JP2008015148A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of outputting an image of high image quality over a long period, in which developing and cleaning are simultaneously performed. <P>SOLUTION: In the image forming apparatus wherein a developing means 4 functions also as a cleaning means for removing the developer not transferred to an intermediate transfer body 10 on transfer, but left on the surface of an image carrier 1, the image forming apparatus includes a transfer cleaning device 9 for removing the developer on the intermediate transfer body 10, and during a non-image formation period, a potential difference between the surface of the image carrier 1 and the developing means 4 is made larger than that during the image formation period to discharge the developer from the developing means 4 onto the image carrier 1, and the developer on the image carrier 1 is electrified with a polarity reverse to the normal one by an electrifying means 2, then, transferred onto the intermediate transfer body 10, and the developer on the intermediate transfer body 10 is recovered by the transfer cleaning device 9. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  In general, in the present invention, an electrostatic latent image formed on an image carrier by an electrophotographic method or the like is used as a developer image (that is, a toner image), and the toner image is applied to a recording material via an intermediate transfer member, or The present invention relates to an image forming apparatus such as a copying apparatus, a printer, and a facsimile machine that directly transfers to a recording material. In particular, according to the present invention, in the developing device, the electrostatic latent image formed on the image carrier is used as a toner image, and at the same time, the transfer residual toner on the image carrier after the transfer process is removed and collected from the image carrier. The present invention also relates to a so-called cleanerless image forming apparatus that is reused.

  In general, an image forming apparatus that records an image on a recording material such as paper, such as a copying apparatus, has the following methods. That is, there is a transfer type image forming apparatus that forms a toner image on an electrophotographic photosensitive member as an image carrier by an electrophotographic process, and then transfers the toner image directly to a recording material through an intermediate transfer member. Widely used.

  The copying apparatus is not used as an office processing copying machine for copying a manuscript, but is used as an information output device linked to other information processing machines through the introduction of digital technology. Or, it is used as a copying machine for creating a new original manuscript, and further as a personal copy for individuals, because the processing and editing of image information is facilitated by the multi-functionalization.

  Therefore, higher speed, higher image quality, smaller size and lighter weight have been pursued, and higher reliability has been strictly pursued.

  Under such circumstances, a corona charging device has been widely used as a means for charging the surface of a photoconductor, which is an image carrier as a charged body, in a copying machine, a printer, a facsimile, etc. using electrophotographic technology. I came. The corona charging device is effective as a means for uniformly charging the surface of the photosensitive member to a predetermined potential, but has problems such as requiring a high voltage power source and generating a large amount of ozone.

  In contrast to such a corona charging device, a contact charging device is recently used in which a charging member (for example, a conductive elastic roller or blade) to which a voltage is applied is brought into contact with the photosensitive member to charge the photosensitive member.

  The contact charging device has advantages such as low power supply and extremely small amount of ozone generation. Therefore, the contact charging device can replace the corona charging device with an image carrier such as a photoconductor and dielectric, and other objects. It has been put to practical use as a means for charging a charged body surface.

  In an image forming apparatus having such a contact charging means, it is possible to uniformly charge and sufficiently transfer a photosensitive member with a relatively low voltage bias as compared with corona charging. It is excellent in terms of suppressing corona discharge products.

  In addition, in order to reduce the running cost when using the device, it solves the problems that conventional electrophotographic photoreceptors have and improves the abrasion resistance and scratch resistance by increasing the film strength. There is a need for highly durable photoconductors. Further, it is desired that the photosensitive member exhibits very little change or deterioration in the photosensitive member characteristics such as an increase in residual potential during repeated use, and exhibits stable performance even during repeated use. As such an electrophotographic photoreceptor, it has been proposed to use a photoreceptor containing a curable resin in a surface layer.

  On the other hand, in view of environmental conservation and effective use of resources, an electrophotographic apparatus has been developed in which transfer residual toner collected by a cleaning device, that is, so-called waste toner is returned to the developing device and reused.

  As one of the methods, a “cleaner-less method” has been proposed in which the cleaning device is eliminated and the transfer residual toner is cleaned simultaneously with the developing process in the developing device.

  That is, normally, the untransferred toner remaining on the photoconductor after transfer is removed from the photoconductor surface by the cleaning device to become waste toner.

  Here, the cleaning device is eliminated, the transfer residual toner on the photosensitive member after transfer is charged to a predetermined polarity by a charging device or an auxiliary charging member, removed from the photosensitive member by a developing device, and collected and recycled by the developing device. There is a cleanerless process electrophotographic apparatus that is configured to be used. (For example, refer to Patent Document 1).

