Image forming apparatus

Abstract

To prevent variations in electrification voltage associated with displacement of a surface potential of a photoconductor drum due to pre-electrification exposure.SOLUTION: An image forming apparatus comprises process cartridges 225y, 225m, 225c, and 225k each having a photoconductor drum 215, an electrifying roller 216 that electrifies a surface of the photoconductor drum 215, and a pre-electrification exposure device 227 that has a light-emitting device 301 and irradiates the photoconductor drum 215 with light emitted from the light-emitting device 301 to eliminate static electricity from the surface of the photoconductor drum 215. When a voltage generation circuit 601 applies electrification voltage to electrifying rollers 216 of the process cartridges 225y, 225m, and 225c, a voltage generation circuit 602 applies electrification voltage to the electrifying roller 216 of the process cartridge 225k. A CPU 209 changes the quantity of light emitted by the light-emitting device 301 from a first light quantity to a second light quantity having a larger quantity of light than the first light quantity, and changes from the second light quantity to a third light quantity having a larger quantity of light than the second light quantity.SELECTED DRAWING: Figure 7

Description

Technical Field

[0001] The present invention relates to an image forming apparatus using an electrophotographic method such as a copying machine or a printer.

Background Art

[0002] In an image forming apparatus using an electrophotographic method such as a copying machine or a printer, an electrostatic latent image is formed by irradiating light corresponding to image data onto a photosensitive drum charged to a uniform potential by a charging means. Then, by attaching toner, which is a developer, from a developing means to the electrostatic latent image formed on the photosensitive drum, the electrostatic latent image is visualized and a toner image is formed. Thereafter, the toner image formed on the photosensitive drum is transferred to a recording material such as recording paper by a transfer means, and the toner image is fixed to the recording material in a fixing device, thereby performing desired image formation.

[0003] In a color image forming apparatus, toner images of different colors are formed on the photosensitive drum in each image forming section provided for each color of toner, and the formed toner images of different colors are sequentially superimposed and transferred onto the same recording material, thereby forming a color image. Therefore, generally, a charging device that charges the photosensitive drums of each image forming section to a uniform potential and a power supply device that supplies power to the charging devices of each image forming section are required. As one means for realizing miniaturization and cost reduction of a color image forming apparatus, for example, in Patent Document 1, a technique of supplying power from a common power supply device to a plurality of charging devices has been proposed.

[0004] Furthermore, from the perspective of miniaturizing image forming apparatuses and eliminating waste, for example, Patent Document 2 proposes a cleanerless system (toner recycling system). In a cleanerless system, a dedicated drum cleaner, which is a cleaning means for removing toner remaining on the photosensitive drum after the transfer process in which the toner image is transferred to the recording material, is eliminated. In a cleanerless system, the transfer residue toner on the photosensitive drum after the transfer process is removed from the photosensitive drum by cleaning it simultaneously with development using a developing means ("simultaneous development cleaning"), and the transfer residue toner can be recovered by the developing means and reused.

[0005] Transfer residue toner mainly consists of toner that is charged with a positive polarity, which is the opposite polarity to the normal negative polarity, or toner that is charged with a negative polarity, which is the normal polarity, but does not have sufficient charge. In a cleanerless system configuration that performs cleaning simultaneously with development, it is necessary to suppress the adhesion of transfer residue toner to the negatively charged charging roller. For this reason, for example, in the image forming apparatus of Patent Document 3, a pre-charging exposure device is installed downstream of the contact point where the photosensitive drum and the transfer roller come into contact with each other, with respect to the rotation direction of the photosensitive drum, and upstream of the contact point where the photosensitive drum comes into contact with the charging roller. The pre-charging exposure device is a device that removes the surface potential of the photosensitive drum by irradiating it with light (hereinafter referred to as pre-charging exposure), and by pre-charging exposure, the photosensitive drum is removed from the charge and the potential difference between the photosensitive drum and the charging roller is increased. As a result, a discharge occurs at the contact point between the photosensitive drum and the charging roller, making it possible to uniformly charge the transfer residue toner with a negative polarity. This makes it possible to suppress the adhesion of transfer residue toner to the charging roller in an image forming apparatus equipped with a cleanerless system.

[0006] Furthermore, if there is residual charge on the photosensitive drum, the surface potential of the photosensitive drum will be disordered. In particular, under low humidity conditions, the potential difference in the charged potential on the photosensitive drum can cause image defects called drum ghosting during the rotation period of the photoreceptor. To suppress drum ghosting, for example, Patent Document 4 proposes a pre-charging exposure apparatus that irradiates the surface of the photosensitive drum with light after the transfer process and before the charging process by the charging roller to remove static charge from the surface potential of the photosensitive drum to a predetermined residual potential level. Also, for example, Patent Document 5 proposes a static elimination configuration in which, during pre-charging exposure, the light irradiated onto the photosensitive drum is guided through a light guide or other light guide to make the surface potential in the rotation axis direction of the photosensitive drum uniform after static elimination. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2016-126252 [Patent Document 2] Japanese Patent Publication No. 2006-301108 [Patent Document 3] Japanese Patent Publication No. 2019-174765 [Patent Document 4] Japanese Patent Publication No. 2001-142365 [Patent Document 5] Japanese Patent Publication No. 2017-58433 [Overview of the project] [Problems that the invention aims to solve]

[0008] However, in a cleanerless system configuration, in order to suppress the adhesion of residual toner to the charging roller and the occurrence of drum ghosting, when performing pre-charging exposure using a pre-charging exposure device, it is necessary to irradiate the photosensitive drum uniformly in the direction of its rotation axis. Figure 11 is a timing chart showing the state of the pre-charging exposure device, photosensitive drum, and charging voltage during printing operation of a color image forming apparatus. In Figure 11, 101y, 101m, 101c, and 101k indicate the amount of light emitted by the light-emitting elements of the pre-charging exposure devices provided in each image forming section with different toner colors. 102y, 102m, 102c, and 102k indicate the transition of the surface potential near the exposure area of ​​the photosensitive drum irradiated with light by the pre-charging exposure device in each image forming section. 103 shows the output waveform of the charging voltage output from the power supply. Furthermore, 104y, 104m, 104c, and 104k indicate the timing when the image forming area on the photosensitive drum, which is irradiated with laser light corresponding to the image data of each image forming section, passes the contact area with the charging roller.

[0009] When pre-charging exposure is performed by a pre-charging exposure device, the surface potential of the photosensitive drum before and after pre-charging exposure changes sharply from -300V to -150V, as shown in 102y, 102m, 102c, and 102k in Figure 11. Hereafter, this sharp change in the surface potential of the photosensitive drum before and after pre-charging exposure will be referred to as "load fluctuation." Therefore, when the irradiated area on the photosensitive drum, which has been illuminated by the pre-charging exposure device, reaches the area close to the charging roller, the high-voltage power supply providing power to the charging roller may not be able to keep up with the load fluctuation of the photosensitive drum, causing the output of the charging voltage to fluctuate. As a result, the voltage level of the charging voltage of the high-voltage power supply that supplies voltage to the charging roller may become unstable. In particular, in a color image forming apparatus in which the charging voltage supplied from the high-voltage power supply and applied to the charging roller is used in common by multiple image forming units, the following phenomenon occurs: That is, the load fluctuation of the photosensitive drum in the image forming unit in which pre-charging exposure has been performed by the pre-charging exposure device causes fluctuations in the output of the charging voltage. Consequently, the charging voltage output applied to the charging rollers of the other image forming units fluctuates at the timings shown by Dy1, Dm1, Dc1, Dy2, Dm2, and Dc2 in Figure 11. Here, Dy1, Dm1, Dc1, Dy2, Dm2, and Dc2 are the timings at which the light irradiation area on the photosensitive drum of each image forming unit, due to pre-charging exposure, reaches the contact area with the charging roller. As a result, the uniform potential state for image formation on the surface of the photosensitive drum (hereinafter referred to as the background potential) is displaced, leading to problems such as uneven density in the images formed on the photosensitive drum of each image forming unit.

