Image forming device

Abstract

To provide an image forming apparatus that can prevent the occurrence of development leak or reduce leak traces to be generated in a two-component development system, and can maintain image quality.SOLUTION: An image forming apparatus comprises: amorphous silicon photoreceptors; electrifying devices; an exposure device; developing devices each having a developer carrier that carries two-component developer including toner and carrier; a developing voltage power supply; and a control unit. When a potential difference between the photoreceptor and the developer carrier is V0-Vdc, the image forming apparatus adjusts V0 or Vdc such that a first potential difference in a direction to move the toner from the developer carrier to the photoreceptor and a second potential difference in a direction to move the toner from the photoreceptor to the developer carrier become the same potential difference and have polarities opposite to each other. A first development current |Ia| flowing between the photoreceptor and the developer carrier at the first potential difference and a second development current |Ib| flowing between the photoreceptor and the developer carrier at the second potential difference satisfy the formula (1).SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine, or a combination machine thereof, which is equipped with an image carrier, and in particular to an image forming apparatus equipped with a two-component developing device that uses a two-component developer containing toner and carrier. [Background technology]

[0002] In an image forming apparatus, an electrostatic latent image formed on an image carrier such as a photoreceptor is developed by a developing device to be visualized as a toner image. One such developing device employs a two-component development system that uses a two-component developer.

[0003] In two-component development systems, low carrier resistance can easily cause carrier migration onto the photoconductor, developer leakage, and charge injection into the electrostatic latent image on the photoconductor. Charge injection changes the electrostatic latent image, resulting in poor image reproducibility. High carrier resistance can also result in insufficient image density, periodic image density variations due to fluctuations in the gap (development gap) between the photoconductor and the developing roller, and edge enhancement, which increases the density at the edges of the image. In particular, two-component development systems using amorphous silicon photoconductor drums with a high dielectric constant are prone to carrier migration and developer leakage. Meanwhile, the higher the carrier resistance, the higher the Vpp required to achieve image density.

[0004] Conventionally, as described in Patent Document 1, the above-mentioned problems have been reduced by increasing the carrier resistance to suppress carrier development and development leakage, and by setting the peak-to-peak value (Vpp) of the AC component of the development voltage higher to counter the decrease in image density caused by the increase in carrier resistance.

[0005] However, increasing the carrier resistance requires increasing the development voltage Vpp, which results in the problem of larger leak diameters when developer leaks first occur. Development leaks occur when the tip of the magnetic brush comes into close proximity with the photosensitive drum, and the resistance of each magnetic brush varies depending on the height and diameter of the magnetic brush. The discharge conditions in the space where discharge begins (the space formed between the photosensitive drum and the tip of the magnetic brush) are the same for both low-resistance and high-resistance carriers, but when the initial microleak occurs, a conductive path is formed in the magnetic brush. Once a conductive path is formed, the current flowing through it increases the greater the applied potential difference. As a result, large leak marks occur with high-resistance carriers.

[0006] When the developing voltage Vpp is increased, the first leaks that occur occur at a level of about one in several thousand sheets, and the leak marks at this time are also small, so the occurrence of leaks is hardly noticeable at this level. As the number of leaks increases, the occurrence of leaks becomes more noticeable. The diameter of the leak marks when they first become noticeable (initial leaks) becomes larger as the carrier resistance increases.

[0007] With amorphous silicon photoconductors, leak marks are more likely to occur in the white areas. The reason for this is that a toner layer is formed on the photoconductor in the image areas, and this toner layer acts as a resistive layer, suppressing the occurrence of leaks. Note that with organic (OPC) photoconductors, the toner layer also acts as a resistive layer in the image areas, but since current does not flow easily in the white areas with organic (OPC) photoconductors, leaks to the white areas are less likely to occur.

[0008] On the other hand, if the toner charge amount is increased to ensure environmental stability, there is a problem that the image density cannot be increased. Therefore, Patent Document 2 discloses a technology that increases the dielectric constant of the developer by adding fine powder of barium titanate, which is a ferroelectric substance, to the coating resin of the carrier, thereby increasing the electric field strength between the developing roller and the photosensitive member, thereby ensuring high developability even with a high charge amount. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-open No. 61-190362 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-33631 Summary of the Invention [Problem to be solved by the invention]

[0010] The carrier in Patent Document 2 has a relatively high coating weight of 5 to 20% of the carrier core weight, but a thicker coating makes it more susceptible to the hygroscopicity of the coating, reducing environmental stability. Furthermore, a thicker coating also creates the problem of carrier particles with particle diameters of 65 μm or less having poor fluidity, which can easily cause aggregation during coating.

[0011] SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide an image forming apparatus that can suppress the occurrence of development leaks in a two-component development system, or can reduce the leak marks that occur, while maintaining image quality. [Means for solving the problem]

[0012] In order to achieve the above object, the first aspect of the present invention is an image forming apparatus including a photoconductor, a charging device, an exposure device, a developing device, a developing voltage power supply, and a control unit. The photoconductor has an amorphous silicon photosensitive layer formed on its surface. The charging device charges the surface of the photoconductor to a predetermined surface potential. The exposure device exposes the surface of the photoconductor to light to form an electrostatic latent image. The developing device is disposed opposite the photoconductor and has a developer carrier that carries a two-component developer containing toner and carrier, and forms a toner image by attaching the toner to the electrostatic latent image formed on the photoconductor. The developing voltage power supply applies a developing voltage, which is a DC voltage superimposed on an AC voltage, to the developer carrier. The control unit controls the developing voltage power supply. In the image forming apparatus, when the surface potential of the photosensitive member during image formation is V0 and the DC component of the development voltage applied to the developer carrier is Vdc, the potential difference V0-Vdc between the photosensitive member and the developer carrier is adjusted so that a first potential difference in a direction that moves toner from the developer carrier to the photosensitive member and a second potential difference in a direction that moves toner from the photosensitive member to the developer carrier are the same potential difference but opposite in polarity, and when the first development current flowing between the photosensitive member and the developer carrier at the first potential difference is |Ia| and the second development current flowing between the photosensitive member and the developer carrier at the second potential difference is |Ib|, the second development current |Ib| satisfies the following formula (1). 1.9×10 -2 ≧|Ib| / N≧5.7×10 -3 [μA / mm] (1) however, N: The axial length of the contact between the magnetic brush on the developer carrier and the photosensitive member [mm] is. [Effects of the Invention]

