Image forming apparatus

Abstract

To provide an image forming apparatus of a two-component development system having a large process linear velocity that can secure an area where a developing condition can be set while extending its life.SOLUTION: An image forming apparatus comprises an image carrier, a charging device, an exposure device, a developing device, a driving unit, a developing voltage power supply, and a control unit. The developing device has a developer carrier that carries a two-component developer including toner and carrier, and supplies the toner in the developer carried on the developer carrier to the image carrier to develop an electrostatic latent image into a toner image. The driving unit drives to rotate the image carrier at a predetermined linear velocity. The developing voltage power supply applies, to the developer carrier, a developing voltage in which an AC voltage is superimposed on an DC voltage. The image forming apparatus has a first printing mode in which the driving unit drives to rotate the image carrier at a linear velocity of 250 mm / sec or more. The image carrier has a single-layer organic photosensitive layer as a photosensitive layer, and a layer thickness of the photosensitive layer at the start of usage is 40 μm or more.SELECTED DRAWING: Figure 5

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 more particularly to an image forming apparatus equipped with a developing device of a two-component development system using a two-component developer. [Background technology]

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

[0003] When a two-component development method is adopted in an image forming apparatus with a high linear process speed, it is preferable for the electric field strength of the photoconductor drum to be weak in order to suppress carrier migration (carrier development) from the developing device to the photoconductor drum. Specifically, an organic photoconductor (OPC) is preferable to an amorphous silicon (a-Si) photoconductor, and a single layer photosensitive layer of an organic photoconductor is preferable to a laminated layer.

[0004] On the other hand, from the viewpoint of extending the life of an image forming apparatus, it is necessary to use a photoconductor drum with a large layer thickness, that is, a photosensitive layer having a certain thickness or more at the start of use. However, when an organic photoconductor with a large layer thickness is used in an image forming apparatus using a two-component development method with a high process linear speed, the setting tolerance (operation window) of the surface potential of the photoconductor and the DC voltage of the development voltage is narrow, and image defects such as uneven discharge and drum ghosts are likely to occur.

[0005] To solve the above problems, for example, Patent Document 1 discloses an image forming apparatus that suppresses carrier development by optimizing the outer diameter and film thickness of the photosensitive drum, development conditions, toner, and carrier conditions. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2008-138881 A Summary of the Invention [Problem to be solved by the invention]

[0007] In order to suppress carrier development while realizing high speed and long life of an image forming apparatus, it is necessary to use a single-layer organic photoreceptor having a layer thickness of 40 μm or more as the photoreceptor drum. In addition, when using a single-layer organic photoreceptor having a layer thickness of 40 μm or more, it is necessary to optimize two-component development conditions according to the layer thickness of the photosensitive layer. Patent Document 1 only describes appropriate conditions when using a laminated organic photoreceptor having a CTL layer (charge transport layer) thickness of 20 to 40 μm, and does not describe any conditions when using a single-layer organic photoreceptor having a layer thickness of 40 μm or more in an image forming apparatus with a high process linear speed.

[0008] SUMMARY OF THE PRESENT DISCLOSURE In view of the above problems, an object of the present invention is to provide an image forming apparatus using a two-component development method with a high process linear speed, which is capable of realizing a long life and ensuring a range in which development conditions can be set. [Means for solving the problem]

[0009] In order to achieve the above object, a first configuration of the present invention is an image forming apparatus including an image carrier, a charging device, an exposure device, a developing device, a driving unit, a developing voltage power source, and a control unit. The image carrier has a photosensitive layer formed on its surface. The charging device charges the surface of the image carrier. The exposure device exposes the surface of the image carrier charged by the charging device to form an electrostatic latent image in which the charge is attenuated. The developing device has a developer carrier that carries a two-component developer containing toner and carrier, and supplies the toner in the developer carried on the developer carrier to the image carrier to develop the electrostatic latent image into a toner image. The driving unit rotates the image carrier at a predetermined linear speed. The developing voltage power source applies a developing voltage, which is a DC voltage superimposed on an AC voltage, to the developer carrier. The control unit controls the driving unit and the developing voltage power source. The image forming apparatus has a first print mode in which the image carrier is rotated at a linear speed of 250 [mm / sec] or more by the driving unit. The image carrier has a single organic photosensitive layer as a photosensitive layer, and the thickness of the photosensitive layer at the start of use is 40 μm or more. Effect of the Invention

