JP2005265910A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus that eliminates the need for a special member for cleaning a photoreceptor and is free from an image defect such as a brush mark. <P>SOLUTION: The image forming apparatus includes: the photoreceptor on which a latent image is formed; a developing roller which is disposed opposite to the photoreceptor and develops the latent image on the surface of the photoreceptor with two-component developer containing toner and magnetic carrier particles. In the image forming apparatus, the developing roller rotates opposite to the direction of the rotation of the photoreceptor in the position where the developing roller faces the photoreceptor. In addition, a change in the magnetic flux density of the developing roller in the direction of the normal line in a development nip that is a gap between the photoreceptor and the developing roller is ±5 mT or less. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus in which no brush mark is generated.

  A photosensitive member whose surface is charged and exposed to form an electrostatic latent image; a developing unit that develops the latent image on the surface by attaching toner to the surface of the photosensitive member; and a toner formed by the developing unit An electrophotographic apparatus having transfer fixing means for transferring and fixing an image onto an appropriate recording medium such as plain paper is widely used as a copying machine or a printer.

  In the image forming apparatus, since the photosensitive member is charged by a discharge device such as a corotron, it is considered that if the operation is continued for a long time, the surface of the photosensitive member is deteriorated or discharge products are accumulated.

  Therefore, every time a certain number of prints are performed, it is widely performed to clean the surface of the photoconductor to remove the deteriorated layer and discharge products on the surface.

  Conventionally, it has been common to clean a photoreceptor by bringing a member such as a blade into contact therewith, but the surface of the photoreceptor may be damaged by the blade. In addition, there is a problem that the cleaning ability decreases when the blade is worn.

As a means for solving the problems of the conventional discharge product removal method, an image forming apparatus provided with a renewal means for bringing a single-foam sponge roller into contact with a photosensitive member at a predetermined cycle via a web has been proposed (Patent Document 1). ).
JP-A-8-123195

  However, in the image forming apparatus, since the web is brought into contact with the surface of the photoconductor to remove the deteriorated layer and the discharge product on the surface of the photoconductor, a special for bringing the web into contact with the photoconductor. Not only is the member required, but the web is in contact with the surface of the photoconductor, resulting in a mark on the surface of the photoconductor, which may cause image defects such as brush marks.

  The present invention has been made to solve the above problems, and provides an image forming apparatus that does not require a special member for cleaning a photosensitive member and that does not cause image defects such as brush marks. For the purpose.

  According to the first aspect of the present invention, a photosensitive member on which a latent image is formed by exposure and a two-component developer that is disposed so as to face the photosensitive member and contains toner and magnetic carrier particles are used for the photosensitive member. An image forming apparatus including a developing roller for developing a latent image on a body surface, wherein the developing roller rotates so that a rotation direction is reversed at a portion facing the photoconductor, and between the photoconductor The present invention relates to an image forming apparatus characterized in that it is formed such that a change in magnetic flux density in the normal direction of the developing roller in a developing nip, which is a gap formed in the above, is within ± 5 mT.

  In an image forming apparatus including a photoreceptor and a developing roller, a magnetic brush is formed on the surface of the developing roller by the magnetic carrier in the two-component developer, and the magnetic brush rubs the surface of the photoreceptor. The toner adheres to the surface of the photoconductor, and the latent image on the surface of the photoconductor is developed.

  In the image forming apparatus, since the change in the normal direction magnetic flux density in the developing nip is small, the change in the shape of the magnetic brush in the developing nip is also small, and the region contributing to development can be widened. As a result, it is possible to achieve both a scraping effect of the discharge product due to the magnetic brush rubbing the surface of the photosensitive member and developability sufficient to obtain a sufficient density in terms of image quality.

  According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the developing roller is formed such that a minimum value of a normal direction magnetic restraining force is located in the developing nip. About.

  In the image forming apparatus, since the minimum value of the normal direction magnetic restraining force is located in the developing nip, the developer, particularly the magnetic carrier particles, easily move in the developing nip. Accordingly, since the magnetic brush easily enters the developing nip, it is possible to prevent the developer pool from being formed on the upstream side of the developing roller in the developing nip, that is, on the downstream side of the rotating direction of the photosensitive member. it can. Therefore, the occurrence of brush marks on the surface of the photoreceptor can be suppressed.