  In the cleaner-less type image forming apparatus for simultaneous development cleaning as described above, a toner charged to a reverse polarity such as a transfer residual toner is returned to normal charging by a charging device, an auxiliary charging member or the like, and collected by the developing device. Is ideal.

  However, if the image is actually passed through 10K sheets and 20K sheets in the cleaner-less type image forming apparatus, there arises a problem that fogging caused by the reverse polarity toner as shown in FIG. 2 increases.

  The cause of the generation of such a reverse polarity toner is the influence of the AC voltage applied from the contact charging device and the deterioration of the toner due to durability.

  As shown in FIG. 3, when the transfer residual toner passes through the contact charging device to which an AC voltage is applied, the toner charge distribution shifts in the reverse polarity direction (in this example, the right positive direction in FIG. 3). This produces uncharged and reversal polarity toner.

  Next, the behavior of the reverse polarity toner generated in this way will be described.

  FIG. 4 shows a toner 2g containing reverse polarity toner after passing through the contact charging device with respect to 200 g of a developer having a toner concentration of 8% prepared with the initial toner and the initial carrier and the initial toner and the deteriorated carrier. The result of measuring the fog is shown below.

  In the developer containing the initial carrier, the fog is hardly changed, whereas in the developer containing the deteriorated carrier, the fog is on the side where the Vback (potential difference between the photoreceptor potential and the developing bias potential) is large, that is, The fog caused by the reverse polarity toner is very deteriorated. This means that the toner that has been reversed when the carrier has deteriorated cannot return to normal charging again.

  Next, the case where the toner is deteriorated will be described in detail.

  FIG. 5 shows a charge distribution when the toner is deliberately deteriorated by applying stress and the deteriorated toner is charged by applying a DC voltage of −1 kV to the contact charging device.

  First, the deteriorated toner is in a state where a normally charged (negatively charged) toner and a reversely charged (positively charged) toner are mixed. Next, when a DC voltage of −1 kV is applied to these, the overall tribo average shifts to the normal charging side, but there is toner that does not return to normal charging. Such toner cannot be returned to normal charge due to the external additive being embedded due to transfer, contact charging member, and stress in the developing device, or excessive addition or deformation of the external additive. It is predicted that

  Patent Documents 2 and 3 are technologies that strengthen the effect of returning to normal charging by applying a duty to the charging bias and suppress the generation of reverse polarity toner. However, as described above, the case where the carrier or toner deteriorates is described. has no effect. Further, the means can sufficiently charge the drum in a low-speed machine with low productivity, but it is difficult to charge the drum sufficiently in a medium-to-high speed machine with high productivity.

Japanese Patent Laid-Open No. 2004-228561 discloses that toner that moves on the drum is collected by applying a bias having a polarity opposite to that at the time of image formation to the transfer device. Since there are many uncharged and weakly charged toners, the transfer efficiency is poor and not effective.
JP 2004-93855 A JP 2003-255675 A Japanese Patent Laid-Open No. 11-72994 JP 2004-252180 A

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus capable of outputting a high-quality image over a long period of time in an image forming apparatus that performs simultaneous development cleaning.

The above object is achieved by the image forming apparatus according to the present invention. In summary, according to the first aspect of the present invention, an image carrier, a charging unit arranged in contact with the surface of the image carrier to charge the surface of the image carrier, and the surface of the image carrier after charging are exposed. An exposure means for forming an electrostatic image, a developing container containing a developer, a developing means for attaching the developer to the electrostatic image to form a developer image, and an intermediate transfer of the developer image Transfer means for transferring to a recording material via a recording medium or a recording material carried and conveyed by a recording material carrier, and not transferred to the intermediate transfer body at the time of transfer, or the recording material carrier In the image forming apparatus in which the developing unit also serves as a cleaning unit that removes the developer remaining on the surface of the image carrier without being transferred to the recording material carried and conveyed by
A transfer cleaning device for removing the developer on the intermediate transfer member or the recording material carrier;
At the time of non-image formation, the potential difference between the surface of the image carrier and the developing unit is made larger than that at the time of image formation, and the developer is discharged from the developing unit onto the image carrier. The developer on the carrier is charged to a polarity opposite to the normal polarity and transferred onto the intermediate transfer member or the recording material carrier, and the developer on the intermediate transfer member or the recording material carrier is transferred to the intermediate transfer member or the recording material carrier. An image forming apparatus is provided that is collected by a transfer cleaning device.

  According to the present invention, in an image forming apparatus that performs development simultaneous cleaning, it is possible to improve image defects due to uncharged and reverse polarity developer, and to provide a good image over a long period of time.

