JP4484901B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus having a function of forming an image on a recording medium such as a sheet, such as a copying machine, a printer, or a facsimile machine.

  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.

  A waste toner collecting container for storing toner remaining on the photosensitive member removed from the photosensitive member surface by the cleaning device is provided in the cleaning device. Therefore, in order to obtain a device having a long durability life, it is necessary to enlarge this container, which is a disadvantage in terms of downsizing the device.

  Therefore, the cleaning device having the waste toner collecting container 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. There is a type of image forming apparatus.

  Simultaneous development cleaning is the process of developing the electrostatic latent image by transferring the residual toner on the photosensitive member after transfer to the subsequent development step, that is, continuously charging the photosensitive member and exposing it to light. A fog removal bias (a fog removal potential difference Vback, which is a potential difference between the DC voltage applied to the developing device and the surface potential of the photosensitive member) during the process, causes the photosensitive member surface portion (non-image portion) that should not be developed with toner. This is a method of collecting the transfer residual toner present in the developing device.

  According to this method, since the transfer residual toner is collected by the developing device and reused for developing the electrostatic latent image in the subsequent steps, the waste toner is eliminated and the maintenance work is reduced. In addition, since there is no waste toner container, it is advantageous for downsizing the image forming apparatus.

  On the other hand, as a charging means, a corona charger has recently been replaced, and a roller charging method using a conductive roller as a contact charging member is particularly preferably used from the viewpoint of charging stability. In roller charging, a conductive elastic roller (charging roller) is brought into pressure contact with a member to be charged and a voltage is applied to the member to charge the member to be charged.

  With respect to this charging method, as disclosed in Patent Document 1, a voltage obtained by superposing an AC component having a peak-to-peak voltage of 2 × Vth or more on a DC voltage corresponding to a desired charged body surface potential Vd is applied to the contact charging member. An AC charging method is proposed and applied to practical use. Due to the leveling effect of the AC, the charge can be made more uniform than in the DC charging method, and the potential of the charged object almost converges to Vd that is the center of the peak of the AC voltage.

In the cleanerless type image forming apparatus that removes and collects the transfer residual toner on the photosensitive member after the transfer process by developing simultaneous cleaning in the developing device, when the contact charging device described above is used as a charging device for the photosensitive member, When the transfer residual toner on the photoconductor passes through the charging portion which is the contact nip portion of the photoconductor and the contact charging device, the toner in which the charge polarity in the transfer residual toner is reversed to a polarity opposite to the normal polarity is detected. The contact charging device adheres to the contact charging device and causes the toner to be contaminated more than allowable, which causes charging failure.

  This is due to the fact that the toner used as the developer contains a mixture of toners whose charging polarity is originally reversed to the opposite polarity to the normal polarity although the amount is small. This is because some toners are affected by the transfer bias, peeling discharge, and the like, and the charge polarity is reversed, and some toners are discharged to reduce the charge amount.

  In other words, the transfer residual toner includes a mixture of a normal polarity charge toner, a reverse polarity reversal toner, and a low charge amount toner, and the reversal toner and the low charge amount toner are included in the photoreceptor and the contact charging device. It is easy to adhere to the contact charging device when passing through the charging portion which is the contact nip portion.

  In addition, in order to remove and collect the transfer residual toner on the photoconductor by the simultaneous development cleaning of the developing device, the charge polarity of the transfer residual toner on the photoconductor that passes through the charging unit and is carried to the developing unit is normal. It is necessary that the toner has a polarity and the charge amount is a charge amount of toner that can develop the electrostatic latent image on the photosensitive member by the developing device. Inverted toner and toner with an inappropriate charge amount cannot be removed and collected from the photosensitive member to the developing device, causing a defective image.

  In view of this, the present applicant has disclosed in Japanese Patent Application Laid-Open No. H11-228707 an image forming system in which a transfer residual toner uniformizing unit is provided downstream of the transfer unit, and a developer charge amount control unit is provided further downstream and upstream of the charging unit for charging the photosensitive member. A device was proposed.

  In this image forming apparatus, the transfer residual toner uniformizing means disperses and distributes the pattern-like residual toner image on the image carrier surface carried on the image carrier from the transfer portion to the developer charge amount control means portion. Thus, it is a means for non-patterning. More specifically, the residual developer image pattern is scraped or disturbed by rubbing the image carrier surface with a rubbing member, and the developer is distributed and distributed on the image carrier surface.

  By having the transfer residual toner uniformizing means, the entire normal polarity charging process to the transfer residual toner is always sufficiently performed by the developer charge amount control means to which the next normal polarity is applied, and It is possible to effectively prevent adhesion to the charging means. Further, the residual toner image pattern is erased, thereby preventing the generation of a ghost image of the residual toner image pattern.

  That is, when there is no means for uniformizing the residual developer image, for example, when the transfer property of the developer image is poor such as a vertical line pattern developer image, environment, paper type (transfer paper), color secondary color, etc. The number of pattern-transfer residual toner images on the body also increases, and the transfer residual toner images are carried as they are to the developer charge amount control means, and the developer concentrates on a part of the developer charge amount control means. As a result, a phenomenon (toner charging failure phenomenon) occurs in which the charge amount of the residual developer cannot be controlled by a part of the developer charge amount control means. As a result, charging member contamination → charging failure → fogging image generation is caused, or a residual toner image pattern is left and a ghost image is generated.