[0010] This invention was made under such circumstances and aims to suppress fluctuations in the charging voltage associated with the displacement of the surface potential of the photosensitive drum due to pre-charging exposure. [Means for solving the problem]

[0011] To solve the above-mentioned problems, the present invention has the following configuration.

[0012] (1) A first image forming unit having a first photosensitive drum, a first charging member that charges the surface of the first photosensitive drum, a first developing unit that develops an electrostatic latent image formed on the first photosensitive drum and forms a toner image, a first transferring unit that transfers the toner image on the first photosensitive drum to a transfer object, and a first static discharge unit that has a light-emitting element and discharges static electricity from the surface of the first photosensitive drum by irradiating the first photosensitive drum with light emitted from the light-emitting element; a second photosensitive drum and a second charging member that charges the surface of the second photosensitive drum. The device comprises: a material; a second developing unit for developing an electrostatic latent image formed on the second photosensitive drum to form a toner image; a second transferring unit for transferring the toner image on the second photosensitive drum to a transfer object; a second static elimination unit having a light-emitting element and irradiating the second photosensitive drum with light emitted from the light-emitting element to eliminate static electricity from the surface of the second photosensitive drum; a power supply unit for applying a charging voltage to the first charging member and the second charging member; and control means for controlling the first static elimination unit and the second static elimination unit. The first image forming unit is positioned upstream of the second image forming unit in the direction of movement of the object to be transferred. The power supply unit is configured such that when the charging voltage is applied to the first charging member, the charging voltage is also applied to the second charging member, and the control means is The second static elimination unit of the second image forming unit The amount of light emitted from the light-emitting element is controlled to change from a first light intensity to a second light intensity greater than the first light intensity, and then to a third light intensity greater than the second light intensity. Furthermore, the control means does not control the amount of light emitted by the light-emitting element of the first static elimination unit of the first image forming unit to change in steps. An image forming apparatus characterized by the following: (2) A first image forming unit having a first photosensitive drum, a first charging member that charges the surface of the first photosensitive drum, a first developing unit that develops an electrostatic latent image formed on the first photosensitive drum and forms a toner image, a first transfer unit that transfers the toner image on the first photosensitive drum to a transfer object, and a first static elimination unit having a light-emitting element that irradiates the first photosensitive drum with light emitted from the light-emitting element to eliminate static electricity from the surface of the first photosensitive drum; a second image forming unit having a second photosensitive drum, a second charging member that charges the surface of the second photosensitive drum, a second developing unit that develops an electrostatic latent image formed on the second photosensitive drum and forms a toner image, a second transfer unit that transfers the toner image on the second photosensitive drum to a transfer object, and a second static elimination unit having a light-emitting element that irradiates the second photosensitive drum with light emitted from the light-emitting element to eliminate static electricity from the surface of the second photosensitive drum. Image forming apparatus comprising: a second image forming unit; a first charging member; a power supply unit for applying a charging voltage to the second charging member; a first static elimination unit; and a control means for controlling the second static elimination unit, wherein the first image forming unit is positioned upstream of the second image forming unit in the direction of movement of the object to be transferred; the power supply unit is configured such that when the charging voltage is applied to the first charging member, the charging voltage is also applied to the second charging member; the control means controls the amount of light emitted by the light-emitting element of the first static elimination unit of the first image forming unit from a first light amount to a second light amount smaller than the first light amount, and from the second light amount to a third light amount smaller than the second light amount; and the control means does not control the amount of light emitted by the light-emitting element of the second static elimination unit of the second image forming unit to change in steps. [Effects of the Invention]

[0014] According to the present invention, fluctuations in the charging voltage due to the displacement of the surface potential of the photosensitive drum caused by pre-charging exposure can be suppressed. [Brief explanation of the drawing]

[0015] [Figure 1] Cross-sectional view showing the schematic configuration of the image forming apparatus in Examples 1 and 2. [Figure 2] Diagram illustrating the schematic configuration of the pre-charging exposure apparatus in Examples 1 and 2. [Figure 3] Circuit diagram showing the circuit configuration of the pre-charge exposure device's light emission control circuit in Examples 1 and 2 [Figure 4] Diagram for explaining the circuit characteristics of the light emission control circuit in Examples 1 and 2 [Figure 5] Diagram for explaining the configuration of the high-voltage power supply of the image forming apparatus in Examples 1 and 2 [Figure 6] Diagram for explaining the outline of the cleanerless system in Examples 1 and 2 [Figure 7] Timing chart showing the states of the pre-charge exposure device and the photosensitive drum during the printing operation in Example 1 [Figure 8] Diagram for explaining the photosensitive characteristics of the photosensitive drum in Example 2 [Figure 9] Timing chart showing the states of the pre-charge exposure device and the photosensitive drum during the printing operation in Example 2 [Figure 10] Diagram for explaining the light emission control method of the pre-charge exposure device in Example 2 [Figure 11] Timing chart showing the states of the pre-charge exposure device and the photosensitive drum during the printing operation in the conventional example

Embodiments for Carrying Out the Invention

[0016] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

Examples

[0017] [Configuration of Image Forming Apparatus] Figure 1 is a cross-sectional view showing the schematic configuration of a color laser beam printer, which is an image forming apparatus to which the present invention is applied. The color laser beam printer 201 shown in Figure 1 is equipped with process cartridges 225y, 225m, 225c, and 225k, which are image forming units that form images with toner colors of yellow (Y), magenta (M), cyan (C), and black (K). The configuration and operation of each process cartridge 225 are substantially the same. Furthermore, the y, m, c, and k appended to the end of the reference numerals of the components constituting the process cartridge 225 indicate that they are components with toner colors of yellow (Y), magenta (M), cyan (C), and black (K), respectively. Similarly, the y, m, c, and k appended to the end of the reference numerals of components other than the process cartridge 225 indicate that they are components corresponding to process cartridges with toner colors of yellow (Y), magenta (M), cyan (C), and black (K), respectively. In the following, the suffixes y, m, c, and k attached to the component names will be omitted unless they refer to a specific component of process cartridge 225.

[0018] In the color laser beam printer 201 (hereinafter referred to as printer 201), each process cartridge 225 has a photosensitive drum 215, which is an image carrier. The photosensitive drum 215 is rotated by a drive source (not shown) in the direction of the arrow in the figure (counterclockwise). Around the photosensitive drum 215 are a charging roller 216, a developing device 217, and a pre-charging exposure device 227 (details will be described later). The charging roller 216, which is a charging component, charges the surface of the photosensitive drum 215 to a uniform potential. In addition, an electrostatic latent image is formed on the photosensitive drum 215 (on the photosensitive drum) by laser light irradiated from the optical device 210, which is the exposure unit described later. The developing device 217, which is the developing unit, develops the electrostatic latent image formed on the photosensitive drum 215 by attaching a developer (toner) to the electrostatic latent image formed on the photosensitive drum 215 using a developing roller 229, thereby forming a toner image. Thus, the process cartridge 225 is composed of an integrated photosensitive drum 215, a charging roller 216, a developing device 217, and a pre-charging exposure device 227.

[0019] In the printer 201, when the video controller 204 receives image data 203 containing print commands and image information from the host computer 202, which is an external computer, it unpacks the image data and generates image data for image formation. Then, based on the generated image data, the video controller 204 generates a video signal 205, which is data in video signal format for controlling the emission of light from the laser diode 211 of the optical device 210, and outputs it to the engine controller 206.