[0010] According to the first configuration of the present invention, in an image forming apparatus using a two-component development method having a first printing mode in which an image carrier is rotated at a linear speed of 250 mm / sec or more, an image carrier having a single organic photosensitive layer with a layer thickness of 40 μm is used as a photosensitive layer, thereby optimizing the development conditions to achieve a high process linear speed and a long life for the image carrier, while also ensuring a sufficient settable range (operation window) for the development voltage and the surface potential of the image carrier. [Brief description of the drawings]

[0011] [Figure 1] 1 is a schematic diagram of a color printer 100 according to an embodiment of the present invention; [Diagram 2] An enlarged view of the image forming section Pa in FIG. [Diagram 3] A block diagram showing an example of a control path used in a color printer 100. [Figure 4]Graph showing the relationship between the thickness of the photosensitive layer 19b of the photosensitive drums 1a to 1d and the number of printable sheets when the photosensitive layer 19b is a single-layer organic photosensitive layer. [Diagram 5] Graph showing the relationship between process linear speed and high-speed responsiveness in a two-component development type color printer 100 using an organic photosensitive layer as the photosensitive layer 19b. [Figure 6] Graph showing the relationship between process linear speed, surface potential, and image defects in a two-component development type color printer 100 using a single organic photosensitive layer as the photosensitive layer 19b. [Figure 7] Graph showing the relationship between process linear speed, surface potential, development voltage, and image defects in a two-component development type color printer 100 using a single organic photosensitive layer as the photosensitive layer 19b. [Figure 8] Graph showing the relationship between development frequency and ghosting in a two-component development type color printer 100 using a single organic photosensitive layer as the photosensitive layer 19b. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a schematic cross-sectional view of a color printer 100 according to one embodiment of the present invention. Fig. 2 is an enlarged view of the vicinity of image forming unit Pa in Fig. 1. Note that image forming units Pb to Pd have a basically similar configuration, so their description will be omitted.

[0013] Four image forming units Pa, Pb, Pc, and Pd are arranged in this order from the upstream side in the transport direction (the left side in FIG. 1) inside the main body of color printer 100. These image forming units Pa to Pd are provided corresponding to images of four different colors (yellow, magenta, cyan, and black), and sequentially form images of yellow, magenta, cyan, and black through the processes of charging, exposing, developing, and transferring, respectively.

[0014] Each of the image forming stations Pa to Pd is provided with a photoconductor drum 1a, 1b, 1c, and 1d that carries a visible image (toner image) of each color. Furthermore, an intermediate transfer belt 8 that rotates counterclockwise in Fig. 1 is provided adjacent to each of the image forming stations Pa to Pd. The intermediate transfer belt 8 is wound around a driving roller 10 on the downstream side and a tension roller 11 on the upstream side, and a belt cleaning device 30 is provided upstream of the image forming station Pa with respect to the rotation direction of the intermediate transfer belt 8, facing the tension roller 11 with the intermediate transfer belt 8 sandwiched therebetween.

[0015] As shown in FIG. 2, a charging device 2a, a developing device 3a, a cleaning device 7a, and a discharging device 20 are arranged around the photosensitive drum 1a in the drum rotation direction (clockwise in FIG. 2), and a primary transfer roller 6a is disposed with an intermediate transfer belt 8 sandwiched therebetween.

[0016] The photosensitive drums 1a to 1d are each composed of a conductive substrate 19a and a photosensitive layer 19b formed on the surface of the conductive substrate 19a. In this embodiment, a single organic photosensitive layer is laminated as the photosensitive layer 19b on the surface of the cylindrical conductive substrate 19a made of aluminum.

[0017] The charging devices 2a to 2d each have a charging roller 21 that contacts the photoconductor drums 1a to 1d and applies a charging voltage (DC voltage+AC voltage) to the drum surface, and a charging cleaning roller 24 for cleaning the charging roller 21.

[0018] The developing devices 3a to 3d are of a two-component development type having two stirring and conveying screws 25 and a developing roller 29, and are filled with a predetermined amount of two-component developer containing toner of each color, yellow, magenta, cyan, and black, and a magnetic carrier. A magnetic brush is formed on the surface of the developing roller 29 using the two-component developer, and the magnetic brush is brought into contact with the surface of the photosensitive drum 1a while a developing voltage of the same polarity as the toner (positive polarity in this case) is applied to the developing roller 29, thereby adhering the toner and forming a toner image. When the ratio of toner in the two-component developer filled in each developing device 3a to 3d falls below a specified value due to the formation of the toner image, toner is replenished from the toner containers 4a to 4d to each developing device 3a to 3d.

