JP2007322634A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To eliminate an abnormal image such as fogging or electrification irregularity caused by the excessive accumulation of toner whose polarity is not normal or toner which is insufficiently electrified on a developing means and a contact electrifying means in an image forming apparatus equipped with: a body to be electrified; an electrifying means for electrifying the body to be electrified; an exposure means for forming an electrostatic latent image on the electrified surface of the body to be electrified; a developing means for developing the electrostatic latent image to be a developer image by sticking developer to the electrostatic latent image; and a transfer means for transferring the developer image to a recording medium, wherein the developing means functions also as a cleaning means for removing the developer left after transfer remaining on the surface of the body to be electrified without moving to the recording medium by the transfer means. <P>SOLUTION: The electrifying means is the contact electrifying means for electrifying the body to be electrified by bringing an electrifying member to which voltage is applied into contact with the body to be electrified. Electrifying bias applied to the electrifying means is bias obtained by superposing alternating voltage on DC voltage when forming an image, and it is switched to DC bias by turning off the alternating voltage and also the electrified surface is exposed by the exposure means when starting operation to discharge the developer from the developing means. <P>COPYRIGHT: (C)2008,JPO&INPIT

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 discarded by reuse. The present invention relates to a cleanerless image forming apparatus.

  2. Description of the Related Art Conventionally, transfer-type image forming apparatuses such as copying machines, printers, and facsimiles using an electrophotographic system are photosensitive drums, which are general image bearing members of a rotating drum type, and the photosensitive drums are made to have a predetermined polarity and potential. In this way, a charging device (charging process) for charging is provided. Further, an exposure apparatus (exposure process) as information writing means for forming an electrostatic latent image on a charged photosensitive member, and the electrostatic latent image formed on the photosensitive member is visualized with toner as a developer. It has a developing device (developing process). In addition, a transfer device (transfer process) that transfers the toner image from the photoreceptor surface to a transfer material such as paper, and a cleaning device (cleaning device) that cleans the photoreceptor surface by removing some residual toner on the photoreceptor after the transfer process. Step). Further, the image forming apparatus includes a fixing device (fixing process) for fixing the toner image on the transfer material. The photoreceptor is repeatedly subjected to an electrophotographic process (charging, exposure, development, transfer, cleaning) to be 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 accumulates in the cleaning device as waste toner. However, such waste toner is generated from the viewpoint of environmental conservation and effective use of resources. Desirably not.

  Therefore, as in Patent Document 1, the cleaning device is eliminated, 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. There are less-type image forming apparatuses.

In the simultaneous development cleaning, the transfer residual toner on the photosensitive member after transfer is transferred to the developing device by the fog removal bias to remove the transfer residual toner present on the surface of the photosensitive member that should not be developed with toner during the subsequent development process. It is a method to collect. The subsequent development process is a process in which the photosensitive member is continuously charged and exposed to form an electrostatic latent image and the electrostatic latent image is developed. The fog removing bias is a potential difference V _back between the DC voltage applied to the developing device and the surface potential of the photoreceptor.

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

  In a cleanerless type image forming apparatus as described above, when toner in which the charging polarity is reversed to the polarity opposite to the normal polarity (hereinafter referred to as reversal toner) is mixed in the developing device, development is performed on the photosensitive member. Those reversal toners are carried around on the photoreceptor without being transferred. Therefore, when the charging device of the photosensitive member is a contact charging device that contacts the photosensitive member and charges the surface of the photosensitive member, the reverse toner on the photosensitive member passes through the charging portion that is the contact nip portion of the photosensitive member and the contact charging device. In doing so, it adheres to the contact charging device. This causes the contact charging device to be contaminated with toner more than permissible, causing a charging failure. Further, when the toner is not adhered to the contact charging device but remains on the surface of the photosensitive member that should not be developed and is transferred by a mechanical force such as pressure or rubbing with the transfer device, so-called Causes image defects called fogging.

  In other words, the toner as the developer contains a small amount of toner, but a toner whose charging polarity is originally reversed to a polarity opposite to the normal polarity. Even if the toner has a regular charge polarity, the charge polarity is affected by the effect of the oscillating electric field between the contact charging device and the photosensitive member by superimposing an alternating voltage on the contact charging device, the transfer bias, the peeling discharge, etc. Some are reversed, and others are neutralized to reduce the charge amount.

  When such non-normal polarity toner accumulates in the developing device or in the contact charging device, it becomes a factor causing image defects and needs to be removed.

  Here, if the oscillating electric field effect between the contact charging device and the photoconductor is eliminated by superimposing the alternating voltage on the contact charging device, that is, if only the DC bias is used without superimposing the alternating voltage, the charging polarity is reversed. It is possible to eliminate those that are neutralized and have a reduced charge amount. In addition, there is an effect that the non-normal polarity toner traveling around the photoconductor generated due to other factors is made normal polarity at the discharge nip between the contact charging device and the photoconductor.