[0013] According to the first configuration of the present invention, in an image forming apparatus of a two-component development type using an amorphous silicon photosensitive member, by adjusting the image forming conditions so that the second development current |Ib| satisfies formula (1), it is possible to suppress the occurrence of development leak or reduce the size of any leak marks that occur, and also to suppress edge enhancement of the image. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a side cross-sectional view showing the internal configuration of an image forming apparatus 100 according to an embodiment of the present invention. [Figure 2] 1 is a side cross-sectional view of a developing device 3a mounted in an image forming apparatus 100. [Figure 3] A diagram showing the configuration and control path of the image forming unit Pa. [Figure 4] 1 is a flowchart showing an example of development voltage control in the image forming apparatus 100 of this embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a cross-sectional view showing the internal structure of an image forming apparatus 100 according to one embodiment of the present invention. Within the main body of the image forming apparatus 100 (here, a color printer), four image forming units Pa, Pb, Pc, and Pd are arranged in this order from the upstream side in the transport direction (left side in Fig. 1). These image forming units Pa to Pd are provided corresponding to images of four different colors (yellow, cyan, magenta, and black), and sequentially form images of yellow, cyan, magenta, and black through the processes of charging, exposure, development, and transfer, respectively.

[0016] Each of the image forming stations Pa through Pd is provided with photosensitive drums (image carriers) 1a, 1b, 1c, and 1d, which carry visible images (toner images) of each color. An intermediate transfer belt (intermediate transfer member) 8, which rotates counterclockwise in FIG. 1 by a belt drive motor (not shown), is provided adjacent to each of the image forming stations Pa through Pd. The toner images formed on the photosensitive drums 1a through 1d are sequentially transferred (primary transfer) onto the intermediate transfer belt 8, which moves while contacting the photosensitive drums 1a through 1d, and then superimposed on each other. The toner images primarily transferred onto the intermediate transfer belt 8 are then secondarily transferred onto a transfer sheet P (an example of a recording medium) by a secondary transfer roller 9. The transfer sheet P onto which the toner images have been secondarily transferred is then fixed in a fixing unit 13, and then ejected from the image forming apparatus 100. While the photosensitive drums 1a through 1d are rotating clockwise in FIG. 1, an image formation process is performed on each of the photosensitive drums 1a through 1d.

[0017] The transfer paper P onto which the toner image is secondarily transferred is stored in a paper cassette 16 located at the bottom of the main body of the image forming apparatus 100, and is transported via a paper feed roller 12a and a pair of registration rollers 12b to the nip between the secondary transfer roller 9 and the drive roller 11 of the intermediate transfer belt 8. A sheet made of dielectric resin is used for the intermediate transfer belt 8, and a seamless belt is usually used. In addition, a blade-shaped belt cleaner 19 is located downstream of the secondary transfer roller 9 to remove toner and other particles remaining on the surface of the intermediate transfer belt 8.

[0018] Next, the image forming units Pa to Pd will be described. Around and below the rotatably arranged photosensitive drums 1a to 1d, there are provided charging devices 2a, 2b, 2c, and 2d that charge the photosensitive drums 1a to 1d, an exposure device 5 that exposes image information onto each of the photosensitive drums 1a to 1d, developing devices 3a, 3b, 3c, and 3d that form toner images on the photosensitive drums 1a to 1d, and cleaning devices 7a, 7b, 7c, and 7d that remove developer (toner) and the like remaining on the photosensitive drums 1a to 1d.

[0019] When image data is input from a host device such as a personal computer, the charging devices 2a-2d first uniformly charge the surfaces of the photosensitive drums 1a-1d. Next, the exposure device 5 irradiates light according to the image data, forming electrostatic latent images on each of the photosensitive drums 1a-1d. The developing devices 3a-3d are filled with a predetermined amount of two-component developer containing yellow, cyan, magenta, and black toner, respectively. If the toner content in the two-component developer in each of the developing devices 3a-3d falls below a specified value due to the formation of a toner image (described below), toner is replenished from toner containers 4a-4d to each of the developing devices 3a-3d. The toner in the developer is supplied to the photosensitive drums 1a-1d by the developing devices 3a-3d and electrostatically adheres to them. This results in the formation of a toner image corresponding to the electrostatic latent image formed by exposure from the exposure device 5.

[0020] Then, primary transfer rollers 6a-6d apply an electric field at a predetermined transfer voltage between the primary transfer rollers 6a-6d and the photosensitive drums 1a-1d, and the yellow, magenta, cyan, and black toner images on the photosensitive drums 1a-1d are primarily transferred onto the intermediate transfer belt 8. These images are formed with a predetermined positional relationship. After that, in preparation for the subsequent formation of a new electrostatic latent image, toner and the like remaining on the surfaces of the photosensitive drums 1a-1d after the primary transfer are removed by cleaning devices 7a-7d.

[0021] The intermediate transfer belt 8 is stretched over a driven roller 10 on the upstream side and a drive roller 11 on the downstream side, and when the intermediate transfer belt 8 starts to rotate counterclockwise as the drive roller 11 is rotated by a belt drive motor (not shown), the transfer paper P is transported at a predetermined timing from the registration roller pair 12b to a nip portion (secondary transfer nip portion) between the drive roller 11 and a secondary transfer roller 9 provided adjacent to it, and the toner image on the intermediate transfer belt 8 is secondarily transferred onto the transfer paper P. The transfer paper P to which the toner image has been secondarily transferred is transported to a fixing unit 13.

[0022] The transfer paper P transported to the fixing unit 13 is heated and pressurized by the fixing roller pair 13a, and the toner image is fixed to the surface of the transfer paper P, forming a predetermined full-color image. The transfer paper P on which the full-color image has been formed is then transported in different directions by the branching unit 14, which branches into multiple directions, and is then discharged directly (or after being sent to the double-sided transport path 18 and having images formed on both sides) onto the discharge tray 17 by the discharge roller pair 15.

[0023] Furthermore, an image density sensor 40 is disposed downstream of the image forming unit 1d at a position facing the intermediate transfer belt 8. The image density sensor 40 is generally an optical sensor equipped with a light-emitting element such as an LED and a light-receiving element such as a photodiode. When measuring the amount of toner adhesion on the intermediate transfer belt 8, the light-emitting element irradiates each reference image formed on the intermediate transfer belt 8 with measurement light, and the measurement light is reflected by the toner and the belt surface and enters the light-receiving element.

