JP5322589B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus for irradiating a pre-transfer toner image carried on an image carrier using a corona charger with charged particles, and more particularly to a structure of a shield electrode of a corona charger.

  A charged toner is attached to the electrostatic image formed on the photosensitive member to form a toner image, and the toner image is transferred to a transfer medium (intermediate transfer member or recording material) that is moved over the photosensitive member by a transfer unit. Image forming apparatuses are widely used. A corona charger is placed between the developing device and the transfer unit, and the toner image before transfer, which is carried on the photosensitive member, is irradiated with charged particles to optimize the toner charge amount of the toner image for transfer. Such an image forming apparatus has been put into practical use.

  In an image forming apparatus in which a corona charger is arranged between the developing device and the transfer unit, various problems occur when toner scattered from the developing device enters the corona charger, and various measures to prevent the toner from entering. Has been proposed.

  Patent Document 1 discloses an image forming apparatus in which a corona charger is disposed between a developing device and a transfer unit, and the amount of charge of a black toner image carried on a photosensitive drum is increased by the corona charger and sent to the transfer unit. Indicated. Here, an exhaust fan is disposed to collect the toner scattered from the developing device and accumulated in the casing.

  In Patent Document 2, in order to eliminate the problem of toner entering from a developing device into a corona charger disposed under the developing device, a shield electrode surrounding three sides of the wire electrode has the same polarity as the charging polarity of the toner. A voltage is applied.

Japanese Patent Application Laid-Open No. 08-110738 JP-A-11-184214

  While the diameter of the photosensitive drum is reduced along with the downsizing of the image forming apparatus, the corona charger is limited in that a predetermined discharge is generated evenly at intervals between the central wire electrode and the shield electrode surrounding the three surrounding directions. It is difficult to reduce the cross section.

  For this reason, the corona charger is disposed between the developing device and the transfer unit so as to open the shield electrode so as to face the photosensitive drum, so that the toner easily falls on the inner surface of the shield electrode. . If insulating toner is deposited on the inner surface of the conductive shield electrode, normal discharge with the central wire electrode is hindered, so that the performance of the corona charger is degraded.

  On the other hand, the exhaust fan shown in Patent Document 1 has no space around the corona charger, and the voltage application shown in Patent Document 2 has little effect on scattered toner with a small charge amount. .

  Therefore, by simply cutting off the edge on the transfer portion side of the shield electrode that opens obliquely upward and retracting it from the photosensitive drum side with respect to the edge on the developing device side, the toner falling from the developing device is placed on the inner surface of the shield electrode. It was proposed to make it difficult to enter (see FIG. 5).

  However, when an experiment was performed by actually cutting off the edge of the shield electrode on the transfer portion side, it was confirmed that a transfer failure of the toner image tends to occur on the transfer medium that has passed through the transfer portion.

  An object of the present invention is to provide an image forming apparatus in which toner does not easily enter the inner surface of a shield electrode of a corona charger without causing a transfer failure of a toner image on a transfer medium.

The image forming apparatus of the present invention includes a photosensitive drum , an image forming unit that carries a toner image on the photosensitive drum , a transfer unit that transfers the toner image carried on the photosensitive drum to a transfer medium, and a longitudinal direction of the photosensitive drum. A corona charger is provided between the image forming unit and the transfer unit in a direction along the axial direction of the drum and charges a toner image before transfer . The corona charger includes a wire electrode and a shield electrode that surrounds the wire electrode and has an opening with respect to the photosensitive drum, and the shield electrode is provided on the image forming means side of the wire electrode. An image forming means side shield electrode and a transfer portion side shield electrode provided on the transfer portion side, and an end portion on the photosensitive drum side in the short direction of the transfer portion side shield electrode is the image forming portion. It is located directly below the end of the photosensitive drum side in the short direction of the means side shield electrode, or is retracted to the opposite side of the photosensitive drum from directly below the vertical direction, and the transfer unit side shield electrode toward the image forming unit side from an end portion of the photosensitive drum side in the lateral direction, the shielding unit for shielding the wire electrode from the transfer medium is formed, the photosensitive de The angle of inclination of the line segment OP connecting the leading end P on the photosensitive drum side and the center O of the photosensitive drum in the image forming means side shield electrode with respect to the horizontal direction on the same plane perpendicular to the axial direction of the photosensitive drum is α, The length of the line segment OP is Lop, the inclination angle of the line segment OQ connecting the tip Q of the shielding part and the center O of the photosensitive drum with respect to the horizontal direction is β, and the length of the line segment OQ is Loq. R is a radius of the photosensitive drum, L is a distance from a position W on a line segment OW connecting the position W of the wire electrode and the center O of the photosensitive drum to the surface of the photosensitive drum, and An angle formed between a line segment QW connecting the tip Q and the position W of the wire electrode and the line segment OW is θ, and a tangent on the transfer portion side connecting the position W of the wire electrode and the surface of the photosensitive drum and the line The angle between the minute OW and θm When ax, it is Lop × cos α <Loq × cos β, and 0 <θ <θmax.