By providing the residual toner image equalizing means, as described above, the pattern-like transfer residual toner image on the image carrier carried from the transfer portion to the developer charge amount control means portion has a large amount of developer. The developer is dispersed and distributed on the surface of the image bearing member and is not patterned, so that the developer is not concentrated on a part of the developer charge amount control means, and the transfer residual by the developer charge amount control means is prevented. The entire normal polarity charging process of the developer is always sufficiently performed to effectively prevent the transfer residual developer from adhering to the charging means. Generation of a ghost image of the residual developer image pattern is also strictly prevented.

  At the same time as charging the photosensitive member to a predetermined potential by the charging means, the charge amount of the transfer residual toner that has been charged with the normal polarity by the developer charge amount control means is used to develop the electrostatic latent image on the photosensitive drum by the developing device. To an appropriate amount of charge that can be developed. As a result, the transfer residual toner was efficiently collected in the developing device, and a good image free from ghosts and poor charging was obtained.

  However, when images are continuously formed with a pattern having a high printing ratio, the amount of toner accumulated in the transfer residual toner uniformizing means and developer charge amount control means increases. As a result, the respective charging ability and function are reduced, the transfer residual toner is not patterned, the transfer residual toner is insufficiently charged, and the toner adheres to the charging means.

  Therefore, in order to maintain the functions of the transfer residual toner uniformizing means and the developer charge amount control means even after a large number of high print patterns are continuously printed, this transfer residual toner image uniformizing means and developer charge amount control means It is necessary to periodically discharge the toner accumulated in the toner. For this purpose, a discharge operation is provided at the time of the initial operation at the time of paper interval or when the power is turned on or at the end of the printing operation. In the discharge operation (discharge mode), a bias that discharges the accumulated toner positively is applied. For example, it is known that it is also effective to apply a bias having a reverse polarity to the voltage applied during the printing operation, or to turn the bias ON / OFF in small increments.

As a result, when images are continuously formed with a pattern with a high printing ratio, even if the toner accumulation amount in the transfer residual toner uniformizing means and developer charge amount control means may increase, the discharge operation is performed each time. By discharging the accumulated toner, the transfer residual toner equalizing means and the developer charge amount control means are always refreshed to prevent deterioration of charging ability and function. As a result, the transfer residual toner can be made non-patterned and the transfer residual toner can be sufficiently charged, and toner adhesion to the charging means can be prevented.
JP-A 63-149669 JP 2001-215798 A

  However, the following problems occurred during the discharging operation from the transfer residual toner image equalizing means and the developer charge amount control means.

  The discharged developer (toner) adheres to the charging member, causing a problem of defective images due to defective charging and uneven charging during the next image formation.

  This is because, in the toner discharged from the transfer residual toner image equalizing means and the developer charge amount control means, in addition to the normal charge toner, the reverse charge of the reverse charge polarity, the charge amount of the normal polarity is the reverse polarity. If there is a small amount of toner and the charging voltage (AC + DC) during printing operation remains applied, reverse polarity toner or toner with a small charge amount will adhere to the charging member when passing through the nip between the contact charging member and the photosensitive member. It will end up.

  Further, the amount of the discharged toner is larger than that of the untransferred toner at the time of normal image formation, and the amount of adhesion to the charging member is increased more than usual. The toner adhering to the charging member causes poor charging and uneven charging, thereby generating an image such as a fog image.

  Therefore, in order to prevent the toner from adhering to the charging member, for example, when the discharging operation from the transfer residual toner uniformizing unit and the discharging operation from the developer charge amount control unit are performed, the DC voltage applied to the charging unit is set. A method such as switching as appropriate is conceivable.

  However, in order to execute these methods, it is necessary to know in detail the potential of the photosensitive drum during the discharging operation, and it is necessary to switch the voltage applied to the charging member in detail.

  Further, there is a problem that a power source for applying a voltage having a polarity opposite to that at the time of printing is required for the charging member, leading to an increase in the size of the entire apparatus.

  The present invention has been made to solve the above-described problems of the prior art, and an object thereof is to efficiently discharge the developer charging means (residual developer uniformization control means or developer charge amount control means). It is an object of the present invention to provide an image forming apparatus capable of performing a stable and high-quality image.

In order to achieve the above object, the present invention provides:
Times and the rolling possible image bearing member,
A charging unit that is charged with a voltage obtained by superimposing an AC voltage on a DC voltage and that contacts and charges the surface of the image carrier;
Developing means for supplying a developer to the electrostatic latent image formed on the image carrier charged by the charging means to visualize the electrostatic latent image into a developer image;
Transfer means for transferring the developer image to a transfer material;
The image carrier is disposed downstream of the transfer unit and upstream of the charging unit in the rotational direction of the image carrier, and remains on the image carrier after the developer image is transferred to a transfer material by applying a voltage. Developer charging means for charging the developer;
In an image forming apparatus comprising:
When performing the discharging operation of discharging the developer accumulated in the developer charging means onto the image carrier,
Only an AC voltage is applied to the charging means, and the potential of the image carrier on which the discharging operation is performed is set to approximately 0 V,
Causing the developer charging means to apply a voltage so that the potential of the surface of the image carrier does not fluctuate to perform a discharge operation on the image carrier;
When the surface on the image carrier on which the discharging operation has been performed is in a charging unit, the AC voltage is off, and control is performed to apply a DC voltage having the same potential as that on the image carrier to the charging means. Control means for performing is provided.

  It is possible to provide an image forming apparatus that can efficiently discharge the developer charging means (residual developer uniformization control means or developer charge amount control means) and obtain a stable high-quality image.

