JP2007219010A - Charging device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce charging unevenness of a corona discharge type charging device and also to reduce the amount of ozone production. <P>SOLUTION: Discharge periods wherein a discharging voltage is applied to discharge electrodes 53a and 53b and discharge stop periods wherein the application of the discharging voltage to the discharge electrodes 53a and 53b is stopped are periodically generated so that each discharge period becomes 10<SP>-5</SP>to 10<SP>-2</SP>sec. Further, the discharge periods of the discharge electrodes 53a and 53b are generated alternately without overlapping each other. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a corona discharge charging device and an image forming apparatus including the same.

  As a charging method in an image forming apparatus adopting an electrophotographic method, an electrostatic recording method, etc., a corona charging method in which ions generated by corona discharge are led to a surface of an electrostatic latent image carrier such as a photosensitive member to be charged has been conventionally used. It has been.

In a corona charging system charging device, discharge products such as ozone O ₃ and nitrogen oxides NOx are generated during corona discharge. That is, due to the energy (electron collision, etc.) associated with the discharge of electrons, the oxygen molecules O ₂ and nitrogen molecules N ₂ are dissociated into oxygen atoms O and nitrogen atoms N, which are combined with the oxygen molecules O _{2. 3.} Nitrogen oxide NO _x is generated.

  When these discharge products adhere to the charging electrode, discharge failure and charging unevenness occur at that portion, and charging failure of the electrostatic latent image carrier and further image failure occur.

As a technique for reducing such charging unevenness, a grid electrode is provided between the discharge electrode and the electrostatic latent image carrier, and a voltage is applied to the grid electrode to control the passing amount of charged particles. There are scorotron charging devices that reduce charging unevenness. For example, in Patent Document 1, in a scorotron charger, the timing for applying a voltage to a grid electrode starts earlier than the timing for applying a voltage to a discharge electrode, and ends later than the end of voltage application to the discharge electrode. A technique is disclosed in which the charging potential of the electrostatic latent image carrier is rapidly increased / decreased by being controlled. Patent Document 1 describes that a plurality of discharge electrodes are provided.
JP-A-5-72873 (published March 26, 1993) JP 2003-151719 A (published May 23, 2003)

  However, although the conventional scorotron charging device can reduce charging unevenness, it cannot reduce the generation amount of the discharge product itself.

  For this reason, it is impossible to prevent a discharge failure or a decrease in discharge performance caused by the discharge product adhering to the discharge electrode. In addition, problems such as generation of ozone odor, harmful effects on the human body, and deterioration of parts due to strong oxidizing power due to ozone generated as a discharge product cannot be prevented.

  The present invention has been made in view of the above-described conventional problems, and an object of the present invention is to reduce charging unevenness and the amount of ozone generated in a corona discharge charging device.

  In order to solve the above problems, the charging device of the present invention includes a plurality of discharge electrodes and voltage applying means for applying a voltage to each of the discharge electrodes, and corona discharge is performed by applying a voltage to each of the discharge electrodes. In the charging device for charging the object to be charged, the voltage application means alternately discharges a discharge period for applying a discharge voltage and a discharge pause period for stopping the application of the discharge voltage to each of the discharge electrodes. A voltage is applied so that the discharge periods of the discharge electrodes do not overlap each other.

  According to the above configuration, the voltage application means applies a voltage to each discharge electrode so that the discharge period in which the discharge voltage is applied and the discharge pause period in which the discharge voltage application to the discharge electrode is suspended do not overlap each other. To do. Thereby, the length of a discharge period can be shortened and ozone generation amount can be reduced. Further, in the above configuration, the voltage applying unit applies a voltage to each discharge electrode so that the discharge periods in each discharge electrode do not overlap each other. Thus, by preventing the discharge periods at the discharge electrodes from overlapping each other, the period during which the discharge rest periods of the discharge electrodes overlap can be shortened, and charging unevenness can be reduced.

  Each of the discharge electrodes has a plurality of lines or needles arranged in a sawtooth shape such that the tip is directed toward the charged object, and arranged in a row so that the tip is directed toward the charged object. Or a wire-like shape having a plurality of protrusions that are arranged so as to extend in parallel to the charging surface of the object to be charged and whose tip protrudes toward the object to be charged. It is good.

  By using the discharge electrode having any one of the above shapes, corona discharge can be stably generated starting from the tip portion, and the object to be charged can be appropriately charged.

  The discharge electrodes may be arranged in parallel to each other, and the positions of the tips of the adjacent discharge electrodes may be different from each other in the parallel direction.

  According to said structure, the interference regarding the discharge of the said front-end | tip parts in each adjacent discharge electrode can be prevented, and the charging nonuniformity about the said parallel direction can be reduced.

  The object to be charged is an electrostatic latent image carrier provided in an image forming apparatus having an image forming speed Vp (mm / s) in the sub-scanning direction, and the voltage applying unit includes a period F ( s) may be set to satisfy the relationship of Vp × F ≦ 0.2.