[0024] The light reflected from the toner and belt surface includes specularly reflected light and diffusely reflected light. This specularly reflected light and diffusely reflected light are separated by a polarization separation prism and then incident on separate light receiving elements. Each light receiving element photoelectrically converts the received specularly reflected light and diffusely reflected light and outputs an output signal to the main control unit 80 (see FIG. 3). The toner amount is then detected from the change in the characteristics of the output signals of the specularly reflected light and diffusely reflected light, and density correction (calibration) is performed by comparing them with a predetermined reference density and adjusting the characteristic values ​​of the developing voltage, etc.

[0025] 2 is a side cross-sectional view of the developing device 3a mounted on the image forming apparatus 100. In the following description, the developing device 3a arranged in the image forming section Pa in FIG. 1 will be exemplified, but the configurations of the developing devices 3b to 3d arranged in the image forming sections Pb to Pd are basically the same, so description thereof will be omitted.

[0026] As shown in FIG. 2, the developing device 3a includes a developer container 20 that contains a two-component developer (hereinafter simply referred to as developer) containing a magnetic carrier and toner. The developer container 20 is partitioned by a partition wall 20a into an agitation / transport chamber 21 and a supply / transport chamber 22. The agitation / transport chamber 21 and the supply / transport chamber 22 are rotatably provided with an agitation / transport screw 25a and a supply / transport screw 25b, respectively, for mixing, agitating, and charging the toner supplied from the toner container 4a (see FIG. 1). In this embodiment, a two-component developer consisting of a positively charged toner and a ferrite / resin-coated carrier is used. The detailed configurations of the toner and carrier will be described later.

[0027] The developer is then transported in the axial direction (the direction perpendicular to the paper surface of FIG. 2) while being agitated by the agitation transport screw 25a and the supply transport screw 25b, and circulates between the agitation transport chamber 21 and the supply transport chamber 22 via developer passages (not shown) formed at both ends of the partition wall 20a. That is, a developer circulation path is formed in the developing container 20 by the agitation transport chamber 21, the supply transport chamber 22, and the developer passages.

[0028] The developing container 20 extends diagonally upward to the right in Figure 2, and the developing roller 31 is disposed diagonally above the right of the supply / conveyor screw 25b within the developing container 20. A portion of the outer circumferential surface of the developing roller 31 is exposed through the opening 20b of the developing container 20, and faces the photosensitive drum 1a across a predetermined gap (development gap). The developing roller 31 rotates counterclockwise in Figure 2 (trail rotation at the position facing the photosensitive drum 1a).

[0029] Developing roller 31 is composed of a cylindrical developing sleeve that rotates counterclockwise in Figure 2 and a magnet (not shown) with multiple magnetic poles fixed inside the developing sleeve. Note that while a developing sleeve with a knurled surface is used here, it is also possible to use a developing sleeve with a large number of recesses (dimples) formed on the surface, a developing sleeve with a blasted surface, or a developing sleeve that has been blasted in addition to being knurled or recessed, or a developing sleeve that has been plated. A developing voltage consisting of a DC voltage Vdc and an AC voltage Vac is applied to developing roller 31 by developing voltage power supply 43 (see Figure 3).

[0030] A regulating blade 27 is attached to the developing container 20 along the longitudinal direction of the developing roller 31 (perpendicular to the plane of the paper in FIG. 2). A small gap is formed between the tip of the regulating blade 27 and the surface of the developing roller 31. In this embodiment, a magnetic blade made of stainless steel (SUS430) is used as the regulating blade 27.

[0031] 3 is a diagram showing the configuration and control path of image forming unit Pa including developing device 3a. In the following explanation, the configuration and control path of image forming unit Pa will be described, but the configurations and control paths of image forming units Pb to Pd are also the same, so their explanation will be omitted.

[0032] The developing roller 31 is connected to a developing voltage power supply 43 that generates an oscillating voltage in which a DC voltage and an AC voltage are superimposed. The developing voltage power supply 43 includes an AC constant voltage power supply 43a and a DC constant voltage power supply 43b. The AC constant voltage power supply 43a outputs a sine wave AC voltage generated from a low voltage DC voltage modulated into a pulse shape using a step-up transformer (not shown). The DC constant voltage power supply 43b outputs a DC voltage obtained by rectifying a sine wave AC voltage generated from a low voltage DC voltage modulated into a pulse shape using a step-up transformer.

[0033] During image formation, the developing voltage power supply 43 outputs a developing voltage in which an AC voltage is superimposed on a DC voltage from the AC constant voltage power supply 43a and the DC constant voltage power supply 43b. The current detection unit 44 detects the value of the DC current flowing between the developing roller 31 and the photosensitive drum 1a.

[0034] Charging voltage power supply 45 applies a charging voltage, which is a DC voltage superimposed on an AC voltage, to charging roller 34 of charging device 2a. The configuration of charging voltage power supply 45 is similar to that of development voltage power supply 43. Transfer voltage power supply 47 applies a primary transfer voltage and a secondary transfer voltage to primary transfer rollers 6a to 6d and secondary transfer roller 9 (see FIG. 1), respectively.

[0035] The cleaning device 7a includes a cleaning blade 32 that removes residual toner from the surface of the photosensitive drum 1a, a rubbing roller 33 that removes residual toner from the surface of the photosensitive drum 1a and also rubs against the surface of the photosensitive drum 1a to polish it, and a conveying spiral 35 that discharges the residual toner removed from the photosensitive drum 1a by the cleaning blade 32 and the rubbing roller 33 to the outside of the cleaning device 7a.

[0036] Next, the control system of image forming apparatus 100 will be described with reference to FIG. 3. Image forming apparatus 100 is provided with a main control unit 80 configured with a CPU and the like. Main control unit 80 is connected to a storage unit 70 configured with ROM, RAM and the like. Main control unit 80 controls each unit of image forming apparatus 100 (charging devices 2a-2d, developing devices 3a-3d, exposure device 5, primary transfer rollers 6a-6d, cleaning devices 7a-7d, secondary transfer roller 9, fixing unit 13, development voltage power supply 43, current detection unit 44, charging voltage power supply 45, transfer voltage power supply 47, voltage control unit 50, drive control unit 51, etc.) based on the control program and control data stored in storage unit 70.