  The image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.

Example 1
FIG. 1 shows a schematic configuration of an embodiment of an image forming apparatus according to the present invention. The image forming apparatus 100 of the present embodiment is an intermediate transfer type electrophotographic laser beam printer (hereinafter simply referred to as “printer”) that employs a contact charging method, a two-component contact development method, and a cleanerless method.

  First, with reference to FIG. 1, the overall configuration of the printer 100 of this embodiment and the image forming means for executing each electrophotographic process will be described.

(Image carrier, charging means)
The printer 100 includes a rotating drum type electrophotographic photosensitive member (hereinafter referred to as “photosensitive drum”) 1 as an image carrier. In this embodiment, the photosensitive drum 1 is an organic photoconductor (OPC) having a negative charging property, and has an outer diameter of 30 mm and an arrow having a process speed (peripheral speed) of 130 mm / sec centering on the central support shaft. It is rotated counterclockwise.

  The printer 100 includes a contact charging device (contact charger) 2 as charging means for uniformly charging the surface of the photosensitive drum 1. In this embodiment, the contact charging device 2 is a charging roller (roller charger), and is charged by utilizing a discharge phenomenon that occurs in a minute gap between the contact charging device 2 and the photosensitive drum 1.

  The charging roller 2 is rotatably supported at both ends of a cored bar 2a as a supporting member by a bearing member (not shown), and is urged toward the photosensitive drum 1 by a pressing spring 2e. 1 is brought into pressure contact with a predetermined pressing force. As a result, the charging roller 2 rotates following the rotation of the photosensitive drum 1. A pressure contact portion between the photosensitive drum 1 and the charging roller 2 is a charging portion (that is, a charging nip portion or a contact portion) a.

  A charging bias voltage of a predetermined condition is applied to the cored bar 2a of the charging roller 2 from the power source S1. As a result, the surface of the rotating photosensitive drum 1 is contact-charged to a predetermined polarity / potential.

  In the present embodiment, the charging bias voltage for the charging roller 2 is an oscillating voltage in which a DC voltage (Vdc) and an AC voltage (Vac) are superimposed. More specifically, it is an oscillating voltage in which a DC voltage of −500 V, a frequency of 1.3 kHz, a peak-to-peak voltage Vpp of 1.7 kV, and a sinusoidal AC voltage are superimposed. By this charging bias voltage, the surface of the photosensitive drum 1 is uniformly contact-charged to −500 V (dark potential Vd), which is the same as the DC voltage applied to the charging roller 2.

(Information writing means)
The printer 100 includes an exposure device 3 serving as an exposure unit as an information writing unit that forms an electrostatic latent image on the surface of the photosensitive drum 1 that has been charged.

  In this embodiment, the exposure apparatus 3 is a laser beam scanner using a semiconductor laser. The laser beam scanner 3 outputs a laser beam L modulated in accordance with an image signal sent from a host processing device such as an image reading device (not shown) to the printer side. The laser beam scanner 3 performs laser scanning exposure (image exposure) on the surface of the rotating photosensitive drum 1 that is uniformly charged with the laser light L at an exposure position (that is, an exposure portion) b. By this laser scanning exposure, the potential irradiated with the laser beam L on the surface of the photosensitive drum 1 is lowered, and electrostatic latent images corresponding to image information are sequentially formed on the surface of the rotating photosensitive drum 1. To go.

(Developing means, developer)
The printer 100 includes a developing device 4 as a developing unit that supplies toner according to the electrostatic latent image on the photosensitive drum 1 and reversely develops the electrostatic latent image as a toner image (developer image). In the present embodiment, the developing device 4 is a developing device that employs a two-component contact developing system that performs development while bringing a magnetic brush made of a two-component developer containing toner and carrier into contact with the photosensitive drum 1.

  The developing device 4 includes a developing container 4a and a nonmagnetic developing sleeve 4b as a developer carrying member. The developing sleeve 4b has a part of the outer peripheral surface thereof exposed to the outside of the developing device 4, and is rotatably arranged in the developing container 4a, and carries and transports the developer. In the developing sleeve 4b, a magnet roller 4c is inserted in a non-rotating manner. A developer coating blade 4d is provided opposite to the developing sleeve 4b. The developer container 4a contains a developer 4e, and a developer stirring member 4f is disposed on the bottom side in the developer container 4a. Further, replenishment toner is accommodated in the toner hopper 4g.

  In this embodiment, the developer 4e in the developing container 4a is a two-component developer mainly including a nonmagnetic toner produced by a polymerization method or a pulverization method and a magnetic carrier, and the developer stirring member 4f. Is stirred.