  According to the above configuration, by setting the period F of the discharge period so as to satisfy the relationship of Vp × F ≦ 0.2, charging is performed in the sub-scanning direction by alternately generating the discharge period and the discharge pause period. Even if unevenness occurs, the pitch of the uneven charging can be reduced to 0.2 mm or less. As a result, it is possible to suppress degradation in image quality due to charging unevenness in the sub-scanning direction to a level that cannot be recognized by human vision.

  Further, the voltage applying means may be configured to perform constant current driving with a constant discharge current. According to the above configuration, since the voltage is applied to the discharge electrode so that the discharge current becomes a constant value, even if the discharge start voltage fluctuates due to changes in environmental factors such as humidity or aging of component characteristics, it is stable. Charging can be performed.

  In addition, the voltage applying means includes a power source provided in common for the discharge electrodes, and voltages applied to the discharge electrodes provided between the power source and the discharge electrodes, respectively. It is good also as a structure provided with the several switching means switched by a discharge rest period.

  According to said structure, a common power supply is provided about each discharge electrode, and each switching means switches the discharge period and discharge rest period of each discharge electrode. Thus, the apparatus cost can be reduced by using a common power source for each discharge electrode.

  The switching means may be composed of a Zener diode and a transistor connected in parallel with each other. According to said structure, the voltage applied to a discharge electrode can be switched easily by controlling conduction | electrical_connection / interruption | blocking of the said transistor.

  Moreover, it is good also as a structure provided with the grid electrode to which a DC voltage is applied between the said discharge electrode and the said to-be-charged object. According to said structure, the passage of a charged particle can be controlled by applying a DC voltage to a grid electrode, and charging uniformity can be improved.

  An image forming apparatus according to the present invention includes any one of the above charging devices. Therefore, the amount of ozone generated due to charging can be reduced.

  As described above, in the charging device of the present invention, the discharge period in which the discharge voltage is applied and the discharge pause period in which the application of the discharge voltage is stopped alternately occur for each discharge electrode, and the discharge in each discharge electrode A voltage is applied so that the periods do not overlap each other. Therefore, charging unevenness can be reduced and the amount of ozone generated can be reduced.

  An embodiment of the present invention will be described. FIG. 2 is an explanatory diagram showing a configuration of the image forming apparatus A according to the present embodiment. Here, as an example of the image forming apparatus A, a laser printer that forms a multicolor or single color image on a sheet (recording paper) based on image data input from the outside or image data obtained by reading a document will be described as an example. To do.

  As shown in the figure, the image forming apparatus A includes an exposure unit 1, a developing device 2 (2a, 2b, 2c, 2d), a photosensitive drum 3 (3a, 3b, 3c, 3d), and a charger 5 (5a, 5d). 5b, 5c, 5d), a cleaner unit 4 (4a, 4b, 4c, 4d), an intermediate transfer belt unit 8, a fixing unit 12, a sheet conveying path S, a paper feed cassette 10, a paper discharge tray 15, and the like.

  The image data of a color image handled in the image forming apparatus A corresponds to a color image using each color of black (K), cyan (C), magenta (M), and yellow (Y). Accordingly, the developing device 2 (2a, 2b, 2c, 2d), the photosensitive drum 3 (3a, 3b, 3c, 3d), the charger 5 (5a, 5b, 5c, 5d), the cleaner unit 4 (4a, 4b, 4c and 3d) are provided four by four so as to form four types of latent images corresponding to the respective colors. The symbols a to d correspond to a being black, b being cyan, c being magenta, and d being yellow. It is configured.

  In the image station, the photosensitive drum 3 is disposed on the upper part of the image forming apparatus A. In this embodiment, the image forming speed (process speed) in the sub-scanning direction with respect to the recording paper is set to 124 mm / s, and the peripheral speed of the photosensitive drum 3 (moving speed of the surface of the photosensitive drum 3) is set to this. Set equal.

  The charger 5 uniformly charges the surface of the photosensitive drum 3 to a predetermined potential. As the charger 5, a charger type as shown in FIG. 2 is used. Details of the charger 5 will be described later.

  The exposure unit 1 includes, for example, an EL or LED writing head in which light emitting elements are arranged in an array, in addition to a method using a laser scanning unit (LSU) including a laser irradiation unit and a reflection mirror as shown in FIG. Some techniques are used. The exposure unit 1 exposes the charged photosensitive drum 3 according to the input image data, thereby forming an electrostatic latent image corresponding to the image data on the surface of the photosensitive drum 3.

  Each developing device 2 visualizes the electrostatic latent image formed on each photoconductor drum 3 with K, C, M, and Y toners. The cleaner unit 4 removes and collects the toner remaining on the surface of the photosensitive drum 3 after the development and image transfer processes. The toner used in each developing device 2 is not particularly limited. For example, a toner to which silica whose surface is hydrophobized with a trimethylsilyl group is externally added can be used.