  The best mode for carrying out the present invention will be exemplarily described in detail below with reference to the drawings. However, the dimensions, materials, shapes, and relative arrangements of the components described in this embodiment should be appropriately changed according to the configuration of the apparatus to which the invention is applied and various conditions. It is not intended to limit the scope to the following embodiments.

  FIG. 1 is a schematic configuration diagram of a main part of an image forming apparatus (image recording apparatus) according to an embodiment of the present invention. FIG. 2 is a schematic configuration diagram of an image forming apparatus (image recording apparatus) according to an embodiment of the present invention. It is.

The color laser printer as the image forming apparatus shown in FIG. 2 is a color laser printer having a maximum sheet passing size of A3 size using a transfer type electrophotographic process, a contact charging method, a reversal development method, and a cleanerless method. A four-drum type (in-line) printer that has multiple process cartridges (hereinafter referred to as P-CRG) 8 and continuously transfers multiple images onto an intermediate transfer belt 9 that is a second image carrier to obtain a full-color print image. . The image forming operation of the image forming apparatus is controlled by the control means. The configuration of the image forming apparatus and the image forming operation will be described below.

  In FIG. 2, an endless intermediate transfer belt 9 is suspended from a drive roller 9e, a tension roller 9f, and a secondary transfer counter roller 10a, and rotates in the direction of the arrow in the figure.

  Four P-CRGs 8 are arranged in series in the order of yellow, magenta, cyan, and black in the intermediate transfer belt 9.

  Hereinafter, the P-CRG 8 will be described with reference to FIG. In the following description, P-CRG 8 for developing yellow toner will be mainly described, but the same applies to other P-CRGs.

  In P-CRG8 for developing yellow toner, reference numeral 1 denotes an electrophotographic photosensitive member (photosensitive drum) as a rotating drum type image carrier as an image carrier. The photosensitive drum 1 is an organic photoconductor (OPC) drum having an outer diameter of 50 mm, and is driven to rotate clockwise with a process speed (circumferential speed) of 100 mm / sec around a central support shaft.

  The photosensitive drum 1 has an undercoat layer for suppressing light interference and improving the adhesion of an upper layer, a photocharge generation layer, and a charge transport layer (thickness 20 μm) on the surface of an aluminum cylinder (conductive drum base). ) Are layered in order from the bottom (center side).

  In the charging step, a voltage of a predetermined condition is applied to the charging roller 2 as a contact charger as a charging means from a power source 20 as a first voltage applying means, and the negative surface is uniformly applied on the surface of the photosensitive drum 1. The charging process is performed.

  The charging roller 2 has a longitudinal length of 320 mm, and has a three-layer structure in which a lower layer, an intermediate layer, and a surface layer 2d are sequentially laminated from the bottom (center side) around the outer periphery of the cored bar (supporting member). . The lower layer is a foamed sponge layer for reducing charging noise, the intermediate layer is a resistance layer for obtaining uniform resistance as a whole charging roller, and the surface layer 2d has defects such as pinholes on the photosensitive drum 1. However, the protective layer is provided to prevent leakage.

The charging roller 2 of the present embodiment uses a stainless steel round bar with a diameter of 6 mm as a core, carbon is dispersed in a fluororesin as the surface layer 2d, the outer diameter as a roller is 14 mm, and the roller resistance is 10 ⁴ Ω to 10 ⁷ Ω.

  The charging roller 2 holds both ends of the cored bar rotatably by bearing members, and is urged toward the photosensitive drum 1 by a pressing pressure spring so as to be pressed against the surface of the photosensitive drum 1 with a predetermined pressing force. It rotates following the rotation of the photosensitive drum 1.

  A predetermined vibration voltage (bias voltage Vdc + Vac) obtained by superimposing an AC voltage having a frequency f on a DC voltage is applied from the power source 20 to the charging roller 2 through a cored bar, so that the peripheral surface of the rotating photosensitive drum 1 is It is charged to a predetermined potential.

  In the present embodiment, a DC voltage: −500 V, an AC voltage; a frequency f = 1150 Hz, a peak-to-peak voltage Vpp = 1400 V, and a vibration voltage superimposed with a sine wave. The peripheral surface of the photosensitive drum 1 is −500 V (dark potential). Vd) is uniformly contact charged.

In FIG. 2, reference numeral 2f denotes a charging roller cleaning member, which is a flexible cleaning film in the present embodiment. 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 via 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, contaminants (fine powder toner, external additives, etc.) on the charging roller surface layer 2d are removed.

  After being uniformly charged to a predetermined polarity / potential by the charging roller 2, image exposure means (not shown) (color separation / imaging exposure optical system for color original image, time-series electric digital pixel signal for image information) Corresponding to the first color component image (yellow component image) in the target color image by receiving image exposure by a scanning exposure system or other information writing means 3 that outputs a laser beam correspondingly modulated) An electrostatic latent image is formed.

  In this embodiment, a laser beam scanner using a semiconductor laser is used as an exposure device, and a laser beam modulated in response to an image signal sent from a host device such as an image reading device (not shown) to the printer side is output. Then, the uniformly charged surface of the rotating photosensitive drum 1 is subjected to laser scanning exposure (image exposure).

  As a result of the laser scanning exposure, the potential of the photosensitive drum 1 surface irradiated with the laser light is lowered, whereby an electrostatic latent image corresponding to the scanned and exposed image information is formed on the surface of the rotating photosensitive drum 1. In the present embodiment, the exposure portion potential is set to -150V.

