JP6488695B2 - Developing device, process cartridge, and image forming apparatus - Google Patents

Description

  The present invention relates to a developing device, a process cartridge, and an image forming apparatus.

  In a developing device used in an electrophotographic image forming apparatus, a latent image on a latent image carrier (photosensitive member) is applied by applying a superimposed voltage obtained by superimposing an alternating current voltage on a DC voltage to a developer carrier (developing roller). A method for developing the toner is already known.

  The developing device stirs and mixes the replenishment toner and the developer to charge the toner to a predetermined value. However, in this type of developing device, there is a problem that the temperature in the developing device increases due to frictional heat generated when the developer in the developer containing container is stirred and conveyed, and the charge amount of the toner decreases. When the charge amount of the toner decreases, the toner adhesion amount to the latent image carrier decreases, and a predetermined image density cannot be obtained.

  Further, when the temperature in the developing device rises, the melted toner adheres to the developer regulating member that regulates the layer thickness of the developer, the developer carrying body, the latent image carrying body, etc., and streaky abnormal images etc. May occur. In recent years, the developing device tends to be hot due to an increase in printing speed, and when a toner having a low melting temperature is used to reduce the fixing energy, an abnormal image due to toner fixation tends to occur.

  The developing device is a very important part to achieve high image quality and high reliability, and the developer container is made of metal with high heat dissipation to facilitate cooling and suppress the temperature rise inside the device. To be done.

  However, in a developing device including a developer container made of metal, charges are accumulated in the container when a developing bias is applied to the developer carrier. Since the developing device is close to the main body of the image forming apparatus, the charge accumulated in the developer container between the developing device and the main body may be discharged, and an abnormality such as a leak may occur. For this reason, the developer container itself is electrically grounded to release accumulated charges.

  Further, in Patent Document 1, in order to effectively discharge the heat generated inside the developing device to the outside, a large number of heat radiating ribs are formed on the lower surface and side surface of the developing case to secure a heat radiating area. The structure which blows air from the direction is disclosed. This has an effect of performing uniform cooling from the side surface to the lower surface by the air blown from the side of the apparatus and the heat radiation rib.

  By the way, in the two-component development method using a non-magnetic toner and a magnetic carrier, if a DC voltage is used as the development bias, an abnormal image such as image roughness (granularity) and uneven image density such as a developing roller pitch occurs. There is. In order to solve this problem, a method of applying a superimposed voltage obtained by superimposing an AC voltage on a DC voltage as a developing bias is used.

  For example, Patent Document 2 discloses a method for controlling an alternating voltage applied to a developer carrying member in order to suppress the occurrence of an image abnormality from the initial use to the end of use of the developing device.

  However, in the case of a developing device including a metal developer container that is electrically grounded, when a superimposed voltage is applied to the developer carrier, the superimposed voltage branches into two parts: a latent image carrier and a developer container. Applied. As a result, a desired voltage cannot be applied to the latent image carrier, and the effect of improving the roughness of the image and the unevenness of the image density is lost, and a new problem that white spots around the solid patch image deteriorate is newly generated.

  The present invention provides a developing device, a process cartridge, and an image forming apparatus that can apply a desired superimposed voltage to a latent image carrier and improve image quality even when a metallic developer container is electrically grounded. The purpose is to do.

In order to solve the above problems, a developer container, an electrically grounded metal part forming the developer container, a non-magnetic toner and a magnetic carrier housed in the developer container A two-component developer containing the developer, a developer-carrying member that is provided in the developer-accommodating container and carries the two-component developer by a magnetic field generation unit, and a superimposed voltage obtained by superimposing an alternating voltage on a direct-current voltage, A developer bias applying mechanism for applying to the developer carrying member, and a developing device in which a current limiting resistor is provided in a facing portion between the developer carrying member and the metal portion, and the developer containing container in the facing portion. A developing device is provided in which a part of the container tip is a separate container tip, the tip of the container is made of resin, and an insulating layer is formed in the facing part .

  In the developing device of the present invention, since the developer container is made of metal, heat is sufficiently dissipated. In addition, since the current flowing through the facing portion between the developer carrier and the developer container can be suppressed, a desired superimposed voltage can be applied to the latent image carrier. Therefore, the occurrence of abnormal images can be suppressed and the image quality can be improved.

1 is a cross-sectional view schematically showing a structure of a color copying machine which is an embodiment of an image forming apparatus. FIG. 2 is a cross-sectional view illustrating a structure of a developing device and a photoreceptor according to the embodiment. It is sectional drawing which shows the structure of a developing roller. It is a schematic diagram of the friction coefficient measuring apparatus using the Euler belt system. It is the figure which plotted the absolute value of the normal direction magnetic flux density of the developing roller. It is an external view of the cell used for the measurement of the volume specific resistance of a magnetic carrier. FIG. 3 is an enlarged view of the vicinity of a development area of the development device according to the first embodiment. It is a graph which shows typically the waveform for about 1 period of a rectangular wave. 5 is a graph for explaining image forming conditions, that is, a post-charge potential (Vd), a post-exposure potential (VL), and a development bias (Vb) of a photoreceptor. FIG. 10 is an enlarged view of the vicinity of a development area of a development device according to a second embodiment. It is an evaluation image (checker image). It is an evaluation image (halftone image). It is a schematic block diagram of a liquid cooling device. It is a schematic block diagram of the process cartridge using a cooling device.

(First embodiment)
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a cross-sectional view schematically showing the structure of a color copying machine which is an embodiment of an image forming apparatus. First, the configuration and operation of the entire image forming apparatus will be described.

  The illustrated image forming apparatus employs an electrophotographic system, and includes an image forming unit 1 in which four process cartridges 11Y, 11M, 11C, and 11K are arranged in parallel. Each process cartridge 11Y, M, C, K is a drum-shaped photosensitive member 18Y, M, C, K, which is a latent image carrier, drum cleaning unit 12Y, M, C, K, charging unit 13Y, M, C, K, two-component developing type development devices 40Y, M, C, K, and the like are provided. These process cartridges 11Y, 11M, 11C, and 11K are detachable from the apparatus main body, and after being removed from the apparatus, consumable parts can be replaced at a time. Further, the process cartridge 11 may integrally include the developing device 40 and at least one of the photosensitive member 18, the charging unit 13 as charging means, and the drum cleaning unit 12 as cleaning means.

  The subscripts Y, C, M, and K in the unit and its components indicate yellow, cyan, magenta, and black colors, respectively. Further, the color order of Y, C, M, and K is not limited to that shown in FIG.

  An exposure unit 9 as a latent image forming unit is provided above the image forming unit 1, and a reading device 10 that scans and reads a document placed on the contact glass is provided above the exposure unit 9. It has been. Below the image forming unit 1, a transfer unit 2 including an intermediate transfer belt 15 as an intermediate transfer member is provided. The intermediate transfer belt 15 is stretched around a plurality of support rollers, and rotates in the clockwise direction in the drawing. A secondary transfer device 4 is provided below the transfer unit 2 and includes a secondary transfer roller 17. The secondary transfer roller 17 is in contact with the transfer counter roller 16 with the intermediate transfer belt 15 interposed therebetween to form a secondary transfer nip. A secondary transfer bias is applied to the secondary transfer roller 17 by a power source (not shown), and the transfer counter roller 16 is electrically grounded. As a result, a secondary transfer electric field is formed in the secondary transfer nip.

  On the left side of the secondary transfer device 4 in the figure, a fixing unit 7 having a heating roller having a heating element therein is provided in order to fix the toner image transferred onto the paper. Between the secondary transfer device 4 and the fixing unit 7, a conveying belt 6 that conveys the paper after the toner image transfer to the fixing unit 7 is provided. Also, below the apparatus, a sheet feeding unit 3 that feeds sheets fed separately from a sheet feeding storage unit (not shown) to the secondary transfer device 4 and sheets that have passed through the fixing unit 7 are displayed. A paper discharge unit 8 is provided for conveying to the outside or the duplex unit 5.

  A series of operations when making a copy in the image forming apparatus configured as described above will be described.