  In the image forming apparatus according to the present invention, the lower shield electrode is more imagewise than the upper shield electrode so that toner or the like that pours from above into the opposing distance between the image carrier and the corona charger does not accumulate in the shield electrode. Retreated from the carrier. The lower shield electrode shields a solid angle that is opened from the wire electrode toward the transfer medium in cooperation with a shield portion formed of the same member or a different member. For this reason, the charged particles generated along with the corona discharge around the wire electrode are difficult to directly irradiate the transfer medium, and the transfer medium entering the transfer portion is hardly charged with the charged particles.

  Therefore, it is possible to make it difficult for the toner to enter the inner surface of the shield electrode of the corona charger without causing a transfer failure of the toner image on the transfer medium.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the present invention, as long as the lower edge of the shield electrode of the corona charger is retracted from the photosensitive drum with respect to the upper edge, part or all of the configuration of the embodiment is replaced with the alternative configuration. Other embodiments can also be implemented.

  Accordingly, the corona charger can be arranged not only in the primary transfer portion that transfers the toner image from the photosensitive drum to the intermediate transfer belt but also in the secondary transfer portion that transfers the toner image from the intermediate transfer belt to the recording material. . Not only an image forming apparatus that transfers a toner image from an intermediate transfer belt to a recording material, but also an image forming apparatus that transfers a toner image to a recording material carried on a recording material conveying belt, to a recording material that is fed in a single-wafer type It can also be implemented in an image forming apparatus that transfers a toner image from a photosensitive drum.

  In the present embodiment, only main parts related to toner image formation / transfer will be described. However, the present invention includes a printer, various printing machines, a copier, a fax machine, a composite machine, in addition to necessary equipment, equipment, and a housing structure. It can be implemented in various applications such as a machine.

  In addition, about the general matter of the image forming apparatus shown by patent document 1, 2, illustration is abbreviate | omitted and the overlapping description is abbreviate | omitted.

<Image forming apparatus>
FIG. 1 is an explanatory diagram of a configuration of the image forming apparatus according to the first embodiment, and FIG. 2 is an explanatory diagram of a configuration of a black image forming unit.

  As shown in FIG. 1, the image forming apparatus 100 is a tandem full-color printer in which yellow, magenta, cyan, and black image forming portions PY, PM, PC, and PK are arranged along the intermediate transfer belt 5.

  In the image forming unit PY, a yellow toner image is formed on the photosensitive drum 1 </ b> Y and is primarily transferred to the intermediate transfer belt 5. In the image forming unit PM, a magenta toner image is formed on the photosensitive drum 1M, and is primarily transferred onto the yellow toner image on the intermediate transfer belt 5. In the image forming units PC and PK, a cyan toner image and a black toner image are formed on the photosensitive drums 1C and 1K, respectively, and similarly, the toner images on the intermediate transfer belt 5 are overlapped with each other and sequentially primary transferred.

  The four-color toner images carried on the intermediate transfer belt 5 are conveyed to the secondary transfer portion T2, and are secondarily transferred collectively to the recording material S which is superimposed on the intermediate transfer belt 5 and sandwiched and conveyed by the secondary transfer portion T2. The The recording material S on which the toner image has been secondarily transferred by the secondary transfer portion T2 is heated and pressurized by the fixing device 9, and the toner image is fixed on the surface, and then discharged to the outside.

  The recording material S (for example, paper, transparent film) drawn from the recording material cassette 16 is separated one by one by the separation roller 13 and fed to the registration roller 15. The recording material S waits at the registration roller 15 and is sent to the secondary transfer portion T2 in time with the toner image on the intermediate transfer belt 5.

The intermediate transfer belt 5 is supported around a driving roller 21, a tension roller 22, and a counter roller 23, and rotates in the direction of arrow R <b> 2 when a driving force is transmitted to the driving roller 21. The driving roller has a conductive rubber layer whose resistance is adjusted to 1 × 10 ³ to 1 × 10 ⁵ Ω on a metal core, and the core is grounded.

The intermediate transfer belt 5 uses a polyimide resin film having a thickness of 85 μm as a base material, disperses carbon black, and has a surface resistivity of 1 × 10 ¹² Ω / □ and a volume resistivity of 1 × 10 ⁹ Ω · cm. The resistance is adjusted so that The rotation speed (process speed) of the intermediate transfer belt 5 is 200 mm / sec.