  However, the DC bias alone is not preferable as the bias applied to the contact charging device at the time of image formation because the charging uniformity of the photosensitive member is inferior to the case where the alternating voltage is superimposed. In addition, even during non-image formation, for example, when the charge uniformity of the photoconductor is required to be comparable to that during image formation, such as when detecting the potential of the photoconductor surface or detecting the amount of developed toner. It is difficult to use a DC bias. Therefore, it is not preferable to use a DC bias for all non-image formation in order to maintain good image formation.

  In addition, as means for normalizing toner that does not have normal polarity around the photoconductor, there is a method using toner charge amount control means as in Patent Documents 2 and 3. This is because transfer residual toner that is carried from the transfer part to the charging part and has a normal polarity, a reverse polarity, and a low charge amount is mixed to the normal polarity by the toner charge amount control means. The charging polarity is made uniform with the normal polarity. In addition, this is a method of reducing toner that is not of a normal polarity by making the charge amount uniform along with this polarity alignment.

  However, the above means are not perfect. Therefore, even when the toner charge amount control means is used, when a printing operation is performed for a long time, the reversal toner that has not been made normal polarity by the charge amount control means is mixed into the developing device. It also adheres to the contact charging device. This causes a defective image such as fogging or charging failure.

Therefore, as in Japanese Patent Application Laid-Open No. H10-260260, there is a configuration in which the voltage applied to the charging roller is not applied with an AC component but is applied with a DC component, so that the toner is discharged from the charging roller and the reverse toner is made more normal polarity. is there.
Japanese Patent Application Laid-Open No. 07-295388 JP 2001-215798 A JP 2001-215799 A JP 2004-126102 A

  However, even though the reversal toner can be made into a normal polarity, there is a deteriorated toner in which the charge amount is insufficient in the developing device, and this toner may cause an image defect. Therefore, it is necessary to sufficiently charge such toner.

  According to the present invention, in the cleanerless image forming apparatus as described above, an abnormal image such as fogging and charging unevenness caused by excessive accumulation of toner having a non-normal polarity with respect to the developing unit and the contact charging unit or toner having insufficient charge amount. Eliminate the occurrence of Accordingly, an object is to maintain good image formation over a long period of time.

  In order to achieve the above object, a typical configuration of an image forming apparatus according to the present invention includes an object to be charged, a charging unit for charging the object to be charged, and an electrostatic latent image on the charging surface of the object to be charged. An exposure means for forming the developer, a developing means for developing a developer image by attaching a developer to the electrostatic latent image, and a transfer means for transferring the developer image to a recording medium. In the image forming apparatus in which the developing unit also serves as a cleaning unit that removes a transfer residual developer remaining on the surface of the member to be charged without moving to the medium, the charging unit includes a charging member to which a voltage is applied. Contact charging means for charging the object to be charged by bringing it into contact with a body. The charging bias applied to the charging means superimposes an alternating voltage on a DC voltage during image formation, and discharges the developer from the developing means. When starting operation, turn off the alternating voltage. The exposure means with switching the flow bias and performs exposure of the charged surface.

  According to the present invention, the polarity of the reversal developer mixed in the discharged developer and the reversal developer attached to the contact charging unit is normalized while discharging the developer from the developing unit. The developing means and the contact charging means are purified. This eliminates the occurrence of abnormal images such as fogging and uneven charging due to excessive accumulation of non-normal polarity toner and insufficiently charged toner with respect to the developing means and contact charging means, and good image formation over a long period of time. Is maintained.

  This will be described more specifically. The developer discharging modes from the developing means are generally the following cases 1) and 2).

1) In order to prevent developer deterioration in the developing means when the frequency of replacement of the developer in the developing means decreases, such as when continuously passing an image with a low image ratio, a small amount mixed with the normal polarity developer 2) In the case where only the reversal developer is discharged by increasing the absolute value of the fog removal bias described above. However, because the reversal developer is mixed in the discharged developer, These reversal developers are not transferred and are carried around on the member to be charged, or are transferred and discarded by making the applied bias polarity of the transfer means reverse.

  In order to make these reversal developers have normal polarity, the superposition of the alternating voltage on the contact charging means at the timing of discharging the developer from the developing means during non-image formation is stopped and the exposed surface is exposed by the exposure means. Do. That is, when the developer discharging operation is started from the developing unit, the alternating voltage is turned off to switch to the DC bias, and the charged surface is exposed by the exposing unit. Thus, while discharging the developer from the developing unit, the reversal developer mixed in the discharged developer and the reverse developer adhering to the contact charging unit are normalized, and the developing unit and the contact are made. The charging means is purified.