[0020] The engine controller 206 has a CPU 209 that controls the image formation operation of the printer 201. When the CPU 209, which is the control means, receives a video signal 205 from the video controller 204, it drives and controls the laser diode 211 located on the laser control board 208 in the optical device 210 to emit light in synchronization with the video signal 205. The laser beam 212 emitted from the laser diode 211 corresponding to each process cartridge 225 is deflected by a rotating polyhedron mirror 207, passes through a lens 213, and is reflected by a folding mirror 214. The laser beam 212 reflected by the folding mirror 214 is irradiated onto the photosensitive drum 215 in the corresponding process cartridge 225.

[0021] The photosensitive drum 215 in each process cartridge 225 is charged to a desired charge (potential) by the charging roller 216. The laser beam 212 emitted from the laser diode 211 irradiates the photosensitive drum 215, partially lowering its surface potential, thereby forming an electrostatic latent image on the surface of the photosensitive drum 215. To visualize the electrostatic latent image formed by the laser beam 212, the developing device 217 applies developer (toner) on the developing roller 229 to the electrostatic latent image, forming a toner image on the photosensitive drum 215 corresponding to the electrostatic latent image.

[0022] The toner image formed on the photosensitive drum 215 is transferred to the intermediate transfer belt 219, which is the transfer target, by applying a primary transfer voltage to the primary transfer roller 218, which is the transfer unit. The intermediate transfer belt 219 rotates in the direction of the arrow in the figure (clockwise). First, the toner image on the photosensitive drum 215y of the process cartridge 225y, which has yellow toner, is transferred to the intermediate transfer belt 219. Then, the toner images formed on the photosensitive drums 215m, 215c, and 215k of the process cartridges 225m, 225c, and 225k, which have magenta, cyan, and black toner, are sequentially transferred to the intermediate transfer belt 219, forming a color image. Viewed from the direction of rotation of the intermediate transfer belt 219, the first image forming unit, process cartridges 225y, 225m, and 225c, are located upstream of the second image forming unit, process cartridge 225k.

[0023] Furthermore, as shown in Figure 1, the printer 201 is equipped with a cassette 220 containing recording material P, which is a recording medium. Synchronized with the image forming operation in the process cartridge 225 described above, the recording material P contained in the cassette 220 is fed into the transport path by the paper feed roller 222, and the transport rollers arranged in the transport path transport the recording material P to the secondary transfer roller 226.

[0024] The secondary transfer roller 226 presses against the opposing roller via the intermediate transfer belt 219, forming a secondary transfer section 223 where the intermediate transfer belt 219 and the secondary transfer roller 226 come into contact. A secondary transfer voltage is applied to the secondary transfer roller 226, and the toner image formed on the intermediate transfer belt 219 is transferred onto the recording material P in the secondary transfer section 223. The charging roller 216, developing roller 229, primary transfer roller 218, and secondary transfer roller 226 are each applied to a desired charging voltage, developing voltage, primary transfer voltage, and secondary transfer voltage, respectively. The CPU 209 controls the supply of voltages from the high-voltage power supply that provides each applied voltage to be appropriate to the characteristics of the recording material P. Subsequently, the recording material P on which the toner image has been transferred is heated and pressurized in the fuser 224, fixing the toner image to the recording material P, and the recording material P with the toner image fixed is discharged to the discharge section outside the printer 201.

[0025] Furthermore, each process cartridge 225 is equipped with a pre-charging exposure device 227, which is a static elimination unit that irradiates the photosensitive drum 215 with light emitted from the light-emitting element to eliminate static electricity from the surface of the photosensitive drum 215 after primary transfer has been performed, thereby smoothing the surface potential. As shown in Figure 1, in each process cartridge 225, the pre-charging exposure device 227 is positioned downstream of the primary transfer roller 218 and upstream of the charging roller 216 with respect to the rotational direction of the photosensitive drum 215. In other words, the pre-charging exposure device 227 is configured to expose the surface of the photosensitive drum 215 downstream of the transfer section, which is the contact point between the photosensitive drum 215 and the intermediate transfer belt 219, and upstream of the charging section, which is the contact point between the photosensitive drum and the charging roller 216.

[0026] [Configuration of pre-charging exposure system] Next, we will explain the configuration of the pre-charging exposure unit 227. For the sake of simplicity, we will only explain the configuration of the pre-charging exposure unit 227y for the yellow toner process cartridge 225y. The pre-charging exposure units 227m, 227c, and 227k installed in the other process cartridges 225m, 225c, and 225k have the same configuration as those for the process cartridge 225y.

[0027] Figure 2 illustrates the configuration of a pre-charging exposure apparatus 227y that performs pre-charging exposure on the photosensitive drum 215y. As shown in Figure 2, the pre-charging exposure apparatus 227y consists of a light-emitting element 301y and a light guide 302y. The light-emitting element 301y is a light-emitting element used for pre-charging exposure and is installed on the main body side of the printer 201. On the other hand, the light guide 302y is a light guide member for irradiating the photosensitive drum 215y with light emitted from the light-emitting element 301y and is installed on the cartridge tray (not shown) that holds the process cartridge 225y. The light guide 302y is positioned downstream of the primary transfer roller 218y shown in Figure 1 in the rotational direction (counterclockwise) of the photosensitive drum 215, and upstream of the charging roller 216y.

[0028] As shown in Figure 2, the light guide 302y is positioned approximately parallel to the axis (rotation axis) of the photosensitive drum 215y, and a light incident part 303y that receives light emitted from the light-emitting element 301y is provided at one end of the light guide 302y in the longitudinal direction. The light-emitting element 301y is controlled at predetermined timings by a light emission intensity control unit (not shown). The light emitted from the light-emitting element 301y and incident on the light guide 302y becomes diffused light from the side of the light guide 302y and irradiates the photosensitive drum 215y, thereby eliminating the static charge on the surface potential of the photosensitive drum 215.

[0029] In this embodiment, the amount of light emitted by the pre-charging exposure device 227 is adjusted to a preset amount, but a mechanism for adjusting the amount of light emitted may be provided. That is, a light-receiving element that detects the amount of light emitted by the pre-charging exposure device 227 is placed near the light-emitting element 301, the light guide 302, and the photosensitive drum 215. Then, a mechanism may be provided to adjust the amount of light emitted based on the amount of light emitted detected by the light-receiving element, in accordance with the deterioration of the light-emitting element 301, the contamination of the light guide 302, and the change in the light-receiving sensitivity of the photosensitive drum 215. In addition, although a configuration in which the light guide 302y is installed on a cartridge tray (not shown) has been described here, for example, the following configurations may also be used. That is, a configuration in which the light guide 302y is placed on the process cartridge 225y, a configuration using an LED array instead of the light guide 302y, or a configuration in which the light guide 302y is eliminated for the sake of simplifying the device and light is directly irradiated onto the photosensitive drum 215.

[0030] [Control circuit for pre-charging exposure equipment] Next, we will describe the circuit that controls the light emission intensity of the light-emitting element 301 of the pre-charging exposure device 227. For the sake of simplicity, we will only describe the circuit that controls the light emission intensity of the light-emitting element 301y of the pre-charging exposure device 227y for the yellow toner process cartridge 225y. Note that the circuits that control the light emission intensity of the light-emitting elements 301m, 301c, and 301k of the other process cartridges 225m, 225c, and 225k have the same configuration as the process cartridge 225y.

[0031] Figure 3 is a circuit diagram showing the circuit configuration of the light emission control circuit of the light-emitting element 301y of the pre-charging exposure apparatus 227y. The light emission control circuit consists of a light-emitting element 301y (hereinafter also referred to as light-emitting diode 301y), resistors 401y and 404y, capacitor 402y, and transistor 403y. The light emission control circuit receives a PWM signal from the CPU 209 (Figure 1) of the engine controller 206 to control the amount of light emitted by the light-emitting element 301y. The PWM signal is smoothed by an RC filter composed of resistor 401y and capacitor 402y and input to the base terminal of transistor 403y. The voltage input to the base terminal of transistor 403y is adjustable according to the onDuty (duty cycle) of the PWM signal.

