JP2007192957A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enable an electrophotographic image forming apparatus to achieve a good image formation over a long period of time. <P>SOLUTION: An AC bias to be applied to a residual toner electrifier 18 is offset to a normal electrification polarity side, so that a bias whose polarity is opposite to the normal electrification polarity of the toner is intermittently applied to the residual toner electrifier 18, the residual toner left after transfer on the photoreceptor drum 11 is temporarily caught by the residual toner electrifier 18, and the residual toner is uniformed. Thereafter, the toner is gradually discharged while adjusting the electrification to the normal electrification polarity side, thereby suppressing the occurrence of image ghost due to that a toner holding unit 17 is insulating material, specially, due to that the width of the image passing through the toner holding unit 17 is narrow. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus, and more particularly to an electrophotographic image forming apparatus that does not have a charge eliminating means for removing a charged potential on an image carrier after transfer.

  In an electrophotographic image forming apparatus, if toner remains on the photoconductor after the transfer process, the residual toner deteriorates the photosensitivity of the photoconductor, resulting in deterioration in image formation quality. Therefore, the transfer residual toner remaining on the photoreceptor is usually removed by a cleaning mechanism such as a cleaning blade.

  On the other hand, with the aim of simplifying the image forming apparatus, the development unit and transfer residual toner are recovered by optimizing the setting conditions of the development bias potential of the development unit without providing a cleaning mechanism such as a cleaning blade. A so-called cleanerless system that performs (cleaning) simultaneously is known.

  In this cleanerless system, residual toner charging means for charging the transfer residual toner to a normal polarity is used. As the residual toner charging means, as shown in FIG. 8, the residual toner charging unit 301 is configured to charge the transfer residual toner on the photosensitive drum 11 to a normal polarity by bringing the conductive fixed brush 301 into contact with the photosensitive drum 11. A toner charger 300 has been proposed (see, for example, Patent Document 1).

  However, in the residual toner charger 300 using the fixed brush 301, it is difficult to remove the discharge products generated when the photosensitive drum 11 is charged by the contact charger 13, so that the image quality may be deteriorated. In particular, in the DC + AC contact charging system in which an AC voltage is superimposed on a DC voltage, a large amount of discharge products adhere to the surface of the photosensitive drum 11 because the photosensitive drum 11 is directly discharged. For this reason, image deletion in a high temperature and high humidity environment often becomes a problem.

  In addition, the photosensitive drum 11 is simultaneously charged by a voltage applied to the fixed brush 301 in order to charge the transfer residual toner to a normal polarity. In the charging of the photosensitive drum 11 by the fixed brush 301 at this time, the charging unevenness is very large. The photosensitive drum 11 is finally charged to a desired potential by the contact charger 13. At this time, if there is uneven charging due to the fixed brush 301, uneven charging will also occur in the charging by the contact charger 13, and thus uneven density or the like will occur in the image such as halftone, and good image formation cannot be performed.

  In order to eliminate such charging unevenness when the photosensitive drum 11 is charged by the fixed brush 301, conventionally, a configuration in which two fixed brushes are arranged side by side has been adopted (for example, see Patent Document 2).

  On the other hand, in the blade method in which the transfer residual toner remaining on the photosensitive drum 11 is scraped off and removed by a cleaning blade, the nonwoven fabric is brought into contact with the photosensitive drum on the downstream side of the cleaning blade, and the nonwoven fabric adheres to the surface of the photosensitive drum. There has been proposed a technique for removing a large amount of discharge products (for example, see Patent Document 3).

JP 2001-215799 A JP 2002-099176 A Japanese Patent Laid-Open No. 2002-258666

  However, if the configuration in which two fixed brushes are arranged side by side as in the prior art described in Patent Document 2, it is necessary to secure a new space for installing the second fixed brush. This hinders the downsizing of the forming apparatus and increases the cost. In addition, charging unevenness is caused by the fact that the tip of the brush is in point contact with the photoconductor drum at a high pressure, which causes a charge injection phenomenon, and the charged potential at that portion increases. Occur. Therefore, even if two fixed brushes are arranged, the cause of the occurrence of charging unevenness cannot be eliminated, and therefore the charging unevenness due to the fixed brush cannot be sufficiently eliminated.