  Next, the electrostatic latent image is developed with yellow toner which is the first color by a first developing device 4 (yellow developing device) as developing means.

  Here, the first developing device 4 (yellow developing device) as a developing device will be described with reference to FIG.

  The developing device 4 is a two-component contact developing device (two-component magnetic brush developing device). Reference numeral 40 denotes a developing container, and reference numeral 41 denotes a non-magnetic developing sleeve, which has a magnet roller (not shown) fixedly arranged inside. The developing sleeve 41 is rotatably disposed in the developing container 40 with a part of its outer peripheral surface exposed to the outside. 42 is a developer regulating blade, 45 is a two-component developer which is a mixture of toner and magnetic carrier contained in the developer container 40, and 43 and 44 are developer agitating members disposed on the bottom side in the developer container 40.

  The developing sleeve 41 is provided with a developer regulating blade 42 having a predetermined gap, and forms a thin developer layer on the developing sleeve 41 as the developing sleeve 41 rotates in the direction of arrow C.

  The developing sleeve 41 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 41 is a developing portion c. The developing sleeve 41 is driven to rotate in the direction opposite to the traveling direction of the photosensitive drum 1 in the developing unit c. The developer thin layer on the developing sleeve 41 comes into contact with the surface of the photosensitive drum 1 in the developing portion c and appropriately rubs the surface of the photosensitive drum. A predetermined developing bias is applied to the developing sleeve 41 from a power source (not shown).

In the present embodiment, the developing bias voltage for the developing sleeve 41 is an oscillating voltage obtained by superimposing a DC voltage (Vdc) and an AC voltage (Vac). More specifically, Vdc = -3
This is an oscillating voltage in which 50 V, Vac = 1800 V, and frequency = 2300 Hz are superimposed.

  Thus, the toner in the developer coated as a thin layer on the surface of the rotating developing sleeve 41 and conveyed to the developing unit c is selected on the surface of the photosensitive drum 1 corresponding to the electrostatic latent image by the electric field due to the developing bias. The electrostatic latent image is developed as a toner image. In the case of the present embodiment, negative polarity toner adheres as a predetermined magnetic pole 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 41 that has passed through the developing portion c is returned to the developer reservoir in the developing container 40 as the developing sleeve 41 continues to rotate.

  In the developing device 4, there are stirring screws 43 and 44 for stirring the developer, which rotate in synchronism with the sleeve rotation, and have a function of stirring the supplied toner and carrier and giving the toner a predetermined tribo. Yes.

  The upstream side wall surface of the screw 44 of the developing device 4 is provided with a sensor unit (not shown) that detects a change in the magnetic permeability of the developer to detect the toner concentration in the developer, and is slightly downstream of the sensor. A toner supply opening 46 is provided. After the developing operation, the developer is carried to the sensor unit, where the toner concentration is detected, and in order to maintain the toner concentration in the developer constant according to the detection result, a developer supply unit (hereinafter referred to as T -CRG) The toner is replenished from here through the toner replenishment opening 46 of the developing device 4 by the rotation of a screw (not shown).

  The replenished toner is conveyed by the screw 44, mixed with the carrier and given an appropriate tribo and then carried to the vicinity of the sleeve 41, where a thin layer is formed on the developing sleeve 41 for development.

In this embodiment, a negatively charged toner having an average particle diameter of 6 μm is used as the toner, and the carrier has a saturation magnetization of 205 emu / cm ³ (magnetization amount per 1000 gauss (0.1 T) of 56.9 Am ² / Kg (however, And a magnetic carrier having a specific gravity of 3.6 g / cm ³ )) and an average particle diameter of 35 μm. Further, a mixture of toner and carrier at a weight ratio of 6:94 was used as a developer.

  The charge amount of the toner developed on the photosensitive drum 1 is −25 μC / g.

  Next, the transfer unit will be described.

  In FIG. 2, the yellow image formed on the photosensitive drum 1 enters the primary transfer nip portion with the intermediate transfer belt 9. In the transfer nip portion, a transfer roller 9g is pressed against the back side of the intermediate transfer belt 9 with a predetermined pressing force. The transfer roller 9g has primary transfer bias sources 9a to 9d as fourth voltage application means so that a bias can be applied independently at each port. The intermediate transfer belt 9 first transfers yellow at the port of the first color, and then performs multiple transfer of each color of magenta, cyan, and black sequentially from the photosensitive drum 1 corresponding to each color through the same process described above.

  In this embodiment, considering the transfer efficiency for the toner developed on the exposed portion Vl (potential −150 V), a voltage of +350 V is applied to the first to fourth colors as the primary transfer bias. The four-color full-color image formed on the intermediate transfer belt 9 is then collectively transferred to the transfer material P sent from the paper supply roller 12 by the secondary transfer roller 10, and is fused and fixed by a fixing device (not shown). Get a print image.

  The secondary transfer residual toner remaining on the intermediate transfer belt 9 is subjected to blade cleaning by the intermediate transfer belt cleaner 11 to prepare for the next image forming process.

  In selecting the material of the transfer belt 9, in order to improve registration at each color port, a material that expands and contracts is not desirable, and a resin-based or metal cored rubber belt or resin + rubber belt is desirable.

In this embodiment, a resin belt in which carbon is dispersed in PI (polyimide) and the volume resistance is controlled to the order of 10 ⁸ Ωcm is used. The thickness is 80 μm, the longitudinal direction is 320 mm, and the entire circumference is 900 mm.

