JP2009053397A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of always providing a stable image even if a time required until first print is shortened, by suppressing failure such as density reduction or density unevenness of an image as soon as image formation is started. <P>SOLUTION: The image forming apparatus 1 includes: a photoreceptor 11; an electrifier 12 electrifying the photoreceptor 11 to predetermined potential; an exposure device 14 exposing the electrified photoreceptor 11 to form an electrostatic latent image; a developing device 16 storing toner and developing the electrostatic latent image formed on the photoreceptor 11 with the toner to form a toner image; a power source 90 applying bias voltage obtained by superposing DC voltage on AC voltage to the developing device 16; a stop time detection part 82 detecting a stop time taken from previous image formation until next image formation; and a number-of-sheet counting part 83 counting the number of printed sheets in image formation. Then, the image forming apparatus 1 is further equipped with a bias correction part 85 correcting the bias voltage applied to the developing device 16 in accordance with the counted result by the number-of-sheet counting part 83 when the stop time detection part 82 detects the stop time equal to or above a predetermined value. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image stabilization technique in an image forming apparatus such as a copying machine, a facsimile machine, or a printer.

  2. Description of the Related Art Conventionally, in an image forming apparatus such as an electrophotographic copying machine or a printer, a surface of a photosensitive member uniformly charged by a charging device is irradiated with a laser by an exposure device such as a laser, thereby electrostatically charging the surface. After forming a latent image and developing the electrostatic latent image as a toner image by a developing unit, the toner image is transferred onto a sheet by a transfer member, and the toner image on the sheet is fixed on the sheet by a fixing unit. Output.

  Then, as a one-component developing type developer, a regulating member is arranged on the developing roller, and when passing through the regulating member, the toner is charged and simultaneously formed into a thin layer, and the developing roller is brought close to the photosensitive member, A developing bias voltage composed of an AC component and a DC component is applied to the developing roller, and development is performed based on a potential difference between the electrostatic latent image on the photosensitive member and the developing roller. It depends on.

  The development characteristics vary greatly depending on the use environment of the apparatus and the number of sheets used. This is mainly because the toner deteriorates or the characteristics change depending on the use environment and the number of sheets used. When the toner deteriorates, the charging ability of the toner changes, and the charge of the toner on the developing roller may change. By continuing to rotate the developing roller and applying a charge to the toner, the toner charge becomes uniform and stable. However, in image formation after standing for a long time, the toner charge on the developing roller is not stable. Since image formation is performed, a problem occurs on the image. Specifically, in image formation after standing for a long time, the toner charge immediately after the start of printing becomes non-uniform, causing fogging due to toner charged to a reverse polarity or insufficiently charged, The charging is lowered and the developing ability is lowered, so that the image density is lowered.

  Therefore, in Patent Document 1, an exposure unit that exposes a charged photosensitive member to form an electrostatic latent image, and a developing unit that develops the electrostatic latent image formed on the photosensitive member to form a toner image; In the image forming apparatus, the stop time and humidity environment from the previous time to the next image formation are detected, and the frequency and / or amplitude of the AC voltage of the development bias voltage is set according to the detection result of the stop time and humidity. At the same time, the optimum gradation correction data is selected according to the detection result of the stop time and humidity. Then, the exposure device controls the exposure amount based on the gradation correction data to form an electrostatic latent image on the photoconductor, but before the development, the development bias voltage is applied to the development roller to perform preliminary rotation, The toner charge is sufficiently stabilized. When the charging of the toner is stabilized, the developing device develops the electrostatic latent image formed on the photoconductor to stabilize the image.

However, in recent years, in the one-component development type image forming apparatus, there is a demand for shortening the time from the operation instruction to start image formation to the first print in response to the increase in operation response and the increase in print speed. In the above-described prior art, if the time until the first print is shortened, it becomes difficult to secure a sufficient time for pre-rotation by applying the developing bias voltage to the developing roller before development, and the charge amount of the toner is reduced. There was a problem that development had to be started from a lowered state. When the time until the first print is shortened in this way, there is a problem that problems such as a decrease in image density and density unevenness occur and it is difficult to sufficiently stabilize the image.
Japanese Patent Laying-Open No. 2005-99260 (paragraph [0012], FIG. 1)

  The present invention has been made to solve the above-described problems. Even if the time until the first print is shortened, problems such as a decrease in image density and density unevenness are suppressed immediately after the start of image formation. An object of the present invention is to provide an image forming apparatus capable of always obtaining a stable image.

