JP2008009148A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress fog from a developing device in an image forming apparatus using a cleaner-less system such as E307, regardless of the use environment of the image forming apparatus or the time for which the image forming apparatus is left, and the contact charging member is not subjected to transfer residual. To provide a stable image forming apparatus which is prevented from being contaminated with toner and free from image defects such as defective charging, ghost, and fog.
When an image forming apparatus is left unused for a certain period of time or more, a copying operation is performed without replenishing toner until the toner concentration in the developer becomes a predetermined threshold value or less after returning. Among the auxiliary charging members disposed in contact with the surface of the photoreceptor, the direct current that is normally applied to the auxiliary charging member that applies a DC voltage having the same polarity as that of the main charging member for a time longer than the time when toner is not supplied. A DC voltage having a value larger than the absolute value of the voltage is applied.
[Selection] FIG.

Description

  The present invention relates to a cleanerless image forming apparatus. More specifically, the developer (toner) remaining on the image carrier after the transfer process is removed and collected from the image carrier by development simultaneous cleaning in the developing device, and the cleaning device is eliminated so that it can be reused. The present invention relates to a type image forming apparatus.

  2. Description of the Related Art Conventionally, transfer type image forming apparatuses such as copying machines, printers, and facsimiles using a transfer type electrophotographic system have a photosensitive drum, which is an image carrier generally used as a rotary drum type, and the photosensitive drum with a predetermined polarity, A charging device (charging process) that uniformly charges the potential, an exposure device (exposure process) as an information writing unit that forms an electrostatic latent image on the charged photosensitive member, and an electrostatic formed on the photosensitive member A developing device (development process) that visualizes the latent image with toner as a developer, a transfer device (transfer process) that transfers the toner image from the surface of the photoconductor to a transfer material such as paper, and a photoconductor after the transfer process. It consists of a cleaning device (cleaning process) that removes some residual toner and cleans the surface of the photoreceptor, and a fixing device (fixing process) that fixes the toner image on the transfer material. True process (charging, exposure, development, transfer and cleaning) is applied is used for image formation.

  The toner remaining on the photoconductor after the transfer process is removed from the surface of the photoconductor by the cleaning device and collected in the cleaning device to become waste toner. However, it is desirable that such waste toner does not come out from the viewpoints of environmental conservation and effective use of resources.

  Therefore, for example, an image forming apparatus has been proposed in which transfer residual toner collected by a cleaning device described in JP-A-2000-010363, that is, so-called waste toner is returned to the developing device and reused. Further, for example, the cleaning device described in Japanese Patent Laid-Open No. 2001-183905 is discarded, and the transfer residual toner on the photoconductor after the transfer process is removed and collected from the photoconductor by “development simultaneous cleaning” in the developing device and reused. A cleanerless image forming apparatus has been proposed.

  In the simultaneous development cleaning, the transfer residual toner on the photoconductor after the transfer process is collected by the developing device during the subsequent development process. That is, the photosensitive member to which the transfer residual toner is attached is continuously charged and exposed to form an electrostatic latent image, and a fog removal bias (DC voltage applied to the developing device and the surface of the photosensitive member is developed during the developing process of the electrostatic latent image. By using a fog removal potential difference Vback which is a potential difference between potentials, transfer residual toner existing on a portion not to be developed (non-image portion) of the transfer residual toner remaining on the surface of the photoreceptor is removed and collected by the developing device. Is the method.

  According to this method, the transfer residual toner is collected by the developing device and reused for development of the electrostatic latent image in the subsequent process. Therefore, waste toner is eliminated and less trouble is caused during maintenance. Can do. Further, the cleaner-less configuration is advantageous for downsizing the image forming apparatus.

  On the other hand, as a charging device, instead of a non-contact charging method using a corona discharge phenomenon such as a scorotron charger, a conductive charging member (mainly a charging roller using a conductive roller) has been used in recent years. A contact charging method in which a charging voltage is applied to a charging member by directly contacting or approaching an image carrier such as the above is becoming mainstream. The contact charging method does not require a large-capacity high-voltage power supply, which is advantageous in terms of cost and downsizing of the device, and generation of ozone due to discharge can be suppressed to a very small amount compared to the corona charging method. It is.

  In a cleanerless type image forming apparatus that removes and collects transfer residual toner on the photoconductor after the transfer process, when a contact charging device is used as the photoconductor charging device, the transfer residual toner on the photoconductor When passing through the contact nip portion between the photosensitive member and the contact charging device, toner in which the transfer polarity in the transfer residual toner is reversed to the polarity opposite to the normal polarity adheres to the contact charging device. Accumulation on the surface layer may cause charging failure and fogging.

  The cause of the occurrence of toner in which the charge polarity is reversed to the normal polarity in the transfer residual toner is that the toner as a developer is small in quantity, but the charge polarity is originally reversed to the reverse polarity. In some cases, there are cases where there is a mixture of toners, and even if the charge polarity is normal polarity, the charge polarity is reversed or the charge is reduced due to the influence of transfer bias or peeling discharge. It is.

  In other words, the transfer residual toner includes a normal charge polarity, a reverse polarity reverse toner, and a low charge amount, and the reverse toner and the low charge amount of the toner are in contact with the photoconductor. It is considered that the toner adheres to the contact charging device when passing through the contact nip portion of the device.

  In addition, in order to remove and collect the transfer residual toner on the photosensitive member by the developing device, the charging polarity of the transfer residual toner on the photosensitive member that passes through the charging portion and reaches the developing portion is a normal polarity. The charge amount needs to be a toner charge amount that can be collected from the photosensitive member by the developing device. Reversal toner and toner with an inappropriate charge amount cannot be removed and collected from the photosensitive member to the developing device, which may cause defective images such as defective charging and fogging.

