Image forming apparatus

Abstract

To prevent the occurrence of fogging in a non-image forming period during which a toner image is not formed on a photoconductor drum.SOLUTION: An image forming apparatus comprises: a photoconductor drum 1; a laser beam source 200 that emits a laser beam; an exposure device 3 that has a BD 206 detecting the laser beam, and exposes a surface of the photoconductor drum 1; a developing voltage power supply 50 that applies developing voltage to a developing roller 42 supplying toner to the photoconductor drum 1; and an engine control unit 205. In a non-image forming period during which a toner image is not formed on the surface of the photoconductor drum 1, the engine control unit 205 controls the exposure device 3 and the developing voltage power supply 50; when an area of the photoconductor drum 1 whose surface potential is changed to -500 V by the exposure device 3 passes through a developing unit, applies a developing voltage of -350 V; when an area of the photoconductor drum 1 whose surface potential is -450 V passes through the developing unit, applies a developing voltage of -300 V, and controls the developing voltage power supply 50 to maintain the potential difference between the surface potential of the photoconductor drum 1 and the developing voltage to be 150 V.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] This invention relates to an image forming apparatus using an electrophotographic method, such as a laser printer, copier, or facsimile. [Background technology]

[0002] Conventionally, in image forming apparatuses using the electrophotographic method, image forming apparatuses equipped with a contact-developing type developing apparatus are known. The contact-developing method is a method in which the developing roller and the photosensitive drum are brought into contact during image formation, and the electrostatic latent image formed on the photosensitive drum is developed to form the image. In the contact-developing method, there are configurations in which the developing roller is separated from the photosensitive drum during the period from the start of the pre-rotation before image formation until the start of the image formation operation, and during the period from the end of the image formation operation until the end of the post-rotation after image formation. Providing a contact and separation mechanism between the developing roller and the photosensitive drum makes the image forming apparatus complex and large. For this reason, in recent years, in order to simplify and miniaturize the mechanism of the image forming apparatus, a configuration without a contact and separation mechanism between the developing roller and the photosensitive drum has been adopted in the developing apparatus.

[0003] In image forming apparatuses that lack a contact / separation mechanism between the developing roller and the photosensitive drum in the developing section, where the photosensitive drum faces the developing roller, the developing roller and the photosensitive drum are always in contact. Therefore, in image forming apparatuses that lack a contact / separation mechanism between the developing roller and the photosensitive drum, toner transfer from the developing roller to the non-image area on the photosensitive drum where an electrostatic latent image has not been formed (hereinafter referred to as "fogging") is more likely to occur compared to image forming apparatuses that have such a mechanism. For this reason, in image forming apparatuses that lack a contact / separation mechanism between the developing roller and the photosensitive drum in the developing section, various technologies are required to prevent the occurrence of fogging.

[0004] For example, Patent Document 1 proposes a configuration in an image forming apparatus that does not have a contact / separation mechanism between the developing roller and the photosensitive drum in the developing unit, in which the surface potential of the photosensitive drum is reduced to 0V after the image forming operation is completed. That is, the surface potential of the photosensitive drum is set to 0V when the photosensitive drum is rotating, and then a positive voltage is applied to the developing roller. As a result, the negatively charged toner on the developing roller is electrically held on the developing roller without being transferred to the photosensitive drum, and even in a configuration that does not have a contact / separation mechanism between the developing roller and the photosensitive drum in the developing unit, it is possible to prevent fogging at the start of the previous rotation operation.

[0005] Furthermore, for example, Patent Document 1 proposes a technique for maintaining a constant potential difference between the surface potential of the photosensitive drum and the development voltage by controlling the charging voltage, development voltage, and laser light intensity during the pre-rotation operation before image formation and the post-rotation operation after image formation. The potential difference between the surface potential of the photosensitive drum and the development voltage is called the back contrast (hereinafter referred to as Vback). Specifically, the potential of the photosensitive drum is set to 0V during the pre-rotation operation before image formation, and then, while maintaining Vback at a constant level, the surface potential of the photosensitive drum is raised to the dark area potential for image formation (hereinafter referred to as Vd). On the other hand, during the post-rotation operation after image formation is completed, the surface potential of the photosensitive drum is lowered from Vd to 0V while maintaining Vback at a constant level. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2020-160361 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, in the conventional control described above, during image formation, the surface potential of the image-forming area on the photosensitive drum where the image is formed is rapidly lowered from Vd to the bright area potential for image formation (hereinafter referred to as V1). Then, during the post-rotation operation, which is performed when no toner image is formed on the photosensitive drum after the image formation operation is completed, the surface potential of the image-forming area on the photosensitive drum where the image was formed is rapidly lowered from V1 to approximately 0V. At this time, because the potential difference that is lowered at once in the surface potential of the photosensitive drum is large, it is difficult to adjust to maintain a constant Vback, and the problem of fogging occurs.

[0008] Furthermore, in order to maintain a constant Vback, it is necessary to use a low laser light intensity to expose the surface of the photosensitive drum. However, if the laser light intensity is low, the Beam Detector (BD) installed in the exposure device that emits the laser light, which detects the laser light and outputs a BD signal, may not be able to detect the laser light, and the BD signal may not be output. In the exposure device, the scanner motor that rotates the rotating multifaceted mirror that deflects the laser light to irradiate the photosensitive drum with the laser light is controlled based on the BD signal. Therefore, if the BD signal is not output correctly, the scanner motor cannot be controlled to the target rotation speed, and as a result, the amount of laser light exposing the photosensitive drum becomes unstable, making it impossible to control the desired surface potential, which can cause fogging.

[0009] This invention was made under such circumstances and aims to suppress the occurrence of fogging during non-image formation, when a toner image is not formed on the photosensitive drum. [Means for solving the problem]

[0010] To solve the above-mentioned problems, the present invention has the following configuration.

[0011] (1) The exposure means comprises a rotatable photoreceptor, a charging member for charging the surface of the photoreceptor, and an exposure means for exposing the surface of the photoreceptor charged by the charging member, the exposure means comprising a light source that emits laser light for exposing the surface of the photoreceptor, and a detection unit that detects the laser light and outputs a detection signal; a developing unit facing the photoreceptor that supplies toner of normal polarity to the photoreceptor to form a toner image; a charging voltage power supply for applying a charging voltage to the charging member; a developing voltage power supply for applying a developing voltage to the developing unit; and a control means for controlling the exposure means, the charging voltage power supply, and the developing voltage power supply to switch the laser light emitted from the light source, the charging voltage, and the developing voltage, wherein the control means, in the non-image formation stage when the toner image is not formed on the surface of the photoreceptor, emits laser light of an amount detectable by the detection unit from the light source to form the photoreceptor Image forming apparatus characterized in that, when the surface of the photoreceptor is exposed to light, thereby changing the surface potential of the photoreceptor, the exposure means and the developing voltage power supply are controlled so that the potential difference formed between the region of the photoreceptor and the developing voltage is maintained at a predetermined value, and when the exposure means changes the surface potential of the photoreceptor formed in the region of the photoreceptor from a first surface potential to a second surface potential which is opposite in polarity to the normal polarity than the first surface potential, the developing voltage power supply is controlled so that the developing voltage is changed by a predetermined voltage range by applying a first developing voltage when the region of the photoreceptor where the first surface potential is formed passes through the developing unit, and applying a second developing voltage which is opposite in polarity to the first developing voltage when the region of the photoreceptor where the second surface potential is formed passes through the developing unit, and the developing voltage power supply is controlled so that the first surface potential is on the normal polarity side than the first developing voltage. [Effects of the Invention]

[0012] According to the present invention, it is possible to suppress the occurrence of fogging during non-image formation, when a toner image is not formed on the photosensitive drum. [Brief explanation of the drawing]

[0013] [Figure 1]Cross-sectional view showing the schematic configuration of the image forming apparatuses of Examples 1 to 6 [Figure 2] Cross-sectional view showing the schematic configuration of the developing apparatuses of Examples 1 to 6 [Figure 3] Diagram for explaining the configuration of the exposure apparatus of Examples 1 to 6 [Figure 4] Timing chart for explaining the print sequence of Example 1 [Figure 5] Graph showing the relationship between the laser exposure amount and the surface potential of the photosensitive drum in Example 1 [Figure 6] Diagram for explaining the relationship between the spot diameter of the laser beam and the scanning distance of the laser beam in Example 1 [Figure 7] Timing chart for explaining the print sequence of Comparative Example 2 for comparison with Example 1 [Figure 8] Diagram for explaining the relationship between the surface potential of the photosensitive drum 1 and the developing voltage in Example 1 and Comparative Example 2 [Figure 9] Diagram for explaining the relationship between the spot diameter of the laser beam and the distance in the sub-scanning direction of the laser beam in Examples 2 and 3 [Figure 10] Timing chart for explaining the print sequence of Example 4 [Figure 11] Graph showing the relationship between the laser exposure amount and the surface potential of the photosensitive drum in Example 4 [Figure 12] Timing chart for explaining other print sequences of Example 4 [Figure 13] Timing chart for explaining the print sequence of Example 5 [Figure 14] Timing chart for explaining the print sequence of Example 6 [Figure 15] Timing chart for explaining the print sequences of other examples

BEST MODE FOR CARRYING OUT THE INVENTION

[0014] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

EXAMPLE

[0015] [Image forming apparatus] Figure 1 is a schematic cross-sectional view showing the general configuration of an image forming apparatus M to which the present invention is applied. The image forming apparatus M in this embodiment is an electrophotographic monochrome laser printer. The photosensitive drum 1, which is the photoreceptor, is rotatably supported on the main body of the image forming apparatus M and is driven to rotate at a process speed (peripheral speed) of 250 mm / second in the direction of the arrow R1 shown in the figure (clockwise direction) by a main motor (not shown), which is the drive source. Around the photosensitive drum 1, along the direction of rotation of the photosensitive drum 1, are a charging roller 2, an exposure device 3, a developing device 4, and a cleaning blade 101. The charging roller 2, which is the charging member, is connected to a charging voltage power supply 52 that applies a high voltage to the charging roller 2, and charges the surface of the photosensitive drum 1 to a uniform potential. The exposure device 3, which is the exposure means, irradiates the surface of the photosensitive drum 1 with laser light L according to the image data and forms an electrostatic latent image on the surface of the photosensitive drum 1. The developing device 4, which is a developing means, has a developing roller 42 that comes into contact with the photosensitive drum 1. By depositing toner from the developing roller 42 onto the electrostatic latent image formed on the photosensitive drum 1 (on the photoreceptor), a visible toner image is formed. The toner image formed on the photosensitive drum 1 then moves toward the transfer nip portion where the photosensitive drum 1 comes into contact with the transfer roller 5, which is a transfer means.