  In this case, it is preferable that the charging bias applied to the contact charging unit during the discharging operation of the developer from the developing unit satisfies the following condition a.

a. | V _DC -Vs |> Vth
V _DC : DC bias on the charging polarity direction side applied to the charging means Vs: On the charging polarity direction side of the object to be charged before being charged by the charging means
Charging potential Vth: minimum potential difference at which discharge starts between the charging unit and the member to be charged. Then, the reversal developer discharged from the developing unit and remaining on the member to be charged and the reversal developer attached to the contact charging unit Can be made normal polarity at the discharge nip between the contact charging means and the member to be charged. Then, only the developer with the normal polarity is positively discharged from the contact charging means to the charged object side by the electric field acting between the contact charging means and the charged object, and the developing means next to the discharged developer. Active collection was possible.

  This prevents the effect that the alternating voltage on the contact charging means causes an oscillating electric field effect between the contact charging means and the member to be charged, and that the transfer residual developer having the normal polarity is turned into a reversal developer. . Further, the reversal developer attached to the contact charging means due to the oscillating electric field effect is not converted into a normal polarity, and the effect of preventing various defective images generated due to discharge on the charged body is also obtained.

  In addition, when discharging the developer, it is not necessary to discharge the developer evenly. For example, the potential of the surface of the charged body is compared with the detection of the potential of the surface of the charged body or the detection of the amount of the developed developer. Does not require much uniformity. For this reason, discharging the developer while charging the object to be charged using a DC bias is suitable for preventing excessive accumulation of developer that is not of a normal polarity in the developing unit or the contact charging unit.

  Further, by using this means, the reverse developer can be made to have a normal polarity even in a contact charging means such as a charging roller that cannot be expected to have a normal polarity of the reverse developer due to the effect of the developer rubbing. Thereafter, it can be sent out from the contact charging means onto the member to be charged.

  Therefore, by discharging the developer in the developing unit during non-image formation, the reversal developer mixed in the discharged developer is made to have a normal polarity so that the developing unit and the contact charging unit can be used even after long-term image formation. Is prevented from being excessively contaminated by the reversal developer. Therefore, a good image can be stably obtained over a long period of time.

  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 of this example is a laser beam printer using a transfer type electrophotographic process, a contact charging method, a counter-expression image method, cleanerless, and a maximum sheet passing size of A3 size.

(1) Overall Schematic Configuration of Printer a) Image carrier 1 is a rotary drum type electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum) as an image carrier (charged member). This photosensitive drum 1 is a negatively chargeable organic photoconductor (OPC), has an outer diameter of 50 mm, and is driven to rotate counterclockwise as indicated by an arrow with a process speed (peripheral speed) of 100 mm / sec centered on the central support shaft. Is done.

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

b) Charging means The charging means is a contact charging device having a contact charging member 2 that contacts the surface of the photosensitive drum 1 as an image carrier and uniformly charges the peripheral surface of the photosensitive drum 1. In this example, the contact charging member 2 is a charging roller (roller charger).

  The charging roller 2 is configured such that both ends of the cored bar 2a are rotatably held by bearing members (not shown) and urged in the direction of the photosensitive drum by a pressing spring 2e to be predetermined with respect to the surface of the photosensitive drum 1. The contact is made with a pressing force of. Then, the charging roller 2 and the photosensitive drum 1 are driven to rotate. A pressure contact portion between the photosensitive drum 1 and the charging roller 2 is a charging portion (charging nip portion) a.

  By applying an oscillating voltage obtained by superimposing a DC voltage and an alternating voltage from the power supply device S1 to the metal core 2a of the charging roller 2, the surface of the photosensitive drum 1 to be rotationally driven is made uniform with a predetermined polarity and potential. Charge it. Here, the alternating voltage means all voltages whose amplitude changes with time, such as a sine wave, a rectangular wave, and a triangular wave.

  The longitudinal length of the charging roller 2 is 320 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. The 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 6 mm in diameter b. Lower layer 2b: Carbon dispersion foamed EPDM, specific gravity 0.5 g / cm ³ ,
Volume resistance value 10 ² to 10 ⁹ Ωcm, layer thickness 3.0 mm,
320mm length
c. Intermediate layer 2c; carbon-dispersed NBR rubber, volume resistivity of 10 ² to 10 ⁵ Ω
cm, layer thickness 700 μm
d. Surface layer 2d: tin oxide and carbon dispersed in a fluororesin resin
Volume resistance value 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.

  In this embodiment, the bias applied to the charging roller at the time of image formation is DC voltage −600 V + AC voltage 2 KV (peak-to-peak voltage).

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. The scanner 3 outputs a laser beam modulated in accordance with an image signal sent from an external host device such as an image reading device / computer (not shown) to the printer side to uniformly charge the surface of the rotating photosensitive drum 1. Is subjected to laser scanning exposure L (image exposure) at the exposure position b. 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. The bright portion potential after exposure is -200V.

d) Developing means 4 is a developing device (developer) as a 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 brush developing type reversal developing device.