  The intermediate transfer belt 5 is pressed against the surfaces of the photosensitive drums 1Y, 1M, 1C, and 1K by primary transfer rollers 6Y, 6M, 6C, and 6K disposed on the inner peripheral surface. As a result, primary transfer portions (primary transfer nip portions) T1 are formed between the photosensitive drums 1Y, 1M, 1C, and 1K and the intermediate transfer belt 5, respectively.

  Further, a secondary transfer outer roller 24 is disposed at a position corresponding to the opposing roller 23 outside the intermediate transfer belt 5. The intermediate transfer belt 5 is pressed against the secondary transfer roller 24 by the counter roller 23, thereby forming a secondary transfer portion (secondary transfer nip portion) T <b> 2 with the secondary transfer roller 24.

  In the fixing device 9, a pressure roller 9b is pressed against a heating roller 9a having a heater at the center to form a fixing nip for a recording material. The belt cleaning device 10 rubs the intermediate transfer belt 5 with a cleaning blade to remove the transfer residual toner attached to the intermediate transfer belt 5 that has passed through the secondary transfer portion T2.

  Note that the image forming apparatus 100 can also form a black single-color image using only the image forming unit PK. In this case, the toner image forming process is performed only in the image forming unit PK, and only the black monochrome toner image is carried on the intermediate transfer belt 5, and the toner image is secondarily transferred to the recording material S and then fixed. In the image forming apparatus 100, the diameter of the photosensitive drum 1K is 84 mm so as to correspond to a black monochrome image that is frequently used and requires a high process speed and high resolution. In contrast, the other photosensitive drums 1Y, 1M, and 1C have a diameter of 30 mm.

  The image forming units PY, PM, PC, and PK are other than the presence or absence of a corona charger 25 that adjusts the toner color used in the attached developing devices 4Y, 4M, 4C, and 4K and the charge amount of the toner image before transfer. The configuration is almost the same. Hereinafter, the image forming unit PK in which the corona charger 25 is disposed will be described. For the other image forming units PY, PM, and PC, K at the end of the reference numerals in the description is read as Y, M, and C. Shall be explained.

  As shown in FIG. 2, the image forming unit PK includes a charging roller 3K, an exposure device 2K, a developing device 4K, a corona charger 25, a primary transfer roller 6K, and a cleaning device 7K around the photosensitive drum 1K. Yes.

  The photosensitive drum 1K is composed of a metal cylinder having a negatively charged photosensitive layer formed on the surface thereof, and rotates in the direction of arrow R1 at a process speed of 200 mm / sec.

As the photosensitive drum 1K, a commonly used organic photoreceptor or the like can be used. Preferably, a photosensitive layer having a surface layer with a resistance value of 10 ⁹ to 10 ¹⁴ Ω · cm, an amorphous silicon photoreceptor, or the like is used. . As a result, charge injection charging can be realized, which is effective in preventing the generation of ozone and reducing power consumption, and the chargeability can also be improved.

  In this embodiment, a negatively charged organic photoconductor is employed, and the photosensitive layer is formed by sequentially providing the following first to fifth layers on an aluminum drum base having a diameter of 84 mm. Forming.

The first layer is an undercoat layer, and is a layer provided for leveling defects and the like of the aluminum substrate, and is made of a conductive layer having a thickness of 20 μm. The second layer is a positive charge injection preventing layer and serves to prevent the positive charge injected from the drum base from canceling the negative charge charged on the surface of the photoreceptor. The positive charge injection preventing layer is a medium resistance layer having a thickness of 1 μm, the resistance of which is adjusted to about 1 × 10 ⁶ Ω · cm with an alamin resin and methoxymethylated nylon.

  The third layer is a charge generation layer, which is a layer having a thickness of about 0.3 μm in which a diazo pigment is dispersed in a resin, and generates positive and negative charge pairs upon exposure. The fourth layer is a charge transport layer, which is obtained by dispersing hydrazone in a polycarbonate resin, and is a P-type semiconductor. Therefore, negative charges charged on the surface of the photosensitive layer cannot move through this layer, and only positive charges generated in the charge generation layer can be transported to the surface of the photosensitive layer.

The fifth layer is a charge injection layer, and is a coating layer made of a material in which SnO ₂ ultrafine particles are dispersed in an insulating resin binder. Specifically, the insulating resin is doped with antimony, which is a light-transmissive insulating filler, to reduce the resistance (conductivity), and 70 wt% of SnO ₂ particles having a particle size of 0.03 μm are dispersed in the resin. It is a coating layer of the material. When mixed in this way, the charge injection layer can be formed by coating the product to a thickness of about 3 μm by a suitable coating method such as dipping, spray coating, roll coating, or beam coating.