[0016] Furthermore, a cassette 7 containing the recording material, paper P, is located at the bottom of the image forming apparatus M. During image formation, the paper P is fed one sheet at a time from the cassette 7 by the feed roller 8, and the paper P fed by the feed roller 8 is transported to the transfer roller 5 by the transport roller 9. A TOP sensor 150 for detecting the paper P is located in the transport path between the transport roller 9 and the transfer roller 5. When the TOP sensor 150 detects the leading edge of the transported paper P, it outputs a TOP signal 210 (see Figure 3) to the engine control unit 205 (see Figure 3), which will be described later. When the TOP signal 210 is input to the engine control unit 205, it starts the image formation operation on the photosensitive drum 1 described above.

[0017] The paper P fed from the cassette 7 is transported to the transfer nip section where the photosensitive drum 1 and the transfer roller 5 come into contact. In the transfer nip section, a high voltage is applied to the transfer roller 5 from a high-voltage power supply (not shown), and the toner image formed on the photosensitive drum 1 is transferred to the paper P. Toner that remains on the photosensitive drum 1 without being transferred to the paper P is removed by the cleaning blade 101, and the removed toner is collected in the waste toner container 102.

[0018] The paper P that has passed through the transfer nip section is transported to the fuser unit 12. In the fuser unit 12, the toner image transferred to the paper P is heated and pressurized to fix it to the paper P. The paper P with the toner image fixed by the fuser unit 12 is discharged by the discharge roller 15 onto the discharge tray 16 located at the top of the image forming apparatus M and stacked there.

[0019] [Developing equipment] Figure 2 is a schematic cross-sectional view showing the general configuration of the developing apparatus 4 shown in Figure 1. As shown in Figure 2, the developing apparatus 4 has a developing roller 42 which is a developing member, a developing agent supply roller 43 which contacts the developing roller 42 to supply the developer, and a developing blade 44 which is a developer regulating member. A stirring rod 45, which is a stirring member for stirring the toner (developer), is provided approximately in the center of the toner container 4a of the developing apparatus 4. A developing voltage power supply 50 that applies a developing voltage to the developing roller 42 is connected to the developing roller 42, and a supply voltage power supply 51 is connected to the developer supply roller 43. Furthermore, the developing apparatus 4 in this embodiment is not provided with a contact / separation mechanism that switches the contact / separation state between the photosensitive drum 1 and the developing roller 42.

[0020] Next, the operation of the developing apparatus 4 in this embodiment will be described. In the developing apparatus 4, as the stirring rod 45 rotates in the direction of the arrow indicated by R2 in the figure (clockwise direction), toner (not shown) is temporarily stored in a region T near the contact area between the developing roller 42 and the developer supply roller 43. The toner stored in region T is supplied to the developing roller 42 by the developer supply roller 43, which rotates in the direction of the arrow indicated by R3 in the figure (counterclockwise direction), so that it is supported on the developing roller 42. The toner supplied to the developing roller 42 is thinned (coated) to an appropriate layer thickness by the developing blade 44 (regulating member) as the developing roller 42 rotates in the direction of the arrow indicated by R4 in the figure (counterclockwise direction). A developing blade voltage power supply 53 that applies a developing blade voltage to the developing blade 44 is connected to the developing blade 44. In this embodiment, a voltage is applied between the developing roller 42 and the developer supply roller 43 so that a potential difference of 100V is generated so that toner with the correct polarity is supplied to the developing roller 42 side. Furthermore, a voltage of 100V is applied between the developing roller 42 and the developing blade 44 to generate a potential difference, so that toner with the correct polarity does not adhere to the developing blade 44 and an electric charge is transferred to the toner carried on the developing roller 42.

[0021] At this time, the toner supplied to the developing roller 42 is frictionally charged to a negative polarity by rubbing against the surface of the developing blade 44. The toner coated on the developing roller 42 is then transported to the developing nip section (not shown) (also called the developing section) where the developing roller 42 contacts the photosensitive drum 1, as the developing roller 42 rotates in the direction of the arrow indicated by R4 in the figure (counterclockwise). In the developing nip section, a portion of the toner coated on the developing roller 42 is transferred to the photosensitive drum 1 by the electric field formed by the potential of the electrostatic latent image formed on the photosensitive drum 1 by the exposure device 3 and the developing voltage applied to the developing roller 42 from the developing voltage power supply 50. In this way, the electrostatic latent image formed on the photosensitive drum 1 is developed (visualized) as a toner image. Toner that does not transfer to the photosensitive drum 1 in the developing nip section and remains on the developing roller 42 is scraped off by the developer supply roller 43 at the contact point between the developing roller 42 and the developer supply roller 43, and toner stored in region T is newly supplied to the developing roller 42.

[0022] In the developing device 4, a developing voltage is applied to the developing roller 42, which carries the toner, from the developing voltage power supply 50. Even in non-image areas on the photosensitive drum 1 where an electrostatic latent image has not been formed, it is necessary to apply a developing voltage to the developing roller 42 in order to suppress fogging, which is caused by toner transfer from the developing roller 42. Here, "fogging" refers to the phenomenon in which toner (fogging toner) adheres to non-image areas (non-image regions) on the surface of the photosensitive drum 1 where an electrostatic latent image has not been formed and no image formation occurs. Fogging is affected by the back contrast (hereinafter referred to as Vback), which is the potential difference between the surface potential of the developing section of the photosensitive drum 1 facing the developing roller 42 and the developing voltage of the developing roller 42. Therefore, in order to suppress the occurrence of fogging, it is necessary to control Vback to an appropriate potential difference.

[0023] For example, when Vback is small, the electric field that keeps the negatively charged toner (which is the normal polarity in this embodiment) on the developing roller 42 weakens, causing the toner to transfer to the photosensitive drum 1 and resulting in fouling of the toner on the non-image areas of the photosensitive drum 1. On the other hand, when Vback is large, the potential difference between the photosensitive drum 1 and the developing roller 42 is large. Therefore, while there is a strong electrical attraction to the negatively charged toner (which is the normal polarity) in the direction of the developing roller 42, toner charged with a positive polarity (which is the opposite polarity) adheres to the non-image-forming areas of the photosensitive drum 1, resulting in fouling.

[0024] If toner fogging occurs during image formation, toner will adhere to areas other than where the image is intended to be formed, causing discoloration in the white areas (non-image areas) of the paper P, resulting in a defective image. On the other hand, if fogging occurs at times other than image formation, the fogging toner is scraped off by the cleaning blade 101 and collected in the waste toner container 102, resulting in wasted toner. Therefore, in this embodiment, Vback is set to a predetermined value of 150V (150 volts) to minimize the amount of fogging toner.

[0025] [Synthesis equipment] Figure 3 illustrates the configuration of the exposure apparatus 3 and the control unit that controls the exposure apparatus 3. The exposure apparatus 3 is controlled by an engine control unit 205 and an image control unit 212. In this embodiment, the engine control unit 205 and the image control unit 212 are each provided on different control boards.

[0026] The exposure apparatus 3 includes a laser light source 200, a collimator lens 203, a rotating polyhedron mirror 204, a photodiode (PD) 202, a BD (Beam Detector) 206, an f-θ lens 217, and a folding mirror 218. The exposure apparatus 3 also includes a laser control unit 201 that controls the emission of light from the laser light source 200 according to a video signal 214 output from an image control unit 212. The laser light source 200 has a light-emitting element and emits laser light in two directions. Of the two laser beams emitted from the laser light source 200, one laser beam is incident on the photodiode 202. The photodiode 202 converts the incident laser light into an electrical signal and outputs it to the laser control unit 201 as a PD signal 215. Based on the input PD signal 215, the laser control unit 201 performs auto power control (APC) of the laser light source 200 so that the laser light emitted from the laser light source 200 is at a predetermined light intensity.

[0027] The other laser beam emitted from the laser light source 200 is incident on the rotating polyhedron mirror 204 via the collimator lens 203. The rotating polyhedron mirror 204 has multiple reflective surfaces and is driven to rotate in the direction of the arrow in the figure (counterclockwise) by a scanner motor (not shown). In this embodiment, the rotating polyhedron mirror 204 has four reflective surfaces. The scanner motor rotates the rotating polyhedron mirror 204 in accordance with the drive signal 220 output from the engine control unit 205. The laser beam incident on the rotating polyhedron mirror 204 is deflected in the direction of the photosensitive drum 1 by the reflective surfaces of the rotating polyhedron mirror 204. As the rotating polyhedron mirror 204 is driven to rotate by the scanner motor, the deflection angle of the laser beam changes, and the deflected laser beam scans the photosensitive drum 1 in the direction of the arrow in the figure. Furthermore, the laser beam's path is corrected by the f-θ lens 217 so that it scans the photosensitive drum 1 at a constant speed, and it is then projected onto the photosensitive drum 1 via the folding mirror 218.

[0028] Furthermore, the detection unit BD206 in this embodiment is positioned to detect the laser beam before it begins scanning the photosensitive drum 1. The laser beam, deflected by the rotating polyhedron mirror 204, is received by BD206 before it scans the photosensitive drum 1. When BD206 detects the laser beam, it outputs a detection signal, the BD signal 207, to the engine control unit 205. The BD signal 207 is, for example, a negative logic signal, and is at the first level (low level) while BD206 is detecting the laser beam, and at the second level (high level) when BD206 is not detecting the laser beam.