The transfer roller 9g is made of a conductive sponge. The resistance is 10 ⁶ Ω or less, the outer diameter is 16 mm, and the longitudinal length is 315 mm.

  In FIG. 1, the developer charge amount control means 6 and the residual developer uniformization control means 7 are in contact with the photosensitive drum 1 where a cleaning blade is normally disposed. Both use brush members made of conductive fibers in the present embodiment.

  Specifically, the developer charge amount control means 6 (second brush) includes a horizontally long electrode plate 62 provided with a brush portion 61, and the residual developer uniformization control means 7 (first brush). Similarly, the electrode plate 72 is provided with a brush portion 71. The brush portions 61 and 71 are disposed in contact with and fixedly supported on the surface of the photosensitive drum 1.

The brush portions 61 and 71 are formed by adding carbon or metal powder to a fiber such as rayon, acrylic, or polyester to control the resistance value. The brush portions 61 and 71 have a thickness of 30 denier or less and a density of 1550 to 77500 / cm ² (1 to 500,000 / inch ² ) so that the surface of the photosensitive drum and the transfer residual toner can be uniformly contacted. The above is preferable. In this embodiment, both the brush portions 61 and 71 are 6 denier, 15500 / cm ² (100,000 / inch ² ), the length of the bristle is 5 mm, and the resistance of the brush is 6 × 10 ³ Ω · cm. .

  The developer charge amount control means 6 and the residual developer equalization control means 7 are brought into contact with the photosensitive drum 1 so that the brush portions 61 and 71 have an intrusion amount of 1 mm with respect to the photosensitive drum 1 surface. The contact nip width with 1 was 5 mm.

  The cleanerless system of the present embodiment is realized by controlling the developer charge amount control means 6 and the residual developer uniformization control means 7 by the control means.

  The cleanerless system is a system in which the transfer residual toner remaining on the surface of the photosensitive drum 1 after the transfer passes through the charging unit a and the exposure unit b and is simultaneously cleaned (collected) by the developing device 3.

  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.

  Residual toners with normal polarity, reverse polarity (reversal toner), and low charge amount are mixed, and the reverse toner and the low charge amount toner pass through the charging part a. When the toner is attached to the charging roller 2, the charging roller 2 is contaminated with toner more than allowable, resulting in a charging failure.

Further, in order to effectively perform the simultaneous cleaning of the transfer residual toner on the surface of the photosensitive drum 1 by the developing device 3, the charging polarity of the transfer residual toner on the photosensitive drum 1 carried to the developing unit c is a normal polarity. And the charge amount of the toner needs to be the charge amount of the toner that can develop the electrostatic latent image on the photosensitive drum 1 by the developing device. Reversal toner and toner with an inappropriate charge amount cannot be removed and collected from the photosensitive drum to the developing device, causing a defective image.

  In addition, with the diversification of user needs in recent years, the above-mentioned problems are further promoted by the generation of a large amount of residual toner at one time by continuous printing operations such as images with high printing rates such as photographic images. It will end up.

  Accordingly, the transfer residual toner on the photosensitive drum 1 is made uniform at a position downstream of the transfer portion d in the rotational direction of the photosensitive drum and upstream of the charging portion a, and the negative polarity having a normal polarity as the charging polarity of the transfer residual toner. Therefore, a residual developer uniformization control means 7 and a developer charge amount control means 6 are provided.

  The residual developer equalization control means 7 is applied with a positive voltage (positive bias = + 350 V) from a power source S4 as a second voltage application means.

  Among transfer residual toners, toners of opposite polarity and those having almost no polarity are mixed, but the toner charged in the vicinity of zero to negative polarity is an electric or physical force. Thus, the residual developer equalization control means 7 sucks the toner. Then, the polarity is reversed (positive polarity) by the positive bias of the residual developer uniformization control means 7, and the toner is gradually detached and reattached onto the photosensitive drum 1 and conveyed.

  By this action, the toner distribution on the photosensitive drum 1 is made uniform, and a large amount of toner is prevented from flowing into the downstream developer charge amount control means 6 at a time. Further, the potential on the photosensitive drum 1 is around 0V. By setting the residual toner to reverse polarity (positive polarity) and setting the potential on the photosensitive drum 1 to around 0 V, the developer charge amount control means 6 can sufficiently charge the toner as will be described later. It also plays the role of

  A negative voltage (negative bias = −800 V) is applied to the developer charge amount control means 6 from a power source S5 as a third voltage application means.

  When the developer charge amount control means 6 is passed, the transfer residual toner on the photosensitive drum 1 is aligned to the negative polarity in which the charge polarity is the normal polarity. With the first brush (residual developer uniformization control means 7), the toner is aligned to a positive polarity, and the potential on the photosensitive drum 1 is also close to 0 V, so that the negative polarity is more effectively aligned. be able to.

  With this toner charge amount control means 6, the charge polarity of the transfer residual toner is aligned with the negative polarity which is the normal polarity, so that the charging unit a located further downstream from the transfer residual toner on the surface of the photosensitive drum 1. When the charging process is performed, the reflection force on the photosensitive drum 1 is increased, and the transfer residual toner is prevented from adhering to the charging roller 2. The toner that has passed without adhering to the charging roller 2 is simultaneously cleaned and collected by the developing device 4.

  In order to collect the transfer residual toner on the photosensitive drum 1 in the developing device 4 by such a method, it is necessary that the toner charge amount is appropriate.