  According to a third aspect of the present invention, in the image forming apparatus according to the first or second aspect, the developing nip of the developing roller has an area closest to the developing roller and the photosensitive member. The present invention relates to an image forming apparatus that is within a range of ± 15 degrees along the rotation direction.

  The range that contributes to development in the developing roller is considered to be a range of ± 15 degrees along the rotation direction of the developing roller with the closest part to the photoconductor interposed therebetween.

  Therefore, in the developing roller, if the change in the normal magnetic flux density is within ± 5 mT within the above range, it is considered that the object of the present invention to suppress the generation of the brush mark on the surface of the photoreceptor can be suppressed. .

  A fourth aspect of the present invention relates to the image forming apparatus according to any one of the first to third aspects, wherein a normal direction magnetic flux density in the developing nip of the developing roller is 90 mT or more.

  If the magnetic flux density in the normal direction at the development nip is too low, the magnetic brush formed on the surface of the developing roller is too soft, and toner does not adhere reliably to the surface of the photoconductor, and the latent image on the surface of the photoconductor May not be stably developed. However, in the image forming apparatus, since the magnetic flux density in the normal direction at the developing nip portion is 90 mT or more, a magnetic brush having a hardness that can reliably adhere toner can be formed on the surface of the photoreceptor. The latent image on the surface of the developing roller is stably developed.

  According to a fifth aspect of the present invention, in the image forming apparatus according to any one of the first to fourth aspects, the developing roller includes a magnet roll in which a plurality of magnetic poles are arranged along a circumferential direction, and the magnet. The present invention relates to an image forming apparatus having a roll inserted therein and a rotatable sleeve.

  In the image forming apparatus of the present invention, the developing mode in which toner is attached to the latent image formed on the surface of the photoconductor and developing, and the discharge product scraping off the discharge product formed on the surface of the photoconductor It is common to operate in either mode. Then, in the discharge product scraping mode, the magnetic carrier particles are spiked on the surface of the developing roller to form a magnetic brush, and a developing bias voltage is applied to the developing roller for at least a certain period of time. By removing the toner from the brush and rotating the photoconductor, the surface of the photoconductor is rubbed with the magnetic brush from which the toner has been removed.

  Here, in the image forming apparatus, since the magnetic pole of the developing roller is fixed, if the magnetic pole is arranged so that a magnetic brush is formed in the developing gap portion of the developing roller, the developing gap portion Therefore, when the image forming apparatus is operated in the discharge product scraping mode, the surface of the photosensitive member can always be rubbed with the magnetic brush. Therefore, it is possible to effectively remove the deteriorated layer and discharge products generated on the surface of the photoreceptor.

  According to a sixth aspect of the present invention, in the image forming apparatus according to the fifth aspect, the magnet roll provided in the developing roller is magnetized so that the change in the normal direction magnetic flux density in the developing nip is within ± 5 mT. The present invention relates to an image forming apparatus that is a cylindrical permanent magnet.

  In the image forming apparatus, since a cylindrical permanent magnet is used as the magnet roll, a magnetic force sufficient to stably form the magnetic brush on the surface of the sleeve can be obtained. Also, it can be manufactured by forming various magnetic materials such as ferrite, NdFeB magnetic powder, and SmFeCo magnetic powder into a cylindrical shape with a predetermined diameter and sintering them while magnetizing them so as to form a specific magnetic pole pattern. Therefore, variations in magnetic pole distribution and normal direction magnetic flux density in the developing nip can be minimized.

  According to a seventh aspect of the present invention, in the image forming apparatus according to the fifth aspect, the magnet roll provided in the developing roller is a columnar or cylindrical permanent magnet, and a change in the normal direction magnetic flux density in the developing nip is achieved. The present invention relates to an image forming apparatus in which a part of the outer peripheral portion is deleted so that is within ± 5 mT.

  In the image forming apparatus, a permanent magnet that has been used in a conventional developing drum can be used as a magnet roll without a predetermined portion.

  As described above, according to the present invention, there is provided an image forming apparatus that does not require a special member for cleaning the photosensitive member and that does not cause image defects such as brush marks.

