JP2010276899A - Developing device and image forming apparatus including the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a developing device for obtaining excellent images over a long period of time without generating roller lines even when accelerating image processing speeds, density deterioration, image inferiority such as density following inferiority and leakage between toner carriers and toner supply members, and to provide an image forming apparatus including the developing device. <P>SOLUTION: A developing roller 23 is formed by laminating a coat layer of silicon modified polyurethane on the outer peripheral face of an aluminum or aluminum alloy made developing sleeve 23a. In addition, a number average particle size of a magnetic carrier composing a two-component developer is 25-50 μm, circularity is 0.95-0.98, and saturated magnetization is 60-80 eum/g. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a developing device mounted on an image forming apparatus such as a copying machine, a facsimile machine, and a printer, and an image forming apparatus including the developing device, and in particular, uses a two-component developer composed of a magnetic carrier and toner, and carries a toner. The present invention relates to a developing device that develops an electrostatic latent image on an image carrier while holding only toner charged on the body.

  Conventionally, as a developing method using dry toner in an image forming apparatus using an electrophotographic process, a one-component developing method using no carrier and a non-magnetic toner using a magnetic carrier (hereinafter also simply referred to as a carrier) are used. There is known a two-component developing system that uses a two-component developer to be charged and develops an electrostatic latent image on a photosensitive member with a magnetic brush composed of toner and a carrier formed on a developing roller.

  The one-component development method is suitable for high image quality because the electrostatic latent image on the image carrier is not disturbed by the magnetic brush, but toner adheres to the regulating blade, resulting in non-uniform layer formation. It sometimes caused image defects.

  In the case of color printing in which color superposition is performed, since the color toner is required to be transparent, it needs to be a non-magnetic toner. Therefore, in a full-color image forming apparatus, a two-component development system in which toner is charged and conveyed using a carrier is often employed. However, the two-component development method can maintain a stable charge amount for a long time and is suitable for extending the life of the toner, but is disadvantageous in terms of image quality due to the influence of the magnetic brush described above.

  As one means for solving these problems, a magnetic roller (toner supply member) is used to transfer the developer onto a developing roller (toner carrier) installed in a non-contact manner with respect to the photoreceptor (image carrier). When developing, a non-magnetic toner is transferred onto the developing roller while leaving the magnetic carrier on the magnetic roller to form a thin toner layer, and the toner is attached to the electrostatic latent image on the photosensitive member by an alternating electric field. A scheme has been proposed.

  In the case of the above developing method, the toner density is not sufficiently formed on the developing roller, so that the toner to be supplied to the electrostatic latent image formed on the photoreceptor is insufficient and the image density is lowered. There is a possibility that density following failure may occur. In addition, there is a problem that a phenomenon in which a part of the immediately developed image appears as an afterimage (ghost) at the next development, that is, so-called history development is likely to occur.

  The mechanism of history development will be described. When a toner consumption area (development area) and a non-consumption area occur in the toner thin layer on the development roller, the toner adhesion state on the development roller and the toner potential difference vary. From the relationship that occurs, only the large-diameter toner with a low charge amount in the toner thin layer remaining in the non-consumed region moves to the magnetic roller side, and the small-diameter toner with a high charge amount remains on the surface of the developing roller.

  When the performance of collecting the toner remaining on the developing roller is low, when a new toner is supplied from the magnetic roller and a thin toner layer is formed on the developing roller, the small diameter toner remains in the developing area in the immediately preceding development. The toner layer thickness is reduced compared to the non-consumed area. When a full-color image or a halftone image is developed in this state, the amount of toner that moves from the immediately preceding developing area to the photosensitive member side is small, and thus the immediately preceding developed image appears as an afterimage.

  Therefore, after passing through the portion facing the photoreceptor (development nip), a thin toner layer that has not been used for development is sufficiently collected using a magnetic brush on the magnetic roller, and a new toner layer is placed on the development roller. It is necessary to form the toner thin layer uniformly.

  Accordingly, various methods for improving the toner recovery performance on the developing roller have been proposed. For example, Patent Document 1 discloses a coating layer (coating layer) containing a conductive material and a resin (silicon-modified polyurethane resin) on the surface of the developing roller. ) Is disclosed. Patent Document 2 discloses a two-component developer having a carrier number average particle diameter of 25 to 50 μm and a circularity of 0.95 to 0.98 and a developing method using the same.

  By forming a coating layer on the surface of the developing roller as in Patent Document 1, the adhesion of the toner to the surface of the developing roller is reduced, so that the toner recoverability can be improved. Further, since the coating layer functions as a charge barrier layer, the leakage resistance of the developing roller is also improved.

  Further, in the two-component developer of Patent Document 2, since the decrease in carrier fluidity is suppressed, the toner can be charged uniformly and with an appropriate charge amount, so that uneven image density, fogging, and toner scattering are generated. Can be suppressed. Further, history development is also suppressed because the toner supply performance to the developing roller and the toner collection performance from the developing roller are improved.

  Furthermore, in the developing roller of Patent Document 1, the film layer on the surface of the developing roller is damaged by a magnetic brush due to long-term use, and vertical stripes (roller stripes) are included in the image, or discharge (leakage) occurs in the part where the coating layer is peeled off. ) And image noise (leak marks) occurs, but by combining the two-component developer of Patent Document 2 using a carrier with a high degree of circularity, roller streaks and leak marks are generated. It can be effectively suppressed.

JP 2008-122952 A JP 2007-264336 A

  By the way, in order to cope with the recent increase in the image processing speed, the peripheral speed of the photosensitive member is remarkably increased. In order to increase the developer conveyance speed, it is necessary to increase the peripheral speed of the developing roller and magnetic roller in the developing device in accordance with the peripheral speed of the photosensitive member. However, when the peripheral speed of the magnetic roller is increased, the contact time of the magnetic brush with the developing roller is shortened, and the toner cannot be sufficiently supplied and collected. As a result, image density reduction and density following failure occur.