  However, as described above, in order to prevent the toner from adhering to the charging roller 2, in order to collect the transfer residual toner charged to the negative polarity by the developer charge amount control means 6 in the developing device 4, It is necessary to do.

  The residual toner that has been negatively charged by the developer charge amount control means 6 is subjected to AC charge removal by the AC voltage (frequency f1150 Hz, Vpp 1400 V) applied to the charging roller 2 and the toner charge amount after passing through the charging portion a. Is substantially the same as the charge amount of the developing toner.

  Then, in the developing process, the transfer residual toner on the unexposed portion of the photosensitive drum 1 where the toner should not be developed is completely negative and is appropriately neutralized by the charging roller 2 and mirrored with the photosensitive drum 1. Since the force can be reduced, it is surely recovered in the developing device 4 in the relationship between the drum potential of −500 V and the DC component of the developing bias of −350 V. In the present embodiment, as described above, the developing sleeve 41 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 the recovery of.

  With the diversification of user needs in recent years, when a continuous printing operation of an image with a high printing rate such as a photographic image is performed, a large amount of transfer residual toner is generated, and the residual developer equalization control means 7 and the developer charging are performed. The amount of toner accumulated in the amount control means 6 increases. If the printing operation is continuously repeated in such a state, the function of the developer charge amount control means 6 is deteriorated, the power is insufficient, and problems such as toner contamination of the charging roller and defective collection of the developing device occur.

  Further, if a bias similar to that during normal printing is applied to the charging roller 2 during the discharging operation, the charging roller 2 is contaminated by the toner discharged from the brush, causing uneven charging and poor charging, and the next after the discharging operation. An image defect occurs on the image.

  Therefore, in order to prevent toner from adhering to the charging roller 2, the discharging operation is performed under the following conditions in the present embodiment.

  When the toner is discharged from the residual developer equalization control means 7 and the developer charge amount control means 6, in order to prevent the discharged toner from adhering to the charging roller 2, the voltage applied to the charging roller 2 is also an AC component (AC The voltage is turned off and only the DC component (Vdc1) is set. By turning off the AC voltage, the charge eliminating effect at the charging portion a is lost, and adhesion to the charging roller 2 is drastically reduced.

  Further, since the discharge toner from the residual developer uniformization control means 7 is mainly normal polarity (negative polarity), and the discharge toner from the developer charge amount control means 6 is mainly reverse polarity (positive polarity). In order to prevent the toner from adhering to the charging roller 2 regardless of which polarity of toner is discharged without complicated sequence switching, the voltage applied to the charging roller 2 during the discharging operation is the DC component Vdc1. Thus, Vdc1 is set to a value satisfying the relationship of the following expression (1) with respect to the potential Vc1 at which the surface of the photosensitive drum 1 discharged by the toner charge amount control means 6 enters the charging roller nip portion.

      Vdc1 = Vc1 (1)

  That is, the DC voltage applied to the charging roller 2 is set to the same potential as the surface of the photosensitive drum 1.

  However, if the discharging operation is performed while the photosensitive drum 1 is charged, the following problem may occur.

When the discharging operation is performed on the photosensitive drum potential after the transfer, a discharge occurs in the residual developer equalization control means 7 and the developer charge amount control means 6, disturbing the potential on the surface of the photosensitive drum 1, and the photosensitive drum. 1 and the charging roller 2 do not have the same potential, and the discharged toner adheres to the charging roller 2.

  This is because the voltage applied to the residual developer equalization control means 7 at the time of discharge is positive, and thus discharge occurs when the photosensitive drum 1 is charged to a negative polarity of a certain potential or more. However, the voltage that is turned ON / OFF by the residual developer equalization control means 7 at the time of discharge can be discharged more efficiently if the voltage is somewhat large.

  Therefore, in the present embodiment, the discharge operation is performed by the following method.

  As a pre-operation for performing the discharging operation of the residual developer uniformization control means 7 and the developer charge amount control means 6 substantially simultaneously, an operation of applying only an AC voltage to the charging roller 2 to make the potential of the photosensitive drum 1 approximately 0V. When the discharge operation is performed, the voltage applied to the residual developer uniformization control means 7 and the developer charge amount control means 6 is a voltage that does not change the potential of the surface of the photosensitive drum 1 of approximately 0 V by the discharge operation. In this embodiment, the charging roller 2 is set to approximately 0 V with respect to the discharged photosensitive drum 1 surface.

  Here, the voltage at which the potential of the surface of the photosensitive drum 1 of approximately 0 V does not fluctuate due to the discharging operation is preferably a voltage equal to or lower than the charging start voltage during the image forming operation.

  Thereby, it becomes possible to satisfy | fill the relationship of Formula (1) continuously during discharge operation.

  Most of the discharged toner is basically recovered by electrostatic and physical rubbing by a developing device using contact two-component counter development. However, there is a case where the developer is not completely recovered for a toner having a small charge amount or a reverse polarity toner even with a normal polarity. However, these are transferred to the intermediate transfer belt from the photosensitive drum by pressure transfer at the transfer nip, and are removed from the belt by the intermediate transfer belt cleaner. During the discharging operation of the developer charge amount control means 6, if the transfer bias is applied with a voltage having a polarity opposite to the normal polarity (negative polarity) by the primary transfer bias sources 9a to 9d, the positive polarity toner is easily transferred. It is effective.

  As a result, it is possible to allow the discharged toner having both normal polarity and reverse polarity to pass through the charging nip portion without adhering to the charging roller 2.