  The carrier particles used in the two-component developer impart good chargeability to the toner particles, develop the toner on the electrostatic latent image at the development pole, and the carrier particles themselves are returned to the developing container and newly toner. It is used repeatedly for a long time in a cycle that is mixed with and imparts good charge. Therefore, the performance required for the carrier particles includes imparting good charge to the toner, and preventing the charge imparting performance from being deteriorated even after repeated use. Conventionally, as a carrier particle, an iron powder carrier, a ferrite carrier, a magnetic material dispersed resin carrier in which magnetic fine particles are dispersed in a binder, and the like are used for a two-component developer for magnetic brush development.

On the other hand, various development systems have been studied in response to the demand for higher image quality. In particular, when a method of applying an alternating electric field is used in the development process, the use of an iron powder carrier has a low resistance, which causes leakage. Will occur, resulting in poor development. Even if a ferrite carrier is used, the carrier particles have a specific resistance of about 10 ⁵ to 10 ⁷ Ω · cm, and a sufficiently good image can be obtained because the halftone latent image is disturbed by the leakage of the developing bias. I can't get it. Therefore, in this embodiment, a good image is obtained by using the magnetic particles dispersed in the binder to increase the specific resistance.

  The developing sleeve 4b is disposed in close proximity to the photosensitive drum 1 while maintaining the closest distance (S-Dgap) to the photosensitive drum 1 at 350 μm. A facing portion between the photosensitive drum 1 and the developing sleeve 4b is a developing portion c.

  The developing sleeve 4b is rotationally driven in the direction opposite to the traveling direction of the photosensitive drum 1 in the developing unit c. Due to the magnetic force of the magnet roller 4c in the developing sleeve 4b, a part of the two-component developer 4e in the developing container 4a is adsorbed and held on the outer peripheral surface of the developing sleeve 4b as a magnetic brush layer. The magnetic brush layer is rotated and conveyed with the rotation of the developing sleeve 4b, and is layered into a predetermined thin layer by the developer coating blade 4d, and comes into contact with the surface of the photosensitive drum 1 at the developing portion c to contact the surface of the photosensitive drum. Rub moderately.

  A predetermined developing bias is applied from the power source S2 to the developing sleeve 4b. In this embodiment, the developing bias voltage for the developing sleeve 4b is an oscillating voltage obtained by superimposing a DC voltage (Vdc) and an AC voltage (Vac). More specifically, it is an oscillating voltage in which a DC voltage of −350 V, a frequency of 8.0 kHz, a peak-to-peak voltage of 1.8 kV, and a rectangular wave AC voltage are superimposed.

  In this way, the developer 4e is coated as a thin layer on the surface of the rotating developing sleeve 4b and is conveyed to the developing unit 4c. The toner in the developer 4e conveyed to the developing unit c is selectively attached to the surface of the photosensitive drum 1 corresponding to the electrostatic latent image by the electric field due to the developing bias, and the electrostatic latent image is developed as a toner image. The In the case of this embodiment, toner adheres to the exposed bright portion of the surface of the photosensitive drum 1 and the electrostatic latent image is reversely developed.

  The developer thin layer on the developing sleeve 4b that has passed through the developing section c is returned to the developer reservoir in the developing container 4a with the subsequent rotation of the developing sleeve 4b.

  In order to maintain the toner concentration of the two-component developer 4e in the developing container 4a within a substantially constant range, the toner concentration of the two-component developer 4e in the developing container 4a is set to, for example, an optical toner concentration sensor (FIG. (Not shown). Then, the toner hopper 4g is driven and controlled according to the detection information, and the toner in the toner hopper 4g is supplied to the two-component developer 4e in the developing container 4a. The toner supplied to the two-component developer 4e is stirred by the stirring member 4f.

  In this embodiment, the toner is triboelectrically charged to a negative polarity by rubbing with the magnetic carrier.

(Transfer means, fixing means)
The printer 100 includes an intermediate transfer member 10 positioned below the photosensitive drum 1. In this embodiment, the intermediate transfer member 10 is an endless belt-like intermediate transfer belt, suspended on support rollers 11, 12, and 13 and rotated and moved in the arrow direction.

  In the present embodiment, a blade-like primary transfer device 5 as a primary transfer unit is disposed at a primary transfer position da facing the photosensitive drum 1, and presses the intermediate transfer belt 10 with a predetermined pressing force. The primary transfer device 5 is applied with a positive transfer bias having a polarity opposite to the negative polarity which is the normal charging polarity of the toner, in this embodiment, +2 kV from the power supply S3a. As a result, the toner image on the surface side of the photosensitive drum 1 is electrostatically transferred sequentially onto the surface of the intermediate transfer belt 10 that moves in the transfer portion d.