[0029] The engine control unit 205 calculates the period of the BD signal 207 output from the BD 206 and controls the rotation of the scanner motor by outputting a drive signal 220 so that the rotation period of the rotating polyhedron 204 indicated by the period of the BD signal 207 becomes a predetermined period. The engine control unit 205 determines that the rotation period of the rotating polyhedron 204, which is driven by the scanner motor, is stable at a predetermined period when the period of the output of the BD signal 207 becomes a predetermined period. In other words, the engine control unit 205 performs feedback control by adjusting the rotation of the scanner motor with the drive signal 220 based on the BD signal 207 so that the rotation of the rotating polyhedron 204 stabilizes at a predetermined period.

[0030] [Print Sequence] Next, the print sequence, which is the control sequence for forming an image on the paper P in this embodiment, will be described. Figure 4 is a timing chart illustrating the relationship between various high voltages (developing voltage, charging voltage), the main motor that drives each roller, the surface potential of the photosensitive drum 1, and the amount of laser light irradiated from the exposure device 3 that exposes the photosensitive drum 1 in the print sequence. In Figure 4, the horizontal axis represents time, and T1 to T8 represent timing (time).

[0031] As shown in Figure 4, the print sequence consists of a "pre-rotation sequence," an "image formation sequence," and a "post-rotation sequence." In the "pre-rotation sequence," control is performed to raise the surface potential of the photosensitive drum 1 from 0V to the dark area potential Vd, which is the surface potential for image formation (hereinafter referred to as pre-rotation). In the "image formation sequence," after the surface potential of the photosensitive drum 1 is raised to the dark area potential Vd, laser exposure corresponding to the image data is performed on a part of the surface of the photosensitive drum 1. An electrostatic latent image is formed on the surface of the photosensitive drum 1 that has been laser-exposed, and the surface potential of the photosensitive drum 1 in the part where the electrostatic latent image is formed decreases to the bright area potential Vl, which is the surface potential of the photosensitive drum 1 for image formation. In the "post-rotation sequence," control is performed to lower the surface potential of the photosensitive drum 1 from the dark area potential Vd to 0V after the image formation sequence is completed (hereinafter referred to as post-rotation). In this embodiment, the dark area potential Vd is -500V and the bright area potential Vl is -250V.

[0032] (Previous rotation sequence) Next, the control in the previous rotation sequence will be explained in detail. When the engine control unit 205 receives a print signal from the host computer (not shown) requesting image formation on the paper P, a development voltage is applied from the development voltage power supply 50 to the development roller 42. As described above, in this embodiment, after the previous image formation is completed, a process is performed to lower the surface potential of the photosensitive drum 1 to 0V during the rotation operation. Therefore, when the main motor that rotates the photosensitive drum 1 is started, the development voltage power supply 50 applies a positive development voltage value of +150V to the development roller 42 in order to maintain Vback at 150V. Note that high-voltage power supplies (for example, the development voltage power supply 50 and the charging voltage power supply 52) require time for the output voltage to reach the target voltage.

[0033] At timing T1, when the developing voltage value applied to the developing roller 42 by the developing voltage power supply 50 rises to +150V and the developing voltage rise is complete, the main motor is started (ON), and the photosensitive drum 1 is rotated by the main motor. The rotation speed of the main motor is controlled to reach a predetermined rotation speed based on the BD signal 207. It also takes time for the rotation speed of the main motor to rise to the target rotation speed and for the main motor startup process to be completed.

[0034] At timing T2, when the main motor startup is complete, the charging voltage power supply 52 applies a first charging voltage S1 (-550V) to the charging roller 2. As shown in Figure 4, following the first charging voltage S1, the charging voltage is increased in a stepwise manner from S1 to S10 with a voltage fluctuation range of 50V (50 volts) every 30ms (milliseconds). In this embodiment, at the start of the first startup control (timing T2), the first charging voltage S1 (-550V) is applied to the charging roller 2. Then, at the start of the 10th startup control (timing T4), the 10th charging voltage S10 (-1000V) is applied to the charging roller 2. In this way, the charging voltage startup control is performed so that the fluctuation range of the charging voltage per step is 50V.

[0035] Next, the surface potential of the exposure area of ​​the photosensitive drum 1, where the laser light is irradiated, will be described. As described above, the surface potential of the photosensitive drum 1 is maintained at 0V until timing T2, when the application of the first charging voltage S1 to the charging roller 2 begins. After the charging voltage rise control is started and the charging voltage is applied to the charging roller 2, the surface potential of the photosensitive drum 1 increases in steps from potential V1 to V10, corresponding to the charging voltages S1 to S10. In this embodiment, the surface potential of the photosensitive drum 1 becomes potential V1 (-50V) at the end of the first charging voltage rise control and potential V10 (-500V) at the end of the tenth charging voltage rise control. In this way, the rise control of the surface potential of the photosensitive drum 1 is performed so that the fluctuation range of the surface potential per step is 50V. In this embodiment, the surface potential of the photosensitive drum 1 is formed by discharge based on Paschen's law that accompanies the application of the charging voltage.

[0036] On the other hand, the developing voltage is controlled so that the Vback is maintained at 150V at the developing nip position where the photosensitive drum 1 and the developing roller 42 come into contact. In this embodiment, at timing T3 when the developing voltage rise control is started, the developing voltage is voltage D1 (+100V), and at the end of the rise control, it becomes voltage D10 (-350V). Therefore, the developing voltage rise control is performed so that the voltage fluctuation range per step is 50V. In this way, by controlling the developing voltage, charging voltage, and surface potential of the photosensitive drum in a stepwise manner, the Vback can be maintained at 150V. As a result, the occurrence of fogging, where toner on the developing roller 42 is transferred onto the photosensitive drum 1, can be suppressed.

[0037] (Image formation sequence) In this embodiment, as described above, the surface potential of the photosensitive drum 1 is set to the optimal potential Vd (-500V) for image formation during image formation. Therefore, during image formation, a charging voltage of -1000V is applied to the charging roller 2 from the charging voltage power supply 52. ​​On the other hand, a developing voltage of -350V is applied to the developing roller 42 from the developing voltage power supply 50. As a result, in this embodiment, the potential difference Vback, which is the potential difference between the developing voltage and the surface potential of the photosensitive drum 1, is maintained at 150V, the same as during the previous rotation sequence.

[0038] During image formation, the engine control unit 205 calculates the rotation speed of the rotating polyhedron mirror 204 based on the period of the BD signal 207 output from the BD 206. Then, based on the calculated rotation speed of the rotating polyhedron mirror 204, the engine control unit 205 outputs a drive signal 220 that controls the rotation of the scanner motor driving the rotating polyhedron mirror 204 so that the rotation speed of the rotating polyhedron mirror 204 becomes a predetermined rotation speed. In this way, the engine control unit 205 provides feedback control of the rotation speed of the rotating polyhedron mirror 204 based on the BD signal 207.

[0039] Meanwhile, when the engine control unit 205 receives a print signal from the host computer (not shown) requesting image formation on the paper P, it rotates the feed roller 8 after a predetermined time has elapsed, and starts the paper feeding operation. The paper P fed from the cassette 7 by the feed roller 8 is transported along the transport path to the transport roller 9, and the transport roller 9 transports the paper P to the transfer roller 5. The TOP sensor 150, which is installed in the transport path between the transport roller 9 and the transfer roller 5 and detects the transported paper P, outputs a TOP signal 210 to the engine control unit 205 when it detects the leading edge of the paper P in the transport direction. Furthermore, the TOP signal 210 is transmitted to the image control unit 212 via the engine control unit 205.

[0040] The image control unit 212 acquires image data for image formation transmitted from the host computer (not shown), performs appropriate image processing, and then converts the image data into a video signal 214. The image control unit 212 then synchronizes the video signal 214 with the TOP signal 210 and BD signal 207 described above and transmits it to the laser control unit 201 of the exposure apparatus 3. By transmitting the video signal 214 to the laser control unit 201 in synchronization with the TOP signal 210, the image control unit 212 synchronizes the position of the formed image with the position of the paper P in the sub-scanning direction, which is the transport direction of the paper P. On the other hand, by transmitting the video signal 214 to the laser control unit 201 in synchronization with the BD signal 207, the image control unit 212 synchronizes the position of the formed image with the position of the paper P in the main scanning direction, which is perpendicular to the transport direction of the paper P.

[0041] Then, the laser control unit 201 of the exposure apparatus 3 controls the laser light source 200 to be on or off in accordance with the video signal 214 transmitted from the image control unit 212. As described above, the surface potential of the photosensitive drum 1 is controlled to -500V until it is exposed to laser light from the laser light source 200, but after exposure to laser light, the surface potential of the photosensitive drum 1 in the area where an electrostatic latent image is formed becomes -250V. Then, as the area of ​​the photosensitive drum 1 where the electrostatic latent image is formed passes through the developing nip, which is the contact area with the developing roller 42, toner is electrically transferred from the surface of the developing roller 42 to the surface of the photosensitive drum 1. As a result, toner adheres to the electrostatic latent image on the photosensitive drum 1, forming a toner image.

[0042] (Post-rotation sequence) Next, the control in the post-rotation sequence will be described. The present invention is characterized by its post-rotation sequence. As shown in Figure 4, the post-rotation sequence is started from timing T5, which is the non-image-forming time when image formation on the paper P in the image formation sequence described above is completed. At timing T5, the charging voltage power supply 52 stops applying the 10th charging voltage S10 (-1000V) to the charging roller 2, and the charging voltage is set to a predetermined potential of 0V. Then, the laser control unit 201 of the exposure apparatus 3 controls the laser light source 200 to start emitting laser light and performs a first exposure in which the surface of the photosensitive drum 1 is exposed with a first exposure amount P1. By performing the first exposure, the surface potential of the exposure area of ​​the photosensitive drum 1 irradiated with laser light is lowered by a predetermined voltage range of 50V from the first surface potential V10 (-500V) to the second surface potential V11 (-450V).