  First, the reading device 10 reads a document placed on a contact glass. In parallel with this document reading, the intermediate transfer belt 15 moves in the clockwise direction in the figure. In the image forming unit 1, the surfaces of the photoconductors 18Y, 18M, 18C, and 18K are charged by the respective charging units 13Y, 13M, 13C, and 13K, and exposure is performed based on the image information of the original read by the reading device 10. It is exposed by the unit 9. Thereby, a latent image is formed on the surface of each photoconductor 18Y, M, C, K.

  Next, the latent images on the surfaces of the photoconductors 18Y, 18M, 18C, and 18K are developed by the developing devices 40Y, 40M, 40C, and 40K to form single-color toner images (visual images). Each of the photoconductors 18Y, 18M, 18C, and 18K sequentially transfers the toner images on the intermediate transfer belt 15 so as to overlap each other, thereby forming a composite toner image on the intermediate transfer belt 15. The residual toner on the photoconductor 18 is removed from the photoconductors 18Y, 18M, 18C, and 18K after the toner image is transferred by the drum cleaning units 12Y, 12M, 12C, and 12K to prepare for the next image formation.

  In parallel with such toner image formation, the paper is fed out one by one from a paper supply housing unit (not shown) and stops against the registration roller 14. Then, the registration roller 14 rotates in synchronism with the formation of the composite toner image on the intermediate transfer belt 15, and a sheet is fed between the intermediate transfer belt 15 and the secondary transfer device 4. Transfer the toner image on top.

  Next, the sheet on which the toner image is transferred is conveyed by the conveyance belt 6 and sent to the fixing unit 7. The fixing unit 7 applies heat and pressure to the paper on which the toner image is transferred to fix the toner image, and sends the paper on which the toner has been fixed to the paper discharge unit 8.

  In the case of single-sided printing, the paper discharge unit 8 guides the paper to a paper discharge tray (not shown) outside the apparatus (on the left side of the apparatus). In the case of duplex printing, the paper discharge unit 8 switches the switching claw to guide the paper to the lower duplex unit 5. The duplex unit 5 reverses the sheet, leads it again to the secondary transfer position (secondary transfer nip position between the secondary transfer device 4 and the intermediate transfer belt 15), fixes the toner image on the back surface, and discharges the sheet. At 8, the paper is discharged onto the paper discharge tray. The intermediate transfer belt 15 after image transfer is prepared for the next image formation by removing residual toner remaining on the belt by the intermediate transfer belt cleaning unit 90. The above is a series of operations when copying.

  In this image forming apparatus, as shown in FIG. 1, the intermediate transfer belt 15 is bent so as to cover the upper surface and the right side surface of the fixing unit 7 together with the higher density of the mechanical device from the viewpoint of reducing the size of the apparatus. In addition, the fixing unit 7 is placed under the transfer unit 2. Thereby, the height direction and the width direction of the apparatus are made compact, and the apparatus size is reduced.

  However, when the fixing unit 7 is brought close to the intermediate transfer belt 15, the intermediate transfer belt 15 may be deformed by heat of the fixing unit 7 that is a heating element, resulting in image defects such as color misregistration. Further, during double-sided printing, the paper heated by the fixing unit 7 comes into contact with the intermediate transfer belt 15 again at the secondary transfer position via the double-sided unit 5, so that the intermediate transfer is now performed by heat conduction from the paper. The temperature of the belt 15 rises. Further, heat is also transmitted to the photoreceptors 18Y, 18M, 18C, 18K, 18K, 18M, 18C, and 18K in contact with the intermediate transfer belt 15 and the developing devices 40Y, M, C, and K in the process cartridge 11Y Problems such as sticking are likely to occur.

  Therefore, in the present embodiment, the heat insulating device 20 is provided between the fixing unit 7 that is a heat generation source and the intermediate transfer belt 15 that is disposed in the vicinity of the fixing unit 7. The heat insulating device 20 includes a heat receiving plate 21 that is a heat receiving member, a heat pipe 22 that is a heat transfer means (heat transport means), a heat radiating plate 23 that is a heat radiating member, a duct 24, and an exhaust fan (not shown). Have.

  The heat receiving plate 21 is formed of a material having high thermal conductivity such as Al or Cu, and is disposed between the fixing unit 7 that is a heat generation source and the transfer unit 2 that is an object to be protected from the influence of the heat. One end portion (lower end portion) of the heat pipe 22 is a heat receiving portion, and is attached to the lower surface of the heat receiving plate 21. The other end (upper end) of the heat pipe 22 is a heat radiating portion, and the heat radiating portion is attached to the heat radiating plate 23 at a position higher than the heat receiving portion.

  The heat radiating plate 23, which is a heat radiating member, is formed of a material having high thermal conductivity such as Al or Cu, and releases heat transported from the heat pipe 22. A heat sink may be added to the heat radiating plate 23 as necessary. The duct 24 extends from the front surface to the back surface of the image forming apparatus main body, and is provided so that the heat radiating plate 23 is positioned inside the duct. An air inlet is provided at the end of the duct 24 on the front side of the apparatus, and an exhaust port is provided at the end of the rear side. An exhaust fan (not shown) is provided at the exhaust port.

  Such a heat insulating device 20 absorbs heat from the heat generating portion (fixing unit 7) by the heat receiving plate 21, and transports the heat to the heat radiating portion (heat radiating plate 23) by the heat pipe 22. The transported heat is released from the heat radiating plate 23 in the duct 24 and is discharged outside the apparatus by an exhaust fan (not shown). In addition, it is good also as natural cooling without providing an exhaust fan, and the method of taking in external air using a duct instead of a heat pipe may be used.

  In this way, the heat insulating device 20 can protect the intermediate transfer belt 15 from the influence of heat of the fixing unit 7 and prevent color misregistration due to deformation of the intermediate transfer belt 15 in advance. Further, it is possible to reduce the heat transmitted to the photoconductors 18Y, 18M, 18C, 18K, and the developing devices 40Y, 40M, 40C, and 40K that are in contact with the intermediate transfer belt 15. Therefore, the present image forming apparatus can be arranged so that the intermediate transfer belt 15 is bent and the fixing unit 7 is inserted under the transfer unit 2, so that the size of the apparatus can be reduced.

  Further, in the developing devices 40Y, 40M, 40C, and 40K, frictional heat due to the friction between the developer accommodated in the developer container and the developer agitating / conveying member in each device, and the friction between the developers. The temperature in the developing devices 40Y, M, C, and K increases due to the frictional heat. Furthermore, frictional heat due to friction between the developer regulating member and the developer that regulates the layer thickness of the developer on the developer carrying member, and frictional heat due to rubbing between developers during regulation by the developer regulating member. However, the temperature in the developing devices 40Y, M, C, and K increases.

  When the temperature in the developing devices 40Y, 40M, 40C, and 40K rises, the charge amount of the toner decreases, the toner adhesion amount to the latent image carrier decreases, and a predetermined image density cannot be obtained. Further, the toner may be melted by the temperature rise and fixed to the developer regulating member, the developer carrying member, the latent image carrying member, etc., and a streaky abnormal image may be generated on the image. In recent years, the developing device tends to be hot due to an increase in printing speed, and an abnormal image due to toner fixation tends to occur even when toner having a low melting temperature is used to reduce fixing energy.

  Thus, the developing devices 40Y, M, C, and K are very important cooling parts because of high image quality and high reliability. Therefore, in this embodiment, the developer containers of the developing devices 40Y, M, C, and K are formed of a material having high thermal conductivity such as Al or Cu, and a large number of heat radiating ribs are provided as cooling members to cool by airflow. Is going.

  FIG. 2 is a cross-sectional view illustrating the structure of the developing device and the photoreceptor according to the embodiment. The configuration and operation of the developing device and the photoreceptor will be described with reference to FIG.

  Since the four process cartridges 11Y, 11M, 11C, and 11K have substantially the same configuration except that the colors of the toners to be handled are different, the suffixes Y, M, C, and K are omitted.