  On the other hand, in this embodiment, a secondary transfer device 15 serving as a transfer roller in this embodiment is disposed at a secondary transfer position db facing the support roller (opposing roller) 12 as a secondary transfer unit. . The secondary transfer roller 15 is pressed against the opposing roller 12 with a predetermined pressing force in such a manner as to sandwich the intermediate transfer belt 10.

  On the other hand, the recording material P is fed to the secondary transfer portion db from a paper feed mechanism portion (not shown) at a predetermined control timing.

  The recording material P fed to the secondary transfer portion db is nipped and conveyed between the moving intermediate transfer belt 10 and the secondary transfer roller 15. Meanwhile, a positive transfer bias having a polarity opposite to the negative polarity that is the normal charging polarity of the toner, +2 kV in this embodiment, is applied to the secondary transfer roller 15 from the power source S3b. As a result, the toner image on the surface side of the intermediate transfer belt 10 is sequentially electrostatically transferred onto the surface of the recording material P that is nipped and conveyed by the secondary transfer portion db.

  At this time, the toner remaining on the intermediate transfer belt 10 without being transferred is removed and cleaned using a transfer cleaning device 9 having a cleaning blade 9a as a cleaning means in contact with the transfer belt 10.

  The recording material P that has received the transfer of the toner image through the secondary transfer portion db is sequentially separated from the surface of the intermediate transfer belt 10 and conveyed to the fixing device 6.

  In this embodiment, the fixing device 6 is a heat roller fixing device, and the recording material P is subjected to a fixing process of a toner image by the fixing device 6 and is output as an image formed product (print, copy).

(Cleanerless system, leveling means)
The printer 100 according to the present embodiment employs a so-called cleanerless system, which is a simultaneous development cleaning, and a small amount of residual transfer toner (residual toner) remaining on the surface of the photosensitive drum 1 after the toner image is transferred to the recording material P. There is no dedicated cleaning device for removal. That is, the developing device 4 also serves as a cleaning device for the photosensitive drum 1.

  The transfer residual toner on the surface of the photosensitive drum 1 after the transfer is transported to the developing unit c through the charging unit a and the exposing unit b with the subsequent rotation of the photosensitive drum 1 and is removed by the developing device 4 by simultaneous development cleaning.・ Recovered. That is, a cleanerless system is adopted.

  In this embodiment, the developing sleeve 4b of the developing device 4 is rotated in the direction opposite to the traveling direction of the surface of the photosensitive drum 1 in the developing unit c as described above. Such rotation of the developing sleeve 4b is advantageous for collecting the transfer residual toner on the photosensitive drum 1.

  Since the untransferred toner on the photosensitive drum 1 passes through the exposure part b, the exposure process is performed on the untransferred toner. Usually, since the amount of the transfer residual toner is small, there is no great influence by performing the exposure process on the transfer residual toner.

  However, the transfer residual toner includes those having a normal polarity (negative polarity in this embodiment), a reverse polarity (positive polarity in this embodiment) (reversing toner), and a toner having a small charge amount. . If the reversal toner or the toner with a small charge amount adheres to the charging roller 2 when passing through the charging portion a, the charging roller 2 may be contaminated with toner more than permissible, resulting in a charging failure. Further, fog and untransferred toner are distributed on the photosensitive drum according to the image pattern.

  Therefore, it is necessary to uniformize the residual toner on the surface of the photosensitive drum and to make the charged polarity of the negative polarity that is a normal polarity so that it can be easily collected at the next development. Therefore, leveling means (toner charging means) 7 is provided at a position downstream of the transfer portion d in the rotation direction of the photosensitive drum 1 and at a position upstream of the charging portion a in the rotation direction of the photosensitive drum 1.

  More preferably, an auxiliary leveling means (uniforming means) 8 is provided upstream of the leveling means (toner charging means) 7 in the rotation direction of the photosensitive drum 1 and downstream of the transfer portion d in the rotation direction of the photosensitive drum 1. It is desirable to provide it. The auxiliary leveling means (homogenizing means) 8 is for uniformizing the toner of various charge amounts remaining on the photosensitive drum 1 after the transfer both in a plane and electrically.

  In general, the untransferred toner remaining on the photosensitive drum 1 without being transferred to the recording material P at the transfer portion d is mixed with reversal toner and toner with an inappropriate charge amount.