  In particular, when a carrier with a high degree of circularity as disclosed in Patent Document 2 is used, the developer slips on the magnetic roller and the magnetic brush cannot be formed densely and uniformly. The reduction in supply and recovery is remarkable.

  In view of the above problems, the present invention does not cause image defects such as roller streaks, density reduction, density tracking failure, and the like, and leakage between the toner carrier and the toner supply member even when the image processing speed is increased. An object of the present invention is to provide a developing device capable of obtaining a good image over a period of time and an image forming apparatus including the developing device.

  In order to achieve the above object, the present invention provides a toner carrier using a toner carrier which is disposed in a non-contact manner and opposed to an image carrier, and a magnetic brush formed of a two-component developer comprising a toner and a magnetic carrier. And a toner supply member for forming a toner layer on the body, wherein a coating layer made of a resin material is formed on the outer peripheral surface of the toner carrier, and the number average particle diameter of the magnetic carrier is 25 to 25. It is characterized by 50 μm, circularity of 0.95 to 0.98, and saturation magnetization of 60 eum / g to 80 eum / g.

  According to the present invention, in the developing device configured as described above, a plurality of grooves extending in the axial direction are formed on the outer peripheral surface of the toner supply member.

  According to the present invention, in the developing device configured as described above, a peripheral speed of the toner supply member is 500 mm / second or more.

  The present invention also provides an image forming apparatus equipped with the developing device having the above-described configuration.

  According to the first configuration of the present invention, a toner carrier having a coating layer made of a resin material formed on the outer peripheral surface, a number average particle diameter of 25 to 50 μm, a circularity of 0.95 to 0.98, By using a magnetic carrier having a saturation magnetization of 60 eum / g to 80 eum / g, a low-resistance magnetic brush sufficiently packed with the carrier can be densely and uniformly formed between the toner supply member and the toner carrier. . As a result, the amount of magnetic brush that comes into contact with the toner carrier increases, and toner supply to the toner carrier and toner recovery from the toner carrier are improved. As well as ensuring the fluidity of the developer, it is possible to suppress the occurrence of image noise such as roller streaks and leak marks by damaging the coat layer on the surface of the toner carrying member.

  According to the second configuration of the present invention, in the developing device of the first configuration, even when a spherical carrier is used by forming a large number of axially extending grooves on the outer peripheral surface of the toner supply member. Slip of the developer on the toner supply member when supplying the developer from the toner supply member to the toner carrier can be suppressed. As a result, the developability from the toner supply member to the toner carrier is not deteriorated, and a uniform toner thin layer can be formed on the toner carrier, so that the density tracking failure can be further suppressed. In addition, since the surface of the toner supply member becomes uniform as compared with the blast treatment, the occurrence of leakage between the toner carrier and the toner supply member can be suppressed.

  According to the third configuration of the present invention, in the developing device having the first or second configuration, the peripheral speed of the toner supply member is set to 500 mm / second or more, and high-speed printing is possible. Since the rubbing ability of the surface of the toner carrying member by the magnetic brush is moderately improved and the decrease in the toner recoverability is suppressed, the developing device can prevent the toner from adhering to the toner carrying member at a high printing rate.

  According to the fourth configuration of the present invention, by mounting the developing device of any one of the first to third configurations, image defects such as roller streaks, density unevenness, history development, and the toner carrier -An image forming apparatus capable of obtaining a good image over a long period of time without causing leakage between toner supply members.

1 is a schematic diagram showing the overall configuration of an image forming apparatus equipped with a developing device of the present invention. Side surface sectional view showing the configuration of the developing device of the present invention The figure which shows an example of the bias waveform applied to a developing roller and a magnetic roller

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view of an image forming apparatus according to the present invention. Here, a tandem color image forming apparatus is shown. In the main body of the color 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. 1). These image forming portions Pa to Pd are provided corresponding to images of four different colors (cyan, magenta, yellow, and black), and cyan, magenta, and yellow are respectively performed by charging, exposure, development, and transfer processes. And a black image are sequentially formed.

  The image forming portions Pa to Pd are provided with photosensitive drums 1a, 1b, 1c, and 1d that carry visible images (toner images) of the respective colors, and are further driven by a driving unit (not shown). The intermediate transfer belt 8 that rotates clockwise is provided adjacent to the image forming portions Pa to Pd. The toner images formed on the photosensitive drums 1a to 1d are sequentially transferred onto the intermediate transfer belt 8 that moves while being in contact with the photosensitive drums 1a to 1d. The image is transferred onto P at a time, and further fixed on the transfer 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 counterclockwise in FIG.

  The transfer paper P onto 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 secondary transfer roller 9 via a paper feed roller 12a and a registration roller pair 12b. A sheet made of a dielectric resin is used for the intermediate transfer belt 8, and a belt in which both ends thereof are overlapped and joined to form an endless shape, or a belt without a seam (seamless) is used. A blade-shaped belt cleaner 19 for removing toner remaining on the surface of the intermediate transfer belt 8 is disposed on the downstream side of the secondary transfer roller 9.

  Next, the image forming units Pa to Pd will be described. There are chargers 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 below the photosensitive drums 1a to 1d that are rotatably arranged. The exposure unit 4 for exposing the toner, the developing devices 3a, 3b, 3c and 3d for forming toner images on the photosensitive drums 1a to 1d, and the developer (toner) remaining on the photosensitive drums 1a to 1d are removed. Cleaning units 5a, 5b, 5c and 5d are provided.

  When the image formation start is input by the user, first, the surfaces of the photosensitive drums 1a to 1d are uniformly charged by the chargers 2a to 2d, and then light is irradiated by the exposure unit 4 to each of the photosensitive drums 1a to 1d. An electrostatic latent image corresponding to the image signal is formed on the top. Each of the developing devices 3a to 3d is filled with a predetermined amount of cyan, magenta, yellow, and black toner by a replenishing device (not shown). The toner is supplied onto the photosensitive drums 1a to 1d by the developing devices 3a to 3d and electrostatically attached, whereby a toner image corresponding to the electrostatic latent image formed by exposure from the exposure unit 4 is formed. It is formed.