  On the other hand, even if a non-woven fabric is used as in the prior art described in Patent Document 3, there is a limit to the discharge products that can be removed, and thus the image flow in a high-temperature and high-humidity environment cannot be sufficiently eliminated. Also, in order to improve the discharge product removal performance, if the contact pressure of the nonwoven fabric to the photosensitive drum is set very high, filming and scratches will occur, and thus good image formation is also possible. Cannot be performed (good images cannot be obtained).

  The present invention has been made in view of the above problems, and an object thereof is to provide an image forming apparatus capable of performing good image formation over a long period of time.

  In order to achieve the above object, an image forming apparatus according to the present invention includes a rotating image carrier, a charging unit for charging the image carrier, and an electrostatic latent image on the image carrier charged by the charging unit. An exposure unit that forms an image; a developing unit that develops the electrostatic latent image to form a toner image; a transfer unit that transfers the toner image to a transfer target; an upstream side of the charging unit; and An insulating toner holding means provided on the downstream side of the transfer means and holding the transfer residual toner remaining on the image carrier after transfer of the toner image on the contact surface with the image carrier; and And a residual toner charging unit provided on the upstream side and downstream of the toner holding unit and having conductivity to charge the toner on the image carrier to a normal polarity. Regular charging polarity and reverse polarity It is characterized by providing a bias intermittently.

  The image forming apparatus having the above-described configuration has a configuration that does not include a static eliminating unit for removing the charged potential on the image carrier after transfer for the purpose of downsizing. Without the neutralizing means, the potential of the image portion remains on the image carrier after the transfer. At this time, since the toner holding means is insulative, an electric field emphasis occurs in a minute gap between the toner holding means and the image carrier, particularly when the width of the image portion is narrow, and the image portion is In addition, since an electric field is generated in the direction in which the transfer residual toner held by the toner holding unit adheres to the image carrier, the toner of the normal charge polarity in the toner holding unit is developed, and after one round of the image carrier. A so-called image ghost, in which an image remains in the same position, is generated.

  On the other hand, the residual toner charging unit intermittently temporarily applies a bias having a polarity opposite to the normal charging polarity of the toner to the residual toner charging unit so that the residual toner charging unit intermittently temporarily captures the transfer residual toner on the image carrier. . The temporarily captured toner is made uniform in the residual toner charging means, the charge is adjusted to the normal charging polarity side, and the toner is gradually discharged. In this way, the temporarily captured toner is made uniform, the charge is adjusted to the normal charge polarity side, and the toner is gradually discharged, thereby suppressing the occurrence of image ghost.

  According to the present invention, the transfer residual toner can be held by the toner holding unit, and the occurrence of the image ghost can be suppressed by the action of the residual toner charging unit, so that a good image formation can be performed over a long period of time. .

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

  FIG. 1 is a schematic configuration diagram schematically showing a basic configuration of an image forming apparatus to which the present invention is applied, for example, a tandem type image forming apparatus.

  An image forming apparatus according to this application example is used as an image output apparatus in a digital copying machine, for example, and is based on image data obtained by reading a document image by an image reading apparatus using a CCD (Charge Coupled Device) image sensor or the like. Thus, a color image is formed on the recording paper by electrophotography.

  However, the present invention is not limited to application to a digital copying machine. For example, image processing is performed based on color image information input from an image data input device such as a personal computer, and the recording sheet is electrophotographic. It can also be applied to an image forming apparatus that forms a color image.

  As shown in FIG. 1, the image forming apparatus 100 includes image forming units 10Y, 10M, and 10C that form toner images for each color material of yellow (Y), magenta (M), cyan (C), and black (K). , 10K. In the following description, when it is necessary to distinguish between the color materials, description will be given with any of Y, M, C, and K after the reference numerals. When it is not necessary to distinguish between the color materials, Y, M , C, and K are omitted.