  The developer container 41 of the developing device 40 is formed by a wall member that forms the agitation conveyance path 44, the recovery conveyance path 47, the supply conveyance path 49, and the like, and a heat radiation rib 41a as a cooling member. A part of the developer container 41 is made of a metal such as Al having high thermal conductivity, and the heat radiation rib 41a is integrally formed on the left side surface of the developer container 41, thereby enhancing the heat radiation effect of the developing device 40. ing. (In the figure, the hatched portion is made of metal.) In addition, in order to prevent electric charges from accumulating in the metal portion of the developer container 41 when a developing bias is applied and discharging to the main body or the like, The metal part is electrically grounded. The material of the developer container is not limited to Al, and other materials such as Cu having high thermal conductivity may be used.

  In the developer container 41 thus formed, a two-component developer containing a nonmagnetic toner and a magnetic carrier is contained.

  The developing device 40 is provided with a developing roller 45 on the left side of the photosensitive member 18 for supplying and developing toner on the latent image on the surface of the photosensitive member 18 while moving in the direction of arrow I. The photosensitive member 18 is an example of a latent image carrier, and the developing roller 45 is an example of a developer carrier.

  FIG. 3 is a cross-sectional view showing the structure of the developing roller. As shown in FIG. 3, the developing roller 45 includes a rotatable developing sleeve 45a, in which a magnetic body 45c (shown in a single shape for convenience) having a plurality of magnetic poles is disposed. The magnetic body 45 c is used to hold the developer on the surface of the developing roller 45.

  On the surface of the developing sleeve 45a, a plurality of circular or elliptical dents in a plan view are provided regularly or irregularly at intervals so as not to overlap each other. As a method for processing the surface of the developing sleeve 45a, a method described in Patent Document 3 may be used.

  A low friction film 45b is formed on the surface of the developing sleeve 45a. The low friction film 45b is made of a material having a lower friction coefficient than the base material of the developing sleeve 45a, such as tetrahedral amorphous carbon (ta-C) or titanium nitride (TiN).

  Here, a method for forming a ta-C film by a filtered cathodic vacuum arc (FCVA) method will be described.

  First, high-purity carbon (graphite) is placed as a target in a substantially vacuum chamber, and arc discharge is performed on the target. Next, the plasma generated by the arc discharge is guided to the base material of the developing sleeve 45a to be deposited by electromagnetic induction. During the induction process, macro particles and neutral atoms / molecules unnecessary for vapor deposition are removed by an electromagnetic spatial filter, and only ionized carbon is extracted. Then, the ionized carbon that has reached the base material of the developing sleeve 45a is agglomerated on the base material surface. In this way, the low friction film 45b made of the ta-C film is formed on the outer surface of the developing sleeve 45a.

  The low-friction film 45b made of a ta-C film can be formed with a uniform thickness as compared with a film formed by plating or coating, and can be formed at a relatively low temperature. The distortion due to the temperature of the sleeve 45a hardly occurs. Therefore, the shape accuracy of the developing sleeve 45a can be increased. In addition, about the vapor deposition technique by a FVCA system, since it is disclosed by patent document 4 etc. and has already been utilized widely, detailed description is abbreviate | omitted.

  Further, the low friction film 45b may be formed of a TiN film by using a hollow cathode method (HCD method: Hollow Cathode Discharge). An ion plating method which is one of physical vapor deposition (PVD) methods can easily obtain a film having excellent adhesion. Among these ion plating methods, in particular, by using the HCD method, it is possible to obtain a coating film that is uniform and has a uniform film thickness and conforms to the surface roughness of the base material. The vapor deposition technique based on the HCD method is disclosed in, for example, Patent Document 5 and Patent Document 6 and has already been widely put into practical use, and detailed description thereof is omitted.

  The low friction film 45b may be any material having a lower friction coefficient than the base material of the developing sleeve 45a. In addition, a material such as titanium carbide (TiC), titanium carbonitride (TiCN), or molybdic acid is used. Also good. The friction coefficient of each material is such that the aluminum alloy is 0.5 or more, TiN is 0.3 to 0.4, and ta-C is about 0.1 or less.

  The developing sleeve 45a is formed with a low friction layer 45b after a dent is formed on the surface. That is, the dent on the surface suppresses a decrease in the developer conveyance amount due to a change with time, and prevents image density unevenness. The low friction layer 45b reduces the frictional force with the developer and prevents the toner from adhering to the non-image portion of the developing sleeve 45a. As a result, it is possible to prevent density unevenness such as a ghost image (the history of one round of the photoreceptor is reflected in the next image).

  A method for measuring the surface friction coefficient of the low friction film 45b will also be described.

  FIG. 4 is a schematic diagram of a friction coefficient measuring apparatus using the Euler belt method. For measuring the friction coefficient of the developing sleeve 45a coated with the low friction film 45b, a friction coefficient measuring device 95 using an Euler belt method is used. On the gantry 96, a developing roller 45 and a force gauge 99 are installed via a pedestal 98. Further, a medium-quality high-quality paper 100 as a belt is stretched around a circumferential quarter portion of the developing roller 45 so that the paper gap is in the longitudinal direction.

For example, a load (weight 97) of 0.98 N (100 g) is applied to one of the belts, and a force gauge 99 is attached to the other. The measured value F of the force gauge 99 at the time when the belt is pulled by the weight 97 and moved is read. Substituting the read measured value F into the following formula (1), the static friction coefficient μ _s can be calculated.
Coefficient of static friction μ _s = (2 / π) × ln (F / 0.98) (1)

  Returning to FIG. 2 again, the description of the developing device and the photoreceptor will be continued.

  A supply conveyance path 49 is formed on the left side of the development roller 45 along the axis of the development roller 45. The developer is supplied to the development roller 45 in the supply conveyance path 49 in the forward direction of the drawing. Is provided.

  A doctor blade 42 that restricts the developer supplied to the developing roller 45 to a thickness suitable for development is provided further downstream from the facing portion of the supply screw 48 and the developing roller 45 in the surface movement direction. The doctor blade 42 is made of stainless steel, whereby the developer on the developing roller 45 is thinned. The doctor blade is an example of a developer regulating unit.

  A collection conveyance path that collects developed toner that has passed through the development area and separated from the surface of the development roller 45 downstream of the development area, which is the facing portion between the development roller 45 and the photoreceptor 18, in the surface movement direction. 47 is formed. A collection screw 46 for conveying the collected toner in the same direction as the supply screw 48 is provided in the collection conveyance path 47. The recovery screw 46 also extends in parallel with the axial direction of the developing roller 45 as with the supply screw 48.

  Note that the developer is detached from the developing roller 45 by setting the magnetic body in the developing sleeve described above to a state where there is no magnetic pole only at a position where the developer is desired to be removed. Further, a magnetic body having a magnetic pole arrangement in which a repulsive magnetic field is formed at a location to be separated may be used.

  FIG. 5 is a plot of the absolute value of the normal direction magnetic flux density of the developing roller. As shown in FIG. 5, the solid line indicated by M1 indicates the value of the normal direction magnetic flux density on the surface of the developing sleeve, and the broken line indicated by M2 indicates the value of the normal direction magnetic flux density at a distance of 1 mm from the surface of the developing sleeve. Indicates.

  Table 1 shows the peak value (mT) of the normal direction magnetic flux density at a position 1 mm away from the developing sleeve surface (distance 0 mm) and the developing sleeve surface of each pole.

  The behavior of the developer will be described with reference to FIG. First, the developer in the supply conveyance path 49 is pumped up to the developing roller 45 by the magnetic force of the pumping pole P3 (N pole), and is transported in the direction of arrow A (clockwise) in the drawing. Next, the conveyed developer is spiked by the restriction electrode P4 (S pole), and the developer amount is restricted by the doctor blade 42. Next, the developer is transported by a transport pole: P5 (N pole), and the toner is developed to face the photoreceptor at a main pole: P1 (S pole). After the development, the developer is transported by a transport pole: P2 (N pole). The developer is separated and separated from the developing roller 45 by the repulsive magnetic field between P2 and P3, and is collected to the collection conveyance path 47.