  The charging roller 2K rotates while being pressed against the photosensitive drum 1K. The power source D3 applies an oscillating voltage in which an AC voltage is superimposed on a DC voltage to the charging roller 2K, and charges the surface of the photosensitive drum 1K to a uniform negative potential.

  The exposure device 3K scans the scanning line image data obtained by developing the image data with a polyhedral mirror, and writes an electrostatic image of the image on the surface of the charged photosensitive drum 1K.

  The developing device 4K stirs the two-component developer in the developing container 41 to charge the nonmagnetic toner to negative polarity and the magnetic carrier to positive polarity. The developing sleeve 42 rotating around the fixed magnet 43 rubs the photosensitive drum 1K while carrying the two-component developer charged on the surface by the magnetic force of the fixed magnet 43 in a spiked state. The power source D4 applies an oscillating voltage obtained by superimposing an AC voltage to a negative DC voltage to the developing sleeve 42, and causes the toner to fly to and adhere to the electrostatic image with the photosensitive drum 1K. Reverse development.

  The toner replenishing container 8K contains black toner for replenishment, and replenishes the developing container 41 with unused toner that is consumed only for developing the electrostatic image on the photosensitive drum 1K by the developing device 4K.

  The corona charger 25 is disposed between the developing device (image forming means) 4K and the primary transfer portion (transfer portion) T1, and irradiates the toner image before transfer with charged particles. The shield electrode 27 surrounds three directions with the wire electrode 26 as the center and opens the photosensitive drum (image carrier) side obliquely upward, and has an opening length of 14 mm and an opening depth of 15 mm. The corona charger 25 is provided with a cleaning mechanism 28 that cleans foreign matter by rubbing the pad along the wire electrode 26, and the external dimensions including the cleaning mechanism 28 are 30 mm × 15 mm × 350 mm.

  In the corona charger 25, the primary transfer portion T1 side (transfer portion side) of the shield electrode 27 is retracted in a planar position to the opposite side of the photosensitive drum (image carrier) 1K from the developing device 4K side (image forming means side). ing. On the primary transfer portion T1 side (transfer portion side) of the shield electrode 27, a shielding portion is formed of the same member so as to shield the wire electrode 26 from the intermediate transfer belt (transfer medium) 5. Specifically, the tip of the shield electrode 27 is bent toward the developing device 4K side (image forming means side).

  The power source D5 performs constant current control on the negative voltage applied to the wire electrode 26 so that the total discharge current through the wire electrode 26 is maintained at 300 to 500 μA. 10% to 20% of the total discharge current becomes charged particles with which the photosensitive drum 1K is irradiated to adjust the charge amount of the toner image.

  A tray (falling toner receiving member) is disposed between the corona charger 25 and the intermediate transfer belt (transfer medium) 5 so that the plane position is overlapped with the facing distance between the corona charger 25 and the photosensitive drum (image carrier). ) 30 is arranged.

  The primary transfer roller 6K is pressed against the intermediate transfer belt 5 to form a primary transfer portion T1 of the toner image between the photosensitive drum 1K and the intermediate transfer belt 5. The transfer power supply DK applies a positive direct current voltage controlled at a constant voltage to the primary transfer roller 6K, and negatively charges the toner image carried on the photosensitive drum 1K for primary transfer to the intermediate transfer belt 5. .

The primary transfer roller 6K includes a conductive foam having a resistance value of 5 × 10 ⁶ [Ω · cm] and a thickness of 4.0 mm around a cylindrical cored bar member having a diameter of 8 mm made of a conductive metal. The outer diameter is 16 mm. As the primary transfer roller 6K, an ion conductive sponge roller formed of a blend of nitrile rubber and ethylene-epichlorohydrin copolymer was used. The resistance value of the primary transfer roller 6K was about 106 to 108Ω when measured with an applied voltage of 2 kV in an environment of a temperature of 23 ° C. and a humidity of 50%.

  The primary transfer roller 6K is pressurized with a total pressure of 5 kPa upwardly in the vertical direction from the inner surface of the intermediate transfer belt 5 by the springs at both ends in order to bring the intermediate transfer belt 5 into contact with the photosensitive drum 6K. The position of the primary transfer roller 6k is shifted by 1.5 mm from the center vertical direction of the photosensitive drum 1K to the downstream side in the rotation direction of the intermediate transfer belt 5.

As the primary transfer roller 6K, it is preferable to use a semiconductive one having a resistance value of 10 ² to 10 ⁸ Ω when 2000V is applied.

  The cleaning device 7K rubs the cleaning blade 7b against the photosensitive drum 1K to remove and collect the transfer residual toner attached to the surface of the photosensitive drum 1K that has passed through the primary transfer portion T1.