  Japanese Patent Laid-Open No. 2001-215798 proposes an image forming apparatus in which charging auxiliary members such as a transfer residual toner uniformizing unit and a toner charge amount control unit are sequentially provided between the transfer unit and the charging unit in the downstream direction from the transfer unit. ing.

  The transfer residual toner uniformizing means is means for dispersing and non-patterning the image pattern of the residual toner image remaining on the image carrier without being transferred at the transfer portion. Specifically, by rubbing the surface of the image carrier with a rubbing member such as a brush, the residual developer image pattern is scraped or disturbed to disperse and distribute the developer on the surface of the image carrier.

  By providing the transfer residual toner equalizing means, it becomes possible to stably carry out the normal polarity charging process of the residual toner on the image carrier by the toner charge amount control means which is the next step. It is possible to effectively prevent adhesion to the charging means. Further, the latent image pattern of the residual toner image on the image carrier is also erased at the same time, thereby preventing the ghost image of the residual toner image pattern from being generated.

  That is, for example, in the case of an image that is difficult to transfer at the transfer portion, such as a vertical line pattern image, the amount of residual toner on the image carrier locally increases. If no transfer residual toner equalizing means is provided at this time, the transfer residual toner is conveyed to the toner charge amount control means without being uniformly dispersed. Therefore, the transfer charge toner is sufficiently charged with normal polarity by the toner charge amount control means. In some cases, the toner cannot be processed and the transfer residual toner adheres to and contaminates the contact charging device, resulting in a charging failure. Further, a ghost image may be generated on the next image due to the pattern of residual toner, that is, the latent image pattern remaining on the image carrier after the transfer process.

  Therefore, by providing the transfer residual toner equalizing means, the transfer residual toner on the image carrier sent to the toner charge amount control means is sufficiently non-patterned. It is possible to process the untransferred toner with a charge amount suitable for the recovery of the toner. As a result, the transfer residual toner can be prevented from adhering to the contact charging device, and the transfer residual toner can be efficiently collected by the developing device, so that a stable image free from charging failure, ghost, fog and the like can be obtained. it can.

  However, when printing an image with a high printing rate such as a photographic image, a part of the transfer residual toner adheres to the contact portion of the transfer residual toner uniformizing means and the toner charge amount control means with the image carrier. -Accumulate. As a result, the resistance value of the contact portion between the transfer residual toner uniformizing means and the toner charge amount control means and the image carrier increases, causing the transfer residual toner uniformizing means and the toner charge amount control means to deteriorate in function. In this case, the non-patterning process and the charging process for the transfer residual toner become insufficient, and problems such as charging failure, ghost, and fogging are caused.

  Therefore, in order to maintain the functions of the transfer residual toner equalizing means and the toner charge amount control means even when an image printing operation with a high printing rate is performed, the transfer residual toner equalizing means and the toner charge amount control means are provided. It is necessary to discharge the accumulated toner periodically.

  In Japanese Patent Laid-Open No. 2003-316202, the voltage applied to the contact charging device is turned off and the transfer residual toner is made uniform at a predetermined timing such as when the image forming apparatus is started, during the printing operation, or at the end of the printing operation. The control which repeats the operation | movement which switches on / off the voltage applied to a means and a toner charge amount control means in a pulse form for a fixed number of times is proposed.

As a result, even when a large amount of transfer residual toner is accumulated in the transfer residual toner uniformizing means and toner charge amount control means, such as when an image is printed with a high printing rate, the above discharge control is performed in a predetermined manner. By performing at the timing, it becomes possible to discharge the toner accumulated in the transfer residual toner uniformizing means and the toner charge amount control means, and to prevent the deterioration of the functions of the transfer residual toner uniformizing means and the toner charge amount control means. Image defects such as defects, ghosts, and fog can be prevented.
JP 2000-010363 A JP 2001-183905 A JP 2001-215798 A JP 2003-316202 A

  However, when the image forming apparatus is allowed to stand for a long time and then used again, there are cases where the amount of toner having a reverse polarity or a toner having almost no charge amount in the toner to be developed increases significantly. This is because, while the image forming apparatus is left unattended, the developer in the developing device electrostatically aggregates and the fluidity becomes very poor, and the newly supplied toner is sufficiently stirred and mixed in the developing device. In other words, the development is performed without reaching the normal polarity and the charge amount.

  In this case, as a fog appears on the image, the transfer residual toner increases in a large amount, so that there is a possibility that a large amount of toner accumulates in the transfer residual toner equalizing means and the toner charge amount control means. As a result, the resistance value of the contact portion between the transfer residual toner uniformizing means and the toner charge amount control means and the image carrier increases, causing the transfer residual toner uniformizing means and the toner charge amount control means to deteriorate in function. In this case, the non-patterning process and the charging process for the transfer residual toner are insufficient. Then, there arises a problem that the contact charging member is contaminated with the transfer residual toner and causes a charging failure, and a problem that the developing device cannot collect the transfer residual toner and rotates around the photosensitive member.

  Especially in an image forming apparatus that employs a cleaner-less system, a charging failure caused by the contact charging device being contaminated with residual transfer toner induces a new fog in the developing device, and cannot be transferred at the transfer section. There is a risk of a vicious circle of increased toner.

  Further, the above-described problem tends to become worse as the usage amount of the image forming apparatus, specifically, the usage amount of the developer in the developing device increases. This is because the ability of the magnetic carrier constituting the developer to impart charge to the toner decreases with the number of sheets used.

  The above-described problems are the worst in the environment where the image forming apparatus is used, specifically in a low humidity environment, and these phenomena are alleviated as the humidity increases. This is because electrostatic charge aggregation is likely to occur because the developer has the highest charge amount in a low-humidity environment and is not relaxed in the air.