[0037] The voltage control unit 50 controls a development voltage power supply 43 that applies a development voltage to the development roller 31, a charging voltage power supply 45 that applies a charging voltage to the charging roller 34, and a transfer voltage power supply 47 that applies a transfer voltage to the primary transfer rollers 6a to 6d and the secondary transfer roller 9. The drive control unit 51 controls a main motor 53 that drives and rotates the photosensitive drums 1a to 1d. The voltage control unit 50 and the drive control unit 51 may be configured by control programs stored in the storage unit 70.

[0038] The main control unit 80 is connected to a liquid crystal display unit 90 and a transmission / reception unit 91. The liquid crystal display unit 90 functions as a touch panel for the user to make various settings for the image forming apparatus 100, and also displays the status of the image forming apparatus 100, the image formation status, the number of printed sheets, etc. The transmission / reception unit 91 communicates with the outside world using a telephone line or an internet line.

[0039] In the image forming apparatus 100 of this embodiment, a carrier with low electrical resistance is used as the carrier in the two-component developer, and the peak-to-peak value (Vpp) of the AC component of the developing voltage is reduced to suppress the occurrence of development leak or reduce the leak marks that occur. Below, a method for setting image formation conditions and the two-component developer to be used will be described in detail.

[0040] [Developing current setting] In the image forming apparatus 100 of this embodiment, the development current flowing between the developing roller 31 of each of the developing devices 3a to 3d and the photosensitive drums 1a to 1d is measured, and the image forming conditions are set so that the measured development current falls within a predetermined range. Specifically, when the surface potential of the photosensitive drums 1a to 1d is V0 and the DC component of the development voltage applied to the developing roller 31 is Vdc, the potential difference (V0-Vdc) between the photosensitive drums 1a to 1d and the developing roller 31 is adjusted so that the potential difference in the direction (during development) in which toner is moved from the developing roller 31 to the photosensitive drums 1a to 1d and the potential difference in the direction in which toner is moved from the photosensitive drums 1a to 1d to the developing roller 31 are the same potential difference but opposite in polarity.

[0041] For example, when V0 is fixed at 450 [V] and Vdc=550 [V] is applied to developing roller 31, the potential difference in the direction to move toner from developing roller 31 to photosensitive drums 1a to 1d (first potential difference -Va) is 350-550=-150 [V]. When Vdc=350 [V] is applied to developing roller 31, the potential difference in the direction to move toner from photosensitive drums 1a to 1d to developing roller 31 (second potential difference Va) is 450-350=150 [V].

[0042] The first potential difference −Va and the second potential difference Va can be set by changing V0 while fixing Vdc, but it is preferable to set them by changing Vdc.

[0043] Next, the second developing current |Ib| that flows when the second potential difference is Va is detected for each of the developing devices 3a to 3d by the current detection unit 44. Then, |Ib| is set to satisfy the following formula (1). 1.9×10 -2 ≧|Ib| / N≧5.7×10 -3 [μA / mm] (1) however, N: The axial length of contact between the magnetic brush on the developing roller and the photosensitive drum [mm] is.

[0044] Equation (1) defines the magnitude of the second development current |Ib|, and defines the current value per unit axial length to eliminate the effect of the axial length of the development roller 31. If the current flowing through the magnetic brush is increased to a level that satisfies equation (i.e., carrier resistance is reduced), image density can be ensured even if the AC component Vpp of the development voltage is reduced. Therefore, when development leaks occur, the leak marks become very small (less than 0.5 mm), and even if a leak occurs, it becomes barely noticeable.

[0045] Furthermore, it is preferable that |Ia| and |Ib| further satisfy the following formula (2). 1.51×|Ia|≧|Ib|≧0.45×|Ia| ···(2) By satisfying the formula (2), it is possible to more effectively improve carrier development in the white background and image areas, half-pitch unevenness, and leakage.

[0046] The first developing current |Ia| and the second developing current |Ib| can be detected by executing a dedicated mode for setting the developing current during non-image formation, and setting the first potential difference -Va and the second potential difference Va by adjusting V0 or Vdc, or they can also be detected in the non-image area (between sheets of paper) during image formation.

[0047] Next, the two-component developer used in the developing devices 3a to 3d of the present invention will be described. The two-component developer contains toner and carrier. The toner concentration (weight ratio of toner to carrier, T / C) in the two-component developer is preferably 5 to 20 parts by weight of toner per 100 parts by weight of carrier.

[0048] [toner] The toner may be, for example, a positively charged toner. The positively charged toner is positively charged due to friction with the carrier. The toner particles contain toner base particles and, if necessary, external additives that adhere to the surfaces of the toner base particles. The composition of the toner base particles is not particularly limited. If not necessary, external additives may not be added. When no external additives are added, the toner base particles correspond to the toner particles.

[0049] The toner base particles contain a binder resin and a colorant. The toner base particles may contain a release agent, a charge control agent, a magnetic powder, etc., as needed. The weight-average particle diameter of the toner base particles is preferably 5 to 12 μm, more preferably 6 to 10 μm. The weight-average particle diameter of the toner base particles is measured using a particle size distribution measuring device (for example, Coulter's Multisizer II). The toner base particles are produced by known methods such as a pulverization classification method, a melt granulation method, a spray granulation method, or a polymerization method.

[0050] When adding an external additive, it is preferable to use inorganic particles with a number-average primary particle diameter of 5 nm to 30 nm in order to obtain a toner with excellent fluidity. To make the external additive function as a spacer between toner particles and obtain a toner with excellent heat-resistant storage stability, it is preferable to use resin particles with a number-average primary particle diameter of 50 nm to 200 nm inclusive as the external additive particles. Examples of external additives include inorganic oxides such as silica, titanium oxide, and alumina, and metal soaps such as calcium stearate. To fully utilize the function of the external additive while suppressing its detachment from the toner base particles, it is preferable to add the external additive in an amount of 1 part by mass to 10 parts by mass per 100 parts by mass of the toner base particles.

[0051] The toner particles may be toner particles without a shell layer (non-encapsulated toner particles), or toner particles with a shell layer (encapsulated toner particles). The encapsulated toner particles include toner base particles with a toner core and a shell layer covering the surface of the toner core. The configuration of the toner core is not particularly limited. The shell layer may be made essentially of a thermosetting resin, may be made essentially of a thermoplastic resin, or may contain both a thermoplastic resin and a thermosetting resin. To obtain a toner suitable for image formation, the volume average particle diameter (D50) of the toner base particles is preferably 4 μm or more and 9 μm or less.