  An intermediate transfer belt unit 8 is disposed above the photosensitive drum 3. The intermediate transfer belt unit 8 includes an intermediate transfer roller 6 (6a, 6b, 6c, 6d), an intermediate transfer belt 7, an intermediate transfer belt driving roller 81, an intermediate transfer belt driven roller 82, an intermediate transfer belt tension mechanism 83, and an intermediate transfer belt. A transfer belt cleaning unit 9 is provided.

  The intermediate transfer roller 6, the intermediate transfer belt drive roller 81, the intermediate transfer belt driven roller 82, the intermediate transfer belt tension mechanism 83, and the like stretch the intermediate transfer belt 7 and rotate it in the direction of arrow B.

  The intermediate transfer roller 6 is rotatably supported by an intermediate transfer roller mounting portion in the intermediate transfer belt tension mechanism 83 of the intermediate transfer belt unit 8. The intermediate transfer roller 6 provides a transfer bias for transferring the toner image on the photosensitive drum 3 onto the intermediate transfer belt 7.

  The intermediate transfer belt 7 is provided so as to be in contact with each photosensitive drum 3. A color toner image (multicolor toner image) is formed on the intermediate transfer belt 7 by sequentially transferring the toner images of the respective colors formed on the photosensitive drum 3 in a superimposed manner. The intermediate transfer belt 7 is formed in an endless shape using a film having a thickness of about 100 μm to 150 μm.

  Transfer of the toner image from the photosensitive drum 3 to the intermediate transfer belt 7 is performed by the intermediate transfer roller 6 in contact with the back side of the intermediate transfer belt 7. A high voltage transfer bias (a high voltage having a polarity (+) opposite to the toner charging polarity (−)) is applied to the intermediate transfer roller 6 in order to transfer the toner image. The intermediate transfer roller 6 is formed based on a metal (for example, stainless steel) shaft having a diameter of 8 to 10 mm, and the surface is covered with a conductive elastic material (for example, EPDM, urethane foam, or the like). With this conductive elastic material, the intermediate transfer roller 6 can uniformly apply a high voltage to the intermediate transfer belt 7. In the present embodiment, a roller-shaped transfer electrode (intermediate transfer roller 6) is used as the transfer electrode, but a brush or the like can also be used.

  As described above, the electrostatic latent images on the respective photosensitive drums 3 are visualized with toners corresponding to the respective hues to become toner images, and these toner images are stacked on the intermediate transfer belt 7. As described above, the stacked toner images are moved to the contact position between the conveyed paper and the intermediate transfer belt 7 by the rotation of the intermediate transfer belt 7, and are transferred onto the paper by the transfer roller 11 disposed at this position. Transcribed. In this case, the intermediate transfer belt 7 and the transfer roller 11 are pressed against each other at a predetermined nip, and a voltage for transferring the toner image onto the paper is applied to the transfer roller 11. This voltage is a high voltage having a polarity (+) opposite to the charging polarity (−) of the toner.

  In order to obtain the nip constantly, either the transfer roller 11 or the intermediate transfer belt drive roller 81 is made of a hard material such as metal, and the other is a soft material such as an elastic roller (elastic rubber roller or foaming resin roller or the like). ).

  The toner adhering to the intermediate transfer belt 7 due to the contact between the intermediate transfer belt 7 and the photosensitive drum 3 and the toner image remaining on the intermediate transfer belt 7 without being transferred when the toner image is transferred from the intermediate transfer belt 7 to the sheet. The toner is removed and collected by the intermediate transfer belt cleaning unit 9 because it causes toner color mixing in the next step. The intermediate transfer belt cleaning unit 9 is provided with a cleaning blade as a cleaning member that comes into contact with the intermediate transfer belt 7. The portion of the intermediate transfer belt 7 in contact with the cleaning blade is supported by an intermediate transfer belt driven roller 82 from the back side.

  The paper feed cassette 10 is for storing sheets used for image formation, such as recording paper, and is provided below the image forming unit and the exposure unit 1. On the other hand, the paper discharge tray 15 provided on the upper part of the image forming apparatus A is for placing printed sheets face down.

  Further, the image forming apparatus A is provided with a sheet conveyance path S for sending the sheets of the paper feed cassette 10 and the sheets of the manual feed tray 20 to the paper discharge tray 15 via the transfer roller 11 and the fixing unit 12. . In a portion from the sheet feeding cassette 10 to the sheet discharge tray 15 in the sheet conveyance path S, a pickup roller 16, a registration roller 14, a transfer unit including a transfer roller 11, a fixing unit 12, a conveyance roller 25, and the like are arranged. Yes.

  The conveyance rollers 25 are small rollers for promoting and assisting conveyance of the sheet, and a plurality of conveyance rollers 25 are provided along the sheet conveyance path S. The pickup roller 16 is a drawing roller that is provided at the end of the paper feed cassette 10 and supplies sheets from the paper feed cassette 10 to the sheet conveyance path S one by one. The registration roller 14 temporarily holds the sheet conveyed in the sheet conveyance path S, and conveys the sheet to the transfer unit at a timing when the leading edge of the toner image on the photosensitive drum 3 and the leading edge of the sheet are aligned.