  In order to achieve the above object, the present invention provides a photoconductor, a charger that charges the photoconductor to a predetermined potential, an exposure device that exposes the charged photoconductor to form an electrostatic latent image, and A developing unit that contains toner and forms a toner image by developing the electrostatic latent image formed on the photoreceptor with toner; and a power source that applies a bias voltage in which a DC voltage is superimposed on an AC voltage to the developing unit. The image forming apparatus includes a stop time detecting unit that detects a stop time from the previous image formation to the next image formation, and a number counting unit that counts the number of printed sheets in the image formation. When a stop time of image formation is detected, a bias correction unit that corrects the bias voltage of the power source according to the counting result of the number counting unit is provided.

  According to this configuration, if the reference value of the developing bias voltage based on the calibration process or the like is corrected according to the stop time of image formation and the number of printed sheets from the start of image formation, the toner is not sufficiently charged. However, the flying performance of the toner onto the photoconductor is substantially the same regardless of the number of printed sheets from the start of image formation. Further, it is not necessary to employ a preliminary rotation of the developing roller that sufficiently stabilizes the charging of the toner before development.

  According to the present invention, in the image forming apparatus configured as described above, the bias correction unit corrects the amplitude of the AC voltage. According to this configuration, regardless of the number of printed sheets from the start of image formation, the flying performance of the toner to the photosensitive member is improved, and the density reduction of the solid image is suppressed and the unevenness of the halftone image is also suppressed. To do.

  According to the present invention, in the image forming apparatus configured as described above, the bias correction unit corrects the amplitude of the AC voltage and the DC voltage value. According to this configuration, image defects such as density reduction and density unevenness are suppressed regardless of the number of printed sheets from the start of image formation.

  According to the first aspect of the present invention, when the reference value of the developing bias voltage based on the calibration process or the like is corrected according to the stop time of image formation and the number of printed sheets from the start of image formation, the toner is charged. Even if it is insufficient, the flying performance of the toner to the photoconductor is almost the same regardless of the number of printed sheets from the start of image formation, so image defects such as density drop and density unevenness are suppressed from the first print. Thus, a stable image can always be obtained. In addition, since it is not necessary to use the pre-rotation of the developing roller that sufficiently stabilizes the charging of the toner, the time until the first printing can be shortened.

  According to the second aspect of the invention, regardless of the number of printed sheets from the start of image formation, the flying performance of the toner to the photoconductor is improved, and the reduction in the density of the solid image is suppressed, and the half image is reduced. Image unevenness of the tone image can be suppressed.

  According to the third aspect of the present invention, image defects such as density reduction and density unevenness can be suppressed regardless of the number of printed sheets from the start of image formation.

  Embodiments of the present invention will be described below with reference to the drawings, but the present invention is not limited to these embodiments. The embodiment of the present invention shows the most preferable form of the invention, and the use of the invention and the terms shown here are not limited thereto.

  FIG. 1 is a schematic cross-sectional view for explaining a schematic configuration of an image forming apparatus according to an embodiment of the present invention.

  The image forming apparatus 1 includes an image forming unit 10, a toner replenishing device 20, an intermediate transfer unit 24, a paper feeding unit 30, a paper transport unit 35, a secondary transfer unit 45, a fixing device 47, and the like.

  As shown in FIG. 1, the image forming unit 10 includes a rotary developing unit 2, a photoreceptor 11, a charger 12, a drum cleaning device 13, an exposure device 14, and the like.

  The photoreceptor 11 is an image bearing member on which an electrostatic latent image is formed, and is driven to rotate approximately at the center of the image forming apparatus 1 (the direction of rotation is indicated by an arrow). The rotation shaft (not shown) is provided so as to extend perpendicular to the paper surface of FIG. The photoreceptor 11 is formed to have a photosensitive layer such as amorphous silicon or an OPC photoreceptor.

  The charger 12 is formed of a roller and is in contact with the photoreceptor 11 so as to uniformly charge the surface of the photoreceptor 11 and is rotatably supported. A cleaner 12 a that cleans deposits adhering to the charger 12 is provided above the charger 12.

  The drum cleaning device 13 is provided on the right side of the photoconductor 11 in FIG. 1 and is for cleaning toner and deposits remaining on the surface of the photoconductor 11.