[0052] Furthermore, the toner particles have hydrophobic silica particles and styrene-acrylic acid resin microparticles attached to the toner base particles. The hydrophobic silica particles are a charge control agent that adjusts the charge amount of the toner. The styrene-acrylic acid resin microparticles are spacers that prevent the silica particles from becoming embedded in the toner base particles. Styrene-acrylic acid resin microparticles usually adhere to the carrier surface during durability testing, causing a decrease in the carrier's charging performance. However, they have weak adhesion to the silicone resin coating layer containing ferroelectric particles (described below), preventing continued accumulation of these microparticles on the carrier. While the detailed mechanism is unknown, it is presumed that this is due to the low adhesion to the ferroelectrics exposed on the surface of the coating layer, making them easy to peel off.

[0053] [Career] The carrier used in the present invention has a coating layer of silicone resin or the like formed on the surface of a carrier core, which is a particle of a magnetic material. Silicone-based resins can be coated in a thin film, which increases the uniformity of the coating layer. Furthermore, a thinner coating layer increases the electrostatic capacitance of the coating layer, making it easier to demonstrate the effects of the ferroelectric material added to the coating layer.

[0054] The carrier can be of any shape, from irregular to spherical. Furthermore, the average particle diameter of the carrier can be 20 μm or more and 65 μm or less. By setting the number-average particle diameter of the carrier to 65 μm or less, the specific surface area of ​​the carrier increases, increasing the amount of toner that the carrier can carry. This allows the toner concentration in the magnetic brush to be maintained at a high level, and sufficient toner is supplied to the developing roller 31, ensuring a sufficient thickness of the toner layer. As a result, a sufficient amount of toner can be dispersed from the toner layer to the electrostatic latent image on the photoreceptor, preventing a decrease in image density and further reducing density unevenness in the image. Furthermore, sufficient toner is supplied to the developing roller 31, preventing toner voids from forming in the toner layer on the developing roller 31 and reducing the occurrence of hysteresis.

[0055] If the average particle diameter of the carrier is smaller than 20 μm, carrier development occurs in which the carrier adheres to the photosensitive drums 1a to 1d, and the adhered carrier transfers to the intermediate transfer belt 8, causing transfer defects, or moves to the belt cleaning device 19, causing cleaning defects. Also, if the average particle diameter of the carrier is larger than 65 μm, the magnetic brush of the two-component developer becomes coarse when transferring the toner in the two-component developer from the development roller 31 to the photosensitive drums 1a to 1d, resulting in reduced image quality.

[0056] Examples of carrier cores include magnetic metals such as iron, nickel, and cobalt, alloys of these metals, alloys containing rare earth elements, soft ferrites such as hematite, magnetite, manganese-zinc ferrite, nickel-zinc ferrite, manganese-magnesium ferrite, and lithium ferrite, iron oxides such as copper-zinc ferrite, and mixtures thereof. Carrier cores are produced by known methods such as sintering and atomization. Among the above, ferrite carriers are preferred from the viewpoints of high image quality and long life because of their good fluidity and chemical stability.

[0057] Barium titanate particles are added to the coating layer as a ferroelectric. Methods for producing barium titanate include hydrothermal polymerization and oxalate processes, but barium titanate has different physical properties depending on the production method. Among these, barium titanate produced by hydrothermal polymerization has internal voids, resulting in a low true specific gravity and a sharp particle size distribution. As a result, it has better dispersibility in the coating resin than those produced by other methods, enabling uniform dispersion. Therefore, the carrier charging performance is also uniform, making it suitable for use in the present invention.

[0058] The volume average particle size of barium titanate is preferably 100 nm or more and 500 nm or less. If the particle size of barium titanate is smaller than 100 nm, the relative dielectric constant of barium titanate drops sharply, and the effect on the relative dielectric constant becomes small. On the other hand, if the particle size of barium titanate is 500 nm or more, it becomes difficult to uniformly disperse the barium titanate in the coating layer.

[0059] Adding 5 parts by weight or more of barium titanate relative to the coating weight begins to stabilize the charge amount, and adding 25 parts by weight or more makes this effect even more pronounced. However, if too much barium titanate is added, it cannot be fully incorporated into the coating layer and becomes liberated from the coating layer. If the liberated barium titanate migrates to the photoreceptor drums 1a-1d and becomes caught in the edge of the cleaning blade 32 of the cleaning devices 7a-7d, it can cause cleaning problems. In particular, in a system in which toner from the toner containers 4a-4d is mixed with a carrier and then replenished to the developing devices 3a-3d, liberated barium titanate is supplied to the developing devices 3a-3d during use, increasing the load on the cleaning blade 32. Therefore, it is preferable to add barium titanate in an amount between 5 parts by weight and 45 parts by weight.

[0060] Carbon black is added to the coating layer as a conductor. If too much carbon black is added, the carbon black released from the coating layer will adhere to the toner, causing the color of non-black toner to become cloudy. On the other hand, if too little carbon black is added, charge transfer from the carrier to the toner is difficult, and the toner charge amount cannot be smoothly increased. In the carrier of the present invention, the carrier resistance is reduced by adding barium titanate (a ferroelectric) to the coating layer, so it is possible to reduce the amount of carbon black added by the amount of the reduced carrier resistance.

[0061] Adding a ferroelectric (barium titanate) to the coating layer increases the carrier's charge retention capacity, allowing for sufficient charge to be imparted to the toner. Adding a conductor (carbon black) to the coating layer also facilitates the transfer of charge from the carrier to the toner. The combined effect of these two factors makes it possible to impart charge up to the saturated charge level of the toner particles, even when the toner concentration increases and the number of toner particles to be charged increases.

[0062] During the manufacturing of the image forming apparatus 100, by setting the image forming conditions that satisfy the formulas (1) and (2) using the above-mentioned two-component developer, it is possible to suppress the occurrence of development leak and reduce the size of leak marks that occur. It is also possible to suppress image fogging, reduction in image density, and edge emphasis.