  The fixing unit 12 includes a heat roller 31, a pressure roller 32, and the like. The heat roller 31 and the pressure roller 32 rotate with a sheet interposed therebetween. The heat roller 31 is controlled by a control unit (not shown) so as to have a predetermined fixing temperature. This control unit controls the heat roller 31 based on a detection signal from a temperature detector (not shown). The heat roller 31 heat-presses the sheet together with the pressure roller 33 to melt, mix, and press the color toner images transferred to the sheet, and heat-fix the sheet. The sheet on which the multicolor toner image (each color toner image) has been fixed is conveyed to the reverse sheet discharge path of the sheet conveyance path S by the plurality of conveyance rollers 25 and is reversed (the multicolor toner image is on the lower side). Are discharged onto the paper discharge tray 15.

  Next, the sheet conveyance operation by the sheet conveyance path S will be described, including processing in each unit. As described above, the image forming apparatus A is provided with the paper feed cassette 10 that stores sheets in advance and the manual feed tray 20 that is used when printing a small number of sheets. The pickup rollers 16 (16-1, 16-2) are respectively disposed in both of these, and these pickup rollers 16 supply sheets one by one to the sheet conveyance path S.

(For single-sided printing)
The sheet conveyed from the sheet feeding cassette 10 is conveyed to the registration roller 14 by the conveyance roller 25-1 in the sheet conveyance path S, and the registration roller 14 and the toner images stacked on the intermediate transfer belt 7 by the registration roller 14. Is conveyed to the transfer section at the timing of alignment with the leading edge of the sheet. In the transfer portion, a toner image is transferred onto the sheet, and this toner image is fixed on the sheet by the fixing unit 12. Thereafter, the sheet is discharged from the discharge roller 25-3 onto the discharge tray 15 through the conveyance roller 25-2.

  Further, the sheet conveyed from the manual feed tray 20 is conveyed to the registration roller 14 by a plurality of conveyance rollers 25 (25-6, 25-5, 25-4). Thereafter, the sheet is discharged to the discharge tray 15 through the same process as that of the sheet supplied from the sheet cassette 10.

(For duplex printing)
As described above, the single-sided printing is completed, and the sheet that has passed through the fixing unit 12 is chucked at the trailing edge by the paper discharge roller 25-3. Next, the sheet is guided to the transport rollers 25-7 and 25-8 by the reverse rotation of the paper discharge roller 25-3, and is printed on the back surface through the registration roller 14, and then discharged to the paper discharge tray 15. Is done.

  Next, the configuration of the charger 5 will be described. Since the four chargers (5a to 5d) provided in the image forming apparatus A shown in FIG. 2 have the same configuration, the following description will be made as the charger 5 without distinction.

  FIG. 3 is a cross-sectional view schematically showing the configuration of the charger 5 and its periphery. The charger 5 is a corona discharge type charger, and has a length substantially equal to the length of the cylindrical photosensitive drum 3 in the longitudinal direction (perpendicular to the paper surface in FIG. 3), and extends along the photosensitive drum 3. is set up.

  The charger 5 includes a charge generation unit 50 that generates charges, a mesh grid electrode 54 that is interposed between the charge generation unit 50 and the photosensitive drum 3, and a grid electrode that fixes the grid electrode 54 to the charge generation unit 50. A holding member 55 is provided.

  The charge generation unit 50 includes a charger case 51, a discharge electrode (first discharge electrode) 53a, a discharge electrode (second discharge electrode) 53b, and a discharge electrode holding member 52 that fixes the discharge electrodes 53a and 53b to the charger case 51. is doing. The charger case 51 is formed in a U-shaped cross section, and is arranged so that the opening portion faces the photosensitive drum 3.

  The grid electrode holding member 55 and the discharge electrode holding member 52 are insulating members and insulate between the charge generation unit 50 and the grid electrode 54 and between the discharge electrodes 53a and 53b and the charger case 51.

  A first power supply 56 is connected to the grid electrode 54 and the charger case 51, and a voltage (DC bias voltage) having a potential difference Vg (Vg <0) with respect to the ground potential is applied.

  A second power source 57 is connected to the discharge electrodes 53a and 53b. The second power source 57 uses a voltage having a pulse waveform (rectangular waveform) in which a discharge period in which a discharge voltage is applied and a discharge pause period in which the application of the discharge voltage is periodically switched, and the discharge period for the discharge electrodes 53a and 53b is alternated. To be generated. Here, the voltage applied to the discharge electrode during the discharge period is a voltage having a potential difference Vc (Vc <Vg <0) with respect to the ground potential. Details of the voltage waveform applied by the second power source 57 and the second power source 57 will be described later.

  Thus, by applying a discharge voltage to the discharge electrodes 53a and 53b, an electric field is generated between the charger case 51 and the discharge electrodes 53a and 53b, and the atmosphere is ionized by this electric field, so that the discharge electrodes 53a and 53b Negative charges (negative ions) are generated in the vicinity. The generated negative charge moves toward the photosensitive drum 3 while being attracted to the grid electrode 54 and spreads, and what passes through the grid electrode 54 reaches the surface of the photosensitive drum 3.