[0032] The collector terminal of transistor 403y is connected to the cathode terminal of light-emitting diode 301y, and the anode terminal of light-emitting diode 301y is connected to the power supply voltage Vcc. On the other hand, the emitter terminal of transistor 403y is connected to ground via resistor 404y. A voltage drop due to the base-emitter voltage, based on the base terminal voltage of transistor 403y, is applied to resistor 404y. This controls the current flowing through the light-emitting element 301y, and the amount of light irradiated onto the photosensitive drum 215y changes according to the current value flowing through the light-emitting element 301y.

[0033] Figure 4 is a graph showing the relationship between the OnDuty (percentage of ON state in one cycle) of the PWM signal output from the CPU 209, the base voltage characteristics of transistor 403y, the control current ratio flowing through the light-emitting element 301y, and the light emission intensity ratio of the light-emitting element 301y. Figure 4(a) is a graph showing the relationship between the OnDuty of the PWM signal and the base voltage input to transistor 403y. In Figure 4(a), the horizontal axis represents the OnDuty of the PWM signal (unit: %), and the vertical axis represents the base voltage of transistor 403y (unit: V). As shown in Figure 4(a), when the OnDuty of the PWM signal is 20%, the voltage at the base terminal of transistor 403y is 0.7V, which is the voltage at which transistor 403y turns ON. Also, when the OnDuty of the PWM signal is 100%, the voltage at the base terminal of transistor 403y is 3.3V.

[0034] Figure 4(b) is a graph showing the relationship between the OnDuty of the PWM signal and the control current ratio flowing through the light-emitting element 301y. In Figure 4(b), the horizontal axis represents the OnDuty of the PWM signal (unit: %), and the vertical axis represents the control current ratio flowing through the light-emitting element 301y (unit: %). The control current ratio represents the ratio of the current flowing through the light-emitting element 301y when the OnDuty of the PWM signal is 100% to 100%. Transistor 403y turns on when the OnDuty of the PWM signal exceeds approximately 20%, and current begins to flow through the light-emitting element 301y, indicating that the light-emitting element 301y of the pre-charging exposure device 227y can be controlled to emit light from low light levels.

[0035] Figure 4(c) is a graph showing the relationship between the onDuty of the PWM signal and the emission intensity ratio of the light-emitting element 301y. In Figure 4(c), the horizontal axis represents the onDuty of the PWM signal (unit: %), and the vertical axis represents the emission intensity ratio of the light-emitting element 301y (unit: %). The emission intensity ratio represents the ratio of the emission intensity of the light-emitting element 301y to the emission intensity when the onDuty of the PWM signal is 100%, with the emission intensity being set to 100%. In this way, the CPU 209 can adjust the amount of static electricity removed from the photosensitive drum 215y when performing pre-charging exposure by varying the onDuty of the output PWM signal and controlling the emission intensity of the light-emitting element 301y used for pre-charging exposure.

[0036] In this embodiment, a method was described in which the base voltage of the transistor 403y is controlled via an RC filter composed of a resistor 401y and a capacitor 402y, thereby controlling the current flowing through the light-emitting element 301y and adjusting the amount of light emitted by the light-emitting element 301y. In this embodiment, the amount of light emitted was adjusted by controlling the current flowing through the light-emitting element 301y, but for example, the amount of surface charge removal of the photosensitive drum 215y may be adjusted by causing the light-emitting element 301y to emit pulsed light.

[0037] [Configuration of high-voltage power supply] Next, the configuration of the high-voltage power supply of the printer 201 in this embodiment will be described. Figure 5 is a schematic cross-sectional view illustrating the configuration of the high-voltage power supply that supplies high voltage to the process cartridges 225y, 225m, 225c, 225k, etc., which perform image formation. Figure 5 schematically shows which high-voltage power supply supplies the voltage applied to the charging roller 216, developing roller 229, primary transfer roller 218, and secondary transfer roller 226 of each process cartridge 225.

[0038] In Figure 5, the voltage generation circuit 601, which is the power supply unit, generates a charging voltage Vc1 and supplies it to the charging rollers 216y, 216m, and 216c of the process cartridges 225y, 225m, and 225c, which are yellow (Y), magenta (M), and cyan (C) toners, respectively. In this embodiment, in order to miniaturize the power supply unit, the charging voltage Vc1 is supplied from a common voltage generation circuit 601 (first power supply) to the charging rollers 216y, 216m, and 216c, which are the charging components of multiple process cartridges 225. When supplying power from the voltage generation circuit 601 to the charging rollers 216y, 216m, and 216c, it is also possible to connect resistors or Zener diodes between each charging roller to supply different voltages to the charging rollers 216y, 216m, and 216c. That is, for example, when attempting to apply a charging voltage to the charging roller 216y, it is sufficient to have a configuration in which at least the charging voltage is also applied to the charging roller 216m. Needless to say, this method is also applicable to the 216c and 216k electrostatic rollers, not just the 216m electrostatic roller.

[0039] Furthermore, a voltage divider circuit, composed of resistor 603 and Zener diode 604, divides the charged voltage Vc1 to generate a developing voltage Vd1. In the voltage divider circuit, one end of resistor 603 is connected to the terminal of voltage generation circuit 601 that outputs the charged voltage Vc1. The other end of resistor 603 is connected to the anode terminal of Zener diode 604 and to the developing rollers 229y, 229m, and 229c of process cartridges 225y, 225m, and 225c. The cathode terminal of Zener diode 604 is connected to ground (ground faulted). The developing voltage Vd1 generated by the voltage divider circuit is supplied to the developing rollers 229y, 229m, and 229c of process cartridges 225y, 225m, and 225c.

[0040] Meanwhile, the voltage generation circuit 602 (second power supply) generates a charging voltage Vc2 and supplies it to the charging roller 216k of the process cartridge 225k, which has a black (K) toner color. In this embodiment, a voltage generation circuit 602 is provided separately from the voltage generation circuit 601 described above in order to supply a charging voltage independently when printing monochrome images. Also, in the voltage generation circuit 602, similar to the voltage generation circuit 601, a voltage divider circuit is provided to divide the charging voltage Vc2 and generate a developing voltage Vd2. The voltage divider circuit consists of a resistor 605 and a Zener diode 606. One end of the resistor 605 is connected to the terminal of the voltage generation circuit 602 that outputs the charging voltage Vc2, and the other end of the resistor 605 is connected to the anode terminal of the Zener diode 606 and the developing roller 229k of the process cartridge 225k. The cathode terminal of the Zener diode 606 is connected to ground (ground faulted). The developing voltage Vd2 generated by the voltage divider circuit is supplied to the developing roller 229k of the process cartridge 225k.

[0041] Voltage generation circuits 601 and 602 each have a voltage detection circuit (not shown) that allows the charging voltage supplied to each charging roller 216 to be varied according to the operating environment of the printer 201 and the changes in the photosensitive drum 215 over time. In this embodiment, the charging voltage Vc1 generated by voltage generation circuit 601 is applied to the charging rollers 216y, 216m, and 216c and used to form a background potential on the surface of the photosensitive drums 215y, 215m, and 215c. On the other hand, the charging voltage Vc2 generated by voltage generation circuit 602 is applied to the charging roller 216k and used to form a background potential on the surface of the photosensitive drum 215k. Note that the charging voltage Vc1 generated by voltage generation circuit 601 and the charging voltage Vc2 generated by voltage generation circuit 602 have the same output value. Meanwhile, the developing voltage Vd1 is applied to the developing rollers 229y, 229m, and 229c, and the developing voltage Vd2 is applied to the developing roller 229k, both used to deposit toner onto the electrostatic latent image formed on each photosensitive drum 215.