  Accordingly, the transfer residual toner is discharged once by the auxiliary leveling means (uniformization means) 8, and then the transfer residual toner is charged again to the normal polarity by the leveling means (toner charging means) 7. Thereby, it is possible to effectively prevent the transfer residual toner from adhering to the charging roller 2 and to completely remove and collect the transfer residual toner in the developing device 4. Therefore, generation of a ghost image of the transfer residual toner image pattern is also effectively prevented.

(Cleaning during non-image formation)
Next, a characteristic configuration of the present embodiment will be described.

  According to this embodiment, as shown in the timing chart of FIG. 6, in the electrophotographic laser beam printer 100 of the intermediate transfer method adopting the so-called contact charging method, two-component contact developing method, and cleaner-less method having the above-described configuration. A cleaning process is performed during non-image formation.

  That is, first, at the time of non-image formation, for example, a DC bias of +400 V is applied to the contact charging device 2 after completion of image formation for every 1000 sheets. Thereby, the uncharged and weakly charged toner brought on the photosensitive drum 1 as shown by the curve A in FIG. 7 can be charged to a polarity opposite to the normal polarity (positive polarity in this embodiment). In this embodiment, the potential of the photosensitive drum 1 is 0V, and the developing bias applied to the developing device is also 0V.

  On the other hand, a DC bias of −700 V having a polarity opposite to that at the time of image formation is applied to the primary transfer device 5 installed downstream of the developing device 4.

  With this configuration, the reverse polarity toner on the photosensitive drum 1 is transferred to the intermediate transfer body 10 and collected by the transfer cleaning means 9.

  Here, as shown in FIG. 10, during the transfer, if the transfer contrast with respect to the photosensitive drum 1 does not exceed 800V, the photosensitive drum 1 is not discharged and the potential of the photosensitive drum 1 is maintained. Therefore, in this embodiment, the potential of the photosensitive drum 1 is maintained at 0 V before transfer even after transfer.

  As shown in FIG. 11, even when charging, if the potential difference between the photosensitive drum potential and the charging bias is 800 V or more, the photosensitive drum is not discharged and the photosensitive drum potential hardly changes. Therefore, as described above, according to the present exemplary embodiment, the uncharged and weakly charged toner brought on the photosensitive drum 1 can be charged to a positive polarity opposite to the normal one.

  FIG. 8 shows the fog on the photosensitive drum when an image having an image density of 30% is passed through the image forming apparatus and the control of this embodiment is used for every 1000 passes. A comparison of fog on the photosensitive drum is shown.

  When the control of this embodiment is not used, the fog due to the reverse polarity toner is deteriorated. However, when the control of this embodiment is used, the fog due to the reverse polarity toner is improved as shown in FIG.

  As described above, the fog caused by the reverse polarity toner can be suppressed for a long period of time by adopting the cleaning process at the time of non-image formation according to the present embodiment to the cleanerless type image forming apparatus.

Example 2
Next, a second embodiment of the image forming apparatus of the present invention will be described.

  The image forming apparatus of the present embodiment is suitably embodied in the electrophotographic laser beam printer 100 of the intermediate transfer method adopting the contact charging method, the two-component contact developing method, and the cleanerless method described in the first embodiment. . Therefore, the description of the first embodiment is used for the overall configuration and operation of the image forming apparatus, and the description of the second embodiment is omitted.

  By adopting the non-image forming cleaning step described in the first embodiment, the fog toner carried on the image carrier, that is, the photosensitive drum, can be removed, but compared with the initial fog level. Sometimes it becomes insufficient.

  Therefore, the inventors charged the photosensitive drum 1 to −500 V after the control of the first embodiment, set the developing bias to −300 V so that Vback becomes 200 V, and developed the toner from the developing device 4 onto the photosensitive drum 1 for 30 seconds. The charge distribution of the toner was measured. The charge distribution of the toner at that time is shown by a curve B in FIG.

  In the control of Embodiment 1, uncharged and reverse polarity toner on the photosensitive drum 1 could be collected, but many such toners still exist in the developing container 4a. For this reason, the control of the first embodiment has not been able to return to the fogging level equivalent to the initial level (see FIG. 8).

  In view of the above, in this embodiment, +650 V is applied to the auxiliary leveling unit 8 and −850 V is applied to the leveling unit 7 even during non-image formation. As a result, the toner on the photosensitive drum 1 is charged in the normal polarity direction.

  Furthermore, a sequence as shown in the timing chart of FIG. 9 is performed during non-image formation. The contact charging device 2 is applied with a oscillating voltage obtained by superimposing a sine wave alternating voltage (Vac) having a frequency of 1.3 kHz and a peak-to-peak voltage (Vpp) of 1.7 kV on a DC voltage (Vdc) of −200 V for T1 seconds. . As a result, the charging potential (Vd) of the photosensitive drum 1 becomes −200V.