[0043] Once the first exposure is complete, a second exposure is performed, exposing the surface of the photosensitive drum 1 with a second exposure amount P2 that is greater than the first exposure amount P1. This causes the surface potential of the photosensitive drum 1 to drop by 50V, from V11 (-450V) to V12 (-400V). Similarly, a stepwise reduction in the surface potential of the photosensitive drum 1 is controlled for a third exposure with a third exposure amount P3, a fourth exposure with a fourth exposure amount P4, a fifth exposure with a fifth exposure amount P5, and a sixth exposure with a sixth exposure amount P6. As a result, the surface potential of the photosensitive drum 1 is reduced by 50V each time, to V13 (-350V), V14 (-300V), V15 (-250V), and V16 (-200V).

[0044] Furthermore, the surface potential of the photosensitive drum 1 is controlled to drop down by the seventh exposure with the seventh exposure amount P7, the eighth exposure with the eighth exposure amount P8, the ninth exposure with the ninth exposure amount P9, and the tenth exposure with the tenth exposure amount P10. As a result, the surface potential of the photosensitive drum 1 is lowered by 50V increments from V17 (-150V), V18 (-100V), V19 (-50V), to V20 (0V), until it drops to the predetermined surface potential of 0V. The tenth exposure is performed for the duration of one rotation of the photosensitive drum, from timing T6 to timing T7. That is, the tenth exposure, which is the final exposure, continues until the surface potential of the photosensitive drum 1 reaches V20 (0V) around its entire circumference. Here, the tenth exposure continues until the photosensitive drum 1 completes one rotation, but it may continue beyond one rotation.

[0045] On the other hand, the developing voltage is controlled to decrease in a stepped manner, similar to the previous rotation sequence, so that Vback, which is the potential difference between the surface potential of the photosensitive drum 1 and the developing voltage, is maintained at 150V at the developing nip position where the photosensitive drum 1 faces the developing roller 42. In other words, in the developing voltage decrease control, a first developing voltage D11 (-300V), a second developing voltage D12 (-250V), D13 (-200V), ..., and a predetermined developing voltage D20 (+150V) are applied to the developing roller 42. In this way, the developing voltage decrease control is performed so that the voltage fluctuation range per step is 50V (50 volts).

[0046] The main motor is then stopped (turned off) at timing T7, when the 10th exposure, which is the final exposure, is completed. The developing voltage power supply 50 stops applying the developing voltage D20 (+150V) to the developing roller 42 at timing T8), when the rotation of the main motor due to inertia has completely stopped.

[0047] [Relationship between surface potential of photosensitive drum and laser exposure dose] Figure 5 is a graph showing the relationship between the surface potential of the photosensitive drum 1 in the post-rotation sequence shown in Figure 4 and the amount of laser light (drum surface light intensity) irradiated onto the photosensitive drum 1 from the exposure apparatus 3. In Figure 5, the vertical axis represents the surface potential of the photosensitive drum 1 (unit: V), and the horizontal axis represents the amount of laser light irradiated onto the surface of the photosensitive drum 1 (indicated as drum surface light intensity in the figure) (unit: μJ / cm²). 2 The figure shows the following. In the figure, P1 to P10 represent the amount of laser light irradiated onto the photosensitive drum 1 in the post-rotation sequence of Figure 4, and V11 to V20 represent the surface potential of the photosensitive drum 1, which ranges from -450V to 0V.

[0048] [Effects of this embodiment] The present invention is characterized by its control of the surface potential drop of the photosensitive drum during the post-rotation sequence. Therefore, the effects of this embodiment will be explained by comparing it with Comparative Examples 1 and 2, which have different control in the post-rotation sequence.

[0049] In Comparative Example 1, the method of controlling the exposure of the photosensitive drum 1 by laser light in the post-rotation sequence is the same as in this embodiment, with a stepped drop-down control that lowers the surface potential of the photosensitive drum 1 by 50V at each step. On the other hand, the method of controlling the exposure of the photosensitive drum 1 by laser light in the post-rotation sequence of Comparative Example 2 uses a different drop-down control than in this embodiment. Specifically, in the post-rotation sequence of Comparative Example 2, in the first exposure, the surface potential of the photosensitive drum 1 is lowered by 300V from -500V to -200V, and the development voltage is lowered by 300V from -350V to -50V, which is different from this embodiment. In addition, Comparative Examples 1 and 2 differ from this embodiment in terms of the exposure amount in the first exposure, the laser light emission pattern, laser brightness, etc.

[0050] Table 1 summarizes the differences in specifications between this embodiment and Comparative Examples 1 and 2 during the first exposure of the post-rotation sequence. Table 1 shows the laser exposure amount (P1), laser emission pattern (ON / OFF), laser brightness, rotation speed accuracy of the rotating polyface mirror, and toner consumption when printing 10,000 sheets of paper P during the first exposure of this embodiment and Comparative Examples 1 and 2.

[0051] [Table 1]

[0052] (Laser light intensity) The laser light intensity (P1) in the first exposure is the amount of laser light initially irradiated onto the photosensitive drum 1 from the exposure apparatus 3 in the post-rotation sequence, and in both this embodiment and Comparative Example 1, it is 0.02 μJ / cm². 2 On the other hand, the laser exposure dose in Comparative Example 2 was 0.18 μJ / cm². 2 This is larger than in Comparative Example 1.

[0053] (Laser emission pattern) The laser emission pattern (ON / OFF) indicates the ratio of the ON time (when the laser light is emitted) to the OFF time (when the laser light is not emitted) during the first exposure, in which the surface of the photosensitive drum 1 is irradiated with laser light. As shown in Table 1, in both Comparative Examples 1 and 2, the ON time (when the laser light is emitted) is 100% during the first exposure. On the other hand, in this embodiment, the ON time (when the laser light is emitted) is 50% and the OFF time (when the laser light is not emitted) is 50%, so the laser emission pattern is different from that of Comparative Examples 1 and 2.

[0054] (Laser beam spot diameter and laser beam scanning distance) Figure 6 illustrates the relationship between the spot diameter of the laser beam emitted from the laser light source 200 of the exposure apparatus 3 in this embodiment and the scanning distance of the laser beam. In Figures 6(a) and (b), the vertical lines indicate the timing of laser beam emission from the laser light source 200, and the circular shapes indicate the spot diameter of the laser beam irradiated onto the photosensitive drum 1. The laser light source 200 is lit and laser beam is emitted at the timings indicated by the vertical lines, but the laser light source 200 is off and no laser beam is emitted during the periods between the vertical lines. The scanning of the laser beam on the photosensitive drum 1 is performed from left to right in the figure.

[0055] Figure 6(a) shows the case where the spot diameter in the scanning direction of the laser beam (main scanning direction) is larger than the scanning distance when the laser beam is off. In this case, because the spot diameter in the scanning direction of the laser beam (main scanning direction) is larger than the scanning distance when the laser beam is off, the areas irradiated by the laser beam when the laser beam is on overlap, and as a result, the surface of the photosensitive drum 1 is uniformly exposed. On the other hand, Figure 6(b) shows the case where the spot diameter in the scanning direction of the laser beam (main scanning direction) is smaller than the scanning distance when the laser beam is off. In this case, because the spot diameter in the scanning direction of the laser beam (main scanning direction) is smaller than the scanning distance when the laser beam is off, there are areas on the surface of the photosensitive drum 1 that are not irradiated by the laser beam, and the surface of the photosensitive drum 1 is not uniformly exposed. Therefore, in Comparative Examples 1 and 2, where the laser beam is emitted (ON) for 100%, the surface of the photosensitive drum 1 is uniformly exposed and the surface potential is maintained at a constant level. On the other hand, in this embodiment, the time the laser light is on and the time it is off are each 50%, which reduces the amount of exposure the surface of the photosensitive drum 1 receives compared to Comparative Examples 1 and 2. Furthermore, by increasing the ratio of time the laser light is off, the amount of exposure the surface of the photosensitive drum 1 receives can be reduced without reducing the laser brightness. In this embodiment, the time the laser light is on (ON) and the time it is off (OFF) during the fall-down phase are each set to 50%, but this is not limited to this. In this embodiment, the time the laser light is on during solid black image printing during image formation is 100%, and it is effective if the proportion of time the laser light is off during the fall-down phase is larger than that during solid black image printing during image formation. In other words, in this embodiment, the ratio of ON time / OFF time of the laser light is changed between image formation and fall-down. And the proportion of OFF time of the laser light during fall-down is larger than that during image formation. For example, the ratio of ON time to OFF time can be set from 50%:50% to 40%:60%, and the exposure amount should be multiplied by 1.25 (=50% / 40%) so that the amount of exposure received by the surface of the photosensitive drum 1 is the same as when it is 50%.In this embodiment, it is preferable that the time spent with the laser light on and the time spent with the laser light off are each 50%.

[0056] However, as shown in Figure 6(b), if the proportion of time the laser light is off is made too large, the surface potential of the photosensitive drum 1 will not be uniform when viewed at a microscopic level. Therefore, it is desirable that the time interval during which the laser light is off be smaller than the spot diameter. In this embodiment, since the proportion of time the laser light is emitted is 50%, when the laser light is irradiated onto the surface of the photosensitive drum 1, the laser light is turned on and off repeatedly for each pixel. Therefore, the spot diameter in the main scanning direction on the photosensitive drum 1 is larger than the size of 2 pixels in the main scanning direction (in this embodiment, the image resolution in the main scanning direction is 600 dpi, so the size of 2 pixels is approximately 84 μm).

[0057] (Laser brightness) The laser brightness shown in Table 1 represents the brightness of the laser beam at the time the laser beam is incident on BD206 during the first exposure. The laser brightness is 0.5mW in this embodiment, 0.25mW in Comparative Example 1, and 2.25mW in Comparative Example 2, showing that the laser brightness differs for each case. Note that if the laser brightness is lower than a predetermined brightness (0.4mW in this embodiment), BD206 of the exposure apparatus 3 will not be able to correctly detect the laser beam. In this embodiment, when the laser beam is irradiated onto the surface of the photosensitive drum 1 during the first exposure, the time ratio during which the laser beam is emitted is 50%. On the other hand, at the time when the photosensitive drum 1 is not being scanned by the laser beam, and the laser beam is irradiated onto BD206, the time ratio during which the laser beam is emitted is 100%.