  The image forming units 10Y, 10M, 10C, and 10K have Y, M, and C with respect to the traveling direction of the endless intermediate transfer belt 23 that is stretched by the backup roll 21 and, for example, two stretch rolls 22A and 22B. , K color materials in order. The intermediate transfer belt 23 is used for the photosensitive drums 11Y, 11M, 11C, and 11K as the image carriers of the image forming units 10Y, 10M, 10C, and 10K, and the photosensitive drums 11Y, 11M, 11C, and 11K. Between the primary transfer rolls 12Y, 12M, 12C, and 12K arranged in pressure contact with each other.

(Image forming unit)
Next, the configuration of the image forming units 10Y, 10M, 10C, and 10K will be described as a representative of the image forming unit 10Y that forms a yellow toner image.

  Around the photosensitive drum 11Y, in addition to the primary transfer roll 12Y, a contact charger 13Y is provided on the opposite side of the primary transfer roll 12Y, downstream of the contact charger 13Y, and upstream of the primary transfer roll 12Y. In addition, an exposure device 14Y and a developing device 15Y are respectively disposed on the downstream side of the primary transfer roll 12Y.

  The contact charger 13Y is made of, for example, a semiconductive roll, and uniformly charges the surface of the photosensitive drum 11Y by a DC + AC contact charging method. The exposure device 14Y forms an electrostatic latent image corresponding to the yellow image on the surface of the photosensitive drum 11Y charged by the contact charger 13Y.

  The developing device 15Y includes a rotating roll 151 (see FIG. 2) to which a developing bias in which an AC voltage is superimposed on a DC voltage is applied. The electrostatic latent image formed on the surface of the photosensitive drum 11Y by the exposure device 14Y. Is developed to form a toner image. Specifically, the electrostatic latent image corresponding to the yellow image becomes a yellow toner image by being developed with, for example, negatively charged toner carried on the rotary roll 151.

  The primary transfer roll 12Y intermediates a yellow toner image formed on the photoconductive drum 11Y by a pressure contact force with respect to the photoconductive drum 11Y and an electrostatic attraction force by a positive potential transfer bias applied to the transfer roll 12Y. Transfer (primary transfer) is performed on the transfer belt 23.

  In the primary transfer by the primary transfer roll 12Y, the entire yellow toner image is not transferred to the intermediate transfer belt 23, and a part thereof remains on the photosensitive drum 11Y as a transfer residual yellow toner. The transfer residual yellow toner is recovered by a toner recovery mechanism described later.

  Here, the configuration of the image forming unit 10Y has been described as a representative, but the image forming units 10M, 10C, and 10K have the same configuration as the image forming unit 10Y. Then, image forming processing is performed in each of the image forming units 10Y, 10M, 10C, and 10K at a timing that takes into account the relative position difference between the image forming units 10Y, 10M, 10C, and 10K. Then, the toner images of the respective color materials of Y, M, C, and K are sequentially superposed to form a full color toner image.

  In order to secondarily transfer the full color toner image onto the recording paper P, the secondary transfer roll 24 is provided in a state of being pressed against the backup roll 21. The secondary transfer roll 24 is a recording that has been conveyed to the secondary transfer position A at a predetermined timing by an electrostatic attraction force by a transfer bias having a positive potential higher than that of the primary transfer roll 12Y applied to the transfer roll 24. The full color toner image on the intermediate transfer belt 23 is transferred onto the sheet P at once.

  The recording paper P on which the full color toner image is transferred is separated from the intermediate transfer belt 23 and conveyed to the fixing device 25. The fixing device 25 fixes the full color toner image on the recording paper P by heat and pressure. Transfer residual toner on the intermediate transfer belt 23 that has not been transferred to the recording paper P is collected by the intermediate transfer belt cleaner 26.