1. Embodiment 1
A color printer as an example of an image forming apparatus that can suitably carry out the image forming method and the discharge product removing method of the present invention will be described below.
(1) Configuration As shown in FIGS. 1 and 2, the color printer 10 includes a print unit 30 that forms a color image with toners of four colors, cyan, magenta, yellow, and black, and a supply unit that is positioned below the print unit 30. A paper tray 16 and a stacking tray 17 located above the print unit 30 are provided.

  As shown in FIGS. 1 and 2, the print unit 30 is developed by developing units 12C, 12M, 12Y, and 12K for cyan, magenta, yellow, and black, and developing units 12C, 12M, 12Y, and 12K, respectively. Photoconductors 13C, 13M, 13Y, and 13K and an intermediate transfer belt 14. The developing units 12C, 12M, 12Y, and 12K and the photosensitive members 13C, 13M, 13Y, and 13K are arranged in parallel so as to face the intermediate transfer belt.

  The color printer 10 is a so-called tandem full-color printer, and the four color toner images developed by the developing units 12C, 12M, 12Y, and 12K are transferred onto the intermediate belt 14 while the intermediate transfer belt 14 makes one round. Superimposed.

  Above the paper feed tray 16, a paper feed roller 18 is arranged so as to abut on the leading end of the paper P set in the paper feed tray 16 in the transport direction. Further, the transport path 19 for transporting the paper P is S-shaped upward from the side of the paper feed tray 16 where the paper feed roller 18 is provided to the stacking tray 17 via a transfer unit 22 and a fixing unit 28 described later. It extends to. Two sets of transport rollers 20 are arranged on the transport path 19. The paper P is fed one by one from the paper feed tray 16 along the transport path 19 along the transport path indicated by the two-dot chain line arrow in FIG. 1 by a paper feed roller 18 and a paper handling means (not shown). It is conveyed upward by the conveyance roller 20 and is conveyed to the stacking tray 17 via the transfer unit 22 and the fixing unit 28.

  The transfer unit 22 is provided with a belt conveyance roller 24A around which the intermediate transfer belt 14 is wound, and a transfer roller 26 pressed against the belt conveyance roller 24A. The intermediate transfer belt 14 is sandwiched between the nip portion of the belt conveyance roller 24A and the transfer roller 26, and the toner image is transferred from the intermediate transfer belt 14 when the paper P passes through the nip portion.

  A fixing unit 28 is disposed above the transfer unit 22. The fixing unit 28 includes a heat roller 28A and a backup roller 28B pressed against the heat roller 28A. When the paper P passes through the nip portion between the heat roller 28A and the backup roller 28B, the toner melts and solidifies. The image is fixed. The paper P on which the toner image has been fixed by the fixing unit 28 is discharged onto the stacking tray 17 by a paper discharge roller 29 disposed on the downstream side of the fixing unit 28 in the transport direction.

  Hereinafter, the print unit 30 will be described in more detail.

  As shown in FIGS. 1 and 2, the intermediate transfer belt 14 includes a belt conveyance roller 24A, a belt conveyance roller 24B disposed below the belt conveyance roller 24A, and an obliquely upper side of the belt conveyance roller 24B. It is wound around and supported by a belt conveyance roller 24C disposed on the opposite side of the conveyance path.

  The outer surface of the intermediate transfer belt 14 is a transfer surface 14A to which a toner image is transferred. The development units 12C, 12M, 12Y, and 12K, and the photoreceptors 13C, 13M, 13Y, and 13K face the transfer surface 14A. Are arranged in parallel, and the photoreceptors 13C, 13M, 13Y, and 13K are in contact with the transfer surface 14A. On the other hand, transfer rollers 32C, 32M, 32Y, and 32K are disposed on the opposite side of the photoreceptors 13C, 13M, 13Y, and 13K across the intermediate transfer belt 14, and the intermediate transfer belt 14 includes the transfer rollers 32C, 32M, and 32Y. , 32K are pressed against the photoreceptors 13C, 13M, 13Y, 13K.

  The photoreceptors 13C, 13M, 13Y, and 13K rotate in the counterclockwise direction as indicated by an arrow b in FIG.

  In FIG. 2, a toner concentration detection sensor 15 is disposed adjacent to the downstream side of the photoreceptor 13K with respect to the rotation direction of the intermediate transfer belt 14 indicated by an arrow a and facing the transfer surface 14A. The toner concentration detection sensor 15 has a function of detecting the concentration of toner transferred to the transfer surface 14A.