[0058] (Rotational accuracy of a rotating polyhedron) The "rotation speed accuracy of the rotating polyhedron mirror" shown in Table 1 will be explained below. As described above, the engine control unit 205 controls the rotation speed of the rotating polyhedron mirror 204, that is, the rotation speed of the scanner motor (not shown) that drives the rotating polyhedron mirror 204, using the drive signal 220 so that the period of the BD signal 207 is a predetermined period. Here, if the brightness of the laser light emitted from the laser light source 200 is lower than a predetermined brightness (0.4 mW in this embodiment), the BD 206 cannot detect the laser light. As a result, the BD signal 207 is not output correctly, so the control of the rotation speed of the rotating polyhedron mirror 204 becomes inaccurate, and the accuracy of the rotation speed of the rotating polyhedron mirror 204 also decreases. The values ​​shown in Table 1 indicate the maximum percentage deviation from the predetermined rotation speed of the rotating polyhedron mirror 204 during the first exposure.

[0059] (Toner consumption) Next, we will explain the "toner consumption" shown in Table 1. The toner consumption shown in Table 1 represents the toner consumption when printing 10,000 sheets of paper P at a print density of 2% in an environment with a temperature of 23°C and a humidity of 50%. As shown in Table 1, the toner consumption in this example was 100g, compared to 120g in Comparative Example 1 and 130g in Comparative Example 2. In this example, it was confirmed that toner consumption could be reduced compared to Comparative Examples 1 and 2.

[0060] (Consideration of toner consumption in Comparative Example 1) Here, we will consider the toner consumption results for Comparative Examples 1 and 2. First, we will consider why the toner consumption of Comparative Example 1 is higher than that of this embodiment. As mentioned above, the rotational speed accuracy of the rotating polyhedron mirror 204 differs significantly between Comparative Example 1 and this embodiment. In Comparative Example 1, the rotational speed of the rotating polyhedron mirror 204 is up to +50% faster during the first exposure. This is because, due to the low laser brightness (0.25mW) of Comparative Example 1, the detection accuracy of the laser light of BD206 decreases, and the rotational speed control of the rotating polyhedron mirror 204, which is based on the period of the BD signal 207 output by BD206, can no longer be performed correctly. As a result, the first exposure amount P1 is reduced by 50%, and the surface potential V11 of the photosensitive drum 1 formed by the first exposure increases by ΔV (≒20V), as shown in Figure 5. Furthermore, because it takes time for the rotation speed of the rotating polyhedron mirror 204 to converge to a predetermined rotation speed, the target surface potential of the photosensitive drum 1 is not achieved in the second and third exposures, and the rotation speed of the rotating polyhedron mirror 204 converges to the predetermined rotation speed in the fourth exposure. As a result, it is estimated that fogging occurs because the ideal Vback value of 150V cannot be maintained during the period from the first to the third exposure, and toner consumption is higher than in this embodiment.

[0061] (Print sequence in Comparative Example 2) Next, we will consider the reason why Comparative Example 2 consumes more toner than this embodiment. As mentioned above, this embodiment and Comparative Example 2 differ in the potential difference used to lower the surface potential of the photosensitive drum 1 during the first exposure. Figure 7 is a timing chart illustrating the print sequence of Comparative Example 2. In Figure 7, the control in the pre-rotation sequence and the image formation sequence is the same as the control in this embodiment shown in Figure 4 above, and therefore the explanation is omitted. As shown in Figure 7, the post-rotation sequence starts from timing T5, when the image formation on the paper P in the image formation sequence is completed. At timing T5, the charging voltage power supply 52 stops applying the 10th charging voltage S10 (-1000V) to the charging roller 2. Then, the laser control unit 201 of the exposure apparatus 3 controls the laser light source 200 to start emitting laser light, and the first exposure amount P1' (0.18 μJ / cm²) is set. 2A first exposure is performed to expose the surface of the photosensitive drum 1. By performing the first exposure, the surface potential of the photosensitive drum 1 is lowered by 300V, from V10 (-500V) to V11' (-200V).

[0062] Once the first exposure is complete, a second exposure is performed, exposing the surface of the photosensitive drum 1 with a second exposure amount P2'. This causes the surface potential of the photosensitive drum 1 to drop from V11' (-200V) to V12' (0V), a drop of 200V. The second exposure is performed for the duration of one rotation of the photosensitive drum, from timing T6 to timing T7, so that the surface potential of the photosensitive drum 1 is V12' (0V) all around. Meanwhile, the developing voltage is controlled to drop down in the same way as in the previous rotation sequence, so that the Vback, which is the potential difference between the surface potential of the photosensitive drum 1 and the developing voltage, is maintained at 150V at the developing nip position where the photosensitive drum 1 faces the developing roller 42. In other words, in the development voltage drop control, a development voltage D11' (-50V) is applied to the development roller 42, and the potential difference is reduced by 300V. Furthermore, a development voltage D12' (+150V) is applied to the development roller 42 in accordance with the second exposure.

[0063] (Consideration of toner consumption in Comparative Example 2) Figure 8 illustrates the relationship between the surface potential and development voltage of the photosensitive drum 1 in Comparative Example 2 and this embodiment. Figure 8(a) shows the relationship between the surface potential and development voltage of the photosensitive drum 1 at the development nip portion where the photosensitive drum 1 and the development roller 42 come into contact in Comparative Example 2. In Figure 8(a), the vertical axis represents potential (unit: V), and the horizontal axis represents time. Ta, Tb, and Tc represent timing (time). Up to timing Ta, when the post-rotation sequence starts, the surface potential and development voltage of the photosensitive drum 1 are -500V and -350V, respectively, and Vback is maintained at 150V. At timing Ta, the first exposure amount P1' (0.18 μJ / cm²) 2 When the first exposure is performed, exposing the surface of the photosensitive drum 1, the surface potential of the photosensitive drum 1 is instantly reduced from -500V to -200V, a drop of 300V.

[0064] On the other hand, in the developing voltage power supply 50, control is performed to lower the developing voltage applied to the developing roller 42 from -350V to -50V, a drop of 300V. However, as mentioned above, in the developing voltage power supply 50, it takes time for the output voltage to fall to the target voltage. In comparative example 2, it takes a period α (300 milliseconds) from timing Ta, when the developing voltage drop begins, to timing Tb for the developing voltage to fall from -350V to -50V. Therefore, the surface potential of the photosensitive drum 1 falls faster than the developing voltage, and as a result, during period α, Vback becomes smaller than the target 150V. In other words, during the period from timing Ta, when the developing voltage drop begins, to timing Tc, the developing voltage is higher than the surface potential of the photosensitive drum 1, causing fouling where toner on the developing roller 42 is transferred to the photosensitive drum 1.

[0065] On the other hand, Figure 8(b) shows the relationship between the surface potential of the photosensitive drum 1 and the developing voltage at the developing nip section where the photosensitive drum 1 and the developing roller 42 come into contact in this embodiment. In Figure 8(b), the vertical axis represents potential (unit: V), and the horizontal axis represents time. Ta and Tb represent timing (time). Up to timing Ta, when the post-rotation sequence starts, the surface potential and developing voltage of the photosensitive drum 1 are -500V and -350V, respectively, and Vback is maintained at 150V. At timing Ta, the first exposure amount P1 (0.02 μJ / cm²) 2 When the first exposure is performed, exposing the surface of the photosensitive drum 1, the surface potential of the photosensitive drum 1 is instantly dropped from -500V to -450V, a drop of 50V. On the other hand, the developing voltage is dropped from -350V to -300V, a drop of 50V, which is a smaller potential difference compared to 300V in Comparative Example 2. As a result, the required period α from timing Ta to timing Tb to drop the developing voltage by 50V is 20 milliseconds in this embodiment, which is shorter than the 300 milliseconds in Comparative Example 2.

[0066] Furthermore, although the Vback in the interval α from the timing Ta when the development voltage starts to fall off to the timing Tb when the development voltage falls off is slightly smaller than the ideal value of 150V, it is possible to suppress it to a potential difference where fogging hardly occurs. Thus, it is estimated that in Comparative Example 2, the toner consumption will be higher compared to this embodiment because the Vback is smaller.

[0067] In this embodiment, the developing roller 42 is in constant contact with the photosensitive drum 1, and in the subsequent rotation sequence, it is possible to maintain a constant Vback and stably lower the surface potential of the photosensitive drum without reducing the detection accuracy of the laser light by the BD206.

[0068] As described above, according to this embodiment, it is possible to suppress the occurrence of fogging during non-image formation, when a toner image is not formed on the photosensitive drum. [Examples]

[0069] In Example 1, during the first exposure, when scanning the surface of the photosensitive drum with laser light, the ratio of the on-time and off-time of the laser light source emitting the laser light was set to 50%, and an example was described in which the on and off of the laser light source was repeated for each pixel. In Example 2, the control of the post-rotation sequence when there are two laser light sources emitting laser light will be described.

[0070] [Laser light source] The laser light source 200 in this embodiment differs from that of Embodiment 1 in that it has two light sources, whereas Embodiment 1 had only one. Therefore, in this embodiment, when scanning the surface of the photosensitive drum 1 with laser light by rotating the multifaceted mirror 204, it is possible to simultaneously expose two pixels in the sub-scanning direction (the direction in which the paper P is transported), i.e., two lines in one scan. The configuration of the other exposure apparatus 3, the image forming apparatus, and the developing apparatus are the same as in Embodiment 1, and the same apparatus and components as in Embodiment 1 are given the same reference numerals, so their explanation here is omitted.

[0071] [Post-rotation sequence] In Example 1, during the first exposure, the laser light source 200 was repeatedly switched on and off for each pixel during the post-rotation sequence when scanning the surface of the photosensitive drum 1 with laser light, thereby exposing the surface of the photosensitive drum 1. However, in this embodiment, the first exposure method is different. Specifically, in this embodiment, during the first exposure, during the post-rotation sequence, one of the two laser light sources is switched on, while the other laser light source is switched off, thereby exposing the surface of the photosensitive drum 1. This allows the amount of laser light irradiated onto the surface of the photosensitive drum 1 to be reduced to 50% of the amount when both laser light sources are switched on, without reducing the brightness of the laser light used when the BD206 detects the laser light.