  By performing the discharging operation under such conditions, it is possible to prevent the charging roller from being contaminated by the discharged toner and to always perform good image formation.

  Next, when the discharging operations of the residual developer uniformization control means 7 and the developer charge amount control means 6 are performed substantially simultaneously, the toner discharged from the residual developer uniformization control means 7 is supplied to the developer charge amount control means 6. In order to prevent adhesion and perform a more efficient discharging operation, the potential Vc2 at which the surface of the photosensitive drum 1 discharged by the residual developer equalization control means 7 enters the nip portion of the developer charge amount control means 6 In contrast, the voltage Vb applied to the developer charge amount control means 6 is set to a value satisfying the relationship of the following expression (2).

      Vc2 = Vb (2)

  That is, the voltage applied to the developer charge amount control means 6 has the same potential as the surface of the photosensitive drum 1 from which the residual developer equalization control means 7 has been discharged. Here, Vc2 = 0V, so Vb = 0V.

In order to perform the discharge operation of the residual developer uniformization control means 7 and the developer charge amount control means 6 substantially simultaneously while satisfying this condition, for example, in this embodiment, the residual developer uniformization means 7 is turned on / off. When the discharging operation is performed by repeating the operation, the developer charge amount with respect to the section of the photosensitive drum 1 surface that has passed through the residual developer uniformizing means 7 when the residual developer uniformizing control means 7 is OFF. For example, a discharge operation of the control means 6 can be considered.

  Specifically, the residual developer equalization control means 7 is turned ON / OFF 10 times at intervals of 50 msec, and the residual developer uniformity control means 7 is discharged, and the residual developer uniformity control means 7 is turned OFF. The discharged portion from the developer charge amount control means 6 was discharged. The state at this time is shown in FIG.

  First, the surface of the photosensitive drum 1 that has passed through the residual developer uniformization control unit 7 for 50 msec when the bias applied to the residual developer uniformization control unit 7 is OFF is the developer charge amount control unit 6 nip portion. The developer charge amount control means 6 was turned on for 30 msec during the passage.

  FIG. 3A shows the timing for turning on / off each unit during the discharging operation of the residual developer equalization control unit 7 and the developer charge amount control unit 6 with the horizontal axis representing time and the vertical axis representing voltage. It is. The time t1 in the figure is the time it takes for the surface on the photosensitive drum 1 on which the residual developer equalization control means 7 has discharged to enter the developer charge amount control means 6, and the residual developer uniformity When the control means 7 is OFF, the developer charge amount control means 6 performs the discharging operation on the surface on the photosensitive drum 1 that has passed through the residual developer uniformization control means 7.

  In FIG. 3 (b), the horizontal axis is the traveling direction of the surface on the photosensitive drum 1, and the vertical axis is the voltage. When the residual developer equalization control means 7 and the developer charge amount control means 6 are discharged, each means is turned on. This shows the timing of turning off / off. With respect to the traveling direction of the photosensitive drum 1, the surface discharged from the residual developer uniformization control unit 7 and the surface discharged from the developer charge amount control unit 6 appear alternately.

  As a result, the toner discharged from the residual developer uniformization control means 7 can be discharged without reattaching to the developer charge amount control means 6.

  Most of the discharged toner is basically recovered by electrostatic and physical rubbing by a developing device using contact two-component counter development. However, there is a case where the developer is not completely recovered for a toner having a small charge amount or a reverse polarity toner even with a normal polarity. However, these are transferred to the intermediate transfer belt 9 from the photosensitive drum 1 by pressure transfer at the transfer nip, and are removed from the belt by the intermediate transfer belt cleaner 11. During the discharging operation of the developer charge amount control means 6, applying a voltage having a polarity (negative polarity) opposite to the normal polarity to the transfer bias is more effective because it facilitates transfer of positive polarity toner.

  By performing the discharge operation under such conditions, the charging roller is prevented from being contaminated by the discharged toner, and the remaining developer uniformization control means 7 and the developer charge amount control means 6 are discharged efficiently to maintain the function. Since the discharge operation time could be shortened while always performing good image formation, it became possible to insert the discharge operation between sheets.

  Specifically, in addition to the initial operation when the power is turned on or at the end of the printing operation, for example, it is possible to input the discharge operation for every 100 A4 images. This makes it possible to prevent toner accumulation in the residual developer uniformization control means 7 and the developer charge amount control means 6 even when several hundred sheets are continuously printed.

As described above, in this embodiment, when discharging from the residual developer equalization control unit and the developer charge amount control unit, the discharging operation is performed after the potential of the photosensitive drum is set to 0 V in advance. At this time, the voltage at which the residual developer uniformization control means and the developer charge amount control means perform the discharge operation is a voltage that does not fluctuate from 0V on the photosensitive drum. Then, by applying a voltage to the developer charge amount control means so as to have the same potential with respect to the surface of the photosensitive drum on which the residual developer uniformizing operation has been performed, the residual developer uniformization control means It is possible to prevent the discharged toner from re-adhering to the developer charge amount control means, and the residual charge is made uniform by setting the contact charging member to 0 V and the photosensitive drum and the contact charging member to the same potential. By discharging the control unit and the developer charge amount control unit at the same time, even if normal toner and reverse polarity toner are mixed in the discharged toner, it is possible to prevent contamination of the contact charging member by the discharged toner. Thus, it is possible to eliminate the occurrence of image defects due to contamination of the contact charging member in the image after the discharge operation. As a result, the printing time can be shortened while maintaining a high-quality image at all times without requiring a large-scale configuration.