  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 its outer peripheral surface exposed to the outside. 4c is a non-rotating fixed magnet roller inserted in 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 developer stirring member provided. Reference numeral 4g denotes a toner hopper, which contains replenishing toner for the developing container 4a.

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 of the magnetic carrier is about 10 ¹³ Ωcm, 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 opposite to the photosensitive drum 1 so that the closest distance (referred to as S-Dgap) to the photosensitive drum 1 is maintained at 350 μm. A facing portion between the photosensitive drum 1 and the developing sleeve 4a is a developing portion 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 on the outer peripheral surface of the developing sleeve 4b as a magnetic brush layer by the magnetic force of the magnet roller 4c in the sleeve. The magnetic brush layer is rotated and conveyed along with the rotation of the sleeve 4a, is layered to 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 unit c to appropriately adjust the surface of the photosensitive drum. Rub on. 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,
DC voltage: -350V
AC voltage: 1600V
Is an oscillating voltage 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.

  At this time, the charge amount of the toner developed on the photosensitive drum is −25 μC / g.

  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). Detected. Then, the toner hopper 4g is driven and controlled according to the detection 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.

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 between the rotating photosensitive drum 1 and the transfer roller 5 and conveyed. During the conveyance, a positive transfer bias having a polarity opposite to the negative polarity which is the normal charging polarity of the toner, +2 kV in this example, is applied to the transfer roller 5 from the power source S3. As a result, 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.

  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 rotating photosensitive drum 1 and conveyed to the fixing device 6 (for example, a heat roller fixing device), where the toner image is fixed. Output as an image formed product (print, copy).

  Reference numeral 100 denotes a control unit of the image forming apparatus, which controls image forming sequence control of the entire image forming apparatus.

(2) Cleanerless system The printer of this embodiment 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 transfer of the toner image to 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 exposing unit b as the photosensitive drum 1 continues to rotate, and simultaneously developed (collected) by the developing device 4. (Cleanerless system).

  In this embodiment, as described above, the developing sleeve 4b of the developing device 4 is rotated in the developing portion c in the direction opposite to the advancing direction of the surface of the photosensitive drum 1, which is the transfer residual toner on the photosensitive drum 1. 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. Adheres to the charging roller 2 when passing through the charging portion a. As a result, the charging roller 2 contaminates the 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 4, the charging polarity of the transfer residual toner on the photosensitive drum 1 carried to the developing unit c is normal polarity. It is necessary to be. The reversal toner and the toner with a small charge amount cannot be removed and collected from the photosensitive drum 1 to the developing device 4 and are rotated along the photosensitive drum 1. Alternatively, even if the image is collected by the developing device 4 by mechanical force, if it is discharged again onto the photosensitive drum 1 during the image formation that is repeatedly formed, the cause of the defective image is also carried along the photosensitive drum 1 in the same manner. End up.

  Therefore, the transfer residual toner (residual) for making the transfer residual toner on the photosensitive drum 1 uniform at a position downstream of the transfer portion d in the rotation direction of the photosensitive drum and upstream of the charging portion a in the rotation direction of the photosensitive drum. Developer image) uniformizing means 8 is provided. Further, at the position downstream of the transfer residual toner equalizing means 8 in the photosensitive drum rotation direction and at the upstream side of the charging portion a in the photosensitive drum rotation direction, the charge polarity of the transfer residual toner is made to be the negative polarity which is the normal polarity. Toner (developer) charge amount control means 7 is provided.

  Generally, the untransferred 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. Therefore, such transfer residual toner is once discharged by the transfer residual toner (residual developer image) uniformizing means 8. Then, the transfer residual toner that has been neutralized is charged again to the normal polarity by the toner charge amount control means 7. Accordingly, it is possible to effectively prevent the transfer residual toner from adhering to the charging roller 2 and to assist the removal and collection by the developing device 4. Therefore, the generation of a ghost image of the transfer residual toner image pattern is strictly prevented.

  In this embodiment, the transfer residual toner uniformizing means 8 and the toner charge amount control means 7 are brush-like members having appropriate conductivity, and are arranged in contact with the photosensitive drum 1 surface. is there. Reference numeral f denotes a contact portion between the transfer residual toner uniformizing means 8 and the surface of the photosensitive drum 1. Reference numeral e denotes a contact portion between the toner charge amount control means 7 and the photosensitive drum 1 surface.

  A positive DC voltage is applied from the power source S5 to the transfer residual toner equalizing means 8, and a negative DC voltage is applied from the power source S4 to the toner charge amount control means 7. Specifically, a DC voltage of +400 V is applied to the transfer residual toner uniformizing means 8 and a voltage of −800 V is applied to the toner charge amount control means 7.

  In the transfer portion d, the transfer residual toner remaining on the photosensitive drum 1 after the transfer of the toner image onto 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. The charge amount is made uniform in the vicinity of 0 μC / g by means 8. 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 is transferred by the toner charge amount control means 7. The remaining toner is arranged to have a negative polarity in which the charging polarity is a normal polarity.