[0042] The voltage generation circuit 607 is used to generate a primary transfer voltage and apply it to the primary transfer rollers 218y, 218m, 218c, and 218k, thereby transferring the toner image formed on the photosensitive drum 215 to the intermediate transfer belt 219. The voltage generation circuit 608 is used to generate a secondary transfer voltage and apply it to the secondary transfer roller 226, thereby transferring the toner image formed on the intermediate transfer belt 219 to the recording material P.

[0043] [Effect of pre-charging exposure in a cleanerless configuration] Next, the cleanerless system will be explained using Figure 6. For simplicity, this explanation will focus on the pre-charging exposure in the cleanerless configuration of process cartridge 225y, which has a yellow toner color. The pre-charging exposure in the cleanerless configurations of other process cartridges, 225m, 225c, and 225k, is the same as that for process cartridge 225y.

[0044] Figure 6 is a cross-sectional view showing the configuration around the photosensitive drum 215y of the process cartridge 225y. In Figure 6, L indicates the laser irradiation position of the laser beam 212y irradiated onto the photosensitive drum 215y from the optical device 210. D indicates the contact area (contact position) between the photosensitive drum 215y and the developing roller 229y. The arrows in the developing roller 229y and the charging roller 216y indicate the rotation direction of the developing roller 229y and the charging roller 216y. Similarly, the arrow indicated by R1 indicates the rotation direction of the photosensitive drum 215y. In the figure, black circles indicate negative polarity toner, and white circles indicate positive polarity toner. In this embodiment, negative polarity is considered the normal polarity.

[0045] In the process cartridge 225y, during the transfer process, the toner image formed on the photosensitive drum 215y is transferred to the intermediate transfer belt 219 by the primary transfer roller 218y. The residual toner 701y that is not transferred to the intermediate transfer belt 219 and remains on the surface of the photosensitive drum 215y is affected by the discharge in the gap 702y upstream of the rotation direction of the photosensitive drum as the photosensitive drum 215y rotates in the R1 direction. Due to the discharge in the gap 702y, the residual toner 701y becomes negatively charged, similar to the surface of the photosensitive drum 215y. At the contact point with the charging roller 216y, the surface potential of the photosensitive drum 215y is approximately -700V, and the potential of the charging roller 216y is approximately -1300V, resulting in a potential difference. Therefore, due to the potential difference, the negatively charged residual toner 703y on the photosensitive drum 215y does not adhere to the surface of the charging roller 216y. As a result, the remaining toner 703y that did not adhere to the surface of the charging roller 216y passes through the charging roller 216y.

[0046] The negative-polarity transfer residue toner 703y on the photosensitive drum 215y, having passed through the charging roller 216y, moves in the R1 direction and reaches the laser irradiation position L where the laser beam 212y is irradiated. Since there is not enough transfer residue toner 703y to shield the laser beam 212y, it does not affect the process of forming an electrostatic latent image on the surface of the photosensitive drum 215y.

[0047] Of the transfer residue toner 703y on the photosensitive drum 215y, the transfer residue toner 703y in the unexposed areas that were not irradiated by the laser beam 212y at the laser irradiation position L is collected by the developing roller 229y in the developing unit D by electrostatic force. On the other hand, the transfer residue toner 703y that was irradiated by the laser beam 212y at the laser irradiation position L is not collected by the developing roller 229y by electrostatic force and continues to exist on the surface of the photosensitive drum 215y.

[0048] In this way, the residual toner 701y that remains on the surface of the photosensitive drum 215y without being transferred to the intermediate transfer belt 219 is generally recovered by the developing roller 229y. The residual toner recovered by the developing roller 229y is then mixed with the toner inside the developing roller 229y and reused.

[0049] In this embodiment of the cleanerless system, there is no mechanism such as a cleaning blade to remove residual transfer toner that remains on the surface of the photosensitive drum 215y and was not transferred to the intermediate transfer belt 219 during the transfer process in the primary transfer section. Therefore, it is necessary to prevent residual transfer toner 701y remaining on the surface of the photosensitive drum 215y after the transfer process from adhering to the charging roller 216y. In order to charge the residual transfer toner 701y to a more negative polarity, the photosensitive drum 215y is discharged by the pre-charging exposure device 227y, and the potential difference between the photosensitive drum 215y and the charging roller 216y is increased. This causes a strong discharge in the void 702y formed near the contact area of ​​the charging roller. This strong discharge then charges the residual transfer toner 701y to a more uniform negative polarity. It is desirable that the exposure potential of the photosensitive drum 215y after discharge by pre-charging exposure be approximately -150V, which is roughly equivalent to the exposure potential by the laser beam 212y.

[0050] [Effect of pre-charging exposure on drum ghosting] Next, we will explain the effect of pre-charging exposure on drum ghosting. After the exposure process by the optical device 210, the development process by the developing roller 229, and the transfer process by the primary transfer roller 218, the surface potential of the photosensitive drum 215y is non-uniform, depending on the image pattern formed on the photosensitive drum 215y. If the charging process by the charging roller 216y is performed in the next drum cycle while the surface potential of the photosensitive drum 215y is in this state, it may not be possible to charge the surface of the photosensitive drum 215y to a uniform potential, depending on the image pattern formed in the previous cycle. As a result, in the exposure process by the optical device 210 in the next cycle, it may not be possible to form the desired electrostatic latent image on the photosensitive drum 215y, and a ghost image may occur. Therefore, after the transfer process by the primary transfer roller 218 and before the charging process by the charging roller 216y, light from the light-emitting element 301y of the pre-charging exposure device 227y is irradiated onto the surface of the photosensitive drum 215y to remove the static charge from the surface potential to a predetermined residual potential level. This makes it possible to equalize the surface potential of the photosensitive drum 215y after the transfer process by the primary transfer roller 218, thereby suppressing the occurrence of drum ghosting.

[0051] [Light emission / extinction control for pre-charging exposure equipment] The light emission control method of the pre-charging exposure device 227 in this embodiment will be explained with reference to Figure 7. Figure 7 shows a timing chart indicating the state of the pre-charging exposure device 227, photosensitive drum 215, charging voltage, etc., during printing operation of the color laser beam printer 201 in this embodiment. The horizontal axis of Figure 7 represents time. In Figure 7, 101y, 101m, 101c, and 101k represent the amount of light emitted from the light-emitting element 301 of the pre-charging exposure device 227 for each process cartridge 225 with toner colors of yellow (Y), magenta (M), cyan (C), and black (K), respectively. Furthermore, 102y, 102m, 102c, and 102k represent the transition of the surface potential near the position (pre-charging exposure area) of the photosensitive drum 215 to which light from the light-emitting element 301 of the pre-charging exposure device 227 for each process cartridge 225 is irradiated. Also, 103 shows the output waveform of the charging voltage (-1300V) of the voltage generation circuit 601 described above. Furthermore, 104y, 104m, 104c, and 104k indicate the timing at which the image forming region on the photosensitive drum 215 of each process cartridge 225 passes the contact portion with the charging roller 216.

[0052] First, the light emission control method of the pre-charging exposure device 227y in the process cartridge 225y, which is the yellow station, will be explained. In the figure, 101y and 102y represent the light emission amount of the light-emitting element 301y of the pre-charging exposure device 227y of the process cartridge 225y, and the transition of the surface potential near the position (pre-charging exposure area) of the photosensitive drum 215y irradiated by light from the light-emitting element 301y, respectively. In this embodiment, when the photosensitive drum 215y is de-charged by light emitted from the light-emitting element 301y of the pre-charging exposure device 227y after the transfer process, the following control is performed. That is, as shown in 101y in the figure, the light emission amount of the light-emitting element 301y of the pre-charging exposure device 227y is increased in eight steps over approximately 300 msec from the off state to a predetermined target light amount, and the light emission amount is controlled accordingly. Hereinafter, this type of light emission amount control will be referred to as step-by-step light emission amount rise control. In this way, by controlling the step-by-step rise of the emitted light intensity, as shown in figure 102y, the surface potential of the photosensitive drum 215y near the pre-charging exposure area does not experience abrupt potential fluctuations, but rather gradually shifts from -300V to -150V in accordance with the step-by-step displacement of the emitted light intensity. Similarly to process cartridges 225m, 225c, and 225k, the emitted light intensity of the light-emitting element 301 of each pre-charging exposure apparatus 227 is controlled in a step-by-step manner, as shown in figures 101m, 101c, and 101k. By controlling the step-by-step rise of the light-emitting element 301, the surface potential of each photosensitive drum 215 of process cartridges 225m, 225c, and 225k can be gradually shifted from -300V to -150V, as shown in figures 102m, 102c, and 102k.