<Corona charger>
FIG. 3 is an explanatory view of the toner charge amount of each color toner, and FIG. 4 is an explanatory view of the behavior of the toner entering the corona charger. FIG. 5 is an explanatory view of a corona charger in which the lower edge of the shield electrode is cut off. FIG. 6 is an explanatory view of the influence of the cut amount of the shield electrode on the primary transfer current.

  As shown in FIG. 3, the behavior of the toner charge amount with respect to the durable number in a high humidity and high temperature environment is different for each color of yellow, magenta, cyan, and black. When the toner charge amount is about 5 to 10 μC / g smaller than the other colors and the toner charge amount is different from the other colors throughout the endurance life of the image forming apparatus 100, the black toner is collectively applied to the recording material S at the secondary transfer portion T2. When performing the secondary transfer, a difference occurs in the transfer characteristics at the secondary transfer portion T2. For this reason, it is necessary to make the toner charge amount of the black toner equal to the toner charge amount of the other color toner at the time of the primary transfer to the intermediate transfer belt 5.

  For this reason, the corona charger 25 is installed only in the black image forming portion PK, and the black toner image carried on the photosensitive drum 1K is irradiated with negative charged particles to change the toner charge amount of the black toner image to other colors. Reinforced like a toner image. The reason why the toner charge amount of the black toner is lower than that of the other color toner is that the black toner contains carbon and has a lower resistance than that of the other color toner, so that the charged charge easily escapes. Particularly, the resistance is remarkably lowered in a high temperature and high humidity environment, and the toner charge amount is markedly lower than that of other color toners.

  For this reason, in this embodiment, the corona charger 25 is operated when the temperature is 23 ° C. or higher or the relative humidity is 50% RH or higher. In the following experiment, an image was formed in an environment of a temperature of 30 ° C. and a relative humidity of 80% RH, and the improvement effect of the corona charger 25 was confirmed.

  As shown in FIG. 4A, when the corona charger 25 is arranged and an experiment of continuous image formation by a halftone image having a reflection density of 0.5 is performed, the vertical direction in the rotation direction is increased after the cumulative number of 50,000 sheets. A stripe-like density unevenness image is generated.

  When the primary transfer portion T1 side of the shield electrode 27 of the corona charging device 25 is observed, scattered toner is accumulated in a concentrated manner so as to correspond to the vertical streak occurrence portion. Therefore, accumulation of dirt due to the scattered toner causes image defects. Conceivable. Therefore, the toner at the density unevenness occurrence location on the intermediate transfer belt 5 was collected and the toner charge amount was measured. Then, it was confirmed that the toner charge amount at the portion where density unevenness did not occur was about 23 μC / g, whereas it was as low as 15 μC / g at the density uneven portion. The toner charge amount was measured by the suction type Faraday cage method described in JP-A-2008-233373.

  From the experimental results, corona discharge is less likely to occur in the portion where the toner is accumulated on the shield electrode 27 than in other portions, charging efficiency is deteriorated, and toner charge amount unevenness is formed on the toner image on the photosensitive drum 1K. It is thought that it was done. It is considered that the uneven toner charge amount becomes the primary transfer unevenness with respect to the intermediate transfer belt 5 and the density unevenness is generated in the final image.

  As shown in FIG. 4B, therefore, the corona charger 25 is disposed close to the photosensitive drum 1K, and the edge of the shield electrode 27 on the developing device 4K side is positioned from the developing position of the photosensitive drum 1K by the developing device 4K. Was bitten into the photosensitive drum 1K side.

  By arranging in this way, it has been confirmed that the toner falling from the developing portion does not easily enter the corona charger 25, and the accumulation of scattered toner on the primary transfer portion T1 side of the shield electrode 27 can be suppressed. However, when the number of image formations is increased in such an arrangement, scattered toner and carriers fall on the upper surface of the shield electrode 27, and the scattered toner randomly adheres to the tip of the shield electrode 27 on the developing device 4K side. As a result, abnormal discharge occurs between the toner image on the photosensitive drum 1K at the portion where the deposit is deposited on the tip of the shield electrode 27, and a vertical stripe-shaped defective image is formed. .

  As shown in FIG. 5, therefore, the corona charger 25 is moved backward until the upper edge P of the shield electrode 27 is located immediately below the developing portion GB of the photosensitive drum 1K, and the upper edge P of the shield electrode 27 and the photosensitive drum 1K. A sufficient distance was ensured. Then, the lower edge (transfer portion side) Q of the shield electrode 27 in the cross section of the corona charger 25 is cut short, so that there is no room for the toner falling from the developing portion GB to fall inside the shield electrode 27.