[0019] The cleaning devices 7a to 7d each have a cleaning blade 31 and a recovery screw 33. The cleaning blade 31 removes toner and other substances remaining on the surfaces of the photoconductor drums 1a to 1d. The recovery screw 33 discharges the toner and other substances removed by the cleaning blade 31 to the outside of the cleaning devices 7a to 7d and recovers them in a waste toner recovery container (not shown). The static eliminator 20 irradiates the surfaces of the photoconductor drums 1a to 1d with static elimination light to remove residual charges.

[0020] When image data is input from a host device such as a personal computer, first, the main motor 40 (see FIG. 3) starts rotating the photoconductor drums 1a-1d. In addition, the belt drive motor 41 (see FIG. 3) starts rotating the intermediate transfer belt 8. Next, the charging devices 2a-2d uniformly charge the surfaces of the photoconductor drums 1a-1d with the same polarity as the toner (positive polarity in this case). Next, the exposure device 5 irradiates light according to the image data, and electrostatic latent images with the charge attenuated according to the image data are formed on the photoconductor drums 1a-1d.

[0021] The developing devices 3a to 3d are filled with a predetermined amount of two-component developer (hereinafter, simply referred to as developer) containing toner of each color of yellow, magenta, cyan, and black by toner containers 4a to 4d, and the toner in the developer is supplied onto the photoconductor drums 1a to 1d by the developing devices 3a to 3d and electrostatically adheres to the photoconductor drums 1a to 1d. As a result, a toner image corresponding to the electrostatic latent image formed by exposure from the exposure device 5 is formed.

[0022] Then, an electric field is applied between the primary transfer rollers 6a-6d and the photoconductor drums 1a-1d by the primary transfer rollers 6a-6d at a predetermined transfer voltage, and the yellow, magenta, cyan and black toner images on the photoconductor drums 1a-1d are primarily transferred onto the intermediate transfer belt 8. Toner and the like remaining on the surfaces of the photoconductor drums 1a-1d after the primary transfer are removed by the cleaning devices 7a-7d. Residual charges remaining on the surfaces of the photoconductor drums 1a-1d after the primary transfer are removed by the charge removing device 20.

[0023] The transfer paper P onto which the toner image is transferred is stored in a paper cassette 16 located at the bottom inside the color printer 100, and is transported at a predetermined timing via a paper feed roller 12a and a pair of registration rollers 12b to a nip portion (secondary transfer nip portion) between the intermediate transfer belt 8 and a secondary transfer roller 9 provided adjacent to the intermediate transfer belt 8. The transfer paper P onto which the toner image has been secondarily transferred is transported to a fixing unit 13.

[0024] The transfer paper P transported to the fixing section 13 is heated and pressurized by the fixing roller pair 13a, so that the toner image is fixed onto the surface of the transfer paper P, and a predetermined full-color image is formed. The transfer paper P on which the full-color image has been formed is discharged onto the discharge tray 17 by the discharge roller pair 15 as is (or after being diverted to the reversing conveyance path 18 by the branching section 14 and images are formed on both sides).

[0025] 3 is a block diagram showing an example of a control path used in color printer 100. Note that, because various controls are performed on each part of color printer 100 when color printer 100 is used, the control paths for color printer 100 as a whole are complex. Therefore, the following description will focus on the parts of the control paths that are necessary for implementing the present invention.

[0026] The charging voltage power supply 52 applies a charging voltage to the charging roller 21 in the charging devices 2a to 2d. The developing voltage power supply 53 applies a developing voltage, which is a DC voltage Vdc superimposed with an AC voltage Vac, to the developing roller 29 in the developing devices 3a to 3d. The transfer voltage power supply 54 applies a predetermined primary transfer voltage and secondary transfer voltage to the primary transfer rollers 6a to 6d and the secondary transfer roller 9, respectively. The voltage control circuit 55 is connected to the charging voltage power supply 52, the developing voltage power supply 53, and the transfer voltage power supply 54, and operates each of these power supplies according to an output signal from the control unit 90.

[0027] The image input unit 60 is a receiving unit that receives image data sent from a personal computer or the like to the color printer 100. The image signal input by the image input unit 60 is converted into a digital signal and then sent to the temporary storage unit 94.