  Subsequently, the main electrode P1 will be described in detail. At the main pole: P1, the developer rises (magnetic brush) by the magnetic force, and the magnetic brush contacts the photosensitive member with a predetermined interval (development gap). The charged toner in the magnetic brush is developed into a latent image on the photosensitive member by the linear velocity difference between the photosensitive member and the developing roller 45 and the developing bias. However, the magnetic force of the main pole and the magnetic brush rub against the photosensitive member. A contact portion (development nip width) and a linear velocity difference generated in a part greatly affect various images.

  This greatly affects the abnormal image of solid white spots that is the subject of the present invention. In particular, white spots at the front end of a solid image occur as follows. The counter charge generated when the toner is used for the development of the solid portion is moved from the solid portion to the halftone portion while being held by the magnetic brush. Then, the magnetic brush electrostatically attracts the toner of the halftone image located at the boundary between the solid and the halftone (the front end of the solid image) from the photosensitive member. In this way, white spots occur on the leading end side of the solid image.

  In order to avoid the influence of the counter charge, the ratio of the linear speed to the photosensitive member is reduced (usually, the developing speed is fast, but here it means close to the photosensitive member linear speed), or the developing roller has a small diameter. It is conceivable to reduce the width of the development nip by making it easier.

  However, if the linear velocity ratio is reduced, the developing ability is lowered and a problem that a sufficient image density cannot be obtained occurs. Further, when the developing roller 45 is reduced in diameter, a magnetic force at the tip of the magnetic brush (photosensitive member side) is weakened because a sufficient magnetism cannot be obtained by reducing the size of the magnet contained therein, and the carrier is moved to the photosensitive member by the electric force from the photosensitive member. So-called carrier adhesion occurs.

  Therefore, as a measure for narrowing the developing nip without making the linear velocity ratio unnecessarily small and also making the developing roller diameter unnecessarily small, the attenuation factor of the main pole: P1 normal direction magnetic flux density is set to 40% or more. To do. Here, the attenuation factor is the difference between the peak value of the normal direction magnetic flux density on the surface of the developing sleeve and the peak value of the normal direction magnetic flux density at a distance of 1 mm from the surface of the developing sleeve. It is defined as the ratio divided by the peak value of the directional magnetic flux density.

  In the case of this embodiment, as shown in Table 1, the peak value of the normal direction magnetic flux density on the surface of the developing sleeve of the main pole: P1 is −120 mT, and the normal direction magnetic flux at a distance of 1 mm from the developing sleeve surface. The peak density value is -68 mT. In addition, the amount of decrease in the magnetic flux density in the normal direction (absolute value of the difference) is 52 mT. Therefore, the attenuation rate is 43%.

  As described above, since the attenuation rate is 40% or more, the developing device of this embodiment can narrow the developing nip width without changing the linear speed ratio and the developing roller diameter, It is possible to improve white spots at the front end of a solid image.

  Returning to FIG. 2, the description of the developing device will be continued.

  A stirring conveyance path 44 is formed below the supply screw 48 and the supply conveyance path 49 in parallel with the collection conveyance path 47. In the agitating and conveying path 44, an agitating screw 43 that conveys the developer to the back side in the figure, which is in the opposite direction to the supply screw 48, is provided. The stirring screw 43 has a configuration in which a spiral stirring blade portion 43b is fixed to a stirring shaft portion 43a.

  The supply conveyance path 49 and the agitation conveyance path 44 are partitioned by the first partition wall 133, but both ends of the first partition wall 133 on the front side and the rear side in the drawing are open, and the supply conveyance is performed at the opening. The path 49 and the agitation transport path 44 communicate with each other. Further, the supply conveyance path 49 and the collection conveyance path 47 are partitioned by the first partition wall 133, and no opening is provided in the first partition wall 133. Furthermore, the two developer conveyance paths, that is, the stirring conveyance path 44 and the collection conveyance path 47, are partitioned by the second partition wall 134. The second partition wall 134 has an opening on the front side in the figure, and the stirring conveyance path 44 and the collection conveyance path 47 communicate with each other in the opening.

  As described above, the heat radiation rib 41a, the lower and left side surfaces of the developer container 41, the first partition wall 133, the second partition wall 134, the recovery transport path 47, and the stirring transport path 44 are formed of Al. The heat radiation effect of the developing device 40 is enhanced. (Refer to the hatched part in Fig. 2)

  The supply screw 48, the recovery screw 46, and the stirring screw 43, which are developer stirring and conveying members, are made of resin or metal. The screw diameters are all 22 (mm), the screw pitch is 50 (mm) for the supply screw 48, and the recovery screw 46 and the stirring screw 43 are 25 (mm) for one winding. The number of revolutions is about 600 (rpm) for all the screws.

  Further, in the developing device 40 of the present embodiment, the total length of the stirring conveyance path 44 is 410 (mm), and the total length of the supply conveyance path 49 is 320 (mm).

  The developing roller 45 is made of an Al or SUS (stainless steel) element tube having a diameter of 25 (mm), and the surface thereof is subjected to V-groove or sandblasting. The gap between the developing roller 45 and the doctor blade 42 and the gap between the developing roller 45 and the photoreceptor 18 are both about 0.3 (mm).

  Although not shown in the drawing, a developing bias applying mechanism is attached to the developing roller 45, and the photosensitive member 18 is electrically grounded.

  In the developing device 40 configured as described above, a toner image is formed on the photoreceptor 18 as follows.

  First, the surface of the photosensitive member 18 is charged by a charging device (not shown) while rotating in the direction of arrow G in the drawing. An electrostatic latent image is formed on the surface of the charged photoreceptor 18 by laser light emitted from an exposure device (not shown). At the same time, the supply screw 48 in the supply conveyance path 49 supplies the developer to the developing roller 45 along the axial direction of the developing roller 45. The supplied developer is regulated to a thickness suitable for development by the doctor blade 42, and a developer layer is formed on the developing roller 45.

  Next, the developing roller 45 is applied with a developing bias by a developing bias applying mechanism while moving the surface in the direction of arrow I in the drawing. As a result, the developing roller 45 supplies the developer to the latent image on the surface of the photoreceptor 18 and performs development. The developed developer that has passed through the developing region and has separated from the surface of the developing roller 45 is collected in the collecting conveyance path 47 and conveyed to the front side of the drawing by the collecting screw 46.

  Then, the developer is transferred to the agitation conveyance path 44 through the opening of the second partition wall 134 provided in the non-image area portion. The agitation transport path 44 transports the developer to the back side in the figure while stirring the developer, and transports the developer to the supply transport path 49 through the opening of the first partition wall 133. A toner supply port (not shown) is provided in the vicinity of the opening in the second partition wall 134 on the upstream side of the agitating / conveying path 44, and toner is replenished to the agitating / conveying path 44 from there.

  This embodiment employs a two-component development system, and uses a magnetic carrier as a medium for conveying toner in addition to non-magnetic toner as a developer.

  As the magnetic carrier of the developer, for example, a magnetic resin carrier in which ferrite, magnetite, or iron powder whose main component is iron oxide is used as a core and is coated with a resin can be used. Examples of such carrier coating materials (coating materials) include urea-formaldehyde resins, melamine resins, benzoguanamine resins, urea resins, and polyamide resins as amino resins.

  Examples of the polyvinyl and polyvinylidene resins include acrylic resins, polymethyl methacrylate resins, polyacrylonitrile resins, polyvinyl acetate resins, polyvinyl alcohol resins, polyvinyl butyral resins, polystyrene resins, and styrene acrylic copolymer resins. Can be mentioned. Further, halogenated olefin resins such as polyvinyl chloride, and polyester resins such as polyethylene terephthalate resin and polybutylene terephthalate resin are also included. Furthermore, polycarbonate resins, polyethylene resins, polyvinyl fluoride resins, polyvinylidene fluoride resins, polytrifluoroethylene resins, polyhexafluoropropylene resins, copolymers of vinylidene fluoride and acrylic monomers, vinylidene fluoride And fluorinated terpolymers such as terpolymers of tetrafluoroethylene, vinylidene fluoride and non-fluorinated monomers, and silicone resins.