  The surface of the photosensitive drum 3 is made of a member having semiconductivity when not exposed and having conductivity when exposed. As described above, the surface of the photosensitive drum 3 is initialized and charged by the negative charge reaching the surface of the photosensitive drum 3, and becomes a predetermined initialization charging potential VO. When the surface of the initialized photosensitive drum 3 is exposed, the portion (bright portion) becomes conductive, and the negative charge moves to the ground. In the portion where the negative charge has moved, the potential changes in the positive direction and becomes the bright portion potential VE. An electrostatic latent image is formed by a portion where the negative charge is present and a portion where the negative charge is not present on the surface of the photosensitive drum 3, that is, a portion of the initialization charging potential VO and a portion of the bright portion potential VE.

  As shown in FIG. 4, the discharge electrodes 53a and 53b of the charger 5 are formed in a sawtooth shape, and discharge is performed from the tip portions (projecting portions) t1 and t2 of the saw blades in both electrodes. That is, the charger 5 is a saw-tooth charger in which the discharge electrodes 53a and 53b have a saw-tooth shape.

  FIG. 5 is an explanatory diagram showing the positional relationship between the sawtooth tips t1 and t2 of both electrodes when the discharge electrodes 53a and 53b are viewed from the photosensitive drum 3 side. The black dots shown in the figure indicate the tip portions t1 and t2.

  As shown in the figure, the respective tip ends t1 of the sawtooth in the discharge electrode 53a are provided at equal intervals with a pitch p. Similarly, with respect to the discharge electrode 53b, the respective sawtooth tip portions t2 are provided at equal intervals with a pitch p. The discharge electrode 53 a and the discharge electrode 53 b are provided substantially parallel to each other along the longitudinal direction of the charger 5, and the positions of the tip portions t 1 and t 2 of both electrodes are relative to the longitudinal direction of the charger 5. They are arranged so as to be shifted from each other by ½ pitch (p × ½).

  Next, the configuration of the second power supply 57 will be described. FIG. 6 is a circuit diagram showing a configuration example of the second power supply 57. The second power source 57 shown in this figure includes a constant current power source unit 70 and switching circuit units Sa and Sb.

  The constant current power supply unit 70 is connected to the discharge electrode 53a through the switching circuit unit Sa, and is connected to the discharge electrode 53b through the switching circuit unit Sb.

  The constant current power supply unit 70 includes a high voltage power supply unit 71, electric resistors 72 and 73, and a capacitor 74. One end of the high-voltage power supply unit 71 is connected to the switching circuit units Sa and Sb, the other end is connected to the ground via the electric resistor 72, and an electric resistor 73 and a capacitor 74 connected in parallel to the electric resistor 72. And is connected to ground through a series circuit. Further, the voltage at the terminal on the electric resistance 73 side of the capacitor 74 is connected to the high voltage power supply unit 71, and the voltage smoothed by the capacitor 74 is fed back to the high voltage power supply unit 71. Based on the fed back voltage, the high voltage power source 71 controls the supply voltage so as to supply a constant current to the discharge electrodes 53a and 53b. Thereby, the constant current power supply unit 70 supplies a constant current to the discharge electrodes 53a and 53b, and drives the charging device at a constant current.

  The switching circuit unit Sa includes a Zener diode 75a and a photocoupler 76a. The photocoupler 76a includes a light emitting diode 77a and a transistor 78a as a light receiving element, and a parallel circuit of the transistor 78a and a Zener diode 75a is connected between the constant current power supply unit 70 and the discharge electrode 53a. Similarly, the switching circuit unit Sb includes a Zener diode 75b and a photocoupler 76b. The photocoupler 76b includes a light emitting diode 77b and a transistor 78b as a light receiving element, and a parallel circuit of the transistor 78b and a Zener diode 75b is connected between the constant current power supply unit 70 and the discharge electrode 53b. In the present embodiment, Zener diodes 75a and 75b having a Zener voltage of 1 kV are used.

  The light emitting diodes 77a and 77b are connected to a control means (not shown), and the light emission state is controlled by a current supplied from the control means. As shown in FIG. 1, the light emitting diodes 77a and 77b are controlled such that the light emitting period and the non-light emitting period are periodically switched. Further, the light emitting diode 77b is controlled to be a non-light emitting period during the light emitting period of the light emitting diode 77a, and the light emitting diode 77a is controlled to be a non light emitting period during the light emitting period of the light emitting diode 77b. In the present embodiment, a rectangular wave current having a frequency of 2 kHz and a duty ratio of 50% is supplied to each of the light emitting diodes 77a and 77b so that the light emission period per time is 250 μs.