  Hereinafter, the developing units 12C, 12M, 12Y, and 12K and the photoreceptors 13C, 13M, 13Y, and 13K will be described. Hereinafter, the developing units 12C, 12M, 12Y, and 12K may be collectively referred to as “developing unit 12”, and the photosensitive members 13C, 13M, 13Y, and 13K may be collectively referred to as “photosensitive member 13”.

  As shown in FIGS. 2 and 3, an intermediate transfer belt 14, a brush roller 34, a charging roller 36, and a developing unit 12 are provided around the photoconductor 13 in order along the rotation direction b. The intermediate transfer belt 14, the brush roller 34, and the charging roller 36 are all in contact with the photosensitive surface 13 </ b> A of the photoreceptor 13. Further, between the charging roller 36 and the developing unit 12, an LED array head 40 that performs line exposure of the photosensitive surface 13A is disposed.

  As shown in FIGS. 3 and 4, the developing unit 12 is positioned below the developing roller 38 and the developing roller 38 disposed so as to face the photosensitive member 13, and the two-component developer is placed on the developing roller 38. Screw feeders 39A and 39B, and a housing 37 for housing the developing roller 38 and the screw feeders 39A and 39B. The two-component developer contains toner and magnetic carrier particles as main components. An opening 37 </ b> A is provided in a portion of the housing 37 that faces the photoreceptor 13.

  As shown in FIG. 4, the developing roller 38 is disposed such that a gap, that is, a developing gap is formed between the developing roller 38 and the photosensitive surface 13A. The developing roller 38 includes a cylindrical magnet roll 38B and a sleeve 38A that covers the magnet roll 38B. The magnet roll 38B has a cylindrical shape and is fixed to the color printer 10, and the sleeve 38A moves around the axis of the magnet roll 38B in the rotation direction b of the photosensitive member 13 as shown by an arrow c in FIG. It rotates in the same counterclockwise direction, in other words, in a direction facing the photoconductor 13 at the portion facing the photoconductor 13, and in other words, it rotates against. Thereby, the transfer efficiency of the toner from the developing roller 38 to the photosensitive member 13 is enhanced.

  The magnet roll 38B is a magnetic roller in which powder of magnetic material such as ferrite or rare earth magnet alloy is formed into a columnar shape or a cylindrical shape, and is sintered while being magnetized so that the N pole and the S pole are arranged in a predetermined pattern. Is formed. As shown in FIGS. 4 and 5, the magnetized pattern has a developing pole S1 at a portion facing the photosensitive member 13, and a pick-off pole N1 adjacent to the developing pole S1 along the rotation direction c of the sleeve 38A. Examples include a pattern in which magnetic poles are arranged in the order of the pickup pole N2, the trimming pole S2, and the transport pole N3 next to the pickup pole N2, but the magnetization pattern is not limited to this. The development pole S1 and the trimming pole S2 are all S poles, and the pickoff pole N1, the pickup pole N2, and the transport pole N3 are all N poles. The portion corresponding to the development nip in the development pole S1 is magnetized so that the change in the normal direction magnetic flux density Br becomes ± 5 mT. Here, in FIG. 4, when the boundary between the conveyance pole N3 and the trimming pole S2 is 0 degree and the clockwise direction is a positive angle, as shown in FIG. And a developing pole S1 is formed so as to include the developing nip portion. As can be seen from FIG. 5, the change in the normal direction magnetic flux density Br is ± 5 mT in the portion of the magnet roll 38 </ b> B at an angle of 240 to 270 degrees. This indicates that the normal direction magnetic flux density Br at the developing nip portion of the magnet roll 38B is magnetized so that the change is ± 5 mT. In FIG. 5, + indicates that the N pole is magnetized, − indicates that the S pole is magnetized, and Bθ indicates the tangential magnetic density. Instead of magnetizing the portion corresponding to the development nip in the development pole S1 so that the change in the normal direction magnetic flux density Br becomes ± 5 mT, the development pole S1 is shaved flat as shown in FIG. The normal direction magnetic flux density Br may be changed to ± 5 mT.