  After an electric field is applied to the intermediate transfer belt 8 at a predetermined transfer voltage, the cyan, magenta, yellow, and black toner images on the photosensitive drums 1a to 1d are transferred to the intermediate transfer belt 8 by the primary transfer rollers 6a to 6d. Transcribed above. 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.

  The intermediate transfer belt 8 is stretched between an upstream conveyance roller 10 and a downstream drive roller 11, and the intermediate transfer belt 8 rotates clockwise as the drive roller 11 is rotated by a drive motor (not shown). When the rotation starts, the transfer paper P is conveyed from the registration roller pair 12b to the secondary transfer roller 9 provided adjacent to the intermediate transfer belt 8 at a predetermined timing, and the full color image is transferred. The transfer paper P onto which the toner image is transferred is conveyed to the fixing unit 7.

  The transfer paper P conveyed to the fixing unit 7 is heated and pressurized by the fixing roller pair 13 so that the toner image is fixed on the surface of the transfer paper P, and a predetermined full color image is formed. The transfer paper P on which the full-color image is formed is distributed in the transport direction by the branching portion 14 that branches in a plurality of directions. When an image is formed on only one side of the transfer paper P, the image is directly discharged onto the discharge tray 17 by the discharge roller pair 15.

  On the other hand, when images are formed on both sides of the transfer paper P, a part of the transfer paper P that has passed through the fixing unit 7 is once protruded from the discharge roller pair 15 to the outside of the apparatus. Thereafter, the transfer paper P is distributed to the paper conveyance path 18 by the branching section 14 by rotating the discharge roller pair 15 in the reverse direction, and is re-conveyed to the registration roller pair 12b with the image surface reversed. Then, after the next image formed on the intermediate transfer belt 8 is transferred to the surface of the transfer paper P on which the image is not formed by the secondary transfer roller 9 and conveyed to the fixing unit 7 to fix the toner image. The paper is discharged from the discharge roller pair 15 to the discharge tray 17.

  FIG. 2 is a side sectional view showing a configuration of a developing device used in the image forming apparatus of the present invention. Here, the developing device 3a disposed in the image forming unit Pa of FIG. 1 will be described, but the configuration of the developing devices 3b to 3d disposed in the image forming units Pb to Pd is basically the same, and thus described. Is omitted.

  As shown in FIG. 2, the developing device 3a includes a developing container 20 in which a two-component developer (hereinafter also simply referred to as a developer) is accommodated. The developing container 20 is divided into a first wall and a second wall by a partition wall 20a. The first and second agitating chambers 20b and 20c are divided into agitating chambers 20b and 20c, and a toner (positively charged toner) supplied from a toner container (not shown) is mixed with a carrier and agitated and charged. The stirring screw 21a and the second stirring screw 21b are rotatably arranged.

  Then, the developer is conveyed in the axial direction while being stirred by the first stirring screw 21a and the second stirring screw 21b, and the first and second are passed through developer passages (not shown) formed in the partition wall 20a. It circulates between the stirring chambers 20b and 20c. In the illustrated example, the developing container 20 extends obliquely upward to the left, a magnetic roller 22 is disposed in the developing container 20 above the second stirring screw 21b, and the developing roller 20 is disposed obliquely upward to the left of the magnetic roller 22. Rollers 23 are arranged opposite to each other. The developing roller 23 faces the photosensitive drum 1 on the opening side (left side in FIG. 2) of the developing container 20, and the magnetic roller 22 and the developing roller 23 rotate clockwise in the drawing.

  Note that a toner concentration sensor (not shown) is disposed in the developing container 20 so as to face the first stirring screw 21a, and the toner supply port 20d is supplied from the supply device according to the toner concentration detected by the toner concentration sensor. The toner is supplied into the developing container 20 via

  The magnetic roller 22 includes a non-magnetic rotating sleeve 22a and a fixed magnet body 22b having a plurality of magnetic poles contained in the rotating sleeve. In the present embodiment, the magnetic poles of the fixed magnet body 22 b have a five-pole configuration including a main pole 35, a regulation pole (head cutting pole) 36, a transport pole 37, a separation pole 38 and a pumping pole 39.

  Further, a spike cutting blade 25 is attached to the developing container 20 along the longitudinal direction of the magnetic roller 22 (front and back direction in FIG. 2), and the spike cutting blade 25 rotates in the rotational direction of the magnetic roller 22 (clockwise in the figure). Around the opposite position between the developing roller 23 and the magnetic roller 22. A slight gap (gap) is formed between the front end of the spike cutting blade 25 and the surface of the magnetic roller 22.

  The developing roller 23 includes a cylindrical developing sleeve 23a and a developing roller-side magnetic pole 23b fixed in the developing sleeve 23a. The magnetic roller 22 and the developing roller 23 are predetermined at a facing position (opposed position). It is facing with a gap of. The developing roller side magnetic pole 23b is different in polarity from the opposing magnetic pole (main pole) 35 of the fixed magnet body 22b.

  The developing roller 23 is connected to a first bias circuit 30 that applies a DC voltage (hereinafter referred to as Vslv (DC)) and an AC voltage (hereinafter referred to as Vslv (AC)). A second bias circuit 31 for applying a voltage (hereinafter referred to as Vmag (DC)) and an alternating voltage (hereinafter referred to as Vmag (AC)) is connected. The first bias circuit 30 and the second bias circuit 31 are grounded to a common ground.

  A voltage variable device 33 is connected to the first bias circuit 30 and the second bias circuit 31, and Vslv (DC), Vslv (AC) applied to the developing roller 23 and Vmag (DC) applied to the magnetic roller 22. ), Vmag (AC) can be varied.

  As described above, the first stirring screw 21a and the second stirring screw 21b circulate in the developing container 20 while the developer is being stirred to charge the toner, and the second stirring screw 21b causes the developer to move to the magnetic roller 22. Be transported. Since the regulation pole 36 of the fixed magnet body 22 b faces the ear cutting blade 25, a nonmagnetic material or a magnetic body having a polarity different from that of the regulation pole 36 is used as the ear cutting blade 25. A magnetic field is generated in a direction attracting the gap with the sleeve 22a.