[0053] Next, we will explain the charging voltage output supplied to each charging roller 216 of process cartridges 225y, 225m, 225c, and 225k. In the figure, 103 shows the output waveform of the charging voltage Vc1 supplied from the voltage generation circuit 601, which is a common high-voltage power supply for the charging rollers 216y, 216m, and 216c of process cartridges 225y, 225m, and 225c. The charging voltage Vc1 is supplied to the charging rollers 216y, 216m, and 216c from the voltage generation circuit 601, which is a common high-voltage power supply. Therefore, if the surface potential of the photosensitive drum 215 changes sharply before and after exposure before charging, the output of the voltage generation circuit 601 will not be able to follow the load changes associated with the sharp surface potential changes of the photosensitive drum 215, and the output of the charging voltage will fluctuate. In the figure, Dy1 indicates the timing when the pre-charging exposed area on the photosensitive drum 215y, illuminated by light from the light-emitting element 301y of the process cartridge 225y, reaches contact with the charging roller 216y. Similarly, Dm1 indicates the timing when the pre-charging exposed area on the photosensitive drum 215m, illuminated by light from the light-emitting element 301m of the process cartridge 225m, reaches contact with the charging roller 216m. Dc1 indicates the timing when the pre-charging exposed area on the photosensitive drum 215c, illuminated by light from the light-emitting element 301c of the process cartridge 225c, reaches contact with the charging roller 216c.

[0054] As shown in the figures for 102y, 102m, 102c, and 102k, the displacement of the surface potential at the position of the pre-charging exposure area where the pre-charging exposure described above begins for each photosensitive drum 215 of process cartridges 225y, 225m, 225c, and 225k is gradual. Therefore, at timings Dy1, Dm1, and Dc1, when the position of the pre-charging exposure area where the pre-charging exposure begins for each photosensitive drum 215y, 215m, and 215c reaches the charging roller 216, the voltage generation circuit 601 is hardly affected by load fluctuations due to pre-charging exposure. As a result, the voltage generation circuit 601 can continue to supply a stable charging voltage Vc1 to each charging roller 216 of process cartridges 225y, 225m, and 225c. Note that for process cartridge 225y, the charging voltage is supplied to the charging roller 216k from a separate voltage generation circuit 602, distinct from the voltage generation circuit 601. Therefore, in the configuration of the power supply in this embodiment, the charging voltage Vc1 supplied from the voltage generation circuit 601 to the process cartridges 225y, 225m, and 225c is not affected.

[0055] In the figure, 104y, 104m, 104c, and 104k indicate the timing at which the image-forming region of each photosensitive drum 215 in process cartridges 225y, 225m, 225c, and 225k passes through the contact area with the charging roller 216. The image-forming region on the photosensitive drum 215 is the region (section) where an electrostatic latent image is formed by the laser beam 212 irradiated from the optical device 210 in Figure 6 described above. When the charging voltage Vc1 supplied from the voltage generation circuit 601 fluctuates in the section where the image-forming region on the photosensitive drum 215 contacts the charging roller 216, the background potential for image formation on the surface of the photosensitive drum 215 is displaced. As a result, density unevenness occurs in the formed image.

[0056] Furthermore, as shown in 104y in the figure, timings Dm1 and Dc1 coincide with the period when the image forming region on the photosensitive drum 215y of the process cartridge 225y passes the contact portion with the charging roller 216y. Timings Dm1 and Dc1 indicate the timing when the pre-charging exposure portion of the photosensitive drums 215m and 215c of the process cartridges 225m and 225c comes into contact with the charging rollers 216m and 216c. In this embodiment, as described above, the stage-by-stage startup control of each light-emitting element 301 of the pre-charging exposure devices 227m and 227c of the process cartridges 225m and 225c is performed. This suppresses sharp fluctuations in the surface potential of the photosensitive drums 215m and 215c of the process cartridges 225m and 225c, and suppresses fluctuations in the output of the charging voltage Vc1 of the voltage generation circuit 601. Furthermore, in this embodiment, the timing for starting the step-by-step startup control of the light-emitting elements 301 of the pre-charging exposure devices 227y, 227m, and 227c of process cartridges 225y, 225m, and 225c is shifted according to the timing at which image formation begins. Therefore, at timings Dy1, Dm1, and Dc1, the charging voltage output does not fluctuate sharply, and a stable charging voltage Vc1 can be continuously supplied to the charging rollers 216 of process cartridges 225y, 225m, and 225c. As a result, the background potential in the image formation region of the photosensitive drum 215 of each process cartridge 225 does not fluctuate, and high-quality image formation without density unevenness can be achieved.

[0057] The light emission control method for the pre-charging exposure apparatus 227 has been described above. However, even when turning off the light-emitting element 301 of the pre-charging exposure apparatus 227, by gradually reducing the light intensity from the target light intensity to turn it off, abrupt fluctuations in the surface potential of the photosensitive drum 215 can be suppressed. That is, as shown in 101y in the figure, the light emission intensity of the light-emitting element 301y of the pre-charging exposure apparatus 227y is controlled to gradually decrease from a predetermined target light intensity to an off state (hereinafter referred to as step-by-step light emission intensity reduction control). By performing step-by-step light emission intensity reduction control in this way, as shown in 102y in the figure, the surface potential of the photosensitive drum 215y near the pre-charging exposure area does not experience abrupt potential fluctuations, but rather shifts gradually from -150V to -300V in accordance with the step-by-step displacement of the light emission intensity. As a result, the occurrence of potential unevenness on the surface of the photosensitive drum 215 can be suppressed.

[0058] In this embodiment, the timing for starting the step-by-step drop-down control of the light-emitting element 301 of the pre-charging exposure units 227y, 227m, and 227c for process cartridges 225y, 225m, and 225c is shifted based on the timing at which image formation is completed. This makes it possible to suppress fluctuations in the output of the charging voltage Vc1 at the timings Dy2, Dm2, and Dc2 when the pre-charging exposure section on each photosensitive drum 215 of process cartridges 225y, 225m, and 225c comes into contact with the charging roller 216 when the light-emitting element 301 is turned off. As a result, the voltage generation circuit 601 can continuously supply a stable charging voltage Vc1 to each charging roller 216 of process cartridges 225y, 225m, and 225c, enabling high-quality image formation without density unevenness.

[0059] In this embodiment, a configuration was described in which the light emission intensity of the light-emitting element 301 of the pre-charging exposure device 227 is controlled in stages for each process cartridge 225. For example, when pre-charging exposure is performed on a certain process cartridge 225, even if the light-emitting element 301 is rapidly increased from an off state to a target light intensity, localized potential unevenness may occur on the surface of the photosensitive drum 215 outside the image formation region. Specifically, this occurs when light emission control for pre-charging exposure is performed on the photosensitive drum 215y of the process cartridge 225y, which is the yellow station and has the earliest image formation start timing in the configuration of this embodiment. In this case, since there is no effect on image formation, it is not necessarily required to implement the above-described staged increase in the light emission intensity of the light-emitting element 301 of the pre-charging exposure device 227. Similarly, when the light-emitting element 301 of the pre-charging exposure device 227 is turned off, localized potential unevenness on the surface of the photosensitive drum 215 may occur outside the image formation region. Specifically, this refers to the case where, among the process cartridges 225y, 225m, and 225c to which the charging voltage Vc1 is supplied from the voltage generation circuit 601, pre-charging exposure control is performed on the photosensitive drum 215c of process cartridge 225c, which has the latest image formation completion time. In this case, it is not necessarily required to perform step-by-step drop-off control of the light emission intensity of the light-emitting element 301 of the pre-charging exposure device 227.