  Then, 50,000 continuous images were formed in a state where the corona charger 25 was operated with different cut amounts of the lower edge Q of the shield electrode 27, and the occurrence of density unevenness in the final image was evaluated. The plane position that overlaps the upper edge P of the shield electrode 27 is set to the origin S2 = 0, and the position S2 of the lower edge Q of the shield electrode 27 is varied according to the seven-stage conditions shown in Table 1.

  In Table 1, S = -3 and -1 are cut-off states, and S = 1, 2, 3, and 4 are lower edges Q (tips Q) projecting toward the photosensitive drum 1K side from the upper edges P (tips P). Thus, there is room for toner to accumulate inside the shield electrode 27. Further, the level of the image density unevenness is determined by x when the density difference Δ between the maximum value and the minimum image density on the paper measured by the reflection densitometer X-rite is 0.3 or more, and the density difference Δ is 0. A case of 3 to 0.15 was evaluated as Δ, and a case of 0.1 or less was evaluated as ○.

  As shown in Table 1, the lower edge Q (tip Q) of the shield electrode 27 overlaps the photosensitive drum 1K side in a planar position with respect to the upper edge P (tip P) (S2 = 0), or the photosensitive drum 1K. Good results can be obtained under conditions 1 to 3 where S is retracted to the opposite side (S = -1, -3). Under conditions 1 to 3, it was confirmed that the toner contamination on the inner surface of the shield electrode 27 was greatly improved and no image density unevenness occurred.

  However, when a solid image was output under the conditions 1 to 3, a defective image having a leopard pattern in which the image density decreased in the form of spots in the final image on the recording material S was generated. Therefore, the voltage-current characteristics at the primary transfer portion T1 were measured under conditions 1, 3, 5, and 7 in Table 1.

  As shown in FIG. 6, as a result, it was found that the discharge start voltage is shifted to the minus side as the lower edge Q of the shield electrode 27 is moved away from the photosensitive drum 1K.

  Next, as shown in FIG. 5B, a neutralizing needle 51 is installed at the position of the intermediate transfer belt 5 upstream of the primary transfer portion T1, and charged particles flowing from the corona charger 25 toward the intermediate transfer belt 5 are disposed. The amount of leakage current was measured. As a result, as shown in Table 1, it was found that the leakage current amount increased as the lower edge Q of the shield electrode 27 moved away from the photosensitive drum 1K.

  It was also confirmed that the image defect of the leopard pattern disappeared by increasing the primary transfer current.

  Accordingly, the image defect of the leopard pattern is caused by negatively charged particles attracted to the positive potential of the primary transfer voltage applied to the primary transfer roller 6K to charge the intermediate transfer belt 5 to the negative polarity, and a necessary transfer current is generated. It turned out that it was raised.

<Example 1>
FIG. 7 is an explanatory diagram of the shield electrode in the corona charger of Example 1, and FIG. 8 is an explanatory diagram of the shield electrode of the comparative example.

  As shown in FIG. 7, in Example 1, the planar positions of the upper edge P and the lower edge Q of the shield electrode 27 are overlapped, and the tip of the lower edge Q toward the upper edge P is slightly L-shaped. It was formed by bending into a mold. An angle formed by a straight line connecting the wire electrode 26 and the lower edge Q and a straight line connecting the center of the photosensitive drum 1K and the wire electrode 26 is defined as θ. Let L be the distance from the position W on the line segment connecting the position W of the wire electrode and the center O of the photosensitive drum to the photosensitive drum 1K.

  At this time, 50,000 continuous images were formed with the angle θ changed and the corona charger 25 operated, and the occurrence of density unevenness in the final image was evaluated.

In Example 1, the outer diameter Φ of the photosensitive drum 1K is 60 mm, and the distance L between the wire electrode 26 and the photosensitive drum 1K is 8 mm. The angle θmax at which the straight line WQ connecting the position W of the wire electrode 26 and the lower edge Q of the shield electrode 27 contacts the outer periphery of the photosensitive drum 1K having the radius R is obtained by the following equation.
θmax = sin−1 (R / (R + L)) = 61.9 degrees

  As shown in FIG. 8, when the angle θ is larger than the angle θmax, charged particles generated around the wire electrode 26 irradiate the intermediate transfer belt 5.

As shown in Table 2, in response to this, when the angle θ is 0 degree or more and less than 62 degrees, a defective image with a leopard pattern does not appear and a good final image can be obtained. Accordingly, when the radius of the photosensitive drum 1K is R and the distance from the position W on the line OW connecting the position W of the wire electrode 26 and the center O of the photosensitive drum 1K to the photosensitive drum 1K is L, the mathematical condition is It becomes as follows.
0 <θ <θmax = sin−1 (R / (R + L))

  As a result, the charged particles flowing out from the corona charger 25 flow toward only the photosensitive drum 1K, and the toner image carried on the photosensitive drum 1K is efficiently charged.