  The object of the present invention is to provide a toner having a reverse polarity to the toner developed in the developing device or an amount of charge when the image forming apparatus is used again after being left unused for a long period of time. In addition to suppressing the inclusion of toner that does not exist, the charge imparting ability to the transfer residual toner by the toner charge amount control means is increased, so that charging failure, ghosting is not caused regardless of the number of image forming apparatuses used and the environment in which the image forming apparatus is used. It is to provide a stable image forming apparatus free from image defects such as fogging.

  A photosensitive drum; charging means for charging the photosensitive drum at a contact portion to form an electrostatic latent image on the photosensitive drum; and exposure means for forming an electrostatic latent image on the photosensitive drum charged by the charging means A two-component developing unit that supplies a toner to an electrostatic latent image formed on a photosensitive drum using a developer composed of toner and a magnetic carrier and visualizes the toner; and a toner replenishing unit that replenishes the toner to the two-component developing unit. And a transfer means for transferring the visualized toner image to the transfer material, and a charging auxiliary member to which a DC voltage having the same polarity as the charging means is applied to the photosensitive drum after transfer, and is not transferred by the transfer means. In an image forming apparatus using a cleanerless system that collects toner remaining on the surface of the photosensitive drum with a two-component developing unit, the image forming apparatus is left unoperated for a predetermined period of time and then again. When image formation is performed, the toner replenishment from the toner replenishing unit to the two-component developing unit is not performed and the toner replenishment is not performed until the toner density with respect to the developer in the two-component developing unit becomes a predetermined threshold value or less. An image forming apparatus characterized in that the absolute value of the DC voltage applied to the auxiliary charging member is larger than a normal value at the same time or more.

  As described above, according to the present invention, the photosensitive drum, the charging means for charging the photosensitive drum at the contact portion to form an electrostatic latent image on the photosensitive drum, and the photosensitive member charged by the charging means. An exposure unit that forms an electrostatic latent image on a drum, a two-component developing unit that uses a developer composed of toner and a magnetic carrier to supply toner to an electrostatic latent image formed on a photosensitive drum, and visualizes the toner. Toner replenishing means for replenishing the two-component developing means, transfer means for transferring the visualized toner image to the transfer material, and a charging auxiliary member for applying a DC voltage having the same polarity as the charging means to the photosensitive drum after transfer And an image forming apparatus using a cleaner-less system that collects toner remaining on the surface of the photosensitive drum without being transferred by the transfer means using a two-component developing means. When the image is formed again after being left unoperated, the toner supply from the toner replenishing unit to the two-component developing unit is continued until the toner concentration with respect to the developer in the two-component developing unit falls below a predetermined threshold. In addition, the absolute value of the DC voltage applied to the auxiliary charging member is made larger than the normal value at the same time or longer than the time when toner is not replenished. It is possible to provide a stable image forming apparatus free from image defects such as charging failure, ghost, and fog, regardless of the use environment of the apparatus.

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

  Hereinafter, the image forming apparatus (image recording apparatus) of the embodiment will be described.

  FIG. 1 is a schematic configuration model diagram of an example of an image forming apparatus according to the present invention. The image forming apparatus 100 in this example is an electrophotographic laser beam printer using a contact charging method, a reversal development method, and a cleaner-less method.

(1) Overall schematic configuration of printer (a) Image carrier 1 is a rotating drum type electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum) as an image carrier. This photosensitive drum 1 is a negatively chargeable organic photoconductor (OPC), has an outer diameter of 60 mm, and is driven to rotate counterclockwise as indicated by an arrow with a process speed (peripheral speed) of 100 mm / sec around the central support shaft. The

  As shown in the layer configuration model diagram of FIG. 2, the photosensitive drum 1 includes an undercoat layer 1 b that suppresses light interference and improves the adhesion of an upper layer on the surface of an aluminum cylinder (conductive drum base) 1 a. The photo-charge generation layer 1c and the charge transport layer 1d are coated in order from the bottom.

(B) Charging means 2 is a contact charging device (contact charger) as a charging means for uniformly charging the peripheral surface of the photosensitive drum 1, and in this example, a charging roller (roller charger).

  The charging roller 2 is rotatably held at both ends of a cored bar 2a by a bearing member (not shown), and is urged toward the photosensitive drum by a pressing spring 2e so as to be predetermined with respect to the surface of the photosensitive drum 1. Is pressed with the pressing force of, and 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 (charging nip portion) a.

  A charging bias voltage of a predetermined condition is applied from the power source S1 to the metal core 2a of the charging roller 2 so that the peripheral surface of the rotating photosensitive drum 1 is contact-charged to a predetermined polarity and potential. In this example, the charging bias voltage for the charging roller 2 is an oscillating voltage obtained by superimposing a DC voltage (Vdc) and an AC voltage (Vac). More specifically, DC voltage: −500 V, AC voltage; frequency f1 kHz, peak-to-peak voltage Vpp 1.5 kV, and vibration voltage superimposed with a sine wave, and the peripheral surface of the photosensitive drum 1 is −500 V (dark potential Vd). Are uniformly charged by contact.

  The longitudinal length of the charging roller 2 is 300 mm, and a lower layer 2b, an intermediate layer 2c, and a surface layer 2d are sequentially laminated from the bottom around the cored bar (support member) 2a as shown in the layer configuration model diagram of FIG. This is a three-layer structure. The lower layer 2b is a foamed sponge layer for reducing charging noise, and the surface layer 2d is a protective layer provided to prevent leakage even if the photosensitive drum 1 has a defect such as a pinhole. .