  In this embodiment, the developing bias is applied to the developing device 4 after T1 seconds. The development bias includes a DC voltage (Vdc) of 0 V, a frequency of 8.0 kHz, and a peak-to-peak voltage (Vpp) of 1.Vback (potential difference between the photosensitive drum potential Vd and the development bias potential Vdc) of 200 V. An alternating voltage (Vac) of 7 kV is applied.

  As a result, reverse polarity toner and uncharged toner are discharged preferentially from the developing container 4a onto the photosensitive drum 1.

  On the other hand, as described above, after the charging bias in which the DC voltage and the AC voltage are superimposed is applied to the charging device 2 for T1 seconds, the charging bias of only DC voltage (Vdc) + 400V is applied. As a result, the toner discharged onto the photosensitive drum is charged in the positive (positive polarity) direction, which has a polarity opposite to that of the normal one.

  Further, a DC bias of −700 V having a polarity opposite to that at the time of image formation is applied to the primary transfer device 5 provided downstream of the developing device 4. As a result, the toner on the photosensitive drum is transferred to the intermediate transfer member 10. The transferred positive toner is collected and removed by the transfer cleaning device 9.

  Here, as shown in FIG. 10, when the transfer contrast with respect to the photosensitive drum 1 does not exceed 800 V during the transfer, the photosensitive drum 1 is not discharged and the potential of the photosensitive drum 1 is maintained. Therefore, in this embodiment, the potential of the photosensitive drum 1 is kept at −200 V before transfer even after transfer.

  Further, as shown in FIG. 11, even when charging, if the potential difference between the photosensitive drum potential and the charging bias is 800 V or more, the photosensitive drum is not discharged and the photosensitive drum potential hardly changes. However, since the photosensitive drum potential is darkly attenuated without receiving light, in this embodiment, the photosensitive drum 1 is again turned on at the time point (T2) when Vback becomes 170 V 10 seconds after the photosensitive drum 1 is charged to -200V. Charge to -200V.

  According to the results of the research experiments conducted by the present inventors, it is found that the following conditions are preferable when discharging uncharged and reversed polarity developers from the developing container 4a onto the photosensitive drum 1 during non-image formation in this embodiment. It was. The condition is that Vback, that is, the potential difference Vback between the potential Vd of the photosensitive drum 1 and the potential Vdc of the developing bias applied to the developing device 4 is not less than 175V and not more than 300V. In this example, as described above, good results were obtained with 200V.

  In this embodiment, the above control is repeated once every 1000 sheets for 1 minute, that is, 6 times.

  When 20000 sheets of an image having an image density of 30% are passed, when the embodiment of the present invention is not used, when the control of the embodiment 1 is performed every 1000 sheets, and the control of the embodiment is performed every 1000 sheets FIG. 12 shows the result of comparison of the fog density when implemented in FIG.

  In the case of using the first embodiment, the deterioration of fog due to the reverse polarity is suppressed to about half, whereas in the case of using the control of this embodiment, the fog due to the reverse polarity is further suppressed. . Further, the fog caused by the uncharged toner is also suppressed, and is at a level almost the same as the initial fog.

  As described above, by using the control of the present embodiment, the initial fogging level can be maintained even when durability is repeated, and a high-quality image can be obtained over a long period of time.

Example 3
In the first and second embodiments, the image forming apparatus of the present invention is described as an intermediate transfer type image forming apparatus. However, the present invention is not limited to the image forming apparatus having such a configuration.

  For example, as shown in FIG. 13, an image forming apparatus including a recording material carrier 10 </ b> A that carries and conveys the recording material P instead of the intermediate transfer body 10 can be used.

  That is, the printer 100 of this embodiment includes the recording material carrier 10 </ b> A located below the photosensitive drum 1. In this embodiment, the recording material carrier 10A is an endless belt-like transfer belt, suspended on support rollers 11, 12, and 13 and rotated and moved in the direction of the arrow.

  In this embodiment, a blade-like transfer device 5 is disposed at a transfer position d facing the photosensitive drum 1 and presses the transfer belt 10A with a predetermined pressing force. The transfer device 5 is applied with a positive transfer bias having a polarity opposite to the negative polarity which is the normal charging polarity of the toner, in this embodiment, +2 kV from the power source S3.

  On the other hand, the toner image on the photosensitive drum 1 is sequentially electrostatically transferred onto the recording material P carried and transported on the transport belt 10A.