  When the light emitting diode 77a is in the light emission period, the transistor 78a is turned on, and a voltage equal to the supply voltage (4 kV in this embodiment) of the constant current power supply unit 70 is applied to the discharge electrode 53a. By setting the supply voltage to a voltage equal to or higher than the discharge start voltage, discharge occurs in the discharge electrode 53a. On the other hand, during the light emitting period of the light emitting diode 77a, the light emitting diode 77b is in a non-light emitting period, and the transistor 78b is in a cut-off state. In this case, the voltage applied to the discharge electrode 53b is a voltage (this embodiment) obtained by subtracting the Zener voltage (1 kV in this embodiment) of the Zener diode 75b from the supply voltage (4 kV in this embodiment) of the constant current power supply unit 70. 3 kV).

  Thereby, as shown in FIG. 1, during the discharge period by the discharge electrode 53a, the discharge electrode 53b is controlled so that no discharge occurs. During the light emission period of the light emitting diode 77b, control is performed so that a discharge occurs in the discharge electrode 53b and no discharge occurs in the discharge electrode 53a.

  Here, the result of the experiment conducted in order to investigate the reduction effect of the ozone generation amount by the charger 5 according to the present embodiment will be described.

  Next, a description will be given of the results of experiments conducted for examining the ozone reduction effect by the charger 5 according to the present embodiment. FIG. 7 is an explanatory diagram showing the configuration of the experimental apparatus used in this experiment.

  As shown in FIG. 7, an acrylic case 61 having a width of 400 mm, a height of 250 mm, and a length of 750 mm is prepared, and the photosensitive drum 3, the charger 5, and the light static eliminator are placed in a substantially sealed state by the acrylic case 61. 65 etc. were arranged. The photosensitive drum 3 is configured to be rotationally driven at a peripheral speed of 124 mm / s by a driving unit (not shown). Further, EG2001F manufactured by Sugawara Jitsugyo Co., Ltd. was used as the ozone measuring device 64 for measuring the ozone concentration in the acrylic case 61.

  As the discharge electrodes 53a and 53b provided in the charger 5, a sawtooth electrode made of a stainless steel plate having a thickness of 0.1 mm subjected to double-side etching and a sawtooth tip pitch (electrode pitch) of 2.0 mm was used. . In addition, the distance between the discharge electrode 53a and the discharge electrode 53b was set to 2 mm, and the tip portions of the saw teeth in both electrodes were arranged so as to be shifted by 1/2 pitch with respect to the main scanning direction.

  The amplifiers 63a and 63b are connected to a pulse generator 62 that outputs a pulse wave voltage, and the amplifiers 63a and 63b are connected to the discharge electrodes 53a and 53b, thereby supplying the pulse wave voltage to both discharge electrodes. did. As the amplifiers 63a and 63b, an AC / DC amplifier HVA4321 manufactured by NF Circuit Design Block Co., Ltd. is used, and a rectangular wave with an output of 0 V (discharge pause period), −6300 V (discharge period), and a frequency of 5 kHz has a duty ratio of 50%. Applied. In addition, the timing of applying a voltage to both electrodes was controlled so that the discharge periods of the discharge electrodes 53a and 53b were alternately switched.

  A first power source 56 was connected to the grid electrode 54 and the charger case 51, and a voltage having a potential difference Vg = −610 V was applied to the ground potential. As the charger case 51, a case having a width of 15 mm, a height of 15 mm, and a length of 390 mm was used.

  Using this experimental apparatus, the ozone concentration in the acrylic case 61 was measured when the photosensitive drum 3 was charged by the charger 5 while rotating at a peripheral speed of 124 mm / s. As a measuring method, the measurement value of the ozone measuring device 64 from immediately after the start of charging to 3 minutes after the start of charging was read every 15 seconds. Further, as a comparative example, the same measurement was performed for the case where a DC voltage of −6300 V was applied to the discharge electrodes 53a and 53b using the experimental apparatus described above.

  Table 1 shows the measurement results for the case of applying a pulse waveform voltage and the case of applying a DC voltage. FIG. 8 shows the ozone concentration 30 seconds after the start of charging, the ozone concentration 3 minutes after the start of charging, and the charging start immediately after the start of charging for each of the case where a pulse waveform voltage is applied and the case where a DC voltage is applied. The graph of the average value of ozone concentration until after 3 minutes is shown.

  As shown in Table 1 and FIG. 8, when a DC voltage was applied, the ozone concentration increased with the lapse of time after the start of charging, whereas when a pulse waveform voltage was applied, the voltage after the start of charging was increased. The ozone concentration decreased with time. In addition, by applying a voltage having a pulse waveform, the average value of the ozone concentration for 3 minutes after the start of charging was reduced by about 85% compared to the case where a DC voltage was applied.

  As described above, the charger according to the present embodiment applies a voltage having a pulse waveform (rectangular waveform) in which the discharge period and the discharge pause period are periodically switched to the discharge electrode, and discharge period per one time. Is set to 250 μs.

  Thus, by shortening the period during which the discharge voltage is applied to the discharge electrode, the amount of ozone generated due to the discharge can be reduced.

  Further, the charger according to the present embodiment includes two discharge electrodes, and the voltage of the pulse waveform (rectangular waveform) in which the discharge period and the discharge pause period are periodically switched is applied to each of the discharge electrodes. The discharge period is applied alternately.