[0028] The operation unit 70 is provided with a liquid crystal display unit 71 and an LED 72. The liquid crystal display unit 71 displays the operating status of the color printer 100, the image formation status, the number of copies to be printed, etc. The LED 72 displays various statuses and errors of the color printer 100. Various settings of the color printer 100 are performed from the printer driver of the personal computer.

[0029] In addition, the operation unit 70 is provided with a start button that the user uses to instruct the start of image formation, a stop / clear button that is used when canceling image formation, and a reset button that is used to reset various settings of the color printer 100 to their default states.

[0030] The internal temperature and humidity sensor 80 detects the temperature and humidity inside the color printer 100, particularly the temperature and humidity around the image forming units Pa to Pd, and is disposed near the image forming units Pa to Pd.

[0031] The control unit 90 at least comprises a CPU (Central Processing Unit) 91 as a central processing unit, a ROM (Read Only Memory) 92 which is a read-only memory unit, a RAM (Random Access Memory) 93 which is a readable and writable memory unit, a temporary memory unit 94 which temporarily stores image data and the like, a counter 95, and multiple (two in this case) I / Fs (interfaces) 96 which send control signals to each device within the color printer 100 and receive input signals from the operation unit 50. The control unit 90 can be placed anywhere inside the color printer 100 main body.

[0032] ROM 92 stores control programs for color printer 100, numerical values ​​necessary for control, and other data that will not change while color printer 100 is in use. RAM 93 stores necessary data generated during the control of color printer 100, and data temporarily required for controlling color printer 100. Temporary storage unit 94 temporarily stores image signals that are input from image input unit 60 and converted into digital signals. Counter 95 cumulatively counts the number of printed sheets.

[0033] Furthermore, the control unit 90 transmits control signals from the CPU 91 through the I / F 96 to each part and device in the color printer 100. Furthermore, signals indicating the state of each part and device and input signals are transmitted to the CPU 91 through the I / F 96 from each part and device. Examples of parts and devices controlled by the control unit 90 include image forming units Pa to Pd, exposure unit 5, intermediate transfer belt 8, secondary transfer roller 9, fixing unit 13, voltage control circuit 55, image input unit 60, operation unit 70, and in-machine temperature and humidity sensor 80.

[0034] When a two-component development method is adopted in a color printer 100 with a high process linear speed (250 mm / sec or more), it is preferable that the electric field strength between the development roller 29 and the photoconductor drums 1a-1d is weak in order to suppress carrier development. Note that the process linear speed is the printing processing speed (the transport speed of the transfer paper P) in the color printer 100, and is the same as the rotation speed of the photoconductor drums 1a-1d.

[0035] For example, the relative dielectric constant ε of an amorphous silicon (a-Si) photosensitive layer is ε = 10, while the relative dielectric constant ε of an organic photosensitive layer (OPC) is ε = 3. Therefore, in a two-component development type color printer 100 with a high process linear speed, an organic photosensitive layer is more advantageous than an a-Si photosensitive layer.

[0036] 4 is a graph showing the relationship between the thickness of the photosensitive layer 19b of the photosensitive drums 1a to 1d and the number of printable sheets (service life) when the photosensitive layer 19b is a single-layer organic photosensitive layer. As shown in FIG. 4, when the photosensitive layer 19b is a single-layer organic photosensitive layer, in order to achieve a long service life such as a number of printable sheets of 300,000 or more, the thickness of the photosensitive layer 19b needs to be 40 μm or more.

[0037] 5 is a graph showing the relationship between the process linear speed and high-speed responsiveness in a two-component development type color printer 100 using an organic photosensitive layer as the photosensitive layer 19b. High-speed responsiveness refers to the degree of reduction in surface potential when the photosensitive drums 1a-1d, which have been charged to a predetermined surface potential, are exposed by the exposure device 5. FIG. 5 compares the potential of the exposed portion (image portion) after exposure by the exposure device 5 when the process linear speed is changed, for a laminated organic photosensitive layer with a layer thickness of 40 [μm] (data series of circles in the figure) and a single-layer organic photosensitive layer (data series of triangles in the figure). In other words, the lower the exposed portion potential, the better the responsiveness, and the higher the exposed portion potential, the worse the responsiveness.