  On the other hand, when the angle θ is 62 degrees or more, charged particles easily flow from the corona charger 25 toward the primary transfer portion T1 to increase the apparent transfer current, and are applied in the direction of the photosensitive drum 1K. The charged particles to be damaged.

<Example 2>
FIG. 9 is an explanatory diagram of a shield electrode in the corona charger according to the second embodiment.

  As shown in FIG. 9, in Example 2, the lower edge Q of the shield electrode 27 of the corona charger 25 was bent obliquely upward. Thereby, compared with Example 1, the distance difference from the wire electrode 26 to each part of the inner surface of the shield electrode 27 became smaller, and the bias of discharge decreased.

  Further, as a prevention of contamination due to scattered toner inside the shield electrode 27, the planar position of the lower edge Q of the shield electrode 27 of the corona charger 25 is shifted from the planar position of the upper edge P to the opposite side of the photosensitive drum 1K.

At this time, the inclination angle with respect to the horizontal direction of the line segment OP connecting the upper edge P of the shield electrode 27 and the center O of the photosensitive drum 1K is α, and the length is Lop. Also. The inclination angle of the line segment OQ connecting the lower edge Q of the shield electrode 27 and the center O of the photosensitive drum 1K with respect to the horizontal direction is β, and the length is Loq. And it defined with the following numerical formula so that the horizontal projection line of line segment OQ may become longer than the horizontal projection line of line segment OP.
Lop × cosα <Loq × cosβ

In addition, a straight line WQ is defined so as to connect the position W of the wire 26 and the lower edge Q, and the lower edge Q is arranged so that an extension line of the straight line WQ has an intersection with the photosensitive drum 1K.
0 <θ <sin−1 (R / (R + L))

  Also in Example 2, it was possible to suppress both charging unevenness due to toner contamination of the corona charger 25 and image deterioration due to abnormal discharge, and obtain a good final image over a long period of time.

<Example 3>
FIG. 10 is an explanatory diagram of a falling toner receiving member in the corona charger according to the third embodiment.

As shown in FIG. 10, in the third embodiment, a tray 30 is disposed as a falling toner receiving member under the corona charger 25 of the second embodiment. In the case of the following formula, the scattered toner falls directly from the developing portion GB of the photosensitive drum 1K onto the intermediate transfer belt 5 through the gap between the photosensitive drum 1K and the corona charger 25.
R <OP × cosα

  For this reason, the scattered toner is collected by arranging the tray 30 with the planar position overlapping the opposing distance between the photosensitive drum 1K and the corona charger 25.

  In the case of the developing device 4K shown in FIG. 2, when the uncharged toner is developed in the developing unit as the two-component developer deteriorates with time, the toner is weakly held by the photosensitive drum 1K and falls in the direction of gravity. Easy to do. In some cases, toner accumulated on the peripheral member of the developing device 4K falls at an unintended timing and falls onto the image on the intermediate transfer belt 5 to generate a defective image. Since these toner drops have a toner charge amount of almost zero, they fall in the direction of gravity and fall onto the corona charger 25 and the intermediate transfer belt 5.

  The third embodiment has been made in view of the above drawbacks, and prevents unevenness of scattered toner stains on the corona charger 25 to maintain the toner charge amount over a long period of time. At the same time, interference of charged particles to the primary transfer portion T2 by the corona charger 25 can be suppressed, and a good image can be obtained.

<Example 4>
Today, image forming apparatuses that can form color images, such as color copiers and color printers, are becoming mainstream in accordance with market demands. One type of color image forming apparatus is a one-drum type and a tandem type. The one-drum type has a plurality of color developing devices around one photosensitive drum, and forms color toner images with these developing devices and superimposes them on an intermediate transfer member, thereby producing a color image on the intermediate transfer member. Is finally transferred to a recording material to obtain a color image.

  Comparing the 1-drum system and the tandem system, the 1-drum system has a single photosensitive drum, so the image forming apparatus can be made relatively small and the cost can be reduced. Since formation is performed, productivity of image formation is low.

  In the fourth embodiment, in such a one-drum type full-color image forming apparatus, a corona charger as in the third embodiment is disposed opposite to the photosensitive drum, and the toner charge amount of the toner image is optimized for transfer.

<Example 5>
A tandem image forming apparatus using a plurality of photosensitive drums includes a direct transfer method and an intermediate transfer method. In the direct transfer tandem image forming apparatus, a plurality of photosensitive drums are arranged along a recording material conveyance body, and images formed on the respective photosensitive drums are sequentially formed on the recording material adsorbed on the recording material conveyance body. Transfer and superimpose.