[0060] Furthermore, process cartridge 225k is supplied with a charging voltage from a different high-voltage power supply (voltage generation circuit) than the other process cartridges 225y, 225m, and 225c to the charging roller 216k. Therefore, even if there are fluctuations in the output of the charging voltage supplied to process cartridge 225k, it does not affect the background potential of the photosensitive drums 215 of the other process cartridges 225y, 225m, and 225c. Even in this configuration, it is not always necessary to implement step-by-step start-up and stop-down control of the light-emitting element 301 of the pre-charging exposure apparatus 227.

[0061] Furthermore, in this embodiment, the control of turning the light-emitting element 301 of the pre-charging exposure apparatus 227 on and off during image formation has been described. For example, in order to suppress potential unevenness that occurs on the surface of the photosensitive drum 215 due to pre-charging exposure, when turning the light-emitting element 301 of the pre-charging exposure apparatus 227 on and off for purposes other than image formation, stepwise rise-up and fall-down control of the light-emitting element 301 may be implemented.

[0062] As described above, in this embodiment, the amount of light emitted by the light-emitting element 301 of the pre-charging exposure device 227 is controlled so that the displacement of the surface potential on the photosensitive drum 215 before and after pre-charging exposure is gradual. This minimizes the output fluctuation of the charging voltage due to fluctuations in the surface potential (load fluctuations) of the photosensitive drum 215. As a result, when pre-charging exposure is performed on the photosensitive drum 215 to suppress the adhesion of residual toner to the charging roller 216 and the occurrence of drum ghosting in a cleanerless system configuration, high-quality image formation without uneven image density can be achieved. In particular, in a color image forming apparatus configured in which power is supplied from a common power supply device to multiple charging members, high-quality image formation without uneven image density can be achieved.

[0063] As described above, according to this embodiment, fluctuations in the charging voltage due to the displacement of the surface potential of the photosensitive drum caused by pre-charging exposure can be suppressed. [Examples]

[0064] In Example 2, we will describe the step-by-step rise and fall control of the light emission intensity of the light-emitting element of the pre-charging exposure apparatus, which differs from that of Example 1. The configuration of the image forming apparatus and the pre-charging exposure apparatus in this example is the same as in Example 1, and the same reference numerals are used for the same apparatus and components as in Example 1, so their explanation here will be omitted.

[0065] [Photosensitive characteristics of the photosensitive drum] Figure 8 shows an example of an EV curve illustrating the photosensitivity characteristics of each photosensitive drum 215 of process cartridges 225y, 225m, 225c, and 225k. In Figure 8, the horizontal axis represents the exposure amount (exposure light amount) E of the light-emitting element 301 of the pre-charging exposure device 227, and the vertical axis represents the surface potential V of the photosensitive drum 215. In the EV curve of Figure 8, when the surface potential (post-transfer potential) of the photosensitive drum 215 at the end of the primary transfer process is V1, irradiating it with exposure light amount E2 from the light-emitting element 301 of the pre-charging exposure device 227 causes the surface potential (pre-charging post-exposure potential) of the photosensitive drum 215 to shift to V2.

[0066] The EV curve shown in Figure 8 indicates that increasing the exposure amount E irradiated by the light-emitting element 301 causes the surface potential of the photosensitive drum 215 to attenuate (decrease). Furthermore, the high-potential areas of the photosensitive drum 215, where the absolute value of the surface potential is large, are in a strong electric field environment, making it difficult for charge carriers (electron-hole pairs) generated by exposure to the light-emitting element 301 to recombine. Therefore, even with a small exposure amount E, the surface potential of the photosensitive drum 215 attenuates significantly. On the other hand, the low-potential areas of the photosensitive drum 215, where the absolute value of the surface potential is small, are in a weak electric field environment, making it easy for charge carriers generated by exposure to the light-emitting element 301 to recombine. Therefore, even with a large exposure amount E, the attenuation of the surface potential of the photosensitive drum 215 is small. In other words, when the photosensitive drum 215 is de-charged by the pre-charging exposure device 227, the surface potential of the photosensitive drum 215 has the following characteristics. The photosensitive drum 215 has the characteristic that in a low light intensity region where the amount of light emitted from the light-emitting element 301 is lower than a predetermined light intensity, the fluctuation of the surface potential of the photosensitive drum 215 in response to the displacement of the light emitted is large. Furthermore, the photosensitive drum 215 has the characteristic that in a high light intensity region where the amount of light emitted from the light-emitting element 301 is higher than a predetermined light intensity, the fluctuation of the surface potential of the photosensitive drum 215 in response to the displacement of the light emitted is small.

[0067] [Light emission / extinction control for pre-charging exposure equipment] Next, the light emission control method of the pre-charging exposure apparatus 227 in this embodiment will be described. Figure 9 shows a timing chart indicating the state of the pre-charging exposure apparatus 227, photosensitive drum 215, charging voltage, etc., during printing operation of the color laser beam printer 201 in this embodiment. The horizontal axis of Figure 9 represents time. The timing chart of the light emission amount of the pre-charging exposure apparatus 227, the surface potential of the photosensitive drum 215 near the pre-charging exposure area, and the output waveform of the charging voltage shown on the vertical axis of Figure 9 is the same as that of Figure 7 described above, so an explanation of how to read the figure will be omitted.

[0068] In this embodiment, when controlling the light emission of the light-emitting element 301 of the pre-charging exposure device 227 for each process cartridge 225, the light emission amount 101 of the light-emitting element 301 is controlled according to the characteristics of the EV curve, which is the photosensitive characteristic of the photosensitive drum 215 described above. Therefore, in this embodiment, a step-up control is performed in which the light emission amount of the light-emitting element 301 is gradually increased in the low-light-intensity region and steeply increased in the high-light-intensity region. As a result, as shown in the figure at 102y, 102m, 102c, and 102k, static electricity can be removed without causing partially steep potential fluctuations in the surface potential of the photosensitive drum 215. In the step-up control in Embodiment 1, the light emission amount of the light-emitting element 301 changed by a constant amount and was controlled linearly in proportion to time. On the other hand, in the step-by-step startup control of this embodiment, compared to Embodiment 1, fluctuations in the charging voltage at timings Dy1, Dm1, and Dc1, when the pre-charging exposure portion on the photosensitive drum 215 of each process cartridge 225 reaches the contact portion with the charging roller 216, can be suppressed more effectively.

[0069] Furthermore, in the step-down control for turning off the light-emitting element 301 of the pre-charging exposure apparatus 227, the turning off of the light-emitting element 301 is controlled according to the characteristics of the EV curve, which is the photosensitive characteristic of the photosensitive drum 215, similar to the light emission control of the light-emitting element 301. In this embodiment, by performing step-down control that sharply reduces the light intensity of the light-emitting element 301 in the high-light intensity region and gradually reduces it in the low-light intensity region, the amount of displacement of the surface potential of the photosensitive drum 215 can be suppressed.

[0070] Figure 10 illustrates an example of the step-by-step control of the light emission intensity of the light-emitting element 301 described above. In Figure 10, the horizontal axis represents time, and the vertical axis represents the light emission intensity of the light-emitting element 301 of the pre-charging exposure device 227. In the step-by-step control shown in Figure 10, in the low-light region where the photosensitive characteristics of the photosensitive drum 215 are more sensitive at the timing of light emission commencement, the light emission intensity of the light-emitting element 301 of the pre-charging exposure device 227 is increased in steps. On the other hand, in the high-light region where the photosensitive characteristics become less sensitive, the light emission intensity of the light-emitting element 301 is increased rapidly. By performing such step-by-step control, fluctuations in the charging voltage due to static discharge in the pre-charging exposure device 227 can be suppressed, while shortening the time required to raise the light emission intensity of the light-emitting element 301 of the pre-charging exposure device 227 to the target light intensity.