  At this time, the toner on the transfer belt 10 </ b> A is removed and cleaned using a cleaning device 9 that has a cleaning blade 9 a that is a cleaning unit in contact with the transfer belt 10.

  The recording material P that has received the transfer of the toner image through the transfer portion d is sequentially separated from the surface of the transfer belt 10A and conveyed to the fixing device 6.

  In this embodiment, the fixing device 6 is a heat roller fixing device, and the recording material P is subjected to a fixing process of a toner image by the fixing device 6 and is output as an image formed product (print, copy).

  Since other configurations and operations are the same as those of the image forming apparatuses according to the first and second embodiments, the same reference numerals are given to components having the same configurations and operations, and the descriptions of the first and second embodiments are incorporated.

  Also for the image forming apparatus of the present embodiment, the same effects as those of the first and second embodiments can be achieved by performing the cleaning control that characterizes the first and second embodiments.

1 is a schematic cross-sectional view of an embodiment of an image forming apparatus according to the present invention. FIG. 4 is a diagram illustrating an example of fog deterioration in a cleanerless type image forming apparatus. FIG. 6 is a toner charge distribution diagram before and after passing through a contact charging device. It is a figure explaining the relationship of carrier deterioration dependence with respect to toner recharging. It is a figure explaining the recharge capability fall by toner deterioration. 6 is a timing chart illustrating cleaning control according to the first exemplary embodiment. FIG. 6 is a charge distribution diagram of toner discharged from a developing container, and a fog toner charge distribution diagram after execution of Example 1. FIG. 3 is a fog toner charge distribution diagram after execution of cleaning control according to the first exemplary embodiment. 6 is a timing chart illustrating cleaning control according to the second exemplary embodiment. FIG. 6 is a relationship diagram between transfer contrast and discharge current amount. FIG. 7 is a relationship diagram between a charging bias potential difference and a discharge current amount with respect to a photosensitive drum potential. It is a figure about the effect which the cleaning control of Example 2 gives to fog. FIG. 5 is a schematic cross-sectional view of another embodiment of an image forming apparatus according to the present invention.

Explanation of symbols

1 Photosensitive drum (image carrier)
2 Charging roller (charging means)
4 Developing device 5 Primary transfer device 7 Leveling means (toner charging means)
8 Auxiliary leveling means (uniformization means)
9 Transfer cleaning device 10 Intermediate transfer belt (intermediate transfer member)
10A transfer belt (recording material carrier)
15 Secondary transfer device

Claims (5)

An image carrier, a charging unit that is placed in contact with the surface of the image carrier and charges the surface of the image carrier, an exposure unit that exposes the surface of the image carrier after charging to form an electrostatic image, and development A developer container containing a developer, and a developing unit that attaches the developer to the electrostatic image to form a developer image; and the developer image is placed on a recording material via an intermediate transfer member or a recording material carrier Transfer means for transferring to a recording material carried and transported by a body, and is not transferred to the intermediate transfer body at the time of transfer, or is not transferred to a recording material carried and transported by the recording material carrier In the image forming apparatus in which the developing unit also serves as a cleaning unit for removing the developer remaining on the surface of the image carrier,
A transfer cleaning device for removing the developer on the intermediate transfer member or the recording material carrier;
At the time of non-image formation, the potential difference between the surface of the image carrier and the developing unit is made larger than that at the time of image formation, and the developer is discharged from the developing unit onto the image carrier. The developer on the carrier is charged to a polarity opposite to the normal polarity and transferred onto the intermediate transfer member or the recording material carrier, and the developer on the intermediate transfer member or the recording material carrier is transferred to the intermediate transfer member or the recording material carrier. An image forming apparatus, wherein the image is collected by a transfer cleaning device.   The image forming apparatus according to claim 1, wherein the potential difference is 175 V or more and 300 V or less when the developer is discharged from the developing unit onto the image carrier during non-image formation.   When the developer on the image carrier is charged to a polarity opposite to the normal polarity by the charging unit during non-image formation, the charging unit has a voltage that does not cause discharge to the image carrier. The image forming apparatus according to claim 1, wherein: is applied.   The transfer at the time of transferring the developer on the image carrier to the intermediate transfer member, or at the time of transferring the developer on the image carrier to the recording material carried and conveyed by the recording material carrier. The image forming apparatus according to claim 1, wherein a voltage having a magnitude that does not cause a discharge to the image carrier is applied to the unit.   The image according to any one of claims 1 to 4, wherein the charging means is a contact charging device, and is connected to a power supply capable of supplying an oscillating voltage in which an AC voltage is superimposed on a DC voltage. Forming equipment.

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