  Thus, by alternately generating the discharge period of each discharge electrode, it is possible to eliminate (or shorten) the period in which no discharge occurs in any discharge electrode, and to reduce charging unevenness in the sub-scanning direction. .

  Further, in the present embodiment, the positions of the tip portions of the respective discharge electrodes are arranged so as to be shifted from each other by ½ pitch with respect to the longitudinal direction of the charger. Thereby, the front end portions of both electrodes are prevented from interfering with each other, and uneven charging in the main scanning direction (longitudinal direction of the photosensitive drum 3) due to the pitch of the front end portions of the respective discharge electrodes can be reduced.

  Further, the charger according to the present embodiment includes a constant current power supply unit that supplies a constant current to each discharge electrode. In general, the discharge voltage varies depending on environmental conditions such as humidity and aging of each member, etc., but it is stable even when there is a variation in the discharge voltage by performing constant current drive like the charger of this embodiment. Charging can be performed.

  The charger according to the present embodiment includes only one high-voltage power supply (high-voltage power supply unit 71), and discharge electrodes are connected in parallel to the common output terminal of the high-voltage power supply via the switching circuits. . Thereby, apparatus cost can be reduced rather than the case where multiple high voltage power supplies are provided.

  Each switching circuit includes a parallel circuit of a Zener diode and a transistor. Thereby, the configuration of each switching circuit can be simplified. Further, by controlling the conduction / cutoff of the transistor, the discharge period and the discharge rest period for the discharge electrode can be switched easily and appropriately.

  Further, the charger according to the present embodiment includes a grid electrode to which a DC voltage is applied between the discharge electrode and the photosensitive drum. Thereby, the amount of charged particles passing in the direction of the photosensitive drum can be controlled, charging unevenness can be reduced, and charging uniformity in the photosensitive drum can be improved.

  In the present embodiment, the discharge period per discharge in each discharge electrode is 250 μs, but the present invention is not limited to this. In this embodiment, a pulse waveform voltage having a duty ratio of 50% is applied so that the length of the discharge period is equal to the length of the discharge pause period. However, the present invention is not limited to this, and the lengths of both periods are different. It may be allowed.

However, if the discharge period is too short, for example, the waveform of the voltage applied to the discharge electrode becomes dull and it becomes difficult to control the charging potential, so that the photosensitive drum 3 cannot be appropriately charged. . For this reason, in order to appropriately charge the photosensitive drum 3, the discharge period is preferably set to 10 ⁻⁵ seconds or more. In addition, if the discharge period is too long, the amount of ozone generated cannot be reduced, so the discharge period is preferably set to 10 ⁻² seconds or less. Although the technical field is different from that of a corona discharger, Patent Document 2 that discloses a technique related to an ion generator provided in an air purifier or the like describes the amount of ozone generated when a voltage application time to an electrode exceeds 0.01 seconds. Is described to increase rapidly.

  In addition, when the discharge period and the discharge rest period are alternately generated, charging unevenness in the sub-scanning direction (the rotation direction of the photosensitive drum 3) may occur. For this reason, assuming that the process speed, that is, the image forming speed in the sub-scanning direction with respect to the recording paper (equal to the peripheral speed of the photosensitive drum in this embodiment) is Vp (mm / s), the pulse cycle of the voltage applied to the discharge electrode F (pps) is preferably set so as to satisfy the relationship of Vp × F ≦ 0.2. If the pulse period F of the applied voltage and the process speed Vp satisfy this relationship, even if charging unevenness occurs in the sub-scanning direction, the pitch of the charging unevenness in the sub-scanning direction can be reduced to 0.2 mm or less. The accompanying deterioration in image quality can be suppressed to a level that cannot be recognized by human vision.

  Moreover, although the charger according to the present embodiment includes two discharge electrodes, the present invention is not limited to this. However, when a plurality of discharge electrodes are provided, it is preferable that the discharge periods of the discharge electrodes are sequentially generated so as not to overlap each other in order to more effectively reduce uneven charging in the sub-scanning direction. For example, when three discharge electrodes 53a, 53b, and 53c are provided, they may be set as shown in FIG. However, the present invention is not limited to this. For example, a part of the discharge periods of the plurality of discharge electrodes may be generated at a timing different from the discharge periods of the other discharge electrodes.

  In the present embodiment, the case where sawtooth electrodes are used as the discharge electrodes 53a and 53b has been described. However, the shape of each discharge electrode is not limited thereto. For example, a wire-like electrode disposed along the longitudinal direction of the charger 5 (arranged so that the extending direction faces the photosensitive drum 3) may be used. In the case where wire-like discharge electrodes 53a and 53b are used, it is preferable to provide projecting portions with leading end portions t1 and t2 projecting toward the photosensitive drum 3 as shown in FIG.