  The residual developer uniformization control means 7 and the developer charge amount control means 6 are fixed brush-like members in the present embodiment, but may be of any form such as a brush rotating body, an elastic roller body, or a sheet-like member. It can be a member.

Further, the image carrier may be of a direct injection charging type provided with a charge injection layer having a surface resistance of 10 ⁹ to 10 ¹⁴ Ω · cm. Even when the charge injection layer is not used, for example, the same effect can be obtained when the charge transport layer is in the above resistance range. An amorphous silicon photoreceptor having a surface layer volume resistance of about 10 ¹³ Ω · cm may be used.

  As the charging means, in addition to the charging roller, a flexible contact charging member having a shape and material such as a fur brush, felt, and cloth can be used. Further, a combination of various materials can provide more appropriate elasticity, conductivity, surface property, and durability.

  Further, as a waveform of an alternating voltage component (AC component, voltage whose voltage value periodically changes) of the oscillating electric field applied to the contact charging member or the developing member, a sine wave, a rectangular wave, a triangular wave, or the like can be used as appropriate. It may be a rectangular wave formed by periodically turning on / off a DC power supply.

  Further, the image exposure means as the information writing means for the charging surface of the photosensitive drum as the image carrier is not only the laser scanning means of the present embodiment but also a digital exposure means using a solid light emitting element array such as an LED. It may be. An analog image exposure unit using a halogen lamp or a fluorescent lamp as a document illumination light source may be used. In short, any device capable of forming an electrostatic latent image corresponding to image information may be used.

  According to the present invention, it is possible to prevent the toner discharged from the residual developer uniformization control unit from re-adhering to the developer charge amount control unit, and the residual developer uniformization control unit and the developer charging unit. Since the discharge from the amount control unit is performed at the same time, and even if the normal polarity toner and the reverse polarity toner are mixed in the discharged toner, it is possible to prevent the charging unit from being contaminated by the discharged toner. The discharge control means and the developer charge amount control means can be efficiently discharged, and it is possible to suppress the occurrence of image defects due to the contamination of the charging means in the image after the discharge operation. Accordingly, it is possible to shorten the printing time while always maintaining a high-quality image without requiring a large-scale configuration.

1 is a schematic cross-sectional view of a main part of an image forming apparatus according to an embodiment. 1 is a schematic cross-sectional view of an image forming apparatus according to an embodiment. It is a figure for demonstrating the mode of the discharge operation | movement in embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Photosensitive drum 2 Charging roller 2d Surface layer 2f Charging roller cleaning member 2g Support member 3 Image exposure means 4 Developer 5 Developer supply unit 6 Developer charge amount control means 7 Transfer residual developer image equalization means 8 Process cartridge 9 Intermediate Transfer belt 9a to 9d Primary transfer bias source 9g Transfer roller 10 Secondary transfer roller 11 Intermediate transfer belt cleaner 12 Paper feed roller 20 Power supply 40 Developer container 41 Developer sleeve 42 Developer regulating blade 43, 44 Developer stirring member 45 Developer 46 Toner supply opening 61, 71 Brush part 62, 72 Electrode plate S4, S5 Power supply a Charging part b Exposure part c Development part d Transfer part

Claims (7)

A rotatable image carrier;
A charging unit that is charged with a voltage obtained by superimposing an AC voltage on a DC voltage and that contacts and charges the surface of the image carrier;
Developing means for supplying a developer to the electrostatic latent image formed on the image carrier charged by the charging means to visualize the electrostatic latent image into a developer image;
Transfer means for transferring the developer image to a transfer material;
The image carrier is disposed downstream of the transfer unit and upstream of the charging unit in the rotational direction of the image carrier, and remains on the image carrier after the developer image is transferred to a transfer material by applying a voltage. Developer charging means for charging the developer;
In an image forming apparatus comprising:
When performing the discharging operation of discharging the developer accumulated in the developer charging means onto the image carrier,
Only an AC voltage is applied to the charging means, and the potential of the image carrier on which the discharging operation is performed is set to approximately 0 V,
Causing the developer charging means to apply a voltage so that the potential of the surface of the image carrier does not fluctuate to perform a discharge operation on the image carrier;
When the surface on the image carrier on which the discharging operation has been performed is in a charging unit, the AC voltage is off, and control is performed to apply a DC voltage having the same potential as that on the image carrier to the charging means. An image forming apparatus comprising a control means for performing. The image forming apparatus according to claim 1 , wherein the developer charging unit is configured to apply a voltage equal to or lower than a charging start voltage to perform a discharging operation. 3. A voltage having a polarity opposite to a normal polarity of the developer is applied to the transfer unit when the surface of the image carrier on which the discharging operation has been performed is in a transfer portion. Image forming apparatus. The control unit discharges the developer accumulated in the developer charging unit by switching a voltage applied to the developer charging unit by a voltage applying unit at a predetermined timing other than during image formation. the image forming apparatus according to any one of claims 1 to 3. The control means includes
2. The developer accumulated in the developer charging unit is discharged by repeatedly performing an on / off operation of a voltage applied to the developer charging unit except during an image forming operation. 5. The image forming apparatus according to any one of 4 above. The developer charging means includes a conductive fiber brush portions, an image forming apparatus according the conductive fiber brush portion to any one of claims 1 to 5, characterized in that contacting with the image bearing member. The developing unit is an image forming apparatus according to any one of claims 1 to 6, characterized in that it is possible to recover the developer discharged from the developer charging means.

Images