  More specifically, the specification of the charging roller 2 of this example is as follows.

a. Cored bar 2a; stainless steel round bar 6mm in diameter b. Lower layer 2b: carbon dispersion foamed EPDM, specific gravity 0.5 g / cm ³ , volume resistivity 10 ² to 10 ⁹ Ωcm, layer thickness 3.0 mm, length 320 mm
c. Intermediate layer 2c: carbon-dispersed NBR rubber, volume resistivity 10 ² to 10 ⁵ Ωcm, layer thickness 700 μm
d. Surface layer 2d: tin resin and carbon dispersed in fluororesin resin resin, volume resistivity 10 ⁷ to 10 ¹⁰ Ωcm, surface roughness (JIS standard 10-point average surface roughness Ra) 1.5 μm, layer thickness 10 μm
In FIG. 2, reference numeral 2f denotes a charging roller cleaning member, which is a flexible cleaning film in this example. The cleaning film 2f is arranged in parallel to the longitudinal direction of the charging roller 2 and fixed at one end to a support member 2g that reciprocates a certain amount in the longitudinal direction. It is arranged to form a contact nip. The support member 2g is driven to reciprocate by a certain amount in the longitudinal direction through a gear train by a drive motor of the printer, and the charging roller surface layer 2d is rubbed with the cleaning film 2f. As a result, the contaminants (fine toner, external additives, etc.) on the charging roller surface 2d are removed.

(C) Information writing means 3 is an exposure apparatus as information writing means for forming an electrostatic latent image on the surface of the charged photosensitive drum 1, and this example is a laser beam scanner using a semiconductor laser. A laser beam modulated in response to an image signal sent from a host process such as an image reading apparatus (not shown) to the printer side is output, and the uniformly charged surface of the rotating photosensitive drum 1 is laser-scanned and exposed at the exposure position b L (image exposure). Due to the laser scanning exposure L, the potential of the surface of the photosensitive drum 1 irradiated with the laser light is lowered, and electrostatic latent images corresponding to the scanned and exposed image information are sequentially formed on the surface of the rotating photosensitive drum 1. Go.

(D) Developing means 4 is a developing device (developing device) as developing means for supplying a developer (toner) to the electrostatic latent image on the photosensitive drum 1 to visualize the electrostatic latent image. This is a reversal developing device of a magnetic brush developing system.

  4a is a developing container. Reference numeral 4b denotes a non-magnetic developing sleeve, and the developing sleeve 4b is rotatably arranged in the developing container 4a with a part of the outer peripheral surface thereof exposed to the outside. 4c is a non-rotating fixed magnet roller inserted into the developing sleeve 4b, 4d is a developer coating blade, 4e is a two-component developer contained in the developing container 4a, and 4f is disposed on the bottom side in the developing container 4a. The provided developer agitating member, 4g, is a toner hopper and contains replenishing toner.

The two-component developer 4e in the developing container 4a is a mixture of toner and a magnetic carrier and is stirred by the developer stirring member 4f. In this example, the resistance value of the magnetic carrier is about 10 ¹³ Ω · cm, and the particle size is 40 μm. The toner is triboelectrically charged to negative polarity by rubbing with the magnetic carrier.

  The developing sleeve 4b is disposed in close proximity to the photosensitive drum 1 while maintaining the closest distance (referred to as S-Dgap) to the photosensitive drum 1 at 350 μm. The facing portion between the photosensitive drum 1 and the developing sleeve 4a is a developer c. The developing sleeve 4b is driven to rotate in the direction opposite to the traveling direction of the photosensitive drum 1 in the developing portion c. A part of the two-component developer 4e in the developing container 4a is adsorbed and held as a magnetic brush layer on the outer peripheral surface of the developing sleeve 4b by the magnetic force of the magnet roller 4c in the sleeve, and is rotated and conveyed as the sleeve rotates. A predetermined thin layer is formed by the developer coating blade 4d, and in contact with the surface of the photosensitive drum 1 at the developing portion c, the surface of the photosensitive drum is rubbed appropriately. A predetermined developing bias is applied to the developing sleeve 4b from the power source S2. In this example, 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 a DC voltage; -350V, an AC voltage; a frequency f8.0 kHz, a peak-to-peak voltage 1.8 kV, and an oscillating voltage in which a rectangular wave is superimposed.

  Thus, the toner in the developer coated as a thin layer on the surface of the rotating developing sleeve 4b and transported to the developing section c corresponds to the electrostatic latent image on the surface of the photosensitive drum 1 by the electric field due to the developing bias. By selectively adhering, the electrostatic latent image is developed as a toner image. In the case of this example, 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.

  In order to maintain the toner concentration of the two-component developer 4e in the developing container 4a within a predetermined substantially constant range, the toner concentration of the two-component developer 4e in the developing container 4a is adjusted by, for example, an optical toner concentration sensor (not shown). The toner hopper 4g is driven and controlled according to the detected information, and the toner in the toner hopper 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.

  The toner concentration in the two-component developer 4e is usually used in the range of 4 to 12 wt%, and in the unused two-component developer 4e, it is used in the range of 6 to 10 wt%. In this embodiment, the toner concentration in the two-component developer 4e in the unused initial state is 8 wt%.

  In general, the toner density in the two-component developer 4e is controlled to decrease as the number of developers used increases. This is because the surface of the magnetic carrier deteriorates with the number of sheets used, so that the ability to pull up the toner replenished from the toner hopper 4g to a desired charge amount, the so-called charge imparting ability, is reduced. In this embodiment, as shown in FIG. 9, the initial toner density is 8 wt%, but after the 40k copy operation is performed, the toner density is controlled to 6 wt%.

(E) Transfer means / fixing means 5 is a transfer device, and this example is a transfer roller. The transfer roller 5 is brought into pressure contact with the photosensitive drum 1 with a predetermined pressing force, and the pressure nip portion is a transfer portion d. A transfer material (a member to be transferred, a recording material) P is fed to the transfer portion d from a paper feeding mechanism portion (not shown) at a predetermined control timing.

  The transfer material P fed to the transfer part d is nipped and conveyed between the rotating photosensitive drum 1 and the transfer roller 5, and during that time, the negative polarity that is the normal charging polarity of the toner from the power source S 3 is transferred to the transfer roller 5. In this example, the positive polarity transfer bias having the reverse polarity is applied with +2 kV, so that the toner image on the surface of the photosensitive drum 1 is sequentially electrostatically transferred onto the surface of the transfer material P that is nipped and conveyed by the transfer portion d. To go.