  Further, as shown in FIG. 10B, a plurality of discharge electrodes 53a and 53b are arranged such that tip portions (projecting portions) t1 and t2 are arranged in a line at a pitch p along the longitudinal direction of the charger 5. You may use the discharge electrode which consists of a needle-like electrode. Alternatively, as shown in FIG. 10 (c), a discharge electrode comprising a plurality of linear electrodes in which tips (projections) t1, t2 are arranged in a line at a pitch p along the longitudinal direction of the charger 5. May be used.

  In the case where a plurality of discharge electrodes are provided, the position of the tip portion of each discharge electrode that is the starting point of discharge in the longitudinal direction of the charger is different from the position of the tip portion of the adjacent discharge electrode in the longitudinal direction of the charger. It is preferable. Specifically, for example, when three discharge electrodes are provided, the position of the tip of each discharge electrode may be varied by 1/3 pitch in the longitudinal direction of each discharge electrode. Thereby, the front ends of the discharge electrodes can be prevented from interfering with each other, and uneven charging in the main scanning direction (longitudinal direction of the photosensitive drum 3) due to the pitch of the front ends of the discharge electrodes can be reduced.

  In the present embodiment, the scorotron charger including the grid electrode has been described. However, the present invention is not limited thereto, and a corotron charger not including the grid electrode may be used.

  The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the claims. That is, embodiments obtained by combining technical means appropriately changed within the scope of the claims are also included in the technical scope of the present invention.

  The present invention can be applied to a corona discharge charging device.

It is explanatory drawing which shows the waveform of the applied voltage with respect to the discharge electrode in the charging device concerning one Embodiment of this invention. 1 is an explanatory diagram illustrating a configuration of an image forming apparatus including a charging device according to an embodiment of the present invention. FIG. 3 is a cross-sectional view schematically showing a configuration around a charging device in the image forming apparatus shown in FIG. 2. It is the perspective view which showed typically the structure of the charging device in one Embodiment of this invention. It is explanatory drawing which shows the positional relationship of each discharge electrode with which the charging device in one Embodiment of this invention is equipped. It is a circuit diagram which shows the structure of the power supply part of the charging device concerning one Embodiment of this invention. It is explanatory drawing which shows schematic structure of the experimental apparatus used for the verification test of the ozone reduction effect. It is a graph which shows the result of the verification test of an ozone reduction effect. It is explanatory drawing which shows the example of the applied voltage waveform with respect to each discharge electrode in the case of providing the three discharge electrodes in the charging device concerning one Embodiment of this invention. It is explanatory drawing which shows the modification of the discharge electrode with which the charging device concerning one Embodiment of this invention is equipped.

Explanation of symbols

3 Photosensitive drum 5 Charger 50 Charge generation unit 51 Charger case 52 Discharge electrode holding members 53a, 53b, 53c Discharge electrode 54 Grid electrode 56 First power source 57 Second power source 70 Constant current power source unit 71 High voltage power source units 72, 73 Electricity Resistor 74 Capacitor 75a, 75b Zener diode 76a, 76b Photocoupler 77a, 77b Light emitting diode 78a, 78b Transistor Sa, Sb Switching circuit part p Pitch t1, t2 Tip (protruding part)

Claims (9)

In a charging device comprising a plurality of discharge electrodes and voltage applying means for applying a voltage to each discharge electrode, and applying a voltage to each discharge electrode to generate a corona discharge to charge an object to be charged.
The voltage applying means includes
A voltage is applied to each discharge electrode so that a discharge period in which a discharge voltage is applied and a discharge pause period in which the application of the discharge voltage is stopped alternately occur, and the discharge periods in the discharge electrodes do not overlap each other. A charging device.   Each of the discharge electrodes has a sawtooth shape arranged such that the tip faces the direction of the object to be charged, a plurality of lines or needles arranged in a row so that the tip faces the direction of the object to be charged, Or a wire-like shape that is arranged so as to extend in parallel to the charging surface of the object to be charged, and that has a plurality of protruding portions whose tip protrudes toward the object to be charged. The charging device according to claim 1. The discharge electrodes are arranged in parallel to each other,
The charging device according to claim 2, wherein the positions of the tips of the adjacent discharge electrodes are different from each other in the parallel direction. The charged object is an electrostatic latent image carrier provided in an image forming apparatus having an image forming speed Vp (mm / s) in the sub-scanning direction.
The voltage application means sets the period F (s) of the discharge period,
Vp × F ≦ 0.2
The charging device according to claim 1, wherein the charging device is set so as to satisfy the relationship.   5. The charging device according to claim 1, wherein the voltage application unit performs constant current driving that sets a discharge current to a constant value. 6. The voltage applying means includes
A plurality of power supplies that are commonly provided for the discharge electrodes, and a plurality of power supplies that are respectively provided between the power supplies and the discharge electrodes and that switch between the discharge periods and the discharge pause periods. The charging device according to claim 1, further comprising a switching unit.   The charging device according to claim 6, wherein the switching unit includes a zener diode and a transistor connected in parallel to each other.   The charging device according to claim 1, further comprising a grid electrode to which a DC voltage is applied between the discharge electrode and the object to be charged.   An image forming apparatus comprising the charging device according to claim 1.

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