[0038] As shown in Fig. 5, when the thickness of photosensitive layer 19b is 40 [μm], the responsiveness of a laminated organic photosensitive layer decreases especially at linear speeds of 250 [mm / sec] or more, so a single-layer organic photosensitive layer is advantageous. Fig. 5 shows that when a two-component development method is adopted in a color printer 100 in which the linear speed of photoconductor drums 1a-1d is 250 [mm / sec] or more, it is preferable to use a single-layer organic photosensitive layer having a thickness of 40 [μm] or more as photosensitive layer 19b.

[0039] However, when a two-component development method is adopted in a color printer 100 with a high linear process speed, if a single organic photosensitive layer having a thickness of 40 μm or more is used as the photosensitive layer 19b of the photosensitive drums 1a to 1d, image defects such as image unevenness and drum ghosts are likely to occur.

[0040] Therefore, in the two-component development type color printer 100 of this embodiment, when the process linear speed is 250 mm / sec or more and a single organic photosensitive layer having a thickness of 40 μm or more is used as the photosensitive layer 19b of the photosensitive drums 1a to 1d, the image defects described above are suppressed by appropriately setting the development conditions. The setting of the development conditions will be described in detail below.

[0041] [Developing voltage settings under different process linear speed conditions] Fig. 6 is a graph showing the relationship between the process linear speed, surface potential, and image defects in a two-component development type color printer 100 that uses a single organic photosensitive layer as the photosensitive layer 19b. In Fig. 6, the horizontal axis represents the process linear speed, and the vertical axis represents the surface potential of the photosensitive drums 1a-1d, and the image evaluation results are shown when two types of layer thickness for the photosensitive layer 19b, 40 [μm] and 20 [μm], are used. In Fig. 6, the upper and lower limit values ​​of the surface potential when the layer thickness is 40 [μm] are indicated by solid lines A and B, and the upper and lower limit values ​​of the surface potential when the layer thickness is 20 [μm] are indicated by dashed lines C and D.

[0042] As shown in Fig. 6, in areas with high surface potential, discharge unevenness occurs due to the high charging voltage applied to the photosensitive drums 1a to 1d. Also, when the thickness of the photosensitive layer 19b is 40 [μm], the charging voltage required to charge the surface to the desired potential is higher than when the thickness is 20 [μm], and discharge unevenness is more likely to occur. That is, when the thickness is 40 [μm], the upper limit of the surface potential is lower than when the thickness is 20 [μm].

[0043] On the other hand, in the area where the surface potential is low, it is easily affected by the primary transfer electric field to the intermediate transfer belt 8 and other potential irregularities, which causes image irregularities and drum ghosts. Also, when the layer thickness of the photosensitive layer 19b is 40 [μm], it is more difficult to cancel the reverse charge injected during the primary transfer than when the layer thickness is 20 [μm], and drum ghosts are more likely to occur. That is, when the layer thickness is 40 [μm], the lower limit of the surface potential is higher than when the layer thickness is 20 [μm].

[0044] For the reasons above, the settable range of surface potential (the distance between solid lines A and B) when using photosensitive drums 1a-1d with thick photosensitive layer 19b (40 μm) is narrower than the settable range of surface potential (the distance between dashed lines C and D) when using photosensitive drums 1a-1d with thin photosensitive layer 19b (20 μm).

[0045] Fig. 7 is a graph showing the relationship between the process linear speed, surface potential, development voltage, and image defects in a two-component development color printer 100 using a single organic photosensitive layer as the photosensitive layer 19b. In Fig. 7, the horizontal axis represents the process linear speed, the left vertical axis represents the development voltage (DC voltage Vdc), and the right vertical axis represents the surface potential. In the two-component development method, in order to avoid fog images and carrier development, the difference V0-Vdc between the DC voltage Vdc and the surface potential V0 (development potential difference) must be kept constant, and here V0-Vdc is set to 150 [V].

[0046] In FIG. 7, the carrier resistance of the two-component developer is 10 9 [Ω cm] and 10 11The figures show the image evaluation results when two types of discharge resistance [Ω·cm] were used. In Fig. 7, solid line A indicates the upper limit of the DC voltage Vdc at which no discharge unevenness occurs, and solid line B indicates the lower limit of the DC voltage Vdc at which no drum ghost or image unevenness occurs.

[0047] As shown in Figure 7, the carrier resistance is 10 9 In the case of [Ω cm] (data series marked with ○ in the figure), the range of change in development voltage when density adjustment and variation due to calibration is taken into account is in the area (between solid lines A and B) where no image defects occur even if the process linear speed changes.