  The transfer material P that has received the transfer of the toner image through the transfer portion d is sequentially separated from the surface of the rotary photosensitive drum 1 and is conveyed to the fixing device 6 (for example, a heat roller fixing device) to receive the toner image fixing process. Output as an image formed product (print, copy).

(2) Cleanerless system The printer of this example is cleanerless, and does not include a dedicated cleaning device that removes a small amount of transfer residual toner remaining on the surface of the photosensitive drum 1 after the transfer of the toner image onto the transfer material P. The transfer residual toner on the surface of the photosensitive drum 1 after the transfer is carried to the developing unit c through the charging unit a and the dew-deposited unit b with the subsequent rotation of the photosensitive drum 1, and simultaneously developed (collected) by the developing device 4 (Cleanerless system)

  In this embodiment, the developing sleeve 4b of the developing device 4 is rotated in the developing portion c in the direction opposite to the traveling direction of the surface of the photosensitive drum 1 as described above. It is advantageous for recovery.

  Since the untransferred toner on the surface of the photosensitive drum 1 passes through the exposure portion b, the exposure process is performed from the untransferred toner. However, since the amount of the untransferred toner is small, no significant influence appears.

  However, as described above, the transfer residual toner includes a mixture of normal polarity, reverse polarity (reversal toner), and low charge amount, among which reverse toner and toner with low charge amount. Adhering to the charging roller 2 when passing through the charging portion a causes the charging roller to be contaminated with toner more than allowable, resulting in a charging failure.

  In addition, in order to effectively perform simultaneous development cleaning of the transfer residual toner on the surface of the photosensitive drum 1 by the developing device 3, the charged polarity of the transfer residual toner on the photosensitive drum carried to the developing unit c is a normal polarity. In addition, the charge amount needs to be a charge amount of toner that can develop the electrostatic latent image on the photosensitive drum by the developing device. Inverted toner or toner with an inappropriate charge amount cannot be removed and collected from the photosensitive drum to the developing device 4, which causes a defective image.

  Therefore, at the position downstream of the transfer portion d in the rotation direction of the photosensitive drum, a transfer residual toner equalizing means 8 is provided as a charging auxiliary member for uniformizing the transfer residual toner on the photosensitive drum 1. Toner charge amount for aligning the charging polarity of the untransferred toner with the negative polarity that is the normal polarity at a position downstream of the toner uniformizing means 8 in the photosensitive drum rotation direction and upstream of the charging portion a in the photosensitive drum rotation direction. Control means 7 is provided.

  Generally, the transfer residual toner remaining on the photosensitive drum 1 without being transferred to the transfer material P at the transfer portion d is mixed with reversal toner and toner with an inappropriate charge amount, and the transfer residual toner equalizing means 8 Then, the transfer residual toner is neutralized once, and the toner charge amount control means 7 again performs the normal polarity charging process on the transfer residual toner, thereby effectively preventing the transfer residual toner from adhering to the charging roller 2. Removal and collection by the developing device 4 can be performed completely. Therefore, the generation of a ghost image of the transfer residual toner image pattern is strictly prevented.

In this embodiment, the transfer residual toner equalizing means 8 and the toner charge amount control means 7 are brush members having appropriate conductivity, and the brush portion is disposed in contact with the surface of the photosensitive drum 1. . More specifically, the brush member has a resistance value adjusted by dispersing a resistance adjusting agent such as carbon or metal powder in fibers such as rayon, acrylic, and polyester. Further, as the brush member, the thickness of one fiber is preferably 30 denier or less and the flocking density is preferably 7750 to 77500 / cm ² (50,000 to 500,000 / inch ² ) or more. In this example, the brush fiber thickness is 6 denier, the flocking density is 15500 / cm ² (100,000 / inch ² ), the length from the fixed end to the free end of the fiber is 5 mm, and the resistance of the brush is 5 × 10 ⁴ Ω. -The thing of cm was used.

  In FIG. 12, f is a contact portion between the transfer residual toner equalizing means 8 and the surface of the photosensitive drum 1, and e is a contact portion between the toner charge amount control means 7 and the surface of the photosensitive drum 1. The width of the contact portion f and the contact portion e in the sub-scanning direction (the width in the sub-scanning direction of the transfer residual toner uniformizing means 8 and the toner charge amount control means 7) is 5 mm. The amount control means 7 is brought into contact with the surface of the photosensitive drum 1 so that the intrusion amount is 1 mm. Further, the transfer residual toner equalizing means 8 and the toner charge amount control means 7 reciprocate with an amplitude of 2.5 mm and a frequency of 5.0 Hz with respect to the longitudinal direction of the surface of the photosensitive drum 1.

  The transfer residual toner uniformizing means 8 is applied with a positive DC voltage having a polarity opposite to that of the charging roller 2 from the power source S5, and the toner charge amount control means 7 is negatively charged with the same polarity as that of the charging roller 2. A voltage is applied from the power source S4. Specifically, a DC voltage is variably applied between +200 V and +400 V to the transfer residual toner uniformizing means 8 for each use environment, and the toner charge amount control means 7 is used between -600 V and -800 V. DC voltage is variably applied for each environment.

  In the transfer portion d, the transfer residual toner remaining on the photosensitive drum 1 after the transfer of the toner image to the transfer material P reaches the contact portion f between the transfer residual toner equalizing means 8 and the photosensitive drum 1, and the transfer residual toner becomes uniform. By means 8, the charge amount is made uniform in the vicinity of 0 μC / g. Further, the transfer residual toner on the photosensitive drum surface made uniform by the transfer residual toner uniformizing means 8 reaches the contact portion e between the toner charge amount control means 7 and the photosensitive drum 1, and the toner charge amount control means 7 makes the transfer residual toner. The toner has a negative polarity in which the charging polarity is a normal polarity.