Further, as shown in FIG. 7, the magnet roll 38B is magnetized so that the minimum value of the normal direction magnetic restraining force Fr is located in the developing nip. Here, the normal magnetic restraining force Fr and the tangential magnetic restraining force Fθ are expressed by the following equations:
Fr = δBr / δr + δBθ / δr
Fθ = δBr / δθ + δBθ / δθ
Given in. In the following, r represents the normal direction coordinate, and θ represents the tangential direction coordinate. In FIG. 7, + indicates the magnetic restraining force in the N-pole direction, and-indicates the magnetic restraining force in the S-pole direction.

  A trimming blade 41 that aligns the height of the magnetic brush in cooperation with the trimming pole S <b> 2 extends toward the developing roller 38 at the opposing portion of the trimming pole S <b> 2. A negative developing bias voltage is applied to the developing roller 38.

(2) Operation The operation of the color printer 10 will be described. The color printer 10 is normally operated in a development mode in which an image is formed on the paper P. However, the color printer 10 is operated in a discharge product scraping mode for a predetermined time, for example, 3 to 5 minutes every 100 sheets. The

  The development mode and the discharge product scraping mode will be described below.

(2-1) Development Mode In the development mode, the photoconductor 13 rotates counterclockwise at a constant speed.

  Accordingly, the photosensitive surface 13A is negatively charged by the charging roller 36. Next, the charged surface of the photosensitive surface 13A is exposed by the LED array head 40, whereby the potential of the exposed portion of the charged surface is lowered to form an electrostatic latent image. Then, in the developing unit 12, the toner charged to a negative voltage is electrically attached to the latent image formed on the photosensitive surface 13 </ b> A, that is, the potential lowering portion of the charged surface by the developing roller 38. The electrostatic latent image is developed to form a toner image. The toner adhering to the photosensitive surface 13A is electrically drawn toward the intermediate transfer belt 14 by the transfer roller 32 to which a positive transfer voltage having a polarity opposite to that of the toner is applied. As a result, the toner image on the photosensitive surface 13A is transferred from the photosensitive member 13 to the intermediate transfer belt.

  Hereinafter, the developing process in the developing roller 38 will be described in detail.

  When the sleeve 38A rotates along the rotation direction c, first, the developer supplied into the housing 37 by the screw feeders 39A and 39B is adsorbed on the surface of the sleeve 38A by the pickup pole N2. Here, on the surface of the sleeve 38A, there are a magnetic field from the transport pole N3 toward the development pole S1, a magnetic field from the pick-off pole N1 toward the development pole S1, a magnetic field in the direction from the pickup pole N2 toward the trimming pole S2, and from the transport pole N3. A magnetic field directed to the trimming pole S2 is formed, and the developer has a structure in which toner adheres to the surface of the magnetic carrier particles. Therefore, as shown in FIG. 8, the developer adsorbed on the surface of the sleeve 38A is arranged in the direction of the lines of magnetic force on the surface of the sleeve 38A, in other words, it rises and forms a magnetic brush.

  As shown by the arrow in FIG. 8, the magnetic brush formed on the surface of the sleeve 38A in the vicinity of the pickup pole N2 is trimming pole S2 → conveying pole N3 → as the sleeve 38A rotates along the rotation direction c. It is conveyed from the right to the left from the developing pole S1 to the pick-off pole N1. The height of the magnetic brush is made uniform when passing through the trimming pole S2, the toner on the magnetic brush is transferred to the photoconductor 13 in the vicinity of the developing pole S1, and the surface of the sleeve 38A is almost only a magnetic carrier. The brush remains. The magnetic brush, which is almost only a magnetic carrier, falls off the surface of the sleeve 38A at the pick-off pole N1 and returns into the housing 37 as the sleeve 38A rotates.

  In the developing mode, the sleeve 38A rotates in the rotational direction c as described above, so that fresh developer is always replenished at the pickup pole N2 and conveyed to the developing pole S1, and toner is transferred to the photosensitive member at the developing pole S1. 13 and the latent image on the photosensitive surface 13A is developed.

(2-2) Discharge Product Scraping Mode In the discharge product scraping mode, charging by the charging roller 36 and exposure by the LED array head 40 are stopped, and the developing roller 38 is also stopped. Therefore, as shown in FIG. 9, the magnetic brush on the sleeve 38A is also stopped, so that a new magnetic brush containing a large amount of toner is not replenished from the casing 37. Here, since a developing bias voltage is applied to the developing roller 38, all of the toner remaining on the magnetic brush when the sleeve 38A is stopped is transferred to the photosensitive member 13 and almost or not on the sleeve 38A. A magnetic brush that consists entirely of magnetic carrier particles remains.