  Next, specifications of main members and main electric specifications in the image forming apparatus having the above configuration will be described.

Photoreceptor drum 11: Organic photoreceptor with a diameter of about 30.0 mm
Non-image part potential: -500V
Image part potential: -100V
Process speed: 104 mm / second development system: dry two-component development system contact charger 13: DC + AC contact charging system
AC component frequency: 614 Hz
DC component voltage value: -520V
Exposure apparatus 14: Laser wavelength = 780 nm
Developing roll 151: Diameter = 16.0 mm
Rotation speed = 208mm / sec
Development bias: DC component voltage value = −400V
AC component voltage value = 1.5 KV (peak to peak)
Frequency = 6KHz
Primary transfer roll 12: Transfer bias = + 2000V
Secondary transfer roll 24: transfer bias = + 1600V

  Next, a specific embodiment of a recovery mechanism that recovers the transfer residual yellow toner on the photosensitive drum 11Y, which is a feature of the present invention, will be described.

[Embodiment]
FIG. 2 is a schematic configuration diagram showing a main configuration of an image forming apparatus according to an embodiment of the present invention. In FIG. 2, the same parts as those in FIG. Here, the configuration of the image forming units 10Y, 10M, 10C, and 10K will be described as a representative of the image forming unit 10Y that forms a yellow toner image.

  In FIG. 2, in addition to the primary transfer roll 12Y, the contact charger 13Y, the exposure device 14Y, and the developing device 15Y, there are an upstream side of the contact charger 13Y and a downstream side of the primary transfer roll 12Y. The toner holder 17Y is arranged on the side, and the residual toner charger 18Y is further arranged on the upstream side of the contact charger 13Y and on the downstream side of the toner holder 17Y.

  The toner holder 17Y is provided in contact with the photosensitive drum 11Y, and after the transfer of the yellow toner image to the intermediate transfer belt 23 by the primary transfer roll 12Y, the transfer residual yellow toner remaining on the photosensitive drum 11Y. Is held on the contact surface with the photosensitive drum 11Y.

  The residual toner charger 18Y charges the toner having a polarity opposite to the normal polarity present on the photosensitive drum 11Y after the toner holder 17Y to a normal polarity (normal charge polarity). At this time, the normal polarity toner is further charged to the normal polarity side.

(Toner holder)
Here, an example of a specific configuration of the toner holder 17 (17Y) will be described with reference to FIG.

  As shown in FIG. 3, in the toner holder 17, a urethane sponge 172 having an insulating property is attached to a base 171 having a square columnar insulating property, and a nonwoven fabric 173 made of insulating fine fibers is formed thereon. The configuration is affixed. As shown in FIG. 2, the toner holder 17 is a fixed toner holder provided with the nonwoven fabric 173 pressed against the photosensitive drum 12 (12Y). Here, the non-woven fabric is literally “a non-woven fabric” and is a sheet in which fibers are bonded by various methods.

  Known non-woven fabrics include dry non-woven fabrics, spound bands, and wet non-woven fabrics. In the present embodiment, a dry nonwoven fabric is used as the nonwoven fabric 173. Specifically, a dry nonwoven fabric is formed by forming fibers with a fiber length of about several centimeters into a thin sheet using a card or an air random machine, and stacking several sheets as necessary. Joining is performed by entanglement of fibers with a high-pressure fine water stream (spanless).

  As shown in FIG. 4, the toner holder 17 is formed such that the width in the longitudinal direction is wider than the image forming width R in the axial direction of the rotation axis K of the photosensitive drum 11.

  From both ends of the base 171 of the toner holder 17, a substantially rectangular columnar shaft 174 protrudes in the same direction as the axial direction of the rotation axis K of the photosensitive drum 11. The shaft 174 is fixed to the base body 171. Both end portions of the shaft 174 are supported by the support portions 175A and 175B by passing through the rectangular holes of the support portions 175A and 175B.