The volume resistivity of the magnetic carrier was measured using a cell 101 made of a fluororesin container as shown in FIG. Inside the cell 101, electrodes 102a and 102b having a surface area of 8 cm ² (2 cm × 4 cm) were arranged at intervals of 2 mm, and a magnetic carrier was filled therebetween. After filling the carrier, the carrier particle layer was kept in a constant state using a Sankyo tapping machine PTM-1 type (manufactured by Piotech). The tapping operation was performed for 1 minute at a tapping speed of 30 times / min. Thereafter, a DC voltage of 1000 V was applied to both electrodes 102a and 102b, the DC resistance was measured by a high resistance meter 4329A (4329A + LJK5HVLVWDQFH0HWHU; manufactured by Yokogawa Hewlett-Packard Co., Ltd.), electric resistivity RΩ · cm was obtained, and LogR was calculated. . As described above, in this embodiment, a magnetic carrier of 10 LogΩ · cm is used, and further an amino resin coating is applied.

  The toner of the developer is not limited as long as it is a toner used as a two-component developer, and toner containing a binder resin, a colorant, a release agent, a charge adjusting agent, an external additive, and the like can also be used. it can.

  As the binder resin, styrene such as polystyrene, poly-p-chlorostyrene, and polyvinyltoluene, and a polymer of its substitution product: styrene-p-chlorostyrene copolymer, styrene-propylene copolymer, styrene-vinyltoluene copolymer Polymer, styrene-vinyl naphthalene copolymer, styrene-methyl acrylate copolymer, styrene-ethyl acrylate copolymer, styrene-butyl acrylate copolymer, styrene-octyl acrylate copolymer, styrene-methacrylic acid Methyl copolymer, styrene-ethyl methacrylate copolymer, styrene-butyl methacrylate copolymer, styrene-α-chloromethyl methacrylate copolymer, styrene-acrylonitrile copolymer, styrene-vinyl methyl ketone copolymer , Styrene-butadiene copolymer Styrene copolymers such as styrene-isoprene copolymer, styrene-acrylonitrile-indene copolymer, styrene-maleic acid copolymer, styrene-maleic acid ester copolymer: polymethyl methacrylate, polybutyl methacrylate, poly Vinyl chloride, polyvinyl acetate, polyethylene, polypropylene, polyester, epoxy resin, epoxy polyol resin, polyurethane, polyamide, polyvinyl butyral, polyacrylic acid resin, rosin, modified rosin, terpene resin, aliphatic or alicyclic hydrocarbon resin , Aromatic petroleum resin, chlorinated paraffin, paraffin wax and the like. These can be used alone or in combination of two or more.

  As the colorant, all known dyes and pigments can be used. For example, carbon black, nigrosine dye, iron black, naphthol yellow S, Hansa yellow (10G, 5G, G), cadmium yellow, yellow iron oxide , Ocher, yellow lead, titanium yellow, oil yellow, Hansa yellow, (GR, A, RN, R), pigment yellow L, benzidine yellow (G, GR), permanent yellow (NCG), Vulcan fast yellow (5G, R) ), Tartrazine Lake, Quinoline Yellow Lake, Anslagen Yellow BGL, Isoindolinone Yellow, Bengala, Red Dan, Lead Zhu, Cadmium Red, Cadmium Mercury Red, Antimon Zhu, Permanent Red 4R, Para Red, Fire Red, Para Chlor ortho nitroaniline red, Sole Fast Scarlet G, Brilliant Fast Scarlet, Brilliant Carmin BS, Permanent Red (F2R, F4R, FRL, FRLL, F4RH), Fast Scarlet VD, Belkan Fast Rubin B, Brilliant Scarlet G, Resol Rubin GX, Permanent Red F5R, Brilliant Carmine 6B, Pigment Scarlet 3B, Bordeaux 5B, Toluidine Maroon, Permanent Bordeaux F2K, Helio Bordeaux BL, Bordeaux 10B, Bon Maroon Light, Bon Maroon Medium, Eosin Lake, Rhodamine Lake B, Rhodamine Lake Y, Alizarin Lake, Thioindigolet B , Thioindigo maroon, oil red, quinacridone red, pyrazolone red, chrome vermil , Benzine orange, perinone orange, oil orange, cobalt blue, cerulean blue, alkali blue rake, peacock blue rake, Victoria blue rake, metal-free phthalocyanine blue, phthalocyanine blue, fast sky blue, indanthrene blue (RS, BC ), Indigo, ultramarine, bitumen, anthraquinone blue, fast violet B, methyl violet lake, cobalt purple, manganese purple, dioxazine violet, anthraquinone violet, chrome green, zinc green, chromium oxide, pyridiane emerald green, pigment green B, Naphthol Green B, Green Gold, Acid Green Lake, Malachite Green Lake, Phthalocyanine Green, Anthraquinone Examples include lean, titanium oxide, zinc white, and lithopone. These can be used alone or in combination of two or more.

  As the external additive, all known inorganic fine particles or hydrophobized inorganic fine particles can be used. For example, silica fine particles, hydrophobic silica, fatty acid metal salts (such as zinc stearate and aluminum stearate), metal oxides, and the like. (Titania, alumina, tin oxide, antimony oxide, etc.), fluoropolymers, etc. may be contained, and hydrophobized silica, titania, and alumina fine particles can be preferably used.

  As inorganic fine particles of external additives, for example, titanium oxide, barium titanate, magnesium titanate, calcium titanate, strontium titanate, iron oxide, copper oxide, zinc oxide, tin oxide, quartz sand, clay, mica, quartz ash Examples thereof include stone, diatomaceous earth, chromium oxide, cerium oxide, bengara, antimony trioxide, magnesium oxide, zirconium oxide, barium sulfate, barium carbonate, calcium carbonate, silicon carbide, and silicon nitride. Of these, silica and titanium dioxide can be preferably used.

  Examples of the release agent include solid paraffin wax, micro wax, rice wax, fatty acid amide wax, fatty acid wax, aliphatic monoketone, fatty acid metal salt wax, fatty acid ester wax, and partially saponified fatty acid ester wax. , Silicone varnish, higher alcohol, carnauba wax and the like. The above conditions are only examples, and the present invention is not limited to these conditions.

  In the two-component development method using the magnetic carrier and the non-magnetic toner determined as described above, a method of applying a superimposed voltage obtained by superimposing an AC voltage on a DC voltage is used as a developing bias. However, as described above, a part of the developer container 41 is made of metal and is electrically grounded. Therefore, when a superimposed voltage is applied to the developer carrying member (developing roller), the superimposed voltage is The image is branched and applied to a body (photoconductor) and a developer container 41. As a result, a desired voltage cannot be applied to the latent image carrier, and the effect of improving the roughness of the image and the unevenness of the image density is lost, and a new problem arises that white spots around the solid patch image deteriorate.

  FIG. 7 is an enlarged view of the vicinity of the developing area of the developing device according to the first embodiment. The characteristic part of this embodiment is demonstrated referring FIG.

  Around the developing roller 45, a supply conveyance path 49 and a supply screw 48 are provided on the left side in the drawing, and a doctor blade 42 is provided in the direction of the rotation direction I therefrom. The developing roller cover E extends from the doctor blade, and the developing roller 45 is exposed from the end thereof. A portion where the developing roller 45 is exposed and the developing roller 45 and the photosensitive member 18 face each other is a developing region A in which toner moves and adheres to the photosensitive member 18 from the developing roller. On the downstream side of the developing area A, a developing portion 45 and the developer container 41 face each other at a facing portion B, and the exposed developing roller 45 is accommodated in the developing device. The downstream of the facing portion B is connected to the collection conveyance path 47. Among these, the supply conveyance path 49 and the developing roller cover E are made of resin, but the doctor blade 42 and the developer container 41 in the facing portion B are made of metal.

  A series of steps from when the developer is supplied to the developing roller 45 to develop the photosensitive member 18 until the developer on the developing roller 45 is collected again is as follows.