  Due to this magnetic field, a magnetic brush is formed between the ear cutting blade 25 and the rotating sleeve 22a. When the thickness of the magnetic brush on the magnetic roller 22 is regulated by the cutting blade 25 and then moves to a position facing the developing roller 23, the magnetic field attracted by the main pole 35 of the fixed magnet body 22b and the developing roller side magnetic pole 23b. Therefore, the magnetic brush contacts the surface of the developing roller 23. Then, a toner thin layer is formed on the developing roller 23 by a potential difference ΔV between Vmag (DC) applied to the magnetic roller 22 and Vslv (DC) applied to the developing roller 23 and a magnetic field.

  The thickness of the toner layer on the developing roller 23 varies depending on the resistance of the developer and the rotational speed difference between the magnetic roller 22 and the developing roller 23, but can be controlled by ΔV. When ΔV is increased, the toner layer on the developing roller 23 is thickened, and when ΔV is decreased, the toner layer is thinned. The range of ΔV at the time of development is generally about 100V to 350V.

  FIG. 3 is a diagram illustrating an example of a bias waveform applied to the developing roller 23 and the magnetic roller 22. As shown in FIG. 3A, the developing roller 23 has a combined waveform Vslv (solid line) in which a rectangular wave Vslv (AC) having a peak-to-peak value of Vpp1 superimposed on Vslv (DC) is a first bias circuit. 30 applied. Further, the magnetic roller 22 has a second combined waveform Vmag (broken line) in which Vmag (DC) has a peak-to-peak value of Vpp2 and a rectangular wave Vmag (AC) having a phase different from Vslv (AC). Applied from the bias circuit 31.

  Accordingly, the voltage applied between the magnetic roller 22 and the developing roller 23 (hereinafter referred to as MS) is a composite waveform Vmag-Vslv having Vpp (max) and Vpp (min) as shown in FIG. Become. Note that Vmag (AC) is set so that the duty ratio is larger than Vslv (AC). Actually, an AC voltage having a partially distorted shape is applied instead of a complete rectangular wave as shown in FIG.

  The toner thin layer formed on the developing roller 23 by the magnetic brush is conveyed to a facing portion between the photosensitive drum 1 a and the developing roller 23 by the rotation of the developing roller 23. Since Vslv (DC) and Vslv (AC) are applied to the developing roller 23, the toner flies due to a potential difference with the photosensitive drum 1a, and the electrostatic latent image on the photosensitive drum 1a is developed. .

  When the rotating sleeve 22a further rotates clockwise, this time, the magnetic brush is pulled away from the surface of the developing roller 23 by the horizontal (roller circumferential direction) magnetic field generated by the different polarity peeling pole 38 adjacent to the main pole 35. The remaining toner that is not used for is collected from the developing roller 23 onto the rotating sleeve 22a. When the rotating sleeve 22a further rotates, a repulsive magnetic field is applied by the peeling pole 38 of the fixed magnet body 22b and the scooping pole 39 having the same polarity, so that the toner is detached from the rotating sleeve 22a in the developing container 20. Then, after being stirred and conveyed by the second agitating screw 21b, a magnetic brush is again formed on the rotating sleeve 22a by the scooping pole 39 as a two-component developer uniformly charged with an appropriate toner concentration, It is conveyed to 25.

  Next, the rotating sleeve 22a constituting the magnetic roller 22 will be described in detail. In the present invention, the outer peripheral surface of the rotating sleeve 22a is blasted, or a number of grooves extending in the axial direction of the outer peripheral surface of the rotating sleeve 22a are formed (knurling). As a result, even when a spherical carrier having a circularity of 0.95 to 0.98 as described later is used, the developer on the rotating sleeve 22a when the developer is supplied from the magnetic roller 22 to the developing roller 23. Since the slip can be suppressed, the developer can be stably supplied to the facing portion between the developing roller 23 and the magnetic roller 22, and a uniform thin toner layer can be formed on the developing roller 23.

  When the developer existing amount on the magnetic roller 22 is too small, a toner thin layer cannot be stably formed on the developing roller 23, and the toner recoverability from the developing roller 23 is also lowered. On the other hand, if the amount of the developer is too large, the developer density at the closest portion between the developing roller 23 and the magnetic roller 22 may increase, and developer (carrier) leakage from the developing device may occur. Accordingly, the surface roughness (maximum height) Rz of the rotating sleeve 22a, or the width, depth, number, etc. of the grooves formed in the rotating sleeve 22a are set according to the number average particle diameter, circularity, etc. of the carrier used. Just do it.

  In particular, when a spherical carrier having a high degree of circularity is used as in the present invention, the density of the magnetic brush formed on the rotating sleeve 22a increases, so that the carrier resistance decreases, and current leakage between the magnetic roller and the developing roller (MS However, by using a knurling process in which the surface of the rotating sleeve 22a is uniform compared to the blasting process, the occurrence of leaks between MSs can be suppressed.

  Further, the blasting region or groove forming region (knurling width) on the surface of the rotating sleeve 22a of the magnetic roller 22 is made longer than the length of the opposing developing roller 23, so that the toner thin layer of the developing roller 23 is magnetized. The formation width of the brush becomes wider, and the entire axial direction of the toner thin layer can be peeled off. Conversely, if the groove forming area is shorter than the developing roller 23, the toner layered on the end of the developing roller 23 cannot be pulled back, and the toner adheres to that portion. When the toner adheres, the carrier adheres to the spot and causes the carrier to jump to the photosensitive drum 1a. Further, if the carrier adhering portion is within the image area, it causes a decrease in image density (whiteout).