  The rotary developing unit 2 is for developing the electrostatic latent image formed on the photosensitive member 11 with toner of each color, and is provided adjacent to the left side of the photosensitive member 11 in FIG. The rotary developing unit 2 includes a rotary rack 15 and four developing devices 16B (black), 16Y (yellow), 16C (cyan), and 16M (magenta) supported by the rotary rack 15. It has a cylindrical shape that can rotate around 15a. The shaft 15 a passes through the center of the rotary rack 15, and the axial direction thereof is parallel to the axial direction of the rotating shaft of the photoconductor 11.

  The rotary rack 15 has four sections that are divided into four equal parts in the circumferential direction by a partition frame 15b extending radially from the center of the shaft 15a. In each section, developing devices 16B, 16Y, 16C, and 16M corresponding to four toner colors of black, yellow, cyan, and magenta are arranged, and the developing devices 16B, 16Y, 16C, and 16M have the same configuration. is there. When the rotary developing unit 2 rotates, the developing rollers 53 of the developing units 16B, 16Y, 16C, and 16M face the photoconductor 11 with a predetermined gap. The detailed configuration of each developing device 16B, 16Y, 16C, 16M will be described later.

  The exposure device 14 is for scanning and exposing the photoconductor 11 based on image information from an external computer or the like, and is provided above the rotary developing unit 2. Data for causing the exposure device 14 to output the laser light 14L is created by the data processing unit 17 that receives image information. Then, a laser beam 14L is emitted from the exposure device 14. The laser beam 14L is irradiated onto the surface of the photoconductor 11 by a reflection mirror 18 provided on the optical path of the laser beam 14L (laser beam 14L is shown by a two-dot chain line). The exposure unit 14 includes a laser light source, a polygon mirror, a polygon mirror driving motor, and the like (not shown).

  The toner supply device 20 includes a toner storage container 21 and a toner supply member 22. The toner storage containers 21 are provided above the photoreceptor 11 and the intermediate transfer belt 25, and four toner containers 21 are arranged side by side in the direction perpendicular to the paper surface of FIG. Each toner storage container 21 is a portion that stores toner to be replenished to the developing devices 16B, 16Y, 16C, and 16M of the rotary developing unit 2. That is, each toner storage container 21 stores toner of any one color of black, yellow, cyan, and magenta. In FIG. 1, the toner container 21 is invisible except for one toner container 21.

  The toner replenishing member 22 is disposed above the photosensitive member 11 in a space between the exposure device 14 and the toner container 21 and is configured to be movable up and down, and four toner replenishing members 22 are provided corresponding to the respective colors. Each toner supply member 22 is connected to the corresponding toner storage container 21 by a pipe, a hose, or the like, and supplies the toner of each color stored in the toner storage container 21 to the corresponding developing devices 16B, 16Y, 16C, and 16M.

  The intermediate transfer unit 24 includes an intermediate transfer belt 25 to which toner images of respective colors formed on the photoconductor 11 are sequentially transferred, a driving roller 26 for stretching and rotating the intermediate transfer belt 25, a tension roller 27, and the like. The primary transfer roller 28 facing the photoconductor 11 with the intermediate transfer belt 25 therebetween, and a driven roller 28a provided between the drive roller 26 and the primary transfer roller 28 in the vicinity of the primary transfer roller 28. And a belt cleaning device 29 for cleaning the intermediate transfer belt 25 and the like. The intermediate transfer belt 25 stretched around each roller rotates and passes through the nip between the photoconductor 11 and the primary transfer roller 28, and the toner images of each color are sequentially primary-transferred. The image is transferred onto the paper P by the transfer unit 45.

  The density sensor 71 faces the intermediate transfer belt 25 and is disposed between the tension roller 27 and the drive roller 26, and measures the density of a patch pattern made up of toner images of each color formed on the intermediate transfer belt 25. It is. The measured density signal is output to an image formation control means described later, and the image formation control means corrects the image density by a calibration process based on the measurement result.

  The paper feed unit 30 includes a paper feed cassette 31, a manual feed tray 32, and the like. A paper feed cassette 31 having a placement plate 31 a on which the paper P is placed is provided at the bottom of the image forming apparatus 1. In addition, the paper feed unit 30 prevents the double feed to the right side of the pickup roller 33 and the pickup roller 33 to the right of the pickup cassette 31 in order to send out the paper P on the loading plate 31 a one by one to the paper conveyance path. A roller pair 34 is provided. Further, a manual feed tray 32 is provided on the right side surface of the apparatus so that paper can be separately fed. The manual feed tray 32 is also provided with a pickup roller 33 for sending the paper P one by one to the paper transport path.