  The developing roller 45 is supplied with the developer from the supply / conveying path 49 and adheres the developer onto the developing sleeve by the magnetic field generated by the internal magnetic material. The developer to be conveyed forms a developer layer, and is regulated to a predetermined height by the doctor blade 42 during the conveyance. The developer layer whose height is regulated is applied with a developing bias by a developing bias applying mechanism in the developing region A, and the toner moves and adheres to the surface of the photoreceptor by electrostatic force. As a result, the latent image on the photoreceptor 18 is developed with toner. Thereafter, the developer layer passes through a facing portion B between the developing roller 45 and the developer container 41 and is separated to the collection conveyance path 47 by a magnetic field generated by a magnetic body inside the developing roller 45.

  When a developing bias is applied, a current flows between the developing roller 45 and the photosensitive member 18 due to the charged toner contained in the developer layer. Similarly, the developing roller 45 and the developer container are disposed at the opposite portion B. A current also flows between 41. Therefore, the superimposed voltage, which is a developing bias, is branched and applied to the photosensitive member 18 and the developer container 41. This makes it impossible to apply a desired developing bias to the photoreceptor 18. Although the doctor blade is also made of metal, there is no problem because it has the same potential as the developing roller 45.

  Therefore, in this embodiment, the developer container 41 in the facing part B is covered with the resistance layer 50, and the developer layer D and the resistance layer 50 are provided between the developing roller 45 and the developer container 41 in the facing part B. A current limiting resistor is formed. As the resistance layer 50, for example, a PTFE tape (No. 5490, thickness 0.09 mm, manufactured by Sumitomo 3M Limited) is used. The resistance layer 50 has a higher resistance than the developer layer D between the developing roller 45 and the facing portion B, and it is desirable to form at least one layer, preferably a plurality of layers.

  With this current limiting resistance, the current flowing between the developing roller 45 and the developer container 41 in the facing portion B can be suppressed, and a desired developing bias can be applied to the photoreceptor 18. Therefore, it is possible to improve roughness of the image, uneven image density, and white spots around the solid patch.

  In order to suppress the current more reliably, the insulating layer may be formed by applying an insulating paint or the like to the developer container 41 in the facing portion B or providing an insulating tape. Furthermore, for example, an insulating paint layer may be provided on the developer container 41 in the facing portion B, an insulating tape layer may be provided, and the developer layer D may be combined with three layers.

  Next, the developing bias to be applied will be described.

  In order to make the latent image on each photoconductor a toner image (a visible image), a developing bias is applied to the developing roller to develop the toner on the photoconductor. In this embodiment, a superimposed voltage obtained by superimposing an alternating voltage (AC) on a direct voltage (DC) is applied as the developing bias.

  The superposed voltage of this embodiment has a frequency of 8 kHz, a peak-to-peak voltage (Vpp) of 800 V, and a + side duty of 50%. Here, in this embodiment, an alternating voltage (AC) is also used as a charging bias for uniformly charging the photosensitive member, and the frequency thereof is 2.65 kHz.

  When the frequency of the charging bias and the frequency of the developing bias deviate from an integral multiple or 1 / (integer multiple), a horizontal streak image due to charging unevenness is generated due to interference (swell) between the charging frequency and the developing frequency. Therefore, the frequency of the developing bias is actually 7.95 kHz, which is three times the charging frequency, but here it is described as 8 kHz for convenience. (Hereafter, the same is described.)

  Next, the AC waveform shape in the superimposed voltage will be described.

  FIG. 8 is a graph schematically showing a waveform of one period of a rectangular wave. The horizontal axis is time (s), and the vertical axis is voltage (V). The offset voltage (DC component of the superimposed voltage) is 0V. A solid line indicates a rectangular wave according to the present embodiment, and a broken line indicates an ideal waveform.

  In the rectangular wave of this embodiment, the rise time and the fall time are each defined as a value between 15 and 20 μs. Here, the rise time is a time interval from 10% of the peak value to 90% of the peak value, as indicated by (1) in the figure. The fall time is a time interval from 90% of the peak value to 10% of the peak value, as indicated by (2) in the figure.

  Table 2 shows the results of evaluating the solid patch peripheral blank image and the image density unevenness when the superimposed voltage rise / fall time is changed in the configuration of the present embodiment. The evaluation method evaluated the output image visually. If there is no abnormality in the image, “◯” is indicated. If the image is slightly abnormal but acceptable level, “Δ” is indicated. If there is abnormality in the image, “X” is indicated.

  As shown in Table 2, it is necessary to make the rise / fall time longer than 10 μs and shorter than 25 μs, preferably 15 to 20 μs, in order to achieve both the image density unevenness and the white spots around the solid patch. .

  In the graph of FIG. 8, in the case of a theoretical waveform indicated by a broken line, the voltage value is flat at the peak voltage (the voltage does not change with time). In order to maintain this flatness, it is necessary to use a switching type in which a + power source and a −power source are switched by a switch, and to further suppress a voltage drop due to heat generation by adding a cooling device or the like. In this case, the cost increases due to the need to select a switch that can withstand the withstand voltage at the time of switch switching, the cost increases due to the addition of a cooling device, and the space increases due to both configurations.

  Therefore, in this embodiment, in order to satisfy all of cost, space, and image quality, a transformer-type power supply is used, and the peak voltage increases or decreases with time and approaches the value of the offset voltage. That is, as shown in (3) in the figure, the voltage decreases toward 0V.

  Regarding the movement of the peak voltage toward the offset voltage (hereinafter referred to as sag), the case where the peak voltage increases is defined as plus, and the case where the peak voltage decreases is defined as minus. Also, the rate of change (%) is defined by dividing the voltage for the movement by the one-side peak voltage.

  For example, when the peak-to-peak voltage (Vpp) of the superimposed voltage is 1000 V and the sag is 1%, the drop (or increase) of the peak voltage is 5 V (1% of the one-side peak voltage). .

  Table 3 shows the result of visual evaluation of white spots around the solid patch when the sag of the superimposed voltage (rectangular wave shown in FIG. 8) in the configuration of the present embodiment is changed. The evaluation method evaluated the output image visually. If there is no abnormality in the image, “◯” is indicated. If the image is slightly abnormal but acceptable level, “Δ” is indicated. If there is abnormality in the image, “X” is indicated.

  As shown in Table 3, the sag needs to be 10% or less, preferably 5% or less, in order to make the solid patch peripheral blank image an acceptable level. However, if the sag is less than 5%, a switching type power supply, a cooling device, and the like are required, which increases both cost and space. Therefore, the rectangular wave sag according to the present embodiment is set to 5% which can achieve both image quality, cost, and space saving.

  In the present embodiment, a rectangular wave is used as the AC voltage waveform, but the present invention is not limited to this. It is good also as a waveform shape which combined the sine wave, the triangular wave, and / or the rectangular wave.

  FIG. 9 is a graph showing the post-charging potential (Vd), post-exposure potential (VL), and developing bias (Vb) of the photoconductor, which are image forming conditions.

  The post-charge potential (Vd) of the photoconductor uniformly charged by the charging device becomes the post-exposure potential (VL) when exposed. Here, the toner image can be developed on the photosensitive member by applying the developing bias Vb. The difference between the post-exposure potential (VL) and the development bias (Vb) is called the development potential.

  When the developing bias is only a DC voltage, it is assumed that the potentials have the relationship shown in FIG. When the developing bias is set to the superimposed voltage, the developing ability is increased, so that the developing potential can be reduced as shown in FIG. 9B or FIG. 9C. Thereby, the edge electric field which generate | occur | produces in the solid part and the halftone part of a solid periphery can be suppressed, and an effect is acquired with respect to a solid peripheral white spot.

  Here, in FIG. 9B, when the developing potential is reduced, the post-charging potential (Vd) is reduced. In this method, since the potential of the photosensitive member is small, a ghost image is likely to occur due to the influence of other biases (for example, a transfer bias when transferring the toner image to the intermediate transfer member), and unevenness in image density. Will occur.

  Therefore, in the present embodiment, as shown in FIG. 9C, when the developing potential is reduced, the post-exposure potential (VL) is larger in absolute value than the VL in FIGS. 9A and 9B. did. As a result, the post-charging potential (Vd) can be set to the same level as when the developing bias is a direct current voltage, so that ghost images can be suppressed and image density unevenness can be improved. Furthermore, since the development potential is small, solid white spots can be improved.