[0072] [Laser beam spot diameter and laser beam scanning distance] Figure 9 illustrates the relationship between the spot diameter of the laser beam emitted from the laser light source 200 of the exposure apparatus 3 in this embodiment and the distance of the laser beam in the sub-scanning direction. In Figures 9(a) and (b), the frames enclosed by vertical and horizontal lines indicate the size of one pixel, and the circular or elliptical shapes indicate the spot diameter of the laser beam irradiated onto the photosensitive drum 1. The thick solid lines indicate the scanning line in the main scanning direction scanned by the laser beam from the lit laser light source, one of the two laser light sources, while the dashed lines indicate the scanning line in the main scanning direction scanned by the laser beam from the unlit laser light source. Note that the scanning of the laser beam on the photosensitive drum 1 in the figure is performed from left to right.

[0073] Figure 9(a) shows the case where the spot diameter of the laser beam in the sub-scanning direction is larger than the size of two pixels in the sub-scanning direction (in this embodiment, the image resolution of the sub-scanning method is 600 dpi, so the size of two pixels is approximately 84 μm). In this case, because the spot diameter of the laser beam in the sub-scanning direction is larger than two pixels, the areas irradiated by the laser beam, which is irradiated every other line, overlap, covering the scanning lines that are not irradiated by the laser beam. As a result, the surface of the photosensitive drum 1 is uniformly exposed. On the other hand, Figure 9(b) shows the case where the spot diameter of the laser beam in the sub-scanning direction is smaller than the size of two pixels in the sub-scanning direction. In this case, there are areas of pixels included in the scanning lines that are not irradiated by the laser beam that are not irradiated by the laser beam, and the surface of the photosensitive drum 1 is not uniformly exposed. Therefore, in this embodiment, the spot diameter of the laser beam in the sub-scanning direction on the photosensitive drum 1 is larger than the size of two pixels in the sub-scanning direction (in this embodiment, the image resolution of the main scanning method is 600 dpi, so the size of two pixels is approximately 84 μm).

[0074] In this embodiment, one laser light source is turned on and the other laser light source is turned off to expose the surface of the photosensitive drum 1. As a result, the time the laser light is on (ON) and the time it is off (OFF) during the falloff state are each set to 50%, but this is not limited to this. For example, instead of setting the time one laser light source is on (ON) to 100%, the ratio of time the laser light is on can be further reduced from 50% by switching each pixel between on (ON) and off (OFF). In this case, the amount of exposure irradiated onto the photosensitive drum 1 needs to be increased according to the reduced ratio of time. In addition, in order to irradiate pixel positions corresponding to the off state where the laser light is not irradiated, it is necessary to make the spot diameter in the main scanning direction to which the laser light is irradiated larger than 2 pixels. In this embodiment, it is preferable that the time the laser light is on (ON) and the time it is off (OFF) are each set to 50%.

[0075] In this embodiment, the developing roller 42 is in constant contact with the photosensitive drum 1, and in the subsequent rotation sequence, it is possible to maintain a constant Vback and stably lower the surface potential of the photosensitive drum without reducing the detection accuracy of the laser light by the BD206.

[0076] As described above, according to this embodiment, it is possible to suppress the occurrence of fogging during non-image formation, when a toner image is not formed on the photosensitive drum. [Examples]

[0077] This embodiment describes an embodiment in which the exposure method of the photosensitive drum in the post-rotation sequence differs from that of Embodiment 1. The configuration of the image forming apparatus, exposure apparatus, and developing apparatus is the same as in Embodiment 1, and the same apparatus and components as in Embodiment 1 are given the same reference numerals, so their explanation here is omitted.

[0078] [Post-rotation sequence] In Example 1, during the first exposure of the post-rotation sequence, when scanning the surface of the photosensitive drum 1 with laser light, the laser light source 200 was repeatedly turned on and off for each pixel to expose the surface of the photosensitive drum 1. On the other hand, in this embodiment, during the first exposure of the post-rotation sequence, when laser light is emitted to a rotating polyhedron 204 having four reflective surfaces, the exposure control is performed in which laser light is emitted to two reflective surfaces and not to the remaining two reflective surfaces. Specifically, when laser light is emitted to the first surface of the rotating polyhedron 204, the exposure control is performed in which laser light is not emitted to the second surface, laser light is emitted to the third surface, and laser light is not emitted to the fourth surface. As a result, the amount of laser light irradiated onto the surface of the photosensitive drum 1 can be reduced to 50% of the amount of light when laser light is emitted to all four reflective surfaces, without reducing the brightness of the laser light used when the BD206 detects the laser light.

[0079] Here, when laser light is emitted from every other of the four reflective surfaces of the rotating polyhedron 204 described above, as shown in Figure 9(b), if the spot diameter in the sub-scanning direction is small, the surface potential of the photosensitive drum 1 becomes non-uniform when viewed at a microscopic level. Therefore, in this embodiment, in order to maintain a uniform surface potential of the photosensitive drum 1, as shown in Figure 9(a), the spot diameter in the sub-scanning direction is larger than the size of 2 pixels in the sub-scanning direction (in this embodiment, the size of 2 pixels is approximately 84 μm).

[0080] In this embodiment, the laser light source is switched between ON and OFF states on every other reflective surface of the rotating polyhedron mirror 204. This results in a 50% ON time and a 50% OFF time for the laser light during the falloff phase, but this is not limited to this. For example, instead of keeping the laser light ON for 100%, the ratio of time the laser light is on can be further reduced from 50% by switching the laser light ON and OFF states for each pixel. In this case, the exposure amount irradiated onto the photosensitive drum 1 must be increased in proportion to the reduced time ratio. Furthermore, to ensure that the laser light irradiates pixels corresponding to the OFF state, it is necessary to make the spot diameter in the main scanning direction where the laser light is irradiated larger than 2 pixels. In this embodiment, it is preferable that the 50% ON time and the 50% OFF time for the laser light are each irradiated.

[0081] In this embodiment, the developing roller 42 is in constant contact with the photosensitive drum 1, and in the subsequent rotation sequence, it is possible to maintain a constant Vback and stably lower the surface potential of the photosensitive drum without reducing the detection accuracy of the laser light by the BD206.

[0082] As described above, according to this embodiment, it is possible to suppress the occurrence of fogging during non-image formation, when a toner image is not formed on the photosensitive drum. [Examples]

[0083] In Example 1, in order for the BD to be able to detect the laser light, the laser light intensity (0.04 μJ / cm²) at which the laser brightness is 0.5 mW is set from the laser light source. 2 ) was being emitted. However, the amount of laser light irradiated onto the surface of the photosensitive drum during the first exposure was 0.02 μJ / cm². 2 Therefore, the control was performed to set the laser light amount to half by setting the on-time ratio of the laser light source that emits laser light to 50%. In Example 4, unlike Example 1, the on-time ratio of the laser light source that emits laser light during the first exposure is set to 100%, and an example is described in which the surface potential of the photosensitive drum can be set to the desired potential of Example 1 even when using a laser light amount that the BD can detect. Note that the configuration of the image forming apparatus, exposure apparatus, and developing apparatus is the same as in Example 1, and the same apparatus and components as in Example 1 are given the same reference numerals, so their explanation here is omitted.

[0084] [Print Sequence] Figure 10 is a timing chart illustrating the relationship between the developing voltage, charging voltage, the main motor driving each roller, the surface potential of the photosensitive drum 1, and the amount of laser light irradiated onto the photosensitive drum 1 in the print sequence of this embodiment. In Figure 10, the horizontal axis represents time, and T1 to T8 represent timing (time). Note that the control in the pre-rotation sequence and the image formation sequence in Figure 10 is the same as the control shown in Figure 4 of Embodiment 1 described above, and therefore the explanation is omitted here.

[0085] In Figure 10, the solid line represents the control of the laser light intensity in the post-rotation sequence, while the dotted line represents the control of Example 1 described above. The first exposure method in Examples 1 to 3 described above was an exposure method in which the time ratio of the laser light emission (on) time and the non-emission (off) time from the laser light source 200 was 50%. On the other hand, the first exposure method in this embodiment differs from Examples 1 to 3 in that the exposure is performed so that the laser light emission (on) time from the laser light source 200 is 100%.

[0086] As shown in FIG. 10, at timing T5, the image forming sequence switches to the post-rotation sequence. At timing T5, control is performed to lower the surface potential of the photosensitive drum 1 to V11 (-450 V) by irradiating laser light from the exposure device 3. In this embodiment, prior to timing T5, the charging voltage applied by the charging voltage power supply 52 to the charging roller 2 is increased in advance from -1000 V to -1050 V, and the surface potential of the photosensitive drum 1 is set to -550 V. In this embodiment, the photosensitive drum 1 is exposed with a laser luminance of 0.50 mW, which is a laser luminance at which the detection accuracy of the laser light by BD206 does not decrease for the first exposure. The laser light quantity when the laser luminance is 0.50 mW is the second exposure quantity P2 (0.04 μJ / cm 2 ) in Example 1, and is twice the first exposure quantity P1 (0.02 μJ / cm 2 ) in Example 1. As described in Example 1, when the surface of the photosensitive drum 1 is exposed with the first exposure quantity P1 (0.02 μJ / cm 2 ), the surface potential of the photosensitive drum 1 is lowered by 50 V. Similarly, when the surface of the photosensitive drum 1 is exposed with the second exposure quantity P2 (0.04 μJ / cm 2 ), the surface potential of the photosensitive drum 1 is lowered by 100 V. Therefore, in order to make the surface potential of the photosensitive drum 1 after exposure the same as in Example 1, in this embodiment, the charging potential of the photosensitive drum 1 is increased by 50 V on the minus side from Example 1 in accordance with the exposure timing, and is set to -1050 V.

[0087] Subsequently, as shown in FIG. 10, while the charging voltage is increased to -1050 V, the second exposure is performed with the third exposure quantity P3 of Example 1. By increasing the charging potential of the photosensitive drum 1 by 50 V on the minus side from Example 1 in accordance with the exposure timing, the surface potential of the photosensitive drum 1 is increased by 50 V on the minus side, and the laser light quantity is set to a quantity larger by 50 V on the plus side than the light quantity in Example 1. Thereby, the surface potential of the photosensitive drum 1 after the second exposure becomes -400 V, which is the same surface potential as in the case of Example 1.