  By aligning the charging polarity of the transfer residual toner with the negative polarity which is the normal polarity, the transfer portion 2 is charged when the surface of the photosensitive drum 1 is charged from the transfer residual toner at the contact portion a between the charging roller 2 and the photosensitive drum 1. The reflection force of the residual toner on the photosensitive drum 1 is increased, and the transfer residual toner is prevented from adhering to the charging roller 2.

  Next, recovery of transfer residual toner in the development process will be described. The developing device 4 is as described above, and is a cleanerless system that cleans the transfer residual toner simultaneously with development. The toner charge amount for collecting the transfer residual toner on the photosensitive drum 1 to the developing device 4 needs to be substantially equal to the toner charge amount at the time of development. When the transfer residual toner is an appropriate amount, As described above, the charge amount of the transfer residual toner can be controlled by the toner charge amount control means 7.

  Thus, the charge amount of the transfer residual toner on the photosensitive drum 1 carried from the transfer portion d to the charging portion a is charged by the toner charge amount control means 7 so as to have a normal polarity negative polarity, and the transfer residual toner is charged. This prevents the toner from adhering to the charging roller 2 and charges the photosensitive drum 1 to a predetermined potential with the charging roller 2, and at the same time, the transfer residual toner charged with the negative polarity having the normal polarity by the toner charge amount control means 7. By controlling the charge amount to an appropriate charge amount by which the electrostatic latent image on the photosensitive drum can be developed by the developing device 4, the transfer residual toner can be efficiently collected by the developing device.

(3) Return control after being left for a long time (toner supply stop control)
(A) Basic control As described above, when the image forming apparatus, specifically, the developing device 4 is left unused for a long time and is used again, the charged polarity is reversed in the toner in the developing device 4. In some cases, the amount of polar toner or toner having almost no charge amount may increase significantly. A large amount of these toners are discharged from the developing device 4 as fog toner to the non-image area on the photosensitive drum 1, and part of the toner is transferred by the transfer device 5 to become image fog, and what remains on the photosensitive drum 1 is untransferred. The toner uniformizing means 8 and the toner charge amount control means 7 cannot be sufficiently charged, and the surface layer of the charging roller 2 is contaminated to cause charging failure.

  This is because, while the image forming apparatus is left unattended, the developer in the developing device electrostatically aggregates and the fluidity becomes very poor, and the newly supplied toner is sufficiently stirred and mixed in the developing device. In other words, the development is performed without reaching the normal polarity and the charge amount.

  FIG. 4 shows a relationship diagram regarding the change in toner fluidity before and after being left for each toner concentration of unused developer. Here, the angle of repose was adopted as an index representing the fluidity of the developer. The angle of repose is the angle between the horizontal plane and the generatrix of the cone when the powder is deposited in a conical shape in a natural state, and is the limit angle at which the powder can be stabilized without collapsing. Using PT-S, the toner concentration in the developer was changed from 2 wt% to 14 wt% and measured. The developer having a toner concentration of 8 wt% used as the initial developer in this embodiment is 36 ° before being left in an environment having an ambient temperature of 23 ° C. and a relative humidity of 50%, whereas it is 42 after being left for 24 hours. It can be seen that it has risen to °. This represents that the fluidity deteriorated due to neglect. Further, the relationship between the toner concentration and the fluidity indicates that the fluidity deteriorates as the toner concentration increases. This is because the flowability of the magnetic carrier in the developer is relatively good, and conversely the flowability of the toner is relatively bad, so the amount of toner carried on the surface of the magnetic carrier increases as the toner is added. As a result, the fluidity of the developer deteriorates.

  FIG. 5 shows the results of image formation with the developer used in FIG. 4 before and after being left standing and the image fog level and the charging roller 2 contamination level examined. As for the developer before being left, neither image fog nor contamination on the surface of the charging roller 2 occurs until the toner concentration is 12 wt%. When the toner density exceeds 14 wt%, a slight occurrence of image fog is first observed, and further exceeds 16 wt%. As a result, slight generation of dirt on the charging roller 2 was observed. On the other hand, in the developer after being left for 24 hours, slight occurrence of both image fogging and charging roller 2 contamination was observed at 8 wt% used as the toner concentration of the initial developer in this embodiment, and the toner was observed at 10 wt% or more. As the density increased, the image fogging and the charging roller 2 contamination were greatly deteriorated. When the above results and the result of FIG. 4 shown above are collated, it can be seen that there is a correlation between developer fluidity, image fogging, and charging roller 2 contamination, and an angle of repose which is an index of fluidity. It was found that the occurrence or non-occurrence of the problem is divided with a threshold value of around 40 °.

  FIG. 6 shows a relationship diagram regarding the change in the fluidity of the developer with respect to the standing time of the developer. The toner concentration of the developer used here is 8 wt%, and the ambient environment is a temperature of 23 ° C. and a relative humidity of 50%. Whereas the angle of repose of the developer before being left is 36 °, the angle of repose during development after about 18 hours is 41 ° exceeding the threshold level at which the problem occurs, which is 40 °, and after 36 hours, the angle of repose is 44 °. It can be seen that it has reached °. That is, it can be seen that a problem may occur after 12 to 18 hours have elapsed after being left unattended.

  In other words, when the initial developer is left in an environment where the ambient temperature is 25 ° and the relative humidity is 50%, and when the developer having a toner concentration of 8 wt% is left for about 12 hours or more, image fogging or charging roller 2 contamination may occur. It becomes a certain liquidity. Therefore, in this embodiment, the time during which image formation is not performed, that is, the standing time is counted, and the following control is performed when 12 hours or more have elapsed.