  The paper transport unit 35 is for transporting the paper P in the image forming apparatus 1. The paper transport unit 35 includes a first transport path 36 from the paper feed unit 30 to the secondary transfer roller 46, and a paper discharge port for discharging the image-formed paper P from the secondary transfer roller 46 through the fixing device 47. And a reverse conveyance path 38 provided so as to connect the middle of the first conveyance path 36 to the middle of the second conveyance path 37 on the downstream side in the sheet conveyance direction from the fixing device 47. The A discharge tray 43 for receiving the discharged paper P is provided at the end of the second transport path 37. Note that the paper transport direction is indicated by arrows.

  A curved portion 39 is provided in the first transport path 36 in order to reverse the transport direction of the paper P sent out from the paper feed cassette 31. In addition, at the upper part of the curved portion 39, the conveyance path from the manual feed tray 32 joins. In front of the secondary transfer roller 46, a registration roller pair 40 for controlling the conveyance timing of the paper P is disposed. A plurality of pairs of conveyance rollers 41 are provided in the conveyance path in which these elements are provided, and a guide plate (not shown) for guiding the paper P is also provided as appropriate.

  The secondary transfer unit 45 includes a secondary transfer roller 46 and a driving roller 26. The secondary transfer roller 46 is for transferring the toner image transferred to the intermediate transfer belt 25 onto the sheet P conveyed so as to be sandwiched between the secondary transfer roller 46 and the intermediate transfer belt 25. Therefore, the secondary transfer roller 46 is supported so as to be rotatable and in contact with and away from the intermediate transfer belt 25 so as to form a nip via the drive roller 26 and the intermediate transfer belt 25. Further, a voltage is applied to the secondary transfer roller 46 so that the toner is transferred onto the paper P at a predetermined timing by a voltage applying means (not shown).

  The fixing device 47 is for melting and fixing the toner transferred onto the paper P, and is provided in the middle of the second transport path 37 and below the rotary developing unit 2. The fixing device 47 includes a heating roller 48 that incorporates a heater and a pressure roller 49 that presses the heating roller 48. The toner is heated and pressurized and fixed on the paper P.

  Next, an image forming operation will be described. The image forming apparatus 1 executes an image forming operation based on the input image data as follows.

  First, the photoreceptor 11 is charged by the charger 12. The exposure unit 14 scans and exposes the surface of the photoconductor 11 in accordance with data from the data processing unit 17 to which image data is input, and an electrostatic latent image is formed on the photoconductor 11. The Next, the rotary developing unit 2 is rotated counterclockwise in FIG. 1 and any one of the corresponding color developing devices 16 faces the photoconductor 11. In this state, the electrostatic latent image on the photoconductor 11 is developed by supplying the corresponding color toner. The developed toner image is transferred to the intermediate transfer belt 25. Toner remaining on the photoconductor 11 after transfer is cleaned by the drum cleaning device 13. The above operation is sequentially repeated for each color, and a full-color toner image is formed on the intermediate transfer belt 25.

  On the other hand, in the paper feed unit 30, one sheet P is taken out from the paper feed cassette 31 or the manual feed tray 32 by the pickup roller 33 and conveyed to the registration roller pair 40. Thereafter, the sheet P is conveyed from the registration roller pair 40 to the toner image on the intermediate transfer belt 25 in time, and is guided to the secondary transfer unit 45. A predetermined voltage is applied to the secondary transfer roller 46 while being in contact with the intermediate transfer belt 25, and the toner image on the intermediate transfer belt 25 is transferred to the paper P. The sheet P is guided to the fixing device 47 through the second conveyance path 37, and the toner image is fixed on the sheet P by the fixing device 47. The fixed sheet P is discharged from the sheet discharge port 42 to the discharge tray 43 through the branching claw 44.

  Next, the developing devices 16B, 16Y, 16C, and 16M supported by the rotary developing unit 2 will be described with reference to FIG. Here, since the configurations of the developing devices 16B, 16Y, 16C, and 16M are the same, the following description and the reference numerals of the drawings are omitted unless otherwise indicated. The description will be made in a unified manner with the vessel 16. Accordingly, the following description applies in common to the developing devices 16B, 16Y, 16C, and 16M unless otherwise specified.