  By aligning the charging polarity of the transfer residual toner to the negative polarity that is the normal polarity, the mirroring force of the transfer residual toner on the photosensitive drum 1 is increased. As a result, at the contact portion a between the charging roller 2 and the photosensitive drum 1, when the surface of the photosensitive drum 1 is charged from above the transfer residual toner, adhesion of the transfer residual toner to the charging roller 2 is reduced.

  The toner of normal polarity that has passed through the charging portion a is collected by the developing device 4 in the developing process, and the transfer residual toner on the photosensitive drum 1 where the toner should not be developed.

  In this way, 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 negative polarity which is a normal polarity, and the transfer residual amount is obtained. The adhesion of toner to the charging roller 2 is reduced. Thereby, the transfer residual toner is efficiently collected in the developing device.

  However, the transfer residual toner equalizing means 8 and the toner charge amount control means 7 are not perfect. Even if these means are used, if a large amount of untransferred toner is generated at once, such as for long-term printing operations or continuous printing of images with a high image ratio, the reversal toner that has not been normalized by the charge amount control means Adheres to the charging roller 2.

  The adhesion of the reversal toner to the charging roller 2 is further increased by applying an alternating voltage to the charging roller 2 due to the oscillating electric field effect between the charging roller 2 and the photosensitive drum 1, and compared with the adhesion of toner of normal polarity. It is remarkable.

  Further, the oscillating electric field effect between the charging roller 2 and the photosensitive drum 1 also has an effect of converting the toner of normal polarity into a reversal toner. As a result, most of the transfer residual toner adhering to the charging roller 2 is a reversal toner. Tend to be.

  Of the transfer residual toner adhering to the charging roller 2, those having normal polarity are discharged onto the photosensitive drum 1 by an electric field acting between the DC component of the bias applied to the charging roller 2 and the charging potential of the photosensitive drum 1. Conversely, the reversal toner remains on the surface of the charging roller 2 without being discharged by the electric field.

  Here, in general, contact charging using the charging roller 2 is difficult to normalize the reversal toner only by rubbing with the transfer residual toner as compared with other contact charging means (for example, magnetic brush charging). is there. For this reason, the reversal toner adhering to the charging roller 2 is easily discharged without being discharged. Since the toner has a relatively high electric resistance, the continuous adhesion of such toner is a factor that increases the electric resistance of the charging roller 2 and decreases the charging ability.

  Therefore, as a countermeasure, when the amount of toner adhering to the charging roller 2 is relatively large, it is necessary to maintain good charge by making the toners have normal polarity and discharging them during non-image formation.

(3) Image ratio and toner adhesion amount (FIG. 3)
Using the image forming apparatus described above, each of the images having different image ratios is continuously formed, and the toner density on the charging roller 2 (toner contamination of the charging roller) is measured every 200 sheets by a reflection densitometer manufactured by X-Rite. It measured using. In the above, it is a horizontal band image in the main scanning direction having a ratio of 100%, 50%, and 30% to the A4 size.

  This is because the density of toner adhering to the surface of the charging roller 2 serves as an index of the amount of reverse toner remaining in the developing device 4 and on the photosensitive drum 1. The toner density, which is a measure of toner contamination on the charging roller, is measured using a reflection densitometer manufactured by X-Rite. FIG. 3 shows the measurement results. This measurement was performed in a low humidity environment at a temperature of 23 ° C. and a humidity of 5%.

  As is apparent from this measurement result, the higher the image ratio, the faster the amount of adhered toner increases and the larger the saturated value. In the charging roller 2 used in this embodiment, when the toner density on the charging roller 2 exceeds 0.25, good charging cannot be performed. Therefore, the image formation is interrupted before good charging cannot be performed, the application of the AC component of the charging bias applied to the charging roller 2 is stopped, and the reversal toner adhering to the inside of the developing device 4 and the surface of the charging roller 2 is discharged. I do. Thus, the amount of reversal toner in the developing device 4 and the surface of the charging roller 2 must be reduced.

  For an image with an image ratio of 30%, the toner density on the charging roller 2 was suppressed to about 0.25 or less, and a good image free from fogging could be obtained.

However, in the images with the image ratios of 50% and 100%, the toner density on the charging roller 2 is increased, and the image formed is fogged. This increases the electrical resistance of the charging roller 2 due to a large amount of toner adhering, thereby reducing the charging ability. As a result, the photosensitive drum 1 is sent to the developing device 4 while leaving a portion with a low surface potential (charging potential), so that it is easily developed in a portion with a low surface potential and appears as fog in the image. It ends up. From FIG. 3, when calculating the number of image formations that the toner density is considered to exceed 0.25 in the document of each image ratio, 50%: 710 sheets,
100%: 400 sheets
It has become.