[0048] On the other hand, when a high resistance carrier is used, the developability is reduced, especially when the process linear velocity is in the high speed region. Therefore, in order to ensure the image density when the process linear velocity is in the high speed region, the development voltage must be set high, and the surface potential of the photoconductor drums 1a to 1d must also be set high in accordance with the development voltage. As a result, when the carrier resistance is 10 11 In the case of [ Ω cm] (data series marked with △ in the figure), when the process linear speed is in the high speed range (200 mm / sec or higher), image defects occur in the range (above solid line A).

[0049] Therefore, in the color printer 100 of the two-component development type using a single organic photosensitive layer as the photosensitive layer 19b, the carrier resistance is set to 10 9 By using a low resistivity carrier such as [Ω·cm], developability in the high process linear velocity range is improved, and the change in the required development voltage between different process linear velocities can be reduced.

[0050] As explained above, when using photoconductor drums 1a-1d in which the thickness of photosensitive layer 19b is 40 [μm], the settable range of surface potential V0 is narrow, so it is necessary to use a low resistance carrier to reduce the difference in development voltage between different linear velocities. Specifically, in a two-component development type color printer 100 having a process linear speed of 250 [mm / sec] or more and using a single organic photosensitive layer of 40 [μm] or more as photosensitive layer 19b, as used in the evaluations of Figures 6 and 7, in order to maintain image quality, it is necessary to suppress the change in development voltage when the process linear speed difference is 180 [mm / sec] to 40 [V] or less.

[0051] As an example of a countermeasure for this, for example, the carrier resistance is 10 9 By using a two-component developer containing a carrier of less than [Ω·cm], the change in development voltage when photoconductor drums 1a-1d, which have a single-layer organic photosensitive layer with a layer thickness of 40 [μm] at the start of use, are used in a printing mode (first printing mode) with a process linear speed of 270 [mm / sec] and a printing mode (second printing mode) with a process linear speed of 90 [mm / sec] (process linear speed difference of 180 [mm / sec]), can be kept to 40 [V] or less.

[0052] 7, the amount of change in the developing voltage when the process linear speed difference is 180 [mm / sec] is suppressed to 40 [V] or less in consideration of the density adjustment and variation due to calibration, but if the variation in the surface potential V0 of the photoconductor drums 1a to 1d can be suppressed, there is no risk of image defects such as discharge unevenness and drum ghosts occurring. Therefore, it is not necessarily necessary to suppress the amount of change in the developing voltage to 40 [V] or less.

[0053] [Setting the development frequency to suppress drum ghosting] Fig. 8 is a graph showing the relationship between the development frequency and drum ghost in a two-component development color printer 100 using a single organic photosensitive layer as the photosensitive layer 19b. In Fig. 7, the horizontal axis represents the frequency of the AC voltage Vac of the development voltage (hereinafter referred to as the development frequency) and the vertical axis represents the image density difference ΔE, and the image evaluation results are shown when the process linear speed is 270 [mm / sec] and two types of layer thickness of the photosensitive layer 19b are used, 40 [μm] and 20 [μm]. In Fig. 8, the density difference ΔE (=1.5) that can be visually recognized as a drum ghost is indicated by a dashed line.

[0054] 8, when the thickness of the photosensitive layer 19b is 40 [μm] (data series indicated by triangles in the figure), ΔE is larger than when the thickness is 20 [μm] (data series indicated by circles in the figure). This is because the thicker the photosensitive layer 19b, the more difficult it is to charge, and so when using photosensitive drums 1a-1d with a thick photosensitive layer 19b, such as one with a thickness of 40 [μm], it is necessary to suppress drum ghosts.

[0055] Also, as is clear from Fig. 8, the higher the development frequency, the larger ΔE becomes. This is because as the development frequency increases, the toner tends to move more easily from the development roller 29 to the photosensitive drums 1a-1d, which increases the developability, and ΔE increases as the image contrast increases. Fig. 8 shows that when a single organic photosensitive layer of 40 [μm] is used in a print mode with a process linear speed of 270 [mm / sec], the development frequency needs to be set to 4 [kHz] or less.

[0056] According to this embodiment, in a two-component development type color printer 100 using photoconductor drums 1a-1d having a single organic photoconductor layer with a thickness of 40 μm or more at the start of use as photoconductor layer 19b, the development conditions can be optimized to increase the process linear speed and extend the life of photoconductor drums 1a-1d, while ensuring a settable range (operation window) for development voltage and surface potential. As a result, image defects can be suppressed and appropriate image quality can be maintained.