  Specifically, the length of the blasting region or groove forming region on the surface of the rotating sleeve 22a is 0.5 to 10 mm, preferably 2 to 6 mm longer than the length of the developing roller 23. At this time, it goes without saying that it is preferable to match the axial centers of the magnetic roller 22 and the developing roller 23. If the difference in length between the blasting region or groove forming region and the developing roller 23 is shorter than 0.5 mm, the toner layered on the end of the developing roller 23 cannot be sufficiently peeled off by the magnetic brush. Further, even if it exceeds 10 mm, the stripping effect does not change, so it is meaningless to make it longer than this.

  When blasting the surface of the rotating sleeve 22a, it is possible to use a metal powder such as stainless steel, aluminum, steel, nickel and brass having a predetermined particle diameter, or various abrasive grains such as ceramic, plastic and glass beads. it can.

  Next, the developing sleeve 23a constituting the developing roller 23 will be described in detail. The developing sleeve 23a is obtained by forming a coat layer made of silicon-modified polyurethane on the outer peripheral surface of an aluminum or aluminum alloy sleeve body. The coat layer suppresses adhesion of the toner supplied on the developing sleeve 23a, and allows the toner to be transferred from the developing roller 23 to the surface of the photosensitive drum relatively easily. Further, since the mechanical stress applied to the developer by the coat layer is less than that on the metal surface, both improvement in toner recoverability from the developing roller 23 and prevention of developer deterioration when the developer transport amount is increased are achieved. Can be achieved.

  Examples of the material of the coating layer include urethane-modified resin, acrylic resin, melamine resin, silicone resin, fluorine resin, etc. in addition to silicone-modified polyurethane. Good urethane resin and acrylic resin are preferable. Among these, the use of a silicon-modified polyurethane resin in which a part of carbon of the urethane resin is replaced with silicon is preferable because the hygroscopic property of the urethane resin is improved and the change in charging characteristics of the coat layer with respect to environmental changes is suppressed.

In addition, the coating layer contains a conductive material having a dielectric constant of 10 or more as a resistance adjusting agent, thereby adjusting the volume resistance value of the coating layer and suppressing resistance unevenness. Examples of the resistance adjusting agent include carbon black, acetylene black, and fiber-shaped potassium titanate. As a result, it is possible to stabilize the charged state of the toner layer and improve the releasability during toner recovery. The volume resistance value is preferably about 10 ⁴ to 10 ⁸ Ω that can appropriately retain the residual charge on the surface of the developing roller 23.

The surface roughness (arithmetic mean roughness) Ra of the coating layer is the surface roughness normally obtained when the coating agent is applied by spraying, dipping, roll coating, or the like, or the surface roughness of urethane beads or the like is adjusted. It is prepared to 0.4 to 1.5 μm by applying an agent. Further, the surface free energy of the silicon-modified polyurethane is 15 to 25 mJ / cm ² .

  Moreover, before forming the coat layer, the outer peripheral surface of the sleeve body may be anodized to form the anodized layer. According to this configuration, since the alumite layer formed on the surface has a function as a charge barrier layer, it is possible to prevent the occurrence of leakage when a developing bias is applied to the developing roller 23.

  Next, the two-component developer used in the developing device of the present invention will be described. The two-component developer contains a toner and a carrier. The weight ratio (T / C) between the toner and the carrier in the two-component developer is preferably 5 to 20 parts by weight, more preferably 8 to 15 parts by weight with respect to 100 parts by weight of the carrier.

  The toner is obtained by adding an external additive to toner base particles. The toner base particles contain a binder resin and a colorant. The toner base particles may contain a release agent, a charge control agent, magnetic powder and the like as necessary. The weight average particle diameter of the toner base particles is preferably 5 to 12 μm, and more preferably 6 to 10 μm. The weight average particle diameter of the toner base particles is measured by a particle size distribution measuring device (for example, Multisizer II type manufactured by Coulter, Inc.). The toner base particles are produced by a known method such as a pulverization classification method, a melt granulation method, a spray granulation method, or a polymerization method.

  Examples of the external additive include inorganic oxides such as silica, titanium oxide, and alumina, and metal soaps such as calcium stearate. The amount of the external additive is usually 0.1 to 5 parts by weight with respect to 100 parts by weight of the toner base particles.

  Examples of the carrier include magnetic particles or resin particles in which a magnetic material is dispersed in a binder resin. Examples of magnetic materials include magnetic metals such as iron, nickel, cobalt, alloys thereof, alloys containing rare earths, hematite, magnetite, manganese-zinc ferrite, nickel-zinc ferrite, manganese-magnesium Examples thereof include soft ferrites such as ferrite and lithium ferrite, iron-based oxides such as copper-zinc ferrite, and mixtures thereof.

  Examples of the binder resin include vinyl resins, polyester resins, epoxy resins, phenol resins, urea resins, polyurethane resins, polyimide resins, cellulose resins, polyether resins, and mixtures thereof. Magnetic particles are produced by a known method such as a sintering method or an atomizing method. The carrier may have a coating layer made of a coating resin on its surface.

  The number average particle diameter of the carrier is 25 to 50 μm, preferably 35 to 45 μm. By setting the number average particle diameter of the carrier to 25 μm or more, a decrease in the fluidity of the two-component developer is suppressed, the toner charge amount is not excessive, and density unevenness is suppressed. By setting the number average particle diameter of the carrier to 50 μm or less, the specific surface area of the carrier increases, and the amount of toner that can be carried by the carrier increases. As a result, the toner concentration in the magnetic brush can be maintained at a high level, and the toner is sufficiently supplied to the developing roller, so that the thickness of the toner layer can be sufficiently secured. As a result, a sufficient amount of toner flying from the toner layer to the electrostatic latent image on the photoreceptor can be secured, image density reduction can be suppressed, and image density unevenness can be suppressed. Further, since the toner is sufficiently supplied to the developing roller 23, it is difficult to form a toner missing portion in the toner layer of the developing roller 23, and the occurrence of history development is suppressed.

  The number average particle diameter of the carrier is measured using a flow type particle image analyzer (for example, FPIA-2100 manufactured by Sysmex). However, fine particles of 10 μm or less are excluded from the measurement.