  From this result, it is understood that the toner density exceeds 0.25 when the image ratio × number of sheets (integrated image ratio)> 35000%.

  Therefore, the control unit 100 of the image forming apparatus accumulates the image ratio calculated from, for example, the laser emission time of the exposure apparatus 3 during continuous image formation, and before the value exceeds 35000%, The toner discharge mode from the charging roller 2 is executed. That is, the discharge mode in which the application of the alternating voltage of the charging bias applied to the charging roller 2 is stopped and the reverse toner is discharged from the developing device 4 and the charging roller 2 is performed. As a result, it is possible to reduce the amount of reversal toner from the developing device 4 and the charging roller 2 and continue to perform good charging.

  The toner is discharged from the developing device 4 effectively by applying a developing bias to the developing device and reducing the surface potential of the photosensitive drum 1 by performing exposure with the exposure device 3.

  After the toner discharge mode is performed and the toner density reaches about 0.1, the integrated value is reset, and the discharge is performed every time the integrated image ratio exceeds 25000%, thereby reducing the toner density to 0.1%. It can be kept below 25.

  Further, it can be determined that the smaller the toner concentration attached to the charging roller 2 is, the smaller the amount of the reversal toner remaining in the developing device 4 and on the photosensitive drum 1 is, and this is necessary for discharging the toner attached to the charging roller 2. Time is also shortened. Therefore, the application of the alternating voltage of the charging bias applied to the charging roller 2 at the time of non-image formation such as during the pre-rotation process, the inter-sheet process, and the post-rotation process of the image forming apparatus is stopped, and the developing apparatus 4 and the charging roller 2 is controlled to execute the toner discharge mode from 2. Alternatively, when the integrated image ratio exceeds a value smaller than 35000% (for example, 20000%), the application of the alternating voltage of the charging bias applied to the charging roller 2 is stopped, and the developing device 4 and the charging roller 2 Control is performed to execute the toner discharge mode. By controlling in this way, the time per discharge toner discharge operation from the developing device 4 and the charging roller 2 is shortened, and as a result, the total time of continuous image formation including the discharge operation can be shortened. It becomes.

  Therefore, in this example, when the toner is discharged from the developing device 4 during non-image formation, particularly during the pre-rotation process, an alternating voltage is not superimposed on the charging bias applied to the charging roller 2, and the following condition a is satisfied. Only a DC bias was applied.

a. | V _DC -Vs |> Vth
V _DC : DC bias on the charging polarity direction side applied to the charging roller 2 Vs: Charging polarity direction side of the photosensitive drum 1 before being charged by the charging roller 2
Charging potential Vth: minimum potential difference at which discharge begins to occur between the charging roller 2 and the photosensitive drum 1 More specifically, V _DC = −1600 V, and the exposure dark portion of the photosensitive drum 1 is approximately −870 V. The DC voltage (Vdc) of the developing bias at that time was −670V.

  The control of the toner discharge from the charging roller 2 during non-image formation is performed by the control unit 100 according to a predetermined sequence.

  Thus, as described above, the reversal toner mixed in the developing device 4 and the reversal toner adhering to the charging roller 2 are normalized in the discharge nip between the charging roller 2 and the photosensitive drum 1. Then, only the normal polarity toner is positively discharged from the charging roller 2 side to the photosensitive drum 1 side by the electric field acting between the charging roller 2 and the photosensitive drum 1. This is because reversal toner attached to the charging roller 2 is discharged without being normal polarity due to an oscillating electric field effect between the charging roller 2 and the photosensitive drum 1 by superimposing an alternating voltage on the charging roller 2. The effect of preventing can be obtained at the same time.

In addition, the larger the above | V _DC −Vs |, the greater the effect of normalizing the reversal toner. Further, as the potential difference between the exposure dark portion of the photosensitive drum 1 and the DC voltage (Vdc) of the developing bias is larger, the reverse toner mixed in the developing device 4 is more easily discharged onto the photosensitive drum 1.

In the present embodiment, the above-mentioned | V _DC -Vs | is increased by setting the above-mentioned V _DC = -1600 V and charging the exposure dark portion of the photosensitive drum 1 to be about -870 V. In addition, by setting the development bias DC voltage (Vdc) to −670 V, carrier adhesion to the photosensitive drum 1 occurs when the potential difference between the exposure dark portion of the photosensitive drum 1 and the development bias DC voltage (Vdc) is too large. The potential difference is increased within the range that does not.

  In other words, at the same time as expelling the reversal toner in the dark exposure area, at the light exposure area, the toner discharge is performed to prevent toner deterioration. It was set appropriately for efficient normal polarity.

  In the present embodiment, the potential of the exposure bright portion is set to −200V. By setting such a potential relationship, the reversal toner can be discharged and the contrast can be increased to sufficiently discharge a toner having a normal polarity including a toner having an insufficient charge amount.