[0088] Then, at timing T6 in Figure 10, specifically in Example 1, the charging voltage is set to 0V in advance to coincide with the timing at which the surface potential of the exposed area on the surface of the photosensitive drum 1, which is irradiated with laser light, is reduced by 50V from -50V (V19) to 0V (V20). This brings the surface potential of the photosensitive drum 1 to -500V, and then the 10th exposure is performed with the 10th exposure amount P10, thereby lowering the surface potential of the photosensitive drum 1 to 0V (V20).

[0089] [Relationship between surface potential of photosensitive drum and laser exposure dose] Figure 11 is a graph showing the relationship between the surface potential of the photosensitive drum 1 and the amount of laser light irradiated from the exposure apparatus 3 in the post-rotation sequence shown in Figure 10. In Figure 11, the vertical axis represents the surface potential of the photosensitive drum 1 (unit: V), and the horizontal axis represents the amount of laser light irradiated onto the surface of the photosensitive drum 1 (indicated as drum surface light intensity in the figure) (unit: μJ / cm²). 2 The figure shows the following. In the figure, P2 to P10 represent the amount of laser light irradiated onto the photosensitive drum 1 in the post-rotation sequence of Figure 10, and V11 to V20 represent the surface potential of the photosensitive drum 1, which is -450V to 0V. Note that in Figure 11, the solid line shows the EV curve when the charging voltage of Example 4 is -1050V, and the dotted line shows the EV curve when the charging voltage of Example 1 is -1000V.

[0090] As described above, even in a configuration where the developing roller 42 is in constant contact with the photosensitive drum 1, the charging voltage can be increased and the surface potential of the photosensitive drum 1 can be controlled to drop in the post-rotation sequence without reducing the detection accuracy of the laser beam by the BD206. This enables control of the post-rotation sequence that suppresses the occurrence of fogging.

[0091] As described above, according to this embodiment, it is possible to suppress the occurrence of fogging during non-image formation, when a toner image is not formed on the photosensitive drum.

[0092] In Example 4, the charging voltage was kept constant at -1050V while increasing the exposure amount. However, since it is sufficient to increase the exposure amount P2 irradiated in the first exposure, the charging voltage after P2 may be decreased in accordance with the exposure amount. In Figure 10, the surface potential of the photosensitive drum 1 decreases by 50V each time, so the charging voltage may be decreased by 50V each time, as shown in Figure 12. The control in Figure 12 is effective because it can suppress discharge in the charging section and also helps to suppress the deterioration of the photosensitive drum 1. [Examples]

[0093] In this embodiment, we will describe an embodiment in which the first exposure of Embodiment 1 described above is applied not only to the post-rotation sequence, but also to the period when no image formation is performed, from the rise of the charging voltage in the pre-rotation sequence to the start of the fall-off of the charging voltage in the post-rotation sequence. The configuration of the image forming apparatus, exposure apparatus, and developing apparatus is the same as in Embodiment 1, and the same apparatus and components as in Embodiment 1 are given the same reference numerals, so their explanation here will be omitted.

[0094] [Print Sequence] First, the control of this embodiment will be described. Figure 13 is a timing chart illustrating the relationship between the developing voltage, charging voltage, the main motor driving each roller, the surface potential of the photosensitive drum 1, and the amount of laser light irradiated onto the photosensitive drum 1 in the print sequence of this embodiment. In Figure 13, the horizontal axis represents time, and T1~T8, Ta, Tb, and Tc represent timing (time). Note that the control in the pre-rotation sequence and the post-rotation sequence in Figure 13 is the same as the control shown in Figure 4 of Embodiment 1 described above, and therefore the explanation is omitted here.

[0095] In this embodiment, during the image formation sequence, exposure control similar to the first exposure in Embodiment 1 is performed during the period when no image formation is taking place between the trailing end of the preceding paper in the transport direction and the leading end of the following paper in the transport direction (hereinafter referred to as the "paper gap"). In Figure 13, the period from timing Ta to timing Tc is the paper gap. Here, we will describe the exposure control in this embodiment during the paper gap when the paper gap is longer than the time it takes for the photosensitive drum 1 to complete one rotation. As shown in Figure 13, the surface potential of the photosensitive drum 1 during image formation is -500V, similar to Embodiment 1. Then, at timing Ta, in order to lower the surface potential of the photosensitive drum 1 for the subsequent paper gap period, the surface of the photosensitive drum 1 is exposed with a first exposure amount P1 similar to the first exposure in Embodiment 1, while maintaining a charging voltage of -1000V. As a result, the surface potential of the photosensitive drum 1 is lowered from -500V to -450V. At this time, it is preferable to lower the developing voltage from -350V to -300V along with the surface potential drop of the photosensitive drum 1, because this allows the Vback to be the same as during image formation.

[0096] Furthermore, during the period from timing Tb to timing Tc, which exceeds the time it takes for the photosensitive drum 1 to complete one rotation, the charging voltage supplied from the charging voltage power supply 52 is switched from -1000V to -950V without exposure by laser light. This allows the surface potential of the photosensitive drum 1 to be maintained at -450V. After timing Tc, the charging voltage supplied from the charging voltage power supply 52 is switched from -950V to -1000V in accordance with the timing of image formation, thereby restoring the surface potential of the photosensitive drum 1 during image formation from -450V to -500V. In addition, if the development voltage between the papers was -300V, it is restored from -300V to -350V at timing Tc.

[0097] In the image formation sequence, it is often unnecessary to apply the charging voltage that corresponds to the surface potential of the photosensitive drum 1 during image formation during periods when image formation is not taking place. For example, during periods when image formation is not taking place, the surface potential of the photosensitive drum 1 does not need to be the surface potential required to obtain the necessary image quality, such as image density and line thickness. Therefore, in such cases, lowering the surface potential of the photosensitive drum 1 below the surface potential during image formation is effective in suppressing the phenomenon of the photosensitive drum 1 being worn down during use and image defects caused by the accumulation of discharge products called drum flow.

[0098] In this embodiment, the first exposure using the laser light source 200 of Embodiment 1, which has one light source, was used for the explanation. For example, this embodiment can also be applied to a laser light source 200 having two light sources, as in Embodiment 2, or to a configuration in which the emission of laser light from the laser light source 200 is switched for each reflective surface of the rotating polyhedron mirror 204, as in Embodiment 3.

[0099] As described above, according to this embodiment, it is possible to suppress the occurrence of fogging during non-image formation, when a toner image is not formed on the photosensitive drum. [Examples]

[0100] In this embodiment, we will describe an example in which the first exposure of Embodiment 4 described above is applied not only to the post-rotation sequence, but also to the period when no image formation is performed, from the rise of the charging voltage in the pre-rotation sequence to the start of the fall-off of the charging voltage in the post-rotation sequence. The configuration of the image forming apparatus, exposure apparatus, and developing apparatus is the same as in Embodiment 4, and the same apparatus and components as in Embodiment 4 are given the same reference numerals, so their explanation here will be omitted.

[0101] [Print Sequence] First, the control of this embodiment will be described. Figure 14 is a timing chart illustrating the relationship between the developing voltage, charging voltage, the main motor driving each roller, the surface potential of the photosensitive drum 1, and the amount of laser light irradiated onto the photosensitive drum 1 in the print sequence of this embodiment. In Figure 14, the horizontal axis represents time, and T1~T8, Ta, Tb, and Tc represent timing (time). Note that the control in the pre-rotation sequence and the post-rotation sequence in Figure 14 is the same as the control shown in Figure 4 of Embodiment 1 described above, and therefore the explanation is omitted here.

[0102] In this embodiment, during the period when no image formation is taking place between the preceding and succeeding sheets of paper in the image formation sequence (hereinafter referred to as the "inter-sheet period"), exposure control similar to that of the first exposure in Embodiment 4 is performed. In Figure 14, the period from timing Ta to timing Tc is the inter-sheet period. Here, we will describe the exposure control of this embodiment during the inter-sheet period when the inter-sheet period is longer than the time it takes for the photosensitive drum 1 to complete one rotation. As shown in Figure 14, the surface potential of the photosensitive drum 1 during image formation is -500V, similar to Embodiment 4. Then, at timing Ta, in order to lower the surface potential of the photosensitive drum 1 during the subsequent inter-sheet period, the charging voltage is increased to -1050V, and the surface of the photosensitive drum 1 is exposed with a second exposure amount P2 similar to that of the first exposure in Embodiment 4. As a result, the surface potential of the photosensitive drum 1 is lowered from -550V to -450V. At this time, it is even more preferable to lower the development voltage from -350V to -300V along with the lowering of the surface potential of the photosensitive drum 1, because this allows the Vback to be the same as during image formation.

[0103] Furthermore, during the period from timing Tb to timing Tc, which exceeds the time it takes for the photosensitive drum 1 to complete one rotation, the charging voltage supplied from the charging voltage power supply 52 is switched from -1050V to -950V without exposure by laser light. This allows the surface potential of the photosensitive drum 1 to be maintained at -450V. After timing Tc, the charging voltage supplied from the charging voltage power supply 52 is switched from -950V to -1000V in accordance with the timing of image formation, thereby restoring the surface potential of the photosensitive drum 1 during image formation from -450V to -500V. In addition, if the development voltage between the papers was -300V, it is restored from -300V to -350V at timing Tc.

[0104] The effects of this embodiment are the same as those of Embodiment 5 described above, so a detailed explanation will be omitted. In addition, the present invention was explained using a monochrome image forming apparatus in each of the embodiments described above. However, the present invention is not limited to monochrome image forming apparatuses, and can also be applied to image forming apparatuses such as tandem-type color image forming apparatuses using a recording material transport belt, or color image forming apparatuses using an intermediate transfer belt.