  FIG. 2 is a cross-sectional view showing a schematic configuration of the developing device 16 according to the embodiment of the present invention. The developing device 16 includes a frame 51, a partition member 52, a developing roller 53, a supply roller 54, a regulating member 55, a stirring member 56, and the like.

  The frame 51 constitutes the outer shell of the developing device 16 with the lower inner wall portion 51a shown in FIG. 2, which is the bottom of the developing device 16, the upper inner wall portion 51b shown in FIG. Each member is provided inside and contains toner.

  The partition member 52 includes a first partition member 61 and a second partition member 62. The partition member 52 is separated into the toner supply unit 58 and the toner storage unit 59. The toner supply unit 58 is a part that supplies toner to the photoreceptor 11, and includes a developing roller 53, a supply roller 54, a regulating member 55, and the like, and a stirring member 56 is arranged in the toner storage unit 59. A portion between the first partition member 61 and the second partition member 62 serves as a passage portion 64 for the toner to move from the toner storage portion 59 to the toner supply portion 58.

  A part of the developing roller 53 is exposed from the frame 51, and is disposed opposite to the photosensitive member 11 (partially shown by a broken line in FIG. 2) and is rotatable about the axis 53 b. Supported and connected to a power supply 90. The power supply 90 applies a bias voltage in which a DC voltage is superimposed on an AC voltage to the developing roller 53. A developing roller 53 on which a toner thin layer charged by a regulating member 55 described later is formed is opposed to the photoconductor 11, and toner is supplied to the photoconductor 11 due to a potential difference between the developing roller 53 and the photoconductor 11. To do. A seal member 53a is provided between the developing roller 53 and the frame 51 to prevent toner leakage.

  The regulating member 55 is formed in a film shape or a blade shape extending in the direction of the axis 53b, one end portion is fixedly held by the support member 66 of the frame body 51, and the other end portion is in contact with the developing roller 53, so that development is performed. When the roller 53 rotates, it is for forming a thin layer of toner on the developing roller 53, and further has a function of charging the toner by friction.

  The supply roller 54 is formed of foamed polyurethane or the like, carries a toner on its surface, and supplies and conveys the toner to the developing roller 53. Therefore, the supply roller 54 is rotatable and supported so that the axis 54b of the supply roller 54 and the axis 53b of the developing roller 53 are parallel to each other, and the developing roller is driven by a driving mechanism (not shown) including a gear, a motor, and the like. Rotate in the same direction as 53 (clockwise in FIG. 2). The supply roller 54 supplies toner to the developing roller 53 at the contact portion between the developing roller 53 and the supply roller 54 in the upper contact portion (near the regulating member 55) in FIG. The portion (near the inner wall portion 51a) functions to scrape off the toner that has not been used for development.

  The agitating member 56 is for agitating the toner in the toner accommodating portion 59, and is provided with an axis parallel to and parallel to the supply roller 54 and the developing roller 53, and includes an axial portion 56a, an agitating blade 56b, and the like. The The shaft portion 56a is provided so as to be bridged in the toner storage portion 59 and is rotatably supported.

  The developing operation will be described. When the agitating member 56 rotates, the toner is agitated and splashed up in the toner accommodating portion 59 by the agitating blade 56b. The splashed toner enters the passage portion 64 and is conveyed to the toner supply portion 58. When the supply roller 54 rotates, the toner is conveyed to the developing roller 53, and when the developing roller 53 rotates while contacting the regulating member 55, a charged toner thin layer is formed on the developing roller 53. Further, when a bias voltage is applied to the developing roller 53 by the power supply 90, the toner on the developing roller 53 is supplied to the photoconductor 11, and an electrostatic latent image on the photoconductor 11 is formed as a toner image. The toner that is not used for development and remains on the developing roller 53 is further rotated by the developing roller 53 and scraped by the supply roller 54 and returned to the toner supply unit 58 to complete the development.

  Next, calibration processing and development bias voltage control will be described with reference to FIGS. FIG. 3 is a block diagram showing image forming control means and power supply means for controlling the developing bias voltage of the image forming apparatus according to the embodiment of the present invention, and FIG. 4 is a diagram showing a correction table for correcting the developing bias voltage. FIG. 5 is a flowchart for controlling the developing bias voltage.

  As shown in FIG. 3, the image formation control unit 80 includes a bias control unit 81, a stop time detection unit 82, and a number counting unit 83.