  The shaft 174 is pressed toward the other end by a spring 176 provided at one end. A movable shaft 178 of a solenoid 177 is coupled to the other end of the shaft 174. The movable shaft 178 moves back and forth (moves) in the axial direction of the rotation axis K of the photosensitive drum 11 (the direction of arrow N in the figure).

  As described above, the movable shaft 178 advances and retreats according to the operation / non-operation of the solenoid 177, whereby the nonwoven fabric 173 of the toner holder 17 moves on the surface of the rotating drum 11 in the axial direction of the rotating shaft K (in the drawing, Slide in the direction of arrow M). Even when such a sliding operation is performed, the nonwoven fabric 173 can maintain a state in which the nonwoven fabric 173 is pressed against the photosensitive drum 11 over the entire area of the image forming width R thereof.

  The drive source of the toner holder 17 in the direction of arrow M is not limited to the solenoid 177, and can be moved and fixed to an arbitrary position in the direction of arrow M using, for example, a cam. It may be.

  Next, the operation of the toner holder 17 will be described.

  Since the non-woven fabric 173 is in contact with the surface of the photosensitive drum 11, the toner holder 17 is not transferred to the intermediate transfer belt 23 by the primary transfer by the primary transfer roll 12 and remains on the photosensitive drum 11. The toner is removed from the surface of the photosensitive drum 11 by temporarily holding the toner on the nonwoven fabric 173.

  The toner holder 17 removes not only the transfer residual toner but also the discharge product generated when the photosensitive drum 11 is charged by the contact charger 13 to remove it from the photosensitive drum 11. This discharge product will be described later. Since the transfer residual toner is held on the nonwoven fabric 173, the held transfer residual toner functions to enhance the discharge product removal capability of the nonwoven fabric 173.

  By the way, as apparent from the above description, the image forming apparatus 100 according to the present embodiment is arranged on the photosensitive drum 11 after the temporary transfer to the intermediate transfer belt 23 by the temporary transfer roller 12 for the purpose of downsizing the apparatus. In this configuration, there is no static elimination means for removing (eliminating) the charged potential.

  As described above, if there is no static elimination means, the potential of the image portion remains on the photosensitive drum 11 even after temporary transfer, as shown in FIG. At this time, since the toner holder 17 is made of an insulating member, especially when the width of the image passing through the toner holder 17 is narrow, as shown in FIG. Electric field enhancement occurs in a minute gap between the nonwoven fabric 173 and the photosensitive drum 11.

  As described above, when electric field emphasis occurs in the minute gap between the nonwoven fabric 173 of the toner holder 17 and the photosensitive drum 11, the toner holder 17 holds the image portion on the photosensitive drum 11. Since an electric field in a direction to adhere to the photosensitive drum 11 is generated with respect to the transfer residual toner that has been transferred, the toner having a normal charging polarity (in this embodiment, negative polarity) in the toner holder 17 is developed. .

  As a result, as shown in FIG. 6, a so-called image ghost is generated in which an image remains at the same position after the first round (second round) of the photosensitive drum 11. In FIG. 6, as an example, it is assumed that an A3-sized image pattern is formed by forming 10 × 90 mm Cin (input coverage) 100% patches at intervals of 10 mm. Incidentally, 90 mm is the circumferential length of the photosensitive drum 11.

  Therefore, the image forming apparatus 100 according to the present embodiment is characterized in that the occurrence of image ghost is suppressed by the action of the residual toner charger 18Y described below.

(Residual toner charger)
In FIG. 2, the residual toner charger 18Y located on the downstream side of the toner holder 17Y is provided in a state of being in uniform contact with the surface of the photosensitive drum 11Y, and rotates around the conductive rotary shaft 181 as an axis. This is a rotary type residual toner charger provided with a non-woven roll 182 having appropriate conductivity.

  As the nonwoven fabric roll 182, for example, a dry nonwoven fabric is used. Moreover, the fiber used for the nonwoven fabric roll 182 is an insulating nylon-polyester mixed cotton fiber, for example, and conductivity is imparted by coating a polypyrrole resin.