[0105] Furthermore, in Example 1, a method was described in which the charging voltage is set to 0V at timing T5, when switching from the image formation sequence to the post-rotation sequence, and then laser exposure such as the first exposure is performed. Even if the charging voltage is maintained at -500V until timing T6, when the 10th exposure, which exposes one rotation of the photosensitive drum 1, is started, and the charging voltage is set to 0V at timing T6, the surface potential of the photosensitive drum 1 can be set to V11 to V20 by the exposure method described above.

[0106] Figure 15 is a timing chart illustrating the relationship between the developing voltage, charging voltage, main motor, surface potential of the photosensitive drum 1, and the amount of laser light irradiated onto the photosensitive drum 1 in a print sequence where the timing for setting the charging voltage to 0V is shifted later than in Example 1. In Figure 15, the horizontal axis represents time, and T1 to T8 represent timing (time). In Figure 15, the solid line showing the voltage change of the charging voltage is the timing chart when the timing for setting the charging voltage to 0V is shifted later than in Example 1. On the other hand, the timing chart shown by the dotted line shows the change in charging voltage as shown in Figure 4 of Example 1. As shown in Figure 15, the same effect as in Example 1 can be achieved by switching the charging voltage to 0V at any timing between timing T5 and timing T6.

[0107] As described above, according to this embodiment, it is possible to suppress the occurrence of fogging during non-image formation, when a toner image is not formed on the photosensitive drum. [Explanation of symbols]

[0108] 1 Photosensitive drum 3. Exposure apparatus 42 Developing roller 50 Developer voltage power supply 200 laser light sources 205 Engine Control Unit 206 BD

Claims

1. A rotatable photoreceptor; a charging member for charging the surface of the photoreceptor; an exposure unit for exposing the surface of the photoconductor to light in order to form an electrostatic latent image on the surface of the photoconductor charged by the charging member; a developing member in a developing section facing the photoconductor, supplying toner of normal polarity to the photoconductor to form a toner image; a developing voltage applying section that applies a developing voltage to the developing member; A control unit that controls the exposure unit and the development voltage application unit; having the control unit is operable to control an image forming operation in which the toner image is formed on a surface of the photoconductor, and a non-image forming operation in which the toner image is not formed on a surface of the photoconductor that is different from the image forming operation, When the control unit changes a surface potential of the photoconductor formed in a region of the photoconductor by the exposure unit from a first surface potential to a second surface potential smaller than an absolute value of the first surface potential during the non-image forming operation, i) controlling the developing voltage application unit to apply a first developing voltage while a region of the photoconductor on which the first surface potential is formed passes through the developing unit, and to apply a second developing voltage smaller than the absolute value of the first developing voltage while a region of the photoconductor on which the second surface potential is formed passes through the developing unit; ii) controlling the exposure unit so that a first exposure amount for exposing the surface of the photoconductor when forming the second surface potential is greater than a second exposure amount for exposing the surface of the photoconductor to form the electrostatic latent image in the image forming operation; 1. An image forming apparatus comprising:

2. the control unit controls the exposure unit to gradually change the surface potential of the photoconductor to a predetermined surface potential on the opposite polarity side to the normal polarity, and controls the development voltage application unit to gradually change the development voltage to the predetermined development voltage on the opposite polarity side, so that a potential difference between the first surface potential and the first developing voltage and a potential difference between the second surface potential and the second developing voltage are maintained at predetermined values ​​during the non-image forming operation.

2. The image forming apparatus according to claim 1,

3. A rotatable photoreceptor; a charging member for charging the surface of the photoreceptor; an exposure unit that exposes a surface of the photoconductor charged by the charging member, the exposure unit having a light source that emits a laser beam that exposes the surface of the photoconductor, and a detection unit that detects the laser beam and outputs a detection signal; a developing member for supplying toner of normal polarity to the photoconductor in a developing section facing the photoconductor to form a toner image; a charging voltage application unit that applies a charging voltage to the charging member; a developing voltage applying section that applies a developing voltage to the developing member; a control unit that controls the exposure unit, the charging voltage application unit, and the developing voltage application unit to switch the laser light emitted from the light source, the charging voltage, and the developing voltage; Equipped with the control unit controls the exposure unit and the developing voltage application unit so that, during a non-image formation period in which the toner image is not formed on the surface of the photoconductor, a potential difference formed between a region of the photoconductor in which a surface potential of the photoconductor has been changed by exposing the surface of the photoconductor to light by emitting the laser light from the light source in an amount detectable by the detection unit and the developing voltage is maintained at a predetermined value; When a surface potential of the photoconductor formed in a region of the photoconductor by the exposure unit is changed from a first surface potential to a second surface potential having a polarity opposite to the normal polarity with respect to the first surface potential, controlling the developing voltage application unit so as to change the developing voltage by a predetermined voltage width by applying a first developing voltage when a region of the photoconductor on which the first surface potential has been formed passes through the developing unit, and applying a second developing voltage having a polarity opposite to that of the first developing voltage when a region of the photoconductor on which the second surface potential has been formed passes through the developing unit; The image forming apparatus according to claim 1, wherein the first surface potential is controlled to be closer to the normal polarity side than the first developing voltage.

4. The image forming apparatus according to claim 3, characterized in that, during a post-rotation operation after image formation is completed, the control unit controls the exposure unit to gradually change the surface potential of the photosensitive member to the predetermined surface potential on the reverse polarity side so that the potential difference is maintained at the predetermined value, and controls the development voltage application unit to gradually change the development voltage to the predetermined development voltage on the reverse polarity side.

5. The image forming apparatus according to claim 4, characterized in that, when emitting the laser light to expose the surface of the photosensitive member, during the post-rotation operation, the control unit increases the time ratio of the light source being turned off compared to during image formation in which the toner image is formed on the surface of the photosensitive member.

6. 6. The image forming apparatus according to claim 5, wherein the control unit controls the charging voltage application unit to set the charging voltage to a predetermined charging potential during the time period until the surface potential of the photoconductor changes from the first surface potential to the predetermined surface potential.

7. The image forming apparatus according to claim 4, characterized in that, during the post-rotation operation, the control unit sets the light source to a turned-on state and does not set it to a turned-off state during the first exposure that changes the surface potential of the photosensitive body from the first surface potential to the second surface potential.

8. 8. The image forming apparatus according to claim 7, wherein the control unit controls the charging voltage application unit to change the charging voltage to the normal polarity side by the predetermined voltage width before starting the first exposure.

9. 9. The image forming apparatus according to claim 8, wherein the control unit controls the charging voltage application unit to set the charging voltage to a predetermined charging potential in accordance with a timing at which exposure that changes the surface potential of the entire circumference of the photoconductor to a predetermined potential begins.

10. 9. The image forming apparatus according to claim 8, wherein the control unit controls the charging voltage application unit to change the charging voltage to the opposite polarity side by the predetermined voltage width in accordance with a timing at which the surface potential of the photoconductor changes stepwise by the predetermined voltage width.

11. The image forming apparatus according to claim 3, characterized in that the control unit controls the exposure unit to change the surface potential of the photosensitive member from the first surface potential to the second surface potential, and controls the development voltage application unit to change the development voltage from the first development voltage to the second development voltage, in the space between the trailing end of a preceding recording material and the leading end of a succeeding recording material after image formation is completed.

12. The image forming apparatus according to claim 11, characterized in that, when emitting the laser light to expose the surface of the photosensitive member, the control unit increases the time ratio of the light source being turned off between sheets of paper compared to during image formation in which the toner image is formed on the surface of the photosensitive member.

13. 13. The image forming apparatus according to claim 12, wherein when the control unit changes the surface potential of the photoconductor from the first surface potential to the second surface potential, the control unit controls the charging voltage application unit to change the charging voltage to the opposite polarity side by the predetermined voltage width.

14. 14. The image forming apparatus according to claim 13, wherein, when the inter-paper period ends, the control unit controls the charging voltage application unit to change the charging voltage by the predetermined voltage width toward the normal polarity side, and controls the developing voltage application unit to change the developing voltage from the second developing voltage to the first developing voltage.

15. 13. The image forming apparatus according to claim 5, wherein the control unit switches the light source between an on state and an off state for each pixel in a main scanning direction in which the laser light scans the photoconductor.

16. 16. The image forming apparatus according to claim 15, wherein a diameter of a spot in the main scanning direction formed on the photoconductor by irradiation with the laser light is greater than twice the size of the pixel in the main scanning direction.

17. the light source has two light sources capable of scanning two lines on the photoconductor in one scan, 13. The image forming apparatus according to claim 5, wherein the control unit sets one of the light sources to an on state and sets the other light source to an off state.

18. the exposure unit has a plurality of reflecting surfaces that deflect the laser light incident from the light source and irradiate the surface of the photoconductor, and has a rotating polygon mirror whose rotation speed is controlled based on the detection signal; 13. The image forming apparatus according to claim 5, wherein the control unit switches the light source between an on state and an off state every time the reflective surface of the rotating polygon mirror onto which the laser light is incident is switched.

19. 19. The image forming apparatus according to claim 17, wherein a spot diameter in a sub-scanning direction perpendicular to a main scanning direction in which the laser light is scanned and formed on the photosensitive member by irradiating the laser light is greater than twice the size of a pixel in the sub-scanning direction.

20. The image forming apparatus according to claim 11, characterized in that the control unit sets the light source to an on state and does not set the light source to an off state when irradiating the laser light to the photoconductor so as to change the surface potential of the photoconductor from the first surface potential to the second surface potential between the sheets.

21. 21. The image forming apparatus according to claim 20, wherein the control unit controls the charging voltage application unit to change the charging voltage to the normal polarity side by the predetermined voltage width before irradiating the photosensitive member with the laser light.

22. 22. The image forming apparatus according to claim 21, wherein the control unit controls the charging voltage application unit to change the charging voltage to the reverse polarity side by twice the predetermined voltage width after irradiating the photosensitive member with the laser light.

23. 23. The image forming apparatus according to claim 22, wherein, when the inter-paper period ends, the control unit controls the charging voltage application unit to change the charging voltage by the predetermined voltage width toward the normal polarity side, and controls the developing voltage application unit to change the developing voltage from the second developing voltage to the first developing voltage.

24. 24. The image forming apparatus according to claim 1, wherein the developing member is disposed in the developing section so as to be in contact with the photoconductor.

Images