  The coefficient of variation (CV) in the particle size distribution of the carrier is 5 to 20%, preferably 7% to 15%. By setting the CV to 20% or less, the particle size of the carrier is uniform (the particle size distribution becomes sharp), so that the toner can be uniformly charged with an appropriate charge amount. As a result, density unevenness of the image is suppressed. Further, by charging the toner uniformly, so-called selective developability, in which toner having a large particle diameter is selectively consumed, can be suppressed. By suppressing the selective developability of the toner, an increase in toner fine powder in the two-component developer is suppressed, and an increase in charge amount is suppressed. In addition, since the increase in the amount of fine powder is suppressed and the decrease in fluidity of the two-component developer is suppressed, the stress on the carrier is reduced, and carrier deterioration such as toner spent and carrier coat peeling is suppressed. The life of the agent can be extended. By setting the CV to 5% or more, the particle size of the carrier is not uniform and the fluidity of the two-component developer does not become excessively high, and the toner can be sufficiently charged. It can be suppressed.

The coefficient of variation CV is obtained from the following equation by measuring the number average particle diameter by a flow image analysis method using a flow particle image analyzer and further obtaining a standard deviation in the particle size distribution.
CV = (standard deviation / number average particle diameter) × 100

  The circularity of the carrier is 0.95 to 0.98, preferably 0.96 to 0.97. By setting the circularity of the carrier to 0.95 or more, the fluidity of the two-component developer is improved and the toner can be charged uniformly. By setting the circularity of the carrier to 0.98 or less, the toner can be sufficiently charged without excessively increasing the fluidity of the two-component developer, and thus occurrence of fogging, toner scattering, and the like can be suppressed. Further, by setting the number average particle diameter of the carrier to 25 to 50 μm and the circularity to 0.95 to 0.98, the magnetic brush becomes a free spike, so that the remaining toner on the developing roller 23 is transferred to the magnetic roller. The magnetic brush 22 can be sufficiently peeled off, and the occurrence of history development can be suppressed.

The circularity is obtained by calculating the circularity of each particle (carrier) from the following equation by a flow image analysis method using a flow particle image analyzer and averaging the circularity of 1000 to 1500 particles. .
Circularity = Perimeter of a circle with the same projected area as the particle image / perimeter of the projected image of the particle

  The saturation magnetization of the carrier is 60 eum / g to 80 eum / g, and preferably 70 to 80 emu / g. By setting the saturation magnetization of the carrier within the above range, it is possible to increase the strength of the magnetic brush and at the same time to reduce the magnetic brush resistance, the developability from the magnetic roller 22 to the developing roller 23 and the magnetic force from the developing roller 23 to the magnetic force. The toner recoverability to the roller 22 can be improved at the same time.

  Further, as the peripheral speed of the magnetic roller 22 increases or as the peripheral speed difference between the magnetic roller 22 and the developing roller 23 increases, the contact time of the magnetic brush with the developing roller 23 decreases. The toner recovery performance is lowered, and history development is likely to occur. According to the configuration of the present invention, by setting the number average particle diameter, the circularity, and the saturation magnetization of the carrier within predetermined ranges, excellent toner recovery can be achieved even in a high-speed machine in which the peripheral speed of the magnetic roller 22 is 500 mm / second or more. The performance can be maintained, and the occurrence of history development can be effectively suppressed.

  In addition, this invention is not limited to the said embodiment, A various change is possible in the range which does not deviate from the meaning of this invention. For example, in the above embodiment, the developing device using positively charged toner whose charging direction is positive (plus side) has been described as an example. However, the developing device using negatively charged toner whose charging direction is negative (minus side) is described. Is equally applicable.

  Further, the present invention is not limited to the tandem type color printer shown in FIG. 1, and includes a developing device such as a digital or analog type monochrome copying machine, a monochrome printer, a rotary developing type color printer, a color copying machine, a facsimile, or the like. The present invention can be applied to various image forming apparatuses. Hereinafter, the effects of the present invention will be described in more detail with reference to examples.

  Using a resin solution obtained by diluting 10 parts by mass of methyl silicone resin (manufactured by Shin-Etsu Silicone Co., Ltd., KR-251) with 500 parts by mass of a solvent (toluene), 1000 parts by mass of carrier core material (spherical ferrite particles, weight average particle diameter 35 μm) is immersed. It was coated by the method. After the carrier core material is coated with the resin liquid, baking is performed at 250 ° C. for 3 hours using a heat treatment apparatus (manufactured by Nippon Pneumatic Co., Ltd., saffusion system), crushing and coarse powder removal are performed. As a result, a carrier having a saturation magnetization of 72 emu / g and a particle size of 35 μm was obtained. This carrier is further classified using a mesh to obtain a carrier having a circularity of 0.967, a saturation magnetization of 72 emu / g, and a particle diameter of 50 μm, and a carrier having a circularity of 0.955, a saturation magnetization of 72 emu / g, and a particle diameter of 25 μm. It was.

  The number average particle diameter and circularity of the carrier were measured with a flow particle image analyzer (FPIA-2100, manufactured by Sysmex Corporation). However, fine particles having a particle size of 10 μm or less were excluded from the measurement. The saturation magnetization of the carrier was measured with a magnetization measuring device (B-H tracer BHU-60, manufactured by Riken Denshi Co., Ltd.).

Reference example 1

  A carrier having a circularity of 0.912, a saturation magnetization of 67 emu / g, and a particle diameter of 45 μm was obtained in the same manner as in Example 1 except that ferrite particles having a weight average particle diameter of 45 μm were used as the carrier core material.

Reference example 2

  A carrier having a circularity of 0.975, a saturation magnetization of 55 emu / g, and a particle size of 38 μm was obtained in the same manner as in Example 1 except that magnetite composite spherical particles having a weight average particle size of 38 μm were used as the carrier core material.

Test example 1

  In the test machine as shown in FIG. 1 equipped with the developing device of the present invention shown in FIG. 2, the carriers having different number average particle diameter, circularity, and saturation magnetization produced in Example 1 and Reference Examples 1 and 2 were used. Using the two-component developer included, the solid image roller streaks, image density, density following failure (history development), and occurrence of leak between MSs were compared when the surface state of the magnetic roller was changed. The test was performed in a cyan image forming portion Pa including the photosensitive drum 1a and the developing device 3a.