  When a density signal of each color is input from the density sensor 71 that measures the density of the patch pattern, the bias control unit 81 obtains the reference density of the toner of each color stored in the memory and the measured density of the input toner of each color. In comparison, a calibration process is performed to obtain the amount of deviation of the measured density of each color toner from the reference density of each color toner. Then, from the next printing, the bias voltage applied to the developing roller 53 is switched so that the deviation amount of the density of the toner image formed on the intermediate transfer belt 25 is eliminated. In the switched bias voltage, the amplitude Vp-p and the duty value of the AC bias voltage are set based on the developability of each developing device 16, and the voltage value Vdc of the DC bias voltage is set to 60 V, for example. The reference value is stored in the memory and transmitted to the bias setting unit 84. The calibration process is performed when the image forming apparatus 1 is turned on after the power is turned on or after a predetermined number of prints are completed.

  The stop time detector 82 detects the stop time from the previous image formation to the next image formation. The stop time detector 82 counts the time from the end of the last development of the developer 16 to the start of the next operation of the developer 16 by the timer, and the detected stop time is used as the bias converter 85. Send to.

  The sheet number counting unit 83 calculates the number of sheets to be printed next at the next image formation. The number counting unit 83 counts the number of printed sheets from the start of the operation of the developing device 16 that has been stopped, calculates the number of printed sheets to be printed next based on the counted value, and determines the number of printed sheets. Data is transmitted to the bias converter 85.

  The bias conversion unit 85 develops based on the reference value of the bias voltage transmitted from the bias setting unit 84, the stop time transmitted from the stop time detection unit 82, and the number of printed sheets transmitted from the number counting unit 83. The bias voltage applied to the device 16 is determined.

  That is, the bias conversion unit 85 reads from the memory a correction table for correcting the reference bias voltage according to the image formation stop time and the number of printed sheets. Based on the correction table, the bias conversion unit 85 corrects the reference value transmitted from the bias setting unit 84 in accordance with the signals transmitted from the stop time detection unit 82 and the number counting unit 83, and the bias conversion unit 85 Determine the voltage. The bias voltage value is transmitted to the power supply 90.

  FIG. 4 shows a correction table. FIG. 4 shows the bias voltage to be corrected according to the stop time and the number of printed sheets, the vertical column indicates the stop time (unit: second) of the developing device 16, and the horizontal column indicates the number of printed sheets. “Vp−p correction” indicates a correction amount from the reference value of the amplitude Vp−p, “Vdc correction” indicates a correction amount from the reference value of the DC voltage value Vdc, and “None” indicates that the bias voltage is This indicates that the reference value transmitted from the bias converter 85 is set.

  As shown in FIG. 4, when the stop time of image formation is less than 60 seconds, the stop time from the previous image formation is short, so the potential of the toner layer does not extremely decrease. Even if the development is performed without correcting the reference value of the development bias voltage based on the calibration process, the image density does not decrease and density unevenness does not occur.

  As shown in FIG. 4, when the image formation stop time is 60 seconds or more and less than 360 seconds, the stop time from the previous image formation is relatively long, so the stop time is less than 60 seconds. Since the potential of the toner layer is much lower than in the case where the number of printed sheets is 1, the AC bias voltage amplitude Vp-p and the DC voltage value Vdc are set to the reference value. When the correction is performed so as to increase, density reduction and density unevenness of the image do not occur. In the second and subsequent prints, the toner potential is increased because correction is performed to increase the development bias voltage when printing the first sheet. Therefore, even if the development bias voltage is set to the reference value, the image Density reduction and density unevenness do not occur.

  Also, as shown in FIG. 4, when the stop time of image formation is 360 seconds or more, the stop time from the previous image formation is longer, so the stop time is 60 seconds or more and less than 360 seconds. Since the potential of the toner layer is further reduced as compared with the case of the above, in the development in which the number of printed sheets is the first and second sheets, the AC bias voltage amplitude Vp-p and the DC voltage value Vdc are set as the reference values. However, in the second printing, even if the DC voltage value Vdc is corrected to be lower than that in the first printing, image density reduction and density unevenness occur with the first printing. It does not occur. In the printing of the third and subsequent sheets, the correction is made to increase the development bias voltage at the time of printing the first and second sheets, so the toner potential is increased, so the development bias voltage is set to a reference value. However, neither density reduction nor density unevenness of the image occurs.