  In this embodiment, the contrast for the exposure dark portion and the exposure light portion with respect to the developing bias in the discharge mode is set larger than that during image formation.

  Here, the voltage is applied at the timing as shown in FIG. 4, and the application time of only the DC bias satisfying the above condition is applied to the charging roller 2 when the toner is discharged from the developing device 4 at the time of non-image formation, particularly in the pre-rotation process. Was established. Then, using the same image forming apparatus as described above, each of the images having different image ratios is continuously formed, and the density of the adhered toner on the charging roller 2 is measured every 200 sheets by a reflection densitometer manufactured by X-Rite. It was performed using. FIG. 5 shows the measurement results. This measurement was also performed in a low humidity environment at a temperature of 23 ° C. and a humidity of 5%. The images having different image ratios are horizontal band images in the main scanning direction that have ratios of 100%, 50%, and 30% with respect to the A4 size.

  As is apparent from the measurement results, even when the image ratio is high, the toner density on the charging roller 2 does not exceed 0.25, and good charging can be performed, and a good image without fogging is obtained. I was able to.

  Incidentally, by providing a non-image forming time in which the exposure is turned off after the positive discharge in FIG. 4, the toner discharged onto the photosensitive drum by this positive discharge can be collected by the developing means. For this reason, it is preferable to set the developing voltage under the same conditions as in the image formation during the non-image formation.

  In this example, when toner is discharged from the developing device 4 during non-image formation, the reversal toner remaining in the developing device 4 and on the charging roller 2 is efficiently converted to normal polarity by the charging roller 2 to which the DC bias is applied as described above. And can be discharged to the photosensitive drum 1.

  Also, by ejecting the reversal toner and the normal polarity toner from the developing device, not only the reversal toner but also the normal polarity toner having a small charge amount can be made the normal polarity, and the shortage of the charge amount can be solved. it can.

  As a result, even when an image with a high image ratio or an image with a high image ratio is formed, the toner density on the charging roller 2 can be suppressed to a predetermined value (for example, 0.25) or less. Stable and good image formation can be performed.

  This embodiment will be described using the image forming apparatus shown in FIG. In this embodiment, each of the images having different image ratios is continuously formed in a high humidity environment at a temperature of 30 ° C. and a humidity of 80%, and the density of the adhered toner is measured on the charging roller 2 every 200 sheets by X-Rite. The measurement was performed using a reflection densitometer manufactured by the company. FIG. 6 shows the measurement results. The images having different image ratios are horizontal band images in the main scanning direction that have ratios of 100%, 50%, and 30% with respect to the A4 size.

  As is apparent from the measurement results, in a high-humidity environment, the toner density adhering to the charging roller 2 increases to about 0.25 or more even when the image ratio is 30%. This is because the charge amount of the toner to be developed is originally lower in the high humidity environment than in the low humidity environment, and the toner remaining on the photosensitive drum 1 after the transfer thereof is the reversal toner compared with that in the low humidity environment. This is because the ratio of the toner increases and tends to adhere to the charging roller 2.

  Then, the image formation (image formation) is performed in a lower humidity environment (temperature 23 ° C., humidity 60%) than this environment while the amount of toner adhering to the charging roller 2 is increased by the image formation (image formation) in the high humidity environment. ), Fogging occurred in the image.

  In this embodiment, even when fogging occurs in an image formed due to such a rapid change in the environment, the same toner discharging operation as that in Embodiment 2 shown in FIG. 4 is performed during non-image formation. . Thus, the reversal toner remaining in the developing device 4 and on the charging roller 2 can be efficiently converted into normal polarity by the charging roller 2 and discharged to the photosensitive drum 1. As a result, the toner density on the charging roller 2 can be suppressed to a predetermined value (for example, 0.25) or less, and good and stable image formation can be performed over a long period of time.

  FIG. 7 shows an operation process diagram of the image forming apparatus.

1) Pre-multi-rotation process This is a start (start) operation period (warming period) of the image forming apparatus. When the main power switch of the image forming apparatus is turned on, the main motor of the image forming apparatus is activated to execute a preparation operation for a required process device.

2) Standby After the predetermined start-up operation period, the main motor is stopped and the image forming apparatus is held in a standby (standby) state until a print job start signal is input.

3) Pre-rotation process This is a period in which the main motor is re-driven based on the input of the print job start signal and the pre-print job operation of the required process equipment is executed.

  More practically, a: the image forming apparatus receives a print job start signal, b: the image is developed by the formatter (the development time varies depending on the image data amount and the formatter processing speed), and c: the pre-rotation process starts. Order.

  If a print job start signal is input during the previous multi-rotation process of a :, after the previous multi-rotation process is completed, the process proceeds to the pre-rotation process without standby of the b :.

4) Execution of print job When the predetermined pre-rotation process is completed, the image forming process is subsequently executed, and an image-formed recording material is output.