  FIG. 3 shows a timing chart of the control performed in this embodiment. In this embodiment, when image formation is performed again after a period of time during which image formation is not performed, that is, a standing time of 12 hours or more, it is expected that the fluidity of the developer has deteriorated. The toner supply from the toner hopper 4g to be performed to the developing device 4 is stopped. The stop timing is a period until the fluidity of the developer in the developing device 4 is lowered to the toner concentration at which the repose angle (toner concentration target value) is equal to or less than the above-described threshold (about 40 °). From the result of FIG. 4, the initial toner concentration of the initial developer is 8 wt%, whereas the replenishment operation is stopped until the toner in the developer in the developing device 4 is consumed to 6 wt%. After the toner is reduced to 6 wt%, the toner supply is resumed, and the operation is performed to return the toner density of the developer to the initial state, that is, to 8 wt%.

  Therefore, in the image forming apparatus after being left, the toner hopper 4g does not supply toner into the developing device 4, and the toner density in the developing device is temporarily reduced, whereby the developer fluidity in the developing device 4 is flowed. As a result, the developer can be returned to a state in which the toner replenished from the toner hopper 4g can be sufficiently stirred and mixed to obtain the required toner charge amount.

  At this time, even if the above control is performed, it is expected that the reversal toner and the toner with a small charge amount are expected to increase slightly than usual, and therefore, it is applied to the toner charge amount control means 7 of the charging auxiliary member. The control for applying the bias with an absolute value larger than usual is performed at the same time. This is because when the amount of toner reaching the nip portion e of the toner charge amount control means 7 increases more than usual, a sufficient amount of charge is given with a bias that is normally applied to the toner charge amount control means 7. It is because it cannot be done. Specifically, in an environment where the ambient temperature is 23 ° C. and the relative humidity is 50%, the bias applied to the toner charge amount control means 7 is normally −750 V, but is increased to −980 V during this control. Here, when considering the potential of the photosensitive drum 1 before entering the toner charge amount control means 7, the amount of current from the toner charge amount control means 7 to the photosensitive drum 1 is about 1.5 times. The upper toner amount can be allowed up to 1.5 times the normal amount. Actually, if the above-described toner supply stop is performed, the toner amount on the photosensitive drum 1 after the transfer is about 1.2 to 1.4 times the normal amount, and the setting of this embodiment is sufficient. On the other hand, when the bias applied to the toner charge amount control means 7 is increased from the normal time, the discharge with the photosensitive drum 1 is increased, which may promote the deterioration of the surface of the photosensitive drum 1 due to the discharge. Therefore, it is necessary to keep it to the minimum necessary.

  Further, the period during which a bias having a larger absolute value than usual is applied to the toner charge amount control means 7 is at least until the toner concentration of the developer becomes equal to or lower than the toner concentration target value. The period from when the toner supply from the hopper 4g to the developing device 4 was resumed until the normal toner density was reached.

  In summary, in this embodiment, when the image formation is not performed, that is, when the leaving time is 12 hours or more, when the image formation is resumed, the toner supply from the toner hopper 4g to the developing device 4 is stopped during the image formation. In the case of the initial developer, control is performed to improve the fluidity of the developer by lowering the toner concentration in the developer from 8 wt% to 6 wt% which is the target value of toner concentration. Further, the bias applied to the toner charge amount control means 7 is applied so that the absolute value increases from the normal -750 V to -980 V at the timing until the toner supply is stopped and the toner density returns to the original amount. Implement control. By performing these controls, it is possible to prevent image fogging and charging roller 2 contamination caused by the fact that the replenishment toner generated after being left for a long time is not sufficiently stirred and mixed in the developing device 4. .

(B) Durability Control FIG. 7 shows a relationship diagram regarding the change in toner fluidity before and after being left for each toner concentration of the developer after 40k images have been formed. Here, the toner density of the developer after 40 k is used is controlled to be 6 wt% in normal time. This is because, as described above, the surface of the magnetic carrier deteriorates with the number of sheets used, so that the ability to pull up the toner replenished from the toner hopper 4g to a desired charge amount, so-called charge imparting ability, is reduced. .

  Therefore, when the developer concentration is 6 wt%, the change before and after being left in an environment where the ambient temperature is 23 ° C. and the relative humidity is 50% is 40 ° before being left, but 48 hours after being left for 24 hours. It can be seen that it has risen to °. Therefore, in this embodiment, as in the case of the initial developer, the toner concentration target value in the toner replenishment stop control after resuming image formation after being left is set to 4 wt%.

  FIG. 8 shows the relationship between the number of developers used and the change in fluidity before and after being left in an environment where the ambient temperature is 23 ° C. and the relative humidity is 50%. Thus, it can be seen that the developer is not only less charged, but also changes the fluidity of the developer according to the number of sheets used. That is, the toner density target value after the toner replenishment stop control needs to be changed according to the number of developers used.

  FIG. 9 shows the relationship between the toner density at the normal time and the toner density target value at the toner replenishment stop control with respect to the number of developers used in this embodiment. The toner density at normal time is 8 wt% at the initial stage, but it is set to 6 wt% after 40 k sheets are used so that it becomes a linear function with respect to the number of used sheets. At the same time, the toner density target value at the time of toner replenishment stop control is set to 4 wt% after 40 k sheets with respect to the initial 6 wt%, and similarly to a linear function with respect to the number of used sheets. However, the toner density target value is fixed at 4 wt% after 40k sheets. This is because, when the toner concentration is less than 4 wt%, the resistivity of the developer is lowered, and the magnetic carrier may be developed together with the toner from the developing device 4 to the photosensitive drum 1 (so-called carrier adhesion). It is.

  In this way, by counting the number of developers used and changing the toner density target value at the time of toner replenishment stop control according to the number of developers used, it occurs after being left for a long time regardless of the number of developers used. It is possible to prevent image fogging and charging roller 2 contamination caused by the replenishment toner not being sufficiently stirred and mixed in the developing device 4.