[0063] Furthermore, by adding barium titanate, which has high hardness as a ferroelectric, to the carrier coating layer, scraping of the coating layer can be reduced, thereby extending the carrier's lifespan. Furthermore, the addition of barium titanate reduces the carrier resistance compared to the addition of carbon black alone, allowing for a reduction in the amount of carbon black added. As a result, color turbidity caused by carbon black adhering to the toner can be suppressed. Furthermore, the carrier's charge-imparting performance is improved, reducing changes in toner charge even when the toner concentration in the developer increases. As a result, the toner charge is stabilized, allowing for stable image density to be maintained.

[0064] Even if the image forming apparatus 100 is set to satisfy formulas (1) and (2) during manufacturing, as durable printing progresses, formulas (1) and (2) may no longer be satisfied due to deterioration of the two-component developer, changes in environmental conditions, etc. Therefore, by detecting the first developing current |Ia| and the second developing current |Ib| each time a predetermined number of printed sheets is reached, and adjusting the developing voltage (particularly the AC component Vpp) according to the detection results so as to satisfy formulas (1) and (2), it is possible to suppress the occurrence of leak marks over a long period of time.

[0065] 4 is a flowchart showing an example of development voltage control in the image forming apparatus 100 of the present invention. The development voltage control procedure for the development devices 3a to 3d will be described in detail along the steps of FIG. 4, with reference to FIGS. 1 to 3 as needed.

[0066] First, the main control unit 80 determines whether a printing command has been received (step S1). If a printing command has been received (Yes in step S1), printing is executed in the image forming units Pa to Pd (step S2), and the number of printed sheets N is counted (step S3).

[0067] Next, the main control unit 80 determines whether printing has ended (step S4). If printing is continuing (No in step S4), the process returns to step S2, and the execution of printing and the counting of the number of printed sheets N are continued (steps S2, S3). If printing has ended (Yes in step S4), the main control unit 80 determines whether the number of printed sheets N has reached a predetermined number N1 (for example, 1000 sheets) (step S5). If N < N1 (No in step S5), the process returns to step S1, and the standby state for the printing command is continued.

[0068] If N ≥ N1 (Yes in step S5), the first developing current |Ia| and the second developing current |Ib| are measured (step S6). Then, it is determined whether the measured |Ia| and |Ib| satisfy both of the formulas (1) and (2) (step S7).

[0069] If either of the formulas (1) and (2) is not satisfied (No in step S7), the main control unit 80 transmits a control signal to the voltage control unit 50 to control the AC constant voltage power supply 43a, thereby adjusting the Vpp of the AC component of the developing voltage so as to satisfy the formulas (1) and (2) (step S8). Thereafter, the number of printed sheets N is reset (N = 0) (step S9), and the process returns to step S1. On the other hand, if both of the formulas (1) and (2) are satisfied in step S7 (Yes in step S7), the number of printed sheets N is reset (N = 0) without adjusting Vpp (step S9), and the process returns to step S1.

[0070] According to the control example shown in FIG. 4, by adjusting the developing voltage Vpp so that the first developing current |Ia| and the second developing current |Ib| satisfy the formulas (1) and (2), it is possible to suppress the occurrence of developing leaks over a long period of time, or to reduce the size of leak marks that occur, and also to suppress carrier development and pitch unevenness, regardless of environmental changes or carrier deterioration during durable printing.

[0071] Furthermore, if the image forming apparatus 100 is a system in which a carrier is mixed with the toner in the toner containers 4a to 4d to form a two-component developer, which is then replenished to the developing devices 3a to 3d, the barium titanate content in the coating layer of the carrier contained in the developer filled in the developing devices 3a to 3d when the developing devices 3a to 3d are first used can be set to a different ratio from the barium titanate content in the coating layer of the carrier contained in the developer in the toner containers 4a to 4d, thereby suppressing fluctuations in the toner charge amount in the developing devices 3a to 3d.

[0072] When the barium titanate content in the carrier is low, the toner charge gradually increases from the beginning, overshoots once, then decreases and stabilizes. On the other hand, when the barium titanate content is high, the above phenomenon does not occur, and the charge gradually increases and eventually stabilizes. At this time, the stable toner charge is lower than when the barium titanate content is low.

[0073] Therefore, by increasing the content of barium titanate in the developer filled in the developing devices 3a to 3d at the start of use and decreasing the content of barium titanate in the developer in the toner containers 4a to 4d, it is possible to suppress the increase in the toner charge amount in the early stages of use of the developing devices 3a to 3d and to keep the toner charge amount (stable charge amount) high during endurance use.

[0074] The present invention is not limited to the above-described embodiment, and various modifications are possible without departing from the spirit of the present invention. For example, in the above-described embodiment, a color printer as shown in FIG. 1 was used as an example of image forming apparatus 100, but the present invention is not limited to color printers and can be applied to various image forming apparatuses equipped with a two-component developing device, such as monochrome and color copiers, monochrome printers, and digital multifunction peripherals. The effects of the present invention will be described more specifically below using examples. [Example]

[0075] [Production of Ferroelectric Particle-Containing Carrier] [Manufacturing Example 1] A coating solution was prepared by dispersing 200 parts by weight of silicone resin (KR-255, manufactured by Shin-Etsu Chemical Co., Ltd., nonvolatile content = 50%), 35 parts by weight of barium titanate (Sakai Chemical Co., Ltd., volume average particle diameter 304 nm), 10 parts by weight of carbon black (Ketjen Black EC, manufactured by Lion Corporation), and 800 parts by weight of toluene using a homomixer. The resulting coating solution was sprayed onto 5 kg of carrier cores (Mn ferrite carrier, volume average particle diameter 34.7 μm, saturation magnetization 80 emu / g, coercive force 8 Oe, manufactured by Dowa IP Creation Co., Ltd.) at 70-80°C using a fluidized bed coating device, coating the carrier cores with the coating solution. The resulting mixture was then baked in an electric furnace at 250°C for 1 hour. After cooling, the mixture was crushed and classified using a sieve to obtain a carrier (Carrier 1) containing ferroelectric particles in the coating layer.

[0076] The volume average particle diameter (D50) of the barium titanate and the carrier core was measured using a laser diffraction / scattering particle size distribution analyzer (LA-950, manufactured by Horiba, Ltd.).

[0077] [Manufacturing Example 2] Carrier 2 was obtained in the same manner as in Production Example 1, except that the amount of carbon black was changed to 9 parts by mass.