  Note that the method of increasing the post-exposure potential with an absolute value can be dealt with by, for example, changing the intensity of the laser light emitted from the exposure apparatus, but is not limited to this method.

(Second Embodiment)
FIG. 10 is an enlarged view of the vicinity of the developing region of the developing device according to the second embodiment. 10, the same components as those in FIG. 7 are denoted by the same reference numerals, and detailed description thereof is omitted.

  As shown in FIG. 10, in the developing device according to the second embodiment, a part of the developer container that faces the developing roller (hereinafter referred to as the container tip 60) is separated from the developer container. Yes. By making the container tip 60 separate from the developer container, the distance between the developing roller and the developer container can be easily adjusted. Therefore, it is possible to easily adjust the airflow sucked into the developer container for suppressing the toner scattering during the developer conveyance.

  When the container tip 60 is made of the same metal material (aluminum in this embodiment) as that of the developer container, a resistance layer (resistance layer 50 in FIG. 7, not shown in FIG. 10) is provided as in the first embodiment. By forming at least one layer, the same effect as that of the first embodiment can be obtained.

  Moreover, since the container front-end | tip part 60 is another body, it is also possible to use a resin material. In this case, since a resin layer (insulating layer) is formed between the developer container and the developing roller, the same effect as that of the first embodiment can be obtained.

(Effect confirmation)
The developing device of the first and second embodiments (hereinafter referred to as Example 1 and Example 2) and a conventional developing device that does not form a resistance layer (tape layer) in the developer container (hereinafter referred to as a comparative example) A paper passing experiment was conducted. The comparative example is the same as the first and second embodiments except that the resistance layer is not formed on the facing portion B of the developing roller 45 and the developer container 41.

<Image creation conditions>
There are three types of image forming conditions: DC voltage, superimposed voltage 1, and superimposed voltage 2. The superimposed voltage 1 is a method of reducing the post-charging potential (Vd) of the photoreceptor. (See FIG. 9B.) The superimposed voltage 2 is a method of increasing the post-exposure potential (VL). (See Fig. 9 (c))

Table 4 shows the voltage values of the image forming conditions.

<Superimposed voltage condition>
A rectangular wave (AC voltage) was superimposed on an offset voltage (DC voltage) as a superimposed voltage to be applied. The rectangular wave had a frequency of 8 kHz, a peak-to-peak voltage (Vpp) of 800 V, a + side duty of 50%, a rise / fall time of 15 μs, and a sag of 5%. The offset voltage was set so that the area average value of the superimposed voltage waveform was the developing bias (Vb). (See Table 4)

<Evaluation image>
As an evaluation image, a checker image (see FIG. 11) in which solid patches and halftones are alternately arranged, and an image having an image area ratio (halftone dot area ratio) of 75% (refer to FIG. 12 hereafter, referred to as a halftone image). Prepared. The roughness of the image and white spots around the solid patch were evaluated by the checker image, and the image density unevenness was evaluated by the halftone image. The halftone image is an image in which the difference in density unevenness is most likely (easy to understand) among the images used by the inventors for evaluation.

<Paper type>
As the paper type, NBS Ricoh type 6000 <70W> was used. This paper has a basis weight of 70 (gsm) and a size of A4.

<Evaluation method>
In the evaluation method, the output image was visually evaluated. If there is no abnormality in the image, “◯” is indicated. If the image is slightly abnormal but acceptable level, “Δ” is indicated. If there is abnormality in the image, “X” is indicated.

The results are shown in Table 5.

  When the developing bias was a DC voltage, an abnormality occurred at an acceptable level together with Comparative Example 1, Example 1, and Example 2, and there was no difference in image quality in any configuration.

  Next, when the developing bias was set to the superimposed voltage 1, the image roughness was not abnormal in any configuration, and was improved from the case of the DC voltage. Solid white spots were improved from the DC voltage in the configurations of Examples 1 and 2, whereas they were worse than the DC voltage in the configuration of Comparative Example 1. The image density unevenness was affected by the ghost image, and was at the same level as in the case of the DC voltage.

  Compared with these two conditions, when the developing bias was set to the superimposed voltage 2, the image roughness was improved from the DC voltage in the same manner as the superimposed voltage 1. Solid white spots were worse than the DC voltage in the configuration of Comparative Example 1 as before, but improved in the configurations of Examples 1 and 2. As for the image density unevenness, no ghost image was generated, and the effect of improving the density unevenness due to the superimposed voltage was obtained. This is presumably because the post-charging potential (Vd) is approximately the same as when the development bias is set to a DC voltage.

  From the above, the effects of improving the image roughness and the solid white spots were confirmed by setting the developing bias to the superimposed voltage 1 in the configurations of Examples 1 and 2. Further, the effect of improving the image density unevenness was confirmed by setting the developing bias to the superimposed voltage 2.

(Modification)
Hereinafter, a configuration capable of more effectively cooling the developing device will be described.

  In the two embodiments described above, the developing device includes a heat radiating rib as a cooling member. However, you may provide the liquid cooling device cooled with a refrigerant | coolant as a cooling member.

  FIG. 13 is a schematic configuration diagram of the liquid cooling device 30. As shown in FIG. 13, the liquid cooling device 30 includes heat receiving parts 32Y, M, C, and K that receive heat from the cooling liquid, three cooling parts 35 that cool the cooling liquid, a circulation pipe 34, and the cooling liquid. A cooling pump 31 that circulates in the circulation pipe, a reserve tank 33 that stores excess coolant, and the like are included.

  The heat receiving portions 32Y, 32M, 32C, and 32K are attached to the developing devices (not shown) of the process cartridges 11Y, 11M, 11C, and 11K, and receive heat generated from the developing devices. Detailed description of the heat receiving portions 32Y, 32M, 32C, and 32K will be described later.

  The cooling unit 35 includes a radiator 35b serving as a heat radiating unit, a cooling fan 35a, and the like, and three are provided beside the four process cartridges 11Y, 11M, 11C, and 11K. The radiator 35b is a heat radiating means that is formed of Al or the like having a high thermal conductivity, and that transfers and dissipates the cooling liquid through an accommodating portion that contains a cooling medium. Depending on the amount of heat radiation, forced air cooling or natural air cooling is performed by the cooling fan 35a. In the case of forced air cooling by the cooling fan 35a, the cooling fan 35a may be provided for each cooling unit 35, or the outside air may be supplied to the radiator 35b of each cooling unit 35 by one cooling fan.

  The circulation pipe 34 connects the heat receiving parts 32Y, 32M, 32C, 32K, three cooling parts 35, the cooling pump 31, and the reserve tank 33 in series. The cooling pump 31 is a drive source that circulates the cooling liquid in the circulation pipe 34 between the heat receiving parts 32Y, 32M, 32C, and 32K and the cooling part 35, and the cooling liquid circulates as indicated by arrows in FIG. The reserve tank 33 is a tank for storing a coolant.

  The coolant is a heat transport medium that transports heat received by the heat receiving units 32Y, 32M, 32C, 32K to the radiator 35b in the cooling unit 35. The coolant is composed mainly of water, and propylene glycol or ethylene glycol is added to lower the freezing temperature, or a rust inhibitor (for example, phosphate-based material: phosphorus to prevent rusting of metal components) Acid potassium salt, inorganic potassium salt, etc.) are used. When the coolant is water, the constant heat capacity is 3000 times or more that of air, and a large amount of heat can be transported with a small flow rate, so that cooling can be performed more efficiently than forced air cooling.

  FIG. 14 is a schematic configuration diagram of a process cartridge using a liquid cooling device. Since the configuration other than the cooling member is the same as that of the above-described embodiment, the description thereof is omitted. In the drawing, one of the four developing devices 40Y, M, C, and K is shown as a representative, and the suffixes Y, M, C, and K are omitted.