(C) Environmental control FIG. 10 shows that the initial developer is left for each toner concentration in three environments of ambient temperature 23 ° C./relative humidity 50%, ambient temperature 30 ° C./relative humidity 80%, and ambient temperature 23 ° C./relative humidity 5%. The relationship figure about a toner fluidity | liquidity change after is shown. The developer has the worst fluidity after being left in an environment where the absolute moisture content is low at an ambient temperature of 23 ° C / relative humidity of 5%, and after standing in an environment where the absolute moisture content is high at an ambient temperature of 30 ° C / 80% relative humidity. It can be seen that the developer has the best fluidity. That is, there is a correlation between the absolute moisture content in the atmospheric environment and the flowability of the developer after standing.

  Therefore, in this embodiment, for each toner environment target value (absolute water amount) as shown in FIG. 11, the toner concentration target value at the time of toner supply stop control at the ambient temperature of 23 ° C./relative humidity of 50% in FIG. Multiplying environmental factors to obtain toner target values for each environment. For example, when the ambient temperature of the initial developer is 23 ° C./relative humidity 50% and the toner density target value is 6 wt%, the ambient temperature 30 ° C./relative humidity 80% is 7.8 wt%, and the ambient temperature 23 ° C./relative humidity 5 % Was 4.8 wt%.

  Further, the amount of current flowing from the toner charge amount control means 7 into the photosensitive drum 1 varies depending on the environment. In other words, the resistance value of the brush forming the auxiliary charging member decreases and the current amount increases in a high humidity environment, whereas the resistance value of the brush increases and the current amount decreases in a low humidity environment. Therefore, in this embodiment, as shown in FIG. 13, the normal application bias applied to the toner charge amount control means 7 and the application bias during the toner replenishment stop control are changed depending on the atmospheric environment (absolute water amount). Yes. Specifically, as described above, the ambient temperature is 23 ° C./relative humidity of 50% and the normal time is −750 V / control time of −980 V, whereas the atmospheric temperature of 30 ° C./relative humidity of 80% is normally −600 V / control. When the ambient temperature is -700V and the ambient temperature is 23 ° C / relative humidity is 5%, the normal time is -800V and the control time is -1100V.

  In this way, the atmospheric environment (absolute water amount) in which the image forming apparatus is used is detected, and applied to the toner concentration target value at the time of toner replenishment stop control and the toner charge amount control means 7 serving as a charging auxiliary member according to the detection result. By changing the bias, regardless of the usage environment of the developer, the replenishment toner generated after being left for a long time is not sufficiently stirred and mixed in the developing device 4 to cause image fogging and charging roller 2 contamination. It becomes possible to prevent.

Schematic configuration model diagram of a conventional image forming apparatus Layered model diagram of photosensitive drum and charging roller Return control timing chart after long-term leave of image forming apparatus Relationship between toner concentration and fluidity (rest angle) before and after unused developer is left Relationship diagram between toner density, fog level and charging roller dirt level before and after unused developer is left Relationship between standing time and fluidity (rest angle) of unused developer with toner concentration 8wt% Relationship diagram between toner density and fluidity (rest angle) before and after leaving developer after using 40k sheets Relationship diagram of fluidity (rest angle) before and after leaving for each usage number of developer with toner concentration 8wt% Relationship diagram between toner density target value during normal operation for each number of used sheets and toner density threshold value used for return control after leaving the image forming apparatus for a long time Relationship diagram between toner density and fluidity (rest angle) before and after leaving unused developer for each usage environment Relationship diagram of toner density threshold coefficient used in return control for each use environment of image forming apparatus Arrangement schematic diagram of auxiliary charging brush (toner charge amount control means) Relationship diagram between normal application bias and return control application bias for each use environment of the image forming apparatus

Explanation of symbols

1 Photosensitive drum (image carrier)
2 Charging roller 3 Laser beam scanner 4 Developing device 5 Transfer roller 6 Fixing device 7 Toner charge amount control means (charging auxiliary member)
8 Transfer residual toner uniformizing means (charging auxiliary member)
10 Arithmetic Circuit with Memory 11 Temperature / Humidity Sensor S1-S5 Bias Voltage Application Power Supply

Claims (6)

  An image carrier, a charging unit for charging the surface of the image carrier at a contact portion to form an electrostatic latent image on the image carrier, and the image carrier charged by the charging unit. An exposure unit that forms an electrostatic latent image; a two-component developing unit that supplies the toner to an electrostatic latent image formed on the image carrier using a developer composed of toner and a magnetic carrier; A toner supply means for supplying toner to the two-component developing means, a transfer means for transferring a visualized toner image to a transfer material, and a DC voltage having the same polarity as that of the charging means is applied to the image carrier after transfer. An image forming apparatus using a cleaner-less system that collects toner remaining on the surface of the image carrier without being transferred by the transfer unit by the two-component developing unit. The device is When the image formation is performed again after being left unoperated for more than a period of time, the two-component replenishing means from the toner replenishing means until the toner concentration with respect to the developer in the two-component developing means falls below a predetermined threshold value. The toner supply to the developing means is not performed, and the absolute value of the DC voltage applied to the auxiliary charging member is greater than the timing value other than the above at the same time as or more than the time when the toner supply is not performed. A featured image forming apparatus.   2. The image forming apparatus according to claim 1, wherein the auxiliary charging member is disposed adjacent to the charging unit on the upstream side in the rotation direction of the image carrier.   3. The image forming apparatus according to claim 1, wherein a threshold value of toner density with respect to the developer of the two-component developing unit is variably controlled according to operation information of the image forming apparatus.   The absolute value of the DC voltage applied to the auxiliary charging member is variably controlled according to operation information of the image forming apparatus at the same time or longer than the time when toner replenishment is not performed. 2. The image forming apparatus according to 2.   The image forming apparatus according to claim 1, wherein the auxiliary charging member is a brush-like member.   The image forming apparatus according to claim 1, wherein the charging unit is a charging roller.

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