  On the other hand, since the photosensitive member 13 rotates in the direction of the arrow b at a constant speed as in the developing mode, the photosensitive surface 13A is rubbed with a magnetic brush on the sleeve 38A. Thereby, the discharge product generated on the photosensitive surface 13A is removed.

  Instead of stopping the sleeve 38A, the sleeve 38A may be rotated intermittently at regular intervals, or it may be rotated at a constant peripheral speed. 5% or less of the peripheral speed of the photoreceptor 13 is preferable.

(2-3) Relationship between Magnetization Pattern of Developing Roller 38 and Development Effect and Discharge Product Scraping Effect As described above, the change in the normal direction magnetic flux density Br in the developing nip of the developing roller 38 is within ± 5 mT. Therefore, the magnetic brush has a small shape change along the normal direction r, in other words, the height is almost equal as shown by the solid line in FIG. In the conventional developing roller in which the change in the normal direction magnetic flux density Br in the developing nip is large, the height distribution of the height distribution of the magnetic brush in the developing nip shows a mountain shape as shown by a two-dot chain line.

  As apparent from FIG. 10, in the developing roller 38, more magnetic brushes are in contact with the photosensitive surface 13A of the photoreceptor 13 than in the conventional developing roller. An image is obtained, and in the discharge product scraping mode, a high discharge product scraping effect can be obtained without making the magnetic brush so hard.

  In addition, since the photosensitive drum 38 is magnetized so that the minimum value of the normal direction magnetic restraining force Fr is located in the developing nip, the developer, in particular, the magnetic carrier particles easily move in the developing nip. . Therefore, since the magnetic brush easily enters the developing nip, as shown by a solid line in FIG. 11, it is difficult to collect the developer upstream of the developing nip with respect to the rotation direction c of the photosensitive drum 38. Therefore, it is possible to effectively suppress the occurrence of brush marks on the photosensitive surface 13A due to the developer pool.

  On the other hand, in the conventional color printer 10, since the minimum value of the normal magnetic restraining force Fr is not magnetized so as to be positioned in the developing nip, the magnetic brush is hard, and therefore the developing in the developing nip is performed. Agents, especially magnetic carrier particles, are difficult to move. Therefore, the magnetic brush does not readily enter the developing nip, and a developer pool is formed on the upstream side of the developing nip with respect to the rotation direction c of the photosensitive drum 38 as indicated by a two-dot chain line in FIG. Here, the developer pool refers to the one in which the two-component developer is hardened without entering the development nip, and the magnetic carrier particles are difficult to move in the developer pool. Therefore, when the developer pool is formed, the photosensitive surface 13A is strongly rubbed by the magnetic carrier particles in the developer pool, and a brush mark is generated.

(3) Features of the present embodiment In the color printer 10 of the first embodiment, as described above, the magnet roll 38B of the developing roller 38 has a change in the normal direction magnetic flux density Br in the developing nip within ± 5 mT, Moreover, since the minimum value of the normal direction magnetic restraining force Fr is magnetized so as to be located in the developing nip, a sufficiently dark toner image is obtained in the developing mode, and in the discharge product scraping mode. Even if the magnetic brush is not so hard, a high discharge product scraping effect can be obtained. Further, since the developer pool is hardly formed on the upstream side of the developing nip with respect to the rotation direction c of the developing drum 38, no brush mark is generated on the photosensitive surface 13A due to the developer pool.

  Further, when operating in the discharge product scraping mode, the magnetic carrier particles are spiked on the surface of the sleeve 38A, and the toner is removed from the magnetic carrier particles by applying a development bias voltage for at least a certain time. The friction coefficient between the magnetic brush and the photoreceptor 13 is large. Therefore, the discharge product generated on the photosensitive surface 13A can be scraped efficiently, and the occurrence of deletion can be effectively prevented.

  In addition, since the developing roller 38 itself is used as a discharge product scraping unit, it is not necessary to provide a special unit for scraping the discharge product. Since the discharge product on the photosensitive surface 13A is scraped only when operating in the discharge product scraping mode, the development is compared with the case where the discharge product is scraped off while developing with the developing roller 38. The packing density of magnetic carrier particles in the gap can be set lower. Therefore, it is preferable in terms of a wide parameter range such as MOS, DRS, and photosensitive drum peripheral speed in which no brush mark is generated by the magnetic brush.