  Next, control of the developing bias voltage in the image forming apparatus of the present invention will be described according to the steps in FIG. 5 with reference to FIGS.

  After the start of the image forming operation, the elapsed time from the previous image formation is detected in step S1, and if the elapsed time is less than 60 seconds, the amplitude of the reference AC bias voltage based on the calibration process is determined in step S2. Vp-p and DC voltage value Vdc are set, and in step S3, the development bias voltage of the reference value is applied to the power source 90, and development is performed.

  If the elapsed time from the previous image formation in step S1 is 60 seconds or more and less than 360 seconds (YES in step S4), the current print number is determined in step S5. If the number of printed sheets is the first (YES in step S5), the toner potential is lowered. Therefore, in step S6, 100V is added to the reference value AC bias voltage amplitude Vp-p, and the reference value DC is set. Correction is performed by adding 15 V to the voltage value Vdc. In step S3, the corrected development bias voltage is applied to the power supply 90, and development is performed.

  If the current number of printed sheets is the second and subsequent sheets in step S5 (NO in step S5), the toner potential has increased due to the correction of the developing bias voltage during the printing of the first sheet. Therefore, in step S7, the reference value is set. The amplitude Vp-p of the AC bias voltage and the DC voltage value Vdc are set, and in step S3, the development bias voltage of the reference value is applied to the power supply 90 and development is performed.

  If it is determined in step S4 that the elapsed time from the previous image formation is 360 seconds or more (NO in step S4), the current number of prints is determined in step S8. If the number of printed sheets is the first sheet (YES in step S8), the toner potential is further lowered because the elapsed time from the image formation is longer, so in step S9, the amplitude of the reference AC voltage is determined. Correction is performed by adding 100 V to Vp-p and adding 30 V to the direct-current DC voltage value Vdc. In step S3, the corrected developing bias voltage is applied to the power supply 90 and development is performed.

  If the current number of printed sheets is not the first sheet at step S8 (YES at step S8) and the second sheet at step S10 (YES at step S10), the elapsed time is longer, but the first developing bias. Since the toner potential is increased due to the voltage correction, in step S11, the correction amount of the amplitude Vp-p of the AC voltage is set to 100 V as in the first printing, and the correction amount of the DC voltage value Vdc is set to the first sheet. In step S3, the corrected developing bias voltage is applied to the power source 90 and development is performed.

  If the current number of prints is the third or later in step S10 (NO in step S10), the toner potential has increased due to the correction of the developing bias voltage at the time of printing the first and second sheets, so in step S12 The reference value AC bias voltage amplitude Vp-p and the DC voltage value Vdc are set, and the development bias voltage of the reference value is applied to the power supply 90 in step S3, and development is performed.

  As described above, when the image formation stop time is long and the development bias voltage is corrected and development is performed, the resulting images shown in FIGS. 6 and 7 are obtained. FIG. 6 is a diagram illustrating a change in image density with respect to the number of printed sheets in a solid image, and FIG. 7 is a diagram illustrating a change in image density with respect to the number of printed sheets in a halftone image (50% halftone). Each figure shows the number of printed sheets (number of printed sheets) on the horizontal axis and the transmission density TD on the vertical axis, with the stop time of 1 minute (1 minute left), 6 minutes (6 minutes left) and 10 minutes (10 minutes left) as parameters. Is.

  As shown in FIG. 6, in the solid image, by performing the above-described correction of the development bias voltage in the printing from the first sheet to the third sheet, it is within the target transmission density TD1.1 to 1.2, The image density is high. For reference, FIG. 8 shows the image density when the development bias voltage is not corrected. As shown in FIG. 8, the first printing with a stop time of 10 minutes (leaving for 10 minutes) greatly deviates from the target image density. Further, in the printing of the first sheet, the difference in transmission density TD is about 0.08 in each printing with a stop time of 10 minutes (leaving for 10 minutes) and a stop time of 1 minute (leaving for 1 minute), as shown in FIG. When the development bias voltage is corrected in the first printing, the difference in transmission density TD is improved to about 0.03.

  In general, in a halftone image, streaks and pitch unevenness may occur in the image because the toner layer potential is reduced in printing at the start of image formation. In order to improve the image, even if only the duty or the DC voltage value of the AC voltage is increased, the toner does not fly sufficiently to the photoconductor 11 and the density unevenness remains, but the AC voltage as in the present embodiment remains. By correcting the amplitude Vp-p of the toner, the flying performance of the toner to the photoreceptor 11 can be improved, and the unevenness of the halftone image in the first to third printing can be suppressed. Concentration is stable.