[0078] [Manufacturing Example 3] Carrier 3 was obtained in the same manner as in Production Example 1, except that the amount of carbon black was changed to 7 parts by mass.

[0079] [Manufacturing Example 4] Carrier 4 was obtained in the same manner as in Production Example 1, except that the amount of carbon black was changed to 5 parts by mass.

[0080] [Manufacturing Example 5] Carrier 5 was obtained in the same manner as in Production Example 1, except that the amount of carbon black was changed to 4 parts by mass.

[0081] [Manufacturing Example 6] Carrier 6 was obtained in the same manner as in Production Example 1, except that the amount of carbon black was changed to 3 parts by mass.

[0082] [Manufacturing Example 7] Carrier 7 was obtained in the same manner as in Production Example 1, except that the amount of carbon black was changed to 2 parts by mass.

[0083] [Manufacturing Example 8] Carrier 8 was obtained in the same manner as in Production Example 1, except that the amount of carbon black was changed to 1 part by mass.

[0084] [Manufacturing Example 9] Carrier 9 was obtained in the same manner as in Production Example 1, except that the blending amount of barium titanate was changed to 5 parts by mass and the blending amount of carbon black was changed to 7 parts by mass.

[0085] [Manufacturing Example 10] Carrier 10 was obtained in the same manner as in Production Example 1, except that the blending amount of barium titanate was changed to 25 parts by mass and the blending amount of carbon black was changed to 7 parts by mass.

[0086] [Manufacturing Example 11] Carrier 11 was obtained in the same manner as in Production Example 1, except that the blending amount of barium titanate was changed to 45 parts by mass and the blending amount of carbon black was changed to 7 parts by mass.

[0087] [Manufacturing Example 12] Carrier 12 was obtained in the same manner as in Production Example 1, except that the volume average particle diameter of barium titanate was changed to 102 nm and the blending amount of carbon black was changed to 7 parts by mass.

[0088] [Manufacturing Example 13] Carrier 13 was obtained in the same manner as in Production Example 1, except that the volume average particle diameter of the barium titanate was changed to 304 nm and the blending amount of carbon black was changed to 7 parts by mass.

[0089] [Manufacturing Example 14] Carrier 14 was obtained in the same manner as in Production Example 1, except that the volume average particle diameter of the barium titanate was changed to 495 nm and the blending amount of carbon black was changed to 7 parts by mass.

[0090] [Manufacturing Examples 15 and 16] Carriers 15 and 16 with different average film weights were obtained in the same manner as in Production Example 1, except that the amount of carbon black was changed to 7 parts by mass and the amount and time of spraying of the coating liquid were changed. [Example]

[0091] [Image evaluation when using a carrier containing ferroelectric particles] An image evaluation was carried out when printing was performed using the ferroelectric particle-containing carrier produced in Example 1. The test method involved filling two-component developers containing carriers 1 to 16 produced in Production Examples 1 to 16 into developing devices 3a to 3d as shown in Figure 2 and mounting them on a test machine.

[0092] Using this testing machine, we performed durability printing of test images with varying print rates from 2% to 50% at a print speed (process speed) of 55 sheets per minute. More specifically, we varied the print rate stepwise from 2% from 0 to 1000 sheets, and then 5% from 1000 to 2000 sheets, with intermittent printing performed every 5 sheets. After printing a total of 100,000 sheets, we evaluated the occurrence of leak marks, carrier development, and pitch unevenness of halftone images. The durability test was performed under three conditions: a low-temperature, low-humidity environment (10°C, 15%), a normal-temperature, normal-humidity environment (22°C, 50%), and a high-temperature, high-humidity environment (32.5°C, 80%).

[0093] The development conditions were as follows: a developing roller 31 with an outer diameter of 20 mm, on whose outer peripheral surface 80 rows of recesses were formed (knurled), and a 1.5 mm thick magnetic blade made of stainless steel (SUS430) was used as the regulating blade 27. The developer transport amount by the developing roller 31 was 275 to 400 g / m 2 The peripheral speed ratio of the developing roller 31 to the photosensitive drums 1a to 1d was 1.8 (trail rotation at the opposing positions), and the distance (developing gap) between the photosensitive drums 1a to 1d and the developing roller 31 was 250 to 400 μm. A developing voltage was applied to the developing roller 31, which was a DC voltage of 50 to 250 V superimposed with an AC voltage of 750 to 1480 V peak-to-peak (Vpp), a frequency of 10 kHz, and a duty of 50%.

[0094] The photosensitive drums 1a to 1d use amorphous silicon (a-Si) photosensitive members with a relative dielectric constant of 11, and the distance between the photosensitive drums 1a to 1d and the developing roller 31 (DS distance) is 0.375±0.025 mm, and the developer transport amount of the developing roller 31 is 350±50 g / m 2 The toner used was a positively charged toner with an average particle diameter of 6.8 μm, and the initial toner concentration in the developer (weight ratio of toner to carrier) was 6%.

[0095] The evaluation method for leak marks was to visually count the number of leak marks that occurred per 100 images, divided into diameters of 1 mm or more and 1 to 0.5 mm.

[0096] The carrier development was evaluated separately for the white background and the image. The carrier development in the white background was evaluated by enlarging the image with a magnifying glass. The evaluation criteria were as follows: x: very noticeable, △: slightly noticeable, and ○: not noticeable at all.

[0097] Carrier development in the image area was evaluated by transfer defects in the solid (solid) image. Transfer defects occur when the polarity of the carrier becomes the same as that of the toner due to charge injection caused by the application of the development voltage, causing the carrier to be developed in the solid image area, and the carrier then exerts a spacer effect, inhibiting transfer. Specifically, the carrier developed in the solid image area creates a distance between the photoreceptor and the transfer material (intermediate transfer belt or paper), preventing the toner around the carrier from being transferred, resulting in white spots in the solid image. The evaluation criteria were as follows: x: very noticeable white spots; △: some white spots were present but not noticeable; and ◯: no white spots or only inconspicuous white spots.

[0098] Pitch unevenness of halftone images was evaluated by evaluating the difference in density at the developing roller pitch of a halftone image with a printing rate of 25%. The evaluation criteria were: × if the pitch unevenness was very noticeable, △ if the pitch unevenness occurred but was not noticeable, and ○ if the pitch unevenness did not occur or was not noticeable. The coating layers and development currents of carriers 1 to 16 are shown in Table 1, and the development conditions and image evaluation results are shown in Table 2. The evaluation results are summarized under three environmental conditions.