  For example, as shown in FIG. 7, an AC (alternating current) bias that is offset by, for example, about −300 V to the normal charging polarity side of the toner is applied to the nonwoven fabric roll 182 through a conductive rotating shaft 181. The AC bias has an amplitude of, for example, about 800 V (peak to peak), so that a bias having a polarity opposite to the normal charging polarity of the toner is intermittently applied to the residual toner charger 18Y. The numerical values of the AC bias offset voltage and amplitude are merely examples, and the present invention is not limited thereto.

  Next, the operation of the residual toner charger 18Y will be described.

  The residual toner charger 18Y intermittently temporarily captures the transfer residual toner on the photosensitive drum 11 by intermittently applying a bias having a polarity opposite to the normal charging polarity (negative polarity in the present embodiment) of the toner. . The temporarily captured toner is made uniform in the residual toner charger 18Y, the charge is adjusted to the normal charging polarity side, and the toner is gradually discharged from the residual toner charger 18Y. In this way, the temporarily captured toner is made uniform, the charge is adjusted to the normal charge polarity side, and the toner is gradually discharged, thereby suppressing the occurrence of image ghost. In addition, the transfer history is eliminated by this charging adjustment.

  The transfer residual toner whose charge is adjusted to the normal charging polarity by the residual toner charger 18Y is sent to the contact charger 13Y as the photosensitive drum 11 rotates. As described above, a charging bias of −520 V is applied to the contact charger 13Y. As a result, repulsive force acts on the transfer residual toner on the photosensitive drum 11 charged to the normal polarity with the contact charger 13Y. Accordingly, the transfer residual toner of normal polarity passes through the contact charger 13Y without adhering to the contact charger 13Y.

  The transfer residual toner of normal polarity that has passed through the contact charger 13Y is sent to a portion (developing unit) facing the developing roll 151 of the developing device 15 as the photosensitive drum 11 rotates. Here, by optimizing the setting condition of the developing bias of the developing roller 151, the developing device 15 performs development, and at the same time, the transfer residual toner having normal polarity is collected (cleaning). The developing roller 151 rotates in the direction opposite to the rotation direction of the photoconductive drum 11Y in order to increase the collection efficiency of the transfer residual toner.

  By the way, when charged by the contact charger 13Y, discharge products (specifically, generated substances such as ozone and nitrogen oxide generated during discharge and reaction products thereof) are generated. In particular, as in the present embodiment, when charging is performed by the contact charger 13Y adopting the DC + AC contact charging method, many discharge products are generated. This discharge product adheres to the surface of the photosensitive drum 11 and lowers the electrical resistance of the surface of the photosensitive drum 11 particularly under high temperature and high humidity.

  When the electrical resistance on the surface of the photoconductive drum 11 is lowered, the electrostatic latent image is disturbed, which causes image deletion. In particular, the setting of the developing bias of the developing roller 151 is optimized without having a special cleaning mechanism for removing the transfer residual toner on the photosensitive drum 11 as in the image forming apparatus according to the present embodiment. In the so-called cleanerless system in which the developing device 15 simultaneously performs development and recovery (cleaning) of the transfer residual toner, the discharge product on the photosensitive drum 11 cannot be recovered by the cleaning mechanism. The effect of the product is large.

  However, in the image forming apparatus according to the present embodiment, the transfer residual toner attached to the nonwoven fabric 173 of the toner holder 17 disposed on the downstream side of the primary transfer roll 12 is temporarily held on the photosensitive drum 11. In order to satisfactorily remove the discharge product along with the rotation, it is possible to prevent a problem caused by the attachment of the discharge product, that is, an image flow without having a special cleaning mechanism such as a cleaning blade.