[0057] Specifically, by suppressing the change in the development voltage (DC voltage Vdc) to 40 [V] or less in two print modes with a process linear speed difference of 180 [mm / sec] or more, it is possible to set the DC voltage Vdc and surface potential V0 such that discharge unevenness, drum ghost, and image unevenness do not occur. 9 Use a two-component developer containing a carrier of Ω·cm or less.

[0058] Furthermore, by setting the development frequency to 4 kHz or less, the occurrence of drum ghosts in print modes with a process linear speed of 250 mm / sec or more can be suppressed. Note that while a lower development frequency can suppress the occurrence of drum ghosts, it also makes foggy images more likely to occur. Therefore, in a print mode (second print mode) with a process linear speed of 90 mm / sec, in which drum ghosts are less likely to occur, it is preferable to set the development frequency to greater than 4 kHz.

[0059] In addition, the present invention is not limited to the above embodiment, and various modifications are possible without departing from the spirit of the present invention. For example, in the above embodiment, an intermediate transfer type color printer 100 was described in which the toner images formed on the photosensitive drums 1a to 1d are primarily transferred to the intermediate transfer belt 8, and then secondarily transferred to the transfer paper P. However, the present invention is not limited to this, and the present invention can also be applied to a direct transfer type color printer in which the toner images formed on the photosensitive drums 1a to 1d are directly transferred to the transfer paper P.

[0060] In addition, in the above embodiment, a tandem type color printer 100 has been described as an example of an image forming apparatus, but the present invention can of course be applied to other image forming apparatuses that use a two-component development system, such as color copiers, color multifunction machines, monochrome printers, and monochrome multifunction machines. [Industrial Applicability]

[0061] The present invention can be used in an image forming apparatus equipped with a two-component developing device. By using the present invention, when a single-layer organic photoconductor is used in an image forming apparatus with a high linear process speed, it is possible to provide an image forming apparatus that can suppress image defects such as carrier development and drum ghosting while achieving a long life. [Explanation of symbols]

[0062] Pa~Pd Image forming section 1a to 1d Photoconductor drum (image carrier) 2a~2d Charging device 3a~3d developing device 5 Exposure equipment 6a~6d Primary transfer roller 7a~7d Cleaning device 8 Intermediate transfer belt 9 Secondary transfer roller 19a Conductive substrate 19b Photosensitive layer 29 Developing roller (developer carrier) 40 Main motor (drive unit) 52 Charge voltage power supply 53 Development voltage power supply 54 Transfer voltage power supply 70 Operation section 80 In-flight temperature and humidity sensor 90 Control section 100 Color printers (image forming devices)

Claims

1. an image carrier having a photosensitive layer formed on its surface; a charging device for charging the surface of the image carrier; an exposure device that exposes the surface of the image carrier charged by the charging device to form an electrostatic latent image in which the charge is attenuated; a developing device having a developer carrier that carries a two-component developer containing a toner and a carrier, and supplying the toner in the developer carried on the developer carrier to the image carrier to develop the electrostatic latent image into a toner image; a drive unit that rotates the image carrier at a predetermined linear speed; a developing voltage power source that applies a developing voltage, which is a DC voltage superimposed on an AC voltage, to the developer carrying member; a control unit that controls the drive unit and the development voltage power supply; and a first print mode in which the drive unit rotates the image carrier at a linear speed of 250 mm / sec or more. The image forming apparatus is characterized in that the image carrier has a single organic photosensitive layer as the photosensitive layer, and the layer thickness of the photosensitive layer at the start of use is 40 [μm] or more.

2. the control unit is capable of switching between a plurality of print modes including the first print mode and a second print mode in which a linear velocity is smaller than that of the first print mode and a linear velocity difference between the first print mode and the second print mode is 180 [mm / sec] or more; 2. The image forming apparatus according to claim 1, wherein a change in the DC voltage of the developing voltage between the first printing mode and the second printing mode is 40 V or less.

3. The resistance value of the carrier in the developer is 10 9 2. The image forming apparatus according to claim 1, wherein the resistance is Ω·cm or less.

4. 4. The image forming apparatus according to claim 1, wherein the frequency of the AC voltage used in the first printing mode is 4 kHz or less.

Images