  In the direct transfer method, it is necessary to arrange a paper feeding device further upstream of a plurality of photosensitive drums arranged side by side and a fixing device downstream, and the size increases in the recording material conveyance direction. On the other hand, in the intermediate transfer system shown in FIG. 1, the paper feeding device and the fixing device can be arranged relatively freely. The paper feeding device and the fixing device are arranged below the photosensitive drum row in the recording material conveyance direction. Miniaturization is possible.

  In the fifth embodiment, a corona charger as shown in the third embodiment is disposed in a black image forming portion in which a photosensitive drum is disposed in contact with a recording material conveyance belt.

It is explanatory drawing of a structure of the image forming apparatus of 1st Embodiment. It is explanatory drawing of a structure of a black image formation part. It is an explanatory diagram of the toner charge amount of each color toner. FIG. 6 is an explanatory diagram of the behavior of toner that has entered a corona charger. It is explanatory drawing of the corona charger which cut off the lower edge of the shield electrode. It is explanatory drawing of the influence of the cutoff amount of the shield electrode with respect to a primary transfer current. 3 is an explanatory diagram of a shield electrode in the corona charger according to Embodiment 1. FIG. It is explanatory drawing of the shield electrode of a comparative example. 6 is an explanatory diagram of a shield electrode in a corona charger according to Embodiment 2. FIG. 6 is an explanatory diagram of a falling toner receiving member in a corona charger according to Embodiment 3. FIG.

Explanation of symbols

1Y, 1M, 1C, 1K Image carrier (photosensitive drum)
2Y, 2M, 2C, 2K Exposure devices 3Y, 3M, 3C, 3K Charging rollers 4Y, 4M, 4C, 4K Developing device 5 Transfer medium (intermediate transfer belt)
6Y, 6M, 6C, 6K Primary transfer roller 25 Corona charger 26 Wire electrode 27 Shield electrode P Image forming means side (upper edge) of shield electrode
PY, PM, PC, PK Image forming part Q Shield electrode transfer part side (lower edge)
T1 transfer section (primary transfer section)

Claims (3)

A photosensitive drum;
Image forming means for carrying a toner image on the photosensitive drum ;
A transfer unit for transferring a toner image carried on the photosensitive drum to a transfer medium;
An image forming apparatus comprising: a corona charger that is disposed between the image forming unit and the transfer unit in a longitudinal direction along the axial direction of the photosensitive drum and charges a toner image before transfer . ,
The corona charger includes a wire electrode, and a shield electrode that surrounds the wire electrode and has an opening with respect to the photosensitive drum,
The shield electrode has an image forming means side shield electrode provided on the image forming means side in the wire electrode and a transfer part side shield electrode provided on the transfer part side,
The end on the photosensitive drum side in the short direction of the transfer unit side shield electrode is located directly below or directly below the end on the photosensitive drum side in the short direction of the image forming means side shield electrode. The wire electrode is moved back from the end on the photosensitive drum side in the short side direction of the transfer unit side shield electrode toward the image forming unit side. A shielding part is formed to shield from ,
On the same plane perpendicular to the axial direction of the photosensitive drum,
The inclination angle with respect to the horizontal direction of the line segment OP connecting the leading end P on the photosensitive drum side and the center O of the photosensitive drum in the shield electrode on the image forming unit side is α, and the length of the line segment OP is Lop.
The inclination angle of the line segment OQ connecting the tip Q of the shielding part and the center O of the photosensitive drum with respect to the horizontal direction is β, the length of the line segment OQ is Loq,
The radius of the photosensitive drum is R,
The distance from the position W on the line segment OW connecting the position W of the wire electrode and the center O of the photosensitive drum to the surface of the photosensitive drum is L,
An angle formed by a line segment QW connecting the tip Q of the shielding part and the position W of the wire electrode and the line segment OW is θ,
When the angle formed between the tangent on the transfer portion side connecting the position W of the wire electrode and the surface of the photosensitive drum and the line segment OW is θmax,
Lop × cosα <Loq × cosβ
And
0 <θ <θmax
An image forming apparatus characterized in that. The shielding unit, according to claim 1, characterized in that it is formed by Rukoto the photosensitive drum side tip in the lateral direction of the shield electrode of the transfer portion is bent toward the image forming unit side Image forming apparatus. Claim 1 or 2, characterized in that a member of Ru undergo falling toner between the corona charger and the photosensitive drum and the corona charger and the transfer medium to a vertical direction immediately below the facing distance of The image forming apparatus described.

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