  As shown in FIG. 14, the heat receiving portion 32 described above is provided on the left side of the developing device 40. The heat receiving part 32 includes a case 32a formed of a member having high thermal conductivity and a flow path 32b provided therein. The case 32a is usually formed based on Cu having a thermal conductivity of about 400 (W / mK) or Al of about 200 (W / mK). A material having higher thermal conductivity (for example, Ag or Au) may be used for the case 32a.

  The heat receiving portion 32 is pressed against the side surface in order to receive heat generated from each developing device 40. However, since the side surface of the developing device 40 is also formed of a member having high thermal conductivity such as Al or Cu, as with the heat receiving portion 32, there is not a little air when the heat receiving portion 32 is brought into close contact with the side surface of the developing device 40. You can layer. Since the air layer reduces the efficiency of heat exchange between the heat receiving portion 32 and the side surface of the developing device 40, in this modification, the pressure contact surface between the heat receiving portion 32 and the side surface of the developing device 40 (hereinafter referred to as the pressure contact surface). The heat conductive sheet 130 is affixed. The heat conductive sheet 130 needs to have high heat conductivity and at the same time, hardness (easiness to be deformed) to crush the surface accuracy between the developing device 40 and the heat receiving unit 32.

  However, the heat conductive sheet 130 has a property of being hard when it has high heat conductivity and soft when it has low heat conductivity. In order to obtain high thermal conductivity, the thermal conductive sheet 130 becomes hard to some extent. Therefore, in this modification, the heat receiving unit 32 is pressed with a large pressing force so that the heat receiving unit 32 is pressed against the side surface of the developing device 40. Accordingly, even if a heat conductive sheet that is hard to some extent is used, the heat conductive sheet 130 is deformed, and the surface accuracy between the developing device 40 and the heat receiving unit 32 can be crushed. Thereby, it is possible to suppress the formation of an air layer between the developing device 40 and the heat receiving unit 32, and to conduct heat of the developing device 40 to the heat receiving unit 32 satisfactorily. Further, the heat conductive sheet 130 may be attached to the side surface of the developing device 40.

  In this modification, the number of cooling units 35 is three, but this may be one or four or more. By providing a plurality of cooling units 35, even if the cooling efficiency of each cooling unit is low, the temperature rise of all the developing devices 40Y, M, C, and K can be satisfactorily suppressed. As a result, it is possible to use a small radiator that has a small heat radiation area and does not have a very high cooling efficiency as compared with one cooling unit 35 that suppresses the temperature rise of all the developing devices 40Y, M, C, and K. The cooling unit 35 can be reduced in size.

  The copying machine using the process cartridge has been described above as an example. However, the present invention is not limited to this, and the process cartridge may have a configuration in which the photosensitive member, the developing device, and the like are separated. It is. The configuration of the image forming apparatus is also arbitrary, and the arrangement order of the color image forming units in the tandem system is arbitrary. Further, the present invention can be applied not only to a four-color machine but also to a full-color machine using three or five or more color toners, a multi-color machine using two-color toners, or a monochrome apparatus. That is, a single developing device or a plurality of developing devices may be provided. Of course, the image forming apparatus is not limited to a copying machine, but may be a printer, a facsimile, or a multifunction machine having a plurality of functions.

DESCRIPTION OF SYMBOLS 1 Image forming part 2 Transfer unit 3 Paper feed unit 4 Secondary transfer device 5 Duplex unit 6 Conveyor belt 7 Fixing unit 8 Paper discharge unit 9 Exposure unit 10 Reading device 11 Process cartridge 12 Drum cleaning unit 13 Charging unit 14 Registration roller 15 Intermediate Transfer belt 16 Transfer counter roller 17 Secondary transfer roller 18 Photoconductor 20 Heat insulating device 21 Heat receiving plate 22 Heat pipe 23 Heat radiating plate 24 Duct 30 Liquid cooling device 31 Cooling pump 32 Heat receiving portion 32a Case 32b Flow path 33 Reserve tank 34 Circulation pipe 35 Cooling unit 35a Cooling fan 35b Radiator 40 Developing device 41 Developer container 41a Radiation rib 42 Doctor blade 43 Conveying screw 43 Stirring screw 43a Stirring shaft 43b Stirring blade 44 Stirring conveyance 45 Developing roller 45a Developing sleeve 45b Low friction film 45c Magnetic body 46 Recovery screw 47 Recovery conveyance path 48 Supply screw 49 Supply conveyance path 50 Resistance layer 60 Container tip 90 Intermediate transfer belt cleaning unit 95 Friction coefficient measuring device 96 Base 97 Weight 97 Table 99 Force gauge 100 Fine paper 101 Cell 102a Electrode 102b Electrode 103 Carrier 133 First partition wall 134 Second partition wall 130 Thermal conductive sheet A Development area A
B Proximity B
D Developer layer D
E Developing roller cover E

JP 2009-103785 A Japanese Patent No. 5267016 JP 2009-080447 A US Pat. No. 6,031,239 JP-A-10-012431 JP 08-286516 A

Claims (13)

A developer container;
An electrically grounded metal part forming the developer container;
A two-component developer containing a non-magnetic toner and a magnetic carrier housed in the developer container;
A developer carrier provided in the developer container and carrying the two-component developer by a magnetic field generating means;
A development bias application mechanism that applies a superimposed voltage obtained by superimposing an alternating voltage on a direct current voltage to the developer carrier, and
In the developing device provided with a current limiting resistor in the facing portion between the developer carrier and the metal part,
A part of the developer storage container in the facing portion is a container tip portion that is a separate body,
The developing device according to claim 1, wherein the container tip is made of a resin and serves as an insulating layer at the facing portion . A developer container;
An electrically grounded metal part forming the developer container;
A two-component developer containing a non-magnetic toner and a magnetic carrier housed in the developer container;
A developer carrier provided in the developer container and carrying the two-component developer by a magnetic field generating means;
A development bias application mechanism that applies a superimposed voltage obtained by superimposing an alternating voltage on a direct current voltage to the developer carrier, and
In the developing device provided with a current limiting resistor in the facing portion between the developer carrier and the metal part,
A part of the developer storage container in the facing portion is a container tip portion that is a separate body,
The container tip is made of metal,
A developing device having a resistance higher than that of a developer layer , wherein at least one resistance layer is formed at the tip of the container . Decay rate of the normal direction magnetic flux density of the main pole of the developer carrier (the peak value of the normal direction magnetic flux density on the surface of the developing sleeve provided in the developer carrier and a method at a distance of 1 mm from the surface of the developing sleeve) 3. The development according to claim 1, wherein a ratio obtained by dividing the difference in the peak value of the linear magnetic flux density by the peak value of the normal magnetic flux density on the surface of the developing sleeve is 40% or more. apparatus. Theoretically, the peak voltage does not change for a certain time.
4. The developing device according to claim 1, wherein the peak voltage increases or decreases with the passage of time and defines a ratio approaching a value of a DC component of the superimposed voltage. 5.   The voltage waveform of the superimposed voltage is a rise time, which is a time interval from 10% of the peak value to 90% of the peak value, and a fall time of 90% of the peak value to 10% of the peak value. 5. The developing device according to claim 1, wherein a time interval until reaching the position is defined.   6. The developing device according to claim 1, wherein the frequency of the superimposed voltage is an integer multiple or 1 / (integer multiple) of the frequency of the charging bias. The potential after exposure of the latent image carrier when the developing bias is a superimposed voltage is
7. The developing device according to claim 1, wherein the developing bias is larger in absolute value than the post-exposure potential of the latent image carrier when the developing bias is only a DC voltage.   The developing device according to claim 1, wherein the developer container includes a cooling member.   The developing device according to claim 8, wherein the cooling member is a heat radiating rib formed integrally with the developer container.   The developing device according to claim 8, wherein the cooling member is a liquid cooling device adjacent to the developer container.   11. The developing device according to claim 1, and at least one of a latent image carrier, a charging unit, and a cleaning unit are integrally provided, and are detachable from the image forming apparatus. Process cartridge.   An image forming apparatus comprising one or a plurality of the developing devices according to claim 1.   An image forming apparatus comprising a plurality of process cartridges according to claim 11.

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