[0099] [Table 1] *Carbon black

[0100] [Table 2]

[0101] As is clear from Table 2, the amount of carbon black blended was 10 parts by mass, and |I b In the case of Carrier 1, where | exceeds the upper limit of the formula (1), the carrier resistance is low, and carrier development in the white background occurs significantly. b In the carrier 8 where | exceeds the lower limit of the formula (1), the charge transfer is difficult to occur, and the leakage traces are generated conspicuously.

[0102] Furthermore, carrier 9, which contained 5 parts by mass of barium titanate, had a small effect on stabilizing the charge amount, and carrier development occurred in the image area. Carrier 12, which contained an average particle diameter of barium titanate of 102 μm, and carrier 14, which contained an average particle diameter of barium titanate of 495 μm, exhibited pitch unevenness in halftone images.

[0103] From the above results, |I b Carriers 2 to 7 and 9 to 16, in which | satisfies formula (1), can suppress the occurrence of leak marks and carrier development in white areas. Furthermore, it was confirmed that by setting the volume average particle diameter of barium titanate to 100 to 500 nm, the blending amount to 5 parts by mass or more and 45 parts by mass or less, preferably 25 parts by mass or more and 35 parts by mass or less, and the blending amount of carbon black to 2 to 9 parts by mass, carrier development in image areas and pitch unevenness in halftone images can be suppressed.

[0104] Although the results shown here are for carrier cores with a volume average particle diameter of 34.7 μm and a saturation magnetization of 80 emu / g, it has been confirmed that similar effects can be obtained when using carrier cores with a volume average particle diameter of 30 to 60 μm and a saturation magnetization of 70 to 90 emu / g. [Industrial Applicability]

[0105] The present invention can be applied to an image forming apparatus equipped with a two-component developing device that uses a two-component developer containing toner and carrier. By using the present invention, it is possible to provide an image forming apparatus that can suppress the occurrence of developer leaks in the two-component developing method or reduce leak marks that occur, while maintaining image quality. [Explanation of symbols]

[0106] Pa~Pd Image forming section 1a to 1d Photosensitive drum (image carrier) 2a~2d Charging device 3a~3d developing device 5 Exposure equipment 31 Developing roller (developer carrier) 43 Development voltage power supply 43a AC constant voltage power supply 43b DC constant voltage power supply 44 Current detection section 45 Charge voltage power supply 47 Transfer voltage power supply 50 Voltage control section 51 Drive control unit 70 Storage section 80 Main control unit (control unit) 90 LCD display section 100 Image forming device

Claims

1. a photoreceptor having an amorphous silicon photosensitive layer formed on its surface; a charging device that charges the surface of the photoreceptor to a predetermined surface potential; an exposure device that exposes the surface of the photoreceptor to light to form an electrostatic latent image; a developing device disposed opposite the photosensitive member, the developing device having a developer carrier that carries a two-component developer containing toner and a carrier, and that forms a toner image by adhering the toner to the electrostatic latent image formed on the photosensitive member; a developing voltage power source that applies a developing voltage to the developer carrier, the developing voltage being a DC voltage superimposed on an AC voltage; a control unit that controls the developing voltage power supply; In an image forming apparatus comprising: The carrier has a resin coating layer formed on the surface of a carrier core, which is a particle of a magnetic material, and the coating layer contains carbon black as a conductor and barium titanate as a ferroelectric material, The coating layer contains 5 to 45 parts by mass of barium titanate per 100 parts by mass of the coating resin that forms the coating layer, When the surface potential of the photosensitive member during image formation is V0 and the DC component of the development voltage applied to the developer carrier is Vdc, a potential difference V0-Vdc between the photosensitive member and the developer carrier is adjusted so that a first potential difference in a direction in which toner is moved from the developer carrier to the photosensitive member and a second potential difference in a direction in which toner is moved from the photosensitive member to the developer carrier are the same potential difference but opposite in polarity, An image forming apparatus characterized in that, when a first developing current flowing between the photosensitive member and the developer carrier when the first potential difference is |Ia| and a second developing current flowing between the photosensitive member and the developer carrier when the second potential difference is |Ib|, the second developing current |Ib| satisfies the following formula (1): 1.9×10 -2 ≧|Ib| / N≧5.7×10 -3 [μA / mm]・・・(1) however, N: The axial length of the contact between the magnetic brush on the developer carrier and the photosensitive member [mm] is.

2. 2. The image forming apparatus according to claim 1, wherein the first developing current |Ia| and the second developing current |Ib| satisfy the following formula (2): |Ia| 1.51×|Ia|≧|Ib|≧0.45×|Ia| ...(2)

3. a current detection unit that detects a DC component of the development current that flows between the photosensitive member and the developer carrier when the development voltage is applied to the developer carrier; The image forming apparatus according to claim 2, characterized in that, when the first developing current |Ia| and the second developing current |Ib| detected by the current detection unit do not satisfy at least one of the formulas (1) and (2), the control unit adjusts the peak-to-peak value of the AC component of the developing voltage so that the first developing current |Ia| and the second developing current |Ib| satisfy both the formulas (1) and (2).

4. The image forming apparatus according to claim 3, characterized in that the control unit detects the first development current |Ia| and the second development current |Ib| each time the number of printed sheets reaches a predetermined number, and adjusts the peak-to-peak value of the AC component of the development voltage according to the detection result.

5. An image forming apparatus according to claim 1, wherein the amount of barium titanate added is 25 to 45 parts by mass per 100 parts by mass of the coating resin.

6. 6. The image forming apparatus according to claim 1, wherein the barium titanate has a volume average particle diameter of 100 nm or more and 500 nm or less.

7. An image forming apparatus according to claim 1, wherein the toner comprises toner base particles to which hydrophobic silica particles and styrene-acrylic acid resin microparticles are attached.

8. A toner container for storing the toner to be replenished to the developing device, a developer supplying system in which the toner in the toner container is mixed with the carrier to prepare the developer, and the developer is then supplied to the developing device, An image forming apparatus as described in any one of claims 1 to 7, characterized in that the content of barium titanate in the carrier in the developer filled in the developing device when the developing device is first used is higher than the content of barium titanate in the carrier in the developer in the toner container.

Images