[0071] As explained above, the amount of light emitted by the light-emitting element 301 of the pre-charging exposure device 227 is controlled according to the photosensitive characteristics of the photosensitive drum 215 so that the displacement of the surface potential of the photosensitive drum 215 before and after pre-charging exposure is gradual. This makes it possible to further reduce the output fluctuation of the charging voltage due to load fluctuations of the surface potential of the photosensitive drum 215. As a result, when pre-charging exposure is performed on the photosensitive drum 215 to suppress the adhesion of residual toner to the charging roller 216 and drum ghosting in a cleanerless system configuration, high-quality image formation without uneven image density can be achieved. In particular, in a color image forming apparatus with a configuration in which power is supplied to multiple charging components from a common power supply unit, high-quality image formation without uneven image density can be achieved.

[0072] As described above, according to this embodiment, fluctuations in the charging voltage due to the displacement of the surface potential of the photosensitive drum caused by pre-charging exposure can be suppressed. [Explanation of symbols]

[0073] 209 CPU 215 Photosensitive drum 216 Electrostatic roller 225 Process Cartridge 227 Pre-charging exposure apparatus 301 Light-emitting element 601, 602 Voltage Generation Circuits

Claims

1. A first image forming unit having a first photosensitive drum, a first charging member that charges the surface of the first photosensitive drum, a first developing unit that develops an electrostatic latent image formed on the first photosensitive drum and forms a toner image, a first transfer unit that transfers the toner image on the first photosensitive drum to a transfer object, and a first static elimination unit that has a light-emitting element and irradiates the first photosensitive drum with light emitted from the light-emitting element to eliminate static electricity from the surface of the first photosensitive drum, A second image forming unit having a second photosensitive drum, a second charging member that charges the surface of the second photosensitive drum, a second developing unit that develops an electrostatic latent image formed on the second photosensitive drum and forms a toner image, a second transferring unit that transfers the toner image on the second photosensitive drum to a transfer object, and a second static elimination unit that has a light-emitting element and irradiates the second photosensitive drum with light emitted from the light-emitting element to eliminate static electricity from the surface of the second photosensitive drum, A power supply unit that applies a charging voltage to the first charging member and the second charging member, Control means for controlling the first static elimination unit and the second static elimination unit, Equipped with, The first image forming unit is positioned upstream of the second image forming unit in the direction of movement of the object to be transferred. The power supply unit is configured such that when the charging voltage is applied to the first charging member, the charging voltage is also applied to the second charging member. The control means controls the amount of light emitted by the light-emitting element of the second static elimination unit of the second image forming unit to change from a first light amount to a second light amount greater than the first light amount, and then to a third light amount greater than the second light amount. The image forming apparatus is characterized in that the control means does not control the amount of light emitted by the light-emitting element of the first static elimination unit of the first image forming unit to change in steps.

2. The image forming apparatus according to claim 1, characterized in that the first light intensity is the light intensity when the light-emitting element is turned off.

3. The image forming apparatus according to claim 2, characterized in that when the control means changes the amount of light emitted by the light-emitting element of the second static elimination unit of the second image forming unit in steps, it increases the amount of light emitted by the light-emitting element by a constant amount.

4. The image forming apparatus according to claim 2, characterized in that the control means increases the amount of light emitted from the light-emitting element more gradually in a low-light-intensity region where the amount of light emitted from the light-emitting element is lower than a predetermined light-intensity region compared to a high-light-intensity region where the amount of light emitted from the light-emitting element is higher than a predetermined light-intensity region.

5. The second image forming unit includes a plurality of image forming units, The image forming apparatus according to claim 4, characterized in that the timing for initiating static discharge by the second static discharge unit in each of the plurality of image forming units differs for each of the image forming units.

6. The second image forming unit includes a plurality of image forming units, The image forming apparatus according to claim 4, characterized in that the control means starts the static discharge by the second static discharge unit in the plurality of image forming units according to the timing at which image forming is started in each of the image forming units.

7. A first image forming unit having a first photosensitive drum, a first charging member that charges the surface of the first photosensitive drum, a first developing unit that develops an electrostatic latent image formed on the first photosensitive drum and forms a toner image, a first transfer unit that transfers the toner image on the first photosensitive drum to a transfer object, and a first static elimination unit that has a light-emitting element and irradiates the first photosensitive drum with light emitted from the light-emitting element to eliminate static electricity from the surface of the first photosensitive drum, A second image forming unit having a second photosensitive drum, a second charging member that charges the surface of the second photosensitive drum, a second developing unit that develops an electrostatic latent image formed on the second photosensitive drum and forms a toner image, a second transferring unit that transfers the toner image on the second photosensitive drum to a transfer object, and a second static elimination unit that has a light-emitting element and irradiates the second photosensitive drum with light emitted from the light-emitting element to eliminate static electricity from the surface of the second photosensitive drum, A power supply unit that applies a charging voltage to the first charging member and the second charging member, Control means for controlling the first static elimination unit and the second static elimination unit, Equipped with, The first image forming unit is positioned upstream of the second image forming unit in the direction of movement of the object to be transferred. The power supply unit is configured such that when the charging voltage is applied to the first charging member, the charging voltage is also applied to the second charging member. The control means controls the amount of light emitted by the light-emitting element of the first static elimination unit of the first image forming unit to change from a first light amount to a second light amount which is less than the first light amount, and then to a third light amount which is less than the second light amount. The image forming apparatus is characterized in that the control means does not control the amount of light emitted by the light-emitting element of the second static elimination unit of the second image forming unit to change in steps.

8. The image forming apparatus according to claim 7, characterized in that the third light intensity is the light intensity when the light-emitting element is turned off.

9. The image forming apparatus according to claim 8, characterized in that when the control means finishes removing static electricity from the first photosensitive drum of the first image forming unit, it gradually reduces the amount of light emitted by the light-emitting element of the first static electricity removal unit from a target amount to the off state.

10. The image forming apparatus according to claim 9, characterized in that when the control means changes the amount of light emitted by the light-emitting element of the first static elimination unit of the first image forming unit in steps, it reduces the amount of light emitted by the light-emitting element by a constant amount.

11. The image forming apparatus according to claim 9, characterized in that the control means reduces the amount of light emitted from the light-emitting element more gradually in a low-light-intensity region where the amount of light emitted from the light-emitting element is lower than a predetermined light-intensity region compared to a high-light-intensity region where the amount of light emitted from the light-emitting element is higher than a predetermined light-intensity region.

12. The first image forming unit includes a plurality of image forming units, The image forming apparatus according to claim 11, characterized in that the timing at which the static discharge by the first static discharge unit is terminated in each of the plurality of image forming units differs for each of the image forming units.

13. The first image forming unit includes a plurality of image forming units, The image forming apparatus according to claim 11, characterized in that the control means terminates the static discharge by the first static discharge unit in the plurality of image forming units according to the timing at which image forming in each of the image forming units is terminated.

14. The first static elimination unit is positioned downstream of the first transfer unit in the rotational direction of the first photosensitive drum, and upstream of the first charging member in the rotational direction of the first photosensitive drum. The image forming apparatus according to any one of claims 1 to 13, characterized in that the second static elimination unit is located downstream of the second photosensitive drum in the rotational direction of the second transfer unit, and upstream of the second charging member in the rotational direction of the second photosensitive drum.

15. The image forming apparatus according to any one of claims 1 to 13, characterized in that the charging voltage applied to the first charging member and the charging voltage applied to the second charging member have the same output value.

Images