  According to the above embodiment, the image forming apparatus 1 forms the electrostatic latent image by exposing the photosensitive member 11, the charger 12 that charges the photosensitive member 11 to a predetermined potential, and the charged photosensitive member 11. An exposure unit 14 that stores toner, a developing unit 16 that develops an electrostatic latent image formed on the photosensitive member 11 with toner to form a toner image, and a DC voltage superimposed on the AC voltage in the developing unit 16. A power supply 90 that applies a bias voltage, a stop time detection unit 82 that detects a stop time from the previous image formation to the next image formation, and a number counting unit 83 that counts the number of printed sheets in image formation are provided. When the stop time detection unit 82 detects a stop time that is equal to or greater than a predetermined value, a bias correction unit 85 that corrects the bias voltage applied to the developing device 16 according to the count result of the number counting unit 83 is provided.

  Thus, when the reference value of the developing bias voltage based on the calibration process is corrected according to the stop time of image formation and the number of printed sheets from the start of image formation, the flying performance of toner on the photoconductor 11 can be determined. Since the first printing at the start can be made substantially the same regardless of the number of printed sheets, image defects such as density reduction and density unevenness are suppressed from the first printing at the start of image formation. , You can always get a stable image. Further, since it is not necessary to employ the preliminary rotation of the developing roller 53 that sufficiently stabilizes the charging of the toner, the time until the first printing can be shortened.

  Further, the bias correcting unit 85 corrects the amplitude Vp-p of the AC voltage, so that the flying performance of the toner from the first printing at the start of image formation to the photoconductor 11 is improved, and the solid image is printed. It is possible to suppress the density reduction and to suppress the image unevenness of the halftone image.

  The bias correction unit 85 corrects the AC voltage amplitude Vp-p and the DC voltage value Vdc to suppress image defects such as density reduction and density unevenness from the first printing at the start of image formation. can do.

  The present invention can be used for an image forming apparatus such as a copying machine, a printer, and a facsimile.

FIG. 1 is a schematic diagram illustrating an overall configuration of an image forming apparatus according to an embodiment of the present invention. FIG. 3 is a side cross-sectional view illustrating a configuration of a developing device according to an image forming apparatus that is an embodiment of the present invention. FIG. 2 is a block diagram showing a configuration for controlling a developing bias voltage of an image forming apparatus according to an embodiment of the present invention. FIG. 4 is a diagram showing a correction table for correcting the developing bias voltage of the image forming apparatus according to the embodiment of the present invention. These are flowcharts for controlling the developing bias voltage of the image forming apparatus according to the embodiment of the present invention. These are diagrams showing development characteristics of the image forming apparatus according to the embodiment of the present invention. These are diagrams showing development characteristics of the image forming apparatus according to the embodiment of the present invention. These are the figures which show the development characteristics of the image forming apparatus according to the prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 2 Rotary type developing unit 11 Photoconductor 12 Charger 14 Exposure device 15 Rotary rack 16, 16B, 16Y, 16C, 16M Developing device 53 Developing roller 54 Supply roller 55 Control member 56 Stirring member 71 Density sensor 80 Image formation Control means 81 Bias control section 82 Stop time detection section 83 Number counting section 84 Bias setting section 85 Bias correction section 90 Power supply

Claims (3)

A photoconductor, a charger that charges the photoconductor to a predetermined potential, an exposure device that exposes the charged photoconductor to form an electrostatic latent image, and a toner that is formed on the photoconductor. A developing unit that develops the electrostatic latent image with toner to form a toner image, a power source that applies a bias voltage in which a DC voltage is superimposed on an AC voltage to the developing unit, and from the previous image formation to the next image formation In an image forming apparatus comprising a stop time detecting unit for detecting a stop time of the image forming unit, and a sheet number counting unit for counting the number of printed sheets in image formation,
An image forming system comprising: a bias correcting unit that corrects a bias voltage of the power source according to a counting result of the number counting unit when the stop time detecting unit detects an image forming stop time that is equal to or greater than a predetermined value. apparatus.   The image forming apparatus according to claim 1, wherein the bias correction unit corrects an amplitude of an AC voltage.   The image forming apparatus according to claim 1, wherein the bias correction unit corrects an amplitude of an alternating voltage and a direct current voltage value.

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