The test machine conditions were a system speed (photosensitive drum peripheral speed) of 169 mm / sec, a developing roller peripheral speed of 270 mm / sec (forward rotation on the surface facing the photoconductor), and a magnetic roller peripheral speed of 406 mm / sec. (Counter rotation on the surface facing the developing roller). In the developing roller, the developing roller side magnetic pole was disposed in a developing sleeve having a diameter of 20 mm in which a coating layer made of a silicon-modified polyurethane resin having a volume resistivity of 10 ⁷ Ω was laminated on the outer peripheral surface. The gap between the photoconductor and the developing roller was 120 μm, and the gap between the magnetic roller and the developing roller was 300 μm.

  For the magnetic roller, a fixed magnet body having a five-pole configuration including a main pole of 85 mT, a regulation pole (hog cutting pole) of 60 mT, a pumping pole, a peeling pole of 35 mT, and a transfer pole of 65 mT is disposed in a sleeve having a diameter of 16 mm. In the case of a magnetic roller subjected to knurling, 50 grooves having a depth of 0.3 mm, a width of 0.3 mm, and an interval of 0.7 mm were formed in the axial direction on the outer peripheral surface of the rotating sleeve. In the case of a magnetic roller subjected to blasting treatment, the surface roughness (maximum height) Rz was 17 μm using ceramic abrasive grains. The surface roughness was measured using SURFCOM 1500DX manufactured by ACPRETECH.

  As the developer, a two-component developer comprising a positively charged toner having an average particle diameter of 6.8 μm and a specific gravity of 1.2 and a coating ferrite carrier having different number average particle diameter, circularity, and saturation magnetization is used. The toner mixing ratio (T / C) was 9% by weight.

  The voltage application conditions to the developing roller were: Vslv (DC) = 75 V, Vpp of Vslv (AC) was 1.3 kV, frequency was 3 kHz, and Duty = 35%. Further, Vmag (DC) = 300 V and Vmag (AC) Vpp of 1.1 kV were applied to the magnetic roller in reverse phase.

  As an evaluation method, after 10,000 test images with a printing rate of 5% are printed, a solid image is visually observed for the roller streaks, and when streaks cannot be confirmed on the image, ○, streaks are confirmed Was marked with x. For the image density, the density (ID; image density) of a solid image is measured using a reflection densitometer (Spectra Eye, manufactured by Gretag Macbeth Co.). Was marked with x. Concerning density follow-up (history development), a reflection densitometer is used to measure the image density (ID) between the leading edge and the trailing edge of a solid image, and the difference between the measured values is 0.05 or less. The case of ˜0.15 was rated as “◯”, and the case of 0.15 or more was marked as “X”. As for leakage between MSs, ◯ indicates that no leak mark can be confirmed in the image, × indicates that the leak mark can be confirmed, and ◎ indicates that no leak mark can be confirmed even if the magnetic roller Vpp is increased to 1.2 kV. . The evaluation results are shown in Table 1 together with the number average particle diameter, the circularity, and the saturation magnetization of the carrier.

  As is apparent from Table 1, the present invention 1 is that a carrier having a number average particle size of 25 to 50 μm, a circularity of 0.955 to 0.967, and a saturation magnetization of 72 eum / g is provided on the developing roller. In -4, no roller streaking and no decrease in image density were observed. Further, the difference in image density between the leading edge and the trailing edge of the solid image was 0.15 or less, and the density tracking was good. Moreover, since the damage of the coat layer by the carrier can be suppressed, the occurrence of leakage between MSs was also suppressed.

  In particular, in the present invention 4 in which the magnetic roller is knurled, it has been confirmed that the density followability is further improved as compared to the present inventions 1 to 3 in which the blast treatment is performed, and the occurrence of leakage between MSs can be further suppressed.

  On the other hand, in Comparative Example 1 using a carrier having a low circularity of 0.912, the coating layer on the surface of the developing roller was damaged by the magnetic brush, and roller streaks were generated in the solid image. In addition, discharge occurred at the part where the coat layer was peeled off, and leakage between MSs was also generated. Further, in Comparative Example 2 using a developing roller on which no coating layer was formed on the surface, a decrease in density due to a decrease in toner recoverability from the developing roller was observed, and leakage between MSs also occurred. Further, in Comparative Example 3 using a carrier having a saturation magnetization of 55 emu / g, the toner recoverability from the developing roller to the magnetic roller was lowered, and density tracking failure occurred.

Test example 2

  Under the conditions of Invention 2 in Test Example 1, the system speed, the developing roller peripheral speed, and the magnetic roller peripheral speed were changed to generate roller streaks and develop after printing 10,000 test images with a printing rate of 20%. Toner adhesion to the roller was observed. In the evaluation of toner adhesion, the developing roller was visually checked, and the case where no toner was adhered was evaluated as ◯, and the case where the toner was clearly adhered was evaluated as x. The results are shown in Table 2.

  As can be seen from Table 2, by setting the speed of the magnetic roller to 500 mm / second or more, an appropriate rubbing force by the magnetic brush can be obtained, so that the toner on the developing roller can be effectively used without generating roller streaks. It was confirmed that the toner can be prevented from adhering to the developing roller. When toner adheres to the developing roller, the amount of toner that can be developed decreases, so that it is necessary to adjust the bias between the magnetic roller and the developing roller.

Test example 3

  Using two-component developers containing carriers with different number average particle diameter, circularity, and saturation magnetization, and changing the peripheral speed of the magnetic roller, the image density of solid images and the occurrence of poor density tracking (history development) Compared. The test machine was the same as in Test Examples 1 and 2, and was performed in the cyan image forming portion Pa including the photosensitive drum 1a and the developing device 3a.

