently, and the quality level that could withstand practicality was satisfied. Further, no density unevenness was observed in the full-color image, and the full-color image that required high-accuracy quality satisfied a level that could sufficiently withstand practicality. Further, in the durability evaluation, fog and streaks caused by the charging rollers 2a to 2d were not recognized even after printing 500,000 sheets.

  In other words, the cleaning power of the charging roller 2d is weakened in the full color mode and strong in the monochrome mode, thereby improving the image quality of the full color image and extending the life of the black charging roller 2d.

  As a result, according to the present invention, the cleaning power of the charging roller 2d can be varied between the full color mode and the monochrome mode, so that it is possible to form a full color image and a monochrome image having a practically sufficient quality level. Further, it was found that the deterioration rate of the charging roller 2d that is frequently used can be suppressed and replaced with the other charging rollers 2a to 2c at substantially the same time. Note that this embodiment is merely an example of the present invention, and the printing ratio, the number of replacement sheets, and the like can be appropriately set by a preliminary test or the like.

  The same effect as above even when the cleaning member 21 of the second embodiment of the present invention is used and the reciprocation cycle of the member is 1 reciprocation / 5 rotations in the color mode and 1 reciprocation / 1 rotation in the monochrome mode. was gotten. Further, when the cleaning member 21 of the third embodiment of the present invention is used and the rotation speed of the charging roller 2d is set to 1 rotation, the rotation speed of the cleaning member 21 is 0.8 rotation in the color mode and 0. 0 in the monochrome mode. Even in the case of 6 rotations, the same effect was obtained.

  The present invention provides an image forming apparatus that includes an image carrier and a charging roller that charges the image carrier, and includes a plurality of image forming units arranged in parallel. A cleaning member for cleaning the charging roller includes: The cleaning force of the cleaning member is variable according to the image to be formed.

  As a result, even if there is a charging roller that is frequently used, it is possible to make the replacement timing substantially the same for all colors and to improve the image quality of a full-color image. Further, the cleaning member is arranged to be able to bite into the charging roller, and the amount of biting of the cleaning member with respect to the charging roller that is frequently used can be varied by varying the amount of biting of the cleaning member to vary the cleaning force. Therefore, it is possible to more effectively set the replacement timing of the charging roller to almost the same time for all colors and to improve the image quality of the full color image.

  Also, the cleaning member can be reciprocated in the axial direction of the charging roller, and the reciprocating cycle of the cleaning member with respect to the charging roller that is frequently used can be varied by changing the cleaning force by changing the reciprocating cycle of the cleaning member. Therefore, it is possible to more effectively set the replacement timing of the charging roller at substantially the same time for all the colors and to improve the image quality of the full color image.

  In addition, the cleaning member is composed of a rotating body that is rotatably supported. By changing the cleaning member's rotational speed, the cleaning member's rotational speed can be changed to change the rotational speed of the cleaning member with respect to the charging roller that is frequently used. Therefore, it is possible to more effectively set the replacement timing of the charging roller at substantially the same time for all the colors and to improve the image quality of the full color image.

  The image carrier is composed of a black image carrier for forming a black image and a color image carrier for forming an image other than black, and the charging roller is provided with a black image carrier. It consists of a charging roller for black to be charged and a charging roller for color to charge the image carrier for color, and the cleaning power of the cleaning member for cleaning the charging roller for black is used for full color image formation and monochrome image formation. In particular, the deterioration rate of the black charging roller, which is generally used frequently in the market, can be suppressed, and a full color image can be maintained with higher image quality, which is more effective.

FIG. 2 is a schematic side view showing a configuration around an image forming unit and a charging roller of the tandem type color image forming apparatus according to the first embodiment of the present invention. FIG. 2 is a schematic perspective view of the periphery of a black charging roller and a photosensitive drum as viewed from the left side of FIG. 1. FIG. 4 is an enlarged plan view of the vicinity of a contact portion around a charging roller and a cleaning member. These are top views which show the cleaning member used for the tandem type color image forming apparatus which concerns on 2nd Embodiment of this invention. These are the schematic perspective views which show the cleaning member and charging roller which are used for the tandem type color image forming apparatus which concerns on 3rd Embodiment of this invention. These are the top views seen from the A direction of FIG. These are the top views seen from the axial direction of FIG. These are the schematic diagrams which show the whole structure of the tandem type color image forming apparatus which concerns on 4th Embodiment of this invention. FIG. 1 is a schematic diagram showing an overall configuration of a conventional tandem type color image forming apparatus. FIG. 3 is a schematic side view showing a configuration around a charging roller and a photosensitive drum of a conventional tandem type color image forming apparatus.

Explanation of symbols

1a to 1d Photosensitive drum (image carrier)
2a to 2d Charging roller 3a to 3d Developing unit 5a to 5d Cleaning unit 8 Intermediate transfer belt 21 Cleaning member 21a Support shaft 21b Brush unit 21c Brush 24 Oscillating member 25 Oscillating shaft 26 Eccentric cam 26a Rotating shaft 28 Bearing member 30 Disc Cam 31 Case member 50 Conveying belt 100 Image forming apparatus P Paper

Claims (5)

In an image forming apparatus including a plurality of image forming units arranged in parallel, including an image carrier and a charging roller for charging the image carrier.
A cleaning member is provided for cleaning the charging roller;
An image forming apparatus, wherein a cleaning force of the cleaning member is varied according to an image to be formed. The cleaning member is arranged to be able to bite into the charging roller,
The image forming apparatus according to claim 1, wherein the cleaning force is varied by varying a biting amount of the cleaning member. The cleaning member is disposed so as to be capable of reciprocating in the axial direction of the charging roller,
The image forming apparatus according to claim 1, wherein the cleaning force is varied by varying a reciprocating cycle of the cleaning member. The cleaning member comprises a rotating body that is rotatably supported.
The image forming apparatus according to claim 1, wherein the cleaning force is changed by changing a rotation speed of the cleaning member. The image carrier comprises a black image carrier for forming a black image and a color image carrier for forming an image other than black,
The charging roller includes a black charging roller for charging the black image carrier, and a color charging roller for charging the color image carrier.
5. The image forming apparatus according to claim 1, wherein a cleaning force of the cleaning member for cleaning the black charging roller is variable between a full color image formation and a monochrome image formation. .