  Further, in order to prevent the transfer residual toner from adhering to the contact charger 13, the residual toner charger 18 that adjusts the charge of the transfer residual toner to the normal polarity also charges the photosensitive drum 11 by the applied voltage. Since the nonwoven fabric roll 182 of the residual toner charger 18 is in uniform contact with the surface of the photosensitive drum 11, there is little charging unevenness, and therefore there is little charging unevenness after the charge adjustment by the contact charger 13.

  Therefore, density unevenness or the like does not occur in an image such as a halftone image, so that favorable image formation can be performed. Further, in order to eliminate charging unevenness caused by the contact charger 13, it is not necessary to increase the charging performance of the contact charger 13 more than necessary. That is, since it is not necessary to increase the voltage applied to the contact charger 13 more than necessary, the generation of discharge products can be suppressed.

  As described above, in the electrophotographic image forming apparatus 100, the toner holder 17 is disposed on the upstream side of the contact charger 13 and the downstream side of the primary transfer roll 12, and the transfer residual toner is transferred by the toner holder 17. Is held temporarily so that the retained transfer residual toner removes the discharge product well as the photosensitive drum 11 rotates, thereby preventing the image from flowing due to the attachment of the discharge product. be able to.

  Further, a residual toner charger 18 is arranged on the upstream side of the contact charger 13 and on the downstream side of the toner holder 17, and the residual toner charger 18 adjusts the charge of the transfer residual toner to a normal polarity by the residual toner charger 18. The transfer residual toner charged to the normal polarity is sent to a portion facing the developing roller 151 of the developing device 15 without adhering to the contact charger 13 and is reliably collected by the developing device 15.

  Furthermore, since the residual toner charger 18 adjusts the charging of the photosensitive drum 11 with the applied voltage without unevenness, there is little charging unevenness after charging by the contact charger 13, and thus there is density unevenness in an image such as a halftone. Therefore, it is possible to form a good image and suppress the generation of discharge products without increasing the voltage applied to the contact charger 13 more than necessary.

  In particular, in the image forming apparatus 100 according to the present embodiment, a bias having a polarity opposite to the normal charging polarity (negative polarity in the present embodiment) of the toner is intermittently applied to the residual toner charger 18. Thus, the residual toner charger 18 acts to intermittently temporarily capture the transfer residual toner on the photosensitive drum 11 and make it uniform, and then charge it to the normal charging polarity side and gradually discharge it. By the action of the residual toner charger 18, it is possible to suppress the occurrence of an image ghost due to the insulating property of the toner holder 17, particularly when the width of the image passing through the toner holder 17 is narrow. Good image formation can be performed over a long period of time.

  In addition, since the toner holder 17 is insulative, even if the distance from the residual toner charger 18 made of a conductive member is shortened, the toner holder 17 and the residual toner charger 18 are electrically connected. Since no leakage occurs, the toner holder 17 and the residual toner charger 18 can be disposed close to each other. Accordingly, the image forming units 10Y, 10M, 10C, and 10K can be downsized, and the entire image forming apparatus can be reduced. Can be miniaturized.

  Further, by applying an AC bias offset to the normal charging polarity side to the residual toner charger 18, it is possible to eliminate problems caused by the residual toner charger 18 excessively charging the transfer residual toner to the normal charging polarity side. There is also an advantage.

  That is, in order to reliably charge the transfer residual toner having a polarity opposite to the normal charge polarity to the normal charge polarity, when the transfer residual toner is adjusted with a DC bias having a high voltage of about −700 V, for example, the toner is normally charged. In some cases, the polar side is excessively charged. Then, since the adhesion force of the transfer residual toner to the photosensitive drum 11 becomes very strong, the toner may not be collected by the developing device 15 in some cases.

  On the other hand, as in the image forming apparatus 100 according to the present embodiment, the residual toner charger 18 has an amplitude of about 800 V (peak to peak) offset by about −300 V to the normal charging polarity side of the toner. By applying an AC bias (see FIG. 7), the transfer residual toner is charged with a bias voltage of −700 V at the peak, but it is instantaneous, and the average bias voltage is about −300 V, and transfer is performed. Since excessive charging with respect to the residual toner can be eliminated, the toner can be collected by the developing device 15.