The test machine conditions were a system speed (photosensitive drum peripheral speed) of 420 mm / sec, a developing roller peripheral speed of 588 mm / sec (forward rotation on the surface facing the photoconductor), and a magnetic roller peripheral speed of 300 mm / sec. , 500 mm / second, 700 mm / second, and 900 mm / second (counter rotation on the surface facing the developing roller). In the developing roller, the developing roller side magnetic pole was disposed in a developing sleeve having a diameter of 20 mm in which a coating layer made of a silicon-modified polyurethane resin having a volume resistivity of 10 ⁷ Ω was laminated on the outer peripheral surface. The gap between the photoconductor and the developing roller was 120 μm, and the gap between the magnetic roller and the developing roller was 300 μm.

  The magnetic roller is placed in a rotating sleeve having a diameter of 25 mm and subjected to blasting with a surface roughness Rz of 17 μm so that the main pole of the fixed magnet body is positioned 5 ° downstream of the closest contact point with the developing roller in the rotational direction. The magnetic pole of the fixed magnet body has a five-pole configuration including a main pole 85 mT, a regulation pole (hobber cutting pole) 60 mT, a pumping pole, a peeling pole 35 mT, and a transport pole 65 mT. The voltage application conditions to the developing roller and the magnetic roller were the same as in Test Example 1.

  As the developer, a positively charged toner having an average particle diameter of 6.8 μm and a specific gravity of 1.2, a number average particle diameter of 35 μm, a circularity of 0.966, a carrier having a saturation magnetization of 72 eum / g, or a number average particle diameter of 45 μm, A two-component developer composed of a carrier having a circularity of 0.912 and a saturation magnetization of 67 eum / g was used, and the mixing ratio (T / C) of the toner to the carrier was 9% by weight. The evaluation results are shown in Table 3.

  In addition, the developing device is driven under the same conditions as the normal printing operation, and the developing device is suddenly stopped at predetermined intervals (near the cutting blade) and two locations near the developing roller-magnetic roller nip. The collodion solution was added dropwise to fix the magnetic brush. The fixed magnetic brush was taken out, the weight and volume were measured, and the magnetic brush density (weight / volume) at two locations was calculated and compared.

  As is apparent from Table 3, in the fifth invention using a carrier having a number average particle diameter of 35 μm, a circularity of 0.966, and a saturation magnetization of 72 eum / g, even when the peripheral speed of the magnetic roller is increased to 900 mm / second, ID1 The image density of 2 or more was maintained and no history development was observed, indicating good density following. Further, the magnetic brush density near the nip portion of the developing roller-magnetic roller was higher than the magnetic brush density near the regulating portion.

  On the other hand, in Comparative Example 5 using a carrier having a number average particle diameter of 45 μm, a circularity of 0.912, and a saturation magnetization of 67 eum / g, history development occurs when the peripheral speed of the magnetic roller exceeds 500 mm / sec, and 900 mm / sec. The image density decreased. The magnetic brush density near the nip portion of the developing roller-magnetic roller was equal to or less than the magnetic brush density near the regulating portion.

  Based on the above results, the toner collection performance from the developing roller is improved by using a developing roller having a coat layer and setting the number average particle diameter, circularity, and saturation magnetization of the carrier in the two-component developer within predetermined ranges. Therefore, it was confirmed that image defects such as roller streaks, density reduction, and history development and leakage between MSs can be effectively suppressed. Further, it was confirmed that the magnetic brush density in the nip portion of the developing roller-magnetic roller can be increased, and the developing performance can be maintained even in a high-speed machine having a high peripheral speed of the magnetic roller. Here, the test was performed using the cyan image forming portion Pa, but it has been confirmed that similar results can be obtained in the magenta, yellow, and black image forming portions Pb to Pd.

  The above embodiment is merely an example of the present invention, and the drum surface potential, conditions for applying voltage to the developing roller and the magnetic roller, and the like can be appropriately set according to the specifications of the apparatus and the usage environment.

  INDUSTRIAL APPLICABILITY The present invention can be used in a developing device that develops an electrostatic latent image on an image carrier by holding only a charged toner on the toner carrier using a toner supply member, and forms a coat layer on the outer peripheral surface. A two-component developer is formed using a magnetic carrier having a number average particle size of 25 to 50 μm, a circularity of 0.95 to 0.98, and a saturation magnetization of 60 eum / g to 80 eum / g. It is a thing.

  Accordingly, it is possible to provide a developing device that does not deteriorate the toner supply property to the toner carrier and the toner recovery property from the toner carrier and does not cause roller streaks, density reduction, history development, and leakage between MSs. . In particular, in a high-speed machine in which the peripheral speed of the toner supply member is 500 mm / second or more, it is possible to effectively suppress carrier pulling and toner recovery performance.

1a to 1d Photosensitive drum (image carrier)
3a to 3d Developing device 21a First stirring screw 21b Second stirring screw 22 Magnetic roller (toner supply member)
22a Rotating sleeve 22b Fixed magnet body 23 Developing roller (toner carrier)
23a Developing sleeve 23b Developing roller side magnetic pole 25 Panicle cutting blade 30 First bias circuit 31 Second bias circuit 33 Voltage variable device 35 Main pole 36 Regulating pole 100 Image forming apparatus Pa to Pd Image forming section

Claims (4)

A toner carrier that is disposed to face the image carrier in a non-contact manner;
A toner supply member that forms a toner layer on the toner carrier using a magnetic brush formed of a two-component developer composed of toner and a magnetic carrier;
In a developing device having
A coat layer made of a resin material is formed on the outer peripheral surface of the toner carrier,
The developing device according to claim 1, wherein the magnetic carrier has a number average particle diameter of 25 to 50 μm, a circularity of 0.95 to 0.98, and a saturation magnetization of 60 eum / g to 80 eum / g.   The developing device according to claim 1, wherein a plurality of grooves extending in the axial direction are formed on an outer peripheral surface of the toner supply member.   The developing device according to claim 1, wherein a peripheral speed of the toner supply member is 500 mm / second or more.   An image forming apparatus on which the developing device according to claim 1 is mounted.

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