  In addition, since the magnet roll 38B is fixed in the developing roller 38, the developing pole S1, the pick-off pole N1, and the pickup so that the developing pole S1 of the magnet roll 38B is located in the developing gap portion of the developing roller 13. By arranging the pole N2, the trimming pole S2, and the transport pole N3, a magnetic brush can always be formed in the development gap portion.

  By rotating the sleeve 38A intermittently or continuously at a low speed, the magnetic brush can be rubbed while the magnetic brush is renewed, so that the magnetic brush rubbing unevenness may occur on the surface of the photoconductor 13. Is prevented. Further, since the two-component developer containing a large amount of toner is not supplied to the development gap one after another, the scraping effect of the discharge product is considered to be almost the same as when the sleeve 38A is stopped. It is done.

FIG. 1 is a schematic cross-sectional view illustrating an outline of the configuration of the color printer according to the first embodiment. FIG. 2 is an enlarged view showing a configuration of a printing unit of the laser printer shown in FIG. FIG. 3 is an enlarged cross-sectional view showing the configuration of the developing unit and the vicinity thereof in the print unit shown in FIG. 4 is an enlarged cross-sectional view illustrating a configuration of a developing roller included in the developing unit illustrated in FIG. FIG. 5 is a graph showing the positional change of the normal direction magnetic flux density and the tangential direction magnetic flux density in the developing roller shown in FIG. FIG. 6 is an enlarged cross-sectional view showing another configuration example of the developing roller provided in the developing unit shown in FIG. FIG. 7 is a graph showing changes in the position of the normal magnetic restraining force and the tangential magnetic restraining force in the developing roller shown in FIG. FIG. 8 is a schematic diagram illustrating how toner moves from the magnetic brush on the developing roller to the photosensitive member when the color printer according to the first embodiment is operated in the developing mode. FIG. 9 is a schematic diagram illustrating a state in which the surface of the photoreceptor is rubbed with a magnetic brush on the developing roller when the color printer according to the first embodiment is operated in the discharge product scraping mode. FIG. 10 is a schematic diagram showing the height distribution of the magnetic brush in the developing nip for the color printer according to Embodiment 1 and the conventional color printer. FIG. 11 is a schematic diagram illustrating a state where the magnetic brush enters the developing nip for the color printer according to Embodiment 1 and the conventional color printer.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Color printer 13 Photoconductor 14 Intermediate transfer belt 34 Brush roller 36 Charging roller 38 Developing roller

Claims (7)

Development that develops the latent image on the surface of the photosensitive member with a two-component developer that is disposed to face the photosensitive member on which a latent image is formed by exposure and contains toner and magnetic carrier particles. An image forming apparatus comprising a roller,
The developing roller rotates so that the rotation direction is reversed at the portion facing the photoconductor, and the normal direction magnetic flux density of the developing roller in the developing nip which is a gap formed between the developing roller and the photoconductor The image forming apparatus is formed so that the change in the range is within ± 5 mT.   The image forming apparatus according to claim 1, wherein the developing roller is formed such that a minimum value of a normal direction magnetic restraining force is positioned in the developing nip.   3. The image according to claim 1, wherein the developing nip of the developing roller is within a range of ± 15 degrees along a rotation direction of the developing roller with a closest portion between the developing roller and the photoconductor interposed therebetween. Forming equipment.   The image forming apparatus according to claim 1, wherein a normal direction magnetic flux density in the developing nip of the developing roller is 90 mT or more.   The image forming apparatus according to claim 1, wherein the developing roller includes a magnet roll in which a plurality of magnetic poles are arranged along a circumferential direction, and a sleeve into which the magnet roll is inserted and is rotatable. apparatus.   The image forming according to claim 5, wherein the magnet roller provided in the developing roller is a columnar or cylindrical permanent magnet that is magnetized so that a change in a normal direction magnetic flux density within a developing nip is within ± 5 mT. apparatus.   The magnet roll provided in the developing roller is a columnar or cylindrical permanent magnet, and a part of the outer peripheral portion is deleted so that the change in the normal direction magnetic flux density in the developing nip is within ± 5 mT. Item 6. The image forming apparatus according to Item 5.

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