  Incidentally, when the technique according to the present embodiment, that is, the technique for suppressing the occurrence of the image ghost due to the action of the residual toner charger 18 is not used, the image ghost is generated even when the number of A4 size prints is about 1000, for example. On the other hand, when this technique is used, it has been confirmed that no image ghost occurs up to at least about 10,000 sheets.

  In the above-described embodiment, the bias of the AC waveform (sinusoidal waveform) is offset to the normal charging polarity side, whereby a bias having a polarity opposite to the normal charging polarity of the toner is intermittently applied to the residual toner charger 18. However, this is only an example. For example, even if the bias of the pulse waveform is offset to the normal charging polarity side, a bias having a polarity opposite to the normal charging polarity of the toner is intermittently applied to the residual toner charger 18. Can be given.

  In the above-described embodiment, the case where the toner holder 17 is a fixed toner holder and the residual toner charger 18 is a rotary residual toner charger has been described as an example. However, this is only an example. The toner holder 17 is a rotary toner holder having the same configuration as the residual toner charger 18, the residual toner charger 18 is a fixed residual toner charger having the same configuration as the toner holder 17, and the like. Combinations such as a fixed mold and a rotating mold, a fixed mold and a fixed mold, and a rotating mold and a rotating mold can be arbitrarily set.

1 is a schematic configuration diagram schematically showing a basic configuration of a tandem image forming apparatus to which the present invention is applied. 1 is a schematic configuration diagram illustrating a configuration of a main part of an image forming apparatus according to an embodiment of the present invention. FIG. 5 is a side view illustrating an example of a specific configuration of a toner holder. FIG. 6 is a view of a state in which a toner holder is pressed against a photosensitive drum as viewed from above. It is explanatory drawing of the generation | occurrence | production cause of an image ghost. It is a figure which shows an example of the image pattern with a narrow image width. FIG. 6 is a waveform diagram of an AC bias applied to a residual toner charger. It is explanatory drawing of the subject of a prior art.

Explanation of symbols

  10 (10Y, 10M, 10C, 10K) ... Image forming unit, 11 (11Y, 11M, 11C, 11K) ... Photosensitive drum, 12 (12Y, 12M, 12C, 12K) ... Primary transfer roll, 13 (13Y, 13M) , 13C, 13K) ... contact charger, 14 (14Y, 14M, 14C, 14K) ... exposure device, 15 (15Y, 15M, 15C, 15K) ... development device, 17 (17Y, 17M, 17C, 17K) ... toner Retainer, 18 (18Y, 18M, 18C, 18K) ... residual toner charger, 21 ... backup roll, 23 ... intermediate transfer belt, 25 ... fixing device, 100 ... image forming apparatus

Claims (3)

A rotating image carrier;
Charging means for charging the image carrier;
An exposure unit that forms an electrostatic latent image on the image carrier charged by the charging unit;
Developing means for developing the electrostatic latent image to form a toner image;
Transfer means for transferring the toner image to a transfer object;
An insulating property provided on the upstream side of the charging unit and the downstream side of the transfer unit, and holds the transfer residual toner remaining on the image carrier after transfer of the toner image on the contact surface with the image carrier. Toner holding means,
A residual toner charging unit provided on the upstream side of the charging unit and on the downstream side of the toner holding unit and having conductivity to charge the toner on the image carrier to a normal polarity;
An image forming apparatus comprising: intermittently applying a bias having a polarity opposite to a normal charging polarity of toner to the residual toner charging unit. 2. The image according to claim 1, wherein the control unit applies an AC bias that is offset to the normal charging polarity side with respect to the residual toner charging unit and intermittently has a polarity opposite to the normal charging polarity. Forming equipment. The image forming apparatus according to claim 1, wherein the toner holding unit includes an insulating nonwoven fabric.

Images