  In the case of a continuous print job, the image forming process is repeated, and a predetermined number of image-formed recording materials are sequentially output.

5) Inter-sheet process In the case of a continuous print job, this is an interval process between the trailing edge of one recording material P and the leading edge of the next recording material P, and is a non-paper passing period in the transfer unit and the fixing device.

6) Post-rotation process After outputting the image-formed recording material in the case of a single print job, or after outputting the last image-formed recording material of the continuous print job in the case of a continuous print job The main motor is continuously driven for a predetermined time. This is a period during which the post-print job operation of the required process device is executed.

7) Standby After completion of the predetermined post-rotation process, the driving of the main motor is stopped, and the image forming apparatus is kept in a standby (standby) state until the next print job start signal is input.

  In the above, the execution time of the print job of 4) is the time of image formation, the time of 1) the pre-multi-rotation process, 3) the pre-rotation process, 5) the inter-sheet process, 6) the post-rotation process. It is during non-image formation.

  In the present invention, the time of non-image formation means at least one of the pre-multi-rotation process, the pre-rotation process, the inter-sheet process, and the post-rotation process, and at least within the process time. It is a predetermined time.

(Other)
1) The exposure means 3 as the information writing means is not limited to the laser beam scanner of the embodiment, and may be another digital exposure apparatus such as a combination of a light source such as an LED array or a fluorescent lamp and a liquid crystal shutter, An analog exposure apparatus that forms and projects a document image may be used.

  2) The image carrier 1 may be an electrostatic recording dielectric. In this case, the dielectric surface is uniformly charged to a predetermined polarity / potential, and is then selectively discharged by a discharging means (information writing means) such as a discharging needle array / electron gun to obtain an electrostatic latent image of image information. Write form.

  The image carrier 1 is not limited to a drum type, and may be a rotating belt (endless belt), a sheet-like member having an end attached to and held on a rotating belt-like support.

  3) The toner developing method and means for the electrostatic latent image are arbitrary. A reversal development method or a regular development method may be used.

  4) The transfer means is not limited to the roller transfer of the embodiment, but may be a blade transfer, belt transfer, other contact transfer charging method, or a non-contact transfer charging method using a corona charger.

  5) The present invention can be applied not only to the formation of a single color image by using an intermediate transfer member such as a transfer drum or a transfer belt, but also to an image forming apparatus that forms a multicolor, full color image by multiple transfer or the like.

  6) As a waveform of the alternating voltage of the bias applied to the contact charging member 2 and the developing device 4, a sine wave, a rectangular wave, a triangular wave, or the like can be used as appropriate. The AC bias includes, for example, a rectangular wave voltage formed by periodically turning on and off a DC power supply.

Schematic configuration model diagram of image forming apparatus of embodiment Layered model diagram of photosensitive drum and charging roller FIG. 10 is a diagram illustrating a relationship between the number of images formed and the toner density on the charging roller with respect to each image ratio in the first embodiment. Control timing chart at the time of toner discharging operation in Embodiment 2 FIG. 10 is a diagram showing the relationship between the number of images formed and the toner density on the charging roller for each image ratio in Example 2 (when discharging operation is added). FIG. 10 is a diagram illustrating the relationship between the number of images formed and the toner density on the charging roller for each image ratio in the third embodiment. Operation process diagram of image forming apparatus

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Photosensitive drum (to-be-charged body, image carrier), 2 ... Charging roller (contact charging means), 3 ... Laser beam scanner, 4 ... Developing apparatus (developing means), 5. Transfer roller (transfer means), 6... Fixing device (fixing means), S1 to S5... Bias voltage application power source

Claims (2)

To-be-charged body, charging means for charging the to-be-charged body, exposure means for forming an electrostatic latent image on the charging surface of the to-be-charged body, and developer image by attaching a developer to the electrostatic latent image Development means for developing the toner image and a transfer means for transferring the developer image to a recording medium, and the transfer residual developer remaining on the surface of the charged body is removed by the transfer means without moving to the recording medium. In the image forming apparatus in which the developing unit also serves as a cleaning unit,
The charging means is a contact charging means for charging the charged body by bringing a charging member to which a voltage is applied into contact with the charged body, and the charging bias applied to the charging means is set to a DC voltage during image formation. An image forming apparatus, wherein an alternating voltage is superimposed and when the developer discharge operation is started from the developing means, the alternating voltage is turned off to switch to a DC bias and the exposure means exposes the charged surface. The charging bias applied to the charging unit during the discharging operation of the developer from the developing unit is as follows: a. | V _DC -Vs |> Vth
V _DC : DC bias on the charging polarity direction side applied to the charging means Vs: On the charging polarity direction side of the object to be charged before being charged by the charging means
2. The image forming apparatus according to claim 1, wherein a charging potential Vth: a minimum potential difference at which discharge starts between the charging unit and the member to be charged is satisfied.